New York, NY – Previous attempts in mice to correct a rare inherited immune disorder, called Hyper IgM X-linked immunodeficiency, have failed because standard gene therapy raised risks for cancer.

Now Weill Cornell Medical College researchers believe they've found a way around that problem.

Reporting in the July 25 issue of Nature Medicine, the investigators used a gene therapy strategy called trans-splicing to successfully correct the disease in mice without increasing malignancy risk. Trans-splicing effectively corrects the mutation at a later step in the genetic process – at the level of messenger RNA (mRNA).

"The next step is to consider moving it to the clinic. Here at Weill Cornell we have the facility for doing that, and as we develop the safety data, our plans are to move it to humans," said lead researcher Dr. Ronald G. Crystal, Chairman of the Department of Genetic Medicine at Weill Cornell Medical College and Chief of the Division of Pulmonary and Critical Care Medicine at NewYork-Presbyterian Hospital/Weill Cornell Medical Center.

Hyper IgM X-linked immunodeficiency is a rare disease affecting a few hundred children and young adults in the United States. Caused by a genetic breakdown in immune antibody production, individuals with this disease "can't make the right array of antibodies to protect against pathogens in the environment," leaving them vulnerable to lethal infections, Dr. Crystal explained.

Although bone marrow transplants can help prolong life, a majority of patients affected with the disorder die before reaching the age of 30.

Luckily, advances in genomics may someday give these patients a new treatment option.

"In the immune system, immune cells are primarily made in the bone marrow, so if we want to attack this by trying to correct it genetically, we know that's where the cells are that we want to correct," Dr. Crystal explained.

Reduced production of an immune cell protein called CD40 ligand lies at the heart of Hyper IgM X-linked immunodeficiency. Previous studies in mice used standard gene therapy – where doctors simply introduce a "normal" gene into bone marrow stem cells – to try and repair the problem.

However, standard gene therapy caused the gene responsible for CD40 ligand production to work overtime, producing excessive amounts of the protein. According to Dr. Crystal, too much CD40 ligand can be as bad as too little, in that it can trigger lymphomas.

"So by putting in the normal gene, but having it turned on all the time, the mice developed cancer," he said.

His team of researchers decided to attack the problem from a different angle. As Dr. Crystal explained, the gene-regulated production of cellular proteins occurs in a step-by-step fashion, with DNA producing a second molecule, called messenger RNA (mRNA), which works to produce cellular proteins like CD40 ligand.

"Unfortunately, individuals with Hyper IgM X-linked immunodeficiency make mRNA that has a mutation in it, so they can't make the CD40 ligand protein," Dr. Crystal said.

With trans-splicing, "we are putting in a gene that makes a piece of correct 'decoy' mRNA that's attractive to the genetic process. This means that the natural mRNA, as it's being processed or 'spliced,' integrates into our decoy instead. In this way it repairs the genetic defect."

Fixing the problem at the mRNA level leaves DNA gene regulation intact, getting around the problem of excessive CD40 ligand production that occurs with standard gene therapy.

The result? In mice bred with X-linked immunodeficiency with Hyper IgM, treatment with this trans-splicing gene therapy "not only repaired the genetic disease, but also avoided cancers," Dr. Crystal said.

Treated mice successfully fended off infection with Pneumocystis carinii, a type of pneumonia that can prove lethal to people with this illness.

Dr. Crystal noted that this gene therapy technique might also be used to treat a wide range of genetic illnesses. "We're simply correcting at the second step, at the level of mRNA," he said.

The next step for his team at Weill Cornell is to test out the technique in humans. According to Dr. Crystal, preparations for clinical trials are already underway.

Study co-researchers include Drs. Minoru Tahara, Robert G. Pergolizzi, and Hiroyasu Kobayashi, of the Weill Cornell Department of Genetic Medicine; Drs. Anja Krause and Karsta Leuttich, of the Weill Cornell Belfer Gene Therapy Core Facility; and Dr. Martin L. Lesser, of the Biostatistics Unit at the North Shore-Long Island Jewish Research Institute, in Manhasset, New York.

This research received funding from the National Heart, Lung, and Blood Institute; the Will Rogers Memorial Fund, Los Angeles; and The Malcolm Hewitt Wiener Foundation, Greenwich, CT.