SEMAPHORIN IV (SEMA IV) resides in 3p21.3 on chromosome 3, a region that exhibits a high loss of heterozygosity in several types of cancers, e.g., small cell lung cancer (SCLC) and ovarian cancer. Recently, the laboratory of Dr. Naylor demonstrated that tumor growth was inhibited in mouse fibrosarcoma (A9) cells transfected with SEMA IV as opposed to control A9 cells where tumors formed in an athymic nude mice assay. Furthermore, SEMA IV confers protection from chemotherapeutic drug-induced apoptosis. Thus, SEMA IV appears to be a putative tumor suppressor gene in 3p21.3. By definition, this would indicate that loss of SEMA IV function would result in tumor growth. Over 90 percent of SCLC have loss a region in chromosome 3 overlapping with the SEMA IV locus, however, no mutations have been found in the remaining locus. Another unexplored possibility by which sema iv may be disrupted is by differential DNA methylation. In neuronal development, Sema IV serves as a chemorepellent protein in axonal guidance, eliciting a response through the neuropilin-2 (np-2) receptor. However, its role in tumorigenesis is unknown. Recently, it has been shown that np-2 as well as, neuropilin-1 (np-1) also serve as coreceptors for vascular endothelial growth factor (VEGF165). VEGF165, one of the most potent angiogenic factors, also binds two receptor tyrosine kinases, KDR and flt-1. Np-1 has been shown to increase the affinity of VEGF165 to KDR and its mitogenic activity. Taken together, one action of SEMA IV in tumor suppression may be to competitively inhibit VEGF action. The goal of this proposal is to determine if DNA methylation is involved in the downregulation of SEMA IV gene expression in SCLC and to elucidate the mechanism(s) by which sema iv suppresses tumor formation.