The delayed activation of post germinal centre memory B cells probably along with marginal zone B cells of the spleen may constitute a cellular origin of hairy cell leukaemia. A majority (90%) of instances may depict a mutant genetic profile of immunoglobulin heavy chain variable region (IGHV). The haematopoietic stem cells (HSCs)and the cellular component of Langerhans Cell Histiocytosis(LCH) and Erdheim Chester Disease (ECD) may elucidate a B raf proto-oncogene( BRAF V600E gene- 7q34). Majority of the mutant alleles accompanying these disorders may be situated in the CD14+ classical monocytes, the CD 16+ non classical monocytes and CD1c+ myeloid dendritic cells, located in the peripheral blood2. The mutant alleles may be dispersed within the HSCs and myeloid progenitors of the bone marrow. In hairy cell leukaemia the mutant alleles may be lacking within monocytes and myeloid cells, although may appear in the normal B lymphocytes and natural killer (NK) cells2.

The patients may display systemic symptoms such as fatigue , infection, splenomegaly besides a constitutional depiction of pancytopenia. An enlarged spleen may be discovered in a majority (90%) of the instances, although the feature may be infrequent in recent times, on account of the earlier discernment of the disorder. Pancytopenia may be the presenting symptom24. The preliminary assessment of disease may mandate a differential leukocyte count with a reappraisal of the peripheral blood smear. Monocytopenia, when delineated, may prove to be a sensitive and specific indicator of hairy cell leukaemia. The appearance of leukaemia cells per se may be exceptional2.

The morphology of hairy cell leukaemia may be specific , in contrast to the variants, on account of an in vitro interaction of the primary hairy cells with BRAF genes and MEK inhibitors, which may incite a prominent MEK - ERK dephosphorylation, thereby curtailing transcriptional outpourings of the RAS- RAF- MEK-ERK pathway. The particular occurrence may induce a deficit in hairy cell leukaemia specific genetic profile signature that may convert the morphology of hairy cells into smooth cells with ultimate apoptosis2. The objective of B actin and leukocyte specific transcript1 ( LST 1) in the establishment of a hairy cell morphology may be uncertain2.

The classic hairy cell is medium sized with a magnitude of 10-14µm. The moderately abundant or variable cytoplasm may be transparent or mildly basophilic. The cellular surface with the characteristic serrated perimeter depicts innumerable fragile or stout extensions of cytoplasm ,particularly discernible on the phase contrast and electron microscopy. The cytoplasm may exhibit vacuoles with occasional azurophilic granules4. The nucleus may be elliptical or reniform, folded or indented with a coarse, reticulated or a finely dispersed chromatin and inconspicuous nucleoli along with infrequent mitosis. Bone marrow aspiration or bone marrow trephine biopsy may be inadequate for diagnosis in 30%-50% individuals4. The trephine sections of the bone marrow may depict a characteristic interstitial pattern of leukaemic infiltration. Generally the bone marrow is hyper-cellular, though it may be hypo-cellular in 10-15% individuals4. The leukaemia cell ingress may be diffuse or partial, although diffuse infiltration is frequent. The partial variety of leukaemic dissemination may be ineptly categorized with an indeterminate differentiation from the uninvolved marrow. The malignant insertions may initially emerge as miniature, undefined, cellular loci. The formalin fixed, paraffin embedded sections may elucidate a crystalline zone or a “halo” appearance of the cells with a circumscribed nucleus on account of the plentiful cytoplasm4. The cellular margins may be intertwined. Fixation of bone marrow smears with Zenker’s fixative may demonstrate a retracted cytoplasm of the hairy cells with a consequent disconnected structure. The bone marrow in the absence of a malignant process may be hypo-cellular or hyper-cellular. Reticulin stains may delineate an enhanced accrual of broad, dense reticulum fibres surrounding the aggregates of leukaemia cells with the fibrous circumlocution of individual malignant cell and fibrotic extensions into the abutting, uninvolved bone marrow4.

The leukaemia cells may enunciate a characteristic immune phenotype, crucial for a confirmatory diagnosis. The peripheral blood mononuclear B cell population may display a kappa or lambda light chain restriction. The phenotype of classic hairy cell leukaemia may be delineated by concurrent, immune reactive CD19+ CD20+,CD 11c+, CD25+, CD103+ and CD123+. An intensely immune reactive CD200+ and a non reactive CD27- antigen may be present24. Evaluation of a trephine bone marrow biopsy and bone marrow aspirate may define the degree of tumour infiltration. A dry tap on account of prominent bone marrow fibrosis may be elucidated at preliminary diagnosis. A decline in the normal haematopoiesis may account for a hypo-cellular marrow in 10% instances. Gradation of cellular infiltrating of the leukaemia within the bone marrow may be appropriately investigated with immune –histochemical stains24. Immune staining for CD20+, annexin 1 and VE1 (a BRAF V600E stain] may validate the diagnosis and precisely analyse the extent of malignant bone marrow infiltration[8]. Determination of BRAF V600E mutation may be critical in therapeutically non responsive individuals with applicable standard therapy or in instances of multitudinous reoccurrences[9]. Deploying inhibitors of BRAF V600E gene may be efficacious in patients impervious to approved therapy. The mutation necessitates a comprehensive scrutiny of the implicated individuals with a sensitive molecular assay which may discern up to < 10% of the hairy leukaemia cells appearing in the peripheral blood smears or bone marrow aspirates diluted with peripheral blood or aspirates elucidating a dry tap[2,4]. Allele specific polymerase chain reaction (PCR) or a next generation sequencing may be optimally employed to circumvent false negative outcomes. If the leukaemia cells are sparse or if particularly sensitive & efficacious molecular techniques are not accessible, the application of appropriate immune histochemical stains to the bone marrow biopsy such as a BRAF V600E mutation stain (VE1) may detect the hairy cells and conclusively diagnose the condition[2,4,10]. Figure 1, Figure 2, Figure 3, Figure 4, Figure 5, Figure 6, Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14.

Hairy cell leukaemia necessitates a distinction from adjunctive disorders such as the hairy cell leukaemia –variant ( HCL –V) and splenic diffuse red pulp lymphoma (SDRPL)2. The hairy cell leukaemia variant (HCL-V) may incorporate a mere 10% of the instances of HCL and as yet remains a conditional form of leukaemia. The disseminated, aberrant lymphoid cells may elucidate an intermediary morphology betwixt a pro-lymphocyte and a hairy cell. Immunological quantification of the HCL-V for aforementioned parameters may be minimal (0-1). An absence of immune reactive CD25- and CD200- may manifest with an inconsistent or poorly enunciated CD1232.The individuals may lack mono-cytopenia. Likewise, the splenic diffuse red pulp lymphoma (SDRPL) may be cogitated as a provisory condition,though dissimilar from HCL-V. Majority (60%) of the small to medium sized, aberrant villous lymphoid cells appear in the peripheral blood and may depict a polar villous disposition with a miniature to absent nucleolus. The monoclonal B cells in the majority (97%)of the implicated SDRPL may exhibit an immune reactive CD11C+ with a discordantly demonstrated CD103+ (38%) and an occasional elucidation of CD123+ (16%) with CD25+ ( 3%)2.

An enlarged spleen beyond 3 centimetres of the left costal margin, leucocytosis with a white blood cell count greater than 10000 cells/µL , circulating hairy cells in the peripheral blood greater than 5000 cells/µL and a β 2 micro-globulin level exceeding twice the normal range of 3 µg/ml may be accompanied by an inferior therapeutic outcome and an emerging resistance to purine analogues (PNAs)2. Simulating the chronic lymphocytic leukaemia , an elucidation of CD38+ may ensure a worse prognosis6. The mutation of immunoglobulin heavy chain variable region (IGHV) gene may dictate prognostic assumptions of the disorder. Individuals lacking an IGHV mutation may depict a reduced overall survival, in contrast to the patients demonstrating the IGHV mutation. An estimated 40% of the hairy cell leukaemia variant( HCL –V) and 10% of classic hairy cell leukaemia may elucidate an IGHV 4-34 immunoglobulin heavy chain variable gene rearrangement13. VH 4- 34 reactive instances may determine a contemporary variant and subset of hairy cell leukaemia with an unfavourable prognosis, an elevated initial disease encumbrance, inadequate response to recognized therapy, a decline in the overall survival and an absence of BRAF V600E mutation2.

The employment of whole exome sequencing (WES) for the detection of a BRAF V600E somatic mutation may be advocated. The B raf proto-oncogene( BRAF gene- 7q34) comprises of 18 exons and a genetic mutation of exon 15 in position 1799 may interchange the thymine and adenine nucleobases, thus engendering a replacement of valine ( v) by glutamate(E) at codon 600 (V 600 E) of the BRAF protein. The mutation may be detected in 80% to 90% of hairy cell leukaemia14 The BRAF V600E mutation may activate the BRAF gene via autophosphorylation of the protein with a consequent decline of the MEK-ERK signalling network, thereby ensuring amplification of genes inciting cellular multiplication and continuation2. Mutant BRAF V 600 E gene may not be elucidated in adjunctive B cell chronic lympho-proliferative conditions7, excluding occasional instances of chronic lymphocytic leukaemia or multiple myeloma. The BRAF mutation is a characteristic molecular feature , a contemporary diagnostic tool and a possible therapeutic approach of targeting BRAF by employing BRAF inhibitors15. A lack of BRAF genetic mutation may arise in 10% -20% of hairy cell leukaemia which comprises of a subcategory of inferior prognosis. The particular instances necessitate an demarcation from a patients with a probable mutation in exon 11(F468C, D449E)16. The BRAF V600E mutation may reappear in diverse solid tumefaction such as the cutaneous melanomas, pulmonary, ovarian, bladder, thyroid or prostate malignancies, cholangiocarcinoma and gastrointestinal stromal tumours/ sarcomas2.

Refractory hairy cell leukaemia may delineate a repetitive neutralization of the cell cycle inhibitor CDKN1B / p27 in a minority (16 %) of the instances. Supplementary mutations of KLF2 genes may be exemplified in an estimated one third (30%) of marginal zone lymphoma ( MZL) and diffuse lymphomas( DLBCL). KLF2 is a transcription factor which regulates the maturation and differentiation of numerous B cell subpopulations, especially marginal zone B cells2. Along with BRAF mutations, the two frequent mutations discerned in hairy cell leukaemia may be the histone methyltransferase KMT2C ( MLL3) in 15% instances and the CDKN1 B mutation appearing in 11% individuals2 (Table 1, Table 2, Table 3, Table 4).