JAK2-gene

The JAK2 gene encodes a non-receptor tyrosine kinase that plays a central role in cytokine and growth factor signaling. The JAK2 gene is a downstream target of the pleiotropic cytokine IL6 (production of B cells, T cells, dendritic cells and macrophages) with the goal of generating an immune response or inflammation.

The primary isoform of the JAK-2 gene product, non-receptor tyrosine kinase has:

• an N-terminal FERM domain, which is required for erythropoietin receptor association,
• an SH2 domain that binds STAT transcription factors,
• a pseudokinase domain, and
• a C-terminal tyrosine kinase domain.

Cytokine binding induces autophosphorylation and activation of this kinase. This kinase then recruits and phosphorylates Signal Transducer and Activator of Transcription (STAT) proteins. Growth factors such as TGF-beta 1 also induce phosphorylation and activation of this kinase and translocation of downstream STAT proteins to the nucleus, where they influence gene transcription. Dysregulation of IL6/JAK2/STAT3 signaling pathways leads to increased cellular proliferation and myeloproliferative neoplasms of hematopoietic stem cells.

Mutations in the JAK2 gene are associated with numerous inflammatory diseases and malignancies. The JAK2-V617F point mutation occurs in various myeloproliferative diseases. In 95 % of cases it is detectable in polycythaemia vera. In 50-60 % of all patients in essential thrombocythaemia and primary myelofibrosis (formerly known as osteomyelosclerosis).

Inflammatory bowel disease (IBD): Furthermore, the JAK2- V617F point mutationis involved in the pathogenesis of IBD (Asadzadeh-Aghdaei H et al. 2019).

A non-synonymous mutation in the pseudokinase domain of the JAK2 gene disrupts the inhibitory action of the domain and results in constitutive tyrosine phosphorylation activity and hypersensitivity to cytokine signaling pathways.

Thrombotic tendencies: Evidence suggests an association between the JAK2 V617F mutation and various forms of thrombosis. This association is comparable to the association between inherited risk factors (factor V Leiden) and thrombotic events, but with a lower prevalence of the mutation (Zerjavic K et al. 2010).

Loss of JAK2 during embryogenesis is lethal.

Melanoma: Of note, evidence of poor clinical response to checkpoint blockade with anti-CTLA-4 and anti-PD-1 antibodies has been associated with acquired IFNγ resistance. This protects tumor cells from antiproliferative and pro-apoptotic cytokine activity. TheseIFNγ-resistant melanoma cells very often lack functional expression of the IFNγ pathway gene JAK2 due to gene deletions or inactivating gene mutations (Zaretsky JM et al. 2016; Horn S et al. 2018).

The following examinations are standard for the diagnosis of "systemic mastocytosis" according to the "WHO criteria 2016":

Serolog. Laboratory parameters: tryptase, albumin, AP, GGT, bilirubin, LDH, ferritin, vitamin D, vitamin B12, folic acid, CRP, ß2-microglobulin, protein electrophoresis, immunofixation, IgE, plasmatic coagulation.

Blood and differential blood count (especially monocytes, eosinophils), possibly mast cells; dysplasia signs (MDS); leuko-/thrombocytosis (MPN)

Imaging: osteodensitometry (only with confirmed diagnosis)

• Abdominal and lymph node sonography
• If necessary CT/MRT (suspicion of osteolysis)

Bone marrow cytology: detection or exclusion of mast cell leukemia (mast cells ≥20%) or ASM-t (mast cells 5-19%) and degree of maturity of mast cells (metachromatic blasts, promastocytes, atypical spindle forms - mature cell or immature cell), dysplasias, blasts

Histology: mast cell load (quantitative), dysplasia, proliferation, AHN (yes/no?), blasts, fibrosis

Immunohistochemistry: tryptase, CD117, CD2, CD14, CD15, CD25, CD30, CD34, CD61

flow cytometry: determination/confirmation of mast cell number and phenotype of KIT+/CD34- mast cells: CD2, CD25, CD30, CD33; in AHN possibly also typing of monocytes, other AHN cells (depending on AHN type) and blasts

Molecular genetics: KIT D816V mutation analysis (qualitative and quantitative) from BM and peripheral blood.

In KIT D816V negative patients sequencing of KIT gene (rarely other KIT mutations possible)

Extended mutation analysis (myeloid NGS panel) in suspected SSM, AdvSM and mast cell sarcoma. Conventional cytogenetics, possibly FISH analysis - especially in case of (suspected) AHN.

Mast cell hyperplasia

Immunocytoma: mast cell hyperplasia may mimic well-differentiated round cell SM

Myeloid neoplasms: Eosinophilia-associated myeloid neoplasms

Myelomastocytic leukemia (MML): as MCL, except that 10% mast cells in the BM smear or blood smear are sufficient to make the diagnosis of MML.

The JAK2 gene as well as its gene product with the IL6/JAK2/STAT3 pathway are a therapeutic target to attenuate exaggerated inflammatory responses to viral infections.

1. Asadzadeh-Aghdaei H et al (2019) V617F-independent upregulation of JAK2 gene expression in patients with inflammatory bowel disease. J Cell Biochem 120:15746-15755.
2. Baxter EL et al.(2005) Acquired mutation of the tyrosine kinase JAK2 in human myeloproliferative disorders. Lancet 365: 1054-1061.
3. Horn S et al (2018) Tumor CDKN2A-Associated JAK2 Loss and Susceptibility to Immunotherapy Resistance. J Natl Cancer Inst 110:677-681.
4. Zaretsky JM et al (2016) Mutations Associated with Acquired Resistance to PD-1 Blockade in Melanoma. N Engl J Med 375:819-829.
5. Zerjavic K et al (2010) Is the JAK2 V617F mutation a hallmark for different forms of thrombosis? Acta Haematol. 124:49-56.