A novel model for exploring causes and treatments of craniofacial birth defects

INTRODUCTION: Most craniofacial anomalies occur as birth defects and/or postnatal growth disturbances of the cranial base and facial bones. A common craniofacial anomaly is cleft lip and/or palate (CL/P), which is a congenital condition that results in severe functional limitations and disfiguration. Part of the craniofacial skeleton comes from the neural crest cells (NCCs). If too few NCC’s are produced or there is a failure of migration to their final destinations, this can result in CL/P as well as infants with relatively small jaws, noses and ears. NCC’s are extremely sensitive to high levels of oxidative stress, which can arise due to environmental factors, such as adverse maternal environments. Alcohol, diabetes and smoking are known risk factors for CL/P, with maternal smoking in particular being repeatedly associated with an increased risk of CL/P. AIM: Previous research has shown that the oxidative stress-inducing compound auranofin (AFN) may cause craniofacial cartilage defects in zebrafish embryos. The aim of this study was to determine how environmental causes of craniofacial birth defects affect the growth and survival of cells contributing to the craniofacial skeleton during embryonic development in a zebrafish model. A second objective was to determine whether factors that enhance cell survival, such as antioxidant molecules, could rescue craniofacial defects. MATERIALS AND METHODS: AFN was applied to zebrafish embryos and the resulting phenotype was characterised at 5 days post-fertilisation (dpf) using light microscopy. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining was used to determine whether craniofacial defects were due to cell death. Photos of embryos were taken at different time points and AFN concentrations, to assess the number of cells stained. An antioxidant, Riboceine (RBC), was added in conjunction with AFN to investigate whether the defect caused by AFN could be rescued via promoting cell survival. The structure of the craniofacial cartilages were analysed quantitatively using a cartilage stain called alcian blue for each of the treatment groups. Quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) was used to analyse the expression of antioxidant genes in the different treatment groups. RESULTS: Application of AFN caused defects in craniofacial cartilage of 5 dpf zebrafish embryos. Higher doses of AFN led to greater numbers of TUNEL-positive cells, indicating the defects are likely due to increased cell death. RBC consistently ‘rescued’ the jaw defect phenotype caused by the application of AFN. The proportions of embryos with normal cartilage were similar in the “rescue” group, where AFN and RBC were both applied to the embryos, to the untreated control group. Application of RBC led to a lower number of TUNEL-positive cells than in embryos treated with AFN only. Treatment with AFN increased the level of antioxidant gene expression and by 48 hours post-fertilisation (hpf) RBC treatment did not return antioxidant gene expression to the level of the untreated control embryos CONCLUSIONS: Oxidative stress results in craniofacial cartilage defects that can be rescued by the antioxidant RBC in a zebrafish model. These findings may have translational significance, as treatment with antioxidants may help to prevent craniofacial defects in children, especially in families where there is an identified genetic or environmental risk