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Inherited platelet function disorders
Cytosolic signaling pathways
Rao and his colleagues were pioneers in identifying candidate proteins in patients with mild bleeding diathe- ses deficient in key signaling pathways and second mes- sengers.83 Gαq, PLC-β2, as well as defects in Ca2+-mobi- lization and protein kinase C-induced phosphorylation were highlighted. Abnormalities extended to granule secretion as well as αIIbβ3 activation. However, most of the early patients still await full genetic analysis. We have chosen to highlight three genes involved in arachidonic acid metabolism in platelets.
The first is a defect of PLA2G4A encoding cytosolic PLA2 in three families. In the first family compound het- erozygous missense mutations induced marked structural changes within cPLA2. In the second family heterozy- gous p.S111P and p.R485H missense variants respectively affected cPLA2 trafficking and abrogated the enzyme’s catalytic activity; while for the third family a homozy- gous p.D575H PLA2G4A variant was present.84-86 In all three cases, platelet aggregation and dense granule secre- tion with collagen and ADP were impaired; gastrointesti- nal bleeding was a feature.
An inherited defect of cyclooxygenase-1 (COX-1) is more specifically linked to defective platelet aggregation with arachidonic acid and absence of the second wave response to epinephrine, ADP and collagen. While failing to respond to arachidonic acid, platelets aggregate nor- mally with TXA2 agonists. Several such patients with mild bleeding have been reported either with autosomal dominant or, more recently, autosomal recessive trans- mission of variants in PTGS1.87-89 Patients may lack COX- 1 or have a functionally defective but normally expressed enzyme. Bastida et al., using a high-throughput sequenc- ing gene panel, found a heterozygous variant with an aspirin-like platelet functional defect linked to a heterozy- gous p.N143S missense mutation in PTGS1.88 Very recently, whole genome sequencing helped to identify a homozygous missense variant (p.W322S) in a large con- sanguineous family of Iranian descent with mild bleed- ing.89
The third disease is Ghosal syndrome, a disorder with increased bone density that was linked to a deletion mapped to chromosome 7q33/34. The deletion includes TBXAS1, the gene for thromboxane synthase, a terminal enzyme in the arachidonic acid cascade converting prostaglandin H2 into TXA2. A key fiding was homozy- gous missense mutations alone in TBXAS1 in consan- guineous families with Ghosal syndrome in ethnic groups; the affected residues were all highly conserved.90 Platelets from the patients had reduced aggregation and secretion in response to arachidonic acid; in contrast the response to U46619 directly activating TPα was normal.
mimetics is not applicable.11 As detailed in Figure 5, strate- gies involve the maintenance of a lifestyle that minimizes risk and being prepared in case of an emergency. Procedures range from local measures to the use of platelet transfusions or recombinant activated FVII to stop major bleeding as well as the establishment of and adherence to standardized protocols for prophylaxis prior to surgery or childbirth.15,91-93 A special case is type I GT in which the absence of αIIbβ3 can lead to the formation of isoantibod- ies, making patients refractory to normal platelets.
Conclusions
Of the disorders covered by this review, GT is the most frequent and widely screened disease.12,15 Genotyping has now identified several hundred disease-causing mutations in a constantly changing spectrum, so the classification of patients into type I and type II subgroups based on resid- ual platelet αIIbβ3 content should be updated to take into account mutation analysis.18,19 From 2001, the year in which P2RY12 was first characterized,10 newly identified genes include FERMT3, ANO6, RASGRP2 and EPHB2 thereby expanding the gene repertoire for IPD.25,26,33,34,36,43 The development of next-generation sequencing plat- forms targeting IPD genes,6,88 together with widening access to whole exome sequencing,4,7,8 and now whole genome sequencing,9 has both increased the number of patients with a genetic diagnosis and the speed at which this is performed. The multiplication of data has under- lined the great diversity of variants responsible for gene dysfunction, the rarity of hot-spot mutations, the frequen- cy of compound heterozygosity within the same gene (limiting the notion of consanguinity for recessive dis- eases); and raised the likelihood of additive effects of het- erozygous variants on different genes modulating the phe- notype. It is also worthy of mention that some inherited thrombocytopenias with associated functional platelet defects (Online Supplementary Table SI) may not have com- plete penetrance and some patients may therefore not be thrombocytopenic (e.g., those with mutations of RUNX1 and FLI1).11 The increasing understanding of the gene spectrum causing IPD is leading to better care of patients and it should be underlined how major anti-ischemic treatments have been initiated or improved as a result of this.
Future strategies
Despite the progress in gene identification a significant proportion of patients still fail to be genotyped. One suc- cessful strategy for the identification of causal genes in patients with inherited thrombocytopenia was by com- parison with mouse models with similar phenotypes;11, 69 Another approach is ongoing screening of local whole exome or whole genome sequencing databases of patients for variants of each newly published IPD gene. A third strategy involves screening large families or those with consanguinity, as done for RASGRP2 and EPHB2, or statis- tical analysis of large cohorts of patients, aligning variant detection in unrelated families with a similar phenotype.26 The final proof that variants in new genes are causal will still require their expression in heterologous cells, the use of in silico modeling to assess changes in protein structure,
in bone remodeling is also indicated. Somewhat surprisingly, none of the patients had a bleed- ing diathesis (distinguishing them from patients with
congenital defects of TBXA2R). Therapy
The management of IPD involves both prevention of bleeding and treatment of mild and severe bleeding episodes and differs little for disorders of platelet count with the exception that the recent use of thrombopoietin
A role for TXA 2
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