The absence of ongoing immunoglobulin gene hypermutation suggests a distinct mechanism for c-myc mutation in endemic Burkitt's lymphoma

Burkitt's lymphoma is a malignancy of mature, immunoglobulin (Ig)-bearing B cells characterized by translocation between c-myc and Ig gene loci. A role for the juxtaposed Ig genes in the mutation and deregulation of c-myc expression typical of endemic Burkitt's lymphoma (eBL) has been proposed, but never proven. Our objective was to determine whether Ig gene hypermutation is ongoing in eBL. We isolated Ig heavy-chain sequences from K962 eBL tumor cells using reverse transcription and polymerase chain reaction (PCR) amplification. The PCR product was ligated into Bluescript II vectors. Multiple subclones were sequenced and the variable regions were compared for evidence of ongoing Ig hypermutation. Six total single base substitutions were observed within four of the nine subclones studied. Four substitutions resulted in amino acid changes and two were silent. There was no clustering of mutations in hypervariable regions, or a high incidence of amino acid replacement or link substitutions, all of which are characteristic of Ig hypermutation. The observed mutations occurred at a rate consistent with Taq polymerase error. Our data indicate that in the eBL tumor sample K962, the mechanism underlying c-myc mutation is distinct from that which gives rise to Ig hypermutation.