Researchers at Tel Aviv University have made a groundbreaking discovery in their understanding of the complex eye disease age-related macular degeneration (AMD), a leading cause of vision loss in advanced age.
They have identified a new genetic risk factor by discovering proteins that are crucial for the development and operation of the tissue affected by the disease. For the first time, the researchers have determined the precise locations of these proteins in the genome and have established a connection between variations in these genomic regions and the risk for AMD.
"The new discovery enhances our understanding of the previously unknown function of genomic regions outside the genes. The method we applied may enable the deciphering of additional genetic mechanisms involved in various complex genetic diseases,” said the researchers.
The study was conducted under the leadership of Professors Ruth Ashery-Padan and Ran Elkon, along with their research teams consisting of Mazal Cohen Gulkar, Naama Mesika, Ahuvit David, and May Eshel from the Department of Human Molecular Genetics and Biochemistry at the Sackler Faculty of Medicine and the Sagol School of Neuroscience at Tel Aviv University.
The results of their findings have been published in PLOS Biology.
Prof. Ashery-Padan explains: "One of the greater challenges in genetic research today is decoding the genetic mechanisms of complex diseases caused by a combination of several different genetic and environmental factors (rather than an identifiable defect in a single gene). Diabetes, bowel diseases, and various mental illnesses are just a few examples. In our study we chose to focus on AMD, which causes degeneration of the central retina - a major cause of loss of vision at an advanced age in developed countries."
Prof. Elkon adds: "AMD has a significant genetic component. Studies comparing the genomes of people with and without AMD (as well as a range of other complex genetic diseases) have found differences in several genomic regions, probably associated with risk factors for the disease. However, these differences were not detected in any specific gene, but rather in the extensive regions that stretch between the genes, whose functions and modes of operation are still largely unknown. In fact, comparative studies have identified whole genomic regions that are probably related to the disease but were unable to pinpoint any specific feature in these regions and define it as a risk factor. Our study addressed this problem."
The study was centered on the examination of retinal pigmented epithelium (RPE) cells, a layer of tissue that supports the photoreceptors in the retina and plays a critical role in their initial development and continued survival throughout a person's life. The researchers stated that this tissue is affected at the earliest stages of AMD.
Prof. Ashery-Padan: "First, we wanted to understand the genetic mechanism that activates and regulates the specific activity of pigmented epithelium cells. Through a series of experiments, knocking down different proteins in both a mouse model and human cells, we identified two key proteins, LHX2 and OTX2, which together dictate the expression of many genes unique to this tissue. The proteins act as transcriptional activators - binding to specific regulatory sites in the genome to determine which genes will be expressed in a particular cell."
The next challenge for the researchers was to determine the specific locations of the two proteins in the genome. They utilized the cutting-edge technology of ChIP-seq, a DNA sequencing technique that detects the binding sites where the proteins attach to the DNA.
Prof. Elkon: "We found that the binding sites of the two proteins were quite close to each other. Moreover, these same sites had previously been identified as related to risk factors for AMD (namely, sequences that showed differences between people with and without AMD). We assume that due to changes in DNA sequences in these genomic regions, transcriptional proteins cannot easily find and bind with their binding sites. This reduces the expression of the nearby gene regulated by the transcriptional proteins, which encodes an ion channel known as important to eye function. The decrease in the gene's activity affects the entire tissue, increasing the risk for development of AMD."
Prof. Ashery-Padan sums up the study: "In our study we identified two proteins related to risk factors for the complex genetic eye disease AMD. In addition, for the first time, we were able to map the exact genomic sites of these proteins and found that they operate in a region previously identified as related to risk factors for AMD. Our findings provide new insight into a previously unsolved issue: the functions and mode of operation of genomic sequences located outside the genes, and how they are involved in complex genetic diseases. We believe that our novel research methodology will enable the identification and mapping of many other genetic mechanisms related to AMD and other complex genetic diseases."