Characterization of the zebrafish atxn1/axh gene family.

In mammals, ataxin-1 (ATXN1) is a member of a family of proteins in which each member contains an AXH domain. Expansion of the polyglutamine tract in ATXN1 causes the neurodegenerative disease, spinocerebellar ataxia type 1 (SCA1) with prominent cerebellar pathology. Toward a further characterization of the genetic diversification of the ATXN1/AXH gene family, we identified and characterized members of this gene family in zebrafish, a lower vertebrate with a cerebellum. The zebrafish genome encodes two ATXN1 homologs, atxn1a and atxn1b, and one ATXN1L homolog, atxn1l. Key biochemical features of the human ATXN1 protein not seen in the invertebrate homologs (a nuclear localization sequence and a site of phosphorylation at serine 776) are conserved in the zebrafish homologs, and all three zebrafish Atxn1/Axh proteins behave similarly to their human counterparts in tissue-culture cells. Importantly, each of the three homologs is expressed in the zebrafish cerebellum, which in humans, is a prominent site of SCA1 pathogenesis. In addition, atxn1a and atxn1b are expressed in the developing zebrafish cerebellum. These data show that in zebrafish, a lower vertebrate, the complexity of the atxn1/axh gene family is more similar to higher vertebrates than invertebrates with a simple central nervous system and suggests a relationship between the diversification of the ATXN1/AXH gene family and the development of a complex central nervous system, including a cerebellum.

Genetic modifier loci affecting survival and cardiac function in murine dilated cardiomyopathy.

BACKGROUND: Understanding the role for genetic factors in human heart failure is difficult because environmental factors cannot be standardized and genetic variation is great. One approach to identify genes that modify disease outcome is to use mouse models that show strong genetic variation of the disease phenotype. METHODS AND RESULTS: In this study, we used transgenic mice that develop severe dilated cardiomyopathy due to the cardiac-specific overexpression of calsequestrin. Transgenic mice showed marked strain-specific variation of cardiac function and survival, independent of transgene expression. A reciprocal backcross strategy was employed using two inbred strains showing distinct differences in survival and cardiac function. To map the genes that modified the heart failure phenotype, progeny from the 2 reciprocal backcrosses were used in a genome-wide scan for linkage. We identified two loci significantly linked to survival with a maximum likelihood ratio statistic of 36.2 (LOD score approximately 7.8) on chromosome 2 and of 26.5 (LOD score approximately 5.7) on chromosome 3. The chromosome 3 locus was also significantly linked to cardiac function with a maximum likelihood ratio statistic of 42.9 (LOD score approximately 9.3). Because only a single strong modifier locus was found in each backcross, we applied a haplotype analysis to map crossovers and successfully narrowed the critical intervals for each locus. CONCLUSION: Using a sensitized mouse model, we identified major modifier loci that affect the genetically complex disease of heart failure. This approach should allow the rapid identification of candidate genes involved in disease susceptibility in human populations and new insights into the pathogenesis of heart failure.

Emerging pathogenic pathways in the spinocerebellar ataxias.

The spinocerebellar ataxias (SCAs) are diseases characterized by neurodegeneration of the spinocerebellum. To date, 28 autosomal dominant SCAs have been described and seventeen causative genes identified. These genes play a role in a broad range of cellular processes. Recent studies focused on the wild type and pathogenic functions of these genes implicate both gene expression and glutamate-dependent and calcium-dependent neuronal signaling as important pathways leading to cerebellar dysfunction. Understanding how these genes cause disease will allow a deeper understanding of the cerebellum in particular as well as neurodegenerative disease in general.

Precocious osteoarthritis in a family with recurrent COL2A1 mutation.

OBJECTIVE: To examine the genotypic and phenotypic characteristics of a Micronesian kindred with autosomal dominant precocious osteoarthritis (OA). METHODS: We reviewed records and radiographs of 3 index patients and their parents, administered questionnaires to 16 additional kindred members, performed whole-genome scans of 24 family members, and sequenced relevant genes from 16 family members. RESULTS: The kindred displayed early onset OA, enlarged epiphyses, platyspondyly, and brachydactyly with dysplastic findings consistent with mild spondyloepiphyseal dysplasia. Genetic analysis revealed an arginine to cysteine substitution at position 75 of the collagen 2A1 gene, a mutation that has been described in 4 other geographically distinct families. The major phenotypic differences among the families were in height (ranging from short to tall) and hearing loss noted in 3 of the 5 families. CONCLUSION: The presence of the COL2A1 Arg75Cys mutation in 5 geographically distinct areas helps to confirm a potential mutational hotspot. The diverse phenotypic spectrum suggests that modifier genes and environmental factors play a role in the expression of this mutation.

QTL mapping in a mouse model of cardiomyopathy reveals an ancestral modifier allele affecting heart function and survival.

The progression from myocardial hypertrophy to heart failure is a complex process, involving genetic and environmental factors. Elucidating the genetic components contributing to heart failure has been difficult, largely because of the heterogeneity of human populations. We have employed a strategy to map genetic loci that modify the heart failure phenotype in a transgenic mouse model of cardiomyopathy caused by cardiac-specific overexpression of calsequestrin. Strain-specific differences in both cardiac function and survival are observed when the transgene is moved into different inbred mouse strains. We have previously reported linkage results from mapping in reciprocal backcrosses between C57/BL6 (BL6) and DBA/2J (DBA) and a backcross between DBA/AKR and AKR. Here we report the results of a genome-wide linkage scan in the reciprocal backcross between DBA/AKR and DBA. We identified one novel locus on Chromosome (Chr) 18 that affects heart function and a second on Chr 3 that shows significant linkage to both survival and heart function. Intriguingly, the Chr 3 allele of AKR shows a susceptibility effect on phenotype, whereas the overall effect of the AKR genetic background is protective. The Chr 3 locus also completely overlaps the Hrtfm2 locus, which was previously mapped in crosses between DBA and BL6. Mapping the same QTL in two different crosses allowed us to use ancestral haplotypes to narrow the candidate gene interval from 9 to 2 Mb. Identification of the genes at these QTLs in the mouse will provide novel candidate genes that can be evaluated for their role in human heart failure.

Multiple quantitative trait loci modify the heart failure phenotype in murine cardiomyopathy.

The variability in outcome of heart failure patients depends on a number of factors including differences in their genetic background. To identify novel genes that modify the human heart failure phenotype, we used a strategy of quantitative trait locus (QTL) mapping in an experimental mouse model of dilated cardiomyopathy induced by cardiac-specific overexpression of calsequestrin and characterized by a strong strain-specific variability in the phenotype. We identified two novel QTLs, Hrtfm3 (heart failure modifier 3) on chromosome (Chr) 4 and Hrtfm4 on Chr 18, significantly linked to survival with likelihood ratio statistics (LRS) of 19.9 and 23.6 respectively (corresponding to LOD scores of 4.3 and 5.1). Two other QTLs, Hrtfm5 on Chr 2 and Hrtfm6 on Chr 13, were significantly linked to cardiac function as measured by echocardiographic fractional shortening (LRS 22.1 and 15.2 respectively, LOD score 4.8 and 3.3) and left ventricular end-diastolic diameter (LRS 23.5 and 18.8, LOD score 5.1 and 4.1). Importantly, Hrtfm5 was not significantly linked to survival. A significant interaction was found between Hrtfm4 and two other QTLs (Hrtfm6 and a QTL near to the marker D19Mit88) for fractional shortening with a LRS of 34.6 and 26.5 respectively (LOD score 7.5 and 5.8). These data show that the effect of genetic background on murine heart failure is complex and result from the action of several loci that differentially modify the cardiac phenotype. The identification of these novel modifier genes will serve as strong candidates for the discovery of modifiers in human heart failure.