Curing Rare Diseases — 3

But with refinements, the fixing can be far more precisely focused. With base editing (fig.8.2), instead of a double-strand DNA break, a single strand is nicked, enabling a single base pair change like an A to a G and C to an A. An even more versatile single-strand editing incorporates an enzyme called reverse transcriptase and a prime editing guide RNA that directly copies edited information to the target DNA site, as used by Verve Therapeutics. David Liu of the Broad Institute has likened it to a word processor: “It allowed researchers to turn any letter into any other letter and delete or insert stretches dozens of base pairs long. In theory, it could debug around 90 percent of all known disease-causing mutations.” These DNA editors with enhanced nondisruptive, fixing capabilities have been called CRISPR 2.0. They include other emerging editing tools for large deletions (CRISPR-Cas 3) and insertions (CRISPR-associated transposases), along with RNA and epigenetic editors (fig. 8.2). Another discovery from nature was “bridge RNA” molecules, a group of programmable recombinases, enabling precise editing of long DNA sequences.

Figure 8.2. CRISPR 2.0. Base editing (upper) with single-strand DNA nick and substitution of intended base pair. Prime editing (lower) with single-stranded DNA nick and insertion of a DNA sequence. Adapted from Joy Wang et al., “CRISPR technology: A decade of genome editing is only the beginning,” Science 379, no. 6629 (January 2023): eadd8643, https://doi.org/10.1126/science.add8643.

In late 2023, just over a decade after CRISPR was discovered, a major milestone was achieved with the regulatory approval by the United Kingdom and United States for genome editing to treat sickle cell anemia, called Casgevy, for relief of debilitating sickle cell crises that affect some one hundred thousand Americans, mostly African Americans. The United Kingdom also approved CRISPR editing for beta-thalassemia, markedly reducing the need for blood transfusions. For both diseases, there had been substantial warm-up using CRISPR 1.0 to demonstrate a disruptive workaround strategy that does not fix the genetic defect in the hemoglobin gene (a simple T for an A switch mutation) but instead targets a gene called BCL11A to restore production of fetal hemoglobin. The marked and durable rise in fetal hemoglobin following CRISPR is what can provide a definitive treatment for both sickle cell anemia and beta-thalassemia.

But it’s an extremely complicated treatment that requires the patient to first undergo a collection process of blood-producing stem cells, using an apheresis machine that separates out blood cells into components, and takes several hours, often requiring transfusions subsequently. Then chemotherapy is given to wipe out the bone marrow (which can lead to infertility) for eventual replenishment with the patient’s edited stem cells. That precipitously drops the patient’s white blood cells, requiring hospitalization under sterile conditions for weeks to months. Eventually, the patient’s blood stem cells exposed fully to CRISPR-Cas9 (via electroporation) are infused, but the patient must stay in the hospital until the immune system recovers.