The small non-coding RNAs, microRNAs (miRNAs), are key players in gene regulation, facilitating gene silencing at the transcriptional level. Their impact extends across a range of cellular processes such as cell survival, growth and proliferation. miRNAs are now also thought to have a role in cancer, in both tumor suppressive and oncogenic capacities. Efforts are therefore underway to better understand this process with the goal of harnessing miRNAs for detection, diagnosis and therapy. In a recent review article in Genome Medicine, Frank Slack and Carlos Stahlhut from Yale University, USA, distil the main methods for miRNA profiling, discuss the miRNAs most often altered in a range of tumors and highlight the clinical applications of this growing field of research. Here Slack explains how miRNAs can be exploited for the diagnosis and treatment of cancer and what the future may hold in this regard.
How can microRNA signatures be used in cancer diagnosis and therapeutics?
One of the earliest clinical uses for microRNAs emerged from the observation that cancers had altered microRNA levels. A number of different groups, including our own, have shown that in fact these changes in microRNA levels can be useful diagnostics in cancer. In fact, one of the first products to come to market that was relevant to clinical cancer medicine turned out to be a panel of microRNAs that one could use to measure the different microRNA levels in tumors versus normal tissue from patients. In fact, that’s probably where these discoveries will have the most immediate clinical benefit.
In development, microRNAs are very tissue- and time-specific. What that’s led to is the observation that certain cancer types have particular levels of microRNAs associated with them. This has allowed certain companies to come up with microRNA profiling arrays that would allow, for example, a physician to profile a metastatic lesion and determine with quite amazing accuracy what the primary site of that tumor would have been. If it had come from the lung, for example, it would have a lung-specific pattern of microRNA expression.
More recently, those sorts of microRNA profiling experiments have shown that specific microRNA profiles are associated with better or worse prognosis. This is work that was pioneered by Carlo Croce and Curtis Harris in the early 2000s. I would imagine that, as time progresses, we’ll see more and more of those sorts of tests coming on the market as well. There are tests in development for determining differences between different tumor types. For example, specific ovarian and uterine cancers are very difficult to distinguish histologically, but you can distinguish them on the basis of their microRNA expression patterns. Tests like that will be on the market in the next few years. So the diagnostic use is already in the market. The therapeutic use is a few years down the road, although human clinical trials are underway.
What will be required to take these techniques to the clinic?
I think the diagnostic use is really just a question of physicians feeling comfortable with the tests and ordering them and actually prescribing them for their patients. In terms of the therapeutic use, well there are two angles to that.
The first angle is that it’s quite well known now that in many different cancer types, certain microRNAs get up-regulated or overexpressed. These are microRNAs that we think of as oncogenic microRNAs. One approach is to identify ways to target or knock out or knock down the levels of those oncogenic microRNAs. That particular approach has been fairly well established in pre-clinical mouse models as being a viable approach to cancer therapeutics. There are human clinical trials with molecules that will target overexpressed microRNAs; not necessarily trials in cancer yet, but there are trials in humans to say that these drugs are pretty well tolerated in humans. I would imagine that in a few years, we’ll see cancer Phase I and Phase II trials of antisense molecules that can target these oncogenic microRNAs.
The second side to the coin is that, in many cancer types, specific microRNAs get lost in cancer or have reduced expression levels. These are the microRNAs that function as tumor suppressors. So basically, when they get lost, the cells find themselves more able to form tumors. The strategy here is to think of ways to re-introduce those lost microRNAs into the tumors. One way to do that is to provide the cells with microRNA mimics. These are molecules that basically resemble the microRNAs and can enter into the RNA-induced silencing complex (RISC) and function to restore the levels of those lost microRNAs. That approach has also been shown to be very effective in preclinical models, in particular in the mouse, and in fact in one case these molecules have found their way into human Phase I trials. I would imagine that in the next few years, once again, we will see more Phase I and Phase II clinical trials using microRNA mimics in cancer patients.
Of course, one strategy that definitely needs to be looked at is whether a combination of antisense molecules knocking out oncogenic microRNAs, combined with microRNA mimics to restore the tumor-suppressor microRNAs might be the most effective route. But we’ll see. My feeling is that in the next five years or so we’ll see the results of safety and efficacy trials using these approaches in cancer patients.
What do you think are the key recent advances in the small RNA field, and where do you see the future of this field with respect to cancer?
With respect to cancer, I think there is this growing optimism – and it’s based on some of the earlier work that I mentioned, using these preclinical models – that in fact these RNAs, which are natural products made from our genes, could be effective in cancer therapy. It’s taken maybe seven or eight years since microRNAs were first shown to be part of cancer pathways, for us to actually get to this exciting point. I would say that it’s this idea that microRNAs, as natural molecules – for which the body has had many, many millions of years to work around any sort of off-targeting effects – could be effective therapeutics.
It’s an important point that because they’re natural molecules and because of their long evolutionary history, we and others have not seen any strong side effects for delivering these particular molecules into preclinical models. In certain cases, where human trials have been done, there have been no real strong side effects. This is extremely encouraging. It’s important not only that these be safe, but that they be safe in humans even for long-term treatment.
That’s one very exciting area that I see. One of the really nice things that the microRNA field has brought to the field of science in general is that we shouldn’t dismiss as uninteresting these RNA products that are being made by cells. For years, people saw these small RNA products at the bottom of their gels and dismissed them as trash or junk. We’re fast realising that the human genome makes a huge array of different RNAs and that we should really pay attention to these. The next up and coming field in cancer biology is probably these longer non-coding RNAs; something that we should keep an eye on because they could also be important diagnostics and maybe therapeutics in cancer, down the road.
Who knows what else we’ll discover next? I think it’s just the tip of the iceberg right now for understanding the roles of RNAs in disease. MicroRNAs are showing promise now, but they may not be the best thing. A few years from now, we might find something that’s even more valuable as a diagnostic or more valuable as a prognostic. But I think microRNAs have shown the route here, if you will – that we need to keep exploring. We need to keep identifying these new things. They could be even better down the road.