ERBB-2 (HER2 NEU) GENE COPY NUMBER, P185(HER-2) OVEREXPRESSION, AND INTRATUMOR HETEROGENEITY IN HUMAN BREAST-CANCER/

Amplification of the ERBB-2 (HER-2/neu) gene is accompanied by overexp ression of its cell surface receptor product, p185(HER-2), Heterogenei ty has been observed for both the gene copy number and the level of ov erexpression of its protein product. To better understand their relati onship, correlation between the level of cellular expression of p185(H ER-2) and ERBB-2 gene amplification was studied in four human breast c ancer cell lines (BT-474, SK-BR-3, MDA-453, and MCF-7) and in a primar y human breast tumor sample. The relative expression of p185(HER-2) wa s measured by immunofluorescence by using flow and/or image cytometry while correlated DNA analysis was performed on the same cells by fluor escence in situ hybridization to determine ERBB-2 gene and chromosome 17 copy numbers. Marked heterogeneity was observed in both protein exp ression and ERBB-2 copy number. Despite this heterogeneity, and in acc ordance with previous studies, the average levels of p185(HER-2) expre ssion correlated well with average ERBB-2 gene copy numbers in the fou r lines examined (r = 0.99), When the relationship between copy number and protein expression was studied on a cell-by-cell basis, p185(HER- 2) expression correlated with both the absolute number of ERBB-2 gene copies/cell (r = 0.59-0.63) and chromosome 17 copy number (r = 0.45-0. 61). It is of interest that there was weak or no correlation between p 185(HER-2) protein expression and the ERBB-2 copy number:chromosome 17 copy number ratio (r = 0.0-0.25). In more than one-half of cells expr essing a high level of p185(HER-2), the chromosome 17 copy number was high (two or three times the average copy number), whereas <2% of an u nselected population had a high chromosome 17 copy number. Bromodeoxyu ridine incorporation indicated that the S-phase-labeling index was hom ogeneous across various p185(HER-2)-expressing subpopulations in the S K-BR-3 cell line. Analysis of the primary breast tumor sample showed r esults similar to the cell lines, supporting the strong possibility of a mechanistic link among p185(HER-2) overexpression, ERBB-2 amplifica tion, and high chromosome 17 copy number.