CRISPR-based CASGEVY therapy uses TIDE

The first-ever FDA-approved CRISPR therapy doesn't use next-generation sequencing (NGS) to validate its potency assay. It uses TIDE, and there's a good reason why.

Delving into older regulatory submissions can offer rare, sometimes surprising insights into the experimental approaches of groundbreaking therapies. That's the case with the first-ever CRISPR-based therapy approved by the U.S. Food and Drug Administration (FDA). Vertex Therapeutics' Biologics License Application (BLA) for CASGEVY provides such a gem, highlighting that validating gene-editing frequency doesn't need to be complex, slow, or costly—accurate simplicity can outperform high resolution.

The FDA approved CASGEVY for sickle cell disease in 2023 and transfusion-dependent β-thalassemia in 2024. The therapy reactivates fetal hemoglobin in the hematopoietic stem and progenitor cells, allowing the patient to produce healthy red blood cells after a one-time autologous transplant.

The approach is elegantly simple: After collecting the patient's blood, the CD34+ hematopoietic stem cells are isolated and expanded. These cells are then exposed to the Cas9/SPY101 ribonucleoprotein complex targeting B-cell lymphoma/leukemia 11A (BCL11A), a gene that represses fetal hemoglobin in adult red blood cells. Disrupting this gene allows fetal hemoglobin to compensate for the defective adult form.

But how do QC teams know that the Cas9 has efficiently and accurately edited the BCL11A before transfusion? What method safeguards the lot quality?

One would intuitively expect that a groundbreaking therapy like CASGEVY would rely on next-generation sequencing (NGS) as the go-to approach for on-target gene-editing validation. After all, NGS has long supported clinical and preclinical research through high-resolution genomic profiling, detecting single-nucleotide variants, indels, and off-target edits across the entire genome.

However, Vertex's public BLA reveals that the company opted for another approach: TIDE (Tracking of Indels by DEcomposition). We can safely assume that extensive internal NGS validation was conducted before choosing TIDE, the simplest solution.

Why TIDE over NGS?

Any gene-editing therapy must show that developers can accurately assess the efficiency of gene-editing tools and the genetic changes they cause. As the traditional gold standard of genomic sequencing, NGS provides precise allele-level indel quantification and is the superior choice at very low editing efficiencies, including relative to TIDE.

However, once the gene-editing efficiency exceeds TIDE's sensitivity threshold (1–2%), the accuracy between NGS and TIDE converges. For example, with CASGEVY routinely generating 80–90% editing in CD34+ stem cells, we expect that a single Sanger sequencing reaction yields TIDE results that almost perfectly match NGS.

With a comparable accuracy profile between TIDE and NGS, practical considerations take over—and here, TIDE's advantages compound.

Assuming labs outsource sequencing, we can estimate that NGS requires a turnaround time of 3–7 days (1–3 days if performed in-house). A Sanger-based TIDE analysis, on the other hand, requires a simple PCR run and, if outsourced, one day to receive the results (2–6 hours in-house). For developers and CMC teams, an alternative that requires less specialized staff and equipment is the decision-maker.

TIDE's suitability for CASGEVY isn't incidental; it aligns with the product's biology. CRISPR/Cas9-induced double-strand breaks repaired via non-homologous end joining produce a limited, predictable spectrum of small indels at the target site. There is no need to scan the entire genome at single-nucleotide resolution when the expected outcome is well-defined and confined. For high-efficiency edits like those in CASGEVY, NGS's higher resolution adds cost, time, and complexity but doesn't provide additional helpful information.

Finally, regulators require sponsors to demonstrate that their data are generated according to GxP-compliant procedures. The FDA-approved CASGEVY's use of TIDE as its on-target editing QC method demonstrates regulatory acceptance of TIDE for GxP-compliant lot-release testing. This matters for sponsors because a validated method reduces the burden of justification and the risk of regulatory pushback. Sometimes, the simplest defensible choice is also the smartest one.

The CASGEVY approval has laid the regulatory groundwork for TIDE in GxP-compliant workflows. For teams developing similar gene-editing therapies, this example offers a clear choice for lot-release testing—one that the FDA has already approved.