Page 34 - 2020_08-Haematologica-web

A.T. Nurden and P. Nurden
affected. One mutation (p.R369W) involved a residue pre- viously described as a somatic mutation in different forms of leukemia and cancer.64
Linkage with skeletal disorders
Genetic defects responsible for inherited thrombocytope- nias can be syndromic and involve a wide range of other organs and tissues as we have already seen for MYH9-relat- ed disease and WAS. Here we provide four examples in which the disease includes well-defined skeletal defects. The Paris-Trousseau syndrome combines macrothrombo- cytopenia and a panoply of developmental defects includ- ing dysmorphogenesis of the face and digits, possible pan- cytopenia, cardiac abnormalities and mental retardation, which are all characteristic of Jacobsen syndrome.21 The inheritance is largely AD with single allele expression of a small deletion in chromosome 11q23 that includes FLI1, an ETS transcription factor family member and a main regula- tor of megakaryopoiesis.65 Genes targeted by FLI1 include MPL, GPIBA, and ITGA2 and a lack of transactivation early in megakaryocyte development due to a transient hemizy- gous loss of the affected allele is responsible for the pathol- ogy.65 Dual populations of normal and immature megakaryocytes are generated, the latter with reduced polyploidization and persistence of the cytoskeletal protein, MYH10 (also see the section on RUNX1). Some of the a- granules in the enlarged platelets appear fused and giant.
In the thrombocytopenia with absent radius (TAR) syn- drome, bilateral radius aplasia and various other develop- mental defects are accompanied at birth by severe thrombo- cytopenia but normal sized platelets. Initial studies identi- fied a chromosome 1.21.1 heterozygous microdeletion in patients with TAR syndrome, but the genetics was only fully resolved when WES showed the necessity of a combi- nation of the microdeletion including the RBM8A gene on one allele with one of two rare single nucleotide polymor- phisms in regulatory regions of RBM8A on the second allele.66 In fact, RBM8A codes for Y14, a nuclear and cyto- plasmic protein that interacts with mRNA produced by splicing; how it relates to the TAR phenotype remains large- ly unknown. Hematopoietic progenitors have a reduced response to thrombopoietin and fail to differentiate into megakaryocytes in the marrow. Other hematologic defects include leukocytosis in some patients; enigmatically, the platelet count may correct with age.67 TAR syndrome is to be distinguished from radio-ulnar synostosis with amegakary- ocytic thrombocytopenia caused by a heterozygous truncat- ing HOXA11 mutation (RUSAT1) or, as determined by WES screening, by de-novo missense mutations in MECOM (RUSAT2) encoding oncoprotein EVI1 leading to altered activator protein-1 and transforming growth factor-b-medi- ated transcriptional responses.68,69 Both HOXA11 and MECOM mutations cause defects in the early phase of megakaryopoiesis and the risk of trilineage marrow aplasia makes the patients candidates for HSC transplantation.
Defects of platelet biogenesis due to cytoskeletal protein defects
We have already highlighted how mutations affecting the myosin IIA heavy chain are a frequent cause of macrothrombocytopenia in MYH9-related disease. This
prompted research for mutations of genes encoding other cytoskeletal proteins in patients with congenital thrombo- cytopenia. X-linked mutations in FLNA, encoding filamin- A (FLNA), cause a syndromic disorder combined with periventricular nodular heterotopia/otopalatodigital syn- dromes and other developmental defects.21 It can also be associated with Ehlers-Danlos syndrome. We first showed that three patients in France also had macrothrombocy- topenia and that bleeding could be severe.70 Characterization of their genotype was aided by reports that mice with FlnA-null megakaryocytes prematurely release large and fragile platelets that are rapidly removed from the circulation.71 Curiously, platelets of some patients had giant a-granules. Cultured megakaryocytes from patients extended proplatelets with large swellings and released giant platelets.71 Filamin A promotes branching of actin filaments as well as anchoring membrane receptors and FLNA mutations associate with a loss of platelet adhe- sive function. Interestingly, a rare gain-of-function mis- sense mutation caused isolated macrothrombocytopenia.71 Initially, FLNA mutations were speculated to cause macrothrombocytopenia through loss of the GPIb-FLNA interaction and involvement of RhoA (a GTPase); more recently an alternative mechanism has been proposed based on the loss of aIIbb3-filamin A signaling.11,72
Hearing loss and macrothrombocytopenia with moder- ate bleeding were reported in patients from two unrelated families with a heterozygous gain-of-function truncating mutation in DIAPH1; a variant identified by WES fol- lowed by crosschecking variants against a database of genes causing deafness.73 DIAPH1 is a member of the formin family of cytoskeletal proteins and is a regulator of actin filament formation. The above gain-of-function mutation resulted in cytoskeletal dysregulation and megakaryocytes with reduced but premature proplatelet formation similar to that seen in MYH9-related disease. The defect was linked to a loss of autoinhibition within the diaphanous autoregulatory domain and was distin- guished from a frameshift modification that produced hearing loss in a group of families in South America but without hematologic problems.73 Brief mention should also be made of moderate macrothrombocytopenia asso- ciated with a cluster of missense mutations detected by next-generation sequencing (NGS) in five out of 13 cases with AD missense mutations in CDC42.74 CDC42 is a small GTPase and a Rho family member and the muta- tions were said to affect the change between the active and inactive states of the molecule. The disease is syn- dromic and is associated with various developmental defects including intellectual disability, brain defects, mus- cle tone abnormalities, facial dysmorphism, and cardiac abnormalities although these were somewhat less appar- ent for patients with mutations causing macrothrombocy- topenia. So far, few platelet function studies appear to have been performed but, on the basis of mouse models, both megakaryocyte maturation and platelet function are likely to be affected.11 So far, bleeding has not been report- ed as a problem for the patients.
Mutations of TUBB1 encoding b1-tubulin cause macrothrombocytopenia through altered association with a-tubulin and defective microtubule assembly.75,76 The defect leads to altered megakaryocyte maturation with the production of fewer proplatelets with large tips but there is a high degree of phenotypic and genotypic vari- ability and while the causal effect of some variants is clear,
2010
haematologica | 2020; 105(8)