Experiment Turns Cancer Cells Healthy
Researchers at UC San Diego using a gene replacement technique have taken a major step toward the development of a new form of cancer therapy.
A team headed by molecular biologist Wen-Hwa Lee has for the first time converted cancer cells grown in a laboratory into healthy cells by replacing a defective gene with a normal gene.
The team reports in today’s Science magazine that when they subsequently implanted the genetically engineered cells into mice, the cells were no longer able to form tumors, indicating that the cancer in this case had been cured.
The defective gene in the laboratory-grown cells causes retinoblastoma, a rare form of eye tumor that affects about 500 children a year in the United States. Recent research has also implicated the gene in bone, breast and lung cancer.
The retinoblastoma gene was discovered two years ago, and its identification triggered a burst of research on the genetics of cancer that has led to new insights into the process of tumor formation. Its discovery also led to the identification of similar genes that play a role in colon tumors.
Researchers are already using the gene as a probe to identify children who are susceptible to retinoblastoma, and others hope to use it to screen for a predisposition to lung cancer. Now, the UCSD work suggests that the gene could be used in cancer therapy.
“I think this whole area is a tremendous breakthrough that, within the next decade, is going to have a tremendous impact on human cancer,” said geneticist John Minna of the National Cancer Institute.
And the UCSD results are “a distinct and significant contribution to the area of genetics and human cancer suppression,” added molecular geneticist Eric Stanbridge of UC Irvine.
The existence of genes like the retinoblastoma (RB) gene was predicted more than 20 years ago by geneticist Alfred Knudson of the Fox Chase Cancer Center in Philadelphia, Pa. But it was only the recent development of biotechnology techniques that have made their identification possible.
The discovery of the RB gene was reported in October, 1986, by a team headed by ophthalmologist Thaddeus Dryja of the Massachusetts Eye and Ear Infirmary in Boston.
The RB gene was like no other cancer-related gene that researchers had ever studied. Previous cancer genes, called oncogenes, could initiate tumor formation when they were activated by exposure to certain viruses or cancer-causing chemicals. The RB gene, in contrast, serves as an anti-cancer sentinel that protects against cancer. Retinoblastoma occurs only when RB genes are inactivated. Some researchers thus dubbed it an anti-oncogene.
Most people are born with two healthy RB genes, one from each parent, that protect them from retinoblastoma. Only rarely, about once in every 40,000 individuals, are both RB genes in a single cell damaged, allowing a tumor to form. This type of the disease is called sporadic retinoblastoma and, as the name suggests, strikes at random.
More common is an inherited form of the disease called familial retinoblastoma. In this case, researchers now know, a child inherits a normal gene from one parent and a defective, or inactive, form from the other one. In this case, only the healthy RB gene must be damaged for a tumor to occur, a process that occurs much more frequently. Many children in such families, in fact, develop multiple tumors.
Dryja and his colleagues reported earlier this year that they had developed a blood test using a genetic probe for the defective gene. And they are now using this probe to identify children predisposed to the disease. These children can then be monitored to detect the early stages of the disease.
“If we determine that an infant has the predisposing gene for retinoblastoma, we can catch the disease soon to treat it by means other than removal of the child’s eye,” such as chemotherapy, said ophthalmologist Janey Lee Wiggs of the Massachusetts infirmary. “We can save vision.”
Prompted by the discovery of the RB gene, epidemiologists began studying the records of adults who had survived retinoblastoma in childhood. They discovered that these adults had an above-normal incidence of osteogenic sarcoma (bone tumors), breast cancer and small cell lung cancer.
Lee also began studying the protein that is produced from the healthy RB gene in hopes of determining how it functions. His group reported this summer that RP protein is of a type that binds to deoxyribonucleic acid (DNA), the cell’s genetic blueprint. Most scientists now believe--although it has not yet been proved--that the protein binds to DNA in cells and prevents them from proliferating. In its absence, the cells proliferate madly, forming a tumor.
Such findings led scientists to conclude that a healthy RB gene could halt the proliferation of malignant cells. UCI’s Stanbridge took a step toward proving this by inserting chromosome 13, the chromosome that contains the proposed RB gene, into osteogenic sarcoma cells. He recently reported that this insertion did, in fact, stop proliferation, but this effect “could result from any one of many genes on the chromosome,” he said.
Lee and his colleagues at UCSD used a retrovirus--a special form of virus that inserts its own genes into those of an infected cell--to insert the normal RB gene into cultured tumor cells from retinoblastomas and bone cancers. The retrovirus was developed by UCSD molecular biologist Theodore Friedmann and his colleagues.
Once the gene had been inserted into the cancerous cells, their proliferation immediately slowed. The appearance of the cells also changed, reverting back to the normal appearance of the cells from which the tumors were derived.
Lee and his colleagues then injected the engineered cells into one flank of nude mice (which have no immune system and thus cannot reject tumor cells) and non-engineered tumor cells into the other flank. None of the engineered cells formed a tumor, while all of the non-engineered cells did.
“This is the first evidence that a single gene can suppress tumor formation,” Lee said in a telephone interview.
Lee speculated that such an engineered virus could be used to treat human tumors in as little as five years. Although the retrovirus would infect all cells of the body, it would have no effect on healthy cells because they already have a normal RB gene. Such therapy would presumably have no side effects, Lee suggested, because neither healthy nor diseased cells would be killed.
Other researchers are more reluctant to infect humans with a retrovirus for fear that the retrovirus will mutate and cause other genetic changes. But as more is learned about the functioning of the RB gene, Minna said, it may become possible to produce proteins or other molecules that can mimic its function in tumor cells without the potential risk of using a virus.
