The role of mtDNA background in disease expression: a new primary LHON mutation associated with Western Eurasian haplogroup J

Leber's hereditary optic neuropathy (LHON) is a maternally transmitted form of blindness caused by mitochondrial DNA (mtDNA) mutations. Approximately 90% of LHON cases are caused by 3460A, 11778A, or 14484C mtDNA mutations. These are designated "primary" mutations because they impart a high risk for LHON expression. Although the 11778A and 14484C mutations unequivocally predispose carriers to LHON, they are preferentially associated with mtDNA haplogroup J, one of nine Western Eurasian mtDNA lineages, suggesting a synergistic and deleterious interaction between these LHON mutations and haplogroup J polymorphism(s). We report here the characterization of a new primary LHON mutation in the mtDNA ND4L gene at nucleotide pair 10663. The homoplasmic 10663C mutation has been found in three independent LHON patients who lack a known primary mutation and all of which belong to haplogroup J. This mutation has not been found in a large number of haplotype-matched or non-haplogroup-J control mtDNAs. Phylogenetic analysis with primarily complete mtDNA sequence data demonstrates that the 10663C mutation has arisen at least three independent times in haplogroup J, indicating that it is not a rare lineage-specific polymorphism. Analysis of complex I function in patient lymphoblasts and transmitochondrial cybrids has revealed a partial complex I defect similar in magnitude to the 14484C mutation. Thus, the 10663C mutation appears to be a new primary LHON mutation that is pathogenic when co-occurring with haplogroup J. These results strongly support a role for haplogroup J in the expression of certain LHON mutations.     

Mitochondrial DNA and genetic disease.

Since the human mitochondrial genome was characterised and sequenced in 1981, it has been viewed as the likely site of genetic diseases showing a maternal inheritance pattern and associated with defects of the respiratory chain, such as the mitochondrial myopathies (MMs). The properties that make it a candidate for the source of such conditions are that it encodes polypeptides involved in electron transport and that it is maternally inherited. However, several of the mtDNA diseases only fulfill one or other of these criteria: the first group of mtDNA diseases showed only sporadic deletions, and the first point mutation in Leber's Hereditary Optic Neuropathy (LHON) is not associated with a clear biochemical defect. Furthermore, it is now clear that both autosomal dominant and probably recessive nuclear genes can cause abnormalities of mtDNA. Each of these major groups will be considered in turn.     

Leber's hereditary optic neuropathy: a model for mitochondrial neurodegenerative diseases.

A number of human diseases have been attributed to defects in oxidative phosphorylation (OXPHOS) resulting from mutations in the mitochondrial DNA (mtDNA). One such disease is Leber's hereditary optic neuropathy (LHON), a neurodegenerative disease of young adults that results in blindness due to atrophy of the optic nerve. The etiology of LHON is genetically heterogeneous and in some cases multifactorial. Eleven mtDNA mutations have been associated with LHON, all of which are missense mutations in the subunit genes for the subunits of the electron transport chain complexes I, III, and IV. Molecular, biochemical, and population genetic studies have categorized these mutations as high risk (class I), low risk (class II), or intermediate risk (class I/II). Class I mutations appear to be primary genetic causes of LHON, while class II mutations are frequently found associated with class I genotypes and may serve as exacerbating genetic factors. Different LHON pedigrees can harbor different combinations of class I, II, or I/II mtDNA mutations, as shown by the complete sequence analysis of the mtDNAs of four LHON probands. The various mtDNA genotypes included an isolated class I mutation, combined class I+II mutations, and combined class I/II+II mutations. The occurrence of such genotypes supports the hypothesis that LHON may result from the additive effects of various genetic and environmental insults to OXPHOS, each of which increases the probability of blindness.     

Genetic heterogeneity in Leber hereditary optic neuroretinopathy revealed by mitochondrial DNA polymorphism.

The presence or absence of a recently observed mitochondrial DNA (mtDNA) mutation associated with Leber hereditary optic neuroretinopathy (LHON) was tested in 19 Finnish families with cases of LHON. Leukocyte and muscle DNA from individuals with optic atrophy, microangiopathy, or normal fundi from maternal lineages were studied by Southern blot analysis, using mouse mtDNA as a hybridization probe. The mtDNA mutation, detected as SfaNI site polymorphism, was seen in 10 of the 19 families. In one family, the mutation was seen only in the two affected individuals, indicating recent origin for the mutation. Nine families and 28 maternally unrelated controls did not show the mutation. The results imply that alternative mtDNA mutations are associated with LHON and that this genetic heterogeneity may be the cause of the interfamilial variation in the clinical expression of LHON. In the families showing the SfaNI site mutation, the mutation was homoplasmic in all individuals irrespective of their disease status, suggesting that the intrafamilial variation in the clinical expression is not due to different ratios of mutant versus normal mtDNA.     

Clinical, biochemical and molecular genetic features of Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) has traditionally been considered a disease causing severe and permanent visual loss in young adult males. In nearly all families with LHON it is associated with one of three pathogenic mitochondrial DNA (mtDNA) mutations, at bp 11778, 3460 or 14484. The availability of mtDNA confirmation of a diagnosis of LHON has demonstrated that LHON occurs with a wider range of age at onset and more commonly in females than previously recognised. In addition, analysis of patients grouped according to mtDNA mutation has demonstrated differences both in the clinical features of visual failure and in recurrence risks to relatives associated with each of the pathogenic mtDNA mutations. Whilst pathogenic mtDNA mutations are required for the development of LHON, other factors must be reponsible for the variable penetrance and male predominance of this condition. Available data on a number of hypotheses including the role of an additional X-linked visual loss susceptibility locus, impaired mitochondrial respiratory chain activity, mtDNA heteroplasmy, environmental factors and autoimmunity are discussed. Subacute visual failure is seen in association with all three pathogenic LHON mutations. However, the clinical and experimental data reviewed suggest differences in the phenotype associated with each of the three mutations which may reflect variation in the disease mechanisms resulting in this common end-point.     

Detection of the G to A mitochondrial DNA mutation at position 11778 in German families with Leber's hereditary optic neuropathy

Summary Leber's hereditary optic neuropathy (LHON) is characterized by acute or subacute bilateral (usually permanent) loss of central vision, caused by neuroretinal degeneration. The maternal inheritance is explained by the mitochondrial origin of the disease. Recently, a single mitochondrial DNA (mtDNA) mutation, a G to A substitution at position 11778 that converts a highly conserved arginine to histidine, has been associated with LHON. The mutation eliminates an SfaNI restriction enzyme recognition site and thus provides a method for detection of the muation by amplification, enzyme digestion and agarose gel electropheresis of polymerase chain reaction (PCR) products. Leukocyte mtDNA from 7 German families with LHON, diagnosed by clinical criteria, was tested for the presence of the G to A mutation at bp 11778. The mtDNa mutation, detected as a loss of the SfaNI site, was seen in one family. The G to A mtDNA mutation is the only known gene alteration associated with LHON so far. It has been identified in patients of different ethnic origin and recent reports strongly support the hypothesis that it represents the most frequent cause of LHON. Identification of the mtDNA replacement mutation using PCR and restriction enzyme digestion requires only a small amount of blood and can be performed rapidly. This method is thus a useful tool in the diagnosis of LHON.     

LHON: Mitochondrial Mutations and More

Leber's hereditary optic neuropathy (LHON) is a mitochondrial disorder leading to severe visual impairment or even blindness by death of retinal ganglion cells (RGCs). The primary cause of the disease is usually a mutation of the mitochondrial genome (mtDNA) causing a single amino acid exchange in one of the mtDNA-encoded subunits of NADH:ubiquinone oxidoreductase, the first complex of the electron transport chain. It was thus obvious to accuse neuronal energy depletion as the most probable mediator of neuronal death. The group of Valerio Carelli and other authors have nicely shown that energy depletion shapes the cell fate in a LHON cybrid cell model. However, the cybrids used were osteosarcoma cells, which do not fully model neuronal energy metabolism. Although complex I mutations may cause oxidative stress, a potential pathogenetic role of the latter was less taken into focus. The hypothesis of bioenergetic failure does not provide a simple explanation for the relatively late disease onset and for the incomplete penetrance, which differs remarkably between genders. It is assumed that other genetic and environmental factors are needed in addition to the 'primary LHON mutations' to elicit RGC death. Relevant nuclear modifier genes have not been identified so far. The review discusses the unresolved problems of a pathogenetic hypothesis based on ATP decline and/or ROS-induced apoptosis in RGCs.     

Profiling the Mitochondrial Proteome of Leber’s Hereditary Optic Neuropathy (LHON) in Thailand: Down-Regulation of Bioenergetics and Mitochondrial Protein Quality Control Pathways in Fibroblasts with the 11778G>A Mutation

Leber’s Hereditary Optic Neuropathy (LHON) is one of the commonest mitochondrial diseases. It causes total blindness, and predominantly affects young males. For the disease to develop, it is necessary for an individual to carry one of the primary mtDNA mutations 11778G>A, 14484T>C or 3460G>A. However these mutations are not sufficient to cause disease, and they do not explain the characteristic features of LHON such as the higher prevalence in males, incomplete penetrance, and relatively later age of onset. In order to explore the roles of nuclear encoded mitochondrial proteins in development of LHON, we applied a proteomic approach to samples from affected and unaffected individuals from 3 pedigrees and from 5 unrelated controls. Two-dimensional electrophoresis followed by MS/MS analysis in the mitochondrial lysate identified 17 proteins which were differentially expressed between LHON cases and unrelated controls, and 24 proteins which were differentially expressed between unaffected relatives and unrelated controls. The proteomic data were successfully validated by western blot analysis of 3 selected proteins. All of the proteins identified in the study were mitochondrial proteins and most of them were down regulated in 11778G>A mutant fibroblasts. These proteins included: subunits of OXPHOS enzyme complexes, proteins involved in intermediary metabolic processes, nucleoid related proteins, chaperones, cristae remodelling proteins and an anti-oxidant enzyme. The protein profiles of both the affected and unaffected 11778G>A carriers shared many features which differed from those of unrelated control group, revealing similar proteomic responses to 11778G>A mutation in both affected and unaffected individuals. Differentially expressed proteins revealed two broad groups: a cluster of bioenergetic pathway proteins and a cluster involved in protein quality control system. Defects in these systems are likely to impede the function of retinal ganglion cells, and may lead to the development of LHON in synergy with the primary mtDNA mutation.     

The epidemiology of Leber hereditary optic neuropathy in the North East of England

We performed the first population-based clinical and molecular genetic study of Leber hereditary optic neuropathy (LHON) in a population of 2,173,800 individuals in the North East of England. We identified 16 genealogically unrelated families who harbor one of the three primary mitochondrial DNA (mtDNA) mutations that cause LHON. Two of these families were found to be linked genetically to a common maternal founder. A de novo mtDNA mutation (G3460A) was identified in one family. The minimum point prevalence of visual failure due to LHON within this population was 3.22 per 100,000 (95% CI 2.47-3.97 per 100,000), and the minimum point prevalence for mtDNA LHON mutations was 11.82 per 100,000 (95% CI 10.38-13.27 per 100,000). These results indicate that LHON is not rare but has a population prevalence similar to autosomally inherited neurological disorders. The majority of individuals harbored only mutant mtDNA (homoplasmy), but heteroplasmy was detected in approximately 12% of individuals. Overall, however, approximately 33% of families with LHON had at least one heteroplasmic individual. The high incidence of heteroplasmy in pedigrees with LHON raises the possibility that a closely related maternal relative of an index case may not harbor the mtDNA mutation, highlighting the importance of molecular genetic testing for each maternal family member seeking advice about their risks of visual failure.     

Biochemical and molecular diagnosis of mitochondrial respiratory chain disorders

Biochemical diagnosis of mitochondrial respiratory chain disorders requires caution to avoid misdiagnosis of secondary enzyme defects, and can be improved by the use of conservative diagnostic criteria. Pathogenic mutations causing mitochondrial disorders have now been identified in more than 30 mitochondrial DNA (mtDNA) genes encoding respiratory chain subunits, ribosomal- and t-RNAs. mtDNA mutations appear to be responsible for most adult patients with mitochondrial disease and approximately a quarter of paediatric patients. A family history suggesting maternal inheritance is the exception rather than the norm for children with mtDNA mutations, many of whom have de novo mutations. Prenatal diagnosis and pre-implantation genetic diagnosis can be offered to some women at risk of transmitting a mtDNA mutation, particularly those at lower recurrence risk. Mutations in more than 30 nuclear genes, including those encoding for respiratory chain subunits and assembly factors, have now been shown to cause mitochondrial disorders, creating difficulties in prioritising which genes should be studied by mutation analysis in individual patients. A number of approaches offer promise to guide the choice of candidate genes, including Blue Native-PAGE immunoblotting and microarray expression analysis.     

Analysis of mitochondrial ND4 gene DNA sequence in Finnish families with Leber hereditary optic neuroretinopathy

A mutation in the mitochondrial DNA at nt 11,778 has recently been found in Leber hereditary optic neuroretinopathy (LHON), a maternally inherited ocular disease. The mutation is located in the ND4 gene encoding subunit 4 of the respiratory chain enzyme NADH dehydrogenase. The mutation was subsequently not found in 9 of the 20 known Finnish families with LHON, implying that there are at least two different mutations associated with the disease. Using direct sequencing of PCR-amplified mtDNA, we have now sequenced the entire ND4 region in the families without the nt 11,778 mutation to find the other mutations. No new mutations in the ND4 region were found, suggesting that the putative mtDNA mutation in these families may be in the coding regions for other subunits of NADH dehydrogenase enzyme. The sequence of ND4 gene as found to be highly homogeneous.     

Clustering of Caucasian Leber hereditary optic neuropathy patients containing the 11778 or 14484 mutations on an mtDNA lineage.

Leber hereditary optic neuropathy (LHON) is a type of blindness caused by mtDNA mutations. Three LHON mtDNA mutations at nucleotide positions 3460, 11778, and 14484 are specific for LHON and account for 90% of worldwide cases and are thus designated as "primary" LHON mutations. Fifteen other "secondary" LHON mtDNA mutations have been identified, but their pathogenicity is unclear. mtDNA haplotype and phylogenetic analysis of the primary LHON mutations in North American Caucasian patients and controls has shown that, unlike the 3460 and 11778 mutations, which are distributed throughout the European-derived (Caucasian) mtDNA phylogeny, patients containing the 14484 mutation tended to be associated with European mtDNA haplotype J. To investigate this apparent clustering, we performed chi2-based statistical analyses to compare the distribution of LHON patients on the Caucasian phylogenetic tree. Our results indicate that, unlike the 3460 and 11778 mutations, the 14484 mutation was not distributed on the phylogeny in proportion to the frequencies of the major Caucasian mtDNA haplogroups found in North America. The 14484 mutation was next shown to occur on the haplogroup J background more frequently that expected, consistent with the observation that approximately 75% of worldwide 14484-positive LHON patients occur in association with haplogroup J. The 11778 mutation also exhibited a moderate clustering on haplogroup J. These observations were supported by statistical analysis using all available mutation frequencies reported in the literature. This paper thus illustrates the potential importance of genetic background in certain mtDNA-based diseases, speculates on a pathogenic role for a subset of LHON secondary mutations and their interaction with primary mutations, and provides support for a polygenic model for LHON expression in some cases.     

A novel mtDNA ND6 gene mutation associated with LHON in a Caucasian family

Leber's hereditary optic neuropathy (LHON) is a frequent cause of inherited blindness. A routine screening for common mtDNA mutations constitutes an important first in its diagnosis. However, a substantial number of LHON patients do not harbor known variants, both pointing to the genetic heterogeneity of LHON and bringing into question its genetic diagnosis. We report a familial case that exhibited typical features of LHON but lacked any of the common mutations. Genetic analysis revealed a novel pathogenic defect in the ND6 gene at 14279A that was not detected in any haplogroup-matched controls screened for it, nor has it been previously reported. This mutation causes a substantial conformational change in the secondary structure of the polypeptide matrix coil and may explain the LHON expression. Thus, it expands the spectrum of deleterious changes affecting ND6-encoding subunit and further highlights the functional significance of this gene, providing additional clues to the disease pathogenesis.     

Functional analysis of lymphoblast and cybrid mitochondria containing the 3460, 11778, or 14484 Leber's hereditary optic neuropathy mitochondrial DNA mutation

Leber's hereditary optic neuropathy (LHON) is a form of blindness caused by mitochondrial DNA (mtDNA) mutations in complex I genes. We report an extensive biochemical analysis of the mitochondrial defects in lymphoblasts and transmitochondrial cybrids harboring the three most common LHON mutations: 3460A, 11778A, and 14484C. Respiration studies revealed that the 3460A mutation reduced the maximal respiration rate 20-28%, the 11778A mutation 30-36%, and the 14484C mutation 10-15%. The respiration defects of the 3460A and 11778A mutations transferred in cybrid experiments linking these defects to the mtDNA. Complex I enzymatic assays revealed that the 3460A mutation resulted in a 79% reduction in specific activity and the 11778A mutation resulted in a 20% reduction, while the 14484C mutation did not affect the complex I activity. The enzyme defect of the 3460A mutation transferred with the mtDNA in cybrids. Overall, these data support the conclusion that the 3460A and 11778A mutants result in complex I defects and that the 14484C mutation causes a much milder biochemical defect. These studies represent the first direct comparison of oxidative phosphorylation defects among all of the primary LHON mtDNA mutations, thus permitting insight into the underlying pathophysiological mechanism of the disease.     

Fifteen novel mutations in the mitochondrial NADH dehydrogenase subunit 1, 2, 3, 4, 4L, 5 and 6 genes from Iranian patients with Leber’s hereditary optic neuropathy (LHON)

Leber’s hereditary optic neuropathy (LHON) is an optic nerve dysfunction resulting from mutations in mitochondrial DNA (mtDNA), which is transmitted in a maternal pattern of inheritance. It is caused by three primary point mutations: G11778A, G3460A and T14484C; in the mitochondrial genome. These mutations are sufficient to induce the disease, accounting for the majority of LHON cases, and affect genes that encode for the different subunits of mitochondrial complexes I and III of the mitochondrial respiratory chain. Other mutations are secondary mutations associated with the primary mutations. The purpose of this study was to determine MT-ND variations in Iranian patients with LHON. In order to determine the prevalence and distribution of mitochondrial mutations in the LHON patients, their DNA was studied using PCR and DNA sequencing analysis. Sequencing of MT-ND genes from 35 LHON patients revealed a total of 44 nucleotide variations, in which fifteen novel variations—A14020G, A13663G, C10399T, C4932A, C3893G, C10557A, C12012A, C13934T, G4596A, T12851A, T4539A, T4941A, T13255A, T14353C and del A 4513—were observed in 27 LHON patients. However, eight patients showed no variation in the ND genes. These mutations contribute to the current database of mtDNA polymorphisms in LHON patients and may facilitate the definition of disease-related mutations in human mtDNA. This research may help to understand the disease mechanism and open up new diagnostic opportunities for LHON.     

The Background of Mitochondrial DNA Haplogroup J Increases the Sensitivity of Leber's Hereditary Optic Neuropathy Cells to 2,5-Hexanedione Toxicity

Leber''s hereditary optic neuropathy (LHON) is a maternally inherited blinding disease due to mitochondrial DNA (mtDNA) point mutations in complex I subunit genes, whose incomplete penetrance has been attributed to both genetic and environmental factors. Indeed, the mtDNA background defined as haplogroup J is known to increase the penetrance of the 11778/ND4 and 14484/ND6 mutations. Recently it was also documented that the professional exposure to n-hexane might act as an exogenous trigger for LHON. Therefore, we here investigate the effect of the n-hexane neurotoxic metabolite 2,5-hexanedione (2,5-HD) on cell viability and mitochondrial function of different cell models (cybrids and fibroblasts) carrying the LHON mutations on different mtDNA haplogroups. The viability of control and LHON cybrids and fibroblasts, whose mtDNAs were completely sequenced, was assessed using the MTT assay. Mitochondrial ATP synthesis rate driven by complex I substrates was determined with the luciferine/luciferase method. Incubation with 2,5-HD caused the maximal loss of viability in control and LHON cells. The toxic effect of this compound was similar in control cells irrespective of the mtDNA background. On the contrary, sensitivity to 2,5-HD induced cell death was greatly increased in LHON cells carrying the 11778/ND4 or the 14484/ND6 mutation on haplogroup J, whereas the 11778/ND4 mutation in association with haplogroups U and H significantly improved cell survival. The 11778/ND4 mutation on haplogroup U was also more resistant to inhibition of complex I dependent ATP synthesis by 2,5-HD. In conclusion, this study shows that mtDNA haplogroups modulate the response of LHON cells to 2,5-HD. In particular, haplogroup J makes cells more sensitive to its toxic effect. This is the first evidence that an mtDNA background plays a role by interacting with an environmental factor and that 2,5-HD may be a risk element for visual loss in LHON. This proof of principle has broad implications for other neurodegenerative disorders such as Parkinson''s disease.     

Sequence analysis of the mitochondrial genomes from Dutch pedigrees with Leber hereditary optic neuropathy

The complete mitochondrial DNA (mtDNA) sequences for 63 Dutch pedigrees with Leber hereditary optic neuropathy (LHON) were determined, 56 of which carried one of the classic LHON mutations at nucleotide (nt) 3460, 11778, or 14484. Analysis of these sequences indicated that there were several instances in which the mtDNAs were either identical or related by descent. The most striking example was a haplogroup J mtDNA that carried the 14484 LHON mutation. Four different but related mitochondrial genotypes were identified in seven of the Dutch pedigrees with LHON, including six of those described by van Senus. The control region of the founder sequence for these Dutch pedigrees with LHON matches the control-region sequence that Macmillan and colleagues identified in the founder mtDNA of French Canadian pedigrees with LHON. In addition, we obtained a perfect match between the Dutch 14484 founder sequence and the complete mtDNA sequences of two Canadian pedigrees with LHON. Those results indicate that these Dutch and French Canadian 14484 pedigrees with LHON share a common ancestor, that the single origin of the 14484 mutation in this megalineage occurred before the year 1600, and that there is a 14484/haplogroup J founder effect. We estimate that this lineage--including the 14484 LHON mutation--arose 900-1,800 years ago. Overall, the phylogenetic analyses of these mtDNA sequences conservatively indicate that a LHON mutation has arisen at least 42 times in the Dutch population. Finally, analysis of the mtDNA sequences from those pedigrees that did not carry classic LHON mutations suggested candidate pathogenic mutations at nts 9804, 13051, and 14325.     

Sequence analysis of the complete mitochondrial genome in patients with Leber's hereditary optic neuropathy lacking the three most common pathogenic DNA mutations

Leber's hereditary optic neuropathy (LHON) is a maternally inherited disorder characterized by central vision loss in young adults. The majority of LHON cases around the world are associated with mutations in the mitochondrial genome at nucleotide positions (np) 3460, 11,778, and 14,484. Usually, these three mutations are screened in suspected LHON patients. The result is important not only in respect to the diagnosis but also as different LHON mutations lead to variations in expression, severity, and recovery of the disease. There are, however, a significant number of patients without any of these primary mutations. In these situations, genetic counselling of a patient and his family can be difficult. We sequenced the complete mitochondrial DNA (mtDNA) in 14 LHON patients with the typical clinical features but without a primary mtDNA mutation to evaluate the potential of extensive mutation screening for clinical purposes. Our results suggest to include the mutation at np 15,257 in a routine screening as well as the ND6 gene, a hot spot for LHON mutations. Screening for the secondary LHON mutations at np 4216 and np 13,708 may also help in making the diagnosis of LHON as these seem to modify the expression of LHON mutations. Although they do not allow to prove the clinical diagnosis, their presence increases the probability of LHON. Sequencing the complete mitochondrial genome can reveal novel and known rare disease causing mutations. However, considering the effort it adds little value for routine screening.     

7例携带线粒体tRNAAlaC5601T突变的Leber遗传性视神经病变家系的相关研究

文章收集了7例携带线粒体tRNAAl。C5601T突变的中国Leber遗传性视神经病变(Leber’s hereditary opticneuropathy,LHON)的家系,通过眼科检查和遗传学分析,发现7个家系的外显率很低,分别为9.5%、14.3%、4.5%、8.3%、10.0%、22.2%和25.0%。用24对有部分重叠的引物对7个先证者线粒体DNA(Mitochondrial DNA,mtDNA)全序列进行扩增,并进行相关的分子生物学分析,结果发现这些家系均未携带G11778A、G3460A和T14484C这3个常见的原发突变位点,而在tRNAAla上发现了C5601T同质性突变,多态性位点分析分别属于东亚线粒体单体型G2、G2a1、G2a1、G2、G2b、G2a1、G2。C5601T突变位于线粒体tRNAAla的高度保守区(通用位点为59位),可能引起tRNA空间结构和稳定性发生改变,继而影响tRNA的代谢,导致线粒体蛋白和ATP合成障碍,最终导致视力损害。因此,tRNAAlaC5601T突变可能是与LHON相关的线粒体突变位点。同时低外显率提示其他因素(包括核修饰基因、环境因素)可能影响这7个中国C5601T突变家系的表型表达。