By Dean Haycock

Special To BioWorld Today

A genetically reengineered virus has the potential to undo some of the damage caused by radiation therapy in patients with cancers of the head and neck.

Each year in the U.S., an estimated 40,000 people get such cancers (brain cancer excluded). Most of these patients receive ionizing radiation therapy, which, unfortunately, often has the serious side effect of destroying the acinar, or fluid-producing, cells in their salivary glands. The consequences of this collateral damage go beyond the discomfort of a chronic dry mouth and inflammation of its mucous membranes. Without the lubricant of saliva, patients have difficulty eating and swallowing. Without its antiseptic properties, they suffer frequent infections and develop tooth cavities. Unlike artificial tears, artificial concoctions prepared to replace saliva just don't work very well.

"It is not clear why, but it has been much easier to make artificial tears than artificial saliva," Bruce Baum, chief of the Gene Therapy and Therapeutics Branch at the National Institute of Dental Research (NIDR), said.

Although no one knows the exact nature of the radiation damage to salivary glands, it is known that acinar cells are more sensitive than ductal cells, which are also part of the salivary gland. Ductal cells, in fact, survive radiation well. Unfortunately, they don't produce or release saliva or other fluid.

There is now one exception to this statement: rats in Baum's lab at the NIDR have ductal cells that produce saliva, or at least a very similar fluid.

Despite damage to their acinar cells induced by radiation, these rats are releasing fluid from their ductal cells. Baum and his co-authors describe how they induced radiation-damaged glands to resume fluid secretion in the April issue of the Proceedings of the National Academy of Sciences. Their article is titled "Increased fluid secretion after adenoviral-mediated transfer of the aquaporin-1 cDNA to irradiated rat salivary glands."

The authors reasoned that it might be possible to manipulate non-fluid producing ductal cells to take over the job of saliva-producing acinar cells. To induce this change, the scientists re-engineered the cells by introducing into them a gene which would, in a sense, provide a channel for them to release fluid. The gene, which was delivered with the help of non-replicating adenoviruses, codes a protein called aquaporin-1, one of a family of recently described water channel proteins. Aquaporin-1 provides ductal cells with an important tool they normally lack, a pore that allows them to release fluid through their largely water-impermeable cell membranes.

In vitro experiments showed that the recombinant adenovirus successfully directed the expression of human aquaporin-1 in several epithelial cell lines.

Then Baum and his team exposed rats to enough radiation to decrease their saliva production by one-third. Results indicated that irradiated rats treated with virus carrying the aquaporin-1 gene secreted two or three times as much fluid as irradiated rats treated with virus that did not carry the gene.

Next, the researchers exposed rats to a higher dosage of radiation, enough to decrease saliva production by two-thirds. Treatment of irradiated rats with the gene-carrying virus doubled fluid production compared to irradiated controls that did not receive the new gene.

"We don't know the exact mechanism of the effect and we are putting a fair amount of effort into detailed mechanistic studies," Baum said.

These will include detailed analysis of the fluid produced and comparison with saliva produced by acinar cells.

Therapy For Humans Is A Long Way Off

Baum cautions that these preliminary results do not mean that a treatment for humans is imminent. Nevertheless, these studies suggest the possibility of one day using a similar gene therapy approach to aid cancer patients whose salivary glands have been damaged by radiation.

One problem that will have to be overcome faces every researcher who uses adenoviruses as vectors to carry genes into cells. The viral infection is transient because the immune system destroys cells infected with the virus. Baum noted that many scientists are working on ways around this problem.

"Deep down inside, I feel this is a transient problem. In five or 10 years, people will have much better ways to deliver foreign genes," Baum said.

The critical point, in Baum's view, is that his group has demonstrated "a proof of principle."

His group now is working on the development of an oral tolerance model, in which rats could be fed adenovirus by mouth and have their immune responses blunted at the same time.

In his paper, Baum notes that there are two "current, significant and presently inescapable limitations to fully understanding the results presented here." These are the facts that ductal cell physiology is poorly understood and the nature of the damage to salivary glands produced by ionizing radiation is unknown."

It is clear, however, that the gene transfer protocol is effective in rats.

"It is an important first step to managing a condition for which no suitable and effective therapy is currently available," Baum said.

As early as May, Baum and his colleagues will extend their gene therapy experiments to primates. The primate experiments will follow the same general approach and will test the effects of the gene transfer technique in the parotid glands of irradiated monkeys. *