Ras genes, Ha-ras, N-ras, and Ki-ras, encode for 21-kDa proteins important in signal transduction. Following binding of growth factors to cell surface receptor tyrosine kinases, p21 is transiently activated by phosphorylation and activates downstream serine/threonine kinases such as mitogen-activated protein (MAP) kinases and Raf-1, which in turn translocate to the nucleus where nuclear transcription factors such as Fos and Jun are phosphorylated/regulated. Activating mutations of ms result in a p21 protein that remains phosphorylated, maintaining active signal transduction pathways , in the absence of external stimuli.
Ras mutations are common in human cancer, occurring in approximately 30% of all cancers combined. However, mutated ras is much more prevalent in some types than in others. Whereas 90% of pancreatic cancers have a mutated ras gene (Almoguera et al., 1988), fewer than 5% of breast cancers do so (Kraus et al., 1984; Kozma etal., 1987;Rochlitzetal., 1989;Prosperietal., 1990; Clark and Der, 1995). Nevertheless, ras may be very important in breast cancer. The p21 ras protein is frequently overexpressed in breast cancer (Horan Hand et al., 1984; DeBortoli et al., 1985) and overexpression is significantly correlated with poor clinical prognosis (Clair etal., 1987; QuerzoU etal., 1988; Dati etal., 1991; Bland etal., 1995).
Over-expression of p21 ras protein occurs early in the development of breast disease (Ohuchi et al., 1986; Spandidos et al., 1987; Agnantis et al., 1992; Going et al., 1992; Pechoux et al., 1994). Normal breast epitheUum (Spandidos et al., 1987) and lactating breast epithelium (Thor et al., 1986) do not express sufficient p21 ras to be identified, and only 1% of cells in fibrocystic disease without hyperplasia stain positive for ras (Ohuchi et al., 1986). Cystic disease associated with apocrine metaplasia is more often positive (Agnantis et al., 1992). As the stage of proliferative disease advances, the proportion of cells positive for ras increases from 18% for PDWA to 34% for atypical hyperplasia, 54% for CIS, and 62% for invasive carcinoma (Ohuchi et al., 1986).
Differences in the Ohuchi study were statistically significant at each sequential stage: fibrocystic disease versus PDWA, P<0.01; PDWA versus atypical hyperplasia, P<0.05; and atypical hyperplasia versus carcinoma (Pooled invasive and CIS), P<0.01; there was no difference in ras expression between CIS and invasive carcinoma. A more recent study confirmed the stepwise increase in ras expression based both on numbers of positive cells and intensity of staining (Going et al., 1992). As in the earlier study, significant overexpression of ras was detected in PDWA.
Since numerous studies implicate ras function in PBD and breast cancer, it is curious that mutated ras is so infrequent. Perhaps breast is very susceptible to even minor disruptions in the ras pathway that could result from a number of genetic events, whereas pancreatic cells, for example, are made malignant only by full ras activation. If we assume that the rate of mutation of ras in breast cells is the same as in pancreatic cells, the number of breast cancers with mutated ras should be similar to the number of pancreatic cancers with mutated ras, suggesting that the low percentage of mutant ras breast cancers reflects the greater number of alternative complementation groups that can lead to this disease.
In the United States, the annual incidence of pancreatic cancers in white women is 7.5 per 100,000 compared to 113 breast cancers per 100,000 women (age adjusted) (Devesa et al., 1995). Thus, the incidence of mutant ras in breast cancer (5.65 cases per 100,000, assuming 5% have mutant ras) is similar to the incidence of mutant ras in pancreatic cancers (6.75 cases per 100,000, assuming 90% have mutant ras). Furthermore, mutations in a number of the components of the ras signal transduction pathway, including upstream receptor tyrosine kinases (eg., PDGFR, EGFR) and downstream serine/threonine kinases or transcription factors, are functionally equivalent to ras mutation (Clark and Der, 1995). Perhaps breast epithehum has a number of genes (e.g., receptor tyrosine kinases) active that pancreatic cells do not so that more genes are at risk in breast cancer cells which, if mutated, are surrogates for activated ras.