Frequent occurrence of deletions and duplications during somatic hypermutation: implications for oncogene translocations and heavy chain disease

Abstract

Human naive and germinal center (GC) B cells were sorted by flow cytometry and rearranged VH region genes were amplified and sequenced from single cells. Whereas no deletions or insertions were found in naive B cells, approximately 4% of in-frame and >40% of out-of-frame rearrangements of GC B cells harbored deletions and/or insertions of variable length. The pattern of deletions/insertions and their restriction to mutated V genes strongly suggests that they result from somatic hypermutation. Deletions and insertions account for approximately 6% of somatic mutations introduced into rearranged VH region genes of GC B cells. These deletions/insertions seem to be the main cause for the generation of heavy chain disease proteins. Furthermore, it appears that several types of oncogene translocations (like c-myc translocations in Burkitt's lymphoma) occur as a byproduct of somatic hypermutation within the GC-and not during V(D)J recombination in the bone marrow as previously thought.

Figure 1

Deletions and insertions in nonfunctional V_H region genes. The figure schematically shows the deletions (Δ) and insertions (ins., insertion; dupl., duplication) within the 12 nonfunctional V_H gene rearrangements amplified from GC B cells. The lengths of the deletions/insertions are indicated. Sequence 4 shows a long deletion; 35 bp of unknown origin are inserted into the deleted region. In sequence 7, the two parts of the 52-bp duplication are separated by a sequence stretch of 27 bp. Sequence 12 must have acquired the duplications before the deletion event happened because the deleted region is still present in both downstream duplicated regions.