Molecular distinctions between variant and classic hairy cell leukemia

Abstract

18002 Background: 10–20% of patients with hairy cell leukemia (HCL) have a variant (HCLv) that presents with high tumor burden in spleen and peripheral blood, and is less responsive to purine analogs. HCLv cells lack CD25 and sometimes CD103. Differentiating HCLv from classic HCL (HCLc) is often difficult and it is unclear whether HCLv and HCLc are different disorders. Detailed molecular distinctions between HCLv and HCLc have not been reported. Methods: Patients with HCL were studied by flow cytometry and PCR to sequence the monoclonal immunoglobulin heavy chain rearrangements. Differences were analyzed by Fisher's exact and Wilcoxon non-parametric tests. Results: 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 with no premature termination codons. 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 (median 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 alone, 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. Conclusions: 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+. No significant financial relationships to disclose.