In 2002, George Calin, Carlo Croce, and colleagues at Thomas Jefferson University provided the first evidence that microRNAs — small noncoding RNAs that can repress gene expression — were linked to cancer.
In the Hot Papers featured here, Croce, now at Ohio State University, and his group tested the link between microRNAs and cancer on a large scale. They carried out a microarray analysis of 363 samples from six frequently found solid-tumor types in humans and 177 controls, revealing that cancer cells have distinct and abnormal microRNA profiles.
A common denominator
Looking at 228 microRNAs in the first Hot Paper, Croce's group found 36 that were overexpressed and 26 that were downregulated in cancer cells versus normal cells. "This and many other groups since have shown that microRNA expression profiles are essentially ubiquitously abnormal in every cancer that has been examined," says Mendell. Croce's group also found that these microRNA profiles allowed the tumors to be grouped based on their tissue of origin. This suggested that a relatively small number of microRNAs could be used as markers to classify and distinguish cancer cells.
Expression-profiling studies can also help determine which of the more than 1,000 predicted human microRNAs might be key regulators, and therefore good candidates for playing a key role in cancer. Croce's group's findings led them to focus on miR-155. In a transgenic mouse that overexpresses this microRNA, expression of this microRNA alone promoted a neoplastic phenotype.3
In addition, work on miR-155 is being extended to other realms, including inflammation. In 2007, David Baltimore and colleagues from California Institute of Technology used microarray analysis to show that miR-155 was upregulated by stimulators of inflammation, providing a potential microRNA link between inflammation and cancer.
More recently, Mendell's group revealed that activating the Myc oncogenic pathway turns off a large subset of microRNAs to drive tumorigenesis.
Indeed, one of the most significant suggestions of these studies is that microRNAs comprise a downstream factor of pathways that lead to cancer. "If they are the downstream targets, and the activity of pathways depend on [the] activity or loss of expression of microRNAs, [this] makes them ideal targets for therapy," says Croce.
"There have been some early glimpses of microRNAs being useful as prognostic and diagnostic markers" for cancer, says Ross. For example, in 2008, Pan-Chyr Yang and colleagues from the National Taiwan University identified a five-microRNA profile that predicted the treatment outcome of lung cancer.
Calin agrees. He also cites the potential role of microRNAs in cancer predisposition. In 2007, Elizabeth Raveche and colleagues from New Jersey Medical School found three loci linked to the spontaneous development of B-cell lymphoproliferative disease in a mouse model for human chronic lymphocytic leukemia.
Mendell hopes that all these aspects of microRNAs will lead to new therapeutic strategies that target them and their pathways, but he adds: "We are still at the very earliest stages of understanding how these microRNAs have such a potent effect on cellular behavior."