Top poker players know that the face mirrors the brain. Specialists in embryonic development wouldn’t disagree. In fact, because the same clumps of primordial cells mold the final features of both, a close look at a child’s face can often yield clues about less visible problems within the skull: a cleft lip or other abnormal facial features can read like a map of brain development gone awry.
Now for the first time, researchers at the Stanford University School of Medicine and the University of California-San Francisco have provided a powerful example of how one genetic pathway can wend its way through an emerging “city” of brain structures and facial features, influencing each phase of development in slightly different ways. Like an architect overseeing a complicated building process, a key protein required early in development for embryonic survival exerts a waning but vital influence throughout the sequential construction of the brain and face. Blocking this protein’s action at varying developmental stages yields very different anatomical results – including one in which only the exterior, or the face, is affected while the scaffolding, or the brain, is left unscathed.
The results not only shed light on a common cause of miscarriage in humans, they also help to untangle a medical mystery: why children born with the same genetic disorder can have vastly different symptoms.
“Everybody recognizes that in some genetic diseases, one person can be much more severely affected than another, and we’ve all wondered ‘Why is that?’” said Jill Helms, DDS, PhD. “We thought that uncovering the gene or genes responsible for the condition might answer the question, but in many cases that has only added more confusion.”
Helms, an associate professor in plastic and reconstructive surgery, recently came to Stanford from UCSF, where the current work was conducted. She is the senior author of the research, which appears in the Aug. 16 issue of The Journal of Clinical Investigation. Helms is supported by the Lucile Packard Foundation for Children’s Health.
Helms, along with former UCSF researcher Dwight Cordero, MD, who is now a perinatologist at Albert Einstein Medical School, and their colleagues studied a birth defect called holoprosencephaly, or HPE, that results when the embryonic brain fails to properly divide into two hemispheres. Although the disorder affects only about one in every 10,000 infants in this country, it’s believed that the initial rates are much higher, occurring about once in every 250 conceptions. Most fetuses are so severely affected that they are miscarried early in pregnancy.
Symptoms range from death within days to severe mental retardation, seizures and an inability to speak. Others, however, suffer only mild learning disabilities. Facial defects can include a cleft lip, a single central incisor or “front tooth,” close-set eyes or even a single eye in the center of the child’s forehead.
Although it’s not known exactly what causes HPE, overexposure to alcohol or other chemicals during early development has been implicated. There’s also a genetic component: children with HPE routinely crop up in some families, although the severity of symptoms within a family can vary widely. This mishmash of possible causes and symptoms has made it difficult for doctors to prevent and treat the disorder.
The researchers turned to chicken embryos to study the problem, capitalizing on the fact that fertilized eggs are, in effect, perfectly isolated, self-contained laboratories. The physical separation of the mother from the embryo means that scientists can add factors they wish to study in precisely controlled amounts at well-defined times during development. And even though a bird beak seems quite different than a human nose or a mouse snout, the cells and pathways involved in brain and facial patterning are conserved between the species.
Cordero used a chemical aptly named cyclopamine to interfere with developmental signals sent by sonic hedgehog, a protein previously implicated in HPE. Although exposure early in development created cyclopic embryos, progressively later exposure mimicked the entire range of symptoms seen in children with HPE. Intriguingly, exposure after sonic hedgehog had already established itself in the divided brain affected only the embryo’s face, and holding off just a bit longer resulted in no detectable malformation of either brain or beak.
“We’ve found that it’s possible to recreate the entire spectrum of HPE defects seen in humans by disrupting sonic hedgehog at different times during development,” said Helms, “which may explain why some children with HPE are more affected than others.”
The scientists believe that HPE may arise from a combination of a faulty sonic hedgehog gene, present from conception, and varying times of exposure to environmental factors that further compromise the protein’s signaling. In this scenario, early exposure leads to more severe defects and sometimes death, while later exposure can leave a child with only minor facial abnormalities.
Understanding how brain and facial features are developmentally linked may help doctors at Lucile Packard Children's Hospital and elsewhere hone new therapies or diagnostic procedures for these severe birth defects. “Twenty years ago, physicians didn’t understand the mechanisms underlying what they were seeing clinically with HPE and other developmental disorders,” said Cordero. “Now with better imaging and molecular diagnostic techniques, we can begin to shed new light on the possible causes of some of these disorders.”