Can microRNAs put the brakes on cancer?

Between 20 and 25 genetic letters long, microRNAs regulate the expression of genes involved in development.

One of the basic premises of biology is that our genetic code lies in our DNA, which, in turn, relies on RNA to transmit that code to build the proteins that carry out the chemical activities necessary for life.

In the early 1990s, however, scientists at Harvard Medical School discovered a genetic switch in the microscopic roundworm C. elegans that called into question long-held beliefs about the role of RNA. Lin-4 was the first of what would become known as microRNAs.

Only 22 genetic letters long, lin-4 is far shorter than a typical 1,000-letter RNA message, and rather than helping to build proteins, it sticks to messenger RNA and shuts down the expression of genes involved in early development.

It would be seven years before Frank Slack, Ph.D., showed that lin-4 was no fluke. In 2000, while a postdoctoral fellow at Harvard, Slack, now associate professor of molecular, cellular and developmental biology at Yale, identified a second microRNA, let-7, that also governs development in C. elegans.

Then the floodgates opened. In the past five years, hundreds of gene-silencing microRNAs have been found in plants and animals, including over 200 in humans that may regulate more than a third of our genes. Because half of the C. elegans genome matches our own, including the genes for let-7, Slack’s research is having an impact on our understanding of human development, aging and illness, especially cancer.

According to Slack, one of the primary roles of microRNAs is to put a brake on cell proliferation during development. “Initially in the human embryo, you’ve got cells just dividing, dividing, dividing—to make as many cells as possible,” he said. “But at some point you want to make an organ. MicroRNAs come on to tell cells to stop dividing and to start differentiating into organs. And they stay on all through life, to keep the cells from dividing again.” Slack believes that the uncontrolled cell division that is a hallmark of cancer might be caused when the check on cell growth imposed by microRNAs is somehow lifted. “In various cancers we’ve looked at, microRNAs have been shut off,” he said. “We think that causes cells to re-enter their cell division program and behave like they’re in the embryo.”

In particular, Slack has found that let-7 is tamped down in human tumors, unleashing Ras, a cell-proliferation gene that has long been implicated in cancers of the lung and pancreas.

Slack is collaborating with Joanne B. Weidhaas, M.D., Ph.D., assistant professor of therapeutic radiology, to develop microRNA-based diagnostic tools and treatments. According to Weidhaas, genomic analyses of tumors and cancer therapies targeting single genes have been largely disappointing, because hundreds of genes are faulty in any given cancer and it has been difficult to discern which mutations are most important. The excitement surrounding microRNAs, she said, stems from their ability to regulate entire suites of genes that underlie biological pathways.

A 2005 study published in the journal Nature found that measuring the levels of just 217 microRNAs could generate clearer genetic signatures for tumors than 16,000 probes for messenger RNA. Encouraged by these results, Slack and Weidhaas hope within two years to perfect a microRNA-based screening device that could help tailor cancer treatments to patients’ tumor types, and they are in the early stages of testing a let-7 inhalant therapy to rein in uncontrolled cell growth in lung cancer. In addition, Weidhaas has shown that raising let-7 levels in C. elegans makes the worm’s cells more sensitive to radiation, leading her to conclude that a let-7 treatment could be a powerful adjunct to standard radiotherapy.

—Peter Farley

Tissue engineering takes a leap forward with new scaffold design

Tissue engineering began in the late 1980s to fill a gap in the treatment of certain diseases—those for which transplants could offer a cure. There isn’t enough natural human restorative tissue to go around, however, and two recent studies at Yale show how clinical applications of tissue engineering are coming within reach.

In one study, researchers created a tiny water-soluble “scaffold” that provides a framework for the growth of new blood vessels; in the other, they demonstrated that the technique for creating new arteries—manipulating telomerase to extend cell life—doesn’t necessarily cause cancer, as had been feared. Both papers appeared in the February 21 issue of the Proceedings of the National Academy of Sciences.

“A microvascular network is fundamentally important for tissue engineering,” said Erin Lavik, Sc.D., an assistant professor of biomedical engineering and author of one of the studies. She added, however, that the “stability of microvascular networks has been a challenge.”

Lavik’s team built scaffolds out of gelatin-like, porous hydrogels that can be chemically treated to form numerous interconnected internal pores. In collaboration with Joseph A. Madri, M.D., HS ’76, Ph.D., professor of pathology and of molecular, cellular and developmental biology, Lavik and her colleagues seeded the gels with blood vessel cells, implanted them under the skin of mice and found that functional and stable vessel networks had formed after six weeks.

Scientists have been able to create new arteries in humans by using the patient’s own cells, but one drawback has been that these replacements haven’t been as effective in older people. A second Yale team found a way around that problem last year.

Researchers had previously used gene therapy to deliver telomerase, an enzyme that extends the cells’ normal life span by lengthening their chromosomes following cell division. The technique worked, even in patients as old as 85, said Laura E. Niklason, M.D., Ph.D., an associate professor of anesthesiology and biomedical engineering. Telomerase, however, is highly active in cancerous cells. “One of the outstanding questions is, ‘How safe is this?’,” she said.

In the latest study, which involved tissue samples from eight elderly patients and one young donor, Niklason’s team took cells obtained during a coronary bypass procedure and increased telomerase expression. They discovered that they had not produced cancerous cells in so doing. “Just turning on telomerase by itself is not enough to create cancer,” she said.

Although Niklason said more work is needed, her findings may one day enable the development of techniques for making replacement tissue.

—John Dillon

et cetera

Fighting a lethal microbe

Scientists at Yale have discerned how the immune system fights the bacterium that causes Legionnaire’s disease.

The bacterium, Legionella pneumophila, hides from immune defenses by living and multiplying in sealed vacuoles inside cells. Craig R. Roy, Ph.D., associate professor of microbial pathogenesis and senior author of a study published in Nature Immunology in March, and colleagues found that a protein called Birc 1e is key to detecting Legionella infection.

Postdoc Dario S. Zamboni, Ph.D., lead author of the study, showed that when bacterial products leave the infected cells, Birc 1e activates a signaling pathway that stimulates a protease called caspase-1. The protease degrades other proteins in the infected cell, starting a cascade of events that results in cell death.

“Identification of Birc 1e and the caspase cascade gives us information about the process of how the body fights off infection by a potentially lethal microbe, as well as possible targets for treatments,” said Roy.

—John Curtis

Gene linked to social aversion

Knocking out a gene in the brains of mice can counteract an aversion to social interactions, according to researchers at Yale and the University of Texas South-western Medical Center at Dallas.

The scientists conditioned mice to avoid mice they didn’t know by exposing them to more aggressive mice. Then, using viral technology developed by Ralph J. DiLeone, Ph.D., assistant professor of psychiatry at Yale and a co-author of the report that appeared in the journal Science in February, the researchers inactivated a gene called brain-derived neurotrophic factor (BDNF).

Mice without BDNF did not develop defeated behavior, DiLeone said, suggesting that the gene is essential to developing social aversion as a response to aggressive behavior.

“The results have implications for a number of psychiatric conditions, including depression and post-traumatic stress disorder, where stressful events can have significant and long-lasting consequences for social behavior and interactions,” DiLeone said.

—J.C.


			Originally published in Yale Medicine, Autumn 2006. Copyright © 2006 Yale University School of Medicine. All rights reserved.