If inhibiting one methyltransferase failed, constraining the methyl donor may not
also called SAM supply hypothesis · MAT2A upstream of DOT1L
Blocking a single methyl-writing enzyme did not help patients. This asks whether limiting the shared chemical those enzymes all consume would do better, and what it would cost.
The same thing, for a scientist
Pinometostat removed H3K79 methylation and produced 2 complete remissions in 51 adults without reaching its maximum tolerated dose. If the failure is redundancy — non-canonical H3K79me2-dependent pathways, or other methyl marks carrying the programme — then constraining SAM supply through MAT2A attacks the shared substrate rather than one active site, and MLLr-specific responses to PF-9366 with enhanced DOT1L and PRMT5 combinations are the existing support. If the failure is instead inadequate exposure, the hypothesis is answering the wrong question.
The three readings of pinometostat, and which one this needs
Pathway not limiting. Then SAM restriction fails for the same reason, only more broadly.
Redundancy. Non-canonical H3K79me2-dependent survival pathways are recorded in this graph, and if other methyl marks carry the programme then removing one writer is insufficient while lowering the shared donor is not. This is the only reading under which the hypothesis is interesting.
Inadequate exposure. The trial never reached its MTD. If the drug was simply underdosed, the correct next experiment is a better DOT1L schedule, not a different target — and this hypothesis would be a detour built on a misread result.
analyses
[ "ana-age-restricted-vulnerability-panel" ]
experiments
[ "exp-methionine-rescue-separates-sam-limitation-from-cytotoxicity" ]
falsifiers
[ "Methionine or SAM supplementation fails to rescue the PF-9366 phenotype in KMT2A-rearranged cells, which would show the effect is not through SAM limitation at all.", "MAT2A inhibition shows the same potency in lineage-matched non-KMT2A leukaemia lines, which would make the MLLr-specificity an artefact of comparing against healthy controls.", "A DOT1L inhibitor given at a genuinely maximum-tolerated exposure produces responses, which would relocate the pinometostat failure to pharmacology and remove this hypothesis's motivation." ]
- Lowering SAM affects DNA methylation maintenance and non-histone methylation as well as the intended marks, in every tissue.
- The named combination partners are themselves broadly required, so a synergy result may report a shared cytotoxic floor rather than a pathway interaction.
- E1weak confidence 2 of 5: Preliminary, small, indirect, or not yet peer reviewed.paper
MAT2A inhibition by PF-9366 gives an MLLr-specific enhanced response in a CRISPR/Cas9-MLLr model, and combination with DOT1L or PRMT5 inhibitors is more pronounced than either alone — the pattern predicted if SAM supply is upstream of several methyltransferases that individually carry redundant load.
What this evidence cannot show
Abstract-level, one model system, one laboratory. 'More pronounced' is not reported as formal synergy in the abstract.
Evidence that argues against this (2)
- Additivity between two agents that both slow proliferation is the default expectation and does not require a shared-substrate mechanism. Demonstrating SAM limitation specifically needs a methionine or SAM rescue arm, which is why this hypothesis carries that as its first falsifier. Oncofusion knowledge graph (internal analysis) 2026
- The pinometostat trial never reached its maximum tolerated dose, so the premise that single-enzyme inhibition failed on the biology is not established. If the failure was exposure, this hypothesis is built on a misreading. Blood 2018