Primary mitochondrial disorders related to thymidine kinase 2 deficiency (TK2d) remain difficult to diagnose, but a study published recently in Mitochondrion indicated that combining clinical evaluation, targeted genetic testing and simple machine learning tools can improve identification and care. The findings highlight both progress and persistent gaps that directly affect patients with TK2d and similar mitochondrial diseases.
“[T]his study offers new insights into oxidative phosphorylation disorders in an underrepresented region and supports a pragmatic diagnostic approach that integrates meticulous clinical phenotyping, targeted molecular assays, and interpretable machine learning,” explained the authors of this research.
Researchers reviewed 48 Indian patients from 42 unrelated families evaluated between March 2016 and January 2024. Patients were classified into mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS; 17 cases), chronic progressive external ophthalmoplegia (14), Leber hereditary optic neuropathy (10) and Leigh syndrome (7). The mean age at presentation was 23.9 years, ranging from nine months to 60 years, reflecting the wide variability also seen in TK2d.
Symptoms varied widely. Patients with MELAS frequently experienced seizures (94.1%), stroke-like episodes (58.8%) and cognitive decline. Those with chronic progressive external ophthalmoplegia commonly had drooping eyelids and eye movement problems (92.9%).
Read more about signs and symptoms of TK2d
Vision loss occurred in all patients with Leber hereditary optic neuropathy. Leigh syndrome often presented earlier in life with developmental regression. These overlapping features mirror the complexity seen in TK2d, where symptoms can affect muscles, brain and energy metabolism.
Laboratory and imaging findings supported the diagnoses. Elevated blood lactate, a marker of impaired energy production, was common across groups, including 76.5% of MELAS cases. Brain imaging was abnormal in 47.9% of patients, with patterns that matched specific syndromes. However, some groups (such as chronic progressive external ophthalmoplegia) often had normal scans, underscoring how easily mitochondrial diseases can be missed.
Genetic testing confirmed well-known mitochondrial DNA mutations in a subset of 21 patients, including MT-TL1 variants in MELAS, m.11778G>A in MT-ND4 in Leber hereditary optic neuropathy, and m.8993T>G in MT-ATP6 in Leigh syndrome. Still, many patients did not undergo testing due to cost or access barriers, a challenge that also affects individuals with TK2d who require specialized sequencing for diagnosis.
To address diagnostic delays, researchers applied decision tree and random forest models. These tools identified key features such as vision loss, eyelid drooping, seizures and family history as strong predictors of specific syndromes. Because these models are transparent and based on clinical signs, they could help doctors prioritize testing in settings where resources are limited.
For patients with TK2d and related mitochondrial disorders, this study suggests earlier recognition may be possible by combining symptom patterns with targeted testing. However, the authors emphasized that expanding access to advanced genetic testing remains critical to ensure accurate diagnosis, timely care and better long-term outcomes.
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