A lot happens in the first month of conception: Between day 17 and 30 after conception, a dynamic process called neural tube closure occurs, giving rise to the baby’s spinal cord, spine, brain and skull.
When the neural tube fails to close properly, the developing brain or spinal cord is left exposed to the amniotic fluid. If the upper end of the neural tube fails to close, the brain either never completely develops or is absent, causing a common neural tube defect called anencephaly. Pregnancies affected by anencephaly often result in miscarriage and infants who are born alive die very soon after birth.
“While neural tube defects—including its most severe form of anencephaly—are common in humans, a genetic etiology of anencephaly in humans has never been established,” said Bruno Reversade, a Research Director at A*STAR’s Genome Institute of Singapore (GIS) and Institute of Molecular and Cell Biology (IMCB). Reversade was a co-corresponding author on the study, which discovered a genetic cause for the devastating condition.
Reversade’s team first learned about a Turkish first-cousin couple that had three consecutive fetuses with anencephaly, suggesting a possible genetic origin. Curiously, no other extended family members had similarly affected offspring. By carrying out exome sequencing of the couple’s DNA and one of their terminated fetuses, they were able to highlight a potential gene candidate: NUAK2. Each parent only had one mutated copy of NUAK2, but the fetus had two, implying that this form of anencephaly was recessive.
Although NUAK2 had previously been studied in the context of cancer, its role in brain development was not known. The NUAK2 gene encodes a kinase, which is an enzyme that catalyzes the transfer of phosphate groups to other proteins. Using in vitro kinase assays, the team observed that the loss of seven critical amino acids in the αC-helix of the NUAK2 protein resulted in the loss of its catalytic activity.
To understand NUAK2’s link with anencephaly, the team mimicked neural development in vitro using 3D cell cultures called cerebral organoids, which were derived from affected fetus skin cells. The researchers found that the loss of NUAK2 enzyme activity led to decreased activity in the downstream Hippo-YAP signaling pathway—a critical pathway that controls cell shape during neural tube folding.
Moving forward, the team intends to explore the relationship between folic acid and NUAK2 function. “Given that folic acid and vitamin supplementation before and during pregnancy has been shown to reduce neural tube defects by 50-70 percent, it would be interesting to use our in vitro models to assess whether the presence of folic acid could rescue the loss of Hippo-YAP signaling in mutant cells,” said Carine Bonnard, currently the Head of Operations at the Skin Research Institute of Singapore (SRIS). Bonnard was both first and co-corresponding author in this study.