Scientists at Johns Hopkins have discovered a potential strategy for cancer therapy by focusing on what's missing in tumors.
Noticing the conspicuous absence of single-stranded genetic snippets called microRNAs in cancer cells, a team of researchers from Johns Hopkins and Nationwide Children's Hospital delivered these tiny regulators of genes to mice with liver cancer and found that tumor cells rapidly died while healthy cells remained unaffected.
Publishing results of the study June 12 in Cell, the researchers say they have provided one of the first demonstrations that microRNA replacement provides an effective therapy in an animal model of human disease.
"This work suggests that microRNA replacement may be a highly effective and nontoxic treatment strategy for some cancers or even other diseases," says Josh Mendell, M.D., Ph.D., an associate professor in the McKusick-Nathans Institute of Genetic Medicine, Johns Hopkins University School of Medicine. "We set out to learn whether tumors in a mouse model of liver cancer had reduced levels of specific microRNAs and to determine the effects of restoring normal levels of these microRNAs to these cancer cells. We were very excited to see that the tumors were, in fact, very vulnerable to microRNA replacement."
His team had considered the possibility that the replacement of a single small RNA might have little if any effect, especially in the setting of all the complex changes that drive the aberrant behavior of a cancer cell. But the tumor cells in the mouse were indeed sensitive to the restoration of the microRNA—so much so that they died, rapidly.
"This concept of replacing microRNAs that are expressed in high levels in normal tissues but lost in diseases hasn't been explored before," Mendell says. "Our work raises the possibility of a more general therapeutic approach that is based on restoring microRNAs to diseased tissues."
Using a "special delivery" virus that can deliver genes to tissues without causing them any disease or harm, the researchers intravenously injected a fluorescent microRNA-containing virus into one group of mice with aggressive liver cancer, and injected a control virus containing no microRNA into another group. The viral delivery system was developed by Mendell's father, Jerry Mendell, M.D., director of the Center for Gene Therapy at The Research Institute at Nationwide Children's Hospital in Columbus, and K. Reed Clark, Ph.D., associate professor and director of the Viral Vector Core Facility at Nationwide Children's Hospital.
After three weeks, six of eight mice treated with the control virus experienced aggressive disease progression with the majority of their livers replaced by cancerous tissue. In contrast, eight of 10 of animals treated with the microRNA were dramatically protected, exhibiting only small tumors or a complete absence of tumors. Liver body weight ratios were significantly lower in the treated mice, further documenting cancer suppression.
"The livers of the mice that received the microRNA virus glowed fluorescent green, showing that the microRNA ended up where it was supposed to go, and the cancer was largely suppressed," Mendell said.
Equally intriguing, he reported, "The tumor cells that received the microRNA were rapidly dying while the normal liver cells were completely spared. These findings, as well as the results of specific tests for liver damage, demonstrated that the microRNA selectively kills the cancer cells without causing any detectable toxic effects on the normal liver or other tissues."
Mendell points out that the microRNA is normally present at high levels in non-diseased tissues, and especially in the liver. Mendell speculates that this is why healthy cells are very tolerant to therapeutic delivery of even higher levels of this microRNA. However, the sensitivity of tumor cells to this microRNA suggests that loss of this molecule is a critical step as normal cells become cancer cells.
"Since we were able to demonstrate such dramatic therapeutic benefit in this extremely aggressive model of human liver cancer, we are hopeful that similar strategies will be effective for patients with this disease," says Mendell.
In addition to Joshua Mendell, authors of the paper are Jerry Mendell, K. Reed Clark, Janaiah Kota and Chrystal L. Montgomery, of The Research Institute at Nationwide Children's Hospital, Columbus, Ohio; and Raghu R. Chivukula, Kathryn A. O'Donnell, Erik A. Wentzel, Hun-Way Hwang, Tsung-Cheng Chang, Perumal Vivekanandan, and Michael Torbenson, all of Johns Hopkins University School of Medicine.
The research was supported by the National Institutes of Health, the Sol Goldman Center for Pancreatic Cancer Research and the Research Institute at Nationwide Children's Hospital.
A Q&A with Josh Mendell of the McKusick-Nathans Institute of Genetic Medicine who tells how microRNAs are a family affair
You started your lab at Hopkins after having earned a medical degree here in 2003 and a Ph.D. here in 2001. At what point in your science career did microRNAs become the focus?
MENDELL: The field was just starting up in 2003 when I started exploring questions of microRNA biology. At the time, very few microRNAs had been studied in detail. Six years later, we now know that there are at least 500 human microRNAs—maybe as many as 1000—but still the functions of only a handful have been worked out fairly rigorously. I think this aspect of the field really drew me in. MicroRNA research is so rich with fundamental questions. It's a dream come true to be in a field that's revolutionizing molecular biology.
MENDELL: We're realizing that there is an entire universe of small RNAs being produced in everything from humans to nematode worms, in which microRNAs were discovered 16 years ago. It used to be thought that small RNAs were junk—degraded products of the big important RNAs that have been studied for decades. Only recently have we begun to appreciate an entirely new layer of complexity involving small RNAs and other types of RNA that do not encode proteins. They have incredibly diverse and important functions. They are critical for normal development and cellular homeostasis, and abnormal expression of these molecules has been linked to several diseases, including cancer.
How does your microRNA research relate to cancer?
MENDELL: Cancer cells often exhibit reduced abundance of numerous microRNAs. Sometimes the production of microRNAs is even globally blocked in cancer cells. We set out to test the efficacy of microRNA replacement therapy: We wanted to know if replacing one microRNA in liver tumors in a mouse model could be a feasible, viable therapeutic paradigm. If this strategy could work for cancer, perhaps it could work for other diseases.
Our main question was, if microRNAs are missing from tumor cells, what happens if we put one back? Do the tumor cells die? Do they care? One might expect that replacement of a single small RNA in the setting of all the complex changes that are driving aberrant behavior of a cancer cell might have little effect.
The answer turns out to be yes, the tumor cells in this mouse model are very sensitive to restoration of this microRNA. They die rapidly when it is delivered.
It must be exciting to find out that delivering just one microRNA into a mouse liver engulfed by tumors yields dramatic therapeutic results.
MENDELL: I think we were all amazed at how dramatic the results were. A few years ago it was entirely unclear—and it would have been incredibly surprising to us—that one microRNA could have effects this potent.
Are there any hitches to microRNA replacement therapy?
MENDELL: Delivery of the molecules is going to be a major barrier for applying this technology for therapy in patients. But I don't think it's going to be an insurmountable barrier. A lot of great minds are working on this.
Two of whom you're well-acquainted with, right? This microRNA replacement therapy idea was cooked up around the Mendell table during Thanksgiving dinner? Both your father and wife are also authors of the paper.
MENDELL: This research was a wonderful opportunity for me to collaborate with my dad, Jerry Mendell, whose focus at Ohio State is human gene therapy for neuromuscular diseases. Without his expertise and the expertise of his research group, it would not have been possible to deliver the microRNA. And my wife, Kate O'Donnell, who's a postdoctoral fellow in Molecular Biology and Genetics here at Hopkins, was integral for her knowledge of the mouse model we used.
By the way, not everybody there at Thanksgiving was appreciating the conversation. But the three of us were really enjoying it. I guess you can view that as either cool or nerdy.
Cool. Very cool!
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