The pro-B arrest state — what a KMT2A::AFF1 blast is actually doing while it sits there
also called pro-B ALL state · CD10-negative pro-B arrest · Il7r+Kit+ blast state · immature oligoclonal Ig arrest
The leukaemia cells are held at an early B-cell step where they are still receiving growth signals but have not built a working antibody receptor. They are not dormant: they are actively proliferating, resistant to steroids, and still able to switch lineage if pushed.
The same thing, for a scientist
The pro-B arrest state is the specific arrest position of KMT2A::AFF1 disease, and it is operationally distinct from the generic differentiation block. Its measurable features are: a CD10-negative CD19-positive CD34-positive immunophenotype with NG2/CSPG4 expression; an immature, oligoclonal, non-expanded immunoglobulin repertoire; sustained IL7R and KIT co-expression in the propagating compartment, which combines B-lineage cytokine drive with retained progenitor identity; intrinsic glucocorticoid resistance mediated by an MLL-AF4 to NG2 to FLT3 to AP-1 to NR3C1 axis; and retained latent myeloid potential that becomes manifest under environmental or therapeutic pressure.
CORRECTION: 'pre-VDJ' was wrong and is withdrawn from this entry
This entry described the KMT2A::AFF1 arrest as pre-VDJ, meaning the blasts had not engaged V(D)J recombination. The immunogenetics say otherwise and the word is withdrawn throughout.
Jansen et al. analysed Ig/TCR rearrangement in 133 infant ALL cases and found IGH rearranged in 91%, IGK in 21%, IGL in 13%, and cross-lineage TCRD in 46%. MLL-AF4 and MLL-ENL cases showed immature rearrangements; MLL-AF9 cases showed more mature ones. Compared with childhood precursor-B-ALL the patterns were less frequent and more oligoclonal — but they exist [E_jansen].
The blasts have engaged the recombinase. The accurate description is immature and oligoclonal Ig rearrangement, not pre-VDJ, and the distinction matters: a pre-VDJ cell and a cell with incomplete DH-JH joins sit at different developmental positions and imply different cells of origin.
Two further things from the same source qualify this entry's framing. Pro-B is about 70% of infant ALL, not all of it — the remainder are common-ALL and pre-B — so 'the KMT2A-r phenotype' is a majority, not a definition. And the maturity of the Ig/TCR pattern tracks with the KMT2A partner, which means arrest position is partner-dependent and a statement about KMT2A::AFF1 should not be generalised across KMT2A-rearranged disease.
The receptor signature of the arrest
In the murine Mll-AF4 system, the pro-B ALL produced by miR-128a was maintained by a highly expanded Il7r-positive Kit-positive blast population 1. That pairing is the arrest expressed in signalling terms:
- IL7R/CD127 is the B-progenitor survival and proliferation receptor that operates before the pre-B-cell-receptor checkpoint. Keeping it on means the cell continues to receive lineage-appropriate growth input.
- KIT/CD117 is a stem and early progenitor receptor that a normally maturing pro-B cell is switching off. Keeping it on means the cell has not surrendered progenitor identity.
Holding both simultaneously is not a state normal B-lymphopoiesis passes through for long. It also names the downstream node where RAS pathway mutations act: IL7R signals through JAK/STAT and RAS/MAPK, which is consistent with the finding that the most refractory infant KMT2A-r cluster is defined by RAS pathway mutation burden.
The arrest is upstream of where the field usually assumes
KMT2A::AFF1 infant disease is CD10-negative, and the immunoglobulin repertoire is immature and oligoclonal: BCR repertoire analysis across 124 uniformly treated infant B-ALL cases found only minor, non-expanded B-cell clones in t(4;11), unlike non-KMT2A-rearranged B-ALL 2. The normal human fetal hierarchy contains a matching cell, the CD10-negative PreProB progenitor, which is upstream of the conventional CD10-positive ProB and shares transcriptomic signatures with CD10-negative B-progenitor infant ALL blasts 3.
So 'pro-B ALL' as a clinical label is one stage coarser than the biology. The arrest sits at a fetal-type, CD10-negative, B-committed position with immature Ig rearrangement for which Cell Ontology has no human term.
The arrest is drug-resistant by construction1 deeper
This state does not merely fail to respond to steroids by accident. NG2 is a direct, epigenetically regulated target gene of MLL-AF4; NG2 negatively regulates the glucocorticoid receptor NR3C1 and confers glucocorticoid resistance, mechanistically by interacting with FLT3 to give ligand-independent FLT3 activation and AP-1-mediated NR3C1 transrepression 4.
That is a closed loop worth stating plainly: the fusion switches on the marker that is used to diagnose the disease, and that same marker switches off the receptor for the drug that forms the backbone of B-ALL therapy. Glucocorticoid resistance in infant KMT2A-r ALL is therefore on-mechanism, not incidental.
The same NG2 is a state marker rather than a hierarchy marker: serial xenotransplantation showed NG2 is malleable and does not enrich leukaemia-initiating or CNS-initiating cells, but is strongly upregulated on blasts infiltrating extramedullary sites and CNS, and specific NG2 blockade caused almost complete loss of engraftment 5.
A therapeutic idea that follows directly
If NG2 causes glucocorticoid resistance by suppressing NR3C1 4, then NG2 blockade should restore steroid sensitivity, which is a more realistic goal than using anti-NG2 to eradicate a clone that NG2 does not mark 5. Combining NG2 blockade with dexamethasone, rather than using either alone, is the experiment the two papers jointly imply and neither performs.
Latent myeloid potential inside the arrest
The arrest is a B-lineage arrest, but it is not a B-lineage commitment. MLL-Af4-transformed human cells can assume a myeloid state under environmental pressure while retaining lymphoid-lineage potential, and the same work noted t(4;11) patients whose leukaemia evaded CD19-directed therapy by undergoing myeloid lineage switch 6. The gene regulatory network built for KMT2A-AFF1 makes the same point from the genomic side: a core set of network nodes is present in both the ALL and the AML forms of the disease 7.
A cell whose regulatory core is shared with the myeloid version of the same disease is a cell for which lineage switch is a short trip, not a transformation.
| state definition | Arrest at a fetal-type, CD10-negative, pre-VDJ B-committed position with sustained IL7R and KIT co-expression, intrinsic glucocorticoid resistance and retained latent myeloid potential |
|---|---|
| immunoglobulin status | pre-VDJ; only minor, non-expanded B-cell clones detectable |
| drug resistance wiring | MLL-AF4 to NG2 to FLT3 (ligand-independent activation) to AP-1 to NR3C1 repression, producing glucocorticoid resistance |
| location | bone marrow, blood, CNS and other extramedullary sites where NG2 is further upregulated |
markers
[ "CD19+", "CD34+", "CD10-", "NG2/CSPG4+", "IL7R/CD127+", "KIT/CD117+ (murine propagating population)", "frequent myeloid antigen co-expression" ]
latent potential
[ "myeloid, inducible under environmental or therapeutic pressure" ]
- The arrest keeps CD19 on the surface, which is what makes CD19-directed therapy attractive and is exactly the pressure under which the reported myeloid lineage switches occurred.
- Glucocorticoid resistance is on-mechanism through NG2, so intensifying steroids in this disease treats a receptor the fusion has already suppressed.
- NG2 is the most KMT2A-specific surface marker but does not mark the leukaemia-initiating cell, so anti-NG2 monotherapy would reduce dissemination without eradicating the reservoir.
- Latent myeloid potential means any successful lymphoid-directed pressure selects for the myeloid escape route.
- Does NG2 blockade restore glucocorticoid sensitivity in vivo? The two NG2 papers jointly imply this combination and neither tests it.
- Is the IL7R-positive KIT-positive propagating population identifiable in human KMT2A::AFF1 marrow, and is it the fraction that persists under menin inhibition?
- Since the arrest is pre-VDJ and CD10-negative, is there a surface protein specific to the fetal PreProB-like state that CD19 is not, and that lineage switch cannot shed?
- The ALL and AML forms share a core regulatory network. Does that mean a single therapy covers both after a lineage switch, or does the switch also change the drug dependency?
- E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In a murine Mll-AF4 model, miR-128a-driven pro-B ALL is maintained by a highly expanded Il7r-positive Kit-positive blast population, and the disease recapitulates human features including CNS infiltration and an MLL-AF4 expression signature.
miR-130b and miR-128a are essential lineage-specific codrivers of t(4;11) MLL-AF4 acute leukemia · Malouf C, Antunes ETB, O'Dwyer M, Jakobczyk H, Sahm F, Landua SL, Anderson RA, Soufi A, Halsey C, Ottersbach K · Blood 2021What this evidence cannot show
MicroRNA overexpression is a strong artificial perturbation and the phenotype may reflect co-driver dose. Mouse Il7r/Kit gating does not translate one-to-one to human CD127/CD117 flow cytometry.
Evidence that argues against this (1)
- The murine Mll-AF4 knock-in gives a different disease. Homologous-recombination Mll-AF4 knock-in mice showed mixed lymphoid/myeloid hyperplasia and, after prolonged latency, mostly B-cell lymphomas rather than a short-latency pro-B ALL, and the authors concluded that Mll-AF4-induced lineage deregulation alone is insufficient for malignancy. A pro-B arrest obtained by adding a second lesion (here miR-128a) to Mll-AF4 may be a property of the cooperating lesion rather than of the fusion. Read: abstract only. Chen W et al. 2006
- E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
BCR repertoire analysis of 124 uniformly treated infant B-ALL cases found only minor, non-expanded B-cell clones in t(4;11) disease, supporting an immature, oligoclonal arrest position, and the transcriptome was closest to the most immature fetal liver HSPCs.
Unraveling the cellular origin and clinical prognostic markers of infant B-cell acute lymphoblastic leukemia using genome-wide analysis · Agraz-Doblas A, Bueno C, Bashford-Rogers R, Roy A, Schneider P, Bardini M, Ballerini P, Cazzaniga G, Moreno T, Revilla C, Gut M, Valsecchi MG, Roberts I, Pieters R, De Lorenzo P, Varela I, Menendez P, Stam RW · Haematologica 2019What this evidence cannot show
Absence of clonal expansion is easier to establish than absence of rearrangement, and detection of minor clones depends on sequencing depth. Interfant-treated cohort.
Evidence that argues against this (1)
- 'Pre-VDJ' overstates what the immunogenetics show. Ig/TCR rearrangement analysis of 133 infant ALL cases found IGH rearrangements in 91%, IGK in 21%, IGL in 13% and cross-lineage TCRD in 46%; MLL-AF4 cases had an immature and oligoclonal pattern, not a germline one. The blasts have engaged the recombinase. The accurate statement is 'immature and oligoclonal Ig rearrangement', not 'pre-VDJ'. Read: abstract only. Jansen MWJC et al. 2007
- E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
The human fetal B-progenitor hierarchy contains a CD10-negative PreProB progenitor upstream of the CD10-positive ProB progenitor, whose fetal-specific expression program is shared with CD10-negative B-progenitor infant ALL blasts.
Discovery of a CD10-negative B-progenitor in human fetal life identifies unique ontogeny-related developmental programs · O'Byrne S, Elliott N, Rice S, Buck G, Fordham N, Garnett C, Godfrey L, Crump NT, Wright G, Inglott S, Hua P, Psaila B, Povinelli B, Knapp DJHF, Agraz-Doblas A, Bueno C, Varela I, Bennett P, Koohy H, Watt SM, Karadimitris A, Mead AJ, Ancliff P, Vyas P, Menendez P, Milne TA, Roberts I, Roy A · Blood 2019What this evidence cannot show
Signature sharing is a state-similarity argument and does not establish that the arrest is at exactly this normal cell's position.
Evidence that argues against this (1)
- Mapping the blasts against a developing bone-marrow reference placed KMT2A-r infant B-ALL in an early lymphocyte precursor (ELP) state carrying hybrid myeloid-lymphoid features and non-physiological antigen combinations, not a clean pro-B or PreProB identity; and the same work showed the rearrangement arose before haematopoietic specification, concluding that cell of origin cannot be read off the transcriptional state. A shared signature with fetal PreProB is therefore compatible with the arrest sitting upstream of, or beside, the B-lineage axis rather than on it. Read: abstract only. Khabirova E et al. 2022
- E4strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
NG2 is an epigenetically regulated direct target gene of MLL-AF4 that negatively regulates the glucocorticoid receptor NR3C1 and confers glucocorticoid resistance, acting by interacting with FLT3 to give ligand-independent FLT3 activation and AP-1-mediated NR3C1 transrepression.
NG2 is a target gene of MLL-AF4 and underlies glucocorticoid resistance in MLLr B-ALL by regulating NR3C1 expression · Lopez-Millan B, Rubio-Gayarre A, Vinyoles M, Trincado JL, Fraga MF, Fernandez-Fuentes N, Guerrero-Murillo M, Martinez A, Velasco-Hernandez T, Falgas A, Panisello C, Valcarcel G, Sardina JL, Lopez-Marti P, Javierre BM, Del Valle-Perez B, Garcia de Herreros A, Locatelli F, Pieters R, Bardini M, Cazzaniga G, Rodriguez-Manzaneque JC, Hanewald T, Marschalek R, Milne TA, Stam RW, Tejedor JR, Menendez P, Bueno C · Blood 2024What this evidence cannot show
Mechanism established largely in cell lines and xenografts; the quantitative contribution to clinical prednisone response in infants has not been prospectively measured, and glucocorticoid resistance in this disease is certainly multifactorial.
Evidence that argues against this (1)
- NG2 is expressed in about 90% of KMT2A-rearranged cases, so in any cohort mixing rearranged and germline patients its associations largely re-report the rearrangement. The two NG2 items in this beat also disagree with each other: one has NG2 conferring glucocorticoid resistance through FLT3 and AP-1, the other has it as a malleable marker that does NOT enrich leukaemia-initiating cells on serial xenotransplantation. The proposed chain — NG2 down-regulates NR3C1 via FLT3 and AP-1 — has three steps each measured separately, and the clinical test of the FLT3 link was negative: lestaurtinib added to chemotherapy in KMT2A-rearranged infant ALL did not improve outcome. Leukemia 2021
- E5strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In MLL-rearranged infant B-ALL, NG2 is a malleable marker that does not enrich leukaemia-initiating or CNS-initiating cells on serial xenotransplantation, but is strongly upregulated on blasts infiltrating extramedullary and CNS sites, with specific NG2 blockade causing almost complete loss of engraftment; clinically, high NG2 associated with lower event-free survival, more circulating blasts and more frequent CNS disease or relapse in the Interfant cohort.
NG2 antigen is involved in leukemia invasiveness and central nervous system infiltration in MLL-rearranged infant B-ALL · Prieto C, Lopez-Millan B, Roca-Ho H, Stam RW, Romero-Moya D, Rodriguez-Baena FJ, Sanjuan-Pla A, Ayllon V, Ramirez M, Bardini M, De Lorenzo P, Valsecchi MG, Stanulla M, Iglesias M, Ballerini P, Carcaboso AM, Mora J, Locatelli F, Bertaina A, Padilla L, Rodriguez-Manzaneque JC, Bueno C, Menendez P · Leukemia 2018What this evidence cannot show
Xenograft homing biology differs from a human marrow and CNS; clinical associations are retrospective and depend on NG2 flow thresholds.
Evidence that argues against this (1)
- NG2 is expressed in about 90% of KMT2A-rearranged cases, so in any cohort mixing rearranged and germline patients its associations largely re-report the rearrangement. The two NG2 items in this beat also disagree with each other: one has NG2 conferring glucocorticoid resistance through FLT3 and AP-1, the other has it as a malleable marker that does NOT enrich leukaemia-initiating cells on serial xenotransplantation. This item is the one that should govern the entry: on serial xenotransplantation NG2 does not enrich leukaemia-initiating cells, which makes it a state marker rather than a property of the arrested compartment. Oncofusion knowledge graph (internal analysis) 2026
- E6strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
MLL-Af4-transformed cells can assume a myeloid state under environmental pressure while retaining lymphoid-lineage potential, an incongruity also observed in t(4;11) patients whose leukaemia evaded CD19-directed therapy by myeloid lineage switch.
Instructive Role of MLL-Fusion Proteins Revealed by a Model of t(4;11) Pro-B Acute Lymphoblastic Leukemia · Lin S, Luo RT, Ptasinska A, Kerry J, Assi SA, Wunderlich M, Imamura T, Kaberlein JJ, Rayes A, Althoff MJ, Anastasi J, O'Brien MM, Meetei AR, Milne TA, Bonifer C, Mulloy JC, Thirman MJ · Cancer Cell 2016What this evidence cannot show
The patient observation is a clinical correlate within a laboratory paper, establishing existence rather than frequency; 'environmental pressure' in the model is culture and xenograft conditions rather than a defined signal.
Evidence that argues against this (2)
- The myeloid state is not a one-way door. An infant with KMT2A-rearranged ALL that switched to AML under blinatumomab spontaneously reverted to the lymphoid phenotype over a timescale of weeks once the immunological stimulus changed. Whatever the 'incongruity' is, it is fast, bidirectional and environmentally contingent, which argues against a stable rewiring of the arrest and in favour of a labile state. Read: abstract only. Wölfl M et al. 2018
- An alternative reading of the same observation is selection, not plasticity. Longitudinal targeted sequencing across a B-ALL-to-AML switch showed the lymphoid and myeloid phases shared ancestral alterations but had distinct mutational profiles, with TP53 and SMC1A newly emerging at switch and only part of the relapse mutation set retained — consistent with expansion of a genetically distinct pre-existing subclone from a common progenitor rather than transdifferentiation of the dominant clone. This case was not KMT2A-rearranged, which limits transfer. Read: abstract only. Kondo K et al. 2026
- E7moderate confidence 3 of 5: One strong study, or several smaller consistent ones.paper
The KMT2A-AFF1 gene regulatory network contains a core set of nodes present in both the ALL and the AML forms of the disease, including RUNX1-centred feed-forward loops and cascades.
A KMT2A-AFF1 gene regulatory network highlights the role of core transcription factors and reveals the regulatory logic of key downstream target genes · Harman JR, Thorne R, Jamilly M, Tapia M, Crump NT, Rice S, Beveridge R, Morrissey E, de Bruijn MFTR, Roberts I, Roy A, Fulga TA, Milne TA · Genome Research 2021What this evidence cannot show
A computationally inferred network; most edges are statistical predictions with only a subset experimentally validated, and input data are cell-line-heavy.
Evidence that argues against this (1)
- A shared regulatory core between the ALL and AML forms is equally consistent with the leukaemia never having been fully committed to B lineage. Single-cell mapping puts KMT2A-r infant blasts in an ELP-like state with hybrid myeloid-lymphoid antigen combinations, so 'ALL nodes and AML nodes overlap' may describe one uncommitted state rather than two programmes held in tension in a pro-B cell. Read: abstract only. Khabirova E et al. 2022
- E_jansenstrong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper
In 133 infant ALL cases, about 70% showed a pro-B immunophenotype with the remainder common-ALL and pre-B; MLL translocations were present in 79% (MLL-AF4 41%, MLL-ENL 18%, MLL-AF9 11%). IGH rearrangements were found in 91%, IGK in 21%, IGL in 13%, and cross-lineage TCRD, TCRG and TCRB in 46%, 17% and 10%. Ig/TCR rearrangements were less frequent and more oligoclonal than in childhood precursor-B-ALL; MLL-AF4 and MLL-ENL cases showed immature rearrangements while MLL-AF9 cases showed more mature ones.
Immunobiological diversity in infant acute lymphoblastic leukemia is related to the occurrence and type of MLL gene rearrangement · Jansen MWJC, Corral L, van der Velden VHJ, Panzer-Grümayer R, Schrappe M, Schrauder A, Marschalek R, Meyer C, den Boer ML, Hop WJC, Valsecchi MG, Basso G, Biondi A, Pieters R, van Dongen JJM · Leukemia 21:633-41 2007What this evidence cannot show
Read as abstract only. 2007 PCR-based rearrangement analysis is less sensitive than current high-throughput immunosequencing, so the 91% IGH figure is a floor rather than a ceiling. The authors' own interpretation invokes transformation of progenitors with germline Ig/TCR genes plus a short latency, so they read the immature pattern as consistent with a very early cell of origin — the disagreement here is about the word 'pre-VDJ', not about the developmental inference.
Evidence that argues against this (1)
- The authors themselves attribute the immature, oligoclonal pattern to transformation of progenitors that had germline Ig/TCR genes at the time the fusion arose, followed by a short latency. So the original entry's developmental intuition may be right about the cell of ORIGIN even though 'pre-VDJ' is wrong about the blasts as sampled at diagnosis. Those are different claims and the entry should keep them apart. Jansen MWJC et al. 2007
the pro-B arrest entry records the MLL-AF4/NG2/FLT3/AP-1/NR3C1 glucocorticoid-resistance axis; this state shows that axis already engaged at diagnosis in the cells that go on to relapse
the same cell placed by two instruments: pro-B by surface immunophenotype, ELP by whole-transcriptome projection; the hybrid myeloid-lymphoid programme reconciles them
- ← testsExperiment: go back to the diagnostic sample and look for the myeloid clone before it existed
tests whether the myeloid competence attributed to the arrested state is present as a pre-existing subclone before immunotherapy rather than acquired by reprogramming
counter-evidence pass adds a clonal-selection reading of the arrest's 'lineage incongruity': in one sequenced post-immunotherapy switch the myeloid phase carried a distinct mutational profile, consistent with outgrowth of a genetically distinct subclone
CD10-negative immature, oligoclonal Ig/TCR rearrangement with IL7R/KIT co-expression and NG2-driven glucocorticoid resistance
the state occupied at this cell type
- ← related toThe differentiation block — what it actually is, operationally, in KMT2A::AFF1 leukaemia
general block; the pro-B arrest is its specific position in KMT2A::AFF1 disease
- ← depends onLineage switch as antigen escape from CD19-directed therapy in KMT2A-rearranged leukemia
the arrest retains latent myeloid potential that antigen pressure selects for
asks whether the arrested pro-B blast resolves to a mature B cell or to a myeloid cell under drug, using per-cell fusion detection to exclude normal myeloid recovery
- ← depends onWhere KMT2A-rearranged leukemia physically sits — marrow niche, CNS, blood, liver/spleen and skin
NG2 is further upregulated on blasts infiltrating extramedullary sites and CNS, so the invasive phenotype is a property of this state
frequent myeloid antigen co-expression on the arrested pro-B blast is the mild end of the same spectrum
the CD10-negative CD19+ CD34+ fingerprint flow reports IS the arrest state this graph models
the stage at which the block lands
Foxp1-null blocks the pro-B to pre-B transition with Rag1/Rag2 down and the Erag enhancer unbound - the same stage at which KMT2A::AFF1 blasts are arrested
HDAC7 maintains B-cell identity by silencing myeloid and T-cell genes at MEF2 sites in B-cell progenitors; its loss dissolves lineage fidelity rather than deepening the arrest (PMIDs 23696748, 27810920)