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Virus–Gene Interaction Triggers Crohn's-Like Pathology in Mouse Model

Virus–Gene Interaction Triggers Crohn's-Like Pathology in Mouse Model

Cell. 2010;141:1114-1116, 1135-1145.

A study led by scientists at Washington University in St. Louis, Missouri, reports that interaction between a mutant allele of Crohn's disease susceptibility gene Atg16L1 and a specific virus causes abnormalities in Paneth cells of mice. Published June 25 in Cell, the findings add to the understanding of Crohn's and other complex diseases caused by combinations of genes and environmental factors.

"Atg16L1 is one component of a protein complex...that functions in autophagy," said senior author Herbert W. Virgin, IV, MD, PhD, professor and chair, Department of Pathology and Immunology, and professor of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri, in an email to Medscape Medical News. "It is not known if it is the autophagy function or Atg16L1 [specifically] that is responsible for the effects of mutation of this gene that we report," Dr. Virgin added.

Autophagy is a process by which cells degrade long-lived or insoluble proteins and microorganisms, and it may also regulate inflammation. Paneth cells, located at the base of crypts in the small intestine, secrete proteins that protect and maintain the intestinal epithelium.

Mice with mutant Atg16L1 typically show hypomorphic protein expression (and so are known as Atg16L1 HM mice) and decreased autophagy; their Paneth cells are abnormal, with increased activation of pathways involved in lipid metabolism and proinflammatory cytokines. These traits resemble those of humans with 2 copies of the ATG16L1 allele associated with Crohn's disease.

The present study was launched when Atg16L1 HM mice failed to show these changes in protein expression and autophagy after the strain was moved (by embryo transfer) to an "enhanced barrier facility." Surprisingly, Paneth cells of Atg16L1 HM mice raised in the enhanced barrier facility looked identical to those of wild-type mice. Apparently, Paneth cell abnormalities previously attributed to mutant Atg16L1 only occurred when murine norovirus (MNV) CR6 was present.

MNVs are present in mouse facilities worldwide and persist long after initial infection. The first MNV was identified in the Washington University facility in 2003, so investigators "considered the possibility" that MNVs might account for the Paneth cell pathology previously seen in Atg16L1 HM mice. To determine what caused the Atg16L1 HM phenotype, the present study tested combinations of genetic, viral, and chemical factors for their influence on intestinal pathology.

When Atg16L1 HM mice were inoculated with MNV CR6, within a week their Paneth cells contained granules of abnormal size, distribution, and quantity (this reaction had been attributed to Atg16L1 HM when mice were raised in a "conventional barrier" facility). Wild-type mice inoculated with active virus did not develop abnormal Paneth cells, nor did Atg16L1 HM mice that received ultraviolet-inactivated MNV CR6.

Substituting MNV CW3 (a nonpersistent norovirus) for CR6 caused mild changes in lysozyme distribution in both wild-type and Atg16L1 HM mice, but neither strain developed the highly abnormal Paneth cells seen with MNV CR6 infection. "The specific molecular mechanisms underlying the difference between 'trigger' virus and other virus are the subject of ongoing studies," noted Dr. Virgin.

Dextran sodium sulfate (DSS), a toxin that induces intestinal injury, was also tested in the 2 mouse strains, with and without CR6 infection. In wild-type mice and in noninfected Atg16L1 HM mice, DSS caused less epithelial cell proliferation, lost epithelial integrity, and ulcers in the transverse colon or near the ano-rectal junction; no ulcers developed in the ileum.

However, Atg16L1 HM mice infected with MNV CR6 a week before DSS administration responded with characteristics resembling human Crohn's disease. Infection with MNV CW3 did not elicit Crohn's-like changes. Interestingly, Atg16L1 HM mice that received MNV CR6 at the same time as DSS, rather than 1 week before, displayed the same mild response as wild-type mice. Apparently, viral strain and time of infection are significant in the virus–gene interaction.

Investigators also considered the role of cytokines and of gut bacteria — which have been implicated previously in inflammatory bowel disease and other diseases "triggered by" viral infection. Antibodies that block tumor necrosis factor–alpha or interferon gamma were given to Atg16L1 HM mice inoculated with MNV CR6 and treated with DSS; the specific pathologies associated with MNV CD6 infection were much reduced.

Broad-spectrum antibiotics also prevented development of DSS-induced abnormalities in the colon and ileum of these Atg16L1 HM–MNV CR6–DSS mice. Thus, not only does the injury response depend on the mutation, the specific virus (MNV CR6), and DSS, but (antibiotic-sensitive) intestinal bacteria are also involved in the intestinal injury response.

"Disease etiology in mice is, as you'd expect, very complex, and it's probably even more complex in humans," John A. Todd, PhD, director, Juvenile Diabetes Research Foundation/Wellcome Trust Diabetes & Inflammation Laboratory, Department of Medical Genetics, Cambridge Institute for Medical Research, University of Cambridge, United Kingdom, said in an email to Medscape Medical News.

"Susceptibility genes and their penetrance or expression are heavily dependent on environmental factors," said Dr. Todd. "We need to understand a lot more about the causes/etiology of common multifactorial diseases before we can develop and target therapeutics that will have a higher likelihood of being safe and effective."

Dr. Todd noted in a Cell commentary that the study's authors "are not arguing that Crohn's disease is caused by infection with norovirus or by any other single microbe. The environmental factors that predispose to and protect from Crohn's disease remain uncertain, but the balance among...gut bacteria and viral infections is likely to be part of the story."
Huh, that's really interesting. Thanks for posting this, Dave. It's funny how complicated the mechanism of disease can be just in mice. At least this might be another piece to the puzzle of how/why the disease happens in humans though.

The whole "virus-gene interaction" thing triggered by environmental factors sounds like the perfect storm. It's interesting how it sounds unlikely to happen, and yet they were actually able to see how it happened in mice.
Dusty I would try to explain it in laymans terms, but unfortunately with a paper this complex it isn't exactly easy to do. I read the paper a few weeks ago so I am somewhat rusty on the details, but it doesn't really offer a cure with their results, just a new perspective on possible causes. The biggest step forward was developing a model mouse that has most/all the major hallmarks of crohns, a feat which has not been previously done. This is a MAJOR step forward in crohns research because the more accurate the model in mice, the better a company can treat or learn about the disease for treating it with medications.

The other thing it does is shows that at least one of the alleles (gene variants) indicated in crohns can in fact lead to interactions with bacteria and viruses that can be treated with antibiotics. This would essentially help explain how some people can respond to antibiotics while others don't. This is probably the first time that a gene-environment interaction has conclusively been linked to crohns and also will allow development of genetic screens to determine courses of action for people (in the future this is). The test won't mean much until we know more about other alleles involved in crohns and how they interact with viruses and bacteria, but it is a start and with this system and knowledge, the field can move forward more rapidly than it did in the past.
Huh, that's really interesting. Thanks for posting this, Dave. It's funny how complicated the mechanism of disease can be just in mice. At least this might be another piece to the puzzle of how/why the disease happens in humans though.

The whole "virus-gene interaction" thing triggered by environmental factors sounds like the perfect storm. It's interesting how it sounds unlikely to happen, and yet they were actually able to see how it happened in mice.
What's even more unlikely is how they found out about this interaction in the first place. This particular norovirus is basically ubiquitous in mice in the lab setting so it wasn't until a special mouse facility that was designed specifically to eliminate this family of viruses was created that it could even be found. Talk about a case of serendipity right there!