Ancient viruses have left behind genetic sequences that make up to 8% of human DNA. These sequences are hundreds of thousands to millions of years old, some even predate the origin of Homo sapiens. They are known as human endogenous retroviruses, or Hervs.
According to recent studies, there may be a connection between some ancient viral DNA sequences in the human genome and the vulnerability to mental illnesses including schizophrenia, bipolar disorder, and major depressive disorder.
Hervs were once thought to be “junk DNA,” or sections of our genome that had no purpose. However, it’s now clear that this so-called junk DNA serves more purposes than once thought as our understanding of the human genome has grown.
Early studies revealed that Hervs control the expression of nearby genes involved in vital biological processes.
In blood and brain samples, hervs have also been shown to generate RNAs and even proteins. Since these molecules may move across different cellular compartments to perform various activities, they have the ability to exert a wide range of actions.
Additionally, research has shown evidence that suggests certain human genes are descended from Hervs. This suggests that Hervs were co-opted for specialised biological roles along the course of evolution.
The researchers attempted to determine if variations in Herv expression were linked to genetic predisposition to certain mental diseases, given the plethora of Hervs in the genome and their potentially various functions.
Researchers were able to identify DNA variants that altered Herv expression in the brain by profiling Herv expression in almost 800 postmortem brain samples.
They then compared the results of these findings with those from extensive genetic studies that examined genetic variations among tens of thousands of individuals, both with and without mental health issues.
The study discovered that the expression of four Hervs was associated with a hereditary predisposition to serious mental illnesses. Of these Hervs, the expression of two was linked to schizophrenia, one to bipolar disorder and schizophrenia, and one to depression. These findings imply that Hervs could be more significant in the brain than previously believed.
This finding is significant because it suggests that these ancient viral DNA sequences may be more important to our brains than previously believed. Though there are many contributing variables to mental diseases, knowing how Hervs function may aid in the development of more effective therapies down the road.