# Base Editing

Canonical URL: https://menfem.com/kb/genomics/concepts/base-editing
Knowledge base topic: [Genomics](https://menfem.com/kb/genomics)
Frontier status: breakthrough
Tags: base-editing, adenine-base-editor, cytosine-base-editor, in-vivo, lnp, gene-therapy, safer-crispr

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Base editing chemically converts a single DNA base into another — **A→G** (adenine base editor, ABE) or **C→T** (cytosine base editor, CBE) — **without creating a double-strand break**. Invented in David Liu's lab (the same lineage as prime editing), it fuses a catalytically impaired Cas to a deaminase enzyme, so the editor "rewrites" one letter at a precise position rather than cutting and relying on the cell's repair machinery. Because there is no double-strand break, base editing avoids the indels and chromosomal rearrangements that cut-based CRISPR-Cas9 can produce — the reason it is widely framed as a "safer CRISPR." With two base editors you can make four of the possible single-base conversions, which covers a large fraction of known pathogenic point mutations.

By 2026 the modality has **broken through in the clinic across multiple indications**, almost all using **lipid nanoparticle (LNP) delivery to the liver** for in vivo correction (Beam, Verve) plus ex vivo programs (sickle cell). The contrast with [Intellia's in vivo *knockout* approach](./crispr-clinical-translation.md) is instructive: base editing *corrects* a disease-causing letter, whereas a Cas9 knockout simply *inactivates* a gene. Both now have human data.

## Key Claims

- **First-ever genetic correction in AATD patients** — Beam BEAM-302 (in vivo ABE, LNP-to-liver) corrects the **PiZ (Z-AAT)** mutation. Single IV doses (15–60 mg) gave dose-dependent, durable increases in total AAT and decreases in Z-AAT; the highest-dose cohort reached AAT levels sufficient to halt disease progression. *Evidence: moderate* ([Beam clinical 2026](../../raw/beam-base-editing-clinical-2026.md))
- **>90% reduction in disease protein, sustained ≥36 weeks (sickle cell)** — Beam BEAM-101 (ex vivo) eliminates the need for busulfan conditioning; a treatment-related death has been reported and is under monitoring. *Evidence: moderate* ([Beam clinical 2026](../../raw/beam-base-editing-clinical-2026.md))
- **FDA clears BEAM-304 IND for PKU (Jun 2026)** — In vivo ABE correcting **PAH** mutations (first the R408W variant); preclinical mouse data showed robust liver editing and normalized plasma phenylalanine. *Evidence: strong (regulatory action)* ([Beam clinical 2026](../../raw/beam-base-editing-clinical-2026.md))
- **Up to 69% LDL reduction (cardiovascular)** — Verve VERVE-102 (PCSK9 base editing) Phase 1b, dose-dependent, no serious adverse events. *Evidence: moderate* ([Beam clinical 2026](../../raw/beam-base-editing-clinical-2026.md))
- **No double-strand break = safer profile** — When VERVE-101 showed thrombocytopenia, it was attributed to the LNP vehicle, not the editing machinery. *Evidence: moderate* ([Beam clinical 2026](../../raw/beam-base-editing-clinical-2026.md))
- **Base editing scales to functional genomics** — Multiplexed in vivo base editing maps gene–variant–context interactions at organism scale, bridging in vitro variant screens to whole-organism biology. *Evidence: strong (preprint)* ([Multiplexed base editing](../../raw/multiplexed-in-vivo-base-editing-biorxiv-2026.md))

## Benchmarks & Data

| Program | Company | Indication | Target | Readout |
|---|---|---|---|---|
| BEAM-302 | Beam | Alpha-1 antitrypsin deficiency | PiZ / Z-AAT | First genetic correction; dose-dependent durable AAT rise (15–60 mg) |
| BEAM-101 | Beam | Sickle cell disease | (ex vivo, fetal Hb) | >90% disease-protein reduction, ≥36 wk; no busulfan |
| BEAM-304 | Beam | Phenylketonuria | PAH (R408W) | FDA IND cleared Jun 2026; Phase 1/2 planned |
| VERVE-102 | Verve | Familial hypercholesterolemia | PCSK9 | Up to 69% LDL reduction, no serious AEs |
| VERVE-201 | Verve | HoFH | ANGPTL3 | First patient dosed |

## Open Questions

- Durability of in vivo base edits over years, and whether liver turnover erodes the correction.
- Long-term off-target and bystander-editing safety as doses escalate.
- Whether the sickle-cell safety signal is editor-related or conditioning/disease-related.
- Pricing and reimbursement for one-time corrective therapies.

## Related Concepts

- [Prime Editing](./prime-editing.md) — The search-and-replace sibling (same Liu lineage); handles insertions/deletions base editing cannot.
- [CRISPR Clinical Translation](./crispr-clinical-translation.md) — Where base editing sits in the broader clinical pipeline, alongside Intellia's in vivo knockout.
- [Gene Therapy Delivery](./gene-therapy-delivery.md) — LNP-to-liver is the dominant in vivo delivery route for these programs.

## Backlinks

*Pages that reference this concept:*
- [Beam Base Editing Clinical 2026](../../raw/beam-base-editing-clinical-2026.md)
- [Multiplexed In Vivo Base Editing](../../raw/multiplexed-in-vivo-base-editing-biorxiv-2026.md)
- [Beam Therapeutics](../entities/beam-therapeutics.md)

## Changelog

- **2026-06-24** — Initial compilation from the Beam 2026 clinical pipeline (BEAM-302/304/101) plus Verve cardiovascular programs and the multiplexed in vivo base-editing preprint. Created to separate base editing from prime editing as a distinct, now clinically-validated modality.

## Sources

- beam-base-editing-clinical-2026
- multiplexed-in-vivo-base-editing-biorxiv-2026

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Cite as: MenFem Knowledge Base — https://menfem.com/kb/genomics/concepts/base-editing