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Hairy Cell Leukemia

OpenAI MONDO:0018935 Model: gpt-5-codex

Hairy Cell Leukemia

Scope

  • This research package is scoped to the disease-level mechanism graph for classic hairy cell leukemia (HCL; MONDO:0018935).
  • Hairy cell leukemia variant (HCL-v) and splenic diffuse red pulp small B-cell lymphoma (SDRPL) are treated as differential diagnoses rather than subtype branches because the literature supports a distinct phenotypic and genetic program without the canonical BRAF-driven classic HCL mechanism (PMID:23347903, PMID:34710243, PMID:35205796).

Disease Overview

  • HCL is a rare indolent mature B-cell leukemia that represents about 2% of leukemias and predominantly affects older men (PMID:38882583).
  • The disease is defined diagnostically by hairy-cell morphology, a characteristic immunophenotype, marrow involvement, and near-universal BRAF V600E in classic HCL (PMID:21663470, PMID:34710243, PMID:38882583).

Diagnostic Signature

  • A practical diagnostic core is morphology plus the HCL score based on CD11c, CD25, CD103, and CD123, together with BRAF V600E testing (PMID:34710243).
  • Classic HCL typically expresses bright CD20, CD22, CD11c, CD25, CD103, and homogeneous CD123 (PMID:23347903).
  • HCL-v differs by loss of CD25, weak/absent CD123, absence of BRAF V600E, and relative resistance to traditional HCL therapy (PMID:23347903).

Core Mechanisms

  • BRAF V600E is the canonical driver lesion of classic HCL and is present clonally in almost all cases across the disease course (PMID:21663470, PMID:28297625).
  • BRAF V600E constitutively activates RAF-MEK-ERK signaling in HCL cells (PMID:21663470).
  • Review literature links this MAPK program to the characteristic hairy morphology and antiapoptotic behavior of HCL cells (PMID:28297625).
  • Hairy cells also overexpress TGF-beta1, which contributes directly to marrow reticulin fibrosis and correlates with marrow infiltration (PMID:14991065).
  • Bone marrow fibrosis and infiltration contribute to multilineage cytopenias, especially pancytopenia, monocytopenia, and neutropenia (PMID:14991065, PMID:27903528, PMID:38882583).

Histopathology And Phenotype

  • Bone marrow biopsy is important to assess the degree of medullary infiltration in HCL (PMID:34710243).
  • Reticulin fibrosis is a characteristic marrow abnormality mechanistically linked to hairy-cell-derived TGF-beta1 (PMID:14991065).
  • Common disease-level clinical manifestations include splenomegaly, pancytopenia, monocytopenia, neutropenia, and infection susceptibility (PMID:27903528, PMID:38882583).

Genetics Beyond BRAF

  • Recurrent cooperating lesions include KLF2 and CDKN1B alterations, but these are secondary to the canonical BRAF-driven disease program rather than replacing it (PMID:28297625, PMID:35454811).

Treatment Landscape

  • Symptomatic first-line HCL is treated with purine nucleoside analogs, classically cladribine or pentostatin (PMID:34710243, PMID:38882583).
  • Adding rituximab to first-line cladribine improves MRD-free complete remission durability (PMID:32109194).
  • Relapsed or refractory classic HCL is now strongly supported by BRAF-targeted therapy, particularly vemurafenib plus rituximab (PMID:33979489).
  • Moxetumomab pasudotox is an active CD22-directed option in multiply pretreated relapsed/refractory HCL (PMID:30030507).
  • BTK inhibition with ibrutinib appears in contemporary reviews as a later-line option, but the strongest curation-grade abstract evidence in this slice is for purine analogs, cladribine-rituximab, vemurafenib-rituximab, and moxetumomab pasudotox (PMID:34710243).