Lack of allelic exclusion and isotype switching in B cell chronic lymphocytic leukemia

Two rearranged bands of the IgH gene were detected in a case with B cell chronic lymphocytic leukemia (B-CLL). The expressed VH gene was only VH1 with no somatic mutations in IgM and IgD. The expressions of C mu, C delta, and C gamma were detected by reverse transcription of RNA followed by the polymerase chain reaction (RT-PCR). Sequence analysis of the CDR3 regions of each PCR reaction product showed that the sequence of one rearranged allele was identical to those of the expressed VH1 gene, C mu and C delta, and the sequence of another rearranged allele was identical to that of C gamma. However, none of the expressed VH genes was detected in IgG. These findings suggest that this is a case of B-CLL lacking allelic exclusion and undergoing a class switch of one allele with the incomplete expression of the VH gene.