The world is full of microbes, and we don’t know anything about many of them. Lots of these microbes, which include bacteria, fungi, and viruses, are harmless. But others can affect us in different ways. Some may infect our cells but have no detectable effects, others could cause a mild illness that our immune system easily clears within a few days, and some could have significant impacts on our health. Scientists are now using advanced genetic tools to learn more about the viruses that can be found in human bodies, and what their effects are.
A new study has attempted to catalogue the human virome–the collection of viruses in the human body. In this work, they analyzed the viral DNA content in blood and saliva samples that were collected from over 900,000 people.
Reporting in Nature, this work showed that there was significant variation in viral DNA content from one person to another that depended on factors like age, lifestyle, sex, and genetics. There were genetic influences that had a strong impact on whether exposure to a virus caused changes that extended far beyond the initial infection.
"We're getting to the point now where we can use human genetics to try to answer fundamental questions about pathology resulting from viruses," such as whether a virus may influence the risk of developing another disease like cancer later on in life, said first author Nolan Kamitaki, research fellow in genetics at the Blavatnik Institute at Harvard Medical School.
There are massive datasets that include DNA sequences that have been captured from human samples taken from large populations. These datasets do not only include human genomic DNA and specific mutations or variants carried by each individual, they also include DNA from the microbes we host in our body, such as the bacteria or viruses that may be living in saliva or blood. Many of these databases also include health data, so patterns and associations between genes, microbes, or health conditions can be identified.
This study was particularly focused on the effects of several viruses including Epstein-Barr virus, two other human herpesviruses (HHV-6 and HHV-7), and common anelloviruses that don’t usually cause illness, but can also be found in most people.
The course of these viruses were significantly different. Most could be found in people in the first years of life, probably when people were infected for the first time. However, Epstein-Barr virus was more prevalent as age increased. And HHV-6 became less prevalent after childhood, maybe because the immune system tends to get better at keeping it under control over time. HHV-7 incidence also tended to decline sharply in middle age.
Epstein-Barr viral load was also found to decline during summertime and increase during wintertime; HHV-7 viral load had the opposite pattern.
For all seven viruses, men carried higher viral levels in blood and saliva than women. Smoking also increased Epstein-Barr viral load.
Viral load was strongly associated with genetic factors that were related to infection response and immune escape of infected cells. The study emphasized how much the immune system does to control viral load, and how its function can vary from one person to another and over time.
EBV has been strongly connected to MS. The study also suggested that the immune response to EBV may affect the risk of developing MS.
It also showed that high levels of Epstein-Barr could increase the risk of developing Hodgkin lymphoma, though more work will be needed to confirm that finding.
Sources: Harvard Medical School, Nature
