Donald R. Johns

Somatic mitochondrial DNA mutations in cortex and substantia nigra in aging and Parkinson's disease

Oxidative damage to mitochondrial DNA (mtDNA) increases with age in the brain and can induce G:C to T:A and T:A to G:C point mutations. Though rare at any particular site, multiple somatic mtDNA mutations induced by oxidative damage or by other mechanisms may accumulate with age in the brain and thus could play a role in aging and neurodegenerative diseases. However, no prior study has quantified the total burden of mtDNA point mutation subtypes in the brain. Using a highly sensitive cloning and sequencing strategy, we find that the aggregate levels of G:C to T:A and T:A to G:C transversions and of all point mutations increase with age in the frontal cortex (FCtx). In the substantia nigra (SN), the aggregate levels of point mutations in young controls are similar to the levels in the SN or FCtx of elderly subjects. Extrapolation from our data suggests an average of 2.7 (FCtx) to 3.2 (SN) somatic point mutations per mitochondrial genome in elderly subjects. There were no significant differences between Parkinsons disease (PD) patients and age-matched controls in somatic mutation levels. These results indicate that individually rare mtDNA point mutations reach a high aggregate burden in FCtx and SN of elderly subjects.

Mitochondrial dysfunction and neuromuscular disease

Mitochondrial diseases are a heterogeneous group of disorders with widely varying clinical features, due to defects in mitochondrial function. Involvement of both muscle and nerve is common in mitochondrial disease. In some cases, this involvement is subclinical or a minor part of a multisystem disorder, but myopathy and neuropathy are a major, often presenting, feature of a number of mitochondrial syndromes. In addition, mitochondrial dysfunction may play a role in a number of classic neuromuscular diseases. This article reviews the role of mitochondrial dysfunction in neuromuscular disease and discusses a rational approach to diagnosis and treatment of patients presenting with a neuromuscular syndrome due to mitochondrial disease.

Somatic mitochondrial DNA mutations in single neurons and glia

Somatic mitochondrial DNA (mtDNA) point mutations reach high levels in the brain. However, the cell types that accumulate mutations and the patterns of mutations within individual cells are not known. We have quantified somatic mtDNA mutations in 28 single neurons and in 18 single glia from post-mortem human substantia nigra of six control subjects. Both neurons and glia contain mtDNA with somatic mutations. Single neurons harbor a geometric mean (95% CI) of 200.3 (152.9-262.4) somatic mtDNA point mutations per million base pairs, compared to 133.8 (97.5-184.9) for single glia (p=0.0251). If mutations detected multiple times in the same cell are counted only once, the mean mutation level per million base pairs remains elevated in single neurons (146.9; 124.0-174.2) compared to single glia (100.5; 81.5-126.5; p=0.009). Multiple distinct somatic point mutations are present in different cells from the same subject. Most of these mutations are individually present at low levels (less than 10-20% of mtDNA molecules), but with high aggregate mutation levels, particularly in neurons. These mutations may contribute to changes in brain function during normal aging and neurodegenerative disorders.

Attenuation of free radical production and paracrystalline inclusions by creatine supplementation in a patient with a novel cytochrome b mutation.

Mitochondrial cytopathies are associated with increased free radical generation and paracrystalline inclusions. Paracrystalline inclusions were serendipitously found in a young male athlete with a very high respiratory exchange ratio during steady‐state exercise; he also had an unusually low aerobic capacity. Direct sequencing of the mitochondrial DNA (mtDNA) coding regions revealed a novel missense mutation (G15497A) resulting in a glycine→serine conversion at a highly conserved site in the cytochrome b gene in the subject, his mother, and sister. Cybrids, prepared by fusion of the subjects platelets with either U87MG ρ° or SH‐SY5Y ρ° cells, generated higher basal levels of reactive oxygen species (ROS), had a lower adenosine triphosphate (ATP) content, and were more sensitive to oxygen and glucose deprivation and peroxynitrite generation compared to control cybrids with wild‐type mtDNA. Cell survival was significantly enhanced with 50 mmol/L creatine monohydrate (CM) administration. The subject was also treated with CM (10 g/d) for a period of 5 weeks and a repeat muscle biopsy showed no paracrystalline inclusions. The results suggest that the development of exercise‐induced paracrystalline inclusions may be influenced by the G15497A mtDNA mutation, and that CM mitigates against the pathological consequences of this mutation. Muscle Nerve 29: 537–547, 2004

Novel SCO2 mutation (G1521A) presenting as a spinal muscular atrophy type I phenotype

Rare cases of suspected spinal muscular atrophy (SMA) have been found to have cytochrome c oxidase (COX) deficiency. To date, four cases with SMA features have been reported in children with mutations in the synthesis of cytochrome oxidase 2 (SCO2) gene. We report a male neonate who was born hypotonic, with persistent lactic acidosis, spontaneous activity with EMG testing, development of respiratory distress in the first few hours of life, and died at 30 days of age with progressive cardiomyopathy. Testing for survival motor neurone (smn) and NAIP deletions were negative and a skeletal muscle biopsy showed neurogenic features with severe reductions of COX enzymatic and histochemical staining intensity. Post‐mortem muscle, heart, and liver biopsies showed severe, moderate, and mild reductions in COX activity, respectively, with parallel findings in the protein content for the mitochondrial DNA (COII) and nuclear DNA (COIV) encoded subunits. DNA sequencing of exon 2 of the SCO2 gene revealed compound heterozygosity with mutations at G1541A (common mutation, E140K) and also at a novel site in the copper binding region (G1521A in the current case (converting a highly conserved cysteine to serine (C133S)); mother heterozygous for G1521A; and father heterozygous for G1541A). This case provides strong support that SCO2 mutations can result in neonatal hypotonia with an SMA 1 phenotype. SCO2 mutations should be screened in suspected SMA cases with normal smn mutation analysis and any one of; cardiomyopathy, lactic acidosis, or COX deficiency in muscle.

Dystonia as a presenting feature of the 3243 mitochondrial DNA mutation

A variety of neurologic phenotypes have been described in patients with mitochondrial disorders. We report a 32‐year‐old man in whom dystonia was the salient and presenting feature of a mitochondrial DNA mutation. He presented at age 23 with writers cramp and progressed over 5 years to exhibit dystonia in facial muscles and lower limbs. He also has exercise intolerance, mild, bilateral ptosis, proximal muscle weakness, and sensorineural hearing loss. Molecular genetic analysis of blood, urine, and muscle biopsy demonstrated the presence of a heteroplasmic point mutation at nucleotide position 3243. The 3243 mtDNA mutation has pleomorphic manifestations, and dystonia should be added to the list of associated clinical features.

Myotonia in colchicine myoneuropathy

Colchicine may induce a myoneuropathy in patients with renal insufficiency. To date, myotonia has not been described in this disorder. We recently studied 4 patients treated with routine doses of colchicine who, in the setting of renal insufficiency, developed a severe myoneuropathy characterized by prominent myotonic discharges on electromyography. In addition, 1 of the 4 patients had profound clinical myotonia. In the 3 patients in whom biopsies were performed, marked myopathic change with intracytoplasmic vacuolization was identified. All 4 patients improved rapidly with discontinuation of the medication. The patient in whom electrophysiologic studies were repeated had a complete resolution of the myotonic discharges. Colchicine myoneuropathy can present with prominent clinical and electrophysiologic myotonia that resolves completely with discontinuation of the medication.

Treatment of Leber's hereditary optic neuropathy: theory to practice

Dramatic advances in our understanding of the molecular genetic basis of Lebers hereditary optic neuropathy (LHON) have revolutionized our ability to diagnose and prognosticate this disease. Unfortunately no corresponding advances in the treatment of LHON have emerged. Glaucoma is a prevalent form of optic neuropathy that has been studied extensively. Lessons learned from treatment of LHON and glaucoma may have important implications for both diseases. LHON presents formidable challenges to the design and conduct of clinical trials. The acutely symptomatic LHON patient with monocular vision loss provides a unique clinical situation in which to test an agent during a critical therapeutic window. Advances in neuroprotection, apoptosis, and neurodegenerative diseases may provide important clues for potential therapeutic agents for LHON. Antioxidants and agents that interfere with the critical steps of mitochondrial-dependent, oxidative stress-induced apoptosis are candidates for future LHON therapy. A variety of neuroprotective agents, under active investigation in other diseases, may be useful in LHON therapy. Effective pharmacotherapy will complement the current management approach that has changed little in the 130 years since LHON was originally described.