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Introduction/Objective Hairy cell leukemia variant (HCL-v) is a rare B cell lymphoproliferative disorder. Although HCL- v shares some overlapping features with classic HCL, it lacks the characteristic immune-morphology and pathogenic BRAF V600E mutation seen in classic HCL and recognized as a distinct entity in the 2017 WHO classification. Approximately half of HCL-v harbors MAPK2 mutation and one third has TP53 mutation. Cytogenetic abnormalities include gain of chromosome 5, loss of chromosome 7q, and deletion of 17p13/TP53 and 11q/22 ATM gene Methods/Case Report We report a case of HCL-v with total loss of ATM gene function. A 71-year-old male with splenomegaly presented with an outside diagnosis of splenic marginal zone lymphoma and underwent splenectomy. Flow cytometry on spleen demonstrated mature clonal B lymphoid cells and was positive for CD11c and CD103. However, unlike classical HCL, it was CD25 negative, which supports the diagnosis of HCL-v. Sections of spleen revealed most of the red pulp replaced by hairy cell leukemic infiltrate with minimal preservation of the normal lymphoid stroma. Peripheral blood smear showed atypical lymphocytosis with prominent nucleoli but most lacking circumferential shaggy contours, seen in classic HCL. Cytogenetic analysis identified a deletion in chromosome 11(q13q23), which deletes the ATM gene. NGS analysis demonstrated a nonsense variant in ATM (p. E888*). No significant sequence variants were identified in BRAF, MAP2K1 or TP53. This is the first case report of HCL-v with a deletion as well as a non-sense mutation in the ATM gene. ATM gene belongs to the PI3/PI4-kinase family, and is an important cell cycle checkpoint kinase that phosphorylates downstream proteins, including TP53, BRCA1 etc. Although detection of the ATM has therapeutic application for prostate cancer patient for poly [ADP]-ribose polymerase inhibitor treatment, the significance of loss of ATM function in hairy cell leukemia variant is unknown. Results (if a Case Study enter NA) N/A Conclusion In conclusion, our case demonstrates that molecular test in hairy cell leukemia variant can be helpful for confirming the diagnosis as well as providing predictive information for potential target therapy. In our case, lack of MAP2K1 mutation precludes the patient for targeted therapy. In addition, the finding of novel mutation might help to understand the molecular pathogenesis in this rare entity.

Objectives The importance of distinguishing mature B-cell lymphoproliferative disorders (B-LPDs) is highlighted by the distinct treatments used for and varying prognoses seen in association with these different diseases. Immunophenotyping allows for accurate and efficient differentiation of many B-LPDs. Recently, we showed that CD200 is highly expressed in hairy cell leukemia (HCL) but not in marginal zone lymphoma (MZL), lymphoplasmacytic lymphoma (LPL), or hairy cell leukemia-variant (HCL-v). Here, we assessed the usefulness of a flow cytometric panel combining CD200 and CD1d with CD25, CD103, and CD11c to distinguish CD10-, CD5- B-LPDs. Methods We analyzed the expression of CD200 and CD1d by flow cytometric analysis in 79 cases of CD10-, CD5- mature B-LPDs. Results Distinct patterns of CD200 and CD1d expression were seen in the examined B-LPDs. HCL showed bright positivity for CD200 along with positive staining for CD1d, whereas HCL-v showed low levels of expression for both markers. LPL demonstrated positive staining for CD200 in combination with dim to negative staining for CD1d. In contrast, MZL was commonly positive for CD1d and negative for CD200. Conclusions Flow cytometric analysis of CD200 and CD1d, along with CD25, CD103, and CD11c, can aid in the diagnosis of CD10-, CD5- mature B-LPDs.

Hairy cell leukemia variant (HCL-V) is a poorly described, rare B-cell lymphoproliferative disorder typically positive for CD103 and CD11c, while lacking CD25. Splenic marginal zone lymphomas (SMZL) also have this unusual phenotype in 15% to 25% of cases, have other overlapping clinical or morphologic features, and are more common than HCL-V. The purpose of our study was to better characterize HCL-V and determine whether most cases could be distinguished from SMZL. Cases with an HCL-V phenotype were identified from our flow cytometry service, and 10 were selected for further study based on bone marrow or splenic tissue availability. All cases had cytologic features consistent with HCL-V, and 9 of 10 patients had lymphocytosis. Bone marrow involvement was mostly interstitial and/or sinusoidal without lymphoid nodules. Coexpression of preswitched with postswitched heavy chain isotypes, an unusual feature of HCL, was seen in 2 of 4 cases. This study better defines HCL-V and establishes that most cases do not represent SMZL.

Objectives: To evaluate CD200 expression in B-cell proliferative disorders. Methods: We analyzed 180 recent specimens of B-cell neoplasms for CD200 expression by flow cytometric immunophenotypic analysis, which is better able to assess relative intensity of staining than immunohistochemical staining. Results: We found that hairy cell leukemia exhibits a high level of staining for CD200 in comparison to other B-cell lymphoproliferative disorders, including hairy cell leukemia-variant (HCL-V), marginal zone lymphoma, and lymphoplasmacytic lymphoma. We confirmed this observation by semiquantitative immunohistochemical staining. Conclusions: Assessment of the CD200 expression level is helpful to distinguish HCL from HCL-V and other B-cell lymphoproliferative disorders and in the differential diagnosis of B-cell neoplasms in general.