Mitochondrial Atpif1 regulates haem synthesis in developing erythroblasts

Atpif1, a mitochondrial ATPase inhibitor, was identified as a zebrafish anemic mutant, pinotage, providing an important link in our understanding of the relationship between mitochondrial homeostasis and haem synthesis and identifying a gene that may have a role in human iron, haem and mitochondrial diseases. Defects in haem biosynthesis in erythrocytes cause anaemia. Many of the enzymes and substrates involved in this important process are known, but it is still unclear how iron transport is regulated in the mitochondria and coordinated with haem homeostasis. Here, Barry Paw and colleagues report the cloning of mitochondrial ATPase inhibitor factor 1 (atpif1) from a zebrafish genetic screen. Atpif1 regulates ferrochelatase, the terminal enzyme in haem biosynthesis, through alteration of mitochondrial pH and redox potential in the presence of a [2Fe–2S] cluster as the iron supply. The authors show that the human orthologue of Atpif1 is vital for normal red-blood-cell differentiation, suggesting that its deficiency may contribute to human diseases such as congenital sideroblastic anaemias and other mitochondriopathies. Defects in the availability of haem substrates or the catalytic activity of the terminal enzyme in haem biosynthesis, ferrochelatase (Fech), impair haem synthesis and thus cause human congenital anaemias1,2. The interdependent functions of regulators of mitochondrial homeostasis and enzymes responsible for haem synthesis are largely unknown. To investigate this we used zebrafish genetic screens and cloned mitochondrial ATPase inhibitory factor 1 (atpif1) from a zebrafish mutant with profound anaemia, pinotage (pnt tq209 ). Here we describe a direct mechanism establishing that Atpif1 regulates the catalytic efficiency of vertebrate Fech to synthesize haem. The loss of Atpif1 impairs haemoglobin synthesis in zebrafish, mouse and human haematopoietic models as a consequence of diminished Fech activity and elevated mitochondrial pH. To understand the relationship between mitochondrial pH, redox potential, [2Fe–2S] clusters and Fech activity, we used genetic complementation studies of Fech constructs with or without [2Fe–2S] clusters in pnt, as well as pharmacological agents modulating mitochondrial pH and redox potential. The presence of [2Fe–2S] cluster renders vertebrate Fech vulnerable to perturbations in Atpif1-regulated mitochondrial pH and redox potential. Therefore, Atpif1 deficiency reduces the efficiency of vertebrate Fech to synthesize haem, resulting in anaemia. The identification of mitochondrial Atpif1 as a regulator of haem synthesis advances our understanding of the mechanisms regulating mitochondrial haem homeostasis and red blood cell development. An ATPIF1 deficiency may contribute to important human diseases, such as congenital sideroblastic anaemias and mitochondriopathies.