Oncofusion

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Cell typestrong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.Q4-cells

Pro-B cell — the stage at which KMT2A::AFF1 leukaemia is arrested, and the phenotype the diagnosis is made on

also called ProB-progenitor · pro-B cell · CD19+CD34+CD10+ B-progenitor · pro-B ALL

The pro-B cell is an early step in making a B lymphocyte, before the cell has built a working antibody receptor. KMT2A::AFF1 leukaemia is stuck at about this step, which is why the blasts look like immature B cells that never finish growing up.

The same thing, for a scientist

The pro-B stage is the committed B-lymphoid progenitor that has begun but not completed immunoglobulin gene rearrangement and does not yet express a pre-B-cell receptor. In human fetal tissue the adult-type CD10-positive ProB-progenitor sits immediately downstream of the CD10-negative PreProB-progenitor. t(4;11)/KMT2A::AFF1 disease is classified as pro-B ALL, and the two most faithful experimental models of it, a retroviral MLL-Af4 in human CD34-positive cells and a murine Mll-AF4 model driven by miR-128a, both converge on a pro-B leukaemia phenotype. This is the differentiation stage where the fusion program holds the cell, and it is the stage whose surface markers (CD19, CD34, CD10-negative, NG2) define the clinical diagnosis and the therapeutic targets.

Go deeper
Two independent models converge on pro-B1 deeper

Human cells. A retroviral MLL-Af4 fusion (murine Af4, highly conserved with human AFF1) in human CD34-positive cells produced a model of t(4;11) pro-B ALL that fully recapitulated the immunophenotypic and molecular aspects of the disease, and did so distinctly from MLL-AF9 because of differential genomic target binding of the two fusion proteins 1.

Mouse cells. In an entirely murine Mll-AF4 model, miR-128a overexpression produced a pro-B ALL maintained by a highly expanded Il7r-positive Kit-positive blast population, whereas miR-130b in the same system produced a mixed B-cell-precursor/myeloid leukaemia propagated by LMPPs 2.

The convergence matters because the two systems share almost no technical assumptions. It also gives the field its cleanest statement of what the fusion does at the cellular level: the fusion is instructive, choosing the lineage and the arrest point, rather than merely permissive 1.

Il7r-positive Kit-positive: the arrest has a receptor signature

The pro-B blast population that maintains murine Mll-AF4 pro-B ALL was Il7r-positive and Kit-positive 2. IL7R is the receptor that drives B-progenitor survival and proliferation before the pre-BCR checkpoint; KIT is a stem/progenitor receptor that a normal committed pro-B cell is downregulating. Holding both simultaneously is a description of the arrest in receptor terms: proliferative input from the B-lineage cytokine, retained progenitor identity from the stem-cell receptor. It also names two surface proteins as candidate handles, and IL7R signalling is the pathway that RAS mutations in this disease feed into.

The clinical phenotype: what the flow cytometrist sees

Infant KMT2A-rearranged B-ALL presents with a pro-B/mixed phenotype and NG2 expression, the latter being near-specific for KMT2A rearrangement and minimally expressed in healthy haematopoietic cells 34. In the Interfant cohort, high NG2 expression was associated with lower event-free survival, higher numbers of circulating blasts, and more frequent CNS disease or relapse 3.

Two cautions belong here, both from the primary data rather than from commentary.

  1. NG2 is not a stem-cell marker. Serial xenotransplantation of primary MLL-AF4-positive leukaemias showed NG2 to be a malleable marker that does not enrich for leukaemia-initiating or CNS-initiating cells; instead NG2 was strongly upregulated on blasts infiltrating extramedullary sites and CNS, and specific NG2 blockade caused almost complete loss of engraftment 3. NG2 marks an invasive state, not a hierarchy position.
  2. NG2 is a fusion target with a drug-resistance job. NG2 is an epigenetically regulated direct target gene of MLL-AF4 that negatively regulates the glucocorticoid receptor NR3C1 and confers glucocorticoid resistance, mechanistically via interaction with FLT3 giving ligand-independent FLT3 activation and AP-1-mediated NR3C1 transrepression 4. So the marker that defines the pro-B phenotype is also part of why the disease resists the backbone of B-ALL therapy.
Definition, ontology, and position

Cell Ontology gives CL:0000826 (pro-B cell), with CL:0002048 (late pro-B cell) and CL:0000817 (precursor B cell) as neighbouring terms 5. In human fetal tissue the CD10-positive ProB-progenitor is the adult-type progenitor, sitting downstream of the CD10-negative PreProB-progenitor; both give rise solely to B-lineage cells and are transcriptionally distinct 6.

The arrest point in infant KMT2A-r disease is therefore slightly upstream of the conventional CD10-positive pro-B: the blasts are CD10-negative. Calling the disease 'pro-B ALL' is clinically correct and developmentally approximate, and the approximation is exactly where the fetal PreProB story lives.

Facts and figures
ontologyCL:0000826
locationEmbryonic and fetal liver in early gestation, then fetal and postnatal bone marrow
arrest stagecommitted B-lymphoid progenitor, before pre-B-cell receptor expression; CD10-negative in infant KMT2A-r disease
atlas refs
MLL-Af4 human CD34+ model…2,016MLL-Af4 human CD34+ model of t(4;11) pro-B ALL: 2016Interfant NG2 clinical an…2,018Interfant NG2 clinical and xenograft analysis: 2018Murine Mll-AF4 miR-128a p…2,021Murine Mll-AF4 miR-128a pro-B ALL, Il7r+Kit+ propagating population: 2021
ontology alternatives
[
  "CL:0002048",
  "CL:0000817"
]
markers
[
  "CD19+",
  "CD34+",
  "CD10+ (adult-type ProB) or CD10- (infant KMT2A-r blasts)",
  "NG2/CSPG4+ in KMT2A-r",
  "IL7R/CD127+",
  "KIT/CD117+ in the murine Mll-AF4 propagating population"
]
lineage potential
[
  "B-lineage; myeloid potential can be re-expressed under environmental pressure in MLL-Af4 models"
]
atlas refs — the numbers behind the chart
[
  {
    "label": "MLL-Af4 human CD34+ model of t(4;11) pro-B ALL",
    "pmid": "27846391",
    "title": "Instructive Role of MLL-Fusion Proteins Revealed by a Model of t(4;11) Pro-B Acute Lymphoblastic Leukemia.",
    "authors": "Lin S, Luo RT, Ptasinska A, et al.",
    "venue": "Cancer Cell",
    "year": 2016,
    "title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
  },
  {
    "label": "Murine Mll-AF4 miR-128a pro-B ALL, Il7r+Kit+ propagating population",
    "pmid": "34111240",
    "title": "miR-130b and miR-128a are essential lineage-specific codrivers of t(4;11) MLL-AF4 acute leukemia.",
    "authors": "Malouf C, Antunes ETB, O'Dwyer M, et al.",
    "venue": "Blood",
    "year": 2021,
    "title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
  },
  {
    "label": "Interfant NG2 clinical and xenograft analysis",
    "pmid": "28943635",
    "title": "NG2 antigen is involved in leukemia invasiveness and central nervous system infiltration in MLL-rearranged infant B-ALL.",
    "authors": "Prieto C, López-Millán B, Roca-Ho H, et al.",
    "venue": "Leukemia",
    "year": 2018,
    "title_provenance": "Title/authors/venue backfilled from PubMed esummary on 2026-09-20 because this citation stored a bare PMID with no title, which made it unscoreable by verify-citations.py. BACKFILLING IS NOT VERIFICATION: it confirms the PMID resolves to a real paper, not that this is the right paper for the claim. That still requires reading."
  }
]
How this could bite us back
  • The pro-B stage is CD19-positive, which is what makes blinatumomab and CD19 CAR-T attractive here and is exactly the pressure under which lineage switch to AML has been reported.
  • NG2, the most KMT2A-specific surface marker available, does not mark the leukaemia-initiating cell, so targeting it would reduce invasiveness while leaving the reservoir.
  • A differentiation-releasing therapy in a cell whose lineage commitment is incomplete may produce myeloid rather than B-cell maturation, converting a response into a lineage switch.
  • The NG2-FLT3 axis means FLT3 is activated ligand-independently, so FLT3 inhibition may be less effective here than the expression level suggests.
Still open
  1. Does menin inhibition release the pro-B arrest into genuine B-cell maturation, or into a myeloid program? The answer determines whether differentiation therapy here risks driving lineage switch.
  2. Is the Il7r-positive Kit-positive propagating population identifiable in human KMT2A::AFF1 marrow, and is it the menin-inhibitor-persistent fraction?
  3. Given that NG2 drives glucocorticoid resistance via NR3C1, does NG2 blockade restore steroid sensitivity in vivo, and would that be a better use of anti-NG2 than trying to eradicate the clone?
  4. Why is the disease CD10-negative when the conventional pro-B stage is CD10-positive? Is CD10/MME actively repressed by the fusion, or is the arrest simply upstream?
Every source behind this entry (6)
  1. E1strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper

    A retroviral MLL-Af4 fusion in human CD34-positive cells generates a model of t(4;11) pro-B ALL that fully recapitulates the immunophenotypic and molecular aspects of the disease, is distinct from MLL-AF9 through differential genomic target binding, and shows that MLL fusion proteins play an instructive role in lineage and arrest choice.

    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 2016
    PMID 27846391DOI
    What this evidence cannot show

    Uses murine Af4 rather than human AFF1 and high-titre retroviral delivery, so it reports what a strongly expressed fusion instructs rather than endogenous t(4;11) behaviour. Species specificity of the oncogene is an explicit theme of the paper.

    Evidence that argues against this (2)
    • Other groups transducing the same human starting cell did not obtain leukaemia. MLL-AF4 in cord-blood CD34+ HSPCs enhanced repopulation, homing and clonogenic potential after intra-bone-marrow transplantation but was insufficient to initiate leukaemogenesis, and the authors raised that either further hits are needed or CD34+ cord-blood HSPCs are not the appropriate target. The difference between that result and a full pro-B ALL model is unexplained and may lie in the fusion construct, the vector or the recipient rather than in the biology. Read: abstract only. Montes R et al. 2011
    • Adding the commonest cooperating human lesion still did not initiate disease from this cell: activated KRAS with MLL-AF4 in cord-blood CD34+ HSPCs enhanced extramedullary engraftment, leukocytosis and CNS infiltration but did not transform, leading the authors to place mutant KRAS in tumour maintenance rather than initiation. Read: abstract only. Prieto C et al. 2016
  2. E2strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper

    In a murine Mll-AF4 model, miR-128a overexpression produces a pro-B ALL maintained by a highly expanded Il7r-positive Kit-positive blast population, while miR-130b produces a mixed B-cell-precursor/myeloid leukaemia propagated by the LMPP population; both recapitulate human features including CNS infiltration.

    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 2021
    PMID 34111240DOI
    What this evidence cannot show

    MicroRNA overexpression is a non-physiological perturbation and the lineage outcome may reflect co-driver dose. Murine Mll-AF4 models do not fully reproduce human t(4;11) infant ALL.

    Evidence that argues against this (1)
    • The same fusion knocked into the mouse germline does not produce pro-B ALL. Mll-AF4 knock-in mice developed mixed lymphoid/myeloid hyperplasia and, after long latency, predominantly B-cell lymphomas; the authors concluded lineage deregulation by the fusion alone is insufficient for malignancy. The pro-B phenotype in the miR-128a model may therefore be attributable to the cooperating lesion. Read: abstract only. Chen W et al. 2006
  3. E3strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper

    KMT2A-rearranged infant B-ALL is associated with a pro-B/mixed phenotype and NG2 expression; in the Interfant cohort high NG2 associated with lower event-free survival, more circulating blasts and more frequent CNS disease or relapse, yet serial xenotransplantation showed NG2 is a malleable marker that does not enrich leukaemia-initiating or CNS-initiating cells, being instead upregulated on blasts infiltrating extramedullary and CNS sites, with NG2 blockade causing almost complete loss of engraftment.

    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 2018
    PMID 28943635DOI
    What this evidence cannot show

    Clinical associations are retrospective within the Interfant cohort and NG2 quantification by flow cytometry is operator- and threshold-dependent. The engraftment-blockade experiment used xenografts, where homing biology differs from a human marrow.

    Evidence that argues against this (1)
    • The pro-B/mixed immunophenotype is not the whole of infant ALL. In 133 infant ALL cases about 70% were pro-B and the remainder common-ALL or pre-B, and the immature Ig/TCR pattern correlated with young age, CD10 negativity and the specific MLL partner — MLL-AF9 cases had more mature rearrangements. 'Pro-B' is therefore partner- and age-dependent rather than a fixed property of KMT2A rearrangement. Read: abstract only. Jansen MWJC et al. 2007
  4. 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 through interaction 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 2024
    PMID 39093982DOI
    What this evidence cannot show

    Mechanism established largely in cell lines and xenografts; the quantitative contribution of the NG2-FLT3-AP-1-NR3C1 axis to clinical prednisone response in infants has not been measured prospectively.

    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 glucocorticoid-resistance chain runs through FLT3, and the randomised clinical test of FLT3 inhibition in this exact population was negative. Leukemia 2021
  5. E5established confidence 5 of 5: Multiple independent lines of evidence; treated as settled in the field.database

    Cell Ontology provides CL:0000826 (pro-B cell), CL:0002048 (late pro-B cell) and CL:0000817 (precursor B cell).

    Cell Ontology term lookup via the EMBL-EBI Ontology Lookup Service OLS4 API · EMBL-EBI Ontology Lookup Service; Cell Ontology consortium · Database 2026
    What this evidence cannot show

    CL B-cell staging largely follows mouse Hardy fractions and maps only approximately onto human B-lymphopoiesis.

    Evidence that argues against this (1)
    • Ontology terms name the stages and do not resolve the disagreement this entry is about. CL:0000826 and CL:0002048 are defined against a staging scheme whose human boundaries are debated, and infant KMT2A-rearranged blasts sit ambiguously across them. OBO Foundry / EMBL-EBI OLS 2026
  6. E6strong confidence 4 of 5: Replicated in peer-reviewed work by more than one group.paper

    In human fetal tissue the adult-type CD10-positive ProB-progenitor lies immediately downstream of a CD10-negative PreProB-progenitor; both give rise solely to B-lineage cells and are transcriptionally distinct.

    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 2019
    PMID 31383639DOI
    What this evidence cannot show

    In vitro differentiation assays report potential under permissive conditions, not obligate in vivo fate.

    Evidence that argues against this (1)
    • Against a developing bone-marrow reference, KMT2A-r infant B-ALL maps to an early lymphocyte precursor state with hybrid myeloid-lymphoid features and non-physiological antigen combinations rather than to a clean PreProB or ProB identity, and the same study concludes that cell of origin cannot be inferred from transcriptional state. Placing the blasts on the PreProB-to-ProB axis may impose more B-lineage structure on them than the data support. Read: abstract only. Khabirova E et al. 2022
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