Messenger RNA (mRNA) serves as a temporary script copied from a cell’s DNA blueprint, providing instructions for building proteins. After the initial transcription of DNA into mRNA, a secondary layer of control can be exerted through mRNA editing. This process allows a cell to make precise changes to the nucleotide sequence of an mRNA molecule before it is translated into a protein. This editing can alter the encoded information, enabling a single gene to produce multiple protein variants or modulating how a gene is expressed.
Understanding the Process of mRNA Editing
mRNA editing modifies the chemical letters, or nucleotides, of an mRNA strand. Alterations include the substitution of one base for another or the insertion and deletion of nucleotides. The most common types of edits in vertebrates are base substitutions, where a single nucleotide is changed into another. This process occurs naturally within cells across all living organisms.
One prevalent type of substitution is adenosine-to-inosine (A-to-I) editing. In this process, an enzyme converts the adenosine (A) base into a different base called inosine (I). When the cellular machinery reads the edited mRNA to build a protein, it interprets inosine as guanine (G), which can change the resulting protein’s amino acid sequence. Another common modification is cytidine-to-uridine (C-to-U) editing, where a cytidine (C) base is converted to a uridine (U).
These natural editing events allow organisms to increase their proteomic diversity, creating different proteins from the same gene. For example, C-to-U editing of the apolipoprotein B gene results in two different proteins: a long form in the liver and a shorter, truncated form in the intestine, which is created when the edit introduces a premature “stop” signal. Scientists are now learning to harness these naturally occurring cellular systems, designing ways to direct these editing enzymes to specific mRNA targets of their choosing.
Key Technologies Enabling mRNA Modification
To direct mRNA edits with precision, scientists have developed molecular tools based on natural biological systems. One technology adapts the CRISPR system, known for its DNA-editing capabilities. Instead of targeting DNA, systems using an enzyme called Cas13 are engineered to recognize and bind to specific RNA sequences.
Guidance for this system comes from a programmable guide RNA (gRNA). The gRNA contains a sequence complementary to the target mRNA, delivering the Cas13 enzyme to a precise location. Once positioned, the system enacts a change by using a Cas13 protein fused to an editing enzyme, such as an adenosine deaminase (ADAR) or a cytidine deaminase.
These fusion systems include REPAIR (for A-to-I editing) and RESCUE (for C-to-U editing). In a REPAIR system, the Cas13-gRNA complex locates the target mRNA, and the attached ADAR enzyme performs the A-to-I conversion on a nearby adenosine nucleotide. This approach combines the targeting precision of CRISPR with natural editing enzymes, providing a programmable platform for altering mRNA transcripts.
Applications in Medicine and Scientific Research
The ability to modify mRNA has applications in medicine and research. Therapeutically, mRNA editing offers a strategy for correcting genetic diseases at the RNA level. This approach is explored for conditions like cystic fibrosis and Duchenne muscular dystrophy, where correcting an error in the mRNA could restore a functional protein.
This method can be applied to neurological and inflammatory disorders by modulating protein function. For example, A-to-I editing of neurotransmitter receptor mRNAs can influence their function and impact synaptic activity.
In scientific research, mRNA editing is used for investigating gene function. Researchers can use these technologies to turn genes “on” or “off” at the protein level or create specific protein variants to study their roles in cellular processes. This allows for exploring a gene’s impact without permanently altering the organism’s genetic code.
Distinctions from DNA Editing Approaches
Both mRNA and DNA editing technologies alter genetic information, but they operate on different molecules with distinct consequences. The primary difference is the permanence of the changes. DNA editing, performed with tools like CRISPR-Cas9, makes permanent alterations to a cell’s genomic DNA that are passed down to subsequent cells.
In contrast, mRNA editing targets the transient mRNA molecule, so any edits are temporary. The effects last only as long as the modified mRNA molecules persist, from hours to days. Once treatment stops, the cell reverts to producing the original, unedited mRNA from its unchanged DNA template.
This transient nature has safety implications. With mRNA editing, the risk of unintended, permanent off-target modifications to the genome is reduced because the DNA is not altered. The reversible effects are an advantage in therapeutic development where transient intervention is preferred. This positions mRNA editing as a complementary approach to DNA editing, suited for different applications.