Reasons Why Blocking microRNAs in Triple Negative Breast Cancer Is Difficult

    While a triple negative score is hardly ever a good thing, for breast cancer it is especially troubling. Triple-negative breast cancer (TNBC) refers to a disease scenario where the cancer cells do not express the genes for estrogen, progesterone, or HER2/neu receptors simultaneously—making this cancer particularly aggressive and difficult to treat as most chemotherapies target one of these receptors.  MicroRNAs (miRNAs) are short, non-coding RNA molecules that play critical roles in human malignancy.

    Over the past several years, researchers have discovered that various microRNAs (miRNAs) underlie the expression of certain genes that can enable cancer cells to proliferate faster. However, the ability to block these miRNAs in TNBC has been met with failure. Yet now, scientists from Thomas Jefferson University in Philadelphia believe they have discovered the reason conventional methods to block these miRNAs have been thus far unsuccessful.

    “Triple-negative breast cancer is one of the most aggressive forms of breast cancer, and there’s been a lot of excitement in blocking the microRNAs that appear to make this type of cancer grow faster and resist conventional treatment,” explained senior author Eric Wickstrom, Ph.D., professor in the department of biochemistry and molecular biology at TJU. “However blocking microRNAs hasn’t met with great success and this paper offers one explanation for why that might be the case.”

    The findings from this study were published online today in PLOS ONE through an article entitled “Non-specific blocking of miR-17-5p guide strand in triple negative breast cancer cells by amplifying passenger strand activity.” Insight from this study may enable new and more effective design of blockers against previously intractable miRNAs.

    “Triple negative breast cancer strikes younger women, tragically killing them in as little as two years,” noted lead author Yuan-Yuan Jin, a doctoral candidate in the department of biochemistry and molecular biology at TJU. “Only chemotherapy and radiation are approved therapies for triple negative breast cancer. We want to treat a genetic target that will keep patients alive with a good quality of life.”

    The investigators targeted the miRNA molecule miR-17, which has been shown previously to cause a surge in TNBC growth by alternating genes that would normally signal a diseased or early cancerous cell to die—specifically, the tumor suppressor genes PDCD4 and PTEN.

    Although, when the TJU researchers tried to reduce the levels of miR-17 in TNBC cells, rather than increase the levels of the tumor suppressor genes, as they had anticipated, they saw an even larger decrease in these genes than unmodified controls.

    The TJU team was acting under the current assumptions that miRNA, which are double stranded, only silence genes using one of their two strands, which is complementary to parts of the messenger RNA coding sequence. The matching, or so-called passenger strand, was thought to be discarded and degraded by the cell.

    Using a method to silence RNAs, involveing flooding the cell with modified RNA sequences that mimic the passenger strand and bind to the single-stranded microRNA before it reaches its target, the TJU team saw more silencing of the PDCD4 and PTEN genes, and after some bioinformatic and folding energy calculations, the authors realized that both strands of miR-17 were active in downregulating the tumor suppressor genes.

    “Rather than blocking miR-17, we were inadvertently boosting its levels, and, therefore, boosting the cell’s cancerous potential,” noted Jin.

    The results of the current study should help to open a pathway to designing specific blockers of one microRNA strand without imitating the opposite strand. Dr. Wickstrom added that “we are now testing new miR-17 blocker designs made possible by these results.”