Oncofusion

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Mechanismmoderate confidence 3 of 5: One strong study, or several smaller consistent ones.Q49-metabolism

Guanine nucleotide supply sustains the LEDGF/menin/MLL-fusion complex - the one place in this graph where metabolism sits UPSTREAM of the menin interface

also called guanine nucleotide - MLL complex axis · IMPDH2 - rRNA - LEDGF/menin axis

The leukaemia's central machine needs two scaffold proteins, menin and LEDGF, to hold it on the DNA. Starving the cell of the raw material for one of the four RNA letters makes it build fewer ribosomes, which makes it make less menin and less LEDGF, which makes the machine fall apart - so a metabolic drug and a menin drug attack the same thing from two ends.

The same thing, for a scientist

The reported chain is: **de novo guanine nucleotide biosynthesis (IMPDH2) -> GTP pools -> rRNA transcription -> LEDGF and menin PROTEIN abundance -> formation and chromatin binding of the MLL-fusion complex -> MLL target gene expression** [E1]. Every existing entry in this graph that touches menin treats the interface as a protein-protein binding problem addressable only by a small molecule that occupies the menin pocket. This mechanism says the same output can be reduced by lowering the *amount* of the scaffold proteins, through a metabolic route that no MEN1 resistance mutation can defend against, and the originating study reports that combining guanine-nucleotide or rRNA-transcription blockade with a menin inhibitor further suppresses MLLr AML [E1]. It is the strongest available answer to the question this beat was dispatched with - is the metabolic phenotype a consequence of the fusion programme, or an independent dependency - because here **the causation runs the other way**: the fusion complex is downstream of metabolic supply. The chain is reported by one laboratory in myeloid models and has two serious unresolved objections, both recorded below.

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Why this matters more than the size of its literature suggests

Menin inhibitors are this disease's first genuinely targeted class, and their principal clinical problem is rapid resistance, much of it through MEN1 point mutations at the drug-binding pocket. A mechanism that lowers menin protein rather than blocking the menin pocket is not defeated by a pocket mutation. That is a structurally different escape profile, and it is the reason this entry exists as a mechanism rather than as a paragraph in gene-impdh2.

The same argument applies to LEDGF (PSIP1), which this graph carries as the chromatin-anchoring partner of the complex. Reducing LEDGF protein reduces complex chromatin binding without needing a LEDGF ligand, and no clinically usable LEDGF ligand exists.

The two objections that have to be stated with the mechanism

1. Specificity. The chain runs through rRNA transcription, which is the most general growth process a cell has. A reduction in LEDGF and menin protein under global translational restraint is not, on its own, evidence of a specific relationship. The published abstract does not state which controls address this 1. Until a manoeuvre that depletes GTP without collapsing ribosome biogenesis reproduces the LEDGF/menin effect, 'specific' is an assumption.

2. Genotype. Two independent 2026 studies of IMPDH inhibition in leukaemia report that sensitivity is independent of KMT2A status, and that the therapeutically meaningful effect runs through SAMHD1 rather than through the MLL complex 23. Those studies measured a different endpoint (viability, nucleoside-analogue synergy) from this one (complex assembly and menin-inhibitor sensitisation), so they do not refute the mechanism - but they do mean this mechanism has no supporting selectivity data and one published null on the closest available proxy.

A third, smaller point: the AML leukaemia-stem-cell literature independently places guanine nucleotides upstream of ribosome biogenesis, TP53 stability and MYC, with no MLL complex in the picture at all 4. Two mechanisms are therefore on offer for the same drug class in the same year, and this entry does not choose between them.

What would settle it

Three experiments, none exotic:

  1. Deplete GTP without inhibiting rRNA transcription (for example by a short guanine-free pulse or an IMPDH degron rather than an inhibitor) and blot LEDGF and menin. If they fall, the relationship survives the specificity objection.
  2. Run the IMPDH-plus-menin-inhibitor combination in a MEN1-mutant, menin-inhibitor-resistant line. The mechanism predicts retained activity; a pocket-mutation-based resistance model predicts it.
  3. Do both in infant KMT2A::AFF1 B-lineage cells. Every result behind this mechanism is myeloid, and this graph's index disease is not.
Facts and figures
direction of causationmetabolism UPSTREAM of the fusion complex - the inverse of the usual framing in this graph
predicted and observedsynergy with menin inhibitors (observed, PMID 40102405)
predicted and untestedretained activity in MEN1-mutant menin-inhibitor-resistant cells; activity in infant KMT2A::AFF1 B-lineage cells
evidence lineagemyeloid only
replication statussingle laboratory for the LEDGF/menin step; no independent replication found 2026-09-20
chain
[
  "IMPDH2 activity",
  "GTP/guanine nucleotide pools",
  "rRNA transcription",
  "LEDGF (PSIP1) and menin (MEN1) protein abundance",
  "MLL-fusion complex formation and chromatin binding",
  "MLL target gene expression"
]
competing mechanisms for the same drug class
[
  "SAMHD1 allosteric deactivation and nucleoside-analogue potentiation",
  "impaired-ribosome-biogenesis checkpoint, TP53 stabilisation and MYC repression",
  "TLR-TRAF6-NF-kB overactivation and VCAM1 upregulation"
]
How this could bite us back
  • The mechanism's own currency is ribosome biogenesis, so its therapeutic window is bounded by the most proliferative normal tissues in a child: marrow, gut and growing bone.
  • A metabolic route to lowering menin protein would be expected to lower menin everywhere, including in the parathyroid, pituitary and pancreatic islets where MEN1 is a tumour suppressor. Persistent menin reduction is not obviously benign.
  • The closest thing to a clinical test of this mechanism - IMPDH inhibitors in leukaemia - has already produced a published null on KMT2A selectivity. A programme built on the mechanism must be designed as a combination from the start, because the single-agent question has been asked and answered.
  • If the effect is genuinely mediated by reduced translation, then any resistance mechanism that restores translational capacity - and leukaemias have several - restores the fusion complex.
Still open
  1. Does GTP depletion reduce LEDGF and menin protein when rRNA transcription is held constant? This one experiment separates a specific mechanism from a translational-capacity artefact and nothing else does.
  2. Does IMPDH inhibition retain activity against MEN1-mutant, menin-inhibitor-resistant leukaemia? If yes this is the most important thing in the metabolic layer; if no, it is a nice mechanism with no clinical niche.
  3. Is the effect present in infant KMT2A::AFF1 B-lineage cells, where the only published IMPDH data are a negative in ALL cell lines?
  4. Which of the four competing mechanisms proposed for IMPDH inhibition in leukaemia is operative in a given cell - and is that determined by SAMHD1 status, TP53 status, or fusion status?
Every source behind this entry (4)
  1. E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper

    Leukaemia stem cells of MLL-rearranged AML exhibit enhanced guanine nucleotide biosynthesis, whose inhibition leads to myeloid differentiation and sensitisation to menin inhibitors; targeting IMPDH2 reduces guanine nucleotides and rRNA transcription, leading to reduced protein expression of LEDGF and menin, so that formation and chromatin binding of the MLL-fusion complex is impaired and MLL target gene expression falls; inhibition of guanine nucleotide biosynthesis or of rRNA transcription further suppresses MLLr AML when combined with a menin inhibitor.

    PMID 40102405DOI
    What this evidence cannot show

    Abstract read this session; full text not retrieved. Single laboratory, myeloid models throughout (MLLr AML, leukaemia stem cells); no lymphoid or infant KMT2A::AFF1 data. The chain passes through rRNA transcription, a general growth axis, and the abstract does not state which controls separate a specific effect on LEDGF and menin from a global reduction in protein synthesis. The menin-inhibitor synergy is reported but the abstract gives no combination index, no resistant-line experiment and no in vivo combination endpoint. Bounded absence: no independent replication of the LEDGF/menin protein result was recoverable from Europe PMC on 2026-09-20 (query IMPDH2 AND (MLL OR KMT2A) AND leukemia, 32 records, 3 passing the title check, none an independent replication).

    Evidence that argues against this (1)
    • A 2026 study that set out specifically to test the KMT2A link found sensitivity to IMPDH inhibitors to be independent of KMT2A status in AML cells and absent in ALL cell lines, and attributed the therapeutically relevant effect to depletion of the allosteric SAMHD1 activators GTP and dGTP rather than to the MLL complex. Klootsema Y et al. 2026
  2. E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper

    Sensitivity to IMPDH inhibitors was independent of KMT2A status in AML cell lines and primary AML samples; IMPDH inhibitors synergised with cytarabine and fludarabine in a SAMHD1-dependent manner in a subset of AML cells but not in acute lymphoblastic leukaemia cell lines.

    Targeting IMPDH to inhibit SAMHD1 in KMT2A-rearranged leukaemia. · Klootsema Y, Tsesmetzis N, Sharma S, ..., Herold N · Cell Cycle 2026
    PMID 41399259DOI
    What this evidence cannot show

    Abstract read this session. The endpoint is viability and nucleoside-analogue synergy, not complex assembly or menin-inhibitor sensitisation, so this is a null on a proxy rather than a direct refutation of E1. Panel sizes are not given in the abstract, so this is an absence of a detected difference rather than a demonstrated equivalence. Cell lines and primary samples; no in vivo arm.

    Evidence that argues against this (1)
    • The mechanism in E1 predicts benefit in combination with a menin inhibitor, not single-agent genotype-selective cytotoxicity; a single-agent viability panel is not the assay that would detect it. Shi X et al. 2025
  3. E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper

    Approved IMPDH inhibitors mycophenolic acid and ribavirin imbalance dNTP pools and increase cytarabine efficacy in SAMHD1-proficient but not SAMHD1-deficient leukaemic cells, establishing an alternative, SAMHD1-centred account of why IMPDH inhibition helps in leukaemia.

    PMID 41711315DOI
    What this evidence cannot show

    Abstract read this session. AML cell lines; no KMT2A claim at all, so this is cited as an alternative mechanism for the drug class, not as evidence in the index disease. Ribavirin is not a clean IMPDH probe.

    Evidence that argues against this (1)
    • A SAMHD1-centred mechanism and a LEDGF/menin-centred mechanism are not mutually exclusive: one concerns nucleoside-analogue potentiation and the other concerns the transcriptional complex. Both can be true of the same drug in the same cell. Shi X et al. 2025
  4. E4strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper

    In AML leukaemia stem cells, upregulated de novo guanine nucleotide biosynthesis sustains ribosome biogenesis; abundant guanine nucleotides suppress the impaired ribosome biogenesis checkpoint, destabilising TP53 and maintaining MYC; IMPDH1/2 inhibition activates the checkpoint, stabilises TP53, represses MYC, impairs a shift to glycolysis and suppresses LSC reconstitution in xenotransplantation regardless of TP53 mutational status.

    PMID 41191529DOI
    What this evidence cannot show

    Abstract read this session. AML-wide LSC study naming no fusion and no KMT2A subgroup; cited here because it independently places guanine nucleotides upstream of ribosome biogenesis, which is the shared step of E1's chain, while reaching the leukaemic phenotype without any MLL complex. It therefore both supports the ribosome step and offers a competing account of the downstream consequence.

    Evidence that argues against this (1)
    • If the ribosome-biogenesis-checkpoint/TP53/MYC account is sufficient to explain the antileukaemic effect of IMPDH inhibition, the LEDGF/menin arm may be a correlate of reduced translation rather than the operative mechanism - which is exactly the specificity objection this entry records. Shi X et al. 2025
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