Mutations in tumor suppressor BRCA1 lead to breast and/or ovarian cancer. Here we show that loss of BRCA1 in mice results in transcriptional derepression of the tandemly repeated satellite DNA. BRCA1 deficiency is accompanied by reduction of condensed DNA regions in the genome and loss of ubiquitylation of histone H2A at satellite repeats. BRCA1 binds to satellite DNA regions in vivo and ubiquitylates H2A in vitro. Ectopic expression of an H2A fused to ubiquitin reverses the effects of BRCA1 loss, suggesting that BRCA1 maintains heterochromatin structure via ubiquitylation of histone H2A. Satellite DNA derepression was also observed mouse and human BRCA1 deficient breast cancers. Ectopic expression of satellite DNA can phenocopy BRCA1 loss in centrosome amplification, cell cycle checkpoint defects, DNA damage and genomic instability. We propose that the role of BRCA1 in maintaining global heterochromatin integrity accounts for many of its tumor suppressor functions.

Chromatin architecture has been implicated in cell-type-specific gene regulatory programs; yet, how chromatin remodels during development remains to be fully elucidated. Here, by interrogating chromatin reorganization during human pluripotent stem cell (PSC) differentiation, we discover a role for the primate-specific endogenous retrotransposon HERV-H in creating topologically associating domains (TAD) in human PSCs. Deleting these HERV-H elements eliminates their corresponding TAD boundaries and reduces transcription of upstream genes, while de novo insertion of HERV-Hs can introduce new TAD boundaries. HERV-H’s ability to create these TAD boundaries depends on high transcription, as transcriptional repression of HERV-H elements prevents formation of these boundaries. This ability is not limited to human PSCs, as these actively transcribed HERV-Hs and their corresponding TAD boundaries also appear in PSCs from other hominids but not in more distantly related species lacking HERV-Hs. Overall, our results provide direct evidence for retrotransposons in actively shaping cell-type- and species-specific chromatin architecture.

Summary The basic determinant of chromosome inheritance, the centromere, is specified in many eukaryotes by an epigenetic mark. Using gene targeting in human cells and fission yeast, chromatin containing the centromere-specific histone H3 variant CENP-A is demonstrated to be the epigenetic mark that acts through a two-step mechanism to identify, maintain and propagate centromere function indefinitely. Initially, centromere position is replicated and maintained by chromatin assembled with the centromere-targeting domain (CATD) of CENP-A substituted into H3. Subsequently, nucleation of kinetochore assembly onto CATD-containing chromatin is shown to require either CENP-A’s amino- or carboxy-terminal tails for recruitment of inner kinetochore proteins, including stabilizing CENP-B binding to human centromeres or direct recruitment of CENP-C, respectively.

Summary In response to DNA double-strand breaks (DSBs), cells sense the DNA lesions and then activate the protein kinase ATM. Subsequent DSB resection produces RPA-coated ssDNA that is essential for activation of the DNA damage checkpoint and DNA repair by homologous recombination (HR). However, the biochemical mechanism underlying the transition from DSB sensing to resection remains unclear. Using Xenopus egg extracts and human cells we show that the tumor suppressor protein CtIP plays a critical role in this transition. We find that CtIP translocates to DSBs, which is dependent on the DSB sensor complex Mre11-Rad50-NBS1, the kinase activity of ATM and a direct DNA-binding motif in CtIP, and then promotes DSB resection. Thus, CtIP facilitates the transition from DSB sensing to processing: It does so by binding to the DNA at DSBs after DSB sensing and ATM activation, and then promoting DNA resection leading to checkpoint activation and HR.

Centrioles organize the centrosome, and accurate control of their number is critical for the maintenance of genomic integrity. Centriole duplication occurs once per cell cycle and is controlled by Polo-like kinase 4 (Plk4). We showed previously that Plk4 phosphorylates itself to promote its degradation by the proteasome. Here we demonstrate that this autoregulated instability controls the abundance of endogenous Plk4. Preventing Plk4 autoregulation causes centrosome amplification, stabilization of p53, and loss of cell proliferation; moreover, suppression of p53 allows growth of cells carrying amplified centrosomes. Plk4 autoregulation thus guards against genome instability by limiting centrosome duplication to once per cell cycle.