Excessive bleeding and platelet dense granule secretion defects are associated with heterozygous mutations in FLI1 and RUNX1. (A-B) Pedigrees showing inheritance of mild bleeding, alopecia totalis, and other clinical features in families 1 (A) and 2 (B). Individuals heterozygous for the c.1009C>T and c.1028A>G transitions in FLI1 are indicated. Lines through symbols indicate deceased individuals. In panel A, individuals with bleeding symptoms, alopecia, and confirmed platelet dense granule secretion defects are indicated by black filled symbols. An asterisk indicates the presence of eczema and a history of recurrent viral infections. The presence of psoriasis is indicated by a “+” sign. In panel B, individuals with bleeding symptoms and alopecia are indicated by black or gray filled symbols. Black filled symbols indicate individuals with confirmed platelet dense granule secretion defect and mild thrombocytopenia. An asterisk indicates a history of infective endocarditis, and the presence of eczema and colitis is indicated by a “+” sign. (C) ATP secretion in response to 100 μM PAR1 peptide in 2 members of F1 with the c.1009C>T transition in FLI1 and 3 members of F4 with the c.508+1G>T transversion in RUNX1 alongside controls. (D) Schematic diagram of RUNX1 and FLI1 showing the regions of the proteins affected by mutations identified in this study. Intronic mutations, predicted to interfere with splicing of the RUNX1 RNA, are shown in italics. (E) HEK293T cells were cotransfected with wild-type (WT) and variant FLI1 constructs, or combinations thereof, and pGL3.10-GP6-luciferase and pRLnull-Renilla reporters as shown, and firefly and Renilla luciferase expression assessed in cell lysates 48 hours later. Data represent the means (± standard error of the mean) of 3 independent experiments; **P < .01; ***P < .001. (F) MYH10 protein expression in platelets from patients with FLI1 mutations (F2.1 and F2.2 with p.Tyr343Cys FLI1 mutation; F3.1 and F3.2 with p.Asn331Thrfs*4 FLI1 mutation) and a healthy control.