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Telomere Protection Without a Telomerase: The Role of Drosophila ATM and Mre11 in Telomere Maintenance
In most organisms studied, the ends of a chromosome are elongated by telomerase, which adds short repeats. In yeast, atm or mre11 mutants have shortened telomeric repeats, implicating abnormal telomerase activity. Previous studies have been largely focused on how ATM and Mre11 regulate telomerase activity. If their function is solely to regulate telomerase activity, one would predict that they have minimal telomere maintenance function for organisms that employ alternative telomere-elongating mechanisms, such as D. melanogaster. Telomeres of Drosophila are enriched with retrotransposons. It is believed that new copies of these transposons elongate Drosophila chromosomes. In addition, artificial chromosome ends that lack all transposons have been created and stably maintained in the lab. It is thus apparent that not only are Drosophila telomeres not elongated by a telomerase, the normal telomere function in Drosophila does not even contain a specific DNA element. We tested whether Drosophila ATM or Mre11 participates in telomere maintenance. We created knockouts of the fly atm and mre11 by using a novel homologous gene targeting method. We showed that both mutations disrupt development and cause lethality in Drosophila. We examined mutant tissues cytologically and discovered a severe telomere defect for both mutants: multiple telomere fusions were observed in all the mitotically active cells. On average, about 20% of the telomeres in a nucleus were engaged in fusions, an extent that is greater than any non-Drosophila case reported. Telomere fusions led to a vicious “fusion-bridge-breakage” cycle in both mutants, which is similar to the one first described in maize by McClintock: telomere fusions lead to the formation of chromosome bridges that connect the separating sister nuclei. These bridges sometimes break creating new broken ends that can subsequently fuse with other ends, including telomeres. In both mutants, this devastating cycle led to widespread genome instability in the forms of chromosome breakage, genome rearrangements, and gross aneuploidy. By double mutant analyses, we suggest that Drosophila ATM and Mre11 function in the same telomere-protecting pathway. Therefore, our findings soundly disprove the hypothesis that Drosophila ATM and Mre11 have minimal telomere-protecting function. Furthermore, we propose the existence of a telomerase-independent function for these proteins that is likely conserved from yeast to human. Our study raises at least two new questions. First, given that similar mutants in yeast cause no telomere fusion, and thus no loss of cell viability, and that human and mouse atm mutants are also viable with much milder telomere dysfunction, why are Drosophila telomeres especially susceptible to telomere fusion? We propose that “popular telomeres” have one additional layer of protection than Drosophila telomeres, which is conferred by the telomerase itself. Not only can telomerase serve as a physical barrier to prevent fusion, it installs multiple binding sites for various telomeric repeat binding proteins, which can also cap the telomeres, preventing them from being repaired as double strand breaks (DSBs). Since Drosophila lack this layer of telomere protection, the function of ATM and Mre11 at the telomeres becomes essential for organismal survival. Secondly, what is the nature of this telomerase-independent mechanism that we propose as conserved throughout evolution? As a first step toward definitely answering these questions, we identified a separate telomere-protecting pathway that is regulated by the ATM-related ATR kinase. This pathway is partly redundant to the one controlled by ATM so that in cells deficient for both ATM and ATR, all telomeres become unprotected and susceptible to fusion (Bi X et al. Proc Natl Acad Sci U S A 102: 15167–72, 2005). Our current model is that ATM and ATR protect telomere integrity by safeguarding chromatin architecture that favors the loading of telomere-elongating and capping proteins. |
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conserved ATM checkpoint kinase and Mre11 DNA repair protein participate in
telomere maintenance for both yeast and mammalian cells. We studied their roles
at Drosophila (Drosophila melanogaster) telomeres, which are not maintained
by a canonical telomerase, in the hope that our studies would shed light on
their telomerase-independent function in telomere protection.