Thrombocytopenia Cargeeg is a rare autosomal dominant disorder and one of three thrombocytopenias caused by mutation of cytochrome c (Online Mendelian Inheritance in Man entry THC4). Our previous observations of platelet-like structures in the marrow space and early platelet production in vitro suggested that the low platelet phenotype in Thrombocytopenia Cargeeg subjects is caused by premature release of platelets into non-vascular regions of the bone marrow. We now show that two processes of platelet release occur in Thrombocytopenia Cargeeg subjects. Circulating platelets have a normal marginal microtubule coil, and cultured megakaryocytes derived from peripheral blood cells of Thrombocytopenia Cargeeg subjects form proplatelets normally and release platelets containing a marginal microtubule coil, consistent with effective platelet release via the proplatelet mechanism. In contrast, platelet-like structures within the extravascular bone marrow space have the dimensions of platelets but lack the marginal microtubule coil, suggesting abnormal proplatelet-independent platelet release. The mechanism of extravascular platelet release remains unclear. The failure to recapitulate this mechanism in vitro implies that the phenotype is not simply an intrinsic property of CYCS mutation-carrying megakaryocytes, but is dependent on the interaction between these cells and their environment.

The naturally occurring human cytochrome c variant (G41S) is associated with a mild autosomal dominant thrombocytopenia (Thrombocytopenia Cargeeg) caused by dysregulation of platelet production. The molecular basis of the platelet production defect is unknown. Despite high conservation of cytochrome c between human and mouse (91.4% identity), introducing the G41S mutation into mouse cytochrome c in a knockin mouse (Cycs G41S/G41S) did not recapitulate the low platelet phenotype of Thrombocytopenia Cargeeg. While investigating the cause of this disparity we found a lack of conservation of the functional impact of cytochrome c mutations on caspase activation across species. Mutation of cytochrome c at residue 41 has distinct effects on the ability of cytochrome c to activate caspases depending on the species of both the cytochrome c and its binding partner Apaf-1. In contrast to our previous results showing the G41S mutation increases the ability of human cytochrome c to activate caspases, here we find this activity is decreased in mouse G41S cytochrome c. Additionally unlike wildtype human cytochrome c, G41S cytochrome c is unable to activate caspases in Xenopus embryo extracts. Taken together these results demonstrate a previously unreported species-specific component to the interaction of cytochrome c with Apaf-1. This suggests that the electrostatic interaction between cytochrome c and Apaf-1 is not the sole determinant of binding, with additional factors controlling binding specificity and affinity. These results have important implications for studies of the effects of cytochrome c mutations on the intrinsic apoptosis pathway.

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Mutations in the cytochrome c gene (CYCS) cause autosomal dominant thrombocytopenia by an unknown mechanism. While attempting to generate megakaryoblastic cell lines exogenously expressing cytochrome c variants, we discovered that endogenous cytochrome c expression increased both upon induction of differentiation with the phorbol ester phorbol 12-myristate 13-acetate (PMA), and as cell density increased. A concomitant increase in cytochrome c oxidase subunit II in response to PMA, but not cell higher cell density, suggests upregulation of the mitochondrial respiratory chain may be a specific feature of differentiation. These results highlight the likely importance of cytochrome c in both differentiating and proliferating cells, and illustrate the unsuitability of megakaryoblastic lines for modeling CYCS-associated thrombocytopenia.