Is chronic wasting disease a threat to humans?

Areas with CWDChronic wasting disease (CWD) is a prion disease of cervids (deer, elk, moose). It was first detected in Wyoming and Colorado, and has since spread rapidly throughout North America (illustrated; image credit). Because prions that cause bovine spongiform encephalopathy (BSE, mad cow disease) are known to infect humans, there is concern that CWD might also cross the species barrier and cause a novel spongiform encephalopathy. Recent experimental results suggest that CWD prions are not likely to directly infect humans.

The prion protein PrPC is encoded by the prnp gene, which is essential for the pathogenesis of transmissible spongiform encephalopathies (TSEs). Transgenic mice have been used to understand the species barrier to prion transmission. When mice are inoculated with human prions, few animals develop disease and the incubation periods are over 500 days. When the mouse prnp gene is replaced with the human gene, the mice become uniformly susceptible to infection with human prions and the incubation period is shorter. The species barrier to prion transmission is therefore associated with differences in the prion protein sequence between host and target species.

Mice have been used to understand whether CWD prions might be transmitted to humans. Mice are not efficiently infected with CWD prions unless they are made transgenic for the cervid prnp gene. Four different research groups have found that  mice transgenic for the human prnp gene are not infected by CWD prions. These findings suggest that CWD prions are not likely to be transmitted directly to humans. However, changing four amino acids in human prnp to the cervid sequence allows efficient infection of transgenic mice with cervid prions.

Another concern is that prions of chronic wasting disease could be transmitted to cows grazing in pastures contaminated by cervids. Prions can be detected in deer saliva and feces, and contamination of grass could pass the agent on to cows. In the laboratory, brain homogenates from infected deer can transmit the disease to cows. Therefore it is possible that cervid prions could enter the human food chain through cows.

A further worry is that BSE prions shed by cows in pastures might infect cervids, which would then become a reservoir of the agent. BSE prions do not infect mice that are transgenic for the cervid prnp gene. However, intracerebral inoculation of deer with BSE prions causes neurological disease,  and the prions from these animals can infect mice that are transgenic for the cervid prnp gene. Therefore caution must be used when using transgenic mice to predict the abilities of prions to cross species barriers.

Although the risk of human infection with CWD prions appears to be low, hunters should not shoot or consume an elk or deer that is acting abnormally or appears to be sick, to avoid the brain and spinal cord when field dressing game, and not to consume brain, spinal cord, eyes, spleen, or lymph nodes. No case of transmission of chronic wasting disease prions to deer hunters has yet been reported.

Transgenic mice susceptible to poliovirus

pvr transgenic mouseYesterday I terminated the last remaining mice in my small colony, including the line of poliovirus receptor transgenic mice that we established here in 1990. Remarkably, I had never written about this animal model for poliomyelitis which has played an important role in the work done in my laboratory.

While I was still working on poliovirus as a postdoctoral fellow with David Baltimore, I became interested in how the virus causes disease. There were no convenient animal models to study poliovirus pathogenesis, so I began to think about the cellular receptor for the virus and how it could be used to make a mouse model for infection. When I moved to Columbia University Medical Center in 1982, I decided to identify the cellular gene for the poliovirus receptor. This work was carried out by the second graduate student in my lab, Cathy Mendelsohn. She identified a gene from human cells that encoded a protein which we believed to be the cellular receptor for poliovirus. When this human gene was expressed in mouse cells, it madepvr model them susceptible* to poliovirus infection (the mouse cells were already permissive for poliovirus replication). The gene encodes a transmembrane glycoprotein (illustrated) that we called the poliovirus receptor (PVR), later renamed CD155. Over the years we worked extensively on PVR, with the goals of understanding its interaction with poliovirus during entry into the cell. In one project we collaborated with Jim Hogle, David Belnap, and Alasdair Steven to solve the structure of poliovirus bound to a soluble form of PVR. The image of that complex decorates the banner at virology blog and twiv.tv.

Shortly after identifying PVR as the cellular receptor for poliovirus, a new student, Ruibao Ren, joined my lab. For his project I suggested he create transgenic mice with the human gene for PVR. We already knew that synthesis of PVR in mouse cells allowed the complete poliovirus replication cycle. Together with Frank Costantini and JJ Lee, Ruibao produced PVR transgenic mice and showed that they were susceptible to poliovirus infection. The illustration at top left shows a PVR transgenic mouse with a paralyzed left hind limb after poliovirus inoculation.

Poliovirus transgenic mice were used for many years in my laboratory to study how the virus causes disease, and to identify the mutations that attenuate the neurovirulence of the Sabin vaccine strains. A good summary of this work can be found in my review, ‘One hundred years of poliovirus pathogenesis‘. But there is a dark side of this story that I wish to briefly recount. When we first developed PVR transgenic mice, my employer decided to patent the animals. Until the patent issued, we could not share the transgenic mice with other researchers. As a consequence, others developed their own lines of PVR transgenic mice. One of these lines has been qualified by the World Health Organization to determine the neurovirulence of the Sabin vaccine strains. However, Columbia University realized little income from the PVR transgenic mice – such animals cannot be patented in Europe. By patenting the mice, we simply delayed research progress. Because of this experience I am personally very wary about patenting biological discoveries.

There are several reasons why I decided to stop doing research with mice. The cost of housing and breeding mice is very high, nearly $1.00 US per cage per day, and I simply don’t have the funds to support such work. More importantly, no one in my laboratory has any interest in working with mice: the last student to do mouse work left years ago. Although there are many interesting experiments to be done using viruses and mice, that line of work ended for the Racaniello lab on 11 July 2011.

*A susceptible cell bears the receptor for the virus; a permissive cell allows viral replication. A susceptible and permissive cell allows the complete viral replication cycle.

 Ren, R., Costantini, F., Gorgacz, E., Lee, J., & Racaniello, V. (1990). Transgenic mice expressing a human poliovirus receptor: A new model for poliomyelitis Cell, 63 (2), 353-362 DOI: 10.1016/0092-8674(90)90168-E

Mouse model for hepatitis C virus infection?

pvrtgNo, not yet. But the recent identification of human occludin as a cell protein required for entry of hepatitis C virus into mouse cells is a huge step in the right direction.

Small animal models for virus infection are of great value for studying viral pathogenesis, testing new vaccines, and developing antiviral drugs. However, mice are often not susceptible or permissive for infection by many important viruses. For example, mice and mouse cells are permissive for poliovirus replication, but are not susceptible to infection with the virus. Mice can be made susceptible to poliovirus infection by transgenic expression of the human poliovirus receptor. In a similar way, a transgenic mouse model for measles virus infection has been produced. But it has been far more difficult to create transgenic mice susceptible to other important viruses, including HIV-1 and hepatitis C virus (HCV).

Mouse cells are permissive for HCV replication (the viral RNA can replicate when transfected into cells), but they are not susceptible: for example, they cannot be infected by lentiviral particles bearing HCV glycoproteins.  Three cellular proteins required for HCV entry into cells were previously identified: CD81, scavenger receptor class B type I or SCARB1, and claudin-1. Most recently a fourth cellular protein, occludin, was found to be essential for HCV entry into cells. HCV can enter mouse cells that synthesize human occludin and CD81; apparently the murine versions of SCARB1 and claudin-1 can promote HCV entry. The human versions of these four proteins was sufficient to confer permissivity to HCV entry in a variety of human, murine, or hamster cell lines.

Because the major block to HCV entry in murine cells can be overcome by synthesis of human CD81 and occludin, will it now be possible to produce transgenic mice susceptible to HCV? Perhaps not yet: there are other blocks to HCV replication in murine cells. For example, viral RNA replication in mouse cells is not efficient, nor has assembly of infectious virions been demonstrated. But these problems can now be readily addressed, and I would not be surprised to find that CD81/occludin transgenic mice already exist and are being tested for susceptibility to HCV in at least one laboratory.

Ploss, A., Evans, M., Gaysinskaya, V., Panis, M., You, H., de Jong, Y., & Rice, C. (2009). Human occludin is a hepatitis C virus entry factor required for infection of mouse cells Nature, 457 (7231), 882-886 DOI: 10.1038/nature07684