For Immediate Release: March 9, 2004
Contact: Bob Kuska, (301) 594-7560
Two years ago, scientists reported exciting news in the development of gene therapy for the salivary gland. They had constructed a new version of a gene-carrying vehicle, or vector, that functioned well in the salivary glands of mice for several weeks. Most significantly, the vector - a stripped down, bioengineered version of the harmless adeno-associated virus (AAV) - had done so without triggering a sustained immune response, a common setback in gene therapy experiments.
Left unanswered, however, was whether the AAV gene vector could keep up the good work for several months or a year. As published recently in the journal Proceedings of the National Academy of Sciences , the scientists report the vector performed admirably in the salivary glands of mice for at least one year. These results now clear the way scientifically to advance the preclinical work into mice bred specifically to model human disease and also into larger animals, which would allow them to better scale the likely therapeutic dosage in people for possible clinical trials.
“The study was scheduled to run for 54 weeks, which really was an arbitrary cut- off point,” said Antonis Voutetakis, a scientist at the National Institute of Dental and Craniofacial Research (NIDCR) and lead author on the study. “We found at the 54-week mark that the salivary glands were still producing relatively large and steady amounts of the replacement protein and showed no immediate signs of slowing down.”
In the latest work, Voutetakis and colleagues created an AAV vector that carried the gene for human erythropoietin, a hormone produced primarily in the kidneys to stimulate the production of red blood cells. After injecting a relatively low-dose of the vector into the submandibular salivary glands of the mice, they began to detect a gradual increase in serum levels of the human erythropoietin over the first 12 weeks, a sign that the salivary gland was producing and pumping the hormone into the bloodstream. Thereafter, serum levels of the human erythropoietin remained relatively stable to the 54- week mark.
“We chose to administer a low dose of the vector because the submandibular gland contains a small volume of encapsulated cells,”said Bruce Baum, an NIDCR scientist and senior author on the paper. “This was a proof-of-principle study, and we thought there was no need to overburden the cells at this point with a moderate or possibly heavy therapeutic doses of vector.”
Baum said the group was also pleased again to find no signs of an immune response in the salivary glands, where about 15 percent of cells contained vector. Importantly, the AAV vector also remained contained within the salivary glands, as the scientists found no signs of the vector in the liver, spleen, or testis. If the vector were to migrate to other organs, it could perhaps lead to complications.
According to Baum, as this study attests, there is nothing unusual about the salivary glands pumping a protein into the bloodstream instead of into the mouth. Frequently overlooked in the medical literature, our salivary glands not only release saliva into the mouth, they routinely secrete digestive enzymes and other proteins into the circulatory system.
With their unique physiology to secrete protein in two directions, the salivary glands are an intriguing target for gene therapy of so-called “single-protein deficiencies,” such as type I diabetes, human growth hormone deficiency, and erythropoietin-responsive deficiencies. As Baum and colleagues already have demonstrated, the salivary glands readily accept gene-carrying vectors. Thereafter, with minimal coaxing, they act as natural protein factories, dutifully manufacturing the encoded replacement protein and pumping it at steady levels into the circulation.
Moreover, the salivary glands offer a way around many of the potential pitfalls of gene therapy in other organs, such as the liver. Salivary glands are easily accessible and non-essential to life should anything go wrong. And, as the Voutetakis study indicated, salivary gland cells are encapsulated to prevent leakage of the vector into the circulation and to other tissues.
“Obviously, gene transfer in the salivary glands will not be ideal for all diseases,” said Baum. “But it clearly has a place in the pantheon of gene therapeutic approaches for systemic, single-protein deficiencies. The approach scientifically is maturing rapidly, and, as the data indicate, there is no doubt that it should be vigorously pursued and utilized.”
The paper is titled, “Reengineered salivary glands are stable endogenous bioreactors for systemic gene therapeutics,” and it appears in the March 2, 2004 issue of the Proceedings of the National Academy of Sciences. The authors are: Antonis Voutetakis, Marc R. Kok, Changyu Zheng, Ioannis Bossis, Jianghua Wang, Ana P. Cotrim, Natanya Marracino, Corrine M. Goldsmith, John A. Chiorini, Y. Peng Loh, Lynnette K. Nieman, and Bruce J. Baum.