Cryptic Polyglutamine Repeat Sequence Variation and Somatic Instability in Huntington’s Disease: Drivers of Pathology?

It is known that in addition to repeat length variation, the exact sequence of the polyglutamine repeat tract and the adjacent polyproline also vary. Likewise, it is known that the expanded CAG is somatically unstable in a process that is age-dependent, tissue-specific and expansion biassed. Notably, very large alleles exceeding greater than 1,000 repeats are observed in a subset of striatal neurons. These data strongly suggest that somatic instability contributes toward the tissue specificity and progressive nature of the symptoms. Indeed, it has been shown that the frequency of large expansions in cortical cells correlates with variation in age at onset not accounted for by inherited repeat length. Most recently, it has been demonstrated that mismatch repair genes lie under some of the association peaks for genome wide analysis of variants contributing to variation in age at onset. Thus, in order to further address these issues, we have developed a high-throughput sequencing pipeline that allows us to determine the precise sequence of the polyglutamine and polyproline tracts. These studies have revealed an unexpectedly high frequency of atypical non-pathogenic alleles. We have also detected novel CCG interruptions within the CAG array in a large ‘premutation’ length allele. The majority of expanded HD alleles retain the expected structure of CAG and CCG repeats, but a subset of atypical pathogenic alleles have been detected. Using this approach we can also estimate the degree of somatic mosaicism present in the blood DNA of each participant. These data confirm that as expected, somatic mosaicism in the blood DNA of HD individuals is age- and allele length-dependent. We are currently attempting to understand how atypical alleles and individual-specific mutational dynamics contribute toward phenotypic variability in HD. We are also investigating how variants in the mismatch repair genes contribute toward variation in somatic mosaicism