Patent Application
20200096525
The present disclosure generally relates to systems and methods for identifying and quantifying lipids. In particular, the present disclosure relates to systems and methods for identifying and quantifying lipids using a chromatography system coupled to a mass spectrometer.
Chromatography involves the flowing of a mobile phase over a stationary phase to effect separation and is often combined with a detector, such as a mass spectrometer. Advances in MS have allowed for more in-depth analysis of lipids; however, unambiguous identification and quantification has proven difficult, as lipids exhibit a high number of isomeric and isobaric lipid species. Furthermore, MS spectra often contain peaks and fragments from multiple compounds, making confident identification and relative quantitation of specific molecular species difficult and time consuming. As a result, the transfer of lipidomic data between laboratories is severely hindered, making it problematic to draw biological interpretation with multiple-site studies.
Identification and quantification of lipids raises a number of challenges due to the high number of isomeric and isobaric lipid species. Technology for unambiguously and efficiently identifying and quantifying lipids would be beneficial and highly desirable.
In one aspect, the present technology relates to a method for screening lipids. The method includes selecting a set of standards to identify at least one class of lipids; combining the standards with a biological sample to form a sample matrix; introducing the sample matrix into a chromatography system to separate the at least one class of lipids; directing the separated lipids to a detector; and quantifying the separated lipids based on a comparison between detector data and a calibration curve generated with a known concentration of the set of standards. In one example embodiment, the detector is configured to perform targeted quantification of the separated lipids using multiple reaction monitoring (MRM) transitions. In another example embodiment, the MRM transitions are performed in both positive and negative ion mode. In another example embodiment, the MRM transitions can be fatty acyl chain fragments, head group fragments, or neutral loss fragments. In another example embodiment, the class of lipids includes monoradylglycerolipids (MG), diradylglycerolipids (DG), triradylglycerolipids (TG), ceramides, lysophosphatidylcholines (LPC), lysophosphatidylethanolamines (LPE), phosphatidylcholines (PC), sphingomyelins (SM), free fatty acids (FFA), lysophosphatidylinositols (LPI), phosphatidic acids (PA), lysophosphatidic acids (LPA), phosphatidylethanolamines (PE), phosphatidylglycerols (PG), phosphatidylinositols (PI), phosphatidylserines (PS), cholesterol, cholesterol ester, hexsocyl ceramides, dihexsocyl ceramides, lipoprotein(a) (LPA), lypopolysaccharides (LPS), or lysyl-phosphatidylglycerol (LPG). In another example embodiment, the chromatography system is a hydrophilic interaction chromatography (HILIC) system.
In another aspect, the present technology relates to a method for identifying potential biomarkers. The method includes selecting a set of standards to identify at least one class of lipids; combining the set of standards with a number of biological samples associated with a medical condition to form a sample matrix; introducing the sample matrix into a chromatography system to separate the at least one class of lipids; directing the separated lipids to a detector; quantifying the separated lipids based on a comparison between detector data and a calibration curve generated with a known concentration of the set of standards; and determining a relationship between the separated lipids and the medical condition. In one example embodiment, the detector is configured to perform targeted quantification of the separated lipids using multiple reaction monitoring (MRM) transitions. In another example embodiment, the MRM transitions are performed in both positive and negative ion mode. In another example embodiment, the MRM transitions can be fatty acyl chain fragments, head group fragments, or neutral loss fragments. In another example embodiment, the class of lipids includes monoradylglycerolipids (MG), diradylglycerolipids (DG), triradylglycerolipids (TG), ceramides, lysophosphatidylcholines (LPC), lysophosphatidylethanolamines (LPE), phosphatidylcholines (PC), sphingomyelins (SM), free fatty acids (FFA), lysophosphatidylinositols (LPI), phosphatidic acids (PA), lysophosphatidic acids (LPA), phosphatidylethanolamines (PE), phosphatidylglycerols (PG), phosphatidylinositols (PI), phosphatidylserines (PS), cholesterol, cholesterol ester, hexsocyl ceramides, dihexsocyl ceramides, lipoprotein(a) (LPA), lypopolysaccharides (LPS), or lysyl-phosphatidylglycerol (LPG). In another example embodiment, the chromatography system is a hydrophilic interaction chromatography (HILIC) system.
In another aspect, the present technology relates to a method for diagnostic screening. The method includes selecting a set of standards to identify at least one class of lipids, wherein an increased or decreased presence of the at least one class of lipids is indicative of a medical condition. The method also includes combining the set of standards with a biological sample to form a sample matrix; introducing the sample matrix into a chromatography system to separate the at least one class of lipids; and directing the separated lipids to a detector to determine an amount of the at least one class of lipids from the biological sample. In one example embodiment, the detector is configured to perform targeted quantification of the separated lipids using multiple reaction monitoring (MRM) transitions. In another example embodiment, the MRM transitions are performed in both positive and negative ion mode. In another example embodiment, the MRM transitions can be fatty acyl chain fragments, head group fragments, or neutral loss fragments. In another example embodiment, the class of lipids includes monoradylglycerolipids (MG), diradylglycerolipids (DG), triradylglycerolipids (TG), ceramides, lysophosphatidylcholines (LPC), lysophosphatidylethanolamines (LPE), phosphatidylcholines (PC), sphingomyelins (SM), free fatty acids (FFA), lysophosphatidylinositols (LPI), phosphatidic acids (PA), lysophosphatidic acids (LPA), phosphatidylethanolamines (PE), phosphatidylglycerols (PG), phosphatidylinositols (PI), phosphatidylserines (PS), cholesterol, cholesterol ester, hexsocyl ceramides, dihexsocyl ceramides, lipoprotein(a) (LPA), lypopolysaccharides (LPS), or lysyl-phosphatidylglycerol (LPG). In another example embodiment, the chromatography system is a hydrophilic interaction chromatography (HILIC) system.
The above aspects of the technology provide numerous advantages. In particular, examples of the techniques described herein provide rapid quantification of 106 choline containing phospholipids (61 PCs, 24 SMs and 21 LPCs), 24 ceramides, 23 hexosylceramides, 24 FFAs, and sphingosine in plasma and serum. Example embodiments disclosed herein also provide a high-throughput (as low as 8 minutes), quantitative and comprehensive LC/MS/MS method for the analysis of polar and non-polar lipid classes in plasma. Example embodiments can also provide methods for fast separation of 16 polar and non-polar lipid classes, and the quantitation of 504 lipid species from plasma samples (250 positive mode injection and 254 negative mode injection). Example embodiments disclosed herein can also improve identification and specificity of phosphatidylcholines (PC) using MRM transitions.
It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the inventive subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the inventive subject matter disclosed herein. It should also be appreciated that terminology explicitly employed herein that also may appear in any disclosure incorporated by reference should be accorded a meaning most consistent with the particular concepts disclosed herein.
One of ordinary skill in the art will understand that the drawings primarily are for illustrative purposes and are not intended to limit the scope of the inventive subject matter described herein. The drawings are not necessarily to scale; in some instances, various aspects of the subject matter disclosed herein may be shown exaggerated or enlarged in the drawings to facilitate an understanding of different features. In the drawings, like reference characters generally refer to like features functionally similar and/or structurally similar elements).
The features and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings.
Following below are more detailed descriptions of various concepts related to, and embodiments of, methodologies, apparatus and systems for identifying and quantifying lipids. It should be appreciated that various concepts introduced above and discussed in greater detail below may be implemented in any of numerous ways, as the disclosed concepts are not limited to any particular manner of implementation. Examples of specific implementations and applications are provided primarily for illustrative purposes.
As used herein, the term “includes” means includes but is not limited to, the term “including” means including but not limited to. The term “based on” means based at least in part on.
The language “biological sample”refers to any solution or extract containing a molecule or mixture of molecules that comprises at least one biomolecule that is subjected to extraction or analysis that originated from a biological source (such as, humans and animals). Biological samples are intended to include crude or purified, e.g., isolated or commercially obtained, samples. Particular examples include, but are not limited to, inclusion bodies, biological fluids, biological tissues, biological matrices, embedded tissue samples, cells (e.g., one or more types of cells), and cell culture supernatants.
Unless specifically stated or obvious from context, the term “or,” as used herein, is understood to be inclusive.
Lipids are essential in maintaining cell structures and play important roles in energy storage and cellular signaling. It is also known that lipids are important in the pathophysiology of diseases such as cancer, neurodegenerative diseases, infections, diabetes etc. Accordingly, the field of lipidomics has emerged when efforts have been devoted in identifying disease biomarkers and probing for disease mechanisms.
Lipids constitute a group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, phospholipids, and others. Major lipid categories include, such as, fatty acids, glycerolipids, glycerophospholipids, sphingolipids, saccharolipids, polyketides, sterol lipids, prenol lipids, fatty acids and their derivatives (including tri-, di-, monoglycerides, and phospholipids), as well as other sterol-containing metabolites such as cholesterol.
It is also appreciated that lipids can be classified in the following groups including, for example, monoradylglycerolipids (MG), diradylglycerolipids (DG), triradylglycerolipids (TG), ceramides, lysophosphatidylcholines (LPC), lysophosphatidylethanolamines (LPE), phosphatidylcholines (PC), sphingomyelins (SM), free fatty acids (FFA), lysophosphatidylinositols (LPI), phosphatidic acids (PA), phosphatidylethanolamines (PE), phosphatidylglycerols (PG), phosphatidylinositols (PI), phosphatidylserines (PS), cholesterol, Hexsocyl ceramides, lipoprotein(a) (LPA), lypopolysaccharides (LPS), and lysyl-phosphatidylglycerol (LPG).
Choline containing lipids such as phosphatidylcholine (PC) and sphingomyelin (SM) are a major component of cell membranes. PCs can account for more than 50% of the phospholipids in eukaryotic membranes. SMs are synthesized by the transfer of a phosphorylcholine group from PC to ceramide, forming the only nonglycerol phospholipid in cell membranes. Interest in measuring these lipid species has increased since they have been identified as being implicated in numerous diseases including multiple sclerosis and Niemann—Pick disease, for example.
Sphingolipids (SLs) are a class of lipid comprising of ceramides (Cer), sphingomyelins (SMs) and more complex glycosphingolipids. A common constituent of all SLs is a “long chain” or “sphingoid” base. These bases are organic aliphatic amino alcohols such as sphingosine (SPH) or a structurally similar compound. They are essential components of cellular membranes and are implicated in a variety of biological functions including cellular signal transduction and apoptosis. SLs can induce tumor cell proliferation and may be resistant to chemotherapy; Cer and hexosylceramide (HexCer) have been found to exhibit elevated levels in Parkinson's disease patients. The rapid metabolic interconversion of SLs as well as the high number of derivatives with similar functions, presents a challenge in terms of generating a profile of functions for individual sphingolipids.
Traditionally, lipids are only associated with cellular roles involving energy storage and used as structural building blocks. Recent developments in biomedical lipid research with the advance of mass spectrometric approach have identified the important role of lipids in different diseases or states including cancer, inflammation and cardiovascular diseases.
The extreme structural diversity of lipids in real biological samples is challenging for analytical techniques used in the lipidomics analysis due to the large difference in physicochemical properties (such as acidity, basicity, neutral, polar and non-polar) of individual lipid species. Currently the two main analytical strategies used for lipidomics are direct infusion MS and high performance reversed phase LC/MS. The direct infusion MS provides high-throughput analysis but can be less convenient for the separation and resolution of isobaric species and the identification of low abundant species because of possible ion suppression. Reversed-phase (RP)-LC is widely used for the separation of individual lipid molecular species based on their lipophilicity which is governed by the carbon chain length and number of double bonds. As a result, lipid species from different classes may co-elute. Normal-phase methods are also used for the separation of lipid classes using solvents such as hexane and chloroform, but their lengthy elution, limited analyte solubility and lack of solvent compatibility with mass spectrometer detection can make the technique less attractive.
Hydrophilic interaction chromatography (HILIC) is a type of normal-phase chromatography that can be used for the separation of lipid classes with a reversed phase solvent system to overcome these potential challenges. The lipid class separation using HILIC is convenient for LC/MS quantitation because lipid class stable isotope labelled (SIL) standards coelute with lipid species inside particular lipid classes under identical mobile phase and matrix composition. This provides identical ionization condition for the SIL standards and determined lipid class. An additional benefit of separating lipid species by class results in fewer SIL standards being required for quantification, thereby saving cost. This disclosure includes examples of HILIC-based comprehensive and high throughput non-polar and polar lipid class separation and quantitation.
A hydrophilic interaction chromatography (HILIC)-based method can offer a number of benefits. For example, a HILIC-based method can use solvents typically used for reversed-phase separations, which are compatible with electrospray ionization (ESI). That is, solvents, such as, acetonitrile, methanol, and water, can be used. Further, in a HILIC method, a highly organic mobile phase (e.g., >80% acetonitrile) can be used, which in turn results in an improved electrospray ionization through efficient mobile phase desolvation and compound ionization.
Fatty acids (FA) are hydrocarbon chains comprising of both carboxyl (—COOH) and methyl functional groups. Traditional nomenclature designates the carbon atom next to the carboxyl group as α and the subsequent one β carbon with the methyl group carbon designated ω. Fatty acid chains may contain one or more double bonds at specific positions (unsaturated and poly unsaturated with cis (Z) or trans (E) configuration) or they may be fully saturated. Branched chain fatty acids with one of more methyl branches on the carbon chain are more commonly found in prokaryotic organism but have been reported in cow's milk fat and the gastrointestinal tract of newborns.
Although free (non-esterified) fatty acids (FFA or NEFA) represent only a small fraction of total fatty acids in plasma, they represent a highly metabolically active lipid class. The most abundant FFAs in plasma consist of oleic acid (18:1), palmitic acid (16:0) and stearic acid (18:0) and together these make up 78% of all FFAs. Some FA species within the unsaturated family are long chain polyunsaturated fatty acids (LC-PUFAs) which are termed essential since they cannot be synthesized de novo. Examples include the main PUFAs, such as linoleic acid (18:2(n-6)), arachidonic acid (20:4(n-6)), alpha-linolenic acid (18:3(n-3)), gamma-linolenic acid (18:3(n-6)), eicosapentaenoic acid (20:5(n-6), EPA) and docosahexaenoic acid (22:6(n-3), DHA). Eicosanoids are locally acting bioactive signalling lipids derived from arachidonic acid and related polyunsaturated fatty acids (PUFAs) that regulate a diverse set of homeostatic and inflammatory processes linked to numerous diseases including metabolic syndrome disorders and cancers.
In step 203, the set of standards is combined with a biological sample to form a sample matrix. Once the sample matrix is formed, the sample matrix is introduced into a chromatography system in step 205 to separate the lipids. In a non-limiting example, the chromatography system can include a HILIC-based system. In some example embodiments, the lipids are extracted from the sample matrix before being introduced to the chromatography system. This step can be performed between steps 203 and 205, in some embodiments.
In step 207, the separated lipids are directed to a detector. In a non-limiting example, the detector is configured to perform a targeted quantification of the lipids using MRM transitions. In some embodiments, the MRM transitions can be performed in both positive and negative ion mode. In another non-limiting example, the MRM transitions can be fatty acyl chain fragments, head group fragments, or neutral loss fragments depending on the lipid class.
As used herein, an “acyl” group refers to a functional group derived by the removal of one or more hydroxyl groups from an oxoacid. The acyl group is usually derived from a carboxylic acid Therefore, it usually has the formula RCO—, where R represents an alkyl group that is attached to the CO group with a single bond.
In step 209, the separated lipids are quantified based on a comparison between detector data and a calibration curve generated with a known concentration of the set of standards. In some embodiments, the calibration curve can be generated prior, or calibration curve data can be retrieved from a database or library. A set of standards is prepared over the target concentration range and the detector response is plotted on the y-axis against the known concentrations of the lipid internal standards on the x-axis. Data is processed using TargetLynx or Skyline and least squares linear regression calculations used to determine the equation of the straight line with the best fit to the data. The equation is then used to convert a measured detector response for the biological sample to the predicted concentration for the biological sample.
In step 223, the set of standards is combined with a number of biological samples associated with a medical condition or a particular disease to form a sample matrix. Once the sample matrix is formed, the sample matrix is introduced into a chromatography system in step 225 to separate the lipids. In a non-limiting example, the chromatography system can include a HILIC-based system. In some example embodiments, the lipids are extracted from the sample matrix before being introduced to the chromatography system. This step can be performed between steps 223 and 225, in some embodiments.
In step 227, the separated lipids are directed to a detector. In a non-limiting example, the detector is configured to perform a targeted quantification of the lipids using MRM transitions. In some embodiments, the MRM transitions can be performed in both positive and negative ion mode. In another non-limiting example, the MRM transitions can be fatty acyl chain fragments, head group fragments, or neutral loss fragments depending on the lipid class.
In step 229, the separated lipids are quantified based on a comparison between detector data and a calibration curve generated with a known concentration of the set of standards. In some embodiments, the calibration curve can be generated prior, or calibration curve data can be retrieved from a database or library.
In step 231, a relationship between the separated lipids and the medical condition can be determined. For example, if the sample matrix formed in step 223 includes a biological sample associated with a particular disease or infection, and if the set of standards selected in step 221 are chosen to identify one particular class of lipids, if that particular class of lipids is detected and quantified at a significantly high rate, a determination could be made that there is a relationship between the detected class of lipids and the particular disease or infection in the biological sample. Such a technique can be used to identify various lipid biomarkers associated with a large number of medical conditions.
In step 243, the set of standards is combined with a biological sample to form a sample matrix. In a non-limiting example, the biological sample can include a plasma sample from a patient, and the techniques described in
In step 247, the separated lipids are directed to a detector to determine an amount of the class of lipids in the biological sample. In a non-limiting example, the biological sample can be a biological sample from a patient, and any increase or decrease of a particular class of lipids detected in step 247 can be indicative of a medical condition. As discussed above, the detector can be configured to perform a targeted quantification of the lipids using MRM transitions. In some embodiments, the MRM transitions can be performed in both positive and negative ion mode. In another non-limiting example, the MRM transitions can be fatty acyl chain fragments, head group fragments, or neutral loss fragments depending on the lipid class.
Example 1 includes an example HILIC-based LC-MS/MS high-throughput targeted phospholipids screen (PC, LPC, SM). This example illustrates the rapid quantification of 106 choline containing phospholipids (61 PCs, 24 SMs and 21 LPCs) in plasma and serum, as well as the improved identification and specificity of PC using MRM transitions from two fatty acyl chain fragments. A HILIC-based approach for the separation of lipids by class prior to MS analysis can reduce identification ambiguity. An additional benefit of separating lipid species by class results in fewer SIL standards being required for quantification, thereby saving cost.
In this example experiment, pooled healthy control plasma was spiked with SIL, (SPLASH LIPIDOMIX™, Avanti Lipids, Alabaster, Ala.) at 9 concentration levels to generate calibration curves for quantification (PC (15:0-18:1) (d7)=16-8000 ng/mL, SM (18:1) (d9)=3-1500 ng/mL and LPC (18:1) (d7)=2.5-1250 ng/mL). Six replicates of human plasma, which is commercially sold as NIST® SRM® 1950 (NIST U.S. Department of Commerce, Gaithersburg Md. 20899), were also spiked with 5% SIL, prior to extraction.
A sample preparation procedure was adopted using protein precipitation with isopropanol (IPA) (1:5, plasma:IPA) and incubated for 2 hours. The extracted samples were centrifuged for 10 mins before transferring the supernatant to glass vials for LC/MS analysis. The LC conditions for the analysis are described below in Table 2, and the MS conditions for the analysis are described below in Table 3.
In this example, a library was generated that contains the LC conditions, MS method, and associated processing method (including retention times). The resulting data can be processed using, for example, TargetLynx or Skyline (MacCoss Lab Software, University of Washington).
A rapid, specific LC-MS method was developed for the analysis of PC, LPC and SM lipids in human plasma employing a HILIC based chromatographic separation and MRM MS detection in both positive and negative ion mode. The HILIC methodology facilitated the elution of lipids in discrete classes. The resulting method was capable of measuring 61 PCs, 21 LPCs and 24 SMs in 8-minutes with a linear dynamic range over 4 orders of magnitude. The method sensitivity easily facilitated the detection of these lipids in human plasma at normal circulating levels from 50 μL of plasma.
The development of the library discussed above allows for the simple download of MRM transitions and chromatographic conditions representing PCs, LPCs and SMs, therefore eliminating manual input of LC-MS methods and reducing possible transcription errors.
In this example embodiment, quantification was achieved using calibration curves of plasma spiked with known concentrations of SIL standards prior to extraction, providing surrogate standards for the quantification of endogenous lipids within the same class. By using surrogate standards prepared and analysed under identical conditions to those of endogenous lipids, the cost of large studies can be significantly reduced. Example calibration curves for SIL standards LPC (18:1) (d7) and SM (18:1) (d9) are shown in
In this example, the method was shown to be linear over 4-orders of magnitude and had sufficient sensitivity to allow for the analysis of lipids at systemic levels in human plasma. Employing HILIC based chromatography allowed lipids to elute according to class, thereby reducing potential isomeric/isobaric interferences and the number of stable label isotopes required for quantification (i.e. cost reductions).
The concentrations of endogenous LPC lipids measured from this example are shown in
Table 4. Readings with CVs <30% were considered acceptable. The back calculated spiked SIL results are included to show accuracy. The actual concentration of LPC (18:1) (d7) spiked into the plasma was 1250 ng/mL.
The concentrations of endogenous PC lipids measured from this example are shown in Table 5. Readings with CVs <30% were considered acceptable. The back calculated spiked SIL results are included to show accuracy. The actual concentration of PC (15:0-18:1) (d7) spiked into the plasma was 8000 ng/mL
The concentrations of endogenous SM lipids measured from this example are shown in Table 6. Readings with CVs <30% were considered acceptable. The back calculated spiked SIL results are included to show accuracy. The actual concentration of SM (18:1) (d9) spiked into the plasma was 1500 ng/mL.
Example 2 includes an example HILIC-based LC-MS/MS high-throughput targeted ceramide and hexosylceramide screen. This example illustrates the rapid quantification of 24 ceramides, 23 hexosylceramide and sphingosine in plasma and serum. As discussed above HILIC-based approach for the separation of lipids by class prior to MS analysis can reduce identification ambiguity. An additional benefit of separating lipid species by class results in fewer SIL standards being required for quantification, thereby saving cost. This example describes the use of a HILIC based approach to perform a targeted screen for Cer, HexCer and SPH.
Pooled healthy control plasma was spiked with SIL, (SPLASH LIPIDOMIX™, Avanti Lipids, Alabaster, Ala.) at 9 concentration levels to generate calibration curves for quantification. SPLASH LIPIDOMIX™ does not contain a suitable surrogate standard for the quantification of Cer or HexCer, but the linearity data can be used to assess the quality of the data generated. Typical R2 values of 0.95 and deviations from the line of best fit (CVs <30%) are routinely achieved. SM (18:1) (d9)=3-1500 ng/mL and TG (15:0/18:1/15:0) (d7)=5.5-2750 ng/mL are used to illustrate this. Six replicates of the NIST® SRM® 1950 were also spiked with 5% SIL, prior to extraction.
A simple sample preparation procedure was adopted using protein precipitation with isopropanol (IPA) (1:5, plasma:IPA) and incubated for 2 hours. The extracted samples were centrifuged for 10 mins before transferring the supernatant to glass vials for LC-MS analysis. The LC conditions for the analysis are described below in Table 7, and the MS conditions for the analysis are described below in Table 8.
In this example, a library was generated containing the LC conditions, MS method, and associated processing method (including retention times). The resulting data were with either TargetLynx or Skyline (MacCoss Lab Software, University of Washington).
This example experiment employs MRM in positive ion mode to identify and quantify Cer and HexCer species contained in human plasma while simultaneously analysing several other lipid classes such as TG, PC and SM. Cer, HexCer and SPH lipids elute as a discrete band (˜0.9-1.0 mins) under HILIC based conditions. A total of 24 Cer, 23 HexCer and SPH were analysed. The method sensitivity easily facilitated the detection of these lipids in human plasma at normal circulating levels from 50 μL of plasma, while demonstrating a linear dynamic range over 4 orders of magnitude.
The Cer and HexCer lipid transitions discussed here use the lipid precursor ion and characteristic long chain base (LCB) product ion (m/z 264). Although the peaks of Cer and HexCer species co-elute under these chromatographic conditions the risk of isobaric effects are minimal since Cer and HexCer precursors have mass ranges of 482.5-610.5 Da and 644.5-772.6 Da respectively. The choline containing SMs produce a characteristic 184 m/z head group fragment and elute at ˜1.92 mins, enabling their analysis alongside other SLs with minimal risk of isomeric/isobaric interferences. Quantification of endogenous SMs is discussed in a separate application note [8] that includes other choline containing lipids (LPC and PC).
The development of the library discussed above allows for the simple download of MRM transitions and chromatographic conditions representing Cer and HexCer lipid species, therefore eliminating manual input of LC-MS methods and reducing possible transcription errors
Table 9 shows the area response of endogenous ceramides lipids in NIST® SRM® 1950 plasma. Readings with CVs <30% were considered acceptable.
Table 10 shows the area response of endogenous hexosylceramides lipids in NIST® SRM® 1950 plasma. Readings with CVs <30% were considered acceptable.
In this example experiment, a rapid quantitative method was demonstrated for the analysis of SLs in conjunction with other lipid classes for plasma and serum. The methodology allowed for the analysis of 24 ceramides, 23 hexosylceramides, and SPH within 8-minutes. The method was shown to be linear over 4-orders of magnitude and had sufficient sensitivity to allow for the analysis of lipids at systemic levels in human plasma.
Example 3 includes an example HILIC-based LC-MS/MS high-throughput (as low as 8 minutes or lower) targeted lipid quantitation platform for the analysis of polar and non-polar lipid classes in plasma. This example illustrates the rapid quantitation of 504 lipid species from plasma sample (250 positive mode injection and 254 negative mode injection).
In this example, pooled healthy control plasma was spiked with SIL standards, (SPLASH® LIPIDOMIX®, Avanti Lipids, Alabaster, Ala.) at 9 concentration levels to generate calibration curves for quantification. Table 13 below shows the list of SIL standards spiked into the NIST® SRM® 1950 plasma extract and other calibration parameters in positive and negative ion mode. Six replicates of the NIST® SRM® 1950 were also spiked with 5% SIL, prior to extraction.
A simple sample preparation procedure was adopted using protein precipitation with isopropanol (IPA) (1:5, plasma:IPA) and incubated for 2 hours. The extracted samples were centrifuged for 10 mins before transferring the supernatant to glass vials for LC/MS analysis. The LC conditions for the analysis are described below in Table 11, and the MS conditions for the analysis are described below in Table 12.
In this example, a library was generated containing the LC conditions, MS method, and associated processing method (including retention times). The resulting data were processed with either TargetLynx or Skyline (MacCoss Lab Software, University of Washington).
Lipidomics involves the analysis of large scale sample sets, and therefore a high throughput method is desirable. A rapid and comprehensive LC-MS method was developed for the analysis of polar and non-polar lipid classes in human plasma employing a HILIC based lipid class separation and MRM MS quantitation in both positive and negative ion mode. Table 13 shows the list of SIL standards spiked into the NIST 1950 plasma extract and calibration parameters in positive and negative ion mode. The different lipid classes measured with their corresponding MRM retention time window, acquisition mode used, and the total number of MRM transitions is shown above in Table 1. A total of 2041 lipid species MRM transition can be measured from 16 polar and non-polar lipid classes with 547 in positive mode and 1494 in negative mode. A mixture of SIL lipid standards representing different lipid classes were used to demonstrate the separation of the lipid classes.
The development of the library mentioned above allows for the simple download of all the MRM transitions and chromatographic conditions representing the different lipid classes with a total of 2041 MRM transitions, therefore eliminating manual input of LC-MS methods and reducing method development training cost and possible transcription errors.
The developed method was used for the quantitative analysis of lipid species using SIL standards for each lipid class. Known concentration SIL standards were added to the plasma sample before extraction.
In this example embodiment, a high-throughput quantitative and comprehensive HILIC method was developed for the analysis of polar and non-polar lipid classes in plasma. The method enabled fast separation of 16 polar and non-polar lipid classes in 8 minutes suitable for the lipidomic analysis of large sample sets. The method was shown to be linear over 4-orders of magnitude and had sufficient sensitivity to allow for the analysis of lipids at systemic levels in human plasma.
Example 4 includes an example HILIC-based LC-MS/MS high-throughput targeted free fatty acid (FFA) screen. This example illustrates the rapid quantification of 24 FFAs in plasma and serum. As discussed above HILIC-based approach for the separation of lipids by class prior to MS analysis can reduce identification ambiguity. This example describes the use of a HILIC based approach to perform a targeted screen for FFA without the need for complex sample preparation and long chromatographic separation.
A simple sample preparation procedure was adopted using protein precipitation with isopropanol (IPA) (1:5, plasma:IPA) and incubated for 2 hours. The extracted samples were centrifuged for 10 mins before transferring the supernatant to glass vials for LC-MS analysis.
The LC conditions for the analysis are described below in Table 14, and the MS conditions for the analysis are described below in Table 15.
In this example, a library was generated which contains the LC conditions, MS method, and associated processing method (including retention times). The resulting data were processed with either TargetLynx or Skyline (MacCoss Lab Software, University of Washington).
Alternatively, reversed-phase (RP) LC-MS can also be applied for the analysis of FFA, but this also requires time consuming sample preparation and the use of toxic organic solvents which are expensive to purchase and dispose of. Reversed-phase chromatography separates lipids according to chain-length and degree of unsaturation. The dual nature of reversed-phase separation (a double bond in the fatty acyl chain reduces retention time and the fatty acyl chain length increases retention time) can hamper the analysis of real samples; due to co-elution the number of components is often so great that identification becomes difficult.
This example method employs pseudo MRM in negative ion mode to identify and quantify 24 FFA species contained in human plasma while simultaneously analysing several other phospholipid classes from the same injection. FFAs elute as a discrete band (˜0.5 mins) under HILIC based conditions, as can be seen in
A hydrophilic interaction chromatography (HILIC) based approach avoids the need for derivatization and complex sample preparation associated with FAME GC-MS. Additionally, since lipids are separated by class, the risk of co-elution associated with RP LC-MS is also minimized. The development of the library discussed above allows for the simple download of MRM transitions and chromatographic conditions representing FFAs, therefore eliminating manual input of LC-MS methods and reducing possible transcription errors.
A sample preparation procedure was adopted using protein precipitation with pre-cooled isopropanol (IPA) (1:5, plasma:IPA). Samples were vortex mixed for one minute and placed at −20° C. for 10 minutes. Samples were vortex mixed again for one minute and placed at 4° C. for two hours to ensure complete protein precipitation. The extracted samples were centrifuged at 10,300 g for 10 minutes at 4° C. and the supernatant was transferred to glass vials for LC-MS/MS analysis. Prepared samples were analyzed in duplicate for both positive and negative ionization modes.
The LC conditions for the analysis are described below in Table 17, and the MS conditions for the analysis are described below in Table 18.
Plasma samples from three biological states of varying phenotype (healthy control, COPD patients, and asthma patients) were analyzed. Samples were prepared using a simple protein precipitation with cooled IPA before targeted LC-MS/MS data were acquired in positive and negative ion electrospray modes. Samples were randomized and two technical replicates per sample were acquired. A large number of lipids were identified from the HILIC-based quantification method MRM library containing highly specific fatty acyl transitions and head group fragments for increased identification confidence and increased specificity. An example of such a library can include the LipidQuan™ method file (Waters Technologies Corp. Milford, Mass. 01757).
The power of the HILIC-based quantification method MRM library is illustrated in
A sample preparation procedure was adopted using protein precipitation with pre-cooled isopropanol (IPA) (1:5, plasma:IPA). Samples were vortex mixed for one minute and placed at −20° C. for 10 minutes. Samples were vortex mixed again for one minute and placed at 4° C. for two hours to ensure complete protein precipitation. The extracted samples were centrifuged at 10,300 g for 10 minutes at 4° C. and the supernatant was transferred to glass vials for LC-MS/MS analysis. A difference between this example and the one described in Example 4, above, is that samples were analyzed for both positive and negative ionization modes. As illustrated below, this example was performed at a different injection volume than the one described in Example 5.
The LC conditions for the analysis are described below in Table 19, and the MS conditions for the analysis are described below in Table 20.
In this example, a rapid and comprehensive LC-MS/MS method was developed for the analysis of polar and non-polar lipid classes in human plasma employing a HILIC-based lipid class separation and MRM MS quantitation in both positive and negative ion modes. Table 21 shows the list of SIL standards spiked into the NIST 1950 plasma extract and the calibration parameters in positive and negative ion mode.
Table 22 below shows the different lipid classes measured with their corresponding MRM retention time window, acquisition mode used, and the total number of MRM transitions, according to this example embodiment. Lipids are mainly separated into lipid classes according to their polarity within about 8 minutes, yielding a quantitative method suitable for the lipidomic analysis of large sample sets. Retention time increases with the increase in lipid polarity, in this example embodiment.
According to this example, the development of a HILIC-based quantification method file allows for the simple download of all the MRM transitions and chromatographic conditions representing the different lipid classes with a total of 2,041 MRM transitions, therefore eliminating manual input of LC-MS methods and reducing method development training costs and possible transcription errors. The developed method was used for the quantitative analysis of lipid species in NIST® SRM® 1950 plasma samples using SIL standards for each lipid class. Known concentration SIL standards were added to the NIST plasma sample before extraction. Table 21 above shows calibration parameters of SIL standards spiked into the NIST plasma extract using positive and negative ion mode. Calibration curves were constructed for each SIL standard, and are linear within tested calibration ranges with average correlation coefficients of greater than 0.992 (positive mode) and 0.980 (negative mode). These correlation coefficients demonstrate advantages of this new method.
Glycerolphospholipids (GPLs) generally consist of two fatty acyl chains and a phosphate head group esterified to a glycerol backbone, with the head group defining GPL. Lysophospholipids are derivatives of phospholipids where one or both of the fatty acyl chains have been removed. Biologically, GPLs are compounds of all biological membranes that act as a protective barrier with selective permeability characteristics to enable cellular metabolism. In recent years, research interest in measuring these lipid species has increased since they are known to be involved in numerous diseases. PE is the second most abundant gycerolphospholipid (after phosphatidylcholines) and includes between about 15-25% of the total GPL content in mammalian cells. Disturbances in PE metabolism have been implicated in both chronic (Alzheimer's and Parkinson's) and infectious (Candidiasis) diseases. Only a small component of PG is observed in eukaryotic mitochondrial membranes. However, PG is the biosynthetic precursor of cardiolipin, which represents a major constituent of mitochondria membrane and is important in maintaining the potential of these membranes. PI plays a small role in membrane structure but plays a major role in membrane-bound signaling processes and vesicular activity.
Although advances in MS have allowed for more in depth lipodomic analysis, unambiguous identification and quantification has proven difficult as lipids have a high number of isomeric and isobaric lipid species. MS spectra often contain multiple peaks and fragments from various compounds making confident identification and relative quantitation of specific molecular species difficult or challenging. As a result, lipidomic data generated during multi-site studies may not be commutative and resulting biological interpretation of the data questionable. This example describes the use of the HILIC-based quantification method discussed herein to perform a targeted screen for LPEs, PEs, PGs, and PIs.
The LC conditions for the analysis are described below in Table 23, and the MS conditions for the analysis are described below in Table 24.
In this example, a rapid and comprehensive LC-MS/MS method was developed for the analysis of LPE, PE, PG, and PI lipids in human plasma employing a HILIC-based chromatographic separation and MRM MS detection. PEs, PG, and PI were analyzed in negative ion mode, while LPE lipids were analyzed in positive ion mode, in this example. The HILIC methodology facilitated the elution of lipids in discrete classes with the PGs eluting first (˜1.21 mins) followed by PEs (˜1.62 mins), LPEs (˜2.34 mins), and PIs (˜2.40 mins). Using this method, 11 LPEs, 47 PEs, 21 PGs, and 33 PIs were quantitatively identified in eight minutes with a linear dynamic range over four orders of magnitude. The method sensitivity easily facilitated the detection of these lipids in human plasma at normal circulating levels from 50 uL of plasma.
The development of a HILIC-based quantification method file allows for the simple download and importing of MRM transitions and chromatographic conditions for LPEs, PEs, PGs, and PIs and eliminates manual input of LC-MS/MS methods reducing possible transcription errors. The method file features highly specific MRM transitions for fatty acyl chain fragments contained in 219 PGs, 279 PEs, and 90 PIs enhancing the specificity of the method and improving lipid identification. Although LPE and PE species share a common head group, the method chromatographically resolves these lipids on the basis of class, thereby reducing potential isomeric and isobaric interferences. Isobaric effects are further minimized as the mass ranges of LPE and PE precursors, 398-476 Da and 686-851 Da, respectively, do not overlap.
Quantification was achieved, in this example, using calibration curves of plasma spiked with known concentrations of SIL standards prior to extraction. These SILs function as surrogate standards for the quantification of endogenous lipids within the same class. By using one surrogate standards per endogenous lipid class, rather than a SIL standard for each measured endogenous lipid, the cost of large studies can be significantly reduced.
Table 25 shows the back calculated concentrations of spiked SIL, as well as the actual concentration of SILs spiked into the NIST plasma in ng/mL. Table 26, below shows the MRM transitions of endogenous LPE lipids in NIST® SRM® 1950 plasma with CVs <30%. The calibration range of LPE (18:1) (d7) was 0.5-250 ng/mL, in this example.
Table 27 shows MRM transitions of endogenous PE lipids, PI lipids, and PG lipids in this example embodiment. The calibration range of PE (15:0-18:1) (d7) was 0.5-250 ng/mL; the calibration range of PI (15:0-18:1) (d7) was 1-500 ng/mL; and the calibration range of PG (15:0-18:1) (d7) was 3-1500 ng/mL.
As demonstrated in this example, the use of the HILIC-based quantification method discussed herein provides the advantage of performing targeted screens for LPE lipids, PE lipids, PG lipids, and PI lipids.
In this example, a rapid, specific LC-MS/MS method was developed for the analysis of PC, LPC, and SM lipids in human plasma employing a HILIC-based chromatographic separation and MRM MS detection in both positive and negative ion mode. The HILIC methodology facilitated the elution of lipids in discrete classes with the PCs eluting first (˜1.52 mins) followed by SMs (˜1.92 mins), and LPCs (˜2.15 mins). The resulting method was capable of measuring 61 PCs, 21 LPCs, and 24 SMs in eight minutes with a linear dynamic range over four orders of magnitude. The method sensitivity easily facilitated the detection of these lipids in human plasma at normal circulating levels from 50 uL of plasma. Product ion scans of these three lipid classes showed the characteristic 184 m/z choline head group fragment in positive ion mode, that results in unambiguous lipid assignment if employed in an MRM transition. To improve identification and specificity, highly specific MRM transitions containing fatty acyl chain fragments for 106 PCs (positive ion) and 278 PCs (negative ion) were developed. Although the MRM transitions for LPC and SM species employ the choline head group fragment, chromatographic separation on the basis of class reduces isomeric and isobaric interferences. Additionally, isobaric effects can be further minimized since LPC and SM precursors have mass ranges of 440-608 Da and 648-776 Da, respectively, and therefore do not overlap.
The LC conditions for the analysis are described below in Table 28, and the MS conditions for the analysis are described below in Table 29.
Table 30 shows MRM transitions of endogenous LPC lipids, Table 31 shows the MRM transitions of PC lipids, and Table 32 shows the MRM transitions of SM lipids in this example embodiment. The calibration range of LPC (18:1) (d7) was 2.5-1250 ng/mL; the calibration range of PC (15:0-18:1) (d7) was 16-8000 ng/mL; and the calibration range of SM (18:1) (d7) was 3-1500 ng/mL.
As demonstrated in this example, the use of the HILIC-based quantification method discussed herein provides the advantage of performing targeted screens for LPC lipids, PC lipids, and SM lipids.
In describing example embodiments, specific terminology is used for the sake of clarity. For purposes of description, each specific term is intended to at least include all technical and functional equivalents that operate in a similar manner to accomplish a similar purpose. Additionally, in some instances where a particular example embodiment includes a plurality of system elements, device components or method steps, those elements, components or steps can be replaced with a single element, component or step. Likewise, a single element, component or step can be replaced with a plurality of elements, components or steps that serve the same purpose. Moreover, while example embodiments have been shown and described with references to particular embodiments thereof, those of ordinary skill in the art will understand that various substitutions and alterations in form and detail can be made therein without departing from the scope of the disclosure. Further still, other aspects, functions and advantages are also within the scope of the disclosure.
Example flowcharts are provided herein for illustrative purposes and are non-limiting examples of methodologies. One of ordinary skill in the art will recognize that example methodologies can include more or fewer steps than those illustrated in the example flowcharts, and that the steps in the example flowcharts can be performed in a different order than the order shown in the illustrative flowcharts.
This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/734,410 filed on Sep. 21, 2018 titled “SYSTEM AND METHOD FOR LIPID QUANTIFICATION,” the entire contents of which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62734410 | Sep 2018 | US |
