Essential tremor (ET) is one of the most common neurological disorders in humans with a prevalence reaching almost four percent. ET is inherited as a dominant trait with incomplete penetrance in most familial cases but complex multi-genic transmission is possible. Three genetic susceptibility loci for familial ET exist on chromosomes 3p13.1 (ETM1), 2p24 (ETM2), and 6p23 (ETM3), but the causal genes have not been identified in the families linked to these loci. Polymorphic ETM1 loci in 16 Icelandic families and a variant in the dopamine receptor D3 gene in 23 French families are associated with an ET phenotype. ETM2 loci are linked to a disease allele in four American families. Further studies suggest a tight allelic association between ETM2 and the ET phenotype in populations from the United States, Singapore, and Korea. Two North American families are linked to ETM3 loci with the largest family showing a mixed phenotype that includes dystonia in addition to ET. An unclear pattern of inheritance due to the presence of phenocopies, incomplete penetrance, and the high prevalence of ET, hinder the search for causal genes. Advances in solid- phase microarrays, and instrumentation have partially alleviated these barriers by permitting large scale DNA sequencing. Advances in genome bioinformatics and the availability of validated normative population databases (e.g. 1000 Genomes SNP database, dbSNP, and HapMap) provide the capability to filter genetic variants from putative mutations. By using this technology, we recently identified missense mutations in a novel gene, feline leukemia virus subgroup C receptor 1 gene, as the cause of the Mendelian disorder, posterior column ataxia and retinitis pigmentosa, in three unrelated families by high density sequencing of the 4.2 megabase (Mb) candidate region. This discovery confirms our expertise in all of the specific technologies required to carry out the proposed genetic studies and provides proof of principle for Specific Aim 1 to identify the genes that cause ET in two large, informative families linked to the ETM2 and ETM3 loci using high- throughput resequencing technology. We have designed a bait tiling library using 120 base-pair bait lengths for the 24.6 Mb ETM2 (48,054 baits) and the 14.4 Mb ETM3 (30,796 baits) loci to include all regulatory regions, exons, splice sites, enhancer, and conserved intergenic regions. A power analysis estimates that the sample size required to detect mutations with 95% confidence are 10 affected individuals with a disease haplotype and 10 unaffected individuals without the disease haplotype from each family linked to the ETM2 and ETM3 loci. Sanger sequencing will reconfirm the ETM2 and ETM3 mutations identified in the 20 affected individuals from the two families. These candidate genes will be Sanger sequenced in 73 additional, unrelated families with ET to identify other ETM2 and ETM3 mutations. Future experiments using in vitro and in vivo models will analyze the functional consequences of ET gene mutations during aging and development.
The identification of ET genes will improve diagnostic accuracy, refine the classification of tremor, and is a pivotal step toward finding effective treatments. Studying the untoward effects of mutant ET genes will increase our knowledge of ET and other movement disorders such as Parkinson disease.
| Liu, Pengfei; Gelowani, Violet; Zhang, Feng et al. (2014) Mechanism, prevalence, and more severe neuropathy phenotype of the Charcot-Marie-Tooth type 1A triplication. Am J Hum Genet 94:462-9 |
