YY1 is a C₂H₂-type zinc finger containing protein [1-3] and essential for cell proliferation. YY1 depleted cells display hypersensitivity to DNA damaging agents, with intact checkpoint responses. YY1-/- MEFs accumulate dramatic chromosome aberrations, such as polyploidy, chromosome breaks, and triradial and quadriradial structures. Those abnormalities are characteristic of HR defects associated with genomic instability. To understand the molecular mechanism, we affinity-purified a nuclear YY1 complex that contains Ino80, which homolog in yeast is an ATP-dependent chromatin remodeler and directly participates in DSB repair via a mechanism independent of transcription [4, 5]. Based on the yeast paradigm, the YY1 association with Ino80 may provide DNA repair connections to chromosome aberration phenotype in mammal. This hypothesis is supported by a reporter-based assay, in which depletion of YY1 and Ino80 strongly reduce the efficiency of HR. Biochemical analyses find that both the YY1 complex and recombinant YY1 protein preferentially bind the recombinant intermediate structure in vitro and suggest a direct role in processing DSBs. In consistent with this model, we observe the recruitment of YY1 at the DNA damage sites in living cells.

I further study YY1 function using spermatogenesis as the model system, where meiotic DSBs are repaired by an HR-mediated mechanism [6, 7]. YY1 expression is specific in the pachytene and diplotene spermatocytes, and localizes to the synaptomenal complex (SC) at the heterochromatic regions. Genetic ablation of YY1 in mouse testis has a severe effect on organ development, and results in the loss of normal pachytene spermatocytes. Moreover, we identify an elevated level of DNA damage signals throughout the pachytene and diplotene stages, suggesting the accumulation of meiotic-induced DSBs. Utilizing an in vivo RNA interference approach, we find that knockdown of YY1 results in abnormal SC structures, missegregation between homologous chromosomes and less defined sex body structures in the pachytene spermatocytes.

In summary, my thesis work discovers an essential role for YY1 in maintaining the genomic stability. We provide multiple lines of evidence that link YY1 and Ino80 complex to HR DNA repair, which is further complemented by demonstration of YY1 function during the meiosis.