By David N. Leff

As many a moviegoer knows, England's King Henry VIII ran through half a dozen wives, several of whom he divorced; one of whom he beheaded. In every case, Henry asserted he was only doing God's will; in other words, the king could do no wrong. When Henry died in 1547, his nine-year-old son, Edward VI, ascended the throne of England, protected by the same aura of royal infallibility.

So whatever the young monarch did wrong, or failed to do right, he couldn't be punished. Instead, a strapping lad by the name of Barnaby Fitzpatrick was hired as royal whipping boy, to be flogged whenever Edward's behavior warranted this proxy punishment.

Laboratory mice are the preferred proxy whipping animals for human failings - physical or behavioral. Transgenic mice mimic these defects at the genetic and cellular levels, in the expectation by researchers that the resulting findings can be extrapolated to the human condition.

One such condition is leukemia, a complex family of cancers in which white blood cells multiply lethally out of control - in mice, as well as people. Transgenic leukemic mouse models also multiply in sophistication to cope with leukemia's complexities.

An article in the January issue of Nature Genetics describes transgenic mice engineered so that a human leukemia oncogene can be switched on or off at will. The paper's title: "Reversibility of acute B-cell leukemia induced by [the oncogene] BCR-ABL." Its first author is molecular oncologist Claudia Huettner, at Harvard Medical School.

"Cancer is thought to arise from multiple genetic events that establish irreversible malignancy," she and her co-authors wrote. "A different mechanism might be present in certain leukemias initiated by a chromosomal translocation. We have taken a new approach to determine if ablation of the genetic abnormality is sufficient for reversion."

Taming A Maverick Cancer Promoter

Whereupon, they generated transgenic mice that express the BCR-ABL oncogene, which arises from a two-way chromosomal swap - a reciprocal translocation - between human chromosomes 9 and 22. "That means it's not like the ras or myc cancer-promoting oncogenes," Huettner told BioWorld Today. "It's not present in the normal human body. But because BCR-ABL is due to a chromosomal translocation, only leukemia patients will have it.

"We knew from previous work done elsewhere in generating transgenic animal models," she went on, "that the expression of BCR-ABL during embryonic development is deleterious. If the transgene - that is, the BCR-ABL - is expressed at a reasonable level, you will not obtain any live offspring, due to embryonic lethality. So any piece of work done in the transgenic field thereafter resulted in generating animals that didn't develop disease reliably.

"Therefore, when I decided on taking over this project," Huettner recalled, "I thought about using a totally different approach that should in theory suppress BCR-ABL expression during embryogenesis. Our thinking was that by so doing we would obtain transgenic animals - bred in sufficient numbers - that would, upon induction of BCR-ABL expression, reliably develop leukemia that mimics the human disease."

BCR-ABL is a fusion gene that links the human BCR gene with the ABL gene. "This Nature Genetics paper," Huettner observed, "reported the pilot experiment in our transgenic model of acute B-cell leukemia, a disease that is fatal in humans. It has an extremely bad prognosis, and all the patients die within a very short time."

The co-authors generated four lines of conditional transgenic mice, which could induce or silence the lethal oncogene. "In order to obtain these mice," Huettner recounted, "we had to generate two different lines - a trans-activator and a trans-responder. Both lines of mice were born totally healthy, normal and fertile."

The team then cross-bred these two murine strains to produce their switchable leukemic models. The oncogene turns the disease on, while the antibiotic tetracycline flips it off.

"Five days before mating them," Huettner recounted, "I started the females on tetracycline, just to be on the safe side. When the newborns grew up, we were then able to induce the lethal BCR-ABL expression, by simply withdrawing tetracycline from their drinking water. In 100 percent of all the oncogene-carrying animals we tested, this induced a type of leukemia also observed in human patients.

"In three of the four transgenic types we created," Huettner continued, "we have a very quick development of leukemia. And the untreated mice - without re-administration of tetracycline - die of acute B-cell leukemia within three to four weeks.

"The fourth line has a much longer latency period," she went on. "In those mice I can detect the effects of BCR-ABL expression in their peripheral blood within 10 to 20 days after tetracycline withdrawal, with the appearance of leukemic cells reflecting a condition also observed in patients during the chronic phase of their disease.

Fourth-Line Mice Die Early, Violent Deaths

"These mice," Huettner said, "develop a violent and accelerated phase of disease, and finally progress to blast crisis and death. I can revert that leukemic phenotype only in the other three lines - by multiple inductions of remission, and the animals will stay healthy as long as they receive tetracycline. But mice from the fourth line will relapse, and they will all die of a leukemia that turned out to be independent of the original oncogene.

"This is something that we also see in human patients. We treat them for BCR-ABL attack, and they can enter remission. But later on, a subset of these patients will come back and relapse with a very violent, fast-progressing acute leukemia that is fatal within a couple of weeks. And this disease," she pointed out, "is independent of BCR-ABL, which indicates that other mutations have taken over.

"In a sense, this work can be extrapolated to human patients," Huettner observed. "These mice were originally generated to test new therapeutic approaches. But my major interest," she concluded, "is in improving this model and generating mice that develop develop different types of human cancers."