In 1993, two post-doctoral researchers named Victor Ambros and Gary Ruvkun independently published back-to-back papers in the December 3 issue of the journal Cell. In their papers, they described how the roundworm Caenorhabditis elegans uses a small RNA molecule to control the production of a protein.
While the work was certainly novel, it did not receive much attention at the time because other scientists thought the phenomenon was unique to worms and of no practical relevance to understanding its role in other life-forms, including humans.
It was not until seven years later that Ruvkun found a similar mechanism existed in nearly all of the animal kingdom. The paper created waves in the scientific community since it represented a whole new paradigm in molecular biology, with potentially far-reaching implications on human health and disease.
Last week, Ambros and Ruvkun were awarded the Nobel Prize in Physiology or Medicine for their discovery of microRNA and the latter’s role in gene regulation, a process universal to all cells.
What, when, where, why
Every cell in an organism contains a copy of its DNA, the blueprint for how to build and maintain that organism. The building and maintenance activities are achieved by molecules called proteins; the DNA contains instructions on how cells can make these proteins.
Every protein has a specific function. For instance, haemoglobin is responsible for carrying oxygen from the air we breathe to the cells in the body. Each set of instructions to make a given protein from the organism’s total DNA is called a gene.
The DNA of humans has between 19,000 and 20,000 genes. While all cells in the body contain all these genes, and thus the information on how to make all the proteins, no cell makes all 20,000 proteins. Gene expression — the process of reading the information in a gene to make a protein — is specific to cell types. A given cell will only make those proteins it needs for its function. Thus the red blood cells make haemoglobin but not the cells that make up the stomach.
When a cell wants to make a protein, it first makes a transient copy of the gene called the messenger RNA (mRNA). The information in the mRNA is then used to make the protein. This process of making an mRNA copy of the information in the gene is called transcription. A gene is transcribed to mRNA to make a protein only in those cells where that protein is required.
Once the mRNA is made, the cell will continue to make proteins until it is stopped. The protein production process must be stopped when enough proteins have been made because if it isn’t controlled, excess protein, apart from being a waste of resources, can be harmful to the cell.
For a long time, this halting of protein production, called post-transcriptional gene regulation, was thought to occur when the mRNA degrades — either on its own (due to its low stability) or aided by special enzymes that the cell makes.
Ambros and Ruvkun essentially identified a new way in which cells regulate protein production. They discovered the existence of tiny RNA molecules, called microRNA (miRNA) that bind to mRNAs and prevent protein synthesis.
A vital cellular process
Chemically, a miRNA is made of the same material that makes up mRNA. The difference lies in their sizes: RNA is composed of a combination of four chemical bases arranged on a sugar-phosphate backbone, rather like a long bead of strings made of four coloured beads arranged at random. Their length is therefore measured in how many beads, or bases, they contain. Thus, mRNAs range from hundreds to lakhs of bases while the average miRNA is just 22 bases long.
The composition of these 22 bases — or the order of arrangement of the beads on the string — depends on which mRNA a given miRNA is going to target. Usually, the sequence of bases of an miRNA is complementary to a stretch of bases on the target mRNA, making it specific to that mRNA. Once the miRNA binds to its target, the target mRNA is either marked for destruction or is unable to serve as a template to produce protein, thus switching protein production off. This way, if needed, miRNAs can inhibit the synthesis of a given protein even before it begins.
Since Ruvkun’s report of the first human miRNA in 2000, researchers have discovered thousands of new miRNAs, playing roles in regulating almost 60% of all human genes.
Switching off protein production at the right time is a vital cellular process. Therefore it was no surprise when researchers found miRNAs to play pivotal roles in animal development, the differentiation of cells into their correct types, cell division, cell death, and — importantly — response to stress and disease, especially in various cancers.
The high specificity of miRNAs made them ideal candidates for targeted therapies for conditions like cancer, which involve abnormal protein production. But despite their potential, the story of the research on the clinical utility of miRNAs does not have a very happy beginning.
RNA is important
The rapid academic progress on miRNAs prompted scientists to test the therapeutic potential of miRNAs. Early experiments in mice gave encouraging results, where researchers were able to inhibit the formation of lung tumours using miRNAs.
The first clinical trial of a human miRNA, called miRNA-34a, soon followed in 2013. But the technology to deliver the mRNA to the target cells was not as well developed then as it is now; as a result scientists had to administer extremely high doses of the molecule to ensure a small amount would reach the target site. This had the unfortunate consequence of triggering an immune response. When four patients died, the investigators immediately stopped the trial.
Scientists later made significant advances in packaging and delivering miRNA, allowing others to test multiple other miRNAs against various diseases — including hepatitis C, multiple cancers, and cardiovascular diseases.
When Ambros and Ruvkun won the Nobel Prize last week, 581 clinical trials involving miRNAs had been registered in the U.S. Of these, 215 had been completed and 20 had been terminated over safety concerns.
Since other alternatives are available for most of these conditions, miRNA’s time in medicine has yet to come. Hopefully the Nobel Prize will change this field’s fortunes: despite the challenges it faces in therapy, miRNAs’ relevance to physiology and medicine is unquestionable. This is why Ambros and Ruvkun were awarded the Nobel Prize despite the absence of therapeutic applications.
This is also the fifth instance of a Nobel Prize being awarded for RNA research: mRNA vaccines won in 2023; RNA interference in 2006; RNA’s role as enzymes in 1989; the discovery of mRNA in 1965. Indeed, scientists are slowly understanding that RNA, not DNA, is at the core of the delicate balance cells must maintain.
Arun Panchapakesan is an assistant professor at the Y.R. Gaithonde Centre for AIDS Research and Education, Chennai.
Published - October 17, 2024 05:30 am IST
