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P. Nurden et al.
Defect of platelet procoagulant activity: Scott syndrome
The original patient reported many years ago in New York had impaired platelet procoagulant activity and fib- rin formation but other platelet functions were normal.35 The affected gene ANO6 (also called TMEM16F) was dis- covered in 2010 when a constitutively active mutant was identified in a cDNA library obtained from a Ba/F3 cell line overexposing phosphatidylserine on the cell surface; a homozygous deleterious variant of ANO6 in a patient with Scott syndrome validated the findings.36
Definition
Scott syndrome is characterized by autosomal recessive transmission, a lack of phosphatidylserine exposure and phosphatidylserine-bearing microparticle production from platelets and red blood cells.37,38 Loss of Ca2+-dependent binding of the intrinsic tenase (FVIIIa, IXa, X) and pro- thrombinase (FVa, Xa, prothrombin) complexes results in reduced thrombin generation.
Clinical phenotype
Only six cases of Scott syndrome have been detailed and all are women.39 Bleeding is predominant after surgery or trauma; menorrhagia and post-partum hemorrhage are marked. Although spontaneous mucocutaneous bleeding is rare, it was severe in one young woman and included epistaxis, gum and gastrointestinal bleeding.39
Biological phenotype
The major diagnostic clues are elevated residual levels of prothrombin in sera and a marked defect of platelet pro- coagulant activity. Full diagnosis requires flow cytometry measurements of platelet phosphatidylserine exposure (annexin V-binding) and microparticle release upon activa- tion by thrombin in combination with collagen or by the ionophore A23187.40 Loss of mitochondrial membrane potential, a major factor in phosphatidylserine exposure, late phase procoagulant ballooning and capping of adhe- sive proteins all still occur but may be reduced. Caspase- dependent apoptotic or necrotic pathways of phos- phatidylserine expression are normal.40 Platelet numbers, morphology, adhesion, secretion and aggregation are largely unaffected.
Genotype
Gene variants of ANO6 encoding a multi-pass trans- membrane protein that is an essential component of the Ca2+-dependent “scramblase” activity cause Scott syn- drome.39 For the original patient a homozygous variant at a splice acceptor site in intron 12 resulted in a premature stop codon.36 Two French siblings showed compound het- erozygosity for a large deletion of exons 1-10 in one allele and a premature stop codon on the other.41 Compound heterozygosity with premature termination was also seen in a Welsh family.42 All mutations result in abrogated TMEM16F expression.
Ephrin type-B receptor 2-related disorder
Recently, we demonstrated that defects in Ephrin type- B receptor 2 (EPHB2) cause a bleeding syndrome and IPD in two siblings from a consanguineous French family.43
Definition
Recurrent bleeding, normal platelet counts but abnor- mal function were linked to a variant of EPHB2, encoding a member of the EPH receptor family of transmembrane tyrosine kinases. The molecular basis of this IPD confirms a specific role for EPHB2 in platelet function via tyrosine phosphorylation of intermediates involved in GPVI and G-protein–coupled receptor-mediated signaling.
Clinical phenotype
Both affected family members showed excessive spon- taneous subcutaneous and heavy bleeding upon minor wounds.43 One sibling developed anemia after chronic gas- trointestinal bleeding requiring iron supplementation. Due to its rarity the clinical phenotype remains poorly defined.
Biological phenotype
Ephrin receptors consist of a N-terminal glycosylated ligand-binding domain, a transmembrane region, and an intracellular kinase domain. Interactions largely occur between neighboring cells; studies using mice revealed a role in thrombus formation and in both contact-depen- dent and -independent signaling.43 Platelets from our fam- ily showed decreased aggregation and secretion in response to ADP, arachidonic acid, collagen, and analogs of thromboxane A2. The mutation did not affect EPHB2- ephrin interactions but phosphorylation of Lyn, Syk and FcRγ, the initial steps in GPVI signaling and of Src were drastically impaired as was inside-out αIIbβ3 activation.
Genotype
A homozygous missense variant (p.R745C) in the EPHB2 gene was identified by whole exome sequencing in both siblings; their asymptomatic parents were het- erozygous. The p.R745C substitution is located within the tyrosine kinase domain.
P2Y12 receptor-related disorder
The use of ticlopidine as an anti-platelet agent in the 1970s showed the benefit of blocking the interaction between ADP and platelets in anti-ischemic treatment.44 The recognition in 1992 of an Italian family with a ticlopi- dine-like platelet function profile and in 1995 of a French family with a similar defect was crucial to identifying how ADP activates platelets.45,46 Some 20 years later only a few such families have been identified.
Definition
P2Y12-related disease affects the full aggregation response of platelets to ADP and the stabilization of aggre- gates. Human platelets possess two ADP receptors that act in synergy: (i) P2Y1, a Gq-coupled receptor, which initiates platelet shape changes and ADP-induced aggregation through the mobilization of internal Ca2+ stores, and (ii) P2Y12, a Gi-coupled receptor linked to adenylyl cyclase and phosphoinositide 3-kinase activation (Figure 3).
Clinical phenotype
The disease is inherited in an autosomal recessive man- ner and is manifested clinically as moderate to severe mucocutaneous bleeding, including after surgery.45,46 Beyond the classical forms, a family with severe hemor- rhage has been recently observed with autosomal domi-
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haematologica | 2021; 106(2)