Reversibility and kinetics — does the program collapse when the fusion is removed, and how fast?
also called degron MLL-AF9 · dTAG fusion degradation · oncogene ablation reversibility · doxycycline-repressible MLL-ENL
If you take the fusion protein away, the cancer program falls apart — the first changes happen within about fifteen minutes, and even an established, genetically messy leukemia regresses. This is the single best reason to think that blocking the fusion's partner could work.
The same thing, for a scientist
Two experiments set the ceiling for what interface disruption can achieve, and both are encouraging. Kinetically, degron-tagged MLL::AF9 degradation changes transcriptional elongation at a core subset of target genes within 15 minutes, with loss of the active chromatin landscape occurring subsequently — elongation changes precede chromatin changes. Therapeutically, a doxycycline-repressible MLL-ENL model shows that leukemic cells which had acquired additional genetic abnormalities and shortened disease latency nonetheless remained dependent on fusion expression in vitro and in vivo, and ablation caused regression of established leukemia. The conclusion the authors drew is the one that matters: even genetically complex leukemias can be reversed on inactivation of the initiating fusion.
The kinetics: elongation first, chromatin second1 deeper
Using FKBP12(F36V)-degron-tagged MLL::AF9 at endogenous levels, transcriptional elongation changes at a core subset of fusion target genes within 15 minutes of degrader addition, and the loss of the active chromatin landscape is subsequent — the paper's framing is decreases in transcriptional elongation prior to changes in chromatin landscape at select loci 1.
This is the cleanest causal ordering available in the field and it reorders the whole mechanism. The fusion's proximate, rate-limiting function is elongation control; the chromatin marks are consequences that then stabilise the state. That is consistent with the DOT1L picture (a mark whose removal is slow and whose loss changes little globally, mechanism-dot1l-h3k79-methylation) and it explains why menin inhibition, which evicts the complex, acts faster and better than a writer inhibitor.
A precision caution the graph should carry. The widely repeated claim that 'H3K79me2 decays more slowly than transcription' is not verbatim supported. The paper says 'active chromatin landscape', not H3K79me2 specifically, and no paper reports an H3K79me2 decay half-life after fusion removal. The correct phrasing is: chromatin-landscape loss lags the transcriptional-elongation change. The half-life remains an open, easily measurable number.
The same work also showed that combined DOT1L plus menin inhibition resembles MLL::AF9 degradation and releases the oncoprotein from chromatin globally, whereas either single agent has more modest effects 1 — the strongest available preclinical rationale for that specific combination.
The complementary experiment on the partner side: acute degradation of ENL suppresses both initiation and elongation of Pol II at active genes genome-wide, with pronounced effects at genes carrying a disproportionate ENL load, and an intact YEATS reader domain is essential for ENL-dependent leukemic growth 2.
Method provenance
Both degron results depend on the dTAG platform, which should be cited properly when the lab plans its own degron experiments: the original FKBP12(F36V)/CRBN system 3, and the VHL-recruiting dTAG-1 molecules that overcome the CRBN system's failure to degrade certain recalcitrant oncoproteins 4. The second matters practically — if a KMT2A::AFF1 degron proves recalcitrant to CRBN recruitment, the VHL route is the documented fallback.
The therapeutic ceiling: established, genetically complex leukemia still regresses
The kinetic result says the program is fusion-dependent moment to moment. The harder question is whether an established leukemia that has accumulated cooperating mutations still needs the fusion — because that is the clinical situation.
In a doxycycline-repressible MLL-ENL model, leukemic cells from primary recipients had acquired additional genetic abnormalities and produced shorter-latency disease in secondary recipients, yet remained dependent on MLL-ENL expression in vitro and in vivo, and its ablation resulted in regression of established leukemias. The authors' conclusion: even genetically complex leukemias can be reversed on inactivation of the initiating MLL fusion 5.
This is the single most important experiment for this project's central hypothesis. It says the fusion is not merely an initiating event that hands off to downstream lesions — it remains the load-bearing dependency even after the genome has drifted. An interface-disrupting drug is therefore aiming at a target that stays valid.
Three honest limits on how far this can be pushed:
- It is MLL-ENL in a transplant model, not KMT2A::AFF1 in an infant. The verification pass for this cycle found no dTAG/degron model of KMT2A::AFF1 in the indexed literature and no Cre-excision (as opposed to tet-off) conditional KMT2A-fusion reversibility model. All verified degron work is MLL::AF9 or ENL. This is a specific, fillable gap and it is the right experiment for this lab.
- Complete removal is not partial occupancy. A degron or dox-off switch abolishes the protein; a drug reduces the function of a protein that is still present. The Bernt-lab finding that heavily pretreated KMT2A::AFF1 samples can be menin-inhibitor-resistant with sustained on-target transcriptional effect 6 shows that target engagement and clinical response can decouple — which a degron experiment can never reveal.
- Regression is not cure in a human. Mouse regression with a genetically clean switch is the optimistic bound, not the expected outcome.
| chain | mechanism-kmt2a-fusion-causal-chain |
|---|---|
| elongation change onset minutes | 15 |
| causal ordering | transcriptional elongation change precedes loss of active chromatin landscape |
| h3k79me2 decay halflife | not reported anywhere in the indexed literature — open, measurable number |
| established disease reversible | true |
| reversible despite cooperating lesions | true |
| model used for reversibility | doxycycline-repressible MLL-ENL, mouse transplant |
| cre excision model missing | true |
| combination rationale | DOT1L + menin inhibition phenocopies fusion degradation and globally evicts the oncoprotein |
| ceiling caveat | complete protein removal is not partial pharmacological occupancy; resistance can occur downstream of sustained target engagement |
degron models available
[ "MLL::AF9 (FKBP12-F36V)", "ENL" ]
degron models missing
[ "KMT2A::AFF1" ]
- Degron and tet-off experiments abolish the protein entirely; a drug only reduces one of its functions, so the optimistic reversibility result is an upper bound rather than a prediction.
- Resistance can arise downstream of sustained target engagement, meaning a menin inhibitor can keep working at the molecular level while failing clinically — and pharmacodynamic biomarkers would look reassuring throughout.
- The reversibility result is from a genetically simple mouse model; human relapsed infant disease carries RAS-pathway and TP53 lesions whose contribution cannot be switched off.
- Because elongation change precedes chromatin change, a partially effective drug may slow transcription without dismantling the chromatin state, leaving a primed locus that can restart when drug pressure is removed.
- Build the missing tool: a dTAG-tagged KMT2A::AFF1 at endogenous levels in a t(4;11) line and in an infant PDX. It does not exist, it is squarely within this lab's reach, and every kinetic claim in the field currently rests on MLL::AF9.
- What is the actual decay half-life of H3K79me2, H3K4me3 and menin occupancy after fusion removal? Nobody has measured it. It sets the minimum achievable speed for any writer- or reader-directed drug.
- Why do heavily pretreated KMT2A::AFF1 samples resist menin inhibitors while target engagement is preserved? This decoupling is invisible in degron models and is the most clinically threatening unexplained observation in this beat.
- Does partial (50-80%) fusion degradation produce partial program collapse, or is there a threshold? A degron titration would directly calibrate what fractional occupancy a drug needs to achieve.
- E1established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
FKBP12(F36V)-degron-tagged MLL::AF9 degradation changes transcriptional elongation at a core subset of target genes within 15 minutes, with loss of the active chromatin landscape occurring subsequently; combined DOT1L and menin inhibition resembles MLL::AF9 degradation and releases the oncoprotein from chromatin globally, whereas single-agent treatment has more modest effects on occupancy and gene expression.
MLL::AF9 degradation induces rapid changes in transcriptional elongation and subsequent loss of an active chromatin landscape · Olsen SN, Godfrey L, Healy JP, et al. (Armstrong SA, senior author) · Molecular Cell 82(6):1140-1155.e11 2022What this evidence cannot show
MLL::AF9 in cell lines; the paper says 'active chromatin landscape', NOT H3K79me2 specifically, so no H3K79me2 decay half-life may be attributed to it. 'Resembles degradation' is a similarity of transcriptional and occupancy profiles, not an equivalence of therapeutic effect. No KMT2A::AFF1 degron equivalent exists.
Evidence that argues against this (1)
- Rapid reversibility in a degron system does not predict reversibility in a pretreated patient's disease. Heavily pretreated KMT2A::AFF1 ALL samples were markedly LESS menin-inhibitor-sensitive than earlier samples from the same patients despite no prior menin-inhibitor exposure, and transcriptomics confirmed sustained ON-TARGET efficacy in the resistant cells - the drug did what it does and the cells did not die. If target engagement and program collapse can be uncoupled in primary human disease, the degron kinetics establish what the fusion does, not what removing it will achieve therapeutically. Mahdavi L et al. 2025
- E2established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
A genome-scale CRISPR screen in an MLL-AF4-positive line identified ENL as specifically required; chemical-genetic targeted degradation showed acute ENL loss suppresses both initiation and elongation of RNA polymerase II at active genes genome-wide with pronounced effects at genes carrying a disproportionate ENL load, and an intact YEATS reader domain is essential for ENL-dependent leukemic growth.
Transcription control by the ENL YEATS domain in acute leukaemia · Erb MA, Scott TG, Li BE, et al. (Bradner JE, senior author) · Nature 543(7644):270-274 2017What this evidence cannot show
Degrades ENL, a SEC component and fusion partner, not the KMT2A fusion protein itself; in an MLL-AF4-positive line ENL is endogenous rather than part of the fusion, so this tests the partner's normal function in the fusion's service.
Evidence that argues against this (1)
- ENL being specifically required in an MLL-AF4 line, with targeted degradation confirming it, is strong — and this graph separately records that ENL PERSISTS at oncoprotein-target loci after the oncoprotein and menin are undetectable. A component that is required and that outlasts the fusion complicates any simple reversibility kinetics. Nature 2017
- E3established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
The dTAG system uses an FKBP12(F36V) tag and heterobifunctional degrader molecules to achieve immediate and target-specific degradation of tagged proteins at endogenous expression levels, enabling kinetic dissection of oncoprotein function.
The dTAG system for immediate and target-specific protein degradation · Nabet B, Roberts JM, Buckley DL, et al. (Gray NS, Bradner JE, senior authors) · Nature Chemical Biology 14(5):431-441 2018What this evidence cannot show
A methods paper cited here for platform provenance only, not for leukemia biology. Tagging can perturb protein function or stability independently of degradation, which every degron experiment must control for.
Evidence that argues against this (1)
- This is a METHOD paper. It establishes that the dTAG system degrades a tagged protein rapidly and specifically, which is a statement about the tool and not about the fusion. An entry on fusion dependency kinetics should cite it for feasibility only, and its presence as an evidence item risks reading as biology. Nature Chemical Biology 2018
- E4established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.paper
VHL-recruiting dTAG molecules (dTAG-1) provide rapid and direct control of target protein levels and overcome a limitation of CRBN-recruiting dTAG molecules, which fail to degrade certain recalcitrant oncoproteins.
Rapid and direct control of target protein levels with VHL-recruiting dTAG molecules · Nabet B, Ferguson FM, Seong BKA, et al. (Stegmaier K, Gray NS, senior authors) · Nature Communications 11(1):4687 2020What this evidence cannot show
Methods paper cited for platform provenance. Which E3 ligase works for a given target is empirical, so the fallback is a practical note rather than a prediction.
Evidence that argues against this (1)
- Also a method paper, and its substantive content is a LIMITATION of the earlier system: CRBN-recruiting degraders fail on certain targets, which is why the VHL variant exists. An experiment designed from this must state which E3 arm it will use and why, because the choice determines whether the target degrades at all. Nature Communications 2020
- E5strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In a doxycycline-repressible MLL-ENL model, leukemic cells from primary recipients had acquired additional genetic abnormalities and gave shorter-latency disease in secondary recipients, yet remained dependent on MLL-ENL expression in vitro and in vivo, and its ablation resulted in regression of established leukemias — showing that even genetically complex leukemias can be reversed on inactivation of the initiating KMT2A fusion.
Acute myeloid leukemia induced by MLL-ENL is cured by oncogene ablation despite acquisition of complex genetic abnormalities · Horton SJ, Walf-Vorderwulbecke V, Chatters SJ, et al. (Williams O, senior author) · Blood 113(20):4922-4929 2009What this evidence cannot show
KMT2A::MLLT1 in a mouse transplant model; tet-off complete transcriptional shutdown rather than Cre excision or partial pharmacological inhibition. 'Cured' refers to the mouse transplant setting. The cooperating abnormalities are karyotypic changes acquired in mice and are not the co-mutation spectrum of human infant KMT2A::AFF1 disease.
Evidence that argues against this (1)
- The entry cites the acquired-abnormalities caveat and not the headline, which runs the other way: MLL-ENL-induced AML is CURED by oncogene ablation DESPITE the acquisition of complex genetic abnormalities. That is the strongest single statement in this beat for continuous requirement, and quoting only its caveat inverts the paper's conclusion. Blood 2009
- E6moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
Heavily pretreated KMT2A::AFF1 ALL samples were markedly less menin-inhibitor-sensitive than earlier samples from the same patients despite no prior menin-inhibitor exposure, with transcriptomics showing sustained on-target efficacy in the resistant cells — i.e. resistance downstream of target engagement; multiple emergent comutations including RAS-pathway and TP53 were present but neither was sufficient to induce resistance in vitro.
Upfront menin-inhibitor resistance in multiply pretreated leukemias · Mahdavi L, Alikarami F, Goodrow H, et al. (Bernt KM, senior author) · Experimental Hematology 153:105268 2025What this evidence cannot show
Two serial patients plus five PDX models — a small but mechanistically pointed dataset. 'Sustained on-target efficacy' is inferred from transcriptional signatures rather than direct chromatin-occupancy measurement in the resistant cells.
Evidence that argues against this (1)
- Resistance arising in multiply pretreated samples WITHOUT menin-inhibitor exposure, with on-target repression preserved, means reversibility kinetics measured in treatment-naive models may not describe the patients who receive the drug. The kinetics question and the resistance question are not separable in that population. Experimental Hematology 2026
prospective revocation predicts hysteresis between occupancy and output
supplies the missing recovery arm to the measured collapse kinetics
a stable truncal driver is the precondition the reversibility work assumes
degron and tet-off reversibility data are the main support
does the block collapse when the fusion is removed
is the fusion needed continuously, or only to establish the state
modifier, not a step - kinetics of the whole chain on fusion removal
turns that entry's unreported-half-life data field into a dated, controlled absence
fills the degron_models_missing entry for KMT2A::AFF1