I was sad to learn that a friend of mine was diagnosed with age-related macular degeneration (AMD) at the age of 62. Doctors told him that the blurriness he was experiencing in the center of his field of vision (see photo)  was a classic symptom of AMD. Millions suffer from this chronic condition that is now the leading cause of blindness in people 60 and older. This debilitating eye disease is caused by the degeneration of the macula, the central portion of the retina important for reading and color vision.
 
There were encouraging findings into the etiology of AMD in the March 11 issue of Nature by Kaneko et al., entitled “DICER1 Deficit Induces Alu RNA Toxicity in Age-Related Macular Degeneration”. The authors not only proposed a molecular mechanism leading to AMD, but also described a new function for the role of the microRNA processing-enzyme, DICER1. This enzyme is an RNase responsible for cutting double-stranded RNA into shorter pieces used in gene-silencing (RNAi) pathways.  Without DICER enzymes, short interfering RNAs (siRNA) or microRNAs (miRNA) cannot be generated in the cell to help control protein expression or antiviral activity.

The authors uncovered a possible cause of geographic atrophy (GA), a more advanced form of AMD, where degeneration of the retinal pigment epithelium (RPE) leads to vision loss.  They observed that the dysregulation of DICER1 causes an accumulation of transcripts of Alu elements (Alu RNA). Alu elements are stretches of DNA about 300bp long, so named because they can be excised using Alu endonucleases (an enzyme originally isolated from Anthrobacter luteus). These elements represent one of the largest classes of repetitive DNA sequences (>1 million Alu elements) interspersed in the human genome that produce non-protein coding RNA. It was this accumulation of Alu RNA which the authors proposed causes RPE cytotoxicity.

Using multiple approaches, the authors investigated whether a decrease in DICER1 expression within RPE was responsible for an increase in Alu RNA. First, they identified that the DICER1 protein was decreased in human donor eyes suffering from GA and not in control, healthy donor eyes. This was confirmed by measuring DICER1 mRNA levels, studying comparative Western blots for DICER1 protein levels, and reviewing histochemical stains of DICER1 protein in RPE tissues of control and GA donor eyes. After breeding DICER1-deficit RPE mice where DICER1 expression was under the control of the BEST1 promoter expressing Cre recombinase, they observed the RPE tissue was severely degenerated, pointing to a DICER1-deficiency pathogenesis. In vitro data using antisense oligonucleotide-mediated knockdown of DICER1 with human RPE cells in culture showed increased cytotoxicity supporting a connection between DICER1 regulation and GA.

Next the authors needed to determine if there was a concomitant increase in Alu RNA that induced RPE degeneration and not some other mechanism. A miRNA-based mechanism was ruled out by testing the depletion of other miRNA processing enzymes (Drosha, Dgcr8 or Ago2) in mice where no RPE degeneration was observed. Immunohistochemistry of human GA donor eyes using an antibody that recognizes long dsRNA indicated an abundance of long dsRNA compared to healthy eye tissue. Furthermore, immunoprecipitation of long dsRNA from GA eye tissue lysates and sequencing of this dsRNA showed the telltale 300-nucleotide long Alu RNA species while healthy control tissue showed none. They proposed that DICER1 normally cuts this long Alu RNA into shorter fragments, rendering the Alu RNA nontoxic. This protective role of DICER1 to eliminate accumulation of Alu RNA had not been previously reported.

DICER1 knockdown studies in cultured human RPE cells were performed to see if this caused Alu RNA build-up and induced cytotoxicity. They observed that DICER1 suppression resulted in an accumulation of Alu RNA in both the nucleus and cytoplasm, inducing apoptosis as evidenced by Caspase-3 cleavage in GA donor eye tissue and DICER1 mice RPE tissue. Since there was no increase in the abundance of RNAs encoded by other retrotransposons (LI.3, hY3, human endogenous retrovirus-W envelope) in human RPE cells, the data supported a biologically specific response to DICER1 depletion in Alu RNA accumulation.

Subretinal injections of Alu RNA into RPE of wild type mice also induced GA. However, when these same Alu RNAs were subjected to DICER1 digestion and then injected, no RPE degeneration was observed. Although there is more extensive data presented in the paper to support an Alu RNA cytotoxicity and not miRNA dysfunction theory to GA, these subretinal experiments lead the authors to propose potential treatments.

A DICER1-deficit induced cytotoxicity of RPE cells could be combated using an antisense remedy. DICER1-knockdown-induced human RPE cytotoxicity was prevented by introducing antisense oligonucleotides targeting the Alu RNA sequences. In addition, subretinal injections of Alu RNA antisense oligonucleotides in DICER1-depleted RPE mice reduced Alu RNA accumulation that prevented RPE degeneration, demonstrating that this method could offer a viable rescue strategy. 

To me this paper offers two intriguing ideas: a new task for DICER1 that expands its role beyond miRNA-dependent processes to a secondary protective role, and the concept of toxic Alu RNA build-up in the pathogenesis of degenerative diseases. I can only image the tip of the iceberg these investigators have uncovered in exploring these news two new mechanisms, and how they can be used as targets for developing therapeutic agents. 

Reference
Kaneko et al. (2011) DICER1 Deficit Induces Alu RNA Toxicity in Age-Related Macular Degeneration. Nature 471: 325.