Researchers at the University of Pennsylvania School of Medicine have discovered part of the reason why cold sores, caused by a herpes virus, come back again and again. The new study, published online last month in Nature, points to a small RNA molecule, called a microRNA (miRNA) as the culprit that keeps the latent virus-infected cell alive. These findings could one day lead to a new way to fight the virus and offers the first target for intervention in the latent infection.
A research team led by Nigel W. Fraser, PhD, Professor of Microbiology, has found that herpes simplex virus-1 (HSV-1), the virus that causes cold sores and ocular keratitis, produces an miRNA molecule. This miRNA is encoded by the Latency-Associated Transcript gene (LAT) in the viral genome and works through a process called RNA interference to prevent normal cell death or apoptosis. Thus, the latent viral infection is maintained for the lifetime of the individual because the latently infected cell does not die.
“Although miRNAs encoded by cellular genes are known to be an important mechanism for controlling gene expression, this is one of the first miRNA found to be encoded by a viral genome,” says Fraser. “Our study helps show how HSV-1 can maintain a latent infection for the lifetime of an infected individual.”
The LAT gene was discovered by Fraser and colleagues in 1984, but a protein product from this gene has never been found. This caused Fraser and his research team to hypothesize that LAT may work through an miRNA molecule, which is a small piece of the LAT gene. It interferes with the translation of two cell proteins that are required for cell death: TGF-b and SMAD-3. The LAT miRNA binds to specific sequences of messenger RNA from these two genes and causes them to be degraded. Thus, the amount of TGF-b and SMAD-3 protein is reduced in the cell and apoptosis is prevented. Because the latent virus is not producing any viral proteins the immune system of the infected individual cannot detect the infected cell.
Latent HSV-1 infections form in neuronal cells of the peripheral nervous system. When a latent infection is reactivated (by stress of many kinds), HSV-1 proteins are synthesized and new infectious virus particles are formed. These virus particles migrate along the neuronal axons to the epithelial cells of the skin. Viral growth in the skin, or other mucous membranes where nerves are found, causes cell damage and an immune reaction that results in a painful sore. Although the latency-to-reactivation process is not fully understood, it is known to involve stress, such as physical damage, ultraviolet light, hormones, or even fever.
Fraser is currently testing whether HSV-2, a relative of HSV-1 that causes genital herpes, also encodes an miRNA molecule in its LAT gene. “MiRNA may be a more general mechanism that latent viruses use to remain alive in the host cell,” suggests Fraser.
Present treatments of HSV-1 rely on acyclovir-based drugs that target the viral polymerase and inhibit viral DNA replication during the acute infection. However, they do not target the latent infection, and thus cold sores return throughout the lifetime of the infected individual. Finding an miRNA that interacts with the cellular TGF-b pathway during latency offers the first target against the latent infection and offers a profoundly different approach to treatment, concludes Fraser.