The Earth emerged 4.5 billion years ago, and life less than a billion years after that. Although life as we know it depends on four major macromolecules – DNA, RNA, proteins and lipids – it is believed that only one was present at the beginning of life: RNA.
It’s no surprise that RNA probably came first. It is the only one of those large macromolecules that can replicate itself and catalyze chemical reactions, both of which are essential for life. Like DNA, RNA is made of individual nucleotides connected in chains. Scientists initially understood that genetic information flows in one direction: DNA is transcribed into RNA and RNA is translated into proteins. That principle is called the central dogma of molecular biology. But there are many deviations.
A major example of an exception to the central dogma is that some RNAs are never translated or encoded into proteins. This fascinating derivation of the central dogma has led me to devote my scientific career to understanding how it works. Indeed, research into RNA has lagged behind other macromolecules. Although several classes of these so-called non-coding RNAs exist, researchers like me have begun to pay close attention to short stretches of genetic material called microRNAs and their potential to treat various diseases, including cancer.
MicroRNAs and diseases
Scientists consider microRNAs to be master regulators of the genome because of their ability to bind to and alter the expression of many protein-coding RNAs. Indeed, a single microRNA can regulate 10 to 100 protein-coding RNAs. Instead of translating DNA into proteins, they can instead bind to protein-coding RNAs to silence genes.
The reason microRNAs can regulate such a diverse collection of RNAs stems from their ability to bind to target RNAs to which they do not match perfectly. This means that a single microRNA can often regulate a collection of targets that are all involved in similar processes in the cell, leading to an enhanced response.
Because a single microRNA can regulate multiple genes, many microRNAs can contribute to disease when they become dysfunctional.
In 2002, researchers first identified the role that dysfunctional microRNAs play in diseases in patients with a type of blood and bone marrow cancer called chronic lymphocytic leukemia. This cancer results from the loss of two microRNAs that are normally involved in blocking the growth of tumor cells. Since then, scientists have identified more than 2,000 microRNAs in humans, many of which are altered by various diseases.
The field has also developed a fairly good understanding of how microRNA dysfunction contributes to disease. Changing one microRNA can alter several other genes, resulting in a plethora of changes that can collectively reshape the cell’s physiology. For example, more than half of all cancers have significantly reduced activity in a microRNA called miR-34a. Because miR-34a regulates many genes involved in preventing the growth and migration of cancer cells, loss of miR-34a may increase the risk of developing cancer.
Researchers are exploring the use of microRNAs as therapies for cancer, heart disease, neurodegenerative diseases and others. Although results in the laboratory are promising, bringing microRNA treatments into the clinic has faced several challenges. Many are related to inefficient delivery into target cells and poor stability, limiting their effectiveness.
Delivering microRNA to cells
One reason why it is difficult to deliver microRNA treatments into cells is because microRNA treatments must be delivered specifically to diseased cells, while healthy cells must be avoided. Unlike mRNA COVID-19 vaccines that are absorbed by clearing immune cells whose job is to detect foreign materials, microRNA treatments must fool the body into thinking they are not foreign to a prevent immune attack and reach the targeted cells.
Scientists are studying different ways to deliver microRNA treatments to their specific target cells. One method receiving a lot of attention is based on directly linking microRNA to a ligand, a type of small molecule that binds to specific proteins on the cell surface. Compared to healthy cells, diseased cells may contain a disproportionately large number of surface proteins or receptors. Ligands can thus help microRNAs target diseased cells specifically, while avoiding healthy cells. The first ligand approved by the US Food and Drug Administration to deliver small RNAs such as microRNAs, N-acetylgalactosamine or GalNAc, preferentially delivers RNAs to liver cells.
Identifying ligands that can deliver small RNAs to other cells requires finding receptors expressed on the surface of target cells at high enough levels. Normally, more than one million copies per cell are needed to achieve sufficient drug release.
One ligand that stands out is folic acid, also called vitamin B9, a small molecule that is crucial during periods of rapid cell growth, such as fetal development. Because some tumor cells have more than a million folate receptors, this ligand provides ample opportunity to deliver enough therapeutic RNA to target different types of cancer. For example, my lab developed a new molecule called FolamiR-34a – folic acid linked to miR-34a – that reduced the size of breast and lung cancer tumors in mice.
Making microRNAs more stable
One of the other challenges in using small RNAs is their poor stability, leading to their rapid degradation. As such, RNA-based treatments in the body are generally short-lived and require frequent doses to maintain a therapeutic effect.
To overcome this challenge, researchers modify small RNAs in several ways. Although each RNA requires a specific modification pattern, successful changes can significantly increase its stability. This reduces the need for frequent dosing, reducing treatment burden and costs.
For example, modified GalNAc siRNAs, another form of small RNAs, reduce dosing from every few days to once every six months in non-dividing cells. My team developed folic acid ligands linked to modified microRNAs for the treatment of cancer, reducing the dosage from once every other day to once a week. For diseases such as cancer where cells divide rapidly and rapidly dilute the microRNA delivered, this increase in activity is a significant advance in the field. We expect that this achievement will facilitate further development of this folate-linked microRNA as a cancer treatment in the coming years.
Although much work remains to be done to overcome the hurdles associated with microRNA treatments, it is clear that RNA holds promise as a therapeutic agent for many diseases.
This article is republished from The Conversation, an independent nonprofit organization providing facts and analysis to help you understand our complex world.
It was written by: Andrea Kasinski, Purdue University.
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Andrea Kasinski receives funding from the National Institutes of Health, the Department of Defense, and the American Lung Association. Kasinski is also the inventor of several patients related to her discoveries in RNA therapy.