The Repeat Expansion Diseases are a large group of human genetic disorders that arise from an increase (expansion) in the number of repeats in a single tandem repeat tract. The mechanism of expansion is unknown. The consequences of expansion depend on a combination of the biochemical properties of the repeat, its location within the affected gene, and the normal function of that gene. My group is particularly interested in those diseases where the repeat is located outside of the open reading frame and thus, where the consequences of expansion are not immediately apparent.
Most recently my group have focused on the Fragile X–related disorders. These disorders result from expansion of a CGG•CCG-repeat in the 5’ untranslated region of the FMR1 gene. Expansion has different consequences depending on the number of repeats in the expanded allele. Carriers of so-called premutation (PM) alleles, which have 55–200 repeats, are at risk of a neurodegenerative disorder known as Fragile X–associated tremor/ataxia syndrome (FXTAS). This disorder involves, in addition to gait and balance difficulties, cognitive decline, loss of executive function skills, autonomic dysfunction, and dementia. Female carriers are also at risk for Fragile X–associated primary ovarian insufficiency (FXPOI), a disorder involving infertility, menstrual abnormalities, and a risk of early menopause. Female carriers are also at risk of transmission of an FMR1 allele with >200 repeats to their offspring. Such alleles are known as full mutation (FM) alleles and in contrast to PM alleles, which are often hyperexpressed, FM alleles are epigenetically silenced. This results in Fragile X Syndrome (FXS), the most common heritable cause of intellectual disability. In addition to moderate to severe intellectual disability, symptoms of this disorder include 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 such sites in other regions of the genome often coincide with deletion or translocation breakpoints in a number of malignancies. In the case of this particular fragile site, fragility may be responsible for the elevated levels of Turner syndrome seen in females who inherit an FM allele.
These diseases are interesting not only because they provide a window into critical processes such as brain and ovarian development, but also because there is evidence to suggest that some aspects of disease pathology may involve a variety of interesting and incompletely understood mechanisms such as RNA toxicity and repeat-mediated chromatin remodeling. We are using a number of approaches to look at both the mechanisms of expansion and the consequences of expansion in the Fragile X-related disorders. Our approaches include in vitro biochemistry as well as the use of various stem cell and animal models for different aspects of these diseases. Our recent work has demonstrated that the expansion mutation results from the combined action of proteins normally involved in three different DNA repair pathways, base excision repair, mismatch repair and transcription coupled repair. However, just how these proteins that normally act to protect the genome against mutation, interact to generate the expansion mutation is unclear. Our current work is aimed at trying to understand this interaction. We are also interested in factors both genetic and environmental that affect mutation risk. For example, we have previously shown that oxidative stress increases the expansion frequency. We are now trying to identify antioxidants and other small molecules that may ameliorate expansion risk. Since there is evidence to suggest that PM carriers have elevated levels of oxidative stress, it may be that antioxidants may have benefits beyond simply reducing the risk of having a child with FXS. We are also currently trying to understand the processes responsible for the repeat-mediated gene dysregulation that is seen in both PM and FM carriers. Amongst our recent findings is the demonstration that the transcript of the FMR1 gene is required for FMR1 gene silencing. The transcript is likely acting via its ability to recruit transcriptional repressors such as Polycomb Repressive Complex 2 (PRC2). However, we have shown that there is a way around this Catch-22 situation. Specifically, we have shown that molecules that interfere with recruitment of PRC2 or that inhibit the enzymatic activity of this complex can prevent gene silencing. Our data suggests that it may be possible to develop gene-specific treatments for the epigenetic abnormality responsible for FXS.
These disorders currently have no effective treatments or cure. We hope that a better understanding of the causes and consequences of the underlying mutation will lead to effective treatments for these disorders and for other disorders like them.