Effects of ERBB2 amplicon size and genomic alterations of chromosomes 1, 3, and 10 on patient response to trastuzumab in metastatic breast cancer.

Trastuzumab is widely used for advanced breast cancer patients with ERBB2-amplified tumors. Nevertheless, over half of these patients do not have an objective response. One reason may be altered expression of genes that might compensate for ERBB2 inhibition. We previously mapped the gene-rich region of chromosome 17 telomeric to ERBB2, and reported considerable variability in the telomeric extent of the ERBB2 amplicon. Here we examined whether the variable amplicon size may be associated with patient response to trastuzumab. In addition, we looked at associations between response and several signaling pathway-related genes unrelated to the ERBB2 amplicon, including AKT3, PTEN, PIK3CA, and PTGS2. In 35 patients with ERBB2-amplified metastatic breast cancer, with 40% overall response to trastuzumab, fluorescence in situ hybridization identified the telomeric extent of the ERBB2 amplicon and the status of the several pathway-related genes. Objective response strongly correlated with the telomeric amplicon size, with 62% of patients with shorter amplicons responding, compared with only 7% of patients with longer amplicons (P = 0.0015). Abnormal copy number of PTGS2 was marginally associated with objective response (P = 0.066), while abnormal copy numbers of two reference loci, 1q25 and the chromosome 10 centromere, were significantly associated with response. Pairwise combinations of copy number status of these loci and ERBB2 amplicon size provided stronger associations and identified a group of patients without responders. These results suggest that patient selection for trastuzumab may be improved by considering ERBB2 amplicon size and genomic status of the 1q25, PTGS2, and centromere 10 loci.