Loss of human CR1- and murine Crry-like exons in human CR2 transcripts due to CR2 gene mutations

Analysis of the coding sequences of the murine Cr2 gene indicated that it contains two distinct regions of homology to the murine Crry, and human CR1 and CR2 genes. The last 15 short consensus repeats (SCR) of Cr2 are very similar to the 15 reported SCR of human CR2. These 15 SCR of Cr2, plus the transmembrane and cytoplasmic domains, make up a protein very similar in organization and sequence to the human CR2 gene product. Another Cr2 transcript contains the sequences encoding the previously described 15 SCR plus those encoding another six amino terminal SCR. These amino terminal SCR are very similar to those of Crry and the first six SCR seen within the CR1 long homologous repeats. Amino acid sequence similarity analysis, however, indicated that the sequences encoding the six amino terminal SCR of Cr2 evolved from a separate lineage of SCR than Crry and CR1, suggesting that the human counterparts to these additional Cr2 SCR had not been identified. Using the cDNA sequences specific for the amino terminal SCR of murine Cr2, the human counterparts were isolated and localized within the human CR2 gene between those nucleotides encoding the signal sequence and those encoding the first SCR of the mature human CR2 protein. Unlike the murine Cr2 gene products, these Crry/CR1-like sequences of the CR2 gene are not maintained in mature CR2 mRNA, and thus represent pseudoexons. DNA sequence analysis of the pseudoexon homologous to that encoding the first SCR of the murine Cr2 gene indicated that a stop codon has been introduced within the human coding sequence but that the 5' and 3' splice recognition sequences appear to be functional. Although the stop codon would block the translation of a transcript with this exon, it would not inhibit the incorporation of the exon within the mature CR2 transcript. Therefore, another mutation must have been introduced within the human CR2 gene such that both this exon and the other CR1/Crry-like exons are removed from mature CR2 transcripts.