SCAI promotes DNA double-strand break repair in distinct chromosomal contexts

Abstract

Summary DNA double-strand breaks (DSBs) are highly cytotoxic DNA lesions, whose accurate repair by non-homologous end-joining (NHEJ) or homologous recombination (HR) is crucial for genome integrity and is strongly influenced by the local chromatin environment. Here, we identify SCAI (Suppressor of Cancer Cell Invasion) as a 53BP1-interacting chromatin-associated protein that promotes the functionality of several DSB repair pathways in mammalian cells. SCAI undergoes prominent enrichment at DSB sites through dual mechanisms involving 53BP1-dependent recruitment to DSB-surrounding chromatin and 53BP1-independent accumulation at resected DSBs. Cells lacking SCAI display reduced DSB repair capacity, hypersensitivity to DSB-inflicting agents and genome instability. We demonstrate that SCAI is a mediator of 53BP1-dependent repair of heterochromatin-associated DSBs, facilitating ATM kinase signaling at DSBs in repressive chromatin environments. Moreover, we establish an important role of SCAI in meiotic recombination, as SCAI deficiency in mice leads to germ cell loss and subfertility associated with impaired retention of the DMC1 recombinase on meiotic chromosomes. Collectively, our findings uncover SCAI as a physiologically important component of both NHEJ- and HR-mediated pathways that potentiates DSB repair efficiency in specific chromatin contexts.

Inhibition of RIF1 by SCAI Allows BRCA1-Mediated Repair

“…Among the multiple repair pathways and processes involving SCAI shown by Hansen et al (2016), our study suggests a 53BP1-dependent function in the regulation of BRCA1-mediated repair pathways, possibly including alt-NHEJ and resectiondependent NHEJ in G1, as well as HDR in S/G2, by counteracting RIF1. It will be interesting to determine the 53BP1-independent function and regulation of SCAI.…”

“…There are some contradictory results between the current analyses and Hansen et al (2016)'s study, including the effect of SCAI depletion on Rad51 foci formation and the HDR reporter assay. Time course analysis revealed that Rad51 foci in S/G2 cells were significantly affected by SCAI depletion at earlier (0.5-1 hr) time points after irradiation, but not at the later (3 hr) time point used by Hansen et al (2016), implying a positive role for SCAI in HDR. The reason for the different HDR reporter assay results in the two studies remains unknown; however, the effect may differ in stable knockout (chronic SCAI depletion) and transient knockdown (acute SCAI depletion) cells.…”

“…In addition, the roles of SCAI in promoting 53BP1-dependent HDR on heterochromatin and PARP inhibitor-induced damage were demonstrated. In contrast, in a reporter assay, Hansen et al (2016) found that the defect in NHEJ was more severe than that in HDR, and the number of gH2AX foci increased in G1 in SCAI À/À mouse embryonic fibroblast (MEF) and U2OS cells, suggesting that SCAI is involved in NHEJ. Collectively, SCAI was assigned as a physiologically important component of the NHEJ-and HDR-mediated pathways and the 53BP1-dependent and 53BP1-independent reactions.…”

“…SCAI has been shown to be a physiologically important component of the DSB repair pathway in vivo (Hansen et al, 2016). SCAI knockout mice exhibited defects in germ cell development and fertility in both males and females, implying that SCAI is involved in 53BP1-independent HDR.…”

How cells ensure correct repair of DNA double-strand breaks

“…Whereas 53BP1-RIF1 promotes certain NHEJ events, 53BP1 bound to SCAI seems to be competent for HR, potentially representing a way in which the balance of repair pathways can be altered. Many HR factors are essential for embryonic development, whereas Scai Ϫ/Ϫ mice are viable but show a defect in 53BP1-dependent repair of heterochromatic DSBs (73). Clearly further work is required to fully understand the role of SCAI in DSB repair.…”