Margaret R. Wallace
Molecular Genetic Studies of Smith-Lemli-Opitz Syndrome and Autosomal Dominant Heart Defects
Our laboratory employs tools of molecular genetics to map, clone, and characterize human genetic disease loci. This involves genetic linkage and characterization of chromosomal abnormalities to pinpoint disease loci; positional cloning to identify the genes involved; and mutation analysis in families with various genetic disorders, in which the disease-causing or susceptibility genes are already known. Currently, three projects are underway in general areas of interest.
Neurofibromatosis type 1 (NF1) is a common autosomal dominant disorder throughout the world in which patients develop neurofibromas and have an increased risk for developing certain cancers. The NF1 gene, a tumor suppressor gene, is over 350 kb and consists of 59 exons. It encodes a large GAP- related protein called neurofibromin. The gene has a high mutation rate, as evidenced by the fact that about half of all newly-diagnosed cases do not have a prior family history of NF1. We have DNA and leukocyte RNA samples from over 200 individuals with NF1 and are analyzing these for germline NF1 gene mutations and examining several aspects of genetics in the inheritance and expression of this disorder. For example, we recently identified the first case of somatic mosaicism in NF1, which may be a common mechanism in the occurrence of new mutation cases. We are also analyzing a variety of benign and malignant tumors from NF1 patients, to test whether the remaining NF1 allele is mutated or deleted in these tissues (i.e. the two-hit hypothesis). The goals of these studies include: identify mutational hot- spots, test and develop improved mutation detection schemes, locate functional domains of the protein product, examine phenotype-genotype correlations, and determine the extent of the role that the NF1 gene plays in tumor formation.
Smith-Lemli-Opitz syndrome (SLOS) is a multi-system birth defect that is associated with a defect in cholesterol synthesis. The symptoms can be relatively mild, or there can be such serious malformations that the child dies in infancy. Mental retardation is inevitable. This autosomal recessive disorder occurs in approximately 1/20,000 births. Although a biochemical defect has been identified, the molecular genetic basis has not been defined. Our work is aimed at cloning the gene(s) responsible, and our primary approaches are based on cloning a chromosome translocation and on linkage studies in a number of small families. We are using positional cloning techniques to map and clone the translocation breakpoint, in concert with linkage analysis to indicate whether other genomic regions may contain related genes. The ultimate test of candidate genes is to identify mutations in SLOS patients. We plan to further characterize the gene(s) involved, and examine the mechanisms and effects of mutations in SLOS patients.
The laboratory is also studying DNA samples from families with autosomal dominant cardiac defects, in particular supravalvular aortic stenosis (SVAS) and restrictive cardiomyopathy (RC). The gene causing RC is unknown, and we are pursuing a genome-wide linkage search in a large 4-generation family to identify the gene region and set the stage for positional cloning or candidate gene analysis. SVAS is caused by a defective or missing elastin gene, and we are searching for mutations in several families. This latter project will address issues such as the mechanism of the defect (dosage problem or dominant negative effect), and also identify the relationship of this gene and surrounding DNA regions to Williams syndrome patients (many of whom have large deletions of the region, causing SVAS as well as facial and neurological features characteristic of Williams syndrome).