Ancient viral DNA appears connected to some psychiatric conditions

Our DNA is a puzzle resulting from millions of years of evolution. An ancient part of it is composed of the genetic material of viruses that integrated in our genome and became part of “us.” Though it may seem strange, about 7% of our genomes are comprised by these old viral sequences. What’s more, recent research indicates that some of these ancient viral DNA may be involved in increased susceptibility to certain psychiatric conditions, such as depression, schizophrenia or bipolar disorder.

How did this viral DNA get stuck into our genome? retroviruses are viruses that insert their genetic material into the DNA of their host cells. This process occurred many times during our evolution, but only those times when the viruses inserted their DNA into reproductive cells (sperm or eggs), the genetic material from these retroviruses was further transmitted over generations, up to the present day.

You may think that this viral DNA is “useless,” but you’d be wrong. It has been shown that some are implicated in the regulation of other genes and that others can get translated into RNA and produce proteins.

A recent study ¹ investigated whether some of this ancient viral DNA could be related to certain mental diseases. To this end, the researchers profiled expression of such viral DNA sequences in nearly 800 autopsy brain samples. This helped identify DNA variations influencing viral DNA expression in the brain.

The information they obtained was contrasted with information coming from large genetic association studies, in which the genome of people having a certain mental disease is compared to that of people not having it. This comparison showed that the expression of four such viral DNA sequences was linked with genetic susceptibility to major psychiatric disorders. The expression of two of them was associated with schizophrenia, one with both schizophrenia and bipolar disorder, and another with depression.

As interesting as this finding is, it is still early days to determine what is the precise role that this ancient viral DNA plays in psychiatric diseases. However, these results indicate two things: one, the intrinsic complexity of mental disease and two, that there is no such thing as junk DNA.