Cholesterol Gallstone Susceptibility Loci: A Mouse Map, Candidate Gene Evaluation, and Guide to Human LITH Genes
2006; Elsevier BV; Volume: 131; Issue: 6 Linguagem: Inglês
10.1053/j.gastro.2006.10.024
ISSN1528-0012
AutoresMalcolm A. Lyons, Henning Wittenburg,
Tópico(s)Drug Transport and Resistance Mechanisms
ResumoCholesterol gallstones are prevalent and costly, chiefly among developed countries. In addition to numerous environmental risk factors, a complex genetic basis determines the risk for developing cholesterol gallstones. Rather than monogenic mutations that are rare and occur in limited populations, genetic predisposition to gallstone susceptibility in general populations arises from polymorphisms in multiple genes, each making a small contribution to overall risk. Because mouse and human genomes are conserved and only a critical subset of homologous genes appears rate limiting for gallstone susceptibility in these species, a genetic map of mouse lithogenic (Lith) loci provides a "roadmap" for the discovery of human LITH genes. Quantitative trait locus (QTL) mapping was employed to identify Lith loci in mice. Repeated detection of colocalizing QTLs among 9 crosses of 12 genetically diverse progenitor strains suggests identification of most major Lith QTLs, including Lith1–Lith23. Therefore, using this knowledge, our priority is to predict human LITH genes. To date, predominantly, prior knowledge of gene-product function was invoked to postulate causal roles for genes in gallstone susceptibility. Unfortunately, few such genes colocalized with empirical Lith loci, suggesting absence of causality or very limited contributions. Consequently, we present a systematic and comprehensive analysis of literature to support or refute the contributions to gallstone susceptibility by genes located within critical regions of empirical QTLs. We envisage that identification of human LITH genes based on our translational approach will provide targets for the development of new means of prevention and nonsurgical management of cholelithiasis. Cholesterol gallstones are prevalent and costly, chiefly among developed countries. In addition to numerous environmental risk factors, a complex genetic basis determines the risk for developing cholesterol gallstones. Rather than monogenic mutations that are rare and occur in limited populations, genetic predisposition to gallstone susceptibility in general populations arises from polymorphisms in multiple genes, each making a small contribution to overall risk. Because mouse and human genomes are conserved and only a critical subset of homologous genes appears rate limiting for gallstone susceptibility in these species, a genetic map of mouse lithogenic (Lith) loci provides a "roadmap" for the discovery of human LITH genes. Quantitative trait locus (QTL) mapping was employed to identify Lith loci in mice. Repeated detection of colocalizing QTLs among 9 crosses of 12 genetically diverse progenitor strains suggests identification of most major Lith QTLs, including Lith1–Lith23. Therefore, using this knowledge, our priority is to predict human LITH genes. To date, predominantly, prior knowledge of gene-product function was invoked to postulate causal roles for genes in gallstone susceptibility. Unfortunately, few such genes colocalized with empirical Lith loci, suggesting absence of causality or very limited contributions. Consequently, we present a systematic and comprehensive analysis of literature to support or refute the contributions to gallstone susceptibility by genes located within critical regions of empirical QTLs. We envisage that identification of human LITH genes based on our translational approach will provide targets for the development of new means of prevention and nonsurgical management of cholelithiasis. Gallstone formation is a common disorder with high prevalence rates throughout Northern Europe and North and South America.1Paigen B. Carey M.C. Gallstones.in: King R.A. Rotter J.I. Motulsky A.G. The genetic basis of common diseases. 2nd ed. Oxford University Press, New York2002: 298-335Google Scholar Most gallstones form and remain in the gallbladder. Many of them remain unnoticed until an upper right quadrant ultrasound is performed as a diagnostic procedure for symptoms or for screening purposes. However, because of pain and complications such as cholecystitis, cholangitis, and biliary pancreatitis, surgery frequently is required as the only definitive treatment of the underlying cholecystolithiasis. Therefore, gallstones are not only a clinical problem but also account for substantial costs to health care systems.2Sandler R.S. Everhart J.E. Donowitz M. Adams E. Cronin K. Goodman C. Gemmen E. Shah S. Avdic A. Rubin R. The burden of selected digestive diseases in the United States.Gastroenterology. 2002; 122: 1500-1511Google Scholar For decades, it was noted that gallbladder disease displayed familial clustering, and, based on these observations, a genetic component to the risk of gallstone formation was suspected (summarized in Paigen and Carey1Paigen B. Carey M.C. Gallstones.in: King R.A. Rotter J.I. Motulsky A.G. The genetic basis of common diseases. 2nd ed. Oxford University Press, New York2002: 298-335Google Scholar). Ultrasound examination of the gallbladder detects both symptomatic and asymptomatic gallstones. Therefore, ultrasound examination determines true gallstone prevalence rates. Systematic ultrasound studies in Europe confirmed a 2- to 5-fold increased risk for cholelithiasis in first-degree relatives of gallstone carriers.3Attili A.F. Capocaccia R. Carulli N. Festi D. Roda E. Barbara L. Capocaccia L. Menotti A. Okolicsanyi L. Ricci G. Lalloni L. Mariotti S. Sama C. Scafato E. Factors associated with gallstone disease in the MICOL experience Multicenter Italian Study on Epidemiology of Cholelithiasis.Hepatology. 1997; 26: 809-818Google Scholar, 4Barbara L. Sama C. Morselli Labate A.M. Taroni F. Rusticali A.G. Festi D. Sapio C. Roda E. Banterle C. Puci A. Formentini F. Colasanti S. Nardin F. A population study on the prevalence of gallstone disease: the Sirmione Study.Hepatology. 1987; 7: 913-917Google Scholar Subsequently, family studies in the United States confirmed the importance of genetic factors for the susceptibility to gallstone formation.5Nakeeb A. Comuzzie A.G. Martin L. Sonnenberg G.E. Swartz-Basile D. Kissebah A.H. Pitt H.A. Gallstones: genetics versus environment.Ann Surg. 2002; 235: 842-849Google Scholar, 6Duggirala R. Mitchell B.D. Blangero J. Stern M.P. Genetic determinants of variation in gallbladder disease in the Mexican-American population.Genet Epidemiol. 1999; 16: 191-204Google Scholar A recent, remarkably large twin study from Sweden revealed significantly higher concordance rates for symptomatic gallstones among monozygotic compared with dizygotic twins,7Katsika D. Grjibovski A. Einarsson C. Lammert F. Lichtenstein P. Marschall H.U. Genetic and environmental influences on symptomatic gallstone disease: a Swedish study of 43,141 twin pairs.Hepatology. 2005; 41: 1138-1143Google Scholar which allowed for a reliable estimation of genetic vs environmental contributions to the risk of gallstone formation. The genetic contribution was estimated to account for 25% of the risk of symptomatic gallstones, whereas shared and individual environmental factors were estimated to account for 13% and 62% of the risk, respectively.7Katsika D. Grjibovski A. Einarsson C. Lammert F. Lichtenstein P. Marschall H.U. Genetic and environmental influences on symptomatic gallstone disease: a Swedish study of 43,141 twin pairs.Hepatology. 2005; 41: 1138-1143Google Scholar It follows from genetic and epidemiologic studies that a complex genetic basis involving multiple gallstone susceptibility genes (lithogenic [LITH] genes) determines the risk for developing gallstones in response to environmental factors. Two gallstone subtypes that form in sterile gallbladder bile can be distinguished: cholesterol gallstones, principally composed of cholesterol monohydrate crystals, and black pigment stones composed of calcium-bilirubinate. In general, all gallstones that form in the same gallbladder display the same constitution. In Western countries, cholesterol gallstones predominate over pigment stones, accounting for 80%–90% of stones found at cholecystectomy,8Portincasa P. Moschetta A. Palasciano G. Cholesterol gallstone disease.Lancet. 2006; 368: 230-239Abstract Full Text Full Text PDF Scopus (513) Google Scholar but the prevalence of cholesterol gallstones may be higher among asymptomatic stone carriers. Consequently, results from epidemiologic and genetic studies of cholelithiasis in Western countries primarily reveal information about cholesterol gallstones. Even though the pathogenesis of both gallstone subtypes most likely includes a genetic predisposition,9Lammert F. Sauerbruch T. Mechanisms of disease: the genetic epidemiology of gallbladder stones.Nat Clin Pract Gastroenterol Hepatol. 2005; 2: 423-433Google Scholar no genetic model exists to identify the underlying genes for bilirubin gallstones. Therefore, in this review, we focus on genetic factors that predispose to the formation of cholesterol gallstones. In the following sections, we review briefly the current limited knowledge of LITH genes that influence the risk to develop cholesterol gallstones in humans. Subsequently, we describe how the inbred mouse model of cholesterol gallstone formation is used to dissect the genetic basis of cholesterol gallstone susceptibility in humans. We summarize the findings from quantitative trait locus (QTL) mapping in crosses of cholesterol gallstone susceptible and resistant inbred mouse strains and discuss candidate genes for each locus. It is possible that Lith genes are pleiotropic, ie, 1 polymorphism affecting multiple traits. This may account for the many regions of the genome at which there exists enrichment of QTLs for physiologically related traits. However, given the magnitude of such a task based on pure speculation that any given QTL is in fact pleiotropic, we restrict our focus entirely to QTLs for cholesterol gallstone susceptibility. Given the large number of genes present within any QTL critical region, we restrict our discussion further to include only those genes that have been the subject of speculation regarding their contributions to gallstone formation in published articles. We put the information from studies of inbred mice into context with findings in humans, highlighting their similarities and disparities, and speculate that the results from studies of inbred mice will translate into the elucidation of human genetic susceptibility to gallstone formation. Because we envisage that the critical evaluation presented in this review will guide researchers in their future experiments, especially in the move to understand the genetics of human gallstone susceptibility, based on available data, we evaluated each candidate gene and determined whether it was likely or unlikely to be a Lith gene. If the case was somewhat less convincing that genes were likely to be Lith genes, we determined whether they were either plausible or equivocal in their contribution to gallstone susceptibility. These classifications are not absolute but reflect the current weight of evidence pertaining to each gene. Knowledge of LITH genes in humans is scarce. To date, one genome-wide linkage study reported several human susceptibility loci associated with cholelithiasis.10Puppala S. Dodd G.D. Fowler S. Arya R. Schneider J. Farook V.S. Granato R. Dyer T.D. Almasy L. Jenkinson C.P. Diehl A.K. Stern M.P. Blangero J. Duggirala R. A genomewide search finds major susceptibility loci for gallbladder disease on chromosome 1 in Mexican Americans.Am J Hum Genet. 2006; 78: 377-392Google Scholar This study was performed in Mexican-American families that participated in the San Antonio Family Diabetes/Gallbladder Study, and, accordingly, a high percentage of individuals were diabetic.10Puppala S. Dodd G.D. Fowler S. Arya R. Schneider J. Farook V.S. Granato R. Dyer T.D. Almasy L. Jenkinson C.P. Diehl A.K. Stern M.P. Blangero J. Duggirala R. A genomewide search finds major susceptibility loci for gallbladder disease on chromosome 1 in Mexican Americans.Am J Hum Genet. 2006; 78: 377-392Google Scholar Two loci on chromosome 1p were linked significantly to symptomatic gallstone disease within the cohort comprising both nondiabetic and diabetic subjects.10Puppala S. Dodd G.D. Fowler S. Arya R. Schneider J. Farook V.S. Granato R. Dyer T.D. Almasy L. Jenkinson C.P. Diehl A.K. Stern M.P. Blangero J. Duggirala R. A genomewide search finds major susceptibility loci for gallbladder disease on chromosome 1 in Mexican Americans.Am J Hum Genet. 2006; 78: 377-392Google Scholar In addition, a number of loci were linked above the suggestive threshold with cholelithiasis in both the nondiabetic plus diabetic subject cohort and the nondiabetic participant only cohort.10Puppala S. Dodd G.D. Fowler S. Arya R. Schneider J. Farook V.S. Granato R. Dyer T.D. Almasy L. Jenkinson C.P. Diehl A.K. Stern M.P. Blangero J. Duggirala R. A genomewide search finds major susceptibility loci for gallbladder disease on chromosome 1 in Mexican Americans.Am J Hum Genet. 2006; 78: 377-392Google Scholar In general, linkage was stronger for symptomatic gallstones compared with symptomatic and asymptomatic gallstones combined, a finding that was explained by higher heritability in symptomatic compared with asymptomatic individuals. Based on concepts of the pathophysiology of cholesterol gallstone formation, some genes were examined for a contribution to the susceptibility to gallstone formation in human association studies based on a "candidate gene approach." In these studies, allele frequencies in gallstone carriers and control populations are compared (summarized in Lammert and Sauerbruch9Lammert F. Sauerbruch T. Mechanisms of disease: the genetic epidemiology of gallbladder stones.Nat Clin Pract Gastroenterol Hepatol. 2005; 2: 423-433Google Scholar). Thus far, associations were observed between gallstones and the genes encoding apolipoprotein E (APOE)11Bertomeu A. Ros E. Zambon D. Vela M. Perez-Ayuso R.M. Targarona E. Trias M. Sanllehy C. Casals E. Ribo J.M. Apolipoprotein E polymorphism and gallstones.Gastroenterology. 1996; 111: 1603-1610Google Scholar (although not in all studies12Hasegawa K. Terada S. Kubota K. Itakura H. Imamura H. Ohnishi S. Aoki T. Ijichi M. Saiura A. Makuuchi M. Effect of apolipoprotein E polymorphism on bile lipid composition and the formation of cholesterol gallstone.Am J Gastroenterol. 2003; 98: 1605-1609Google Scholar, 13Jiang Z.Y. Han T.Q. Suo G.J. Feng D.X. Chen S. Cai X.X. Jiang Z.H. Shang J. Zhang Y. Jiang Y. Zhang S.D. Polymorphisms at cholesterol 7α-hydroxylase, apolipoproteins B and E and low-density lipoprotein receptor genes in patients with gallbladder stone disease.World J Gastroenterol. 2004; 10: 1508-1512Crossref Scopus (59) Google Scholar), apolipoprotein B (APOB)13Jiang Z.Y. Han T.Q. Suo G.J. Feng D.X. Chen S. Cai X.X. Jiang Z.H. Shang J. Zhang Y. Jiang Y. Zhang S.D. Polymorphisms at cholesterol 7α-hydroxylase, apolipoproteins B and E and low-density lipoprotein receptor genes in patients with gallbladder stone disease.World J Gastroenterol. 2004; 10: 1508-1512Crossref Scopus (59) Google Scholar, 14Han T. Jiang Z. Suo G. Zhang S. Apolipoprotein B-100 gene XbaI polymorphism and cholesterol gallstone disease.Clin Genet. 2000; 57: 304-308Google Scholar (although not in all studies15Juvonen T. Savolainen M.J. Kervinen K. Kairaluoma M.I. Lajunen L.H. Humphries S.E. Kesaniemi Y.A. Polymorphism at the apoB, apoA-I, and the cholesterol ester transfer protein gene loci in patients with gallbladder disease.J Lipid Res. 1995; 36: 804-812Google Scholar, 16Singh M.K. Pandey U.B. Ghoshal U.C. Srivenu I. Kapoor V.K. Choudhuri G. Mittal B. Apolipoprotein B-100 XbaI gene polymorphism in gallbladder cancer.Hum Genet. 2004; 114: 280-283Google Scholar), and cholesteryl ester transfer protein (CETP).15Juvonen T. Savolainen M.J. Kervinen K. Kairaluoma M.I. Lajunen L.H. Humphries S.E. Kesaniemi Y.A. Polymorphism at the apoB, apoA-I, and the cholesterol ester transfer protein gene loci in patients with gallbladder disease.J Lipid Res. 1995; 36: 804-812Google Scholar In addition to the genetic association of polymorphic genes to complex cholelithiasis in general populations, in rare instances, gallstone formation is linked to single gene mutations, for instance, CCKAR,17Miller L.J. Holicky E.L. Ulrich C.D. Wieben E.D. Abnormal processing of the human cholecystokinin receptor gene in association with gallstones and obesity.Gastroenterology. 1995; 109: 1375-1380Google Scholar, 18Schneider H. Sanger P. Hanisch E. In vitro effects of cholecystokinin fragments on human gallbladders Evidence for an altered CCK-receptor structure in a subgroup of patients with gallstones.J Hepatol. 1997; 26: 1063-1068Google Scholar encoding the cholecystokinin A receptor (although subsequent findings were inconsistent19Miyasaka K. Takata Y. Funakoshi A. Association of cholecystokinin A receptor gene polymorphism with cholelithiasis and the molecular mechanisms of this polymorphism.J Gastroenterol. 2002; 37: 102-106Google Scholar, 20Nardone G. Ferber I.A. Miller L.J. The integrity of the cholecystokinin receptor gene in gallbladder disease and obesity.Hepatology. 1995; 22: 1751-1753Google Scholar) and CYP7A1.21Pullinger C.R. Eng C. Salen G. Shefer S. Batta A.K. Erickson S.K. Verhagen A. Rivera C.R. Mulvihill S.J. Malloy M.J. Kane J.P. Human cholesterol 7α-hydroxylase (CYP7A1) deficiency has a hypercholesterolemic phenotype.J Clin Invest. 2002; 110: 109-117Google Scholar, 22Pullinger C.R. Eng C. Malloy M.J. Kane J.P. Cholesterol 7α-hydroxylase (CYP7A1) gene mutation: another cause for gallstone formation.in: Adler G. Blum H.E. Fuchs M. Stange E.F. Gallstones: pathogenesis and treatment. Kluwer Academic Publishers, Dordrecht2004: 14-23Google Scholar CYP7A1 encodes cholesterol 7α-hydroxylase, an enzyme catalyzing a rate-limiting step in bile salt synthesis, which was rendered dysfunctional by the genetic mutation.21Pullinger C.R. Eng C. Salen G. Shefer S. Batta A.K. Erickson S.K. Verhagen A. Rivera C.R. Mulvihill S.J. Malloy M.J. Kane J.P. Human cholesterol 7α-hydroxylase (CYP7A1) deficiency has a hypercholesterolemic phenotype.J Clin Invest. 2002; 110: 109-117Google Scholar Mutations that cause severe disorders are rare, likely reflecting selective pressure, ie, the gene is required for survival and very few severe mutations result in viable offspring. Given the importance of such a gene, it was postulated that less severe gene mutations (ie, polymorphisms) that alter subtly the function of the encoded protein, but do not ablate its activity, could be common variants that account for disease susceptibility in general populations.23Cohen J.C. Kiss R.S. Pertsemlidis A. Marcel Y.L. McPherson R. Hobbs H.H. Multiple rare alleles contribute to low plasma levels of HDL cholesterol.Science. 2004; 305: 869-872Google Scholar Recently, this concept was confirmed for genes involved in high-density lipoprotein (HDL) homeostasis.23Cohen J.C. Kiss R.S. Pertsemlidis A. Marcel Y.L. McPherson R. Hobbs H.H. Multiple rare alleles contribute to low plasma levels of HDL cholesterol.Science. 2004; 305: 869-872Google Scholar Indeed, a recent study in China detected polymorphisms in the CYP7A1 promoter that were associated with cholelithiasis; however, the pathophysiologic mechanism by which this polymorphism affects gallstone formation remains elusive.13Jiang Z.Y. Han T.Q. Suo G.J. Feng D.X. Chen S. Cai X.X. Jiang Z.H. Shang J. Zhang Y. Jiang Y. Zhang S.D. Polymorphisms at cholesterol 7α-hydroxylase, apolipoproteins B and E and low-density lipoprotein receptor genes in patients with gallbladder stone disease.World J Gastroenterol. 2004; 10: 1508-1512Crossref Scopus (59) Google Scholar Recently identified polymorphisms in the ABCB11 gene, encoding the canalicular bile salt export pump, were associated with cholelithiasis.24van Mil S.W. van der Woerd W.L. van der Brugge G. Sturm E. Jansen P.L. Bull L.N. van den Berg I.E. Berger R. Houwen R.H. Klomp L.W. Benign recurrent intrahepatic cholestasis type 2 is caused by mutations in ABCB11.Gastroenterology. 2004; 127: 379-384Abstract Full Text Full Text PDF Scopus (304) Google Scholar A large population-based study confirmed an increased risk of gallstone formation in women with a history of intrahepatic cholestasis of pregnancy, a finding that can be linked indirectly to genetic variation of ABCB4,25Ropponen A. Sund R. Riikonen S. Ylikorkala O. Aittomaki K. Intrahepatic cholestasis of pregnancy as an indicator of liver and biliary diseases: a population-based study.Hepatology. 2006; 43: 723-728Google Scholar the gene encoding the canalicular phospholipid transporter. Furthermore, mutations in ABCB4 were confirmed to cause a unique form of low phospholipid-associated cholelithiasis (LPAC).26Rosmorduc O. Hermelin B. Boelle P.Y. Parc R. Taboury J. Poupon R. ABCB4 gene mutation-associated cholelithiasis in adults.Gastroenterology. 2003; 125: 452-459Google Scholar Thus, to date, polymorphisms in 3 genes were associated with cholesterol gallstones in general populations, whereas rare mutations in an additional 4 genes were associated with gallstones in specific subpopulations. Clearly, given the strength of genetic contributions to gallstone formation, many LITH genes await discovery. For detailed review of genetic associations with gallstone disease among humans, readers are directed elsewhere (see references 1, 8, 9, and 27–32 and citations therein). When different inbred mouse strains are challenged with a diet that is enriched in cholesterol, cholic acid, and dairy fat, some strains form cholesterol gallstones in their gallbladders within a few weeks, whereas other strains exhibit gallstone resistance.33Khanuja B. Cheah Y.C. Hunt M. Nishina P.M. Wang D.Q.-H. Chen H.W. Billheimer J.T. Carey M.C. Paigen B. Lith1, a major gene affecting cholesterol gallstone formation among inbred strains of mice.Proc Natl Acad Sci U S A. 1995; 92: 7729-7733Google Scholar, 34Bouchard G. Johnson D. Carver T. Carey M.C. Paigen B. Multiple new murine models of cholesterol gallstones (ChGS): The Jackson Laboratory-Brigham and Women's Hospital strain survey (abstr).Gastroenterology. 2001; 120: A72Google Scholar All mice from the same inbred mouse strain are genetically identical, whereas different inbred mouse strains vary in the number and location of single nucleotide polymorphisms (SNPs) across their genomes.35Beck J.A. Lloyd S. Hafezparast M. Lennon-Pierce M. Eppig J.T. Festing M.F. Fisher E.M. Genealogies of mouse inbred strains.Nat Genet. 2000; 24: 23-25Google Scholar, 36Wade C.M. Kulbokas III, E.J. Kirby A.W. Zody M.C. Mullikin J.C. Lander E.S. Lindblad-Toh K. Daly M.J. The mosaic structure of variation in the laboratory mouse genome.Nature. 2002; 420: 574-578Google Scholar, 37Frazer K.A. Wade C.M. Hinds D.A. Patil N. Cox D.R. Daly M.J. Segmental phylogenetic relationships of inbred mouse strains revealed by fine-scale analysis of sequence variation across 4.6 Mb of mouse genome.Genome Res. 2004; 14: 1493-1500Google Scholar Because studies in inbred mice allow control of the experimental environment, the difference in gallstone prevalence rates can be attributed to the genetic variation among strains. Once a genetic basis for a complex human disorder has been confirmed in an inbred mouse model, the standard procedure to detect genomic regions that harbor the causative genetic variations is to perform QTL mapping (for detailed discussion, readers are directed to references 1, 28, 29, and 38–41). In addition to the ability to control the experimental environment and inbreeding, which leads to strains that are fully homozygous for genetic markers and genes, the ease and rapidity of breeding renders genetic studies in mice advantageous compared with linkage analyses in humans.42Paigen K. A miracle enough: the power of mice.Nat Med. 1995; 1: 215-220Google Scholar Furthermore, the mouse model provides the opportunity for genetic modification by means of knock out and knock in techniques, transgenesis, and mutational analysis and gives access to tissue for expression studies.43Abiola O. Angel J.M. Avner P. Bachmanov A.A. Belknap J.K. Bennett B. Blankenhorn E.P. Blizard D.A. Bolivar V. Brockmann G.A. Buck K.J. Bureau J.F. Casley W.L. Chesler E.J. Cheverud J.M. Churchill G.A. Cook M. Crabbe J.C. Crusio W.E. Darvasi A. de Haan G. Dermant P. Doerge R.W. Elliot R.W. Farber C.R. Flaherty L. Flint J. Gershenfeld H. Gibson J.P. Gu J. Gu W. Himmelbauer H. Hitzemann R. Hsu H.C. Hunter K. Iraqi F.F. Jansen R.C. Johnson T.E. Jones B.C. Kempermann G. Lammert F. Lu L. Manly K.F. Matthews D.B. Medrano J.F. Mehrabian M. Mittlemann G. Mock B.A. Mogil J.S. Montagutelli X. Morahan G. Mountz J.D. Nagase H. Nowakowski R.S. O'Hara B.F. Osadchuk A.V. Paigen B. Palmer A.A. Peirce J.L. Pomp D. Rosemann M. Rosen G.D. Schalkwyk L.C. Seltzer Z. Settle S. Shimomura K. Shou S. Sikela J.M. Siracusa L.D. Spearow J.L. Teuscher C. Threadgill D.W. Toth L.A. Toye A.A. Vadasz C. Van Zant G. Wakeland E. Williams R.W. Zhang H.G. Zou F. The nature and identification of quantitative trait loci: a community's view.Nat Rev Genet. 2003; 4: 911-916Google Scholar These features are unique to model organisms, and they facilitate definitive identification of genes underlying QTLs based on recently proposed guidelines.43Abiola O. Angel J.M. Avner P. Bachmanov A.A. Belknap J.K. Bennett B. Blankenhorn E.P. Blizard D.A. Bolivar V. Brockmann G.A. Buck K.J. Bureau J.F. Casley W.L. Chesler E.J. Cheverud J.M. Churchill G.A. Cook M. Crabbe J.C. Crusio W.E. Darvasi A. de Haan G. Dermant P. Doerge R.W. Elliot R.W. Farber C.R. Flaherty L. Flint J. Gershenfeld H. Gibson J.P. Gu J. Gu W. Himmelbauer H. Hitzemann R. Hsu H.C. Hunter K. Iraqi F.F. Jansen R.C. Johnson T.E. Jones B.C. Kempermann G. Lammert F. Lu L. Manly K.F. Matthews D.B. Medrano J.F. Mehrabian M. Mittlemann G. Mock B.A. Mogil J.S. Montagutelli X. Morahan G. Mountz J.D. Nagase H. Nowakowski R.S. O'Hara B.F. Osadchuk A.V. Paigen B. Palmer A.A. Peirce J.L. Pomp D. Rosemann M. Rosen G.D. Schalkwyk L.C. Seltzer Z. Settle S. Shimomura K. Shou S. Sikela J.M. Siracusa L.D. Spearow J.L. Teuscher C. Threadgill D.W. Toth L.A. Toye A.A. Vadasz C. Van Zant G. Wakeland E. Williams R.W. Zhang H.G. Zou F. The nature and identification of quantitative trait loci: a community's view.Nat Rev Genet. 2003; 4: 911-916Google Scholar, 44Glazier A.M. Nadeau J.H. Aitman T.J. Finding genes that underlie complex traits.Science. 2002; 298: 2345-2349Google Scholar For those involved in mapping genes for complex traits, a community perspective detailing standards of proof was developed. These standards may be summarized as follows: (1) identification of a DNA sequence variant, (2) observation of a functional link between the gene and trait of interest, (3) differential allelic effects using in vitro functional assays, (4) confirmation of genetic effect using transgenesis, (5) confirmation of allelic effect using knock-in models, (6) confirmation of allelic effect by complementation of the null allele with functional variants, (7) confirmation of genetic effect using a series of mutations, and (8) QTL homology across multiple species or concordance. It was agreed that achievement of a combination, but not a full complement, of these criteria was satisfactory to prove quantitative trait gene (QTG) identity.43Abiola O. Angel J.M. Avner P. Bachmanov A.A. Belknap J.K. Bennett B. Blankenhorn E.P. Blizard D.A. Bolivar V. Brockmann G.A. Buck K.J. Bureau J.F. Casley W.L. Chesler E.J. Cheverud J.M. Churchill G.A. Cook M. Crabbe J.C. Crusio W.E. Darvasi A. de Haan G. Dermant P. Doerge R.W. Elliot R.W. Farber C.R. Flaherty L. Flint J. Gershenfeld H. Gibson J.P. Gu J. Gu W. Himmelbauer H. Hitzemann R. Hsu H.C. Hunter K. Iraqi F.F. Jansen R.C. Johnson T.E. Jones B.C. Kempermann G. Lammert F. Lu L. Manly K.F. Matthews D.B. Medrano J.F. Mehrabian M. Mittlemann G. Mock B.A. Mogil J.S. Montagutelli X. Morahan G. Mountz J.D. Nagase H. Nowakowski R.S. O'Hara B.F. Osadchuk A.V. Paigen B. Palmer A.A. Peirce J.L. Pomp D. Rosemann M. Rosen G.D. Schalkwyk L.C. Seltzer Z. Settle S. Shimomura K. Shou S. Sikela J.M. Siracusa L.D. Spearow J.L. Teuscher C. Threadgill D.W. Toth L.A. Toye A.A. Vadasz C. Van Zant G. Wakeland E. Williams R.W. Zhang H.G. Zou F. The nature and identification of quantitative trait loci: a community's view.Nat Rev Genet. 2003; 4: 911-916Google Scholar In addition, according to the International Committee on Standardized Genetic Nomenclature for Mice, QTLs are assigned with a name only if significant. When suggestive, QTLs may be named if there is substantial overlap with a previously identified suggestive QTL. If a QTL has been reported previously, the old name is used if the crosses share 1 parental strain and a new name chosen if crosses do not share 1 parental strain. These detailed guidelines are recent. As such, in the past, researchers sometimes failed to name both significant and colocalizing suggestive QTLs. As the nomenclature is implemented, the catalogue of mouse QTLs across all traits including cholesterol gallstone susceptibility loci should become systematic and more easily interrogated and interpreted. For readers with limited familiarity with the methodology of QTL mapping, it must be pointed out that QTL mapping alone cannot identify genes that affect biologic traits. Theoretic calculations revealed that, for a QTL of modest effect detected within an intercross population, the number of mice required to reduce the 95% confidence interval (CI) to 1 cM (∼2 Mb in mouse), which harbors approximately 20 genes, approached 20,000.45Darvasi A. Experimental strategies for the genetic dissection of complex traits in animal models.Nat Genet. 1998; 18: 19-24Google Scholar QTL mapping is the first step in a process that is far from facile. Questions remain regarding the ability to detect QTLs with small effects of 1%–2% of overall phenotypic vari
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