Respiratory chain deficiency in nonmitochondrial disease

RESULTS

Patient 1 presented with generalized muscular hypotonia, failure to thrive, liver dysfunction, atrial septal defect, and severe immunodeficiency. Based on the biochemical defect of complex I (50%) and IV (80% rest-activity) in skeletal muscle, mitochondrial disease was suggested. Exome sequencing identified a homozygous mutation c.587T>C, p.Leu196Pro in the ORAI1 gene, which is predicted to be deleterious with 6 different prediction programs (table e-2) and is located next to a pathogenic mutation (p.Leu194Pro).³ ORAI1 encodes a calcium release-activated calcium channel protein that allows cellular calcium influx³ and may alter mitochondrial calcium metabolism, leading to RC deficiency. Autosomal recessive mutations in ORAI1 have previously been described in association with severe combined immunodeficiency and congenital myopathy and in dominantly inherited tubular aggregate myopathy with hypocalcemia.³

Patient 2 presented with a juvenile-onset slowly progressive myopathy and limb-girdle muscle weakness. The clinical diagnosis of a mitochondrial myopathy was based on the detection of cytochrome c oxidase–negative fibers and decreased complex IV activity in muscle (80% rest-activity). A known pathogenic homozygous c.1715G>A, p.Arg572Gln mutation was detected in CAPN3 encoding a skeletal muscle-specific intracellular calcium-activated neutral protease, and its deficiency leads to oxidative stress and potentially a secondary RC deficiency.⁴ This is a previously published pathogenic mutation and the only variant that segregated with the disease in the family. CAPN3 mutations are common causes of limb-girdle muscular dystrophy type 2A (LGMD2A).⁴ Muscle biopsy shows a dystrophic pattern; however, disorganized mitochondria and reduced complex V levels were occasionally reported in a few patients with LGMD2A.⁴

Patient 3 presented with axial muscle weakness, ptosis, respiratory failure requiring ventilation, and reduced IV (80% rest-activity) and borderline complex I activities. We detected a homozygous frameshift mutation c.636delA, p.Lys212fs in the COLQ gene. Mutations of the acetylcholinesterase collagen-like tail subunit gene (COLQ) cause congenital myasthenic syndromes due to the deficiency of acetylcholine esterase at the synaptic endplate.⁵ The clinical phenotype includes hypotonia, muscle weakness, delayed motor development, ophthalmoparesis, and respiratory failure. No involvement of mitochondria has been suggested in previous patients to date.⁵

Patient 4 belonged to a large Hungarian Roma pedigree presenting with hypomyelination, hypoplasia of the cerebellum and corpus callosum, and spinal muscular atrophy. Muscle biopsy revealed reduced complex I (60% rest-activity) and IV (80% rest-activity). We identified a homozygous c.815G>C, p.Ser272Thr mutation in a novel disease gene EXOSC8 encoding an exosome subunit involved in AU-rich element (ARE)-containing messenger RNA (mRNA) degradation, and the altered myelination was due to abnormal degradation of ARE myelin proteins.⁶ The pathogenicity of the mutation was supported by the absence of EXOSC8 protein in myoblasts of the patient and by segregation with the disease in the large pedigree.⁶ A secondary RC defect in these patients may be related to abnormal degradation of ARE mRNAs of mitochondrial disease genes.

Patient 5 presented in childhood with epilepsy and learning difficulties and developed cerebellar ataxia, brisk reflexes, and nystagmus from age 45 years. Complex III defect (60% rest-activity) and low coenzyme Q₁₀ (CoQ₁₀) (60% of low normal) were detected in her muscle. We identified compound heterozygous pathogenic mutations in ANO10 (c.132_133insT, p.Asp45Arg fs*53 and c.1843G>A, p.Asp615Asn).⁷ The pathogenicity of the missense variant c.1843G>A, p.Asp615Asn has been confirmed by its detection in additional patients.⁸ CoQ₁₀ supplementation resulted in clinical improvement, emphasizing the importance of low CoQ₁₀ in the pathomechanism.⁷ ANO10 encodes an anoctamin protein forming calcium-activated chloride channels, implying that an abnormal calcium metabolism may cause mitochondrial dysfunction in ANO10 deficiency.⁷