Bone morphogenetic protein 7 is widely overexpressed in primary breast cancer

Bone morphogenetic proteins (BMP) make up a family of extracellular signaling molecules that play a critical role in vertebrate development and both inhibit and stimulate growth in cancer cells. BMP7 was recently identified in our genomewide copy number and expression survey as being activated through amplification in breast cancer cell lines. In the present study, we further explored BMP7 gene copy number and expression changes in 22 breast cancer cell lines and 146 primary breast tumors. FISH analysis revealed that BMP7 copy number varied greatly from one cell line to another, with three cell lines showing extremely high-level amplification. Among primary tumors, BMP7 copy number was increased in 16% of the cases. BMP7 mRNA expression was determined in the cell lines and in a subset of 44 tumor samples by RT-PCR or quantitative real-time RT-PCR, respectively. Despite elevated mRNA levels in cancer cells, there was no significant association between copy number increase and mRNA expression, even though the highest expression was seen in cell lines and tumors with increased BMP7 copy number. Most interestingly, immunohistochemical analysis revealed BMP7 protein staining in all 11 breast cancer cell lines examined and strongly elevated BMP7 protein expression in 71.4% of the tumor samples as compared to normal mammary epithelium. Our results illustrate the frequent involvement of BMP7 alterations in breast cancer and especially highlight overexpression of the BMP7 protein in a very large fraction of primary breast tumors, thus suggesting a possible functional role for BMP7 in breast cancer development.