2.3. mtDNA MUTATIONS IN DEGENERATIVE DISEASE
A large number of pathogenic mutations have now been identified in the mtDNA. These include both base substitution and rearrangement mutations. The base substitution mutations fall into two classes: missense mutations in mRNA genes and protein synthesis mutations tRNA and rRNA genes.
Common missense mutations are those, which result in Leber's hereditary optic the neuropathy (LHON), movement disorders including dystonia, and Leigh's disease the neurogenic muscle weakness and retinitis pigmentosa (NARP). LHON stereotypically presents as acute or subacute onset control vision loss, ultimately leading to central scotoma and permanent blindness. Patients can also have other clinical manifestations such as cardiac conduction defects. The most common LHON mutations occur at np 11778 in the NO4 gene (19), at np 3460 in ND1 gene (20, 21) and at np 14484 in the ND6 gene (22, 23).
Occasionally, patients harboring the LHON np 11778 and 3460 mutations can exhibit movement disturbances including generalized dystonia. This feature of mtDNA missense mutations is more dramatically demonstrated by the np 14459 mutation in the ND6 gene; which frequently results in generalized dystonia when close to homoplasmic mutant. Individuals harboring the 14459 mutation can experience gait disturbances, rigidity, pseudobulbar syndrome, impaired intelligence, short stature and myopathic features. These individuals generally exhibit a bilateral striated necrosis on CT or MRI scan (24-27). In some pedigrees, the np 14459 mutation can also manifest as LHON. Hence, LHON and movement disorders seem to be closely aligned through mitochondrial complex I defects.
Missense mutations at np 8993 in the ATP6 gene have been associated with retinitis pigmentosa (degeneration of the pigmentary retinal epithelium) and peripheral nerve disease resulting in weakness. In more severe cases, this same mutation can cause olivopontocerebellar atrophy, macular degeneration, and mental retardation. The most severe cases are associated with Leigh's disease, a sudden onset, frequently lethal childhood disease associated with degeneration of the basal ganglion. The variability of symptoms caused by this mutation is the product of its being heteroplasmic and segregating in families. Individuals with 75% mutant mtDNAs frequently only manifest retinitis pigmentosa, while those with 95%, mutant may die of Leigh's disease (28-30).
MtDNA protein synthesis mutations can occur in both the rRNA and tRNA genes. The best-known rRNA mutation occurs at np 1555 in the 12s rRNA. This (40) mutation is associated with maternally inherited sensory neural hearing loss, and confers increase sensitivity to aminoglycoside antibiotic-induced deafness (31, 32).
The best known tRNA mutations are the np 8344 mutation in tRNALys gene associated with myoclonic epilepsy and ragged red fiber (MERRF) disease (33, 34) and the np 3243 mutation in tRNALeu(UUR) gene associated with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes (MELAS) (35, 36). Both of these mutations are associated with a wide spectrum of symptoms, the nature and severity of which seem to be determined by subtle differences in the percentage of mutant and normal mtDNAs. When the percentage of mutant mtDNAs varies above the expression threshold of 85%, symptoms can include epilepsy, stroke-like episodes, migraine and sensory neural hearing loss. Progressive dementia, mitochondrial myopathy including ragged red muscle fibers (RRF) and the proliferation of abnormal mitochondria, ophthalmoplegia, cardiomyopathy, diabetes mellitus, short stature, lactic acidosis and nephropathy (18).
In addition to these common mutations, a large number of novel mtDNA mutations have been reported, ranging from severe to mild phenotypic effects. The most severe mutation reported is the deletion of a single T in the anticodon stem of the tRNALeu(UUR) gene at np3271. This mutation resulted in sensory neural deafness and seizures in childhood with progressive encephalomyopathy leading to retinitis pigmentosa, glaucoma, seizures, and severe cerebral calcification (Fahr's disease) by age 27. The individual then died of septicemia and renal failure. Interestingly, the patient harbored over 70%, mutant mtDNAs while the mother had l00%, normal mtDNAs in blood (37). This demonstrates that deleterious mtDNA mutation can segregate rapidly in families.
The mildest mtDNA tRNA mutation reported to date is at np 4336 in the tRNAGln gene. This mutation is associated with between 5 and 7% of late onset Alzheimer's disease patients, but less than 0.7% of controls. The mutation is always associated with a particular mtDNA background haplotype, implying that the mutation arose once and has become established by genetic drift in the European population. Two additional novel mutations have been associated with the tRNAGln mutation, an NDI missense mutation at np 3397, and a 5 bp insertion in the 12S rRNA at nps 956-965 (38, 39).
Generally, the milder the mtDNA mutation, the later the onset of symptoms, the more stereotyped are the symptoms, and the higher the proportions of mutant mtDNA required to give the symptoms. The more severe the mutation, the more variable the phenotype and the more likely the mutation is to be heteroplasmic (40).
MtDNA rearrangement mutations have been associated with maternally inherited diabetes and deafness, the ocular myopathies including chronic progressive external ophthalmoplegia (CPEO) and the Kearns-Sayre Syndrome (KSS), and with the Pearson's marrow pancreas syndrome. One three-generation pedigree has been reported with maternal transmission of adult onset sensory neural hearing loss and diabetes mellitus. The proband and probands' mother had also experienced stroke-like episodes, and the skeletal muscle mitochondria had a significant defect in multiple OXPHOS enzyme complexes, but without mitochondrial myopathy and its associated RRF and abnormal mitochondria. These symptoms were caused by a heteroplasmic rearrangement involving some molecules with a 10.4 kb deletion, others with a reciprocal 6.3 kb insertion, and normal molecules (41, 42).
The ocular myopathies are generally associated with systemic deletions or insertion/deletion mutations. These patients present with ophthalmoplegia (paralyses of the extraocular eye muscles), ptosis (droopy eyelids), and mitochondrial myopathy associated with RRF and abnormal mitochondrial. More severe cases can also have seizures, cerebellar ataxia, hearing loss, cardiac conduction defects, diabetes mellitus, etc. (43, 44). A wide variety of rearrangements have been associated with CPEO and KSS (45). However, about one-third to one-half of the cases harbor the "common" 5 kb (4977 bp deletion) which occurs between a pair of direct repeats (46, 47). In the skeletal muscle of CPEO and KSS patients, the muscle fibers show alternating COX-positive and COX-negative regions.
The COX-negative regions also correspond to the RRF; and these regions contain high concentrations of the deleted mtDNAs, high levels of transcripts from the non-deleted regions of the mtDNA and low levels of transcripts from the deleted regions (48, 49). Hence, it appears that early in development, relatively few deleted mtDNA molecules are present, but they are distributed widely throughout the body and along the muscle fibers. However, as the individual ages, the deleted mtDNAs are selectively amplified, resulting in regional OXPHOS defects. Hence, these diseases appear to progress due to a compensatory but futile proliferation of mutant mtDNAs resu1ting in segmental respiratory failure along the muscle fibers.
The Pearson's syndrome is associated with childhood pancytopenia (loss of blood cells), resulting in transfusion dependence and frequently death. Pearson's individuals have deletions in their blood cells, as well as distributed systemically throughout the body (50, 51). Pearson's children, who survive the pancytopenia, progress to KSS (52). Hence, Pearson's is a more severe form of the KSS and the CPEO.