CHOP, Penn researchers develop gene editing approaches for PKU treatment

Phenylketonuria (PKU) is a rare newborn genetic disease that impacts between 1 in 10,000 to 1 in 20,000 people, depending on the individuals’ genetic ancestry. PKU causes an amino acid—called phenylalanine (Phe)—to build up in the bloodstream. Uncontrolled PKU can lead to intellectual disability, psychiatric issues, and seizures. While current therapies can partially improve outcomes, they require meticulous, lifelong compliance that is very difficult for most patients.

Now, new research from the Perelman School of Medicine at the University of Pennsylvania and Children’s Hospital of Philadelphia (CHOP) provides insights into potential future treatments using gene editing—specifically, two newer forms of CRISPR gene editing, prime editing and base editing.

Two separate studies, one in The American Journal of Human Genetics and the other in Human Genetics and Genomics Advances, were recently published. 

The findings were also presented at the American Society of Human Genetics (ASHG) annual meeting in Washington, D.C. by Dominique Brooks, a graduate student in the laboratory of Kiran Musunuru, MD, PhD, a professor of Cardiovascular Medicine and Genetics at Penn.

In the first study, the researchers explored a "prime editing" approach to correct the genetic variant responsible for the condition. Prime editing, often compared to a word processor, allows for precise changes to be made in the DNA by rewriting specific genetic sequences. Researchers discovered that patients with a specific genetic variant called c.1222C>T in the phenylalanine hydroxylase (PAH) gene—the most common PKU-causing variant worldwide— had very poor control over their metabolic condition. Using prime editing in mice expressing this variant, they were able to lower build up of Phe while not impacting the liver.

The second study also focused on the most frequent genetic cause of PKU, the same PAH c.1222C>T  variant. Using base editing—a gene-editing technique that precisely modifies a specific DNA sequence by replacing one DNA letter with another DNA letter, reminiscent of a pencil and eraser—they first tested this method in lab-grown liver cells and then in mouse models. The researchers found that when the selected base editor and guide RNA were delivered into the mice via lipid nanoparticles, using similar technology as the COVID-19 mRNA vaccines, Phe levels were normalized within 48 hours.

“These findings are a significant step forward in the treatment of PKU,” said Rebecca C. Ahrens-Nicklas, MD, PhD, an assistant professor of Pediatrics at Children’s Hospital of Philadelphia and a senior author of both studies. “While our results with animal models point us in the right direction, future research is needed to move these advances forward. For example, next, we will focus on refining the base-editing approach and comparing its effectiveness to other gene-editing methods.”

The team of researchers recently received a $26 million grant from the National Institutes of Health to further their efforts to bring a PKU base-editing treatment to the clinic.

You can read more details about the study in a press release from the Perelman School of Medicine at the University of Pennsylvania.