Variants of the Gamma Chain of AMPX, DNA Sequences Encoding the Same, and Uses Thereof

Abstract

Variants of the gamma chain of vertebrate AMP-activated kinase (AMPK), as well as nucleic acid sequences encoding the variants and use thereof for the diagnosis or treatment of dysfunction of energy metabolisms.

Description

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the results of linkage disequilibrium analysis done with the markers shown in Table 1 and a random sample of 68 breeding boars from the Swedish Hampshire population, scored for the RN phenotype by measuring glycogen content in muscle. (22:11-13)

FIG. 2 is a sequence of a pig cDNA sequence (SEQ ID NO:1) having highly significant sequence similarity to various AMP-activated protein kinase γ sequences, including the yeast SNF4 sequence. 5′ UTR: 5′ untranslated region; 3′ UTR: 3′ untranslated region; CDS: coding sequence; ***; stop codon; ‘-’identity to master sequence; ‘.’: alignment gap. (23:12-26)

FIG. 3 is an alignment of human (SEQ ID NO:30) and porcine (SEQ ID NO:28) PRKAG3 sequences with other AMPK γ sequences. Sequences used: HumG1: Genbank U42412; MusG1: Genbank AF036535; HumG2: Human PRKAG2 (Genbank AJ249976); PigG3: pig PRKAG3 (this study); HumG3: human PRKAG3 (this study); Dros: Drosophila (Genbank AF094764); SNF4 (yeast): Genbank M30470. *: stop codon; ‘-’: identity to master sequence; ‘.’: alignment gap. The four CBS domains are overlined and the position of the RN⁻ mutation is indicated by an arrow. (8:20-22; 24:3-24)

FIG. 4 shows a Neighbor-Joining phylogenetic tree constructed using yeast SNF4 as outgroup; support for branch orders obtained in bootstrap analysis with 1,000 replicates are indicated, and the scale of the tree is indicated at the bottom. (25:2-9)

FIG. 5 shows the result of hybridisation analysis of a human multiple tissue northern blot using human PRKAG1, human PRKAG2, and porcine PRKAG3 probes. H: Heart; B: Brain; Pl: Placenta; L: Lung; Li: Liver; M: Skeletal muscle; K: Kidney; Pa: Pancreas; S: Spleen; Th: Thymus; P: Prostate; T: Testis; O: Ovary; I: Small intestine; C: Colon (mucosal lining); PBL: Peripheral Blood Leukocyte. (26:9-25)

FIG. 6 is a series of graphs showing typical results of oligonucleotide ligation assays for the three possible genotypes of the R41Q mutation (A/A, A/G, and G/G). Genotypes were determined using DNA samples collected from 68 Swedish Hampshire animals phenotyped as either RN⁻ or rn⁺ based on their glycolytic potential value. All RN⁻ animals were scored as homozygous A/A (n=28) or heterozygous A/G (nq36) at nucleotide position 122, whereas the rn⁺ animals were homozygous G/G (n=4) at this position. (31:29-32:2).

The present invention relates to new variants of the γ chain of AMP-activated protein kinase (AMPK), to genes encoding said variants and to uses thereof.

AMPK has a key role in regulating the energy metabolism in the eukaryotic cell (HARDIE et al., Annu. Rev. Biochem., 67, 821-855, 1998; KEMP et al., TIBS, 24, 22-25, 1999). Mammalian AMPK is a heterotrimeric complex comprising a catalytic α subunit and two non-catalytic β and γ subunits that regulate the activity of the α subunit. The yeast homologue denoted SNF1) of this enzyme complex is well characterised; it comprises a catalytic chain (Snf1) corresponding to the mammalian α subunit, and regulatory subunits: Sip1, Sip2 and Gal83 correspond to the mammalian β subunit, and Snf4 correspond to the mammalian γ subunit. Sequence data show that AMPK homologues exist also in Caenorhabditis elegans and Drosophila.

It has been observed that mutations in yeast SNF1 and SNF4 cause defects in the transcription of glucose-repressed genes, sporulation, thermotolerance, peroxisome biogenesis, and glycogen storage.

In the mammalian cells, AMPK has been proposed to act as a “fuel gauge”. It is activated by an increase in the AMP:ATP ratio, resulting from cellular stresses such as heat shock and depletion of glucose and ATP. Activated AMPK turns on ATP-producing pathways (e.g. fatty acid oxidisation) and inhibits ATP-consuming pathways (e.g. fatty acid and cholesterol synthesis), through phosphorylation of the enzymes acetyl-CoA carboxylase and hydroxymethylglutaryl-CoA (HMG-CoA) reductase. It has also been reported to inactivate in vitro glycogen synthase, the key regulatory enzyme of glycogen synthesis, by phosphorylation (HARDIE et al., 1998, supra); however, whether glycogen synthase is a physiological target of AMPK in vivo remained unclear.

Several isoforms of the three different AMPK subunits are present in mammals. In humans, PRKAA1 on human chromosome (HSA) 5p12 and PRKAA2 on HSA1p31 respectively encode isoforms α1 and α2 of the α subunit, PRKAB1 on HSA12q24.1 and PRKAB2 (not yet mapped) respectively encode isoforms β1 and β2 of the β subunit, and PRKAG1 on HSA12q13.1 and PRKAG2 on HSA7q35-q36 respectively encode isoforms γ1 and γ2 of the γ subunit (OMIM database, available on the World Wide Web at ncbi.nlm.nih.gov/omim, July 1999). HARDIE et al., [1998, supra] also mention the existence of a third isoform (γ3) of the γ subunit of AMPK but do not provide any information about it. Analysis of the sequences of these γ subunits shows that they are essentially composed of four cystathione β synthase (CBS) domains whose function is unknown. No phenotypic effect resulting from a mutation in either of the AMPK subunits has yet been documented.

On the other hand, it has been observed that most Hampshire pigs have a high intramuscular glycogen concentration. In these pigs, glycogenolysis which occurs after slaughtering leads to an important decrease of the pH, resulting in acid meat having a reduced water-holding capacity and giving a reduced yield of cured cooked ham.

The locus (named RN) associated with high muscular content of glycogen was first identified by family segregation analysis of phenotypic data from Hampshire pigs (LE ROY et at., Genet. Res., 55, 33-40, 1990). A fully dominant allele, RN, correlated with high glycogen content occurs at a high frequency in most Hampshire populations while pigs from other breeds are assumed to be homozygous for the normal, recessive rn⁺ allele. Subsequent studies showed that RN⁻ carriers have a large increase (about 70%) of glycogen in skeletal muscle but not in liver (MONIN et al., in 38^th ICoMST, Clermont-Ferrand, FRANCE, 1992).

The large difference in glycogen content between RN⁻ and rn⁺ pigs leads to marked differences in meat quality and technological yield (ENFÄLT et al., J. Anim. Sci., 75, 2924-2935, 1997). The RN⁻ allele is therefore of considerable economical significance in the pig industry and most breeding companies would like to reduce or eliminate this dominant mutation.

The RN phenotype can be determined by measuring the glycolytic potential in muscle biopsies from live animals, or after slaughter (MONIN et al., Meat Science, 13, 49-63, 1985). However, this method has severe limitations for application in practical breeding programs. The accuracy of the test is not 100%: as there is some overlap in the phenotypic distribution of RN⁻ and rn⁺, the test is not able to distinguish RN⁻/RN⁻ homozygotes and RN⁻/rn⁺ heterozygotes. Further, the sampling of muscle biopsies on live animals is invasive and costly.

Thus, there is a strong need for the development of a simple diagnostic DNA test for the RN locus. Moreover, the dramatic phenotypic effect of the RN gene in pigs implies that this gene has an important role in the regulation of carbohydrate metabolism in skeletal muscle in other vertebrates, in particular mammals.

Skeletal muscle and liver are the two major reservoirs of glycogen in mammals and the observation of an increased muscular glycogen while liver glycogen is normal suggests that the RN⁻ phenotype maybe due to a mutation in a gene expressed in muscle but not in liver. The inventors have previously reported that the RN gene is located on pig chromosome 15 (MILAN et al., Mamm. Genome, 7, 47-51, 1996; MARIANI et al., Mamm. Genome, 7, 52-54, 1996; LOOFT et al., Genetics Selection Evolution, 28, 437-442, 1996). They have now discovered that the RN⁻ allele is associated with a non-conservative mutation in a gene encoding a new muscle-specific isoform of the AMP-activated protein kinase (AMPK) γ chain.

The various aspects of the present invention are based upon the discovery and characterisation of this mutation and the identification and isolation of the mutant gene.

According to the invention it is shown that a mutation in a γ chain of AMPK results in an altered regulation of carbohydrate metabolism, demonstrating that AMPK is an essential component of said metabolism. It is also provided a nucleic acid sequence encoding a muscle-specific isoform of the γ chain of AMPK. Thus it is provided means to regulate carbohydrate metabolism, more specifically to detect and/or correct potential or actual dysfunctions of the regulation of carbohydrate metabolism, in particular in skeletal muscle.

The invention provides a polypeptide comprising an amino acid sequence having at least 70% identity or at least 85% similarity, preferably 80% identity or at least 90% similarity, more preferably at least 90% identity or at least 95% identity or at least 99% similarity, with the polypeptide SEQ ID NO: 2. The invention also provides an isolated nucleic acid sequence encoding said polypeptide, as well as the complement of said nucleic sequence.

Said polypeptide represents a new muscle-specific isoform of the γ chain of AMPK, and will also be hereinafter referred as PRKAG3; the gene encoding said polypeptide will also be hereinafter referred as PKAG3.

According to a preferred embodiment of the invention, said polypeptide comprises an amino acid sequence having at least 75% identity, preferably at least 80% identity with the polypeptide SEQ ID NO: 28.

“Identity” of a sequence with a reference sequence refers to the percent of residues that are the same when the two sequences are aligned for maximum correspondence between residues positions. A polypeptide having an amino acid sequence having at least X % identity with a reference sequence is defined herein as a polypeptide whose sequence may include up to 100-X amino acid alterations per each 100 amino acids of the reference amino acid sequence. Amino acids alterations include deletion, substitution or insertion of consecutive or scattered amino acid residues in the reference sequence.

“Similarity” of a sequence with a reference sequence refers to the percent of residues that are the same or only differ by conservative amino acid substitutions when the two sequences are aligned for maximum correspondence between residues positions. A conservative amino acid substitution is defined as the substitution of an amino acid residue for another amino acid residue with similar chemical properties (e.g. size, charge or polarity), which generally does not change the functional properties of the protein. A polypeptide having an amino acid sequence having at least X % similarity with a reference sequence is defined herein as a polypeptide whose sequence may include up to (100-X) non-conservative amino acid alterations per each 100 amino acids of the reference amino acid sequence. Non-conservative amino acids alterations include deletion, insertion, or non-conservative substitution of consecutive or scattered amino acid residues in the reference sequence.

For instance:

• • searching the “GenBank nr” database using BLASTp (ALTSCHUL et al., Nucleic Acids Res., 25, 3389-3402, 1997) with default settings and the whole sequence SEQ ID NO: 2 as a query, the higher percents of identity or similarity with SEQ ID NO: 2 were found for:
  • γ1 subunit of human AMPK: 65% identity or 82% similarity (score: 399);
  • γ1 subunit of rat AMPK: 65% identity or 82% similarity (score: 399);
  • γ1 subunit of murine AMPK: 64% identity or 80% similarity (score: 390);
  • γ subunit of Drosophila AMPK: 53% identity or 75% similarity (score: 332);
  • Yeast Snf4: 33% identity or 56% similarity (score: 173);
  • searching the “GenBank nr” database using BLASTp with default settings and the whole sequence SEQ ID NO: 28 as a query, the higher percents of identity or similarity were found for:
  • γ1 subunit of human AMPK: 64% identity or 80% similarity (score: 403);
  • γ2 subunit of human AMPK: 62% identity or 83% similarity (score: 425);
  • γ1 subunit of rat AMPK: 61% identity or 77% similarity (score: 404);
  • γ1 subunit of murine AMPK: 63% identity or 79% similarity (score: 394);
  • γ subunit of Drosophila AMPK: 52% identity or 76% similarity (score: 340).

Polypeptides of the invention include for instance any polypeptide (whether natural, synthetic, semi-synthetic, or recombinant) from any vertebrate species, more specifically from birds, such as poultry, or mammals, including bovine, ovine, porcine, murine, equine, and human, and comprising, or consisting of, the amino acid sequence of either:

• • a functional PRKAG3; or
  • a functionally altered mutant of PRKAG3.

“Functional” refers to a protein having a normal biological activity. Such a protein may comprise silent mutations inducing no substantial change in its activity, and having no noticeable phenotypic effects. Non-limitative examples of functional PRKAG3 are:

• • a porcine PRKAG3 comprising at least the sequence represented in the enclosed sequence listing under SEQ ID NO: 2; this includes, for instance the polypeptide SEQ ID NO: 28;
  • a human PRKAG3 comprising at least the sequence represented in the enclosed sequence listing under SEQ ID NO: 4; this include for instance the polypeptide SEQ ID NO: 30.

The invention also includes splice variants of PRKAG3: for instance, the nucleotide sequence SEQ ID NO: 27, and the corresponding amino-acid sequence SEQ ID NO: 28 on one hand, and the nucleotide sequence SEQ ID NO: 31 and the corresponding amino-acid sequence SEQ ID NO: 32 on the other hand represent two different splice variants or porcine PRKAG3.

A “functionally altered mutant” of a protein comprises one or several mutations inducing a change in its activity. Such mutations include in particular deletions, insertions, or substitutions of amino acid residues in a domain essential for the biological activity of said protein. They may result for instance in a partial or total loss of activity, or conversely in an increase of activity, or in an impairment of the response to regulatory effectors. Deletions, insertions, or non-conservative substitutions are more likely to result in a critical effect on the biological activity; however conservative substitutions may also induce a noticeable effect, if they occur at an important position of an active site of the protein.

Non-limitative examples of functionally altered mutants of PRKAG3 are:

• • the R41Q variant resulting from the non-conservative substitution of an arginine residue in position 41 of SEQ ID NO: 2 or SEQ ID NO: 4 by a glutamine residue (this substitution results in an important increase of the glycogen content, inducing an increased glycolytic potential of the skeletal muscle);
  • the V40I variant resulting from the substitution of a valine residue in position 40 of SEQ ID NO: 2 or SEQ ID NO: 4 by an isoleucine residue (this substitution results in a decrease of the glycogen content and thus of the glycolytic potential of the skeletal muscle).

These substitutions occur inside a portion of the first CBS domain that is highly conserved between PRKAG3 and the previously known isoforms of the γ subunit of AMPK.

Residue numbers for PRKAG3 refer to the amino acid numbering of SEQ ID NO: 2 or SEQ ID NO: 4. Alignment of human and porcine PRKAG3 sequences with previously known γ1 and γ2 isoforms is shown in FIG. 3.

The invention also provides mutants of PRKAG3 which may for instance be obtained by deletion of part of a PRKAG3 polypeptide. Said mutants are generally functionally altered. They may have identity with the overall PRKAG3 sequence lower than 70%. However, the identity of the non-deleted sequences of said mutants, when aligned with the corresponding PRKAG3 sequences and more specifically with the corresponding sequences from SEQ ID NO: 2, should remain higher than 70%. Said mutants may for instance result from the expression of nucleic acid sequences obtained by deletion or insertion of a nucleic acid segment, or by a punctual mutation introducing a nonsense codon; in a nucleic acid sequence encoding a functional PRKAG3.

The invention also provides a functionally altered mutant of a γ subunit of AMPK, wherein said mutant comprises at least one mutation responsible for said functional alteration located within the first CBS domain, and preferably within the region thereof aligned with the region spanning from residue 30 to residue 50 of SEQ ID NO: 2 or SEQ ID NO: 4. Said mutation may result from the insertion, deletion, and/or substitution of one amino-acid or of several amino-acids, adjacent or not. More preferably the mutation is located within the region aligned with the region spanning from residue 35 to residue 45 of SEQ ID NO: 2 or SEQ ID NO: 4, for instance within the region spanning from residue 65 to residue 75 of the γ1 isoform.

According to a particular embodiment, said mutation is a non-conservative substitution, preferably a R→Q substitution. According to another particular embodiment, said mutation is a conservative substitution, preferably a V→I substitution.

Advantageously, the mutation is located at a residue corresponding to residue 41 of SEQ ID NO: 2 or SEQ ID NO: 4, for instance in the case of the γ1 isoform, at residue 70, or at a residue corresponding to residue 40 of SEQ ID NO: 2 or SEQ ID NO: 4, for instance in the case of the γ1 isoform, at residue 69.

The invention also provides a heterotrimeric AMPK wherein the γ subunit consists of a polypeptide of the invention.

The invention also provides isolated nucleic acid sequences encoding any of the above-defined functional or functionally altered PRKAG3 or functionally altered mutants of the γ subunit of AMPK, and nucleic acid sequences complementary of any one of these nucleic acid sequences.

This includes particularly any isolated nucleic acid having the sequence of any of the naturally occurring alleles of a PRKAG3 gene, as well as any isolated nucleic acid having the sequence of an artificial mutant of a PRKAG3 gene, provided that said nucleic acid does not consist of the EST GENBANK AA178898.

This also includes any isolated nucleic acid having the sequence of a natural or artificial mutant of a PRKAG1 or a PRKAG2 gene, wherein said mutant encodes a functionally altered γ1 or γ2 subunit of the AMPK as defined above.

Nucleic acids of the invention may be obtained by the well-known methods of recombinant DNA technology and/or of chemical DNA synthesis. These methods also allow to introduce the desired mutations in a naturally occurring DNA sequence.

Examples of the nucleic acids encoding naturally occurring alleles of a PRKAG3 gene are represented by SEQ ID NO: 1, which encodes a naturally occurring allele of the porcine gene and SEQ ID NO: 3, which encodes a naturally occurring allele of the human gene. These sequences may be used to generate probes allowing the isolation of PRKAG3 from other species or of other allelic forms of PRKAG3 from a same species, by screening a library of genomic DNA or of cDNA.

The invention also includes genomic DNA sequences from any vertebrate species, more specifically from birds, such as poultry, or mammals, including in particular bovine, ovine, porcine, murine, equine, and human, comprising at least a portion of a nucleic acid sequence encoding a polypeptide of the invention, preferably a portion of a PRKAG3 gene, and up to 500 kb, preferably up to 100 kb of a 3′ and/or of a 5′ adjacent genomic sequence.

Such genomic DNA sequences may be obtained by methods known in the art, for instance by extension of a nucleic acid sequence encoding a polypeptide of the invention, employing a method such as restriction-site PCR (SARKAR et al., PCR Methods Applic., 2, 318-322, 1993), inverse PCR (TRIGLIA et al., Nucleic Acids Res., 16, 8186, 1988) using divergent primers based on a PRKAG3 coding region, capture PCR (LAGERSTROM et al., PCR Methods Applic., 1, 111-119, 1991), or the like.

The invention also includes specific fragments of a nucleic acid sequence encoding a polypeptide of the invention, or of a genomic DNA sequence of the invention as well as nucleic acid fragments specifically hybridising therewith. Preferably these fragments are at least 15 bp long, more preferably at least 20 bp long.

“Specific fragments” refers to nucleic acid fragments having a sequence that is found only in the nucleic acids sequences encoding a polypeptide of the invention, and is not found in nucleic acids sequences encoding related polypeptides of the prior art. This excludes the nucleic acid fragments that consist of a sequence shared with one of the known PRKAG1 or PRKAG2 genes.

“Specifically hybridising fragments” refers to nucleic acid fragments which can hybridise, under stringent conditions, only with nucleic acid sequences encoding a polypeptide of the invention, without hybridising with nucleic acid sequences encoding related polypeptides of the prior art. This excludes the nucleic acid fragments that consist of the complement of a sequence with one of the known PRKAG1 and PRKAG2 genes.

Nucleic acid fragments that consist of the EST GENBANK AA178898 or the EST GENBANK W94830 or the complements thereof are also excluded.

Said specific or specifically hybridising nucleic acid fragments may for example be used as primers or probes for detecting and/or amplifying a nucleic acid sequence encoding a polypeptide of the invention. The invention encompasses set of primers comprising at least one primer consisting of a specific or specifically hybridising nucleic acid fragment as defined above.

The invention also provides recombinant vectors comprising a nucleic acid sequence encoding a polypeptide of the invention. Vectors of the invention are preferably expression vectors, wherein a sequence encoding a polypeptide of the invention is placed under control of appropriate transcriptional and translational control elements. These vectors may be obtained and introduced in a host cell by the well-known recombinant DNA and genetic engineering techniques.

The invention also comprises a prokaryotic or eukaryotic host cell transformed by a vector of the invention, preferably an expression vector.

A polypeptide of the invention may be obtained by culturing the host cell containing an expression vector comprising a nucleic acid sequence encoding said polypeptide, under conditions suitable for the expression of the polypeptide, and recovering the polypeptide from the host cell culture.

A heterotrimeric AMPK wherein the γ subunit consists of a polypeptide of the invention may be obtained by expressing, together or separately, a nucleic acid sequence encoding a polypeptide of the invention, a nucleic acid sequence an α subunit, and a nucleic acid sequence encoding β subunit, and reconstituting the heterotrimer.

The polypeptides thus obtained, or immunogenic fragments thereof may be used to prepare antibodies, employing methods well known in the art. Antibodies directed against the whole PRKAG3 polypeptide and able to recognise any variant thereof may thus be obtained. Antibodies directed against a specific epitope of a particular variant (functional or not) of PRKAG3 or antibodies directed against a specific epitope of a functionally altered mutant having a mutation in the first CBS domain of a γ subunit of AMPK, and able to recognise said variant or functionally altered mutant may also be obtained.

As shown herein, mutations in a γ subunit of AMPK, and particularly mutations in the first CBS domain of a γ subunit of AMPK are likely to cause disorders in the energy metabolism (e.g. diabetes, obesity) in vertebrates, including humans. Further, mutations in the first CBS domain or other parts of the PRKAG3 gene are likely to cause disorders in the muscular metabolism leading to diseases such as myopathy, diabetes and cardiovascular diseases.

The present invention provides means for detecting and correcting said disorders.

More specifically, the present invention is directed to methods that utilise the nucleic acid sequences and/or polypeptidic sequences of the invention for the diagnostic evaluation, genetic testing and prognosis of a metabolic disorder.

For example, the invention provides methods for diagnosing of metabolic disorders, more specifically carbohydrate metabolism disorders, and preferably disorders correlated with an altered, in particular an excessive, glycogen accumulation in the cells, resulting from a mutation in a gene encoding a γ subunit of AMPK, wherein said methods comprise detecting and/or measuring the expression of a functionally altered PRKAG3 gene, or of a functionally altered mutant of a γ subunit of AMPK having a mutation within the first CBS domain in a nucleic acid sample obtained from a vertebrate, or detecting a mutation in the PRKAG3 gene or in a sequence encoding the first CBS domain of a γ subunit of AMPK in the genome of a vertebrate suspected of having such a disorder.

According to a preferred embodiment of the invention, the disorder is correlated with an altered, in particular an excessive, glycogen accumulation in the muscular cells and results from the expression of a functionally altered PRKAG3 gene.

The expression of a functionally altered Prkag3, or of a functionally altered mutant of a γ subunit of AMPK having a mutation within the first CBS domain may be detected or measured using either polyclonal or monoclonal antibodies specific for the functionally altered polypeptides of the invention, as defined above. Appropriate methods are known in the art. They include for instance enzyme-linked immunosorbent assay (ELISA), radioimmunoassay (RIA), and fluorescence activated cell sorting (FACS).

The nucleotide sequences of the invention may be used for detecting mutations in the PRKAG3 gene or in a sequence encoding the first CBS domain of a γ subunit of AMPK, by detection of differences in gene sequences or in adjacent sequences between normal, carrier, or affected individuals.

The invention provides a process for detecting a mutation in the PRKAG3 gene or in a sequence encoding the first CBS domain of a γ subunit of AMPK wherein said process comprises:

• • obtaining a nucleic acid sample from vertebrate;
  • checking the presence is said nucleic acid sample of a nucleic acid sequence encoding a mutant Prkag3, or a mutant of a γ subunit of AMPK having a mutation within the first CBS domain, as defined above.

According to a preferred embodiment of the invention there is provided a method for detecting a nucleic acid sequence comprising a mutation in the PRKAG3 gene or in a sequence encoding the first CBS domain of a γ subunit of AMPK wherein said process comprises:

• • obtaining a nucleic acid sample from a vertebrate;
  • contacting said nucleic acid sample with a nucleic acid probe obtained from a nucleic acid of the invention and spanning said mutation, under conditions of specific hybridisation between said probe and the mutant sequence to be detected;
  • detecting the hybridisation complex.

Preferably, the process of the invention further comprises, prior to hybridisation, PCR amplification from the nucleic acid sample, of a sequence comprising at least the portion of the PRKAG3 sequence or the sequence encoding the first CBS domain of the γ subunit of AMPK wherein the mutation is to be detected.

Methods allowing the specific hybridisation of a probe only with a perfectly matching complementary sequence, and useful for the detection of punctual mutations are known in the art. They include for instance Allele Specific PCR (GIBBS, Nucleic Acid Res., 17, 2427-2448, 1989), Allele Specific Oligonucleotide Screening (SAIKI et al., Nature, 324, 163-166, 1986), and the like.

A mutation in the PRKAG3 gene may also be detected through detection of polymorphic markers closely link to said mutation.

The invention also provides means for identifying said polymorphic markers, and more specifically polymorphic markers comprised within a genomic DNA sequence comprising at least a portion of a PRKAG3 gene, and up to 500 kb, preferably 300 kb, more preferably up to 100 kb of a 3′ and/or of a 5′ adjacent sequence.

Said polymorphic markers may be obtained for instance, by screening a genomic DNA library from a vertebrate with a probe specific for the PRKAG3 gene, in order to select clones comprising said nucleic acid sequence and flanking chromosomal sequences, and identifying a polymorphic marker in said flanking chromosomal sequences. The allele(s) of a polymorphic marker associated with a given mutant allele of the PRKAG3 gene may also easily be identified by use of a genomic DNA library from an individual wherein the presence of said mutant allele has previously been detected by hybridisation with a nucleic acid probe of the invention.

Polymorphic markers include for instance, single nucleotide polymorphisms (SNP), microsatellites, insertion/deletion polymorphism and restriction fragment length polymorphism (RFLP). These polymorphic markers may be identified by comparison of sequences flanking the PRKAG3 gene obtained from several individuals. Microsatellites may also be identified by hybridisation with a nucleic acid probe specific of known microsatellite motifs.

Once a polymorphic marker has been identified, a DNA segment spanning the polymorphic locus may be sequenced and a set of primers allowing amplification of said DNA segment may be designed.

The invention also encompasses said DNA primers.

Detection of a mutation in the PRKAG3 gene may be performed by obtaining a sample of genomic DNA from a vertebrate, amplifying a segment of said DNA spanning a polymorphic marker by polymerase chain reaction using a set of primers of the invention, and detecting in said amplified DNA the presence of an allele of said polymorphic marker associated with said mutation.

By way of example, polymorphic markers which may be obtained according to the invention, and DNA primers allowing the detection of polymorphic markers closely linked to the RN⁻ allele of porcine PRKAG3 gene are listed in Table 1 hereinafter.

According to a preferred embodiment of the invention, the vertebrate is a mammal, preferably a farm animal and more preferably a porcine, and the mutation to be detected produces a functionally altered PRKAG3. The detection of said mutation allows to predict whether said mammal or the progeny thereof is likely to have an intramuscular glycogen concentration higher or lower than the average. An example of such a mutation produces a functionally altered PRKAG3 having a R41Q substitution, and resulting in an increased glycogen content in the skeletal muscle.

Another example of such a mutation produces a functionally altered PRKAG3 having a V40I substitution, and resulting in a decreased glycogen content in the skeletal muscle. In farm animals having such a mutation, glycogenolysis which occurs after slaughtering is less important than in normal animals, resulting in a higher pH and in a potential better quality of meat.

The present invention also includes kits for the practice of the methods of the invention. The kits comprise any container which contains at least one specific fragment of a nucleic acid sequence of the invention, or at least one nucleic acid fragment able to specifically hybridise with a nucleic acid sequence of the invention. Said nucleic acid fragment may be labeled. The kits may also comprise a set of primers of the invention. They may be used in conjunction with commercially available amplification kits. They may also include positive or negative control reactions or markers, molecular weight size markers for gel electrophoresis, and the like.

Other kits of the invention may include antibodies of the invention, optionally labeled, as well as the appropriate reagents for detecting an antigen-antibody reaction. They may also include positive or negative control reactions or markers.

The invention further provides means for modulating the expression of vertebrate genes encoding a γ subunit of AMPK, and more specifically of the PRKAG3 gene and/or the synthesis or activity of the products of said genes.

A purified AMPK heterotrimer comprising wild-type or mutant PRKAG3 subunit, or a functionally altered mutant γ subunit having a mutation in the first CBS domain, may be used for screening in vitro compounds able to modulate AMPK activity, or to restore altered AMPK activity. This may be done, for instance, by:

• • measuring the binding of the compound to said heterotrimer, using for example high-throughput screening methods; or,
  • measuring changes in AMPK kinase activity, using for example high-throughput screening methods.

High throughput screening methods are disclosed, for instance, in “High throughput screening: The Discovery of Bioactive Substances”, J. P. DEVLIN (Ed), MARCEL DEKKER Inc., New York (1997).

Nucleic acids of the invention may be used for therapeutic purposes. For instance, complementary molecules or fragments thereof (antisense oligonucleotides) may be used to modulate AMPK activity, more specifically in muscular tissue.

Also, a nucleic acid sequence encoding a functional Prkag3 may be used for restoring a normal AMPK function.

Transformed cells or animal tissues expressing a wild-type or mutant PRKAG3, or a functionally altered mutant of a γ subunit of AMPK as defined above, or expressing an AMPK comprising said mutant Prkag3, or said functionally altered mutant of a γ subunit of AMPK, may be used as in vitro model for elucidating the mechanism of AMPK activity or for screening compounds able to modulate the expression of AMPK.

The screening may be performed by adding the compound to be tested to the culture medium of said cells or said tissues, and measuring alterations in energy metabolism in said cells or said tissues using methods such as measurements of glucose concentrations (levels), glucose uptake, or changes of the ATP/AMP ratio, glycogen or lipid/protein content.

The invention provides animals transformed with a nucleic acid sequence of the invention.

In one embodiment, said animals are transgenic animals having at least a transgene comprising a nucleic acid of the invention.

In another embodiment, said animals are knockout animals. “Knockout animals” refers to animals whose native or endogenous PRKAG3 alleles have been inactivated and which produce no functional Prkag3 of their own.

In light of the disclosure of the invention of DNA sequences encoding a wild-type or mutant Prkag3, or a functionally altered mutant of a γ subunit of AMPK, transgenic animals as well as knockout animals may be produced in accordance with techniques known in the art, for instance by means of in vivo homologous recombination.

Suitable methods for the preparation of transgenic or knock-out animals are for instance disclosed in: Manipulating the Mouse Embryo, 2^nd Ed., by HOGAN et al., Cold Spring Harbor Laboratory Press, 1994; Transgenic Animal Technology, edited by C. PINKERT, Academic Press Inc., 1994; Gene Targeting: A Practical Approach, edited by A. L. JOYNER, Oxford University Press, 1995; Strategies in Transgenic Animal Science, edited by G. M. MONASTERSKY and J. M. ROBL, ASM Press, 1995; Mouse Genetics: Concepts and Applications, by Lee M. SILVER, Oxford University Press, 1995.

These animals may be used as models for metabolic diseases and disorders, more specifically for diseases and disorders of glycogen metabolism in muscle. For instance they may be used for screening test molecules. Transgenic animals may thus be used for screening compounds able to modulate AMPK activity. Knockout animals of the invention may be used, in particular, for screening compounds able to modulate energy metabolism, more specifically carbohydrate metabolism, in the absence of functional PRKAG3.

The screening may be performed by administering the compound to be tested to the animal, and measuring alterations in energy metabolism in said animal using methods such as glucose tolerance tests, measurements of insulin levels in blood, changes of the ATP/AMP ratio, glycogen or lipid/protein content in tissues and cells.

Transgenic or knock-out farm animals with modified meat characteristics or modified energy metabolism may also be obtained.

The present invention will be further illustrated by the additional description which follows, which refers to examples of obtention and use of nucleic acids of the invention. It should be understood however that these examples are given only by way of illustration of the invention and do not constitute in any way a limitation thereof.

EXAMPLE 1

Isolating the PRKAG3 Gene

We have screened a porcine Bacterial Artificial Chromosome (BAC) library (ROGEL-GAILLARD et al., Cytogenet and Cell Genet, 851, 273-278, 1999) and constructed a contig of overlapping BAC clones across the region of pig chromosome 15 harbouring the RN gene. These BAC clones were in turn used to develop new genetic markers in the form of single nucleotide polymorphisms (SNPs) or microsatellites (MS) as described in Table 1 below.

	TABLE 1
				Size
	Name			of PCR
	of	BAC		product	Marker
	marker	clone	Primer sequences	(bp)	typea	Allelesb
1	H3	115B9,	F; 5′-GGAATTTCAAGTCAGCCAAC-3′	114-138	MS	114, 126, 128,
		156E6,	(SEQ ID NO:5)			132*, 134*, 136
		361B4,	R; 5′-CTTCAAAAGACCGTGCTACT-3′			138
		90A9	(SEQ ID NO:6)
2	MS982H1	982H11	F; 5′-CTGGGAACCTCTATATGCTG-3′	114-157	MS	114, 140,142*,
			(SEQ ID NO:7)			144, 146, 150,
			R; 5′-TAGGGAAATACAAATCACAG-3′			158
			(SEQ ID NO:8)
3	MS479L3	479L3,	F; 5′-CTCCAGCTCACAGGATGACA-3′	150-164	MS	150*, 160, 162,
		297D7,	(SEQ ID NO:9)			164
		852B5,	R; 5′-GTTTCTGCAGCTTTAGCATCTATTCC-3′
		153B5	(SEQ ID NO:10)
4	MS997M35	997F12	F; 5′-GAAGTATCCTGGGCTTCTGA-3′	138-160	MS	138, 144, 152,
			(SEQ ID NO:11)			154, 160*
			R; 5′-GTTTCTCCAGGTTTCCAGACATCCAC-3′
			(SEQ ID NO:12)
5	MS482H65	482E7	F; 5′-GCTTCTGTCTGCCCCTACTT-3′	78-90	MS	78, 80, 88*, 90
			(SEQ ID NO:13)
			R; 5′-GTTTCTAAGTTCTACTGTAAGACACC-3′
			(SEQ ID NO:14)
6	MS337H2	808G10,	F; 5′-CCAAGCTGTGGTGGCTGAAT-3′	145-165	MS	145, 149, 155,
		947E5,	(SEQ ID NO:15)			161*, 165*
		337G11	R; 5′-CAGCACAGCAGTGCCACCTA-3′
			(SEQ ID NO:16)
7	MS127B1	127G6,	F; 5′-CAAACTCTTCTAGGCGTGT-3′	94-108	MS	94, 100, 108*,
		134C9	(SEQ ID NO:17)			114
			R; 5′-GTITCTGGAACTTCCATATGCCATGG-3′
			(SEQ ID NO:18)
8	CMKAR2	128A3,	F; 5′-AGGGTGGATGGTAGGCTTCA-3′	208	SNP	112A*, 112T;
		37G11,	(SEQ ID NO:19)			158A*, 158G
		808G10,	R; 5′-GTCTCGCTCCTGAAGGAAGT-3′			176A*, 176G
		947E7,	(SEQ ID NO:20)
		1110H12
9	127G63	127G6,	F; 5′-AGTCACGTGGCCATGCTATC-3′	409	SNP	234A*, 234C
		134C9,	(SEQ ID NO:21)
		170D7,	R; 5′-CTCAACTGGATTGAGTCAGT-3′
		1030A5,	(SEQ ID NO:22)
10	VIL1	1088F2	F; 5′-TTGGCGGAACTGTTATTTCT-3′	270	SNP	90T, 90G, 120A,
			(SEQ ID NO:23)			120G, 166C, 166T
			R; 5′-AGGCAAAGGAAGAGCACAG-3′
			(SEQ ID NO:24)
11	NRAMP1	315F7,	F; 5′-AGCCGTGGGCATCGTTGG-3′	1300	RFLP	1: 100 + 1200 bp
		530A6,	(SEQ ID NO:25)		(Styl)	2: 100 + 200 + 1000 bp
		651C12,	R; 5′-AGAAGGAGACAGACAGGGCGA-3′
		1088F2,	(SEQ ID NO:26)
		1095H3
aMS = microsatellite; SNP = nucleotide polymorphism.
bMicrosatellite alleles are designated according to the length of the amplified fragment while SNPs are denoted according to the polymorphic nucleotide. Alleles associated with the RN allele are marked with an asterisk.

The new markers were used together with some previously described markers to construct a high-resolution linkage map. Standard linkage analysis using pedigree data comprising about 1, 000 informative meioses for segregation at the RN locus made it possible to exclude RN from the region proximal to MS479L3 and distal to microsatellite Sw936. Linkage Disequilibrium (LD) analysis was done with the same markers and a random sample of 68 breeding boars from the Swedish Hampshire population, scored for the RN phenotype by measuring glycogen content in muscle. The results of LD analysis using the DISMULT program (TERWILLIGER, Am. J. Hum. Genet., 56, 777-787, 1995) are shown in FIG. 1. They reveal a sharp LD peak around the markers MS127B1 and SNP127G63. These markers appeared to show complete linkage disequilibrium with the RN⁻ allele, i.e., RN⁻ was associated with a single allele at these two loci. The most simple interpretation of this finding is that the RN⁻ mutation arose on a chromosome carrying these alleles and that the two markers are so closely linked to the RN locus that the recombination frequency is close to 0%. The two markers are both present on the overlapping BAC clones 127G6 and 134C9 suggesting that the RN gene may reside on the same clone or one of the neighbouring clones.

A short-gun library of the BAC clone 127G6 was constructed and more than 1,000 sequence reads were collected giving about 500,000 base pair random DNA sequence from the clone. The data were analysed and sequence contigs constructed with the PHRED, PHRAP and CONSED software package (University of Washington Genome Center, available online at bozeman.mbt.washington.edu). The sequence data were masked for repeats using the REPEATMASKER software (available online at ftp.genome.washington.edu/cgi.bin/RepeatMasker) and BLAST searches were carried out using the NCBI web site (available on the World Wide Web at ncbi.nlm.nih.gov). Three convincing matches to coding sequences were obtained. Two of these were against human cDNA sequences/genes, KIAA0173 described as being similar to pig tubulin-tyrosine ligase and located on HSA2q (UniGene cluster Hs. 169910, available on the World Wide Web at ncbi.nlm.nih.gov/UniGene) and CYP27A1 located on HSA2q33-ter (UniGene cluster Hs. 82568). The results strongly suggested that the pig coding sequences are orthologous to these human genes as it is well established that the RN region is homologous to HASA2q33-36 (ROBIC et al., Mamm. Genome, 10, 565-568, 1999). However, none of these sequences appeared as plausible candidate genes for RN. The third coding sequence identified in BAC 127G6 showed highly significant sequence similarity to various AMP-activated protein kinase γ sequences including the yeast SNF4 sequence. The cDNA sequence of this gene was determined by RT-PCR and RACE analysis using muscle mRNA from an rn⁺/rn⁺ homozygote. This sequence is shown in FIG. 2 and in the enclosed sequence listing under SEQ ID NO: 1.

Legend of FIG. 2:

5′ UTR: 5′ untranslated region

3′ UTR: 3′ untranslated region

CDS coding sequence

***: stop codon

‘-’: identity to master sequence

‘.’: alignment gap

The frame of translation was determined on the basis of homology to other members in the protein family and assuming that the first methionine codon in frame is the start codon. The polypeptidic sequence deduced on this basis is shown in the enclosed sequence listing under SEQ ID NO:2.

The complete nucleotidic sequence of pig PRKAG3 cDNA is shown in the enclosed sequence listing under SEQ ID NO:27 and the complete polypeptidic sequence is shown in the enclosed sequence listing under SEQ ID NO:28 and in FIG. 3.

FIG. 3 shows an amino acid alignment constructed with the CLUSTAL W program (THOMPSON et al., Nucleic Acids Research, 22, 4673-4680, 1994) with representative AMPK γ sequences in the nucleotide databases.

Legend of FIG. 3:

Sequences used:

HumG1: Genbank U42412

MusG1: Genbank AF036535

HumG2: Human PRKAG2 (Genbank AJ249976)

PigG3: pig PRKAG3 (this study)

HumG3: human PRKAG3 (this study)

Dros: Drosophila (Genbank AF094764)

SNF4 (yeast): Genbank M30470

Both the PRKAG2 and Drosophila sequences have longer aminoterminal regions but they do not show significant homology to the aminoterminal region of PRKAG3 and were not included.

Abbreviations:

*: stop codon

‘-’: identity to master sequence

‘.’: alignment gap

The four CBS domains are overlined and the position of the RN⁻ mutation is indicated by an arrow.

Table 2 below shows the amino acid (above diagonal) and nucleotide sequence (below diagonal) identities (in %) among mammalian, Drosophila and yeast AMPKG/SNF4 sequences. In the case of pig PRKAG3 and human PRKAG3, the identities were calculated referring to the portions thereof represented respectively by SEQ ID NO:1 and SEQ ID NO:3, for the nucleotide sequences, and by SEQ ID NO:2 and SEQ ID NO:4, for the amino acid sequences.

	TABLE 2
	PigG3	HumG3	HumG1	RatG1	MusG1	HumG2	Dros	SNF4
PigG3	—	97.0	64.2	64.2	63.9	62.6	53.2	34.0
HumG3	90.7	—	63.6	63.6	63.6	62.6	53.5	34.4
HumG1	64.2	64.5	—	96.7	96.3	75.6	60.9	33.5
RatG1	65.8	65.8	88.0	—	97.4	75.3	61.1	33.5
MusG1	65.3	64.8	87.2	92.8	—	74.6	61.7	33.5
HumG2	61.6	61.6	68.1	67.8	65.9	—	63.1	34.5
Dros	58.4	58.4	59.0	59.3	59.0	60.0	—	36.2
SNF4	44.0	44.2	45.4	44.6	45.3	45.7	44.8	—

FIG. 4 shows a Neighbor-Joining phylogenetic tree constructed with the PAUP software (SWOFFORD, Phylogenetic analysis using parsimony (and other methods), Sinauer Associates, Inc. Publishers Sunderland, Mass., 1998) using yeast SNF4 as outgroup; support for branch orders obtained in bootstrap analysis with 1,000 replicates are indicated, scales of tree is indicated at the bottom. The result showed that the pig gene located in the RN region is distinct from mammalian PRKAG1 AND PRKAG2 isoforms and most likely orthologous to a human gene represented by the human EST sequence AA178898 (GenBank) derived from a muscle cDNA library. This gene is herein denoted PRKAG3 since it is the third isoform of a mammalian AMP-activated protein kinase γcharacterised so far.

The cDNA sequence of this gene was determined by RT-PCR and 5′ RACE analysis using human skeletal muscle cDNA (Clontech, Palo Alto, Calif.). This sequence is shown in FIG. 2 and in the sequence listing under SEQ ID NO:3. The deduced polypeptidic sequence having 97% identity with the porcine sequence SEQ ID NO:2 (cf. Table 2) is shown on FIG. 2 and in the sequence listing under SEQ ID NO:4.

The complete cDNA sequence is also shown in the enclosed sequence listing under SEQ ID NO:29; the deduced polypeptidic sequence is shown in the enclosed sequence listing under SEQ ID NO:30 and in FIG. 3.

Using the high resolution human TNG radiation hybrid panel: (available online at shgc-www.stanford.edu/RH/TNGindex.html) we mapped the human homologs of PRKAG3, CYP27A1 and KIAA0173, all present in the porcine BAC127G6. The three genes are also very closely linked in the human genome. PRKAG3 was mapped at a distance of 33 cR_50.000 from KIAA0173 and 52 cR_50.00 from CYP27A1, with lod score support of 6.8 and 4.5, respectively.

The established role of AMPX in regulating energy metabolism, including glycogen storage, and its location in the region showing maximum linkage disequilibrium made PRKAG3 a very strong candidate gene for RN. This was further strengthened by hybridisation analysis of a human multiple tissue northern blots (CLONTECH, Palo Alto, Calif.) using human PRKAG1 (IMAGE clone 0362755 corresponding to GenBank entry AA018675), human PRKAG2 (IMAGE clone 0322735 corresponding to GenBank entry W15439) and a porcine PRKAG3 probe. The results are shown in FIG. 5.

Legend of FIG. 5:

H: Heart, B: Brain, Pl: Placenta, L: Lung, Li: Liver, M: Skeletal muscle, K: Kidney, Pa: Pancreas, S: Spleen, Th: Thymus, P: Prostate, T: Testis, O: Ovary, I: Small intestine, C: Colon (mucosal lining), PBL: Peripheral Blood Leukocyte.

While the PRKAG1 and PRKAG2 probes showed a broad tissue distribution of expression, PRKAG3 showed a distinct muscle-specific expression. This result is also supported by the human EST database where multiple ESTs representing PRKAG1 and PRKAG2 have been identified in various cDNA libraries whereas a single EST (GenBank entry AA178898) representing PRKAG3 has been obtained from a muscle cDNA library. The muscle-specific expression of PRKAG3 and the lack of expression in liver are entirely consistent with the phenotypic effect of RN⁻, namely that glycogen content is altered in muscle but normal in liver (ESTRADE et al., Comp Biochem. Physiol. 104B, 321-326, 1993).

PRKAG3 sequences were determined from rn⁺/rn⁺ and RN⁻/RN⁻ homozygotes by RT-PCR analysis. A comparison revealed a total of seven nucleotide differences four of which were nonsynonymous substitutions was found between the sequence from rn⁺ and RN⁻ animals, as shown in Table 3 below. Screening of these seven SNPs with genomic DNA from additional rn⁺ and RN⁻ pigs of different breeds revealed five different PRKAG3 alleles, but only the R41Q missense substitution was exclusively associated with RN⁻. This nonconservative substitution occurs in CBS1 which is the most conserved region among isotypic forms of the AMPK γ chain and arginine at this residue (number 70 in PRKAG1) is conserved among different isoforms of mammalian AMPK γ sequences as well as in the corresponding Drosophila sequence (FIG. 3). A simple diagnostic DNA test for the R41Q mutation was designed based on the oligonucleotide ligation assay (OLA; LANDEGREN et al., Science, 241, 1077-1080, 1988). Screening a large number of RN⁻ and rn⁺ animals from the Hampshire breed as well as large number of m⁺ animals from other breeds showed that the 41Q allele was present in all RN⁻ animals but not found in any rn⁺ animals, as shown in Table 4 below. The absence of the 41Q allele from other breeds is consistent with the assumption that the RN⁻ allele originated in the Hampshire breed; the allele has not yet been found in purebred animals from other breeds. In conclusion, the results provide convincing evidence that PRKAG3 is identical to the RN gene and that the R41Q substitution most likely is the causative mutation.

TABLE 3
Comparison of the PRKAG3 sequences associated with the
rn+ and RN− alleles in different pig populationsa
RN	Codon
allele	nt83	nt152	34	35	40	41	213	Populationb
RN−	ACC	CTC	GCC	CTG	GTC	CAA	TCT	H
	T	L	A	L	V	Q	S
rn+	---	---	---	---	---	-G-	---	L, LW, WB
	-	-	-	-	-	R	-
rn+	---	-C-	--T	T--	---	-G-	--C	H, L, LW,
	-	P	-	-	-	R	-	M, WB
rn+	-A-	-C-	--T	T--	---	-G-	--C	D, H
	N	P	-	-	-	R	-
rn+	---	-C-	--T	T--	A--	-G-	--C	H, LW, WB,
	-	P	-	-	I	R	-	D, L
Nucleotide and codon numbers refer to the numbering of the sequence SEQ ID NO: 1
H = Hampshire, L = Landrace, LW = Large White, M = Meishan, WB = Wild Boar, D = Duroc
ND = not determined, “-” indicates identity to the top sequence.

	TABLE 4
	Genotype at nucleotide 593d
	RN phenotype	A/A	G/A	G/G	Total
	RN− Hampshirea	40	87	0	127
	RN− Hampshirea,b	0	13	0	13
	rn+ Hampshirea	0	0	60	60
	rn+, other breedsc	0	0	488	488
	arepresent both French and Swedish Hampshire populations
	bheterozygosity RN−/rn+ deduced using pedigree information
	cbreeds: Angler Saddleback, n = 31; Blond Mangalitza, n = 2; Bunte Bentheimer, n = 16; Duroc, n = 160; Göttinger Minipig, n = 4; Landrace, n = 83; Large White, n = 72; Meishan, N = 8; Piétrain, n = 75; Red Mangalitza, n = 5; Rotbunte Husumer, n = 15; Schwalbenbauch Mangalitza, n = 7; Schwäbisch Hällische, n = 2; European Wild Boar, N = 5; Japanese Wild Boar, n = 3.
	drefers to the nucleotide numbers of SEQ ID NO: 1

Without being bound to any particular mechanism, it may be hypothesised that the AMPX heterotrimer including PRKAG3 is involved in the regulation of glucose transport into skeletal muscle.

It has recently been reported that AMPX activation induced by the AMP analogue AICAR or by muscle contraction leads to an increased glucose uptake in skeletal muscle (BERGERON et al., Am. J. Physiol., 276, E938-944, 1999; HAYASHI et al., Diabetes. 47, 1369-1373, 1998). If this is the function of the AMPK heterotrimer including PRKAG3, R41Q may be a gain-of-function mutation causing a constitutively active holoenzyme, for instance due to the loss of an inactivating allosteric site. If so, the reduced AMPK activity in RN⁻ animals is likely to reflect feed-back inhibition due to the high-energy status of the muscle. An increased uptake of glucose to skeletal muscle is expected to lead to an increase in muscle glycogen content as observed in RN⁻ animals. It has been shown that overexpression of glucose transporter 4 (GLUT4) in transgenic mice leads to increased uptake of glucose and increased glycogen storage (TREADWAY et al., J. Biol. Chem., 269, 29956-29961, 1994). This type of gain-of-function model is consistent with the dominance of RN⁻ as the presence of a single unregulated copy would have a large effect on AMPK enzyme activity.

An alternative hypothesis on the functional significance of the R41Q substitution associated with the RN⁻ allele may also be proposed. Based on the established roles of the yeast SNF1 enzyme in utilisation of glycogen and of mammalian AMPK for inhibiting energy-consuming pathways and stimulating energy-producing pathways, activated AMPK is expected to inhibit glycogen synthesis and stimulate glycogen degradation. If this is the functional role of the isoform(s) containing the PRKAG3 product, the R41Q substitution would be a loss-of-function mutation or a dominant-negative mutation locking the AMPK heterotrimer in an inactive state, and thus inhibiting AMP activation and glycogen degradation. In these cases the phenotypic effect should be explained by haplo-insufficiency, since RN⁻ appears fully dominant.

R41 Q may thus be a dominant negative mutation, but only if it interferes with multiple isoforms since the major AMPK activity in muscle appears to be associated with the PRKAG1 and 2 isoforms [CHEUNG; et al. Biochem. J. 246, 659 (2000)].

The distinct phenotype of the RN⁻ mutation indicates that PRKAG3 plays a key role in the regulation of energy metabolism in skeletal muscle. For instance, PRKAG3 is likely to be involved in the adaptation to physical exercise, which is associated with increased glycogen storage. It is also conceivable that loss-of-function mutations in PRKAG3 (or other AMPK genes) may predispose individuals to noninsulin-dependent diabetes mellitus, and AMPK isoforms are potential drug targets for treatment of this disorder.

EXAMPLE 2

Detection of the R41Q Substitution in PIG PRKAG3

A part of PRKAG3 including codon 41 was amplified in 10 μl reactions containing 100 ng genomic DNA, 0.2 mM dNTPs, 1.5 mM MgCl₂, 4.0 pmol of both forward (AMPKG3F3:5′-GGAGCAAATGTGCAGACAAG-3′) (SEQ ID NO:33) and reverse (AMPKG3R2:5′-CCCACGAAGCTCTGCTTCTT-3′) (SEQ ID NO:34) primer, 10% DMSO, 1 U of Taq DNA polyrnerase and reaction buffer (ADVANCED BIOTECH, London, UK). The cycling conditions included an initial incubation at 94° for 5 min followed by 3 cycles at 94° C. (1 min), 57° C. (1 min) and 72° C. (1 min), and 35 cycles of 94° C. (20 sec), 55° C. (30 sec) and 72° C. (30 sec). Allele discrimination at nucleotide position 122 was done using the oligonucleotide ligation assay (OLA, LANDEGREN, et al., Science, 241, 1077-1080, 1988). The OLA method was carried out as a gel-based assay. Each 10 μl OLA reaction contained 0.5 pmol of each probe SNPRN-A (5′Hex-TGGCCAACGGCGTCCA-3′) (SEQ ID NO:35), SNPRN-G (5′ROX— GGCCAACGGCGTCCG-3′) (SEQ ID NO:36) and SNRPN-Common (5′phosphate-AGCGGCACCTTTGTGAAAAAAAAAA-3′) (SEQ ID NO:37), 1.5 U of thermostable AMPLIGASE and reaction buffer (EPICENTRE TECHNOLOGIES, Madison, Wis.) and 0.5 μl of the AMPKG3F3/AMPKG3R2 PCR product. After an initial incubation at 95° C. for 5 min, the following thermocycling profile was repeated 10 times: denaturation at 95° C. (30 sec), and probe annealing and ligation at 55° C. (90 sec). After OLA cycling, 1 μl of product was heat denatured at 94° C. (3 min), cooled on ice, and loaded onto 6% polyacrylamide denaturing gel for electrophoresis on an ABI377 DNA sequencer (PERKIN ELMER, Foster City, USA). The resulting fragment lengths and peak fluorescence were analyzed using GENESCAN software (PERKIN ELMER, Foster City, USA).

The OLA-based method for the R41 Q mutation was used to determine the genotype of DNA samples collected from 68 Swedish Hampshire animals phenotyped as either RN⁻ or rn⁺ based on their glycolytic potential (GP) value. FIG. 6 illustrates typical OLA results from the three possible genotypes. All RN⁻ animals were scored as homozygous A/A (n=28) or heterozygous A/G (n-36) at nucleotide position 122 whereas the rn⁺ animals were homozygous G/G (n=4) at this position.

EXAMPLE 3

Predicting the Presence of the RN⁻ Allele Using a Closely Linked Microsatellite, MS127B1

A microsatellite 127B1 (MS127B1) was cloned from BAC 127G7 containing pig PRKAG3. The BAC clone was digested with Sau3AI and the restriction fragments subcloned into the BamHI site of pUC18. The resulting library was probed with a (CA)₁₅ oligonucleotide probe labeled with [γ-32P]-dATP. Strongly hybridizing clones were sequenced and primers for PCR amplification of microsatellite loci were designed. Ten μl PCR reactions were performed containing 100 ng genomic DNA, 0.2 mM dNTPs, 1.5 mM MgCl₂, 4.0 pmol of both forward (MS127B1F: 5′ -Fluorescein-CAAACTCTTCTAGGCGTGT-3′) (SEQ ID NO:38) and reverse (MS127B1R: 5′-GTTTCTGGAACTTCCATATGCCATGG-3′) (SEQ ID NO:39) primers, and 1 U of Taq DNA polymerase and reaction buffer (ADVANCED BIOTECH, London, UK). The cycling conditions included an initial incubation at 94° C. for 5 min followed by 3 cycles at 94° C. (1 min), 57° C. (1 min) and 72° C. (1 min), and 35 cycles of 94° C. (20 sec), 55° C. (30 sec) and 72° C. (30 sec). The PCR products (0.3 μl) were separated using 4% polyacrylamide denaturing gel electrophoresis on an ABI377 DNA sequencer (PERKIN ELMER, Foster City, USA). The resulting fragment lengths were analyzed using the GENESCAN and GENOTYPER software (PERKIN ELMER, Foster City, USA). The resulting fragment lengths were analysed using the GENESCAN and GENOTYPER software [PERKIN ELMER, Foster City, USA).

The method was used to determine the genotype of DNA samples collected from 87 Swedish Hampshire animals phenotyped as either RN⁻ or rn⁺ based on their glycolytic potential (GP) value. Allele 108 (bp) showed a complete association to the RN⁻ allele in this material as all RN⁻ (RN⁻/RN⁻ or RN⁻/rn⁺ animals were homozygous or heterozygous for this allele while no rn⁺ (rn⁺/rn⁺) animals carried this allele, as shown in Table 5 below.

	TABLE 5
	Genotype
Animals	N	94/94	94/108	94/114	100/108	108/108
RN−	80	0	37	0	2	41
rn+	7	3	0	4	0	0

EXAMPLE 4

Detecting the Presence of the RN⁻ Allele Using a PCR-RFLP Test

The RN⁻ mutation inactivates a BsrBI site GAGˆCGG/CTCˆGCC (BsrBI RE site is not palindromic). At that site, the RN⁻ sequence is AAGCGG instead of GAGCGG.

A 134 bp long fragment of the RN gene is amplified from porcine genomic DNA. The rn⁺ allele is identified after BsrBI digestion, by detection of two fragments of 83 and 51 bps.

The test is performed as follows:

1° Primer sequences:

Sequence of primers used to amplify the RN mutation region:

RNU: 5′ GGGAACGATTCACCCTCAAC 3′ (SEQ ID NO:40)
RNL: 5′ AGCCCCTCCTCACCCACGAA 3′ (SEQ ID NO:41)

To provide an internal control of digestion, a BsrBI site has been added at the extremity of one of the two primers within a 20 bp long tail. The tail permits both creation of a BsrBI site (a shorter tail might be sufficient), and an easy discrimination of uncut fragment from other fragments. The use of tailed primers does not affect efficiency and specificity of amplification.

The sequence of the RNL modified primer including a control tail with a BsrBI site is: RNLBsrA14: 5′ A₅C₂A₇CCGCTCAGCCCCTCCTCACCCACGAA 3′ (SEQ ID NO:42)

20 PCR reaction mixture used:

• • 50 ng DNA
  • 0.5 Unit Taq polymerase (GIBCO BRL)
  • 1.5 mM mgCL²
  • 200 mM dNTP
  • 0.2 μM each primer
  • Total reaction volume: 25 μl

30 PCR conditions used (on OMNIGENE HYBAID thermocycler):

• • 1× (5 min 95° C.)
  • 35× (45 sec 57° C., 45 sec 72° C., 45 sec 95° C.)
  • 1× (45 sec 57° C., 15 min 72° C.)

4° Restriction enzyme digestion performed at 37° C. for 2 hours:

• • 10 μl PCR product
  • 1× BsrBI BIOLABS buffer
  • 5 U BsrBI restriction enzyme (BIOLABS)
  • Total reaction volume: 15 μl

5° Size of fragments produced after PCR using primers with control tail and digestion with BsrBI:

• • Uncut fragment from RN⁻ or rn⁺ allele: 154 bp
  • After digestion of fragment amplified from RN⁻ allele: 137 bp+17 bp
  • After digestion of fragment amplified from rn⁺ allele: 83 bp+54 bp+17 bp

Size difference can be identified either after polyacrylamide, agarose/NUSIEVE or agarose gel electrophoresis.

EXAMPLE 5

Effect of V40I Polymorphism on Glycolytic Potential

Further, a set of 181 rn⁺/rn⁺ homozygous animals (R/R at position 41 of SEQ ID NO:2) were analyzed for the V40I polymorphism (referring to position 40 of SEQ ID NO:2) by PCR-RFLP using FokI restriction enzyme. The glycolytic potential was determined in parallel according to the method disclosed by MONIN et al., (Meat Science, 13, 49-63, 1985).

The results are shown in Table 6 below:

TABLE 6
Genotype at	Average glycolytic	Standard	Number of typed
position 40	potential	Deviation	animals
I/I	178.30	31.13	13
V/I	204.15	37.73	164
V/V	210.83	38.21	104

These results show that the V40I polymorphism has a significant effect on the glycolytic potential in skeletal muscle.

Claims

1. A gamma subunit of a vertebrate AMP-activated kinase (AMPK), wherein said gamma subunit is a polypeptide comprising at least a sequence having at least 70% identity with the polypeptide of SEQ ID NO: 2.

2. The polypeptide of claim 1, wherein said polypeptide comprises a sequence having at least 95% identity with the polypeptide of SEQ ID NO:2.

3. The polypeptide of claim 1, wherein said polypeptide comprises a sequence having at least 75% identity with the polypeptide of SEQ ID NO: 28.

4. A polypeptide of claim 1, wherein said polypeptide comprises the sequence of SEQ ID NO: 2 or SEQ ID NO: 4.

5. A polypeptide of claim 1, wherein said polypeptide comprises the sequence of SEQ ID NO: 28, SEQ ID NO: 30 or SEQ ID NO: 32.

6. A polypeptide which is a functionally altered mutant of a gamma subunit of a vertebrate AMPK, wherein said polypeptide has at least a mutation located within a first CBS domain of said gamma subunit.

7. The polypeptide of claim 6, wherein the mutation is located within the region of the first CBS domain aligned with the region of a polypeptide of SEQ ID NO: 2 spanning from residue 30 to residue 50.

8. The polypeptide of claim 7, wherein the mutation is a R→Q substitution or a V→I substitution.

9. The polypeptide of claim 8 selected from the group consisting of: a polypeptide having a sequence resulting from a R→Q substitution at a position corresponding to position 41 in SEQ ID NO: 2; and a polypeptide having a sequence resulting from a V→I substitution at the position corresponding to position 40 of SEQ ID NO: 2.

10. A polypeptide which is a mutant of a gamma subunit of a vertebrate AMP-activated kinase, wherein said polypeptide results from a deletion of a part of a polypeptide of claim 1.

11. A nucleic acid sequence encoding a polypeptide of claim 1, or the complement thereof, provided that said nucleic acid sequence does not consist of the EST GENBANK AA178898, or of the EST W94830.

12. The nucleic acid sequence of claim 11, having the sequence SEQ ID NO: 1, SEQ ID NO:3, SEQ ID NO: 27, SEQ ID NO: 29, SEQ ID NO: 31, or the complement thereof.

13. A nucleic acid sequence comprising at least a portion of a nucleic acid sequence encoding a polypeptide of claim 1, and up to 500 kb of a 3′ or 5′ adjacent genomic DNA sequence, or the complement thereof.

14. A nucleic acid fragment selected from the group consisting of: (a) a specific fragment of a nucleic acid sequence encoding a polypeptide comprising at least a sequence having at least 70% identity with the polvpeptide of SEQ ID NO: 2; (b) a specific fragment of a nucleic acid sequence comprising at least a portion of a nucleic acid sequence encoding a polypeptide of comprising at least a sequence having at least 70% identity with the polypeptide of SEQ ID NO: 2, and up to 500 kb of a 3′ or 5′ adjacent genomic DNA sequence, or the complement thereof; and (c) a nucleic acid fragment which specifically hybridizes under stringent conditions with a nucleic acid sequence encoding a polypeptide of comprising at least a sequence having at least 70% identity with the polypeptide of SEQ ID NO: 2; provided that said nucleic acid fragment does not consist of the EST GENBANK AA178898 or of the EST GENBANK W94830.

15. A set of primers for amplifying a nucleic acid sequence of or a portion thereof, comprising a primer consisting of a nucleic acid fragment of claim 14.

16. A recombinant vector comprising a nucleic acid sequence encoding a polypeptide of claim 1.

17. An isolated host cell transformed by a nucleic acid sequence encoding a polypeptide of claim 1.

18. A transgenic animal transformed by a nucleic acid sequence encoding a polypeptide of claim 1.

19. A knockout animal, wherein the gene encoding a polypeptide of claim 1 is inactive.

20. A heterotrimeric AMPK wherein the γ subunit consists of a polypeptide of claim 1.

21. A method of detecting a metabolic disorder resulting from a mutation in a gene encoding a γ subunit of AMPK, wherein said process comprises: obtaining a nucleic acid sample from a vertebrate; and checking said nucleic acid for the presence of a nucleic acid sequence encoding a polypeptide of claim 1, wherein said polypeptide is functionally altered.

22. The method of claim 21 wherein the disorder is correlated with an altered glycogen accumulation in the muscular cells and results from the expression of a functionally altered allele of a polypeptide of claim 1.

23. The method of claim 21 wherein the presence of the nucleic acid sequence encoding said functionally altered allele of said polypeptide is checked for by contacting said nucleic acid sample with a nucleic acid probe obtained from a nucleic acid of claim 14 and comprising said mutation, under conditions of specific hybridization between said probe and the sequence to be detected, and detecting the hybridization complex.

24. A process for detecting a dysfunction of carbohydrate metabolism resulting from the expression of a functionally altered allele of a polypeptide of claim 1 in a vertebrate, wherein said process comprises: obtaining a sample of genomic DNA from said vertebrate; contacting said DNA with a pair of primers under conditions allowing PCR amplification; and analyzing the PCR product to detect if an allele of a polymorphic marker linked to a nucleic acid sequence encoding a functionally altered allele of a polypeptide of claim 1 is present.

25. The process of claim 24, wherein said functionally altered allele comprises a R41Q substitution in SEQ ID NO: 2.

26. The process of claim 24, wherein said vertebrate is a mammal.

27. The process of claim 26 wherein said mammal is a pig.

28. The process of claim 27 wherein the pair of primers is selected from the group consisting of: a pair of primers consisting of SEQ ID NO: 5 and SEQ ID NO: 6; a pair of primers consisting of SEQ ID NO: 7 and SEQ ID NO: 8; a pair of primers consisting of SEQ ID NO: 9 and SEQ ID NO: 10; a pair of primers consisting of SEQ ID NO: 11 and SEQ ID NO: 12; a pair of primers consisting of SEQ ID NO: 13 and SEQ ID NO: 14; a pair of primers consisting of SEQ ID NO: 15 and SEQ ID NO: 16; a pair of primers consisting of SEQ ID NO: 17 and SEQ ID NO: 18; a pair of primers consisting of SEQ ID NO: 19 and SEQ ID NO: 20; a pair of primers consisting of SEQ ID NO: 21 and SEQ ID NO: 22; a pair of primers consisting of SEQ ID NO: 23 and SEQ ID NO: 24; and a pair of primers consisting of SEQ ID NO: 25 and SEQ ID NO: 26.