SAN FRANCISCO—About 4,000 human diseases have a genetic cause, and many such diseases are untreatable or poorly treated by conventional medicine, said R. Michael Blaese, MD, chief of the Clinical Gene Therapy Branch at the NIH National Center for Human Genome Research. In theory, many of these diseases could be treated by adding, deleting, or altering genes.
Clinical trials in adenosine(Drug information on adenosine) deaminase (ADA) deficiency have demonstrated the practicality of inserting genes to cure disease, Dr. Blaese said in the Neel Distinguished Research Lecture given at the 101st Annual Meeting of the American Academy of Otolaryngology-Head and Neck Surgery.
The missing ADA gene has successfully been inserted into several patients by infecting autologous bone marrow or stem cells with an altered retrovirus that contains the missing genetic material. The infected cells are cultured and infused back into the patient where they begin replicating and producing the missing enzyme.
Because the altered cells have a selective survival advantage over native cells, they eventually replace enough of the patient’s original marrow or stem cells to produce normal cellular immunity and antibody responses.
Cancer Applications
Now gene therapy is moving toward oncology. “It has been a number of years since we have seen a major new tool to treat cancer,” Dr. Blaese said. The theoretical framework to turn genes against cancer is in place, he noted, and, “the potential of gene therapy is enormous,” but it is a long way from being realized.
Dr. Blaese pointed to several avenues of research for gene therapy in oncology. Adjuvant gene therapy might increase the ability of drugs to bind selectively to malignant cells. A tumor vaccine might make cancers more susceptible to drug therapy. Chemoprotective therapy might make nonmalignant cells more resistant to chemotherapeutics, allowing higher treatment thresholds for conventional drugs.
There are also gene therapies that attack cancer directly. Suicide genes show great therapeutic potential. The idea, Dr. Blaese explained, is to insert into cancer cells a gene that converts a relatively benign drug into a toxic product that selectively kills altered cells.
An early candidate is the herpes simplex virus thymidine kinase gene (HSV-tk). The gene phosphorylates ganciclovir(Drug information on ganciclovir) (Cytovene) into a highly toxic compound that kills only altered cells.
Another possibility is inserting genes for bacterial proteins that convert 5-fluorocytosine into 5-fluorouracil, killing malignant cells without exposing the rest of the body to the highly toxic drug.
In murine models, inserting the HSV-tk gene into just 40% of tumor cells is enough to destroy the entire cancer through the “bystander effect.” Gap junctions between malignant cells allow the transfer of molecules between cells, Dr. Blaese said, greatly expanding the killing range of the altered cells. Because gap junctions do not extend into normal tissue, the rest of the body is unaffected by the treatment.
From Theory to Practice
Reducing theory to practice is the slow work of biophysics and biomechanics, Dr. Blaese said. The retrovirus used to treat ADA is too unstable to infect cancer cells in vivo, but murine cells containing the altered virus have been used in early human trials.
Mouse cells carrying a retrovirus altered to code for HSV-tk have been injected directly into brain and breast tumors on multiple parallel tracks. Once the virus has had time to infect tumor cells, patients are given ganciclovir.
Among 12 patients with inoperable refractory brain tumors, Dr. Blaese reported, there were two complete re-sponses and two partial responses to this form of gene therapy. The complete responders were still alive at 52 and 47 months post-treatment.
These early trials revealed several problems, Dr. Blaese noted. The murine cells and viruses are too large to diffuse easily into the tumor, so relatively few tumor cells are actually infected with the HSV-tk gene. It also turns out that there is a significant variation in the expression of gap junction between tumors. Depending on the frequency of gap junctions, the bystander effect ranges from lethal to almost nil.
Researchers are already looking for motile vector-producer cells that might diffuse throughout a wider area, as well as different vectors such as adenovirus vectors, Dr. Blaese said. He also noted that work is under way with replicating viruses engineered to replicate only within tumor tissue. A virus has already been developed, he said, that replicates only in cells containing a mutated form of p53 that is unique to cancerous cells.
“Genes are big, big molecules,” Dr. Blaese said. “It’s our challenge to find better ways to deliver them.”
