Effective gene editing by high-fidelity base editor 2 in mouse zygotes

Targeted point mutagenesis through homologous recombination has been widely used in genetic studies and holds considerable promise for repairing disease-causing mutations in patients. However, problems such as mosaicism and low mutagenesis efficiency continue to pose challenges to clinical application of such approaches. Recently, a base editor (BE) system built on cytidine (C) deaminase and CRISPR/Cas9 technology was developed as an alternative method for targeted point mutagenesis in plant, yeast, and human cells. Base editors convert C in the deamination window to thymidine (T) efficiently, however, it remains unclear whether targeted base editing in mouse embryos is feasible. In this report, we generated a modified high-fidelity version of base editor 2 (HF2-BE2), and investigated its base editing efficacy in mouse embryos. We found that HF2-BE2 could convert C to T efficiently, with up to 100% biallelic mutation efficiency in mouse embryos. Unlike BE3, HF2-BE2 could convert C to T on both the target and non-target strand, expanding the editing scope of base editors. Surprisingly, we found HF2-BE2 could also deaminate C that was proximal to the gRNA-binding region. Taken together, our work demonstrates the feasibility of generating point mutations in mouse by base editing, and underscores the need to carefully optimize base editing systems in order to eliminate proximal-site deamination.

Correction of β-thalassemia mutant by base editor in human embryos

β-Thalassemia is a global health issue, caused by mutations in the HBB gene. Among these mutations, HBB -28 (A>G) mutations is one of the three most common mutations in China and Southeast Asia patients with β-thalassemia. Correcting this mutation in human embryos may prevent the disease being passed onto future generations and cure anemia. Here we report the first study using base editor (BE) system to correct disease mutant in human embryos. Firstly, we produced a 293T cell line with an exogenous HBB -28 (A>G) mutant fragment for gRNAs and targeting efficiency evaluation. Then we collected primary skin fibroblast cells from a β-thalassemia patient with HBB -28 (A>G) homozygous mutation. Data showed that base editor could precisely correct HBB -28 (A>G) mutation in the patient's primary cells. To model homozygous mutation disease embryos, we constructed nuclear transfer embryos by fusing the lymphocyte or skin fibroblast cells with enucleated in vitro matured (IVM) oocytes. Notably, the gene correction efficiency was over 23.0% in these embryos by base editor. Although these embryos were still mosaic, the percentage of repaired blastomeres was over 20.0%. In addition, we found that base editor variants, with narrowed deamination window, could promote G-to-A conversion at HBB -28 site precisely in human embryos. Collectively, this study demonstrated the feasibility of curing genetic disease in human somatic cells and embryos by base editor system.

Clinical analysis of preimplantation genetic diagnosis with HLA matching for beta-thalassemia / 中华妇产科杂志

Objective To investigate the efficacy and feasibility of preimplantation genetic diagnosis (PGD) with human leukocyte antigen (HLA) matching for beta-thalassemia. Methods A total of 43 referred beta-thalassemia couples, with at least on child in need of hematopoietic stem cell transplantation (HSCT), underwent PGD for HLA matching at the First Affiliated Hospital of Sun Yat-sen University from 2010 to 2015. PGD cycles of these couples were retrospectively analyzed, and 15 infants born from PGD-HLA were followed up. Results A total of 84 oocyte retrieval cycles were performed, providing 14±7 oocytes per cycle. Fifty nine embryos biopsied cycles were done, including 24 cleavage stage and 35 blastocyst stage biopsy cycles. In cleavage stage, 259 embryos were biopsied, 93.4% (242/259) of them with conclusive molecular diagnosis, and the percentage of unaffected embryos (normo-homozygote and heterozygote) was 71.4%(185/259). The percentage of HLA matched unaffected embryos was 9.3%(24/259). In blastocyst stage, 306 embryos were biopsied, while 93.8% (287/306) of them were conclusive, and the percentage of unaffected embryos was 70.6% (216/306). The percentage of HLA matched unaffected embryos in blastocyst stage biopsy was 14.4%(44/306), which was higher than in cleavage stage biopsy (P<0.05). Forty three female carriers underwent 48 embryo transfer cycles including 3 fresh and 45 frozen-thawed embryo transfer cycles. Three fresh embryo transfer cycles were done after cleavage stage biopsy, resulted in a birth of healthy twins born at 36 weeks′gestation. All the embryos were frozen after blastocyst biopsied. Totally, 54 frozen-thawed embryos that were transferred in 45 frozen-thawed embryo transfer cycles included 25 embryo from cleavage stage biopsy and 29 embryo from blastocyst stage biopsy, and 42 of them were HLA matched. Clinical pregnancy rate and implantation rate per cycle in frozen-thawed embryo transfer were 38%(17/45) and 37%(20/54) respectively. A total of 15 infants were born, 2 were from a fresh embryo transfer cycle, and 13 were from frozen-thawed embryo transfer cycles. Results of prenatal diagnosis from delivered cases were matched to that of PGD. Four sick children have been cured by HSCT from these HLA matched born siblings. Conclusion PGD with HLA matching is an established method for conceiving a child who may donate hematopoietic stem cells to save an ill sibling.

Preimplantation genetic diagnosis for carriers of thalassemia and chromosomal abnormality / 中华医学遗传学杂志

<p><b>OBJECTIVE</b>To provide preimplantation genetic diagnosis(PGD) for two couples carrying thalassemia mutations and chromosomal abnormalities.</p><p><b>METHODS</b>Couple 1 were both carriers of β 41/42 thalassemia mutations, while the husband has carried a reciprocal translocation with a karyotype of 46,XY,inv(9)(p11;q13),t(11;22)(q25;q13). Couple 2 were both carriers of α (-SEA) thalassemia mutation. Their chromosome karyotypes were both normal, but had two spontaneous abortions. The couples had received 1 and 3 blastocysts respectively through in vitro fertilization(IVF) cycles. Following the biopsy, the cells underwent whole genome amplification, and the amplified DNA from each embryo was subjected to genetic testing and a 23-chromosome single nucleotide polymorphism(SNP) microarray assay.</p><p><b>RESULTS</b>The embryo of couple 1 was diagnosed as carrier of β 41/42 thalassemia with euploid chromosomes. The embryo was transferred and resulted in intrauterine pregnancy. Similarly, an embryo of couple 2 was verified as carrier of α (-SEA) thalassemia with euploid chromosomes.</p><p><b>CONCLUSION</b>PGD for aneuploidy coupled with testing for single gene disorders via trophectoderm biopsy and whole genome amplification is feasible. The approach can attain diagnosis with minimal damage with sound clinical outcome.</p>

CRISPR/Cas9-mediated gene editing in human tripronuclear zygotes

Genome editing tools such as the clustered regularly interspaced short palindromic repeat (CRISPR)-associated system (Cas) have been widely used to modify genes in model systems including animal zygotes and human cells, and hold tremendous promise for both basic research and clinical applications. To date, a serious knowledge gap remains in our understanding of DNA repair mechanisms in human early embryos, and in the efficiency and potential off-target effects of using technologies such as CRISPR/Cas9 in human pre-implantation embryos. In this report, we used tripronuclear (3PN) zygotes to further investigate CRISPR/Cas9-mediated gene editing in human cells. We found that CRISPR/Cas9 could effectively cleave the endogenous β-globin gene (HBB). However, the efficiency of homologous recombination directed repair (HDR) of HBB was low and the edited embryos were mosaic. Off-target cleavage was also apparent in these 3PN zygotes as revealed by the T7E1 assay and whole-exome sequencing. Furthermore, the endogenous delta-globin gene (HBD), which is homologous to HBB, competed with exogenous donor oligos to act as the repair template, leading to untoward mutations. Our data also indicated that repair of the HBB locus in these embryos occurred preferentially through the non-crossover HDR pathway. Taken together, our work highlights the pressing need to further improve the fidelity and specificity of the CRISPR/Cas9 platform, a prerequisite for any clinical applications of CRSIPR/Cas9-mediated editing.

Comparison of CGH array and SNP array in preimplantation genetic diagnosis / 实用医学杂志

Objective To compare two kinds of strategies of preimplantation genetic diagnosis (PGD) to evaluate embryos for reciprocal and robertsonian translocation carriers. Methods A total of 152 PGD cycles for chromosomal translocation were performed from April 2012 to June 2014 , including 60 aCGH-PGD cycles using blastomere biopsy and fresh embryo transfer, and 92 SNP-PGD cycles using blastocyst biopsy and thawed embryo transfer. The diagnosis results and clinical outcome with these two kinds of strategies were compared. Results No significant difference was found in the cycles of no embryo transfer between SNP-PGD and aCGH-PGD. The normal rate in SNP-PGD was 33.8%, which was significant higher than that of aCGH-PGD. The clinical pregnancy rate per embryo transfer in SNP-PGD was higher than that in aCGH-PGD, but the misscarrage rate and embryo damage rate were lower than those in aCGH-PGD. Conclusions The PGD strategy of applying blastocyst biopsy, SNP array, embryo cryopreservation and thawed ET leads to a better clinical outcome. It may be a promising choice for future PGD treatment for carriers with chromosomal translocation.