Scientists at the Francis Crick Institute have made a surprising
discovery about the Vaccinia virus, a large DNA virus belonging to
the pox family that was used as the vaccine to eradicate
smallpox.
Today, the Vaccina virus is being developed and used in vaccines
for a number of other diseases and in immunotherapy to treat
cancer.
Contrary to previous assumptions, the Crick team found that the
Vaccinia virus needs proteins from the host cells it infects to
replicate.
Michael Way of the Crick explains: "Poxviruses, unlike other DNA
viruses such as herpesviruses, do not replicate in the cell
nucleus, where the host's DNA replication machinery is located.
Instead, it was previously thought that Vaccinia replicates its DNA
outside the nucleus, in the host cell's cytoplasm because it does
not require any factors from the cell as it only uses viral
proteins."
Dr Way's team infected cells with Vaccinia and used microscopy
to look at where a number of host cell proteins involved in
replicating and repairing DNA were localised after infection. They
also looked at what happened to the virus's ability to replicate
when they removed proteins involved in the host cell's usual
response to DNA damage.
They discovered that Vaccinia does in fact use host proteins,
alongside viral proteins, to replicate its DNA.
One of these proteins, called ATR, is involved in overcoming
damage to our DNA caused by chemicals, UV light or replication
errors. The scientists found that ATR might promote viral DNA
replication in a similar way to its normal role in preventing
damage caused by errors in DNA replication.
Dr Way says: "From a basic science point of view, our work has
now opened up using Vaccinia as a new model system to provide
further insights into the role of ATR in regulating DNA replication
and in maintaining DNA integrity during the DNA damage
response.
"With the eradication of smallpox, the development of
anti-poxviral strategies has not evolved much. Yet, outbreaks of
monkeypox in the USA, for instance, highlight the continued risk
this family of viruses might cause in the future. Therefore, having
access to anti-poxviral agents remains of interest. So far, such
agents have always targeted pox viral proteins. Our findings
indicate that drugs targeting host molecules involved in DNA
replication, damage and repair might be repurposed to protect
against poxvirus infection."
The research also has implications for cancer immunotherapy
using the Vaccinia virus. Dr Way adds: "Recent thinking favours
combining Vaccinia immunotherapy with conventional anti-tumour
therapies for maximal effect. However, such therapies should not
decrease the virus's ability to replicate inside a tumour, which is
a key factor for efficacy against cancer. Our work therefore
suggests that poxvirus therapy should not be combined with chemical
inhibitors of ATR, and possibly other inhibitors of the DNA damage
response, as this may weaken viral replication."
The paper, Cytoplasmic ATR Activation Promotes Vaccinia Virus
Genome Replication, is published in Cell Reports.