BDPLT16 is an autosomal dominant form of congenital macrothrombocytopenia associated with platelet anisocytosis. It is a disorder of platelet production. Affected individuals may have no or only mildly increased bleeding tendency. In vitro studies show mild platelet functional ... BDPLT16 is an autosomal dominant form of congenital macrothrombocytopenia associated with platelet anisocytosis. It is a disorder of platelet production. Affected individuals may have no or only mildly increased bleeding tendency. In vitro studies show mild platelet functional abnormalities (summary by Kunishima et al., 2011 and Nurden et al., 2011).
Gross et al. (1960) reported a family in which affected members over 3 generations had petechiae, bleeding from mucous membranes, prolonged bleeding after injury, and severe anemia. Studies revealed prolonged bleeding time, abnormal capillary fragility, and a normal ... Gross et al. (1960) reported a family in which affected members over 3 generations had petechiae, bleeding from mucous membranes, prolonged bleeding after injury, and severe anemia. Studies revealed prolonged bleeding time, abnormal capillary fragility, and a normal or an increased number of platelets, with giant platelets. Alteration in the concentration of several platelet enzymes was found. Hardisty et al. (1992) reported a young Italian man with a lifelong history of bleeding from gums and mucocutaneous tissues. Laboratory studies showed modest thrombocytopenia, platelet anisocytosis, and large platelets. Platelet aggregation was decreased, but clot retraction was normal. His platelets had decreased levels of the GPIIb/IIIa complex compared to controls (40-50% by crossed immunoelectrophoresis), and further decreased surface expression (12-20% using monocloncal antibodies). Surface expression of GPIIb/IIIa increased upon platelet stimulation, suggesting a substantial amount of these proteins in the internal platelet store. His father also showed platelet anisocytosis and large platelets. This family was also studied by Peyruchaud et al. (1998) and Nurden et al. (2011). Ghevaert et al. (2008) reported a family in which 5 individuals had macrothrombocytopenia. GPIIb/IIIa expression on platelets was normal, and none of the affected individuals had bleeding abnormalities; the defect in the proband was an incidental finding. Gresele et al. (2009) reported 2 unrelated Italian families with autosomal dominant BDPLT16. Clinical features included lifelong bleeding tendency, particularly mucosal bleeding, and macrothrombocytopenia. Patient platelets showed decreased expression of the GPIIb/IIIa complex. Functional studies showed several abnormalities, including impaired platelet aggregation to physiologic agonists but not to ristocetin, normal clot retraction, reduced fibrinogen binding and expression of activated GPIIb/IIIa upon stimulation, normal platelet adhesion to immobilized fibrinogen but reduced platelet spreading, and decreased tyrosine phosphorylation, indicating defective outside-in signaling. Jayo et al. (2010) reported a Spanish woman with a lifelong history of mucocutaneous bleeding tendency associated with moderate thrombocytopenia and platelet anisocytosis. Functional studies showed decreased platelet agglutination to physiologic agonists and impaired spreading on fibrinogen. There was no family history of a similar disorder. Kunishima et al. (2011) reported 11 patients from 4 unrelated Japanese families with congenital macrothrombocytopenia. Bleeding tendency was mild or absent. Platelet aggregation was decreased, but bleeding time was normal, and platelet spreading on fibrinogen was partially impaired. Patient platelets showed decreased surface expression of GPIIb/IIIa (50-70% of controls). Kobayashi et al. (2013) reported a 4-generation Japanese kindred in which 10 individuals had mild bleeding tendencies, such as nasal bleeding and purpura, associated with macrothrombocytopenia and platelet anisocytosis. Laboratory studies showed decreased platelet aggregation by physiologic agonists. Studies of patient platelets showed decreased expression of the GPIIb/IIIa complex and evidence of spontaneous partial activation, including increased PAC-1 binding and increased fibrinogen binding potential. After treatment with the agonist ADP, patient platelets did not show significantly increased fibrinogen binding potential compared to controls, suggesting that they could not be fully activated in the presence of such signals.
In an Italian man with macrothrombocytopenia reported by Hardisty et al. (1992), Peyruchaud et al. (1998) identified a heterozygous mutation in the ITGA2B gene (R995Q; 607759.0017). In vitro functional expression ... - Mutations in the ITGA2B Gene In an Italian man with macrothrombocytopenia reported by Hardisty et al. (1992), Peyruchaud et al. (1998) identified a heterozygous mutation in the ITGA2B gene (R995Q; 607759.0017). In vitro functional expression studies in CHO cells indicated that the R995Q mutation would give rise to an integrin complex that is more easily activatable compared to wildtype. In 11 patients from 4 Japanese families with BDPLT16, Kunishima et al. (2011) identified a heterozygous mutation in the ITGA2B gene (R995W; 607759.0018). The disease haplotype was unique in each family, indicating independent occurrence. In vitro studies indicated that mutant protein assumed a constitutive, activated conformation, but did not induce platelet activation. Transfection of the mutation into CHO cells and mouse liver-derived megakaryocytes resulted in abnormal membrane ruffling and cytoplasmic protrusions, as well as defect proplatelet formation. The findings were reminiscent of the activating D723H mutation in ITGB3 (173470.0018), and Kunishima et al. (2011) concluded that activating mutations in ITGA2B and ITGB3 are responsible for a subset of congenital macrothrombocytopenias. - Mutations in the ITGB3 Gene In 5 members of a family with autosomal dominant BDPLT16 manifest as macrothrombocytopenia, Ghevaert et al. (2008) identified a heterozygous mutation in the ITGB3 gene (D723H; 173470.0018). Molecular modeling indicated that the mutation changed the electrostatic surface potential, causing disruption of a conserved salt bridge between D723 in ITGB3 and residue R995 in the ITGA2B gene. In vitro functional expression assays showed that the mutant protein was constitutively active. Cells transfected with the mutation exhibited spontaneous and specific increased binding of the antibody PAC-1, increased adhesion to von Willebrand factor (VWF) in static conditions, and increased binding to fibrinogen under shear stress compared to wildtype, all consistent with a gain of function. The mutant protein also led to the formation of large proplatelet-like protrusions in CHO cells and in patient megakaryocytes in the presence of fibrinogen. The findings suggested that constitutive partial activation of the mutant receptor caused incorrect sizing of platelets during formation, resulting in thrombocytopenia due to increased platelet turnover. In affected members of 2 unrelated Italian families with autosomal dominant BDPLT16, Gresele et al. (2009) identified a heterozygous splice site mutation in the ITGB3 gene (173470.0019). Haplotype analysis suggested a founder effect. In vitro studies suggested defective GPIIb/IIIa outside-in signaling. The concomitant presence of both the normal and a mutant ITGB3 allele in patient platelet lysates suggested a loss-of-function hypothesis with a dominant-negative effect. In a Spanish woman with a bleeding disorder, thrombocytopenia, platelet anisocytosis, and reduced platelet aggregation, Jayo et al. (2010) identified a de novo heterozygous mutation in the ITGB3 gene (L718P; 173470.0020). Kobayashi et al. (2013) identified a heterozygous L718P mutation in the ITGB3 gene in affected members of a 4-generation Japanese family with BDPLT16. In CHO cells, the mutation promoted the generation of proplatelet-like protrusions by downregulation of RhoA (165390) activity. The findings suggested that this mutation contributes to thrombocytopenia through a gain of function.