First, catch your mouse. Next, shave bare a small patch of skin.Then, from a micropipette, lay a drop of carcinogenicbenzo(a)pyrene solution on a square cm of nude skin. Repeatonce a week for up to a year. Then analyze the resultingmalignancies by PCR and monoclonal antibody technologies forpatterns of mutation in the gene sequence of tumor-suppressing p53.
Histopathologist A. J. P. Klein-Szantos and his research team atFox Chase Cancer Center in Philadelphia did all of the above.They reported their results in Monday's PNAS (February 1,1993) in a paper headed: "Benzo[a]pyrene-induced murineskin tumors exhibit frequent and characteristic G to Tmutations in the p53 gene."
Their findings confirm a number of current concepts regardingmutated p53, cancer and tobacco smoke, which they are thefirst to test in vivo, in a living animal model. Notably:
-- Benzo[a]pyrene B(a)P, one of several hundred products ofincomplete cigarette-smoke combustion, is a strong skin-contact carcinogen, widely used to induce experimental tumorsin laboratory mice.
-- It works by mutating the gene for p53, which perches onchromosome 17. The p53 gene encodes a 53 kiloDalton nuclearphosphoprotein, which P besides ordinary cell-division chores Ppatrols the cell to suppress incipient tumors.
%But once mutated P by heredity, virus, ultra-violet rays,dietary carcinogens (e.g., aflatoxin), or in this case B(a)P P p53turns into a strong oncogene. The gene most commonlymutated in human cancers, oncogenic p53 leaves itsfingerprints on at least 75% of all solid tumors.
The bull's-eye for cancer-causing agents targeting the p53 geneis one of several "hot spots" or fragile sites along its exonsequence. At the nittiest grittiest base-pair level, they wreak atransversion in the DNA. (That is, not merely a point mutationbetween two compatible nucleotides, but across the barrierbetween purines and pyramidines.)
Specifically, the p53 mutation most often takes the form ofknocking out guanine (G) a purine, and substituting thymine(T), a pyramidine.
Various carcinogens, aiming at various organs in the body,shoot their mutagenic arrows at specific, selected base pairsalong the p53 gene's hot-spot sequence. "Human tobacco-related cancers," says the Fox Chase PNAS report, "exhibit ahigh fre-quency of guanine to thymine transversions in themutation hot-spot region of the p53 tumor suppressor gene."This, surmises, Curtis C. Harris, a leading p53 researcher at theNational Cancer Institute, "is possibly the result of specificmutagens in tobacco smoke, most notably B(a)P."
Klein-Szantos finds "of particular clinical and epidemiologicalinterest...the high frequency of G to T transversions among p53mutations in tobacco-related human neoplasias," especiallylung and esophageal cancers. But he adds that while his dataprovide in vivo evidence that B(a)P targets the p53 gene,"comparing animal models to human cancers must nonethelessbe made cautiously."
"The new thing in Klein-Szantos' paper," Harris toldBioWorld,"is to find that the mutations one would predict are infact what one finds in the animal model."
Since submitting their paper to PNAS last May, Klein-Zsantosand his group have taken a step toward human testing. He toldBioWorld that six months ago they began a long-termexperiment using human tissue in a xenograft in vivo. First,they grafted human bronchial epithelium cells into the tracheasof rats. Then they transplanted these cultures into nude mice,together with beeswax cyclinders encapsulating a tumorigenicdose of B(a)P. They exposed another set of animals to adifferent tobacco-smoke mutagen, nitrosamine.
"We have found changes in the epithelium," Klein-Szantos toldBioWorld," but no p53 mutations yet. We will keep looking foranother half year."020393P53
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.