A Powerful Weapon for Battling Breast Cancer

Columnist
Graduate Division

Cancer begins from a single mutation in a single cell. By the time it is a full-blown tumor, it carries hundreds of genetic and molecular alterations that allow it to grow uncontrollably outside the constraints of normal biology.

Cancer therapy today falls into one of three groups: classic chemotherapy, which kills actively-dividing cells, targeted therapy, which blocks one cancer-specific target molecule, and immunotherapy, which unleashes the natural cancer-killing power of the immune system. Most therapeutic approaches to treating cancer face at least one of the following problems: severe side effects, lack of response, or a temporary response followed by eventual relapse.

Consider the frustration when, after years of research, a drug designed to target a specific molecule on or inside a cancer cell achieves initial responses in patients, but a few months later the tumors find a way around the blockade and come back in full force. In this event, more research must be done to identify the mechanism of resistance. If an explanation is found, a second drug is brought in to deal with the new problem, patients respond, but then may relapse again. The whole process turns into a deadly game of whack-a-mole.

MicroRNA is a relatively new addition to our understanding of genetics and molecular biology - the first one was only discovered in the 1990’s - but leveraging it for cancer therapy could prove to be a powerful way to improve the outcomes of targeted therapies.

Unlike messenger RNA, which is information copied from our DNA and used to make functional proteins inside our cells, microRNA is a tiny form of non-coding RNA - it’s never translated into protein. Instead, a cell uses microRNA to turn other genes off, by blocking messenger RNA’s from being expressed into protein. But the power of microRNA doesn’t lie solely in its ability to block other genes from being expressed. The clincher is that a single tiny microRNA - only about 22 nucleotides long - can turn off a huge set of genes by blocking a common target sequence on an array of messenger RNAs.

Many microRNAs have already been implicated in cancer. Similarly to other genetic alterations, microRNAs that increase in cancer are classified as oncogenic microRNAs (oncomiRs) and those that are lost are classified as tumor suppressive microRNAs. Because a single microRNA blocks a large set of genes, an outstanding question is whether microRNA can be exploited as a gene therapy in cancer to target a genetic program without which a tumor can’t survive. This enticing possibility could reinvigorate the targeted therapy field, which has been riddled with resistance and relapse due to the tendency of utilizing drugs that only hit one target at a time.

“The therapeutic potential of [microRNA] in cancer is limited by the lack of efficient delivery vehicles,” begins a provocative new article published on December 7 in the journal Nature Materials. The group of Natalie Artzi, assistant professor at Harvard Medical School as well as research scientist at MIT and the Broad Institute in Cambridge Massachusetts, has published an innovative study on how to deliver microRNA to highly aggressive breast tumors in mice.

The delivery strategy begins with a self-assembly process that produces an RNA-triple-helix structure consisting of three strands of RNA: a pair called an antagomiR used to inhibit a target microRNA (miR-221 antagomiR) and a microRNA mimic used to replace a lost microRNA in the cancer cells (miR-205 mimic). These triplex structures are then formed into a sponge-like nanoparticle by the addition of a molecule called polyamidoamine, and subsequently formed into a hydrogel by adding another molecule called dextran aldehyde.

The group then ran tests to ensure that their microRNA nanoparticles would be compatible with physiologic conditions, confirming stability as well as recognition by cellular microRNA processing machinery. Using fluorescent tags, they also confirmed that these structures could be taken up by cells in vitro, specifically by the process of macropinocytosis in which a cell brings in solutes from its environment.

After observing that uptake of their microRNA triplex by breast cancer cells caused decreases in cell viability, growth, and migration (an indicator of invasiveness), the real question was whether or not the triplex could be delivered to a tumor in a host animal. The authors chose to work with triple-negative breast cancer, a highly aggressive form of the disease that does not currently have good options for targeted therapy. After implanting human triple-negative breast cancer cells into immunodeficient host mice and allowing small tumors to form, the authors implanted hydrogels - loaded with either their microRNA nanostructures or with standard breast cancer drugs - adjacent to the growing tumors..

The impressive result from this paper was the profound effect on tumor size from adjacent microRNA triplex-loaded hydrogel. These tumors never grew beyond their tiny size, and the host mice survived for an extra 40 days beyond untreated control mice. For immunosuppressed mice carrying extremely aggressive tumors, which normally succumb to the disease after just one month, this is indeed an impressive result. Conversely, the drugs had a modest effect on tumor growth, slowing growth but not arresting it, and hardly affecting mouse survival at all.

These powerful results come from delivering just two microRNAs - one that rescues a tumor suppressor microRNA and one that blocks an oncogenic microRNA. The main goal of the work was to demonstrate microRNA delivery, but the therapeutic effects on a highly aggressive form of breast cancer were a massive bonus.

Proving that microRNA delivery to tumors is feasible opens doors for the future of targeted cancer therapy. “There are so many microRNAs that are involved in metastasis. It’s really an underexplored field,” said João Conde, postdoc at MIT’s Institute for Medical Engineering and Science, and lead author on the paper. Loading a hydrogel with some microRNAs that inhibit primary tumor growth, and others that inhibit metastasis, could prove to be a killer combination.