Human genome carries 40 million-year-old virus

Posted by Emma Woollacott

About eight percent of human genetic material comes from a virus and not from our ancestors - and could be causing mutations and psychiatric disorders such as schizophrenia.

Research from the University of Texas shows that the genomes of humans and other mammals contain DNA derived from the insertion of bornaviruses, RNA viruses whose replication and transcription takes place in the nucleus.

The assimilation of viral sequences into the host genome is a process referred to as endogenization. It occurs when viral DNA integrates into a chromosome of reproductive cells and is subsequently passed from parent to offspring.

Until now, retroviruses were the only viruses known to generate such endogenous copies in vertebrates. But Feschotte said that scientists have found that non-retroviral viruses called bornaviruses have been endogenized repeatedly in mammals throughout evolution.

Bornaviruses are unique because they infect only neurons, establishing a persistent infection in their host's brain, and their entire life cycle takes place in the nucleus of the infected cells.

Feschotte said this intimate association of BDV with the cell nucleus prompted researchers to investigate whether bornaviruses may have left behind a record of past infection in the form of endogenous elements. They searched the 234 known eukaryotic genomes for sequences similar to that of BDV.

"The researchers unearthed a plethora of endogenous Borna-like N (EBLN) elements in many diverse mammals, " Feschotte said.

The scientists were also able to recover spontaneous BDV insertions in the chromosomes of human cultured cells persistently infected by BVD. Based on these data, Feschotte proposes that BDV insertions could be a source of mutations in the brain cells of infected individuals.

"These data yield a testable hypothesis for the alleged, but still controversial, causative association of BDV infection with schizophrenia and mood disorders," Feschotte said.

Feschotte's research appears in Nature.