RESUMO
Correction of disease-causing mutations in human embryos holds the potential to reduce the burden of inherited genetic disorders and improve fertility treatments for couples with disease-causing mutations in lieu of embryo selection. Here, we evaluate repair outcomes of a Cas9-induced double-strand break (DSB) introduced on the paternal chromosome at the EYS locus, which carries a frameshift mutation causing blindness. We show that the most common repair outcome is microhomology-mediated end joining, which occurs during the first cell cycle in the zygote, leading to embryos with non-mosaic restoration of the reading frame. Notably, about half of the breaks remain unrepaired, resulting in an undetectable paternal allele and, after mitosis, loss of one or both chromosomal arms. Correspondingly, Cas9 off-target cleavage results in chromosomal losses and hemizygous indels because of cleavage of both alleles. These results demonstrate the ability to manipulate chromosome content and reveal significant challenges for mutation correction in human embryos.
Assuntos
Alelos , Proteína 9 Associada à CRISPR/metabolismo , Cromossomos Humanos/genética , Embrião de Mamíferos/metabolismo , Animais , Sequência de Bases , Blastocisto/metabolismo , Ciclo Celular/genética , Linhagem Celular , Deleção Cromossômica , Quebras de DNA de Cadeia Dupla , Reparo do DNA por Junção de Extremidades/genética , Implantação do Embrião/genética , Proteínas do Olho/genética , Fertilização , Edição de Genes , Rearranjo Gênico/genética , Loci Gênicos , Genoma Humano , Genótipo , Heterozigoto , Células-Tronco Embrionárias Humanas/metabolismo , Humanos , Mutação INDEL/genética , Camundongos , Mitose , Fases de Leitura Aberta/genética , Polimorfismo de Nucleotídeo Único/genéticaRESUMO
DNA double-strand breaks (DSBs) are especially toxic DNA lesions that, if left unrepaired, can lead to wide-ranging genomic instability. Of the pathways available to repair DSBs, the most accurate is homologous recombination (HR), where a homologous sequence is used as a donor template to restore genetic information at the break site. While much of the biochemical aspects of HR repair have been characterized, how the repair machinery locates and discriminates between potential homologous donor templates throughout the genome remains elusive. We use Drosophila melanogaster to investigate whether there is a preference between intrachromosomal and interhomolog donor sequences in mitotically dividing cells. Our results demonstrate that, although interhomolog HR is possible and frequent if another donor template is not available, intrachromosomal donor templates are highly preferred. This is true even if the interhomolog donor template is less diverged than the intrachromosomal donor template. Thus, despite the stringent requirements for homology, the chromosomal location of the donor template plays a more significant role in donor template choice.