Differences in herpes virus symptoms may relate to variations in strain gene expression
Why do some people with cold sores around their lips experience painful lesions, while others have no symptoms at all, yet still spread the virus? A new study conducted at Penn State finds that these differences could be due to variations in the way certain strains of herpes simplex (HSV-1) — the virus that causes cold sores, as well as genital herpes — activate gene expression in neurons.
“HSV-1 occurs in more than half the global population,” said Moriah Szpara, associate professor of biology and biochemistry and molecular biology. “Not only does it cause recurrent problems, such as cold sores and genital herpes, but recent research has implicated chronic HSV-1 infection with the development of disease later in life, including neurodegenerative diseases like Alzheimer’s.”
Szpara explained that the HSV-1 lifecycle begins upon contact with mucosal surfaces, where it invades skin cells, replicates, and can induce local lesion formation. The virus also enters local nerve endings in the skin, and transits into neurons in the nervous system. There the virus can lie dormant until it reactivates on future occasions. Neuronal damage and host immune responses triggered by viral reactivations are thought to contribute to long-term neurodegeneration.
“Since every person carries a subtly different version of HSV-1, this might explain some of the variation in human responses to infection; for example, why people have different triggers for their outbreaks or why some people experience more painful sores. Differences in the frequency of viral outbreaks, or in virus-induced gene expression patterns, might also affect the different rates at which people with chronic infections go on to develop neurodegenerative diseases.”
To investigate the causes of this variation in responses, Szpara and her colleagues infected human neuronal cells with one of three HSV-1 strains that are known to differ in their ability to cause disease in the nervous system. Next, they used deep sequencing to identify and quantify the transcriptomes — the entire set of messenger RNAs (mRNAs) made in a cell at any given time — of the neurons during infection by HSV-1.
According to Szpara, when a neuronal cell is infected with HSV-1, the resulting transcriptome includes the whole collection of mRNAs produced by both the human neuron and the HSV-1 virus. By looking at the timing and amount of mRNAs expressed during infection, scientists can gain insights on the proteins that will soon be produced from those mRNAs. It is the viral proteins and new viral progeny produced during infection that ultimately lead to health problems.