Somatic mutation analysis of IgH variable regions reveals that tumor cells of most parafollicular (monocytoid) B-cell lymphoma, splenic marginal zone B-cell lymphoma, and some hairy cell leukemia are composed of memory B lymphocytes.

The cell of origin of parafollicular (monocytoid) B cell lymphoma (PBCL), splenic marginal zone lymphoma (SMZL), and hairy cell leukemia (HCL) is controversial. To better understand the relationship between these low-grade B-cell neoplasms, we analyzed the nucleotide sequences of the rearranged immunoglobulin heavy (IgH) chain variable (V) region of the clonal population of cells in five cases of PBCL, four cases of SMZL, and seven cases of HCL to determine whether these neoplasms could be differentiated by the degree of somatic mutation in the IgH V gene or by the IgH V gene family usage. DNA was extracted from diagnostic material and clonality confirmed by PCR. The DNA was reamplified using V heavy chain family specific primers, and the amplicons were sequenced. Sequences were compared with germline IgH V gene sequences, and base changes were determined to be silent or to represent amino acid replacements by using three different methods. Four of five (80%) cases of PBCL, three of four (75%) cases of SMZL, and three of seven (43%) cases of HCL showed evidence of antigen selection, suggesting that these neoplasms involved clonal expansions of postgerminal center memory lymphocytes. Only SMZL showed a preferential usage of V(H)1 family genes.