The three known human raf family proto-oncogenes, raf-1, A-raf, and B-raf map to chromosomal regions, 3p25, Xp11.2, and 7q33-36, respectively, which have been associated with human cancers either in the form of deletions or translocations in tumor tissue or genetic linkage in hereditary cancer syndromes. The genes consist of 16-18 exons and are of variable size with raf-1 and B-raf being relatively large (>100 kilobase pairs) and A-raf much smaller (<11 kilobase pairs). Their tissue-specific patterns of expression in mice and their overexpression in human tumors make it clear that raf-1, A-raf, and B-raf are independently and differentially regulated at the level of transcription or post-transcriptional processing by developmental, tissue-specific, and/or tumor-specific factors. Since overexpression of ras and raf is a frequent occurrence in a variety of tumors and overexpression of both wild-type human ras and raf proto- oncogenes results in the cooperative transformation of NIH 3T3 cells, gene products which coordinately regulate proto-oncogene expression are likely to be critical determinants in tumor cell growth. From analyses of their individual promoter regions, we are beginning to see how these genes are regulated at the transcriptional level. For example, the putative Wilms' tumor suppressor gene (WT1) binds in vitro and transcriptionally down- regulates the raf-1 gene promoter in vivo. These observations are highlighted by overexpression of raf-1 gene transcripts in Wilms' tumors. Similarly, we have obtained evidence for regulation of A-raf promoter activity by steroid hormones. These observations may provide us with opportunities to learn how to downregulate their expression and hopefully the tumor cell phenotype in human tumors assuming we can obtain suitable animal models for in vivo studies. Towards this goal, we have developed transgenic mice expressing either wild-type or oncogenic versions of human raf-1 under the transcriptional control of human raf promoters.
