The role of the Met98Lys optineurin variant in inherited optic nerve diseases

Comparison of the clinical and genetic features of autosomal dominant optic atrophy and normal tension glaucoma in young Chinese adults

BACKGROUND/OBJECTIVES

To compare the clinical and optical coherence tomography (OCT) characteristics of autosomal dominant optic atrophy (ADOA) and normal tension glaucoma (NTG) in Chinese patients.

SUBJECTS/METHODS

Twenty-four unrelated patients with ADOA and 21 unrelated patients with NTG, younger than 30 years, were enrolled in this study. Data regarding the demographic and clinical characteristics of the patients were collected, and their peripapillary retinal nerve fibre layer (RNFL) and macular ganglion cell complex (GCC) thicknesses were evaluated using OCT. Sequencing of genes associated with neuro-ophthalmic disorders was performed for all patients.

RESULTS

The average age at onset of the ADOA group (13.92 ± 10.73 years) was significantly younger than that of the NTG group (23.67 ± 4.98 years, P = 0.002). Best-corrected visual acuity was significantly poorer in the ADOA group (0.75 ± 0.32) than in the NTG group (0.16 ± 0.19, P < 0.001). The average peripapillary RNFL thickness and the RNFL thicknesses in the temporal upper, temporal lower, and nasal lower sectors were significantly thinner in the ADOA group than in the NTG group (all P < 0.05). Moreover, the macular GCC thickness of the ADOA group was significantly thinner than that of the NTG group (P < 0.001). Twenty-three OPA1 variants (11 novel OPA1 variants) and one OPA3 variant were detected in 24 patients with ADOA.

CONCLUSIONS

Our study revealed a distinct difference between the patterns of RNFL and GCC loss in ADOA and NTG, which will help to differentiate ADOA from NTG in young patients. Additionally, this study expanded the genetic spectrum of ADOA.

Establishing risk of vision loss in Leber hereditary optic neuropathy

We conducted an updated epidemiological study of Leber hereditary optic neuropathy (LHON) in Australia by using registry data to establish the risk of vision loss among different LHON mutations, sex, age at onset, and mitochondrial haplogroup. We identified 96 genetically unrelated LHON pedigrees, including 56 unpublished pedigrees, and updated 40 previously known pedigrees, comprising 620 affected individuals and 4,948 asymptomatic carriers. The minimum prevalence of vision loss due to LHON in Australia in 2020 was one in 68,403 individuals. Although our data confirm some well-established features of LHON, the overall risk of vision loss among those with a LHON mutation was lower than reported previously-17.5% for males and 5.4% for females. Our findings confirm that women, older adults, and younger children are also at risk. Furthermore, we observed a higher incidence of vision loss in children of affected mothers as well as in children of unaffected women with at least one affected brother. Finally, we confirmed our previous report showing a generational fall in prevalence of vision loss among Australian men. Higher reported rates of vision loss in males with a LHON mutation are not supported by our work and other epidemiologic studies. Accurate knowledge of risk is essential for genetic counseling of individuals with LHON mutations. This knowledge could also inform the detection and validation of potential biomarkers and has implications for clinical trials of treatments aimed at preventing vision loss in LHON because an overestimated risk may lead to an underpowered study or a false claim of efficacy.

Incomplete penetrance in mitochondrial optic neuropathies

Incomplete penetrance characterizes the two most frequent inherited optic neuropathies, Leber's Hereditary Optic Neuropathy (LHON) and dominant optic atrophy (DOA), due to genetic errors in the mitochondrial DNA (mtDNA) and the nuclear DNA (nDNA), respectively. For LHON, compelling evidence has accumulated on the complex interplay of mtDNA haplogroups and environmental interacting factors, whereas the nDNA remains essentially non informative. However, a compensatory mechanism of activated mitochondrial biogenesis and increased mtDNA copy number, possibly driven by a permissive nDNA background, is documented in LHON; when successful it maintains unaffected the mutation carriers, but in some individuals it might be hampered by tobacco smoking or other environmental factors, resulting in disease onset. In females, mitochondrial biogenesis is promoted and maintained within the compensatory range by estrogens, partially explaining the gender bias in LHON. Concerning DOA, none of the above mechanisms has been fully explored, thus mtDNA haplogroups, environmental factors such as tobacco and alcohol, and further nDNA variants may all participate as protective factors or, on the contrary, favor disease expression and severity. Next generation sequencing, complemented by transcriptomics and proteomics, may provide some answers in the next future, even if the multifactorial model that seems to apply to incomplete penetrance in mitochondrial optic neuropathies remains problematic, and careful stratification of patients will play a key role for data interpretation. The deep understanding of which factors impinge on incomplete penetrance may shed light on the pathogenic mechanisms leading to optic nerve atrophy, on their possible compensation and, thus, on development of therapeutic strategies.

Mitochondrial optic neuropathies - disease mechanisms and therapeutic strategies

Leber hereditary optic neuropathy (LHON) and autosomal-dominant optic atrophy (DOA) are the two most common inherited optic neuropathies in the general population. Both disorders share striking pathological similarities, marked by the selective loss of retinal ganglion cells (RGCs) and the early involvement of the papillomacular bundle. Three mitochondrial DNA (mtDNA) point mutations; m.3460G>A, m.11778G>A, and m.14484T>C account for over 90% of LHON cases, and in DOA, the majority of affected families harbour mutations in the OPA1 gene, which codes for a mitochondrial inner membrane protein. Optic nerve degeneration in LHON and DOA is therefore due to disturbed mitochondrial function and a predominantly complex I respiratory chain defect has been identified using both in vitro and in vivo biochemical assays. However, the trigger for RGC loss is much more complex than a simple bioenergetic crisis and other important disease mechanisms have emerged relating to mitochondrial network dynamics, mtDNA maintenance, axonal transport, and the involvement of the cytoskeleton in maintaining a differential mitochondrial gradient at sites such as the lamina cribosa. The downstream consequences of these mitochondrial disturbances are likely to be influenced by the local cellular milieu. The vulnerability of RGCs in LHON and DOA could derive not only from tissue-specific, genetically-determined biological factors, but also from an increased susceptibility to exogenous influences such as light exposure, smoking, and pharmacological agents with putative mitochondrial toxic effects. Our concept of inherited mitochondrial optic neuropathies has evolved over the past decade, with the observation that patients with LHON and DOA can manifest a much broader phenotypic spectrum than pure optic nerve involvement. Interestingly, these phenotypes are sometimes clinically indistinguishable from other neurodegenerative disorders such as Charcot-Marie-Tooth disease, hereditary spastic paraplegia, and multiple sclerosis, where mitochondrial dysfunction is also thought to be an important pathophysiological player. A number of vertebrate and invertebrate disease models has recently been established to circumvent the lack of human tissues, and these have already provided considerable insight by allowing direct RGC experimentation. The ultimate goal is to translate these research advances into clinical practice and new treatment strategies are currently being investigated to improve the visual prognosis for patients with mitochondrial optic neuropathies.

Optineurin coding variants in Ghanaian patients with primary open-angle glaucoma

PURPOSE

Coding variants in the optineurin gene (OPTN, GLC1E) have been reported to play a role in primary open-angle glaucoma (POAG) in various populations. This study investigated the role of OPTN sequence variants in patients with POAG in Ghana (West Africa).

METHODS

This is a case-control study of unrelated Ghanaian POAG cases and non-glaucomatous controls. Ascertainment criteria for POAG included the presence of glaucomatous optic nerve neuropathy, associated visual field loss, and elevated intraocular pressure (IOP) in both eyes, all in the absence of secondary causes of glaucoma. Controls had normal optic nerves, visual fields, and IOP. All the coding exons of OPTN were polymerase chain reaction (PCR) amplified and sequenced in all 140 cases and 130 controls using an ABI 3730 DNA analyzer.

RESULTS

All the coding exons of OPTN were sequenced in 140 POAG patients and 130 controls. Several coding variants were identified including M98K, A134A, V147L, P292P, A301G, S321S, and E322K. Three coding variants (V147L, P292P, and A301G) have not been reported previously. There were no significant differences on the frequencies of all the identified variants between POAG cases and controls in this population.

CONCLUSIONS

This is the first comprehensive study of OPTN in a single West African population. Our results suggest that coding variants in OPTN may not contribute to the risk for POAG in persons of West African descent.

The genetics of primary open-angle glaucoma: a review

Glaucoma is the major cause of irreversible blindness worldwide. Primary open-angle glaucoma (POAG), as the most prevalent form of glaucoma, is a complex inherited disorder and affects more than 2 million individuals in the United States. It has become increasingly clear that a host of genetic as well as environmental factors are likely to contribute to the phenotype. A number of chromosomal and genetic associations have been reported for POAG. This review examines what is currently known about the underlying genetic structure, what remains to be learned, and how this may affect our medical management of this major blinding disease.

Absence of optineurin (OPTN) gene mutations in Taiwanese patients with juvenile-onset open-angle glaucoma

PURPOSE

To investigate sequence variants in the optineurin (OPTN) gene in patients with juvenile-onset open-angle glaucoma (JOAG) in Taiwan.

METHODS

We analyzed the sequence variants of OPTN in 51 unrelated Taiwanese probands with JOAG and in 51 control group subjects who did not have JOAG. Genomic DNA was extracted from the individuals and subjected to polymerase chain reaction (PCR) to amplify all 16 exons and flanking introns of OPTN. The amplified products were then screened for base variants by autosequence. Data from the two study groups were then compared using Fisher's exact test and Armitage's trend test.

RESULTS

Fifteen variants of OPTN were found in the 51 JOAG patients and 51 unrelated normal controls. Two were missense variants (M98K and K322E), one was a synonymous codon change (T34T), and 12 were changes in the noncoding sequences. Seven of the variants have been reported and eight were novel. All of the sequence changes were found in patients with JOAG and in the normal controls except for variant c.-233+25C>G, which was found only in the control group. Allelic frequencies of these sequence changes did not differ significantly between patients and controls (p>0.05) except for the variant c.-233+25C>G (p<0.001). Genotype frequencies of c.-233+25C>G was shown to be significant between the two groups using Fisher's two-tailed exact test (p<0.001) and Armitage's trend test (p=6.815e(-06)).

CONCLUSIONS

Our data indicate that none of the mutations in OPTN are associated with JOAG. The variant M98K is not a risk factor and the variant c.-233+25C>G may be protective against glaucoma in Taiwanese.

Variation in optineurin (OPTN) allele frequencies between and within populations

PURPOSE

To evaluate the extent to which mutations in the optineurin (OPTN) glaucoma gene play a role in glaucoma in different populations.

METHODS

Case-controlled study of OPTN sequence variants in individuals with or without glaucoma in populations of different ancestral origins and evaluate previous OPTN reports. We analyzed 314 subjects with African, Asian, Caucasian and Hispanic ancestries included 229 cases of primary open-angle glaucoma, 51 cases of juvenile-onset open-angle glaucoma, 33 cases of normal tension glaucoma, and 371 controls. Polymerase chain reaction-amplified OPTN coding exons were resequenced and case frequencies were compared to frequencies in controls matched for ancestry.

RESULTS

The E50K sequence variant was identified in one individual from Chile with normal tension glaucoma, and the 691_692insAG variant was found in one Ashkenazi Jewish individual from Russia. The R545Q variant was found in two Asian individuals with primary open-angle glaucoma; one of Filipino ancestry and one of Korean ancestry. In addition to presenting OPTN allele frequencies for Caucasian and Asian populations that have been the subject of previous reports, we also present information for populations of Hispanic and black African ancestries.

CONCLUSIONS

Our study contributes additional evidence to support the previously reported association of the OPTN E50K mutation with glaucoma. After finding an additional 691_692insAG OPTN variant, we can still only conclude that this variant is rare. Combined analysis of our data with data from more than a dozen other studies indicates no association of R545Q with glaucoma in most populations. Those same studies disagree in their conclusions regarding the role of M98K in glaucoma. Our analysis of the combined data provides statistically significant evidence of association of M98K with normal tension glaucoma in Asian populations, but not in Caucasian populations; however, the validity of this conclusion is questionable because of large differences in allele frequencies between and within populations. It is currently not possible to tell how much of the underlying cause of the allele frequency difference is attributable to demographic, technical, or ascertainment differences among the studies.