We've only just discovered that they're in our blood, but tiny strands of nucleic acid might hold the key to early disease detection.
Illustration by Olivia Baenziger
You've probably given blood at some point in your life. Blood banks hold donation drives to stock their stores for patients. We think nothing of it. Someone with a bad wound, a haemophiliac or a person needing surgery can expect to get a transfusion of somebody else's blood and expect it to go flawlessly.
If you look back a hundred years, however, transfusions weren’t so reliable. Transferring blood from one person to another was a risky business and often killed the recipient. Nobody knew why. It wasn’t until the early 1900s that blood was sorted into types — A, B, and O — based on whether it clotted upon mixing. The molecules that cause the deadly reaction weren’t found for decades more. But with that critical knowledge of how formerly-invisible ingredients in blood interact, health workers made swift improvements and learned to save many more lives.
A century later, our blood still has microscopic surprises for us.
One such surprise is micro-RNA, tiny strands of nucleic acids which are usually found inside our cells. Recently, they were also discovered floating around in blood and other fluids. A healthy level of micro-RNA keeps our cells working normally, but if there’s too much or too little, we get sick. Researchers have learnt that when diseases as serious as cancer appear in a body, they can find abnormal levels of micro-RNA as well. But what’s the connection?
It’s early days, but scientists might be on the verge of leveraging micro-RNA to detect disease earlier. Or maybe even treat it.
Meet micro-RNA
It wasn’t until the early 1990s that Victor Ambros and Gary Ruvkun described these very short fragments of ribonucleic acids (RNA) for the first time. The ensuing 20 years saw biochemists reveal the micro-RNA’s function — keeping a variety of cell processes in check. It works by sticking to other messenger RNA molecules that the cell uses to communicate, either destroying them or making them too hard to read.
Nucleic acids like DNA are made out of individual molecular units called base pairs. A strand of DNA in your cells might have millions of base pairs, where a piece of mRNA that codes for a single protein might have a thousand. In comparison, micro-RNA is typically only about 22 base pairs long.
Ribonucleic acid (RNA) is a nucleic acid similar to DNA. Rather than a paired double-strand, it exists as a single-strand folded onto itself. Micro-RNA is then small fragments of the RNA chain. Vossman/Wikimedia Commons (CC BY-SA 3.0)
At first, micro-RNA studies focused on what it was doing inside the cell. These early investigations in the late 1990s and early 2000s revealed micro-RNA’s role as a regulator of cell processes. The next logical question surfaced: what happens when micro-RNA isn't where it should be? Researchers turned their gaze to cancer. Sure enough, specific kinds of micro-RNA can often be found inside cancer cells in either much higher or much lower amounts than normal. The same story holds for heart disease. Alcoholism. Obesity.
The associations between micro-RNA and diseases are now so various and appearing so rapidly that there’s a public database dedicated to keeping track of them. As of July 21, 2017, the miR2Disease database cites nearly 3300 micro-RNA-disease relationships across 163 diseases and 349 micro-RNA types.
Then the focus widened again, as, in 2007, Professor Siu Chung Stephen Chim’s team found micro-RNA outside of cells, floating in other biological tissues like blood. No longer confining their search within cells, the following year saw three independent groups find that cancer patients had elevated levels of specific micro-RNA fragments circulating in their blood. Somewhat strangely, they often found these far away from the site of cancerous tumours.
This baffled scientists. Generally, cell biologists have a pretty good idea about how RNA works inside a cell, but when it comes to micro-RNA or RNA outside of cells, our understanding gets a little foggy. We now know that cells can actually secrete small chains of RNA into our bloodstream and do so all the time. But what this circulating RNA is doing there is still an open question. If micro-RNA’s job is to interfere with communication between the cell nucleus and the ribosome, what’s it doing in blood?
Early research suggests circulating micro-RNA might be an important part of how cancer spreads from one body part to another. Scientists have watched healthy cells absorb micro-RNA from foreign sources and use it as if it was their own. Some researchers think that cells communicate with one another via the blood using circulating micro-RNA as a language. Cells in healthy tissue might do this for a few reasons.
Cancer cells also secrete micro-RNA, and when healthy cells absorb the cancerous micro-RNA their defences against cancer go down. If the predictions are correct, disrupting micro-RNA messages from cancer cells could be a way to stop it from spreading.
A red flag for disease detection?
Scientists might not have nailed down just what the micro-RNA in your blood is doing, but they do know how to find it when they look for it. One of the most intriguing aspects of micro-RNA research examines how levels of micro-RNA in the blood change while someone is sick. If we find that a particular piece of RNA shows up reliably in the same place and in similar concentrations for people with a specific disease, then it could be used as a biological indicator of that disease — a biomarker.
This is where micro-RNA could really come in useful. Circulating micro-RNA decomposes less readily than many currently used disease biomarkers, which are mostly proteins and larger nucleic acids. It also has the benefit of being easily measured via a blood test and more specific than current biomarkers — which would mean less stressful tests for patients.
“Biofluid-based biomarkers that actually work in the clinic to favourably change patient care, have historically been hard to come across,” says Prof Tewari from the University of Michigan. But the presence or absence of specific micro-RNA fragments could be used as biomarkers for disease. (The green strands are micro-RNA.) Ryan Jeffs/Wikimedia Commons (CC BY-SA 3.0)
But according to Associate Professor Muneesh Tewari from the University of Michigan, connecting the dots between a micro-RNA’s presence in the blood and a particular disease is only the first step. There’s a lot more science to be done before micro-RNA biomarkers can be useful to detect diseases clinically. “To make that a reality,” Tewari said, “there are certainly biological questions that still need answers, like how variable the candidate [biomarkers] are between people and what underlies that variability.”
Tewari has harboured a curiosity about micro-RNA for a decade. His team was one of the first to connect circulating micro-RNA to cancer, and in 2008 his group established a 'proof-of-principle' for using a specific kind of micro-RNA to tell the difference between a patient with prostate cancer and a healthy one via biomarkers in their blood. The method works because prostate tumours produce this fragment of RNA and release it into surrounding fluids like blood and semen, but it isn’t present in such high amounts in those without the disease.
So, now there’s a new blood test for cancer?
Not quite. “The bar for that is higher than what we did,” said Tewari.
The first crop of studies made it look like micro-RNA biomarkers were going to be a smash hit, but as more research surfaced, the picture became much more complicated. Associate Professor Kenneth Witwer from Johns Hopkins University pointed out in 2014 that the same micro-RNA Tewari used in the prostate cancer study also increases in the blood of pregnant people, as well as in people with breast, lung, and colon cancer. Many micro-RNA targets share the same problem: the information provided by the micro-RNA is just not specific enough and is too variable between people for a broad clinical application.
Basically, we still need to know more.
For more precision, up the frequency
Disease detection normally relies on physical observations of the disease (like a tumour in an expensive MRI) or on the symptoms it creates in a patient. While it might not be on the cards just yet, the promise of a molecule that that doesn't degrade easily, that can be easily measured and that also detects what disease might be at play, is still worth chasing for micro-RNA researchers.
A single snapshot of the micro-RNA in your blood isn't likely to be helpful, says Assoc Prof Muneesh Tewari from the University of Michigan. Instead, he thinks we should measure how this changes over time in order to be more accurate. Yellowstone National Park/Flickr (Public domain)
But Tewari doesn’t think we’ll be able to do it with a single test. A one-time test relies on being able to see a huge change above the baseline for what’s healthy. Getting accurate information about whether or not there is a disease, or whether a treatment is working, requires more data: a picture of the moon doesn’t tell you whether it’s waxing or waning. Tewari believes that moving away from the prevailing 'snapshot' model of testing patients and into a strategy where biomarkers are measured much more often, could be a huge step forward for all biomarkers, including micro-RNA. Tewari suggests that smaller fluctuations observed over a longer period might contain important information that scientists are currently overlooking.
To do this, clinicians would use cheap, routine tests like blood tests and urine samples to collect information on micro-RNA biomarkers. Tewari suggests that patients might also collect their own blood or urine samples at home. Patients like diabetics who already measure their glucose levels with blood glucometers could submit data that way. Tewari even envisions a device attached to a home toilet that could measure biomarkers in urine. While maybe not popular with everyone, the samples from that particular method could always be frozen and sent to a lab for low-cost automated analysis.
Tewari acknowledges that the idea is futuristic — and certainly a non-traditional way of collecting samples. But with a bank of data collected in such a way, Tewari hopes we'll be able to learn more about the fundamental biochemistry of biomarkers as well as develop care strategies for patients. “I believe this strategy will add to our biological knowledge of the tempo of disease evolution.”
The micro-RNA research frontier is an exciting one, and one with great promise if the high variance patient-to-patient can be worked out. There's still so much we don't know: including why micro-RNA is getting into our blood in the first place. Once we know that, we might begin to target these short, tangled strands in order to fight the diseases they came from.
Edited by Tessa Evans
