In December 2023, the U.S. Food and Drug Administration approved Casgevy, a treatment for sickle cell disease that works by editing a patient's own bone marrow cells with a molecular tool borrowed from bacteria. It was the first CRISPR-based therapy ever approved for clinical use, and it turned a technique that started as a curiosity in microbial immune systems into a one-time, potentially curative medicine.
Genetic engineering, the deliberate modification of an organism's DNA, has moved from science fiction to hospital pharmacy in under two decades.
The underlying idea is old; the tools have gotten dramatically better. Scientists have been altering DNA since the 1970s, but the precision, speed, and low cost of the newest methods have changed what is practically possible - and what ethical questions the technology now forces society to answer.
The Early Tools: Restriction Enzymes and Recombinant DNA
Genetic engineering became possible once scientists learned to cut and paste DNA with precision. In the early 1970s, researchers discovered restriction enzymes - proteins, originally found in bacteria, that recognize specific short DNA sequences and cut the DNA strand at that exact point. Bacteria use these enzymes as a primitive immune defense, slicing up the DNA of invading viruses.
Herbert Boyer and Stanley Cohen used restriction enzymes in 1973 to cut a gene out of one organism's DNA and splice it into a bacterial plasmid - a small circular piece of DNA that bacteria can carry and copy independently of their main chromosome. This created the first recombinant DNA molecule: genetic material assembled from two different sources.
Because bacteria multiply rapidly, inserting a gene into a plasmid and letting the bacteria reproduce is an efficient way to manufacture large quantities of a specific protein - a technique that became the basis of the biotechnology industry, most famously producing synthetic human insulin starting in the early 1980s, replacing insulin previously extracted from animal pancreases.
Amplifying DNA: PCR
A second foundational tool, developed by Kary Mullis in 1983, is the polymerase chain reaction (PCR), a method for making millions of copies of a specific DNA sequence from a tiny starting sample. PCR made genetic engineering, DNA sequencing, and diagnostic testing dramatically faster and cheaper, and it remains one of the most widely used techniques in molecular biology laboratories worldwide.
CRISPR-Cas9: Programmable Editing
Restriction enzymes cut DNA at fixed sequences they naturally recognize, which limits precision. CRISPR-Cas9, developed into a gene-editing tool by Jennifer Doudna and Emmanuelle Charpentier in 2012, solved this by making the cutting sequence programmable.
CRISPR - Clustered Regularly Interspaced Short Palindromic Repeats - is itself a bacterial immune system, a way bacteria store snippets of viral DNA from past infections so they can recognize and destroy the same virus if it attacks again. Doudna and Charpentier, along with their teams, showed that this natural system could be repurposed: a synthetic guide RNA can be designed to match almost any DNA sequence, and the Cas9 protein - a molecular pair of scissors - will cut the DNA precisely where that guide RNA leads it.
“We can programme these bacterial cells - or programme this protein - to bind to any sequence of interest in the DNA, wherever we want, and once it's bound there, it triggers a cut in the DNA.” - Jennifer Doudna, describing the mechanism of CRISPR-Cas9 in interviews following the discovery published with Emmanuelle Charpentier
Once Cas9 makes a double-strand break, the cell's own repair machinery takes over. If left to repair itself without a template, the cell often makes small errors at the cut site, disrupting the gene - useful for turning a gene off. If scientists supply a DNA template alongside the cut, the cell can be induced to repair the break using that template, making a specific, deliberate edit. Doudna and Charpentier received the 2020 Nobel Prize in Chemistry for this work.
CRISPR Compared to Earlier Gene-Editing Tools
| Method | How it targets DNA | Relative cost and speed |
|---|---|---|
| Restriction enzymes (1970s) | Fixed, naturally occurring sequences only | Limited flexibility |
| Zinc finger nucleases (1990s-2000s) | Custom-engineered proteins per target | Expensive, slow to design |
| TALENs (2010-2011) | Custom-engineered proteins per target | Faster than zinc fingers, still costly |
| CRISPR-Cas9 (2012) | Programmable guide RNA, redesigned by changing a short RNA sequence | Cheap, fast, widely accessible |
From Lab Tool to Medicine: Gene Therapy
Gene therapy uses genetic engineering to treat or prevent disease by correcting, replacing, silencing, or adding genes in a patient's cells. Several gene therapies have reached approval and clinical use over the past decade, moving the field from experimental to established medicine, though treatments remain extremely expensive:
- Zolgensma - a gene replacement therapy for spinal muscular atrophy, delivered using a modified virus to carry a working copy of a missing gene into cells.
- Casgevy - approved by the FDA on December 8, 2023, for sickle cell disease, the first therapy based on CRISPR-Cas9 to reach approval. It edits a patient's own blood stem cells outside the body to increase production of fetal hemoglobin, which prevents the red blood cell sickling that causes the disease's painful complications.
- Hemgenix - a gene therapy for hemophilia B.
- Luxturna - a gene therapy for an inherited form of retinal blindness.
- CAR-T cell therapies such as Kymriah and Yescarta - genetically engineered immune cells used to treat certain blood cancers.
These treatments are described as somatic gene therapy: the edits are made in specific cells of a living patient's body, affect only that individual, and cannot be passed on to children.
Somatic Versus Germline Editing
The distinction between somatic and germline editing is central to how genetic engineering is regulated and debated. Somatic editing changes genes in the cells of an existing person - bone marrow, blood, liver, eye tissue - and the changes die with that individual. Germline editing changes genes in eggs, sperm, or early embryos, meaning the change would be inherited by every subsequent generation.
Germline editing in humans is either banned or under moratorium in most of the world specifically because of this permanence, and because it raises questions of consent that cannot be resolved: a future generation cannot agree to have its genome altered before it exists. The dangers of proceeding without international consensus were made concrete in November 2018, when the Chinese scientist He Jiankui announced he had created the first gene-edited babies, twin girls whose CCR5 gene had been altered in an attempt to confer resistance to HIV.
The experiment was conducted secretively, violated established ethical and regulatory norms, and drew swift, near-universal condemnation from the international scientific community; He was later sentenced to prison in China for illegal medical practice.
Precision Improves: Base Editing and Prime Editing
Standard CRISPR-Cas9 works by cutting both strands of the DNA double helix, which is effective but carries some risk of unintended errors during repair. Researchers in the laboratory of David Liu at the Broad Institute developed two refinements that reduce this risk.
Base editing, introduced in 2016, converts one DNA letter directly into another - for example, changing a C to a T - using a modified Cas9 fused to a chemical editing enzyme, without cutting both DNA strands at all.
Prime editing, published by the Liu lab in 2019, functions more like a word processor's search-and-replace: it can make insertions, deletions, or any single-letter conversion with a single tool, and generally produces fewer unintended edits than the original cut-and-repair approach.
Off-Target Effects
A central technical challenge in CRISPR editing is the off-target effect: the guide RNA can occasionally bind to a DNA sequence that closely resembles, but is not identical to, its intended target, causing an unintended cut elsewhere in the genome. Early CRISPR tools had measurable off-target rates that raised safety concerns for clinical use.
Newer variants - high-fidelity Cas9 proteins, base editors, and prime editors - have substantially reduced, though not entirely eliminated, this risk, and detecting and minimizing off-target activity remains an active area of research for any therapy intended for clinical use.
Genetic Engineering Outside Medicine: Agriculture
The same underlying tools are used to modify crops, most commonly to confer resistance to pests, herbicides, or drought, or to improve nutritional content. Genetically modified (GM) crops have been grown commercially since the mid-1990s and now make up a large share of global soy, corn, and cotton production.
On food safety specifically, the scientific consensus is not ambiguous. A comprehensive 2016 report from the National Academies of Sciences, Engineering, and Medicine, along with assessments from the World Health Organization and virtually every major scientific body that has reviewed the evidence, found no credible evidence that currently approved GM foods pose greater health risks than their conventional counterparts. Public debate about GMOs concerns real issues, but they are largely separate from food safety: the environmental effects of herbicide-resistant weeds, corporate concentration in the seed industry, and consumer labeling rights.
The Ethical Questions Genetic Engineering Raises
Most bioethicists and scientific bodies draw a working distinction between treating disease and enhancing normal human traits, and that distinction shapes most current policy. Somatic therapy for serious, well-understood genetic diseases is broadly supported. Germline editing for disease prevention remains highly restricted given the consent problem described above.
Editing aimed at enhancement - selecting for height, intelligence, or appearance rather than treating illness - is the most contested territory, raising concerns that such capabilities, if they become technically feasible, would be accessible primarily to the wealthy, potentially hardening existing inequality into biology itself, and that genetic selection could shade into discrimination against people with disabilities.
None of these questions are fully settled, and they are likely to become more pressing rather than less as base editing, prime editing, and their successors continue to make precise genetic changes cheaper and more reliable.
Sources & Further Reading
- Jinek, M., Chylinski, K., Fonfara, I., Hauer, M., Doudna, J. A., & Charpentier, E. (2012). A Programmable Dual-RNA-Guided DNA Endonuclease in Adaptive Bacterial Immunity. Science, 337(6096), 816-821. DOI: 10.1126/science.1225829
- U.S. Food and Drug Administration. (2023). FDA Approves First Gene Therapies to Treat Patients with Sickle Cell Disease. fda.gov
- Anzalone, A. V., Randolph, P. B., Davis, J. R., et al. (2019). Search-and-Replace Genome Editing Without Double-Strand Breaks or Donor DNA. Nature, 576, 149-157. DOI: 10.1038/s41586-019-1711-4
- National Academies of Sciences, Engineering, and Medicine. (2016). Genetically Engineered Crops: Experiences and Prospects. The National Academies Press. DOI: 10.17226/23395
- Cohen, S. N., Chang, A. C. Y., Boyer, H. W., & Helling, R. B. (1973). Construction of Biologically Functional Bacterial Plasmids In Vitro. Proceedings of the National Academy of Sciences, 70(11), 3240-3244. DOI: 10.1073/pnas.70.11.3240
Frequently Asked Questions
How does CRISPR-Cas9 actually edit genes?
CRISPR-Cas9 works in two parts: a guide RNA (gRNA) that acts as a molecular GPS, it is designed to match a specific DNA sequence in the genome, and the Cas9 protein, a molecular scissors that cuts DNA. The gRNA leads Cas9 to the target location; Cas9 makes a precise double-strand break in the DNA. The cell then repairs the break, either imperfectly (disrupting the gene) or using a provided template (making a specific edit). The system is adapted from a bacterial immune defense mechanism.
What is the difference between somatic and germline genetic editing?
Somatic editing changes genes in specific cells of a living person, the changes affect only that individual and cannot be inherited. Most current gene therapies are somatic. Germline editing changes genes in embryos, eggs, or sperm, the changes would be inherited by all subsequent generations. Germline editing is highly controversial and largely banned or suspended internationally. In 2018, Chinese scientist He Jiankui controversially created the first gene-edited babies (CCR5 modification intended to confer HIV resistance), drawing global condemnation.
What diseases can gene therapy treat?
Approved gene therapies (as of 2024) include treatments for spinal muscular atrophy (Zolgensma), beta-thalassemia and sickle cell disease (Casgevy, the first CRISPR therapy approved), hemophilia B (Hemgenix), retinal dystrophy (Luxturna), and some cancers (CAR-T cell therapies like Kymriah and Yescarta). Hundreds of clinical trials are ongoing for conditions including Huntington’s disease, Duchenne muscular dystrophy, and various cancers. Gene therapy has moved from experimental to clinically validated, though treatments remain extremely expensive.
Are GMO foods safe to eat?
The scientific consensus, supported by the National Academies of Sciences (2016 report), WHO, and virtually every major scientific organization, is that currently approved genetically modified foods are as safe to eat as their conventional counterparts. No credible evidence exists of harm from consuming GM foods that have passed regulatory review. The debate about GMOs largely concerns environmental effects (herbicide-resistant weeds, biodiversity), corporate control of food systems, and labeling rights, not food safety per se.
What are off-target effects in CRISPR editing?
Off-target effects occur when CRISPR’s guide RNA binds to sequences similar (but not identical) to the intended target, causing unintended cuts elsewhere in the genome. These unintended edits could potentially disrupt important genes or affect gene regulation. Early CRISPR tools had meaningful off-target rates; newer variants (high-fidelity Cas9, base editors, prime editors) have substantially reduced but not eliminated this problem. Off-target detection and minimization is a major area of ongoing research for clinical applications.
What is base editing and prime editing?
Base editing (David Liu, 2016) makes precise single-letter changes to DNA without cutting both strands, converting one DNA base to another (e.g., C to T) using a modified Cas9 fused to a chemical editing enzyme. Prime editing (Liu lab, 2019) is even more precise: a ‘search and replace’ system that can make insertions, deletions, or any base-to-base conversion with lower off-target effects. These ‘next-generation’ editing tools extend CRISPR’s capabilities and reduce risks compared to original cut-and-repair approaches.
What ethical issues surround genetic engineering?
Key ethical concerns include: germline editing creates permanent heritable changes without consent of future generations; ‘designer babies’ could exacerbate inequality if genetic enhancements are available only to the wealthy; genetic selection raises concerns about discrimination against disability; enhancement vs. treatment boundaries are unclear. Most ethicists and scientific bodies support somatic therapy for serious disease while calling for a moratorium or extreme caution on germline enhancement. The distinction between treating disease and enhancing normal traits is central to the bioethical debate.