Saccharomyces cerevisiaeH2A copies differentially contribute to recombination and CAG/CTG repeat maintenance, with a role for H2A.1 threonine 126
Authors: House Nealia C.M. , Polleys Erica J. , Quasem Ishtiaque , Joyce Cailin E. , Takacsi-Nagy Oliver , Hayman Ryan B. , Krebs Jocelyn E. , Fuchs Stephen M. , Freudenreich Catherine H. 
Published: May 2018

DNA are sites of genomic instability. Long CAG/CTG repeats form hairpin structures, are fragile, and can expand during DNA repair. The chromatin response to DNA damage can influence repair fidelity, but the knowledge of chromatin modifications involved in maintaining repair fidelity within repetitive DNA is limited. In a screen for CAG repeat fragility inSaccharomyces cerevisiae, histone 2A copy 1 (H2A.1) was identified to protect the repeat from increased rates of breakage. To address the role of H2A in CAG repeat instability, we tested the effect of deleting each histone H2 subytpe. Whereas deletion ofHTA2, HTZ1, HTB1, andHTB2did not significantly affect CAG repeat maintenance, deletion ofHTA1resulted in increased expansion frequency. Notably, mutation of threonine 126, unique to H2A.1, to a non-phosphorylatable alanine increased CAG repeat instability to a similar level as thehta1Δmutant. CAG instability in the absence of HTA1 or mutation tohta1-T126Awas dependent on the presence of the homologous recombination (HR) repair proteins Rad51, Rad52, and Rad57, and the Polδ subunit Pol32. In addition, sister chromatid recombination (SCR) was suppressed in thehta1Δandhta1-T126Amutants and this suppression was epistatic topol32Δ.Finally, break-induced replication (BIR) is impaired in thehta1Δmutant, resulting in an altered repair profile. These data reveal differential roles for the H2A subtypes in DNA repair and implicate a new role for H2A.1 threonine-126 phosphorylation in mediating fidelity during HR repair and promoting SCR. Using a fragile, repetive DNA element to model endogenous DNA damage, our results demonstrate that H2A.1 plays a greater role than H2A.2 in promoting homology-dependent repair, suggesting H2A.1 is the true homolog of mammalian H2AX, whereas H2A.2 is functionally equivalent to mammalian H2A.

Author Summary

CAG/CTG trinuncleotide repeats are fragile sequences that when expanded can cause human disease. To evaluate the role ofS. cerevisiaehistone H2A copies in DNA repair, we have measured instability of an expanded CAG/CTG repeat tract and repair outcomes in H2A mutants. Although the two copies of H2A are nearly identical in amino acid sequence, we found that the CAG repeat is more unstable in the absence of H2A copy 1 (H2A.1) than H2A copy 2, and that this role appears to be partially dependent on a phosphorylatable threonine at residue 126 in the C-terminal tail of H2A.1. Further, we show through a series of genetic assays that H2A.1 plays a role in promoting homologous recombination events, including sister chromatid recombination and break-induced replication. Our results uncover a role for H2A.1 in mediating fidelity of repair within repetitive DNA, and demonstrate that modification of its unique Thr126 residue plays a role in regulating SCR. Given the dependence of HR repair on H2A.1 but not H2A.2, we conclude that H2A.1 plays a greater repair-specific role in the cell and therefore would be the true homolog of mammalian H2AX.