TGME49_261580

Toxoplasma gondii H2AX is a histone H2A variant that couples the DNA damage response to chromatin-based transcriptional silencing (PMID:19607843). Unlike the canonical H2A1, which carries only a partial SQ motif, H2AX bears a complete SQ(E/D)φ motif that is phosphorylated upon DNA damage (PMID:19607843), and this phosphorylation is written by ATM kinase and promoted upstream by the MYST-family acetyltransferase TgMYST-B (PMID:20159970). H2AX assembles into nucleosomes of distinct composition, being excluded from interaction with the H2Bv variant that partners H2AZ and H2A1 (PMID:19607843). Genome-wide, H2AX is enriched at repressed genes and the silent TgIRE repeat rather than active genes, and it accumulates at silenced chromatin during bradyzoite differentiation, when its transcript is upregulated (PMID:19607843). Beyond these findings, the structural and biochemical details of how H2AX phosphorylation is coordinated with silencing have not been characterized in the available corpus.

1. 2009 Medium

   It was unknown whether T. gondii encodes a bona fide DNA-damage-responsive H2A variant; showing H2AX carries a complete SQ(E/D)φ motif phosphorylated after DNA damage established it as the parasite's γ-H2AX equivalent and distinguished it from canonical H2A1.

   Evidence sequence analysis of the SQ motif plus detection of H2AX phosphorylation after DNA damage in the parasite

   PMID:19607843

   Open questions at the time

   • Does not identify the kinase responsible
   • Does not resolve the functional consequence of phosphorylation at damage sites
2. 2009 Medium

   Whether H2AX forms compositionally distinct nucleosomes was unresolved; demonstrating that the H2Bv variant partners H2AZ and H2A1 but not H2AX established that H2AX-containing nucleosomes are biochemically distinct from other H2A variant nucleosomes.

   Evidence Co-immunoprecipitation between H2A variants and H2Bv with H2AX as an explicit negative

   PMID:19607843

   Open questions at the time

   • Single Co-IP without reciprocal or reconstitution validation
   • Does not define which H2B partners H2AX
3. 2009 Medium

   The chromatin context of H2AX was unknown; ChIP mapping showed H2AX marks repressed genes and the silent TgIRE element while H2AZ marks active genes, and it accumulates at silenced loci with upregulated transcript during bradyzoite differentiation, establishing a role in transcriptional silencing and stage conversion.

   Evidence ChIP genomic mapping, quantitative RT-PCR, and localization during in vitro and in vivo bradyzoite differentiation

   PMID:19607843

   Open questions at the time

   • Does not establish whether H2AX deposition is a cause or consequence of silencing
   • Does not connect the silencing role to the DNA damage phosphorylation role
4. 2010 Medium

   The writer and upstream regulator of H2AX phosphorylation were undefined; placing TgMYST-B upstream and ATM kinase as the writer established the signalling axis for H2AX phosphorylation in the parasite DNA damage response.

   Evidence TgMYST-B overexpression with phospho-H2AX readout and pharmacological ATM inhibition epistasis

   PMID:20159970

   Open questions at the time

   • Relies on pharmacological inhibition rather than genetic ablation of ATM
   • Does not define how TgMYST-B acetylation feeds into ATM activation
5. 2024 Medium

   It was unclear whether infection perturbs host genome integrity; γ-H2AX quantification showed T. gondii induces host DNA double-strand breaks and ATM-dependent homologous recombination in a strain-dependent manner, with RH most potent.

   Evidence γ-H2AX microscopy, comet assay, and HR/NHEJ marker analysis in infected primary endothelial cells (host-cell H2AX readout)

   PMID:38524184

   Open questions at the time

   • γ-H2AX here reports host-cell, not parasite, H2AX
   • Does not identify the parasite effector driving host DSBs

• How the DNA-damage phosphorylation function of H2AX is mechanistically integrated with its role in transcriptional silencing, and the structural basis of its distinct nucleosome composition, remain open.
• No structural model of H2AX nucleosomes
• No genetic loss-of-function characterization of parasite H2AX
• Mechanistic link between silencing and DNA damage roles unresolved