Research into the genetics of glaucoma is helping us better understand the disease. Discoveries are enabling improved screening for and stratification of disease, and leading to new treatments. One day, we may even find cures.
Glaucoma is a group of conditions characterised by progressive optic neuropathy. It is the second most common cause of irreversible vision loss in Australia, affecting up to 3% of the population over 40 years old. Primary open-angle glaucoma (POAG) is the most common type of glaucoma and its exact cause and pathogenesis remains to be determined. Raised intraocular pressure (IOP) is an important risk factor for the development of glaucoma and lowering IOP remains the only way to alter the clinical course, regardless of whether the patient’s IOP is elevated or not.
Anyone with bilateral glaucoma who has a visual field showing central involvement, or a mean deviation <-22dB in the worst eye, can be referred to ANZRAG
As far back as 1842, having a family history of glaucoma was recognised as a risk factor, pointing to a genetic basis for glaucoma. The Glaucoma Inheritance Study Tasmania found that almost 60% of people with glaucoma had at least one family member with the disease, of which 39% had a first-degree relative affected. Studies internationally found similar results. The population-based Rotterdam Eye Study found that first-degree relatives of those with POAG had a nine times higher risk of developing glaucoma.
This strong association between family history and glaucoma is key and Glaucoma Australia has sought to increase awareness of this among affected individuals and their family members. An increased uptake of screening examinations in these higher-risk populations should improve the rate of diagnosis; currently 50% of people with glaucoma remain undiagnosed.
COMPLEX DISEASE GENETICS
While a family history of glaucoma confers a higher risk of disease, the genetic basis of glaucoma is generally complex rather than Mendelian. With a Mendelian disorder, a single genetic defect from one (autosomal dominant, x-linked) or both (autosomal recessive) parents results in disease. The patterns of inheritance of such diseases can be studied with sufficiently large family pedigrees, from which individuals at risk can be identified, provided the genetic defect is known and screening for the disease can be undertaken.
In complex disease genetics, rather than a single genetic defect, multiple genes are involved and there may also be interactions between these genes and the environment. The onset of a disease is the culmination of many different genes, of which the genes involved may differ from one individual to another.
Sequencing the entire genome was not a feasible proposition until more recently. However, this was not a limitation to genetic research provided the genetic defect was associated with an inheritable disease in large family groups. Identification of these genes used a methodology known as linkage analysis. This required only around 400 genetic markers across the entire genome. Linkage analysis identified some of the first genes associated with POAG such as myocilin (MYOC), optineurin (OPTN), WD repeat domain 36 (WDR36), and, more recently, Tank Kinase 1 (TBK1). Although mutations in these genes underlie the development of glaucoma in affected families, collectively mutations in these genes are thought to account for only 6% of glaucoma in the population.
Recent advances in gene-sequencing technology have made it possible to efficiently and economically undertake genome-wide genotyping in large populations. This has enabled identification of genes associated with glaucoma without the need for families. Instead genome-wide association studies (GWAS) use traditional case-control epidemiological study design in which the genetic profile across several million genetic markers of a group with glaucoma is compared to an unrelated control group without glaucoma. Statistical analysis is used to identify possible differences between the two groups at each of these genetic markers. Given the small effect on disease of each marker, these studies require massive sample sizes in the order of thousands of cases to have sufficient statistical power to detect a difference between the groups. More recent discoveries were only possible by pooling subjects from multiple large studies. The need for multiple statistical tests also necessitates a need for a more stringent p-value. Rather than a p-value of 0.05, p-values need to be less than 5×10-8 for an association to reach statistical significance in GWAS studies. A similar result in at least one other independent group is required for any genetic marker to be valid. LOXL1 for pseudoexfoliation syndrome was the first gene associated with glaucoma discovered using GWAS. The rapid progress in the area is reflected by the number of genomic regions found to be associated with glaucoma. From just 16 in 2017, there are now at least 74 for POAG alone.
UNDERSTANDING PATHOGENESIS
These new genetic discoveries have led to a better understanding of possible mechanisms for the pathogenesis of glaucoma and the role of previously established risk factors. While genetic associations identified by GWAS do not necessarily indicate a specific genetic mechanism, they enable further follow-up laboratory studies using animal and human cell lines looking for candidate genes and their mechanism of action.
The established dogma is that POAG is primarily a disease of the trabecular meshwork, but this is being increasingly challenged by results from GWAS. While the trabecular meshwork comprises epithelial cells, Schlemm’s canal and the collector channels are derived from endothelial cells. A number of studies have found associations between glaucoma and the genes involved with vascular endothelial cell morphology and angiogenesis, which suggests a crucial role for post-trabecular meshwork structures in the development of glaucoma.
GWAS discoveries have helped to better understand the role of central corneal thickness and glaucoma. A thinner central cornea is a recognised risk factor for conversion of ocular hypertension to glaucoma and glaucoma progression and is associated with a higher incidence of POAG. What has been uncertain is whether this association is biological or mediated through measurement artefact, with true IOP being higher than that measured when the cornea is thinner. In 2013, a GWAS study of central corneal thickness found 16 genetic loci associated with corneal thickness but only one was associated with POAG. The direction of effect of this one gene was also opposite to that expected, with the allele for a thinner cornea being protective for glaucoma. Subsequent studies in European and Asian populations have also failed to find any association between corneal thickness and glaucoma. These results suggest that the association between a thinner cornea and glaucoma is due to measurement artefact.
ADVANCING DETECTION AND TREATMENT
Progress in research into the genetics of glaucoma will, in the future, have possible application with treatment and personalised management. A better understanding of the genes involved with the development and progression of glaucoma, coupled with the decreasing cost of gene sequencing, will one day allow practical screening of individuals. This could determine individuals’ risk of developing glaucoma and stratify those with glaucoma in terms of their risk of rapid progression of disease and potential blindness.
The Australian and New Zealand Registry of Advanced Glaucoma (ANZRAG) is a local initiative that is making significant contributions to international glaucoma genetics research. ANZRAG aims to amass the world’s largest collection of advanced glaucoma cases and with corresponding clinical and genetic profiles, contribute to a better understanding of the genetic determinants of glaucoma. Anyone with bilateral glaucoma who has a visual field showing central involvement, or a mean deviation <-22dB in the worst eye, can be referred to ANZRAG. Eligible participants will receive free genetic testing and counselling.
Ultimately it is hoped that these new genetic discoveries may herald new therapies for glaucoma beyond lowering IOP. A better understanding of the pathogenic pathways should enable development of new therapies to target the causal pathways in glaucoma. Recent studies found that TEK mutations were associated with early-onset glaucoma and that improving TEK signalling may be therapeutic. Besides drugs to target abnormalities, new developments such as CRISPR technology have also made genetic therapy using gene-replacement and editing possible.
Rapid advances in delineating the role of genetics in glaucoma is improving our understanding of the cause and pathogenesis of the disease. In the near future this should help us better identify people at risk of developing glaucoma to target screening services and reduce the 50% rate of undiagnosed glaucoma in the community. These new genetic discoveries may also allow us to better allocate health resources to those at most risk of rapid progression and blindness. One day, gene editing technology may even result in cures for glaucoma.
Dr Jonathon Ng serves on the Ophthalmology Committee of Glaucoma Australia and is a former member of the RANZCO Glaucoma Curriculum Standards Review Panel and the RANZCO Clinical Audit Working Group.
Dr Ng is actively engaged in research at the Eye and Vision Epidemiology Research Group and is a Clinical Senior Lecturer at The University of Western Australia’s School of Population Health and Global Health. He is an investigator on research grants worth more than $2 million and author of 57 peer-reviewed papers. A comprehensive ophthalmologist, Dr Ng consults with Perth Eye Surgeons in Midland and Geraldton, and operates at the Perth Eye Hospital and St John of God Hospital in Midland and Geraldton.
Professor David Mackey AO is an NHMRC practitioner Fellow in Ophthalmology at the University of Western Australia. He is the former Managing Director of the Lions Eye Institute in Perth, a councillor of the Royal Australian and New Zealand College of Ophthalmologists (RANZCO) and current RANZCO representative on the Council of the Asia Pacific Academy of Ophthalmology. He is past president of the International Society for Genetic Eye Disease and Retinoblastoma. Prof. Mackey is a renowned international researcher in the genetics of eye disease and has published over 400 peer reviewed papers since 1989. He is the world’s most published author in glaucoma genetics and is a lead investigator in the International Glaucoma Genetics Consortium (IGGC) and the Consortium for Refractive Error and Myopia (CREAM). In 1993, Professor Mackey initiated the Glaucoma Inheritance Study in Tasmania (GIST), thereby creating one of the largest glaucoma biobanks in the world, with over 5,000 DNA samples and clinical material from familial and sporadic cases of glaucoma.
