Variability of CR2 gene products is due to alternative exon usage and different CR2 alleles

The gene encoding human complement receptor 2 (CR2) lies within a cluster of genes on human chromosome 1. Polymorphisms have been described for several of these genes or gene products. To examine any possible polymorphisms in the CR2 gene product we have analyzed the CR2 transcriptional products for variations via alternative exon usage and for the presence of different CR2 alleles within the human population. This analysis has suggested that an exon encoding a 60 amino acid short consensus repeat can be alternatively spliced. Evidence for two transcriptional products has been found in a variety of transformed human B cells and two human tonsils. Interestingly the ratio of the two forms of mRNA, that which includes the alternative exon and that which does not, is not constant when examining the RNA from these sources. In addition to the variation introduced by the alternatively spliced product, there appears to exist in the human population at least three different alleles for CR2. These alleles have been defined by multiple nucleotide changes which are not silent but which alter the amino acid encoded at that site. However, we have not identified allelic variants of CR2 which would produce a product varying in the number of long homologous repeats, as has been identified for the various alleles of the closely related protein, human complement receptor 1.