By David N. Leff
¿Everybody grumbles about the weather,¿ grumbled Mark Twain a century ago, ¿but nobody does anything about it!¿
In those days, nobody did anything much about deaths from cancer, or the short human life span. Nowadays, thanks to intensive and extensive research, cancer mortality rates are edging down somewhat, while investigations on how to extend life expectancy are just getting off the ground.
Both of these Grails are connected by a recently discovered gene called Sir2.
¿Sir¿ is an acronym that stands for ¿silent information regulator,¿ explained molecular biologist Ed Cannon, who is president and CEO of Elixir Pharmaceuticals Inc. in Cambridge, Mass. ¿That name,¿ he continued, ¿derives from Sir¿s function in yeast, where Leonard Guarente at MIT first announced his discovery early last year that by overexpressing Sir2, he could extend the life span of yeast cells. He has since shown that the Caenorhabditis elegans ¿ nematode worm ¿ version of Sir2 also prolonged the life span of that organism.
¿Now that Sir has turned up in everything from Archaebacter to humans,¿ Cannon observed, ¿a lot of researchers have started looking at crystal structure, enzymology, genetics and cell pathways ¿ plus a little drug screening.¿
This tsunami of scientific interest in Sir is reflected in today¿s issue of the journal Cell, dated Oct. 19, 2001. It carries two separate but similar papers titled, respectively:
(1) ¿Negative control of p53 by Sir2a promotes cell survival under stress.¿ Guarente is a co-author, and cancer geneticist Wei Gu, at Columbia University¿s College of Physicians & Surgeons in New York, is senior author.
(2) ¿hSIR2SIRT1 functions as an NAD-dependent p53 deacetylase.¿ Its senior author is molecular biologist Robert Weinberg, at the Whitehead Institute for Biomedical Research in Cambridge.
Cells Sustain Stresses Akin To Aging Process
¿Our paper¿s finding is rather simple,¿ Weinberg told BioWorld Today. ¿Its essence is that Sir2 acts on the tumor-suppressor protein p53, into which many of the cells¿ stress signals ¿ damage to DNA, inadequate oxygen, metabolic imbalances, oncogenes and so forth ¿ are funneled. In response to these incoming alarms, p53 can call a temporary halt to the progress of the cell through its growth cycle, or induce the cell to kill itself. This irreversible apoptotic suicide leads to senescence ¿ aging.
¿P53 protein is an important impediment to development of cancer cells,¿ Weinberg pointed out, ¿because a number of stress responses are activated during the process of turning a cell cancerous.¿
Guarente made the point, ¿There¿s been a lot of work done on aging in worms and yeast. What¿s going to happen in mammals? From a biotech point of view I think that question frames Sir2 as a potential therapeutic target.
¿The connection between Sir and p53,¿ Guarente recounted, ¿was discovered by Wei Gu at Columbia. About a year and a half ago he called our lab when we had just discovered that Sir2 was a deacetylase. Gu said, Hey, maybe we should see if p53 might be a substrate.¿ So we started these experiments.
¿Wei Gu discovered that p53 was an acetylated protein, which raised the possibility that it could be a substrate for Sir2. In one of two critical experiments, he took mouse embryo fibroblasts, and increased the amount of Sir2 in them by putting extra copies of the Sir2 gene in several different cell types, so there was more Sir2 activity. He found that if Sir2 deacetylates p53, it raised the activity of Sir2, lowered p53 activity and prevented apoptosis.
¿The converse experiment was done in human skin and lung fibroblast cells. It lowered the amount of Sir2 activity by means of a dominant-negative construct. Some of these experiments were done at Columbia, some at the Whitehead. Wei Gu did both the overexpression experiment and the inhibition experiment. The Weinberg people did the dominant-negative inhibition experiments.
¿The next step is to go from in vitro to in vivo,¿ Guarente stated, ¿to start manipulating levels of Sir2 in mice, see what happens to their cell death rate and how it plays out in their aging profile. Will they age faster? More slowly?
¿We¿re setting up the mice now,¿ he observed, ¿but these are long-term experiments. Mice normally live three years. After we make the genetic variants, we¿ll have to wait till they get older. We¿re making knockout mice to get rid of the Sir2 genes, and mice with extra copies of that gene. Then we¿ll breed them, get pure colonies, and see how they fare.¿
Knockout Mice May Stretch Human Life Span
¿The anticancer therapy connection is less clear,¿ Guarente pointed out. ¿It poses the question: Is this connection between Sir2 and p53 an important aspect of cancer surveillance? We don¿t know the answer to that; it, too, will come out of the transgenic mice.
¿How will we know how this plays out in humans? What would happen if we raised the level of Sir2 in these altered mice? The prediction is it would repress apoptosis and make the animals live longer. But would they get more cancer? We don¿t know. Might there be a window of p53 activity sufficient to prevent tumors, but prevent extra apoptosis, too?
¿The second avenue of possible human relevance,¿ Guarente went on, ¿is to ask: Are there Sir2 SNPS ¿ single nucleotide polymorphisms ¿ in the human population that correlate with longevity? That¿s something we¿ve started. We¿re looking for SNPs. We¿re sequencing Sir2 genes now in several individuals to see what alleles are out there. Let¿s say, for this gene there¿s a common allele and a rare SNP that¿s present in 10 percent of the general population. If we go to long-lived families, will that frequency change significantly? That¿s one way of directly linking the gene to life span in humans ¿ if we found such a linkage.
¿The third avenue is to look for drugs that would modulate Sir2 activity,¿ Guarente concluded. ¿If we had such a compound we could isolate in the test tube, we could start feeding it to mice and see what it does to their aging. That¿s something we won¿t be doing here in the lab. Ed Cannon¿s company, Elixir Pharmaceuticals, of which I¿m a co-founder, will be doing that.¿