DNA coiled around histones

Image by Eric Smith

Researchers say they have discovered critical details that explain how a cellular response system tells the difference between damage to the body’s own DNA and the foreign DNA of an invading virus.

The team believes this discovery could aid the development of new cancer-selective viral therapies, and it may help explain why aging, cancers, and other diseases

seem to open the door to viral infections.

“Our study reveals fundamental mechanisms that distinguish DNA breaks at cellular and viral genomes to trigger different responses that protect the host,” said Clodagh O’Shea, of the Salk Institute for Biological Studies in La Jolla, California.

“The findings may also explain why certain conditions like aging, cancer chemotherapy, and inflammation make us more susceptible to viral infection.”

Dr O’Shea and Govind Shah, PhD, also of the Salk Institute, reported these findings in Cell.

The pair described how a cluster of proteins known as the MRN complex detects DNA breaks and amplifies its response through histones.

MRN starts a domino effect, activating histones on surrounding chromosomes, which summons a cascade of additional proteins and results in a cell-wide, all-hands-on-deck alarm to help mend the DNA.

If the cell can’t fix the DNA break, it will induce apoptosis—a self-destruct mechanism that helps to prevent mutated cells from replicating and therefore prevents tumor growth.

“What’s interesting is that even a single break transmits a global signal through the cell, halting cell division and growth,” Dr O’Shea said. “This response prevents replication so the cell doesn’t pass on a break.”

Drs O’Shea and Shah also found that, when it comes to defending against DNA viruses, the cell’s response system begins the same way—with MRN detecting breaks. But it never progresses to the global alarm signal in the case of the virus.

Typically, a common DNA virus enters the cell’s nucleus and turns on genes to replicate its own DNA. The cell detects the unauthorized replication, and the MRN complex grabs and selectively neutralizes viral DNA without triggering a global response that would arrest or kill the cell.

So the MRN response to the virus stays localized and only selectively prevents viral, but not cellular, replication.

When both threats to the genome are present, MRN will activate the massive response at the DNA break, and no MRN is left to respond to the virus. This means the virus is effectively ignored while the cell responds to the more massive alarm.

“The requirement of MRN for sensing both cellular and viral genome breaks has profound consequences,” Dr O’Shea said.

“When MRN is recruited to cellular DNA breaks, it can no longer sense and respond to incoming viral genomes. Thus, the act of responding to cellular genome breaks inactivates the host’s defenses to viral replication.”

Dr O’Shea said this may explain why people who have high levels of cellular DNA damage—such as cancer patients—are more susceptible to viral infections.

“Having damaged DNA compromises our cells’ ability to fight viral infection, while having healthy DNA boosts our cells’ ability to catch viral DNA,” Dr Shah said. “Our work implies that we may be able to engineer viruses that selectively kill cancer cells.”

The researchers aim to use this new knowledge to create viruses that are destroyed in normal cells but replicate specifically in cancer cells.

Unlike normal cells, cancer cells almost always have very high levels of DNA damage. In cancer cells, MRN is already so preoccupied with responding to DNA breaks that an engineered virus could sneak in undetected.

“Cancer cells, by definition, have high mutation rates and genomic instability even at the very earliest stages,” Dr O’Shea said. “So you could imagine building a virus that could destroy even the earliest lesions and be used as a prophylactic.”

Discovery reveals potential for viral cancer treatment