Multiple acyl-CoA dehydrogenase deficiency (MADD) is considered an autosomal recessive disorder; yet, recent findings suggest up to 10% of cases may result from heterozygous electron transfer flavoprotein dehydrogenase (ETFDH) variants exhibiting dominant or dominant-like effects. Here, a novel heterozygous ETFDH variant (c.1798A>C, p.Asn600His) was identified within a three-generation family. The grandfather presented with muscular weakness at age 35, and the father developed similar symptoms at 19 following a tonsillectomy. Both were diagnosed with MADD based on muscle biopsies revealing neutral lipid accumulation and acylcarnitine profiles and responded fully to riboflavin therapy (150 mg/day). The two siblings, aged 8 and 10, carry the same mutation and show increased acyl-carnitine levels but remain asymptomatic due to early riboflavin treatment. Skin fibroblasts from affected individuals were immortalized and subjected to normal and reduced riboflavin levels. Gene expression analysis demonstrated unchanged ETFDH RNA but reduced protein levels in mutant cells, particularly under low riboflavin. Structural modelling suggested the Asn600His substitution destabilizes the protein, diminishing its mitochondrial function. Proximity ligation assays indicated a decreased interaction with mitochondrial complex III, while oxygen consumption via fatty acid oxidation was impaired, especially at reduced riboflavin. The novel ETFDH variant found in this family gives a possible dominant pattern of inheritance for MADD, where a single mutant allele impairs the mitochondrial metabolism, particularly under riboflavin-deficient conditions, and highlights the importance of early riboflavin supplementation in preventing clinical symptoms.

Pathogenic variants in MT-RNR1 are classically associated with aminoglycoside-induced sensorineural hearing loss through disruption of the mitochondrial 12S rRNA decoding center. Their contribution to pediatric neurological disease remains poorly defined. We identified a novel homoplasmic MT-RNR1 variant, m.1014A>G, in a patient with drug-resistant focal epilepsy beginning at 9 months of age who remained undiagnosed after trio whole-exome sequencing filtered using an extensive epilepsy gene panel. The variant was also detected in her mother. Structural modeling suggests that the variant, affecting a highly conserved region of the mitochondrial 12S rRNA, may alter the mitochondrial small ribosomal subunit assembly. To investigate its pathogenicity, we performed comprehensive functional studies in patient-derived fibroblasts. Unlike canonical MT-RNR1 variants, m.1014A>G did not confer aminoglycoside toxicity. Functional in vitro studies on patients’ fibroblasts demonstrated increased 12S/16S rRNA ratios, reduced abundance of both mitoribosomal subunit proteins, decreased levels of mitochondrially encoded respiratory-chain subunits, and impaired oxidative phosphorylation. These abnormalities were accompanied by mitochondrial network fragmentation, increased mitochondria–lysosome colocalization suggestive of enhanced mitophagy and reduced mitochondrial DNA copy number. Altogether, the findings are consistent with defective mitoribosome assembly and altered mitochondrial translation, resulting in a bioenergetic defect. Our study points to a previously unrecognized candidate pathogenic mechanism associated with MT-RNR1 and may expand the phenotypic spectrum of MT-RNR1-related disorders to include maternally inherited infantile-onset epileptic encephalopathy. These findings highlight the importance of dedicated mitochondrial DNA analysis in children with unexplained early-onset epilepsy despite negative nuclear genomic testing.

The BCS1L gene encodes a mitochondrial chaperone which inserts the Fe₂S₂ iron–sulfur Rieske protein into the nascent electron transfer complex III. Variants in the BCS1L gene are associated with a spectrum of mitochondrial disorders, ranging from mild to severe phenotypes. Björnstad syndrome, a milder condition, is characterized by sensorineural hearing loss (SNHL) and pili torti. More severe disorders include Complex III Deficiency, which leads to neuromuscular and metabolic dysfunctions with multi-systemic issues and Growth Retardation, Aminoaciduria, Cholestasis, Iron Overload, and Lactic Acidosis syndrome (GRACILE). The severity of these conditions varies depending on the specific BCS1L mutation and its impact on mitochondrial function. This study describes a 27-month-old child with SNHL, proximal renal tubular acidosis, woolly hypopigmented hair, developmental delay, and metabolic alterations. Genetic analysis revealed a homozygous BCS1L variant (c.38A>G, p.Asn13Ser), previously reported in a patient with a more severe phenotype that, however, was not functionally characterized. In this work, functional studies in a yeast model and patient-derived fibroblasts demonstrated that the variant impairs mitochondrial respiration, complex III activity (CIII), and also alters mitochondrial morphology in affected fibroblasts. Interestingly, we unveil a new possible mechanism of pathogenicity for BCS1L mutant protein. Since the interaction between BCS1L and CIII is increased, this suggests the formation of a BCS1L-containing nonfunctional preCIII unable to load RISP protein and complete CIII assembly. These findings support the pathogenicity of the BCS1L c.38A>G variant, suggesting altered interaction between the mutant BCS1L and CIII.