• Mitochondrial DNA mutations
• Replicative segregation
• Expression threshold
• High mutation rate
• Accumulation of mutations with age
Major Phenotypic Features
• Age at onset: Childhood through adulthood
• Myoclonic seizures
History and Physical Findings
R.S., a 15-year-old boy, was referred to the neurogenetics clinic for myoclonic epilepsy; his electroencephalogram was characterized by bursts of slow wave and spike complexes. Before the seizures developed, he had been well and developing normally. His family history was remarkable for a maternal uncle who had died of an undiagnosed myopathic disorder at 53 years; a maternal aunt with progressive dementia who had presented with ataxia at 37 years; and an 80-year-old maternal grandmother with deafness, diabetes, and renal dysfunction. On examination, R.S. had generalized muscle wasting and weakness, myoclonus, and ataxia. Initial evaluation detected sensorineural hearing loss, slowed nerve conduction velocities, and mildly elevated blood and cerebrospinal fluid lactate levels. Results of a subsequent muscle biopsy identified abnormal mitochondria, deficient staining for cytochrome oxidase, and ragged-red fibers—muscle fibers with subsarcolemmal mitochondria that stained red with Gomori trichrome stain. Molecular testing of a skeletal muscle biopsy specimen for mutations within the mitochondrial genome (mtDNA) identified a heteroplasmic mutation (8344G>A, tRNAlys gene), a mutation known to be associated with myoclonic epilepsy with ragged-red fibers (MERRF), in 80% of the mtDNA from muscle. Subsequent testing of blood samples from R.S.'s mother, aunt, and grandmother confirmed that they also were heteroplasmic for this mutation. A review of the autopsy of the deceased uncle identified ragged-red fibers in some muscle groups. The physician counseled the family members (R.S.'s sibs and his mother's sibs) that they were either manifesting or nonmanifesting carriers of a deleterious mtDNA mutation compromising oxidative phosphorylation. No other family members chose to be tested for the mutation.
Disease Etiology and Incidence
MERRF (MIM 545000) is a rare panethnic disorder caused by mutations within the mtDNA in the tRNAlys gene. More than 90% of patients have one of three mutations within this gene: 8344G>A accounts for 80% and 8356T>C and 8363G>A together account for an additional 10% (see Fig. 12-26). The disease is inherited maternally because mitochondria are inherited almost exclusively from the mother. Patients with MERRF are nearly always heteroplasmic for the mutant mitochondria (see Chapters 7 and 12).
Mitochondria generate energy for cellular processes by producing adenosine triphosphate (ATP) through oxidative phosphorylation. Five enzyme complexes, I to V, compose the oxidative phosphorylation pathway. Except for complex II, each complex has some components encoded within the mtDNA and some in the nuclear genome. The mtDNA encodes 13 of the polypeptides in the oxidative phosphorylation complexes as well as two ribosomal RNAs and 22 transfer RNAs (tRNAs) (see Fig. 12-26).
In MERRF, the activities of complexes I and IV are usually most severely reduced. The tRNAlys mutations associated with MERRF reduce the amount of charged tRNAlys in the mitochondria by 50% to 60% and thereby decrease the efficiency of translation so that at each lysine codon, there is a 26% chance of termination. Because complexes I and IV have the most components synthesized within the mitochondria, they are most severely affected.
Because each mitochondrion contains multiple mtDNAs and each cell contains multiple mitochondria, a cell can contain normal mtDNAs or abnormal mtDNAs in varying proportions; therefore expression of the MERRF phenotype in any cell, organ, or individual ultimately depends on the overall reduction in oxidative phosphorylation capacity. The threshold for expression of a deleterious phenotype depends on the balance between oxidative supply and demand. This threshold varies with age and among individuals, organ systems, and tissues.
The threshold for expression of the MERRF phenotype in an individual tissue heteroplasmic for a tRNAlys can be exceeded either by an accumulation of mutations in the normal mtDNA or by increasing the proportion of mutant mtDNAs. Compared to nuclear DNA, mtDNA has a 10-fold higher mutation rate; this may result from exposure to a high concentration of oxygen free radicals from oxidative phosphorylation, a lack of protective histones, and ineffective DNA repair. Because mtDNA has no introns, random mutations usually affect coding sequences. Consistent with this increased mutation rate, mitochondrial efficiency declines gradually throughout adulthood, and as reserve oxidative phosphorylation activity declines, expression of defects in the oxidative phosphorylation pathway becomes increasingly likely.
Increases in the proportion of mutant mtDNA can occur by a combination of inheritance, preferential replication of mutant mtDNA, and selection. First, the children of heteroplasmic mothers have widely varying proportions of mtDNA genotypes because of replicative segregation, that is, random partitioning of mitochondria during expansion of the oogonial population, particularly because of the mitochondrial “genetic bottleneck” that occurs during oogenesis. Second, as heteroplasmic cells in an individual undergo mitosis, the proportion of mtDNA genotypes in daughter cells changes from that of the parent cell by replicative segregation. Third, because changes in the proportion of mtDNA genotypes affect the cellular phenotype, the mtDNA is subject to strong selective pressures; the selective pressures vary among tissues and result in different mtDNA populations in different tissues of the same person. Thus, both intercellular and intergenerational mtDNA transmission follow the principles of population genetics.
Phenotype and Natural History
The classic MERRF phenotype includes myoclonic epilepsy and mitochondrial myopathy with ragged-red fibers (Fig. C-33). Other associated findings include abnormal brainstem evoked responses, sensorineural hearing loss, ataxia, renal dysfunction, diabetes, cardiomyopathy, and dementia. Onset of symptoms can be in childhood or adult life, and the course can be slowly progressive or rapidly downhill.
FIGURE C-33 Quadriceps muscle histology. A, Modified Gomori trichrome stain illustrating ragged-red fiber (×525). B, Cytochrome oxidase stain illustrating absence of cytochrome oxidase in an affected muscle fiber, consistent with a mitochondrial DNA defect (×525). See Sources & Acknowledgments.
Because mtDNA genetics follows quantitative and stochastic principles, clinical features of affected relatives vary in pattern and severity and do not have an easily defined clinical course. The absence of ragged-red fibers in a muscle biopsy specimen does not exclude MERRF. Within pedigrees, phenotypes generally correlate well with the severity of the oxidative phosphorylation deficit, but correlation with the percentage of mutant mtDNA in skeletal muscle requires adjustment for age. In one pedigree, a young adult with 5% normal mtDNA in skeletal muscle, had a severe clinical and biochemical phenotype; other young adults with 15% normal mtDNA had normal phenotypes; and an older adult with 16% normal mtDNA had a severe phenotype. This expression pattern demonstrates that symptoms accumulate progressively as oxidative phosphorylation capacity drops below organ expression thresholds and that age-related declines in oxidative phosphorylation play a critical role in the appearance and progression of symptoms.
Treatment is symptomatic and palliative. No specific therapies are currently available. Most patients are given coenzyme Q10 and L-carnitine supplements to optimize the activity of the oxidative phosphorylation complexes.
The risk to children of affected males is essentially zero because, with only one known exception, children do not inherit paternal mtDNA. The risk to children of affected or unaffected females with a MERRF mutation cannot be estimated accurately by prenatal testing because the critical parameters defining disease in the child (replicative segregation, tissue selection, and somatic mtDNA mutations) cannot be predicted in advance.
Similarly, molecular testing of blood samples from at-risk family members is complicated by two general problems. First, because of replicative segregation and tissue selection, the mutation may not be detectable in blood; therefore a negative result does not exclude a family member as a carrier of the mtDNA mutation. Second, because of replicative segregation, a positive result predicts neither the proportion of mutant mtDNA in other tissues nor the expected severity of disease.
Questions for Small Group Discussion
1. How does a mutant mtDNA molecule, arising de novo in a cell with hundreds of normal molecules, become such a significant fraction of the total that energy-generating capacity is compromised and symptoms develop?
2. How could mitochondrial mutations affecting oxidative phosphorylation accelerate the mutation rate of mtDNA?
3. How would mitochondrial mutations affecting oxidative phosphorylation accelerate aging?
4. In the fetus, oxygen tension is low and most energy is derived from glycolysis. How could this observation affect the prenatal expression of deleterious oxidative phosphorylation mutations?
Abbott JA, Francklyn CS, Robey-Bond SM. Transfer RNA and human disease. Front Genet. 2014;5:158.
DiMauro S, Hirano M. MERRF. [Available from] http://www.ncbi.nlm.nih.gov/books/NBK1520/.
Suzuki T, Nagao A, Suzuki T. Human mitochondrial tRNAs: biogenesis, function, structural aspects and diseases. Ann Rev Genet. 2011;45:299–329.