αIIbβ3 variants in ten families with autosomal dominant macrothrombocytopenia: Expanding the mutational and clinical spectrum

Abstract

Background Rare pathogenic variants in either the ITGA2B or ITGB3 genes have been linked to autosomal dominant macrothrombocytopenia associated with abnormal platelet production and function, deserving the designation of Glanzmann Thrombasthenia-Like Syndrome (GTLS) or ITGA2B/ITGB3-related thrombocytopenia. Objectives To describe a series of patients with familial macrothrombocytopenia and decreased expression of αIIbβ3 integrin due to defects in the ITGA2B or ITGB3 genes. Methods We reviewed the clinical and laboratory records of 10 Portuguese families with GTLS (33 patients and 11 unaffected relatives), including the functional and genetic defects. Results Patients had absent to moderate bleeding, macrothrombocytopenia, low αIIbβ3 expression, impaired platelet aggregation/ATP release to physiological agonists and low expression of activation-induced binding sites on αIIbβ3 (PAC-1) and receptor-induced binding sites on its ligand (bound fibrinogen), upon stimulation with TRAP-6 and ADP. Evidence for constitutive αIIbβ3 activation, occurred in 2 out of 9 patients from 8 families studied, but also in 2 out of 12 healthy controls. We identified 7 missense variants: 3 in ITGA2B (5 families), and 4 in ITGB3 (5 families). Three variants (αIIb: p.Arg1026Trp and p.Arg1026Gln and β3: p.Asp749His) were previously reported. The remaining (αIIb: p.Gly1007Val and β3: p.Thr746Pro, p.His748Pro and p.Arg760Cys) are new, expanding the αIIbβ3 defects associated with GTLS. The integration of the clinical and laboratory data allowed the identification of two GTLS subgroups, with distinct disease severity. Conclusions Previously reported ITGA2B and ITGB3 variants related to thrombocytopenia were clustered in a confined region of the membrane-proximal cytoplasmic domains, the inner membrane clasp. For the first time, variants are reported at the outer membrane clasp, at the transmembrane domain of αIIb, and at the membrane distal cytoplasmic domains of β3. This is the largest single-center series of inherited macrothrombocytopenia associated with αIIbβ3 variants published to date.

Inherited Platelet Disorders: An Updated Overview

“…Because it is a recessive autosomal disorder, only homozygote subjects exhibit moderate-severe mucocutaneous bleeding starting in childhood, whereas heterozygote individuals tend to be asymptomatic [ 1 , 8 , 163 ]. On the other hand, some ITGA2B and ITGB3 heterozygous variants can cause autosomal dominant inherited thrombocytopenia [ 39 ].…”

Profiling the Genetic and Molecular Characteristics of Glanzmann Thrombasthenia: Can It Guide Current and Future Therapies?

“…Early MK maturation was normal, but proplatelets were short and had enlarged tips. Recently, Morais et al 73 have reported more European cases with AD variants affecting the salt bridge and all with mild forms of MTP; again the authors emphasized the phenotypic variability of such cases and which extended to the degree of spontaneous activation of αIIbβ3.…”

“…76 Platelet anisotropy was again associated with enlarged "fused" α-granules. Recently, Morais et al 73 have added p.G976V in αIIb and p.T720P, p.H722P and p.R734H (p.…”

“… 76 Platelet anisotropy was again associated with enlarged “fused” α-granules. Recently, Morais et al 73 have added p.G976V in αIIb and p.T720P, p.H722P and p.R734H (p.R760H) in β3 to the list causing MTP with GT-like functional defects ( Figure 1 ).…”

CalDAG-GEFI Deficiency in a Family with Symptomatic Heterozygous and Homozygous Carriers of a Likely Pathogenic Variant in RASGRP2

“…Surface expression of PLT glycoproteins (GP) was measured by FCM using citrated PB and a direct immunofluorescence technique, as previously described in detail [12]. Monoclonal antibodies (mAbs) used, conjugated with fluorescein isothiocyanate (FITC) or with phycoerythrin (PE) were from Becton Dickinson, San Jose, CA, USA (BD) or from Dako, Glostrup, Denmark (DK): FITC-conjugated anti-CD41a (clone HIP8) (BD, 333147) FITC-conjugated anti-CD61 (GPIIIa) (clone RUU-PL7F12) (BD, 347407), and PE-conjugated anti-CD42b (GPIb) (clone AN51) (DK, R7014).…”