Some heritable diseases manifest during the fetal stage, causing considerable morbidity or mortality after birth. Editing the genome and correcting these mutations in utero before the onset of disease could be a new treatment to cure these diseases, particularly in conditions that currently have no treatments. Now, new research by Kiran Musunuru, William Peranteau and colleagues provides proof-of-concept work showing that gene editing disease-causing genes during fetal development in mice could be a viable therapy option.

To perform the gene editing, the researchers used a CRISPR-based base editor (BE3) instead of standard CRISPR–Cas9 genome editing, as BE3 does not cause double-strand breaks and is more efficient at single base pair changes than CRISPR–Cas9. “We initially focused on PCSK9, which encodes a protein that is crucial in the regulation of plasma cholesterol homeostasis, as a proof-of-concept, and then turned our attention to a grievous liver disorder that affects neonates, hereditary tyrosinaemia type 1 (HT1),” explains Musunuru.

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One of the consequences of the loss of function of PCSK9 in humans is reduced levels of cholesterol. To test whether knockout of murine Pcsk9 would result in reduced levels of PCSK9 and cholesterol in postnatal mice, the researchers injected a BE3 construct that would introduce a nonsense mutation into Pcsk9 via the vitelline vein, which directly delivers the construct to the entire fetal liver, at embryonic day 16. At postnatal day 1, the mice that were injected with the viral construct showed base editing of Pcsk9 that was restricted to the liver. The in utero edited mice showed decreased levels of PCSK9 and total cholesterol.

Following the success of the proof-of-concept experiments, the researchers used BE3 as a therapeutic treatment in a mouse model of HT1. HT1 is caused by mutations in FAH, which disrupts the metabolism of tyrosine, leading to liver and kidney failure. Treatment with nitisinone inhibits an upstream enzyme (HPD) in the tyrosine metabolism pathway, preventing the accumulation of toxic metabolites.

To treat HT1 using base editing, the researchers introduced a nonsense mutation into Hpd in Fah−/− mice (a mouse model of HT1) using the in utero editing technique. The Fah−/− mice die shortly after birth if they are not treated with nitisinone. “What we were surprised to see is that the fetal gene editing not only cured the disease in the Fah−/− mice, but the edited mice also did much better than nitisinone-treated Fah−/− mice, gaining more weight and thriving well into adulthood,” notes Musunuru.

The team will investigate other heritable liver disorders that can also be treated using this technique. “In utero gene editing has the potential to offer a new treatment approach for select diseases that cause significant morbidity and mortality and for which treatments do not currently exist,” concludes Peranteau.