Biomol

Meni Melek and Dorothy E. Shippen
Summary

Telomeres are protective caps for chromosome ends that are essential for genome stability. Broken chromosomes missing a telomere will not be maintained unless the chromosome is ‘healed’ with the formation of a new telomere. Chromosome healing can be a programmed event following developmentally regulated chromosome fragmentation, or it may occurspontaneously when a chromosome is accidentally broken. In this article we discuss the consequences of telomere loss and the possible mechanisms that the enzyme telomerase employs to form telomeres de novo on broken chromosome ends.
Introduction The essential function of the telomere, the DNA-protein structure that comprises the physical terminus of the eukaryotic chromosome, has been established formore than 60 years. However,the molecular basis for telomere behavior is still a mystery. In the late 1930s and early ’ ~ O S , Hermann Muller and Barbara McClintock demonstrated that broken chromosomes were highly unstable(lS2), cycling through repeated breakage-fusion-bridging eventd2). On occasion a broken chromosome became stabilized and could be propagated normally. Subsequent work has shownthat these ’healed’ chromosomes had acquired telomeres. Research in many laboratories over the ensuing years confirmed that the natural ends of chromosomes provide a protective cap that imparts stability to chromosomes by protecting against endto-end fusion, recombination and exonucleolytic degradation. The importance of the telomere is further highlighted by recent studies in yeast demonstratingthat the loss of even a single telomere causes cell-cycle arrest(3). Human chromosomes lose a small amount of telomeric DNA following each cell division. This phenomenon is thought to result from the inability of the conventional DNA replication machinery to replicate the extreme 5’terminus of each DNA strand(4). Consequently, terminal DNA sequences are lost with each round of replication, and overtime chromosomes gradually shorten at their ends. The striking correlation between telomere shortening and cellular aging led to the telomere hypothesis of aging, which proposes that telomeres represent a biological clock(5). According to this hypothesis, once telomeres shorten below a critical threshold, they lose the capacity to cap chromo-

Accepted 12 January 1996

somes effectively andactivate a damaged DNA response pathway that causes cell-cycle arrest@). The loss of telomeric DNA does not always culminate in cell death. Telomere function may be restored if the terminal DNA sequences are replenished by the action of telomerase. Telomerase is the specialized DNA polymerase that synthesizes telomeric DNA. This enzyme not only maintains pre-existing telomeres, but also addstelomeres directly onto broken chromosome ends in a process known as chromosome healing (Fig. 1). In this review we focus on the consequences and mechanisms of chromosome healing, beginning with an overview of telomere structure and synthesis. More extensive treatises on telomeres and their maintenance can be found el~ewhere(~1~).
Telomere formation by telomerase In the majority of organisms surveyed,the chromosome terminus consists of short repeated DNA sequence arrays, typified by a clustering and segregation of G-residues on one DNA strand(g). The terminal segments of mammalian telomeres, for example, comprise hundreds to thousands of consecutive TTAGGG repeats(l0).This DNA is complexed with specific telomere-binding proteins(ll). The G-rich DNA strand of the telomere forms a 3’singlestranded overhang and this protrusion is the substrate onto which telomerase synthesizes additional G-rich repeats. Telomerases represent a novel class of DNA polymerases in that they are ribonucleoproteins.The essential RNA subunit within telomerase carries a sequence complementary to the G-rich telomeric DNA strand. This domain functions as the template to specify the sequences added to chromo-...