A gene drive biases the transmission of one of two copies of a gene such that it is inherited more frequently than by random segregation. Highly efficient gene drive systems were recently developed in insects, which leverage the sequence-targeted DNA cleavage activity of CRISPR/Cas9 and endogenous homology directed repair mechanisms to convert heterozygous genotypes to homozygosity 1 – 4 . If implemented in laboratory rodents, similar systems would enable the rapid assembly of currently impractical genotypes that involve multiple homozygous genes (e.g., to model multigenic human diseases). However, such a system has not yet been demonstrated in mammals. Here, we utilize an “active genetic” element that encodes a guide RNA embedded in the mouse Tyrosinase gene to evaluate whether targeted gene conversion can occur when CRISPR/Cas9 is active in the early embryo or in the developing germline. Although Cas9 efficiently induces double strand DNA breaks in the early embryo and male germline, these breaks are not resolved by homology directed repair. However, Cas9 expression limited to the female germline forms double strand breaks that are resolved by homology directed repair, which copies the active genetic element from the donor to the receiver chromosome and increases its rate of inheritance in the next generation. These results demonstrate feasibility of CRISPR/Cas9-mediated systems that bias inheritance in mice, which have potential to transform the use of rodent models in basic and biomedical research.
A gene drive biases the transmission of a particular allele of a gene such that it is inherited at a greater frequency than by random assortment. Recently, a highly efficient gene drive was developed in insects, which leverages the sequence-targeted DNA cleavage activity of CRISPR/Cas9 and endogenous homology directed repair mechanisms to convert heterozygous genotypes to homozygosity. If implemented in laboratory rodents, this powerful system would enable the rapid assembly of genotypes that involve multiple genes (e.g., to model multigenic human diseases). Such complex genetic models are currently precluded by time, cost, and a requirement for a large number of animals to obtain a few individuals of the desired genotype. However, the efficiency of a CRISPR/Cas9 gene drive system in mammals has not yet been determined. Here, we utilize an active genetic "CopyCat" element embedded in the mouse Tyrosinase gene to detect genotype conversions after Cas9 activity in the embryo and in the germline. Although Cas9 efficiently induces double strand DNA breaks in the early embryo and is therefore highly mutagenic, these breaks are not resolved by homology directed repair. However, when Cas9 expression is limited to the developing female germline, resulting double strand breaks are resolved by homology directed repair that copies the CopyCat allele from the donor to the receiver chromosome and leads to its super-Mendelian inheritance. These results demonstrate that the CRISPR/Cas9 gene drive mechanism can be implemented to simplify complex genetic crosses in laboratory mice and also contribute valuable data to the ongoing debate about applications to combat invasive rodent populations in island communities. Main Text:A cross of mice that are heterozygous for each of three unlinked genes must produce 146 offspring for a 90% probability that one will be triple homozygous. The likelihood decreases further if any of the three genes are close together, or genetically linked, on the same
Highly efficient gene conversion systems have the potential to facilitate the study of complex genetic traits using laboratory mice and, if implemented as a “gene drive,” to limit loss of biodiversity and disease transmission caused by wild rodent populations. We previously showed that such a system of gene conversion from heterozygous to homozygous after a sequence targeted CRISPR/Cas9 double-strand DNA break (DSB) is feasible in the female mouse germline. In the male germline, however, all DSBs were instead repaired by end joining (EJ) mechanisms to form an “insertion/deletion” (indel) mutation. These observations suggested that timing Cas9 expression to coincide with meiosis I is critical to favor conditions when homologous chromosomes are aligned and interchromosomal homology-directed repair (HDR) mechanisms predominate. Here, using a Cas9 knock-in allele at the Spo11 locus, we show that meiotic expression of Cas9 does indeed mediate gene conversion in the male as well as in the female germline. However, the low frequency of both HDR and indel mutation in both male and female germlines suggests that Cas9 may be expressed from the Spo11 locus at levels too low for efficient DSB formation. We suggest that more robust Cas9 expression initiated during early meiosis I may improve the efficiency of gene conversion and further increase the rate of “super-mendelian” inheritance from both male and female mice.
Multipotent progenitor populations are necessary for generating diverse tissue types during embryogenesis. We show the Polymerase Associated Factor 1 Complex (Paf1C) is required to maintain multipotent progenitors of the neural crest (NC) lineage in zebrafish. Mutations affecting each Paf1C component result in near-identical NC phenotypes; alyron mutant embryos carrying a null mutation in paf1 were analyzed in detail. In the absence of zygotic paf1 function, definitive premigratory NC progenitors arise but fail to maintain expression of the sox10 specification gene. The mutant NC progenitors migrate aberrantly and fail to differentiate appropriately. Blood and germ cell progenitor development is affected similarly. Development of mutant NC could be rescued by additional loss of Positive Transcription Elongation Factor b (P-TEFb) activity, a key factor in promoting transcription elongation. Consistent with the interpretation that inhibiting/delaying expression of some genes is essential for maintaining progenitors, mutant embryos lacking the CDK9 kinase component of P-TEFb exhibit a surfeit of NC progenitors and their derivatives. We propose Paf1C and P-TEFb act antagonistically to regulate the timing of the expression of genes needed for NC development.
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