An intracytoplasmic β3 Leu718 deletion in a patient with a novel platelet phenotype

Abstract

Key Points A novel heterozygous ITGB3 Leu718del shows loss of synchronization between the intracytoplasmic tail of β3 with that of αIIb. Decreased activation of αIIbβ3 accompanies enlarged platelets that contain giant granules and give a poor aggregation response.

Mutations of the integrin αIIb/β3 intracytoplasmic salt bridge cause macrothrombocytopenia and enlarged platelet α‐granules

“…This was surprising as another cytoplasmic domain mutation involving a near‐neighbor Arg724Ter truncating mutation in β3, while not preventing αIIbβ3 expression gave a full GT phenotype . We recently reported a heterozygous intracytoplasmic β3 Leu718del that resulted in loss of synchronization between the cytoplasmic tails of β3 and αIIb; changes that gave moderate MTP, a reduced platelet aggregation response and, unexpectedly, enlarged α‐granules . It is in this context that we now report our studies on a second European family with a heterozygous β3D723H variant as well as the first two families to be described outside of Japan with a heterozygous αIIbR995W substitution.…”

“…Glycoprotein expression on unstimulated platelets was assessed using citrated PRP according to our standard protocols . On occasion, platelet surface labeling for αIIb, β3, GPIbα and P‐selectin was quantified using the PLT Gp/Receptors kit (Biocytex, Marseille, France) at room temperature before and after stimulation with 10 µM ADP and 50 µM TRAP using the Beckman Coulter Navios flow cytometer (Beckman Coulter, Villepinte, France).…”

“…Rare gain‐of‐function mutations of the ITGA2B or ITGB3 genes encoding αIIbβ3 also cause macrothrombocytopenia (MTP) with a low platelet count and platelets of increased size . Mostly heterozygous with autosomal dominant (AD) expression these include D621_E660del, L718P, L718del and D723H mutations in β3, and G991C, G993del, R995Q or W in αIIb (Supporting Information Table S1) . While D621_E660del affects the extracellular cysteine‐rich βA domain of β3, the others affect transmembrane or intracellular cytoplasmic domains and in particular the salt bridge linking the negatively charged D723 of β3 with the positively charged R995 of the much studied GFFKR sequence of αIIb .…”

ITGA2B and ITGB3 gene mutations associated with Glanzmann thrombasthenia

“…Also highlighted is a biallelic β3C586R substitution in the β3 extracellular disulfide-rich EGF-3 domain that spontaneously converts residual αIIbβ3 into an active conformation able to bind Fg but with platelets refractory to aggregation [reviewed in 6]. Also shown in Figure 2 are membrane-proximal or cytoplasmic tail missense mutations or deletions, often affecting single alleles (with possible autosomal dominant inheritance) and provoking limited conformational changes in αIIbβ3 (detected by the activation-dependent monoclonal antibody, PAC-1 but without spontaneous Fg binding) that lead to a moderate macrothrombocytopenia combined with a severe platelet aggregation deficiency [reviewed in 6,16]. This is associated with a reduced capacity of MKs to form proplatelets with release of platelets with anisotrophy and according to a recent report enlarged α-granules [16,17].…”

“…Loss-of-function mutations (in blue) in the β3 extracellular head prevent binding of fibrinogen or other adhesive proteins to the opened integrin headpiece following platelet activation, while those in the β3 cytoplasmic tail prevent binding of kindlin-3 and/or talin, steps essential for integrin activation [14,15]. In red are gain-of-function mutations mostly leading to at least partial activation of αIIbβ3 and mostly associated with macrothrombocytopenia often with severe loss of αIIbβ3 function [6,16].…”