Re-entry of tumour B cells into the cycle of somatic mutation and isotype switching in follicular lymphoma

Sequencing of the immunoglobulin (Ig) gene transcripts of the tumour B cells in lymph node (LN) and bone marrow (BM) from a follicular lymphoma (FL) patient associated with multiple myeloma identified two dominant clones. One of the clones, present in both LN and BM, had somatic mutations and extensive clonal diversity. Among the diversified clones, two dominant populations of identical sequences (group I and II) were present. Group II was a descendant population of group I and had nine more somatic mutations. Group I contained μ-, δ-, γ- and α-expressing clones. Group II clones contained mainly μ- and δ-expressing clones. These findings showed that somatic mutation and isotype switching occurred repeatedly in this patient.