The mutations m.5628T>C and m.8348A>G in single muscle fibers of a patient with chronic progressive external ophthalmoplegia

Progressive external ophthalmoplegia

Progressive external ophthalmoplegia (PEO), characterized by ptosis and impaired eye movements, is a clinical syndrome with an expanding number of etiologically distinct subtypes. Advances in molecular genetics have revealed numerous pathogenic causes of PEO, originally heralded in 1988 by the detection of single large-scale deletions of mitochondrial DNA (mtDNA) in skeletal muscle of people with PEO and Kearns-Sayre syndrome. Since then, multiple point variants of mtDNA and nuclear genes have been identified to cause mitochondrial PEO and PEO-plus syndromes, including mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) and sensory ataxic neuropathy dysarthria ophthalmoplegia (SANDO). Intriguingly, many of those nuclear DNA pathogenic variants impair maintenance of the mitochondrial genome causing downstream mtDNA multiple deletions and depletion. In addition, numerous genetic causes of nonmitochondrial PEO have been identified.

Ancient mitochondrial DNA pathogenic variants putatively associated with mitochondrial disease

Mitochondrial DNA variants associated with diseases are widely studied in contemporary populations, but their prevalence has not yet been investigated in ancient populations. The publicly available AmtDB database contains 1443 ancient mtDNA Eurasian genomes from different periods. The objective of this study was to use this data to establish the presence of pathogenic mtDNA variants putatively associated with mitochondrial diseases in ancient populations. The clinical significance, pathogenicity prediction and contemporary frequency of mtDNA variants were determined using online platforms. The analyzed ancient mtDNAs contain six variants designated as being "confirmed pathogenic" in modern patients. The oldest of these, m.7510T>C in the MT-TS1 gene, was found in a sample from the Neolithic period, dated 5800-5400 BCE. All six have well established clinical association, and their pathogenic effect is corroborated by very low population frequencies in contemporary populations. Analysis of the geographic location of the ancient samples, contemporary epidemiological trends and probable haplogroup association indicate diverse spatiotemporal dynamics of these variants. The dynamics in the prevalence and distribution is conceivably result of de novo mutations or human migrations and subsequent evolutionary processes. In addition, ten variants designated as possibly or likely pathogenic were found, but the clinical effect of these is not yet well established and further research is warranted. All detected mutations putatively associated with mitochondrial disease in ancient mtDNA samples are in tRNA coding genes. Most of these mutations are in a mt-tRNA type (Model 2) that is characterized by loss of D-loop/T-loop interaction. Exposing pathogenic variants in ancient human populations expands our understanding of their origin and prevalence dynamics.

Panel-Based Nuclear and Mitochondrial Next-Generation Sequencing Outcomes of an Ethnically Diverse Pediatric Patient Cohort with Mitochondrial Disease

Mitochondrial disease (MD) is a group of rare inherited disorders with clinical heterogeneous phenotypes. Recent advances in next-generation sequencing (NGS) allow for rapid genetic diagnostics in patients who experience MD, resulting in significant strides in determining its etiology. This, however, has not been the case in many patient populations. We report on a molecular diagnostic study using mitochondrial DNA and targeted nuclear DNA (nDNA) NGS of an extensive cohort of predominantly sub-Saharan African pediatric patients with clinical and biochemically defined MD. Patients in this novel cohort presented mostly with muscle involvement (73%). Of the original 212 patients, a muscle respiratory chain deficiency was identified in 127 cases. Genetic analyses were conducted for these 127 cases based on biochemical deficiencies, for both mitochondrial (n = 123) and nDNA using panel-based NGS (n = 86). As a pilot investigation, whole-exome sequencing was performed in a subset of African patients (n = 8). These analyses resulted in the identification of a previously reported pathogenic mitochondrial DNA variant and seven pathogenic or likely pathogenic nDNA variants (ETFDH, SURF1, COQ6, RYR1, STAC3, ALAS2, and TRIOBP), most of which were identified via whole-exome sequencing. This study contributes to knowledge of MD etiology in an understudied, ethnically diverse population; highlights inconsistencies in genotype-phenotype correlations; and proposes future directions for diagnostic approaches in such patient populations.

Genetic and clinical features of Chinese patients with mitochondrial ataxia identified by targeted next-generation sequencing

AIM

To characterize the mutations in mitochondrial DNA (mtDNA) and mitochondrion-related nuclear genes (nDNA), and clinical features in Chinese patients with mitochondrial ataxia.

METHODS

Targeted next-generation sequencing (NGS) technology was performed to screen the whole mtDNA sequence and nDNA genes in a cohort of 33 unrelated ataxia patients.

RESULTS

A total of 5 pedigrees were finally genetically diagnosed as mitochondrial ataxia, with 3 pathogenic mutations (m.8344A>G, m.9176T>C, and m.9185T>C), one likely pathogenic mutation (m.3995A>G) in mtDNA, and one pathogenic mutation (c.1159_1162dupAAGT, p.Ser388Terfs) in PDHA1. The prevalence of mitochondrial ataxia in our patient cohort is 15.2%. In addition, all 4 patients with mtDNA mutations experienced symptoms of ataxia with age at onset ranging from 12 to 39 years (21 ± 12.2) and developed extrapyramidal symptoms during the disease course. One male patient with pyruvate dehydrogenase deficiency showed an acute intermittent ataxia phenotype.

CONCLUSIONS

Our results implicate that mitochondrial ataxia might not be as rare in Chinese as previously assumed. This study firstly defines the mutations of mitochondrial ataxia in a Chinese population by targeted NGS, which broadens the clinical spectrum of mtDNA mutations and highlights the importance of screening mtDNA and nDNA mutations among undefined ataxia patients.

High glucose uptake unexpectedly is accompanied by high levels of the mitochondrial ß-F1-ATPase subunit in head and neck squamous cell carcinoma

A hallmark of solid tumors is the consumption of large amounts of glucose and production of lactate, also known as Warburg-like metabolism. This metabolic phenotype is typical for aggressive tumor growth, and can be visualized by ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) uptake detected by positron emission tomography (PET). High ¹⁸F-FDG uptake inversely correlates with survival and goes along with reduced expression of the catalytic beta-subunit of the H⁺-ATP synthase (β-F1-ATPase) in several tumor entities analyzed so far.

For this study we characterized a series of 15 head and neck squamous cell carcinoma (HNSCC) by (i) determining ¹⁸F-FDG-uptake; (ii) quantitative expression analysis of β-F1-ATPase (Complex V), NDUF-S1 (Complex I) and COX1 (Complex IV) of the mitochondrial electron transport chain (ETC), as well as Hsp60 (mitochondrial mass) and GAPDH (glycolysis) in tumor cells; (iii) sequencing of the mtDNA of representative tumor samples.

Whereas high ¹⁸F-FDG-uptake also correlates with poor prognosis in HNSCC, it surprisingly is accompanied by high levels of β-F1-ATPase, but not by any of the other analyzed proteins.

In conclusion, we here describe a completely new phenotype of metabolic adaptation possibly enabling those tumors with highest levels of β-F1-ATPase to rapidly proliferate even in hypoxic zones, which are typical for HNSCC.