When a cell in your body turns up dead, the molecular-biologist prosecutor looks for one of two suspects:necrosis or apoptosis. The former spells murder; thelatter, suicide.
Infections kill cells. So do extremes of temperature,lacerations, or other acts of violence. Apoptosis, asordered by programmed cell death, prompts the cell tofall on its own sword. But what prompts the prompter?
Among the usual suspects so far are a family of genesfeaturing in particular, Bcl2, BclX and Bax, all closelyimplicated in apoptosis. But their assisted-suicide roles,in wellness or illness, are only now being defined: Whichgene perpetrates apoptosis? Which inhibits it? Do theyact as loners or as a gang?
In search of such evidence, molecular geneticist StanleyKorsmeyer encountered a surprise. His report appears inthe Oct. 6, 1995, issue of Science under the title, "Bax-deficient mice with lymphoid hyperplasia and male germcell death."
Korsmeyer is a senior investigator at the Howard HughesMedical Institute at Washington University in St. Louis.
He told BioWorld Today: "To get an idea of how thosegenes play out in the development of an organism, andwhat their true in vivo roles are, one critical thing you cando is take them away _ by making a knockout mouse."
When researchers previously knocked out Bcl2 and BclX,he added, "the result was increased cell death." That hungthe question mark around the Bax gene, which Korsmeyerand his group discovered in 1993.
"That question," he said, is: "How would Bax, as the firstcell-death-promoting member of that apoptosis family,play out if you took it away?" He and his co-authorsproceeded to do just that; they created a strain ofknockout mice deficient in the Bax gene.
The heterozygotes, with only one parental dose of BaxDNA, "are fine," Korsmeyer said, and even thecompletely Bax-lacking homozygotes "make it throughdevelopment to adulthood."
Molecule Of Prenatal Castration
"The unexpected finding," Korsmeyer continued, "wasthat the males are sterile, due to massive cell death duringspermatogenesis. So their adult testes are atrophic, withan empty epididymis and vas deferens, indicatingcomplete cessation of sperm production, and clusteredapoptotic germ cells.
"That dramatic phenotype suggests that the role of Baxmay vary, depending upon the context of the cell."
He explained: "We know that, at least in part, thesepositive and negative regulators of cell death, namely,Bcl2 and Bax, will heterodimerize," that is, form a doublemolecule comprising one of each gene. Also, Bax/Baxhomodimers _ two of the same.
That leaves the question: Which dimer is principallyactive in the apoptotic tug-of-war?
"If for example," Korsmeyer pointed out, "Baxhomodimers were the bad guys, and activated the cell-death pathway, one might well expect that Bax knockoutmice would take this path away, and so develophyperplasia. That is, excess numbers of cells that shouldhave died, linger on."
Sure enough, his team's Bax-minus mice piled upredundant cells in lymphoid tissues and in ovarianfollicles, but paradoxically the opposite in testiculartissues. "It may be," Korsmeyer speculated, "that thisprocess is particularly active to patrol the germline, toeliminate damaged DNA, which would of course affectthe next generation."
This doesn't mean that no-Bax mice go through lifeshielding every cell in their bodies from apoptosis, or viceversa. "In certain cell types in the animals," Korsmeyerobserved, "like those involved in making sperm, we seecell death, but in other cell types, we see excess cellbuild-up."
Which of course suggests cancer. "I suspect," he said,"that for the biotech world, what will be most interestingis to continue to survey these knockout animals to seewhich cell types have had their vulnerability to deathmodified by removing Bax. That might argue that, if youwanted to either kill cancer cells or, by contrast, saveneurodegenerative neurons, the gene would be ameaningful place to interfere."
Such efforts to interfere neatly describe in large part whatIdun Pharmaceuticals Inc., of La Jolla, Calif., is up to.(See BioWorld Today, Aug. 9, 1994, p. 1.)
Idun Tools Up For Preclinical Drug Discovery
Korsmeyer is a co-founder of the company, which haslicensed the patent to his Bax discovery from WashingtonUniversity.
"Bax is important to us for particular disease models,"Idun's vice-president of research and development, LarryFritz, told BioWorld Today. "Dr. Korsmeyer's knockoutmice are a useful development, enabling us to compareresponses to particular diseases."
Meanwhile, Idun has Bcl2, which encodes an anti-apoptosis (hence oncogenic) protein, "definitely in thepreclinical drug-discovery stage. We envision moleculeswith oral-delivery potential," Fritz concluded, "as sites oftherapeutic action."
Like Korsmeyer, molecular geneticist Craig Thompson,University of Chicago, is a member of Idun's scientificadvisory board. He discovered BclX, the third leadinggene contender in the apoptosis game. It sits on humanchromosome 20, Thompson told BioWorld Today; Bcl2on chromosome 18. Bax's genomic residence, he said, "isstill uncertain."
Dimerization is the name of the game that he, Korsmeyerand others are playing to clarify the mechanisms ofapoptosis. "It's like two kinds of dance cards," Thompsonsaid. "Who partners whom in the dimerization dancegives the final decision of who lives, who dies." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.