Different IGH Rearrangement and Somatic Hypermutation Patterns in Hairy-Cell Leukemia, Hairy-Cell Leukemia Variant and Splenic Marginal Zone Lymphoma.

Hairy cell leukemia (HCL) and its variant form (HCL-v) are rare B-cell disorders, with different and distinct morphology, immunophenotype, clinical behaviour and response to treatment. HCL-v patients do not respond to purine analogues and have a median survival of seven years compared to over 20 years in HCL, a disease potentially curable with purine analogues in up to 80% of cases. Despite these differences, the resemblance between these two malignancies and splenic marginal zone lymphoma (SMZL) has led to the suggestion that they all may share a common clonal progenitor that homes within the marginal zone of the splenic white pulp. Since the study of immunoglobulin heavy chain gene (IGH) rearrangements provides crucial information regarding the differentiation stages at which particular B-cells are transformed, we have investigated the configuration of the IGH genes in a series of well defined HCL (n=17), HCL-v (n=28) and SMZL (n=82) samples. VDJH rearrangements were PCR-amplified and sequenced using an automated 3130xL sequencer (ABI). The sequences obtained were compared to the closest germline segments using the IMGT database (http://imgt.cines.fr) in order to study the level of somatic hypermutation as well as gene segment usage and CDR3 composition in IGH rearrangements. We were able to amplify a functional VDJH rearrangement in all 127 cases. A significant proportion of SMZL (31/82; 38%) and HCL-v (7/28; 25%) cases were found to be unmutated (range: 0–1.8%). In contrast, all but one HCL cases 16/17 (94%) showed more than 2% deviation from the germline sequence. This suggests that HCL-v is more similar to SMZL than HCL with respect to IGH mutational status. There was a significant over-representation of the VH1 family in the SMZL cases (38%; p<0.01). Regarding specific VH gene segment usage, VH1-02 and V4-34 were significantly over-represented in the SMZL group (28% and 12%, respectively; p<0.001). This was especially striking in the unmutated SMZL cases, the majority (20/31; 64%) using either V4-34 or V1-02. Regarding specific VH segment usage, those SMZL cases carrying a VH1-02 rearrangement showed a similar CDR3 length and composition, with an average isoelectric point of 10.5, and 43% of these cases associated with DH3-03 usage. Within the HCL-v group, there was a significant over-representation of the VH4 family (39%; p<0.001). VH4-34 and VH4-59 were significantly over-represented (18% and 11%, respectively; p<0.001), and mainly restricted to the unmutated cases, 71% of them using VH4-34 or VH4-59. In contrast, there was no significant over-representation of any VH family or gene segment in the typical HCL cases.

CONCLUSIONS: Our findings show that in SMZL specific VDJH stereotypes can be identified, likely as a consequence of antigen interaction, which are not present in HCL or HCL-v. Furthermore, HCL-v shares more similarities with SMZL, in terms of IGH rearrangement and somatic hypermutation patterns, than with typical HCL and suggest that different processes occur in the pathogenesis of these three disorders. The clinical implications of these findings are being addressed.