High levels of frataxin overexpression leads to mitochondrial and cardiac toxicity in mouse models

Brahim Belbellaa, Laurence Reutenauer, Nadia Messaddeq, Laurent Monassier, Helene Puccio; bioRxiv 2020.03.31.015255; doi:10.1101/2020.03.31.015255

AAV-mediated gene therapy constitutes a promising approach for FA, as demonstrated in cardiac and neurological mouse models. While the minimal therapeutic level of FXN protein to be restored and biodistribution have recently been defined for the heart, it is unclear if FXN overexpression could be harmful. Indeed, depending on the vector delivery route and dose administrated, the resulting FXN protein level could reach very high levels in the heart, cerebellum, or in off-target organs such as the liver. The present study demonstrates safety of FXN cardiac overexpression up to 9-fold the normal endogenous level, but significant toxicity to the mitochondria and heart above 20-fold. We show gradual severity with increasing FXN overexpression, ranging from subclinical cardiotoxicity to left ventricle dysfunction. This appears to be driven by impairment of mitochondria respiratory chain, ultrastructure and homeostasis, which lead to myofilaments alteration, cell death and fibrosis. Overall, this study underlines the need, during the development of gene therapy approaches, to consider appropriately vector potency, long term safety and biomarkers to monitor such events.

High levels of frataxin overexpression leads to mitochondrial and cardiac toxicity in mouse models