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A. Veninga et al.
CHIP-associated CVD still remains to be determined. Few studies have shown that there is higher expression of pro- inflammatory cytokines in p53-deficient murine leuko- cytes, which may accelerate the development of CVD.9 However, there is no evidence directly linking platelet traits to CVD development.
WAS
The Wiskott-Aldrich syndrome (WAS) protein is selectively expressed in hematopoietic cells, where it regulates actin cytoskeleton rearrangements. In the classical X-linked Wiskott-Aldrich syndrome, patients suffer from thrombo- cytopenia with smaller sized platelets and recurrent infec- tions, due to an impaired functionality or availability of WAS.74 A milder phenotype is that of X-linked thrombo- cytopenia, in which patients only suffer from bleeding because of low platelet count.74 Rare inherited mutations that instead cause constitutive WAS activation are seen in patients with X-linked neutropenia, experiencing recur- rent bacterial infections while having normal platelet count and size.75 In addition, these patients show an increased predisposition for AML or MDS.76
In classical Wiskott-Aldrich syndrome patients, the prevalence of malignancy is 13-22%, mostly due to devel- opment of lymphoma, but also to lymphoblastic leukemia, MDS or MPD.76 The thrombocytopenia is likely caused by increased platelet removal. In Was-deficient mice, platelet turnover was shortened, with proteomic evidence for alterations in proteins of metabolic and pro- teasomal pathways.77 Furthermore, in both the mutant mice and patients, there is evidence for a hyperactivation status of the platelets, thus explaining the higher elimina- tion rate. Several groups reported on alterations in integrin activation in the patient's platelets.78,79 However, one patient study concluded platelet activation properties were normal.80
There is limited evidence for the presence of somatic mutations in the WAS gene. This mainly concerns gain-of- function mutations, associated with poor outcome in patients with juvenile myelomonocytic leukemia.81 This suggested a clonal role of the gene in the pre-malignant state.
Section 3: Clonal mutations in other genes
For the genes FLI1 and GATA1 encoding for transcrip- tion regulators, whether the mutation is gain-of-function or loss-of-function likely determines its respective effect on platelet count and/or function. Mutations in TET2 have been associated with increased inflammation-induced atherosclerosis and thrombotic disease, although possible effects on platelets remain to be established (Table 1 and Figure 1).
FLI1
The protein Friend leukemia virus integration 1 (FLI1) is a member of the ETS transcription factor family, which is highly expressed in the hematopoietic lineage and endothe- lial cells. Due to a faulty vasculature, Fli1 knockout mice die during embryonic development, but heterozygous mice are viable without apparent phenotype.82 Detailed studies indi- cate that FLI1 plays an important role in both erythropoiesis and megakaryopoiesis by regulating the expression of mul- tiple genes.83 It acts together with the transcription factor
GABPA, especially in later phases of megakaryopoiesis. This is exemplified by the fact that, in Fli1 knockout mice, megakaryocytes are specifically reduced in the expression of late-stage genes, e.g. genes encoding for glycoprotein (GP)Iba, GPIX and platelet factor 4.82
In humans, heterozygous mutations in FLI1 are com- monly grouped together as 'Bleeding disorder platelet- type 21' (Phenotype MIM 617443). Examples are the Jacobsen syndrome and Paris-Trousseau syndrome, which are characterized by a heterozygous partial deletion of chromosome 11, encompassing the FLI1 gene. Such patients characteristically suffer from abnormal growth and mental retardation, accompanied by thrombocytope- nia, most likely due to impaired megakaryopoiesis.84 In the Paris-Trousseau syndrome, platelets are enlarged and contain large fused a-granules.84 In patients with a mutat- ed FLI1 gene, presenting with congenital macrothrombo- cytopenia, also an impaired agonist-induced platelet aggregation has been reported.85
In the case of somatic mutations, FLI1 can become fusion partner with the transcriptional repressing gene EWSR1, a condition known as Ewing sarcoma.86 The effect on platelets is unclear. On the other hand, in vitro studies have indicated that the overexpression of FLI1 in stem cells enhances megakaryopoiesis, thrombopoiesis, and platelet functionality.87 Furthermore, deregulated high lev- els of FLI1 are found in various types of cancer. In agree- ment with this, a predisposition to pre-T-cell lymphoblas- tic leukemia and lymphoma is described for transgenic mice over-expressing Fli1 in the hematopoietic progenitor cells.83 It remains to be established whether FLI1 is a main contributing gene in CHIP-related CVD.
GATA1
The transcription factor GATA-binding protein 1 (GATA1) controls the development and production of megakary- ocytes, platelets and erythrocytes. In mouse studies, the loss of Gata1 in the megakaryocyte lineage resulted in smaller size megakaryocytes and a defect in proplatelet formation. The Gata1-deficient platelets were larger in size, showed an excess in rough endoplasmic reticulum, and contained fewer a-granules.88 Furthermore, the defi- cient mice were impaired in red blood cell development, and often died because of anemia.89 Consistent with this, GATA1 is highly expressed in human megakaryocytes and erythroid cells. Mutations in GATA1 can appear as germline or somatic. Inherited mutations associate with hematopoietic disorders, characterized by low blood cell counts. On the other hand, somatic mutations often result in the production of shorter GATA1 variants, for example, in cases of AMKL (acute megakaryoblastic leukemia) or Down syndrome.54 Here, platelets tend to be low in counts and display an atypical morphology.
A common consequence of germline GATA1 mutations in hematopoietic disorders is the altered interaction of GATA1 with its co-factor FOG1, i.e. a zinc finger protein co-operating with GATA1 to regulate cell differentiation. This has been reported for patients with X-linked throm- bocytopenia, or other forms of macrothrombocytopenia, who experience bleeding diatheses.90 The patient's megakaryocytes are abnormal in structure and the platelets show decreased numbers of a-granules.90 Patients with primary myelofibrosis, having upstream driver mutations resulting in low GATA1 levels in megakary- ocytes, show an increased risk of both thrombosis and
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haematologica | 2020; 105(8)