The wall above that photocopier located at the University of Pennsylvania may eventually feature a plaque that reads something like 鈥渋t was here that Katalin Kariko and Drew Weissman met in 1998 and forged a partnership that would lead to millions of lives being saved by modified RNA Covid-19 vaccines and result in the duo being awarded that 2023 Nobel Prize in Medicine and Physiology.鈥澛�
Dr. Weissman was a young physician who had trained under Dr. Anthony Faucci at the National Institutes of Health (NIH) where he had developed an interest in immunology and found himself waiting to use a photocopier alongside Dr. Kariko, a Hungarian biochemist who had come to the U.S. in 1985. The two started to chat and Weissman mentioned that he was looking into the possible use of messenger RNA to develop a vaccine against AIDS to which Kariko responded that she had become adept at making such molecules. That chance meeting would lead to a collaboration that resulted in a finding that turned out to be critical step in the march towards developing a successful COVID vaccine. That march makes for a fascinating story.聽
Just about everything that happens in our body relies on the family of molecules we call proteins. Indeed, the term 鈥減rotein鈥� derives from the Greek for 鈥渋n first place.鈥� Proteins are polymers, long chain-like molecules with the component links being amino acids. Since the chains can be of various lengths with all sorts of twists and turns, and feature twenty amino acids that can be incorporated in diverse sequences, the number of possible protein structures is immense. Some proteins are the building blocks of muscles, some make up our hair and skin, some play key roles in immune function, some are hormones and others act as enzymes, the biochemical catalysts that control the myriad chemical reactions that go on in our body all the time and together constitute life. The instructions for making all these proteins are encoded in molecules of deoxyribonucleic acid (DNA) found in the nucleus of every cell. DNA is indeed the 鈥渂lueprint of life.鈥澛�
DNA does not synthesize proteins directly, rather it transfers the instructions for assembling the amino acids to molecules of 鈥渕essenger ribonucleic acid (mRNA)鈥�. These molecules then migrate out of the cell鈥檚 nucleus into the cytoplasm where they serve as a template for constructing proteins by little factories called ribosomes. The requirement for protein synthesis can now be seen as the presence of the appropriate mRNA in the cytoplasm.聽
What if that mRNA could be synthesized outside the body, in a lab, and be delivered directly into the cell? Then the protein for which that RNA codes would be produced just as if the mRNA had come from the cell鈥檚 nucleus bearing instructions from DNA. If that synthetic mRNA codes for the production of the 鈥渟pike protein鈥� found on the surface of the SARS-CoV-2 virus, then this protein will be produced and will go on to stimulate the production of antibodies against it by B cells of the immune system. Should the coronavirus then at some future time invade the body, the antibodies will recognize its spike proteins and latch onto them preventing them from interacting with receptors on the surface of cells. Since that interaction is a necessary prelude for the virus鈥� entry into a cell, infection is avoided. This is the principle behind both the Pfizer and Moderna vaccines.聽
The name 鈥淢oderna鈥� derives from 鈥渕odified RNA,鈥� which brings up the question of what 鈥渕odified鈥� means in this context. RNA is a polymer, the building blocks of which are simple molecules called nucleotides, analogous to how proteins are composed of amino acids. It is the sequence in which the four possible nucleotides, namely adenosine, guanosine, uridine, and cytidine are strung together that constitutes the code for protein synthesis by ribosomes.聽
When Chinese researchers published the entire genome of the SARS-CoV-2 virus, attention was immediately drawn to the sequence of nucleotides that code for the spike protein. This was regarded as the best candidate as the target for a potential vaccine since it was a good bet that the virus uses this protein to gain entry into cells where it can then hijack the cell鈥檚 reproductive machinery and begin to replicate. Now the task was to synthesize the mRNA that codes for the spike protein and introduce it into the body, essentially tricking the immune system into believing it has been attacked by a virus resulting in antibody formation.聽
These days, building RNA from nucleotides in the lab is not a major problem. What is needed is a DNA template that can be derived from the decoding of the genome of an organism, a supply of nucleotides and some RNA polymerase enzymes, the same enzymes cells themselves use to synthesize RNA. There is no doubt that science has come a long way since Dr. Kelvin Ogilvie and associates at 捆绑SM社区 built a 鈥済ene machine鈥� in the 1980s capable of joining a few nucleotides together using automated chemical techniques. That first historic machine now sits in a display case just outside my office and I get to admire it every day!聽
Synthesis of the required mRNA was only the first step towards a vaccine. The challenge was to deliver this relatively unstable molecule into cells intact without causing any adverse reaction, given that any foreign substance, including synthetic mRNA, that intrudes into the body can trigger an attack by the immune system. Indeed, mice reacted adversely when in initial experiments they were treated with synthetic mRNA. Katalin Kariko was intrigued by this immune reaction. She wondered whether it was a specific part of the RNA molecule that was recognized by immune cells as the enemy.聽
It turned out that one of the nucleosides, uridine, was the problem. Could simply replacing this with the chemically similar pseudouridine, which also occurs in the body, be the solution? Working with Drew Weissman, Kariko found that this alteration did the trick. The ability of this 鈥渕odified鈥� mRNA to synthesize proteins was actually enhanced and the immune reaction to it was significantly reduced. Furthermore, the modified RNA was more resistant to enzymes that normally degrade mRNA. Modifying RNA in this fashion was a major breakthrough and led to other similar modifications that were critical to the development of the Pfizer and Moderna vaccines.聽
While RNA can exit from a cell鈥檚 nucleus, it cannot enter it. However, even if it could, there is no way for our genes to be 鈥渕odified,鈥� since mRNA does not incorporate into DNA. Aside from RNA鈥檚 instability and possible activation of unwelcome immune reactions, there was another problem. RNA does not easily penetrate the cell membrane which is made of fatty substances. That problem was addressed by Dr. Pieter Cullis of the University of British Columbia and MIT biomechanical engineer Robert Langer, who was a co-founder of Moderna. They independently found that encapsulating RNA in various fatty substances would allow passage into the cell鈥檚 cytoplasm. This paved the way for the lipid nanoparticle encapsulated mRNA used in the novel COVID-19 vaccines. The components of the nanoparticles, as well as the mRNA are sensitive to heat, which is why these vaccines have to be maintained at a very low temperature.聽
When Isaac Newton was once asked how he had been able to come up with his 鈥渂reakthrough鈥� discoveries, he replied that had taken advantage of standing on the shoulders of giants. I think Weissman and Kariko would give a similar answer when asked about their contribution. The successful COVID vaccines were the culmination of years and years of research by numerous scientists who all added pieces to the puzzle. Weissman and Kariko certainly added a key piece and there is no question that they merit the Nobel Prize, but there were undoubtedly vigorous discussions among Nobel committee members about whether a third person should be included. My guess is that there were so many equally worthy candidates that the choice was too difficult and they decided to just go with Weissman and Kariko whose work was certainly pivotal.聽聽
Katalin Kariko deserves recognition not only for the impact of her research but for perseverance! Her position at the University of Pennsylvania as an adjunct professor was always on shaky grounds and she had such poor success with grant applications that she had to rely on the support of colleagues for funding. In the face of numerous stumbling blocks she persevered, pushing the envelope of mRNA research. At one point, the University had so little regard for her research that she was politely asked to retire. Not the usual career path for a Nobel Laureate! But eventually both Moderna and Pfizer recognized the value of her research and the rest, as they say, is history. Dr. Kariko鈥檚 grit and love of science is set to infect generations of students.聽
This story does not end here. Messenger RNA is destined to find numerous applications beyond vaccines. Cancer cells, for example, express proteins on their surface that are different from ones on normal cells and mRNA molecules can be designed to code for these proteins and stimulate an immune reaction that will then also target the cancer cells. Stay tuned for more future Nobel Prizes in this area. Just one more thing. Hopefully the recognition of the benefits of mRNA vaccines by the Nobel Prize Committee made up of some of the world鈥檚 top scientists will blunt the evidence-starved attacks of the anti-vaxxers on a technology that has already saved millions of lives and will save millions more in the future.聽聽
