Gene deletions force nonsecretory alpha-chain disease plasma cells to produce membrane-form alpha-chain only.

We studied a case of nonsecretory alpha-chain disease. The proliferating plasma cells contained a short transcript coding for a truncated membrane-form alpha 1-chain. The productive alpha-gene bore several noncontiguous deletions affecting the VHDJH and CH1 regions. Two deletions were accompanied with peculiar insertions containing duplications. The first insertion contained an acceptor splice site and was present in part in the mature transcript, thus coding for an abnormal aminoterminal peptide. Another deletion located 3' to CH3 eliminated the polyadenylation site of secreted-form alpha-mRNA. As a result, only membrane-form alpha mRNA was present in the tumoral plasma cells, thus explaining the nonsecretory phenotype of the disease. Comparison of cDNA and genomic sequences showed that the previously undescribed human alpha membrane region is encoded by a single exon, beginning with two alternate acceptor splice sites, and comprises either 65 or 71 amino acids.