RRM2B encodes the p53-inducible small subunit (p53R2) of ribonucleotide reductase, a key protein for mitochondrial DNA (mtDNA) synthesis. Pathogenic variants in this gene result in familial mitochondrial disease in adults and children, secondary to a maintenance disorder of mtDNA. This study describes two patients, mother and son, with early-onset chronic progressive external ophthalmoplegia (PEO). Skeletal muscle biopsy from the latter was examined: cytochrome c oxidase (COX)-negative fibres were shown, and molecular studies revealed multiple mtDNA deletions. A next-generation sequencing gene panel for nuclear-encoded mitochondrial maintenance genes identified two unreported heterozygous missense variants (c.514 G > A and c.682 G > A) in the clinically affected son. The clinically affected mother harboured the first variant in homozygous state, and the clinically unaffected father harboured the remaining variant in heterozygous state. In silico analyses predicted both variants as deleterious. Cell culture studies revealed that patients' skin fibroblasts, but not fibroblasts from healthy controls, responded to nucleoside supplementation with enhanced mtDNA repopulation, thus suggesting an in vitro functional difference in patients' cells. Our results support the pathogenicity of two novel RRM2B variants found in two patients with autosomal recessive PEO with multiple mtDNA deletions inherited with a pseudodominant pattern. Four phenotypes comprise the RRM2B mitochondrial DNA maintenance defects (RRM2B-MDMDs): RRM2B encephalomyopathic MDMD, the most severe phenotype, usually manifesting shortly after birth as hypotonia, poor feeding, and faltering growth requiring hospitalization. Subsequent assessments are likely to reveal multisystem involvement including sensorineural hearing loss, renal tubulopathy, and respiratory failure. Autosomal dominant progressive external ophthalmoplegia (adPEO), typically adult onset; other manifestations can include ptosis, bulbar dysfunction, fatigue, and muscle weakness. RRM2B autosomal recessive progressive external ophthalmoplegia (arPEO), a typically childhood-onset predominantly myopathic phenotype of PEO, ptosis, proximal muscle weakness, and bulbar dysfunction. RRM2B mitochondrial neurogastrointestinal encephalopathy (MNGIE)-like, characterized by progressive ptosis, ophthalmoplegia, gastrointestinal dysmotility, cachexia, and peripheral neuropathy. To date, 78 individuals from 52 families with a molecularly confirmed RRM2B-MDMD have been reported. The diagnosis of an RRM2B-MDMD is established in a proband with suggestive findings and either biallelic RRM2B pathogenic variants or a heterozygous RRM2B pathogenic variant identified by molecular genetic testing. Treatment of manifestations: To date, there are no known cures and few effective treatments for any forms of mitochondrial disease, including the RRM2B-MDMDs. Treatment modalities focusing on symptomatic management and supportive care are best implemented by a multidisciplinary team. Surveillance: Because most infants and young children with the encephalomyopathic phenotype are severely affected and are hospitalized for prolonged periods, monitoring typically occurs regularly by senior clinical specialists. Individuals with the other phenotypes warrant routine monitoring based on their clinical findings, rate of disease progression, and response to interventions. Agents/circumstances to avoid: Valproic acid should be used only in exceptional circumstances. Use of prescription drugs should always take into consideration the specific needs of and potential risks for the affected individual. Evaluation of relatives at risk: It is appropriate to clarify the genetic status of at-risk relatives of an affected family member so that those with the RRM2B pathogenic variant(s) can undergo timely routine surveillance for disease complications and avoid possible precipitating factors. With the exception of autosomal dominant progressive external ophthalmoplegia, RRM2B mitochondrial DNA maintenance defects – RRM2B encephalomyopathic MDMD, RRM2B MNGIE-like, and RRM2B-arPEO – are inherited in an autosomal recessive manner. Autosomal recessive inheritance. If both parents are known to be heterozygous for an RRM2B pathogenic variant, each sib of an affected individual has at conception a 25% chance of inheriting biallelic RRM2B pathogenic variants and being affected, a 50% chance of being heterozygous, and a 25% chance of inheriting neither of the familial RRM2B pathogenic variants. Autosomal dominant inheritance. If a parent of the proband is affected and/or is known to have the RRM2B pathogenic variant identified in the proband, the risk to sibs of inheriting the pathogenic variant is 50%. Once the RRM2B pathogenic variant(s) have been identified in an affected family member, prenatal and preimplantation genetic testing for RRM2B-MDMD are possible.

To determine the pathogenicity of a novel POLG mutation in a man with late-onset autosomal recessive progressive external ophthalmoplegia using clinical, molecular, and biochemical analyses. A multipronged approach with detailed neurologic examinations, muscle biopsy analyses, molecular genetic studies, and in vitro biochemical characterization. The patient had slowly progressive bilateral ptosis and severely reduced horizontal and vertical gaze. Muscle biopsy showed slight variability in muscle fiber size, scattered ragged red fibers, and partial cytochrome c oxidase deficiency. Biallelic mutations were identified in the POLG gene encoding the catalytic A subunit of POLγ. One allele carried a novel mutation in the exonuclease domain (c.590T>C; p.F197S), and the other had a previously characterized null mutation in the polymerase domain (c.2740A>C; p.T914P). Biochemical characterization revealed that the novel F197S mutant protein had reduced exonuclease and DNA polymerase activities and confirmed that T914P was inactive. By deep sequencing of mitochondrial DNA (mtDNA) extracted from muscle, multiple large-scale rearrangements were mapped and quantified. The patient's phenotype was caused by biallelic POLG mutations, resulting in one inactive POLγA protein (T914P) and one with decreased polymerase and exonuclease activity (F197S). The reduction in polymerase activity explains the presence of multiple pathogenic large-scale deletions in the patient's mtDNA.

The POLG gene encodes the mitochondrial DNA polymerase that is responsible for replication of the mitochondrial genome. Mutations in POLG can cause early childhood mitochondrial DNA (mtDNA) depletion syndromes or later-onset syndromes arising from mtDNA deletions. POLG mutations are the most common cause of inherited mitochondrial disorders, with as many as 2% of the population carrying these mutations. POLG-related disorders comprise a continuum of overlapping phenotypes with onset from infancy to late adulthood. The six leading disorders caused by POLG mutations are Alpers-Huttenlocher syndrome, which is one of the most severe phenotypes; childhood myocerebrohepatopathy spectrum, which presents within the first 3 years of life; myoclonic epilepsy myopathy sensory ataxia; ataxia neuropathy spectrum; autosomal recessive progressive external ophthalmoplegia; and autosomal dominant progressive external ophthalmoplegia. This Review describes the clinical features, pathophysiology, natural history and treatment of POLG-related disorders, focusing particularly on the neurological manifestations of these conditions.