GRM7 variants confer susceptibility to age-related hearing impairment.

Age-related hearing impairment (ARHI), or presbycusis, is the most prevalent sensory impairment in the elderly. ARHI is a complex disease caused by an interaction between environmental and genetic factors. Here we describe the results of the first whole genome association study for ARHI. The study was performed using 846 cases and 846 controls selected from 3434 individuals collected by eight centers in six European countries. DNA pools for cases and controls were allelotyped on the Affymetrix 500K GeneChip for each center separately. The 252 top-ranked single nucleotide polymorphisms (SNPs) identified in a non-Finnish European sample group (1332 samples) and the 177 top-ranked SNPs from a Finnish sample group (360 samples) were confirmed using individual genotyping. Subsequently, the 23 most interesting SNPs were individually genotyped in an independent European replication group (138 samples). This resulted in the identification of a highly significant and replicated SNP located in GRM7, the gene encoding metabotropic glutamate receptor type 7. Also in the Finnish sample group, two GRM7 SNPs were significant, albeit in a different region of the gene. As the Finnish are genetically distinct from the rest of the European population, this may be due to allelic heterogeneity. We performed histochemical studies in human and mouse and showed that mGluR7 is expressed in hair cells and in spiral ganglion cells of the inner ear. Together these data indicate that common alleles of GRM7 contribute to an individual's risk of developing ARHI, possibly through a mechanism of altered susceptibility to glutamate excitotoxicity.

Identification of two major loci that suppress hearing loss and cochlear dysmorphogenesis in Eya1bor/bor mice.

The Eya1(bor) mutant hypomorph contains an intracisternal A particle insertion in intron 7 of the Eya1 gene that results in a 50% reduction in wild-type mRNA levels. The homozygous mutants have middle and inner ear defects and variable kidney abnormalities. The severity of the disorder is affected by genetic background. In contrast to complete deafness and cochlear agenesis in the C3HeB/FeJ strain, F2 Eya1(bor/bor) mutants from an intercross between C3HeB/FeJ-Eya1(bor/+) and C57BL/6J showed variable auditory brain-stem responses and cochlear coiling. In this study, using these F2 Eya1(bor/bor) mutants, we have identified two major loci, Mead1 (modifier of Eya1-associated deafness 1) and Mead2, that are responsible for suppression of the original phenotypes. We have narrowed these two loci to 5.4 and 4.4 cM, respectively, in congenic lines. Quantitative PCR demonstrated that this modifying effect did not result from an increase in wild-type Eya1 mRNA, suggesting Mead1 and Mead2 are interacting directly or indirectly with Eya1 during inner ear development.

Genomic analysis of metabolic pathway gene expression in mice.

BACKGROUND: A segregating population of (C57BL/6J x DBA/2J)F2 intercross mice was studied for obesity-related traits and for global gene expression in liver. Quantitative trait locus analyses were applied to the subcutaneous fat-mass trait and all gene-expression data. These data were then used to identify gene sets that are differentially perturbed in lean and obese mice. RESULTS: We integrated global gene-expression data with phenotypic and genetic segregation analyses to evaluate metabolic pathways associated with obesity. Using two approaches we identified 13 metabolic pathways whose genes are coordinately regulated in association with obesity. Four genomic regions on chromosomes 3, 6, 16, and 19 were found to control the coordinated expression of these pathways. Using criteria that included trait correlation, differential gene expression, and linkage to genomic regions, we identified novel genes potentially associated with the identified pathways. CONCLUSION: This study demonstrates that genetic and gene-expression data can be integrated to identify pathways associated with clinical traits and their underlying genetic determinants.

Thioredoxin-interacting protein deficiency disrupts the fasting-feeding metabolic transition.

Through a positional cloning approach, the thioredoxin-interacting protein gene (Txnip) was recently identified as causal for a form of combined hyperlipidemia in mice (Bodnar, J. S., A. Chatterjee, L. W. Castellani, D. A. Ross, J. Ohmen, J. Cavalcoli, C. Wu, K. M. Dains, J. Catanese, M. Chu, S. S. Sheth, K. Charugundla, P. Demant, D. B. West, P. de Jong, and A. J. Lusis. 2002. Positional cloning of the combined hyperlipidemia gene Hyplip1. Nat. Genet. 30: 110-116). We now show that Txnip-deficient mice in the fed state exhibit a metabolic profile similar to fasted mice, including increased levels of plasma ketone bodies and free fatty acids, decreased glucose, and increased hepatic expression of peroxisome proliferator-activated receptor-gamma coactivator-1alpha, phosphoenolpyruvate carboxykinase, glucose-6-phosphatase, and acyl-CoA oxidase. Dramatic differences in the expression of key metabolic enzymes were also observed in other tissues, and the fat-to-muscle ratio of Txnip-deficient mice was increased by approximately 40%. We demonstrate an effect of Txnip on the redox status, as the Txnip-deficient mice in the fed state had a significant increase in the ratio of NADH to NAD(+). Surprisingly, we observed that Txnip-deficient mice and wild-type mice had similar levels of thioredoxin activity, suggesting that the effects of Txnip deficiency may be mediated in part by other interactions. These results indicate a role for Txnip in the metabolic response to feeding and the maintenance of the redox status.

Increased lipogenesis and fatty acid reesterification contribute to hepatic triacylglycerol stores in hyperlipidemic Txnip-/- mice.

The effect of decreased fatty acid oxidation on liver lipid metabolism in HcB-19 mice, a mouse model of hyperlipidemia (Txnip(-/-)), was investigated using metabolic labeling. De novo cholesterol synthesis and de novo lipogenesis were quantified using 1-(13)C(1) acetic acid, and liver triacylglycerol (TAG) derived from dietary fatty acids was quantified using dietary glyceryl tri(hexandecanoate-d(31)). Tissue samples were analyzed for TAG, free cholesterol (FC), and cholesterol ester (CE) content. Txnip(-/-) mice had significantly elevated (P < 0.05) serum nonesterified fatty acids compared with wild-type (WT) littermates; their livers weighed more and contained more TAG and total cholesterol. Txnip(-/-) liver also contained measurable CE; CE was not detectable in WT mice. Liver CE content was elevated despite lower cholesterol fractional synthesis rates (16 vs. 31%/d in Txnip(-/-) and WT mice, respectively). FC absolute synthesis rate (ASR) in WT mice (0.28 +/- 0.0 micromol/d) was similar to the combined synthesis rates of FC (0.13 +/- 0.10 micromol/d) and CE (0.10 +/- 0.00 micromol/d) in Txnip(-/-) mice. Lipogenesis, as assessed by TAG-palmitate ASR, was significantly greater in Txnip(-/-) mice (1.47 +/- 0.08 vs. 0.49 +/- 0.06 micro mol/d) and liver fatty acid synthase activity was also higher (7.96 +/- 2.53 vs. 4.83 +/- 1.44 U/mg protein). Both elevated lipogenesis and increased fatty acid reesterification to glycerol and cholesterol contributed to fat in the livers of Txnip(-/-) mice. These data support elevated fatty acid synthesis as the primary contributor to liver TAG in Txnip(-/-) mice, although increased esterification of fatty acids also contributed to excess liver TAG. The absolute total cholesterol synthesis rate was not altered, but esterification of fatty acids to cholesterol provided an additional means to buffer physiologically the negative results of excess fatty acid availability.

Mice lacking thioredoxin-interacting protein provide evidence linking cellular redox state to appropriate response to nutritional signals.

Thioredoxin-interacting protein (Txnip) is a ubiquitous protein that binds with high affinity to thioredoxin and inhibits its ability to reduce sulfhydryl groups via NADPH oxidation. HcB-19 mice contain a nonsense mutation in Txnip that eliminates its expression. Unlike normal animals, HcB-19 mice have approximately 3-fold increase in insulin levels when fasted. The C-peptide/insulin ratio is normal, suggesting that the hyperinsulinemia is due to increased insulin secretion. Fasted HcB-19 mice are hypoglycemic, hypertriglyceridemic, and have higher than normal levels of ketone bodies. Ablation of pancreatic beta-cells with streptozotocin completely blocks the fasting-induced hypoglycemia/hypertriglyceridemia, suggesting that these abnormalities are due to excess insulin secretion. This is supported by increased hepatic mRNA levels of the insulin-inducible, lipogenic transcription factor sterol-responsive element-binding protein-1c and two of its targets, acetyl-CoA carboxylase and fatty acid synthase. During a prolonged fast, the hyperinsulinemia up-regulates lipogenesis but fails to down-regulate hepatic phosphoenolpyruvate carboxykinase mRNA expression. Hepatic ratios of reduced:oxidized glutathione, established regulators of gluconeogenic/glycolytic/lipogenic enzymes, were elevated 30% in HcB-19 mice, suggesting a loss of Txnip-enhanced sulfhydryl reduction. The altered hepatic enzymatic profiles of HcB-19 mice divert phosphoenolpyruvate to glyceroneogenesis and lipogenesis rather than gluconeogenesis. Our findings implicate Txnip-modulated sulfhydryl redox as a central regulator of insulin secretion in beta-cells and regulation of many of the branch-points of gluconeogenesis/glycolysis/lipogenesis.

Understanding atherosclerosis through mouse genetics.

During the past year studies with mouse models have significantly clarified our understanding of atherosclerosis. Noteworthy achievements include: the discovery of a number of novel genes and pathways; new evidence emphasizing the role of lymphocytes in atherogenesis; the development of mouse models exhibiting advanced lesions with evidence of thrombosis; and new results indicating an anti-atherogenic effect of testosterone.

Positional cloning of the combined hyperlipidemia gene Hyplip1.

Familial combined hyperlipidemia (FCHL, MIM-144250) is a common, multifactorial and heterogeneous dyslipidemia predisposing to premature coronary artery disease and characterized by elevated plasma triglycerides, cholesterol, or both. We identified a mutant mouse strain, HcB-19/Dem (HcB-19), that shares features with FCHL, including hypertriglyceridemia, hypercholesterolemia, elevated plasma apolipoprotein B and increased secretion of triglyceride-rich lipoproteins. The hyperlipidemia results from spontaneous mutation at a locus, Hyplip1, on distal mouse chromosome 3 in a region syntenic with a 1q21-q23 FCHL locus identified in Finnish, German, Chinese and US families. We fine-mapped Hyplip1 to roughly 160 kb, constructed a BAC contig and sequenced overlapping BACs to identify 13 candidate genes. We found substantially decreased mRNA expression for thioredoxin interacting protein (Txnip). Sequencing of the critical region revealed a Txnip nonsense mutation in HcB-19 that is absent in its normolipidemic parental strains. Txnip encodes a cytoplasmic protein that binds and inhibits thioredoxin, a major regulator of cellular redox state. The mutant mice have decreased CO2 production but increased ketone body synthesis, suggesting that altered redox status down-regulates the citric-acid cycle, sparing fatty acids for triglyceride and ketone body production. These results reveal a new pathway of potential clinical significance that contributes to plasma lipid metabolism.