Progressive external ophthalmoplegia in southwestern Finland: a clinical and genetic study

Incidence and prevalence of mtDNA-related adult mitochondrial disease in Southwest Finland, 2009-2022: an observational, population-based study

Background

Mitochondrial diseases are common inherited metabolic disorders. Due to improved case ascertainment and diagnosis methods, the detection of new diagnoses of mitochondrial disease can be expected to increase. In December 2009, the prevalence of mitochondrial DNA (mtDNA)-related mitochondrial disease was 4.6/100 000 (95% CI, 2.7 to 7.2) in the adult population of Southwest Finland. We investigated the number of new diagnoses and the incidence of mitochondrial disease in Southwest Finland between December 2009 and December 2022.

Methods

We collected data on all adult patients from Southwest Finland diagnosed with mitochondrial disease on 31 December 2009 and 31 December 2022. Most patients had been diagnosed at the Turku University Hospital (TUH) neurology outpatient clinic. Patients were also identified by searching the TUH electronic patient database for relevant International Classification of Diseases, Tenth Revision codes and conducted mtDNA analyses.

Results

42 new patients were diagnosed giving a mean annual rate of 3.2 new diagnoses. In 2022, the minimum prevalence estimate of adult mtDNA-related mitochondrial disease was 9.2/100 000 (95% CI, 6.5 to 12.7). The prevalence of adult mtDNA disease associated with m.3243A>G was 4.2/100 000 (95% CI, 2.5 to 6.7), and that with large-scale mtDNA deletions was 1.3/100 000 (95% CI, 0.4 to 2.9). During the 13-year period, the annual incidence of adult mtDNA disease was 0.6/100 000 and that of adult m.3243A>G-related disease 0.3/100 000.

Conclusion

Our results suggest that improved means of diagnostics and dedicated effort increase the detection of mitochondrial disease.

Retrospective assessment of the most common mitochondrial DNA mutations in a large Hungarian cohort of suspect mitochondrial cases

Prevalence estimations for mitochondrial disorders still vary widely and only few epidemiologic studies have been carried out so far. With the present work we aim to give a comprehensive overview about frequencies of the most common mitochondrial mutations in Hungarian patients. A total of 1328 patients were tested between 1999 and 2012. Among them, 882 were screened for the m.3243A > G, m.8344A > G, m.8993T > C/G mutations and deletions, 446 for LHON primary mutations. The mutation frequency in our cohort was 2.61% for the m.3243A > G, 1.47% for the m.8344A > G, 17.94% for Leber's Hereditary Optic Neuropathy (m.3460G > A, m.11778G > A, m.14484T > C) and 0.45% for the m.8993T > C/G substitutions. Single mtDNA deletions were detected in 14.97%, while multiple deletions in 6.01% of the cases. The mutation frequency in Hungarian patients suggestive of mitochondrial disease was similar to other Caucasian populations. Further retrospective studies of different populations are needed in order to accurately assess the importance of mitochondrial diseases and manage these patients.

Peripheral neuropathy predicts nuclear gene defect in patients with mitochondrial ophthalmoplegia

Mitochondrial ophthalmoplegia is a genetically heterogeneous disorder. Horga et al. investigate whether peripheral neuropathy can predict the underlying genetic defect in patients with progressive external ophthalmoplegia. Results indicate that neuropathy is highly predictive of a nuclear DNA defect and that it is rarely associated with single mitochondrial DNA deletions.

Progressive external ophthalmoplegia is a common clinical feature in mitochondrial disease caused by nuclear DNA defects and single, large-scale mitochondrial DNA deletions and is less frequently associated with point mutations of mitochondrial DNA. Peripheral neuropathy is also a frequent manifestation of mitochondrial disease, although its prevalence and characteristics varies considerably among the different syndromes and genetic aetiologies. Based on clinical observations, we systematically investigated whether the presence of peripheral neuropathy could predict the underlying genetic defect in patients with progressive external ophthalmoplegia. We analysed detailed demographic, clinical and neurophysiological data from 116 patients with genetically-defined mitochondrial disease and progressive external ophthalmoplegia. Seventy-eight patients (67%) had a single mitochondrial DNA deletion, 12 (10%) had a point mutation of mitochondrial DNA and 26 (22%) had mutations in either POLG, C10orf2 or RRM2B, or had multiple mitochondrial DNA deletions in muscle without an identified nuclear gene defect. Seventy-seven patients had neurophysiological studies; of these, 16 patients (21%) had a large-fibre peripheral neuropathy. The prevalence of peripheral neuropathy was significantly lower in patients with a single mitochondrial DNA deletion (2%) as compared to those with a point mutation of mitochondrial DNA or with a nuclear DNA defect (44% and 52%, respectively; P < 0.001). Univariate analyses revealed significant differences in the distribution of other clinical features between genotypes, including age at disease onset, gender, family history, progressive external ophthalmoplegia at clinical presentation, hearing loss, pigmentary retinopathy and extrapyramidal features. However, binomial logistic regression analysis identified peripheral neuropathy as the only independent predictor associated with a nuclear DNA defect (P = 0.002; odds ratio 8.43, 95% confidence interval 2.24–31.76). Multinomial logistic regression analysis identified peripheral neuropathy, family history and hearing loss as significant predictors of the genotype, and the same three variables showed the highest performance in genotype classification in a decision tree analysis. Of these variables, peripheral neuropathy had the highest specificity (91%), negative predictive value (83%) and positive likelihood ratio (5.87) for the diagnosis of a nuclear DNA defect. These results indicate that peripheral neuropathy is a rare finding in patients with single mitochondrial DNA deletions but that it is highly predictive of an underlying nuclear DNA defect. This observation may facilitate the development of diagnostic algorithms. We suggest that nuclear gene testing may enable a more rapid diagnosis and avoid muscle biopsy in patients with progressive external ophthalmoplegia and peripheral neuropathy.

Juvenile parkinsonism, hypogonadism and Leigh-like MRI changes in a patient with m.4296G>A mutation in mitochondrial DNA

Leigh syndrome is a mitochondrial disease with considerable clinical and genetic variation. We present a 16-year-old boy with Leigh-like syndrome and broad developmental retardation, parkinsonism and hypogonadism. Sequencing of the entire mitochondrial DNA from blood revealed the m.4296G>A mutation in the MT-TI gene. The mutation was heteroplasmic with a 95% proportion of the mutant genome, while the proportion was 58% in the blood of the patient's clinically healthy mother. Our results suggest that m.4296G>A is pathogenic in humans, and that the phenotype related to this change includes Leigh-like syndrome in adolescence with parkinsonism and hypogonadism, in addition to the previously reported early infantile Leigh syndrome.