Ancient Viruses Offer Future Promises

Imbedded in the genomes of creatures as varied as mouse and man are retroviral remnants. These artifacts of ancient infections result from RNA viruses inserting DNA copies of themselves into their hosts' genomes. Sometimes they hit the jackpot and make their way into the germ line. Sorted and shuffled over the eons, some of these ancient endogenous viruses have serendipitously developed the ability to shield cells against new viruses. Research on retroviruses and resistance to them in mice and o

May 13, 2002
Brendan Maher
Imbedded in the genomes of creatures as varied as mouse and man are retroviral remnants. These artifacts of ancient infections result from RNA viruses inserting DNA copies of themselves into their hosts' genomes. Sometimes they hit the jackpot and make their way into the germ line. Sorted and shuffled over the eons, some of these ancient endogenous viruses have serendipitously developed the ability to shield cells against new viruses.

Research on retroviruses and resistance to them in mice and other organisms surged when the war on cancer was declared in 1971. As scientists began to realize, however, that retroviruses played less of a role in cancer than previously thought, research diminished—only to be rekindled by the AIDS blight in the 1980s. "We'd be nowhere on HIV if there hadn't been years studying these weird mouse viruses," says David Sanders, associate professor, Markey Center for Structural Biology, Purdue University department of biosciences, who works with the Fv-4 resistance gene. Though questions remain—especially about the immune system's role in resistance—Sanders and other researchers hope that the information they uncover could lead to gene therapies that can fight the spread of HIV.

A Game of MCAT and Mouse

Ecotropic viruses, which can only infect the organism type from which they originate, all gain cell entry through the same receptor. In mice the receptor is mCAT-1. The Fv-4 gene, a remnant from an ancestral relative to modern ecotropic viruses, can block infection from ecotropic murine leukemia viruses like Moloney and Friend. Fv-4 encodes an envelope protein capable of binding to mCAT-1. Somewhere in the history of either murine or viral evolution, a deletion occurred knocking out the 5' upstream region of the endogenous viral gene, says Hidetoshi Ikeda, section head, department of infectious diseases, National Institute of Animal Health, Japan, who has been studying Fv-4 since the mid-1970s. This truncated allele wards off other viruses, thereby benefiting the mouse. The most pervasive current theory is that the faulty envelope product made by the truncated gene inextricably binds to the receptor cells, prohibiting viruses from attaching to the cell surface.

Sanders and his team believe they have pinpointed the amino acid sequence that causes this interference.1 In addition, according to Sander's paper, if per chance a cell is infected, Fv-4 has a dominant negative effect that hobbles the progeny. New virions will incorporate some of the faulty Fv-4 envelope protein, rendering them less capable of membrane fusion. Says Sanders, "These mice, through evolution, have figured out how both to prevent viruses from getting in, and, if a virus [does so], to reduce the infectivity of the rare virus that happens to make it out." His lab's focus on the structural basis for resistance in vitro has yielded some compelling answers, but problems arise with the receptor-mediated schema as in vitro and in vivo experiments do not appear to match up.

"I think there's still a lot of questions about Fv-4. I think there are probably several components to the mechanism of Fv-4 function," says Christine Kozak, viral biology section head, National Institute of Allergy and Infectious Diseases (NIAID). Mice heterozygous at Fv-4 are resistant, but heterozygous fibroblasts are more susceptible, says Ikeda. "The resistance is very strong in vivo," comments Kozak. "You can inoculate these mice with the virus, and the next day, it's gone. In vitro, it's a different story."

Adding to the muddle are mixed results in immunosuppressed mice. Nude mice, heterozygous for Fv-4, are more susceptible to Friend virus, and resistance drops further when immunosuppressant drugs are added.2 The evidence points to an important role that the immune system plays. Immune function could merely be batting cleanup to eradicate viruses not turned away by receptor-mediated interference, or it could be taking a more proactive role—already prepped by its recognition of the endogenous envelope protein. Jonathan Silver, senior NIAID investigator, says that a few experiments support the latter. He made bone marrow chimeras that express Fv-4 and challenged mice at various times after transplant. "It looked like the resistance was stronger several months after transplant, which might suggest some sort of change in the immune system because of exposure to Fv-4," says Silver, though he stresses the data are far from conclusive.

Sanders says that his results suggest that the primary mechanism is receptor-mediated, and that discrepancies are simply due to the lower values of defective envelope protein in heterozygotes. "We try in the paper to indicate that the issues here are probably numerical rather than that Fv-4 is actually stimulating the immune system." Silver says that, whatever the immune response's function, it is largely an issue of semantics, and doesn't diminish the applicability to HIV research. "The Friend virus can be an overwhelming infection, which can lead to immunosuppression like HIV does. If you could [transplant] cells that were ... resistant to the virus and possibly as a consequence resistant to immunosuppression, they might be able to provide a more normal immune response."

Outwitting the Invaders


Courtesy of Whitehead Institute for Biomedical Research

Be Sure to Wear a Coat: A coat protein from Moloney murine leukemia virus

Instead of a gene therapy that uses viruses to infect a good copy of a malfunctioning gene in the human genome, here the intent is to infect with a malfunctioning copy of a retroviral gene in hopes that the harmless proteins produced will confound the invaders' actions. "I think HIV is a perfectly legitimate target for this," says Sanders. "The mechanism that we propose for Fv-4 would allow this to be useful both for people who are not infected with HIV and also for people infected. You could potentially prevent or reduce the spread of it within the patient." Ikeda and others recently demonstrated a gene therapy model involving bone marrow transplant.3 "We don't know the mechanism, but so far if the envelope gene is expressed in hematopoietic cells transferred to the mice ... the mice become resistant to the Friend virus leukemia," says Ikeda.

Studies of systems like Fv-4 resistance have brought about a better understanding of the battles between retroviruses and the cells they infect. "I think—and this is what we're trying to show—that there's still neat things that the mouse system can teach us about," says Sanders. A mouse is not a human, however. Jonathan Stoye, head, Division of Virology, National Institute for Medical Research, UK, has done significant work on Fv-1, a distantly related gene that blocks retrovirus replication. Stoye explains, "You would find people that would say that if you want to treat a human disease, the proper form of study is humans and the human retrovirus. You'd find other people who'd be less dogmatic than that, who would say that the information we gather from studying the mouse viruses may yet prove useful in terms of trying to design strategies to combat human disease. I think the jury's out."

Brendan A. Maher can be contacted at bmaher@the-scientist.com.

References
1. G.W. Taylor et al., "Fv-4: Identification of the defect in env and the mechanism of resistance to ecotropic murine leukemia virus," Journal of Virology, 75:11244-8, November 2001.

2. F. Zhang et al., "Resistance to Friend murine leukemia virus infection conferred by the Fv-4 gene is recessive but appears dominant from the effect of the immune system," Journal of Virology, 74:6193-7, July 2000.

3. M. Kitagawa et al., "A gene therapy model for retrovirus-induced disease with a viral env gene: Expression-dependent resistance in immunosuppressed hosts," Leukemia, 15:1779-84, November 2001.