Novel mutations in Thai patients with glanzmann thrombasthenia

OBJECTIVES

Glanzmann thrombasthenia (GT) is an autosomal recessive platelet disorder, caused by defects of the platelet integrin αIIbβ3 (GPIIb/IIIa) resulting from pathogenic mutations in either ITGA2B or ITGB3. It is characterized by spontaneous mucocutaneous bleeding. The molecular features of GT in Thailand have not been identified. This study aimed to determine the clinical and molecular features of unrelated Thai patients with GT.

METHODS

Four patients with clinically suspected GT were recruited at the Division of Pediatric Hematology/Oncology, King Chulalongkorn Memorial Hospital. The diagnosis was based on clinical and hematological parameters as well as genetic analysis. Whole exome sequencing (WES) was performed in all cases.

RESULTS

Of the four patients studied, the median age at first suspicion of GT was 2.5 years. All presented with severe bleeding symptoms (WHO bleeding scale 3). Flow cytometry to assess the surface GPIIb/IIIa complex showed reduced expression. By WES, we successfully identified seven mutant alleles in ITGA2B. One alteration, the c.2915dup (p.Leu973Alafs*63), was detected in two unrelated families. One patient was homozygous for the c.617T>A (p.Val206Asp). Of the five different mutations, three have never been previously described. These include a missense, c.617T>A (p.Val206Asp), a deletion, c.1524_1533del (p.Gln508Hisfs*3), and a nonsense, c.2344C>T (p.Arg782Ter).

CONCLUSION

This study reported three novel mutations expanding the genotypic spectrum of ITGA2B causing GT.

Simultaneous occurrence of intussusception and duodenal haematoma in a patient with Glanzmann's thrombasthenia

Glanzmann's thrombasthenia (GT) is a rare autosomal recessive disorder in which the platelets have anomalies of fibrinogen receptors causing bleeding tendencies. The disease usually presents with mucosal bleeding, petechial haemorrhages or gastrointestinal (GI) bleeding. Only sporadic cases of intussusception or duodenal haematoma have been reported with GT. We present a case of 5-year-old girl admitted with non-bilious vomiting, watery diarrhoea and abdominal pain. She is a known patient of Glanzmann's thrombasthenia.

C560Rβ3 caused platelet integrin αII b β3 to bind fibrinogen continuously, but resulted in a severe bleeding syndrome and increased murine mortality

BACKGROUND AND OBJECTIVES

β(3)-Deficient megakaryocytes were modified by human β(3)-lentivirus transduction and transplantation to express sufficient levels of a C560Rβ(3) amino acid substitution, for investigation of how an activated αII b β(3) conformation affects platelets in vivo in mice.

PATIENT/METHODS

As in our previous report of an R560β(3) mutation in a patient with Glanzmann thrombasthenia, R560β(3) murine platelets spontaneously bound antibody that only recognizes activated αII b β3 bound to its ligand, fibrinogen.

RESULTS

With this murine model, we showed that αII b -R560β3 mutation-mediated continuous binding of fibrinogen occurred in the absence of P-selectin surface expression, indicating that the integrin was in an active conformation, although the platelets circulated in a quiescent manner. Remarkably, only 35% of R560β(3) 'mutant' mice survived for 6 months after transplantation, whereas 87% of C560β(3) 'wild-type' mice remained alive. Pathologic examination revealed that R560β(3) mice had enlarged spleens with extramedullary hematopoiesis and increased hemosiderin, indicating hemorrhage. R560β(3) megakaryocytes and platelets showed abnormal morphology and irregular granule distribution. Interestingly, R560β(3) washed platelets could aggregate upon simultaneous addition of fibrinogen and physiologic agonists, but aggregation failed when platelets were exposed to fibrinogen before activation in vitro and in vivo.

CONCLUSIONS

The results demonstrate that continuous occupancy of αIIb β3 with fibrinogen disrupts platelet structure and function, leading to hemorrhagic death consistent with Glanzmann thrombasthenia rather than a thrombotic state.

Demonstration of novel gain-of-function mutations of αIIbβ3: association with macrothrombocytopenia and glanzmann thrombasthenia-like phenotype

Integrin αIIbβ3 is indispensable for normal hemostasis, but its role for thrombopoiesis is still controversial. Recently, αIIb and β3 mutations have been identified in patients with congenital macrothrombocytopenia. We analyzed three unrelated Japanese families with congenital macrothrombocytopenia. Expression and activation state of αIIbβ3 in platelets was examined by flow cytometry and immunoblotting. Sequence of whole coding region and exon-intron boundaries of ITGA2B and ITGB3 genes was performed. The effects of mutations on αIIbβ3 activation state and phosphorylation of FAK were analyzed in transfected cells. We newly identified three mutations: two mutations in highly conserved Gly-Phe-Phe-Lys-Arg sequence in juxtamembrane region of αIIb, p.Gly991Cys and p.Phe993del, and one donor site mutation of intron 13 of ITGB3 leading to 40 amino acids deletion, p.(Asp621_Glu660del), in the membrane proximal β-tail domain of β3. One patient, who showed Glanzmann thrombasthenia-like marked reduction in surface αIIbβ3 expression (3-11% of normal control), was a compound heterozygote with ITGA2B p.Gly991Cys and a novel nonsense mutation, ITGA2B p.Arg422*. All three mutations, ITGA2B p.Gly991Cys, ITGA2B p.Phe993del, and ITGB3 p.(Asp621_Glu660del), led to highly activated conformation of αIIbβ3 and spontaneous tyrosine phosphorylation of FAK in transfected cells. These results suggest that gain-of-function mutations around membrane region of αIIbβ3 lead to abnormal platelet number and morphology with impaired surface αIIbβ3 expression.

Novel Mutations in the GPIIb and GPIIIa Genes in Glanzmann Thrombasthenia

BACKGROUND: Glanzmann thrombasthenia (GT) is an inherited autosomal recessive platelet disorder characterized by a complete or partial lack, or mutation, of the GPIIb/IIIa complex (integrin α(IIb)β(3)) on the thrombocytes' surface, leading to a severe bleeding syndrome. MATERIAL AND METHODS: Molecular genetic analysis was performed in patients with suspected GT. The aim of the present study was the identification of new natural variants, their impact on platelet function, and their relation to the risk of bleeding. RESULTS: Expression of the platelet integrin α(IIb)β(3) was determined by flow cytometry. Mutations were identified through sequencing of cDNA and genomic DNA. In addition, platelet function studies (PAC-binding, aggregations) were implemented. The study included 25 patients revealing 13 mutations (GPIIb: n = 9; GPIIIa: n = 4). Two of the 13 mutations were previously described (T207I; L214P). The remaining mutations have not been published yet, whereas 1 mutation in 2 unrelated families was identical (3062 T→C). CONCLUSION: All patients with less than 25% of present α(IIb)β(3) have a medical history of bleeding.

Three novel beta-propeller mutations causing Glanzmann thrombasthenia result in production of normally stable pro-alphaIIb, but variably impaired progression of pro-alphaIIbbeta3 from endoplasmic reticulum to Golgi

BACKGROUND

Glanzmann thrombasthenia (GT) is an autosomal recessive bleeding disorder characterized by lack of platelet aggregation in response to most physiological agonists and caused by either a lack or dysfunction of the platelet integrin alpha(IIb)beta3 (glycoprotein IIb/IIIa).

OBJECTIVES

To determine the molecular basis of GT and characterize the mutations by in vitro expression studies.

PATIENTS

We studied three unrelated patients from southern India whose diagnosis was consistent with GT.

RESULTS

Immunoprecipitation of the cell lysates and immunoblotting showed no detectable mature alpha(IIb) in the G128S mutant, in contrast to 6% and 33% of the normal amount of mature alpha(IIb) in the S287L and G357S mutants, respectively. Pulse-chase analysis demonstrated pro-alpha(IIb) in the mutants comparable with the normal pro-alpha(IIb), but no conversion to mature alpha(IIb) in the G128S mutant, and only trace conversion to mature alpha(IIb) in the S287L and G357S mutants. The disappearance of pro-alpha(IIb) in the three mutants was similar to that in cells expressing normal alpha(IIb)beta3 or alpha(IIb) only. All three mutants demonstrated pro-alpha(IIb)beta3 complexes and co-localized with an ER marker by immunofluorescence. The G128S mutant showed no co-localization with a Golgi marker, and the other two mutants showed minimal and moderate co-localization with the Golgi marker.

CONCLUSIONS

These three beta-propeller mutations do not affect the production of pro-alpha(IIb), its ability to complex with beta3, or its stability, but do cause variable defects in transport of pro-alpha(IIb)beta3 complexes from the endoplasmic reticulum to the Golgi.