Research introduction

African trypanosomes

The African trypanosome Trypanosoma brucei causes African Sleeping Sickness in humans, which is endemic to subSaharan Africa and is spread by tsetse flies.  Trypanosomiasis is normally fatal in humans if left untreated. 

Trypanosomes are flagellated unicellular eukaryotes with a genome size about three-fold that of bakers yeast.  Trypanosomes are easily cultured in the laboratory both in vitro and in vivo, and are straightforward to genetically modify. In addition, RNA interference (RNAi) is a highly effective means of inducibly interfering with gene function. Trypanosomes provide a very manipulable experimental system for investigating immune evasion and pathogen-host adaptation.




Bloodstream form T. brucei among red blood cells (left) Scanning electron micrograph of bloodstream form T. brucei from Susan Vaughan and Keith Gull (right)

Antigenic variation in African trypanosomes

Trypanosomes are unusual parasites in that they multiply extracellularly within the blood of the mammalian host where they remain fully exposed to continuous immune attack.  Their success as a pathogen is a consequence of their highly sophisticated strategy of antigenic variation of a Variant Surface Glycoprotein (VSG) coat. As trypanosomes multiply in the bloodstream, the host eventually mounts an antibody response against trypanosomes expressing a given VSG.  However, as trypanosomes can switch to new VSG coat variants not recognised by host antibodies, these can evade antibody mediated lysis and form the next wave of infection.  As trypanosomes have more than one thousand antigenically distinct VSGs, a chronic infection can be mounted lasting for years.  The bloodstream form trypanosome is therefore continuously balancing on the brink of destruction.  It is only through its ability to change its surface coat that it can survive.

Above: Trypanosomes multiply in the blood of the host until an antibody response results in lysis of recognised variants.  Switched variants have a selective growth advantage until a host antibody response is mounted against these too.

The active Variant Surface Glycoprotein gene is transcribed from a single VSG expression site.

The active VSG gene is located in one of about 20 VSG expression sites located at telomeres (chromosome ends).  VSG expression sites are polycistronic (multiple gene containing) transcription units.  In addition to the telomeric VSG gene, there are a number of families of expression site associated genes, most of unknown function.  It is thought that these expression site associated genes could play a role in adaptation of the trypanosome to life in different species of mammalian host.

gene

Above: Schematic of a typical Variant Surface Glycoprotein (VSG) gene expression site transcription unit.  Expression sites are located at telomeres (telomere repeats indicated with triangles).  The promoter is indicated with a flag, and transcription with an arrow.  The coloured boxes indicate different families of expression site associated genes.  

Switching VSG gene expression

Switching the active VSG can involve one of three predominant mechanisms.  Individual trypanosomes have many hundreds of silent VSG genes in tandem arrays.  DNA rearrangements can move a previously silent VSG gene into the active VSG expression site transcription unit.  The most important DNA rearrangement mechanism during a chronic infection is duplicative gene conversion.  This results in a previously silent VSG gene being copied into the active VSG expression site and replacing the old copy.  Alternatively, a reciprocal exchange can occur between two telomeres.  Lastly, a VSG switch can be mediated by transcriptional control.  In this case the trypanosome silences one VSG expression site, and activates a new one.

vsgswitching

Above: Schematic of the major mechanisms for switching the active Variant Surface Glycoprotein (VSG). The large boxes indicate trypanosomes, with the colour indicating the colour of the expressed VSG gene. The small filled boxes represent VSG genes. The active VSG is transcribed from one active telomeric VSG expression site (ES). The ES promoter is indicated with a flag, and transcription with an arrow. During a switch mediated by gene conversion, a copy of a previously silent VSG is duplicated into the active ES replacing the active VSG gene. After a telomere exchange, another telomeric VSG replaces the active VSG, but there is no loss of DNA sequences. During a transcriptional switch, the active ES is silenced and a previously silent ES is activated.

Many questions regarding VSG switching remain unanswered. Analysis of the T. brucei genome sequence has lead to the surprising discovery that the majority of the silent VSG genes present within tandem arrays are in fact nonfunctional pseudogenes. How is the trypanosome able to correct these disfunctional VSG genes during the VSG gene switching process? VSG switching is not completely random, and there appears to be a preferential hierarchy of VSG activation. To what extent is this preferential hierarchy of expression hard-wired within the trypanosome genome? Lastly, how is the monoallelic exclusion operating on VSG expression sites maintained?