Background: Correcting splicing defects in an FHL3 model

Cytosine base editing (CBE) restored Unc13d splicing and cytotoxic T-cell function in a mouse model of familial hemophagocytic lymphohistiocytosis type 3 (FHL3). Protection against virus-induced hyperinflammation was also observed in mice that received edited hematopoietic stem cells. However, genotoxicity comparisons revealed different patterns depending on the editing platform and cell type.

Base editing is an approach for correcting point mutations without requiring DNA double-strand breaks. That feature alone, however, does not explain all of its effects on the genome. The researchers used CBE to disrupt a cryptic splice site in the Unc13d gene that causes aberrant splicing in Jinx mice, an FHL3 model, and assessed both functional recovery and genotoxicity.

Key findings: 62%–89% editing and functional recovery, alongside genotoxicity findings

The researchers reported editing rates of 62%–89% in fibroblasts, T cells, and hematopoietic stem cells (HSCs). After editing, Unc13d splicing was restored, as was cytotoxic T-cell function. Mice transplanted with edited HSCs were protected from virus-induced hyperinflammation. This editing-rate range describes genomic editing in cells; it does not represent animal survival rates or treatment success rates.

In the genotoxicity comparison, the hyperactive CBE used in this study produced broader off-target activity and more structural variants than the CRISPR-Cas9 gene-editing system. Off-target editing refers to editing that occurs outside the intended target. This result should be interpreted as a comparison involving the platform used in this study; it does not justify generalizing that all CBEs carry the same risks.

Off-target sequence edits persisted, but the stability of CBE-induced chromosomal translocations varied by cell type. Persistence of sequence-level edits should be distinguished from the persistence of translocations, in which chromosome fragments move and attach to another chromosome. The supplied abstract does not specify in which cells translocations persisted longer or what mechanism underlies the difference.

Implications and outlook: Assessing therapeutic potential alongside safety

This study showed that gene editing in an FHL3 mouse model can extend beyond molecular restoration of splicing to recovery of T-cell function and protection against hyperinflammation after transplantation. It also provides evidence that safety assessments of therapeutic editors should consider the cell type to be edited as well as the platform.

The material reviewed, however, consists of the paper’s abstract and bibliographic information. Cell-specific editing rates, animal numbers, virus type, the magnitude of the protective effect, and follow-up duration cannot be confirmed from these materials alone. The findings should not be interpreted as clinical evidence of efficacy, a cure, or long-term safety in humans.

The study appeared in the June 2026 issue of Cell Stem Cell. Its key findings and bibliographic information are available in the abstract indexed in Europe PMC.