Gene editing

Gene editing is a form of genetic engineering that involves the insertion, deletion, or replacement of DNA at specific sites in the genome. This technology allows for precise manipulation of genetic material.

Key Tools and Techniques:

• CRISPR-Cas9 System: This is a more recent genome-editing tool developed in 2009 and revolutionized gene editing due to its simplicity, speed, cost-effectiveness, and high accuracy.

  • Guide RNA (gRNA): Researchers create a short RNA template (typically 20 bases long) that is complementary to a specific target DNA sequence in the genome. The gRNA guides the Cas9 enzyme to the targeted DNA sequence.

  • Cas9 Enzyme: This is a nuclease enzyme (specifically a restriction endonuclease) that cuts double-stranded DNA at the location directed by the gRNA. It has two active sites to cut both strands of DNA.

  • Mechanism: After Cas9 cuts the DNA, natural DNA repair mechanisms can repair the break by either adding or deleting nucleotides to change the base sequence, or by inserting a short length of prepared double-stranded DNA with a specific sequence to correct a faulty allele. This allows for the removal of faulty alleles or parts of them, or their replacement with functioning ones.

• Earlier Technologies:

  • Zinc-finger nucleases (ZFN) and transcription activator-like effector nucleases (TALENs): These were developed earlier (late 1900s and early 2000s) but are more difficult, time-consuming, expensive, and require more expertise in molecular biology to produce modified endonuclease enzymes compared to CRISPR. They are still used in research labs and were first clinically used in humans in 2015 to modify T-cells to treat leukemia.

Applications:

• Treating Diseases: Gene editing offers the possibility of treating diseases by cutting out or replacing faulty (mutated) genes.

  • Genetic Disorders: It is particularly aimed at inherited disorders caused by recessive alleles. Examples include Severe Combined Immunodeficiency (SCID) and inherited eye diseases like LCA.

  • Cancer and HIV: Large-scale trials are underway in China to tackle diseases like cancer and HIV. Scientists have attempted to delete part of the CCR5 gene, which HIV uses to enter cells, in early IVF embryos to make children resistant to HIV. Gene editing may also be used to make white blood cells produce tumor necrosis factor (TNF) to kill cancer cells, or to insert antisense genes for HIV proteins to prevent viral reproduction.

  • Hunter Syndrome: The first human to receive in vivo gene editing treatment in 2017 suffered from Hunter syndrome, where viruses carried gene-editing tools and correct gene copies to vital organs.

• Research: It is widely used in research labs to study how gene changes affect animal health (e.g., mice and zebrafish) and to predict effects on human health. It also helps understand complex genetic systems.

• Customized Drugs: The ability to sequence individual human genomes allows for the preparation of customized drugs to treat cancers.

• Industrial Protection: It can be used to provide bacteria in industry with protection against viruses.

• "Designer Proteins": It is possible to make "designer proteins" by altering gene sequences, although commercial applications are not yet widely available.

Advantages over Older Methods:

• Precision: Gene editing tools offer much greater precision in cutting and pasting genes or parts of genes compared to older vector-based methods, which had no control over where DNA was inserted, potentially disrupting other genes.

• Efficiency and Cost: CRISPR is significantly simpler, faster, and cheaper than previous gene editing technologies like ZFNs and TALENs.

• Versatility: It allows for insertion, deletion, or replacement of DNA. Also, it can target multiple genes simultaneously.

Challenges and Ethical Considerations:

• Safety and Unforeseen Consequences: Concerns exist about the safety of the treatment, as changes to DNA are permanent, and long-term effects, which could be passed on to future generations, are unknown.

• Specificity: While precise, it is still in the experimental stage.

• Accessibility: If expensive, gene editing treatments may only be accessible to wealthier individuals.

• Ethical Boundaries: There are strong ethical and moral issues, particularly concerning gene editing in human embryos.

  • Germ Line Therapy: Altering genes in sex cells (germ line therapy) means the changes are inherited by offspring. This is currently illegal in the UK and many other countries due to concerns about "designer babies" and potential for eugenics.

  • Human Embryos: The Nuffield Council on Bioethics in the UK agreed in 2018 that changing human embryo DNA could be "morally permissible" if in the child's best interests, but work on human embryos remains illegal in the UK and many other countries.

Despite the challenges, gene technology, including gene editing, offers numerous practical benefits in biotechnology, medicine, and agriculture, as well as for scientific study.