Rare Daily Staff

A hybrid approach combining elements of gene therapy with gene editing converted an experimental model of a rare genetic disease into a milder form, significantly enhancing survival, according to a study  

led by researchers at the University of Pennsylvania and Children’s National Hospital in Washington, D.C.

The findings, published online in Science Advances, could offer hope for children and adults with a variety of inborn errors of metabolism.

The study focused on a condition called ornithine transcarbamylase deficiency (OTCD), the most common disease in a family of conditions called urea cycle disorders, which affect about 1 in 30,000 people and impair how the body breaks down dietary proteins. Normally the amines in excess proteins in the body are converted into ammonia, which the liver then converts into urea to be excreted. When a person is deficient in urea cycle enzymes, ammonia builds up in the body causing vomiting, lethargy, and if left untreated, coma and death. While the mother can provide protection to affected fetuses during pregnancy, babies born with this condition often deteriorate in the first week of life and may die before the problem is even diagnosed.

For children and older patients, treatment options are not optimal. They include a heavily restricted protein diet, daily medication that scavenges nitrogen from the blood, or liver transplantation for the most severe cases.

“Through these therapies, we’ve turned this fatal disease into a chronic one for most patients,” said Mark Batshaw, Children’s National executive vice president, physician-in-chief and chief academic officer, and the study’s co-senior author.  “But there’s still no curative approach other than liver transplantation.”

Because these conditions are caused by genetic mutations, Batshaw and James Wilson at the University of Pennsylvania, and co-senior author of the study, have tried different gene therapy approaches to treat OTCD using an experimental model of this condition that carries a mutation that replicates a moderate form of this disease in humans.

This approach involves infecting the preclinical models with a virus that carries a properly functioning form of the OTC gene, which provides the instructions to make the missing essential enzyme, ornithine transcarbamylase. While this method worked well in older animals, it didn’t have long-lasting effects in newborn animals due to their rapid liver growth.

But a different approach, known as gene editing, can effectively correct mutations in the genome, said Batshaw. Wilson’s and Batshaw’s labs successfully treated OTCD in an experimental model with gene editing, reporting this advance in 2017. However, because this approach requires a tailored vector to correct a specific mutation, it’s not universally applicable for the more than 400 different mutations that can cause OTCD.

Seeking a long-lasting way to help patients with this condition regardless of their mutation type, Wilson, Batshaw, and colleagues tested a new approach that combined elements of gene therapy with CRISPR/Cas9-mediated gene editing. The researchers created a viral vector that carried an enzyme necessary to create a targeted break in DNA, a step used in gene editing. Then, rather than simply correct a faulty gene, a second vector carried a copy of the correct OTC gene sequence. The two vectors were given at the same time.

Their results show that for newborn experimental models given this treatment, the new gene successfully integrated in cells and expanded in patches in their livers as they grew, producing successively more of the necessary detoxifying enzyme. These effects were in contrast to animals given a vector that wasn’t targeted to the affected gene or those that went completely untreated.

When the animals were challenged with a nitrogen load, those that had been effectively treated by the combined strategy had about 60 percent lower ammonia levels in their bloodstreams compared with untreated animals. While all the treated animals survived the seven-day test, only about one-quarter of the untreated ones did.

This approach still has multiple hurdles to cross before it can become available in the clinic – including safety studies in other preclinical models and safety and efficacy studies in people.  

“Theoretically, this could be a curative approach for OTCD,” said Batshaw. “And if it worked for that, we could create similar templates to treat other related disorders.”


Photo: Mark Batshaw, Children’s National executive vice president, physician-in-chief and chief academic officer

 

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