theRTK I family in thedevelopmentandprogressionof themalignant

Recent in vitro studies of the epidermal growth factor receptor (EGFR) family have revealed complex signaling interactions involving the produc lionofligand-mediatedheterodimersynergisticforthetransformationof cells in vitro. In a series of7O patients with childhood medulloblastoma, we haveusedimmunohistochemistry and Westernblottinganalysisto inves tigate the expression patterns of all four EGFR family members (EGFR, HER2, HER3, and HER4) and heregulin-a, a ligand for the HER) and HER4receptors.The majorityof cases expressedtwo or more receptor proteins; coexpression ofthe HER2 and HER4 receptors occurred in 54%. Expression of the ligand heregulin-a was detected in 31% of tumors. To investigate whether coexpression results in receptor heterodimerization, we havealsoperformedimmunoprecipitation analysisof proteinextracts from primary tumors, and we demonstrate various patterns of receptor interaction including between HER2 and HER4. In multivariate25-yearsurvivalanalysiswith clinicopathological dis easefeatures,noindividual receptoror heregulin-aachievedsignificance. In contrast, when considered together in the multivariate model, coax pression of HER2 and HER4 demonstrated independent prognostic sig nificance (P = 0.006). These data suggest the hypothesis that HER2-HER4 receptorheterodimerizationis of particularbiologicalsignificancein this disease, and this report is the first to demonstrate potential clinical significance ofEGFR family heterodimerization in human cancer Finally, we havealso analyzedexpressionof the AP.2 transcriptionfactor1mph rated in the positive regulation of HER2 and HER3 gene transcription in malignant cells and reveal an association between AP-2 expression and not only HER2 and HER3, but also HER4 leveis in medulloblastoma primary tumors.