Reanalysis of genetic testing uncovered a rare inherited MGME1 variant that disrupts the same mitochondrial DNA maintenance pathway as thymidine kinase 2 deficiency (TK2d), helping explain years of unexplained symptoms and severe heart disease in two siblings, according to a study published recently in the American Journal of Medical Genetics.
These results place this family’s condition within the growing spectrum of disorders related to TK2-dependent mitochondrial function and highlight the real-world impact of revisiting genetic data.
The siblings were found to carry a homozygous MGME1 variant, NM_052865.4:c.818 T>A; p.(Val273Glu). Both presented with ptosis (drooping eyelids), muscle weakness, scoliosis and chronic gastrointestinal symptoms.
These features closely resemble those seen in TK2d, a mitochondrial disorder caused by impaired nucleotide supply needed to maintain mitochondrial DNA. MGME1 works in the same biological pathway, helping process and repair mitochondrial DNA, which explains the clinical overlap.
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Despite sharing the same genetic change, the siblings were affected very differently. One developed progressive dilated cardiomyopathy, a condition also reported in severe TK2d. The heart failure advanced despite treatment and ultimately required orthotopic heart transplantation. The other sibling experienced neuromuscular and gastrointestinal symptoms but did not develop advanced cardiac disease, illustrating the variable expressivity typical of TK2-related mitochondrial disorders.
“[T]his report underscores the importance of periodic genomic data reanalysis and highlights the variable expressivity that may occur even within the same family,” explained the authors of this study.
Importantly, the MGME1 variant was not identified during the initial whole-exome sequencing analysis. Only after reanalysis, using updated databases and improved understanding of mitochondrial disease genes, was the variant detected. Sanger sequencing confirmed that both siblings carried two copies. Although the clinical picture overlaps with known MGME1– and TK2-related disease, the variant remains classified as a variant of uncertain significance, meaning functional studies are still needed to confirm its exact role.
For patients, this report underscores that mitochondrial disorders linked to the TK2 pathway can be missed on first-pass genetic testing. A negative result does not rule out a genetic cause, particularly in rare diseases where knowledge is evolving. Periodic reanalysis can uncover diagnoses that change monitoring, treatment decisions and family planning.
Clinically, the findings suggest that patients with suspected TK2d or related mitochondrial disease who have muscle weakness, digestive symptoms or unexplained cardiomyopathy may benefit from ongoing genetic review. Early recognition of involvement in the TK2 pathway could prompt closer heart surveillance and earlier intervention, potentially improving outcomes for patients and their families.
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