Rare missense variants in Tropomyosin-4 (TPM4) are associated with platelet dysfunction, cytoskeletal defects and excessive bleeding

Background
A significant challenge is faced for the genetic diagnosis of inherited platelet disorders where candidate genetic variants can be found in over 100 Bleeding, Thrombotic and Platelet disorder genes, especially within families where there is both normal and low platelet counts. Genetic variants of unknown clinical significance (VUS) are found in a significant proportion of such patients where functional studies are required to prove pathogenicity.

Objective
To identify the genetic cause in patients with a suspected platelet disorder and subsequently perform a detailed functional analysis of the candidate genetic variants found.

Methods
Genetic and functional studies was undertaken in 3 patients in 2 unrelated families with a suspected platelet disorder and excessive bleeding. A targeted gene panel of previously known bleeding and platelet genes was used to identify plausible genetic variants. Deep platelet phenotyping was performed using platelet spreading analysis, transmission electron microscopy, immunofluorescence and platelet function testing using lumiaggregometry and flow cytometry.

Results
We report rare conserved missense variants (p.R182C and p.A183V) in TPM4 encoding Tromomyosin-4, in 3 patients. Deep platelet phenotyping studies revealed similar platelet function defects across the 3 patients including reduced platelet secretion, and aggregation and spreading defects suggesting that TPM4 missense variants impact platelet function and show a disordered pattern of Tropomyosin staining.

Conclusions
Genetic and functional TPM4 defects are reported making TPM4 a diagnostic grade tier 1 gene and highlights the importance of including TPM4 in diagnostic genetic screening for patients with significant bleeding and undiagnosed platelet disorders, particularly for those with a normal platelet count.