Deciphering molecular basis of holoprosencephaly – HOLOPRO

Deciphering molecular basis of holoprosencephaly

Our research objective is to elucidate genetic and physiopathological mechanisms leading to holoprosencephaly (HPE), a remarkably common congenital anomaly characterized by failure to define the midline of the forebrain and midface. It is now currently admitted that HPE is a multi-hit model, in which the penetrance and expressivity of an HPE mutation is enhanced by a second mutation. The SHH and NODAL pathways have been shown to be the main signalling cascades implicated in human HPE, but, other pathways like FGF have also been described to be involved. Importantly, we recently showed that NOTCH signalling could also be implicated. HPE is thus a complex malformative syndrome involving genes from more than one signalling pathway and whose genetic basis is still unknown in 70% of the human HPE cases. At the Institute of Genetics and Development of Rennes (IGDR), we have initiated, 13 years ago, a genetic study of human HPE. From the beginning, the activity of the group went from research to diagnosis. In order to elucidate molecular mechanisms leading to HPE, we have recently set up an ex ovo chick embryos culture system in presence of pharmacological inhibitors known to block the different signalling pathways involved in forebrain development (i.e. SHH, NODAL, FGF and NOTCH). The originality of this approach is to be able to perform simultaneous inhibition by using various combinations of drugs at various concentrations. The strategy consists in creating an HPE-like phenotype using two or more chemical treatments. During this proposal, the molecular reprogramming of the brain will be tested in these various conditions, using a transcriptomic approach and in situ hybridization. Overall, these data will reveal the molecular mechanisms by which HPE can be produced through disruption of several molecular pathways. Additionally, these HPE-like models will allow us to study the interactions between putative candidate genes previously isolated by genetics approaches and by the different methods cited above. Ultimately, we will design a gene network dedicated to HPE that will give a global view of all genes potentially involved in HPE and their interconnections. In a context of a multigenic-hit pathology, it will represent an essential tool for diagnosis. Altogether, this project will contribute to understand the molecular mechanisms leading to HPE and it will also participate to a better knowledge of the normal forebrain development.