Thousands of genetic sequences with origins in ancient viral infections are active in the human brain and contribute to susceptibility to psychiatric disorders such as schizophrenia, bipolar disorder and major depressive disorder, researchers said in a study released on Friday.

Their study has identified two sequences from human endogenous retroviruses (HERVs) associated with a risk for schizophrenia, one HERV sequence associated with the risk of both bipolar disorder and schizophrenia, and one associated with the risk of
depression.

Although earlier research had hinted that HERV sequences may play a role in major psychiatric disorders, the new study is the first to demonstrate that a specific set of HERV sequences active in the human brain enhances the risk of the disorders. The findings were published on Wednesday in the journal Nature Communications.

“Our results suggest that these viral sequences play a more important role in the human brain than originally thought,” said Timothy Powell, a study team member and senior lecturer in genetics and neuroscience at King’s College.

He said that specific HERV activity profiles appear associated with an increased susceptibility for some psychiatric disorders.

Scientists have known for two decades that 8 per cent of the human genome is made up of HERV sequences that originated from ancient retroviral infections over 1 million years ago.

The sequences inserted themselves in the genome and multiplied through a copy-paste mechanism. Earlier studies have collectively catalogued some 14,968 HERV sequences, representing over 60 retroviral families.

Genome researchers have hypothesised that some of these so-called fossil virus sequences, which were once dismissed as “junk DNA”, may control the activity of nearby genes.

The first hint of a role for HERV sequences in psychiatric disorders emerged in
2001 when medical researchers at the Johns Hopkins School of Medicine in the US identified retroviral genetic material in the cerebrospinal fluid and brains of individuals with schizophrenia.

The evidence has grown over the past two decades through studies hinting at HERV sequences influencing bipolar disorder and schizophrenia among other neurological disorders. Earlier studies, however, relied on small sample sizes, limiting the reliability of their findings.

Powell and his colleagues analysed data from genetic studies that involved tens of thousands of people, both with and without psychiatric disorders and from 792 post-mortem brain samples to look for connections between HERV activity and psychiatric disorders.

“Our study was better able to circumvent the limitations of earlier studies because we used HERV expression with better precision alongside data from large genetic studies and analysed thousands of people with mental health conditions and unaffected individuals,” Powell told The Telegraph. This is something no other study has done, he said.

Rodrigo Duarte, a research fellow at King’s College and the study’s first author, said the HERV sequences are not just “junk DNA”.

“We find nearly 5,000 HERV sequences expressed in the brain. The expression of HERVs correlates with genetic risk to (some) psychiatric disorders,” Duarte said.