Organization of variable region segments of the human immunoglobulin heavy chain: duplication of the D5 cluster within the locus and interchromosomal translocation of variable region segments

We have studied the organization of variable region (V) genes of the human immunoglobulin heavy chain (H) by cosmid cloning. We isolated two independent immunoglobulin D5 clusters (D5‐a and D5‐b) from cosmid libraries of the human genome. Restriction maps of these two regions showed that downstream 15 kb portions of the 55 kb overlap were different although upstream 40 kb portions were almost identical. Four more D segments, (DM, DXP, DA and DK) were found around the D5 segment in the conserved region of each cluster. Nucleotide sequences of the corresponding D segments from each cluster were almost identical and they encoded potentially functional D regions. Analysis using human‐rodent somatic cell hybrids demonstrated that both clusters were located in the immunoglobulin heavy chain (H) locus on chromosome 14, suggesting that the D5‐a and D5‐b regions evolved by internal duplication within this locus. We also isolated a 60 kb DNA region carrying four VH segments, designated as VH‐F region, which was located on chromosome 16. Nucleotide sequences of the four VH segments were determined. Two of them encoded potentially functional VH segments, and the other two were pseudogenes. Some more VH segments were found to be located outside chromosome 14, by Southern blot hybridization of human‐rodent hybrid cell DNAs. These results provide further evidence that the human VH locus has undergone recent reorganization.