When setting out on a vacation, we usually plan in advance the route we will be taking and the time that we will arrive at our destination. Similarly, in biological systems, cells must arrive at defined locations and at specific times for normal organ development to occur. However, the roadmap for organ development remains incomplete, and while model organisms such as mice are useful for exploring the genes involved in organ development, some interspecies differences may not be accounted for.
“It's becoming increasingly obvious that human and mouse embryology are not the same,” said Ray Dunn at the Institute of Medical Biology (IMB). For example, in humans, the loss of one functional copy of the GATA6 gene results in an extremely rare condition called pancreatic agenesis, where the person is born without a functional pancreas. In mice, however, pancreatic agenesis only occurs when both copies of the GATA6 gene—and both copies of another highly related gene, GATA4—are non-functional.
Seeking to better model and understand pancreatic development in humans, Dunn’s team at IMB studied human induced pluripotent stem cells from GATA6-heterozygous individuals (provided by Professor Andrew Hattersley’s lab at the University of Exeter, UK) and a human embryonic stem cell line in which they introduced a range of different GATA6 mutations.
The researchers then evaluated the ability of the various stem cells to generate definitive endoderm (DE), from which the pancreas and other organs in the gastrointestinal tract emerge, using markers unique to different stages of DE differentiation. They found that GATA6 mutant stem cells showed significantly lower expression of the DE markers CXCR4 and SOX17, and this corresponded with fewer DE cells forming. “This means that GATA6 is required very early on in the formation of the human pancreas,” explained Dunn.
The team further used techniques such as RNA sequencing and chromatin immunoprecipitation sequencing to show that the GATA6 protein cooperates with other proteins—EOMES and Smad2/3—to activate the expression of genes promoting DE formation in the various stem cell lines.
“There are still many different holes in that story,” Dunn said. “We're trying to decode what are the direct transcriptional targets of GATA6 and its partners so we can sequentially understand and mirror in vitro the probabilities of what happens during human gestation. It's sort of like a relay race, figuring out who comes first, and who the baton is passed on to.”
The A*STAR-affiliated researchers contributing to this research are from the Institute of Medical Biology (IMB).