An ethnic-specific polymorphism in the catalytic subunit of glutamate-cysteine ligase impairs the production of glutathione intermediates in vitro.

Glutathione plays a crucial role in free radical scavenging, oxidative injury, and cellular homeostasis. Previously, we identified a non-synonymous polymorphism (P462S) in the gene encoding the catalytic subunit of glutamate-cysteine ligase (GCLC), the rate-limiting enzyme in glutathione biosynthesis. This polymorphism is present only in individuals of African descent. Presently, we report that this ethnic-specific polymorphism (462S) encodes an enzyme with significantly decreased in vitro activity when expressed by either a bacterial or mammalian cell expression system. In addition, overexpression of the 462P wild-type GCLC enzyme results in higher intracellular glutathione concentrations than overexpression of the 462S isoform. We also demonstrate that apoptotically stimulated mammalian cells overexpressing the 462S enzyme have increased caspase activation and increased DNA laddering compared to cells overexpressing the wild-type 462P enzyme. Finally, we genotyped several African and African-descent populations and demonstrate that the 462S polymorphism is in Hardy-Weinberg disequilibrium, with no individuals homozygous for the 462S polymorphism identified. These findings describe a glutathione production pathway polymorphism present in individuals of African descent with significantly decreased in vitro activity.

The X chromosome in quantitative trait locus mapping.

The X chromosome requires special treatment in the mapping of quantitative trait loci (QTL). However, most QTL mapping methods, and most computer programs for QTL mapping, have focused exclusively on autosomal loci. We describe a method for appropriate treatment of the X chromosome for QTL mapping in experimental crosses. We address the important issue of formulating the null hypothesis of no linkage appropriately. If the X chromosome is treated like an autosome, a sex difference in the phenotype can lead to spurious linkage on the X chromosome. Further, the number of degrees of freedom for the linkage test may be different for the X chromosome than for autosomes, and so an X chromosome-specific significance threshold is required. To address this issue, we propose a general procedure to obtain chromosome-specific significance thresholds that controls the genomewide false positive rate at the desired level. We apply our methods to data on gut length in a large intercross of mice carrying the Sox10Dom mutation, a model of Hirschsprung disease. We identified QTL contributing to variation in gut length on chromosomes 5 and 18. We found suggestive evidence of linkage to the X chromosome, which would be viewed as strong evidence of linkage if the X chromosome was treated as an autosome. Our methods have been implemented in the package R/qtl.

Lack of association between autism and four heavy metal regulatory genes.

Autism is a common neurodevelopmental disorder with genetic and environmental components. Though unproven, genetic susceptibility to high mercury (Hg) body burden has been suggested as an autism risk factor in a subset of children. We hypothesized that exposure to "safe" Hg levels could be implicated in the etiology of autism if genetic susceptibility altered Hg's metabolism or intracellular compartmentalization. Genetic sequences of four genes implicated in the transport and response to Hg were screened for variation and association with autism. LAT1 and DMT1 function in Hg transport, and Hg exposure induces MTF1 and MT1a. We identified and characterized 74 variants in MT1a, DMT1, LAT1 and MTF1. Polymorphisms identified through screening 48 unrelated individuals from the general and autistic populations were evaluated for differences in allele frequencies using Fisher's exact test. Three variants with suggestive p-values <0.1 and four variants with significant p-values <0.05 were followed-up with TaqMan genotyping in a larger cohort of 204 patients and 323 control samples. The pedigree disequilibrium test was used to examine linkage and association. Analysis failed to show association with autism for any variant evaluated in both the initial screening set and the expanded cohort, suggesting that variations in the ability of the four genes studied to process and transport Hg may not play a significant role in the etiology of autism.

Genome-wide linkage identifies novel modifier loci of aganglionosis in the Sox10Dom model of Hirschsprung disease.

Hirschsprung disease (HSCR) is a complex disorder that exhibits incomplete penetrance and variable expressivity due to interactions among multiple susceptibility genes. Studies in HSCR families have identified RET-dependent modifiers for short-segment HSCR (S-HSCR), but epistatic effects in long-segment (L-HSCR) and syndromic cases have not been fully explained. SOX10 mutations contribute to syndromic HSCR cases and Sox10 alleles in mice exhibit aganglionosis and pigmentary anomalies typical of a subset of HSCR patients categorized as Waardenburg-Shah syndrome (WS4, OMIM 277580). Sox10 mutant alleles in mice exhibit strain-dependent variation in penetrance and expressivity of aganglionic megacolon analogous to the variation observed in patients with aganglionosis. In this study, we focused on enteric ganglia deficits in Sox10Dom mice and defined aganglionosis as a quantitative trait in Sox10Dom intercross progeny to investigate the contribution of strain background to variation in enteric nervous system deficits. We observe that the phenotype of Sox10Dom/+ mutants ranges over a continuum from severe aganglionosis to no detectable phenotype in the gut. To systematically identify genes that modulate Sox10-dependent aganglionosis, we performed a single nucleotide polymorphism-based genome scan in Sox10Dom/+ F1 intercross progeny. Our analysis reveals modifier loci on mouse chromosomes 3, 5, 8, 11 and 14 with distinct effects on penetrance and severity of aganglionosis. Three of these loci on chromosomes 3, 8 and 11 do not coincide with previously known aganglionosis susceptibility genes or modifier loci and offer new avenues for elucidating the genetic network that modulates this complex neurocristopathy.

Interactions between Sox10 and EdnrB modulate penetrance and severity of aganglionosis in the Sox10Dom mouse model of Hirschsprung disease.

Cumulative evidence suggests that Hirschsprung disease (HSCR) is the consequence of multiple gene interactions that modulate the ability of enteric neural crest (NC) cells to populate the developing gut. One of the essential genes for this process is the NC transcription factor Sox10. Sox10Dom mice on a mixed genetic background show variation in penetrance and expressivity of enteric aganglionosis that are analogous to the variable aganglionosis seen in human HSCR families. The phenotype of Sox10Dom mice in congenic lines indicates this variation arises from modifiers in the genetic background. To determine whether known HSCR susceptibility loci are acting as modifiers of Sox10, we tested for association between genes in the endothelin signaling pathway (EdnrB, Edn3, Ece1) and severity of aganglionosis in an extended pedigree of B6C3FeLe.Sox10Dom mice. Single locus association analysis in this pedigree identifies interaction between EdnrB and Sox10. Additional analysis of F2 intercross progeny confirms a highly significant effect of EdnrB alleles on the Sox10Dom/+ phenotype. The presence of C57BL/6J alleles at EdnrB is associated with increased penetrance and more severe aganglionosis in Sox10Dom mutants. Crosses between EdnrB and Sox10 mutants corroborate this gene interaction with double mutant progeny exhibiting significantly more severe aganglionosis. The background strain of the EdnrB mutant further influences the phenotype of Sox10/EdnrB double mutant progeny implying the action of additional modifiers on this phenotype. Our data demonstrates that Sox10-EdnrB interactions can influence development of the enteric nervous system in mouse models and suggests that this interaction could contribute to the epistatic network producing variation between patients with aganglionosis.