New research reveals how the common virus that causes cold sores – herpes simplex 1 – invades our cells
and takes control of their machinery to replicate itself and hide from the immune system.
Viruses insert their DNA into host cells and use their machinery to make copies of themselves.
The discovery is significant because it offers new information about how the virus affects processes in the cell
rather than the immune system.
Researchers from the University of Cambridge in the UK, and two German institutions: the Julius-Maximilians-
Universität Würzburg and the Ludwig-Maximilians-Universität München, report their findings in the journal
Nature Communications.
They describe how HSV-1 inserts its DNA into
our cell DNA, takes over and disrupts the production of cell proteins while using it to make perfect versions of its own proteins.
Herpes simplex virus 1 (HSV-1) is very common and usually harmless. Most people become infected by their
early 20s, and after the first infection, the virus remains dormant in the facial nerve tissues.
Now and again, the virus flares up and causes mild cold sore symptoms. But there are also cases where HSV-1 can lead to life-threatening disease. For instance, in intensive care units, it can cause severe lung
problems.
And even in healthy people, there are rare instances where HSV-1 can spontaneously cause inflammation
of the brain, resulting in irreversible brain damage.
From genes to proteins: transcription and translation
Our DNA code contains the instructions for making all the cells of our whole body, and the work that they do.
Each cell contains the DNA for the whole individual, but not every cell needs to obey all the instructions in
all of the DNA.
DNA on its own does not do anything. Imagine a cell is like a factory – its DNA holds the map of the factory and the instruction manuals for its production line. For the factory to work it needs
workers and machinery. The cellular equivalent of these are proteins – the workhorses that carry out all the
cell functions.
To create the workhorses or proteins, cells make working copies of the genes in the DNA that are
relevant to them. Thus, liver cells copy genes in the DNA that are relevant to liver cells and ignore genes and
bits of code relevant to lung and other types of cell.
Taking a working copy of a gene for making a protein is called transcription. The transcription is held in
RNA molecules – they decide which proteins should be made by the cell.
The next stage, where RNA molecules instruct the cell machinery to make proteins is called translation. So
the process of using DNA to make proteins has two stages – transcription and translation.
HSV-1 causes transcription of host cell proteins to ignore ‘end codes’
Viruses are not like cells – they have DNA, but no machinery. They survive by inserting their DNA into the
DNA of host cells so that instead of making proteins for the cells, the cell machinery makes virus proteins and
copies of the virus. Eventually, the host cell bursts and releases all the new virus particles so they are free
to go and hijack other host cells.
Within a few hours of invading a host cell, HSV-1 has seized the protein-making machinery and is producing
its own proteins and copies of itself on a massive scale.
The new study investigates in detail how HSV-1 manages to do this and evade detection by the immune system,
which normally spots cells behaving abnormally and kills them before they can do too much damage.
The study team used cultures of fibroblasts – a type of human connective tissue cell. They note this is an
ideal cell for investigating how HSV-1 seizes control of RNA molecules and gene transcription.
The researchers found that within 4 hours of entering the cell, HSV-1 does something quite unexpected.
Usually, the process of transcribing the DNA into RNA stops when it reaches the end of the gene. A section of
code that marks the end of the gene tells the transcription process to stop.
But in a host cell controlled by HSV-1, the transcription process ignores these end codes and goes
on, blindly transcribing from DNA into RNA thousands of irrelevant sections of A, C, G and T code – sometimes
from neighboring genes. The result is masses of unusable RNA products that cannot be translated into
proteins.
One of the senior investigators, Professor Lars Dölken from Cambridge’s department of medicine and Würzburg’s
institute of virology, describes the effect HSV-1 has on the host’s transcription process:
“It’s like someone transcribing a short story, but instead of stopping at ‘The End,’ they carry on and
transcribe all the copyright and publication details and ISBN numbers at the beginning and end of the book. This
produces lots of meaningless, confusing and useless information.”
Meanwhile, virus proteins are transcribed perfectly
Another interesting discovery is that although the host DNA is transcribed into nonsense, it transcribes the
viral DNA perfectly throughout infection.
Thus, HSV-1 produces two results that are to its advantage. The effect of producing nonsense from the host
cell DNA causes the cell to shut itself off – preventing the immune system from attacking the virus. And this in
turn helps increase the production of viral proteins and the production of new virus particles.
The researchers note that their discovery may explain why so many previous studies have suggested HSV-1
activates large numbers of genes that normal host transcription would overlook. They suggest something else is
happening: because the transcription does not stop at the end codes, it may appear that hundreds of additional
genes are being transcribed, but these are never translated into proteins. Prof. Dölken explains:
“Unlike previous studies, which only studied single genes, we also found no indication that the virus
generally impedes the processing of RNA in the cell nucleus, known as splicing. Instead, it causes unusual
splicing events, many of which have never before been observed.”
The study also achieves a significant step forward in the method for studying viruses in cells. It shows how
a single experimental approach can be used to observe and record all the changes to host RNA transcription and
their effect on protein production.
Viruses like HSV-1 can remain dormant in their hosts for a long time and then flare up again. Why this
happens is somewhat of a mystery, but in June 2014, Medical News Today learned about a study published
in the journal Science that showed how interactions between different
viruses can trigger dormant viruses.
In that study, the researchers investigated how human herpes virus 8 reactivates. Working first with a mouse
model, and then repeating the results in humans, the team found that after initial infection, the host protein
interferon gamma kept the virus dormant in the body. But this effect was canceled following an infection from a
worm parasite.