Scientists have just found a way to edit the genes in human embryos to prevent genetic diseases without causing harmful mutations.
Scientists have just achieved a massive breakthrough by successfully modifying the DNA in human embryos without introducing harmful mutations. An international team of scientists has published a paper in the journal Nature detailing a way to edit DNA, which could help stop genetic diseases dead in their tracks.
Scientists used a gene-editing technique to correct a genetic defect that can cause a heart disorder in young people. They were able to correct the defect in an astonishing two thirds of embryos, all without causing a mutation that could prove dangerous.
If the technique could be confirmed through further tests to be safe and effective, it could be used in the future to stop genetic diseases that plague mankind.
The abstract and intro to the paper follow below.
Genome editing has potential for the targeted correction of germline mutations. Here we describe the correction of the heterozygous MYBPC3 mutation in human preimplantation embryos with precise CRISPR–Cas9-based targeting accuracy and high homology-directed repair efficiency by activating an endogenous, germline-specific DNA repair response. Induced double-strand breaks (DSBs) at the mutant paternal allele were predominantly repaired using the homologous wild-type maternal gene instead of a synthetic DNA template. By modulating the cell cycle stage at which the DSB was induced, we were able to avoid mosaicism in cleaving embryos and achieve a high yield of homozygous embryos carrying the wild-type MYBPC3 gene without evidence of off-target mutations. The efficiency, accuracy and safety of the approach presented suggest that it has potential to be used for the correction of heritable mutations in human embryos by complementing preimplantation genetic diagnosis. However, much remains to be considered before clinical applications, including the reproducibility of the technique with other heterozygous mutations.
More than 10,000 monogenic inherited disorders have been identified, affecting millions of people worldwide. Among these are autosomal dominant mutations, where inheritance of a single copy of a defective gene can result in clinical symptoms. Genes in which dominant mutations manifest as late-onset adult disorders include BRCA1 and BRCA2, which are associated with a high risk of breast and ovarian cancers1, and MYBPC3, mutation of which causes hypertrophic cardiomyopathy (HCM)2. Because of their delayed manifestation, these mutations escape natural selection and are often transmitted to the next generation. Consequently, the frequency of some of these founder mutations in particular human populations is very high. For example, the MYBPC3 mutation is found at frequencies ranging from 2% to 8%3 in major Indian populations, and the estimated frequency of both BRCA1 and BRCA2 mutations among Ashkenazi Jews exceeds 2%4.
HCM is a myocardial disease characterized by left ventricular hypertrophy, myofibrillar disarray and myocardial stiffness; it has an estimated prevalence of 1:500 in adults5 and manifests clinically with heart failure. HCM is the commonest cause of sudden death in otherwise healthy young athletes. HCM, while not a uniformly fatal condition, has a tremendous impact on the lives of individuals, including physiological (heart failure and arrhythmias), psychological (limited activity and fear of sudden death), and genealogical concerns. MYBPC3 mutations account for approximately 40% of all genetic defects causing HCM and are also responsible for a large fraction of other inherited cardiomyopathies, including dilated cardiomyopathy and left ventricular non-compaction6. MYBPC3 encodes the thick filament-associated cardiac myosin-binding protein C (cMyBP-C), a signalling node in cardiac myocytes that contributes to the maintenance of sarcomeric structure and regulation of both contraction and relaxation2.
Current treatment options for HCM provide mostly symptomatic relief without addressing the genetic cause of the disease. Thus, the development of novel strategies to prevent germline transmission of founder mutations is desirable. One approach for preventing second-generation transmission is preimplantation genetic diagnosis (PGD) followed by selection of non-mutant embryos for transfer in the context of an in vitro fertilization (IVF) cycle. When only one parent carries a heterozygous mutation, 50% of the embryos should be mutation-free and available for transfer, while the remaining carrier embryos are discarded. Gene correction would rescue mutant embryos, increase the number of embryos available for transfer and ultimately improve pregnancy rates.
Recent developments in precise genome-editing techniques and their successful applications in animal models have provided an option for correcting human germline mutations. In particular, CRISPR–Cas9 is a versatile tool for recognizing specific genomic sequences and inducing DSBs7, 8, 9, 10. DSBs are then resolved by endogenous DNA repair mechanisms, preferentially using a non-homologous end-joining (NHEJ) pathway. Obviously, NHEJ is inappropriate for gene correction applications because it introduces additional mutations in the form of insertions or deletions at the DSB site, commonly referred to as indels. In some cases, however, targeted cells activate an alternative DNA repair pathway called homology-directed repair (HDR) that rebuilds the DSB site using the non-mutant homologous chromosome or a supplied exogenous DNA molecule as a template, leading to correction of the mutant allele11, 12. At present, CRISPR–Cas9 is predominantly used to introduce mutations and in the generation of gene knockouts using intrinsic NHEJ. Because HDR efficiency is relatively low8, applications of genome editing for gene therapy have been limited.
In early attempts, the introduction of genome editing constructs into one-cell embryos (zygotes), resulted in multicellular embryos or offspring with mosaic outcomes in individual cells13, 14. Also, off-target mutations that could be introduced into the developing embryo remained an undesirable possibility.
We sought to investigate human gamete and embryo DNA repair mechanisms activated in response to CRISPR–Cas9-induced DSBs. In an effort to demonstrate the proof-of-principle that heterozygous gene mutations can be corrected in human gametes or early embryos, we focused on the MYBPC3 mutation that has been implicated in HCM. Although homozygous mutations with no PGD alternative would have been most desirable for gene correction, generating homozygous human embryos for research purposes is practically impossible. Homozygous MYBPC3 mutations in adults are extremely rare owing to the severity of the clinical symptoms and early onset of the disease. Therefore, we specifically targeted the heterozygous four-base-pair (bp) deletion in the MYBPC3 gene in human zygotes introduced by heterozygous, carrier sperm while oocytes obtained from healthy donors provided the wild-type allele. By accurate analysis of cleaving embryos at the single-cell level, we show high targeting efficiency and specificity of preselected CRISPR–Cas9 constructs. Moreover, DSBs in the mutant paternal MYBPC3 gene were preferentially repaired using the wild-type oocyte allele as a template, suggesting an alternative, germline-specific DNA repair response. Mechanisms responsible for mosaicism in embryos were also investigated and a proposed solution to minimize their occurrence developed—namely the co-injection of sperm and CRISPR–Cas9 components into metaphase II (MII) oocytes.