Age-, tissue- and length-dependent bidirectional somatic CAG•CTG repeat instability in an allelic series of R6/2 Huntington disease mice.

The expansion of simple sequence CAG•CTG repeats is associated with a number of inherited disorders including Huntington disease (HD), myotonic dystrophy type 1 and several of the spinocerebellar ataxias. Inherited disease-associated alleles usually exceed 40 repeats and may be in excess of 1,000 repeats in some disorders. Inherited allele length is inversely proportional to age at onset, and frequent germline expansions account for the striking anticipation observed in affected families. Expanded disease associated alleles are also somatically unstable via a pathway that is age dependent and tissue specific, and also appears to be expansion biased. Somatic expansions are thought to contribute toward both tissue specificity and disease progression. Here we have examined the somatic mutational dynamics in brain and peripheral tissues from an allelic series of R6/2 HD transgenic mice inheriting from 52 to >700 CAG repeats. We found age-dependent, tissue-specific somatic instability, with particularly large expansions observed in the striatum and cortex. We also found a positive increase in somatic instability with increasing allele length. Surprisingly, however, the degree of somatic variation did not increase in a linear fashion, but leveled off with increasing allele length. Most unexpectedly, the almost exclusive bias toward the accumulation of expansions observed in mice inheriting smaller alleles was lost, and a high frequency of large somatic contractions was observed in mice inheriting very large alleles (>500 repeats). These data highlight the bidirectional nature of CAG•CTG repeat instability and the subtle balance that exists between expansion and contraction in vivo. Defining the dynamics and tissue specificity of expansion and contraction is important for understanding the role of genetic instability in pathophysiology and in particular the development of novel therapies based on suppressing expansions and/or promoting contractions.