Today, the CRISPR/Cas9 is the technique for the genetic modification of cells. Many years ago, researchers found the information for the important protein Cas in the neighbourhood of frequently occurring sequences that provide the same information when reading forwards or backwards (palindromes). It originates from a bacterial defense system with which the organism can defend itself against viruses by recognizing the invading sequences, cutting them up and making them harmless. But it also works in all other cells.

Scientists have made use of this mechanism by inserting the RNA for the Cas protein and a guide sequence into the cell with information on where the cell's own DNA is to be cut. The natural repair mechanism then completes the missing piece of DNA and reassembles the ends. DNA building blocks can be removed, exchanged or inserted using this method.

It was said that unintentional cuts in the DNA strand are rare, but they do exist. The CRISPR gene scissors were used to cut the DNA at the target site, but the cell's own reverse transcriptase correction system did not always correct the DNA as precisely as desired. The correction area was often "heterogeneous". Therefore, the procedure for therapeutic purposes for the treatment of human hereditary diseases is too delicate. However, this could also mean that countless genetically modified animals, so-called "disease models", may have had a questionable genotype.

The new method developed by the talent factory of the Broad Institute led by Prof. David Liu, Vice Chairman of the Faculty and Director of the Merkin Institute of Transformative Technologies in Healthcare at the Broad Institute and Dr. Andrew Anzalone, Postdoc at Lius Laboratory of the Broad Institute, is intended to provide a remedy: they simply supply the reverse transcriptase and do not use the cell's own, potentially error-prone transcriptase. CRISPR-Cas9 is combined with reverse transcriptase in one tool. The molecular complex uses a strand of the target DNA site to bring the direct transcript of the processed genetic information into the genome. Using Prime Editing, the scientists returned the mutation of sickle cell anemia directly to the normal sequence and removed four additional DNA bases that cause Tay-Sachs disease, they reported in the journal Nature, where they published their work.

Andrew V. Anzalone, Peyton B. Randolph, Jessie R. Davis, Alexander A. Sousa, Luke W. Koblan, Jonathan M. Levy, Peter J. Chen, Christopher Wilson, Gregory A. Newby, Aditya Raguram & David R. Liu (2019). Search-and-replace genome editing without double-strand breaks or donor DNA. Nature. 2019 Oct 21 DOI: 10,1038/s41586-019-1711-4

Sources:
https://www.broadinstitute.org/news/new-crispr-genome-editing-system-offers-wide-range-versatility-human-cells
https://www.pharmazeutische-zeitung.de/prime-editing-als-hochpraezise-variante-des-crispcas9-systems/

Further information:
https://www.spektrum.de/wissen/gen-editing-die-5-wichtigsten-fragen-zu-crispr-cas9/1441060