AML with Mutated NPM1: Cup-Like Blasts, Cytoplasmic NPM1, FLT3-ITD and Prognosis, with MCQs

Subtitle: AML with mutated NPM1: cup-like blasts, the CD34-negative HLA-DR-negative immunophenotype, cytoplasmic NPM1 on immunohistochemistry, the FLT3-ITD interaction, measurable residual disease monitoring and the WHO 5th edition blast-count change, with 10 MCQs.

Author: PathologyMCQ Editorial Board

Category: Haematopathology

Last Updated: October 2026

Read Time: 7 minutes

At-a-Glance

Exon 12 frameshift insertion in almost all cases; type A is the commonest.
Cup-like blasts with nuclear invaginations of at least a quarter of the nuclear diameter.
CD34 negative, HLA-DR negative, CD33 bright; CD117, CD123 and CD110 often positive.
Cytoplasmic NPM1 on immunohistochemistry is a surrogate for the mutation; wild type is nucleolar.
Strong association with myelomonocytic and monocytic differentiation; female predominance.
Favourable when the karyotype is normal and there is no FLT3-ITD.
WHO 5th edition: diagnosed irrespective of blast count.

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Morphology

AML with mutated NPM1 accounts for roughly a third of adult AML and about half of AML with a normal karyotype. There is a female predominance, and it is largely a disease of adults.

The characteristic finding is the cup-shaped blast: a nucleus with a deep invagination involving at least 25% of the nuclear diameter, giving a cup or fish-mouth profile. Cup-like nuclei are suggestive rather than diagnostic, and are also described with FLT3-ITD. They are helpful because they can be spotted on the day the smear is screened, prompting the right molecular test.

Cup-shaped blasts in NPM1-mutated acute myeloid leukaemia — Cup-shaped blasts in NPM1-mutated AML. The nuclear invagination involves at least a quarter of the nuclear diameter.

There is a strong association with acute myelomonocytic and acute monocytic leukaemia. Two broad subgroups are described: one with an immature myeloid profile, and one with a monocytic profile expressing CD36, CD64 and CD14.

Immunophenotype

The immunophenotype is distinctive and is often the first clue after morphology.

CD33 bright, with variable and often low CD13.
CD34 negative and HLA-DR negative in most cases.
CD117 (KIT), CD123 and CD110 expression are common.
Monocytic cases add CD36, CD64 and CD14.

The CD34-negative, HLA-DR-negative combination overlaps with acute promyelocytic leukaemia, which is the single most important differential to exclude urgently. APL is separated by its hypergranular promyelocytes, faggot cells, coagulopathy and PML::RARA.

Immunophenotyping of AML with NPM1 mutation.

Cytoplasmic NPM1 on Immunohistochemistry

Nucleophosmin shuttles between nucleolus and cytoplasm but is predominantly nucleolar in normal cells. A mutation in exon 12 both disrupts the nucleolar localisation signal and creates a new nuclear export signal, so the mutant protein is aberrantly retained in the cytoplasm.

Cytoplasmic NPM1 staining is therefore a reliable surrogate for the mutation, and is useful where molecular testing is delayed or unavailable. Nucleolar-only staining argues against it. Molecular confirmation remains the standard, because the mutation type and allele burden matter for monitoring.

Diagnostic algorithm for NPM1-mutated AML — A diagnostic algorithm for AML with NPM1 mutation. Cytoplasmic staining prompts confirmatory molecular testing.

Genetics and Prognosis

Almost all mutations are small frameshift insertions in exon 12; the 4-base duplication known as type A accounts for most, with types B and D making up much of the remainder. The mutation is usually heterozygous and is considered a founding lesion.

Prognosis turns on what accompanies it. Mutated NPM1 with a normal karyotype and no FLT3-ITD is a favourable-risk group in the ELN 2022 recommendations. A concurrent FLT3-ITD removes much of that benefit and moves the patient towards intermediate risk, which changes the discussion about transplant in first remission. Co-mutated DNMT3A and adverse cytogenetics also worsen outcome.

Blast Count and WHO 5th Edition

The WHO Classification of Haematolymphoid Tumours, 5th edition (2022), removed the 20% blast requirement for most genetically defined AMLs. AML with mutated NPM1 can be diagnosed irrespective of blast count, so a marrow with 12% blasts and an NPM1 mutation is AML, not MDS with excess blasts. Within the genetically defined group, only BCR::ABL1 and CEBPA still require at least 20% blasts.

Measurable Residual Disease

The mutation is usually stable between diagnosis and relapse, which makes quantitative PCR for the mutant transcript a standard measurable residual disease marker. This matters practically because these cases are often CD34 negative and can shift immunophenotype, so flow-based monitoring is less dependable. A minority of cases do lose the mutation at relapse, so a suspected relapse is still confirmed morphologically.

NPM1-mutated AML at a glance

Feature	Finding
Mutation	Exon 12 frameshift insertion; type A commonest
Morphology	Cup-like blasts; myelomonocytic or monocytic differentiation
Immunophenotype	CD33 bright, CD13 variable, CD34−, HLA-DR−, CD117+, CD123+
Monocytic subgroup	CD36+, CD64+, CD14+
IHC	Cytoplasmic NPM1 (wild type is nucleolar)
Sex	Female predominance; adults
Favourable	Normal karyotype, no FLT3-ITD
Less favourable	FLT3-ITD, adverse cytogenetics
Blast count	Not required under WHO 5th edition
MRD	Quantitative PCR for mutant transcript

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NPM1-mutated AML: 10 MCQs

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Exam Pearls

Cup-like blasts + CD34− + HLA-DR− + normal karyotype = test for NPM1.
Exclude APL first: both are CD34− and HLA-DR−, but APL carries a coagulopathy and needs ATRA on suspicion.
Cytoplasmic NPM1 on IHC means mutated; nucleolar means wild type.
Favourable only without FLT3-ITD.
No blast threshold under WHO 5th edition; BCR::ABL1 and CEBPA are the exceptions that keep one.
Use molecular, not flow, for MRD in these CD34-negative cases.

For more concise notes, see the Concise Hematopathology Notes →

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