Sci. Aging Knowl. Environ., 16 January 2002
How a clk Ticks
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/sageke;2002/2/nf2
Research groups coming from different directions have converged on the same longevity gene. The new work offers tantalizing clues about possible connections between life-span, DNA damage, and the health of chromosome ends. Although the gene's sequence suggests that it controls the length of telomeres--chromosome caps that shrink each time a cell copies its DNA--the different teams disagree about that potential function. Additional clues about the gene's capabilities, however, suggest that DNA-mending machinery can modulate life-span. "This is the first implication in worms of a direct role for DNA repair mechanisms in specifying longevity," says geneticist Tom Johnson of the University of Colorado, Boulder. "That's intriguing."
Scientists had previously identified a mutation in worms called rad-5 that confers heightened sensitivity to radiation and other DNA-damaging treatments: Mutant animals exposed to such agents die when they receive doses that normal animals can withstand, and worm cells with the rad-5 mutation fail to stop cell division in response to marred DNA. These results suggested that the gene that carries the rad-5 mutation encodes a protein that monitors DNA damage.
Now, scientists have identified that gene, according to work published in the 11 December issue of Current Biology. By measuring the genetic reshuffling between rad-5 and other mutations with known chromosomal positions, Shawn Ahmed of the University of North Carolina, Chapel Hill, and his colleagues narrowed down rad-5's position. Further experiments revealed that another mutation known to extend worm life-span--called clk-2--resides in the same region and that the two mutations likely dwell in the same gene. Scientists originally identified clk-2 in a hunt for worm mutants that develop slowly as embryos and later showed that clk-2 animals enjoy a 25% extension of life-span. It's not yet clear whether the rad-5 mutation also slows aging. The new results are the first to indicate that clk-2 might help keep DNA intact.
If a single gene contains both mutations, a normal copy should correct both defects. To test this idea, Ahmed's group inserted fragments of DNA from the rad-5/clk-2 region of a normal chromosome into mutant worms and looked for pieces that could reverse the effects of the two mutations. They found such bits and zeroed in on the gene. Comparison of the normal gene sequence with the corresponding genes in strains that carry the clk-2 or rad-5 versions revealed that each mutation results from a single amino acid change--at opposite ends of the gene.
Further experiments support the notion that the two mutations alter the same gene. Like rad-5 cells, clk-2 cells continue to divide after exposure to radiation, whereas wild-type cells stop (Fig. 1). It isn't yet clear why a mutation that increases sensitivity to damaging agents would extend--rather than shorten--life-span. "That's a bit surprising," says Ahmed. "We don't know how to explain it." Johnson suggests that response to acute stress might not correlate with the ability to deal with chronic stress--the kind that likely speeds aging. Recent work supports the idea that links between life-span and cellular injury do not adhere to intuitive rules. A mutation in the tumor suppressor gene p53--which normally protects from acute assaults--drastically reduces the occurrence of cancer in mice yet curtails life-span (see Campisi Perspective).
Further sequence analysis revealed that the rad-5/clk-2 gene resembles a yeast gene called TEL2 that helps maintain telomere length. So Ahmed and his colleagues measured telomeres in clk-2 and rad-5 mutants. Initial results revealed that clk-2 and rad-5 mutants harbored longer telomeres than wild-type animals did. But Ahmed has previously observed variation in telomere length among Caenorhabditis elegans strains, so he performed additional experiments to ensure that his wild-type and mutant worms descended from the same grandparents and therefore were genetically similar. Through breeding, he generated a collection of wild-type and mutant siblings. He then compared multiple wild-type and mutant worms to cancel out remaining genetic differences aside from the clk-2 or rad-5 mutation. In this analysis, clk-2 and rad-5 mutant worms displayed telomeres of the same length as those in their wild-type siblings. The results suggest that the life-extending capability of the clk-2 mutation doesn't depend on a change in telomere length, says Ahmed. Given the faulty DNA repair capabilities conferred by this genetic lesion, he suggests that the doorway to long life might hinge on the gene's ability to sound alarms in response to corrupted DNA.
But two other groups that each independently isolated the clk-2/rad-5 gene--because of their respective interests in embryonic development and possible links between telomere length and life-span--have reached two different conclusions about its effect on telomere length. Worms with mutated clk-2 carry telomeres that are twice as long as those in wild-type animals, reported developmental biologist Siegfried Hekimi of McGill University in Montreal, Canada, in the 15 October 2001 issue of Development. Confounding the situation further, molecular biologist Judith Campisi of Lawrence Berkeley National Laboratory in Berkeley, California, discovered that the clk-2 mutant had shorter telomeres than wild-type worms did, according to work published in the 30 October 2001 issue of Current Biology.
The discrepancy isn't easily resolved, and each group stands by its data. Ahmed points to his correction for strain differences as crucial. Hekimi counters that worms must be inbred for several generations before the telomere effect appears. And Campisi says that her group has best proven that it is measuring telomere ends rather than unknowingly analyzing sequences in more central portions of chromosomes. Other scientists scratch their heads, unable to explain the conflicting conclusions. "The researchers working on clk-2 are going to have to resolve the telomere problem," says Johnson.
Although clk-2/rad-5 seems to be racking up cellular roles--in DNA repair and possibly telomere biology--nailing down how it influences life-span still eludes researchers. Previous evidence supports Ahmed's conclusion about the lack of connection between telomeres and longevity in the clk-2 mutant. "I'd be very surprised if the effects on telomere length were relevant to the extended life-span of clk-2 mutants," says Richard Wang, a molecular biologist at Rockefeller University in New York City. Telomere shortening has been linked to the demise of cells in culture, but possible connections to aging of entire organisms remain murky (see "More Than a Sum of Our Cells"). Because worms live for only 2 weeks and because most of the animals' cells stop dividing after embryonic development, it's hard to imagine that telomere shortening with successive cell divisions would trigger aging, he says. In contrast, alterations in mammalian proteins that monitor genome integrity can trigger premature aging disorders (see "Of Hyperaging and Methuselah Genes", "Deadly Network", "ATR Takes ATRIP to Fix Defects", and "Protective Pas de Deux?").
The new studies provide a jumping-off point for researchers who aim to discern the function of the clk-2/rad5 gene. Future studies will probe where the protein encoded by clk-2/rad-5 resides in the cell, whether it binds to telomeres or to DNA that is damaged in specific ways, and whether similar proteins from other species can replace clk-2/rad-5, says Campisi. In addition, because clk-2/rad-5 resembles yeast TEL2, the well-defined DNA repair pathways in yeast might provide a place to tease apart the worm gene's machinations. Together, such findings should illuminate how the specific tasks undertaken by the protein lead to the characteristics of the clk-2 and rad-5 mutant worms. This insight should help clarify the confusion that has converged on clk-2.
January 16, 2002
R. John Davenport is an associate editor of SAGE KE who writes about science and rides killer waves in Santa Cruz, California. He hopes that the DNA damage from frequent sunburns won't shorten his life-span.
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