Forebrain development–an intricate balance decides between health and disease

Abstract Patients carrying pathogenic gene variants encoding factors linked to the sonic hedgehog (SHH) pathway suffer from severe congenital brain malformations including holoprosencephaly (HPE). A poorly understood feature of these common anomalies is the highly variable penetrance, even amongst family members, carrying the same mutation. Modifier genes–genetic variants that can affect the phenotypic outcome of the primary disease-causing gene–contribute to this variability within pedigrees. Modifier genes can confer resilience or susceptibility to a disease, but are difficult to identify in humans. Studying the complex genetic interactions in mouse models of human congenital disorders can be instrumental in the identification of genes, that powerfully modulate SHH signaling pathway capacity and ultimately the penetrance of genetic disturbances. Understanding the underlying complex molecular mechanisms of disease aetiology and can support directing future genetic linkage studies in humans.