The natural history of infantile mitochondrial DNA depletion syndrome due to RRM2B deficiency.

PURPOSE: Mitochondrial DNA (mtDNA) depletion syndrome (MDDS) encompasses a group of genetic disorders of mtDNA maintenance. Mutation of RRM2B is an uncommon cause of infantile-onset encephalomyopathic MDDS. Here we describe the natural history of this disease. METHODS: Multinational series of new genetically confirmed cases from six pediatric centers. RESULTS: Nine new cases of infantile-onset RRM2B deficiency, and 22 previously published cases comprised a total cohort of 31 patients. Infants presented at a mean of 1.95 months with truncal hypotonia, generalized weakness, and faltering growth. Seizures evolved in 39% at a mean of 3.1 months. Non-neurological manifestations included respiratory distress/failure (58%), renal tubulopathy (55%), sensorineural hearing loss (36%), gastrointestinal disturbance (32%), eye abnormalities (13%), and anemia (13%). Laboratory features included elevated lactate (blood, cerebrospinal fluid (CSF), urine, magnetic resonance (MR), spectroscopy), ragged-red and cytochrome c oxidase-deficient fibers, lipid myopathy, and multiple oxidative phosphorylation enzyme deficiencies in skeletal muscle. Eight new RRM2B variants were identified. Patients with biallelic truncating variants had the worst survival. Overall survival was 29% at 6 months and 16% at 1 year. CONCLUSIONS: Infantile-onset MDDS due to RRM2B deficiency is a severe disorder with characteristic clinical features and extremely poor prognosis. Presently management is supportive as there is no effective treatment. Novel treatments are urgently needed.

Cognitive impairment and reduced life span of oculopharyngeal muscular dystrophy homozygotes.

OBJECTIVE: To assess the evolution and life expectancy in patients with oculopharyngeal muscular dystrophy (OPMD) who are homozygotes for two (GCN)13 expansions in the PABPN1 encoding gene. BACKGROUND: OPMD is particularly frequent among French Canadians (FCs) and Uzbek Jews (UJs), who carry a same size, (GCN)13, PABPN1 mutation. The high rate of consanguinity among UJs together with late disease onset and normal fertility results in homozygous cases. METHODS: For 15 to 20 years, we followed 4 FC and 6 UJ homozygotes with OPMD and compared them with their heterozygous parents and siblings. In addition to clinical evaluation, electrodiagnostic tests, psychological tests, and brain imaging studies were performed. RESULTS: In all (GCN)13-(GCN)13 patients, OPMD started before age 35 years, with bilateral ptosis and dysphagia; external ophthalmoparesis and dysphonia followed within a few years, as well as weakness in proximal limb muscles. All patients had recurrent aspirations and lost weight; 4 patients required surgical interventions to alleviate dysphagia, and 5 required feeding gastrostomies. Most patients were followed by psychiatrists due to cognitive decline, recurrent depression, or psychotic episodes. Six patients died at ages 50, 51, 53, 56, 56, and 57 years. The eldest patient is now 51 years old; she is cachectic and requires special diet and psychiatric care for paranoid psychosis and uninhibited behavior. CONCLUSIONS: Oculopharyngeal muscular dystrophy progresses faster in homozygote compared with heterozygote patients. It is not restricted to the muscles, but also involves the CNS with cognitive decline and psychotic manifestations and leads to a reduced life expectancy.