Dissecting the biological functions of various isoforms of ferredoxin reductase for cell survival and DNA damage response

The ferredoxin reductase (FDXR) gene is expressed as seven isoforms: 1-6 by alternative splicing and 7 by alternative promoter. FDXR, primarily the reference mitochondrial isoform 1, is required for biosynthesis of sterols, heme and iron-sulfur clusters, but isoforms 2-7 are undefined. Here, we found that isoform 1 is the most abundant one, accounting for ~70% of total FDXR whereas isoforms 4 and 7 account for ~10% and ~7%, respectively. We found that isoforms 1 and 4 are mainly localized in the mitochondria whereas isoform 7, which lacks a mitochondria localization signal, is expressed in the cytosol. We also found that like the promoter for isoforms 1-6, the P2 promoter for isoform 7 can be induced by DNA damage in a p53-dependent manner. To determine isoform-specific activity, we generated multiple MCF7 cell lines in that one or more isoforms are knocked out. While total FDXR-KO MCF7 cells are non-viable, cells deficient in isoforms 1-6, isoforms 4 or 7 remain viable but are weak in cell proliferation, DNA damage response and repair. These data suggest that each FDXR isoform contributes to cell survival and that isoform 7 has an extra-mitochondrial activity sufficient for cell survival.