Earth formed 4.5 billion years ago, and life formed less than a billion years later. Although life as we know it depends on four major macromolecules (DNA, RNA, proteins, and lipids), only one is thought to be present at the beginning of life: RNA.
It’s no surprise that RNA probably came first. It is one of the large macromolecules that can both replicate itself and catalyze the chemical reactions necessary for life. Like DNA, RNA is made of individual nucleotides linked in chains. Scientists initially understood that genetic information flows in one direction: DNA is copied into RNA, and RNA is translated into proteins. This principle is called the central dogma of molecular biology. But there are many deviations.
An important exception to the central dogma is that some RNAs are never translated or encoded into proteins. This fascinating departure from central dogma is what led me to devote my scientific career to understanding how this works. Indeed, research on RNA has lagged behind other macromolecules. Although there are multiple classes of these non-coding RNAs, researchers like me have begun to show great interest in short pieces of genetic material called microRNAs and their potential to treat a variety of diseases, including cancer.
MicroRNAs and diseases
Scientists consider microRNAs to be master regulators of the genome due to their ability to bind to and alter the expression of many protein-coding RNAs. In fact, a single microRNA can regulate between 10 and 100 protein-coding RNAs. Instead of translating DNA into proteins, they can silence genes by binding to protein-coding RNAs.
The reason microRNAs can regulate such a diverse pool of RNAs is due to their ability to bind to target RNAs to which they do not match perfectly. This means that a single microRNA can regulate a pool of targets that are often 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.
Researchers first identified the role dysfunctional microRNAs play in the disease in 2002 through patients with a type of blood and bone marrow cancer called chronic lymphocytic leukemia. This cancer is caused by the loss of two microRNAs that normally play a role in blocking tumor cell growth. Since then, scientists have identified more than 2,000 microRNAs in humans, many of which are altered in various diseases.
The field has also developed a fairly solid understanding of how microRNA dysfunction contributes to disease. Changing one microRNA can cause many other genes to change, resulting in numerous changes that can collectively reshape the physiology of the cell. For example, more than half of all cancers have significantly reduced activity in a microRNA called miR-34a. Because miR-34a regulates many genes that play a role in preventing the growth and migration of cancer cells, losing miR-34a may increase the risk of developing cancer.
Researchers are exploring using microRNAs therapeutically for cancer, heart disease, neurodegenerative disease, and others. While results in the laboratory have been promising, bringing microRNA therapies to the clinic has faced many challenges. Many are associated with inefficient delivery to target cells and poor stability, limiting their effectiveness.
Delivery of microRNA to cells
One reason microRNA therapies are difficult to deliver into cells is that microRNA therapies must be delivered specifically to diseased cells while avoiding healthy cells. Unlike mRNA COVID-19 vaccines, which are received by clearing immune cells whose job it is to detect foreign substances, microRNA treatments need to trick the body into thinking they are not foreign in order to prevent immune attack and reach the targeted cells.
Scientists are investigating various ways to deliver microRNA therapies to their specific target cells. One method that has attracted great attention is based on the direct binding of microRNA to a ligand, a type of small molecule that binds to specific proteins on the surface of cells. Compared to healthy cells, diseased cells may have a disproportionate number of certain surface proteins or receptors. That is, the ligands may help microRNAs specifically localize to diseased cells 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 at sufficiently high levels on the surface of target cells. Typically more than a million copies per cell are needed to ensure adequate delivery of the drug.
One prominent ligand is folate, also called vitamin B9, a small molecule that is critical during periods of rapid cell growth, such as fetal development. Since more than one million folate receptors are present in some tumor cells, this ligand provides ample opportunity to deliver sufficient amounts of therapeutic RNA to target different types of cancer. For example, my laboratory developed a new molecule called FolamiR-34a (miR-34a-bound folate) that reduced the size of breast and lung cancer tumors in mice.
Making microRNAs more stable
One of the other difficulties in using small RNAs is their poor stability, which leads to their rapid degradation. Therefore, RNA-based therapies are generally short-lived in the body and require frequent doses to maintain therapeutic effect.
To overcome this challenge, researchers are modifying small RNAs in various ways. Although each RNA requires a specific modification pattern, successful modifications can significantly increase their stability. This reduces the need for frequent dosing, thereby reducing treatment burden and cost.
For example, modified GalNAc-siRNAs, another form of small RNAs, reduce dosage in non-dividing cells to every six months instead of every few days. My team developed folate ligands linked to modified microRNAs that reduce dosing from every other day to once a week for cancer treatment. For diseases such as cancer, where cells divide rapidly and delivered microRNA is rapidly diluted, this increase in activity is a significant advance in the field. We predict that this success will facilitate the further development of this folate-related microRNA as a cancer treatment in the coming years.
While significant work still needs to be done to overcome the hurdles associated with microRNA therapies, it is clear that RNA holds promise as a therapeutic tool for many diseases.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and authoritative analysis to help you understand our complex world. The Conversation has a wide range of fascinating free newsletters.
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 associated with his discoveries in the field of RNA therapeutics.