The National Institute of Diabetes & Digestive & Kidney Diseases

Repeat Expansion Diseases arise from an increase in the number of repeats in a specific tandem repeat tract. My group works on the diseases resulting from expansion at 2 different genomic loci. Friedreich ataxia is caused by the expansion of a GAA•TTC repeat in the first intron of the frataxin gene. This results in reduced levels of frataxin mRNA which leads to sensory motor neuron degeneration, diabetes and cardiomyopathy. Expansion of a CGG•CCG-repeat in the 5’ untranslated region of the FMR1 gene has different consequences depending on the number of repeats in the expanded allele. Carriers of so-called premutation alleles which have 55-200 repeats are at risk of a neurodegenerative disorder known as Fragile X associated tremor and ataxia syndrome (FXTAS). Female carriers are also at risk for Fragile X associated primary ovarian insufficiency. These individuals make more FMR1 mRNA than individuals with normal alleles. Female carriers are also at risk of transmission of an FMR1 allele with a greatly expanded repeat tract to their offspring. Alleles with more than 200 repeats are known as full mutation alleles. Carriers of such alleles are at very high risk of a developmental disorder known as Fragile X Syndrome in which the FMR1 gene is silenced. Symptoms of this disorder include moderate to severe intellectual disability, autistic behavior, connective tissue abnormalities, digestive difficulties, and occasionally hyperphagia and obesity. Expansion also results in the appearance of a folate-sensitive fragile site coincident with the expansion. Fragile sites are prone to breakage in vivo and these sites often coincide with deletion or translocation breakpoints in a number of malignancies.

These diseases are interesting not only because they provide a window into critical processes such as learning and memory, but also because there is evidence to suggest that some aspects of disease pathology may involve a variety of interesting and incompletely understood mechanisms including RNA toxicity and repeat-mediated chromatin remodeling. We are using a number of approaches to look at both the mechanism of expansion and the consequences of expansion in these two disorders. These include biochemical studies of the unusual nucleic acid structures formed by disease associated repeats, in situ hybridization and immunocytochemistry to examine the molecular basis of the chromosome fragility seen in individuals with Fragile X Syndrome, chromatin immunoprecipitation and qRT-PCR to understand the gene misregulation responsible for both groups of disorders, as well as the development of various in vitro, bacterial, tissue culture, and animal models for different aspects of these diseases.

Kumari, D., Biacsi, R.E. and Usdin, K. Repeat expansion affects both transcription initiation and elongation in Friedreich ataxia cells. Journal of Biological Chemistry. In press. [ Full Text ]

Kumari, D. and Usdin, K. The distribution of repressive histone modifications on silenced FMR1 alleles provides clues to the mechanism of gene silencing in fragile X syndrome Human Molecular Genetics. 19: 4634-4642, 2010. [ Full Text ]

Entezam, A., Lokanga, A. R., Le, W., Hoffman, G. and Usdin, K. Potassium bromate, a potent DNA oxidizing agent exacerbates germline repeat expansion in a Fragile X premutation mouse model. Human Mutation. 31: 611-616, 2010. [ Abstract ]

Kumari, D. , Somma, V, Nakamura, A.J., Bonner, W.M., D'Ambrosio, E. and Usdin, K. The role of DNA damage response pathways in chromosome fragility in Fragile X syndrome. Nuc. Acids. Res. 37: 4385-92, 2009. [ Full Text / Abstract ]

Kumari, D. and Usdin, K. Chromatin remodeling in the noncoding repeat expansion diseases. J Biol Chem. 284: 7413-7, 2009. [ Full Text / Abstract ]

Entezam, A. and Usdin, K. ATM and ATR protect the genome against two different types of tandem repeat instability in Fragile X premutation mice. Nucleic Acids Research. 37: 6371-6377. [ Full Text ]

Usdin, K. The biological effects of simple tandem repeats: lessons from the repeat expansion diseases. Genome Res . 18: 1011-9, 2008. [ Full Text/Abstract ]

Entezam, A. and Usdin, K. ATR protects the genome against CGG.CCG-repeat expansion in Fragile X premutation mice. Nucleic Acids Res 36: 1050-6, 2008. [ Full Text / Abstract ]

Biacsi, R., Kumari, D., and Usdin, K. SIRT1 inhibition alleviates gene silencing in Fragile X mental retardation syndrome. PLoS Genetics 4:e1000017, 2008. [ Full Text / Abstract ]

Entezam, A., Biacsi, R., Orrison, B., Saha, T., Hoffman, G.E., Grabczyk, E., Nussbaum, R.L., Usdin, K. Regional FMRP deficits and large repeat expansions into the full mutation range in a new Fragile X premutation mouse model. Gene 395: 125-34, 2007. [ Full Text] / Abstract ]

Greene, E., Mahishi, L., Entezam, A., Kumari, D., Usdin, K. Repeat-induced epigenetic changes in intron 1 of the frataxin gene and its consequences in Friedreich ataxia. Nucleic Acids Res 35: 3383-90, 2007. [ Full Text] / Abstract ]

Mahishi, L. and Usdin, K. NF-Y, AP2, Nrf1 and Sp1 regulate the fragile X-related gene 2 (FXR2). Biochem J 400: 327-35, 2006. [ Full Text / Abstract ]

Greene, E., Entezam, A., Kumari, D. and Usdin, K. Ancient repeated DNA elements and the regulation of the human frataxin promoter. Genomics 85: 221-30, 2005. [ Full Text / Abstract ]

Handa, V., Goldwater, D., Stiles, D., Cam, M., Poy, G., Kumari, D. and Usdin, K. Long CGG-repeat tracts are toxic to human cells: implications for carriers of Fragile X premutation alleles. FEBS Lett 579: 2702-8, 2005. [ Full Text / Abstract ]

Handa, V., Yeh, H.J., McPhie, P. and Usdin, K. The AUUCU repeats responsible for spinocerebellar ataxia type 10 form unusual RNA hairpins. J Biol Chem 280: 29340-5, 2005. [ Full Text / Abstract ]

Handa, V., Saha, T. and Usdin, K. The fragile X syndrome repeats form RNA hairpins that do not activate the interferon-inducible protein kinase, PKR, but are cut by Dicer. Nucleic Acids Res 31: 6243-8, 2003. [ Full Text / Abstract ]