Grainyhead-like genes and mammalian neural tube defects

During early pregnancy, a crucial event in the developing embryo is the formation of the neural tube, which will later develop into the brain and spinal cord. Failure of the neural tube to form correctly leads to a group of birth defects called neural tube defects (NTDs), in which the brain and/or spinal cord of the fetus become irreversibly damaged, resulting in death before or shortly after birth, or handicap in surviving babies. Overall, NTDs occur in around 1 per 1,000 pregnancies although the rate varies and is significantly higher in some regions (e.g. Northern Ireland and Scotland). Worldwide, approximately 130,000 cases occur every year. The risk of NTDs depends on both inherited genetic factors and non-genetic factors such as diet, but the exact causes are not well understood. We are studying mouse strains that are predisposed to develop NTDs that resemble the corresponding human birth defects, with the aim of understanding why the defects develop and finding ways to prevent them. In one of these strains, curly tail, we found that reduced expression of a gene called grainyhead-like-3, causes NTDs. We now have evidence to suggest that increased expression of the same gene or a related gene, grainyhead-like-2, can also cause NTDs, and we propose to use genetic approaches to test this idea. We will then investigate how the altered expression of the grainyhead-like genes changes cellular behaviours and thereby alters the mechanical properties of the developing embryo such that the neural tube fails to close. Finally, we will test whether dysregulation of grainyhead-like-2 and -3 cause NTDs through similar or differing effects on the expression of other ?downstream? genes. Identification of the genes that cause NTDs in mice may indicate genes that may be causative in humans. Knowledge of the genes responsible for NTDs in humans may then allow more accurate counselling for affected families who are considering a further pregnancy, and may allow development of novel therapeutic strategies.

Neural tube defects (NTDs), such as spina bifida and anencephaly, are severe congenital malformations caused by failure of closure of the embryonic neural tube. Folic acid supplementation can prevent some NTDs but a significant proportion (at least 30%) are unresponsive, and these defects remain a major health issue in the UK and worldwide. The causes of human NTDs are still largely unknown, but appear to involve multiple genetic and environmental factors. Therefore, our long term aim is to use detailed knowledge of the molecular and developmental basis of the pathogenesis of NTDs in mouse models, to assist identification of genetic risk factors for the corresponding human birth defects and development of primary preventive strategies. Several lines of evidence suggest that spinal neurulation depends on precise regulation of expression of members of the grainyhead-like (Grhl) gene family, and that their dysregulation confers susceptibility to NTDs. We previously found that reduced expression of Grhl3 causes spinal NTDs in curly tail mice, a model for folate-resistant and inositol-responsive NTDs. We now have preliminary data to suggest that over-expression of Grhl3 can also cause spinal NTDs, and that this may involve a distinct mechanism from that in the curly tail model. Furthermore, mapping and expression studies in another partially penetrant model for spinal NTDs, the Axial defects (Axd) mouse, suggest that the causative genetic defect involves a regulatory mutation resulting in over-expression of another Grhl member, Grhl2. We will build on these findings by, (1) testing the hypothesis that Grhl2 over-expression is the direct cause of NTDs in Axd. Suppression of Grhl2 function in Axd mice is predicted to rescue neurulation defects, whereas, transgenic over-expression of Grhl2 in wild-type embryos is predicted to cause NTDs. Next, (2) a series of studies will investigate the cellular and developmental basis of NTDs in Grhl2 and Grhl3 over-expression models, to determine whether the mechanism is shared with, or distinct from, that which we previously found to operate in the curly tail model. We will test whether NTDs caused by over-expression of Grhl3 involve a re-opening defect, which would represent a novel mechanism for development of spinal NTDs. Finally, (3) we will test the idea that over-expression of Grhl2 results in suppression of Grhl3 function, through disruption of transcription factor dimerisation, such that NTDs in the Axd (Grhl2 over-expression) and curly tail (Grhl3 under-expression) models, share a similar molecular basis in terms of aberrant expression of downstream genes.