Taking it to the max: The genetic and developmental mechanisms coordinating midfacial morphogenesis and dysmorphology

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Abstract
The rapid proliferative expansion and complex morphogenetic events that coordinate the development of the face underpin the sensitivity of this structure to genetic and environmental insult and provide an explanation for the high incidence of midfacial malformation. Most notable of these malformations is cleft lip with or without cleft palate (CLP) that, with an incidence of between one in 600 and one in 1000 live births, is the fourth most common congenital disorder in humans. Despite the obvious global impact of the disorder and some recent progress in identifying causative genes for some prominent syndromal forms, our knowledge of the key genetic factors contributing to the more common isolated cases of CLP is still remarkably patchy. The current understanding of the molecular and cellular processes that orchestrate morphogenesis of the midface, with emphasis on events leading to fusion of the lip and primary palate, is detailed in this review. The roles of crucial factors identified from relevant animal model systems, including BMP4 and SHH, and the likely events perturbed by key genes pinpointed in human studies [such as PVRL1, IRF6p63, MID1, MSX1, and PTCH1] are discussed in this light. New candidates for human CLP genes are also proposed.