Newly identified gene plays major role in breast cancer metastasis, chemoresistance

Metadherin was responsible for an animal metastasis burden seven-fold higher than controls.

Metadherin or MTDH, a metastases gene that is amplified and over-expressed in 30% to 40% of breast cancers, promotes metastatic seeding and enhances chemoresistance, according to data published in Cancer Cell. According to researchers, MTDH should be therapeutically targeted to improve the efficacy of chemotherapy and reduce the risk for metastases.

“This is probably one of the first examples of a novel class of dual functional breast cancer genes that cause both metastasis and chemoresistance,” said Yibin Kang, PhD, senior corresponding author of the study and assistant professor of molecular biology at Princeton University, in a press release.

To chart minimum recurrent genomic alterations related to poor-prognosis breast cancer, researchers developed the analysis of CNAs by expression data (ACE) algorithm. They tested the efficacy of ACE and once it was validated they used the method to examine data sets from three previous studies that detected two poor-prognosis gene sets of 70 and 76 genes that could be used to predict the clinical outcome of human breast cancer.

8q22 and poor prognosis

The researchers observed five common genomic gains in at least two data sets and 15 other gains from one of the three sets. Out of the five genomic events, 8q22 was the most consistently observed genomic gain in all three data sets. The researchers then calculated the regional expression pattern using a neighborhood score for the genomic loci. Patients with a high 8q22 neighborhood score had a lower probability of metastasis-free survival compared with controls from all three data sets. According to the researchers, this suggests that 8q22’s genomic gain is a strong predictor of poor prognosis breast cancer.

The 8q22 genomic gain was confirmed using FISH and genomic DNA quantitative PCR (qPCR). Researchers then went on to confirm the association between 8q22 genomic gain and the elevated expression of genes located in the same region. They examined the expression patterns of PTDSS1, MTDH and LAPTM4beta that are located in 8q22. The researchers reported a strong association between 8q22 and a high expression level of all three genes after analyzing a panel of microdissected breast tumors.

The role of MTDH in metastasis, chemoresistance

The researchers tested six genes based on their potential involvement in disease progression, as demonstrated in statistical analyses, and their known biological functions: UQCRB, PTDSS1, TSPYL5, MTDH and LAPTM4beta; the genes were over-expressed in at least two of the three data sets. The sixth gene tested, SDC2, has been reported to mediate cell adhesion and proliferation in colon cancer, according to the researchers.

The genes were over-expressed in the SCP28 cell line — one that is weakly metastatic to lung and bone when injected into mice — to determine their role in metastasis. The researchers reported that in mice that received tumor cell xenografts, MTDH hastened the development of lung metastasis and shortened survival. MTDH over-expression caused an animal metastasis burden that was almost seven-fold higher than controls six weeks after the injection.

Using nude mice, the researchers then used short-hairpin RNA constructs to destroy MTDH expression in the LM2 cell line, which was selected in vivo for its high lung metastasis propensity, according to the study. Knocking down MTDH reduced the lung metastasis burden of LM2 by three-to-five-fold and extended survival by one to two weeks.

The researchers identified a potential role of MTDH in promoting chemoresistance by analyzing the pharmacogenomic data of the NCI60 panel of cancer cell lines. They then used genetically modified LM2 cell lines used in vivo metastasis assays to determine the function of MTDH in chemoresistance. The cell lines were treated with chemotherapy or stress agents, which included paclitaxel, doxorubicin, cisplatin and hydrogen peroxide with or without coculture with the HMVEC-L endothelial cell line. The researchers found that inhibiting MTDH expression made the cell lines sensitive to chemotherapeutic and stress agents, while overexpression made SCP28 cells more resistant to them. Co-culture with endothelial cells further enhanced MTDH-dependent chemoresistance.

“Not only has a new metastasis gene been identified, but this also is one of a few such genes for which the exact mode of action has been elucidated,” said Michael Reiss, MD, director of the Breast Cancer Research Program at The Cancer Institute of New Jersey and a study coauthor, in a press release. “That gives us a real shot at developing a drug that will inhibit metastasis.”

Cancer Cell. 2009;15:9-20.