Unraveling the Consecutive Recombination Events in the Human IGK Locus

Abstract

In addition to the classical Vκ-Jκ, Vκ-κ deleting element (Kde), and intron-Kde gene rearrangements, atypical recombinations involving Jκ recombination signal sequence (RSS) or intronRSS elements can occur in the Igκ (IGK) locus, as observed in human B cell malignancies. In-depth analysis revealed that atypical JκRSS-intronRSS, Vκ-intronRSS, and JκRSS-Kde recombinations not only occur in B cell malignancies, but rather reflect physiological gene rearrangements present in normal human B cells as well. Excision circle analysis and recombination substrate assays can discriminate between single-step vs multistep rearrangements. Using this combined approach, we unraveled that the atypical Vκ-intronRSS and JκRSS-Kde pseudohybrid joints most probably result from ongoing recombination following an initial aberrant JκRSS-intronRSS signal joint formation. Based on our observations in normal and malignant human B cells, a model is presented to describe the sequential (classical and atypical) recombination events in the human IGK locus and their estimated relative frequencies (0.2–1.0 vs <0.03). The initial JκRSS-intronRSS signal joint formation (except for Jκ1RSS-intronRSS) might be a side event of an active V(D)J recombination mechanism, but the subsequent formation of Vκ-intronRSS and JκRSS-Kde pseudohybrid joints can represent an alternative pathway for IGK allele inactivation and allelic exclusion, in addition to classical Cκ deletions. Although usage of this alternative pathway is limited, it seems essential for inactivation of those IGK alleles that have undergone initial aberrant recombinations, which might otherwise hamper selection of functional Ig L chain proteins.