Inborn and Induced Defects of Mitochondria

NEUROLOGICAL REVIEW Inborn and Induced Defects of Mitochondria

Anthony H. V. Schapira, DSc, MD, FRCP

itochondria play a pivotal role in cellular metabolism and in energy production in particular. Predictably, defects of mitochondrial metabolism have a deleterious ef- fect on cell function and survival, especially in highly energy-dependent tissues such as brain and skeletal muscle. Although a multitude of biochemical reactions occurM within mitochondria, the oxidative phosphorylation (OXPHOS) system is the most important in terms of adenosine triphosphate generation and in its association with human disease.

The OXPHOS system is located on the in- or mitochondrial genomes, or second- ner mitochondrial membrane and com- ary and induced by either endogenous prises the respiratory chain (complexes I- or exogenous toxins. In addition, IV) and adenosine triphosphate synthase defects of OXPHOS enzymes and/or (complex V). It is responsible for proton mutations of mtDNA have been identi- pumping, producing the transmembra- fied in a number of neurodegenerative nous electrochemical gradient (␺⌬m), and diseases. It would be premature at pres- generating adenosine triphosphate by ent to classify these as primary or sec- aerobic metabolism. The 5 protein com- ondary abnormalities. plexes of the OXPHOS system comprise approximately 82 subunits, 13 of which PRIMARY DEFECTS: mtDNA are encoded by mitochondrial DNA (mtDNA). Human mtDNA is a 16.5- Each mitochondrion contains 2 to 10 mol- kilobase circular double-stranded molecules of mtDNA. Mitochondrial DNA is ecule that codes for 22 transfer RNAs virtually exclusively maternally inher- (tRNAs), 2 ribosomal RNAs (rRNAs), sub- ited. This feature, together with its high units ND1 through ND6 and ND4L of polymorphism rate, has enabled mtDNA complex I, cytochrome b of complex III, to be used in genetic studies of evolution COI-III of complex IV (cytochrome oxi- and population migration, as well as more dase [COX]), and subunits 6 and 8 of com- recent applications in forensic science. The plex V. Thus, complexes I, III, IV, and V first report of mtDNA mutations in hu- are unique in that they are the products man disease described deletions in pa- of 2 different genomes—nuclear and mi- tients with chronic progressive external tochondrial (Figure). Nuclear-encoded ophthalmoplegia or Kearns-Sayre syn- subunits are translated on cytoribosomes drome.2,3 The deleted mtDNA molecules and transported to mitochondrial com- coexist with normal wild-type mole- partments via a complex import and cules, a situation referred to as hetero- intramitochondrial sorting system incor- plasmy. The proportion of mutant mol- porating targeting sequences and mem- ecules is known to vary between patients brane receptors (see Ryan and Jensen1 and between different tissues of the same for review). patient, although deletions are rarely found Deficiencies of the OXPHOS system in blood. It appears from in vitro studies can be considered as primary, ie, inborn that for deletions, a threshold of 60% mu- genetic defects affecting either the nuclear tant-mtDNA must be exceeded for biochemical deficiency to ensue.4 However, From the University Department of Clinical Neurosciences, Royal Free Hospital School there is often little correlation in patients of Medicine, and University Department of Clinical Neurology, Institute of Neurology, between deletion load and clinical pre- London, England. sentation. The reasons for this are ob-

ARCH NEUROL / VOL 55, OCT 1998 1293

©1998 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/25/2021 consequence of mutations involving tRNA genes. Leigh Mitochondrial Respiratory Chain and disease may be associated with other mtDNA muta- Oxidative Phosphorylation System tions, pyruvate dehydrogenase deficiency, and other biochemical abnormalities.10 Point mutations in tRNA genes are found in a number of clinical syndromes, including myopathy, en- cephalopathy, lactic acidosis, and strokelike episodes (MELAS), myoclonic epilepsy with ragged red fibers, and encephalomyopathies encompassing limb myopathy, de- mentia, ataxia, and deafness. In addition, syndromes including cardiomyopathy, diabetes mellitus, and multiple lipomatosis have been described. The relationship Complex I II III IV V between genotype and phenotype is not strict, although Schematic representation of the mitochondrial respiratory chain and MELAS is most often associated with tRNALeu(UUR) mu- oxidative phosphorylation system. Each hexagon is the polypeptide product tations (particularly the A3243G mutation) and myo- of a single gene. Yellow hexagons are encoded by nuclear genes, and green clonic epilepsy with ragged red fibers with the A8344Gbp hexagons represent subunits encoded by mitochondrial DNA. tRNALys mutation. Mutations in tRNA are heteroplas- mic and can usually be detected in blood. In vitro stud- scure. Mitochondrial DNA deletions in muscle are asso- ies suggest that greater than 90% mutant-mtDNA load ciated with ragged red, succinate dehydrogenase (SDH)– is required before biochemical defects appear, although positive and COX-negative fibers. In situ hybridization this may vary from one tissue to another.11 The mecha- studies have demonstrated that the deleted molecules are nisms by which tRNA mutations cause cell dysfunction present in highest proportion in these abnormal fibers. are unknown but inevitably must involve translational Deletions may also be found in association with mtDNA abnormalities. duplications, although recent evidence suggests that the A polymorphism at 1555 in the 12S rRNA of mtDNA duplicated molecules do not cause respiratory chain de- has been associated with increased sensitivity to deaf- ficiency.5 ness induced by aminoglycoside antibiotics.12 Aminogly- Point mutations in protein coding genes have been cosides target the 16S rRNA of Escherichia coli to impair found in Leber hereditary optic neuropathy (LHON), neu- “housekeeping” translational activity, the 1555 polymor- rogenic weakness ataxia and retinitis pigmentosa, and phism probably results in enhanced binding of aminogly- Leigh disease. Leber hereditary optic neuropathy is char- cosides to the human mtDNA 12S analog in the cochlear acterized by bilateral sequential visual loss with onset in cells, and consequent enhanced toxicity. Aminoglycoside- young adulthood, optic atrophy, and variable recovery. induced deafness is relatively common and it has been Three mutations in complex I genes (G11778Abp in ND4, estimated that 17% of affected patients have this poly- T14484Cbp in ND6, and A3460Tbp in ND1) are consid- morphism. Interestingly, this same polymorphism has ered primary as they are found only in patients with been identified in families with maternally inherited non- LHON, while a number of secondary (possibly modify- syndromic deafness. ing) base changes have been identified in LHON, but also in some control subjects.6 Mutations in LHON are sys- PRIMARY DEFECTS: NUCLEAR DNA temically distributed, often in high proportion to wild- type, and readily detected in the blood. The majority of Several families with autosomal dominant mitochon- persons with LHON have disease confined to the optic drial myopathy have been described.13 Typically, auto- nerve although a multiple sclerosis–like illness has been somal dominant mitochondrial myopathy is character- described in some women with the G11778Abp muta- ized by chronic progressive external ophthalmoplegia with tion,7 and patients with LHON-dystonia mutations8,9 have limb myopathy and ragged red fibers on biopsy. Mito- been described. Several features of LHON remain unex- chondrial DNA analysis shows multiple deletions in tis- plained, eg, restriction of clinical deficit to the optic nerve, sues from patients, which contrasts with the single de- the excess of male patients, and the presence of asymp- letions of patients with sporadic chronic progressive tomatic carriers (usually females) often with as high lev- external ophthalmoplegia or Kearns-Sayre syndrome. To els of mutation in the blood as affected siblings. These date, 2 loci on chromosomes 3 and 10 have been iden- factors have led to the suggestion that there may be modi- tified by linkage analysis but the respective genes have fying factors on the biochemical expression of the mtDNA not been identified, although a number of likely candi- mutations, eg, X-linked susceptibility genes, environ- dates have been excluded.14,15 mental factors, or autoimmunity. Neurogenic weakness Mitochondrial DNA depletion syndrome presents in ataxia and retinitis pigmentosa and Leigh disease may early infancy with hepatic failure, hypotonia, and fail- be associated with point mutations in the adenosine tri- ure to thrive; death is common before 1 year of age.16 Mi- phosphate synthase 6 gene at position 8993. Interest- tochondrial DNA levels in severely affected individuals ingly, muscle biopsy specimens from patients with may be 10% or less of age-matched control subjects. Fam- LHON, neurogenic weakness ataxia and retinitis pig- ily studies are limited but have suggested autosomal in- mentosa, or Leigh disease are not commonly associated heritance, probably recessive in most cases. Nuclear in- with ragged red fibers, leading to the suggestion that volvement has been confirmed in recent studies on 2 these morphological abnormalities are more often the different patients that demonstrated restoration of mtDNA

ARCH NEUROL / VOL 55, OCT 1998 1294

©1998 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/25/2021 levels following fusion of cells containing remnant pa- cal features strikingly similar to those of Huntington tient mtDNA with mtDNA-less (␳0) cells containing nor- disease (HD). Malonate, another complex II inhibitor, mal nuclei.17,18 The molecular mechanisms involved in induces similar effects to 3-nitropropionic acid. Severe mtDNA depletion are not known and may vary from one deficiency of complex II and III activity has been identi- family to another. However, a genetic defect, perhaps in fied in HD caudate24,25 and thus provides another a developmentally regulated gene controlling mtDNA copy example of a direct link between a mitochondrial toxin number, seems at least one likely explanation. and human neurodegenerative disease. In HD, the The only mutation identified in a nuclear gene en- mitochondrial defect must be secondary to the abnor- coding a respiratory chain subunit is a missense base mal CAG expansion mutation in the huntingtin gene change in the flavoprotein of SDH (complex II).19 The (IT15). The absence of the complex II and III defi- mutation was identified in 2 sisters with Leigh disease ciency in tissues that express mutant huntingtin indi- and severe SDH deficiency in muscle and cultured tis- cates that it must be related to specific biochemical or sues. To date, this mutation has not been identified in pharmacological features in the striatum. Nitric oxide other patients with SDH defects. synthase–positive neurons and fibers are present in the striatum in addition to glutaminergic fibers and MITOCHONDRIAL DYSFUNCTION this has led to the suggestion that excitotoxicity may IN NEURODEGENERATION participate in neuronal cell death in HD. Nitric oxide is a potent inhibitor of complex II and III and so the Parkinson Disease mitochondrial defect may be secondary to an excito- toxic mechanism. Numerous toxins inhibit mitochondrial function and many of these are used in agriculture as herbicides and Friedreich Ataxia pesticides. Attention has focused on mitochondrial toxins that cause human disease and several of these are now Clinically, Friedreich ataxia (FA) is characterized by on- known to produce models of neurodegenerative dis- set in early adolescence of ataxia, areflexia, and pyrami- ease. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine dal features in association with kyphoscoliosis and a car- (MPTP) is probably the most studied of these; it is known diomyopathy. Patients usually die of cardiac failure within to induce parkinsonism in humans and other primates. 15 years of diagnosis. Pathologically, the main feature in MPTP targets the dopaminergic neurones of the substan- FA is a “dying back” neuropathy affecting dorsal sen- tia nigra through specific uptake and conversion path- sory neurons. Friedreich ataxia is an autosomal reces- ways and is then concentrated within mitochondria.20 sive disorder caused by an expanded GAA repeat in in- 1-Methyl-4-phenylpyridinium (MPP+), the active me- tron 1 of the X25 gene on chromosome 9.26 The repeat tabolite of MPTP, is a selective complex I inhibitor and causes reduced transcription of the X25 gene and there- is thought to induce cell death through adenosine tri- fore decreased expression of its product, frataxin. phosphate depletion. However, there is good evidence Frataxin is widely expressed but its function is not known. that free radicals, including nitric oxide, participate with However, frataxin has significant sequence homology with complex I inhibition in MPP+ toxicity. A remarkable par- a yeast protein now termed YFH-1.27 Deletion of the gene allel between MPTP toxicity and idiopathic Parkinson dis- for YFH-1 results in defective energy production and ease (PD) was drawn when complex I deficiency was iden- mtDNA depletion.28 This yeast knockout model also ac- tified in the substantia nigra of patients who had died with cumulates iron in the mitochondrial matrix. Studies with PD. This defect, together with other biochemical abnor- fluorescent tags have localized frataxin to the mitochon- malities, including oxidative stress and damage, is thought drion.29 Thus, there is overwhelming evidence that to participate in the pathogenesis of nigral cell loss in PD. frataxin is a mitochondrial protein and that in yeast, at The complex I defect appears to be isolated to the sub- least, its deficiency causes defective OXPHOS, a loss of stantia nigra within PD brain, although several reports mtDNA, and accumulation of iron. Some of these fea- have also documented a decrease in complex I activity tures are reminiscent of the situation in the PD substan- (with and without defects in complexes II-IV) in plate- tia nigra (see above). lets from patients with PD.21 The presence of an OXPHOS defect in PD skeletal muscle is more conten- Alzheimer Disease tious. One recent report has suggested that the platelet complex I deficiency in some patients with PD may be Several reports have demonstrated COX deficiency both caused by mtDNA mutations.22 This finding implies that histochemically and biochemically in Alzheimer dis- complex I deficiency may be of primary etiological im- ease (AD). One group has also demonstrated COX defi- portance, at least in some patents with PD. ciency in AD platelets.30 ␳0 cells have been used to show that the platelet COX defect is transmitted with AD Huntington Disease mtDNA transfer and that this deficiency is associated with certain base changes in mtDNA COX genes.31 Although Accidental ingestion of 3-nitropropionic acid, a con- these “mutations” were also found in controls, they were taminant of mildewed sugar cane, was found to cause present at higher frequency in patients with AD. If con- striatal necrosis and chorea with dystonia in survivors.23 firmed, these observations could have important impli- 3-Nitropropionic acid is a complex II inhibitor and cations for etiology and inheritance, in at least a propor- when injected into primates produces striatal pathologi- tion of patients with AD.

ARCH NEUROL / VOL 55, OCT 1998 1295

©1998 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 09/25/2021 Dystonia nally inherited Leber hereditary optic neuropathy and dystonia. Proc Natl Acad SciUSA.1994;91:6202-6210. 9. De Vries DD, Went LN, Bruyn GW, et al. Genetic and biochemical impairment of Three groups have reported on platelet mitochondrial mitochondrial complex I activity in a family with Leber hereditary optic neuropa- function in platelets from patients with focal or gener- thy and hereditary spastic dystonia. Am J Hum Genet. 1996;58:703-711. alized dystonia. The first study found marked complex I 10. Rahman S, Blok RB, Dahl HHM, et al. Leigh syndrome: clinical features and bio- deficiency in both groups of patients32; the second showed chemical and DNA abnormalities. Ann Neurol. 1996;39:343-351. 33 11. Dunbar DR, Moonie PA, Jacobs HT, Holt IJ. Different cellular backgrounds con- normal mitochondrial function. The third study looked fer a marked advantage to either mutant or wild-type mitochondrial genomes. at sporadic focal dystonia and DYT-1–positive and –nega- Proc Natl Acad SciUSA.1995;92:6562-6566. tive families with generalized dystonia; a complex I de- 12. Prezant TR, Agapian JV, Bohlman C, et al. Mitochondrial ribosomal RNA muta- ficiency was seen in the focal group, but no abnormality tion associated with both antibiotic-induced and non-syndrome deafness. Nat was detectable in the patients with generalized dystonia Genet. 1993;4:289-294. 34 13. Zeviani M, Petruzella V, Carrozzo R. Disorders of nuclear-mitochondrial interge- linked or not to DYT-1. nomic signalling. J Bioenerg Biomembr. 1997;29:121-130. 14. Suomalainen JM, Kaukonen J, Amati P, et al. An autosomal locus predisposing NEURODEGENERATION: CONCLUSIONS to deletions of mitochondrial DNA. Nat Genet. 1995;9:146-151. 15. Kaukonen JA, Amati P, Suomalainen A, et al. An autosomal locus predisposing to multiple deletions of mtDNA on chromosome 3p. Am J Hum Genet. 1996;58: Mitochondrial abnormalities in HD and FA are undoubt- 763-769. edly secondary to the respective primary molecular de- 16. Tritschler HJ, Andreetta F, Moraes CT, et al. Mitochondrial myopathy of child- fects in the huntingtin and frataxin genes. The mecha- hood onset associated with depletion of mitochondrial DNA. Neurology. 1992; nisms by which the mitochondrial defects in HD and the 42:209-217. 17. Bodnar AG, Cooper JM, Holt IJ, Leonard JV, Schapira AHV. Nuclear comple- putative abnormalities in FA are induced and the part they mentation restores mtDNA levels in cultured cells from a patient with mtDNA play in disease pathogenesis are unknown. In HD, how- depletion. Am J Hum Genet. 1993;53:663-669. ever, the pattern of respiratory chain defect supports the 18. Taanman J-W, Bodnar AG, Cooper JM, et al. Molecular mechanisms in mito- role of excitotoxicity. In PD, AD, and dystonia, the situ- chondrial DNA depletion syndrome. Hum Mol Genet. 1997;6:935-942. ation is much less clear. Data from mtDNA transfer stud- 19. Bourgeron T, Rustin P, Chretien D, et al. Mutation of a nuclear succinate dehydrogenase gene results in mitochondrial respiratory chain deficiency. Nat Genet. ies suggest a primary role for mtDNA mutations in PD 1995;11:144-149. and AD, but whether this is relevant to the majority or 20. Tipton KF, Singer TP. Advances in our understanding of the mechanisms of the only a small subgroup of patients is not clear. neurotoxicity of MPTP and related compounds. J Neurochem. 1993;61:1191- Since the manuscript was accepted for publication, 1206. 21. Schapira AHV. Evidence for mitochondrial dysfunction in Parkinson’s disease: a the mitochondrial DNA mutations reported to be pres- critical appraisal. Mov Disord. 1994;9:125-138. 31 ent at increased frequency in AD have subsequently been 22. Swerdlow RH, Parks JK, Miller SW, et al. Origin and functional consequences of identified as artifacts of polymerase chain reaction am- the complex I defect in Parkinson’s disease. Ann Neurol. 1996;40:663-671. plification of nuclear DNA, ie, nuclear-embedded mtDNA 23. Ludolph AC, Ludolph AG, Sabri MI, et al. 3-Nitropropionic acid-abundant xeno- pseudogenes.35 A second nuclear respiratory chain DNA biotic excitotoxin linked to putaminal necrosis and tardive dystonia. Ann Neurol. 1991;30:253-262. mutation in the gene encoding the 18-kd subunit of com- 24. Gu M, Cooper JM, Gash M, Mann VM, Javoy-Agid F, Schapira AHV. Mitochon- 36 plex I has also recently been described. drial defect in Huntington’s disease caudate nucleus. Ann Neurol. 1996;39:385- 389. Accepted for publication November 26, 1997. 25. Browne SE, Bowling AC, MacGarvey U, et al. Oxidative damage and metabolic dysfunction in Huntington’s disease: selective vulnerability of the basal ganglia. Corresponding author: Anthony H. V. Schapira, DSc, Ann Neurol. 1997;41:646-653. MD, FRCP, Clinical Neurosciences, Royal Free Hospital 26. Campuzano V, Montermini L, Molto MD, et al. Friedreich’s ataxia: autosomal re- School of Medicine, Rowland Hill Street, London NW3 2PF, cessive disease caused by an intronic GAA triplet repeat expansion. Science. 1996; England. 217:1423-1427. 27. Koutnikova H, Campuzano V, Foury F, Dolle P, Cazzalini O, Koenig M. Studies of human, mouse and yeast homologues indicate a mitochondrial function for REFERENCES frataxin. Nat Genet. 1997;16:345-351. 28. Wilson RB, Roof DM. Respiratory deficiency due to loss of mitochondrial DNA in yeast lacking the frataxin homologue. Nat Genet. 1997;16:352-357. 1. Ryan KR, Jensen RE. Protein translocation across mitochondrial membranes: 29. Priller J, Scherzer CR, Faber PW, MacDonald ME, Young AB. Frataxin gene of what a long, strange trip it is. Cell. 1995;83:517-519. Friedreich’s ataxia is targeted to mitochondria. Ann Neurol. 1997;42:265-269. 2. Holt IJ, Harding AE, Morgan-Hughes JA. Deletions of mitochondrial DNA in pa- 30. Parker WD, Mahr NJ, Filley CM, et al. Reduced platelet cytochrome c oxidase tients with mitochondrial myopathies. Nature. 1988;331:717-719. activity in Alzheimer’s disease. Neurology. 1994;44:1086-1090. 3. Moraes CT, DiMauro S, Zeviani M, et al. Mitochondrial DNA deletions in pro- 31. Davis RE, Miller S, Herrnstadt C, et al. Mutations in mitochondrial cytochrome c gressive external ophthalmoplegia and Kearns-Sayre syndrome. N Engl J Med. oxidase genes segregate with late-onset Alzheimer disease. Proc Natl Acad Sci 1989;320:1293-1299. USA.1997;94:4526-4531. 4. Hayashi J-I, Ohta S, Kikuchi A, Takemitsu M, Goto Y-I, Nonaka I. Introduction of 32. Benecke R, Stru¨mper P, Weiss H. Electron transfer complex I defect in idio- disease-related mitochondrial DNA deletions into HeLa cells lacking mitochon- pathic dystonia. Ann Neurol. 1992;32:683-686. drial DNA results in mitochondrial dysfunction. Proc Natl Acad SciUSA.1991; 33. Reichmann H, Naumann M, Hauck S, Janetsky B. Respiratory chain and mito- 88:10614-10618. chondrial deoxyribonucleic acid in blood cells from patients with focal and gen- 5. Manfredi G, Vu T, Bonilla E, et al. Association of myopathy with large-scale mi- eralized dystonia. Mov Disord. 1994;9:597-600. tochondrial DNA duplications and deletions: which is pathogenic? Ann Neurol. 34. Schapira AHV, Warner T, Gash MT, Cleeter MJW, Marinho CFM, Cooper JM. Com- 1997;42:180-188. plex I function in familial and sporadic dystonia. Ann Neurol. 1997;41:556-559. 6. Howell N. Leber hereditary optic neuropathy: how do mitochondrial DNA mutations 35. Hirano M, Shtilbans A, Mayeux R, et al. Apparent mtDNA heteroplasmy in Alz- cause degeneration of the optic nerve? J Bioenerg Biomembr. 1997;29:165-173. heimer’s disease patients and in normals due to PCR amplification of nucleus- 7. Harding AE, Sweeney MG, Miller DH, et al. Occurrence of a multiple sclerosis- embedded mtDNA pseudogenes. Proc Natl Acad SciUSA. 1997;94:14894- like illness in women who have a Leber’s hereditary optic neuropathy mitochon- 14899. drial DNA mutation. Brain. 1992;112:979-989. 36. Van den Heuvel L, Ruitenbeek W, Smeets R, et al. Demonstration of a new patho- 8. Jun AS, Brown MD, Wallace DC. A mitochondrial DNA mutation at nucleotide genic mutation in human complex I deficiency: a 5-bp duplication in the nuclear pair 14459 of the NADH dehydrogenase subunit 6 gene associated with mater- gene encoding the 18-kD (AQDQ) subunit. Am J Hum Genet. 1998;62:262-268.

ARCH NEUROL / VOL 55, OCT 1998 1296