XLF deficiency results in reduced N-nucleotide addition during V(D)J recombination

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Abstract

Repair of DNA double-strand breaks (DSBs) by the nonhomologous end-joining pathway (NHEJ) is important not only for repair of spontaneous breaks but also for breaks induced in developing lymphocytes during V(D)J (variable [V], diversity [D], and joining [J] genes) recombination of their antigen receptor loci to create a diverse repertoire. Mutations in the NHEJ factor XLF result in extreme sensitivity for ionizing radiation, microcephaly, and growth retardation comparable to mutations in LIG4 and XRCC4, which together form the NHEJ ligation complex. However, the effect on the immune system is variable (mild to severe immunodeficiency) and less prominent than that seen in deficiencies of NHEJ factors ARTEMIS and DNA-dependent protein kinase catalytic subunit, with defects in the hairpin opening step, which is crucial and unique for V(D)J recombination. Therefore, we aimed to study the role of XLF during V(D)J recombination. We obtained clinical data from 9 XLF-deficient patients and performed immune phenotyping and antigen receptor repertoire analysis of immunoglobulin (Ig) and T-cell receptor (TR) rearrangements, using next-generation sequencing in 6 patients. The results were compared with XRCC4 and LIG4 deficiency. Both Ig and TR rearrangements showed a significant decrease in the number of nontemplated (N) nucleotides inserted by terminal deoxynucleotidyl transferase, which resulted in a decrease of 2 to 3 amino acids in the CDR3. Such a reduction in the number of N-nucleotides has a great effect on the junctional diversity, and thereby on the total diversity of the Ig and TR repertoire. This shows that XLF has an important role during V(D)J recombination in creating diversity of the repertoire by stimulating N-nucleotide insertion.

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Most cited references 48

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Hairpin opening and overhang processing by an Artemis/DNA-dependent protein kinase complex in nonhomologous end joining and V(D)J recombination.

Yunmei Ma, Ulrich Pannicke, Klaus Schwarz … (2002)

Mutations in the Artemis protein in humans result in hypersensitivity to DNA double-strand break-inducing agents and absence of B and T lymphocytes (radiosensitive severe combined immune deficiency [RS-SCID]). Here, we report that Artemis forms a complex with the 469 kDa DNA-dependent protein kinase (DNA-PKcs) in the absence of DNA. The purified Artemis protein alone possesses single-strand-specific 5' to 3' exonuclease activity. Upon complex formation, DNA-PKcs phosphorylates Artemis, and Artemis acquires endonucleolytic activity on 5' and 3' overhangs, as well as hairpins. Finally, the Artemis:DNA-PKcs complex can open hairpins generated by the RAG complex. Thus, DNA-PKcs regulates Artemis by both phosphorylation and complex formation to permit enzymatic activities that are critical for the hairpin-opening step of V(D)J recombination and for the 5' and 3' overhang processing in nonhomologous DNA end joining.

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IMGT(®) tools for the nucleotide analysis of immunoglobulin (IG) and T cell receptor (TR) V-(D)-J repertoires, polymorphisms, and IG mutations: IMGT/V-QUEST and IMGT/HighV-QUEST for NGS.

Eltaf Alamyar, Patrice Duroux, Marie-Paule Lefranc … (2012)

IMGT/V-QUEST is the highly customized and integrated online IMGT(®) tool for the standardized analysis of the immunoglobulin (IG) or antibody and T cell receptor (TR) rearranged nucleotide sequences. The analysis of these antigen receptors represents a crucial challenge for the study of the adaptive immune response in normal and disease-related situations. The expressed IG and TR repertoires represent a potential of 10(12) IG and 10(12) TR per individual. This huge diversity results from mechanisms that occur at the DNA level during the IG and TR molecular synthesis. These mechanisms include the combinatorial rearrangements of the variable (V), diversity (D) and joining (J) genes, the N-diversity (deletion and addition at random of nucleotides during the V-(D)-J rearrangement) and, for IG, somatic hypermutations. IMGT/V-QUEST identifies the V, D, J genes and alleles by alignment with the germline IG and TR gene and allele sequences of the IMGT reference directory. The tool describes the V-REGION mutations and identifies the hot spot positions in the closest germline V gene. IMGT/V-QUEST integrates IMGT/JunctionAnalysis for a detailed analysis of the V-J and V-D-J junctions and IMGT/Automat for a complete annotation of the sequences and also provides IMGT Collier de Perles. IMGT/HighV-QUEST, the high-throughput version of IMGT/V-QUEST, implemented to answer the needs of deep sequencing data analysis from Next Generation Sequencing (NGS), allows the analysis of thousands of IG and TR sequences in a single run. IMGT/V-QUEST and IMGT/HighV-QUEST are available at the IMGT(®) Home page, http://www.imgt.org.

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XLF interacts with the XRCC4-DNA ligase IV complex to promote DNA nonhomologous end-joining.

Peter Ahnesorg, Philippa Smith, Stephen P. Jackson (2006)

DNA nonhomologous end-joining (NHEJ) is a predominant pathway of DNA double-strand break repair in mammalian cells, and defects in it cause radiosensitivity at the cellular and whole-organism levels. Central to NHEJ is the protein complex containing DNA Ligase IV and XRCC4. By searching for additional XRCC4-interacting factors, we identified a previously uncharacterized 33 kDa protein, XRCC4-like factor (XLF, also named Cernunnos), that has weak sequence homology with XRCC4 and is predicted to display structural similarity to XRCC4. We show that XLF directly interacts with the XRCC4-Ligase IV complex in vitro and in vivo and that siRNA-mediated downregulation of XLF in human cell lines leads to radiosensitivity and impaired NHEJ. Furthermore, we establish that NHEJ-deficient 2BN cells derived from a radiosensitive and immune-deficient patient lack XLF due to an inactivating frameshift mutation in its gene, and that reintroduction of wild-type XLF into such cells corrects their radiosensitivity and NHEJ defects. XLF thus constitutes a novel core component of the mammalian NHEJ apparatus.