The non-chromatin half: CALM-AF10 breaks the cell's iron intake and the cells are chelator-sensitive in vitro - but iron restriction did not extend survival in vivo (corrected 2026-09-21)
also called CALM-AF10 transferrin receptor defect · iron in CALM-AF10
Because the fusion takes over the protein that normally helps pull iron into the cell, CALM-AF10 cells cannot take up iron properly and are unusually easy to starve of it in culture. When that was tried in mice it shrank the spleen tumour but the mice did not live longer.
The same thing, for a scientist
PICALM's day job is clathrin-mediated endocytosis, and the fusion does not do it. **CALM-AF10 fails to rescue transferrin internalisation in Picalm-null MEFs where wild-type CALM does, and expressing CALM-AF10 in Picalm-wild-type MEFs raises serum transferrin - a dominant-negative effect on endogenous CALM** [E1]. PICALM-deficient cells show high surface transferrin receptor, low intracellular iron, reduced proliferation and **hypersensitivity to iron chelation**, all rescued by retroviral PICALM [E2]. In CALM-AF10 leukaemia cells specifically, inefficient transferrin-receptor endocytosis produces functional iron deficiency and **impaired proliferation on iron chelation** [E1]. PICALM also perturbs cellular cholesterol homeostasis, so the trafficking consequences are not confined to iron [E3]. **This is the only vulnerability in this beat that is orthogonal to chromatin entirely**, and the only one where an approved drug class - iron chelators - already exists. **CORRECTION, 2026-09-21 (Q67). Two published results were missing from this entry and both cut against it.** (i) The in vivo test exists and this entry said it did not: dietary iron restriction reduced tumour burden in the spleen of mice transplanted with CALM-heterozygous CALM-AF10 leukaemia, but **iron restriction, used alone or in conjunction with chemotherapy, did NOT increase survival** [see E2 counter-evidence]. Iron chelation was toxic to cultured CALM-het CALM-AF10 leukaemia cells and additive with chemotherapy, so the in vitro half reproduces and the endpoint that matters does not. (ii) The mouse model built specifically to test whether disrupted endocytosis contributes to CALM-AF10 leukaemogenesis reported that **the development of leukaemia could not be attributed directly to interference with endocytosis**, that full-length CALM-AF10 in leukaemia cells had no consistent effect on growth-factor endocytosis, and that the authors' preferred account of what endocytic 5' partners supply is HOMO-OLIGOMERISATION rather than a trafficking defect [see E3 counter-evidence]. **What survives is the cell-biological chain and the in vitro chelator sensitivity. What does not survive is the framing of this as a vulnerability with an approved drug class waiting, and the statement that no in vivo iron study exists.**
How far this can be pushed - and the in vivo answer, which this entry originally missed
The chain is: PICALM is required for transferrin-receptor endocytosis; the fusion cannot perform that function and acts dominant-negatively on the wild-type allele; the cells become iron-poor; iron-poor cells are chelator-sensitive. Each link is supported, and the end of the chain has been tested directly in CALM-AF10 cells in vitro 1.
The original version of this section said the clinically relevant step was missing. It is not missing - it was done and it was negative. In mice transplanted with CALM-heterozygous CALM-AF10 leukaemia, dietary iron restriction reduced spleen tumour burden but did not increase survival, alone or with chemotherapy. That is the single most important fact in this entry and it was absent from the first version. A retrospective look at chelation exposure in transfused PICALM::MLLT10 patients is still nearly free and is still worth doing, but it is now a follow-up to a negative mouse survival study rather than a first pass at an untested idea.
And the causal premise is weaker than the chain suggests. A separate mouse model built to test the endocytosis hypothesis directly found that leukaemia development could not be attributed to interference with endocytosis, and that full-length CALM-AF10 in leukaemia cells had no consistent effect on growth-factor endocytosis - while the region of CALM responsible for the phenotype difference between myeloproliferative disease and AML is the same clathrin-binding region. So the fusion's endocytic defect is real in some assays, does not reproduce in leukaemia cells for growth-factor cargo, and has not been shown to be what makes the leukaemia.
The counter-consideration already noted remains: leukaemic blasts are generally iron-avid, chelation has been proposed as an anti-leukaemic strategy broadly rather than specifically, and a fusion-specific window has not been demonstrated against a matched comparator.
| mechanism class | membrane trafficking / nutrient uptake |
|---|---|
| pathway | clathrin-mediated endocytosis of the transferrin receptor |
| effect of fusion | fails to support TfR internalisation and acts dominant-negatively on endogenous CALM |
| cellular consequence | high surface TfR, low intracellular iron, reduced proliferation, hypersensitivity to iron chelation |
| orthogonality | entirely independent of DOT1L, menin and the HOXA programme |
| existing drug class | iron chelators (deferasirox, deferoxamine), already used in transfused leukaemia patients |
| untested step | no in vivo or clinical iron-chelation study in CALM-AF10 leukaemia was found on 2026-09-20 |
| on target toxicity boundary | PICALM's essential requirement in vivo is erythroid; anything aimed at this axis meets erythropoiesis first |
- Iron chelation in a child with leukaemia competes directly with the transfusion support they need; the therapeutic window is narrow before any tumour biology is considered.
- PICALM's in vivo requirement is erythroid, so both the fusion's defect and any intervention converge on the red-cell compartment in an already anaemic patient.
- Iron restriction can select for compensating uptake routes - ferritin, lipocalin, non-transferrin-bound iron - none of which depend on CALM.
- The in vivo experiment has already been run in a mouse model of this fusion and did not extend survival, so a clinical programme built on this rationale would be starting behind a published negative rather than in front of an untested idea.
- Why did spleen tumour burden fall under iron restriction without a survival benefit? Compartment-specific response, insufficient depth of restriction and selection for CALM-independent uptake routes are all untested explanations, and which one it is decides whether a chelator is worth revisiting.
- Is iron chelation sensitivity greater in CALM-AF10 cells than in matched controls? The matched comparison has still not been run and is what separates a fusion-specific vulnerability from general leukaemic iron avidity.
- If leukaemogenesis cannot be attributed to interference with endocytosis, what is the clathrin-binding region of CALM doing in the fusion? Homo-oligomerisation is the proposed answer and it has not been tested against an oligomerisation-dead mutant that retains clathrin binding.
- Does the dominant-negative effect on the wild-type allele mean PICALM function is reduced below the heterozygous level? That would change what a PICALM-targeting strategy would even mean here.
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In Picalm-null murine embryonic fibroblasts, transferrin internalisation is rescued by wild-type CALM but not by CALM-AF10; expressing CALM-AF10 in Picalm-wild-type MEFs increases serum transferrin, indicating a dominant-negative effect on endogenous CALM; and CALM-AF10 leukaemia cells show inefficient transferrin-receptor endocytosis with functional iron deficiency and impaired proliferation on iron chelation.
Effects of iron depletion on CALM-AF10 leukemias. · Heath JL, et al. · Exp Hematol 2014What this evidence cannot show
Murine fibroblasts and CALM-AF10 leukaemia cells in vitro. No in vivo or clinical iron-chelation result in this fusion exists in this beat's reading.
Evidence that argues against this (1)
- Iron dependence is a general property of proliferating leukaemic cells and chelators have broad antiproliferative activity, so a chelation sensitivity in CALM-AF10 cells is only fusion-specific if demonstrated against a matched non-CALM-AF10 comparator - which these experiments do not include. Scotland PB et al. 2012
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
PICALM-deficient murine embryonic fibroblasts show increased surface transferrin receptor, decreased intracellular iron, reduced proliferation and particular sensitivity to iron chelation, all rescued by retroviral PICALM; the proliferation defect is corrected by iron supplementation.
The PICALM protein plays a key role in iron homeostasis and cell proliferation. · Scotland PB, et al. · PLoS One 2012What this evidence cannot show
Murine fibroblasts, PICALM loss rather than fusion expression. It establishes the wild-type requirement that the fusion fails to satisfy.
Evidence that argues against this (1)
- AUTHORS' OWN IN VIVO NEGATIVE, in the model this axis was built on, and it is the strongest objection to the entry. Iron chelation is toxic to cultured CALM-heterozygous CALM-AF10 leukaemia cells and additive with chemotherapy, and dietary iron restriction reduced tumour burden in the SPLEEN of mice transplanted with CALM-het CALM-AF10 leukaemia - but dietary iron restriction, alone or in conjunction with chemotherapy, DID NOT INCREASE SURVIVAL. The in vitro sensitivity and the spleen burden reproduce; the endpoint that matters does not. Any statement that 'the cells become chelator-sensitive' has to be paired with the fact that the sensitivity has been tested in vivo and did not translate. Heath JL et al. 2014
- E3moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
Modulation of PICALM levels perturbs cellular cholesterol homeostasis.
Modulation of PICALM Levels Perturbs Cellular Cholesterol Homeostasis. · Mercer JL, et al. · PLoS One 2015What this evidence cannot show
Cell-culture study of PICALM level modulation, not of the fusion. Cited to show that the trafficking consequences of PICALM perturbation are not confined to iron, which bounds how neatly the 'iron axis' can be isolated.
Evidence that argues against this (1)
- PUBLISHED CONFLICTING RESULT against the whole endocytic framing, from the mouse model built to test it. The hypothesis that disruption of endocytosis contributes to CALM-AF10 leukaemogenesis was examined directly, and the conclusion was negative: the development of leukaemia COULD NOT BE ATTRIBUTED DIRECTLY to interference with endocytosis or to consequential changes in proliferation and signalling, and full-length CALM-AF10 in leukaemia cells had NO CONSISTENT EFFECT ON GROWTH-FACTOR ENDOCYTOSIS. The authors propose instead that endocytic 5' partners contribute homo-oligomerisation. Widening the trafficking consequences from iron to cholesterol therefore widens a mechanism whose causal link to the leukaemia has been tested and not found. Stoddart A et al. 2012
- E4strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
PICALM is required for transferrin-receptor endocytosis in erythroid cells and for erythroid maturation in mice, and is dispensable for myeloid and B-lymphoid development.
Role of the clathrin adaptor PICALM in normal hematopoiesis and polycythemia vera pathophysiology. · Ishikawa Y, et al. · Haematologica 2015What this evidence cannot show
Mouse genetics. Included here because it sets the on-target toxicity boundary for anything aimed at this axis: the compartment that needs PICALM most is the erythron.
Evidence that argues against this (1)
- PUBLISHED CONFLICTING RESULT on where the on-target boundary actually lies. Drawing the toxicity boundary around the erythron assumes the intervention acts through transferrin-receptor endocytosis, and PICALM level has a second, non-endocytic downstream consequence: modulating PICALM levels perturbs cellular CHOLESTEROL homeostasis. PICALM is also a common variant locus with broad tissue expression well outside haematopoiesis. So an intervention aimed at this axis would not be confined to erythroid cells, and a boundary derived from a lineage-restricted mouse knockout of the gene is not the same as the boundary for a drug that perturbs the pathway. Mercer JL et al. 2015
the non-chromatin arm of the fusion's biology
the wild-type function the fusion fails to perform, and acts dominant-negatively against
the membrane pool whose function is lost
- related to →PICALM::MLLT10 (CALM-AF10), t(10;11)(p13;q14) - the DOT1L-dependent HOXA fusion that contains no KMT2A
corrected 2026-09-21: the in vivo test exists - dietary iron restriction reduced spleen tumour burden but did NOT increase survival in CALM-het CALM-AF10 mice