RESEARCH HIGHLIGHTS Nature Reviews Molecular Cell Biology | AOP, published online 9 April 2014; doi:10.1038/nrm3792

D N A DA M A G E R E S P O N S E

Mitosis: don’t ‘repair’ the telomere! DNA double-strand breaks (DSBs) are not repaired during mitosis, for hitherto unknown reasons. Orthwein et al. now show that reactivating DSB repair during mitosis results in telomere fusions and aneuploidy. Following damage, mediator of DNA damage checkpoint protein 1 (MDC1) and the E3 ubiquitin ligase RING finger protein 8 (RNF8) are recruited to DSBs and promote their repair. The authors show that inactivation of DSB repair during mitosis is achieved by blocking RNF8 recruitment through the suppression of RNF8–MDC1 interaction. This requires RNF8 phosphorylation at Thr198, as preventing it with a mutation (T198A) restored RNF8 recruitment to DSBs in mitotic U2OS cells. The authors tested whether the T198A mutation could also restore the recruitment of p53‑binding protein 1 (53BP1) and BRCA1, which are major

components of the DSB repair pathways non-homologous end-joining (NHEJ) and homologous recombination, respectively. BRCA1, but not 53BP1, was recruited to mitotic DSB sites in cells that express RNF8-T198A. Two 53BP1 residues — Thr1609 and Ser1618 — were found to be phosphorylated during mitosis, and introducing phosphomimetic mutations to both residues markedly decreased 53BP1 recruitment to mitotic DSB sites; conversely, 53BP1 mutated in both residues (53BP1-TASA) was active during mitosis in RNF8-T198A‑expressing cells. Moreover, mitotic cells that express RNF8-T198A and 53BP1-TASA could repair DSBs, which demonstrates that RNF8 pathway restoration reactivates DSB repair in mitosis. What are the consequences of reactivating DSB repair during mitosis? The authors observed an increase in micronuclei in cells

that express RNF8-T198A and 53BP1-TASA after DSB formation, which is evidence of chromosomal missegregation and aneuploidy. This effect was suppressed by the depletion of PTIP (PAX transactivation activation domain-interacting protein; also known as PAXIP1) and RIF1 (Rap1‑interacting factor 1 homologue), the 53BP1 effectors in DSB repair, which promote the formation of telomere fusions. Using fluorescence in situ hybridization, the authors observed an increase in sister telomere fusions and the resulting formation of dicentric chromosomes in cells that express RNF8-T198A and 53BP1-TASA. This was caused by telomere uncapping and seems to be Aurora B kinase-dependent. Together, these results indicate that cells inactivate mitotic DSB repair to prevent uncapped telomeres from fusing and causing aneuploidy. Eytan Zlotorynski

Telomere fusions are formed in cells with reactivated mitotic DNA double-strand break repair. Image courtesy of A. Orthwein, the Lunenfeld-Tanenbaum Research Institute, Mount Sinai Hospital, Toronto, Canada.

ORIGINAL RESEARCH PAPER Orthwein, A. et al. Mitosis inhibits DNA double-strand break repair to guard against telomere fusions. Science http:// dx.doi.org/10.1126/science.1248024 (2014)

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DNA damage response: Mitosis: don't 'repair' the telomere!

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