By David N. Leff

Editor's note: Science Scan is a roundup of recently published biotechnology-relevant research.

Is the generation of colorectal tumors a crapshoot, or Russian roulette with most revolver chambers loaded? Experimental pathologists at Roswell Park Cancer Institute in Buffalo, N.Y., found approximately 11,000 aberrant genomic events per colon carcinoma cell - "considerably more abundant than expected."

Even less expected, and more disquieting, was their further finding that precancerous adenomatous polyps "showed similar numbers of events." To reach this mutational data, the group subjected 58 colon cancers and 14 polyps to PCR analysis.

Their report, titled "The onset and extent of genomic instability in sporadic [non-familial] colorectal tumor progression," appears in the Proceedings of the National Academy of Sciences (PNAS) dated Dec. 21, 1999. In this paper, the Roswell Park co-authors conclude: "Together, our results support the model of genomic instability being a cause rather than an effect of malignancy, facilitating vastly accelerated somatic cell evolution, with the observed orderly steps of the colon cancer progression pathway reflecting the consequences of natural selection."

A commentary accompanying the PNAS paper, by cancer researchers at the University of California, San Diego, bears the apt title: "How many mutations does it take to make a tumor?" They point out, "Human cells experience a certain number of mutations each day as a consequence of exposure to carcinogens or ordinary 'wear and tear,' which alters nucleotide sequences. Errors will also occur during new DNA synthesis. By chance," they add, "an occasional genomic misadventure might create a growth advantage for a cell, permitting increased net cellular growth (because of increased proliferation or reduced cell death). A second genomic alteration might then occur within this expanded clone, which would eventually overgrow neighboring cells. The accumulation of these growth-promoting mutations," they point out, "is the basis of multistep carcinogenesis."

The commentators raise the question: "What accounts for genomic instability in hyperplastic [non-tumorous cell proliferation] polyps, which are not considered neoplasms?" and suggest, "If they are truly not neoplasms, we must revise our thinking about polyps."

Binational Group Engineers Synthetic, Transparent, Transplantable Corneas That Reproduce Human Orbs

People who receive donor transplant organs pay a lifetime price in fighting off graft rejection by their unwitting immune defenses. One exception to this bar is the human cornea, the transparent window at the front of the eyeball. This ocular component is immune-privileged, and devoid of blood vessels, so it presents no antigenic target to prowling immune system cells. In consequence, upwards of 10,000 corneal transplants a year take place in the U.S. alone, with eyes from autopsied donors who died from causes not affecting their ocular tissues. Still, the demand exceeds the supply.

At the University of Ottawa, Ontario, cellular and molecular biologists led a four-center Canadian-U.S. consortium (including the Procter & Gamble Co. Laboratories in Cincinnati) that developed an artificial, transplantable cornea. Their paper in Science dated Dec. 10, 1999, is titled: "Functional human corneal equivalents constructed from cell lines."

The co-authors began by isolating human eye-bank cells from the three cellular layers of the cornea. They rendered these cultured cells immortal by infecting them with recombinant retroviruses containing herpesvirus genes, or mammalian expression vectors encoding SV40 and adenovirus antigens. Thus assured that these constructs would grow, and that they mimicked the structural, genetic and electrophysiological properties of normal human corneal cells, they built polymeric scaffolding to provide a growth matrix.

The resulting corneal equivalents, they report, "resembled human corneas in gross morphology, transparency and histology." In ocular toxicology tests, the team saw "changes in light transmission after exposure to chemicals, with increased cell death, similar to those observed in human and rabbit corneas."

So far, the co-authors have not reported any in vivo preclinical testing, but conclude, "Future research could lead to readily available, complex engineered tissues that reproduce their natural human counterparts, and are suitable for implants, transplants, and biomedical research."

African Frog Gets Upgrade As Transmission Belt For Expressing Transgenes Of Interest In Progeny

In any popularity contest among laboratory animal models - from fruit flies to primates - the African clawed frog (Xenopus laevis) rates fairly near the bottom. Still, with its large and easily manipulated embryo, it's the most widely used amphibian for studies of early vertebrate development and metamorphosis. But going against X. laevis has been its limited ability to transmit foreign genes to its offspring, owing to the high level of genetic abnormalities caused by the DNA integration procedure.

Now, embryologists at the Carnegie Institution of Washington in Baltimore have developed a way to generate African frogs that transmit transgenes quickly and efficiently - yea even unto the third generation. Their demo DNA was the gene for green fluorescent protein (GFP), a visible reporter of gene expression. The co-authors' report in the Proceedings of the National Academy of Sciences (PNAS), dated Dec. 7, 1999, bears the title: "Germ-line transmission of transgenes in Xenopus laevis."

All of the 19 animals expressing GFP in their tissues - even their eyes - were raised to sexual maturity. The team's method involved integrating the DNA of interest into frog sperm nuclei stripped of their membrane coat, then transplanted into unfertilized eggs. The transgenic frogs were crossed with wild-type animals, and their progeny screened by fluorescence during the first week of development. By counting the GFP-positive and negative progeny, it was possible to estimate the number of integration events in the founder frogs. Transgene expression ranged from 50 percent to 99 percent. In one case tested, expression was unaltered by a second passage through the germ line - i.e., third-generation transmission.