A molecular basis of Ferredoxin Reductase (FdxR) mutations that result in mitochondriopathies

Inherited mutations in the Ferredoxin Reductase (FdxR) gene can result in a spectrum of disorders that include auditory and optic neural atrophies as well as adrenal insufficiency. FdxR (also referred to as Adrenodoxin Reductase) is a flavoprotein located in the inner mitochondrial membrane. It is responsible for mediating electron transfer from NADPH to either Fdx1 (Adrenodoxin), which is the sole reductant for all seven mitochondrial cytochromes P450, or to the related ferredoxin Fdx2, which is a component in the FeS cluster biogenesis pathway. In most cases, the mechanistic causes that underpin FdxR-related neuropathies and steroid imbalances remain unknown. In this study, we investigate three clinically relevant variants of FdxR (R211Q, R275C, and R355Q) that exhibit classic FdxR-related disease phenotypes and are widely distributed in the protein. We use a combination of biophysical and biochemical techniques to evaluate both the FdxR:Fdx1 complex and the FdxR:Fdx2 complex since these redox complexes represent an important branch point in FdxR function. Two key findings from this study are that i) all three mutants alter the recognition of Fdx1 and Fdx2, despite R275C and R355Q being located distally from the expected site of interaction, and ii) R275C and R355Q disrupt the functional complex with Fdx1, but not with Fdx2. These findings are supplemented with 2D NMR data of each mutant FdxR complex. In summary, this work implicates protein instability and degradation as the proximal cause of FdxR-related disease, with a secondary cause being the disruption of cytochrome P450-mediated metabolism in mitochondria.