Chromatin and the CAG trinucleotide repeat: The contribution of histone H2A and the chromatin remodeling complex Isw1 to the maintenance of CAG/CTG repeat stability and chromatin structure

Human trinucleotide repeat diseases arise when repeats in the genome expand past a stable threshold. CAG repeats can form unusual secondary structures and in order to correctly process such events, the cell must work within the context of the chromatin structure at the CAG repeat tract to prevent expansions. CAG/CTG repeats are a strong nucleosome positioning element and may exhibit special chromatin features that influence resolution of breakage intermediates at the CAG repeat. In a genetic screen for CAG repeat instability in S. cerevisiae, both the histone H2A.1 and the ATPase chromatin remodeler Isw1 were implicated in maintaining CAG repeat number. To further characterize the role of these proteins in maintaining CAG repeat stability, a genetic approach using a PCR-based stability assay identified mutants with an increased frequency of CAG contractions and expansions. Through use of a series of H2A.1 C-terminal tail point mutants, H2A.1 appears to be regulating CAG stability through phosphorylation of threonine-126 in the C-terminal tail and the position of this modifiable residue is important for preventing CAG repeat expansions. Isw1 is working predominantly through the Isw1b complex to maintain CAG repeat integrity, but Isw1a can contribute as well. This role is likely independent of the previously defined role of Isw1 in gene regulation and therefore may be through a repair process, such as transcription-coupled repair. Observation of nucleosome positioning flanking the CAG repeat tract did not reveal significantly altered nucleosome positioning in the absence of H2A.1 or Isw1. Thus, both H2A.1 and Isw1 may contribute to CAG repeat stability through processes other than steady state regulation of nucleosome occupancy at the trinucleotide repeat.