When present at levels 20 times higher than normal, frataxin became toxic and caused heart problems in a mouse model of Friedreich’s ataxia (FA), a study found.
According to the study authors, these findings underscore the importance of selecting an appropriate viral vector for gene therapy to ensure the production of frataxin remains within a harmless threshold.
The study, “High levels of frataxin overexpression leads to mitochondrial and cardiac toxicity in mouse models,” was published in the journal Molecular Therapy Methods & Clinical Development.
FA is a progressive disease caused by mutations in the sequence of the FXN gene, leading to a significant reduction in the production of frataxin, a protein involved in iron metabolism that usually is found in mitochondria, which are the small cell compartments responsible for producing energy.
The lack of frataxin leads not only to defects in iron metabolism, but also compromises mitochondria’s ability to produce energy. This is particularly problematic in high energy-demanding cells, including those found in the heart, and is the main reason why FA patients often develop heart disease that can progress to heart failure.
Previous studies in animal models have shown the neurological and heart problems associated with FA may be eased by forcing cells in the central nervous system (CNS, composed of the brain and spinal cord) and heart to produce frataxin.
This can be attained through gene therapy approaches, which use harmless adeno-associated virus (AAV) vectors to transport and deliver a normal copy of the FXN gene to cells in those tissues.
However, depending on the AAV vector and the dose administered, levels of frataxin can vary significantly in the heart, CNS, or in other off-target organs where the protein is not intended to be produced.
“While the minimal therapeutic level of FXN [frataxin] protein to be restored and biodistribution have recently been defined for the heart, it is unclear if FXN overexpression could be harmful,” the investigators wrote.
To investigate if high levels of frataxin driven by gene therapy could have a negative impact on heart health, researchers in France gave intravenous injections containing increasing doses of a frataxin gene construct to mice.
Researchers created two constructs with the sequence of the human FXN gene, which then were transported and delivered to cells in the animals’ hearts by an AAV vector called AAVRh.10.
These experiments were performed in healthy animals (wild-type mice) and in mice that had been genetically engineered to be unable to produce frataxin in their cardiomyocytes (heart cells). After treatment, levels of frataxin in the heart were measured.
Overall, results demonstrated that when frataxin was within levels ranging from 0.5 to 9 times higher than normal in healthy animals it had no negative effects in their iron metabolism or heart health.
Within those levels, the human version of the protein normalized iron metabolism in animals unable to produce frataxin, as well as the function and organization of their cardiomyocytes.
However, when found at levels 20 times higher than normal, human frataxin caused metabolic issues in both healthy and sick animals. It also induced tissue scarring in the heart and affected the overall function and organization of cardiomyocytes, which started to die. Ultimately, these alterations led to heart problems of different levels of severity.
“The severity of this cardiotoxicity appeared to be proportional to the level of FXN overexpression, but also to the proportion and extent of cardiomyocytes affected throughout the heart,” the researchers wrote.
They also discovered these abnormalities in the function of cardiomyocytes seemed to be driven by defects in mitochondria function and internal organization.
“Overall, this study underlines the need, during the development of gene therapy approaches, to consider appropriately vector expression level, long term safety and biomarkers to monitor such events,” the investigators concluded.
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