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17. Tolias, A.S., Moore, T., Smirnakis, S.M.,Tehovnik, E.J., Siapas, A.G., and Schiller, P.H.(2001). Eye movements modulate visualreceptive fields of V4 neurons. Neuron 29,757–767.

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suppression in humans. Curr. Biol. 14,386–390.

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Visual Sciences, Group and ARC Centreof Excellence in Vision Science,Australian National University, Canberra,ACT 2601, Australia.E-mail: Michael.Ibbotson@anu.edu.au

DOI: 10.1016/j.cub.2009.05.010

Current Biology Vol 19 No 12R496

Antigenic Variation: Extending theReach of Telomeric Silencing

Immune evasion in the parasitic African trypanosome relies upon the silencingof variant surface glycoprotein genes that are found adjacent to telomeres.Work on the RAP1 telomere-binding protein now indicates that silencingspreads over a sufficient distance to repress these genes.

David Horn

Promoters drive gene expression byrecruiting RNA polymerase but, in thevicinity of telomeres, promoters andtheir associated genes are silenced.This ‘telomere position effect’phenomenon was first described inyeast [1] and was subsequentlydemonstrated in human cells and alsoin trypanosomes [2]. There are distincttypes of telomeric silencing that aremediated by different factors and,depending upon the cell type, differin the distance that the effect spreadsfrom the telomere. A recent study onthe trypanosome telomere-bindingprotein repressor/activator protein 1(RAP1) now reveals that a far-reachingtype of silencing extends overa sufficient distance to silence all butone of the promoters associated withvariant surface glycoprotein (VSG)genes [3].

The African trypanosome,Trypanosoma brucei, is a protozoanparasite of major medical andeconomic importance. These highlymotile cells circulate in the mammalianhost bloodstream and are spreadamong mammals by tsetse flies.Mammalian host immune mechanismswill likely eliminate any infectiousagent that exposes a common setof epitopes for several days and thisis why many pathogens have evolvedstrategies for phenotypic, clonalvariation of surface proteins. Africantrypanosomes provide a sophisticatedexample of an immune evasionstrategy that allows the establishmentof a persistent infection inimmunocompetent hosts. In

trypanosomes, the VSG coats the celland has the capacity to shield othersurface molecules from immune attack[4]. There are many potential telomericVSG expression sites (ESs; Figure 1)but only a single VSG is expressed byeach cell and a co-ordinated switch cantransfer active transcription from onetelomere to another [5]. Importantly,silencing at all other ESs is critical tomaintain monoallelic expression andthe integrity of the evasion strategy.

The discovery of telomeric silencingoffered a potential mechanism for thecontrol of VSG genes in trypanosomes,as well as for the control of othersubtelomeric gene families that aresubject to monoallelic expression,such as those encoding olfactoryreceptors in mammals [6] and variantsurface proteins (var genes) in themalaria parasite [7]. Crucially, thesilencing effect would have to extenda long way from the telomere in theseorganisms if these genes were to beaffected. Most of what we know abouttelomeric silencing mechanismscomes from studies in yeast wherethe repressive activity of RAP1 isexhibited at telomeres [8], via bindingto the characteristic short telomericDNA repeats and recruitment ofa silent information regulator (SIR)complex. This SIR complex spreadsbeyond the telomeric repeats andmodifies histones, thus forming silentchromatin or heterochromatin atsubtelomeres [9]. The SIR2 componentof this complex is a histonedeacetylase [10], and disruption ofSIR2-related proteins results in lossof telomeric gene silencing in yeast[11] and loss of telomeric var gene

silencing in the malaria parasite [7].These results also indicated that thesilencing effect can spread greatdistances from the telomere. In thetrypanosome, however, disruption oftbSIR2 resulted in only partial loss ofsilencing close to the telomere andVSG ES promoters were unaffected[12] (Figure 1A).

So how are telomeric VSG genessilenced? Telomere position effectsthat are mechanistically related tothose described in yeast have obviousappeal and sequencing of thetrypanosome genome presented newopportunities for research in this area.Yang and colleagues [3] now providedirect evidence that telomeres areinvolved in the regulation of VSGcontrol by showing that depletion oftrypanosome RAP1 increases VSGexpression from all ESs by up to50-fold (Figure 1B,C). tbRAP1 wasinitially identified via an interactionwith a telomere repeat-binding factor(tbTRF1) and was then shown tocolocalise with this protein and toassociate with telomeric DNA, possiblydirectly via Myb-like DNA-bindingdomains. Depletion of tbRAP1 usingan inducible RNA interferenceapproach indicated that the proteinis essential for growth but provideda window of opportunity to exploreVSG expression. To provide goodcoverage, the authors exploiteda recently published complementof ES sequences [13] and usedquantitative real-time PCR to examinethe expression of all fourteen VSGslocated in telomeric ESs in the strainanalysed. Importantly, VSG-specificantisera detected multiple VSGs onthe surface of individual cells followingtbRAP1 depletion, indicating thatmonoallelic VSG expression wasseverely compromised. Other recentstudies support the idea that multiplechromatin modifiers cooperate tosilence VSG ESs; deletion of a histonemethyltransferase gene (tbDOT1B,named after yeast disruptor oftelomeric silencing 1, DOT1) led to

DispatchR497

VSG1 VSG2 VSG3 VSG1-RAP1

B

A

C

Telomere-binding: Possibly tbRAP1

Spreading: tbSIR2rp1

Telomere-binding: tbRAP1

Spreading: Unknown

Current Biology

Figure 1. Model for telomeric silencing and VSG expression control in trypanosomes.

Our current understanding of telomeric silencing involves separate telomere-binding andspreading factors. (A,B) Two different forms of silencing, indicated by blue triangles, operatein trypanosomes (adapted from Figure 6 in [3]). (A) Telomere-proximal silencing is tbSIR2rp1-dependent and may depend upon recruitment of silencing regulators by tbRAP1. (B) tbRAP1-dependent silencing extends sufficiently far from the telomere to silence VSG ESs. Thissilencing is tbSIR2rp1-independent but spreading may require another (as yet unknown)factor. The flag represents the promoter, the light grey arrows represent ES-associated genesand the dark grey arrow represents a VSG flanked by 70-bp repeats (hatched box) and telo-meric repeats (arrowheads). (C) Fluorescent images representing three trypanosomes withdifferent VSG coats that are normally expressed in a monoallelic manner. The schematic repre-sentation on the right is of VSG expression on the surface of a RAP1-depleted trypanosome;note that the ‘active’ VSG, VSG1 in the example shown, remains the most abundant followingloss of RAP1, but that multiple other VSGs can also be detected at the surface. (The fluores-cent trypanosome image was provided by Sam Alsford.)

a slight increase in VSG transcription[14], while depletion of a chromatinremodeler (tbISWI) led to increasedVSG ES promoter activity, but did notincrease VSG transcription [15].

Telomeric silencing appears to beindependent of the promoter orpolymerase type and VSG ESpromoters actually recruit RNApolymerase I, a polymerase restrictedto nucleolar ribosomal RNAtranscription in other eukaryotes; thesingle active VSG ES localises to anextra-nucleolar focus enriched in RNApolymerase I [16]. Yang and colleagues[3] also explored RNA polymerase Idistribution in tbRAP1-depleted cells

and found additional RNA polymeraseI foci. Taken together, their resultssuggest that a tbRAP1-dependentmechanism blocks RNA polymerase Ielongation at silent ESs, while another(as yet unknown) mechanism booststranscription at the active ES(Figure 1C). The consequence isa 10,000-fold difference in expressionfrom silent and active ESs.

The work of Yang and colleagues [3]reveals a related theme in telomericsilencing in yeast and trypanosomes.Over the past decade, histonemodification has also emerged asa common mechanism of geneexpression control. Of particular

relevance here, a second histonedeacetylase has been linked tosilencing of subtelomeric genes inmultiple independent studies [11,17]and in each case the deacetylasesilenced genes encoding surface-exposed proteins. As with mostexciting new findings, new questionsfollow. It will be particularly interestingto determine which trans-acting factorsand, possibly, subtelomeric sequencescontrol the spreading mechanism intrypanosomes (Figure 1); tbRAP1 mayspread silencing by recruiting histonemodifiers or histone variants to VSGESs, for example. In addition, VSG ESscontain a DNA-base modificationknown as ‘J’ that is restricted to a stageof the life cycle that expresses VSG ina monoallelic manner [18]. Since VSGES silencing appears to be reinforcedin this life-cycle stage [2,19], it will beinteresting to determine whether J isresponsible. It was recognized morethan 25 years ago that the active VSGwas adjacent to a telomere [20] and,as its name suggests, RAP1 activatesas well as silences genes in yeast [8].Could tbRAP1 also play a role in VSGactivation?

Finally, it is important to rememberthat African trypanosomiasis isa neglected tropical disease (http://www.who.int/). Since VSG silencingis so important for immune evasionand persistent infection, a betterunderstanding of the silencingmechanism may reveal attractivetargets for intervention strategies.There are many possible drug targets,but a target involved in VSG silencingand possibly other essential cellularfunctions could be particularlyattractive.

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Billington, B.L., and Zakian, V.A. (1990).Position effect at S. cerevisiae telomeres:reversible repression of Pol II transcription.Cell 63, 751–762.

2. Glover, L., and Horn, D. (2006). Repression ofpolymerase I-mediated gene expression atTrypanosoma brucei telomeres. EMBO Rep.7, 93–99.

3. Yang, X., Figueiredo, L.M., Espinal, A.,Okubo, E., and Li, B. (2009). RAP1 is essentialfor silencing telomeric variant surfaceglycoprotein genes in Trypanosoma brucei.Cell 137, 99–109.

4. Barry, J.D., Ginger, M.L., Burton, P., andMcCulloch, R. (2003). Why are parasitecontingency genes often associated withtelomeres? Int. J. Parasitol. 33, 29–45.

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London School of Hygiene & TropicalMedicine, Keppel Street,London WC1E 7HT, UK.E-mail: david.horn@lshtm.ac.uk

DOI: 10.1016/j.cub.2009.04.049