Search

Intoduction: Two scaffold/matrix attachment regions (5'- and 3'- MARs Eµ ) flank the intronic core enhancer (c Eµ ) within the immunoglobulin heavy chain locus ( IgH ). Besides their conservation in mice and humans, the physiological role of MARs Eµ is still unclear and their involvement in somatic hypermutation (SHM) has never been deeply evaluated., Methods: Our study analyzed SHM and its transcriptional control in a mouse model devoid of MARs Eµ , further combined to relevant models deficient for base excision repair and mismatch repair., Results: We observed an inverted substitution pattern in of MARs Eµ -deficient animals: SHM being decreased upstream from c Eµ and increased downstream of it. Strikingly, the SHM defect induced by MARs Eµ -deletion was accompanied by an increase of sense transcription of the IgH V region, excluding a direct transcription-coupled effect. Interestingly, by breeding to DNA repair-deficient backgrounds, we showed that the SHM defect, observed upstream from c Eµ in this model, was not due to a decrease in AID deamination but rather the consequence of a defect in base excision repair-associated unfaithful repair process., Discussion: Our study pointed out an unexpected "fence" function of MARs Eµ regions in limiting the error-prone repair machinery to the variable region of Ig gene loci., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Martin, Thomas, Marquet, Bruzeau, Garot, Brousse, Bender, Carrion, Choi, Vuong, Gearhart, Maul, Le Noir and Pinaud.)

Activation-induced cytidine deaminase (AID) generates U:G mismatches in Ig genes that can be converted into untemplated mutations during somatic hypermutation or DNA double-strand breaks during class switch recombination (CSR). Null mutations in UNG and MSH2 demonstrate the complementary roles of the base excision repair (BER) and mismatch repair pathways, respectively, in CSR. Phosphorylation of AID at serine 38 was previously hypothesized to regulate BER during CSR, as the AID phosphorylation mutant, AID(S38A), cannot interact with APE1, a BER protein. Consistent with these findings, we observe a complete block in CSR in AID S38A/S38A MSH2 -/- mouse B cells that correlates with an impaired mutation frequency at 5'Sμ. Similarly, somatic hypermutation is almost negligible at the JH4 intron in AID S38A/S38A MSH2 -/- mouse B cells, and, consistent with this, NP-specific affinity maturation in AID S38A/S38A MSH2 -/- mice is not significantly elevated in response to NP-CGG immunization. Surprisingly, AID S38A/S38A UNG -/- mouse B cells also cannot complete CSR or affinity maturation despite accumulating significant mutations in 5'Sμ as well as the JH4 intron. These data identify a novel role for phosphorylation of AID at serine 38 in mismatch repair-dependent CSR and affinity maturation., (Copyright © 2019 by The American Association of Immunologists, Inc.)