| Trinucleotide repeat disorders, such as Huntington's disease, are a set of genetic diseases caused by expansions of trinucleotide repeats. In the past two decades, extensive studies have been done to understand the molecular mechanism of the repeat expansions. Several DNA metabolic processes, including DNA replication, repair and transcription, have been implicated in generating repeat expansions. Here, we demonstrated that histone modification, particularly histone acetylation, also plays a role in maintaining stability of repeats. This idea came initially from two genome-wide yeast screens done in the lab to identify genes affecting repeat fragility and/or stability. From the screens, cells overexpressing SDS3 (overexpression screen) or deleted for SNT1 had increased repeat instability and/or fragility. Interestingly, both genes encode a component of histone deacetylase complexes, Rpd3 and Set3 respectively. To investigate whether histone acetylation has a role in repeat stability, we performed a stability assay in strains which have defects in histone acetylation or deacetylation. Significantly increased expansions were detected in mutants affecting proper acetylation of histone H4, including ESA1 and HAT1 (histone acetyltransfereases), and HST1 and HOS2 (histone deacetylases). These expansions were generated by a Rad52-dependent repair process. On the other hand, cells with hyper or hypoacetylation at histone H3 lysine 56 showed significantly increased repeat contractions, which were largely Rad52-independent and most likely due to defective nucleosome assembly at the replication fork. Therefore, we conclude that histone acetylation plays an important role in maintaining CAG/CTG repeat stability during both DNA replication and repair.;In a separate study (Chapter 5), we tested whether one copy of Rad27 is enough to maintain CAG/CTG tract stability in diploid yeast cells. Previous studies using mice heterozygous for FEN1 (mammalian homolog of yeast Rad27) and harboring an expanded CAG-120 repeat within the human HD gene showed that loss of one allele of FEN1 had a minor effect on repeat stability (Spiro and McMurray 2003), whereas yeast containing a repeat tract of similar length deleted for RAD27 have a much more severe effect on repeat stability. We found that CAG/CTG repeats are stable or exhibit a slightly increased expansion frequency in RAD27 +/- cells when the tract is short or medium size (from 70 to130 repeats). However for longer tracts, such as a CAG-155 tract, the repeat expansion frequency is significantly increased in heterozygous cells. Our data indicates that cells containing longer CAG/CTG repeats need more Rad27 protein to maintain tract stability and that maintenance of long CAG/CTG repeats is particularly sensitive to Rad27 protein levels. |
