Genetic diseases that manifest themselves in a systemic way (e.g., cystic fibrosis or genetic mitochondrial disease), in a single but widespread tissue (e.g., muscular or myotonic dystrophy), or in a tissue not easily accessible (e.g., the basal ganglia in Huntington's disease) may all be resistant to somatic cell–based genome-editing approaches. The mouse model of muscular dystrophy is an important example of the limitation of the zygote-editing strategy, as only a fraction of offspring exhibited sufficient chimerism in the muscle tissue to mitigate the clinical impact of mutations in the dystrophin gene.Ī critical aspect of the discussion surrounding the potential of germ-line genome editing is that it may be the only way to cure certain genetic diseases. This variegation precludes any rational prediction of the resulting phenotype of an offspring. Finally, after editing reagents are injected into zygotes, the subfraction of cells that have undergone genome editing exhibit heterogeneity in the molecular nature of the resulting alleles (insertions/deletions, homologous recombination, or both). In subsequent generations, only a fraction of the offspring-possibly none-would be derived from a germ-line cell that had been edited. Furthermore, the resulting offspring would be chimeric, with only a fraction of somatic and germ cells carrying edited genomes. This strategy is simply not possible in humans. Tens to hundreds of zygotes would need to be injected and implanted into several surrogate mothers to generate viable, genetically modified offspring. First, only a fraction of injected zygotes give rise to viable offspring. However, several important details preclude the adoption of zygote injection in humans, irrespective of any ethical concerns. Gene Editing: Technology & Applications.
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