While BRAF V600E is identified in nearly all cases of cHCL, it is rare in vHCL.18 The variant type of HCL was classified as a distinct clinical entity in 2008 and can now often be distinguished from cHCL on the basis of BRAF mutational status, among other differences. Interestingly, in the rare cases of BRAF V600E–negative cHCL, other mutations in BRAF or downstream targets as well as aberrant activation of the RAF-MEK-ERK signaling cascade are observed, indicating that this pathway is critical in HCL and may still represent a viable therapeutic target. Expression of the IGHV4-34 immunoglobulin rearrangement, while more common in vHCL, has also been identified in 10% of cHCL cases and appears to confer poor prognosis.19 Other mutated genes that have been identified in HCL include CDKN1B, TP53, U2AF1, ARID1A, EZH2, and KDM6A.20
Classic HCL is characterized by the immunophenotypic expression of CD11c, CD25, CD103, and CD123, with kappa or lambda light chain restriction indicating clonality; HCL B cells are generally negative for CD5, CD10, CD23, CD27, and CD79b. In contrast, vHCL often lacks expression of CD25 and CD123.18 The B-cell receptor (BCR) is expressed on hairy cells and its activation promotes proliferation and survival in vitro.21 The role of BCR signaling in B-cell malignancies is increasingly recognized, and therapies that target the BCR and associated signaling molecules offer an attractive treatment strategy.22 HCL B cells also typically express CD19, CD20, CD22, CD79a, CD200, CD1d, and annexin A1. Tartrate-resistant acid phosphatase (TRAP) positivity by immunohistochemistry is a hallmark of cHCL. Interestingly, changes to the patient’s original immunophenotype have been observed following treatment and upon disease recurrence, highlighting the importance of tracking immunophenotype throughout the course of disease.
Diagnosis
Prior to the advent of annual screening evaluations with routine examination of complete blood counts (CBC), patients were most often diagnosed with HCL when they presented with symptoms of the disease such as splenomegaly, infections, or complications of anemia or thrombocytopenia.23 In the current era, patients are more likely to be incidentally diagnosed when they are found to have an abnormal value on a CBC. Any blood lineage may be affected and patients may have pancytopenia or isolated cytopenias. Of note, monocytopenia is a common finding in cHCL that is not entirely understood. The cells typical of cHCL do not usually circulate in the peripheral blood, but if present would appear as mature lymphocytes with villous cytoplasmic projections, pale blue cytoplasm, and reniform nuclei with open chromatin (Figure).9 Even if the morphologic examination is highly suggestive of HCL, additional testing is required to differentiate between cHCL, vHCL, and other hematologic malignancies which may also have cytoplasmic projections. A complete assessment of the immunophenotype, molecular profile, and cytogenetic features is required to arrive at this diagnosis.
The international Hairy Cell Leukemia Foundation recently published consensus guidelines for the diagnosis and treatment of HCL.24 These guidelines recommend that patients undergo examination of the peripheral blood for morphology and immunophenotyping and further recommend obtaining bone marrow core and aspirate biopsy samples for immunophenotyping via immunohistochemical staining and flow cytometry. The characteristic immunophenotype of cHCL is a population of monoclonal B lymphocytes which co-express CD19, CD20, CD11c, CD25, CD103, and CD123. Variant HCL is characterized by a very similar immunophenotype but is usually negative for CD25 and CD123. It is notable that CD25 positivity may be lost following treatment, and the absence of this marker should not be used as the sole basis of a cHCL versus vHCL diagnosis. Because marrow fibrosis in HCL may prevent a marrow aspirate from being obtained, many of the key diagnostic studies are performed on the core biopsy, including morphological evaluation and immunohistochemical stains such as CD20 (a pan-B cell antigen), annexin-1 (an anti-inflammatory protein expressed only in cHCL), and VE1 (a BRAF V600E stain).
As noted above, recurrent cytogenetic abnormalities have now been identified that may inform the diagnosis or prognosis of HCL. Next-generation sequencing and other testing of the genetic landscape are taking on a larger role in subtype differentiation, and it is likely that future guidelines will recommend evaluation for significant mutations. Given that BRAF V600E mutation status is a key feature of cHCL and is absent in vHCL, it is important to perform this testing at the time of diagnosis whenever possible. The mutation may be detected via VE1 immunohistochemical staining, allele-specific polymerase chain reaction, or next-generation sequencing. Other less sensitive tests exist but are utilized less frequently.