ANALYSIS OF THE FREQUENCY AND PATTERN OF SOMATIC MUTATIONS WITHIN NONPRODUCTIVELY REARRANGED HUMAN VARIABLE HEAVY-CHAIN GENES

Somatic hypermutation plays an essential role in avidity maturation of Ab. To characterize the effects of hypermutation without the imposed bias of Ag-mediated selection, the mutational pattern of 37 nonproduct ively rearranged V-H genes amplified from individual human B cells was analyzed. A high frequency of mutations as well as frequent replaceme nt mutations were observed in the complementarity-determining regions (CDR) and in the framework regions of nonproductive V(H)DJ(H) rearrang ements. Comparison with 57 productive V-H rearrangements indicated tha t replacement mutations, especially those occurring in the framework r egions, were less frequent in productively rearranged V-H genes, sugge sting that they were deleted from the expressed repertoire. A number o f factors contributed to the nonrandom localization of mutations, incl uding: the targeting of specific motifs, such as AGY, GCY, GTA, TAY, a nd RGYW; an increased frequency of some commonly mutated motifs in the CDRs; and an apparent increased likelihood of mutations of CDR nucleo tides. Each of these appeared to bias the mutational machinery, result ing in an increased frequency of replacement mutations in the CDRs of nonproductive V-H rearrangements.