Ways to unwind with HROB, a new player in homologous recombination

Abstract

Homologous recombination (HR) is an important route for repairing DNA double-strand breaks (DSBs). The early stages of HR are well understood, but later stages remain mysterious. In this issue of Genes & Development, Hustedt and colleagues (pp. 1397-1415) reveal HROB as a new player in HR required for recruitment of the MCM8-9 complex, which is paralogous to the MCM2-7 replicative helicase. HROB functions closely with MCM8-9 to promote postsynaptic DNA repair synthesis. This study sheds valuable light on late events in HR and suggests that HROB may load MCM8-9 onto HR intermediates to facilitate the DNA unwinding required for DNA repair synthesis.Homologous recombination (HR) is a major route for repairing DNA double-strand breaks (DSBs) (Prakash et al. 2015). HR starts with resection of broken DNA ends, involving nucleases and helicases. The resulting overhangs, coated by RAD51 to yield a nucleoprotein filament, invade the intact sister chromatid, searching for a homologous sequence. The pairing of RAD51-coated single-stranded DNA (ssDNA) with the donor strand (termed synapsis) results in the displacement of the strand complementary to the donor (forming a structure called a "D loop") and the establishment of a RAD51-bound heteroduplex species that primes DNA repair synthesis (Fig. 1). The postsynaptic stages of HR are poorly understood, but a number of proteins are involved at this stage specifcially. For example, the MCM8 and MCM9 proteins, which are related to the subunits of the hexameric MCM2-7 replicative helicase, have been implicated in postsynaptic DNA synthesis. In contrast to MCM2-7, the MCM8-9 complex is dispensable for bulk DNA replication (Griffin and Trakselis 2019). The residual DNA synthesis that is maintained in MCM2-depleted cells requires MCM8-9, but this reflects DNA repair synthesis occurring during HR-mediated repair of the high level of DSBs that occur upon MCM2 depletion (Natsume et al. 2017). Importantly, MCM8-9 was shown to act downstream from RAD51 in the HR-mediated repair of DSBs induced by nuclease overexpression, in meiotic HR, and during the HR stage of DNA interstrand cross-link (ICL) repair (Lutzmann et al. 2012;Nishimura et al. 2012;Natsume et al. 2017). The dominant mode of ICL repair is initiated by collision of replisomes with ICLs, resulting in programmed formation of DSBs that are repaired by HR. Crucially, point mutations in MCM8-9 predicted to abolish helicase activity suppress ICL repair, suggesting that helicase activity is important for HR (Nishimura et al. 2012). One explanation is that MCM8-9 might facilitate postsynaptic DNA repair synthesis by unwinding D loops and enabling extension of the invading RAD51-coated DNA end (Fig. 1).In this issue of Genes & Development, Hustedt et al. (2019) identify a new factor-HROB (HR OB-fold)-that functions closely with MCM8-9 in HR. Rationalizing that HR defects should cause sensitivity to inhibitors of ATR kinase and PARP, the investigators mined published CRISPR screens to identify new genes whose deletion sensitizes to both...

DNA replication: Mechanisms and therapeutic interventions for diseases

“…Substantive results revealed that MCM8 and MCM9 play a vital role in HR repair as MCM8-9 complex. 126 Lee et al 127 found that incapable mutation of MCM8-9 complex could not recognize MRN complex, leading to degressive HR efficiency. Moreover, some research proved that the depletion of MCM9 is attributed to reduced proliferation, which may be modulated by ATM-CHK2 pathway.…”

“…Substantive results revealed that MCM8 and MCM9 play a vital role in HR repair as MCM8‐9 complex. 126 Lee et al. 127 found that incapable mutation of MCM8‐9 complex could not recognize MRN complex, leading to degressive HR efficiency.…”

“…Substantive results revealed that MCM8 and MCM9 play a vital role in HR repair as MCM8‐9 complex 126 . Lee et al 127 .…”

Mechanism of DNA unwinding by hexameric MCM8-9 in complex with HROB

“…Most reports to date suggest that MCM8-9 functions in meiosis and in DNA repair, particularly in homologous recombination (HR) in response to DNA interstrand crosslinks (ICLs), as well as to maintain replication fork stability 17,18,22,[32][33][34][35][36] . Mutant cell lines show only minor sensitivity to ionizing radiation or bleomycin, suggesting that MCM8-9 is not a universal DNA break repair factor 37,38 . Rather, MCM8-9 may function in recombination assisting ICL repair in the context of stalled replication forks, possibly alongside Fanconi anemia proteins [37][38][39] .…”

Interaction with C21ORF2 controls the cellular functioning of the NEK1 kinase

“…RAD51 coats the ssDNA to form a nucleoprotein filament which searches for sequence homology in the intact chromatid, invades the homologous duplex and directs annealing to the homologous sequences. The invading strand is then extended using a form of non-canonical DNA synthesis (Bonilla et al ., 2020; Saredi & Rouse, 2019). The final stages of HR can result in the formation of structures that entangle the two chromatids, which are enzymatically removed to complete repair (Lilley, 2017).…”