Neurological manifestations and genotype-phenotype correlations in NDUFAF6-associated mitochondrial disease.

NDUFAF6 encodes a mitochondrial complex I assembly factor essential for the proper biogenesis and stability of the nicotinamide adenine dinucleotide (NAD) + hydrogen (H) (NADH)-ubiquinone oxidoreductase complex. Pathogenic variants in NDUFAF6 have been increasingly recognized as a cause of mitochondrial disease, particularly Leigh syndrome, a severe neurodegenerative disorder characterized by bilateral symmetrical lesions in the central nervous system. To date, fewer than 50 patients with NDUFAF6-related mitochondrial disease have been reported, displaying a broad phenotypic spectrum ranging from early-onset neurodevelopmental regression to milder, more chronic presentations. The molecular mechanisms underlying these phenotypes are linked to impaired complex I assembly and reduced enzymatic activity, highlighting the critical role of NDUFAF6 in mitochondrial function. Here we present a cohort of 27 patients (14 males and 13 females) from 18 families harbouring biallelic variants in the NDUFAF6 gene. The patient's mean age was 9.15 ± 8.30 years (range: 4 weeks to 25 years); 12 patients (37%) had died by the time the data were collected for this article. The clinical presentation showed wide phenotypic variability, from mild to severe psychomotor regression (74%) most commonly before the age of 5 years, hypotonia (22%), movement disorders (30%), and hypertonia (15%). Bilateral striatal necrosis lesions were the most characteristic features on cranial MRI (67%) although white matter abnormalities were also noted (15%), occasionally accompanied by cystic formations, suggestive of early neurodevelopmental anomalies. Genomic sequencing was applied, leading to the identification of 19 distinct variants in the NDUFAF6 gene, including nine novel variants not previously reported and either absent or extremely rare in public population databases. Functional studies confirmed the pathogenicity of these variants, demonstrating a deleterious effect on NDUFAF6 protein expression and a consequent impairment in complex I assembly and stability. To date, this represents the largest reported cohort of patients with NDUFAF6-associated mitochondrial disease. Our findings provide a comprehensive overview of clinical characteristics-including age of symptom onset, phenotypic variability, and patient outcomes-aiming to improve prognostic information and facilitate genetic counselling in clinical practice.