The events that occur during early mammalian development have to be carefully orchestrated and timed perfectly. Controlling gene expression is integral to the process; certain genes have to be turned on or off at critical times, to regulate the mechanisms underlying the growth of an organism. Now researchers have developed a tool for studying human development with stem cells and the CRISPR gene-editing tool. Published in Cell Reports, the work can not only provide new research insights, it can help investigators study developmental disorders.
Scientists at the Morgridge Institute for Research added a fluorescent tag to a gene that plays a critical role in timing in development, creating a kind of clock in a dish. Stem cells growing in the lab that have been engineered this way generate a flash of color every five hours, as the genes that cycle through developmental stages repeat their role. This confirms the timing of oscillating genes as they are expressed during human development.
Until now, most studies have used model organisms to study oscillating genes. "It is so early in the development states that in humans, there is no way you could even approach this process," explained Li-Fang Chu, who works in the lab of Morgridge and University of Wisconsin-Madison stem cell researcher James Thomson. "That's why it was attractive for us to use stem cells to see if we can create it in the lab."
In this work, the researchers focused on a stage of development called somitogenesis, when the segments of the body develop, about twenty days into development. Pairs of structures called somites form in precise pattens over about two weeks. This enabled the scientists to observe distinct physiological characteristics, noted Chu, so they could link gene activity with a specific phase of development. A well-known gene mutation also exerts its impact during this stage, leading to a debilitating rare genetic disorder called spondylocostal dysostosis (SCDO).
The HES7 gene is one of four genes that can cause SCDO, and when it carries a mutation, the vertebrae don’t form correctly and fuse together. The researchers used CRISPR to target HES7, which would oscillate once every five hours normally. When the HES7 gene carries the mutation linked to SCDO, the oscillation was disrupted, showing how the disorder arises.
"Like an airplane crash, we really don't know what happened or what went wrong without the information in the black box," Chu said. "I believe our system provides that black box."
Development is a complex web of events involving many genes and different cell types. This research was a simplification, in which only mesodermal cells were used, but that allowed the scientists to zero in on what happened when HES7 activation was disrupted. Understanding the timing of development in cells can also improve their clinical applications in cell-based therapeutics.
"If we understand in this case why mouse cells oscillate faster and human ones slower, we could apply that principle to other cell lineages and potentially speed things up," added Chu.