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A. Veninga et al.
JAK2
The non-receptor tyrosine kinase Janus kinase 2 (JAK2) is one of the general regulators of cell survival and prolifera- tion, by controlling, for example, cytokine receptor signal- ing pathways. Also in hematopoiesis, JAK2 controls pre- cursor cell maintenance and function.34 Inherited muta- tions of JAK2 are detected in patients with hereditary thrombocytosis,35 while somatic mutations of the gene link to various phenotypes including erythrocytosis.
A well-known acquired JAK2 variant is the mutation V617F, which is carried by the majority of patients with MPN, i.e. in nearly all PV patients and half of patients with ET or primary myelofibrosis. In general, the V617F muta- tion affects the proliferation of myeloid cells and leads to increased inflammatory responses.34 However, this somat- ic mutation as such is not considered to enhance the risk of thrombotic events in patients with ET or PV.36 Nevertheless, platelets from JAK2 V617F-positive patients demonstrated an enhanced activation status and procoag- ulant potential. In addition, the fraction of immature platelets, which can be more active than mature platelets, was higher in carriers of the JAK2 V617F mutation versus non-carriers.37
Transgenic mice have been generated carrying the human JAK2 V617F mutation in the megakaryocyte line- age.38 The JAK2 V617F megakaryocytes responded better to thrombopoietin, and displayed a greater migratory abil- ity, proplatelet formation and increased ploidy. The pro- duced platelets responded stronger to multiple agonists.
In aging healthy individuals, the prevalence of the JAK2 V617F variant is only 1%, but carriers have a 10-fold increased risk of CVD.16 Depending on the degree of mutation expansion, subjects may develop MPN instead of CHIP.8 How or under which conditions the thrombocy- tosis is linked to somatic JAK2 mutations aggravating CVD is still a matter of debate and requires further inves- tigation.
SF3B1
The gene splicing factor 3B subunit 1 (SF3B1) encodes for a core component of the RNA spliceosome machinery.39 Somatic mutations in this gene, along with other genes of the spliceosome, have been identified in over half of MDS patients.39 Common mutations in the SF3B1 gene are those of K700E, K666N and R625C.40 To study the impact of the frequent K300E mutation, a conditional knock-in mouse model was developed, which revealed a RNA splicing defect similar as suggested in MDS patients har- boring this mutation.39 Regarding the thrombotic risk, studies revealed that patients carrying a mutated SF3B1 gene had higher platelet counts and were more prone to develop CVD than patients without mutation,40 although the altered molecular players are unclear. Furthermore, a sequencing study identified SF3B1 mutations in 5% of ET patients.41 In the aging healthy population, clonal hematopoietic mutations of SF3B1 appear to be infre- quent, ranging from 2 to 5%.42
SH2B3
The signaling adaptor protein Src homology 2 B3 (SH2B3, also named LNK) acts as an interactor of JAK2, and nega- tively regulates thrombopoietin-induced megakary- opoiesis. An associated inherited disease is B-precursor acute lymphoblastic leukemia. Somatic mutations in the SH2B3 gene are found in >5% of MPN patients. These
concern frameshift and missense mutations throughout the whole gene, often co-existing with mutations in driver genes, including JAK2, CALR and MPL.43 The loss-of-func- tion of SH2B3 can lead to a higher expansion of hematopoietic stem cells, acting by increased thrombopoi- etin signaling and megakaryopoiesis.44 The higher platelet and leukocyte counts may worsen atherosclerosis and the thrombotic risk.43 In Sh2b3 knockout mice, it was found that hyperlipidemia aggravated both atherosclerosis and thrombosis, likely due to positive platelet priming.45 Whether this priming event due to SH2B3 mutations also occurs in humans, is not known.
Section 2: Clonal mutations in genes associat- ed with decreased platelet count and/or function
Several mutations in genes encoding for transcription reg- ulators (ETV6, GATA2, GFI1B, SMAD4), cell signaling pro- teins (TP53, WAS), and other proteins (FANCA, FANCC) are linked to impaired hematopoiesis, causing thrombocytope- nia of varying severity with evidence for concomitant platelet function defects (Table 1 and Figure 1).
ETV6
The transcriptional repressor E26 transformation-specific variant 6 (ETV6) serves to maintain the development of hematopoietic stem cells in the BM as a continuous sur- vival signal. It acts by inhibiting other transcription fac- tors, such as FLI1. However, it appears not to be required for embryonic stem cell expansion.46
ETV6 is known as a proto-oncogene, since it can be a fusion partner with over 30 other genes, but in case of truncating mutations it acts as a tumor suppressor gene. Depending on the fusion site, the fused protein can alter the transcription levels of ETV6 target genes, which may support the development of leukemia.46,47 On the other hand, germline heterozygous ETV6 mutations have been identified in some patients with dominantly inherited thrombocytopenia.48 Such patients seem to have a predis- position to hematologic malignancies, most commonly ALL, AML or MDS. Given that the complete loss of ETV6 is lethal, truncating or protein-inactivating mutations are mainly found as somatic events, and rarely as germline variants.46
As a transcriptional repressor, ETV6 has an established role in megakaryocyte and platelet (patho)physiology. Patients with germline ETV6 variants show a large expan- sion of immature megakaryocyte colony-forming units, accompanied by a reduced formation of proplatelets, thus explaining the thrombocytopenia. The mutant platelets are of normal size, although characterized by aberrant cytoskeleton organization, lower levels of small GTPases, and defective clot retraction.49 Evidence is lacking to link clonal variants of ETV6 to thrombotic phenotypes; how- ever, a related bleeding tendency has been described.
FANCA, FANCC
The proteins Fanconi anemia complementation group A and
C (FANCA/C) are repair factors after DNA damage or apoptosis. Inherited mutations in either gene are seen in the disorder Fanconi anemia, where patients in 60-70% of the cases show a mutation in FANCA and in 15% a muta-
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haematologica | 2020; 105(8)