By David N. Leff
Global warming and the dwindling ozone layer have come in for increased concern as the 21st century gets going. Like a stratospheric sunshade, the ozone layer shields the earth and its denizens - mainly people - from the damaging effects of ultraviolet radiation on the skin.
Malignant melanoma in particular seems to correlate with the intensity of the sun's rays hitting our planet's surface. Two geographic comparisons make the point: "Recently published statistics," observed molecular biologist Maria Soengas, "show that one out of 74 people per year in North America is diagnosed with melanoma. In Australia," she added, "there is something greater to worry about because of their problem with the ozone layer. So one of 30 persons down under will be diagnosed."
But Soengas pointed out, "Depletion of the ozone layer is not the only cause of melanoma. Rather, it's a factor that increases the risk for developing it. But it's the one that has been proven to increase the incidence of that skin cancer. The mortality rate is high, and survival prospects are not very good." Current figures report melanoma as the fifth commonest cancer in the U.S., with over 40,000 new cases diagnosed yearly and more than 7,000 deaths recorded. (See BioWorld Today, May 24, 2000, p. 1.)
The protein p53 is known as "Guardian of the Genome," because it patrols the DNA of dividing cells and orders imperfect ones - notably mutant precancerous cells - to commit suicide on the spot by apoptosis. But that normal p53 tumor suppressor is itself subject to mutation, and its imperfect version promotes cancer growth. In fact, from 50 percent to 70 percent of all tumors reveal mutated p53 in their cellular makeup. Melanoma cells are not among them.
In fact, though metastatic melanoma contains normal p53, it is resistant to chemotherapeutics, which attack the disease by causing its tumors to abort by performing apoptosis - programmed cell death.
Soengas, a post-doctoral fellow at the Cold Spring Harbor Laboratory in New York, is principal author of a paper in today's issue of Nature, dated Jan. 11, 2001, titled: "Inactivation of the apoptosis effector Apaf-1 in malignant melanoma."
"We have identified a new putative tumor suppressor," she told BioWorld Today. "It's a protein called Apaf-1, whose function is to induce endogenous cell death. Apaf-1," she explained, "stands for 'apoptosis-activation factor-1.' Our laboratory didn't discover Apaf-1; it was identified a few years ago by other groups. What we discovered was that it is actually inactivated in a high percentage of metastatic melanoma tumors. And that inactivation of the Apaf-1 gene can contribute to the chemoresistance of melanoma - one of the cancers most aggressively resistant to chemotherapy. Our final finding," Soengas continued, "was that in tumor cell culture we could somehow rescue, at least partially, the chemoresistance in these cell lines by reintroducing the missing Apaf-1 gene back into the tumor cells."
Caspase, Lord High Executioner Of Apoptosis
"What this protein does," she explained, "is a critical factor in promoting the execution of cell death. Afap-1 binds and activates another protein called caspase-9, the key player that will ultimately induce the apoptotic disintegration of the cell."
Soengas described the double-barreled in vitro experiments she and her co-authors carried out to reach these conclusions: "We used tumor sample material and melanoma cell lines we grew in culture from a patient," she recounted. "In the tumors, we looked for genes that are inactivated, to identify them. In cell lines we tried to find the function in consequence of this inactivation. With the tumors," she went on, "we identified Afap-1 as the gene that is inactivated. Then in cell lines we found that the consequence of this inactivation is that the cells don't die; they proliferate, and accumulate as tumors, which now don't respond to therapy."
To explore this recalcitrance to chemotherapy, Soengas narrated, "we turned to adriamycin, a chemotherapeutic drug. We found that when cells that don't have this Apaf-1 protein are treated with adriamycin they are resistant to it, and its apoptotic command. The tumor cells that do have Afap-1 respond to therapy and die."
Looking to the future, the Cold Spring Harbor group is planning to move from in vitro to in vivo. "We cannot work with clinical patients, of course," Soengas observed, "so we have to work with something. This study reported in Nature was done in patients at a very late stage of their disease, when the melanoma is metastatic. So we are trying now to look earlier and see what's happening when the disease is in its initial stage - whether Afap-1 is or is not present then.
"The other thing we want to do is develop animal models to study in vivo the function of Afap-1 and other proteins that work together with it. Finding an ideal model is the critical issue. There are several mouse models of melanoma developed already by other groups. The best one for us would be a mouse that somehow recapitulates the human disease, in which we could try to see whether our Afap-1 gene is also playing a role in its development of melanoma.
"As for potential clinical benefits of this work," Soengas pointed out, "in principle, the first thing we have to do is extend these results to a larger number of cases. Then one could imagine that if we find Afap-1 is frequently inactivated in metastatic melanoma, we could use it as a tumor marker to maybe predict prognosis - like a prognostic factor. That could be very important."
Surgery Still First Best Hope For Melanoma
"Farther along the road," she added, "our hope would be to exploit what we know now about apoptosis and proteins involving the execution of cell death, so we might develop better anticancer therapies. The most efficient treatment at present is early surgical excision. At late stages, all treatments are experimental - meaning no single one is sufficient. Combinations of therapy are unfortunately very inefficient, and very late stages of the disease defy any treatment.
"We are still in the early stages of basic research in trying to apply our findings to the clinic. It's very interesting, and relevant for the communities that are studying tumor-suppressor mechanisms. It's important," Soengas concluded, "but there's still lots that we don't know."