DNA polymerase beta participates in DNA End-joining

Abstract

DNA double strand breaks (DSBs) are one of the most deleterious lesions and if left unrepaired, they lead to cell death, genomic instability and carcinogenesis. Cells combat DSBs by two pathways: homologous recombination (HR) and non-homologous end-joining (NHEJ), wherein the two DNA ends are re-joined. Recently a back-up NHEJ pathway has been reported and is referred to as alternative NHEJ (aNHEJ), which joins ends but results in deletions and insertions. NHEJ requires processing enzymes including nucleases and polymerases, although the roles of these enzymes are poorly understood. Emerging evidence indicates that X family DNA polymerases lambda (Pol λ) and mu (Pol μ) promote DNA end-joining. Here, we show that DNA polymerase beta (Pol β), another member of the X family of DNA polymerases, plays a role in aNHEJ. In the absence of DNA Pol β, fewer small deletions are observed. In addition, depletion of Pol β results in cellular sensitivity to bleomycin and DNA protein kinase catalytic subunit inhibitors due to defective repair of DSBs. In summary, our results indicate that Pol β in functions in aNHEJ and provide mechanistic insight into its role in this process.

A minimal threshold of FANCJ helicase activity is required for its response to replication stress or double-strand break repair

“…To explain the separation-of-function incurred by the R707C mutation, we surmise FANCJ-R707C retains the ability to catalyze unwinding of relatively short DNA duplexes in order to remodel stalled forks during replication or to resolve secondary DNA structure that impedes DNA synthesis (Figure 7 ). In addition, FANCJ-R707C confers partial resistance to Bleo, a drug that induces single-strand breaks and DSBs characterized by 5′-staggered ends or blunt ends ( 48 ) that are repaired at least in part by nonhomologous end-joining ( 49 ). Consistent with the Bleo rescue, FANCJ-R707C was able to recruit to DSBs, albeit less efficiently than FANCJ-H396D or FANCJ-WT ( 24 ).…”

Pol μ ribonucleotide insertion opposite 8-oxodG facilitates the ligation of premutagenic DNA repair intermediate

“…that pol β can perform inefficient ribonucleotide insertion (~4 orders of magnitude less than dNTPs) during BER, and unlike pol µ, the enzyme undergoes large conformational changes in protein subdomains upon dNTP binding or catalysis [41][42][43] . Moreover, the role of pol β in NHEJ has been reported 44 .…”

Complex genetic interactions between DNA polymerase β and the NHEJ ligase

“…Consistent with this view, Almohaini et al [35] have reported that BER can interfere with NHEJ when thymine glycol is formed at the fifth base from the 3' terminus of one DNA end. In this respect, it is important to mention that human POLb was recently reported to participate in DNA end-joining, likely alternative end-joining [37], which is a POLh-dependent DSB repair pathway mechanistically distinct from Lig4-dependent NHEJ [13,[38][39][40]. As POLB À/À cells were slightly more resistant to x-ray than WT cells, we expected that LIG4 À/À POLB À/À cells would exhibit similarly increased radioresistance relative to LIG4 À/À cells.…”

“…5), suggesting the possibility that POLb may have a role in repairing xray-induced DNA damage when LIG4 is absent. In this respect, it is important to mention that human POLb was recently reported to participate in DNA end-joining, likely alternative end-joining [37], which is a POLh-dependent DSB repair pathway mechanistically distinct from Lig4-dependent NHEJ [13,[38][39][40]. Although that study only employed knockdown experiments and NHEJ inhibitors (but not gene-knockout cells), a possible involvement of POLb in alternative end-joining is quite intriguing in light of the genetic interaction of POLb and POLh in BER of chicken Fig.…”