Change of grounds
A new method of editing mitochondrial DNA has been developed
Tatiana Matveeva, "Scientific Russia"
Korean scientists have developed a new platform for gene editing – TALED. With its help, they were able to exchange adenine for guanine – the nitrogenous bases of DNA – in mitochondria, reports Institute of Fundamental Sciences (Institute for Basic Science). This discovery will make it possible to treat some human genetic diseases. The development is described in detail in the journal Cell (Cho et al., Targeted A-to-G base editing in human mitochondrial DNA with programmable deaminases).
Mitochondria are tiny organelles in cells that serve as factories for energy production. They have their own genome. Mutations in mitochondrial DNA cause serious genetic diseases associated with energy metabolism: for example, hereditary Leber optic neuropathy (LHON), which causes sudden blindness in both eyes, or mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS), which slowly destroys the patient's brain. Some studies even suggest that abnormalities in mitochondrial DNA may also be the cause of degenerative diseases such as Alzheimer's disease and muscular dystrophy.
The mitochondrial genome is inherited through the maternal line. There are 90 known point mutations of mitochondrial DNA that cause diseases. Many existing genome editing tools could not be used in mitochondria. For example, the "molecular scissors" of CRISPR-Cas cannot edit mutations in this organelle, since the guide RNA cannot penetrate into the mitochondria itself. In addition, there are not enough animal models of these mitochondrial diseases.
The new TALED technology makes it possible to overcome these difficulties. The "Editor" includes three different components. The first component is an effector similar to a transcription activator (TALE), which is able to target a DNA sequence. The second component is TadA8e –adenine deaminase, which facilitates the conversion of adenine into guanine. The third component, DddAtox, is cytosine deaminase, which makes DNA more accessible to TadA8e.
One of the interesting aspects of TALED is the ability of TadA8e to perform A–G editing in mitochondria that possess double-stranded DNA (dsDNA). This is a mysterious phenomenon, since TadA8e is a protein characteristic only of single—stranded DNA. No one had ever thought of using TadA8e to edit bases in mitochondria before, the authors note. And it is assumed that DddAtox makes double-stranded DNA available due to short-term unwinding of the double chain. This allows TadA8e, an ultra-fast enzyme, to make the necessary changes quickly.
The group showed how the new technology works by creating a single cell clone containing the desired modifications of mitochondrial DNA (mtDNA). In addition, it was found that TALEDS are not cytotoxic and do not cause mtDNA instability. In addition, TALED's "scissors" did not affect nuclear DNA. Now researchers are striving to improve the technology by increasing the efficiency and specificity of editing. Ultimately, this will make it possible to correct disease-causing mtDNA mutations in an embryo, fetus, newborn or adult patient. The group is also developing TALED, suitable for editing bases in the DNA of chloroplasts, which encodes important photosynthesis genes in plants.
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