XLF/Cernunnos: An important but puzzling participant in the nonhomologous end joining DNA repair pathway

Abstract

DNA double strand breaks (DSBs) are one of the most deleterious DNA lesions that promote cell death, genomic instability and carcinogenesis. The two major cellular mechanisms that repair DSBs are Nonhomologous End-Joining (NHEJ) and Homologous Recombination Repair (HRR). NHEJ is the predominant pathway, in which XLF (also called Cernunnos) is a key player. Patients with XLF mutation exhibit microcephaly, lymphopenia, and growth retardation, and are immunodeficient and radiosensitive. During NHEJ, XLF interacts with XRCC4-Ligase IV, stimulates its ligase activity, and forms DNA-binding filaments of alternating XLF and XRCC4 dimers that may serve to align broken DNA and promote ligation of noncomplementary ends. Despite its central role in NHEJ, the effects of XLF deficiency are surprisingly variable in different biological contexts, and different individual cell lines. This review summarizes the role of XLF in NHEJ, and the unexpected complexity of its interplay with other repair factors in supporting radiosurvival and V(D)J recombination.

XLF and APLF bind Ku80 at two remote sites to ensure DNA repair by non-homologous end joining

“…Indeed, we found that separate or even combined Ku80 mutations do not completely abolish XRCC4 and XLF recruitment and do not radiosensitize cells as much as XLF complete defect that abolishes all XLF functions in NHEJ 51 . In that view, only a mild, if any, repair defect in human cells has been associated with X-KBM deletion 27 or APLF depletion 9,24–26 : in case of individual absence of XLF C-terminal tail or of APLF, the other intact partner would still bind to Ku80 and be able, although with a slower kinetics, to recruit the other components to achieve ligation.…”

Mechanistic modelling supports entwined rather than exclusively competitive DNA double-strand break repair pathway

“…To be able to emulate the results seen in the XLF-deficient system either further mechanisms for un-repaired DNA ends to form should be present within HR, or XLF-deficiency is not an absolute determinant of synapsis stabilisation, but does perturb the ability to ligate the break. Whilst XLF is known to be important within NHEJ repair, its purpose is not as clearly defined 60 as that of Lig4; this may be why the simulation reproduces better the Lig4-deficient system than the XLF-deficient system. Therefore, the choice to attempt modelling XLF-deficiency in this study required extra consideration.…”

C-NHEJ without indels is robust and requires synergistic function of distinct XLF domains

“…This finding is consistent with the activity of XLF to promote XRCC4-LIG4 activity in vitro, as well as the substantial radiosensitivity of XLF-deficient cells 11 , 17 , 28 . However, in vitro experiments have found that XLF is dispensable for XRCC4-LIG4 joining of blunt ends, and instead may be particularly important for non-complementary ends 15 , 30 . Since these studies appear to contradict our findings, we suggest that protection and bridging of DSB ends to avoid indels may have a more stringent requirement for XLF in the context of the cell, compared to in vitro ligation assays.…”