Testing the reverse transcriptase model of somatic mutation

Somatic hypermutation of the variable (V) regions of rearranged immunoglobu lin genes leads to antibody affinity maturation. Although this process has been extensively studied, the mechanisms responsible for these multiple poi nt mutations are still elusive. One mechanism that was proposed over 10 yea rs ago by Steele and Pollard was that an intrinsic reverse transcriptase (R T) copies the nascent mRNA creating the large number of observed point muta tions due to its high error rate. A cDNA copy of the mutated V region would then replace the endogenous DNA through a gene conversion-like event, thus integrating these point mutations into the genome. This model of hypermuta tion would account for the very high mutation rate, the presence of hotspot s, strand bias, the requirement for transcription and localization of mutat ion within the immunoglobulin V region. Using AZT and ddC to inhibit endoge nous RTs, we have assayed for somatic mutation using a murine in vivo model . Somatic mutation occurred at similar frequencies and with the same charac teristics with or without treatment of RT inhibitors, suggesting that stand ard reverse transcription is not required for antibody V region hypermutati on in the mouse. (C) 2001 Elsevier Science Ltd. All rights reserved.