Treatment of a metabolic liver disease by in vivo genome base editing in adult mice

CRISPR–Cas-based genome editing holds great promise for targeting genetic disorders, including inborn errors of hepatocyte metabolism.

Precise correction of disease-causing mutations in adult tissues in vivo, however, is challenging. It requires repair of Cas9-induced double-stranded DNA (dsDNA) breaks by homology-directed mechanisms, which are highly inefficient in nondividing cells. Here we corrected the disease phenotype of adult phenylalanine hydroxylase (Pah)enu2 mice, a model for the human autosomal recessive liver disease phenylketonuria (PKU)1, using recently developed CRISPR–Cas-associated base editors.

These systems enable conversion of C∙G to T∙A base pairs and vice versa, independent of dsDNA break formation and homology-directed repair (HDR). We engineered and validated an intein-split base editor, which allows splitting of the fusion protein into two parts, thereby circumventing the limited cargo capacity of adeno-associated virus (AAV) vectors. Intravenous injection of AAV-base editor systems resulted in Pahenu2 gene correction rates that restored physiological blood phenylalanine (L-Phe) levels below 120 µmol/l. We observed mRNA correction rates up to 63%, restoration of phenylalanine hydroxylase (PAH) enzyme activity, and reversion of the light fur phenotype in Pahenu2 mice.

Our findings suggest that targeting genetic diseases in vivo using AAV-mediated delivery of base-editing agents is feasible, demonstrating potential for therapeutic application.

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