Tuesday, 11 July 2017

Human Herpesvirus 6 and Multiple Sclerosis: A Possible Link

Human herpesvirus 6 (HHV-6) is a very common virus. Researchers estimate that more than 80% of us were exposed to the virus in childhood, usually without realizing it. The immune system can generally fight the active form of HHV-6, but the virus has a technique to avoid complete destruction. It has the ability to become latent. This means that it’s present in our cells but is hiding from the immune system and is apparently inactive. Researchers think that the latent virus isn’t completely inactive, however, and that it may be involved in some cases of multiple sclerosis.
Illustration by LadyofHats, public domain license

Multiple sclerosis or MS is a demyelinating disorder. Myelin is a fatty material that covers and insulates the nerves. For an unknown reason, in MS patients the immune system destroys myelin surrounding the nerve cells in the brain and spinal cord. This disrupts the flow of nerve impulses through the body and can cause a wide variety of debilitating symptoms, including muscle weakness, coordination problems when moving, and problems with sensation.

As long ago as 2003, researchers found latent HHV-6 in the brain cells of people with severe multiple sclerosis. HHV-6 may not be a direct cause of MS, but it’s thought that the virus prevents the body from repairing loss of myelin. When myelin is damaged in our body, oligodendrocyte progenitor cells normally migrate to the injured area and produce oligodendrocytes. These cells then make myelin to repair the injury. This process may not happen in someone with MS.

HHV-6 becomes latent by incorporating its own DNA into the DNA of its host. The virus then produces a protein called U94. This protein helps the virus to remain in the DNA and prevents it from being detected by the immune system. Researchers from the University of Rochester in the United States have discovered that when the HHV-6 virus is present in the DNA of human oligodendrocyte progenitor cells placed in lab animals, the U94 protein that’s made prevents the cells from migrating to nerves with injured myelin. As a result, the myelin of the animals disappears and the nerves are damaged.

                                           Possible symptoms of multiple sclerosis 
                               Illustration by Mikael Haggstrom, public domain license

The discovery about the possible relationship between the virus and MS is interesting, but there are some questions that need to be answered. 
  • One question is whether the inability of oligodendrocyte progenitor cells to migrate when they are infected by the virus is true in human cells as well as in the cells of lab animals. 
  • Another is whether the latent virus is harmful in people with MS or whether it needs to become active in order to be harmful. 
  • Yet another topic to investigate is the location of the virus in the bodies of healthy people and in the bodies of those with MS. 
  • We also need to know how the body of people with MS responds to the latent and active virus compared to the body of someone without MS. 
It certainly seems that the potential link between human herpesvirus 6 and multiple sclerosis should be investigated further. It’s sad that we still don’t know the cause or causes of the disorder. Understanding how the disease arises could enable us to treat it better and perhaps prevent it. 

References

The possible role of HHV-6 in multiple sclerosis
Facts about multiple sclerosis