Nobel Prize in Physiology or Medicine 2024: MicroRNA and Gene Regulation

This note covers the Nobel Prize in Physiology or Medicine 2024: who won it, what microRNA is and how it switches genes off after they have already been copied into messenger RNA, how the discovery unfolded in a tiny roundworm, why it matters for human health, and quick facts for exams.

What was the Nobel Prize in Physiology or Medicine 2024 awarded for?

The Nobel Assembly at Karolinska Institutet awarded the prize jointly to Victor Ambros and Gary Ruvkun, with the official citation: "for the discovery of microRNA and its role in post-transcriptional gene regulation".

In plain words, the two scientists found a brand new class of very small RNA molecules, now called microRNA, that cells use to control which genes are active.

This control happens after a gene has already been copied into messenger RNA (mRNA), which is why the committee called it "post-transcriptional" regulation; it is not about switching a gene on at the DNA level but about silencing the mRNA message once it already exists.

The official name of this award is the Nobel Prize in Physiology or Medicine. It was announced on 7 October 2024 and is one of the oldest and most prestigious science prizes in the world, given since 1901 to discoveries of greatest benefit to humankind.

Who are the laureates?

Victor Ambros

Victor Ambros was born on 1 December 1953 in Hanover, New Hampshire, USA. At the time of the award he was affiliated with UMass Chan Medical School, Worcester, Massachusetts, USA, where he holds the title of Silverman Professor of Natural Science. He received one half of the prize.

Ambros did his PhD with David Baltimore on poliovirus, then joined Robert Horvitz's laboratory as a postdoctoral fellow, where he began studying genes that control the timing of development in a tiny roundworm.

He later set up his own laboratory at Harvard University, where his group cloned the lin-4 gene and discovered that it produced a short RNA molecule rather than a protein, the first microRNA ever found.

Gary Ruvkun

Gary Ruvkun was born in 1952 in Berkeley, California, USA. At the time of the award he was affiliated with Massachusetts General Hospital and Harvard Medical School, both in Boston, Massachusetts, USA, where he is Professor of Genetics. He also received one half of the prize.

Ruvkun completed his PhD in bacterial genetics, then worked as a postdoctoral fellow jointly in the laboratories of Walter Gilbert and Robert Horvitz.

He set up his own laboratory at Massachusetts General Hospital and Harvard Medical School, where he showed that the lin-14 gene was regulated not at the level of RNA production but at a later stage, through the shutdown of protein-making.

In 2000 his laboratory also discovered a second, highly conserved microRNA called let-7.

What problem were Ambros and Ruvkun trying to solve?

Every cell in the human body carries the same chromosomes and therefore exactly the same set of genes. Yet a muscle cell and a nerve cell look and behave very differently.

The press release explains that this puzzle is solved by gene regulation, which allows each cell to use only the instructions it needs, so that each cell type switches on its own particular set of genes.

By the mid-twentieth century scientists already knew one major way this happens: proteins called transcription factors bind to specific regions of DNA and control which mRNAs get made in the first place. This mechanism itself had earned François Jacob and Jacques Monod the Nobel Prize in Physiology or Medicine in 1965.

This was so well established that, for a long time, researchers believed the main principles of gene regulation had already been worked out, since thousands of transcription factors had by then been identified across many organisms.

Ambros and Ruvkun were not originally hunting for a new regulatory mechanism. Victor Ambros had completed a PhD with David Baltimore on the poliovirus genome before joining Robert Horvitz's laboratory, while Gary Ruvkun had finished a PhD in bacterial genetics under Frederick Ausubel and grew curious about worm genetics while travelling, later discussing it with Martin Chalfie and Horvitz before starting postdoctoral work jointly in the laboratories of Walter Gilbert and Horvitz.

In the late 1980s, as postdoctoral fellows in Horvitz's laboratory, they were simply curious about how different cell types develop at the right time in a small, 1 mm long roundworm called Caenorhabditis elegans (C. elegans), an organism first introduced to genetics by Sydney Brenner decades earlier.

Despite its tiny size, this worm has specialised cell types such as nerve and muscle cells, making it a useful model for studying development in more complex animals generally.

They focused on two mutant worm strains, lin-4 and lin-14, both of which had defects in the timing of their genetic developmental programmes; lin-4 mutant worms reiterated earlier developmental stages and failed to form a vulva, while lin-14 mutants lacked proper larval development altogether. Ambros had already shown that lin-4 seemed to switch off lin-14, but nobody knew how.

Solving that mystery is what led, unexpectedly, to the discovery of an entirely new layer of gene control.

How does microRNA actually work?

To understand microRNA, it helps to first picture the normal route that genetic information takes.

The press release describes a flow of genetic information from DNA to messenger RNA (mRNA) to protein: DNA is first copied into mRNA in a process called transcription, and the mRNA is then read by the cell's protein-making machinery, which assembles proteins according to the genetic instructions.

Draw a strand of DNA inside a cell nucleus, an arrow labelled "transcription" leading to a messenger RNA (mRNA) strand, and a second arrow labelled "translation" leading to a protein chain being built outside the nucleus.

Add a small microRNA molecule binding to the mRNA strand with a "block" symbol, showing that it stops the mRNA from being turned into protein.

What Ambros and Ruvkun found disturbed this simple picture. When Ambros's laboratory cloned the lin-4 gene, they discovered it did not code for a protein at all.

Instead it produced an unusually short RNA molecule, eventually measured at just 22 nucleotides long, that was never meant to be translated. Meanwhile, Ruvkun showed that lin-14 was not being blocked at the stage of mRNA production; the block happened later, at the stage of protein-making itself, through elements located in the untranslated end of the lin-14 mRNA.

When the two laboratories compared their sequence data, on the evening of 11 June 1992, they found the explanation: the short lin-4 RNA had a sequence that was complementary to a region in the lin-14 mRNA, meaning the two could stick together like matching puzzle pieces.

The lin-4 microRNA was binding directly onto the lin-14 mRNA and physically preventing it from being used to build protein. In his presentation speech, Professor Rickard Sandberg of the Nobel Committee compared microRNAs to unusual conductors of a cellular orchestra, saying they do not amplify the music but instead tell certain genes "play softer" or to take a break.

The general steps by which a microRNA silences a target gene can be summarised as follows:

1. A microRNA gene is transcribed into a long "primary" RNA with a hairpin-shaped fold.
2. This hairpin is cut down inside the cell nucleus into a shorter precursor microRNA, roughly 60 to 70 nucleotides long.
3. The precursor is moved out of the nucleus into the cytoplasm and cut again, by an enzyme called Dicer, to produce the short, mature microRNA.
4. The microRNA is loaded onto a protein complex that scans mRNA molecules for a matching, complementary sequence, usually through a short stretch called the "seed" region.
5. Once the microRNA finds and binds its target mRNA, the complex either blocks protein production from that mRNA or causes the mRNA itself to be broken down.

A single microRNA can act on many different target genes, and a single gene can be controlled by several different microRNAs at once, which lets cells fine-tune whole networks of genes rather than switching single genes on or off one at a time.

What is the lin-4 and let-7 story, step by step?

The discovery did not happen all at once; it built up through two separate but connected worm genes, studied years apart.

This table shows why the let-7 discovery mattered so much: it proved microRNA was not just an oddity of one small worm, but a mechanism shared across very different animals, including people.

The let-7 gene itself encodes a short, 21-nucleotide RNA that pairs with the untranslated regions of several timing genes in the worm, including lin-14 itself. When Ruvkun's laboratory compared its sequence against databases of other species, matching sequences turned up in the fruit fly and in humans, and the gene later proved to be active, at the right developmental stage, in zebrafish, molluscs and other animals with left-right body symmetry.

That wide conservation convinced the scientific community that microRNAs were not a one-off worm curiosity. Within a few years, several laboratories began systematically cloning small RNAs from humans, fruit flies and worms, and the tally of known human microRNA genes grew rapidly from a handful to the over one thousand now recognised.

How did the discovery unfold?

Why does microRNA matter for health and disease?

Scientists now know that the human genome contains more than a thousand different microRNA genes, and this type of gene regulation is universal among multicellular organisms, from plants to animals. According to the press release, microRNA regulation has been essential for the evolution of increasingly complex, multicellular life.

When microRNA regulation goes wrong, serious problems can follow. Abnormal regulation by microRNA can contribute to cancer, and mutations in genes coding for microRNAs have been linked in humans to congenital hearing loss and to certain eye and skeletal disorders.

Mutations in the seed sequence of the miR-96 gene are associated with progressive hearing loss, and mutations in miR-184, which cause a rare eye condition called EDICT syndrome, marked by iris abnormalities, corneal changes and congenital cataract. Mutations in miR-140-5p have likewise been linked to a congenital skeletal disorder.

Mutations in one of the proteins needed to produce microRNAs cause a rare but severe condition called DICER1 syndrome, which is linked to cancers in several different organs and tissues, including the kidney, thyroid, ovary, cervix, testicle, brain, eye and lung.

The committee also noted that understanding gene regulation in general has long been an important scientific goal, because faulty regulation can underlie diseases such as cancer, diabetes and autoimmune conditions.

The discovery of microRNA added a completely new, previously unknown layer to this picture, one that works after the mRNA has already been made rather than at the DNA stage, and work continues on using microRNAs as diagnostic markers and possible treatment targets for conditions including heart disease and nervous system disorders.

How does this connect to related Nobel Prizes?

This discovery sits alongside several other Nobel-recognised breakthroughs in gene regulation. In 1965, François Jacob and Jacques Monod won the Nobel Prize in Physiology or Medicine for showing how transcription factors regulate genes at the DNA level, the mechanism that was considered the main story before 1993.

In 2002, Robert Horvitz, together with Sydney Brenner and John Sulston, won the same prize for using C. elegans to work out how cell division and cell death are genetically controlled, the very research tradition in which Ambros and Ruvkun trained as postdoctoral fellows. Brenner had introduced C. elegans as a model organism decades earlier because it grows and reproduces quickly, is easy to see through under a microscope, and is simple to modify genetically.

In 2006, Andrew Z. Fire and Craig C. Mello won the prize for discovering RNA interference, a related but distinct process in which double-stranded RNA silences specific genes. The scientific background notes that, while this mechanism shares much of its molecular machinery with microRNA silencing, RNA interference works mainly as a defence against viruses and unwanted mobile genetic elements, whereas microRNAs instead carry out everyday fine-tuning of normal gene networks throughout development and adult life.

Taken together, these three prizes, spanning 1965 to 2024, trace a widening picture of gene regulation: first regulation at the DNA stage by transcription factors, then the genetic control of cell division and death discovered through the same small worm, and finally this entirely unexpected layer of post-transcriptional control carried out by microRNAs, which Ambros and Ruvkun first revealed through patient, curiosity-driven work on two obscure worm mutants.

How does this connect to what you study?

If you study biology topics such as the cell, DNA, genes and protein synthesis, this discovery extends exactly that chain: DNA makes mRNA, mRNA makes protein, and microRNA is an extra control switch sitting on top of that chain.

The worm C. elegans, used by Ambros and Ruvkun, is also widely used in biology as a simple model organism for studying how genes control development, since many of its cell types resemble those found in larger animals.

Understanding microRNA also connects to topics on genetic disorders and cancer biology, since faults in microRNA genes or in the machinery that produces them, such as DICER1 syndrome, show how a tiny change in gene regulation can have large effects on health throughout the body.

How is a microRNA produced and loaded onto its target?

Beyond the lin-4 story, later research by several laboratories worked out the general machinery that every microRNA passes through before it can silence a gene.

Many microRNA genes sit on their own in the genome, sometimes in clusters, while others are hidden inside the introns of ordinary protein-coding genes. Either way, the gene is first copied by the same enzyme that makes most mRNA, producing a long "primary" RNA that folds back on itself into a hairpin shape.

Inside the nucleus, a protein complex containing an enzyme called Drosha trims this hairpin down to a shorter precursor microRNA, which is then carried out of the nucleus into the cytoplasm by a transport protein.

In the cytoplasm, a second cutting enzyme called Dicer (first identified in Greg Hannon's laboratory) trims the precursor further, producing a short double-stranded microRNA. One strand of this pair is then loaded onto a silencing complex built around a protein called Argonaute, while the other, "passenger" strand is discarded.

Once loaded, the Argonaute-containing complex uses the microRNA as a guide to find matching mRNA sequences. Helper proteins then shorten the tail of the target mRNA, which leads either to the mRNA being broken down or to its translation into protein being blocked, depending on the cell type and stage of development.

The same general silencing machinery is also used by cells, and by plants in particular, to produce other small defensive RNAs that protect against virus infection, showing that nature reused one toolkit for several related jobs.

Quick facts for exams

The Nobel Prize in Physiology or Medicine 2024 was awarded jointly to Victor Ambros and Gary Ruvkun for discovering microRNA and its role in post-transcriptional gene regulation.

The prize was announced on 7 October 2024 by the Nobel Assembly at Karolinska Institutet, which consists of 50 professors and awards this prize every year. Each laureate received one half of the prize, which totalled 11,000,000 Swedish kronor.

Ambros was affiliated with UMass Chan Medical School, USA, while Ruvkun was affiliated with Massachusetts General Hospital and Harvard Medical School, USA.

Their 1993 discovery in a tiny roundworm revealed that short RNA molecules can silence genes by binding to messenger RNA, a mechanism now known to involve over a thousand genes in humans.

Note: Source. The prize facts in this note are from the Nobel Prize's official site, nobelprize.org.

Glossary

• Gene regulation — the set of mechanisms a cell uses to control which genes are active and when.
• Transcription — the process by which the DNA sequence of a gene is copied into a messenger RNA (mRNA) molecule.
• Messenger RNA (mRNA) — the RNA copy of a gene that carries instructions from DNA to the cell's protein-making machinery.
• Translation — the process by which mRNA instructions are used to build a protein.
• MicroRNA — a tiny RNA molecule that does not code for protein but instead binds to mRNA to block or degrade it.
• Post-transcriptional regulation — control of gene activity that happens after mRNA has already been made, rather than at the DNA stage.
• Transcription factor — a protein that binds to specific DNA regions to switch genes on or off during transcription.
• Complementary sequence — a sequence of RNA or DNA that can pair exactly or partially with another sequence, like matching puzzle pieces.
• C. elegans — a tiny roundworm about 1 mm long, widely used as a model organism to study animal development.
• lin-4 and lin-14 — two genes in C. elegans whose study led to the discovery of the first microRNA.
• let-7 — the second microRNA discovered, found to be conserved across many animal species including humans.
• DICER1 syndrome — a rare inherited disorder caused by a mutation affecting microRNA production, linked to tumours in several organs.

Common errors and misconceptions

• Misconception: MicroRNA controls genes by switching DNA on or off. Correct: MicroRNA acts after transcription, on the mRNA message, not directly on DNA.
• Misconception: MicroRNA was discovered in human cells first. Correct: It was first discovered in the tiny roundworm C. elegans, through the lin-4 gene.
• Misconception: The discovery was immediately recognised as important. Correct: The 1993 findings were initially met with little interest, and it was the 2000 discovery of let-7 that showed microRNA was widespread and important.
• Misconception: One microRNA controls only one gene. Correct: A single microRNA can regulate many genes, and one gene can be controlled by several microRNAs.
• Misconception: Ambros and Ruvkun worked entirely independently of each other. Correct: They were both postdoctoral fellows in the same laboratory and later compared sequence data that led directly to their joint breakthrough.
• Misconception: MicroRNA and RNA interference (RNAi) are exactly the same mechanism. Correct: They are related but distinct; RNAi, recognised by the 2006 Nobel Prize, mainly defends against viruses, while microRNA regulates normal gene networks.

Exam-style questions with model answers

Q1. What was the official citation for the Nobel Prize in Physiology or Medicine 2024? [2 marks]

1. The citation was "for the discovery of microRNA and its role in post-transcriptional gene regulation".
2. The prize was awarded jointly to Victor Ambros and Gary Ruvkun.

Q2. Name the two laureates and state one affiliation each at the time of the award. [2 marks]

1. Victor Ambros was affiliated with UMass Chan Medical School, USA.
2. Gary Ruvkun was affiliated with Massachusetts General Hospital and Harvard Medical School, USA.

Q3. Explain what a microRNA is and how it differs from a normal messenger RNA. [4 marks]

1. A microRNA is a tiny RNA molecule that does not code for a protein, unlike a normal messenger RNA (mRNA), which carries instructions for building a protein.
2. Instead of being translated, a microRNA binds to a target mRNA through a complementary sequence.
3. This binding either blocks the mRNA from being translated into protein or leads to the mRNA being broken down.
4. In this way, microRNA acts as a regulator that fine-tunes gene activity after transcription has already occurred.

Q4. Describe the roles of lin-4 and lin-14 in the discovery of microRNA. [4 marks]

1. Both lin-4 and lin-14 are genes in the roundworm C. elegans that showed defects in the timing of development when mutated.
2. Ambros had shown that lin-4 acted as a negative regulator of lin-14, but the mechanism was unknown.
3. Ambros's laboratory cloned lin-4 and found it produced a short RNA rather than a protein, while Ruvkun's laboratory showed lin-14 was regulated at the protein-making stage.
4. Comparing their data revealed that the lin-4 RNA sequence was complementary to part of the lin-14 mRNA, explaining how lin-4 silences lin-14, which was the discovery of the first microRNA.

Q5. Discuss why the 2000 discovery of let-7 was important for the field of microRNA research. [5 marks]

1. After the 1993 discovery of lin-4, the scientific community largely ignored microRNA, treating it as a quirk specific to the C. elegans worm.
2. In 2000, Ruvkun's laboratory discovered a second microRNA gene, let-7, in the same worm.
3. Unlike lin-4, let-7 was found to be highly conserved across many different animal species, including humans.
4. This conservation suggested that microRNA regulation was not a worm-specific oddity but a general feature of multicellular animals.
5. The finding sparked renewed interest, leading over the following years to the identification of hundreds and eventually over a thousand human microRNA genes, firmly establishing microRNA as a universal layer of gene regulation.

Q6. How can faults in microRNA regulation affect human health? [5 marks]

1. Because microRNA fine-tunes which genes are active, abnormal microRNA regulation can disturb normal cell behaviour.
2. Abnormal regulation by microRNA can contribute to cancer.
3. Mutations in specific microRNA genes have been linked to congenital hearing loss and to certain eye and skeletal disorders in humans.
4. Mutations in one of the proteins required for producing microRNAs cause DICER1 syndrome, a rare but severe condition linked to tumours in several organs and tissues, including the kidney, thyroid and lung.
5. These examples show that a regulatory system first found in a tiny worm has direct, serious consequences for human disease when it malfunctions.

Key takeaways

• Victor Ambros and Gary Ruvkun won the 2024 Nobel Prize in Physiology or Medicine for discovering microRNA.
• MicroRNA regulates genes after transcription, by binding to messenger RNA rather than to DNA.
• The discovery began with the lin-4 and lin-14 genes in the tiny roundworm C. elegans.
• The two researchers found in 1993 that lin-4 produces a short RNA that binds a complementary sequence in lin-14 mRNA.
• The 1993 finding was largely overlooked until the 2000 discovery of the conserved let-7 microRNA.
• Humans are now known to carry over a thousand different microRNA genes.
• Faults in microRNA regulation are linked to cancer, hearing loss, and rare syndromes such as DICER1 syndrome.
• The discovery added a completely new layer to the previously known DNA-to-protein regulation story.

Test yourself

What does "post-transcriptional" mean in the prize citation?

It means the regulation happens after mRNA has already been transcribed from DNA, acting on the mRNA itself rather than on the gene.

Which worm did Ambros and Ruvkun study?

They studied Caenorhabditis elegans (C. elegans), a tiny roundworm about 1 mm long used as a model organism.

What did Ambros's laboratory find when it cloned the lin-4 gene?

It found that lin-4 produced a short RNA molecule instead of a protein, which turned out to be the first microRNA discovered.

In what year were the first microRNA findings published, and in which journal?

The findings were published in 1993 in two papers in the journal Cell.

Why was the discovery of let-7 in 2000 significant?

Let-7 was found to be highly conserved across many animal species, including humans, showing microRNA regulation was universal, not unique to one worm.

Name one human disease or syndrome linked to microRNA problems.

DICER1 syndrome, a rare disorder linked to tumours in several organs, caused by a mutation affecting microRNA production.