Genetic Basis of the Antibody Repertoire in Xenopus: Analysis of the Vh Diversity

The Xenopus IgH locus includes various variable (VH) families, several putative diversity (DH) and at least seven joining (JH) elements, but--although structurally very similar to the mammalian locus--it contributes to a restricted antibody repertoire. The largest three VH families contain 15-30 VH elements which are interspersed at the VHI-VHII and VHII-VHIII boundaries. Twenty-nine genomic and eight expressed VH regions have been sequenced. Each VH family has distinct promoter elements with up to three octamers (ATGCCTAAAT) in either orientation. The incidence of pseudogenes ranges from less than 15% in VHI and VHII to approximately 50% in VHIII, consistent with their relative expression. CDR1 and CDR2 show low overall diversity with nucleotide divergence limited to parts of the CDRs. Randomly selectedly VH elements share CDR1 and CDR2, in some cases also with expressed VH regions. Thus, the complexity of VH elements is not maximal. Patterns of sequence similarities or identities indicate recombination or gene conversion events; sets of direct and inverted repeats flank the sites of, or lie within FR or CDR sequences where these genetic events may occur. Restricted antibody diversity in Xenopus seems therefore to be at least partially related to low complexity of VH elements, frequence of pseudogenes and expression regulated by specific promoter elements; diversity may potentially be increased by (non)homologous recombination events.