DNA double strand break position leads to distinct gene expression changes and regulates VSG switching pathway choice

Abstract

Antigenic variation is an immune evasion strategy used by Trypanosoma brucei that results in the periodic exchange of the surface protein coat. Underlying this process is the movement of variant surface glycoprotein genes in or out of a specialized locus known as bloodstream form expression site by homologous recombination, facilitated by blocks of repetitive sequence known as the 70-bp repeats, that provide homology for gene conversion events. DNA double strand breaks are potent drivers of antigenic variation, however where these breaks must fall to elicit a switch is not well understood. To understand how the position of a break influences antigenic variation we established a series of cell lines to study the effect of an I-SceI meganuclease break in the active expression site. We found that a DNA break within repetitive regions is not productive for VSG switching, and show that the break position leads to a distinct gene expression profile and DNA repair response which dictates how antigenic variation proceeds in African trypanosomes.

VSGs expressed during natural T. b. gambiense infection exhibit extensive sequence divergence and a subspecies-specific expression bias

“…The fact that only very short stretches of homology could be found within the N-terminal domain, however, is consistent with recombination through microhomology-mediated end joining, a DNA repair mechanism that uses short stretches of homology (5-20bp) to repair DNA damage (59). This appears to be the favored form of DNA repair in the VSG expression site and has been hypothesized to play a role in VSG switching (59, 60). The data presented here suggest this mechanism, or a similar one, may play a role in diversification of the VSG repertoire as well.…”