Gene editing is a technique that allows for the repair of mutated genes, as opposed to adding new genes. To identify and cleave the mutated sequence, combinations of DNA-binding molecules (proteins or RNA) and endonucleases are used. This cleavage activates the repair of double-stranded DNA breaks through homologous recombination (refer to page 429), which integrates DNA containing the correct sequence into the gene. It is worth noting that gene editing in human cell lines has been achieved by using an endonuclease guided to a specific DNA sequence with a custom-designed RNA. This technique is based on the CRISPR-Cas9 system found in prokaryotic cells, which identifies and cleaves foreign DNA in bacterial cells. Although CRISPR is currently used in laboratory settings, it is not yet utilized in clinical applications.

Transgenic animals can be created by injecting a cloned foreign gene (referred to as a transgene) into a fertilized egg. If the gene integrates randomly and stably into a chromosome, it will be present in the germline of the resulting animal and can be passed on to subsequent generations. An example of this is the production of a giant mouse known as "Supermouse," where the gene for rat growth hormone was injected into a fertilized mouse egg. It is important to note that transgenic animals have been developed to produce therapeutic human proteins in their milk, a process called "pharming." For instance, transgenic goats were able to produce antithrombin, an anticlotting protein (refer to online Chapter 35), and it was approved for clinical use in 2009. If the functional transgene is specifically inserted, it leads to the creation of a knockin (KI) mouse that expresses the gene. On the other hand, targeted insertion of a nonfunctional version of the transgene results in a knockout (KO) mouse that does not express the gene. These genetically engineered animals can be utilized as models for studying corresponding human diseases.

To summarize, gene editing allows for the repair of mutated genes by utilizing DNA-binding molecules and endonucleases. Transgenic animals can be produced by injecting a foreign gene into fertilized eggs, and if the gene integrates randomly and stably, it can be passed on to future generations. Additionally, transgenic animals can be designed to produce therapeutic human proteins in their milk. Genetically engineered animals can serve as valuable models for studying human diseases.
Gene editing, as opposed to gene addition, allows a mutated gene to be repaired. Combinations of DNA-binding molecules (proteins or RNA) and endonucleases are used to identify and cleave the mutated sequence. Cleavage activates homologous recombination repair of dsDNA breaks (see p. 429) that integrates DNA containing the correct sequence into the gene. [Note: An endonuclease guided to a specific DNA sequence by a custom-designed RNA has been used in gene editing in human cell lines. The technique is based on (and named for) the prokaryotic CRISPR-Cas9 (clustered regularly interspaced short palindromic repeats [CRISPR]-associated protein) system that identifies and cleaves foreign DNA in bacterial cells. CRISPR is currently used in the laboratory but not in the clinic.]
X. TRANSGENIC ANIMALS
Transgenic animals can be produced by injecting a cloned foreign gene (a transgene) into a fertilized egg. If the gene randomly and stably integrates into a chromosome, it will be present in the germline of the resulting animal and can be passed from generation to generation. A giant mouse called “Supermouse” was produced in this way by injecting the gene for rat growth hormone into a fertilized mouse egg. [Note: Transgenic animals have been designed that produce therapeutic human proteins in their milk, a process called “pharming.” Antithrombin, an anticlotting protein (see online Chapter 35), was produced by transgenic goats and approved for clinical use in 2009.] If the functional transgene undergoes targeted (not random) insertion, a knockin (KI) mouse that expresses the gene is created. Targeted insertion of a nonfunctional version of the transgene creates a knockout (KO) mouse that does not express the gene. Such genetically engineered animals can serve as models for the study of a corresponding human disease.
XI. CHAPTER SUMMARY