The type 1 growth factor receptor family: new ligands and receptors and their role in breast cancer.

The type 1 family of growth factor receptors, which consist of the epidermal growth factor receptor, c-erbB-2, c-erbB-3, and c-erbB-4, are expressed in normal breast ductal epithelial cells and in some breast cancers. Nine genes have now been identified which code for ligands. In some cases the genes are spliced into a series of proteins which differ in structure, but all retain an EGF-like element responsible for receptor recognition. The EGF receptor is expressed in normal breast and in some cancers, but is apparently reduced in expression in other cases. Cancers with EGF receptors appear to represent a greater threat to patients as in most studies they are associated with a shorter time to relapse and overall survival. The c-erbB-2 protein is overexpressed at very high levels in about one fifth of breast cancers and is indicative of poor prognosis. Other cancers may express lesser degrees of overexpression but it is not clear if this is biologically or clinically significant. The c-erbB-3 protein is expressed in normal breast epithelial cells and has been reported to be present at high levels in some cancers but at normal levels or at lower than normal levels in some others. The limited studies to date suggest that when measured on its own c-erbB-3 expression is not predictive. c-erbB-4 is also expressed in normal breast and in some cancers but no studies have yet been performed to address whether it is associated with disease behaviour. In the future it is likely that a greater understanding of the function of this complex family of interacting proteins will assist in gaining the maximum predictive power from measurement of their expression in human breast cancer.