Ancient Primate Virus Silenced a Gene Crucial for Human Brain Development
An ancient retrovirus that infected the ancestors of chimpanzees millions of years ago left behind genetic remnants that actively switch off a gene still vital to human brain development, according to a study published in Science Advances. Researchers at Lund University discovered that while humans and chimpanzees share nearly identical protein-coding DNA, 158 copies of a viral insertion known as PTERV1 populate the chimpanzee genome while remaining entirely absent in humans. This viral fossil methylates and permanently silences a non-coding RNA gene called LINC00662 in chimpanzees, whereas human cells leave the corresponding DNA unmethylated and fully active.
Researchers Study How Retroviruses Influence Human Brain Size
For decades, evolutionary biologists have investigated why human brains grow to roughly three times the size of chimpanzee brains despite an overwhelming genetic overlap exceeding 96 percent in protein-coding regions. The Lund University team, led by postdoctoral researcher Patricia Gerdes and neurogeneticist Johan Jakobsson alongside collaborators in Gothenburg, Uppsala, and the United States, focused on gene regulation rather than the genes themselves. Retroviren insert their genetic code into host cells, and when such an infection strikes germ cells, the viral DNA passes vertically to offspring as endogenous retroviruses. At least eight percent of both human and chimpanzee genomes consist of these ancient viral remnants.
The specific retrovirus identified in the study, designated PTERV1 (Pan troglodytes endogenous retrovirus 1), invaded the lineage of African apes approximately five to seven million years ago. Researchers found 158 complete copies of PTERV1 in the chimpanzee genome, spanning about 7,500 base pairs each. This ancient viral spread likely swept through ancestral ape populations as a severe pandemic that created a genetic bottleneck. Humans likely escaped this viral invasion either by occupying distinct geographic territories or through the protective action of an antiviral protein called TRIM5-alpha, which blocks PTERV1 infections in human cells.
Laboratory Organoids Reveal LINC00662 Activity Differences
Because early embryonic chimpanzee brain tissue is inaccessible for direct study, the research team generated induced pluripotent stem cells from two humans and two chimpanzees. They cultivated these cells under identical conditions to produce millimeter-sized brain organoids that mimic early neurodevelopment. Using long-read sequencing technology, the team mapped chemical methyl tags across the genomes and found that dense methylation covered nearly all 158 PTERV1 copies in the chimpanzee organoids. This cellular defense mechanism spilled over into neighboring sequences, specifically shutting down the LINC00662 gene on chromosome 19.
Human organoids displayed strong activity for LINC00662, whereas chimpanzee organoids kept the gene completely dormant. The insertion of a roughly 7,000-base-pair PTERV1 sequence directly into the middle of the chimpanzee LINC00662 gene drives this silencing effect. When the researchers deployed CRISPR-Cas9 gene-editing tools to excise the viral segment from chimpanzee stem cells, LINC00662 transcription immediately resumed. Additional tests across neural cell models confirmed that the gene remains inactive in bonobos, orangutans, and rhesus macaques, while gorillas and macaques lack the sequence entirely.
Cellular Mechanics of Early Human Neurogenesis
Human brain tissue data analyzed by the Lund team indicates that LINC00662 expression peaks roughly ten to twelve weeks post-fertilization before tapering off in adulthood. Researchers grew brain organoids to investigate the role of the LINC00662 gene.
Frequently Asked Questions About the PTERV1 Study
How did researchers prove the virus caused the gene silencing?
The team used CRISPR-Cas9 gene editing to physically cut out the 7,000-base-pair PTERV1 viral insert from chimpanzee stem cells. Once the viral fragment was removed, the LINC00662 gene resumed transcription, confirming that the retroviral insertion directly caused the shutdown.
Which primate species carry the PTERV1 viral insertions?
Researchers identified complete copies of PTERV1 in the genomes of chimpanzees, bonobos, and gorillas. The specific viral insertion that silences LINC00662 is found exclusively in chimpanzees and bonobos, while humans lack the virus entirely.
Does a single gene explain the difference in brain size between humans and chimpanzees?
No single gene accounts for the threefold difference in brain volume. Researchers emphasize that LINC00662 represents one specific regulatory mechanism among many genetic and environmental factors influencing primate brain evolution.