Differential transcript level of <i>ANKRD26</i> and clinical phenotype among the <i>ANKRD26</i> variants in the Japanese registry for congenital thrombocytopenia

Ankyrin repeat domain 26 (ANKRD26)-related thrombocytopenia (ANKRD26-RT) is an autosomal dominant disorder caused by variants in the 5'-untranslated region of the ANKRD26 gene, leading to its overexpression and impaired megakaryocyte maturation. We identified a novel ANKRD26 variant (c.-129_-112del) and investigated its transcriptional impact, platelet function, and haematopoietic effects. We analysed ANKRD26 transcription in megakaryocytes derived from 21 ANKRD26-RT patients using quantitative reverse transcription droplet digital polymerase chain reaction (PCR). Furthermore, platelet function, megakaryocyte morphology and RNA sequencing were assessed. The novel variant exhibited significantly higher ANKRD26 transcription levels than previously reported variants (p < 0.001), but did not show any characteristic clinical phenotypes. Megakaryocytes with the novel variant showed reduced proplatelet formation and lower nuclear ploidy. RNA sequencing revealed upregulation of cell proliferation genes, including dedicator of cytokinesis 2 (DOCK2), cluster of differentiation (CD38) and interleukin-1 receptor accessory protein (IL1RAP). No platelet-related or megakaryocyte-related genes other than ANKRD26 were detected in these variants. The new variant disrupted the runt-related transcription factor 1 (RUNX1)-binding site, leading to ANKRD26 overexpression and possible haematopoietic abnormalities, though it did not directly correlate with clinical severity. However, genes linked to cell proliferation (CD38, DOCK2 and IL1RAP) showed increased expression. Our results suggested a potential link to leucocytosis, although further research is required to confirm this.