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.
Highlights
Repair of DNA double-strand breaks by the nonhomologous end joining pathway is central for proper development of the adaptive immune system
Mice were born at a Mendelian ratio and did not present any morphological defects at birth, confirming bypass of the embryonic lethality seen in the constitutive Paxx/Xlf double KO (DKO)
Xlf⌬CD21 mice confirmed the dispensability of PAXX in class switch recombination (CSR), first proposed in vitro using the CH12 model [15], as shown by the absence of an additive effect of Paxx deficiency over Xlf KO alone
Summary
Repair of DNA double-strand breaks by the nonhomologous end joining pathway is central for proper development of the adaptive immune system. Xlf؊/؊ and Paxx؊/؊ mice are viable and exhibit only a mild immunophenotype 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. XLF and PAXX appear to be an exception because their loss of function only marginally affects V(D)J recombination; Xlf or Paxx KO mice present with a mild immunophenotype characterized by a slightly reduced number of B and T lymphocytes (16 –21). E18.5 fetuses recovered from Xlf/Paxx double KO (DKO) mice revealed complete block of B and T cell development [19]
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