The IgH Eμ-MAR regions promote UNG-dependent error-prone repair to optimize somatic hypermutation

Two scaffold/matrix attachment regions (5- and 3-MARsE) flank the intronic core enhancer (cE) within the immunoglobulin heavy chain locus (IgH). Besides their conservation in mice and humans, the physiological role of MARsE is still unclear and their involvement in somatic hypermutation (SHM) has never been deeply evaluated. By analysing a mouse model devoid of MARsE, we observed an inverted substitution pattern: SHM being decreased upstream from cE and increased downstream of it. Strikingly, the SHM defect induced by MARsE-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 cE 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. Our study pointed out an unexpected "fence" function of MARsE regions in limiting the error-prone repair machinery to the variable region of Ig gene loci.