Opinion TRENDS in Parasitology Vol.17 No.9 Septem ber 2001 425
46 Ozwara, H. et al. (1997) Flow cytometric analysis 48 Aucan, C. et al. (2000) High immunoglobulin preventing malaria. Cochrane Database Syst. Rev.
on reactivity of human T lymphocyte-specific and G2 (IgG2) and low IgG4 levels are associated CD000129 (http://hiru.mcmaster.ca/cochrane/
cytokine-receptor-specific antibodies with with human resistance to Plasmodium cochrane/revabstr/ccsales.htm)
peripheral blood mononuclear cells of chimpanzee falciparum malaria. Infect. Immun. 51 Smith, P.G. and Morrow, R.H., eds (1996) Study
(Pan troglodytes), rhesus macaque (Macaca 68, 1252–1258 Size. In Field Trials of Health Interventions in
mulatta) and squirrel monkey (Saimiri sciureus). 49 Nosten, F. et al. (1996) Randomised double-blind Developing Countries: A Toolbox (2nd edn),
J. Med. Primatol. 26, 164–171 placebo-controlled trial of SPf66 malaria pp. 42–71, MacMillan Education
47 Van de Berg, J.L. (1995) Genetics of nonhuman vaccine in children in northwestern Thailand. 52 Institute of Laboratory Animal Resources,
primates. In Nonhuman Primates in Biomedical Shoklo SPf66 Malaria Vaccine. Lancet National Research Council (2000) Guide for the
Research: Biology and Management (Bennett, 348, 701–707 Care and Use of Laboratory Animals, National
B.T. et al., eds), pp. 129–146, Academic Press 50 Graves, P. and Gelband, H. (2000) Vaccines for Academy Press, Washington, DC, USA
Telomere biology of organisms such as yeast and protozoa, as well as in
highly proliferative human cells and in 85% of
tumors2. The absence of enzyme activity in most
Trypanosomatids: human somatic cells seems responsible for the
progressive shortening of telomeres as cells age,
indicating that telomere length can predict cell fate2,5.
more questions than Immortalization by the ectopic expression of TERT is
clearly cell-type-specific and could be dependent on
whether the cells were also virus-transformed. There
answers are, however, cases where immortalization by TERT
will never occur5.
Despite the extensive progress in the telomere
biology of many eukaryotes, very little headway has
M aria Isabel N. Cano been made in understanding the implications of
telomeres on the biology of trypanosomes.
Trypanosomatids are severe pathogens in developing countries, w here they Telomere structure of trypanosomatids
affect both humans and domestic animals. Factors intrinsic to the host, the The trypanosomatids include parasites of medical
toxicity or subcurative effects of the available antiparasite medication and the and epidemiological importance, such as Leishmania
low perspective of potential vaccines favor research on novel candidates for species, Trypanosoma brucei and Trypanosoma cruzi.
drug target. Telomeres are essential for the survival of most eukaryotes. In These parasites, which branched early in the
trypanosomatids, events such as antigenic variation and/ or gene conversion eukaryotic lineage, possess unique biological
and duplication occur at telomeric positions, possibly facilitating genome features6. For this reason, trypanosomes can be
rearrangement. Understanding the role that telomere maintenance might play useful models for studying telomeres in the context of
in the cell life span of trypanosomatids has important implications for evolution.
therapeutics of parasitic diseases. Since the 1980s, work on trypanosomatids
telomeres has concentrated on exploring variant
For the past 15 years, telomeres and the mechanisms surface glycoprotein (VSG) antigenic variation in
involved in telomere replication and function have T. brucei, a relevant mechanism used by the parasite
been subject to intense research. Despite variations in to evade the host immune system6,7. VSG expression
length and sequence, all telomeres comprise is exclusively subtelomeric and occurs either by in
noncoding sequences, which are repeated in tandem, situ activation of a silent VSG gene or by DNA
associated with proteins. These nucleoproteic rearrangement that inserts an internal silent copy of
structures, which occur in both double- and single- a VSG gene into an active telomeric expression site7,8.
stranded conformations, are the physical ends of the These events are partly responsible for the extensive
chromosomes and play a crucial role in maintaining polymorphism in chromosome size and, consequently,
genome stability by capping the chromosome termini for the polymorphism found at each telomere1,8. A
and protecting them from degradation, end-to-end further curiosity is the presence of an unusual base J
fusion and rearrangement events1. (β-D-glucosyl-hydroxymethyluracil) in the place of
M aria Isabel N. Cano Most of the cells, including trypanosomatids, thymidine in sequences located upstream of a
Depart. de Genética e maintain telomeres using telomerase reverse telomeric VSG and at T. brucei telomeres. Whether
Evolução, Instituto de transcriptase (TERT, see Glossary)2,3. In contrast, some this arrangement means that J controls the
Biologia, Universidade
Estadual de Cam pinas,
insects replenish telomeres by a retrotransposon- transcription of telomeric VSG and/or recombination
UNICAM P, Cidade mediated elongation, and human alternative is still unknown7.
Universitária Zeferino lengthening of telomere (ALT) cells maintain The average length of T. brucei telomeres is
Vaz, Cam pinas,
telomeres through recombination/gene conversion approximately 10–20 kilobase pairs (kbp)8. The
13083-970, Brazil.
e-m ail: among tandem repeats4. Telomerase is constitutively telomeres extend towards the end of the chromosome
m icano@unicam p.br expressed in normally immortal unicellular as a single strand or as 3′ G-overhangs that are
http://parasites.trends.com 1471-4922/01/$ – see front m atter © 2001 Elsevier Science Ltd. All rights reserved. PII: S1471-4922(01)02014-1
, 426 Opinion TRENDS in Parasitology Vol.17 No.9 Septem ber 2001
Glossary
BC-VSG: Basic copy variant surface glycoprotein (VSG). This is a nontranscribed, SRA: An ESAG that confers resistance to hum an serum .
internal copy of several hundred VSGs dispersed around the T. brucei genom e. SZ23: A SIRE-associated sequence.
Chromosome healing: New ly form ed telom eres added to free DNA ends (usually Telomere capping: Telom eres as nucleoprotein structures can be found in tw o states
G-rich sequences) that w ere generated during events of double-strand during cell life span: capped (protected by telom eric chrom atin factors) or uncapped
chrom osom e breakage and repair. (deprotected).
ORFF: Open reading fram e F. Encodes a protein im portant for the survival of TER: Telomerase RNA component, forms with TERT the catalytic core of telomerase.
Leishm ania spp. Its secondary structure presents sites for protein binding, and sites involved in
ORFG: Open reading fram e G. Encodes a putative biopterin transporter of modulation of telomerase activity, in addition to a template sequence region used by
Leishm ania spp. telomerase to add telomeric repeats at the 3’ G-rich strand present at the chromosome
Sir: Silencing information regulator. Comprises a conserved gene family that, in yeast, end teminus.
controls the expression of genes located at different chromosome position, including TERT: Telomerase reverse transcriptase. With TER, TERT forms the catalytic core of the
telomeres. telomerase holoenzyme. In vivo , and depending on the species, TERT can form dimers
SIRE: Short interspersed repetitive elem ent. Present in m ultiple copies dispersed in or multimers with other protein subunits.
the genom e of T. cruzi. 3’UTR: Untranslated region located at the 3’end of a trypanosom e gene.
21–250 nucleotides (nt) long (M. Cano et al., gp85 is found only a few kilobase pairs distant from
unpublished and Ref. 9). These 3′ overhangs are found the telomeric repeats10. Determining whether
at both telomeres in the same chromosome and, like telomere maintenance in T. cruzi is directly involved
those in humans and ciliates, can form t-loops9. The in the generation of new copies of gp85 gene members
structure of the T. brucei telomeric/subtelomeric will provide valuable insights into the genetic events
region (Fig. 1a) is highly polymorphic and shares that govern parasite growth and survival in the
several similarities with the same region in other mammalian host.
eukaryotes1,8. Telomeres extending from the 3′ G- The telomeric architecture of Leishmania species
overhang to an internal position on the chromosome seems to be an exception among the trypanosomatids.
show a telomeric hexameric repeat (5′-TTAGGG-3′)n of The chromosomes of different species of Leishmania do
variable size, followed by a large subtelomeric region not present copies of genes encoding surface antigens
containing active or silent bloodstream (B) or at telomeric positions. Instead, Leishmania telomeres
metacyclic (M) VSG genes. The expression of VSG are composed of the conserved 5′-TTAGGG-3′
genes at subtelomeric polycistronic expression sites sequence, although a nonconserved telomere repeat
(ES) occurs under the control of a promoter located was also found in L. braziliensis12. Leishmania
40–60 kbp upstream of the B-VSG gene or just a few conserved telomere-associated sequence (LCTAS) or
hundred base pairs upstream of the M-VSG gene. A Leishmania subtelomeric sequences (LSTS) are very
minisatellite (177 bp repeat) is also found at telomeres conserved but, depending on the chromosome or the
in minichromosomes8 (Fig. 1a). Minichromosomes are species, they show different organization (Fig. 1c)12,13.
transcriptionally inactive and could provide the LCTAS are mainly responsible for the ~20 kb size
parasite with a repertoire of silent VSG genes7. difference between the two homologues of
The cloning and characterization of T. cruzi chromosome 1 of L. major13. The organization of the
telomeres revealed that they are very similar to subtelomeric regions of some species of Leishmania
T. brucei in composition and organization10,11. The also contain other nonconserved elements6,13.
average length of the 3′ G-overhang of T. cruzi Leishmania DNA 1 (LD1), an amplified DNA element
chromosomes is approximately 9–50 nt and ends in a found in different species of Leishmania, is a
consensus terminus (5′-GGGTTAGGG-3′)10 identical multigenic locus located near one telomere of
to the antisense sequence of the hypothetical chromosome 35. Various portions of LD1 arise
template region of T. brucei telomerase RNA (TER)3. spontaneously from LCTAS and form
This suggests that T. cruzi and T. brucei TER have a extrachromosomal circles or linear minichromosomes
template sequence very similar to that used by TERT that usually contain genes encoding proteins of
to elongate telomeres. Adjacent to the hexamer is the important function such as ORFF and ORFG. For
double-stranded telomeric repeat and a conserved example, in L. donovani (strain LSB-51.1), these genes
189 bp junction10,11. Telomeres of different strains of are overexpressed and duplicated by gene conversion
T cruzi are highly polymorphic in size11, which is into the rRNA gene locus on chromosome 27 (Ref. 13).
probably due to gene conversion/duplication at The fact that trypanosome subtelomeres are the
subtelomeric positions2. There is a strong indication most variable region on their chromosomes is not a
for recombination events occurring at parasite surprise because the subtelomeric regions of other
telomeres. Entire copies or parts of members of the eukaryotes are as diverse as they are dynamic1.
multigene family of gp85-sialidase, the main parasite
virulence factor, are found at subtelomeric positions Telomere replication in trypanosomatids
in different chromosomes of T. cruzi10 (Fig. 1b). A few studies have investigated the molecular
Although no promoters for gp85 genes have been features that control telomere replication and
found close to telomeres, subtelomeric copies of gp85 dynamics3,8,14. In 1975, it was noted that the terminal
genes are expressed10,11. In addition, the subtelomeric sequence of T. brucei telomeres increases in length by
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