ComBio: making and unmasking faces

Paul Trainor investigates the molecular interactions during normal mammalian development that regulate cranial and facial development in his lab at the Stowers Institute for Medical Research in Kansas City, Missouri. Anomalies in craniofacial development manifest one-third of all congenital birth defects in humans, and knowing exactly what happens in normal development is key to understanding how these defects arise.

Neural crest cells are a migratory cell population formed early during embryogenesis that are central to craniofacial development. They give rise to most of the bone, cartilage, connective and peripheral nerve tissues in the head and face, so not surprisingly then, abnormal patterning of these cells in the embryo is behind most craniofacial defects.

“In simplest terms, we think of most craniofacial birth defects as being a neural crest cell-related problem,” Trainor says. “This can be a formation, migration or differentiation problem, and depending on what phase of crest cell development is disrupted, you can end up with very different craniofacial anomalies.”

These different defects produce diverse facial phenotypes and complications of the phenotype, and accordingly, quite different syndromes in humans. “To understand any of these syndromes, or in fact to start thinking about ways to prevent them and exactly when to intervene, we first need to understand the origin of the problem in each case, and then the pathogenesis of that syndrome.”

A disorder of craniofacial development that Trainor is particularly interested in is Treacher Collins syndrome. “We work on it as an example of how understanding the basic biology of a problem using animal models can explain a lot of what is going on in the human condition,” he says.

This rare human condition is characterised by developmental anomalies such as hypoplasia of the facial bones, middle and external ear defects and cleft palate. Most of the structures affected in this syndrome arise, at least in part, from cranial neural crest cells, and the syndrome seems to involve a problem early on in crest cell formation.

The gene mutation that causes Treacher Collins syndrome is in the Tcof1 gene, which encodes the nucleolar protein Treacle 1. Using a combination of lineage tracing, cell transplantation and cDNA library screening experiments, Trainor is currently investigating the role of Tcof1 in neural crest cell development for a clue to the origins of Treacher Collins syndrome as well as possible avenues for prevention and repair.

---PB--- Fixated on faces

Trainor has been fixated on faces since his days as a postgraduate with Professor Patrick Tam in the Embryology Unit at the Children’s Medical Research Institute in Sydney. During this time he became a guru on the trials and tribulations of neural crest cells in mouse embryos through specialised techniques such as cell grafting and labelling.

“Our work then reached the point of knowing a lot about what regulates the different phases of neural crest cell development,” he says. “We knew the genes controlling the formation of these cells, and those for regulating the migration, as well as a third group that control different components of the differentiation.”

What they then needed to do was put it all together in a mechanistic context. On completing his PhD in 1996, Trainor took up a postdoctoral fellowship at the National Institute for Medical Research in London with renowned developmental biologist, Professor Robb Krumlauf. There, he continued his highly productive work on the developing craniofacial region in mouse embryos.

In 2000, Krumlauf was appointed as scientific director of the newly established Stowers Institute for Medical Research in Kansas City, and 18 months later, Trainor was recruited there to start his own group. Around that time, Trainor was pondering whether to head home to Australia to launch his independent research career.

However, as is often the case with talented scientists early in their career and about to blossom, he realised that the opportunities in Australia were very limited and so decided to stay in the north. Plus, the offer from a well-funded and supported institute specifically set up to do the sort of work that Trainor does was too good an opportunity to ignore.

Scientifically, it was also a time when Trainor shifted focus slightly. “Based on the genetic knowledge that we and others had amassed on neural crest cells and craniofacial development, we realised the time was right to find a specific syndrome to work on, for which there was an animal model where you could try to put all that information into perspective and then maybe try and understand how some of the defect mechanisms develop in humans.”

Serendipitously, developmental biologists at the University of Manchester, Michael and Jill Dixon, had just made a mouse model of Treacher Collins. These scientists were part of the collaboration that identified the gene responsible for the condition in humans. Trainor knew the Dixons from his time in the UK and realised that their mouse model would be a great tool for his work as well.

“Mike and Jill had the clinical background and this mouse model, but didn’t have the basic biology, and we were coming at it from the other end, so it was the perfect matching of two groups for the same problem.”

---PB--- p53 signalling

At ComBio, Trainor will be discussing his group’s recent work surrounding novel ways to potentially prevent Treacher Collins syndrome by inhibiting the death of specific neuroepithelial cells during early craniofacial development. Some time ago, Trainor showed that p53 signalling is the major downstream pathway affected in this syndrome, thus identifying a potential target for prevention or treatment.

“The problem in Treacher Collins syndrome is that this pathway does a lot of good things during development and potentially a lot of bad things. Everyone needs a little bit of p53 to prevent the risk of developing tumours and cancer, but we also know that if we remove p53 function during embryogenesis, we can prevent some craniofacial birth defects, because the neural crest cells end up surviving during the formation stage so can go on to make normal bone and cartilage and connective tissue of the head and face.”

Of course, the risk of this action long term is that while facial development might be normal, tumourigenesis or some form of cancer appearing soon after birth is more likely.

“So, what we are looking for now is a different point at which we can intervene. We know many of the p53 targets activated in the syndrome as part of the mechanism that is killing off the cells, and so we are looking for another avenue to block the cell death part of the process, but not remove the additional repair function of p53.”

Trainor points out that to make any intervention really effective, an individual would have to be treated in the first couple of months of pregnancy and in a way, that is where the biggest problem lies for such an approach.

This is firstly because Treacher Collins syndrome arises from a spontaneous mutation in 50 per cent of cases, making it impossible to identify those individuals. Ultrasound imaging may identify facial abnormalities even in the very early stages of the pregnancy, but there are roughly 300 different syndromes that could account for such features and without genetic testing it is impossible to distinguish which one it might be.

Secondly, familial cases must be caught by genetic testing early in the pregnancy and prior to the mutation to have any chance of reversing the defects.

Trainor’s ultimate aim therapeutically is to “find something almost akin to folic acid that will provide a general beneficial effect without any side effects – something that could be taken as a prenatal vitamin for instance”. This eventuality is possible, he says, but not without a lot more work to identify the exact point downstream in the p53 pathway where interventions would be most effective, and to find out just exactly what all the genes in that pathway are doing.

It will take two to three years just to fully characterise each single gene target. Work towards this goal was published earlier this year by Trainor’s group in the prestigious journal, Nature Medicine.

Now an old hand at the Stowers Institute, Trainor continues to find it a great place for doing basic science. “We have extremely good support, which is fairly unique. This is not just in terms of funding, and in fact most of our money comes from peer-funded sources such as the NIH or the Birth Defects foundations, but we also have solid internal support. The thing is that rather than just giving us start funding that runs out after two or three years, it is designed to be spread out over five years or over the term of your contract.”

Although his work is purely basic science, Trainor’s group enjoys close contact with the societies and groups that support people with craniofacial birth defects such as Treacher Collins, to update them on advances in research and possible hopes for heading off the problem in the future.