Browsing by Author "Lokanga, Rachel Adihe"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
- ItemOpen AccessHeterozygosity for a hypomorphic polβ mutation reduces the expansion frequency in a mouse model of the fragile x-related disorders(Public Library of Science, 2015) Lokanga, Rachel Adihe; Senejani, Alireza Ghodsi; Sweasy, Joann Balazs; Usdin, KarenAuthor Summary Unstable microsatellites are responsible for a number of debilitating human diseases known as the Repeat Expansion Diseases. The unstable microsatellites, which consist of tandem arrays of short repeat units, are prone to increase in length (expand) on intergenerational transmission and during the lifetime of the individual. Unlike the typical microsatellite instability seen in disorders like Lynch syndrome that arise from mutations in mismatch repair (MMR) genes, expansions of these microsatellites are abolished when MMR is lost. However, how MMR, which normally protects the genome against microsatellite instability, actually promotes microsatellite expansions in these diseases is unknown. There is evidence to suggest that a second DNA repair process, base excision repair (BER), may be involved, but whether the nicks generated early in the BER-process are subverted by an MMR-dependent pathway that generates expansions or whether some MMR proteins contribute to a BER-based expansion process is unclear. Here we show that a mutation that reduces the activity of Polβ, an essential BER enzyme, also reduces the expansion frequency. Since Polβ is essential for key events in BER downstream of the generation of nicks, our data favor a model in which expansions occur via a BER-dependent pathway in which MMR participates.
- ItemOpen AccessSomatic expansion of premutation alleles and the role of the mismatch repair and base excision repair proteins on repeat expansion in a mouse model of the fragile X-related disorders(2016) Lokanga, Rachel Adihe; Parker, Iqbal; ; Usdin, KarenThe Fragile X-related disorders arise from an unusual mutation in the X-linked FMR1 gene. The mutation involves expansion, or an increase in the number of repeats, in a CGG•CCG repeat tract located in its 5' untranslated region. FMR1 alleles carrying 55-200 repeats are called Premutation (PM) alleles, and cause Fragile X associated tremor/ataxia syndrome (FXTAS) and Fragile X-associated primary ovarian insufficiency (FXPOI). FMR1 alleles having more than 200 repeats are referred to as full mutation (FM) alleles and cause Fragile X syndrome (FXS). These different alleles arise by intergenerational expansion of the repeat tract from smaller unstable alleles by a mechanism that is unknown. We have shown that in addition to germ line expansion, somatic expansion also occurs in a human cell line in vivo and in a FX PM mouse model. In the mouse model, we found that the extent of somatic instability is dependent on age, gender and tissue. Specifically, organs such as brain, liver and gonads are susceptible to expand more than heart and kidney and expansion is much more frequent in males than in females. No differences were found between male and female mice in the levels of the DNA repair proteins that had already been implicated in repeat expansion in model systems of other disorders thought to arise via a similar mechanism. Neither were there any differences between males and females in the amounts of proteins produced from X-linked DNA repair genes. We also showed that estrogen did not protect against expansion. However, we found that PM alleles expanded exclusively when they were located on the active X chromosome. Thus some of the differences between males and xii females in the level of somatic expansion might be due to the fact that females undergo X inactivation and thus have the PM allele on the inactive X chromosome in half (~50%) of their cells. It also indicates that transcription and/or an open chromatin configuration is required for expansion in the FX PM mouse.