Recent studies suggest that one or more genes on chromosome 5q21 are important for the development of colorectal cancers, particularly those associated with familial adenomatous polyposis (FAP). To facilitate the identification of genes from this locus, a portion of the region that is tightly linked to FAP was cloned. Six contiguous stretches of sequence (contigs) containing approximately 5.5 Mb of DNA were isolated. Subclones from these contigs were used to identify and position six genes, all of which were expressed in normal colonic mucosa. Two of these genes (APC and MCC) are likely to contribute to colorectal tumorigenesis. The MCC gene had previously been identified by virtue of its mutation in human colorectal tumors. The APC gene was identified in a contig initiated from the MCC gene and was found to encode an unusually large protein. These two closely spaced genes encode proteins predicted to contain coiled-coil regions. Both genes were also expressed in a wide variety of tissues. Further studies of MCC and APC and their potential interaction should prove useful for understanding colorectal neoplasia.

Previous studies suggested that one or more genes on chromosome 5q21 are responsible for the inheritance of familial adenomatous polyposis (FAP) and Gardner's syndrome (GS), and contribute to tumor development in patients with noninherited forms of colorectal cancer. Two genes on 5q21 that are tightly linked to FAP (MCC and APC) were found to be somatically altered in tumors from sporadic colorectal cancer patients. One of the genes (APC) was also found to be altered by point mutation in the germ line of FAP and GS patients. These data suggest that more than one gene on chromosome 5q21 may contribute to colorectal neoplasia, and that mutations of the APC gene can cause both FAP and GS. The identification of these genes should aid in understanding the pathogenesis of colorectal neoplasia and in the diagnosis and counseling of patients with inherited predispositions to colorectal cancer.

Raf appears to suppress its own activation by a novel feedback loop, such that inhibition is always counterbalanced by reactivation. These observations imply that some agonists reported to trigger the cellular activation of c-Raf might actually be inhibitors of this enzyme, and that compounds which inhibit the kinase activity of Raf might not be useful as anticancer drugs. The binding sites for ZM 336372 and SB 203580 on Raf and SAPK2/p38 are likely to overlap.

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The inhibition of SAPK2a/p38 (a mitogen activated protein (MAP) kinase family member) by SB 203580 depends on the presence of threonine at residue 106. Nearly all other protein kinases are insensitive to this drug because a more bulky residue occupies this site (Eyers et al., 1998). Raf is one of the few protein kinases that possesses threonine at this position, and we show that SB 203580 inhibits c-Raf with an IC 50 of 2 mM in vitro. However, SB 203580 does not suppress either growth factor or phorbol ester-induced activation of the classical MAP kinase cascade in mammalian cells. One of the reasons for this is that SB 203580 also triggers a remarkable activation of c-Raf in vivo (when measured in the absence of the drug). The SB 203580-induced activation of c-Raf occurs without any increase in the GTP-loading of Ras, is not prevented by inhibitors of the MAPK cascade, protein kinase C or phosphatidylinositide 3-kinase, and is not triggered by the binding of this drug to SAPK2a/p38. The paradoxical activation of cRaf by SB 203580 (and by another structurally unrelated c-Raf inhibitor) suggests that inhibitors of the kinase activity of c-Raf may not be e ective as anti-cancer drugs.