Former viral DNA integrated into the human genome may increase people’s susceptibility to neuropsychiatric disorders, such as depressionbipolar disorder and schizophrenia.
A study published in May in the journal Natural communications zoomed in on human endogenous retroviruses (HERV) — extracts of DNA which constitute approximately 8% of the modern human genome.
Psychiatric disorders tend to be hereditary and twin studies have also suggested that genetics play a role in their development. Estimates suggest schizophrenia and bipolar disorder could impact heritability up to 80%meaning most of the variability seen in these disorders it comes down to differences in people’s genetics.
Specific versions of genes, or genetic variants, have been linked to these disorders, but little is known about the influence of HERVs.
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“We were fascinated by the concept that (HERVs) existed in the human genome and not much was known about them,” study co-author Timothy Powellneuroscientist and molecular geneticist at King’s College London, told Live Science.
HERVs are fragments of viruses that have been integrated into the human genome through evolution, with the oldest examples having been introduced by our ancestors. more than 1.2 million years ago. Some HERVs are known to be activated in cancer cells and may contribute to disease; others are active in healthy tissues or play an important role early in development, so they are not necessarily all bad. Some HERVs are even active in the brain, but it is not yet clear what they do.
Previously, scientists have studied the role of HERVs in psychiatric disorders by comparing the genetic material of individuals without such disorders with that of people with a given disorder. A drawback of this method, however, is that it does not take into account the influence of environmental factors or other conditions that a person may have. It is therefore difficult to say with certainty that a given part of DNA, taken in isolation, is strongly associated with disease.
The new study used a different approach to assess the effects of thousands of HERVs. The researchers accessed genetic data from previous studies involving tens of thousands of people, as well as post-mortem brain tissue samples collected from nearly 800 patients with and without psychiatric disorders. They then studied the genetic variants carried by different individuals, noting whether they appeared to affect nearby HERVs.
They found that specific genetic variants were associated with a higher risk of three psychiatric disorders: schizophrenia, depression and bipolar disorder. These variants also affected whether and to what extent HERVs in the brain were “activated.”
“This (association) gives us much more certainty that the genetic differences we observe between cases and controls are more likely to be an accurate reflection of the biology of the disease,” Rodrigo Duarteresearcher at King’s College London, told Live Science.
The team is the first to identify five new HERVs strongly linked to psychiatric disorders. Two were associated with schizophrenia, one was common to schizophrenia and bipolar disorder, and one was specific to major depressive disorder. These five HERVs are distinct from all those previously linked to each of the conditions.
“This is a major step forward,” said Dr Avindra Nath, clinical director of the National Institute of Neurological Disorders and Stroke, who was not involved in the study. “The same way we study all these other neurological diseases, we need to re-examine them using their technique,” Nath told Live Science.
The study suggests that these HERVs increase the chances of developing these disorders, but at this point, not much can be said about the extent to which these genetic extracts increase an individual’s risk. Carrying one of the HERVs does not necessarily guarantee that a person will be affected by the associated disorder.
In the future, the group plans to manipulate HERV activity in brain cells in laboratory dishes to see if they affect how neurons grow and make connections.
“From a genetic point of view, this is a breakthrough in the field,” Nath said. “But from a pathogenesis perspective, there is a lot left to answer” about how HERVs actually contribute to disease.
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