The National Institute of Diabetes & Digestive & Kidney Diseases

The Repeat Expansion Diseases are a group of human genetic disorders that arise from an expansion or increase in the length of a specific tandem repeat tract. More than 20 diseases are known to belong to this group including Huntington Disease and many spinocerebellar ataxias. My lab works on Friedreich ataxia and the Fragile X-related disorders where disease pathology results from expansions of repeat tracts that are located outside the coding sequence of the affected gene. Friedreich ataxia is caused by the expansion of a GAA•TTC repeat in the first intron of the frataxin (FXN) gene. This results in heterochromatinization of the 5’ end of the FXN gene and reduced levels of the FXN gene product, frataxin. This leads to sensory motor neuron degeneration, diabetes and hypertrophic 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 (PM) alleles have 55-200 repeats and 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 (FXPOI), a condition that accounts for ~11% of familial cases of infertility and ~4% of idiopathic cases. Female carriers are also at risk of transmission of much larger repeat tract to their offspring. PM carriers show repeat-mediated chromatin changes that are associated with hyper-expression of the PM allele. Alleles containing >200 repeats are known as full mutation (FM) alleles and most carriers of such alleles have Fragile X Syndrome (FXS), the most common heritable cause of intellectual disability and the most common monogenic cause of autism. Expansion to the FM results in epigenetic silencing of the FMR1 gene. FM alleles also display a folate-sensitive fragile site coincident with the expanded repeat. Fragile sites are prone to breakage in vivo and these sites often coincide with deletion or translocation breakpoints in different malignancies. FM alleles are associated with a high risk of loss of the affected X chromosome in females resulting in Turner Syndrome.

In situ hybridization of metaphase chromosome spreads from a patient with Fragile X Syndrome showing the chromosomes in blue and the region of the X chromosome containing the FMR1 gene in yellow.

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, induced pluripotent stem cell (iPSC) and animal models for different aspects of these diseases.

Hoffman GE, Le W, Entezam A, Otsuka N, Tong Z-B, Nelson, L., Flaws, JA, McDonald, JH, Jafar, S and Usdin, K. (2012) Ovarian abnormalities in a mouse model of Fragile X-associated primary ovarian insufficiency. J Histochem and Cytochem 60: 439-456. [Full Text / Abstract]

Kumari D, Lokanga R, Yudkin D, Zhao XN, Usdin K (2012) Chromatin changes in the development and pathology of the Fragile X-associated disorders and Friedreich ataxia. Special Issue of Biochim Biophys Acta 1819: 802-810. [Full Text / Abstract]

Lokanga RA, Entezam A, Kumari D, Yudkin D, Qin M, et al. (2012) Somatic expansion in mouse and human carriers of Fragile X premutation alleles. Hum Mutat. [Full Text / Abstract]

Qin M, Entezam A, Usdin K, Huang T, Liu ZH, et al. (2011) A mouse model of the fragile X premutation: effects on behavior, dendrite morphology, and regional rates of cerebral protein synthesis. Neurobiol Dis 42: 85-98. [Full Text / Abstract]

Kumari, D., Biacsi, R.E. and Usdin, K. (2011) Repeat expansion affects both transcription initiation and elongation in Friedreich ataxia cells. Journal of Biological Chemistry. 286: 4209-4215. [ Full Text ]

Kumari, D. and Usdin, K. (2010) 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. [ Full Text ]

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

Kumari, D. , Somma, V, Nakamura, A.J., Bonner, W.M., D'Ambrosio, E. and Usdin, K. (2009) The role of DNA damage response pathways in chromosome fragility in Fragile X syndrome. Nuc. Acids. Res. 37: 4385-4392. [ Full Text / Abstract ]

Kumari, D. and Usdin, K. (2009) Chromatin remodeling in the noncoding repeat expansion diseases. J Biol Chem. 284: 7413-7417, [ Full Text / Abstract ]

Entezam, A. and Usdin, K. (2009) 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. (2008) The biological effects of simple tandem repeats: lessons from the repeat expansion diseases. Genome Res . 18: 1011-1019. [ Full Text / Abstract ]

Entezam, A. and Usdin, K. (2008) ATR protects the genome against CGG.CCG-repeat expansion in Fragile X premutation mice. Nucleic Acids Res 36: 1050-1056. [ Full Text / Abstract ]

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

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

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

Greene, E., Entezam, A., Kumari, D. and Usdin, K. (2005) Ancient repeated DNA elements and the regulation of the human frataxin promoter. Genomics 85: 221-320. [Full Text / Abstract ]

Handa, V., Goldwater, D., Stiles, D., Cam, M., Poy, G., Kumari, D. and Usdin, K. (2005) Long CGG-repeat tracts are toxic to human cells: implications for carriers of Fragile X premutation alleles. FEBS Lett 579: 2702-2708. [Full Text / Abstract ]

Handa, V., Saha, T. and Usdin, K. (2005) 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: 6243-6248. [ Full Text / Abstract ]