by The Earth was created 4.5 billion years ago, and life 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 have been 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 to life. Like DNA, RNA is made of single nucleotides linked in chains. Scientists first realized 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.
One major example of an exception to the central dogma is that RNAs are not translated or encoded into proteins. It is this wonderful diversion from the central dogma that led me to dedicate my scientific career to understanding how it works. In fact, research on RNA has lagged behind other macromolecules. Although there are many classes of these so-called non-coding RNAs, researchers like myself have begun to focus a lot of attention on short stretches of genetic material called microRNAs and their potential to cause various diseases. treatment, including cancer.
MicroRNAs and disease
Scientists view microRNAs as master regulators of the genome because of their ability to bind to and alter many protein-coding RNAs. In fact, a single microRNA can regulate anywhere from 10 to 100 protein-coding RNAs. Besides translating DNA into proteins, they can bind to protein-coding RNAs to silence genes.
The reason that our microRNAs can regulate such a wide variety of RNAs, stems from their ability to bind to target RNAs with which they are not a perfect match. This means that a single microRNA can often regulate a pool of targets that are all involved in similar processes in the cell, resulting in an enhanced response.
Because a single microRNA can regulate multiple genes, many microRNAs can contribute to disease when they are dysfunctional.
In 2002, researchers first identified the role that dysfunctional microRNAs play in disease through 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 inhibiting 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 relatively strong understanding of how microRNA dysfunction contributes to disease. Altering a single microRNA can alter several other genes, resulting in a multitude of changes that can collectively reshape the physiology of the cell. For example, over half of all cancers have decreased activity in a microRNA called miR-34a. Because miR-34a regulates many genes involved in inhibiting the growth and migration of cancer cells, loss of miR-34a may increase the risk of developing cancer.
Researchers are looking to use microRNAs as therapies for cancer, heart disease, neurodegenerative disease and others. Although the 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, which limits their effectiveness.
Delivery of microRNA to cells
One reason microRNA treatments are difficult to deliver into cells is because microRNA treatments must be delivered specifically to diseased cells while avoiding healthy cells. Unlike COVID-19 mRNA vaccines that are taken by scavenging immune cells that aim to detect foreign materials, microRNA treatments must trick the body into thinking they are not foreign to avoid immune attack and the intended cells achieve them.
Scientists are studying different ways to deliver microRNA treatments to their specific target cells. One method of gaining a lot of attention relies on linking the microRNA directly to a ligand, a type of small molecule that binds to specific proteins on the surface of cells. Compared to healthy cells, diseased cells can have a disproportionate number of certain surface proteins, or receptors. Thus, ligands can help microRNAs home specifically to diseased cells and avoid healthy cells. The first ligand approved by the US Food and Drug Administration to deliver small RNAs such as microRNAs, N-acetylgalactosamine, or GalNAc, delivers RNAs to liver cells.
To identify ligands that can deliver small RNAs to other cells, it is necessary to find receptors that are expressed at high enough levels on the surface of target cells. Typically, over a million copies per cell are required to achieve adequate delivery of the drug.
One ligand that stands out is folate, also known as vitamin B9, a small molecule that is critical 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 – folate 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 with the use of small RNAs is their poor stability, which causes them to degrade rapidly. Therefore, RNA-based treatments tend to be short-lived in the body and require frequent doses to maintain a therapeutic effect.
To overcome this challenge, researchers are modifying small RNAs in a variety of ways. Although each RNA requires a specific modification pattern, successful changes can significantly increase their stability. This reduces the need for frequent dosing, which in turn reduces treatment burden and cost.
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 folate ligands linked to modified microRNAs for cancer treatment that reduced doses from once every other day to once a week. For diseases such as cancer where cells are rapidly dividing and rapidly depleting the delivered microRNA, 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 of microRNA treatments, it is clear that RNA shows promise as a therapeutic for many diseases.
This article is republished from The Conversation, a non-profit, independent news organization that brings you reliable facts and analysis to help you make sense of our complex world. The Conversation has a variety of free newsletters.
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 multiple patients related to her discoveries in the field of RNA therapy.
