Introduction: Telomere dynamics — causes and effectsHoward Cooke

ALTHOUGH HISTORICALLY the first level of genomeorganization observed, an understanding of themolecular biology of chromosome structure andfunction has been more elusive than that of the gene.Both of these aspects relate to the size and complexityof the chromosome. Light microscopy has been andremains a powerful tool to investigate the largermetaphase chromosomes and in combination with in-situ hybridization and antibody probes of particularsequences and proteins is widely used to map theinterphase nucleus.

A crucial additional tool in this area has been theuse of yeast genetics and biochemistry to manipulatechromosomal components, both DNA and protein,with the ultimate proof that we understand chromo-somes in S cerevisiae at a primary level provided bythe development and widespread use of the yeastartificial chromosome cloning system.

The approaches taken in yeast to define replicationorigins and centromeres have not, however, beentransferable to mammalian systems. Walking across ahuman centromere from flanking marker to flankingmarker as was done in Schizosaccharomyces pomberemains a daunting problem due to the distances andrepetitive nature of DNA from the region. Even thehuge amount of data and number of technicaladvances from the genome project has not yet madethis a feasible approach. Cross-species comparison ofcentromeric sequences from a number of mammalshas shown a wide range of sequences at the cen-tromeres of different organisms with, to date only onesignificant conserved motif recognized — the bindingsite for a protein first detected using autoimmunesera. Reintroduction of sequences containing thismotif into mammalian cells can result in sites ofintegration of the DNA which mimic some aspects ofcentromeres but do not provide complete function.

The genetic approaches used in yeast to thedefinition of origins of replication when applied tomammalian systems have not provided functional orwell-defined DNA sequences capable of being used in

the way that the yeast autonomously replicatingsequences, many of which are demonstrably origins ofreplication in vivo, have been used to provide replica-tion functions enabling extrachromosomal replica-tion of DNA molecules. This failure has itself castdoubt on the existence of discrete origins of replica-tion although there is strong evidence from a numberof genes that there may be favoured regions in whichreplication initiates.

The success story in this field is the telomere.Perhaps this is because the involvement of theribonuclear protein telomerase in the replication oftelomeres provides a link to an ancient RNA catalysedworld with the result that most species of plant andanimal use a common strategy to protect and replicatethe ends of their chromosomes. Apart from indrosophila (which utilises a telomere specific trans-poson strategy) telomeres consist of arrays of shortrepeated sequences with single repeat lengths typi-cally between five and fifteen nucleotides and highlyvariable array lengths ranging from a few hundredbase pairs in the yeast Saccharomyces cerevisiae to asmuch as a hundred kilobase pairs on some mousetelomeres. The sequence of the repeats can be highlyhomogenous or degenerate but has as a commonfeature a deoxyguanosine-rich strand running 5' to 3'towards the end of the chromosome.

Much of the work which laid the foundation of ourcurrent view of telomeres derived from studies ontractable organisms such as ciliates where chromo-some fragmentation in the macronucleus gives rise toa genome which is very rich in both telomeric DNAand the proteins which interact with it. The realiza-tion that mammalian telomeres have much the samestructure as that of ciliates and the demonstration thatthe sequences involved are the same as those oftrypanosomes made possible the cloning of mammal-ian telomeres using yeast systems and the detailedanalysis of these regions of the human and mousechromosomes. The demonstration that thesesequences can function again as telomeres on reintro-duction into mammalian cells has opened the way tomanipulate mammalian chromosomes in vivo (Farr,this issue) and provides a tool for dissecting our other

From MRC Human Genetics Unit, Chromosome Biology Section,Western General Hospital, Crew Road, Edinburgh EH4 2XU, UK

©1996 Academic Press Ltd

seminars in CELL & DEVELOPMENTAL BIOLOGY, Vol 7, 1996: pp 3–4

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essential chromosome elements such as centromeresand origins of replication.

The telomere hypothesis of cellular senescence (seereviews by Bachetti and Villepointeau, this issue)argues that in the absence of telomerase telomeresprovide a counter for cell division by progressive lossand that at some point this loss becomes criticalresulting in the inability of the cell to divide. Thecentral role that this hypothesis places on telomereshas resulted in a recent focus on the presence orabsence of telomerase activity in different cell typesand has intensified the efforts to understand thebiochemistry of the enzyme, to clone its componentsfrom a range of organisms and to search for com-pounds capable of modifying its activity in vivo andhence with potential theraputic uses.

If telomeres have a critical role in the aging of cellsand are a limiting factor in cellular proliferation thisrole is as a part of a chromosome. As such a telomereconsists not only of DNA but also has proteincomponents, not only telomerase but also otherproteins which have as yet unknown functions. Someof these proteins must be involved in the interactionsof telomeres with other nuclear components asexemplified by the formation of ‘bouquet’ arrange-ments of chromosomes during meiosis resulting fromattachment of telomeres to the nuclear envelope.Localizations such as this may have an important rolein meiosis and must be driven by a set of telomere-nuclear envelope interacting proteins. Vertebratetelomere binding proteins have only recently beenisolated and are reviewed here by de Lange. Theseproteins will include those which set up the hetero-chromatic domain shown to be present at yeasttelomeres and the idea of a silenced region around atelomere has been invoked to explain human geneticdiseases as well as being proposed as one mechanismby which cellular senescence results from telomere

shortening. Before the idea of telomere as a silencedcompartment of the genome is accepted as universalit is worth pointing out that trypanosomes use atelomeric site from which to express surface antigengenes and plasmodium (reviewed by Scherf) appearsto concentrate its surface antigen genes towards itstelomeres. In both these cases genome rearrange-ments may provide variation which allows the organ-ism to evade the hosts response to infection.

These sporadic genome rearrangements are anexample of processes which occur as a function of atelomeric location. There is evidence of exchangesbetween chromosomes in a wide range of species.Another form of telomeric genome flexibility is thatof programmed chromosome fragmentation. I havealready mentioned its role in making the ciliates thefavoured source of telomere components but anotherform of genome rearrangement associated with telo-meres is the chromosome fragmentation and healingassociated with DNA loss in the somatic tissues ofsome nematodes such as Ascaris. Caenorhabditis-— which does not eliminate DNA from its somatictissues — has in common with Ascaris centromereswhich in mitosis extend the whole length of thechromosome but during meiosis the centromerefunction is provided by one telomere. Given thegenetics available in this organism and the effort onsequencing its genome C. elegans may become apowerful organism for chromosome structure andfunction studies.

The focus of this issue of essays in cell biology isintended to be the biological role of change at thetelomeres of chromosomes, the protein and RNAcomponents involved in these processes and theinformation about other aspects of chromosomestructure and function which we can derive frommaking changes in chromosomes with telomeres. Ihope the reader finds it useful.

Introduction

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