Written by Bethany Hoernfeldt, Science Writer
Researchers have successfully facilitated A-to-G base conversion in mitochondrial DNA, ushering in a new era in genome engineering.
Earlier this week, details of a breakthrough mitochondrial DNA editing platform were revealed to the world in Cell. The technology, developed by a team of scientists from the Centre for Genome Engineering, facilitates A-to-G nucleotide conversion. This method draws upon decades of genetics research and could correct 41% of the known disease-causing mitochondrial DNA variants.
Mitochondrial DNA (mtDNA) is maternally inherited and houses genes that are critical to cellular respiration. 90 pathogenic variants have been recorded to date, and they cause disease in at least 1 in 5,000 individuals. As mitochondria are tasked with powering diverse biochemical reactions within the body, these highly specialised organelles are more numerous in energy-demanding tissues such as the muscles, brain, nerves, heart, and eyes. For that reason, mitochondrial genetic disorders can be serious, and they may even afflict multiple organ systems simultaneously.
“There are some extremely nasty hereditary diseases arising due to defects in mitochondrial DNA,” said Director Jin-Soo, the corresponding author on the study. “For example, Leber hereditary optic neuropathy (LHON) – which causes sudden blindness in both eyes – is caused by a simple single point mutation in mitochondrial DNA.”
Ground-breaking techniques such as polymerase chain reaction (PCR) and CRISPR-Cas9 allowed us to study and manipulate DNA on a monumental scale. However, mtDNA editing has proven challenging due to limitations in the method of delivery of genome editing tools to the organelle. An example of an unusable method in this case is CRISPR-Cas9, as guide RNA is unable to penetrate the mitochondrial membrane. This constraint also hinders the development of targeted therapeutics.
“…It is currently not possible to engineer mitochondrial mutations necessary to create animal models,” Director Jin-Soo added. “Lack of animal models makes it very difficult to develop and test therapeutics for these diseases.”
Ushering in a new era of genome engineering
In 2020, researchers from the Broad Institute developed a novel tool that could perform C-to-T conversion in mtDNA without severing the DNA. The technology, named DddA-derived cytosine base editors (DdCBEs), was mostly limited to the TC motif, essentially rendering it a TC-TT converter. Nonetheless, it was an immense achievement that inspired the first author of this week’s published study to accomplish what was long considered impossible.
“We began to think of ways to overcome these limitations,” Dr Sung-Ik said. “As a result, we were able to create a novel gene-editing platform called TALED that can achieve A-to-G conversion. Our new base editor dramatically expanded the scope of mitochondrial genome editing. This can make a big contribution, not only to making a disease model, but also to developing a treatment.”
TALE-linked deaminases (TALEDs) are composed of three components: A transcription-activator-like effector (TALE), which can target a specific DNA sequence; TadA8e, an adenine deaminase that performs A-to-G editing; and a DddA variant, which briefly unwinds the double-stranded DNA so that TadA8e can make the necessary edits. Since TadA8e only binds to single-stranded DNA, no one considered using the protein to perform base editing, until now.
Altogether, the components form an unprecedented instrument in the fight against rare genetic disorders. Even more importantly, the findings remind us that, when armed with human intellect, resolve, and a healthy dose of curiosity, anything is possible.
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