Mental illnesses linked to ‘junk DNA’ embedded with viruses inherited from our ancestors

Man and DNA chains

Ancient viruses embedded in human DNA increase susceptibility to psychiatric disorders, scientists believe.

About 8 per cent of the human genetic code consists of sequences called Human Endogenous Retroviruses (Hervs), the remnants of ancient infections caught by our ancestors hundreds of thousands of years ago.

These mysterious pieces of code were thought to be junk DNA, having little impact on the body.

But King’s College London has discovered that some of the sequences appear to contribute to the likelihood of developing conditions such as schizophrenia, bipolar disorder and depression.

Dr Timothy Powell, co-senior author of the study and senior lecturer at the King’s Institute of Psychiatry, Psychology and Neuroscience, said: “These viral sequences probably play a more important role in the human brain than originally thought.

“Retroviruses infected our lineage multiple times throughout our evolutionary history, embedding their genetic material within infected cells.

“When these infections happened in sperm or egg cells that created offspring, the retroviral genetic material was inherited by future generations, eventually becoming a fixed part of our genetic make-up.”

The study analysed data from large genetic studies involving tens of thousands of people – both with and without mental health conditions – as well as information from autopsy brain samples from 800 individuals.

Researchers found that in people who were genetically susceptible to psychiatric disorders, parts of the ancient virus DNA were being either ramped up or dialled down which may affect brain function.

A substantial genetic component

Two virus codes linked to schizophrenia were found, one associated with bipolar and schizophrenia and a third linked to the risk of depression.

Dr Rodrigo Duarte, the first author and research fellow, said: “We know that psychiatric disorders have a substantial genetic component, with many parts of the genome incrementally contributing to susceptibility.

“In our study we were able to investigate parts of the genome corresponding to Hervs, which led to the identification of five sequences that are relevant to psychiatric disorders.

“While it is not clear yet how these Hervs affect brain cells to confer this increase in risk, our findings suggest that their expression regulation is important for brain function.”

The sequences were inserted into human DNA from ancient infections hundreds of thousands to millions of years ago but it was assumed the code was there to help viruses themselves to hijack cells and replicate.

But in recent years, scientists have realised that they also have an impact on other functions in the body, such as helping the placenta to form.

The team is hopeful that targeting the ancient viral sequences might help treat psychiatric conditions.

Dr Douglas Nixon, the co-senior author on the study and researcher at Northwell Health in the US, said: “We think that a better understanding of these ancient viruses, and the known genes implicated in psychiatric disorders, have the potential to revolutionise mental health research and lead to novel ways to treat or diagnose these conditions.”

The research was published in the journal Nature Communications.

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