Hip dysplasia in your beloved dog? Prime editing may offer a future solution

If you are like any of the few million or so people in the world who have a Labrador retriever gazing at you right now with big, dopey, droopy, love-filled eyes, you know that these animals are more than just household pets.

Labs are vivacious, spirited, and smart. They make friends with breezy ease, let babies tug on their ears, and can be trained to guide people who have trouble seeing. While they do more than their fair share of shedding, they are great companions and are so good-natured you’d have to work hard to get one to bite you. Yet, as any lab-endowed family can tell you, all is not perfect in the world of Canis lupus familiaris: Like many large, purebred dogs, labs are prone to a serious and debilitating ailment: hip dysplasia.

Hip dysplasia is a deformation of the dog’s hip joint. In this circumstance, the femur bone does not precisely fit into the hip socket, and instead of a smooth ball-and-socket slide, the pieces of the joint rub together painfully. Over time, the cartilage disappears, chronic pain develops, a limp is noticeable, and no one in the house is as happy as they were before. For years, the veterinarian field has had a number of solutions, including everything from triple pelvic osteotomy to total hip replacement. The cost? Thousands of dollars per hip.

Ouch is right.

Could prime editing save dogs from hip dysplasia?

No one wants to see their pet undergo a complicated surgical procedure, and while for now there may be no other viable alternative to dealing with hip dysplasia, prime editing is offering a tantalizing glimpse into the future. Prime editing is seen as a significant update to CRISPR, the gene editing tool that is doing everything in research settings from treating Alzheimer’s to pumping tomatoes full of vitamin D.

Prime editing, first described in 2019 by Anzalone, et al., has been described as a “search and replace” editor for genome editing. Using prime editing, it is possible to directly write new genetic information into a specific DNA site without creating a double-stranded break in the target, and no donor templates are needed to carry the desired changes to the target region. Anzalone, et al. used prime editing in a research setting to edit cultured cells through targeted insertions and deletions to efficiently correct the primary genetic causes of sickle cell disease and Tay-Sachs disease.

Prime editing involves three main parts:

• A prime editing guide RNA, or pegRNA, that finds the target nucleotide sequence to be edited and encodes new genetic information that replaces the targeted sequence
• A Cas9 nickase fused to a reverse transcriptase
• A single guide RNA, or sgRNA, that directs the Cas9 nickase part of the fusion to cut the non-edited DNA strand

Since its announcement, prime editing has undergone a number of developments, the most recent being the creation of twin prime editing that allows the editing of large DNA sequences and addresses one of the method’s main drawbacks.

Prime editing in purebred dogs

While prime editing is still in the development phase when it comes to human applications, researchers continue to push its boundaries with an eye on correcting other genetic problems. Most recent among those is use in purebred dogs.

There are more than 400 breeds of dogs, with the creation of different breeds due to intense artificial selection from a handful of founders—a fact which exposes purebreds to genetic disorders at a higher rate than any other species. Major disorders include cranial cruciate ligament rupture, brachycephalic obstructive airway syndrome, degenerative myelopathy, heart disease, dilated cardiomyopathy, and myxomatous mitral valve disease. Few of these, however, are as disheartening—or prevalent—as canine hip dysplasia.

Hip dysplasia is a common skeletal condition in large dogs, incusing Newfoundlands, great Danes, rottweilers, Bernese mountain dogs, and mastiffs. While the inherited disorder is based in genetics, other factors can pile on to make it worse, including excessive growth, obesity, and a lack of exercise. Treatments for hip dysplasia depend on the severity, running from chondroitin and glucosamine to cold laser treatments, physical therapy, stem cell therapy, and the dreaded total hip replacement.

Recently, however, a team of Korean researchers evaluated another method—prime editing—with the goal of developing an animal model to study the disease and applications.

As described in Nature by Kim, et al., researchers used prime editing in this first-of-its-kind research study. Here, the team collected skin cells from the ear of an 18-month-old lab. Then, using prime editing, a single strand of DNA was cut out and corrected sequencing was inserted in the laboratory. Somatic cell nuclear transfer was then used where the nucleus of a corrected cell was injected into an egg that had previously had its nucleus removed. The end results? Two ridiculously cute puppies named Gene and Geny whose hips look pretty darn normal.

What’s next for prime editing?

Kim, et al. Looked at off-target effects and noted that no off-target mutations were identified in any of the analyzed loci. They wrote in their conclusion that “The off-target analysis results revealed that the PE system is specific in canine cells. These findings are in line with previous studies demonstrating that the PE-mediated base conversion is highly specific.”

According to the study’s authors, this work should be seen only as a starting point for overcoming hip dysplasia in purebred dogs, and they plan additional studies to correct other single nucleotide polymorphisms, or SNPs, while noting there could be a human application.

“We did not find any indel mutation from our sequencing results; however, a more stable form of prime editing, such as RNP, can be recommended in further studies,” they wrote. “Precise editing of pathogenic SNP in dog also provides valuable information for understanding the role of each SNP as it relates to hip dysplasia. Since canine hip dysplasia is remarkably similar in clinical expression and pathogenesis to that of human hip dysplasia, information gleaned from gene-corrected dogs may be very useful for understanding human hip dysplasia. Thus, prime editing may be a very useful tool for generating genome-edited dog models to study human diseases.”

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