Bone morphogenetic protein 4 (BMP4) is involved in the development and progression of gastric cancer—a finding that could lead to targeted gastric cancer treatment and alternative treatment strategies for patients with tumours resistant to cisplatin.

Although some mechanisms by which different cancers evoke cisplatin resistance have been elucidated, there is currently no way to determine accurately which patients with gastric cancer are most likely to respond to cisplatin. “Our overall goal was to identify biomarkers that might aid clinicians in tailoring gastric cancer treatments ... and highlight novel targeted strategies for overcoming cisplatin resistance in gastrointestinal cancer,” explains Patrick Tan, corresponding author.

The team's first step was to treat 20 gastric cancer cell lines with increasing cisplatin concentrations. The cell lines varied greatly in their level of cisplatin sensitivity, and extending drug incubation times generated similar results. Comparing the gene expression profiles of the four most cisplatin-sensitive and four most cisplatin-resistant cell lines identified 291 differentially expressed genes.

The identification of such a large number of genes, and the proposal that aberrant DNA methylation patterns can serve as molecular markers predictive of prognosis and sensitivity to chemotherapy, motivated Tan and colleagues to study CpG (cytosine–phosphate–guanine) methylation levels across the cell lines. 1,505 CpG sites corresponding to 807 genes were quantified and the results validated. Combining the lists of differentially methylated and differentially expressed genes between the cisplatin-sensitive and cisplatin-resistant cell lines revealed five genes (BMP4, CD9, DSC2, CDH17 and TFPI2) that were both differentially expressed and differentially methylated.

Credit: © Wa Li | Dreamstime.com

Focusing on BMP4—which encodes a member of the TGFβ superfamily—the team found that its methylation −199 bp and −123 bp (BMP4−199 and BMP4−123) from the transcription start site was inversely correlated with BMP4 expression in the cisplatin-sensitive and cisplatin-resistant cell lines and across the original 20 cell lines (the correlation was strongest for BMP4−199). BMP4 methylation was also inversely correlated with cisplatin resistance and BMP4 expression levels positively correlated with cisplatin resistance.

Additional work with the gastric cancer cells lines further elucidated the role of BMP4: its expression is probably regulated by methylation of its promoter; high BMP4 expression levels at baseline in cisplatin-resistant cell lines can activate downstream components of the BMP signalling pathway; suppression of cell proliferation and invasion (by BMP4 silencing) and their enhancement (by BMP4 overexpression) support a pro-oncogenic effect of BMP4 in gastric cancer; and reciprocal regulation of mesenchymal genes by BMP4 silencing and overexpression suggests BMP4 expression could aid the epithelial–mesenchymal transition in gastric cancer cells.

Moving to the clinical setting, the team confirmed that BMP4 expression was significantly upregulated in primary gastric cancers compared with nonmalignant gastric tissue. Analysis of a subset of tumours designated as having 'low' or 'high' BMP4 expression showed that BMP4 methylation significantly correlated with BMP4 expression. High BMP4 expression levels were also found to correlate with poor patient survival. Several gene sets were found to be enriched in BMP4 'high' tumours, including some related to the epithelial–mesenchymal transition (for example, TGF-β signalling) and others related to cisplatin resistance (for example, nucleotide excision repair).

In the final part of their study, Tan and colleagues considered the therapeutic implications of their findings. They found that targeted inhibition of BMP4 signalling could be one way to sensitize gastric cancer cells to cisplatin. Encouragingly, they also found that cisplatin-resistant cell lines did not exhibit resistance to oxaliplatin and had the same response as cisplatin-sensitive cell lines.

The researchers are now validating the utility of BMP4 as a cisplatin resistance marker in a series of patient cohorts. One aspect of their continuing efforts involves prospectively collecting gastric cancer samples from patients receiving a fluorouracil prodrug plus either cisplatin or oxaliplatin as part of an ongoing phase II clinical trial. “Analysis of the genomic profiles of these samples upon completion of the trial should allow us to correlate BMP4 expression levels to cisplatin and oxaliplatin responses, respectively,” they conclude.