January 2019

NEWS & OPINION

Presentation spotlight
Landmark achievements in ocular genetics


by Stefanie Petrou Binder, MD, EyeWorld Contributing Writer

Most cited papers from 2016–18 reveal high level of progress in ocular
genetics

A systematic look at the most frequently cited papers from 2016–2018 revealed a number of standout reports on gene discovery, stem cells, gene therapy with AAV and CRISPR, and understanding disease mechanisms. Although only two of these papers appeared in ophthalmology journals, the reports are specifically relevant to eye medicine and include new technologies such as large scale genome-wide association studies (GWAS), next-generation sequencing (NGS), adeno-associated virus gene therapy (AAV), induced pluripotent stem cells (iPPSC), and gene auditing (CRISPR/CAS9).

AMD

According to David Mackey, MD, Lions Eye Institute, University of Western Australia, Perth, Western Australia, who co-moderated a session on landmark achievements in ophthalmology at the 2018 World Ophthalmology Congress, one of the highest cited studies in the past 2 years investigated AMD variants. As a leading cause of blindness with limited therapeutic options, breakthroughs in AMD could mean preventing visual loss in millions of aging patients. The study included 16,144 patients and 17,832 controls who were clinically classified and analyzed for common/rare genetic variants. The investigators found 52 independently associated common/rare gene variants distributed across 34 loci. They noted that both wet and dry AMD subtypes largely exhibited shared genetics, and they identified the first genetic association signal specific to wet AMD, near MMP9.1
“The genes themselves are not just markers of the disease but have multiple changes within the genes that seem to predispose to the disease, and thus are likely part of the pathogenic pathway,” Dr. Mackey said. “A few very rare coding variants in CFH, CFI, and TIMP3 suggest causal roles for these genes.”
The report analyzed more than 12 million variants. The results support the hypothesis that rare coding variants can pinpoint causal genes within known genetic loci and illustrate that applying a systematic approach to detect new loci requires extremely large sample sizes.

POAG

A meta analysis of GWAS incorporating eight independent U.S. studies, two Australian, three European, and one Singaporean study that included more than 6,000 cases and 42,000 controls identified new pathways underlying primary open angle glaucoma (POAG) susceptibility and suggested new targets for preventive therapies. The study identified three new glaucoma genes (loci). It revealed, in addition, that although FOXC1 was known to be associated with anterior segment dysgenesis, it was also seen to be linked to POAG in the general population.2

PACG

Primary angle closure glaucoma (PACG) is another major cause of blindness worldwide. According to new evidence from a meta analysis that combined a large number of patient samples (10,503 PACG patients/29,567 controls) from around the world, investigators identified five new genes (loci) associated with PACG, as well as some previously described loci. The study outcomes revealed some genetic overlap between PACG and general glaucoma, which was not expected.3

Retinal dystrophy

Retinal dystrophies (RD) constitute a group of blinding diseases that are characterized by clinical variability and pronounced genetic homogeneity. The different non-syndromic and syndromic forms can be attributed to mutation in more than 200 genes. Consequently, next generation sequencing (NGS) technologies are among the most promising approaches to identify mutations in RD.
In terms of NGS, a major paper that looked at inherited RD focused on more than 200 genes already known to be associated with RD and analyzed the full genomes of a large series of independent index cases from 89 families with various subforms of RD. The analysis found the causative mutation 61% of the time, including mutations in 34 known and two novel RD genes.4
“Many of these genes are known, but they still pull up new genes that have not yet been described as being associated with diseases,” Dr. Mackey explained. “This technology will be more in practice for our inherited retinal disease families. Sixty-nine distinct mutations were identified in this study including 39 novel mutations. Genetic findings in several families were not coincident with clinical diagnoses, so the families’ disease label was refined based on this genetic testing.”

LncRNA and miRNA

“Diseases such as diabetes were investigated to see if there is a genetic predisposition for diabetic retinopathy,” Dr. Mackey said. “The noncoding regions of RNA, which are involved in regulation rather than in protein expression, in particular the noncoding region MEG3, showed an association with the expression of retinopathy in an animal model, making this a good therapeutic target to treat and prevent diabetes-related microvascular complications. MEG3 is mainly mediated by activation of PI3K/Akt signaling, in endothelial function, making it a therapeutic strategy point.”
Microvascular dysfunction is an important characteristic of diabetic retinopathy. In a study that focused on the role of the long noncoding RNA-MEG3 (lncRNA-MEG3) in diabetes-related microvascular dysfunction, MEG3 expression was shown to be significantly down regulated in retinas of STZ-induced diabetic mice, and in endothelial cells upon high glucose and oxidative stress. MEG3 knockdown aggravates retinal vessel dysfunction in vivo as shown by serious capillary degeneration and increased microvascular leakage and inflammation. MEG3 knockdown also regulates retinal endothelial cell proliferation, migration, and tube formation in vitro.5
Another study group that investigated microRNA (miRNA) segments found that two of the miRNAs previously implicated in angiogenesis, miR-27b and miR-320a, were significantly associated with the incidence and progression of diabetic retinopathy. The study focuses on circulating miRNAs in patients with type 1 diabetes and their association with diabetic retinopathy. Twenty-nine miRNAs were quantified in 300 serum samples using nested case control study design in two prospective cohorts of the Diabetic Retinopathy Candesartan Trial (DIRECT): PROTECT-1 and PREVENT-1. The study’s findings open up therapeutic targets using potential biomarkers for the treatment of diabetes and retinopathy.6
A final frequently cited paper that Dr. Mackey discussed described 3,000 miRNAs, some for the first time. The report revealed a very specific miRNA enrichment pattern in the retina that was distinct from those in the retinal pigment epithelium and choroid, two tissues known to be crucial for retinal homeostasis. These findings open up interesting new therapeutic opportunities, he said. In the study, investigators carried out deep sequencing analysis on 16 individuals and established a catalogue of retina-expressed miRNA, in order to characterize the miRNome of the human retina. They found that a small number of miRNA accounted for almost 90% of the retina miRNome. The investigation was the first to reveal the complexity of the human retina miRNome at nucleotide resolution, constituting a unique resource to assess the contribution of miRNAs to the pathophysiology of the human retina.

References

1. Fritsche LG, et al. A large genome-wide association study of age-related macular degeneration highlights contributions of rare and common variants. Nat Genet. 2016;48:134–43.
2. Baily JN, et al. Genome-wide association analysis identifies TXNRD2, ATXN2 and FOXC1 as susceptibility loci for primary open-angle glaucoma. Nat Genet. 2016;48:189–94.
3. Khor CC, et al. Genome-wide association study identifies five new susceptibility loci for primary angle closure glaucoma. Nat Genet. 2016;48:556–62.
4. Weisschuh N, et al. Mutation detection in patients with retinal dystrophies using targeted next generation sequencing. PLoS One. 2016;11:e0145951.
5. Qiu GZ, et al. Long noncoding RNA-MEG3 is involved in diabetes mellitus-related microvascular dysfunction. Biochem Biophys Res Commun. 2016;471:135–41.
6. Zampetaki A, et al. Angiogenic microRNAs linked to incidence and progression of diabetic retinopathy in type 1 diabetes. Diabetes. 2016;65:216–27.
7. Karali M, et al. High-resolution analysis of the human retina miRNome reveals isomiR variations and novel microRNAs. Nucleic Acids Res. 2016;44:1525–40.

Editors’ note: Dr. Mackey has no financial interests related to his comments.

Contact information

Mackey
: david.mackey@uwa.edu.au

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