Project Summaries

Research in Dr. Finnell's laboratory focuses on the interaction between specific genes and environmental toxicants as they influence normal embryonic development. The laboratory currently has four major research areas, all of which focus on determining susceptibility to environmentally induced birth defects. These four areas are summarized below.

Using genetically modified mice to understand the mechanism by which folic acid prevents birth defects. We have used homologous recombination in embryonic stem cell technology in order to successfully knockout both of the murine folate binding protein genes (Folbp1 and Folbp2), as well as the reduced folate carrier gene (RFC1). The heterozygous Folbp1 animals have significantly lower serum folic acid levels and very high serum and brain homocysteine levels when the dams are maintained on a low folate diet. Embryos homozygous for the Folbp1 null allele fail to develop beyond gestational day 10, and these embryos all have neural tube defects (NTDs) and other serious congenital malformations, including conotruncal heart defects and cranofacial malformations. In the nullizygous animals, there is an increase in apoptosis and an inhibition of cellular proliferation in the neuroepithelium and craniofacial tissues. We have also observed aberrant expression of the embryonic patterning genes Pax3 and Shh in the nullizygous embryos. Utilizing microarrays we are identifying additional downstream targets that might provide insight into the protective effects of folic acid supplementation, as well as reveal candidate genes for mutation analysis in our human population studies.

Candidate genes for susceptibility to neural tube defects (NTDs), heart defects, and orofacial clefts. The laboratory serves as the primary mutation analysis laboratory for the States of California and Texas for their participation in the National Birth Defects Prevention Study, one of eight CDC recognized Centers of Excellence. We use a variety of approaches to screen population based case-control samples for polymorphisms in several different candidate genes to identify significant gene-environment interactions. Many of these samples are stratified as to the vitamin status of the mother to enable us to focus in on various hypotheses concerning folates and birth defects.

We are currently testing this hypothesis examining the gene coding for the 5-methyltetrahydrofolate receptor (hFRa) as well as MTHFR, MTHFD, NAT1, NAT2, SHH, MTRR and RFC1, BHMT, BHMT2, TS, cited2, and a variety of other gene candidates using high-throughput approaches as well as direct DNA sequencing. We have also examined genes related to energy metabolism (UPC2), obesity (leptin and its receptor), and drug metabolism (mEH, GST). As an adjunction to these molecular epidemiology studies, the laboratory is actively cloning new human folate receptor and transporter genes that might serve as new targets for these epidemiological investigations.

Antiepileptic drug therapy and congenital malformations. The laboratory has a long-standing interest in the teratogenicity of anticonvulsant medications. Using animals models, we are trying to identify specific alterations in gene expression and function that can explain the craniofacial dysmorphia and other congenital defects observed in human infants exposed in utero to these compounds. In addition to testing new compounds for their teratogenic potential, we have finally established solid linkage with a gene that confers susceptibility to anti-epileptic drug induced NTDs. We are in the process of positionally cloning this gene and learning more about its characteristics. We are working with a critical region of less than one cM. Once the gene is positionally cloned, we will clone its human ortholog and then examine the clinical samples that have been collected which include sibships with both adverse and normal pregnancy outcomes in the face of maternal anticonvulsant drug therapy during pregnancy.

Environmental Toxicogenetics. The final area in which we work combines many aspects of the above-mentioned studies yet focuses the technology on addressing problems of true environmental significance. Funded through the Superfund Research Program, we are working with a variety of investigators at Texas A&M University to study the consequences of complex mixtures and petrochemical pollution on birth defects using both genetically modified mouse model systems and human molecular epidemiology studies. The mouse work involves test compounds (arsenic, TCDD, BAP) and embryos that should be genetically sensitive (Folbp1) or resistant (AhR-) to these exogenous agents. We will define the responses on both the morphological and molecular levels, using genetic microarray technology. In parallel, we are establishing the infrastructure to perform molecular epidemiology studies in the petrochemical producing country, Azerbaijan, which has very high anecdotal rates of congenital defects. New studies of an environmental toxicant nature are being conducted in northern China, in collaboration with the Peking University Health Science Center.