Mapping and identification of genes involved in human craniofacial syndromes

Abstract

Craniofacial development is a complex developmental process requiring many cell types, cell movements, cell interactions, and the orchestrated expression of many genes. Numerous human syndromes are associated with defects in craniofacial development. This thesis is a molecular study of two such syndromes, craniofrontonasal syndrome (CFNS) and holoprosencephaly (HPE). By identifying the genetic causes of these syndromes, we hope to further our understanding of the developmental processes participating in the normal formation of the brain and face. We have accomplished several goals towards the identification of specific genes involved in CFNS and HPE. First, we mapped CFNS by linkage analysis to Xp22. Subsequent mapping of a cytogenetic deletion in a CFNS patient narrowed the CFNS critical region defined by the linkage analysis. Through detailed phenotypic analysis, we also showed that females are more severely affected than males. This is an unusual phenotypic pattern for X-linked inheritance. On the HPE project, the HPE3 region was cytogenetically defined in relation to Sonic Hedgehog (SHH). Deletions or translocations involving position effects and SHH were shown to cause HPE. Also on the HPE project, the HPE2 minimal critical region (MCR) was defined. Because the SIX3 gene was an excellent candidate gene, it was cloned from the region. After screening the entire SIX3 gene for mutations in HPE patients, we determined it to be the HPE2 gene as mutations in the homeodomain were identified. Translocations were also found to map outside the coding region of SIX3 indicating a position effect with SIX3 similar to that observed with SHH. Functional studies of Six-3 in the developing chick embryo were attempted, but no conclusions could be made from the results. Finally, a literature review of the molecular genetics of HPE was compiled. Through this review it became more evident how all the identified HPE genes and likely candidate genes related to each other via signaling pathways such as the SHH, TGFβ, and retinoic acid pathways. These studies have contributed to our understanding of the growing complexities of craniofacial development