Human endogenous retroviruses (HERVs) have been the subject of a great deal of investigation in recent years. It has been established that HERVs cause some genetic diseases and there is strong circumstantial evidence for a link to some types of cancer and autoimmune diseases, but what else do we need to know about these unique organisms?
In the December issue of the Journal of the Royal Society of Medicine, Dr Frank Ryan discusses HERVs in the context of medicine, human evolution and physiology.
Although retroviruses were discovered in animals much earlier, their infectious potential for humans was not realized until the emergence of the leukaemia viruses, HTLV-I and II and, most dramatically of all, with the AIDS pandemic caused by viruses HIV1 and HIV2. As Dr Ryan explains, endogenous retroviruses (ERVs) 'have the unique ability to invade the germ cells of every species of vertebrate' where they are transmitted as part of normal reproduction (germline transmission), meanwhile interacting with their host's evolution over millions of years. 'Genome sequencing reveals that 8% of the human genome consists of HERVs and roughly half of our DNA is made up of viral genes or their genetic products.' So what are they really doing there?
The HIV-1 retrovirus pandemic is taking the lives of millions of people across the globe. However, HERVs also play a very important role in the normal human development and day to day existence. Three separate retroviruses supply vital components to every human pregnancy. If, as the public might imagine, we should remove these viruses from our chromosomes, the human species would become extinct. How do we reconcile these strange contradictions implicit in the behaviour of the viruses?