TopBP1 and ATR Colocalization at Meiotic Chromosomes: Role of TopBP1/Cut5 in the Meiotic Recombination Checkpoint

Abstract

Mammalian TopBP1 is a BRCT domain-containing protein whose function in mitotic cells is linked to replication and DNA damage checkpoint. Here, we study its possible role during meiosis in mice. TopBP1 foci are abundant during early prophase I and localize mainly to histone ␥-H2AX-positive domains, where DNA double-strand breaks (required to initiate recombination) occur. Strikingly, TopBP1 showed a pattern almost identical to that of ATR, a PI3K-like kinase involved in mitotic DNA damage checkpoint. In the synapsis-defective Fkbp6 ؊/؊ mouse, TopBP1 heavily stains unsynapsed regions of chromosomes. We also tested whether Schizosaccharomyces pombe Cut5 (the TopBP1 homologue) plays a role in the meiotic recombination checkpoint, like spRad3, the ATR homologue. Indeed, we found that a cut5 mutation suppresses the checkpoint-dependent meiotic delay of a meiotic recombination defective mutant, indicating a direct role of the Cut5 protein in the meiotic checkpoint. Our findings suggest that ATR and TopBP1 monitor meiotic recombination and are required for activation of the meiotic recombination checkpoint.

DNA double strand break repair, chromosome synapsis and transcriptional silencing in meiosis

“…TOPBP1 and at least one component of the 9-1-1 complex, RAD1, normally also accumulate on the XY body. 104,106,107 RPA-and damage-independent recruitment of ATR to chromatin in somatic cells has also been reported. For example CDC6, a protein required for replication initiation, may recruit ATR to stalled replication forks.…”

“…These include BRCA1, BRCA2, BLM, MDC1, TOPBP1, 53BP1 and RAD1. [102][103][104][105][106][107][108] The above-mentioned post replication repair enzyme HR6B and its E3 ligase partner RAD18 also accumulate on the XY body. 109 All of these proteins, except BLM, 53BP1 and the RAD18/ HR6A/B complex, also accumulate on the meiotic DSBs shortly after their formation in leptotene.…”

Clamping down on mammalian meiosis

“…[30][31][32] In mammalian germ cells, ATR, TOPBP1, and RAD1 proteins have been reported to localize along meiotic chromosomes. [33][34][35] However, many details remain to be resolved regarding the mode of ATR activation in germ cells, and to the best of our knowledge, the Rad9a 14 and Hus1 studies are the first to suggest that meiotic ATR activation is, at least in part, 9-1-1-independent. 15 A variety of studies in somatic cells indicate that the 9-1-1 complex not only contributes to ATR activation but also participates directly in DNA repair.…”

“…34,35,76,77 In particular, both factors localize to sites undergoing meiotic silencing of unsynapsed chromatin (MSUC), which occurs on the autosomes, and meiotic sex chromosome inactivation (MSCI), which occurs in the XY body, 76,78 suggesting that ATR kinase activation occurs in response to asynapsis. Currently it is thought that ATR phosphorylates histone H2AX at sites of MSCI and MSUC, including the XY body domain, while the ATM kinase phosphorylates H2AX at DSBs.…”

“…Additionally, many of the same proteins involved in somatic cell checkpoints, including ATM, H2AX, ATR, TOPBP1, RAD9A, RAD1, and HUS1, are expressed during meiosis, and most are known to localize along meiotic chromosomes. 5,14,15,[33][34][35]76,83 A variety of meiotic defects have been shown to lead to cell cycle arrest and apoptosis between zygonema and midpachynema in stage IV of the seminiferous epithelium. 77 These include cases of complete absence of DSBs (such as in Spo11 mutants), 88 impaired DSB repair (such as in Dmc1 and Msh5 mutants), [89][90][91] autosomal asynapsis, 92,93 or impaired MSCI.…”