Telomere biology of trypanosomatids: beginning to answer some questions Cristina B.B. Lira 1, 2 , Miriam A. Giardini 1, 2 , Jair L. Siqueira Neto 1, 2 , Fa ´ bio F. Conte 3 and Maria Isabel N. Cano 1 1 Laborato ´ rio de Telo ˆ meros, Departamento de Gene ´ tica, Instituto de Biocie ˆ ncias, Universidade Estadual Paulista Ju ´ lio de Mesquita Filho (UNESP), Botucatu, SP, Brazil 2 Instituto de Biologia, Universidade Estadual de Campinas (UNICAMP), Campinas, SP, Brazil 3 Departamento de Gene ´ tica Me ´ dica, Faculdade de Cie ˆ ncias Me ´ dicas, Universidade Estadual de Campinas (UNICAMP), Campinas, SP, Brazil Studies of telomere structure and maintenance in trypanosomatids have provided insights into the evol- utionary origin and conservation of some telomeric components shared by trypanosomes and vertebrates. For example, trypanosomatid telomeres are maintained by telomerase and consist of the canonical TTAGGG repeats, which in Trypanosoma brucei can form telo- meric loops (t-loops). However, the telomeric chromatin of trypanosomatids is composed of organism-specific proteins and other proteins that share little sequence similarity with their vertebrate counterparts. Because telomere maintenance mechanisms are essential for genome stability, we propose that the particular fea- tures shown by the trypanosome telomeric chromatin hold the key for the design of antiparasitic drugs. An overview of trypanosomatid telomere biology Trypanosomatid protozoan parasites can cause devastating human and animal diseases, for which there is currently no adequate treatment (http://www.who.int/tdr; http:// www.who.int/en/). Because trypanosomatids branched from the vertebrate evolutionary tree millions of years ago [1], studying their telomeres should provide insight in to how these structures have been transformed during evolution. This article is aimed both at emphasizing some recent findings and at comparing them with the established knowl- edge from other eukaryotes; it does not contain a compi- lation of all available data. Five years ago, our knowledge of trypanosomatid telomeres was limited mainly to antigenic variation in Trypanosoma brucei and to the architectural organization of the chromosomal terminus in Trypanosoma cruzi and Leishmania [2]. Recent advances in telomeric research have shown that telomeres have important biological func- tions in all organisms, including in cell proliferation, and in cellular and organismal aging (see Ref. [3] and references therein). As a result, telomere proteins (and telomere maintenance mechanisms) have been increasingly recog- nized as potential drug targets. The completion of several trypanosomatid genomes, in addition to the detailed analyses of antigenic variation in Opinion TRENDS in Parasitology Vol.23 No.8 Glossary Cdc13: cell division control protein 13, a G-strand telomere-binding protein that acts as a regulator of telomere length in yeast and coordinates C-strand re-synthesis by DNA polymerase alpha. End-replication problem: a problem caused by the conventional replicative machinery that synthesizes DNA only in the 5 0 -3 0 direction and is therefore unable to complete DNA replication at the end of the lagging strand. In the absence of telomerase, organisms with linear chromosomes lose telomeric DNA with each cell division. KU70–KU80 (or KU70–KU86): a pair of proteins that, together with the catalytic subunit of the DNA-dependent protein kinase (DNA PKcs), form a complex called DNA-PK that is involved in non-homologous end joining, V(D)J recombination and telomere maintenance. MRE11: meiotic recombination 11, a conserved member of the RAD52 epistasis group that forms the MRX/N complex (MRX in yeast, comprising MRE11, RAD50 and XRS2, or MRN in mammals, comprising MRE11, RAD50 and NBS1). This protein is involved in general homologous recombination, in DSB repair and in telomere stability. POT1: protection of telomere 1, a protein that binds G-rich telomeric DNA and is a co-effector of the TRF1 complex by transducing information to the telomere terminus. RAD51: the eukaryotic homolog of bacterial RecA and participates in DNA damage-response pathway, homologous recombination and DSB repair. Rap1: repressor activator protein 1, a multifunctional protein first described as a transcriptional factor, which is involved in telomere length homeostasis, and telomere-position effect in yeast and humans. Rbp38: RNA-binding protein 38, a trypanosomatid protein that binds single- and double-stranded kinetoplast RNAs. It is involved in T. brucei kinetoplast DNA replication and has also been described as a telomere- and GT-rich DNA-binding protein. Rpa-1: replication protein A subunit 1, involved in DNA replication and recombination, and in telomere maintenance. This protein contributes to chromosome stability during DNA metabolism. TAZ1: telomere-associated in Schizosaccharomyces pombe. Regulates telo- mere length and function; required for the repression of telomere-adjacent gene expression and for normal meiosis or sporulation. TERT: the protein component of telomerase, the enzyme that elongates telomeres by copying a template sequence within its RNA component (TER). TER: the telomerase RNA component that contains a short sequence encoding the cognate telomere repeat that serves as the template for reverse transcription by TERT. TRF1 and TRF2: TTAGGG repeat-binding factors 1 and 2, which bind to telomeric double-stranded DNA as homodimers and to other telomeric proteins. TRF1 is involved in the cis-inhibition of TERT, whereas TRF2 protects chromosomes against end-to-end fusion and has a role in cell cycle progression and in t-loop formation. t-loops: telomeric loops, duplex loops generated by invasion of the G-rich strand in double-stranded telomeric DNA. The loops are maintained by proteins, such as human TRF2, and mask telomeres from the cellular machinery that detects DSB. T. brucei chromosomes: these are organized as 11 pairs of 1–5 Mb chromosomes containing most of the VSG and other essential genes, in addition to a variable number of intermediate chromosomes ranging in size from 200–500 kb and 100 minichromosomes varying from 50–150 kb, which contain mainly nontranscribed VSG genes and the 177 bp minisatellite. Corresponding author: Cano, M.I.N. (micano@ibb.unesp.br). Available online 18 June 2007. www.sciencedirect.com 1471-4922/$ – see front matter ß 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.pt.2007.06.005