We have been characterizing the molecular structure, expression and function of the raf oncogene family and examining the role they may play in the etiology of human neoplasia. Two functional genes, c-raf-1 and A-raf-1, related to v-raf (the active oncogene from 3611 MSV) have been identified. c-raf-1 is located on human chromosome 3p25, a site specifically altered in several human neoplasms including small cell lung carcinoma, familial renal carcinoma and mixed parotid gland tumors. The c-raf-1 gene is composed of 16 protein coding exons spanning more than 40 kb. It is expressed as a 3.4 kb and a 3.1 kb mRNA in human and murine cells, respectively, and is found in most cells and tissues examined. The c-raf protein is composed of 648 amino acids (73,000 daltons) and is located in the cytoplasm. A-raf is located on human chromosome Xp21-q11 near the locus for testicular feminization syndrome and Menkes syndrome. The A-raf-1 gene is expressed as a 2.6 kb mRNA in human and murine cells and shows a highly restricted tissue distribution of expression with highest levels attained in the epididymis of mice. The A-raf protein is composed of 606 amino acids (67,500 daltons) and its amino acid sequence shows 60% identity with c-raf. A role for raf family oncogenes in malignant transformation is indicated by the following evidence: (1) When incorporated into a retrovirus expression vector, truncated versions of c-raf and A-raf are transforming in vitro and in vivo. (2) c-raf-1 can be activated by promoter insertion in vitro. (3) Activated versions of raf family oncogenes have been identified in primary human stomach cancer, human glioblastoma and rat hepatocellular carcinoma. Functional assays of raf family oncogenes and their gene products have demonstrated that (1) amino terminal truncation of c-raf and A-raf protein enhances their serine/threonine kinase activity and their transformation potential, and (2) raf acts independent of ras in the signal transduction pathway of growth factors.
