Molecular Characteristic of Variant and Classic Hairy Cell Leukemia.

10–20% of patients with hairy cell leukemia (HCL) have a variant (HCLv) and present with high tumor burden in spleen and peripheral blood, and are less responsive to purine analogs. HCLv cells lack CD25 and sometimes CD103. Differentiating HCLv from classic HCL (HCLc) is sometimes difficult and it is unclear whether HCLv and HCLc are different disorders. Detailed molecular distinctions between HCLv and HCLc have not been reported. Patients with HCL were studied by flow cytometry and PCR to sequence the monoclonal immunoglobulin heavy chain rearrangements. 50 and 21 VH-D-JH rearrangements were obtained from 49 HCLc and 19 HCLv patients, respectively. All rearrangements except 2 each for HCLc and HCLv were productive. The incidence of VH4 usage was higher in HCLv than in HCLc (57% vs 20%, p=0.0041). VH4-34 was the most common VH gene in HCLv and was more common in HCLv than in HCLc (33% vs 8%, p=0.012). The percentage of rearrangements which were unmutated (defined as < 2% somatic mutations) was higher in HCLv than in HCLc (43% vs 18%, p=0.038), but % homology was similar in both groups by rank order (94.9 vs 94.3% p=0.22). However, in comparing 11 VH4-34 with 60 other rearrangements, homology was higher with VH4-34 (median 99.2 vs 95.2%, p < 0.0001). In fact, the higher frequency of unmutated rearrangements in HCLv vs HCLc was due to VH4-34 cases, since the 7 HCLv VH4-34 rearrangements were all unmated and had higher homology than the other 14 HCLv rearrangements (median 99.6 vs 94.2%, p=0.006). Moreover, unmutated rearrangement incidence was similar between HCLv and HCLc for non-VH4-34 cases (14 vs 13%, p=1.0, median homology 94.2% vs 95.3%, p=0.5). Of the 4 VH4-34+ HCLc (CD25+) patients, 3 (75%) had unmutated rearrangements, and these 3 all had clinical features of HCLv, including presenting with lymphocytosis, large splenomegaly, absent cytopenias, and primary failure of purine analog treatment. Our data shown that homology of monoclonal immunoglobulin rearrangement to the germline sequence of HCL patients appears more related to VH4-34 status than to whether patients have HCLv or HCLc. In fact, no molecular distinctions between HCLv and HCLc were observed in VH4-34-negative patients. Our data suggest VH4-34-positive HCL is itself a variant of HCL affecting ~15% of our patients. Such patients have features of HCLv, but their HCL cells can be CD25+.