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P. Nurden et al.
Antiplatelet therapy
P2Y12 identification was key to the development of next- generation therapies including the orally available thienopyridines (clopidogrel and prasugrel). These are pro- drugs requiring cytochrome P450-based conversion to a short-lived active metabolite that binds covalently to P2Y12 although drug-resistance to clopidogrel can affect up to a third of patients.14 Ticagrelor and cangrelor are direct-acting and rapidly reversible antagonists; cangrelor given intra- venously has the advantage of acting immediately.
Other platelet primary receptor defects
Glycoprotein VI-related disease
Rare inherited autosomal recessive defects of the GP6 gene cause a mild bleeding syndrome. GPVI is a receptor for collagen as well as other extracellular matrix compo- nents and, as recently identified, fibrin.54,55 A member of the immunoglobulin superfamily, GPVI forms a non-cova- lent complex with FcRγ and acts in synergy with integrin α2β1. Early evidence of an inherited GPVI deficiency came from a Japanese woman with lifelong although mild mucocutaneous bleeding and platelets refractory to colla- gen despite responding to other agonists; immunological studies confirmed the specific absence of GPVI.56 The first genotyped patients with GP6 variants belonged to fami- lies from France and Belgium with autosomal recessive compound heterozygous mutations.57,58 However, most studies have centered on a cluster of patients from Chile with a homozygous 2 bp insertion followed by a stop at position 242.59 All evidence suggests a founder effect. The Chilean patients had mild bleeding and heterozygous family members were asymptomatic. A defective platelet interaction with fibrin and immobilized fibrinogen helps to explain the diminished thrombus build up under flow.58
) 2
antibody blocking GPVI, ACT017, have exhibited excel- lent safety and tolerability profiles in a phase 1 study in humans.65 An exciting challenge for anti-GPVI drugs will be to block thrombus build up through inhibition of the GPVI-fibrin interaction while preserving the initial interac- tion with collagen. Anti-GPVI agents may also have a role in cancer therapy by inhibiting platelet-tumor cell interac- tions; galactin-3 is a novel counter-receptor for GPVI on tumor cells.66 Attempts at treating ischemic disease based on inhibition of TXA2R have largely failed compared to the efficiency of aspirin and related drugs.14
Classic dense granule defects
Much information has been obtained from studies on disorders of platelet lysosome-related organelles.67,68
Hermansky-Pudlak syndrome
Hermansky-Pudlak syndrome (HPS) is named after the Czechoslovakian physicians who in 1959 described two patients with oculocutaneous albinism, prolonged bleed- ing and pigmented macrophages in the bone marrow.67 Many of the advances regarding HPS have been linked to the numerous mouse and other animal models of storage pool diseases.68,69
Definition
HPS is a heterogeneous autosomal recessive multisys- tem disorder resulting from defects of ten genes encoding proteins essential for the biogenesis of platelet dense gran- ules, a member of the lysosomal-related organelle fami- ly.67-69 Association with albinism reflects defects in the bio- genesis of melanosomes in the melanocytes of the skin, hair and choroid of the eye while pigment epithelial cells of the retina can also be affected.
Clinical phenotype
The bleeding diathesis includes easy bruising, epistaxis, gingival bleeding, menorrhagia, postpartum hemorrhage and prolonged bleeding after surgery.67 Platelet transfu- sions prevent the hemorrhagic risk during surgery, while tranexamic acid is often used preventively or to stop mod- erate bleeding. Oculocutaneous albinism involves hypopigmentation of the skin and hair associated with characteristic ocular findings. Granulomatous colitis, neu- tropenia, immunodeficiency and fatal pulmonary fibrosis can feature according to the subtype.67-69
Biological phenotype
Platelet aggregation is characterized by an impaired sec- ond wave.68,69 The specific absence of dense granules means that the platelet secretory pool of ADP, ATP, sero- tonin, calcium, and polyphosphate is reduced or lacking. Dense granules can be evaluated by whole mount electron microscopy due to their opacity to electrons (Figure 4). Total platelet ADP and ATP and the secretion of ATP dur- ing platelet aggregation is now mostly measured by biolu- minescence. Quantifying mepacrine uptake by flow cytometry is an indirect method for assessing dense gran- ule content. Granule secretion can also be measured by flow cytometry based on the translocation of P-selectin and lysosomal-associated membrane protein (LAMP-3, CD63) to the platelet surface; exposure of LAMP-3 can be spontaneous.
Thromboxane prostanoid type α receptor-related disease The TBXA2R gene encodes the thromboxane prostanoid
type α receptor (TPα), the major thromboxane A2 (TXA
receptor on platelets.60 Its activation leads to Ca2+ mobi-
lization and platelet aggregation. It associates mainly with
G and principally modulates platelet activation through q
the PLCβ pathway. Platelet aggregation is characterized by a decreased response to arachidonic acid or TXA
2 analogs such as U46619 and I-BOP.61 From as early as 1994, Japanese families were described with a bleeding disorder due to a non-responsive TPα receptor.62,63 A p.R60L variant in the first intracytoplasmic loop of TPα with autosomal dominant transmission predominated, but it was homozygous in one member with mild muco- cutaneous bleeding. Globally, three more families were later reported with autosomal dominant transmission; two had missense mutations with a dominant-negative effect linked to impaired receptor dimerization while a third had a frameshift variant, a stop codon and reduced
TPα expression.60,64
Antiplatelet therapy
The apparent preservation of hemostasis in the absence of GPVI has prompted research for anti-ischemic drugs and, in particular, treatment for stroke.14 Revacept is a recombinant soluble dimeric GPVI-Fc fusion protein blocking GPVI-reactive sites in collagen and is the object of ongoing clinical trials. Fab fragments of a monoclonal
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haematologica | 2021; 106(2)