Clonal evolution in B-lineage acute lymphoblastic leukemia by contemporaneous V_H-V_H gene replacements and V_H-DJ_H gene rearrangements

B-cell acute lymphoblastic leukemia (B-ALL), more frequently than any other B-lineage neoplasm, exhibits oligoclonal Ig heavy chain (IgH) gene rearrangement in 15% to 43% of all cases studied. To study the molecular processes that promote multiple IgH rearrangements, a comprehensive sequence analysis of a B-ALL case was performed in which seven clonal IgH gene rearrangements were identified. The genetic profiles suggested that a single leukemic progenitor clone evolved into several subclones through dual processes of variable (V_H) to preexisting diversity-joining (DJ_H) gene segment rearrangement and V_H to V_H gene replacement. Predominant IgH-V usage and the uniquely rearranged clonotype-specific V_HDJ_H region gene sequences were identified using a novel DNA-based gene amplification strategy. Polymerase chain reaction (PCR) was directed by an IgH-J generic primer and a complement of family-specific IgH-V primers that defined the major B-cell IgH-V gene usage. Clonality of rearranged V_HDJ_H bands was substantiated by high resolution denaturant gel electrophoretic analysis. Sequence patterns of the amplified V_HDJ_H fragments segregated into two groups defined by common DJ_H sequences. Partial N region homology at the V_HD junction as well as shared DJ_H sequences firmly established V_H to V_HDJ_H gene replacement as a mechanism generating clonal evolution in one group. In the second subset, oligoclonality was propagated by independent V_H gene rearrangements to a common DJ_H precursor. The contributions of all clonal Ig-V_HDJ_H repertoires for each group was approximately 50% and reflected a symmetric distribution of leukemic subclones generated by either process. Thus, oligoclonal rearrangements evolved by two independent, yet seemingly contemporaneous molecular genetic mechanisms. All seven clones displayed nonfunctional Ig-V_HDJ_H recombinations. These observations may have relevance to the recombinatorial opportunities available during normal B-cell maturation.