Sci. Aging Knowl. Environ., 7 July 2004
A Twist in an Early End
Snafus in telomere unwinding foster a premature aging syndrome
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/2004/27/nf63
Patients with Werner syndrome show signs of rapid aging, perhaps because their chromosomes come to an abrupt end, two new studies suggest. Mice that lack the Werner syndrome protein (WRN) deteriorate quickly only if they also harbor short telomeres, protective caps that guard the ends of chromosomes. Furthermore, WRN flocks to telomeres and unravels structures there. The work solidifies the idea that telomere malfunction contributes to the disease.
"There are a lot of [biochemical] data out there on WRN, what it does and what it interacts with, but it's been a real struggle for everybody in the field to know where and in what pathway it's acting," says biochemist David Orren of the University of Kentucky in Lexington. "The great thing about these papers is that they do point us specifically to the telomeres."
Werner syndrome patients develop wrinkled skin, gray hair, and slumped spines at an unusually young age (see "Of Hyperaging and Methuselah Genes"). In addition, their eyes cloud with cataracts, their metabolism falters, and they suffer from unusual types of cancer. Discovering how the disease develops might help treat or prevent the affliction and reveal mechanisms that contribute to normal aging. Eight years ago, researchers pinpointed mutations that spur the condition. The gene in which these defects reside normally produces the WRN protein, a helicase that untwists DNA strands. Unlike most helicases, WRN also chews up DNA ends. Scientists have since discovered that WRN participates in DNA copying, damage repair, and recombination, the swapping of DNA segments from one chromosome to another (see Fry Review and Monnat and Saintigny Review).
When WRN fails, cells suffer. They carry fused and broken chromosomes and in culture, they divide fewer times than normal, entering a limbo state called replicative senescence. Furthermore, they carry short telomeres, and cranking up production of telomerase, an enzyme that lengthens telomeres, delays senescence in Werner cells. These observations suggest that WRN normally helps keep telomeres robust. But researchers hadn't shown that shortened telomeres were necessary to incite the disease in animals.
Mice without the WRN gene develop and age normally, a discovery that surprised scientists and has frustrated their effort to decipher the disease. But mice differ from humans in a crucial way. The rodents generate telomerase throughout their bodies, whereas humans produce the enzyme only in particular tissues. As a result, mice carry longer telomeres, which could explain why they don't show signs of illness.
In the new work, geneticist Sandy Chang of the University of Texas M. D. Anderson Cancer Center in Houston and colleagues generated mice that lack telomerase and WRN. Then the researchers raised the animals for several generations to allow their telomeres to decay. In the first two generations, the mutant mice aged at a normal rate. But in the fourth, fifth, and sixth generations, the animals died young, the team reported online 5 July in Nature Genetics. Some animals showed symptoms similar to those of Werner syndrome patients. They developed cataracts, lost hair, and their gonads shrank. In addition, they suffered from thin bones, diabetes, and slow wound healing. They also developed the types of cancers, such as lymphomas and soft-tissue sarcomas, that typically strike Werner syndrome patients but are rare in the general population. Except for heart problems--which are unusual in mice compared with people--"they get everything that Werner patients get," says Chang. Only a subset of the mutant animals developed these Werner-like symptoms; Chang attributes the variability to the fact that individual animals lose telomeres at different rates.
The finding argues that WRN and related helicases play an important role when telomeres shorten, says molecular biologist Brad Johnson of the University of Pennsylvania School of Medicine in Philadelphia: "If you don't have them, you might be more susceptible to pathology from dysfunctional telomeres." His team has also studied mice without WRN and telomerase and obtained results--currently under review for publication--consistent with the findings of Chang and colleagues.
But the mouse results are "perplexing," says oncologist Robert Marciniak of the University of Texas Health Science Center in San Antonio. Werner syndrome cripples so-called mesenchymal tissues, such as hair and bone, but leaves other types--such as the outer layer of skin and gut lining--relatively unscathed, and the mice reflect this pattern. Unlike humans, however, who don't produce telomerase in mesenchymal cells but do in others, the genetically altered rodents lack the enzyme in all tissues, says Marciniak. The paper drives home the point that "there must be something unique about telomere biology in mesenchymal cells," he says. For instance, WRN's job at telomeres might be more important in those types of cells, or other tissues might carry another enzyme that steps in if WRN is absent or malfunctions. After several generations, mice without telomerase develop cancer in rapidly dividing tissues such as blood and skin, says pathologist Raymond Monnat of the University of Washington, Seattle, and scientists might have predicted that, if WRN plays a role at telomeres, removing the protein in addition to telomerase would exacerbate those defects. Instead, the lack of WRN seems to block those problems and create new ones in different tissues. "So there's a more interesting, intriguing story going on here," he says.
The study solves the mystery of why mice missing only WRN age normally, but the protein's function at telomeres still baffles researchers. In a second study, biochemist Vilhelm Bohr of the National Institute on Aging branch in Baltimore, Maryland, and colleagues investigated WRN's biochemical interactions with telomeres. Telomeres end in a single strand of DNA; this segment loops back, slithers into a double-stranded portion of the telomere, and binds to one of those strands, creating a so-called D loop. Bohr and colleagues created an artificial DNA molecule that resembles a D loop. WRN yanked out the invading strand and chewed it off, the scientists reported on 18 June in Molecular Cell. That talent might allow WRN to break up D loops in telomeres that block DNA-copying machinery, thus allowing telomere duplication and perhaps retarding cell senescence. D loops also form when chromosomes intertwine during cell division, and by unwinding the structures, WRN might unravel improper pairings, thereby preventing chromosomes from fusing or breaking.
Alternatively, WRN might promote DNA exchanges that refresh telomeres when telomerase is absent, and additional studies by Bohr and colleagues support that idea. When cells lack telomerase, they can lengthen their chromosomes through a second pathway known as ALT, for alternative lengthening of telomeres. Through recombination, this process borrows one chromosome's telomeric DNA and uses it as a template to stitch telomeres onto another chromosome. Bohr and colleagues showed that cells that employ ALT harbor clusters of DNA-repair proteins at their telomeres and that WRN is part of this bunch; normal cells are less likely to carry WRN at telomeres. That difference suggests that WRN jumps in when cells fire up the ALT pathway. The researchers propose that WRN helps untwist D loops to enable ALT to occur and then untangles the chromosomes when the job is complete. "It's really nice," says Chang. "What their data imply is that Werner patients lose a protein that potentially resolves crazy DNA intermediates."
Together the results suggest that WRN performs important tasks at telomeres and that WRN deficits spur tissue malfunction when telomeres get too short. The protein's involvement with the ALT pathway could explain why Werner patients and the mutant animals show problems such as cancer mostly in certain tissues. That pathway might be more important in mesenchymal cells, says Marciniak, but more work is needed to test the idea. Researchers have seen ALT in tumors and cultured cells with ramped-up recombination activity, but no one knows whether the phenomenon occurs at low frequency in healthy cells within animals. "The question is, in a normal cell, does recombinational repair go on at telomeres and is WRN there?" says Johnson. Further experiments that uncover WRN's workings at telomeres could help researchers understand how its absence promotes Werner syndrome. Those efforts might also reveal whether the same mechanisms underlie normal aging, a possibility that could reveal ways to postpone the end for everyone.
July 7, 2004
R. John Davenport is an associate editor of SAGE KE. He twists on waves in Santa Cruz, California.
Science of Aging Knowledge Environment. ISSN 1539-6150