A new method for how viruses ensure their maintenance in dividing cells has been identified by researchers at the University of Wisconsin McArdle Laboratory for Cancer Research and Carbone Cancer Center.
The study compared two closely related herpesviruses, Kaposi’s Sarcoma Herpesvirus (KSHV) and Epstein-Barr Virus (EBV). It showed that despite their similarities, each virus has evolved different ways to ensure their retention in cells. The work suggests that more mechanisms of viral propagation have yet to be discovered, and has implications for treating viral-associated diseases.
Like all viruses, herpesviruses can infect host cells, replicate their genome and produce hundreds of viral progeny per cell. The proteins KSHV or EBV use in that process are nearly interchangeable. Herpesviruses can additionally establish latency, where viral DNA persists in host cells as small, circular plasmids.
Latency is usually asymptomatic, but can lead to cancers such as Kaposi’s Sarcoma for KSHV and B-cell lymphomas for EBV, most commonly in immunocompromised individuals. Cancer cells that lose the virus die, indicating there is some way these viruses are essential for the cancers.
“In this work, we identified a mode of viral partitioning to dividing cells that is without precedent,” said Bill Sugden, professor of oncology.
Sugden’s group previously identified how latent EBV is maintained in lymphomas by visually tagging plasmid DNA and looking at “spots” of it under the microscope. As each plasmid is copied into two, the spot doubles in intensity. Then one copy of each attaches to DNA bound for different cells, assuring the replicating cancer cells hold on to EBV. Because the process is not 100 percent efficient, it is called “quasi-faithful partitioning.”
“Nearly 10 years ago a new graduate student joined the lab and I said, ‘As a beginning project, why don’t you try to do the same thing with KSHV because it, too, is known to be maintained as a plasmid and it’ll probably be just like EBV,’” Sugden said. “Now, we know I was completely wrong.”
The first hint Sugden was wrong came when graduate student and study co-author Kathryn Fox visually tagged the KSHV DNA and compared the results to those of EBV.
“When Kathryn repeated the experiment with KSHV, there was clearly a much greater distribution of signal intensities than just two-fold, indicating several plasmids ‘clustered’ together,” Sugden said. “And as the cells divided, you could see a cluster of KSHV signals all go from one parent cell into one daughter cell, meaning they partitioned randomly instead of quasi-faithfully.”
Next, the researchers asked how clustering and random partitioning happens. In the case of KSHV, they already knew it moved to new cells by attaching itself to histones, or proteins around which both host and viral DNA wrap. They also knew that a KSHV protein, LANA1, attaches itself directly to KSHV DNA and to histones. This setup means that LANA1 could bind to multiple copies of KSHV by attaching itself to both viral DNA and a histone around which another plasmid is wrapped. By engineering a new virus that replaces a part of LANA1 with a part of the EBV equivalent, they showed that clustering was gone but the plasmids were still partitioned.
“One end of LANA1 binds histones and the other end attaches to KSHV DNA, which itself is bound to histones, and what that means is that LANA1 at least in part mediates cluster formation,” Sugden said.
With a clearer picture of the KSHV partitioning mechanism worked out, an inhibitor drug is more likely to be developed to treat Kaposi’s Sarcoma, a disease mostly associated with HIV-positive individuals. But more generally, it indicates that even two closely related viruses have evolved different partitioning mechanisms.
“I think viruses have evolved to sample all possibilities and the ones that work, are kept; the ones that don’t work, are never seen by virologists,” Sugden said.
Other members of Sugden’s group who contributed to the work include Ya-Fang Chiu, Mitchell Hayes and Arthur Sugden. The study was published in The Journal of Cell Biology. This work was funded by grants from the National Cancer Institute (grants P01 CA022443, R01 CA133027, and R01 CA070723), the Ministry of Science and Technology (MOST-105-2320-B-182-033), the National Health Research Institute (NHRI-EX106-10623BI), and the Morgridge Institute for Research.