Of all the viruses that bedevil humanity with every ill from the common cold to HIV to the uncommon Ebola, the reovirus remains unfamiliar.
Yet this unusual, double-stranded RNA virus is ubiquitous throughout the animal kingdom. Among mammals, only whales don't harbor it.
"Most people - 80 to 90 percent - have antibodies to the reovirus by the age of 15 or 16," observed virologist and cancer biologist Patrick Lee, at the University of Calgary, in Alberta. "In fact," he added, "reovirus is more common in the human population than herpesvirus," which annoyingly infects lips and genitals.
Reovirus infections cause humans little or no annoyance at all. They occur, mostly unnoticed, in intestines and respiratory tracts. But they do kill baby mice.
The euphonic acronym "reo" stands for "respiratory enteric [i.e., gut] orphan." That last word signifies that the virus knows no other parental sources. Under electron microscopy, a reovirus virion looks a lot like a golf ball, its pocked surface studded with 92 capsomere spikes.
Lee has been studying reoviruses for 20 years. He is now researching ways to unleash this mild-mannered pathogen against human cancer cells.
"Initially," he told BioWorld Today, "we were looking at the way the virus infects these cells. How it binds to receptors on their surface. Viruses are parasites, so they have to get inside the cell in order to reproduce.
"It turned out that the receptor to which the reovirus particle binds," he went on, "is sialic acid, a simple sugar. It's very similar to the influenza virus, which also uses sialic acid as its receptor. Since sialic acid is so common, the reovirus, of course, bound all the cell lines that we tested. But, interestingly, only certain cell lines proved susceptible to reovirus infection.
"So, we asked," Lee continued, "'What is wrong with these reovirus-resistant cells?' The virus can even get inside them, can even undergo transcription. It does not go any further to translation. However, when we introduced Ras oncogenes into these resistant cells - boom - there's no problem. Then the translation - that is, the synthesis of viral proteins - can go on. That is how we identified the Ras signaling pathway as the guide that is used by the reovirus for infection."
Lee is senior author of a paper in the current issue of Science, dated Nov. 13, 1998. Its title: "Reovirus therapy of tumors with activated Ras pathways."
He explained: "The Ras oncogene receives signals from the cell surface and transmits them all the way to the nucleus via a phosphorylation cascade. The end result is telling the nucleus to make more genes that regulate growth, or shut off certain other genes. So, basically, the Ras pathway is a major pathway regulating cell growth."
When Ras Oncogene Pathway Goes Hyper
"Now, in human cancers," Lee continued, "since there are so many elements along the Ras pathway, any mutation in any of them could cause hyperactivity of the pathway. Take, for example, Ras itself. There are two sites on Ras that are highly susceptible to mutations. But, when these sites are mutated, the Ras pathway will be highly activated all the time, nonstop."
Direct mutations of Ras are prevalent in 90 percent of pancreatic cancers, 50 percent of colorectal cancers, 40 percent of lung tumors and 30 percent of myeloid leukemias.
"Now, mind you," Lee pointed out, "the Ras does not have to be mutated itself. Even mutations upstream of that oncogene can activate the pathway as well."
He cited two upstream tumor proteins which are frequently overexpressed in human breast cancer and glioblastomas (brain cancer).
"When double-stranded reoviral RNA enters a tumor cell," Lee observed, "the transcript it makes triggers a feedback cell-suicide loop. In lay terms, it turns out that the virus also uses the same hyperactive Ras pathway.
"At that point," he said, "the reovirus actually takes over the entire machinery inside the cell. No more host-cell proteins are made - none. All the proteins the cell makes are viral proteins - strictly. In other words, the virus is able to shut off the tumor cell mechanism at the same time it is reproducing itself like crazy."
Preclinical Results 'Beyond Wildest Dreams'
In one of several critical preclinical experiments, Lee and his co-authors put this mechanism to the test in severe combined immunodeficient (SCID) mice implanted with human glioblastoma cells. Once the engrafted tumors became palpable, the team injected them with a single dose of reovirus. "In four of five mice," their paper reported, "tumor growth was substantially suppressed by the fourth week after treatment."
SCID mice, having no functioning immune system, couldn't reject such foreign tumor grafts. Next, Lee and his co-workers injected other cancer cells into nine immunocompetent rodents. The cells grew into sizable tumors within two weeks, when the team began injecting them with repeated booster shots of reovirus over 21 days, to overwhelm incipient graft immune rejection.
Six of the nine mice had complete regression of their tumors, which persisted for the four months after treatment ended. "These results are beyond our wildest dreams," Lee said at the time.
Why does reovirus act so mildly in the human intestine and airway?
"If you think about it," Lee explained, "the cells that line our guts have to be replaced continually, for obvious reasons. In some ways, these cells are very similar to cancer cells, because they have to grow all the time. And guess what they have? A highly activated Ras pathway."
Lee is waiting for approval of Phase I human trials, which he hopes will come in six to 12 months. "The first clinical studies would definitely be in Calgary and other Canadian centers," he said. "The patients enrolled would have cancers that are readily detectable, such as breast, head and neck tumors, which you can actually measure quite easily. But, at the same time, we are not ruling out any other primary targets."
Last August, the university filed worldwide patent applications covering the use of reovirus against cancer. And, Lee concluded, "I've been getting quite a few calls from pharmaceutical companies interested in developing our reovirus anticancer approach further." n