Transcription Dependent Loss of an Ectopically Expressed Variant Surface Glycoprotein during Antigenic Variation in Trypanosoma brucei.

In the mammalian host, Trypanosoma brucei is coated in a single-variant surface glycoprotein (VSG) species. Stochastic switching of the expressed allows the parasite to escape detection by the host immune system. DNA double-strand breaks (DSB) trigger VSG switching, and repair via gene conversion results in an antigenically distinct being expressed from the single active bloodstream-form expression site (BES). The single active BES is marked by exclusion 2 (VEX2) protein. Here, we have disrupted monoallelic expression by stably expressing a second telomeric from a ribosomal locus. We found that cells expressing two contained one VEX2 focus that was significantly larger in size than the wild-type cells; this therefore suggests the ectopic is expressed from the same nuclear position as the active BES. Unexpectedly, we report that in the double -expressing cells, the DNA sequence of the ectopic copy is lost following a DSB in the active BES, despite it being spatially separated in the genome. The loss of the ectopic is dependent on active transcription and does not disrupt the number or variety of templates used to repair a BES DSB and elicit a VSG switch. We propose that there are stringent mechanisms within the cell to reinforce monoallelic expression during antigenic variation. The single-cell parasite Trypanosoma brucei causes the fatal disease human African trypanosomiasis and is able to colonize the blood, fat, skin, and central nervous system. Trypanosomes survive in the mammalian host owing to a dense protective protein coat that consists of a single-variant surface glycoprotein species. Stochastic switching of one VSG for an immunologically distinct one enables the parasite to escape recognition by the host immune system. We have disrupted monoallelic antigen expression by expressing a second and report that following DSB-triggered VSG switching, the DNA sequence of the ectopic is lost in a transcription-dependent manner. We propose that there are strict requirements to ensure that only one variant antigen is expressed following a VSG switch, which has important implications for understanding how the parasite survives in the mammalian host.