Gene deletions in the human immunoglobulin heavy chain constant region locus: molecular and immunological analysis.

Single and extensive multigene deletions have been described in the Ig CH immunoglobulin heavy-chain constant region genes, some of these encompassing up to 160 kilobases. To date six different multigene deletion haplotypes have been identified, designated I to VI according to the chronological order of their findings; deletion I (del G1-EP1-A1-GP-G2-G4), II (del EP1-A1-GP), III (del A1-GP-G2-G4-E), IV (del EP1-A1-GP-G2-G4), V (del GP-G2-G4-E-A2), VI (del G1-EP1-A1-GP-G2). Individuals were found either homozygous for one type of deletion or heterozygous for two different deletions, mainly in the Mediterranean area. The high level of consanguinity in the Tunisian population accounts for the high frequency of individuals homozygous for one or the other of these multigene deletions which involve highly homologous regions as hot spots of recombinations, outside of the switch sequences, in the Ig CH locus. In 15 cases out of 16, these multigene deletions have been observed in healthy people, although these individuals lacked several immunoglobulin subclasses and, even, one class. Such an immunological situation makes it possible to study the importance of these subclasses for the overall immunity, and to analyse the specific immune responses by the retained IgG and IgA subclasses.