Correcting Just Half the Heart Muscle Cells Fully Restores Cardiac Function in Friedreich’s Ataxia Mice

A gene therapy approach to increase the amount of the critical frataxin protein shows that correcting only half of the heart muscle cells, or cardiomyocytes, is enough to fully restore the cardiac function in a mouse model of Friedreich’s Ataxia (FA)

The study, “Correction of half the cardiomyocytes fully rescue Friedreich Ataxia mitochondrial cardiomyopathy through cell-autonomous mechanisms,” was published in the journal Human Molecular Genetics.

FA is characterized by reduced level of the frataxin protein due to a mutation in both copies of the FXN gene. Frataxin deficiency causes a deficit in iron sulfur clusters (Fe-S, crucial for functions such as iron metabolism and energy production in cells), impaired activity of specific enzymes, dysfunction of mitochondria (the cellular power plants), and iron overload.

Cardiac dysfunction and anomalies are common in patients with FA. The research team previously showed that delivering the normal FXN gene to cardiac cells via modified, harmless viral vectors prevents and rapidly reverses cardiac alterations in a mouse model (Mck) of cardiomyopathy (heart muscle disease) in FA.

These mice showed levels of frataxin 24-fold greater than normal, which may carry safety issues in clinical trials and may not be required in patients, as asymptomatic carriers of the mutated FXN gene have nearly half the normal amount of protein. As such, finding the therapeutic thresholds for these vectors in the heart is key, the team from France considered.

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Aiming to address this, the scientists used the MCk mouse model to evaluate the extent of cardiac function rescue following the administration of lowering doses of an adeno-associated viral vector (AAV) carrying the human FXN gene. To assess how disease progression affected therapeutic success, the researchers used mice with early (five weeks of age) or advanced (seven weeks) cardiac dysfunction.

The team further characterized the vector’s pharmacological profile in the heart, and established the minimal cardiac biodistribution — meaning the vector copies per cell (VCN) and the percentage of cardiomyocytes rescued — to restore cardiac function.

The mice were given one of six 2-2.5-fold decreasing doses and were followed for seven to eight weeks. When treated with 5 x 10¹³ and 2.5 x 10¹³ vg/kg doses at five weeks, the mice survived beyond the median survival of untreated mice, and showed normal body weight and echocardiography parameters.

Mice given lower vector doses had partial or no rescue, but the also lower 5 x 10¹² vg/kg dose provided meaningful therapeutic effects, including stabilized cardiac function and hypertrophy (enlargement), and decreased severity of co-morbidities associated with heart failure, such as arrhythmia, or changed heart rhythm.

Mice administered with the vector at seven weeks showed full therapeutic benefit only when treated with 2.5 x 10¹³ vg/kg, higher than the minimum dose having the same effect in younger mice (1 x 10¹³ vg/kg). However, this lower dose still extended lifespan by 57%, despite being associated with severe cardiac dysfunction. Assessments of heart fibrosis, or scarring, and changes within the cardiomyocytes showed the same pattern in regards to effective doses.

Then, the team found a dose-dependent increase in VCN as well as in levels of frataxin protein and messenger RNA (generated from DNA to give rise to proteins). Restored cardiac function was seen with levels of vector-derived frataxin in the heart lower than the normal amount in healthy mice, suggesting “that only a moderate increase in [frataxin] is necessary,” the scientists wrote.

Importantly, the scientists subsequently found that to fully rescue cardiac function, only half the cardiomyocytes needed to receive the vector (transduction), as evidenced by the correlation among the vector’s distribution in the heart, survival, cardiac function, and cardiac biomarkers of disease.

Also, they observed that the dose of vector was positively correlated with the heart surface with restored enzyme activity, the clearance or prevention of iron deposits in cardiomyocytes, and corrected plasma level of GDF15 — a molecule whose increased levels result from mitochondrial failure.

The data further showed that meaningful benefit was achieved with as little as 30% of cardiomyocytes treated in mice at both early and advanced cardiac dysfunction. The restored benefit in iron metabolism and mitochondrial function was mediated via cell-autonomous processes — meaning that only cells receiving the vector showed improvements.

“This appears as a crucial consideration for the clinical trial design of cardiac gene therapy or other similar therapeutic strategies, as their success might also depend directly on the percentage of cardiomyocytes treated,” the scientists wrote.

Overall, the results “provide a comprehensive framework for the preclinical development of FA cardiac gene therapy protocols” and subsequently to the “successful design of clinical trial[s],” they added.