From the outside, humans are pleasingly symmetrical, with arms, legs, and eyes that have matching right and left sides. But inside, it’s a different story: our heart is on the left; our liver is on the right. Lungs and kidneys are also asymmetric. Now researchers have pinned down a gene that helps developing organs find their proper place.
Scientists have identified other genes that break the initial symmetry of a developing round embryo, and help organs pick sides. But the way researchers tracked this one down was unique, says Daniel Grimes, a developmental biologist at the University of Oregon who was not involved with the work but calls it “exceptional.” The research, he says, could lead to a better understanding of why organ formation goes awry, as it does in some people.
Developmental biologists have long known that the off-center placement of the heart and other organs is linked to a group of cells called the left-right organizer, which transiently forms in an early embryo. In 1998, based on studies in mice, Japanese researchers proposed that twirling cilia—hairlike appendages on a subset of organizer cells—send embryonic fluid to the left but not to the right, helping organs form in the correct place. The flow activates certain genes just on that left side, altering what grows next, they and others have speculated. The same thing happens in fish and frogs, researchers later found.
But surprisingly, there are no such cells with twirling cilia in developing chicks and pigs, even though their hearts still form to one side. There have been “many confusing results in the literature that are hard to reconcile,” Grimes says. He and others think these so-called motile cilia evolved early in animal evolution but were lost in the branches of the animal family tree leading to birds and to the “even-toed” mammals such as pigs, but not humans.
Developmental biologists Bruno Reversade from the Genome Institute of Singapore and Christopher Gordon from the Imagine Institute in Paris wondered whether this disparity could hint at a way to track down new genes responsible for breaking body symmetry. They and their colleagues simply looked for genes active in developing mice, fish, and frogs, but inactive at the stage of development in pigs and birds where there was no longer any fluid flow and thus no need for those genes.
The researchers discovered five such genes, they report this month in Nature Genetics. Reversade knew his team was on the right track because three of these genes were already known to be important in flow-induced loss of symmetry. Of the two new genes, the researchers focused first on one called CIROP, which no one had ever described before.
By engineering this gene to make a fluorescent tag when active, the scientists determined that CIROP turned on for a few hours just as the organizer formed in zebrafish, mice, and frog embryos. The gene in chicks, pigs, and reptiles was either missing or had so many mutations there was no way for the researchers to use the gene editor CRISPR to reconstitute it. When they used CRISPR to deactivate this gene in fish and frogs, they determined CIROP was only needed on the left side of the embryo to ensure the heart, intestines, and gallbladder formed correctly. Thus, the gene appeared to play a key role in setting the stage for these organs to form on just one side or the other.
At least one in every 10,000 people are born with organs in the wrong place, misformed, or missing altogether, conditions that range from being benign to fatal. So, Reversade and his team sequenced CIROP from 186 individuals with varying degrees of this so-called heterotaxy. Twenty-one people from 12 families had a mutation.
“This paper clearly demonstrates that CIROP is involved in [the placement] of human organs,” says Kyosuke Shinohara, a cell biologist at Tokyo University of Agriculture & Technology who was not involved with the work. It’s not yet clear whether this discovery can be used to prevent or treat heterotaxy, Reversade says, but “the confirmation in human patients was the cherry on the cake.”
Shinohara is pleased, too, with clues from CIROP about how asymmetry happens. Its sequence, he notes, suggests the gene’s protein makes use of zinc ions and operates outside the organizer to help kick off the transition to asymmetric development.
Still, there are many unknowns. Emmanuelle Szenker-Ravi of Singapore’s Agency for Science, Technology and Research and colleagues don’t understand exactly how the CIROP protein works and which proteins or genes it interacts with. “But,” Grimes says, “that’s really for the next paper.”