Thrombocytopenia and CD34 expression is decoupled from α‐granule deficiency with mutation of the first growth factor‐independent 1B zinc finger

SummaryBackground Mutation of the GFI1B fifth DNA-binding zinc-finger domain causes macrothrombocytopenia and α-granule deficiency leading to clinical bleeding. The phenotypes associated with GFI1B variants disrupting non-DNA-binding zinc-fingers remain uncharacterized. Objectives To determine the functional and phenotypic consequences of GFI1B variants disrupting non-DNA-binding zinc-finger domains. Methods The GFI1B C168F variant and a novel GFI1B c.2520+1_2520+8delGTGGGCAC splice variant were identified in four unrelated families. Phenotypic features, DNA binding properties and transcriptional effects were determined and compared to individuals with a GFI1B H294fs mutation of the fifth DNA-binding zinc-finger. Patient-specific induced pluripotent stem cell (iPSC)-derived megakaryocytes were generated to facilitate disease modeling. Results The DNA-binding GFI1B variant, C168F, predicted to disrupt the first non-DNA-binding zinc-finger domain is associated with macrothrombocytopenia without α-granule deficiency or bleeding symptoms. A GFI1B splice variant, c.2520+1_2520+8delGTGGGCAC that generates a short GFI1B isoform that lacks non-DNA-binding zinc-fingers 1 and 2 is associated with increased platelet CD34 expression only, without quantitative or morphological platelet abnormalities. GFI1B represses the CD34 promoter and this repression is attenuated by different GFI1B zinc-finger mutations suggesting that deregulation of CD34 expression occurs at a direct transcriptional level. Patient-specific iPSC-derived megakaryocytes phenocopy these observations. Conclusions Disruption of GFI1B non-DNA-binding zinc-finger 1 is associated with mild to moderate thrombocytopenia without α-granule deficiency or bleeding symptomatology indicating the site of GFI1B mutation has important phenotypic implications. Platelet CD34 expression appears to be a common feature of perturbed GFI1B function and may have diagnostic utility. This article is protected by copyright. All rights reserved.