JMed Genet 1995;32:81-87

Review article

Leber's hereditary optic neuropathy: the clinicalrelevance of different mitochondrial DNAmutations

Paul Riordan-Eva, Anita E Harding

Leber's hereditary optic neuropathy (LHON)was first described in 1858 by von Graefe.'It was later named after Theodore Leber inrecognition of his paper published in vonGraefe' Archives of Ophthalmology in 1871.2Characteristically the disease presents in thesecond or third decades oflife as a subacute opticneuropathy, developing sequentially in the twoeyes, with vision reduced to worse than 6/60,large central visual field defects, poor colourvision, and little recovery ofvision. The import-ance of LHON lies in its ability to cause suchsevere and usually permanent visual loss ingenerally fit young people who are about tostart or have recently begun employment orhigher education, and the ramifications of diag-nosing an inherited disease to other membersof the patient's family. The recent discovery ofvarious mutations of mitochondrial DNA infamilies with LHON has provided both animportant diagnostic tool and also a means toimprove our ability to determine prognosis andto understand the pathogenesis of the disease.

Department ofNeuro-Ophthalmology,National Hospitalfor Neurology andNeurosurgery,Queen Square,London WCIN 3BG,UKP Riordan-Eva

UniversityDepartment ofClinical Neurology(NeurogeneticsSection), Instituteof Neurology,Queen Square,London WClN 3BG,UKA E Harding

Correspondence to:Mr Riordan-Eva.

The inheritance of LHONLeber recognised the two unusual features ofthe inheritance of ILHON: maternal trans-mission and the propensity to affect males.Many theories have been proposed to explainthe maternal inheritance pattern. Imai and Mo-riwaki3 first suggested cytoplasmic inheritancein 1936 and Erikson4 suggested a mutationof mitochondrial DNA (mtDNA) in 1972. In1988 Wallace et al5 identified a mutation atbase pair (bp) 11778 of mitochondrial DNAthat is exclusively found in families withLHON.

Subsequently many mutations of mtDNAhave been reported in LHON but the pa-

thogenic significance of several of them is dis-puted.

Evaluating the pathogenicity ofmtDNAmutations in LHONAll the mutations so far identified inLHON arepoint mutations ofmtDNA. They are generallydetected by amplification of the relevant regionof mtDNA and digestion by a restriction en-

zyme, for which the mutation causes loss or

gain of a site. The criteria which are appliedto establish the pathogenicity ofindividual mut-ations include the following.(1) Identification ofthe mutation in a numberof families known to have LHON, preferablyfamilies of independent origin with large welldocumented pedigrees showing a clear patternof maternal transmission.(2) Identification of the mutation exclusivelyin affected families and not in either normalcontrols or subjects with other types of opticneuropathy. Such a mutation is referred to asa primary mutation.(3) The mutation should not coexist withother known pathogenic mutations.(4) The mutation should be heteroplasmic insome people from affected families, as het-eroplasmy does not seem to be a feature ofharmless polymorphisms.(5) The mutation should change a highly con-served amino acid residue.The criteria are not absolute but the more

criteria that are fulfilled the more certain thepathogenicity of the mutation under con-sideration.

The mtDNA mutations in LHONThe causative role for the Wallace mutation,which is the most commonly identified muta-tion in LHON families from Europe, the USA,Japan, and Australia, was originally establishedby identification of the mutation in two differ-ent pedigrees with LHON, one of an Americanblack family and the other of a European whitefamily.6 The mutation is not found in controlsand it is heteroplasmic in certain subjects. Themutation results in substitution of histidine fora highly conserved arginine in the structure ofNADH dehydrogenase (ND 4) in the complex Isubunit of the mitochondrial respiratory chain.Both arginine and histidine are basic aminoacids and thus this substitution is relativelyconservative. A reduction in complex I activityhas been shown in people with the 1 1778 muta-tion but only to the level found in smokerswithout the mutation.7 The 11778 mutation isnot found in other optic neuropathies, such asanterior ischaemic optic neuropathy.Other primary mutations, found exclusively

in LHON families, include the mutations at

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bp 3460, 14484, 4160, 4136, and 5244.8-12 Allof these mutations also result in amino acidchanges in complex I polypeptides. The 3460and 14484 mutations are now generally ac-cepted as causative, having also been foundexclusively in multiple LHON families of in-dependent origin, and always being associatedwith the same clinical disorder. The recognitionof the 14484 mutation as being of pathogenicimportance was delayed because it only pro-duces a conservative amino acid substitution(valine for methionine) in a relatively poorlyconserved residue of the ND6 subunit of com-plex I. The 3460 mutation results in a sub-stitution of a hydrophilic threonine for ahydrophobic alanine at a highly conserved res-idue in the ND1 subunit of complex I. Boththe 3460 and 14484 mutations are found inthe absence of other primary and secondarymutations and both are associated with changesin complex I activity. About 90% of patientswith LHON worldwide carry either the 11778,3460, or 14484 mutations.13The mutations at 4160 and 4136 were iden-

tified in a single family from Australia." It wassuggested that the mutation at 4160 was theprimary mutation and that the mutation at4136 influenced the clinical syndrome, the fam-ily being characterised by unusual neurologicalfeatures (see below). It has now been shownthat this family also has the 14484 mutation,'4which is more likely to be the primary pa-thogenic mutation with respect of the opticnerve disease. The mutation at 5244 was alsoidentified only in a single pedigree from theUSA.'2 It is heteroplasmic and affects a con-served region of the ND2 subunit of complexI. A mutation at 13730 has been identified,in the absence of other previously describedprimary or secondaryLHON mutations, in oneperson with clinical findings consistent withLHON but without a family history. The muta-tion was not present in any other members ofthe family, even the mother.'5 The mutationwas heteroplasmic and results in a substitutionof glutamic acid for glycine in what is thoughtto be an extramembrane hydrophobic domainof the ND5 protein of complex I.Numerous mutations have been identified

that occur in both LHON and control familiesbut possibly at a higher frequency in theformer.' 1216-20 These secondary mutations areoften found in association with other secondaryor primary mutations, making it difficult toassess their significance. It has been suggestedthat in the absence of one of the three majorprimary pathogenic mutations (11778, 3460,or 14484) LHON only occurs if there aremultiple secondary mutations, and that if aprimary pathogenic mutation is present thenthe coexistence of secondary mutations willaffect the disease manifestations.'2162' NormalmtDNA is highly polymorphic and thus thesetheories are difficult to validate or refute dir-ectly.The secondary mutations include the muta-

tion at bp 15257 which produces a substitutionof asparagine for a highly conserved aspartatein apocytochrome b of complex III, resultingin a conformational change in a haem binding

site. The mutation has been homplasmic inall cases tested, is found in 0 3% of normalcontrols, and is very frequently found in as-sociation with the 13708 or the 14484mutations.'222 A study from the Netherlandsof three pedigrees with LHON and the 15257mutation showed that in each of the pedigreeseither the 11778, 3460, or 14484 mutation wasalso present and that the 15257 mutation didnot influence the clinical features or the prob-ability ofthe development ofLHON.23 A NorthAmerican study of patients with LHON andthe 15257 mutation found a 42% prevalenceof spinal cord and peripheral neurologicalsymptoms, although the aetiology of thesesymptoms was not investigated.24The 15257 mutation has also been associated

with retinal pigment epithelial disease, saidmost closely to resemble the inherited retinaldystrophy Stargardt's disease.25 Three affectedpatients were reported of whom one also hadthe 14484 mutation. The sister of the patientwith both mutations developed unilateral visualloss for which the diagnosis of LHON wasconsidered. Otherwise no details were providedas to the existence of a family history of visualloss or of any similar retinal changes in otherrelatives of the three probands.The 13708 mutation produces a substitution

of threonine for alanine in ND5 of complex1.16 It is found in up to 8% of normal controlsand is likely to be a common northern Europeanpolymorphism.26 Assessing the significance ofthe secondary mutations is complicated bydifferences in their prevalence around theworld. For instance the 9438 mutation is foundin up to 18% of Africans but less than 0-05%of controls from non-African backgrounds.2728The detection of any of these secondary mut-ations cannot be used to make a diagnosis ofLHON.

Male predominance in LHONThe predominance of males among affectedsubjects with LHON is still unexplained. Theconcept of an X linked visual loss susceptibilitygene was suggested by linkage studies29 but itsexistence was not confirmed by other authors30and has now been refuted by reassessment ofthe original data.3'

The relevance ofmtDNA mutations inLHONThe identification of mtDNA mutations inLHON has led to a major reappraisal of thedisease. In this respect it is imperative that thediagnostic significance ofthe various mutationsis understood if this reappraisal is to be worth-while, and if incorrect diagnoses with theirundesirable consequences to other membersof the family are to be avoided. Before theavailability of mtDNA testing, diagnosis ofLHON was based upon clinical findings, whenpossible supported by a family history, and theexclusion of other causes of optic neuropathy.Now the identification in a patient with an opticneuropathy of one of the primary pathogenicmutations, at bp 11778, 3460, or 14484,

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Leber' hereditary optic neuropathy: the clinical relevance of different mitochondrial DNA mutations

virtually establishes the diagnosis whether thereis a family history or not. Occasionally falsepositive results are obtained, as a result ofnon-pathogenic polymorphisms occurring veryclose to the mutation being investigated andcoincidentally changing the relevant restrictionsite."2 Consideration should also be given towhether other factors have precipitated theonset of visual loss, as will be discussed later.Difficulty arises when the patient with an opticneuropathy is found to have one or more sec-ondary mutations. Most clinicians would re-gard this as a negative result and rely on theclinical picture and the presence of a familyhistory to diagnose LHON.

Clinical spectrum ofLHONThe availability of mitochondrial DNA studieshas emphasised that LHON may present fromthe first to the seventh decade of life, may beunilateral, may be associated with significantrecovery of vision, and may be associated withneurological disease. All these relatively un-common features have been identified pre-viously in family studies but when they manifestin isolated cases the diagnosis of LHON isnow much easier to substantiate. It is alsoapparent that the diagnosis ofLHON can easilybe missed in isolated cases if factors point toother diagnoses. For example a history ofheavyalcohol and tobacco consumption may lead toa diagnosis of tobacco/alcohol amblyopia. Thegreater accessibility to and use ofmtDNA test-ing in the investigation of optic neuropathiesshould reduce the risk of such misdiagnosis.Of increasing interest to clinicians is the

potential for correlating disease manifestationswith particular mutations. A number of reportsof the clinical manifestations in families withthe three primary pathogenic mutations havenow been published. 22 23 33-37 There is relativelylittle difference in most of the features ofLHON between patients possessing the threeprimary pathogenic mutations. Men are morecommonly affected than women, in a ratio ofabout 3:1. Only about 50% of index cases havean appropriate family history. The mean age atonset of visual loss is in the third decade of life.Virtually all patients suffer bilateral symmetricalvisual loss with the two eyes being affectedsequentially in about 50%, the median delaybetween the two eyes being affected in suchcases ranging from 6 to 22 weeks. Usuallyvision declines in each eye over a period ofeight to 16 weeks with the visual acuity fallingto counting fingers only.

The ocular fundus in LHONLHON is associated with characteristic al-though not entirely specific fundalabnormalities.2 38A0 The small superficial ves-sels in the retina adjacent to the optic discappear dilated and telangiectatic. The retinalnerve fibre layer in the peripapillary regionappears swollen as may the optic disc itself. Onfluorescein angiography there is no leakage ofdye from the optic disc, in comparison to mostcauses of acquired optic disc swelling in which

fluorescein leakage from the optic disc is anearly feature. Also there is no leakage of dyefrom the retinal vessels.Fundal abnormalities were reported to be

universal in the acute phase of LHON suchthat their absence was said to exclude thediagnosis.40 Now it is clear that the fundalabnormalities are frequently not present" andthis may be particularly the case in the 14484and 3460 mutation groups.2225The pathogenesis ofthe fundal abnormalities

in LHON is unclear. It has been suggested thatthey indicate a primary retinal microvasculardisorder but they may also be a non-specificresponse to changes within the retinal nervefibre layer or within the anterior optic nerve.Also very similar features may be seen in thefundi of normal children and teenagers. Con-sequently no conclusions can yet be drawnabout the relevance of the possibile paucity offundal changes in LHON not associated withthe Wallace mutation, except that it is likely tohinder the diagnosis being made in such cases.

Visual outcome in LHONThe most important and very consistent findingof recent publications is that the 14484 muta-tion is associated with a much better, andindeed relatively good, prognosis for vision.Studies from Australia, the USA, and the Neth-erlands have documentated a significant rateof recovery of vision among LHON patientswith the 14484 mutation.22363 Fifty per centof patients in these three series recovered visionand the overall median final visual acuity inthe better eye was 6/24 compared to 3/60 forthe 11778 and 3460 mutation groups. Fur-thernore a younger age of onset of visual loss,for practical purposes before the age of 20, isstrongly correlated with a better visual out-come. Among 14 patients at the National Hos-pital with LHON and the 14484 mutation forwhom follow up at more than 24 months fromthe time of onset ofvisual loss is available, 50%have recovered vision to 6/24 or better in thebetter eye. Six out of seven with onset of visualloss before 20 years of age have a final visualacuity of 6/9 or better in the better eye, whichshould allow the possession of a standard Brit-ish driving licence (Riordan-Eva et al, sub-mitted). The better visual outcome in the14484 mutation group is clearly because ofvisual recovery and not relatively mild diseasefrom the outset. This recovery does not occurfor many months or even years after the initialvisual loss, and usually more than one yearfrom the onset. The 3460 mutation may alsobe associated with a better visual outcome thanthe 11778 group but this has not yet beenfirmly established.'5

Mechanism of optic neuropathy inLHONHistopathological studies of LHON, carriedout several years after the onset of visual loss,showed both axonal degeneration and myelinpallor involving the whole of the anterior visualpathways to the lateral geniculate bodies.4142

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As discussed above, the fundal abnormalitieshave been interpreted as indicating a primarilyintraocular disorder but this is not generallyaccepted. The clinical features suggest in-volvement of the retinal ganglion cells andtheir axons within the retina and optic nerves,possibly with selective involvement of smallercells and axons explaining the characteristiclarge central visual field defects. In some peoplethe nature of the field defect indicated in-volvement of the optic chiasm, which was alsosuggested by the findings at surgicalexploration.4344 Electrodiagnostic studies havenot as yet shown any specific features althoughfurther studies using pattern electro-retinography (PERG) are warranted to explorethe potential involvement of the retinal gan-glion cells. MRI abnormalities were originallyinterpreted as indicating disease in the orbitalportion of the optic nerve45 but this is nowthought to be a non-specific sign of Walleriandegeneration and gliosis.

Before the identification of a mitochondrialDNA mutation in LHON, biochemical studiesrelated to a possible abnormality of cyanidemetabolism with its potential relationship tocigarette smoking, which has been implicatedin the precipitation of visual loss in the disease.Plasma cyanocobalamin concentrations werefound to be raised in LHON patients, com-patible with an inborn error of cyanide meta-bolism.46 Plasma and urinary levels ofthiocyanate, the product ofthe detoxification ofcyanide by the enzyme rhodanese (thiosulphatesulphur transferase), were found to be lowerin LHON patients who smoked compared tocontrol smokers.4 Measurement of rhodaneseactivity has produced conflicting results withsome studies showing reduced levels in liverand rectal mucosa and others showing normallevels in liver and muscle.4"" There is nodefinite evidence that patients with LHONhave been exposed to excessive cyanide levels,even through cigarette smoking, and the role ofcyanide intoxication in the causation of isolatedoptic nerve disease is disputed.The identification of mtDNA mutations in

LHON has shifted interest towards functionalabnormalities of the mitochondrial respiratorychain. As discussed above, all the mtDNAmutations identified as of primary pathogenicimportance in LHON families, and most of thethe other mutations, produce changes in thestructure of components of complex I of themitochondrial respiratory chain. In the Aus-tralian family with the 14484 and 4160 bpmutations a deficiency of complex I activity, asmeasured by rotenone sensitive NADH-CoQreductase activity, was found in platelets ofaffected members.50 Reductions in complex Iactivity have been reported in muscle and isol-ated platelet mitochondria ofboth affected andunaffected members of families with the 11778bp and the 3460 bp mutations.5 52 In the 11778mutation group the mild reduction (ap-proximately 10%) was equivalent to that foundin smokers without the mutation but the de-ficiency is more marked with the 3460 muta-tion. Thus the presence of a mitochondrialDNA mutation is associated with reduction in

complex I activity but there is not a clearcorrelation with development of visual loss.There has been continuing interest in the

relationship between tobacco and excessive al-cohol consumption in the precipitation ofvisualloss in LHON. There are certain similarities inthe clinical features of LHON and the opticneuropathy seen in tobacco/alcohol amblyopia,and it has been suggested that among LHONpatients there is a greater than normal pre-valence of tobacco and alcohol consumption,although this has not been formally sub-stantiated. Among the 56 adult patients withLHON at the National Hospital for whom atobacco and alcohol history was available, 11%of those with the 11778 mutation, 58% withthe 14484 mutation, and 50% with the 3460mutation consumed both tobacco and largeamounts (more than 40 units per week) ofalcohol but these data were not controlled(Riordan-Eva et al, submitted).The true relationship between optic nerve

disease and tobacco/alcohol abuse is still de-bated. Patients with the syndrome known astobacco/alcohol amblyopia are usually verypoorly nourished and vitamin deficiency, prin-cipally ofmembers of the B vitamin group suchas B12 and Bi, which is known to cause opticnerve disease, may be a more important ae-tiological factor. The one published study thatlooked for the three primary pathogenicmtDNA mutations in patients with tobacco/alcohol amblyopia found a mutation in two outof 12 (17%) patients.53 One patient had the11778 mutation and one the 3460 mutation.Neither patient had a family history of LHON.Their clinical features did not appear to differsignificantly from the patients without theLHON mutations.

Further studies are needed to examine therelevance of tobacco and alcohol consumptionto the pathogenesis of visual loss in LHON,particularly whether patients with LHON dosmoke and drink more than their unaffectedmaternal relatives and whether any sus-ceptibility to tobacco, alcohol, or vitamin de-ficiencies may differ from one mutation groupto another. In the mean time it seems prudentto advise subjects at risk of developing LHONthat tobacco smoking and heavy alcohol con-sumption should be avoided.

The clinical significance of heteroplasmyHeteroplasmy, the presence of a mixture ofnormal and mutant mtDNA, is commonlyfound in leucocytes from LHON patients andtheir unaffected relatives. Usually there are onlytrace amounts of normal mtDNA, but about14% of patients and unaffected relatives withthe 11778 mutation exhibit more obvious het-eroplasmy.54 There is variability in the degreeof heteroplasmy between families and betweendifferent generations of the same family. Rapidchanges from very low proportions of mutantDNA to almost complete homoplasmy for mut-ant DNA can occur within two or threegenerations."" Heteroplasmy may be a phasein the establishment of a pathogenic mutation.

Heteroplasmy has been implicated as the

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Leber' hereditary optic neuropathy: the clinical relevance of different mitochondrial DNA mutations

basis for the incomplete penetrance inLHON,5457 only about 50% ofmales and about10% of females possessing a primary patho-genic mutation suffering visual loss, andthe variability in severity of visual loss amongmembers of the same family.5758 Althoughvisual loss is more likely if the patient is homo-plasmic for mutant DNA, the correlation be-tween the degree of heteroplasmy and theophthalmological features is not good. For ex-

ample many people who are homoplasmic formutant DNA do not suffer visual loss. Thismay be a reflection of discrepancies betweenthe degree of heteroplasmy in leucocytes andthat in the optic nerve and retina,59 althoughthis seems relatively unlikely.

LHON and neurological diseaseAn association between a maternally inheritedoptic neuropathy identical to LHON and otherneurological disease has long been recognised.The associated neurological diseases includemultiple sclerosis, Charcot-Marie-Tooth dis-ease, and dystonic syndromes.6"2 In 1992Harding et al6" reported six patients, all fe-males, with the 11778 bp mutation, who pre-sented with bilateral optic neuropathy and a

family history consistent with LHON, and laterdeveloped a multiple sclerosis-like illness ac-

cording to clinical, MRI, and CSF findings.The combination of LHON and a multiplesclerosis-like illness has also been reported inone male patient with the 11778 bp mutation64and two women with the 3460 mutation.26The risk ofwomen with the 11778 mutation

developing a multiple sclerosis-like illness hasnot been defined prospectively. At the NationalHospital, among 11 women with LHON andthe 11778 mutation (including three of thepatients reported by Harding et al 63) five hada multiple sclerosis-like illness (Riordan-Eva etal, submitted).This association between multiple sclerosis

and LHON has prompted speculation aboutthe role of immunological factors in the patho-genesis of LHON.65 Systemic steroid therapydoes not appear to have any benefit in LHON.Whether other forms ofimmunological therapywill be helpful in improving vision or preventingsecond eye involvement has yet to be de-termined.The Australian family from Queensland with

the 14484, 4160, and 4136 mutations appearsto be unique in being associated with both asubacute infantile encephalopathy and otherneurological abnormalities including tremor,ataxia, dysarthria, posterior column signs, andspasticity.66 The aetiology of these variousneurological features is not clear but the en-

cephalopathy does resemble Leigh's syndrome,which has been reported in association withother mutations of mtDNA.67 This en-

cephalopathy apparently has occurred very in-

frequently, ifat all, since the original descriptionin 1970.1' A mutation at bp 4160 of mtDNA,which results in the substitution of proline fora highly conserved leucine and disruption of a

small alpha helix in a hydrophilic loop of theND1 protein, was reported in this kindred."

It was also suggested that the mutation at bp4136, present in some but not all branches ofthe family, which produces a substitution ofcysteine for tyrosine within the ND1 protein,acted as an intragenic suppressor mutation,ameliorating the neurological abnormalities.Subsequently this kindred was found to havethe 14484 mutation.'4 The ophthalmologicalfeatures in this family resemble those of otherfamilies with the 14484 mutation, with a rel-atively good visual prognosis and a correlationbetween good visual outcome and an earlyonset of visual loss. Interestingly all males ap-pear to develop some degree of visual loss. Theneurological features still seem to be associatedwith the presence ofthe 4160 mutation. A largepedigree showing maternal transmission ofvariable features, including a LHON-like opticneuropathy and dystonia, was recently iden-tified as having a mutation at position 14459.68

LHON and cardiac diseaseElectrocardiographic abnormalities that havebeen associated with LHON include pre-ex-citation syndromes in eight pedigrees reportedfrom Finland,69 prolonged Q-T invervals in anAmerican black family with the 11778 muta-tion,70 and a combination of deep Q waves andtall R waves in the anterior chest leads in twopatients reported from England.7' Informationon the relative prevalence of elec-trocardiographic abnormalities between thethree primary pathogenic mutations groups hasnot been published.

Genetic counsellingGenetic counselling in LHON is not straight-forward as transmission of a mutation is notalways associated with transmission of the dis-ease. Although mtDNA heteroplasmy may be afactor determining penetrance of the disease,57most patients and their unaffected relativesfrom LHON families have very high amountsof mutant mtDNA (>95%), and many malesat risk remain unaffected in this context. Forthis reason, analysis of mtDNA in relatives atrisk is not very useful, particularly in youngmales at risk. Finding homoplasmy for therelevant mtDNA mutation, the most likely re-sult, serves largely to confirm maternity andnearly always enhances family anxieties, evenif the likelihood of this outcome is explainedbeforehand. This particularly applies to testingchildren, which is also dubious from the ethicalpoint of view and thus best avoided. Althoughit is possible that some affected males mayrepresent fresh mutations, which would be sug-gested by absence of the mutation in theirmothers and other matrilineal relatives, this hasonly been described for the recently identified13730 mutation which is of uncertain pa-thogenetic significance.'5 It seems reasonableto analyse mtDNA in female relatives whoseek genetic advice, partly for this theoreticalpossibility, and also because they may be atlower risk of transmitting the disease if theyhave substantial (>30%) proportions ofnormalmtDNA. However, this is again more theor-

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etical than proven, and heteroplasmy in bloodmay not reflect the situation in their ocytes.There have been no published data con-

cerning recurrence risks in genetically definedLHON, and it is not clear whether they vary,depending on the mutation. Preliminary ana-

lysis of 87 LHON pedigrees with the 11778(most common), 3460, and 14484 mutationsstudied at the National Hospital suggests risksof 34, 14, 42, 9, 40, and 2% to the brothers,sisters, nephews, nieces, and male and femalefirst cousins of patients respectively (Hardingand Riordan-Eva, unpublished data). Theserisks can be modified taking age related pene-trance into account; the median age at onsetfor all three mutations is approximately 20 yearsfor males and 28 years for females, ranging from3 to 65 years.The difficulties referred to above in the use

ofmtDNA analysis in relatives at risk also applyto molecular genetic prenatal diagnosis. As itis not known what proportion of mutantmtDNA in chorionic villus material reducesthe genetic risk, or whether this proportion maychange during pre-and postnatal development,the only option for female carriers who wishto avoid a high risk pregnancy is abortion ofmale fetuses. However, the daughters of thesewomen have a tangible risk of being affectedand some may choose alternatives such as invitro fertilisation of donated ova.

The futureThe development ofmtDNA testing forLHONhas made an immense difference to the cer-

tainty with which the diagnosis can be madein subjects without a family history, in women,and in patients with atypical optic neuropathies.There has been a very useful re-evaluation ofthe clinical spectrum of the disease, whichhas actually led to very little change in theconclusions of previous family studies. Theplethora of mtDNA mutations identified infamilies in LHON has resulted in confusion as

to the pathogenic significance ofeach mutation,but it has been established that the three prim-ary mutations at bp 11778, 3460, and 14484are present in at least 90% of families withLHON. The correlation between the 14484mutation and a good visual prognosis not onlyprovides hope for affected patients, but an

avenue for further research into the patho-genesis of LHON. Hopefully this will directresearch into methods to prevent and treat thedisease.

1 von Graefe A. Ein ungewohnlicher Fall von hereditareAmaurose. Graefes Arch Ophthalmol 1858;4:266-8.

2 Leber T. Ueber hereditare und congenital-angelegte Sehner-venleiden. Graefes Arch Ophthalmol 1871;17:249-9 1.

3 Imai Y, Moriwaki D. A probable case of cytoplasmic in-

heritance in man. A critique of Leber's disease. J7 Genet

Hum 1936;3:163-7.4 Erikson RP. Leber's optic atrophy: a possible example of

mitochondrial inheritance. Am Hum Genet 1972;24:348-9.

5 Wallace DC, Singh G, Lott MT, et al. Mitochondrial DNAmutation associated with Leber's hereditary optic neuro-

pathy. Science 1988;242:1427-30.6 Singh G, Lott MT, Wallace DC. A mitochondrial DNA

mutation as a cause of Leber's hereditary optic neuropathy.N Engl Med 1989;320:1300-5.

7 Smith PR, Cooper JM, Govan GG, Harding AE, SchapiraAHV. Platelet mitochondrial function in Leber's hereditaryoptic neuropathy. Neurol Sci 1994;122:6-14.

8 Huoponen K, Vilkki J, Aula P, Nikoskelainen EK, Savontaus

ML. A new mtDNA mutation associated with Leber

hereditary optic neuroretinopathy. Am J Hum Genet 1991;48:1147-53.

9 Howell N, Bindoff LA, McCullough DA, et al. Leber her-editary optic neuropathy: identification of the same mito-chondrial ND1 mutation in six pedigrees. Am J HumGenet 1991;49:939-50.

10 Johns DR, Neufeld MJ, Park RD. An ND-6 mitochondrialDNA mutation associated with Leber hereditary opticneuropathy. Biochem Biophys Res Commun 1992;187: 1551-7.

11 Howell N, Kubacka I, Xu M, McCullough DA. Leberhereditary optic neuropathy: involvement of the mito-chondrial ND 1 gene and evidence for an intragenic sup-pressor mutation. Am J Hum Genet 199 1;48:935-42.

12 Wallace DC, Brown MD, Lott MT, Voljavec AS, TorroniA, Yang CC. Multiple mitochrondrial DNA mutationsassociated with Leber's hereditary optic neuropathy. Cy-togenet Cell Genet 1991;58:2121.

13 Newman NJ. Leber's hereditary optic neuropathy. Newgenetic considerations. Arch Neurol 1993;50:540-8.

14 Mackey DA. Three subgroups of patients from the UnitedKingdom with Leber hereditary optic neuropathy. Eye1994;8:431-6.

15 Howell N, Halvorson S, Bums J, McCullough DA, PoultonJ. When does bilateral optic atrophy become Leber her-editary optic neuropathy?Am_JHum Genet 193;53:959-63.

16 Johns DR, Berman J. Alternative simultaneous complex Imitochondrial DNA mutations in Leber's hereditary opticneuropathy. Biochem Biophys Res Commun 1991;174: 1324-30.

17 Johns DR, Neufeld MJ. Cytochrome c oxidase mutationsin Leber hereditary optic neuropathy. Biochem Biophys ResCommun 1993;196:810-15.

18 Huoponen K, Lamminen T, Juvonen V, Aula P, Ni-koskelainen E, Savontaus ML. The spectrum of mito-chondrial DNA mutations in families with Leberhereditary optic neuroretinopathy. Hum Genet 1993;92:379-84.

19 Brown MD, Yang CC, Trounce I, Torroni A, Lott MT,Wallace DC. A mitochondrial DNA variant, identified inLeber hereditary optic neuropathy patients, which extendsthe amino acid sequence of cytochrome c oxidase subunitI. Am JfHum Genet 1992;51:378-85.

20 Johns DR, Neufeld MJ. Cytochrome b mutations in Leberhereditary optic neuropathy. Biochem Biophys Res Commun1991;181: 1358-64.

21 Shoffner JM, Wallace DC. Mitochondrial genetics: prin-ciples and practice. Am J Hum Genet 1992;51:1179-86.

22 Johns DR, Heher KL, Miller NR, Smith KH. Leber'shereditary optic neuropathy. Clinical manifestations of the14484 mutation. Arch Ophthalmol 1993;111:495-8.

23 Oostra RJ, Bolhuis PA, Zom-Ende I, de Kok-Nazaruk MM,Bleeker-Wagemakers EM. Leber's hereditary optic neuro-

pathy: no significant evidence for primary or secondarypathogenicity of the 15257 mutation. Hum Genet 1994;94:265-70.

24 Johns DR, Smith KH, Savino J, Miller NR. Leber's her-editary optic neuropathy. Clinical manifestations of the15257 mutation. Ophthalmology 1993;100:981-6.

25 Heher KL, Johns DR A maculopathy associated with the15257 mitochondrial DNA mutation. Arch Ophthalmol1993;111: 1495-9.

26 Kellar-Wood H, Robertson N, Govan GG, Compston DAS,Harding AE. Leber's hereditary optic neuropathy mito-chondrial DNA mutations in multiple sclerosis. Ann Neurol1994;36: 109-12.

27 Brown MD, Torroni A, Huoponen K, Chen YS, Lott MT,Wallace DC. Pathological significance of the mtDNACOX III mutation at nucleotide pair 9438 in Leber her-editary optic neuropathy. Am J Hum Genet 1994;55:410.

28 Johns DR. Reply to Brown et al. AmJHum Genet 1994;55:410-12.

29 Vilkki J, Ott J, Savontaus ML, Aula P, Nikoskelainen EK.Optic atrophy in Leber hereditary optic neuroretinopathyis probably determined by an X-chromosomal gene closelylinked to DXS7. Am J Hum Genet 1991;48:486-91.

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