A novel variant of the CYCS gene causes thrombocytopenia through deregulated Tumor Necrosis Factor signaling

The human CYCS gene encodes for Cytochrome C (Cytc), a small mitochondrial protein with important cellular functions including energy production and apoptosis. Mutations in the CYCS gene have been linked to the development of mild autosomal dominant thrombocytopenia, which is included in the group of inherited thrombocytopenias. Here we report the identification of a novel heterozygous variant of the CYCS gene (c.292T>C (p.Tyr98His)) found in a Greek family with thrombocytopenia. Its functional role in several cellular processes has been examined to delineate the mechanisms underlying the thrombocytopenic phenotype of the patients. The novel variant has been associated with increased megakaryocyte apoptosis, and has led to decreased ATP levels through the reduced superoxidase activity of Cytc in megakaryocytic cells. RNA-Seq has provided information about the effect of the CYCS variant on Tumor Necrosis Factor (TNF) signaling, revealing a dual mechanism through the negative enrichment of TNF via nuclear factor kappa B (NFKB) signaling in combination with the positive enrichment of the TNF via p38 . Downregulation of the TNF via NFKB pathway has led to increased apoptosis observed in megakaryocytic cells, probably through a decrease in anti-apoptotic signaling mechanisms. Furthermore, the positive enrichment of the TNF via p38 signallng has also contributed to the induced apoptosis of megakaryocytic cells caused by the variant and the blockade of megakaryocyte differentiation determined in patient samples. Collectively, this study has revealed a connection between the novel CYCS variant and the thrombocytopenic phenotype through deregulated TNF mediated pathways in megakaryocytic cells, providing novel therapeutic targets for CYCS-related thrombocytopenias