A new review published in The American Journal of Medicine examines why patients with mitochondrial diseases, including thymidine kinase 2 deficiency (TK2d), may present very differently depending on their age at diagnosis, with adults often experiencing milder symptoms than children.
Mitochondrial diseases arise from mutations in either mitochondrial DNA or nuclear DNA that disrupt the body’s ability to produce adenosine triphosphate (ATP), the molecule that powers virtually every cellular process. There are over 350 known mutations that can cause this disruption, and they do not affect all patients equally.
Read more about signs and symptoms of TK2d
The type of DNA affected differs significantly between age groups. According to the review, approximately 80% of mitochondrial metabolic dysfunction in adults is caused by mutations in mitochondrial DNA, whereas in children, 80% of cases involve mutations in nuclear DNA.
Nuclear DNA exists in only two copies per cell, meaning mutations in both copies leave no functional backup. Mitochondrial DNA, by contrast, is present in thousands of copies per cell. This allows a mixture of healthy and mutant copies to coexist, a state known as heteroplasmy.
Research explains that the percentage of heteroplasmy is a major factor in determining clinical severity in mitochondrial DNA mutations. Full disease manifestation typically occurs only when mutant copies exceed 80% of the total, while minimal to no abnormalities are found at levels below 50%.
Even when mutant DNA is present, a decrease in mitochondrial function may not always be apparent, as the body can draw on several compensatory mechanisms. When normal energy production is disrupted, cells can partially compensate by activating alternative energy sources. Cells also continuously remove damaged mitochondria and replace them with healthy ones, helping maintain overall mitochondrial function.
For adults, however, these compensatory mechanisms are not indefinitely reliable. The review notes that adult variants of mitochondrial disease can become significantly more severe during concurrent stressful illnesses, as conditions such as systemic infection, stroke or a heart attack may increase mutant gene expression and impair the body’s ability to compensate.
This pattern is particularly relevant for older adults, in whom mitochondrial DNA naturally accumulates additional mutations and deletions over time, further reducing the margin of compensation.
For patients and families living with TK2d and related mitochondrial conditions, these findings carry a practical message. When symptoms first appear in adulthood, the milder symptoms reflect a balance between genetic mutation load and the body’s ability to compensate, a balance that can shift with age and illness.
Adults with previously silent or mild mitochondrial dysfunction who develop complications during stressful illnesses may benefit from newly approved medications, as detecting “hidden” mitochondrial dysfunction in this population remains an evolving area of medicine.
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