MechanismestablishedQ192-the-rpa-heterotrimer

RPA-coated single-stranded DNA is the structure that licenses ATR: ATRIP reads it, ATR is brought to the lesion, and CHK1 is activated - which is how this corpus's ATR and CHEK1 entries connect to the clamp that had no entry until now

also called RPA-ssDNA-ATRIP-ATR-CHK1 axis · ATR licensing by RPA

A cell cannot see a broken chromosome directly. What it sees is a stretch of DNA that has become single-stranded and has been coated by the RPA clamp - and that coated stretch is the signal that switches on ATR, the kinase that halts the cell cycle. RPA is therefore not just a protective coating; it is the thing that makes damage visible.

The same thing, for a scientist

**The licensing structure is RPA-ssDNA, not ssDNA.** RPA is required for recruitment of ATR to damage sites and for ATR-mediated Chk1 activation in human cells; RPA stimulates ATRIP binding to ssDNA in vitro; ATRIP binding to RPA-coated ssDNA lets ATR-ATRIP associate with DNA and stimulates phosphorylation of DNA-bound Rad17; and the budding-yeast ATRIP orthologue Ddc2 is recruited to double-strand breaks in an RPA-dependent manner, with the checkpoint-deficient `rfa1-t11` allele defective for that recruitment [E1]. **The quantity that matters clinically is that the RPA pool is finite**: in ATR-deficient cells, unscheduled origin firing generates enough ssDNA to exhaust nuclear RPA, and stalled forks then break nucleus-wide; lowering RPA accelerates this and raising RPA delays it even without ATR activity [E2]. **This is the mechanistic bridge between two things this corpus already holds separately** - `gene-atr` and `gene-chek1` on one side, and a replication-stress literature in KMT2A-rearranged disease on the other, including ATR/ATM kinase inhibition showing antitumour activity in MLL-rearranged AML mouse models [E3]. **It is a bridge, not a result in this disease: no arm of any cited work is a KMT2A-rearranged cell, and the RPA step has never been measured in one** [E4]. **ETAA1, the RPA-binding ATR activator that runs parallel to ATRIP, has no entry anywhere in this corpus** and is named here as a gap [E1].

Go deeper

The chain, and which links this corpus already owns

Resection or fork uncoupling generates ssDNA. Owned here by gene-rbbp8 (CtIP), gene-exo1, mechanism-ledgf-dna-end-resection-and-homologous-recombination and the menin-SETD2-LEDGF-CtIP cascade.
RPA coats it. Was owned by nothing until protein-rpa-heterotrimer, gene-rpa1, gene-rpa2 and gene-rpa3 were written in cycle 160. This is the link the corpus was missing.
ATRIP reads RPA-ssDNA and brings ATR. 1 ATRIP has no entry here either.
ATR phosphorylates CHK1. Owned by gene-atr and gene-chek1.
RAD51 displaces RPA to make the recombinogenic filament. Owned by gene-rad51, which also holds the one locus-level observation where RPA, RAD51 and INO80 all over-load at the KMT2A breakpoint hotspot.

Alternative activator, also owned: gene-topbp1. Alternative activator, not owned: ETAA1, which binds RPA directly and activates ATR by a route distinct from ATRIP. That matters for drug reasoning: if the ATR dependence reported in MLL-rearranged AML runs through ETAA1 rather than ATRIP, the RPA-side determinant is a different surface and a different protein 1 3.

Why the finite-pool property changes how a combination should be read

The usual framing is that ATR inhibition sensitises cells with high replication stress. The mechanism is sharper than that: ATR inhibition kills by causing RPA exhaustion 2.

So RPA abundance is not a bystander, it is the variable:

Lowering RPA and inhibiting ATR are the same lever, not two levers. An RPA-directed agent plus an ATR inhibitor is not a rational combination on this mechanism; it is one intervention applied twice.
A large RPA reserve is a predicted resistance mechanism to ATR inhibitors, and nobody has looked for it in this disease or, on this beat's reading, in any patient material.
The assay exists: single-cell imaging of chromatin-bound RPA is the readout in the original work 2.

This corpus argues that KMT2A-rearranged cells carry transcription-replication conflict and replication stress 4. If that is right, the RPA reserve of a KMT2A-rearranged blast is a measurable, therapeutically relevant, and entirely unmeasured quantity - which is the actionable content of this entry.

Facts and figures

shown for fusion	NO for the RPA step - nobody has measured RPA, in any form, in a KMT2A-rearranged cell. YES for the downstream ATR/ATM step, in MLL-rearranged AML MOUSE MODELS only (PMID 27625305), which is the partial-credit form rule 86 asks for: the axis is drug-validated at its far end in a non-index fusion in a mouse, and entirely unmeasured at its near end in any system.
epigenetic readout	NONE intrinsic to this axis. The chromatin input is upstream: menin recognises gamma-H2A.X, recruits SETD2 to write H3K36me3, LEDGF reads it, CtIP resects - and only then does RPA coat the product (PMID 42578372). gamma-H2A.X itself is the mark that makes this axis visible experimentally and is the readout in this corpus's existing menin-inhibition HR experiments.
absence class	never-measured
absence class note	For the RPA step in a KMT2A-rearranged cell. But the cost of closing is LOW for a never-measured claim, because this corpus already designed the experiment - exp-hr-competence-under-menin-inhibition-in-kmt2a-rearranged-blasts uses RPA2 foci and phospho-RPA2 as its readout. The instrument is specified and unrun, not absent.

chain

chain — the numbers behind the chart

[
  {
    "step": 1,
    "event": "resection or fork uncoupling generates ssDNA",
    "corpus_entries": [
      "gene-rbbp8",
      "gene-exo1",
      "mechanism-ledgf-dna-end-resection-and-homologous-recombination"
    ]
  },
  {
    "step": 2,
    "event": "RPA heterotrimer coats the ssDNA (8-10 nt then ~30 nt mode)",
    "corpus_entries": [
      "protein-rpa-heterotrimer",
      "gene-rpa1",
      "gene-rpa2",
      "gene-rpa3"
    ],
    "note": "written cycle 160; nothing held this step before"
  },
  {
    "step": 3,
    "event": "ATRIP binds RPA-coated ssDNA and brings ATR",
    "corpus_entries": [],
    "note": "ATRIP HAS NO ENTRY IN THIS CORPUS"
  },
  {
    "step": 4,
    "event": "ATR phosphorylates CHK1; Rad17 phosphorylation is stimulated",
    "corpus_entries": [
      "gene-atr",
      "gene-chek1"
    ]
  },
  {
    "step": 5,
    "event": "RAD51 displaces RPA to form the recombinogenic filament",
    "corpus_entries": [
      "gene-rad51"
    ],
    "note": "INO80 implicated in the exchange at the KMT2A BCR (PMID 21048951)"
  }
]

alternative activators

{
  "in_corpus": [
    "gene-topbp1"
  ],
  "absent_from_corpus": [
    "ETAA1",
    "ATRIP"
  ],
  "why_it_matters": "If the ATR dependence reported in MLL-rearranged AML runs through ETAA1 rather than ATRIP, the RPA-side determinant is a different surface on a different protein, and an RPA-directed intervention designed against the ATRIP route would miss. Neither ETAA1 nor ATRIP has an entry here."
}

the finite pool

{
  "claim": "nuclear RPA is exhaustible; exhaustion converts stable stalled forks into nucleus-wide breakage",
  "consequence_1": "ATR inhibition and RPA reduction are the SAME lever, not a combination",
  "consequence_2": "high RPA reserve is a predicted resistance mechanism to ATR inhibitors, never looked for in this disease",
  "consequence_3": "the RPA reserve of a KMT2A-rearranged blast is measurable by single-cell chromatin-bound RPA imaging and has never been measured",
  "pmid": "24267891"
}

cell type

{
  "answer": "NOT LINEAGE-SPECIFIC. Every proliferating cell runs this axis. The disease-relevant arm, MLL-rearranged AML, exists only in mouse models and only at the ATR/ATM step."
}

cell state

{
  "state": "REPLICATION STRESS AND DAMAGE, and the axis is quantitative in that state: the amount of ssDNA determines whether the RPA pool holds."
}

measurements

[
  {
    "quantity": "count",
    "subject": "steps of this five-step axis with an entry in this corpus before cycle 160",
    "value": 3,
    "units": "steps",
    "n": 5,
    "universe": "the five steps enumerated in data.chain, each checked against the node id list of 3,086 entries on 2026-09-24",
    "null": 5,
    "note": "steps 1, 4 and 5 were held; step 2 (RPA) had nothing and is now written; step 3 (ATRIP) still has nothing. null is what a complete axis would give."
  }
]

How this could bite us back

An ATR inhibitor and any agent that lowers RPA are the same lever pulled twice, not a rational combination - the exhaustion model says so explicitly. A combination designed on 'both target replication stress' reasoning would be expected to add toxicity faster than efficacy.
RPA is pan-essential, so the RPA node itself is not a target and the therapeutic window belongs entirely to the ATR step. Any proposal that reaches RPA directly inherits a window problem no interactome or occupancy result addresses.
RPA2 phosphorylation is the field's standard replication-stress readout, so any drug that raises it appears to engage this axis whether or not it does. A trial reading out phospho-RPA2 alone cannot distinguish target engagement from generic replication stress, and needs a mechanistically independent second endpoint.
The one locus-level RPA observation in this disease points at DISEASE GENESIS rather than treatment: RPA over-loads at the KMT2A breakpoint hotspot after etoposide when ATM is deficient. Anything that lowers the RPA reserve during etoposide exposure - including an ATR inhibitor - is therefore a candidate risk factor for therapy-related KMT2A-rearranged leukaemia, which is a bite-back running opposite to the intended effect.

Still open

What is the RPA reserve of a KMT2A-rearranged blast, measured by single-cell chromatin-bound RPA imaging as in PMID 24267891? If this disease carries intrinsic replication stress, the reserve is a determinant of ATR-inhibitor sensitivity and it has never been measured in any leukaemia.
Does the ATR dependence of MLL-rearranged AML run through ATRIP or through ETAA1? Neither protein has an entry here, and the answer decides which RPA surface, if any, is the relevant one.
Is high RPA abundance a resistance mechanism to ATR inhibitors in patient material? The exhaustion model predicts it directly and this beat found no paper testing it.
Would adding an RPA readout to exp-hr-competence-under-menin-inhibition-in-kmt2a-rearranged-blasts change what that experiment can conclude? It already uses RPA2 foci as a resection readout - but chromatin-bound RPA INTENSITY (the exhaustion readout) is a different measurement from RPA2 FOCI (the resection readout), and conflating them is a live risk.

Every source behind this entry (4)

E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
RPA is required for the recruitment of ATR to sites of DNA damage and for ATR-mediated Chk1 activation in human cells; RPA stimulates the binding of ATRIP to ssDNA in vitro; the binding of ATRIP to RPA-coated ssDNA enables the ATR-ATRIP complex to associate with DNA and stimulates phosphorylation of DNA-bound Rad17; and Ddc2, the budding-yeast ATRIP homologue, is recruited to double-strand breaks in an RPA-dependent manner, with the checkpoint-deficient RPA mutant rfa1-t11 defective for recruiting Ddc2 to ssDNA both in vivo and in vitro.
Sensing DNA damage through ATRIP recognition of RPA-ssDNA complexes. · Zou L, Elledge SJ · Science 300(5625):1542-8 2003
PMID 12791985 DOI
What this evidence cannot show
Abstract level via PubMed efetch; full text not opened. HeLa plus purified proteins plus Saccharomyces cerevisiae. The allele that pins the requirement to the large subunit, rfa1-t11, is a YEAST coordinate and is not mapped onto human RPA1 numbering anywhere in this beat. No haematopoietic cell, no fusion, no disease context. This is a 2003 result and the ATR-activation field has grown substantially since; the requirement has not been overturned but it is no longer the whole story.
Evidence that argues against this (1)
- RPA-ssDNA is necessary for ATRIP-mediated recruitment but is not the only route to ATR activation, and presenting this axis as the mechanism would be incomplete. TOPBP1 is an independent activator with an ATR-activating domain and has an entry in this corpus. ETAA1 activates ATR through a DIRECT RPA contact that is mechanistically distinct from the ATRIP route - so a second, RPA-dependent but ATRIP-independent branch exists, and ETAA1 has NO entry anywhere here. Any inference from an ATR-inhibitor result back to RPA has to survive both alternatives. This repository 2026
E2established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
ATR's local fork protection and its global suppression of origin firing are mechanistically coupled through the RPA pool: stalled forks in ATR-deficient cells are initially stable but undergo nucleus-wide breakage after unscheduled origin firing generates an excess of ssDNA that exhausts nuclear RPA; partial reduction of RPA accelerated fork breakage; forced elevation of RPA delayed replication catastrophe even in the absence of ATR activity; and unscheduled origin firing induced breakage of stalled forks even in cells with active ATR.
ATR prohibits replication catastrophe by preventing global exhaustion of RPA. · Toledo LI, Altmeyer M, Rask MB, Lukas C, Larsen DH, Povlsen LK, Bekker-Jensen S, Mailand N, Bartek J, Lukas J · Cell 155(5):1088-103 2013
PMID 24267891 DOI
What this evidence cannot show
Abstract level; full text not opened. Human tumour cell lines with single-cell imaging of chromatin-bound RPA as the primary readout - no haematopoietic system, no primary cells, no fusion. The paper's closing move is explicitly to offer this as 'a molecular rationale' for cancer cells' hypersensitivity to ATR inhibitors, which is a rationale rather than a demonstrated clinical mechanism, and the distinction should be kept whenever this is cited toward a combination.
Evidence that argues against this (1)
- The model's own prediction - that RPA abundance modulates ATR-inhibitor sensitivity, and therefore that high RPA is a resistance mechanism - is not, in the sources this beat read, confirmed in any patient material. And the RPA pool of a cycling tumour line is not obviously the same quantity as that of a primary progenitor, which is the cell type this project cares about. Searched for a paper disputing the exhaustion model and found none; the phrase 'RPA exhaustion' returns 18 title/abstract records and every one this beat saw builds on the model rather than challenging it, which is corroboration by extension rather than by independent test. NCBI PubMed E-utilities 2026
E3moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
Targeting the kinase activities of ATR and ATM exhibits antitumoral activity in mouse models of MLL-rearranged AML.
Targeting the kinase activities of ATR and ATM exhibits antitumoral activity in mouse models of MLL-rearranged AML. · Morgado-Palacin I, Day A, Murga M, Lafarga V, et al. · Science Signaling 9(445):ra91 2016
PMID 27625305 DOI
What this evidence cannot show
Title-level claim only; already cited four times in this corpus (gene-atr, cell-state-replication-stress-icl-repair, exp-replication-stress-readout-kmt2a-r, mech-transcription-replication-conflict-kmt2a-r) and those entries hold its content. Cited here to mark that the druggable end of this axis is already in this disease, NOT to add a new claim. Mouse models of MLL-rearranged AML, not the index B-lineage KMT2A::AFF1 disease. The expected_absent_terms field records that RPA is NOT expected to appear in this paper: the axis is inferred from the ATR biology, not measured here.
Evidence that argues against this (1)
- Reading an ATR-inhibitor result backwards onto RPA is exactly the inference this entry must not license, and it is the same shape as reading a prey row as an interaction. ATR has at least two RPA-independent or RPA-differently-dependent activation routes (TOPBP1, ETAA1), so an antitumour effect of ATR inhibition in MLL-rearranged AML is consistent with the RPA axis and does not evidence it. Nothing in that paper measures RPA. This repository 2026
E4strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.database
The RPA step of this axis has never been measured in a KMT2A-rearranged cell: no paper names replication protein A or any RPA subunit together with MLL or KMT2A except one ChIP study of the breakpoint cluster region in cells containing no fusion, and RPA1, RPA2 and RPA3 had no entry in this corpus before cycle 160 despite being named in 7, 21 and 5 node files respectively.
PubMed esearch sweep via scripts/pubmed_safe.py and scripts/beat-inventory.py plus scripts/is-symbol-covered.py, all run 2026-09-24 by Q192; fifteen RPA-versus-KMT2A/MLL queries with every querytranslation read back, and a word-boundary corpus count over 3,086 node files. Full working in ana-q192-the-complex-form-query-the-rpa2-decline-left-unrun-finds-rpa-at-the-kmt2a-breakpoint-hotspot. · NCBI PubMed E-utilities and this repository 2026
What this evidence cannot show
Bounded by titles and abstracts (rule 80) and by the name list recorded in the analysis entry. The corpus count is a word-boundary match on node files and counts a symbol appearing anywhere in an entry, including inside an assay description - which is precisely why the count was high and the coverage was zero.
Evidence that argues against this (1)
- The 21 corpus mentions of RPA2 are not nothing, and treating this as a blank slate would be wrong: this corpus already PROPOSES to measure RPA2 foci and RPA2 phosphorylation as the resection readout in KMT2A-rearranged blasts, in exp-hr-competence-under-menin-inhibition-in-kmt2a-rearranged-blasts. So the measurement is designed and unperformed rather than unimagined - which is a much cheaper gap than 'never-measured' implies, and the experiment entry already exists. This repository 2026

Connected to

depends on →The replication protein A heterotrimer (RPA1/RPA2/RPA3) - the ssDNA clamp that every resection and replication-stress claim in this corpus silently depends on, and which had no entry until cycle 160
the licensing structure is RPA-coated ssDNA
regulates →ATR - the replication-stress kinase with the best preclinical case in this disease and the best published reason not to inhibit it
RPA-ssDNA recruits ATR via ATRIP
regulates →CHEK1 (CHK1) - the node where this graph's chidamide synergy actually lands, and the reason that synergy is an HR-impairment argument
ATR-mediated Chk1 activation requires RPA
related to →TOPBP1 - a real KMT2A contact that no abstract-level query can see, because it is a control panel in a paper about a different protein
the alternative ATR activator already in this corpus; ETAA1, the RPA-binding activator, has no entry
related to →RBBP8 / CtIP - the only unwritten core-complex partner with a DIRECT-interaction record, and the graph already held its mechanism without holding the protein
CtIP-driven resection is the upstream step that generates the ssDNA RPA coats
related to →Transcription–replication conflict as the candidate reason KMT2A-rearranged cells need the Fanconi pathway — a chain with two measured links and three inferred ones
the replication-stress context in this disease that this axis presupposes and never names