Genetic modification of the human germ line that will pass to children, and to future generations,has sparked a lively debate in the international scientific community.
Genome editing consists of the modification or removal of specific DNA sequences in order, for example, to correct a disease-causing mutation. Early approaches were based on the recognition of specific sequences using oligonucleotides, small molecules or self-splicing introns.New techniques were later developed based on DNA sequence recognition by proteins, such as site-directed zinc finger nucleases (ZFNs) and transcription activator-like effector nucleases (TALENs). These methods are based on the binding of a protein DNA cleavage domain to a zinc finger or TALE DNA binding domain, respectively, which has been modified to target the desired DNA sequence. However, difficulties in the design, synthesis and validation of these proteins are an obstacle to the widespread adoption of these artificial nucleases.
One new genome editing technique is CRISPR (clusteredregularlyinterspaced short palindromicrepeat)/Cas9,which is based on a system discovered in bacteria that gives them adaptive immunity against viruses.In natural bacterial systems, some genome sequences of viruses that infect the bacterium are incorporated into the bacterial genome between the CRISPR sequences, so that if the same virus attacks it again, the bacterium produces an immune response that includes a copy of the “remembered” sequences, called crRNA, to bind to the virus DNA, and a second RNA, called tacrRNA, which recruits a Cas endonuclease to cut the virus DNA.The technique, patented by Jennifer Doudna and Emmanuelle Charpentier in 2012, consists of modifying the tacrRNA:crRNA pair as a single guide RNA (sgRNA) with a sequence at the 5′ end that determines the DNA target sequence and a duplex RNA structure at the 3′ end, which binds to Cas9.Thus, the leader sequence can be modified to carry the Cas9 endonuclease to any DNA sequence.
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