On October 7, 2024, the Nobel Prize in Physiology or Medicine was awarded to Professor Victor Ambros from the University of Massachusetts Medical School and Professor Gary Ruvkun from Harvard Medical School for their groundbreaking discovery of microRNA (miRNA) and its role in post-transcriptional gene regulation. This marks the second time that RNA-mediated post-transcriptional regulation has been recognized with a Nobel Prize, following the 2006 award to Andrew Z. Fire and Craig Mello for their discovery of RNA interference (RNAi).

Figure 1. The Nobel Prize in Physiology or Medicine

This year's award has sparked debate among researchers. Some argue that the mechanisms of miRNA and small interfering RNA (siRNA) significantly overlap. Both are small RNA molecules that bind to messenger RNA (mRNA) to regulate gene expression by inhibiting translation or causing degradation, thus interfering with gene expression at the post-transcriptional level. Furthermore, both miRNA and siRNA rely on proteins like Dicer and Argonaute to form the RNA-induced silencing complex (RISC), crucial for their function. Therefore, considering that RNAi research was awarded the Nobel Prize 18 years ago, the novelty of miRNA, in terms of its biological mechanism, may seem less revolutionary.

Differences between siRNA and miRNA

However, despite their many mechanistic similarities, RNAi and miRNAs differ significantly in their origins, functions, and biological roles within cells.

Origins and Structure

RNAi: RNAi is initiated by the presence of exogenous double-stranded RNA (dsRNA) which is then processed by enzymes like Dicer into smaller fragments called siRNA; these siRNA molecules then bind to complementary mRNA, leading to its degradation and effectively silencing the expression of the corresponding gene.

miRNA: miRNAs are endogenous single-stranded RNA molecules, usually 20-24 nucleotides in length. They are encoded by the cell’s own genes and initially exist as longer precursor molecules (known as pri-miRNA) in the nucleus. After processing, they become mature miRNAs. Unlike siRNAs, miRNAs typically only partially match their target mRNA, preventing complete degradation and instead reducing translation efficiency.

Mechanism of Action

RNAi: siRNA binds with the RISC, which degrades the target mRNA through a fully complementary sequence match. This precise matching allows RNAi to silence specific genes completely and effectively, a key mechanism in antiviral defense that helps cells recognize and destroy invading viral RNA.

miRNA: miRNA’s regulation is more subtle, as it reduces protein production without fully degrading the target mRNA. miRNA typically binds to partially complementary sequences in the 3' untranslated region (3' UTR) of mRNA, inhibiting translation or making the mRNA unstable. Due to this partial matching, a single miRNA can regulate multiple different genes, performing multilayered control within complex gene networks.

Figure 2. Comparison of siRNA-mediated RNA interference and miRNA-mediated mechanisms. Source: Int. J. Mol. Sci. 2016, 17(5), 719

Function and Applications

RNAi: RNAi mainly targets foreign RNA, especially in plants where it’s critical for antiviral defense. In research and medicine, RNAi is widely used for its precise gene-silencing ability, helping scientists study gene function and develop treatments aimed at specific genes.

miRNA: miRNA plays a crucial role in normal cell development, differentiation, proliferation, and apoptosis. Abnormal miRNA regulation is linked to diseases such as cancer and cardiovascular disorders, making miRNA an important target for potential therapies and diagnostic biomarkers.

Roles in Living Organisms

RNAi: RNAi usually activates under specific conditions, like viral infections. Cells use this mechanism to recognize foreign dsRNA and inhibit its expression by cutting and silencing its genes. RNAi is particularly common in plants and some invertebrates, like C. elegans, but in mammals, it’s primarily used for research and treatment rather than as a natural gene regulation system.

miRNA: miRNA is an endogenous regulator widely present in multicellular organisms, helping maintain cellular balance and manage complex biological processes. miRNA genes are highly conserved through evolution, with homologous miRNA genes shared between humans and other species, highlighting their importance in evolutionary biology.

In summary, while both RNAi and miRNA mechanisms rely on small RNA molecules for gene regulation, they differ significantly in origin, mechanism, and biological function. These differences highlight the independent scientific significance of miRNA’s discovery and widespread application, explaining why miRNA research was awarded the 2024 Nobel Prize, despite its association with the 2006 Nobel Prize-winning RNAi research.

Chanlleges in miRNA Development

Despite considerable advancements in preclinical research, the field of miRNA-based diagnostic and therapeutic applications is still in its early stages. Only a few of these miRNA-based therapies have progressed to clinical development. There are two chanlleges ahead.

miRNA presents a high degree of diversity and complexity. The human genome encodes thousands of miRNAs, each potentially regulating multiple target genes. Furthermore, miRNA expression and regulation are influenced by a range of factors, such as genetic variation, epigenetic modifications, and environmental conditions, adding to the complexity and difficulty of miRNA research. Additionally, although miRNA is associated with various diseases, the specific mechanisms and regulatory networks involved remain incompletely understood.

Delivering miRNA-based drugs to target tissues or cells is a major challenge in drug development. Although several delivery strategies exist, including liposomes, nanoparticles, and conjugate modifications, these methods face practical limitations in application, such as issues with bioavailability, tissue distribution, and immunogenicity. Even with accurate delivery, ensuring that miRNA drugs function effectively within target tissues or cells remains a significant hurdle.

References:

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