The Complete Nucleotide Sequence of the Human Immunoglobulin Heavy Chain Variable Region Locus

The complete nucleotide sequence of the 957-kb DNA of the human immunoglobulin heavy chain variable (V_H) region locus was determined and 43 novel V_H segments were identified. The region contains 123 V_H segments classifiable into seven different families, of which 79 are pseudogenes. Of the 44 V_H segments with an open reading frame, 39 are expressed as heavy chain proteins and 1 as mRNA, while the remaining 4 are not found in immunoglobulin cDNAs. Combinatorial diversity of V_H region was calculated to be ∼6,000. Conservation of the promoter and recombination signal sequences was observed to be higher in functional V_H segments than in pseudogenes. Phylogenetic analysis of 114 V_H segments clearly showed clustering of the V_H segments of each family. However, an independent branch in the tree contained a single V_H, V4-44.1P, sharing similar levels of homology to human V_H families and to those of other vertebrates. Comparison between different copies of homologous units that appear repeatedly across the locus clearly demonstrates that dynamic DNA reorganization of the locus took place at least eight times between 133 and 10 million years ago. One nonimmunoglobulin gene of unknown function was identified in the intergenic region.