Expanding the genetic spectrum of mitochondrial diseases in Tunisia: novel variants revealed by whole-exome sequencing

Introduction: Inherited mitochondrial diseases are the most common group of metabolic disorders caused by a defect in oxidative phosphorylation. They are characterized by a wide clinical and genetic spectrum and can manifest at any age. In this study, we established novel phenotype-genotype correlations between the clinical and molecular features of a cohort of Tunisian patients with mitochondrial diseases. Materials and methods: Whole-exome sequencing was performed on five Tunisian patients with suspected mitochondrial diseases. Then, a combination of filtering and bioinformatics prediction tools was utilized to assess the pathogenicity of genetic variations. Sanger sequencing was subsequently performed to confirm the presence of potential deleterious variants in the patients and verify their segregation within families. Structural modeling was conducted to study the effect of novel variants on the protein structure. Results: We identified two novel homozygous variants in NDUFAF5 (c.827G>C; p.Arg276Pro) and FASTKD2 (c.496_497del; p.Leu166GlufsTer2) associated with a severe clinical form of Leigh and Leigh-like syndromes, respectively. Our results further disclosed two variants unreported in North Africa, in GFM2 (c.569G>A; p.Arg190Gln) and FOXRED1 (c.1261G>A; p.Val421Met) genes, and we described the first case of fumaric aciduria in a Tunisian patient harboring the c.1358T>C; p.Leu453Pro FH variant. Conclusion: Our study expands the mutational and phenotypic spectrum of mitochondrial diseases in Tunisia and highlights the importance of next-generation sequencing to decipher the pathomolecular mechanisms responsible for these disorders in an admixed population.

Α rare case of myopathy, lactic acidosis, and severe rhabdomyolysis, due to a homozygous mutation of the ferredoxin-2 (FDX2) gene

Mitochondrial myopathy is a severe metabolic myopathy related to nuclear or mitochondrial DNA dysfunction. We present a rare case of mitochondrial myopathy, presented with multiple episodes of proximal muscle weakness, lactic acidosis, and severe rhabdomyolysis (CPK 319,990 U/L, lactic acid 22.31 mmol/L, and GFR 3.82 mL/min/1.73m2 ). She was hospitalized in the pediatric intensive care unit due to acute kidney injury, elevated blood pressure, and deterioration of respiratory and cardiac function. Investigation for inherited metabolic disorders showed elevated levels of ammonia, lactic acid to pyruvic acid ratio, and urine ketone bodies. Exome sequencing detected a homozygous pathogenic variant in FDX2 (ENST00000541276:p.Met4Leu/c.10A > T) and a heterozygous variant of uncertain significance in MSTO1 (ENST00000538143:p.Leu137Pro/c.410 T > C). After Sanger sequencing, the p.Met4Leu pathogenic variant in FDX2 (ENST00000541276:p.Met4Leu/c.10A > T) was identified in a heterozygous state in both her parents and sister. Recently, pathogenic variants in the FDX2 gene have been associated with mitochondrial myopathy, lactic acidosis, optic atrophy, and leukoencephalopathy. Only four reports of FDX2-related rhabdomyolysis have been described before, but none of the previous patients had hyperammonemia. This is a rare case of severe mitochondrial myopathy in a pediatric patient related to a pathogenic FDX2 variant, suggesting the need for genetic analysis of the FDX2 gene in cases of suspicion of mitochondrial myopathies.

Rare presentation of FDX2-related disorder and untargeted global metabolomics findings

We present the case of a 20-year-old male with a history of myopathy and multiple episodes of rhabdomyolysis, and lactic acidosis. He needed hemodialysis for severe rhabdomyolysis-related acute renal failure at the time of initial presentation (age 10 years). Exome sequencing detected a homozygous likely pathogenic variant in FDX2 (c.12G>T, p.M4I). The FDX2 gene encodes a mitochondrial protein, ferredoxin 2, that is involved in the biogenesis of Fe-S clusters. Biallelic pathogenic variants in FDX2 have previously been associated with episodic mitochondrial myopathy with or without optic atrophy and reversible leukoencephalopathy. Only two cases with FDX2-related rhabdomyolysis as a predominant feature have been reported in medical literature. Here, we report a third patient with FDX2-related recurrent, severe episodes of rhabdomyolysis and lactic acidosis. He does not have optic atrophy or leukoencephalopathy. This is the oldest patient reported with FDX2-related disorder and he has significantly elevated CK during episodes of rhabdomyolysis. In addition, we describe untargeted global metabolomic findings during an episode of metabolic decompensation, shedding light on the biochemical pathway perturbation associated with this ultra-rare genetic disorder.

Whole-exome sequencing in adult patients with developmental and epileptic encephalopathy: It is never too late

Developmental and epileptic encephalopathies (DEE) encompass rare, sporadic neurodevelopmental disorders and usually with pediatric onset. As these conditions are characterized by marked clinical and genetic heterogeneity, whole-exome sequencing (WES) represents the strategy of choice for the molecular diagnosis. While its usefulness is well established in pediatric DEE cohorts, our study is aimed at assessing the WES feasibility in adult DEE patients who experienced a diagnostic odyssey prior to the advent of this technique. We analyzed exomes from 71 unrelated adult DEE patients, consecutively recruited from an Italian cohort for the EPI25 Project. All patients underwent accurate clinical and electrophysiological characterization. An overwhelming percentage (90.1%) had already undergone negative genetic testing. Variants were classified according to the American College of Medical Genetics and Genomics guidelines. WES disclosed 24 (likely) pathogenic variants among 18 patients in epilepsy-related genes with either autosomal dominant, recessive or X-linked inheritance. Ten of these were novel. We obtained a diagnostic yield of 25.3%, higher among patients with brain malformations, early-onset epilepsy and dysmorphisms. Despite a median diagnostic delay of 38.7 years, WES analysis provided the long-awaited diagnosis for 18 adult patients, which also had an impact on the clinical management of 50% of them.

The Mitochondrial Dysfunction Hypothesis in Autism Spectrum Disorders: Current Status and Future Perspectives

Autism spectrum disorders (ASDs) constitute a set of heterogeneous neurodevelopmental conditions, characterized by a wide genetic variability that has led to hypothesize a polygenic origin. The metabolic profiles of patients with ASD suggest a possible implication of mitochondrial pathways. Although different physiological and biochemical studies reported deficits in mitochondrial oxidative phosphorylation in subjects with ASD, the role of mitochondrial DNA variations has remained relatively unexplored. In this review, we report and discuss very recent evidence to demonstrate the key role of mitochondrial disorders in the development of ASD.