An in vivo study of the impact of deficiency in the DNA repair proteins PAXX and XLF on development and maturation of the hemolymphoid system

Abstract

Repair of DNA double-strand breaks by the nonhomologous end joining pathway is central for proper development of the adaptive immune system. This repair pathway involves eight factors, including XRCC4-like factor (XLF)/Cernunnos and the paralog of XRCC4 and XLF, PAXX nonhomologous end joining factor (PAXX). Xlf−/− and Paxx−/− mice are viable and exhibit only a mild immunophenotype. However, mice lacking both PAXX and XLF are embryonic lethal because postmitotic neurons undergo massive apoptosis in embryos. To decipher the roles of PAXX and XLF in both variable, diversity, and joining recombination and immunoglobulin class switch recombination, here, using Cre/lox-specific deletion to prevent double-KO embryonic lethality, we developed two mouse models of a conditional Xlf KO in a Paxx−/− background. Cre expressed under control of the iVav or CD21 promoter enabled Xlf deletion in early hematopoietic progenitors and splenic mature B cells, respectively. We demonstrate the XLF and PAXX interplay during variable, diversity, and joining recombination in vivo but not during class switch recombination, for which PAXX appeared to be fully dispensable. Xlf/Paxx double KO in hematopoietic progenitors resulted in a shorter lifespan associated with onset of thymic lymphomas, revealing a genome caretaking function of XLF/PAXX.

Leaky severe combined immunodeficiency in mice lacking non-homologous end joining factors XLF and MRI

“…Previously, we showed that mice lacking XLF, PAXX and p53 were live-born and had nearly no B and T cells, reduced size of spleen and hardly detectable thymus [20] (Figure 5). Consistent with this model, a conditional AGING knockout mouse model, which results in doubledeficiency of XLF/PAXX in early hematopoietic progenitor cells, was also able to overcome the embryonic lethality of Xlf -/-Paxx -/mice [33]. With this model, impairment of V(D)J recombination in Xlf -/-Paxx -/cells, as well as the resulting depletion of mature B cells and lack of a visible thymus could also be observed in vivo [33].…”

“…Consistent with this model, a conditional AGING knockout mouse model, which results in doubledeficiency of XLF/PAXX in early hematopoietic progenitor cells, was also able to overcome the embryonic lethality of Xlf -/-Paxx -/mice [33]. With this model, impairment of V(D)J recombination in Xlf -/-Paxx -/cells, as well as the resulting depletion of mature B cells and lack of a visible thymus could also be observed in vivo [33]. Our new data provide evidence that Xlf -/-Paxx -/-Trp53 +/and Xlf -/-Paxx -/-Trp53 -/mice possess a very small number of mature B cells in the spleen and bone marrow, as well as very minor fractions of single positive T cells in thymus and spleen (Figures 2, 5 and Supplementary Figure 1).…”

Leaky Severe Combined Immunodeficiency in Mice Lacking Non-homologous End Joining Factors XLF and MRI

“…Consistent with this model, a conditional knockout mouse model, which results in doubledeficiency of XLF/PAXX in early hematopoietic progenitor cells, was also able to overcome the embryonic lethality of Xlf -/-Paxx -/mice [32]. With this model, impairment of V(D)J recombination in Xlf -/-Paxx -/cells, as well as the resulting depletion of mature B cells and lack of a visible thymus could also be observed in vivo [32].…”

“…Consistent with this model, a conditional knockout mouse model, which results in doubledeficiency of XLF/PAXX in early hematopoietic progenitor cells, was also able to overcome the embryonic lethality of Xlf -/-Paxx -/mice [32]. With this model, impairment of V(D)J recombination in Xlf -/-Paxx -/cells, as well as the resulting depletion of mature B cells and lack of a visible thymus could also be observed in vivo [32]. Our new data provide evidence that Xlf -/-Paxx -/-Trp53 +/and Xlf -/-Paxx -/-Trp53 -/mice possess a very small number of mature B cells in the spleen and bone marrow, as well as very minor fractions of single positive T cells in thymus and spleen ( Figure 2, 5 and Supplementary Figure 1).…”

Leaky severe combined immunodeficiency in mice lacking non-homologous end joining factors XLF and MRI

“…6). Moreover, a conditional knockout mouse model, which results in double-deficiency of XLF/PAXX in early hematopoietic progenitor cells only, was also able to overcome the embryonic lethality of Xlf -/-Paxx -/mice [37]. With this model, impairment of V(D)J recombination in Xlf -/-Paxx -/cells, as well as the resulting depletion of mature B cells and lack of a visible thymus could also be observed in vivo [37].…”