Analysis of Immunoglobulin Gene Mutation Status in Lymphoplasmacytic and Splenic Marginal Zone Lymphomas with Bone Marrow Infiltration.

Splenic Marginal-Zone lymphomas (SMZL) and Lymphoplasmacytic lymphomas (LPL) correspond to entities that shared clinical, morphological and immunohistochemical features and are not always easy to discriminate. Immunoglobulin variable heavy chain gene (VH) usage and somatic mutation pattern were analyzed to clarify the relationship and the pathogenesis of these 2 entities.

Bone marrow frozen samples from 27 patients, 16 LPL and 11 SMZL, were studied. B-cell clonality was first analysed using the standard PCR method with the Biomed 2 primers. Informative PCR products were then purified, sequenced and analyzed on IMGT and Ig-Blast websites.

Overall, median rate of somatic mutations was 9% in LPL and 4% in SMZL (p=0.013).

All cases of LPL were mutated. Mutation rate was always higher than 5%. A preferential usage of VH3 gene was observed in 13/16 cases (80%), with a predominance of VH3–23 and VH3–74, both representing 60% of cases. VH4 gene was used only in 2/16 cases (13%). Regarding JH usage, a predominance of JH4 gene rearrangement was observed in 11/16 cases (69%). JH5 and JH6 were used in 20% and 13% of cases respectively.

In the group of SMZL, mutation rates and usage of VH and JH genes were more heterogeneous. Thirty six percent of SMZL were unmutated and 19% had a mutation rate lower than 5%. VH3 was used in 4/11 cases (36%), VH4 in 3/11 cases (27%) and VH1 in 4/11 cases (36%). JH4 and JH6 were used in 4/11 cases (36%) and the JH5 in 1/11 cases (9%).

Therefore, the profile of somatic mutations and the VH and JH segment preferentially used are different in SMZL and LPL, suggesting a different process of antigen-driven selection and of transformation of the initial B-cell clone. These findings consolidate the existence of LPL as a genuine entity, different of SMZL despite their histopathological overlaps.