Article: Switching trypanosoma coats: what's in the wardrobe?

Question 1

Trypanosoma brucei background

Answer 1

Trypanosoma brucei: flagellated unicellular protozoa / extracellular / causes African sleeping sickness
Zoonosis: transmitted with tsetse flies from humans to mammalian reservoirs
Since its extracellular: its open to the attacks of immune system

Question 2

How Trypanosome survives in blood / explain VSG coat briefly

Answer 2

• Trypanosoma cells are coated with dense/tightly packed variant surface glycoprotein (VSG) homodimers + bound with GPI anchors
• VSG protects from complement lysis for some time, eventually, the host builds an Ab response against the coat and drills it, then changes the antigen and survives for years resulting in a chronic infection
• VSG coat expression is essential 4 blood form Trypanosoma, even in vitro
• The coat replaces itself with antigenic variation

Question 3

VSG genes

Answer 3

• There are more than a thousand VSG genes in the genome, found in subtelomeres/telomeres/minichromosomes + also in expression sites
• Not in the telomere repeats tho: between cap and chromatin, so sequence does not shorten down, but recombination happens often here
• 80% is not functional /pseudogenes
• Only 1 VSG is expressed at a time, on Bloodstream form expression sites (BES) while others are silent.

Question 4

Gene conversion:

Answer 4

Type of homologous recombination.

Take a gene to an active replication site, dump the previous one

Question 5

Editing VSG gene:

Answer 5

Gene conversion/insitu switch/telomere exchange

Question 6

Telomere exchange

Answer 6

Type of homologous recombination.
DNA exchange of two chromosome ends/telomeres = crossover.
AXB = BXA , If B was transcribed, now A is
-can preserve newly generated VSG on minichromosomes

Question 7

In situ switch

Answer 7

Activating another transcription site/silence old one

-doesnt create new VSG, expression sites are rather unstable.

Question 8

Pseudogenes of VSG/segmental gene conversion

Answer 8

silent VSG genes are pseudogenes (only %7 works)
%66: in frame stopcodons/frameshift
%18: gene segments
%9: random

They are believed to be involved in more chronic infections = different pseudogenes are sewn together to create diverse VSGs. = segmental gene conversion/mosaic VSGs

• happens during the crossover/antigenic variation
• generates chimeric VSG genes from different VSG pseudogenes
• random generation of new VSGs, gene often contains more than 2 donors
• once replaced: they often get destroyed

They arent expressed unless they are brought to BES: evolutionarily good not to show the variants to immune system

Question 9

RAD51 knockout:

Answer 9

Repairs dsbreaks in DNA
Disruption of RAD51 leads to impairing of
the VSG switching

Question 10

Why its advantageous/not advantageous to keep VSG on pseudogenes?

Answer 10

-Genetic drift: relaxed selection on VSG genes: distruptive mutations accumulate
Since not transcribed, not repaired
-VSG genes don’t exist on homologous chromosomes: no template for mismatch repair available to fix them
-Having such large VSG piece repertoire can be advantageous = antigenic diversity on chronic infection

-Gene conversion here can lead to cell cycle arrest due to too much errors
To save this: replace gene conversion: only can happen if cell goes hyperrecombinogenic before it dies without VSG.
-Also VSG mosaic pattern is predictable = some pseudogenes could be directly selected instead to increase variants in population