Novel homozygous frameshift mutation of ITGB3 in the Glanzmann thrombasthenia patient with abnormal bone metabolism and congenital bone defects.

Abstract

Background: Glanzmann thrombasthenia (GT) is a rare inherited bleeding disorder caused by dysfunction of the integrin αIIbβ3 in platelets. The subunit β3, encoded by ITGB3 also plays a significant role in bone metabolism. Whether GT patients with β3 deficiency also suffer from bone pathology remains unclear.

Method: The 21-year-old female patient presenting with bleeding diathesis and multiple congenital bone defects in her right hand, and her seven family members were included in the study. Whole exome sequencing as well as Sanger sequencing were conducted to identify GT-associated mutations within the family. The platelet function of the family was detected by the platelet aggregation test and thromboelastography (TEG). The expression levels of CD41 (αIIb) and CD61 (β3) on the platelet surface and total in platelet were detected by flow cytometry and Western blot. Bioinformatics analysis was used to evaluate the pathogenicity of mutation sites and their effects on protein structure and function. X-ray imaging, bone densitometry and bone metabolism index were performed to evaluate bone development and metabolism.

Result: A novel homozygous frameshift mutation c.2143_2158delinsCT (p.Lys715Leufs*36) of ITGB3 was found in the proband. Platelet aggregation by ADP, collagen, epinephrine, and arachidonic acid was absent, TEG showed hypocoagulability and decreased platelet function, and the expression levels of αIIb and β3 on the platelet surface and total in platelet were significantly reduced (< 5%) in the proband. The parents, second elder sister and grandmother of proband were heterozygous carriers without bleeding symptoms and had normal platelet aggregation function and αIIb/β3 protein expression. Structural modeling strongly suggested that the mutation creates a truncation in cytoplasmic domains of β3, resulting in the mutant β3/αIIbβ3 inactivated and low expression. The proband was born with partial absence of phalanges in digits 2-4 and the deformity of fingers 1 and 5 in her right hand, bone densitometry indicated significant osteopenia and increased risk of fracture in her right radius, and no other gene mutations related to bone pathology were identified.

Conclusion: A novel mutation of ITGB3 which results in GT was identified. This is the third reported case of GT combined with bone defect. Our work expands ITGB3 mutation spectrum and provide further insights into the potential association between GT and bone development and metabolism.