Clinical characteristics: SUCLA2-related mitochondrial DNA (mtDNA) depletion syndrome, encephalomyopathic form with methylmalonic aciduria is characterized by onset of the following features in infancy or childhood (median age of onset 2 months; range of onset birth to 6 years): psychomotor retardation, hypotonia, dystonia, muscular atrophy, sensorineural hearing impairment, postnatal growth retardation, and feeding difficulties. Other, less frequent, features include distinctive facial features, contractures, kyphoscoliosis, gastroesophageal reflux, ptosis, choreoathetosis, ophthalmoplegia, and epilepsy (infantile spasms or generalized convulsions). The median survival is 20 years; approximately 30% of affected individuals succumb during childhood. Affected individuals may have hyperintensities in the basal ganglia, cerebral atrophy, and leukoencephalopathy on head MRI. Elevation of methylmalonic acid (MMA) in the urine and plasma is found in a vast majority of affected individuals, although at levels that are far below those typically seen in individuals with classic methylmalonic aciduria.
Diagnosis/testing: The diagnosis of SUCLA2-related mtDNA depletion syndrome is established in a proband by the identification of biallelic pathogenic variants in SUCLA2 on molecular genetic testing.
Management: Treatment of manifestations: Physical therapy to maintain muscle function and prevent joint contractures; antiepileptic drugs for seizures; nasogastric or gastrostomy tube as needed to assure adequate caloric intake; chest physiotherapy, aggressive antibiotic treatment of chest infections, and respiratory aids such as assisted nasal ventilation or use of a tracheostomy and ventilator when indicated; bracing to treat scoliosis or kyphosis; blepharoplasty for significant ptosis; and hearing aids/cochlear implantation for sensorineural hearing loss. Surveillance: Routine monitoring of development, growth, and hearing; periodic ophthalmologic evaluations; routine skeletal evaluations for kyphoscoliosis and joint contractures.
Genetic counseling: SUCLA2-related mtDNA depletion syndrome is inherited in an autosomal recessive manner. At conception, each sib of an affected individual has a 25% chance of being affected, a 50% chance of being an asymptomatic carrier, and a 25% chance of being unaffected and not a carrier. Carrier testing for at-risk relatives and prenatal testing for pregnancies at increased risk are possible once the pathogenic variants in the family have been identified.
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