Education

How mRNA Vaccines Work – Gene Transcription And Translation

Conventional vaccines typically involve growing a virus in various cell lines, injecting a part of the virus into the host, and relying on the host to mount an immunologic response (ie, create antibodies and “memory”) for the next time the virus is encountered. This process can be time consuming, expensive, and hazardous when scaling up production.

In contrast, the COVID-19 vaccines by Pfizer and Moderna are messenger RNA (mRNA) vaccines. This approach is faster, safer, cheaper, and widely scalable… all important for a pandemic which has infected over 55 million people worldwide at the time of this writing. The genetic sequence of a portion of the virus (in this case, SARS-CoV-2’s spike protein) is determined and the corresponding mRNA sequence is generated in laboratories. This mRNA relies on the cell’s normal machinery (namely ribosomes) to translate it into the protein.

DNA is transcribed into RNA in a cell’s nucleus. RNA is translated into proteins by the cell’s ribosomes. That’s what gene expression boils down to.


TRANSCRIPTION – THE LANGUAGE

The human genome consists of the entire set of nucleic acid sequences spread over 23 paired chromosomes in every single one of our cells. The genome has over 20,000 genes which each code for proteins like receptors, enzymes, ion channels, etc. Much of the genome is not actually involved in coding for protein but provides stability and regulatory functions . Additionally, not all genes are expressed in every cell (ie, kidney cells express different genes than brain cells).

When a gene needs to be translated to a protein, the relevant portion of the double-stranded DNA helix is unwound and an enzyme called RNA polymerase creates a complementary strand of pre-mRNA. This is called transcription. The initial pre-mRNA undergoes processing (sequences called introns are spliced out, a poly(A) tail is added to one end, and a 7-methylguanosine cap is added to the other end). These events signal the mRNA is ready to be exported out of the nucleus into the cell’s cytosol for the next step – translation! There are untranslated regions (UTRs) of mRNA which are very important in regulating the translation process too.


TRANSLATION TO PROTEINS

mRNA is a language of four “letters” (or what are known as nucleotides): adenine (A), uracil (U), guanine (G), and cytosine (C). DNA shares the same language except thymine (T) replaces uracil. A and U (or T) pair with two hydrogen bonds. G and C pair with three hydrogen bonds. These letters make a language of three-letter words (called codons). For example: AAU-AUG-UAC-GGA-GGG-GAC-UGA-CUU (24 nucleotides, 8 codons). In this language, AUG is the start codon and UAA, UGA, or UAG are all stop codons. In other words, AUG starts the sentence and UAA, UGA, or UAG are all punctuation options at the end.

Now let’s introduce a group of translators: transfer ribonucleic acids (tRNAs). These molecules contain different anticodon regions which complements the codons on the mRNA. For example, a codon that reads CCA will “link” to a tRNA anticodon which reads GGU. The other part of tRNA carries an amino acid – the building blocks of proteins. The following diagram shows how different codons code for different amino acids.

The genetic code (Image: Khan Academy)

For example, the start codon (AUG) codes for the amino acid methionine (Met). The codons GUU, GUC, GUA, and GUG all code for the amino acid valine. In other words, there are four different ways to write for valine in the mRNA language. As the ribosome moves from codon-to-codon, tRNAs are adding amino acids to a growing polypeptide chain. Ultimately when the ribosome arrives at a stop codon, the protein is released.


mRNA Vaccines

The upcoming mRNA vaccines rely on the aforementioned machinery which our cells normally use to create proteins from genetic material. In fact, they’ll be the first ever approved mRNA vaccines by the Food and Drug Administration. Keep in mind that the accelerated timeline is primarily due to parallel research, development, and distribution funded heavily by the government to minimize financial risk.

mRNA is fairly unstable and easily degraded by the body, so much of the vaccine development involves transporting the mRNA vaccine into the cell for ribosomes and tRNAs to read. This is done by using lipid nanoparticles to “shield” the mRNA and modifying the nitrogenous base/pentose sugar structure of the nucleotides (remember, the letters in the mRNA language). Furthermore, the vaccines are stored and transported at very cold temperatures (-70°C for the Pfizer vaccine, -20°C for the Moderna vaccine) for additional stability.

This mRNA codes for receptor binding domains of SARS-CoV-2’s spike protein. After this mRNA has been translated by ribosomes, the synthesized protein creates a robust CD4+ and CD8+ T cell response as well as antibodies! Furthermore, the initial mRNA vaccine ultimately is degraded and is not incorporated into the cellular genome.

Many questions remain to be answered regarding the duration of antibody titers and long term side effects, but hopefully you found this overview helpful! Drop me a comment below with questions! 🙂

Related Articles

16 Comments

  1. Hi Dr. Kumar, I was wondering if you had any insight into the mutated covid 19 strain that is being talked about in the UK and in parts of the US. Will the vaccine be affective against this new strain? Thanks for all of your educational materials, I really appreciate it!

    1. Short answer is we don’t know. It’s hard to believe that the vaccine will be rendered completely ineffective given that the goal is to form antibodies to various domains on the spike protein. If the mutations involved some of these regions, perhaps the antibodies will still be effective if they’re targeting another part of the protein. Time will tell!

  2. Hi, My questions relates to mRNA translation. I have not been able to find any information on translation errors where mRNA may be misinterpreted by tRNA resulting in a error of translation in creating amino acids / protein, and the potential implications of translation errors for vaccine effectiveness / safety? I know that translation errors can occur randomly in general (not vaccine specific) but have not seen anything about this in relation to the vaccines?

    1. Theoretically, each time the mRNA sequences are translated, there’s an independent chance of errors occurring (codon-anticodon matching incorrectly). As rare as this is, it’s possible. I guess hypothetically, you may have an amino acid that’s substituted for another one creating a protein that “looks” like a spike protein. There have already been several modifications made to the wild type spike protein mRNA to improve stability in the vaccine. You’re going to make antibodies to different regions of the spike protein in general, so if an error does occur at a single amino acid location within the peptide chain, I really doubt it would make a huge difference in the grand scheme of things since you’re having plenty of properly synthesized spike protein.

      I think it would be ridiculously hard to study this question in vivo. You’d essentially have to vaccinate, allocate a sample of the translated spike proteins, map all of their sequences, and see which ones were aberrantly translated.

  3. Once the ribosome transcribes the mRNA, where is the s-protein expressed? If the spike protein is a cell surface protein, is it expressed on the surface of the cell which hosted the mRNA? And, is the nano-encapsulation specific to certain body cells, or is this RNA able to be incorporated into all body cells

    1. From my understanding, it’s expressed on the surface of deltoid muscle cells (the same cells into which the mRNA was delivered). The expressed spike protein generates the immune response, but the original mRNA will be degraded (ie, it does NOT incorporate into the host’s genome).

      1. But, as a new cell surface protein, does that not make the host cell itself now a target for autoimmune attack?

  4. Hi Rishi, Do you think a person with monoclonal gammopathy is safe to take the COVID vaccine? My mother is 82, has MGUS, diabetes, and HTN. I am concerned about the abnormal m protein confusing the vaccine protein and maybe causing harm. I’ll have labs drawn to see if I also have MGUS. Both planned on getting the vaccine. Thoughts? And thank you so much for your always awesome educational discussions!

    1. Hey Katherine! MGUS is something way beyond my scope (I think I’ve seen it in the ICU < 5 times, remember more about it from med school, haha), so I would definitely ask your mother's specialist (hematologist, oncologist, or whoever has been overseeing her MGUS). Stay safe! 🙂

  5. Amazing read Dr. Kumar. I do have a question. How are the pentose sugars or nitrogenous bases modified for greater stability? Are the lipid vesicles phagocytized by macrophages due to their foreign nature? How do they guarantee that the mRNA will be translated before getting degraded?

  6. Hi Dr. Rishi Kumar, can you please post a link to some sources talking about the post translational life of the mRNA as well as the lipid nanoparticle and nitrogenous base/pentode sugar modification aspect of the vaccine?

Leave a Reply

Your email address will not be published. Required fields are marked *

Check Also
Close
Back to top button