The explosive growth of human genomic data has revealed unprecedented numbers of disease-causing point mutations. Repairing such mutations may offer the best, and in some cases, only cure for genetic diseases. We and other groups have sought to correct disease mutant by combining CRISPR/Cas9 and homology directed repair (HDR) in human tripronulcear zygotes and diploid zygotes. However, low efficiency, mosaicism, off-target cleavage, and unintended homologous recombination (between target site and endogenous homologous genomic DNA sequence) remain obstacles that hamper the clinical applications of such approaches (Kang et al., 2016; Liang et al., 2015; Tang et al., 2017). In a recent report, it was found that diploid human zygotes, distinct from pluripotent cells, tends to repair DNA double strand break (DSB) using endogenous homologous sequence (Ma et al., 2017), consistent with what we have found in human tripronuclear zygotes (Liang et al., 2015). In the study, highly efficient repair of the mutant allele was achieved using the wild-type (WT) allele in heterozygous human zygotes through CRISPR/Cas9 (Ma et al., 2017). However, homozygous mutant embryos could not be repaired in way because of the lack of WT alleles. Additionally, recombination may occur with similar but not identical endogenous sequences, leading to unexpected mutations, as we found in human tripronuclear zygotes in which HBB recombined with HBD (Liang et al., 2015). Using base editors to directly repair point mutations may represent an efficient and highly specific alternative.
The base editor is a RNA-protein complex, adapted from the CRISPR/Cas9 system and cytidine deaminase (Komor et al., 2016). The effector protein is composed of cytidine deaminase (rAPOBEC1), Cas9, and uracil DNA glycosylase inhibitor (UGI). It can deaminate cytidine (C) to uridine (U) without inducing DNA DSB, and finally result in C-to-T (or G-to-A) conversion in the target DNA sequence (Hohmann, 2017; Komor et al., 2016; Liang et al., 2015). Efficient base editing at single-base resolution has been reported in plant, yeast, human cells, mouse zygotes, and human tripronuclear zygotes (Chen et al., 2017; Kim et al., 2017b, c; Komor et al., 2016; Li et al., 2016, 2017a, b; Liang et al., 2017; Lu and Zhu, 2016; Ren et al., 2017; Zhou et al., 2017); Zong et al., 2017). Intriguingly, mouse embryos and pups with 100% point mutation efficiency (free of mosacism), as well as human tripronuclear zygotes has been generated (Kim et al., 2017c; Li et al., 2017a; Liang et al., 2017). However, whether base editors can repair homozygous T>C (or A>G) disease mutant in human embryos remains to be tested.
β-Thalassemia, a common genetic disease in Mediterranean countries, North Africa, the Middle East, India, Central Asia, and Southeast Asia, is a major problem of global health (Cao and Galanello, 2010; Galanello and Origa, 2010; Weatherall, 2010). Genetic mutations, which will lead to the reduction of hemoglobin β chain (β-globin) and erythrocytes, finally cause oxygen shortage, bone deformity, organ dysfunction and even organ failure in many parts of the human body (Cao and Galanello, 2010). Based on the severity of the disease, β-thalassemia can be classified into β-thalassemia minor (also called β-thalassemia carrier), β-thalassemia intermedia, and β-thalassemia major (Cooley’s anemia) (Cao and Galanello, 2010). Without treatment, patients with β-thalassemia major usually die before age 5. Thalassemia major patients require lifelong blood transfusion and iron chelation treatment to survive, often accompanied by numerous complications, including arrhythmia, congestive heart failure, hypothyroidism, hypoparathyroidism, hypogonadism, diabetes, osteoporosis, liver cirrhosis, and infection (Chern et al., 2007; Wu et al., 2017). To date, allogeneic bone marrow transplantation (BMT) is the only curative therapy, but BMT is limited by human leukocyte antigen (HLA) compatibility. β-Thalassemia is mainly caused by mutations in the HBB gene, of which −28 (A>G) mutation is a common defect reducing the transcription of HBB (Orkin et al., 1983). Patients with homozygous or compound heterozygous −28 (A>G) mutation may develop severe anemia or intermedia anemia (Cao and Galanello, 2010; Orkin et al., 1983). Correcting the −28 (A>G) mutation by base editing should help to ameliorate anemia. Here, we report the efficient correction of −28 (A>G) mutation in human primary cells and human embryos by base editors.