Patients with primary mitochondrial disease — a category that includes thymidine kinase 2 deficiency (TK2d) — may eventually benefit from a new gene therapy strategy that sharply improved heart function in a severe form of mitochondrial cardiomyopathy, according to preclinical research published recently in Nature Communications.
This study showed that restoring even a small amount of missing mitochondrial activity can reverse heart damage that otherwise greatly increases the risk of early death.
Primary mitochondrial disease patients who develop cardiomyopathy (heart muscle disease) are about twice as likely to die as those without heart involvement. One such condition is caused by null mutations in the SLC25A4 gene, also known as ANT1, which disrupts energy exchange inside mitochondria.
Similar to TK2d, this disorder impairs mitochondrial DNA maintenance and energy production, leading to progressive muscle and heart failure. Disease severity is further shaped by mitochondrial DNA background, helping explain why some patients develop hypertrophic cardiomyopathy, which thickens the heart muscle, while others develop dilated disease, which causes the ventricles to stretch.
Read more about signs and symptoms of TK2d
To model this spectrum, researchers engineered mice lacking Ant1 and combined the defect with a mitochondrial DNA mutation known as ND6P25L. Newborn mice received a single injection of an adeno-associated virus carrying a normal Ant1 gene, delivered directly into the pericardium surrounding the heart. The therapy restored only about 10% of normal Ant1 protein levels, yet this modest increase was enough to markedly improve heart structure and performance.
“Our model system demonstrates the feasibility of AAV [adeno-associated virus]-mediated mitochondrial gene therapy to treat life-threatening PMD [primary mitochondrial disease] cardiomyopathy,” explained this study’s authors.
Over 12 to 14 months of follow-up, treated mice showed higher stroke volume, ejection fraction, fractional shortening and cardiac output, along with reduced left ventricular mass — all signs of a healthier heart. Their heart size stabilized over time, while untreated mice developed worsening enlargement. About 20% of heart muscle cells were corrected, indicating that partial repair can benefit the whole organ. Importantly, the procedure caused no excess inflammation, cell death or tissue injury.
Molecular analyses explained why small gains mattered. Gene and protein studies showed reversal of abnormal mitochondrial metabolism and contractile pathways, including restoration of PGC1α signaling, a master regulator of energy production. Single-cell sequencing confirmed that the most dramatic improvements occurred in ventricular cardiomyocytes, a type of cell within the ventricles, with secondary benefits seen across other heart cell types through cell-to-cell signaling.
For patients, including those with TK2d-associated cardiomyopathy, these findings suggest that future treatments may not need to fully replace a missing gene to be effective. A therapy that safely boosts mitochondrial energy just enough could one day slow or reverse heart failure, improve quality of life and reduce mortality.
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