Comparison of IgH Gene Rearrangement Configuration between Hairy Cell Leukemia (HCL) and Hairy Cell Leukemia Variant (HCL-v).

Hairy cell leukemia (HCL) is a rare B-cell disorder, with a variant form, which differs from the former in clinical behaviour, morphology and immunophenotype. The resemblance of these malignancies to splenic marginal zone lymphoma (SMZL), has led to the suggestion that they may share a common clonal progenitor that homes within the marginal zone of the splenic germinal centres. The immunophenotype and recent global gene expression results offer some support for this view, with a similar profile seen in HCL, and a subset of memory B-cells that are localized to the marginal zone. However, studies of the mutational status of the Ig genes are controversial. The majority of HCL cases are mutated, whereas up to 30% of SMZL cases are unmutated. Further investigation to understand these differences and to identify the cell of origin in these disorders is needed. In addition, there are no data on gene expression or VH mutation in HCL-variant (HCL-v), an entity with a poorer outcome and for which there is as yet no effective therapy.

We have analysed a series of 13 HCL and 23 HCL-v, for the configuration of the Ig heavy chain rearrangements. VDJH rearrangements were PCR-amplified and sequenced using an automated 3130xL sequencer (Applied Biosystems, Warrington, UK). The sequences obtained were compared to the closest germline segments using V-base (CRI, Cambridge, UK) in order to study the level of somatic hypermutation as well as gene segment usage in IgH rearrangements.

The majority of the HCL (92%) showed more than 2% deviation from the closest germline VH gene segment, compared to only 17/23 (74%) in the HCL-v cases. In the mutated cases, the percentage of mutation was similar for HCL and HCL-v (mean 5.8% and 5.9%, respectively), and none of the cases appeared to show ongoing hypermutation. Within the HCL-v group, 4 out of the 6 unmutated cases used VH4-34 segment, and the remaining 2 used VH1-03. Both genes are known to be associated with autoimmune diseases, and they are excluded from the normal and malignant memory plasma cell repertoires. Also, JH5 segment was found in 47% of the HCL-v, while it was absent in the HCL repertoire. This finding is particularly interesting, as JH5 segment is normally under-represented in most normal and malignant B-cells, except for a characteristic group of IgG-expressing chronic lymphocytic leukemias. Our data suggest that HCL-v is, unlike HCL, heterogeneous regarding IgVH mutations, but has restricted IgH repertoire that, at least in some cases, might be derived from activated, self-reactive B-lymphocytes that have not experienced the germinal-centre reaction.