Prenatal inhibition of an enzyme called TOP2A blunts the sociability of both zebrafish and mice, a new study suggests. The findings may be the first to implicate this enzyme, which unwinds DNA, in social behaviors.

TOP2A belongs to a family of enzymes called topoisomerases, which help cells express extremely long genes in neurons. More than a quarter of “ultra-long” genes are linked to autism.

The team behind the new study homed in on TOP2A after testing more than 1,100 compounds in zebrafish. These fish are useful models for this kind of work, says lead investigator Randall Peterson, dean of the College of Pharmacy at the University of Utah, because the animals have developed social behaviors, share about 70 percent of their genes with people and absorb chemicals through their scales, directly into the body.

“When we started this project, our primary interest was social development, [and] the question of how animals and humans develop their social instincts,” Peterson says. “A big takeaway for us was this apparently important role of TOP2A in social development.”

TOP2A appears to stymie the action of a large protein conglomerate called polycomb repressive complex 2 (PRC2), Peterson and his colleagues found, and PRC2 suppresses more than two dozen genes linked to neurological conditions, including autism. Blocking PRC2 with an inhibitor normalized the social behavior of zebrafish exposed prenatally to compounds that block TOP2A. The study appeared in Science Advances in November.

“In general, with any animal model, it’s important not to overinterpret face validity, whether it be with fish or mammals,” says Ellen Hoffman, associate professor at Yale University’s Child Study Center, who was not involved in the work. Instead, scientists should “use this as a launch point to understand the circuitry underlying that behavior.”

Peterson’s team conducted their initial chemical screen in zebrafish embryos. They exposed each fish to one of the chemicals for three days and then measured how much time it spent near other fish at three weeks of age, using a semi-automated video-based device called Fishbook. The device can screen and classify the behaviors of more than 1,000 animals per day, Peterson says.

“This screen really, for the first time, enables systematic and high-throughput assessment of environmental factors, like chemicals and drugs, on social development,” he says.

Four compounds, all fluoroquinolones that inhibit TOP2A, diminished zebrafish social behaviors, causing the animals to spend more time in open water and less time near other fish, as they typically would. The same changes happened when the team deleted a single copy of the TOP2A gene from the fish’s genome.

TOP2A has previously been shown, in mice, to downregulate the long, noncoding RNA implicated in Angelman syndrome, says Mark Zylka, professor of cell biology and physiology at the University of North Carolina at Chapel Hill, who was not involved in the study. But it hasn’t been linked to autism in human sequencing studies.

Depletion of TOP2A in zebrafish downregulated more than 5,000 genes that have human orthologs, according to RNA-sequencing data. Many of the downregulated genes have PRC2 binding sites, and some have been previously linked to autism in human studies.

This repressor complex, which is present in insects, fish and mammals, is built from four components that come together in the cell, bind to DNA and compress chromatin. PRC2 methylates histone H3 proteins, thus shutting down the expression of specific genes. With less TOP2A, PRC2 may somehow be more active than usual, repressing genes linked to autism or social development, the findings suggest.

The diminished social behaviors were also reversed in zebrafish by inhibiting PRC2. “After a single overnight treatment with this inhibitor, these fish became social,” Peterson says. “That was a really shocking and interesting finding.”

TOP2A and its role in social development is conserved in mammals, the new study also shows. Pregnant mice treated with ICRF-193, a TOP2A inhibitor, had pups with diminished social behaviors but that otherwise showed typical exploratory behaviors, stress responses and motor coordination.

Finding genes or compounds that reverse social behaviors in zebrafish could help pinpoint other circuits and pathways involved in their development, Peterson says. “Moving forward, that’s an area of great interest to us.”