Single Large-Scale Mitochondrial DNA Deletion Syndromes

Excerpt

Clinical characteristics: Single large-scale mitochondrial DNA deletion syndromes (SLSMDSs) comprise overlapping clinical phenotypes including Kearns-Sayre syndrome (KSS), KSS spectrum, Pearson syndrome (PS), chronic progressive external ophthalmoplegia (CPEO), and CPEO-plus.

KSS is a progressive multisystem disorder with onset before age 20 years characterized by pigmentary retinopathy, CPEO, and cardiac conduction abnormality. Additional features can include cerebellar ataxia, tremor, intellectual disability or cognitive decline, dementia, sensorineural hearing loss, oropharyngeal and esophageal dysfunction, exercise intolerance, muscle weakness, and endocrinopathies. Brain imaging typically shows bilateral lesions in the globus pallidus and white matter.

KSS spectrum includes individuals with KSS in addition to individuals with ptosis and/or ophthalmoparesis and at least one of the following: retinopathy, ataxia, cardiac conduction defects, hearing loss, growth deficiency, cognitive impairment, tremor, or cardiomyopathy. Compared to CPEO-plus, individuals with KSS spectrum have more severe muscle involvement (e.g., weakness, atrophy) and overall have a worse prognosis.

PS is characterized by pancytopenia (typically transfusion-dependent sideroblastic anemia with variable cell line involvement), exocrine pancreatic dysfunction, poor weight gain, and lactic acidosis. PS manifestations also include renal tubular acidosis, short stature, and elevated liver enzymes. PS may be fatal in infancy due to neutropenia-related infection or refractory metabolic acidosis.

CPEO is characterized by ptosis, ophthalmoplegia, oropharyngeal weakness, variable proximal limb weakness, and/or exercise intolerance.

CPEO-plus includes CPEO with additional multisystemic involvement including neuropathy, diabetes mellitus, migraines, hypothyroidism, neuropsychiatric manifestations, and optic neuropathy.

Rarely, an SLSMDS can manifest as Leigh syndrome, which is characterized as developmental delays, neurodevelopmental regression, lactic acidosis, and bilateral symmetric basal ganglia, brain stem, and/or midbrain lesions on MRI.

Diagnosis/testing: The diagnosis of an SLSMDS is established in a proband with characteristic clinical features by identification of a mitochondrial DNA (mtDNA) deletion ranging in size from 1.1 to 10 kb on molecular genetic testing. SLSMDSs can be identified in DNA from blood, buccal cells, and urine in affected children; analysis of skeletal muscle tissue may be required to detect an SLSMDS in an affected adult.

Management: Targeted therapy: Folinic acid supplementation in individuals with KSS with low 5-methyltetrahydrofolate in CSF or white matter abnormalities on brain MRI.

Supportive care: Consider mitochondrial supplement therapies such as coenzyme Q₁₀ and antioxidants; optimize nutrition and exercise regimen to prevent acute decompensation; physical and occupational therapy for myopathy and/or ataxia; standard treatment with anti-seizure medication; hearing aids or cochlear implants for sensorineural hearing loss; developmental and educational support; feeding therapy; consider gastrostomy tube placement if poor weight gain, choking, or aspiration risk is present; dilation of the upper esophageal sphincter to alleviate cricopharyngeal achalasia; prophylactic placement of cardiac pacemaker in individuals with cardiac conduction block, with consideration of an implantable cardioverter defibrillator; hormone replacement therapy per endocrinologist; electrolyte monitoring and replacement for renal tubular acidosis; eyelid slings and/or ptosis repair for severe ptosis; eye ointment for dry eyes; eyeglass prisms for diplopia; transfusion therapy for individuals with PS with sideroblastic anemia; replacement of pancreatic enzymes for exocrine pancreatic insufficiency; ventilatory support for respiratory abnormalities that may occur in individuals with Leigh syndrome; standard treatment of anxiety and/or depression; social work support and care coordination as needed.

Surveillance: Annual neurology assessment for ataxia, neuropathy, seizures, and myopathy; annual audiology evaluation; annual assessment of developmental progress, educational needs, and cognitive issues; annual evaluation by a neuro-ophthalmologist and/or retinal specialist and oculoplastics; measurement of growth parameters and evaluation of nutritional status and safety of oral intake at each visit; annual assessment of mobility and self-help skills with physical medicine, occupational therapy, and/or physical therapy; EKG and echocardiogram every six to 12 months; annual assessment with an endocrinologist; BUN and creatinine, with consideration of cystatin C in those with low muscle mass; complete blood count in those with PS to assess transfusion needs with additional labs per hematologist, and ferritin for those needing recurrent transfusions as needed; annual complete blood count in those with other SLSMDSs; fecal fat and fecal elastase as needed based on symptoms; monitor for evidence of aspiration and respiratory insufficiency at each visit; assess family needs at each visit.

Agents/circumstances to avoid: Volatile anesthetic hypersensitivity may occur. Avoid prolonged treatment with propofol (>30-60 minutes). Medications should be reviewed with a physician familiar with mitochondrial disorders including a thorough individualized assessment of risk vs benefit as several medications may be toxic to mitochondria.

Genetic counseling: SLSMDSs are almost never inherited, suggesting that these disorders are typically caused by a de novo single large-scale mitochondrial DNA deletion (SLSMD) that occurs in the mother's oocytes during germline development or in the embryo during embryogenesis. If the mother is clinically unaffected and the proband represents a simplex case (i.e., a single affected family member), the empiric risk to the sibs of a proband is very low (at or below 1%). If the mother is affected, the recurrence risk to sibs is estimated to be approximately 4% (one in 24 births). Maternal transmission to more than one child has not been reported to date. Prenatal testing for a pregnancy at increased risk and preimplantation genetic testing are scientifically possible but technically prohibitive as next-generation sequencing methodology does not accurately quantify heteroplasmy level of an SLSMD and droplet digital quantitative PCR cannot reliably detect less than 10% heteroplasmy levels of an SLSMD. Further, prenatal testing is not clinically available due to the inability to accurately interpret the clinical prognosis based on prenatal diagnostic results of an SLSMD.