Tandem Substitutions in Somatic Hypermutation

Abstract Upon antigen recognition, activation-induced cytosine deaminase initiates affinity maturation of the B-cell receptor by somatic hypermutation (SHM) through error-prone DNA repair pathways. SHM typically creates single nucleotide substitutions, but tandem substitutions may also occur. While have been described in mice and other species, incidence this phenomenon its underlying mechanism humans is currently unknown. We investigated sequence context massive parallel sequencing V(D)J repertoires healthy human donors generated unbiased ARTISAN PCR. Selection unique, clonally unrelated, antigen-experienced sequences carrying up to 5% mutations yielded 13.532 VDJ, 7.952 VJ-kappa 7.598 VJ-lambda. Comparison closest germline allele allowed for identification a total 122.878 (SNS), 10.735 dinucleotide (TDNS) 2.615 longer contiguous substitutions. After correcting expected clusters adjacent SNS, comprised 5,7% all AID-induced mutations. The mutation more than one event, was shown overcome amino acid codon redundancy therefore enhance adaptive immune response. Clustering such around AID hotspots their overall distribution indicates that are an integral part spectrum. In majority mutated be identified directly reference context. Tandem represent juxtalocations. Such juxtalocations appear favored polydipyramidine stretches. These observations could confirmed patients with MSH2/6 deficiency, were absent VDJ library from UNG-deficient patient, indicating strict dependence on abasic sites as instigating mechanism. Together, these findings delineate model where predominantly translesion synthesis across apyramidinic site created UNG. During replication, apyrimidinic transiently adapt extruded configuration, causing skipping base. Consequent strand decontraction leads juxtalocation, after which exonucleases any mismatched base pairs. mismatch pathway appears account remainder Our study shows significant portion acquired during caused expedite response overcoming degeneracies or mutating two residues simultaneously. Figure legend. Corrected donors. (A) Dinucleotide unique IGHV, IGKV IGLV corrected silico predictions did not occur tandem. Burgundy cells inversions, light dark purple 5' 3' pair (as seen non-transcribed strand), respectively. For unshaded cells, juxtalocation assessed due nucleotides matching sequence. (B) Relative contribution inversions 1 1. Disclosures No relevant conflicts interest declare.