PAXX and Xlf interplay revealed by impaired CNS development and immunodeficiency of double KO mice

Abstract

The repair of DNA double-stranded breaks (DNAdsb) through non-homologous end joining (NHEJ) is a prerequisite for the proper development of the central nervous system and the adaptive immune system. Yet, mice with Xlf or PAXX loss of function are viable and present with very mild immune phenotypes, although their lymphoid cells are sensitive to ionizing radiation attesting for the role of these factors in NHEJ. In contrast, we show here that mice defective for both Xlf and PAXX are embryonically lethal owing to a massive apoptosis of post-mitotic neurons, a situation reminiscent to XRCC4 or DNA Ligase IV KO conditions. The development of the adaptive immune system in Xlf − / − PAXX −/− E18.5 embryos is severely affected with the block of B-and T-cell maturation at the stage of IgH and TCRβ gene rearrangements, respectively. This damaging phenotype highlights the functional nexus between Xlf and PAXX, which is critical for the completion of NHEJ-dependent mechanisms during mouse development. Cell Death and Differentiation (2018) 25, 444-452; doi:10.1038/cdd.2017; published online 27 October 2017All living organisms are subjected to multitude sources of DNA damage during their lifespan, either as a result of external assault or endogenous physiological processes.1 Among endogenous sources of physiological DNAdsb is the somatic rearrangement of immunoglobulin (Ig) and TCR genes in B and T lymphocytes, respectively, during the diversification of the adaptive immune system through V(D)J recombination.2 DNA double-stranded breaks (DNAdsb) are considered the most toxic lesions. DNAdsbs are repaired by two main mechanisms: the homologous recombination (HR) in cycling cells, when a sister chromatid is available as DNA repair template, and the non-homologous end joining (NHEJ) during all phases of the cell cycle.NHEJ proceeds via the simple religation of DNA ends without the need for a repair template.3 Briefly, the NHEJ is composed of seven core factors comprising the Ku70/80/ DNA-PKcs (DNA-dependent protein kinase catalytic subunit) complex, which recognizes and protects the broken DNA ends, the Artemis endo/exonuclease, which participates, when needed, in processing the DNA ends and the XRCC4/ DNA-Ligase IV/Xlf complex, which ultimately reseals the DNA break. The critical function of the NHEJ apparatus in various aspects of higher eukaryote development has been extensively perceived in several animal and human pathological conditions. As emblematic examples, loss of function of either XRCC4 or DNA ligase IV results in embryonic lethality in mice 4,5 and mutations in Artemis or DNA-PKcs result in severe combined immunodeficiency conditions in both men and mice, owing to aborted V(D)J recombination. 6 In addition, defects in NHEJ results in genetic instability and the propensity to develop various types of cancers, notably leukemia and lymphomas. Recently, a new DNA repair factor, PAXX (PAralog of XRCC4 and Xlf, also known as C9orf142 or XLS), has been identified independently by three laboratories based on bioinformatic...

Normal development of mice lacking PAXX, the paralogue of XRCC4 and XLF

“…TUNEL assay revealed that the levels of apoptosis were almost undetectable in both Paxx −/− and WT brains. We concluded that inactivation of Paxx alone did not affect growth, size, fertility, and development of central nervous system in mice, which is in line with recently published three independent Paxx ‐knockout mouse models .…”

“…Finally, we analyzed brain sections of WT and Paxx À/À mice and found no difference in neurodevelopment using two independent methods, Nissl staining and TUNEL assay (not shown). While our mouse model was produced and analyzed, three other groups independently reported Paxx-deficient mice [17][18][19]. In contrast to our mouse model, those mice had the entire Paxx locus deleted.…”

“…While our mouse model was produced and analyzed, three other groups independently reported Paxx ‐deficient mice . In contrast to our mouse model, those mice had the entire Paxx locus deleted.…”

“…In experiments based on knockout chicken and murine cells, PAXX was shown to have some functional redundancy with its paralogue XLF [12][13][14][15][16]. In addition, three knockout mouse models were generated at different laboratories and published recently [17][18][19]. These studies suggest that PAXX has an overlapping function with XLF and is required for embryonic development and maintenance of central nervous system.…”

Loss of H3K36 Methyltransferase SETD2 Impairs V(D)J Recombination during Lymphoid Development

“…In mice, loss of core C-NHEJ factors leads to apoptosis of post-mitotic neurons and embryonic lethality (Gao et al, 1998;Gu et al, 2000). Moreover, the same synthetic lethal combinations that result in severe lymphocyte developmental blocks (e.g., MRI and XLF) also display similar post-mitotic neuronal apoptosis Abramowski et al, 2018), indicating the critical role of end-ligation and C-NHEJ in the repair of DSBs during neurogenesis.…”