LOW-RATE OF SOMATIC HYPERMUTATIONS CHARACTERIZE PROGRESSIVE B-CELL LYMPHOMAS

Immunoglobulin heavy (IgH) chain gene rearrangements were characterize d in 40 samples from 15 patients with B-cell lymphomas at different ti me points during tumour progression. Using polymerase chain reaction ( PCR) amplification and single strand conformation polymorphism (SSCP) analysis of variable heavy (V-H) chain gene segments, we found that 6 cases displayed alterations in their IgH chain rearrangements at relap se. These alterations were mainly observed in follicular or transforme d lymphomas, but no association to clinical features was found. Nucleo tide sequence analysis revealed a low frequency of mutations in 3 case s, whereas 1 case displayed an extensive mutation rate in a compartmen t with transformed morphology at relapse. The mutations observed most probably resulted from somatic hypermutations. Further, the mutations were scattered randomly over the V-H gene segment and no significant b ias favouring amino acid substitutions was observed in 3 cases, sugges ting that the tumour cells had not been subjected to antigen-driven se lection. In 1 case, however, the mutation pattern indicated that the t umour cells had been affected by an antigen selection process. Ln the 2 remaining cases, the original V(H)DJ(H) rearrangement could no longe r be detected by VH gene family specific PCR at relapse,but using prim ers specific for the framework region 2 or 3 altered rearrangements we re demonstrated, implying that mutations had been introduced in framew ork region 1. However, the majority of the tumour cell clones analysed were relatively stable during tumour progression, which make them eli gible for analysis of minimal residual disease using the V-H gene regi ons as molecular markers.