Human PMS2 deficiency is associated with impaired immunoglobulin class switch recombination

Abstract

Ig class switch recombination (CSR) defi ciencies are rare primary immunodefi ciencies characterized by normal or increased serum IgM levels and a contrasting, marked decrease or absence of IgG, IgA, and IgE. As a result of this molecular defect, defective CSR may be associated with faulty generation of somatic hypermutations (SHMs) in the Ig variable (V) region. The molecular identifi cation and analysis of several CSR defi ciencies has made it possible to better describe the mechanisms underlying CSR and SHM, which are both key elements in the maturation of antibody responses.CSR results in the production of antibodies of diff erent isotypes (IgG, IgA, and IgE) with the same V(D)J specifi city and, therefore, the same antigen affi nity. SHM primarily introduces stochastic mutations into the V region of the Ig, a genetic modifi cation followed by positive selection of B cells harboring a B cell receptor with high antigen affi nity.Mutations in the gene encoding CD40L ( 1 ) or CD40 ( 2 ) result in a CSR defi ciency generally associated with reduced SHM generation, demonstrating the essential role of the B cell ' s CD40 signaling pathway for both CSR and SHM. Other CSR defi ciencies are a consequence of an intrinsic defect in the CSR machinery. The activation-induced cytidine deaminase Immunoglobulin (Ig) class switch recombination (CSR) defi ciencies are rare primary immunodefi ciencies characterized by the lack of switched isotype (IgG/IgA/IgE) production. In some cases, CSR defi ciencies can be associated with abnormal somatic hypermutation. Analysis of CSR defi ciencies has helped reveal the key functions of CSR-triggering molecules, i.e., CD40L, CD40, and effector molecules such as activation-induced cytidine deaminase and uracil N-glycosylase. We report a new form of B cell -intrinsic CSR deficiency found in three patients with deleterious, homozygous mutations in the gene encoding the PMS2 component of the mismatch repair machinery. CSR was found partially defective in vivo and markedly impaired in vitro. It is characterized by the defective occurrence of double-strand DNA breaks (DSBs) in switch regions and abnormal formation of switch junctions. This observation strongly suggests a role for PMS2 in CSR-induced DSB generation.

PMS2 endonuclease activity has distinct biological functions and is essential for genome maintenance

“…Although the present study and previous work have shown that PMS2 plays a role in postreplicative MMR and in the repair of double-stranded DNA breaks in Ig switching regions, deficiency of the protein has a modest or no effect during SHM of the Ig locus (16)(17)(18)(19). Considering the possibility that other factors (e.g., MLH3 or apurinic/apyrimidinic endonuclease) might efficiently compensate for the absence of PMS2 and fulfill the endonucleolytic requirements of V-region hypermutation, we investigated SHM in our mutant mice.…”

“…Indeed, our Pms2 −/− mice had Sμ-Sγ3 junctional DNA segments with extended microhomology (≥5 nts) and a longer average length compared with WT mice (5.07 ± 1.4 vs. 2.6 ± 0.44; P = 0.04) ( Table 1). Abnormal formation of switch junctions also has been identified in truncated forms of human PMS2 (16). In contrast, although we detected a tendency toward a preferential use of blunt junctions in Pms2 EK/EK mice compared with WT mice (Table 1), we found no significant differences in the relative frequencies of the different types of junctions (i.e., blunt junctions, microhomologies, or insertions) or in the average length of microhomology (2.54 ± 0.53 vs. 2.6 ± 0.44; P = 0.47) between the Pms2 EK/EK and the WT mice (Table 1).…”

“…Moreover, Pms2 −/− male mice are sterile, suggesting an important role for PMS2 in spermatogenesis. PMS2 also plays a role in class switch recombination (CSR) of Ig genes, because Pms2 −/− mice (14,15) and patients with deleterious homozygous mutations in PMS2 (16) have a significant reduction in isotype switching. Despite numerous reports (16)(17)(18)(19), however, the role of PMS2 in somatic hypermutation (SHM), if any, has not been resolved.…”

Impaired induction of DNA lesions during immunoglobulin class-switch recombination in humans influences end-joining repair

“…of the DNA repair pathway for S region end-joining is made at the start of the CSR process (i.e., as soon as DNA lesions have been induced). We chose to analyze Sμ-Sα junctions because these junctions are known to be strongly affected by deficiencies in DNA ligase 4 or PMS2, in contrast to Sμ-Sγ junctions, which are little influenced by these deficiencies (17,31). We also attempted to amplify Sμ-Sα junctions from homozygous AIDdeficient (AID −/− ) patients but failed, most likely due to the complete absence of CSR in these patients (1).…”

“…Interestingly, AID, UNG, and PMS2 involved in the generation of DNA lesions or breaks seem to exert further activity during S region end-joining. Why microhomology-mediated repair occurs in PMS2-deficient cells remains unclear (31,41). A recent study with PMS2 endonuclease-deficient mice indicated that the presence of the PMS2 protein, but not its endonuclease activity, was required for proper S region end-joining, but that PMS2 endonuclease activity was still necessary for efficient CSR (42).…”