David N. Leff
Editor's note: Science Scan is a roundup of recently published, biotechnology-relevant research.
A mouse's tail is farther from its head than any other part of its body. Yet a gene therapy package injected into the tail veins of mice with brain cancer made its way from caudal appendage to cerebral space - and delivered the anticancer transgenes to the malignant tumors.
The malignancy in question, glioblastoma multiforme (GBM), is the commonest solid tumor in children. Yearly in the U.S., 19,000 new primary brain cancers are diagnosed. GBM kills 5,000 to 6,000 Americans annually - young and old. It is virtually untreatable and inevitably fatal.
GBM cells keep several jumps ahead of the brain surgeon's extirpation of the visible, accessible tumor mass. By the time it's been excised, the surrounding edges of the wound are rife with GBM cells, expanding, infiltrating surrounding tissue, and metastasizing widely into normal brain. The brain-wide invasion leaves further resection, radiation treatment, chemotherapy and putative gene therapy far behind.
The only type of brain cell that can keep up with this loose-cannon migration is the neural stem cell (NSC) - ancestor of all the other cells in the central nervous system.
This parallel behavior - suggestive of hypothetical gene therapy strategies - did not escape the notice of Harvard neurobiologist Evan Snyder. He is senior author of a paper in the Nov. 1, 2000 issue of PNAS - Proceedings of the National Academy of Sciences - titled: "Neural stem cells display extensive tropism for pathology in adult brain: Evidence from intracranial gliomas."
Snyder's is not the first attempt to deputize neural stem cells into posses sent out to track, corner, round up and shoot down the murderous, outlaw GBM cells. But such experimental efforts to date have focused on stem cells in newborn animals - consistent with the assumption that NSCs don't have the same long-distance migratory prowess in adult brains. The PNAS co-authors injected adult NSCs harboring foreign (including human glioblastoma) tumor-fighting genes not only into tail veins but also into remote regions of the brain, and the tumor mass itself. They first rendered their rat and mouse models cancerous by inoculating highly malignant GBM cells into their brains.
The antitumor transgenes expressed cytosine deaminase, which converts to the powerful oncolytic chemotherapy compound 5-fluorouracil. This zeroes in preferentially on rapidly dividing - ergo, malignant - cells. In rodents it killed surrounding tumor cells, even when outnumbered four to one. "Moreover," the PNAS paper recounted, "the foreign gene-expressing neural stem cells seem to follow or 'track' - virtually ride 'piggy-back' - upon those aggressively infiltrating tumor cells escaping into normal tissue."
A commentary accompanying this PNAS article posed numerous caveats to its approach, notably "the increased recognition that treatment with chemotherapeutic agents used to treat CNS tumors may themselves cause injury [to the brain]." However, the commentary also said: "For the patient with a malignant glioma, concern over [such] questions is an unrealistic luxury."
Transgenic Spuds Potentially Immunize Mice With Edible Vaccine Against Hepatitis B Virus Infection
Picture a bunch of hungry children pressing their noses against the windowpane of a house where a sumptuous meal is in progress. Now picture well over 100 million people worldwide who are infected with the hepatitis B virus, but can't afford the highly effective, extremely expensive vaccines available in industrial countries.
Hepatitis B is only occasionally fatal, but its debilitating and pathological effects drastically lower the well being of this infected global population. Besides their out-of-reach cost, vaccines are barred from underdeveloped countries by the sophisticated technologies that attend their production, and the need to keep vaccines refrigerated until use - and ideally administered by mouth rather than needle.
A preliminary report in the November 2000 issue of Nature Biotechnology suggests a picture of genetically engineered fruits and vegetables loaded with edible vaccines aimed at vaulting these barriers. Its title: "Production of hepatitis B surface antigen in transgenic plants for oral immunization." The article's senior author is plant molecular biologist Hugh Mason, at the Boyce Thompson Institute of Plant Research Inc. on the campus of Cornell University in Ithaca, N.Y. Their prototype edible vaccine informs and invests the common potato (Solanum tuberosum).
By definition, a typical vaccine designed to ward off infection consists of immune system cells - antibodies and T lymphocytes - programmed to "remember" target immunogens on or in the specific pathogen. On its surface, the hepatitis B virus (HBV) sports an antigenic protein, HBsAg, which is copiously produced in the livers of infected individuals. Liver cells secrete HBsAg into the bloodstream in the form of highly immunogenic, virus-like particles. These form the active ingredient of current, commercially available, injectable, HBV vaccines. They are so effective that they lower infectivity by an average 90 percent - for those who can afford it.
The co-authors designed a gene-delivery vector containing the potato-specific patatin promoter, to drive transcription of HBsAg. The transgenic plants they raised accumulated 1.1 micrograms of the antigen per gram of fresh potato. They fed mice on raw, cubed, peeled transgenic spuds for three weekly doses. Three weeks after the last dose, the animals' blood contained a primary serum antibody response that peaked at 73 IU (international units) per liter. It persisted at this protective level for the next three weeks. An injection of commercial HBV vaccine triggered an immediate high-level antibody response.
The paper concluded, "The resulting information opens the opportunity for future preclinical and clinical studies of HBsAg as an oral vaccine, with more detailed focus on communication between humoral and mucosal branches of the immune system, which may be important for the commercial development of plant-based vaccines."
"A human trial of our vaccine has recently ended at the Roswell Park Cancer Institute in Buffalo, N.Y.," Mason told BioWorld Today. "A score or so of volunteers, all health workers at Roswell, ingested a somewhat higher dose of the transgenic potatoes than we fed the mice. The results have been submitted for publication."
His laboratory is now "working on an optimal delivery system, currently in tomatoes, but eventually perhaps in leafy vegetables, corn or soybeans - conceivably a candy bar. We are looking for ways to process the material into a uniform and quantifiable product,' Mason observed, "but have not yet identified an industrial partner."