c-myc hypermutation is ongoing in endemic, but not all Burkitt's lymphoma

Deregulation of c-myc oncogene secondary to chromosomal translocation appears to play an essential role in the genesis of both endemic (African) Burkitt's lymphoma (eBL) and sporadic Burkitt's lymphoma (sBL). In most eBL, mutations in or near exon 1 disrupt normal c-myc regulatory sites. We examined c-myc sequences from a patient with sBL and two patients with eBL to determine (1) whether mutation is ongoing as the tumor clone expands, (2) the nature of mutations in the protein-coding exons 2 and 3, and (3) the extent of c-myc hypermutation in the two clinical forms of BL. Using the polymerase chain reaction (PCR), we amplified segments of c-myc from bulk tumor samples, cloned the products into plasmid vectors, and sequenced multiple subclones of each segment. The mutation frequencies in the control (remission bone marrow) and sBL tumor subclones were 0.65 x 10(-4) and 3.0 x 10(-4) (mutations/base), respectively (P greater than .25). Subclones from the two eBLs exhibited mutation frequencies of 20 x 10(-4) and 16 x 10(-4), respectively (P less than .001 v control). In addition to the consensus mutations seen in one eBL, a random pattern of unshared mutations was observed throughout c-myc in both samples, demonstrating that mutations may be introduced in a stepwise fashion. We noted a clear excess of transitions over transversions (30:9), which is qualitatively similar to the pattern observed in diverse examples of eukaryotic gene mutation. These data demonstrate that c-myc hypermutation is an ongoing process as the eBL tumor clone expands, is qualitatively different from immunoglobulin gene hypermutation, and is not a universal feature of BL, perhaps reflecting the nature of the translocation or the stage of tumor cell maturation.