Activation of multiple cancer-associated genes at the ERBB2 amplicon in breast cancer

During the past decade the role of the ERBB2 (neu/HER2) oncogene as an important predictor of patient outcome and response to various therapies in breast cancer has been clearly established. This association of ERBB2 aberrations with more aggressive disease and poor clinical outcome, together with the high prevalence of such alterations in breast cancer, has also made ERBB2 an attractive target for therapy. A specific antibody-based therapy, Herceptin, directed against the extracellular domain of the ERBB2 receptor tyrosine kinase, was recently developed and several clinical trials have shown the therapeutic efficacy of this drug against ERBB2-positive breast cancer. However, a relatively large fraction of patients does not benefit from Herceptin treatment, indicating that other factors beyond ERBB2 itself must influence therapy response in ERBB2-positive tumors. It is well known that amplification of the 17q12-q21 region is the most common mechanism for ERBB2 activation in breast cancer and that it leads to simultaneous activation of several other genes. These co-amplified and co-activated genes may have an impact on disease progression and the clinical behavior of ERBB2-positive tumors and thus represent important targets of research. In this paper we discuss the current knowledge on the structure of the ERBB2 amplicon, the genes involved, and their possible contribution to breast cancer pathogenesis.

New amplified and highly expressed genes discovered in the ERBB2 amplicon in breast cancer by cDNA microarrays

Amplification of the ERBB2 oncogene at 17q12 has been well documented in breast cancer and has been shown to contribute to a poor clinical outcome. However, systematic surveys of copy number and expression levels of all genes within the 17q12 region have not been performed. Here, we used cDNA and comparative genomic hybridization microarray technologies to undertake a broad survey of genes involved in the 17q12 amplification in breast cancer. A chromosomal region-specific cDNA microarray containing 217 expressed sequence tag (EST) clones from 17q12 was constructed and used for parallel analysis of gene copy numbers and expression levels in seven breast cancer cell lines allowing direct identification of genes whose expression is elevated because of an increase in copy number in this chromosomal region. The copy number and expression survey identified 12 transcripts that showed a consistent pattern of increased copy number and expression in three or more of the 17q12-amplified cell lines. As expected, these included ERBB2 as well as the GRB7 and MLN64 genes previously shown to be coamplified with ERBB2. In addition, five other known genes and four uncharacterized ESTs were also found to be consistently activated by amplification in these breast cancer cell lines. Amplicon mapping by fluorescence in situ hybridization revealed a minimal common region of amplification containing four highly expressed genes, ERBB2, GRB7, MLN64, and an uncharacterized EST 48582. Furthermore, several other genes, although not located in the minimal common region of amplification, showed a correlated pattern of amplification and expression indicating that they might play a role in breast cancers with the 17q12 amplification. In conclusion, parallel analysis of gene copy number and expression levels by cDNA microarray can be used to directly identify candidate target genes involved in amplifications. Our results show that the 17q12 amplification in breast cancer leads to the simultaneous elevation of expression levels of several genes.

High topoisomerase IIalpha expression associates with high proliferation rate and and poor prognosis in oligodendrogliomas

The role of molecular markers predicting the prognosis and the selection of patients for further adjuvant therapies is not well established in oligodendroglioma patients. A potential prognostic as well as a therapeutically predictive factor, topoisomerase IIalpha (topoIIalpha), is a molecular target for certain cytotoxic drugs. Its expression has been shown to correlate with the prognosis in a number of different cancers and with the chemosensitivity of cancer cells in vitro. The expression of topoIIalpha was evaluated immunohistochemically in 59 oligodendrogliomas and in 29 mixed gliomas with a predominating oligodendroglioma component by the use of a tissue microarray technique. In the gliomas, the percentage of topoIIalpha immunopositive cells protein expression varied from 0.0 to 49.1% (5.2 +/- 8.3%, mean+/- SD). In oligoastrocytomas, the mean topoIIalpha score was significantly higher in the oligodendroglioma than in the astrocytoma component of the tumour (5.37 +/- 5.58% vs. 1.89 +/- 2.49%, P = 0.018). A significant association was found between the high proportion of topoIIalpha positive cells and high grade of the tumour (P < 0.0001), high tumour proliferation rate (P < 0.0001), p53 overexpression (P = 0.01) and high expression of tumour suppressing retinoblastoma protein (P = 0.023). TopoIIalpha expression was not associated with the age or sex of patient, and the rate of apoptosis. TopoIIalpha expression associated highly significantly with patient prognosis; a significantly higher proportion of patients with low rather than with high topoIIalpha score was alive at the end of the 5-year follow-up (P = 0.03). Cox analysis was used to demonstrate that topoIIalpha had an independent prognostic value for survival (P = 0.034). In conclusion, high topoIIalpha expression characterizes oligodendrogliomas and oligoastrocytomas which are poorly differentiated, have high proliferation rate, and has prognostic value for overall survival of these patients. Therefore, topoIIalpha may be a useful marker for better targeted selection of poor prognosis oligodendroglioma patients for adjuvant therapy.

MYB oncogene amplification in hereditary BRCA1 breast cancer

Comparative genomic hybridization analysis has demonstrated that breast tumors from BRCA1 and BRCA2 germ-line mutation carriers contain a large number of chromosomal copy number gains and losses. A high regional copy number gain at 6q22-q24 was observed in one BRCA1 tumor, and fluorescence in situ hybridization analysis indicated a strong amplification of the MYB oncogene (15 copies of MYB compared with 1 copy of chromosome 6 centromere). Fluorescence in situ hybridization analysis revealed amplification of MYB in 5 (29%) of 17 BRCA1 breast tumors, whereas none of 8 BRCA2 tumors and 13 breast cancer cell lines, and only 2 of 100 sporadic breast tumors exhibited altered MYB copy numbers. Gene amplification resulted in mRNA overexpression as determined by Northern blot and cDNA microarray analysis, and protein overexpression by immunohistochemical staining. We conclude that MYB amplification is infrequent in sporadic breast cancer but common in breast tumors from BRCA1 mutation carriers, suggesting a role of this cell cycle regulator and transcription factor in the progression of some BRCA1 tumors. However, we cannot rule out the significance of other genes in the 6q22-q24 amplicon.