The use of genetically humanized animal models for personalized medicine approaches

Abstract

For many genetic diseases, researchers are developing personalized medicine approaches. These sometimes employ custom genetic interventions such as antisense-mediated exon skipping or genome editing, aiming to restore protein function in a mutation-specific manner. Animal models can facilitate the development of personalized medicine approaches; however, given that they target human mutations and therefore human genetic sequences, scientists rely on the availability of humanized animal models. Here, we outline the usefulness, caveats and potential of such models, using the example of the hDMDdel52/mdx model, a humanized model recently generated for Duchenne muscular dystrophy (DMD).

Keywords: Exon skipping; Genetic therapies; Genome editing; Muscular dystrophy; Pre-clinical studies.

Fig. 1.

Dystrophin mutations underlie both Duchenne and Becker muscular dystrophy. The DMD gene encodes the dystrophin protein, which links the actin-cytoskeleton to the extracellular matrix with its actin binding domain (Actin-BD) and dystroglycan binding domain (DBD), respectively (upper panel). In Duchenne patients, mutations, generally deletions involving one or more exons, disrupt the reading frame. In the example in the bottom left panel, an exon 52 deletion causes a frameshift and premature truncation of protein translation, and a non-functional dystrophin. In Becker patients, deletions maintain the reading frame. In the example in the bottom right panel a deletion of exon 51-52 does not disrupt the reading frame, thus allowing the production of an internally deleted dystrophin that contains both crucial domains and that, consequently, is partially functional.

Fig. 2.

Schematic depiction of therapeutic exon skipping and genome editing approaches for Duchenne muscular dystrophy. Exon skipping (left panel) interferes in the pre-mRNA splicing process using antisense oligonucleotides (AON) that target a specific exon (exon 51 in this example). Thus the target exon is hidden from the splicing machinery and ‘skipped’ from the mature mRNA. This enlarges the deletion, but restores the reading frame, thus allowing the production of an internally deleted Becker-like dystrophin. Genome editing (right panel) acts on the DNA level, using guide RNAs that guide the Cas9 enzymes (scissors) to specific locations in the gene. This will result in double-stranded DNA breaks, which are repaired by non-homologous end joining in postmitotic cells (such as skeletal muscle fibers), leading to a larger deletion. Consequently, all transcripts produced have an in-frame deletion, thus allowing the production of internally deleted Becker-like dystrophin. Actin-BD, actin binding domain; DBD, dystroglycan binding domain.

Fig. 3.

Overview of the mouse models most commonly used to evaluate genetic therapies.