Overlapping functions between XLF repair protein and 53BP1 DNA damage response factor in end joining and lymphocyte development

Abstract

Nonhomologous end joining (NHEJ), a major pathway of DNA double-strand break (DSB) repair, is required during lymphocyte development to resolve the programmed DSBs generated during Variable, Diverse, and Joining [V(D)J] recombination. XRCC4-like factor (XLF) (also called Cernunnos or NHEJ1) is a unique component of the NHEJ pathway. Although germ-line mutations of other NHEJ factors abrogate lymphocyte development and lead to severe combined immunodeficiency (SCID), XLF mutations cause a progressive lymphocytopenia that is generally less severe than SCID. Accordingly, XLF-deficient murine lymphocytes show no measurable defects in V(D)J recombination. We reported earlier that ATM kinase and its substrate histone H2AX are both essential for V(D)J recombination in XLF-deficient lymphocytes, despite moderate role in V(D)J recombination in WT cells. p53-binding protein 1 (53BP1) is another substrate of ATM. 53BP1 deficiency led to small reduction of peripheral lymphocyte number by compromising both synapse and end-joining at modest level during V(D)J recombination. Here, we report that 53BP1/XLF double deficiency blocks lymphocyte development at early progenitor stages, owing to severe defects in end joining during chromosomal V(D)J recombination. The unrepaired DNA ends are rapidly degraded in 53BP1 −/− XLF −/− cells, as reported for H2AX −/− XLF −/− cells, revealing an end protection role for 53BP1 reminiscent of H2AX. In contrast to the early embryonic lethality of H2AX −/− XLF −/− mice, 53BP1 −/− XLF −/− mice are born alive and develop thymic lymphomas with translocations involving the T-cell receptor loci. Together, our findings identify a unique function for 53BP1 in end-joining and tumor suppression.Ataxia-Telangiectasia-Mutated | XRCC4-like factor | classical nonhomologous end joining L ymphocyte development requires the ordered assembly of variable region of antigen receptor genes from individual Variable, Diverse, and Joining gene segments through V(D)J recombination (1). V(D)J recombination is initiated by the RAG endonuclease (RAG), which introduces DNA double-strand breaks (DSBs) between the conserved recombination signal sequence (RSS) and the participating germ-line V, D, or J gene segments (1). RAG cleavage generates two types of DNA ends, the 5′-phosphorylated blunt signal ends (SEs) and the hairpin sealed coding ends (CEs) (1). In the next step, ubiquitously expressed classical nonhomologous end-joining (C-NHEJ) factors directly join the two SEs to form a signal join (SJ), and process (via hairpin opening, de novo synthesis, and loss of nucleotides) and join the two CEs to form a coding join (CJ) (1).There are seven C-NHEJ factors in mammalian cells. Ku70 and Ku80 (or KU86 in human) form heterodimers (KU) that bind DSB ends and, among other functions, recruit and activate other NHEJ factors (2). DNA-bound KU interacts with the DNA-PK catalytic subunit (DNA-PKcs), and together they activate the endonuclease function of Artemis for end processing, including opening hairpin-sealed CEs during V(D)J reco...

Peripheral subnuclear positioning suppresses Tcrb recombination and segregates Tcrb alleles from RAG2

“…53BP1 foci form within minutes of a DSB and may remain at DSB sites for several hours after their repair (17). 53BP1 also has been implicated in V(D)J recombination and class-switch recombination, functioning to protect DNA ends and to support long-range DNA interactions (18)(19)(20).…”

XRCC4/XLF Interaction Is Variably Required for DNA Repair and Is Not Required for Ligase IV Stimulation

“…There is evidence that XRCC4/XLF filaments affect both the precise alignment of DNA ends, in that XLF promotes ligation of noncohesive ends in vitro (10,11) (Fig. 1), as well as the maintenance of ends in "repair centers," in that XLF deficiency is exacerbated when factors that function in assembling ␥H2AX foci are disrupted (20)(21)(22)(23)(24). We first assessed, in living cells, whether L115A's deficit in resolving coding ends was exacerbated if the ends are not compatible.…”

Synthetic lethality between PAXX and XLF in mammalian development

“…Furthermore, Xlf −/− B cells perform V(D)J recombination at almost wild-type levels, which explains the lack of significant immunodeficiency in these mice and suggests that compensatory mechanisms can mitigate loss of XLF in developing lymphocytes. One of these mechanisms comprises the ATM-H2AX-53BP1 axis of DSB repair, as combined loss of any one of these factors with XLF deficiency causes profound defects in V(D)J recombination and lymphocyte development even though loss of any of these proteins individually is not significantly detrimental to these processes (Zha et al 2011;Liu et al 2012;Oksenych et al 2012).…”