Genetics
To understand the role of craniofacial development in the evolution of primates, we must understand the developmental genetic basis of contemporary variation in head morphology among primate species, including humans. Our research is initially centered on analysis of variation in the unique, very large baboon genealogy of the Southwest Foundation for Biomedical Research (SFBR), and in specific crosses of laboratory mice. Due to the conservation of development in mammalian evolution, gene networks and developmental processes discovered in mice can be informative about primates. The results will be immediately comparable to existing (or collectible) data on variation in other species and will be added to existing catalogs of human variation.
Gene mapping studies in baboons and mice
Natural craniofacial variation is present in the SFBR baboon colony, in a two-way intercross genealogy maintained by Dr. Cheverud’s lab at Washington University, and in recombinant inbred mouse lines that are commercially or collaboratively available.
Animals from these genealogical resources have been typed for genome-spanning markers, and in this project, mapping studies are used to identify candidate genome regions that are statistically associated with morphometric variation. The details of statistical methodology differ, but the essential logic of the search is the same for both species. We use QTL (Quantitative Trait Locus) mapping to identify associations between variation in measured traits and in genetic markers.
Candidate Genes
1. Studies of nominal developmental processes. The expression of substantial numbers of genes and gene networks has been identified in the normal development of craniofacial tissues in mice or other species and catalogued in published articles and websites.
2. Studies of craniofacial pathology and reported major variation. Studies of many diseases in humans and pathological mutations in other animals (most often mice but sometimes fish) have led to the
identification of genes whose mutation is associated with pathology. We will look for non-pathology producing variation in these genes that may account for normal craniofacial variation.
3. Bioinformatics techniques (see Bioinformatics). Comparison of whole genome sequences among human, chimpanzee, mouse and macaque reveals many genes or chromosome regions that appear to have undergone rapid (or unusually static) evolutionary histories in these lineages. Whole genome sequences of other mammals are continually being added to the public databases and will be used in these comparisons as they become available. With the growing gene ontology resources now available (which categorize genes by their function), we can identify genes that may be expressed in the development of craniofacial tissues.
Expression and Intervention Studies
When candidate genes have been identified, the first order of business is to determine at what developmental stages they are expressed in mice. In particular, we are looking for variation in gene expression that might be relevant to the craniofacial variation that led to identification of that gene as potentially causal.
In broad candidate genome regions, after informatics methods have identified plausible genes, we can test gene expression in mouse embryos using molecular laboratory methods known as RT-PCR and in situ hybridization to determine which of the genes have relevant craniofacial expression patterns. In different ways, these methods allow us to identify the tissues in which a candidate gene is expressed in the embryo at selected developmental ages.
We will follow up positive results in a variety of ways. Depending on the situation, one possible approach is to obtain deceased baboon infants, abortuses, or embryos experimentally obtained to determine expression patterns in primates (there are various resources including the SFBR where tissue may be obtained, but this is always ad hoc rather than experimental). More importantly, we can engineer transgenic mice to over- or under- or ectopically express candidate genes. Since most of the relevant genes are likely to be signaling factors, receptors, and responders, we will use a variety of standard molecular genetic approaches to identify genome regions that regulate the expression of, or are themselves regulated by, the candidate genes.
Preliminary Results
Our initial QTL mapping in the baboon sample has revealed LOD scores for two distance measures within the face that relate to muzzle length or protrusion. The LOD score for the measure along the alveolus (see figure) was statistically significant while the LOD score for the other distance was nearly significant. Both distances map to a similar region on chromosome 4, indicating that there might be a candidate gene in that region that is related to facial length.
Lines drawn on CT reconstruction of baboon skull indicate distances with significant (in blue) and highly suggestive (in red) LOD scores. The top graph with the red line displays the LOD scores along chromosome 4 for the distance drawn in red on the baboon. Likewise, the bottom graph represents the LOD scores along chromosome 4 for the blue distance.
