December 2013, Issue 64

A Glimpse into Epigenetic Regulation of Age-Related Macular Degeneration

Rajesh C. Rao, MD Assistant Professor Department of Ophthalmology & Visual Sciences W.K. Kellogg Eye Center University of Michigan Medical School Ann Arbor, MI Gaurav K. Shah, MD Professor of Clinical Ophthalmology The Retina Institute, St. Louis MO Washington University School of Medicine, St. Louis, MO

The emerging field of epigenetics is revolutionizing our understanding of development, inflammation, stem cell biology, cancer and metabolism.¹ The term "epigenetics" refers to processes that regulate gene expression but that do not involve changes in DNA sequence. Epigenetic processes often involve changes in chromatin, which represents the nucleic acid-protein structure of DNA as it wraps around histone proteins in the nucleus of the cell. Examples of epigenetic processes include DNA and histone methylation, histone phosphorylation, sumoylation, ubiquitination, and others, including regulation of gene expression by small RNAs. These processes can silence or activate genes in many ways, for instance by making regulatory areas of genes (such as promoters and enhancers) more or less accessible by transcriptional machinery which transcribes DNA into RNA, a crucial step in gene expression. In DNA methylation, methyl groups are catalytically added to cytosine and guanine nucleotides by enzymes known as DNA methyltransferases. DNA methylation causes silencing of specific genes. These processes remain dynamic, and rapid changes in these modifications can occur with environmental stimuli, such as growth factors that induce activation of intracellular cascades, such as cell survival associated-insulin signaling.² Small molecules that target epigenetic modifiers have shown promise as experimental therapies for some oncologic, infectious, and retinal disorders.^3-6

Age-related macular degeneration (AMD) is the leading worldwide cause of irreversible blindness in the older population. Genetic analyses from patients affected by AMD have revealed mutations in genes involved in the complement pathway,⁷ as well as genes involved in lipid metabolism and others.^8,9 Indeed, based on mutations in some of these genes, some commercial companies offer genetic testing to estimate risk of progression of AMD. However, the American Academy of Ophthalmology currently recommends against testing for genetically complex disorders such as AMD until published clinical trials demonstrate that particular treatment or surveillance strategies are beneficial to patients with specific genotypes.¹⁰ Given this recommendation and given that environmental factors such as age, tobacco use, and diet have been associated with AMD, the field of epigenetics seems a natural place to further investigate non-genetic mechanisms of AMD pathogenesis. To this end, a recent publication provides the first glimpse into how DNA methylation, a key epigenetic mechanism, contributes to AMD.¹¹

In this report, the authors identified three sets of twins (one monozygotic and two dizygotic), in which one twin developed AMD and the other did not. Nucleated white cells were collected from blood from the affected and non-affected twins, and the entire DNA methylome (i.e. an assay that detects DNA nucleotides that have been methylated) was analyzed. Among the genes that were methylated differentially in the serum of affected twin sibs was the IL17RC promoter, a portion of the gene that stimulates gene expression for the receptor for interleukins (IL) IL-17A and IL-17F. Interestingly, the authors had previously reported elevated levels of the IL-17A and IL-22 proteins in the serum of AMD patients. The epigenetic results were confirmed by finding differential methylation patterns among seven pairs of non-twin siblings, as well as in 202 AMD patients and 96 non-AMD controls. In AMD-affected patients, the IL17RC promoter was hypomethylated. Intriguingly, IL17RC promoter hypomethylation was noted both in patients with wet AMD and in patients with dry AMD. Since DNA methylation corresponds to gene silencing, relative hypomethylation of the promoter meant that IL17RC was more highly expressed in AMD patients versus non-affected patients. Indeed, in the peripheral blood of the affected patients, more IL-17RC+ monocytes were found compared to control patients, and these IL-17RC+ cells expressed higher levels of other proteins specifically involved in the IL-17 signaling pathway. Finally, antibody staining in autopsy maculae showed increased staining of IL-17RC protein in AMD patients with choroidal neovascularization (CNV) or geographic atrophy (GA) compared to non-AMD control samples. This finding makes sense as hypomethylation of the IL-17RC promoter stimulates higher transcription of the gene, leading to increased translation of the protein, which was confirmed in the study by increased antibody staining of IL-17RC in the maculae of AMD-affected patients.

This report opens the door to a potentially new direction of research in understanding AMD pathogenesis. Importantly, the data identifies a serum-related biomarker that may predict which patients have an increased epigenetic risk for developing AMD. Finally, this study offers clues into new pharmacologic interventions for both wet and dry AMD. It is possible that the next generation of targeted epigenetic drugs, such as DNA methyltransferase modifiers,¹² may be harnessed for AMD therapy.


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