Adding to the evidence of gene-disease association of RAP1B and syndromic thrombocytopenia.

Syndromic constitutive thrombocytopenia encompasses a heterogeneous group of disorders characterised by quantitative and qualitative defects of platelets while featuring other malformations. Recently, heterozygous, de novo variants in RAP1B were reported in three cases of syndromic thrombocytopenia. Here, we report two additional, unrelated individuals identified retrospectively in our data repository with heterozygous variants in RAP1B: NM_001010942.2(RAP1B):c.35G>A, p.(Gly12Glu) (de novo) and NM_001010942.2(RAP1B):c.178G>A, p.(Gly60Arg). Both individuals had thrombocytopenia, as well as congenital malformations, and neurological, behavioural, and dysmorphic features, in line with previous reports. Our data supports the causal role of monoallelic RAP1B variants that disrupt RAP1B GTPase activity in syndromic congenital thrombocytopenia.