Authors
B.G.M. van Engelen, M.D., Ph.D., Professor of Neuromuscular Diseases
J.A.M. Smeitink, M.D., Ph.D., Professor of Pediatrics
B.W. Smits, M.D.
Clinical features
Mitochondria are subcellular organels found in all nucleated cells. Every cell contains hundreds or thousands of mitochondria.Their main function is to provide the cell with adenosine triphosphate (ATP), a small molecule which serves as a source of energy needed for cellular function. Mitochondrial disorders comprise a group of perhaps several hundred different genetic diseases, with a large variety in clinical features. Each individual disorder is rare, but collectively they account for substantial use of health care resources. In recent years, the role of mitochondria in aging, apoptosis and cancer has been a new field of study.
A characteristic feature in recognizing mitochondrial disorders is the diversity in signs and symptoms. Patients with mitochondrial disease can present at any age to any medical specialist, but neurological disorders are relatively common. Although a large proportion of patients with mitochondrial disease can not be classified according to specific disease entity, several syndromes have been described.1 A variety of frequently encountered symptoms are listed in Table 1:2
| Brain | Convulsions Stroke like episodes Hypotonia, hypertonia, spasticity Transient paresis Decreased consciousness, coma Psychomotor retardation Extrapyramidal features Ataxia Central hypoventilation Cortical blindness Oculomotor Deafness |
| Skeletal muscle | Exercise Intolerance Weakness Ptosis Myaglia, cramps Myoglobulinuria |
| Peripheral nerve | Polyneuropathy Senosory ataxia |
| Heart | Cardiomyopathy Conduction disorders |
| Eye | Cataract Retinopathy |
| Kidney | Tubulus dysfunction Interstitial nephritis |
| Endocrine | Diabetes mellitus Diabetes insipidus Hypothyroidism Hypoparathyroidism Exocrine pancreas dysfunction Primary ovary dysfunction |
| Gastrointestinal | Diarrhea Intestinal pseudo obstruction Hepatopathy |
| Other | Small stature |
Differential diagnosis
Chronic progressive external ophthalmoplegia (CPEO)
CPEO is characterised by slowly progressive bilateral ptosis and paresis of eye musculature. Some patients also have mild proximal muscle weakness and cardiac conduction defects. Age at onset is variable, with many patients noticing ptosis in the third or fourth decade of life. The commonest genetic defect is a single deletion at 4977 bp, but other single mitochondrial DNA (mtDNA) deletions, mitochondrial tRNA mutations and nucleair DNA mutations (dominant and recessive) have been found.
Kearns-Sayre syndrome
The onset of ophthalmoparesis and pigmentary retinopathy before the age of 20 is characteristic of a more clearly defined disorder known as Kearns-Sayre syndrome. This sporadic condition is the result of large single deletions, complex rearrangements, or point mutations of mtDNA. Other clinical features include cerebellar ataxia, proximal myopathy, complete heart block, and raised cerebrospinal fluid (CSF) protein concentrations.
Pearson's syndrome
Pearson's syndrome results from large-scale rearrangements of mtDNA. In this disease, clinical features of sideroblastic anaemia with pancytopenia and exocrine pancreatic dysfunction are evident in early life and commonly result in death during infancy. If the patient survives childhood, anaemia improves but characteristic features of KSS then develop. This change is thought to indicate a shift away from mutated (duplicated or deleted) mtDNA in rapidly dividing tissues (bone marrow) with a drift toward heteroplasmy of mutated mtDNA in postmitotic tissues (brain and muscle).
Mitochondrial neurogastrointestinal encephalopathy syndrome
This multisystem disorder is characterised by the onset of CPEO, ptosis, gastrointestinal dysmotility (pseudo-obstruction), diffuse leucoencephalopathy, peripheral neuropathy, and myopathy in the second to fifth decades of life. Inheritance seems to be autosomal recessive rather than maternal. Defective thymidine metabolism due to mutations in the gene for thymidine phosphorylase impairs the replication and repair of mtDNA in this disorder.
Mitochondrial DNA depletion
In the case of mitochondrial DNA depletion, the defect is quantitative rather than qualitative. This autosomal recessive disorder is fatal in most cases by the age of 3 years, irrespective of its presentation. Liver, muscle, and kidney (Fanconi syndrome) are the organs most commonly affected with distinct hepatocerebral and myopathic forms of this disorder. Encephalopathy and respiratory failure are the two commonest causes of death. These quantitative mtDNA disorders probably result from imbalances in the mitochondrial nucleotide pool which, in turn, may impair mtDNA replication and repair.
Mitochondrial encephalopathy lactic acidosis and stroke-like episodes (MELAS)
This clinical syndrome is defined by stroke-like episodes, generally in the parieto-occipital regions, that are not restricted to a single recognised vascular territory in many cases. The origin of these strokes is unclear, but succinate dehydrogenase activity in the subintimal cells of cerebral arteries is increased, suggesting that proliferation of mitochondria occurs in these cells in much the same way as it does in ragged red muscle fibres. Consequently, one hypothesis has been that aberrant vascular tone in some vessels results in local ischaemia and a stroke-like episode ensues. Additional features of MELAS include intermittent episodes of encephalopathy associated with high plasma and CSF lactate concentrations, vomiting, migraine, dementia, and focal or generalised epilepsy. The A3243G mutation in the mitochondrial tRNALeu(UUR) gene was the first mtDNA mutation to be associated with this clinical phenotype and is the most frequently described. However, other mutations have been described in association with this phenotype, and the A3243G mutation also causes other distinct clinical phenotypes such as diabetes and deafness. These variations in phenotype can occur between individuals in the same family carrying identical mutations.
Myoclonus epilepsy with ragged red fibres (MERRF)
MERRF was initially reported in association with an adenine to guanine mutation in the gene encoding mitochondrial tRNALys at position 8344. Although other mutations in the same tRNA gene have now been reported, this mutation remains the most common cause of this devastating disease. MERRF is a severe neuromuscular disorder that causes progressive myoclonus, focal and generalised epilepsy, muscle weakness and wasting, hypertrophic cardiomyopathy, dementia, deafness, and cerebellar ataxia.
Leigh's syndrome
Leigh's syndrome is a progressive neurodegenerative condition of infancy and childhood. Symmetric necrotic lesions occur in the brainstem, diencephalon, and basal ganglia. The clinical presentation and course can vary substantially, but symptoms commonly include signs of brainstem or basal-ganglia dysfunction such as respiratory abnormalities, nystagmus, ataxia, dystonia, hypotonia, and optic atrophy. Developmental delay, and particularly developmental regression, are prominent clinical features. Inheritance can be X-linked recessive, autosomal recessive, or maternal, depending on the causative mutation. Although several mutations in mtDNA have now been described in association with this syndrome, point mutations in the ATPase 6 gene (T8993G/C and T9176G/C) are the most common.
Neuropathy, ataxia, and retinitis pigmentosa (NARP)
First described as a variable combination of developmental delay, retinitis pigmentosa, dementia, seizures, ataxia, proximal neurogenic muscle weakness, and sensory neuropathy in four members of a single family, this phenotype has now been expanded to include cardiomyopathy and Leigh's syndrome. The original family studied had a heteroplasmic thymidine to guanine transversion at nucleotide pair 8993, probably resulting in impaired ATP synthesis. There is no histochemical evidence of mitochondrial myopathy in patients with NARP syndrome.
Leber's hereditary optic neuropathy (LHON)
The first mitochondrial disease to be associated with a point mutation in mtDNA is an acute or subacute, bilateral, painless, central visual loss and is the commonest cause of blindness in young men. Most families with LHON have the same mtDNA mutation (G11778A). Three mtDNA mutations (G11778A, G3460A, and T14484C) are present in at least 90% of families. The disease commonly has its onset in the third or fourth decade of life, with initial unilateral involvement. Sequential involvement of the other eye commonly occurs within 2 months, and subsequent decline in visual acuity of both eyes may be either very sudden or slowly progressive over a period of several years.
Mitochondrial myopathy
In addition to these well-recognised syndromes, some patients with mitochondrial disease present with non-specific disorders in which weakness and exercise intolerance are the main clinical features. Muscle weakness can either be dynamic (weakness is more profound after exercise or increases during the day) or static (weakness is independent of exercise or time of day). Some of these presentations progress to involve other organ systems and become more obviously mitochondrial in origin, whereas others remain confined to muscle with gradual deterioration in power and involvement of muscle groups outside the shoulder and hip girdles, including the diaphragm.
Ancillary investigations
Several non specific investigations can support the diagnosis of a mitochondrial disorder. Lactate levels are often raised in blood or CSF. Electrocardiography will provide information about cardiac conduction velocity. Electromyography (EMG) can reveal non specific myopathic changes mainly in proximal muscles: motor units of short duration, reduced amplitude and increased polyphasia. Computed tomography and magnetic resonance imaging are particularly useful in Leigh’s syndrome (symmetrical hypodensities of brainstem, thalamus and basal ganglia) or MELAS (basal ganglia calcification). Functional tests include exercise tests, near infra red spectroscopy, which measures oxygen consumption , and magnetic resonance spectroscopy, which can measure lactate levels in the brain and phosphocreatine and ATP in muscle. These non invasive tests give quanitative and in-vivo information about mitochondrial function and are promising screening methods.
Investigation of skeletal-muscle biopsy ensures a correct and accurate diagnosis for the vast majority of patients with mitochondrial disorders. Histochemical assessment with Gomori trichome stain often reveals subsarcolemmal accumulation of mitochondria (ragged red fibers, RRF). This finding is not specific for mitochondrial disease. It is also found in dystrophic myotonia, inclusion body myositis and acid-maltase deficiency, although high percentage of RRF is suggestive of a mitochondrial disorder. Staining for cytochrome c oxidase, reduced nicotinamide adenine dinucleotide or succinate dehydrogenase can also identify affected muscle fibers.
Specialized centres can measure individual respiratory chain complex activities in a sample of fresh skeletal muscle. If a single complex is deficient, this points to a genetic defect in the relevant coding region of mtDNA or nuclear DNA or a gene involved in the assembly of that particular complex. If there are multiple complex defects, that suggests a generalised defect of protein synthesis, and an underlying mtDNA defect involving a tRNA gene (including deletions that remove tRNA genes) or perhaps a nuclear gene defect with secondary effects on mtDNA.3
With long-range polymerase chain reaction and Soutern blotting it is possible to detect duplications or deletions in mtDNA. If no duplications or deletions is found the mtDNA is screened for common point mutations. Although many laboratories have the capability of sequencing the entire mitochondrial genome, the results must be interpreted with caution. Mitochondrial DNA is highly polymorphic, with any two individuals differing at up to 60 base pairs. Only if the criteria stated by DiMauro4 are met it is safe to state that a sequence variation contribes to the pathogenesis of the disease. These include the absence of the mutation from a control population; the presence of mtDNA heteroplasmy and its correlation in other family members with the severity of clinical symptoms; its ability to alter a functionally conserved base pair or amino acid; and its correlation with an observable biochemical defect.
Genetics
Mitochondrial DNA (mtDNA)
Mitochondria are the only organelles of the cell besides the nucleus that contain their own DNA (called mtDNA) and their own machinery for synthesizing RNA and proteins.
The human mtDNA is a 16,569-bp, double-stranded, circular molecule containing 37 genes. Of these, 24 are needed for mtDNA translation, and 13 encode subunits of the respiratory chain: seven subunits of complex I, one subunit of complex III (cytochrome b), three subunits of cytochrome c oxidase and two subunits of ATP synthase. Mitochondrial genetics differs from mendelian genetics in three major aspects: maternal inheritance, heteroplasmy, and mitotic segregation.
Maternal Inheritance
As a general rule, all mitochondria (and all mtDNAs) in the zygote derive from the ovum. Therefore, a mother carrying an mtDNA mutation passes it on to all her children, but only her daughters will transmit it to their progeny. Recent evidence of paternal transmission of mtDNA in skeletal muscle (but not in other tissues) in a patient with a mitochondrial myopathy8 serves as an important warning that maternal inheritance of mtDNA is not an absolute rule, but it does not negate the primacy of maternal inheritance in mtDNA-related diseases.
Heteroplasmy and the Threshold Effect
There are thousands of mtDNA molecules in each cell, and in general, pathogenic mutations of mtDNA are present in some but not all of these genomes. As a result, cells and tissues harbor both normal (wild-type) and mutant mtDNA, a situation known as heteroplasmy. Heteroplasmy can also exist at the organellar level: a single mitochondrion can harbor both normal and mutant mtDNAs. In normal subjects, all mtDNAs are identical (homoplasmy). Not surprisingly, a minimal number of mutant mtDNAs must be present before oxidative dysfunction occurs and clinical signs become apparent: this is the threshold effect. The threshold for disease is lower in tissues that are highly dependent on oxidative metabolism, such as brain, heart, skeletal muscle, retina, renal tubules, and endocrine glands. These tissues will therefore be especially vulnerable to the effects of pathogenic mutations in mtDNA.
Mitotic Segregation
The random redistribution of organelles at the time of cell division can change the proportion of mutant mtDNAs received by daughter cells; if and when the pathogenic threshold in a previously unaffected tissue is surpassed, the phenotype can also change. This explains the age-related, and even tissue-related, variability of clinical features frequently observed in mtDNA-related disorders.
Nuclear DNA
Nuclear DNA contains about 70 genes coding for proteins involved in mitochondrial function. Although only few mutations have been described in these nuclear genes coding, their existence is supported by families with mendelian transmission of respiratory chain complex deficiencies. Defects in certain nuclear proteins can interfere with replication or maintenance of mtDNA, importation of mitochondrial protein or promoting of mtDNA replication, which normally occurs in early development.
Still, there are more questions than answers about the relationship between genotype and phenotype in mitochondrial disorders. Even within one family large differences in symptoms exist between affected siblings. Both nuclear and mitochondrial DNA mutations can cause exactly the same phenotype (for example Leigh’s syndrome), while on the other hand the same genetical defect can cause very different phenotypes (for example, the A3243G mtDNA mutation can present with classical MELAS as well as with CPEO or diabetes-deafness syndrome). Futhermore, it is largely unknown why some mutations seem to be highly organ specific (Leber hereditary optic neuropathy), while others cause multi-organ diseases.
Therapy
Coenzyme Q10 supplementation therapy can dramatically reverse disease in coenzyme Q10 deficiency but apart from this rare disorder, there is no cure available for mitochondrial disorders. Small clinical trials and case reports suggested benefical effects of dichloroacetate, co-enzyme Q10, nicotinamide, riboflavin, vitamin E and corticosteroids, but none of these agents have been studied in larger clinical trials. Futhermore, since we do not have a clear picture of the natural history of mitochondrial disorders the reported clinical improvements may just reflect a fluctuating disease course rather than a real effect of treatment.
