From Bacterial Defence to Medicine: The CRISPR Gene-Editing Revolution

The discovery came while researchers were sequencing a gene in <em>Escherichia coli</em>. They noticed an unusual pattern: short DNA sequences repeated at regular intervals, separated by unique fragments. Nothing at the time revealed that these sequences formed part of a microbial defence system. The observation mattered because it created the first record of the structure later called CRISPR, even though its biological purpose would remain unclear for years.

Researchers proposed the name CRISPR for this family of regularly spaced repeats and identified nearby genes that became known as Cas, for CRISPR-associated. A common vocabulary did not explain what the system did, but it allowed observations from different microorganisms to be compared. That helped turn several disconnected findings into one research field.

Francisco Mojica, Alexander Bolotin and Christine Pourcel independently noticed, using bioinformatic comparisons, that the unique 'spacer' sequences sitting between CRISPR repeats matched fragments of bacteriophage and plasmid DNA. That match was the conceptual leap: if bacteria were storing snippets of the viruses that had attacked them, CRISPR might function as a form of acquired immune memory. The hypothesis reframed a structural curiosity as a defence system, though it would take until 2007 to prove experimentally.

Researchers inserted fragments of viral DNA into bacterial CRISPR arrays and showed that those changes altered resistance to later infection. The experiment provided direct evidence that the system stores a genetic memory of past invaders and uses it to recognise them again. CRISPR was therefore not merely an unusual DNA pattern; it was part of an adaptive microbial immune system.

While studying Streptococcus pyogenes, Emmanuelle Charpentier's team discovered a small RNA molecule, tracrRNA, that base-pairs with the CRISPR repeat sequences and is essential for processing crRNA into its mature, functional form. The finding revealed a missing piece of the Cas9 system's machinery. A year later, that same tracrRNA would be fused with crRNA into a single guide RNA, the design that let Doudna and Charpentier turn Cas9 into a programmable tool.

Jennifer Doudna, Emmanuelle Charpentier and colleagues showed that an engineered guide RNA could direct the Cas9 enzyme to cut a chosen DNA sequence. The crucial simplification was programmability: researchers could change the guide sequence instead of designing a different protein for every target. The study established the core mechanism, but precise cutting in a test system was still far from a safe human treatment.

Independent teams demonstrated that CRISPR-Cas9 could edit selected genes inside mammalian, including human, cells. Moving from purified molecules into living cells rapidly expanded its usefulness for studying gene function, building disease models and exploring agricultural and medical applications. It also exposed practical problems such as delivery, unintended edits and variable results, all of which matter before clinical use.

A team led by Junjiu Huang at Sun Yat-sen University injected CRISPR-Cas9 into 86 non-viable tripronuclear human embryos, aiming to correct the mutation behind beta-thalassaemia. Only 4 of 71 surviving embryos carried the intended edit, and the procedure produced mosaicism and off-target mutations. Nature and Science had declined to publish the work over ethical concerns before Protein & Cell did; the study nonetheless proved human embryos could be edited and forced the field to confront, years before He Jiankui's 2018 experiment, how unready the technology was for anything heritable.

Alexis Komor and David Liu's team fused a catalytically disabled Cas9 to a cytidine deaminase enzyme, creating a 'base editor' able to convert a C:G base pair directly into a T:A pair at a chosen site. Because the DNA backbone was never cut, the approach produced far fewer random insertions and deletions than standard Cas9. It gave researchers a second precision tool alongside the original cut-and-repair method, one better suited to correcting the many genetic diseases caused by single-letter mutations.

At West China Hospital in Chengdu, oncologist Lu You's team extracted T cells from a patient with advanced non-small-cell lung cancer, used CRISPR-Cas9 to knock out the gene for PD-1, a protein that normally restrains immune attacks, then re-infused the edited cells. It was the first time a CRISPR-edited human cell product had ever been given to a person, months ahead of comparable US trials. The Phase 1 study, eventually enrolling 22 patients, would later confirm the approach was feasible and safe, opening the door to CRISPR-based cell therapies more broadly.

Chinese researcher He Jiankui announced the birth of children whose embryos had been edited, without a compelling medical need or acceptable oversight. Because changes made to embryos may be inherited, any unintended effect could extend beyond one patient. The episode drew international condemnation and showed that technical capability can advance faster than the ethical rules, transparency and governance needed to control its use.

Andrew Anzalone in David Liu's lab fused a Cas9 nickase to an engineered reverse transcriptase, guided by a 'prime editing guide RNA' that both finds the target site and encodes the desired edit. The system can perform targeted insertions, deletions and all 12 possible base-to-base conversions without cutting both DNA strands or needing a separate donor template. Prime editing is estimated to be able to correct roughly 90% of known pathogenic human genetic variants, making it one of the most versatile editing tools developed to date.

The Nobel committee recognised Charpentier and Doudna's 2012 work reprogramming CRISPR-Cas9 as a genome-editing method, describing it as one of gene technology's sharpest tools. It was the first Nobel Prize in the sciences shared by two women, awarded within a decade of the discovery it honoured, a notably short gap that reflected how quickly CRISPR had reshaped biology, agriculture and medicine.

Unlike Casgevy, which edits cells outside the body, NTLA-2001 is infused directly into the bloodstream, packaged in lipid nanoparticles that carry it to the liver, where it permanently switches off the gene producing a misfolded protein behind transthyretin amyloidosis. A single dose lowered serum TTR protein by up to 87% within weeks. It was the first published clinical evidence that CRISPR components could be delivered into a living patient and edit a target gene in place, without ever removing a single cell.

The FDA approved Casgevy for certain patients aged 12 and older with sickle-cell disease. Clinicians collect a patient’s blood-forming stem cells, edit them outside the body to increase fetal haemoglobin, and return them after intensive conditioning. The approval proved that CRISPR can deliver clinical benefit, but the process remains demanding, expensive and available only through specialised treatment centres.

Weeks after its sickle-cell approval, the FDA cleared Casgevy for transfusion-dependent beta thalassaemia, a disease that shares the same underlying gene target. The European Commission followed in February 2024, and UK and other regulators granted authorisations across 2023 and 2024, together making more than 8,000 additional patients eligible. The rapid, multi-geography rollout showed that a single approved CRISPR therapy could scale into a genuine clinical product line rather than a one-off treatment.