By David N. Leff
MIAMI — The virulent, diarrhea-dealing strain of Escherichia coli that led to the recall of $15 million worth of hamburger patties last year has a 20-percent bigger genome than the K12 version of the bacterium that dwells benignly in our gut, and in biotechnology research laboratories.
Molecular geneticist Frederick Blattner, University of Wisconsin at Madison, told the functional genomics session of the Miami Winter Symposia Sunday that the fast-food-poisoning version of E. coli has genomic similarities not only to K12, but to the Yersinia pestis bacterium — the rat-borne, flea-bite-injected pathogen that causes the plague.
Offering "a little anecdote along those lines," Blattner informed his audience, "there's a Y. pestis gene that is responsible for aggregating the pestis bacteria in the esophagus of fleas. This causes the flea to regurgitate the clump of bacteria into the wound of the human that's been bitten."
"That gene," he added, "is present in the genome of E. coli K12." Early last year Blattner announced having sequenced the complete 4,639,221-base pair genome of K12. (See BioWorld Today, Sept. 5, 1997, p. 1.)
He noted that K12 has 5,093 gene products, 1,985 of them of known function; 1,246 "attributed," the remaining 1,862 unknown.
So far, Blattner and his team of genome mappers, have found "approximately a megabase of DNA not present in K12. And conversely, K12 has 300 kilobases that are not present on O157:H7" — the virulent version's serial number.
"There's a lot more difference between the pathogenic and the benign strain," Blattner continued, "than a lot of people had expected — not only large genomic chunks but smaller chunks. So the process of whole-genome sequencing of O157:H7 is going to make a difference here. What we're finding is that the K12 genome is smaller by approximately one megabase out of five — almost 20 percent. And virulence segments are of all sizes, including some large chunks, such as plasmids and phages."
Diarrhea is often belittled as "Montezuma's revenge," or "tourists' complaint," but Blattner pointed out that "diarrhea kills 3 million people a year worldwide, 1 million more than those who die of malaria."
Answering a question as to when E. coli's virulence factor will be discovered, Blattner replied: "I would have to say that it is being discovered even as we speak."
Conquest Of Disease Is Focus Of 1998 Miami/Nature Symposia
This year's Miami Winter Biotechnology Symposia is centered on "Advances in Gene Technology: Molecular Biology in the Conquest of Disease." The four-day gathering, which started Sunday and ends Wednesday, drew 547 attendees, including 40 speakers and journalists.
The event is celebrating two anniversaries: 30 years since the first symposia, held in 1968, and 10 years since the annual event, created and sponsored by the University of Miami, joined forces with Nature Publishing Company, notably its monthly Nature Biotechnology.
First order of business Sunday morning was the annual Feodor Lynen Lectures, divided this year between Mario Capecchi, of the University of Utah, in Salt Lake City, and Rudolf Jaensch, of Massachusetts Institute of Technology's Whitehead Institute for Biomedical Research, in Cambridge, Mass.
In past years, 37 bioscientists have been honored with the Lynen lectureship, including James Watson, Francis Crick, Cesar Milstein, Paul Berg, Thomas Cech, and other Nobelists too numerous to mention.
In Fruit Fly, House Mouse, Humans, Capecchi Traces 39 Genetic Tools For Building Bodies, Head to Toe
Mario Capecchi is best known for his pioneering work in gene targeting in stem cells derived from mouse embryos. His present focus, and topic of his lecture, was "How do HOX genes specify our body plan?"
In a growing embryo, whether fruit fly, mouse or human, it takes 39 HOX genes, no more, no less, to make the body, and its parts," Capecchi said, "going from the head all the way down to the feet."
By moving a specific HOX gene, he pointed out, one can endow a fruit fly with an extra set of wings or legs, for example. "There are equivalent genes in the mouse," he observed, adding, "These genes are all transcription factors, so they act like master genes, controlling the activation and turning off of other genes.
Capecchi sees as "serendipity that there are pathologies in the mouse that are very similar to known diseases in humans. One HOX gene mutation," he observed, "has similar pathologies to what's known as Möbius syndrome in cancer."
He explained: "In children, all the muscles in the face, controlling what are called facial expressions — the ability to perk your lips, or to smile or to frown and so on. All of these require particular muscles in the face, and these muscles aren't innervated with particular nerves, the facial nerves. Lacking the HOX B1, they're nonfunctional.
"It turns out that in the mouse, by inactivating this same gene, we see the same pathology."
Another example that works the same in mouse and men is the HOX C13 gene, which masterminds the embryonic development of hair and nails. When Capecchi mutated HOX C13, he got hairless mice, akin to Alopecia universalis in humans (See BioWorld Today, Jan. 30, 1998, p. 1.)
"In the long run," he observed, "I think this is going to have a fundamental effect on human medicine, because man will be able to not work empirically in terms of trying to develop a drug. Instead of simply trying it to see if it works or doesn't work, we will actually be able to direct drugs at particular processes, known to cause the pathology."
DNA Methylation And Cancer Topic Of Jaensch's Lynen Lecture.
"Methyl," Rudolph Jaensch explained, "is a chemical group, which is put onto cytosine, one of the four bases of DNA. DNA methylation, if you will," Jaensch continued, "is a way to format the genome. It's something reversible; it makes the genome more readable or unreadable."
He offered "a very simple example: The text you have in a word processor can be either formatted or not. If it's formatted — with paragraphs, bold-face type and things like that — it's easy to read. If not, it's not easy to read, but the information content is the same. That's what methylation does — format the DNA.
"It turns out," he went on, "that this type of modification is important for gene expression, particularly for gene expression in cancer. The cancer cell uses this modification, for example, to turn off genes that inhibit tumor growth. That's very well known."
Jaensch, who is recognized as an originator of transgenic mice, recalled, "One of the first pieces of evidence came from our work with a strain of mouse that is prone to intestinal tumors. This animal has a mutation in the same gene that, when humans have it, leads to multiple polyps in the colon, which lead to cancer.
"The gene that encodes the enzyme methyltransferase," he explained, "is a tumor suppressor gene. But we have seen, contrary to all expectations, that when we inhibit the enzyme — in other words, when we lower the level of modification, formatting, if you will — these mice are significantly protected against polyps and cancer.
"When we did this, the wild-type mice, which normally would get, say, 1,000 polyps, got only 10 or so. This is significant because, in principle humans have similar mutations to this particular mouse model. Importantly, this can be done not only genetically but also by drugs, which interfere with the activity of the methyltransferase, the enzyme that does the modification.
"So I think there is potential for therapeutic intervention," Jaensch concluded. *