ACTN1‐related thrombocytopenia: Homozygosity for an ACTN1 variant results in a more severe phenotype

ACTN1-related thrombocytopenia is a rare disorder caused by heterozygous variants in the ACTN1 gene characterized by macrothrombocytopenia and mild bleeding tendency. We describe for the first time two patients affected with ACTN1-RT caused by a homozygous variant in ACTN1 (c.982G>A) with mild heart valve defects unexplained by any other genetic variants investigated by WES. Within the reported family, the homozygous sisters have moderate thrombocytopenia and marked platelet macrocytosis with giant platelets, revealing a more severe haematological phenotype compared to their heterozygous relatives and highlighting a significant effect of allelic burden on platelet size. Moreover, we hypothesize that some ACTN1 variants, especially when present in the homozygous state, may also contribute to the cardiac abnormalities.