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Pathogenic mutations in NDUFAF4 have been reported in very few cases. Here we present new data to further delineate the phenotypic spectrum of NDUFAF4 deficiency. We describe two siblings presenting with facial dysmorphia and lactic acidosis in the neonatal period. Later on, they developed fatal early encephalopathy with apneic episodes, irritability, central hypoventilation, liver involvement and hyperammonemia. Abnormality of the cerebral white matter was demonstrated in one case, and cardiomyopathy in the other. Urine organic acid profile showed an increased excretion of lactate, Krebs cycle metabolites and 3-methylglutaconate. Whole-exome sequencing identified a novel homozygous nonsense mutation in NDUFAF4 (c.478G > T; p.Glu160Ter), encoding a mitochondrial complex I assembly factor. The disruptive effect of the mutation was corroborated by the absence of NDUFAF4 expression in patient fibroblasts. OXPHOS assembly studies demonstrated almost undetectable levels of fully assembled complex I and complex I–containing supercomplexes and an abnormal accumulation of SCIII2IV1 supercomplexes. Morphologically, fibroblasts showed rounder mitochondria and a diminished degree of branching of the mitochondrial network. Cellular respiratory capacity in fibroblasts was also markedly reduced. In sum, we provide insights into the physiopathological mechanisms underlying NDUFAF4 deficiency and expand the knowledge about the clinical and biochemical spectrum of this disorder.

3-Methylglutaconic aciduria (3-MGA-uria) syndromes comprise a heterogeneous group of diseases associated with mitochondrial membrane defects. Whole-exome sequencing identified compound heterozygous mutations in TIMM50 (c.[341 G>A];[805 G>A]) in a boy with West syndrome, optic atrophy, neutropenia, cardiomyopathy, Leigh syndrome, and persistent 3-MGA-uria. A comprehensive analysis of the mitochondrial function was performed in fibroblasts of the patient to elucidate the molecular basis of the disease. TIMM50 protein was severely reduced in the patient fibroblasts, regardless of the normal mRNA levels, suggesting that the mutated residues might be important for TIMM50 protein stability. Severe morphological defects and ultrastructural abnormalities with aberrant mitochondrial cristae organization in muscle and fibroblasts were found. The levels of fully assembled OXPHOS complexes and supercomplexes were strongly reduced in fibroblasts from this patient. High-resolution respirometry demonstrated a significant reduction of the maximum respiratory capacity. A TIMM50-deficient HEK293T cell line that we generated using CRISPR/Cas9 mimicked the respiratory defect observed in the patient fibroblasts; notably, this defect was rescued by transfection with a plasmid encoding the TIMM50 wild-type protein. In summary, we demonstrated that TIMM50 deficiency causes a severe mitochondrial dysfunction by targeting key aspects of mitochondrial physiology, such as the maintenance of proper mitochondrial morphology, OXPHOS assembly, and mitochondrial respiratory capacity.

Recessive HACE1 mutations are associated with a severe neurodevelopmental disorder (OMIM: 616756). However, the physiopathologycal bases of the disease are yet to be completely clarified. Whole-exome sequencing identified homozygous HACE1 mutations (c.240C>A, p.Cys80Ter) in a patient with brain atrophy, psychomotor retardation and 3-methylglutaconic aciduria, a biomarker of mitochondrial dysfunction. To elucidate the pathomechanisms underlying HACE1 deficiency, a comprehensive molecular analysis was performed in patient fibroblasts. Western Blot demonstrated the deleterious effect of the mutation, as the complete absence of HACE1 protein was observed. Immunofluorescence studies showed an increased number of LC3 puncta together with the normal initiation of the autophagic cascade, indicating a reduction in the autophagic flux. Oxidative stress response was also impaired in HACE1 fibroblasts, as shown by the reduced NQO1 and Hmox1 mRNA levels observed in H2O2-treated cells. High levels of lipid peroxidation, consistent with accumulated oxidative damage, were also detected. Although the patient phenotype could resemble a mitochondrial defect, the analysis of the mitochondrial function showed no major abnormalities. However, an important increase in mitochondrial oxidative stress markers and a strong reduction in the mitophagic flux were observed, suggesting that the recycling of damaged mitochondria might be targeted in HACE1 cells. In summary, we demonstrate for the first time that the impairment of autophagy, mitophagy and oxidative damage response might be involved in the pathogenesis of HACE1 deficiency.

Isolated complex I (CI) deficiency is the most common cause of oxidative phosphorylation (OXPHOS) dysfunction. Whole-exome sequencing identified biallelic mutations in NDUFA8 (c.[293G > T]; [293G > T], encoding for an accessory subunit of CI, in two siblings with a favorable clinical evolution. The individuals reported here are practically asymptomatic, with the exception of slight failure to thrive and some language difficulties at the age of 6 and 9 years, respectively. These observations are remarkable since the vast majority of patients with CI deficiency, including the only NDUFA8 patient reported so far, showed an extremely poor clinical outcome. Western blot studies demonstrated that NDUFA8 protein was strongly reduced in the patients' fibroblasts and muscle extracts. In addition, there was a marked and specific decrease in the steady-state levels of CI subunits. BN-PAGE demonstrated an isolated defect in the assembly and the activity of CI with impaired supercomplexes formation and abnormal accumulation of CI subassemblies. Confocal microscopy analysis in fibroblasts showed rounder mitochondria and diminished branching degree of the mitochondrial network. Functional complementation studies demonstrated disease-causality for the identified mutation as lentiviral transduction with wild-type NDUFA8 cDNA restored the steady-state levels of CI subunits and completely recovered the deficient enzymatic activity in immortalized mutant fibroblasts. In summary, we provide additional evidence of the involvement of NDUFA8 as a mitochondrial disease-causing gene associated with altered mitochondrial morphology, CI deficiency, impaired supercomplexes formation, and very mild progression of the disease.

Congenital disorders of glycosylation (CDG) are a heterogeneous and rapidly growing group of diseases caused by abnormal glycosylation of proteins and/or lipids. Mutations in genes involved in the homeostasis of the endoplasmic reticulum (ER), the Golgi apparatus (GA), and the vesicular trafficking from the ER to the ER-Golgi intermediate compartment (ERGIC) have been found to be associated with CDG. Here, we report a patient with defects in both N- and O-glycosylation combined with a delayed vesicular transport in the GA due to mutations in TRAPPC11, a subunit of the TRAPPIII complex. TRAPPIII is implicated in the anterograde transport from the ER to the ERGIC as well as in the vesicle export from the GA. This report expands the spectrum of genetic alterations associated with CDG, providing new insights for the diagnosis and the understanding of the physiopathological mechanisms underlying glycosylation disorders.

We report the case of a 10-year-old Spanish girl with mutations in NADK2. Prenatal central nervous system abnormalities showed ventriculomegaly, colpocephaly, and hypoplasia of the corpus callosum. At birth, axial hypotonia, uncoordinated movements, microcephaly, and generalized cerebellar atrophy were detected. Metabolic investigations revealed high lysine, lactate, and pipecolic acid levels in blood and cerebrospinal fluid. Pyruvate carboxylase and pyruvate dehydrogenase activity in fibroblasts were normal. Beginning at birth she received biotin, thiamine, and carnitine supplementation. A lysine-restricted diet was started when she was 1 month old. Because pipecolic acid was high, pyridoxine was added to treatment. At 3 years old, astatic myoclonic epilepsy appeared, with no response to levetiracetam. We switched pyridoxine to pyridoxal phosphate, with electroclinical improvement. Because the activity of mitochondrial respiratory chain complexes III and IV was slightly low in muscle, other cofactors such as ubidecarenone, idebenone, vitamin E, and creatine were added to the treatment. At 8 years old, plasma acylcarnitine testing was performed, and high levels of 2-trans, 4-cis-decadienoylcarnitine were found. Whole exome sequencing identified a homozygous splice site mutation in NADK2 (c.956+6T>C; p.Trp319Cysfs*21). This substitution generates exon skipping, leading to a truncated protein. In fact, NADK2 messenger RNA and the corresponding protein were almost absent. Now, at 10 years of age she presents with ataxia and incoordination. She has oromotor dysphasia but is able to understand fluid language and is a very friendly girl. We hypothesize that the patient’s clinical improvement could be due to her lysine-restricted diet together with cofactors and pyridoxal phosphate administration.