The Genom of Homo sapiens.pdf

Genomic Variation in Multigenic Traits: Hirschsprung DiseaseA.S. MCCALLION,* E.S. EMISON,* C.S. KASHUK,* R.T. BUSH,* M. KENTON,*M.M. CARRASQUILLO,* K.W. JONES, † G.C. KENNEDY, † M.E. PORTNOY, ‡ E.D. GREEN, ‡AND A. CHAKRAVARTI**McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University School of Medicine, Baltimore,Maryland 21205; † Affymetrix, Santa Clara, California 95051; ‡ Genome Technology Branch, National HumanGenome Research Institute, National Institutes of Health, Bethesda, Maryland 20892Conflict-of-Interest Disclosure Statement: Aravinda Chakravarti isa paid member of the Scientific Advisory Board of Affymetrics. Theterms of this arrangement are being managed by the Johns Hopkins Universityin accordance with its conflict-of-interest policies.Recent advances in genomic technology and the availabilityof a finished human genome sequence havegreatly facilitated the identification of genes underlyinghuman Mendelian disorders. The contemporary challengelies in the elucidation of complex disorders. Classically,the transmission of a Mendelian disorder is explainedby the exact co-segregation of a single mutationwith the phenotype. Such mutations are absent in controlsand, most frequently, involve conserved coding sequences.These observations are in stark contrast to thecomplex non-Mendelian diseases, wherein mutations atsingle genes do occur in unaffecteds, and variants withweak or moderate quantitative effects on the phenotypeplay a significant role. Consequently, such mutationsmay exist at relatively high frequency in the generalpopulation.Complex organisms tolerate genetic variation, suppressingthe potential expression of deleterious alleles byrequiring the combined influence of hypomorphic allelesat multiple loci. Thus, a disease phenotype is revealedonly in some multilocus genotypes and physiological andgenetic homeostasis is maintained. However, this complicatesgenetic dissection of complex disease. First, multiplegenes contribute to clinical expression, yet they maynot contribute equally to the phenotypic variation. Onemust therefore identify multiple contributing genes to explainthe majority of the phenotypic variation. This maybe further complicated by locus heterogeneity whereincomponents of multiple pathways or multiple componentswithin a single pathway may contribute to a phenotype.Second, the presence of disease-causing variants incases and controls necessitates functional analyses to establishtheir respective contributions to disease transmission.Notably, the ultimate proof lies in the synthesis ofthe corresponding complex phenotype in a model organism.Consequently, understanding the genetic basis ofcomplex traits necessitates integration of multiple geneticstrategies, including studies of human populations, comparativegenomics, and model organism-based approaches.We report a synergistic set of approachesaimed at understanding the genetic mechanisms underlyingHirschsprung disease (HSCR), a complex trait andrelatively common birth defect.HSCR is a congenital malformation with an incidencein the general population of 1 per 5,000 live births, and ischaracterized by an absence of neural crest (NC)-derivedintrinsic ganglia along a variable length of the distal intestinaltract. Patients typically present in the neonatal periodwith intestinal obstruction and abdominal distensionresulting from an inability to propagate peristaltic wavesin the distal gut. HSCR also displays several hallmarks ofcomplex genetic disease, including incomplete penetranceand pleiotropic effects of mutant genotypes, amarked sex-difference in clinical expression, and variationin penetrance with extent of aganglionosis. Pathologicalexamination permits classification as long (L)- orshort (S)-segment HSCR dependent on whether aganglionosisextends beyond the upper descending colon (L-HSCR) or not (S-HSCR). Significantly, the observed sexbias differs between these classifications, with a fourfoldexcess of males among S-HSCR patients compared totheir twofold excess in L-HSCR patients (Badner et al.1990). Although it is assumed to be a sex-modified multifactorialtrait (Bodian and Carter 1963; Passarge 1967),molecular genetic studies have shown HSCR to be oligogenic,the reduced penetrance arising from the segregationof one or a few genes. Genetic interaction betweenhypomorphic mutations at known genes explains whysingle-gene inheritance is rare and non-Mendelian patternsof inheritance are common in HSCR.To date, mutations in at least 8 genes encoding membersof the RET receptor tyrosine kinase (RET) pathway,and the endothelin receptor type B (EDNRB) pathway,and the SOX10 and ZFHX1B transcription factors, havebeen reported in patients with HSCR (Fig. 1). The RETand EDNRB receptors are expressed in the same NC-derivedcell populations during development (McCallion etal. 2003), contrasting with their corresponding ligandsthat are expressed in the mesenchymal cell populations ofthe developing gut. Consequently, although HSCR is primarilyrecognized as a defect in NC-derived enteric neurons,it may not be described as a cell-autonomous defect.Targeted and spontaneous mutations of HSCR genes inmice, embryologic analyses of enteric development, andfunctional analyses of gene mutations have also been criticalin illuminating the molecular processes contributingCold Spring Harbor Symposia on Quantitative Biology, Volume LXVIII. © 2003 Cold Spring Harbor Laboratory Press 0-87969-709-1/04. 373