Background: The contractile protein non-muscle myosin heavy chain IIA, encoded by the MYH9 gene, binds to filamentous actin and generates biomechanical forces. Heterozygous defects in this gene lead to different autosomal dominant syndromes in humans, which are characterized among others by macrothrombocytopenia and a mild to moderate bleeding tendency.

Aims: We hypothesized that reduced platelet force generation is responsible for the increased bleeding risk in MYH9 patients.

Methods: We analyzed three mouse lines each with one point mutation in the Myh9 gene at the positions 702, 1424, or 1841, which have been described to recapitulate defects found in patients. We characterized the basic platelet function and tested the biophysical properties of the mutant platelets with atomic force spectroscopy and micropost arrays.

Results: Myh9 mutant mice displayed a macrothrombocytopenia, but only slightly altered glycoprotein expression. αIIbβ3 integrin activation and P-Selectin surface exposure of mutant platelets was overall comparable to controls. The capacity to assemble actin after activation was partially reduced in Myh9 mutant platelets, although the G- to F-actin ratio was unaltered in resting platelets. Phosphorylation of the myosin light chain after activation with thrombin was strongly reduced. In line with this, biophysical analysis revealed that Myh9 mutant platelets generate lower adhesion forces to collagen, lower interaction forces between platelets and reduced traction forces when spread on fibrinogen-coated micropost arrays. Clot retraction of mutant samples was delayed, further reflecting less force generation of Myh9 mutant platelets. Finally, we observed more unstable thrombi, when blood of Myh9 mutant mice was perfused ex vivo over collagen fibers.

Conclusions: We show that Myh9 mutant platelets generate lower forces. These data suggest that reduced platelet-substrate and platelet-platelet forces lead to the increased bleeding tendency found in MYH9 patients. We are currently testing platelets from humans with MYH9 mutations to test whether they show the same changes as mouse platelets.

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ISTH Congress Abstracts - https://abstracts.isth.org/abstract/low-adhesion-and-interaction-forces-of-myh9-mutant-platelets-lead-to-impaired-clot-retraction-and-unstable-thrombus-formation/