Abstract
Holoprosencephaly (HPE) is the most common developmental disorder of the human forebrain, where there is incomplete or failed division of the cerebral hemispheres. This causes neurological and facial anomalies, with more severe forms resulting in fetal loss or death. As both genetic and environmental factors can be determinants, along with intricate inheritance patterns, it makes diagnosis and specifying the exact cause difficult. The cohesin protein complex is essential for mediating chromatid cohesion, repairing DNA damage and regulating gene expression through three-dimensional chromatin organisation. Mutations in genes encoding the subunits of the cohesin complex have recently been found to be associated with HPE. STAG2 is one of the four core subunits of this complex, and variants of this gene have previously been identified to cause growth and developmental defects. Clinical and genetic features of individuals with cohesin-associated HPE have been described, however, further investigations on the exact causes of how these mutations lead to this disorder are lacking. Forebrain and early eye development are tightly linked processes, with brain growth directly affecting face morphogenesis. Both facial and ocular malformations are fundamental elements of the HPE phenotypic spectrum, and will frequently correlate with the severity of the brain malformation.
Previous research has highlighted Sonic hedgehog signalling and Wnt signalling as key pathways in development, including brain and eye formation. Disruptions to these signalling pathways can cause midline patterning defects such as HPE, but the exact mechanisms behind this are unclear. Zebrafish are a prominent model for studying development and disease due to their transparent embryos, rapid development and genetic similarity to humans. Previously, stag2b mutant zebrafish were shown to have increased sensitivity to Wnt signalling, however it is unclear what causes this Wnt hyperactivation. As ophthalmological anomalies are frequently present in HPE patients, analysing both brain and eye development in zebrafish with stag2b mutations was favourable in investigating how these variants, combined with an environmental factor to stimulate aberrant signalling, cause developmental defects. It was found a stag2b mutation caused an increase in susceptibility to the effects of a drug treatment on eye and brain morphology. Through in situ hybridisation, reduced expression of optic stalk marker pax2a was also seen in those treated with a drug, highlighting a synergistic effect between genetic and environmental factors. A combination of stag2b mutation with drug treatment caused symptoms commonly seen in HPE, indicating a signalling pathway essential in forebrain development had been affected. This finding suggests although these environmental causes are already risk factors for HPE alone, individuals genetically predisposed with cohesin mutations influencing certain signalling pathways may be more susceptible. This emphasises gene-environmental interactions are significant in the multifactorial aetiology of HPE.