By Rob Mitchum
Among the most hyped cancer therapies for the future, microRNA looms large. While much smaller than the RNA produced by protein-coding genes, these tiny transcripts play an important regulatory role in cells by acting as a brake on the process of making proteins from genes. MicroRNAs bind to their relatives, the messenger-RNAs, which are used by the cell as a recipe for making new proteins. When this binding occurs, the protein builders (called ribosomes) can no longer attach to messenger-RNA, essentially halting production.
Because many kinds of cancers are caused by the excessive production of protein from various “oncogenes,” researchers have fixated upon the interrupting power of microRNAs as a potential targeted therapy. In theory, treating a patient with the right microRNA for their over-expressed oncogene could bring the out-of-control protein back to normal levels, preventing the unrestrained cellular growth that characterizes cancer. But a new discovery published this week in Nature Communications by researchers from the University of Chicago Medicine Section of Hematology/Oncology cautions that microRNAs are not straightforward weapons against cancer.
One promising anti-cancer target is a microRNA called miR-196b, which is associated with certain types of leukemia associated with translocations of the mixed lineage leukemia (MLL) gene. In a translocation, two chromosomes are accidentally broken and the pieces are put back together incorrectly, leaving two unnatural hybrid genes. In this Frankenstein manner, the MLL gene can be abnormally combined with 60 different partner genes, and this “fusion protein” boosts the transcription of a handful of genes, such as HOXA9 and MEIS1, that cause white blood cells to grow and proliferate uncontrollably.
In 2009, a team of researchers including assistant professor of medicine Jianjun Chen and University of Chicago legend Janet Rowley discovered that miR-196b expression is also boosted by MLL fusions, and that the microRNA is necessary for the immortality and proliferation of the leukemic cells. The finding suggested that reducing levels of miR-196b could be an effective therapeutic strategy in fighting leukemia, while raising the levels of the microRNA would accelerate the disease.
But when Chen’s laboratory tested the second part of that hypothesis by experimentally boosting the levels of miR-196b in mice, they found the exact opposite effect: higher levels of the supposedly cancer-promoting microRNA actually delayed the onset of leukemia.
“It was a surprise result for us, because people already reported that by knocking down expression you delay leukemogenesis, so we expected overexpression would promote leukemogenesis,” said first author Zejuan Li, research associate assistant professor at the University of Chicago Medicine. “We didn’t believe the result, we thought something had gone wrong during our experiment. So we repeated and repeated and repeated several times and we got the same result. Finally, we found this mechanism.”
The miR-196b mystery boiled down to looking at the full set of gene transcripts that the microRNA targets and represses. Surprisingly, the researchers discovered that miR-196b reduces the expression of HOXA9 and MEIS1, two oncogenes also upregulated in MLL-related forms of leukemia. But this anti-cancer action is opposed by another target of the microRNA — a tumor suppressor gene called Fas. Since like many microRNAs, miR-196b has many different targets in the genome (41 were detected by this paper alone), it’s a fallacy to consider the factor as either one-dimensionally pro-cancer or anti-cancer.
“I think this is a very common phenomenon, and this should cause caution for the basic research scientists,” Chen said. “When doing research [on one microRNA function], you could be ignoring half of the potential targets.”
So why in the world would a cell need a regulatory factor with seemingly contradictory effects?
The answer, Chen believes, lies in how cells must be delicately controlled to undergo normal growth and proliferation without tipping over to the unrestrained multiplication of cancer. To do so, the expression of growth-promoting genes such as HOXA9 and MEIS1 must be carefully calibrated.
“That’s why we believe microRNA regulation is very important: they can fine tune, not just totally turn genes on or off,” Chen said. “During development, these genes are highly expressed at first, then this microRNA can be upregulated to clean up the transcripts so that the system can develop or differentiate. This kind of regulation is very important for normal development, which is why it is highly evolutionarily conserved and seen in other species.”
This custodial role explains why leukemia can result both when too little and too much miR-196b is around. If levels of the microRNA are too low, HOXA9 and MEIS1 run amok. If levels of the microRNA are too high, it starts repressing its other targets, such as the tumor suppressor gene Fas.
In cases of leukemia caused by MLL fusion, HOXA9 and MEIS1 expression levels can be up to 100 times higher than normal. Even though miR-196b levels are also boosted as much as 50 times above baseline, that’s not enough to keep up with the runaway oncogenes…but more than enough to silence the tumor suppressors, which remain at their normal levels.
The nuanced role of miR-196b means that simply increasing or decreasing the microRNA in an attempt to stop leukemia would likely have unforeseen and dangerous consequences. But the finding doesn’t mean microRNAs can’t be effective weapons against cancer, the authors said. Instead, the delicate balance of these factors and how they control their various targets must be fully understood and respected before they can be translated from laboratory favorite to clinical tool.
“Dosage is very important,” Li said. “We need to pay attention to how much we should use to treat the cells. You can’t just overwhelm the cell, this is a more critical factor that we should think about.”
“People now are talking about using microRNA as a tool or a target for kinds of therapy. But right now our study really calls for bigger caution,” Chen said. “When you study a particular microRNA to do therapy in the future, you should do this kind of mouse model to verify the effect on knockdown or restoration of repression, and whether it can really affect your therapy.”
[Illustration created by Jianjun Chen]
Li, Z., Huang, H., Chen, P., He, M., Li, Y., Arnovitz, S., Jiang, X., He, C., Hyjek, E., Zhang, J., Zhang, Z., Elkahloun, A., Cao, D., Shen, C., Wunderlich, M., Wang, Y., Neilly, M., Jin, J., Wei, M., Lu, J., Valk, P., Delwel, R., Lowenberg, B., Le Beau, M., Vardiman, J., Mulloy, J., Zeleznik-Le, N., Liu, P., Zhang, J., & Chen, J. (2012). miR-196b directly targets both HOXA9/MEIS1 oncogenes and FAS tumour suppressor in MLL-rearranged leukaemia Nature Communications, 2 DOI: 10.1038/ncomms1681