# Dissecting the Biological Functions of Various Isoforms of Ferredoxin Reductase for Cell Survival and DNA Damage Response

## TL;DR

This study explores the roles of different FDXR protein isoforms in cell survival and DNA repair, finding that each isoform contributes uniquely to these processes.

## Contribution

The study characterizes the biological functions of previously unexplored FDXR isoforms and identifies their roles in cell survival and DNA damage response.

## Key findings

- FDXR isoform 1 is the most abundant (~70%), while isoforms 4 and 7 account for ~10% and ~7%, respectively.
- Isoform 7, lacking a mitochondrial localization signal, is cytosolic and can be induced by DNA damage in a p53-dependent manner.
- Knockout of specific FDXR isoforms leads to defects in cell proliferation and DNA repair, indicating their essential roles in cell survival.

## Abstract

The ferredoxin reductase (FDXR) gene is expressed as seven isoforms: 1–6 by alternative splicing and 7 by an alternative promoter according to the Entrez Gene Database. Previous studies showed that FDXR, primarily the mitochondrial isoform 1, plays a role in biosynthesis of sterols, heme, and iron–sulfur clusters. However, the biological functions of FDXR isoforms 3–7 have not been characterized. Here, we first examined the expression profile of various FDXR isoforms. 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 (MLS), is expressed in the cytosol. We also found that like the promoter 1 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 which one or more FDXR isoforms are knocked out. While total FDXR-KO MCF7 cells are non-viable, cells deficient in isoforms 1–6, isoform 4, or isoform 7 remain viable but are defective in cell proliferation, DNA damage response, and repair. These data suggest that each FDXR isoform contributes to cell survival and that isoform 7 has extra-mitochondrial activity that may be sufficient for cell survival.