While head and neck radiation therapy kills cancerous cells, it also often destroys the acinar (fluid-producing) cells of salivary glands that lie within the field of radiation. When this occurs, patients are unable to produce adequate saliva, and as a result suffer a host of long-term problems including xerostomia (dry mouth), inflammation of the mucous membranes lining the mouth, dental caries, frequent infections of the mouth and pharynx, and difficulty with swallowing, speech, and taste. Although clinicians and researchers have recognized these side effects for nearly a century, they have had little to offer patients in the way of treatment.
Now NIDR researchers may have found a way around the problem by coaxing cells into doing what doesn’t come naturally. Unlike acinar cells, ductal cells in salivary glands frequently are not destroyed by irradiation. But ductal cells lack the ability to make or secrete saliva. The researchers sought to re-engineer ductal cells into fluid-producing cells by giving them the gene for an aquaporin protein. Aquaporins are a recently discovered family of proteins that form pores in cell membranes, through which fluid can pass.
The scientists inserted an aquaporin gene into an adenovirus—similar to a cold virus—that had been genetically altered so it could not reproduce. After irradiating the salivary glands of rats to significantly diminish saliva production—mimicking what happens to head and neck cancer patients following radiation therapy—the researchers infected the animals’ salivary glands with the adenovirus carrying the aquaporin gene. Remarkably, the rats’ salivary glands produced fluid. The researchers reported their results in the April 1 issue of the Proceedings of the National Academy of Sciences.
Although the investigators caution that it may be several years before this technique can be tried in humans, they are optimistic about the potential use of the therapy for restoring salivary gland function. "It is an important first step to managing a condition for which no suitable and effective therapy is currently available," said Dr. Bruce Baum, chief of the NIDR Gene Therapy and Therapeutics Branch and principal investigator on the study.
In the first stage of their study, the researchers irradiated a group of rats with a single dose of radiation that, in rats, causes moderate salivary gland damage. A second group of rats served as controls and were sham irradiated—that is, they were placed in the irradiator, but it was not activated. Three months later, the irradiated rats experienced a 30 percent reduction in salivary flow.
The investigators then infected the salivary glands of both irradiated and control rats with an adenovirus. Some animals received the adenovirus containing the aquaporin gene; others were given a control virus without the aquaporin gene. Several days later, the animals’ salivary flow was measured. Irradiated rats who received the virus containing the aquaporin gene secreted nearly 2-3 times as much saliva as irradiated rats infected with the control virus.
In the next stage of the study, one group of rats was exposed to an even higher level of radiation, and a control group was sham irradiated. Four months later, salivary secretion decreased by 64 percent in the irradiated rats, but not in the control animals. The researchers note that this level of salivary hypofunction is similar to what head and neck cancer patients can experience following radiation therapy. In addition, the salivary glands of the irradiated rats showed a significant loss of acinar cells. When the irradiated rats’ salivary glands were infected with an adenovirus containing the aquaporin gene, their salivary secretion increased two-fold, approaching a normal level.
Findings from this study suggest the possibility of someday using gene therapy to correct the salivary gland defects that result from head and neck radiation therapy. The researchers point out, though, that the fluid-producing effect from the adenovirus infection is a transient one, since the viral infection does not last indefinitely. The next step, they say, will be to conduct similar studies in nonhuman primates. The investigators also will assess the composition of the fluid produced by aquaporin-infected glands to see if it functions like normal saliva.
Working with Dr. Baum on the study were Drs. Christine Delporte, Brian C. O’Connell, Xinjun He, Henry Lancaster, and Anne C. O’Connell from the NIDR; and Dr. Peter Agre, the discoverer of aquaporins, from the Johns Hopkins University School of Medicine.
The NIDR, one of the National Institutes of Health, conducts and funds research to improve oral, dental, and craniofacial health. To learn more about the Institute and its programs, visit us on the World Wide Web at http://www.nidr.nih.gov.