Leber's hereditary optic neuropathy: implications of the sex ratio for linkage studies in families with the 3460 ND1 mutation

Poor visual prognosis of Asian patients with 3460 mitochondrial DNA mutation in Leber's hereditary optic neuropathy

BACKGROUND

Among the 3 primary mutations of Leber's hereditary optic neuropathy (LHON), the incidence of LHON with a mutation at nucleotide position 3460 is the lowest in Asians. Therefore, information about the clinical manifestations of LHON mutations in Asians with the 3460 mutation is limited.

OBJECTIVE

To determine the clinical manifestations including visual prognosis of Asians with the LHON 3460 mutation.

METHODS

We performed a retrospective study of 5 Korean LHON patients with the 3460 mutation.

RESULTS

All patients were male, and the age of onset for visual impairment varied from 17 to 35 years, with an average of 25.4 ± 7.16 years. Among the 10 affected eyes, only 1 eye of 1 patient showed visual improvement to 20/50 at 2 years after onset. The remaining patients had a visual acuity of worse than 20/200.

CONCLUSION

The visual prognosis of Korean patients with the LHON 3460 mutation was generally poor. Further studies regarding Asian patients with the LHON 3460 mutation are necessary.

Prevalence of Leber hereditary optic neuropathy in the Community of Madrid (Spain), estimation with a capture-recapture method

BACKGROUND

Leber hereditary optic neuropathy (LHON) typically presents in young adults as bilateral painless subacute visual loss. Prevalence data are scarce. The aim of this study was to examine the validity of different ascertainment sources used in population-based rare diseases registries to detect cases, and to explore the impact of a capture-recapture method in the estimation of the prevalence of LHON in the Autonomous Community of Madrid (ACM) in 2022.

METHODS

Descriptive cross-sectional population-based study. Potential LHON cases were detected by automatic capture from the healthcare information sources usually explored for the Regional Registry for Rare Diseases (SIERMA). Ophthalmologists provided data from their clinical registry. Positive predictive values (PPV) and sensitivity with 95% confidence intervals (CI) were estimated. Global and by sex prevalences were calculated with confimed cases and with those estimated by the capture-recapture method.

RESULTS

A total of 102 potential LHON cases were captured from healthcare information sources, 25 of them (24.5%) finally were confirmed after revision, with an overall PPV of 24.5% (95%CI 17.2-33.7). By source, the electronic clinical records of primary care had the highest PPV (51.2, 95%CI 36.7-65.4). The ophthalmologists clinical registry provided 22 cases, 12 of them not detected in the automatic capture sources. The clinical registry reached a sensitivity of 59.5% (95%CI 43.5-73.6) and the combination of automatic capture sources reached a 67.6% (95%CI: 51.5-80.4). The total confirmed cases were 37, with a mean age of 48.9 years, and a men: women ratio of 2.4:1. Genetic information was recovered in 27 cases, with the m.3460 mutation being the most frequent (12 cases). The global prevalence was 0.55 cases/100,000 inhabitants (95%CI 0.40-0.75), and with the capture-recapture method reached 0.79 cases/100,000 (95%CI 0.60-1.03), a 43.6% higher, 1.15 cases/100,000 (95%CI 0.83-1.58) in men and 0.43 cases/100,000 (95%CI 0.26-0.70) in women.

CONCLUSIONS

The prevalence of LHON estimated in the ACM was lower than in other European countries. Population-based registries of rare diseases require the incorporation of confirmed cases provided by clinicians to asure the best completeness of data. The use of more specific coding for rare diseases in healthcare information systems would facilitate the detection of cases. Further epidemiologic studies are needed to assess potential factors that may influence the penetrance of LHON.

Genetic investigation of 211 Chinese families expands the mutational and phenotypical spectra of hereditary retinopathy genes through targeted sequencing technology

BACKGROUND

Hereditary retinopathy is a significant cause of blindness worldwide. Despite the discovery of many mutations in various retinopathies, a large number of patients remain genetically undiagnosed. Targeted next-generation sequencing of the human genome is a suitable approach for the molecular diagnosis of retinopathy.

METHODS

We describe a cohort of 211 families from central China with various forms of retinopathy; 95 patients were investigated using multigene panel sequencing, and the other 116 with suspected Leber hereditary optic neuropathy (LHON) were tested by Sanger sequencing. The detected variation of targeted sequencing was verified by PCR-based Sanger sequencing. We performed a comprehensive analysis of the cases using sequencing data and ophthalmologic examination information.

RESULTS

Potential causal mutations were identified in the majority of families with retinopathy (57.9% of 95 families) and suspected LHON (21.6% of 116 families). There were 68 variants of a certain significance distributed in 31 known disease-causing genes in the 95 families; 37 of the variants are novel and have not been reported to be related to hereditary retinopathy. The NGS panel solution provided a 45.3% potential diagnostic rate for retinopathy families, with candidate gene mutations of undefined pathogenicity revealed in another 12.6%of the families.

CONCLUSION

Our study uncovered novel mutations and phenotypic aspects of retinopathy and demonstrated the genetic and clinical heterogeneity of related conditions. The findings show the detection rate of pathogenic variants in patients with hereditary retinopathy in central China as well as the diversity and gene distribution of these variants. The significance of molecular genetic testing for patients with hereditary retinopathy is also highlighted.

Childhood-onset Leber hereditary optic neuropathy

BACKGROUND

The onset of Leber hereditary optic neuropathy (LHON) is relatively rare in childhood. This study describes the clinical and molecular genetic features observed in this specific LHON subgroup.

METHODS

Our retrospective study consisted of a UK paediatric LHON cohort of 27 patients and 69 additional cases identified from a systematic review of the literature. Patients were included if visual loss occurred at the age of 12 years or younger with a confirmed pathogenic mitochondrial DNA mutation: m.3460G>A, m.11778G>A or m.14484T>C.

RESULTS

In the UK paediatric LHON cohort, three patterns of visual loss and progression were observed: (1) classical acute (17/27, 63%); (2) slowly progressive (4/27, 15%); and (3) insidious or subclinical (6/27, 22%). Diagnostic delays of 3-15 years occurred in children with an insidious mode of onset. Spontaneous visual recovery was more common in patients carrying the m.3460G>A and m.14484T>C mutations compared with the m.11778G>A mutation. Based a meta-analysis of 67 patients with available visual acuity data, 26 (39%) patients achieved a final best-corrected visual acuity (BCVA) ≥0.5 Snellen decimal in at least one eye, whereas 13 (19%) patients had a final BCVA <0.05 in their better seeing eye.

CONCLUSIONS

Although childhood-onset LHON carries a relatively better visual prognosis, approximately 1 in 5 patients will remain within the visual acuity criteria for legal blindness in the UK. The clinical presentation can be insidious and LHON should be considered in the differential diagnosis when faced with a child with unexplained subnormal vision and optic disc pallor.

Epidemiology and penetrance of Leber hereditary optic neuropathy in Finland

We have performed an entire-population-based survey of the epidemiology and penetrance of Leber hereditary optic neuropathy (LHON) in Finland - a country that is among the best-studied genetic isolates in the world. During our long-term clinical follow-up period since 1970, we have so far identified 36 LHON families in Finland, comprised of almost 1000 family members. Counting the unaffected family members has been possible thanks to accessible genealogical records, and this has improved the accuracy of our penetrance figures by minimizing the sample bias. Our results, although confirming some well-known features of LHON, indicate that the overall penetrance of LHON is lower than previously estimated, and that affected females have a higher incidence of affected offspring compared to the unaffected females. The prevalence of LHON in Finland is 1:50 000, and one in 9000 Finns is a carrier of one of the three LHON primary mutations.

Human mitochondrial diseases: answering questions and questioning answers

Since the first identification in 1988 of pathogenic mitochondrial DNA (mtDNA) mutations, the mitochondrial diseases have emerged as a major clinical entity. The most striking feature of these disorders is their marked heterogeneity, which extends to their clinical, biochemical, and genetic characteristics. The major mitochondrial encephalomyopathies include MELAS (mitochondrial encephalopathy with lactic acidosis and stroke-like episodes), MERRF (myoclonic epilepsy with ragged red fibers), KSS/CPEO (Kearns-Sayre syndrome/chronic progressive external ophthalmoplegia), and NARP/MILS (neuropathy, ataxia, and retinitis pigmentosum/maternally inherited Leigh syndrome) and they typically present highly variable multisystem defects that usually involve abnormalities of skeletal muscle and/or the CNS. The primary emphasis here is to review recent investigations of these mitochondrial diseases from the standpoint of how the complexities of mitochondrial genetics and biogenesis might determine their varied features. In addition, the mitochondrial encephalomyopathies are compared and contrasted to Leber hereditary optic neuropathy, a mitochondrial disease in which the pathogenic mtDNA mutations produce a more uniform and focal neuropathology. All of these disorders involve, at some level, a mitochondrial respiratory chain dysfunction. Because mitochondrial genetics differs so strikingly from the Mendelian inheritance of chromosomes, recent research on the origin and subsequent segregation and transmission of mtDNA mutations is reviewed.

Recent developments in the molecular genetics of mitochondrial disorders

Rapid progress has been made in the identification of mitochondrial DNA mutations which are typically associated with diseases of the nervous system and muscle. The well established mitochondrial disorders are maternally inherited and males and females are equally affected. An exception is Leber's hereditary optic atrophy (LHON) which is observed much more frequently in males than in females. There are three common point mutations in LHON which can be homoplasmic or heteroplasmic. In mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) most mutations are single base changes and lie within the tRNA-Leu gene. Point mutations in myoclonic epilepsy with ragged red fibres (MERRF) usually occur within the tRNA-Lys gene but mutations of the tRNA-Leu gene are also observed. MELAS and MERRF mutations are heteroplasmic and there is considerable clinical overlap between these diseases. Point mutations within the ATPase6 gene result in either neuropathy, ataxia and retinitis pigmentosa (NARP) or in Leigh's syndrome. The latter occurs if the mutation is present in the majority of mitochondria (extreme heteroplasmy). Finally, mitochondrial DNA deletions are the cause underlying Kearns-Sayre syndrome (KSS). Apart from the well-established mitochondrial diseases, there is increasing evidence that mitochondrial mutations may also play a role in the neurodegenerative disorders Parkinson, Alzheimer and Huntington disease. The complex I defect found in Parkinson disease is especially interesting in this respect. However, no causative mitochondrial mutation has as yet been established in any of these three common disorders.

Leber hereditary optic neuropathy: mitochondrial mutations and degeneration of the optic nerve

The predominant manifestation of Leber hereditary optic neuropathy (LHON) is a sudden and usually severe bilateral loss of central vision, most often in the mid-20s, that is due to a degeneration of the ganglion cell layer and optic nerve. LHON is an inherited form of blindness in which a mutation in the mitochondrial genome (mtDNA) is the primary etiological event. More than 95% of the LHON pedigrees in peoples of Northern European descent harbor one of the three mitochondrial mutations at nucleotides 3460, 11,778 and 14,484, although there are other rare primary mutations. In addition, there may be mtDNA mutations that have a secondary etiological role. The penetrance of the optic neuropathy is incomplete in LHON families, and males are affected much more often then females. The incomplete penetrance indicates that secondary etiological factors are necessary for the development of the optic neuropathy, although they are poorly understood at the present time. Several types of studies suggest that optic nerve function in LHON patients is impaired in the presymptomatic phase, probably as a result of a mitochondrial respiratory chain abnormality, although visual acuity is not compromised. In some family members, the presence of secondary etiological factors triggers a wave of optic nerve dysfunction in which vision is lost (the acute phase). Depending upon the particular primary LHON mutation that the patient carries, a variable proportion of the dysfunctional ganglion cells and optic nerve axons die during the atrophic phase, probably through an apoptotic pathway. In 11,778 LHON patients, retinal ganglion cell degeneration occurs almost without exception, and recovery of vision is extremely rare. In contrast, activation of the cell death pathway is less frequent, or less extensive, in 14,484 LHON patients and there is often a substantial recovery of vision.

A gene for X-linked optic atrophy is closely linked to the Xp11.4-Xp11.2 region of the X chromosome

The aim of this study was to identify the chromosomal location of the disease-causing gene in a family apparently segregating X-linked optic atrophy. A large family of 45 individuals with a four-generation history of X-linked optic atrophy was reexamined in a full ophthalmic as well as electrophysiological examination. A DNA linkage analysis of the family was undertaken in order to identify the chromosomal location of the disease-causing gene. Linkage analysis was performed with 26 markers that spanned the entire X chromosome. The affected males showed very early onset and slow progression of the disease. Ophthalmic study of the female carriers did not reveal any abnormalities. Close linkage without recombination was found at the MAOB locus (maximum LOD score [Zmax] 4.19). The Zmax - 1 support interval was found at a recombination fraction of .076 distal and .018 proximal to MAOB. Multipoint linkage analysis placed the optic atrophy-causing gene in the Xp11.4-p11.21 interval between markers DXS993 and DXS991, whereas any other localization along the X chromosome could be excluded.

On the many faces of Leber hereditary optic neuropathy

Leber hereditary optic neuropathy (LHON) is a maternally inherited disorder, associated with mutations in the mitochondrial DNA, which is notorious for its aspecific presentations. Two pedigrees are described with cases that are atypical for LHON with respect to sex, age of onset, interval between the eyes becoming affected, course of the disease, concomitant disorders, additional test results, final visual acuity, and/or results of mtDNA analysis. Moreover, the pedigrees themselves did not suggest maternal inheritance. We analysed the diagnostic and clinical genetic difficulties related to the atypical aspects of these pedigrees. We conclude that mtDNA analysis is justified in every case of optic nerve atrophy with no clear cause. Identification of one of the three LHON specifically associated mtDNA mutations is essential to confirm the diagnosis.

Prospects for DNA-based prenatal diagnosis of mitochondrial disorders

Mitochondria have their own DNA which is maternally inherited. Mitochondrial DNA (mtDNA) diseases are extremely variable because of the genetics of mtDNA and the unique pathogenesis of these disorders. This makes predicting the prognosis and the transmission of mtDNA disorders difficult. While mtDNA polymorphisms at a single base position are common, the overwhelming majority of the mitochondrial genomes within a single human individual are usually identical. When there is a point mutation difference between a mother and her offspring, there may be a complete switching of mtDNA type within a single generation. It is generally assumed that there is a genetic bottleneck whereby a single or small number of founder mtDNA(s) populate the organism, but it is not known at which stages the restriction/amplification of mtDNA subtype(s) occur, and this uncertainty impedes antenatal diagnosis for mtDNA disorders. Autosomally inherited disorders of mitochondrial function may be caused by mutations in genes for the components of the respiratory chain and for the machinery of mitochondrial biogenesis, which are nuclear-encoded. Accurate diagnosis of these disorders is important as prenatal diagnosis is available in a minority of cases.

Leber's hereditary optic neuropathy: heteroplasmy is likely to be significant in the expression of LHON in families with the 3460 ND1 mutation

AIM

To assess the effect of heteroplasmy on the expression of Leber's hereditary optic neuropathy (LHON) in a large family with the 3460 LHON mutation.

METHODS

Mutation detection was performed by restriction enzyme digestion of polymerase chain reaction (PCR) products. Heteroplasmy was estimated by quantitation of wild type:mutant product ratios.

RESULTS

There is a significant association between levels of mutant mtDNA and manifestation of the disease phenotype.

CONCLUSION

As a high proportion of families with the 3460 mutation demonstrate heteroplasmy; this is likely to be a significant factor in disease expression.

No evidence for 'skewed' inactivation of the X-chromosome as cause of Leber's hereditary optic neuropathy in female carriers

Leber's hereditary optic neuropathy (LHON) is a maternally inherited disorder of the optic nerves. It has been proposed that the specific mutations in the mitochondrial DNA (mtDNA) that are associated with LHON require and X-chromosomally encoded permissive factor in order to become expressed. This would explain both the preponderance of male patients and the fact that most carriers of specific mtDNA mutations remain unaffected. Although linkage studies have been negative so far, the existence of such a factor has not been ruled out. We investigated the genealogical data of 24 large LHON pedigrees and concluded that the presumed X-linked factor would be recessively inherited and that at least 57% of the affected females would be heterozygous. Therefore, these females must be the victim of nonrandom X-chromosomal inactivation (skewed lyonization). However, analysis of X-chromosomal methylation patterns in 16 LHON-affected females revealed substantial skewing in only 15%-20% of cases, which is not significantly different from the patterns in 49 controls. Moreover, we found the frequency of LHON in daughters of affected heterozygous females to be twice to three times as high as in daughters of unaffected heterozygous females, which cannot be explained by an X-chromosomally inherited factor. We concluded that the results of our investigations do not support the hypothesis that LHON is a digenic disease with an X-linked factor being the main cause of loss of vision in the presence of relevant mtDNA mutations.