Monoclonal mast cell activation syndrome is the entity defined by a threshold rather than by a lesion, and that is the whole difficulty of curating it. The patient has severe, recurrent, systemic mast cell mediator symptoms - classically hymenoptera-venom anaphylaxis without urticaria - and a demonstrably clonal mast cell population, carrying KIT D816V or aberrant CD25 or CD2 expression, or a clonal androgen-receptor assay result. What they do not have is enough mast cell burden to meet the WHO criteria for systemic mastocytosis. MMAS is what is left when the clonality is present and the criteria are not met. So the mechanism is the same mechanism as systemic mastocytosis, arrived at with a smaller cell population: an activating KIT mutation lowers the mast cell activation threshold and drives constitutive signalling, and the clinical picture is mediator release rather than organ infiltration. That is why the disease behaves as an anaphylaxis disorder while being, molecularly, a clonal neoplastic one. This entry is separate from `Systemic_Mastocytosis` on a decision the knowledge base has already recorded rather than one made here. `kb/groupings/Mastocytosis.yaml` states that MMAS "carries a clonal KIT D816V mast cell population but does not meet the mast cell burden that the proliferation and mastocytosis-finding operands require", and its membership criteria deliberately exclude it. The nosology agrees: the ECNM-AIM classification separates confirmed MCAS from mast cell activation disorder not fulfilling MCAS criteria, and places clonal and non-clonal MCAS in different categories. Two boundaries move under this entry and curators should know it. The 2021 refined criteria for bone marrow mastocytosis carve out a variant that is older, more male, has lower tryptase and lower neoplastic mast cell burden, and has *more* hymenoptera-triggered allergic reactions than typical indolent systemic mastocytosis - which is the same clinical description MMAS was built from, on the other side of the SM threshold. And MCAS as a category has never been uncontroversial; the 2010 criteria paper says so in its own abstract. Neither is a reason to avoid the entry, but both mean its extension depends on which criteria set is in force, and no prevalence figure here should be read as stable.
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name: Monoclonal Mast Cell Activation Syndrome
creation_date: "2026-09-03T16:40:00Z"
category: Complex
synonyms:
- MMAS
- monoclonal MCAS
- clonal mast cell activation syndrome
description: >
Monoclonal mast cell activation syndrome is the entity defined by a threshold rather than
by a lesion, and that is the whole difficulty of curating it. The patient has severe,
recurrent, systemic mast cell mediator symptoms - classically hymenoptera-venom
anaphylaxis without urticaria - and a demonstrably clonal mast cell population, carrying
KIT D816V or aberrant CD25 or CD2 expression, or a clonal androgen-receptor assay result.
What they do not have is enough mast cell burden to meet the WHO criteria for systemic
mastocytosis. MMAS is what is left when the clonality is present and the criteria are not
met.
So the mechanism is the same mechanism as systemic mastocytosis, arrived at with a
smaller cell population: an activating KIT mutation lowers the mast cell activation
threshold and drives constitutive signalling, and the clinical picture is mediator
release rather than organ infiltration. That is why the disease behaves as an
anaphylaxis disorder while being, molecularly, a clonal neoplastic one.
This entry is separate from `Systemic_Mastocytosis` on a decision the knowledge base has
already recorded rather than one made here. `kb/groupings/Mastocytosis.yaml` states that
MMAS "carries a clonal KIT D816V mast cell population but does not meet the mast cell
burden that the proliferation and mastocytosis-finding operands require", and its
membership criteria deliberately exclude it. The nosology agrees: the ECNM-AIM
classification separates confirmed MCAS from mast cell activation disorder not fulfilling
MCAS criteria, and places clonal and non-clonal MCAS in different categories.
Two boundaries move under this entry and curators should know it. The 2021 refined
criteria for bone marrow mastocytosis carve out a variant that is older, more male, has
lower tryptase and lower neoplastic mast cell burden, and has *more* hymenoptera-triggered
allergic reactions than typical indolent systemic mastocytosis - which is the same
clinical description MMAS was built from, on the other side of the SM threshold. And MCAS
as a category has never been uncontroversial; the 2010 criteria paper says so in its own
abstract. Neither is a reason to avoid the entry, but both mean its extension depends on
which criteria set is in force, and no prevalence figure here should be read as stable.
disease_term:
preferred_term: monoclonal mast cell activation syndrome
term:
id: MONDO:0033954
label: monoclonal mast cell activation syndrome
parents:
- Mast cell activation syndrome
genetic:
- name: KIT
gene_term:
preferred_term: KIT
term:
id: hgnc:6342
label: KIT
association: CAUSATIVE
relationship_type: SOMATIC_DRIVER
variant_origin: SOMATIC
features: >
Somatic activating KIT mutation, characteristically D816V, in the mast cell compartment.
Where present it is the same driver as in systemic mastocytosis, and what differs is
burden rather than the variant. Demonstrating it requires a sensitive assay on bone
marrow, because the clonal population is small by definition, which is why the diagnosis
cannot be made on peripheral blood or on symptoms.
It is not universal in this disorder, and that distinguishes it from systemic
mastocytosis. In a 38-patient diagnostic work-up, KIT D816V was found in every SM patient
but in only two of four with MMAS, at mutated-cell fractions as low as 0.007 per cent -
so half of that MMAS group were established as clonal on aberrant CD25 expression alone.
Whether that reflects genuinely KIT-negative clonal disease or assay sensitivity at very
low burden is not resolved by the sources here.
evidence:
- reference: PMID:20434205
reference_title: "Clinical, biological, and molecular characteristics of clonal mast cell disorders presenting with systemic mast cell activation symptoms."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Patients showing clonal BM MCs were grouped into indolent systemic mastocytosis without skin lesions (ISMs(-); n = 48) and other c-MCADs (n = 3)-both with CD25(++) BM MCs and either positive mast/stem cell growth factor receptor gene (KIT) mutation or clonal human androgen receptor assay (HUMARA) tests-and nc-MCAD (CD25-negative BM MCs in the absence of KIT mutation; n = 32) and compared for their clinical, biological, and molecular characteristics."
explanation: >-
Defines the clonality criteria this entry rests on, and shows the clonal-but-not-SM group
is small even within a referred cohort: three of eighty-three patients.
- reference: PMID:28262030
reference_title: "Highly sensitive assays are mandatory for the differential diagnosis of patients presenting with symptoms of mast cell activation: diagnostic work-up of 38 patients."
supports: REFUTE
evidence_source: HUMAN_CLINICAL
snippet: "In bone marrow, the KIT D816V mutation was detected in all SM patients but in only 2 patients with MMAS (range: 0.007-9% mutated cells)."
explanation: >-
Graded REFUTE against treating KIT D816V as universal in this disorder: it was found in
every systemic mastocytosis patient but in only two of four with MMAS, the rest being
established as clonal on CD25 expression alone. This is the source for the 38-patient
figures in features above.
- reference: PMID:34298172
reference_title: "Pathogenic and diagnostic relevance of KIT in primary mast cell activation disorders."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Here, we review the relevant aspects related to the pathogenesis of MCAS, with special emphasis on the prevalence and diagnostic relevance of KIT mutations."
explanation: >-
Establishes KIT mutation status as the pathogenetically and diagnostically relevant axis
in mast cell activation syndromes.
pathophysiology:
- name: Somatic Activating KIT Mutation in a Small Mast Cell Clone
biological_scale: MOLECULAR
description: >
An activating KIT variant, characteristically D816V, arises somatically in the mast
cell lineage. The variant is the same one that drives systemic mastocytosis, and the
distinguishing feature of this disorder is quantitative: the clone stays below the mast
cell burden that the systemic mastocytosis criteria require. Nothing about the variant
itself is different, which is why the entry does not claim a distinct molecular lesion.
genetic_context:
gene:
preferred_term: KIT
term:
id: hgnc:6342
label: KIT
allele_type: SNV
variant_origin: SOMATIC
functional_impact_category: GAIN_OF_FUNCTION
description: >-
Somatic gain-of-function KIT variant, characteristically D816V, restricted to the mast
cell compartment.
molecular_functions:
- preferred_term: protein tyrosine kinase activity
term:
id: GO:0004713
label: protein tyrosine kinase activity
modifier: GAIN_OF_FUNCTION
downstream:
- target: Constitutive KIT Downstream Signaling
causal_link_type: DIRECT
evidence:
- reference: PMID:30948489
reference_title: "Oncogenic D816V-KIT signaling in mast cells causes persistent IL-6 production."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "Furthermore, mast cell lines expressing D816V-KIT, but not those expressing normal KIT or other KIT variants, produced constitutively high IL-6 amounts at the message and protein levels."
explanation: >-
Shows the D816V variant specifically, and not normal KIT or other KIT variants, produces
constitutive downstream output, which is the causal step this edge asserts.
evidence:
- reference: PMID:30948489
reference_title: "Oncogenic D816V-KIT signaling in mast cells causes persistent IL-6 production."
supports: SUPPORT
directness: INDIRECT
evidence_source: HUMAN_CLINICAL
snippet: "Since systemic mastocytosis often associates with the presence in hematopoietic cells of a somatic gain-of-function variant in KIT, D816V-KIT, we examined its potential role in IL-6 upregulation."
explanation: >-
Establishes D816V-KIT as a somatic gain-of-function variant in haematopoietic cells.
Graded INDIRECT because the framing is systemic mastocytosis rather than MMAS, and the
shared driver is the point being relied on.
- name: Constitutive KIT Downstream Signaling
biological_scale: CELLULAR
description: >
D816V-KIT signals constitutively through STAT5 and PI3K, with STAT5A and STAT5B
activation mediated by JAK2 and also by MEK and ERK1/2, the latter driving both STAT5
phosphorylation and its long-term transcription. STAT3 and STAT4 are not involved. The
functional consequence demonstrated in this pathway is persistent IL-6 production by
the mast cell itself, which links the clone to a systemic inflammatory output
independent of degranulation.
cell_types:
- preferred_term: mast cell
term:
id: CL:0000097
label: mast cell
downstream:
- target: Lowered Mast Cell Activation Threshold
causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
evidence:
- reference: PMID:30948489
reference_title: "Oncogenic D816V-KIT signaling in mast cells causes persistent IL-6 production."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "We further demonstrate that aberrant KIT activity and signaling are critical for the induction of IL-6 and involve STAT5 and PI3K pathways but not STAT3 or STAT4."
explanation: >-
Identifies the pathways carrying the constitutive signal, and excludes two others.
- reference: PMID:30948489
reference_title: "Oncogenic D816V-KIT signaling in mast cells causes persistent IL-6 production."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: "Activation of STAT5A and STAT5B downstream of D816V-KIT was mediated by JAK2 but also by MEK/ERK1/2, which not only promoted STAT5 phosphorylation but also its long-term transcription."
explanation: >-
Details the kinases through which the constitutive signal reaches STAT5.
- name: Lowered Mast Cell Activation Threshold
biological_scale: CELLULAR
description: >
The clonal mast cells activate and degranulate on stimuli that would not trigger normal
mast cells, which is what converts a small clone into a systemic clinical syndrome. This
is the node the entry is least able to source directly: the concept was articulated at
the consensus conference that classified systemic mastocytosis variants, as a
hypothesis, and the evidence curated here is that statement of the hypothesis rather
than a measurement in MMAS patients. The `directness: INDIRECT` grading and this note
are the honest description of its status.
cell_types:
- preferred_term: mast cell
term:
id: CL:0000097
label: mast cell
biological_processes:
- preferred_term: mast cell activation
term:
id: GO:0045576
label: mast cell activation
modifier: INCREASED
downstream:
- target: Systemic Mast Cell Mediator Release
causal_link_type: DIRECT
evidence:
- reference: PMID:21035176
reference_title: "Mast cell activation syndrome: Proposed diagnostic criteria."
supports: SUPPORT
directness: INDIRECT
evidence_source: OTHER
snippet: "There also may be unknown diseases of mast cell activation, in which mast cells either activate at a lower threshold of stimulation, or perhaps resist the usual stimuli to degranulate. The explanation for why some patients would have unexplained flushing or anaphylaxis would then be an abnormal sensitivity of their mast cells to activation"
explanation: >-
The consensus-conference formulation of the lowered-threshold hypothesis, quoted in a
criteria paper. Graded INDIRECT and evidence_source OTHER because it is expert reasoning
about a then-hypothetical disease class, not a measurement; it is curated because it is
the origin of this node's claim rather than confirmation of it.
- name: Systemic Mast Cell Mediator Release
biological_scale: ORGANISM
description: >
Degranulation releases histamine, tryptase and other mediators, producing severe,
recurrent, systemic symptoms across skin, gastrointestinal tract and cardiovascular
system. In clonal disease the episodes have a characteristic shape - cardiovascular
collapse predominating, frequently without urticaria - which is what makes hymenoptera
anaphylaxis in a patient with no skin lesions the classic presentation.
cell_types:
- preferred_term: mast cell
term:
id: CL:0000097
label: mast cell
biological_processes:
- preferred_term: mast cell degranulation
term:
id: GO:0043303
label: mast cell degranulation
modifier: INCREASED
downstream:
- target: Anaphylaxis
causal_link_type: DIRECT
- target: Flushing
causal_link_type: DIRECT
- target: Hypotension
causal_link_type: DIRECT
evidence:
- reference: PMID:28740494
reference_title: "Insights in Anaphylaxis and Clonal Mast Cell Disorders."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Although the release of mast cell (MC) mediators upon MC activation might present with a wide variety of symptoms, particular clinical features typically characterize MC mediator release episodes in patients with clonal MCD without skin involvement."
explanation: >-
Establishes that mediator release episodes in clonal disease without skin involvement have
a characteristic clinical shape, which is the claim this node makes.
- reference: PMID:35623575
reference_title: "Global Classification of Mast Cell Activation Disorders: An ICD-10-CM-Adjusted Proposal of the ECNM-AIM Consortium."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "When the symptoms are severe, systemic, and recurrent, and accompanied by a diagnostic increase in the serum tryptase level or other mast cell mediators, an MCA syndrome (MCAS) may be diagnosed."
explanation: >-
States the severity, distribution, recurrence and mediator-rise pattern that defines the
syndrome and that this node produces.
- name: Sub-threshold Clonal Burden
biological_scale: TISSUE
description: >
The clonal population does not reach the mast cell burden required by the systemic
mastocytosis criteria. This node records the defining negative, and it is the reason the
entry exists: the disorder is constituted by failing a threshold, so the negative is
part of the mechanism rather than a caveat about it. It has no downstream edges by
design. Note that where the threshold sits has moved - the 2021 refined bone marrow
mastocytosis criteria describe an SM variant with lower neoplastic mast cell burden and
more hymenoptera-triggered reactions than typical indolent systemic mastocytosis - so
this node's extension depends on which criteria set is applied.
cell_types:
- preferred_term: mast cell
term:
id: CL:0000097
label: mast cell
evidence:
- reference: PMID:35623575
reference_title: "Global Classification of Mast Cell Activation Disorders: An ICD-10-CM-Adjusted Proposal of the ECNM-AIM Consortium."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "In this article, we discuss diagnostic features and criteria and propose a ICD-10-CM-adjusted classification for disorders associated with MCA, herein referred to as MCA disorders (MCADs), with special emphasis on the delineation between confirmed MCAS, MCAD not fulfilling MCAS criteria, and suspected MCAD that is not present."
explanation: >-
Establishes that the classification turns on which criteria are and are not fulfilled,
which is the threshold logic this node encodes.
- reference: PMID:28262030
reference_title: "Highly sensitive assays are mandatory for the differential diagnosis of patients presenting with symptoms of mast cell activation: diagnostic work-up of 38 patients."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Flow cytometric analysis of bone marrow showed CD25 expression of MCs in all patients with SM and MMAS (range: 0.002-0.3% of cells)."
explanation: >-
The only direct measurement of the burden this node names, made in MMAS patients rather
than in a neighbouring category: aberrant CD25-expressing mast cells are 0.002 to 0.3 per
cent of bone marrow cells.
- reference: PMID:34545185
reference_title: "Refined diagnostic criteria for bone marrow mastocytosis: a proposal of the European competence network on mastocytosis."
supports: SUPPORT
directness: INDIRECT
evidence_source: HUMAN_CLINICAL
snippet: "BMM patients were significantly older, predominantly male, had lower tryptase and lower burden of neoplastic mast cells, and displayed a higher frequency of allergic reactions, mainly triggered by Hymenoptera, than patients with typical ISM."
explanation: >-
Describes an SM variant with low mast cell burden and hymenoptera-triggered reactions,
which is the moving boundary this node sits against. Graded INDIRECT because the cohort is
bone marrow mastocytosis, that is patients who do meet SM criteria.
phenotypes:
- category: Immunologic
name: Anaphylaxis
description: >
Severe recurrent anaphylaxis, frequently hymenoptera-venom triggered and frequently
without urticaria, and commonly the presenting event. Anaphylaxis prevalence in clonal
mast cell disease is clearly higher than in the general population.
phenotype_term:
preferred_term: Anaphylaxis
term:
id: HP:0100845
label: Anaphylactic shock
temporality: RECURRENT
frequency: VERY_FREQUENT
diagnostic: true
evidence:
- reference: PMID:28740494
reference_title: "Insights in Anaphylaxis and Clonal Mast Cell Disorders."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The prevalence of anaphylaxis among patients with clonal mast cell disorders (MCD) is clearly higher comparing to the general population."
explanation: >-
Establishes raised anaphylaxis prevalence in clonal mast cell disease, the group this
entry belongs to.
- reference: PMID:28740494
reference_title: "Insights in Anaphylaxis and Clonal Mast Cell Disorders."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Due to a lower frequency of symptoms outside of acute episodes, clonal MCD in the absence of skin lesions might sometimes be difficult to identify which may lead to underdiagnosis, and anaphylaxis is commonly the presenting symptom in these patients."
explanation: >-
States that anaphylaxis is commonly the presenting symptom and that the disorder is
underdiagnosed for lack of interval symptoms.
- category: Cardiovascular
name: Hypotension
description: >
Hypotension during mediator release episodes, part of the cardiovascular-predominant
episode pattern that characterises clonal disease without skin involvement.
phenotype_term:
preferred_term: Hypotension
term:
id: HP:0002615
label: Hypotension
temporality: RECURRENT
evidence:
- reference: PMID:28740494
reference_title: "Insights in Anaphylaxis and Clonal Mast Cell Disorders."
supports: SUPPORT
directness: INDIRECT
evidence_source: HUMAN_CLINICAL
snippet: "Although the release of mast cell (MC) mediators upon MC activation might present with a wide variety of symptoms, particular clinical features typically characterize MC mediator release episodes in patients with clonal MCD without skin involvement."
explanation: >-
Supports a characteristic episode phenotype in clonal disease without skin involvement.
Graded INDIRECT because the abstract does not itemise which features, so the specific
attribution of hypotension rests on the anaphylaxis phenotype above.
- category: Dermatologic
name: Flushing
phenotype_term:
preferred_term: Flushing
term:
id: HP:0031284
label: Flushing
temporality: RECURRENT
evidence:
- reference: PMID:23179866
reference_title: "Mast cell activation syndrome: a review."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Earlier proposed criteria for the diagnosis of MCAS included episodic symptoms consistent with mast cell mediator release affecting two or more organ systems with urticaria, angioedema, flushing, nausea, vomiting, diarrhea, abdominal cramping, hypotensive syncope or near syncope, tachycardia, wheezing, conjunctival injection, pruritus, and nasal stuffiness."
explanation: >-
Lists flushing among the episodic mediator-release symptoms in the diagnostic criteria.
- category: Gastrointestinal
name: Diarrhea
phenotype_term:
preferred_term: Diarrhea
term:
id: HP:0002014
label: Diarrhea
temporality: RECURRENT
evidence:
- reference: PMID:23179866
reference_title: "Mast cell activation syndrome: a review."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Earlier proposed criteria for the diagnosis of MCAS included episodic symptoms consistent with mast cell mediator release affecting two or more organ systems with urticaria, angioedema, flushing, nausea, vomiting, diarrhea, abdominal cramping, hypotensive syncope or near syncope, tachycardia, wheezing, conjunctival injection, pruritus, and nasal stuffiness."
explanation: >-
Lists diarrhoea among the episodic mediator-release symptoms in the diagnostic criteria.
Re-sourced from the criteria list rather than from a sentence describing how the label is
applied, which is a different claim.
- category: Gastrointestinal
name: Abdominal pain
phenotype_term:
preferred_term: Abdominal pain
term:
id: HP:0002027
label: Abdominal pain
temporality: RECURRENT
evidence:
- reference: PMID:23179866
reference_title: "Mast cell activation syndrome: a review."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Earlier proposed criteria for the diagnosis of MCAS included episodic symptoms consistent with mast cell mediator release affecting two or more organ systems with urticaria, angioedema, flushing, nausea, vomiting, diarrhea, abdominal cramping, hypotensive syncope or near syncope, tachycardia, wheezing, conjunctival injection, pruritus, and nasal stuffiness."
explanation: >-
Lists abdominal cramping among the episodic mediator-release symptoms in the diagnostic
criteria. Re-sourced for the same reason as diarrhoea.
- category: Dermatologic
name: Absence of skin lesions
description: >
The absence of cutaneous mastocytosis lesions is part of the diagnostic setting rather
than a symptom, and it is what makes the disorder hard to find: patients have few
symptoms between episodes and no skin sign to prompt investigation. Curated because the
negative carries diagnostic weight, left unbound, and deliberately so: see the note below.
phenotype_term:
preferred_term: absence of cutaneous mastocytosis lesions
evidence:
- reference: PMID:20434205
reference_title: "Clinical, biological, and molecular characteristics of clonal mast cell disorders presenting with systemic mast cell activation symptoms."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "To analyze the clinical, biological, and molecular characteristics of adult patients presenting with systemic MC activation symptoms/anaphylaxis in the absence of skin mastocytosis who showed clonal (c) versus nonclonal (nc) MCs and to provide indication criteria for bone marrow (BM) studies."
explanation: >-
Establishes absence of skin mastocytosis as the defining clinical setting of the cohort in
which clonal and non-clonal disease were separated.
biochemical:
- name: Serum baseline tryptase
notes: >
Raised serum baseline tryptase is the mediator marker that prompts investigation and is
part of the diagnostic increase required for a mast cell activation syndrome diagnosis.
It is also the axis on which the moving SM boundary is drawn: the refined bone marrow
mastocytosis criteria use a tryptase level below 125 ng/mL as one of their defining
features, so tryptase does double duty as a mediator readout and as a burden proxy.
biomarker_term:
preferred_term: Elevated total serum tryptase
term:
id: HP:0031901
label: Elevated total serum tryptase
presence: INCREASED
readouts:
- target: Systemic Mast Cell Mediator Release
relationship: READOUT_OF
direction: POSITIVE
endpoint_context: DIAGNOSTIC
interpretation: >-
Raised baseline tryptase reports the mast cell mediator burden and is part of the
diagnostic increase the syndrome definition requires.
evidence:
- reference: PMID:20434205
reference_title: "Clinical, biological, and molecular characteristics of clonal mast cell disorders presenting with systemic mast cell activation symptoms."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Systemic mast cell activation disorders (MCADs) are characterized by severe and systemic mast cell (MC) mediators-related symptoms frequently associated with increased serum baseline tryptase (sBt)."
explanation: >-
Establishes raised serum baseline tryptase as a characteristic feature of systemic mast
cell activation disorders.
- reference: PMID:34545185
reference_title: "Refined diagnostic criteria for bone marrow mastocytosis: a proposal of the European competence network on mastocytosis."
supports: SUPPORT
directness: INDIRECT
evidence_source: HUMAN_CLINICAL
snippet: "These data support the proposal to define BMM as a separate SM variant characterized by SM criteria, absence of skin lesions, absence of B-Findings, and tryptase levels <125 ng/mL."
explanation: >-
Shows tryptase being used as a criteria threshold in the neighbouring category. Graded
INDIRECT because the criteria are for bone marrow mastocytosis, not for this disorder.
treatments:
- name: Epinephrine for Acute Anaphylaxis
description: >
First-line treatment of the acute episode, and given the frequency of hymenoptera-venom
anaphylaxis as the presenting event, self-injectable epinephrine is the practical
mainstay. No trial evidence specific to this disorder exists; this is standard
anaphylaxis management applied to a population with an unusually high anaphylaxis rate.
treatment_term:
preferred_term: Pharmacotherapy
term:
id: NCIT:C15986
label: Pharmacotherapy
therapeutic_agent:
- preferred_term: epinephrine
term:
id: CHEBI:33568
label: adrenaline
therapeutic_modality: SMALL_MOLECULE
target_phenotypes:
- preferred_term: Anaphylaxis
term:
id: HP:0100845
label: Anaphylactic shock
target_mechanisms:
- target: Systemic Mast Cell Mediator Release
description: >-
Counteracts the haemodynamic and airway effects of released mediators. It does not act
on the clone or on the activation threshold.
evidence:
- reference: PMID:28740494
reference_title: "Insights in Anaphylaxis and Clonal Mast Cell Disorders."
supports: SUPPORT
directness: INDIRECT
evidence_source: HUMAN_CLINICAL
snippet: "In this article, we address the main triggers for anaphylaxis, risk factors, clinical presentation, diagnosis, and management of patients with MC activation syndromes (MCASs), with special emphasis on clonal MCAS [systemic mastocytosis and mono(clonal) MC activations syndromes]."
explanation: >-
Establishes that management of anaphylaxis in monoclonal mast cell activation syndrome is
an addressed clinical topic. Graded INDIRECT because the abstract states the scope rather
than the specific recommendation, so this citation supports the treatment being relevant
rather than reporting its effect.
- name: Hymenoptera Venom Immunotherapy
description: >
The most disorder-specific management point here, and the reason establishing clonality
changes care rather than only classification. Patients with venom allergy and a clonal mast
cell disorder are treated with immunotherapy against the venom they are sensitised to, and
for this high-risk subgroup the recommendation is to continue it beyond five years or
indefinitely and to carry at least three epinephrine autoinjectors rather than one. Both
departures from standard venom-allergy practice follow from the clonal diagnosis.
treatment_term:
preferred_term: Immunotherapy
term:
id: NCIT:C15262
label: Immunotherapy
therapeutic_modality: OTHER
target_mechanisms:
- target: Lowered Mast Cell Activation Threshold
description: >-
Raises the threshold for venom-triggered degranulation by inducing tolerance to the
sensitising allergen. It does not act on the clone.
evidence:
- reference: PMID:39187156
reference_title: "Mast Cell Disorders and Hymenoptera Venom-Triggered Anaphylaxis: Evaluation and Management."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "For this high-risk subgroup of patients with HVA, it is recommended to continue immunotherapy for more than 5 years or indefinitely and to carry at least three epinephrine autoinjectors."
explanation: >-
States both the indefinite duration and the three-autoinjector recommendation that
distinguish management in clonal disease from standard venom-allergy care.
- reference: PMID:39187156
reference_title: "Mast Cell Disorders and Hymenoptera Venom-Triggered Anaphylaxis: Evaluation and Management."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Patients with HVA and a clonal MCD should be treated with immunotherapy directed against the Hymenoptera venom for which they are sensitized."
explanation: >-
States the indication for venom immunotherapy in clonal mast cell disease specifically.
- name: Anti-mediator Therapy
description: >
H1 and H2 histamine receptor antagonists, anti-leukotrienes and mast cell stabilisers such
as cromolyn, used to suppress mediator production or block mediator effects between and
during episodes. Response to these agents is not merely therapeutic: a decrease in
frequency or severity of symptoms on anti-mediator therapy is itself one of the diagnostic
criteria for the syndrome, so treatment and diagnosis are entangled here in a way they are
not in most entries.
treatment_term:
preferred_term: Pharmacotherapy
term:
id: NCIT:C15986
label: Pharmacotherapy
therapeutic_modality: SMALL_MOLECULE
target_mechanisms:
- target: Systemic Mast Cell Mediator Release
description: >-
Blocks mediator receptors or stabilises the mast cell against degranulation. It does not
act on the clone.
evidence:
- reference: PMID:23179866
reference_title: "Mast cell activation syndrome: a review."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Other criteria included a decrease in the frequency, severity, or resolution of symptoms with anti-mediator therapy including H(1) and H(2)histamine receptor antagonists, anti-leukotrienes, or mast cell stabilizers."
explanation: >-
Names the anti-mediator agents and records that response to them is part of the diagnostic
criteria, which is the entanglement this treatment description notes.
- reference: PMID:35623575
reference_title: "Global Classification of Mast Cell Activation Disorders: An ICD-10-CM-Adjusted Proposal of the ECNM-AIM Consortium."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "In these patients, the symptoms typically respond to drugs suppressing MCA, mediator production in mast cells, or mediator effects."
explanation: >-
Confirms symptomatic response to the three classes of mediator-directed drug.
diagnosis:
- name: Bone marrow study for clonality assessment
description: >
The step the diagnosis depends on. Establishing clonality moves a patient from idiopathic
anaphylaxis to a clonal mast cell disorder with different surveillance, and it cannot be
done on peripheral blood or on symptoms. The recommendation is that it be performed in
reference centres. A sensitive assay is needed because the clone is small by definition.
markers: KIT D816V, aberrant CD25 or CD2 expression, or a clonal HUMARA result, on bone marrow mast cells
evidence:
- reference: PMID:28740494
reference_title: "Insights in Anaphylaxis and Clonal Mast Cell Disorders."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Final diagnosis requires a bone marrow study, and it is recommended that this should be done in reference centers."
explanation: >-
States that bone marrow study is required for diagnosis and should be done in reference
centres.
- reference: PMID:20434205
reference_title: "Clinical, biological, and molecular characteristics of clonal mast cell disorders presenting with systemic mast cell activation symptoms."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "To analyze the clinical, biological, and molecular characteristics of adult patients presenting with systemic MC activation symptoms/anaphylaxis in the absence of skin mastocytosis who showed clonal (c) versus nonclonal (nc) MCs and to provide indication criteria for bone marrow (BM) studies."
explanation: >-
Establishes that indication criteria for bone marrow study in this population were the
explicit object of the defining cohort.
- name: Red Espanola de Mastocitosis (REMA) score
description: >
The instrument that decides who is sent for a bone marrow biopsy, calculated from
anaphylaxis clinical features, baseline serum tryptase and sex. It sits upstream of the
clonality assessment: the recommendation is biopsy at a REMA score of 2 or more, or with
monomorphic maculopapular cutaneous mastocytosis lesions, or an elevated baseline tryptase
given the patient's tryptase genotype. Note the genotype step, which exists because
hereditary alpha-tryptasemia raises baseline tryptase independently of any clone and would
otherwise send the wrong patients to biopsy.
markers: anaphylaxis clinical features, baseline serum tryptase, tryptase genotype, sex
evidence:
- reference: PMID:39187156
reference_title: "Mast Cell Disorders and Hymenoptera Venom-Triggered Anaphylaxis: Evaluation and Management."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "A bone marrow biopsy should be performed in patients with monomorphic maculopapular cutaneous mastocytosis, a Red Española de Mastocitosis score of 2 or greater, or an elevated BST based on tryptase genotype."
explanation: >-
States the biopsy threshold this record captures, including the tryptase-genotype
qualifier.
- reference: PMID:39187156
reference_title: "Mast Cell Disorders and Hymenoptera Venom-Triggered Anaphylaxis: Evaluation and Management."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "The pretest probability of clonal MCD can be assessed in a stepwise fashion starting with examination of the skin for typical monomorphic maculopapular cutaneous mastocytosis lesions; measurement of the baseline serum tryptase (BST) and tryptase genotyping for patients with BST greater than 11 ng/mL; followed by the Red Española de Mastocitosis score, which is calculated using anaphylaxis clinical features, BST, and the patient's sex."
explanation: >-
Gives the score's components and its place in the stepwise pretest-probability assessment.
- name: Mast cell activation syndrome mediator criterion
description: >
Diagnosis of the syndrome layer requires severe, systemic and recurrent symptoms
accompanied by a diagnostic rise in serum tryptase or another mast cell mediator, and
response to drugs suppressing mast cell activation or mediator effects. The monoclonal
qualifier is then added by the clonality finding above. Note that consensus criteria must
be fulfilled for the syndrome diagnosis at all, which is what makes this disorder a
conjunction of two separate criteria sets rather than one.
evidence:
- reference: PMID:35623575
reference_title: "Global Classification of Mast Cell Activation Disorders: An ICD-10-CM-Adjusted Proposal of the ECNM-AIM Consortium."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "When the symptoms are severe, systemic, and recurrent, and accompanied by a diagnostic increase in the serum tryptase level or other mast cell mediators, an MCA syndrome (MCAS) may be diagnosed."
explanation: >-
States the mediator-rise criterion this diagnosis record captures.
- reference: PMID:35623575
reference_title: "Global Classification of Mast Cell Activation Disorders: An ICD-10-CM-Adjusted Proposal of the ECNM-AIM Consortium."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "In each case, diagnostic consensus criteria must be fulfilled to diagnose MCAS."
explanation: >-
Establishes that the syndrome diagnosis is criteria-gated, which is what makes this
disorder a conjunction of a syndrome criterion and a clonality criterion.
definitions:
- name: Clonality-based separation of monoclonal from non-clonal mast cell activation syndrome
definition_type: DIAGNOSTIC_CRITERIA
derivation_basis: ESTABLISHED_CRITERIA
attaches_to:
- pathophysiology#Sub-threshold Clonal Burden
- genetic#KIT
description: >
The operative distinction is made on bone marrow mast cells: aberrant CD25 expression
together with either a KIT mutation or a clonal human androgen receptor assay result
defines clonal disease, and CD25-negative mast cells without a KIT mutation define
non-clonal disease. Monoclonal mast cell activation syndrome is then the clonal group
that does not meet systemic mastocytosis criteria. Note the practical asymmetry: in the
cohort this criterion set comes from, the clonal group was dominated by indolent
systemic mastocytosis without skin lesions, and only three of eighty-three patients fell
into the clonal-but-not-SM category. So applying the criterion mostly reclassifies
patients as systemic mastocytosis, and this diagnosis is what remains.
validation_status:
status: UNVALIDATED
rationale: >-
The criteria are consensus- and cohort-derived and are in clinical use, but no diagnostic
accuracy study establishes operating characteristics for the monoclonal category
specifically, and the neighbouring systemic mastocytosis threshold has been redefined
since, which moves this category's boundary without revalidating it.
evidence:
- reference: PMID:20434205
reference_title: "Clinical, biological, and molecular characteristics of clonal mast cell disorders presenting with systemic mast cell activation symptoms."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Patients showing clonal BM MCs were grouped into indolent systemic mastocytosis without skin lesions (ISMs(-); n = 48) and other c-MCADs (n = 3)-both with CD25(++) BM MCs and either positive mast/stem cell growth factor receptor gene (KIT) mutation or clonal human androgen receptor assay (HUMARA) tests-and nc-MCAD (CD25-negative BM MCs in the absence of KIT mutation; n = 32) and compared for their clinical, biological, and molecular characteristics."
explanation: >-
States the clonality criteria and the group sizes this definition records.
discussions:
- discussion_id: mmas_boundary_moves_with_sm_criteria
kind: KNOWLEDGE_GAP
prompt: >-
How much of what was called monoclonal mast cell activation syndrome is now bone marrow
mastocytosis, and does the category retain a distinct population?
attaches_to:
- pathophysiology#Sub-threshold Clonal Burden
- definitions#Clonality-based separation of monoclonal from non-clonal mast cell activation syndrome
rationale: >
This entity is defined by failing the systemic mastocytosis criteria, so redefining those
criteria redefines it, without anybody restudying it. The 2021 refined criteria carve out
bone marrow mastocytosis as an SM variant that is older, more male, has lower tryptase and
lower neoplastic mast cell burden, and has more hymenoptera-triggered allergic reactions
than typical indolent systemic mastocytosis. That is the same clinical description this
category was assembled from, on the other side of the threshold. No study has
reclassified an MMAS cohort under the refined criteria to see what is left, and the
original cohort had three such patients, so the question is whether the category survives
as a population or only as a residual definition. Until that is answered, no prevalence or
frequency figure for this entry is stable.
- discussion_id: threshold_hypothesis_unmeasured
kind: KNOWLEDGE_GAP
prompt: >-
Has a lowered mast cell activation threshold ever been measured in monoclonal mast cell
activation syndrome patients, as opposed to inferred?
attaches_to:
- pathophysiology#Lowered Mast Cell Activation Threshold
rationale: >
The lowered-threshold model is the link between a very small clone and a systemic clinical
syndrome, and it is the load-bearing step in the mechanism. Its origin is a
consensus-conference conjecture about a then-hypothetical class of disease, quoted in the
2010 criteria paper, and the sources curated here contain no measurement of activation
threshold in patients with this diagnosis. The alternative account - that mediator output
scales with clone size and the syndrome is simply the low end of systemic mastocytosis -
is not excluded by anything cited, and would predict that MMAS and low-burden systemic
mastocytosis are one continuum, which is what the boundary question above asks from the
other direction.
- discussion_id: mcas_category_contested
kind: KNOWLEDGE_GAP
prompt: >-
Is mast cell activation syndrome, as a clinical entity, established enough for its
monoclonal subset to be a stable disease concept?
attaches_to:
- disease#Monoclonal Mast Cell Activation Syndrome
rationale: >
The 2010 criteria paper states in its own abstract that MCAS as a distinct clinical entity
has not been generally accepted and that definitive diagnostic criteria do not exist, and
it proposes criteria explicitly as a basis for further study and validation. The 2022
ECNM-AIM classification is a substantial advance but is itself framed as a proposal and
notes that validated diagnostic criteria for implicating suspected mast cell activation are
lacking. The monoclonal subset is the best-defined part of that landscape, because clonality
is a demonstrable laboratory fact rather than a symptom pattern, which is why this entry
curates it rather than the broader category. The contestedness is recorded because it bounds
how much weight any epidemiology or phenotype frequency here can carry.
prevalence:
- population: Adults with systemic mast cell activation symptoms or anaphylaxis and no skin mastocytosis, referred for bone marrow study
measure_type: POINT_PREVALENCE
prevalence_class: UNKNOWN
notes: >-
Three of 83 such patients fell into the clonal-but-not-systemic-mastocytosis category; 48
had indolent systemic mastocytosis without skin lesions and 32 were non-clonal. This is a
proportion within a highly selected referred population, not a population prevalence, and
it predates the refined bone marrow mastocytosis criteria that have since moved the
boundary. prevalence_class is UNKNOWN deliberately: no population figure exists and this
proportion should not be converted into one.
evidence:
- reference: PMID:20434205
reference_title: "Clinical, biological, and molecular characteristics of clonal mast cell disorders presenting with systemic mast cell activation symptoms."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Patients showing clonal BM MCs were grouped into indolent systemic mastocytosis without skin lesions (ISMs(-); n = 48) and other c-MCADs (n = 3)-both with CD25(++) BM MCs and either positive mast/stem cell growth factor receptor gene (KIT) mutation or clonal human androgen receptor assay (HUMARA) tests-and nc-MCAD (CD25-negative BM MCs in the absence of KIT mutation; n = 32) and compared for their clinical, biological, and molecular characteristics."
explanation: >-
Gives the numerator and denominator behind this proportion, and the composition of the
cohort it comes from.
Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.
Create: Monoclonal Mast Cell Activation Syndrome · 2026-09-03T16:23:23Z · View source
De novo curation of monoclonal mast cell activation syndrome (MONDO:0033954) as a Disease entry. entry_type DISEASE, and the decision was already half-made in the repository: kb/groupings/Mastocytosis.yaml states in its grouping_rationale that MMAS 'carries a clonal KIT D816V mast cell population but does not meet the mast cell burden that the proliferation and mastocytosis-finding operands require', and its NECESSARY_AND_SUFFICIENT criteria deliberately exclude it. So the KB had already ruled MMAS is not a mastocytosis and is not a member of that grouping, which is the argument for a separate entry rather than a has_subtypes line on Systemic_Mastocytosis. Process error worth recording: my first coverage check for mastocytosis in the KB used git grep piped through head, on an alphabetical file list, and I concluded from the truncated output that only Maculopapular_Cutaneous_Mastocytosis existed. Systemic_Mastocytosis.yaml and kb/groupings/Mastocytosis.yaml both exist and both bear directly on this lump/split call. Caught before writing YAML, but the near-miss was a truncated grep, not a hard question. Deep research: one openscientist run (research/Monoclonal_Mast_Cell_Activation_Syndrome-deep-research-openscientist.md, 23 citations). It shipped with no reference_validation or term_validation block, term validation having aborted on a 5-second EBI OLS read timeout on HP:0002018; this is dismech#10396 and four of the five runs in this batch failed the same way. The retro-fitted reference section is the cleanest of the five: 23/23 references resolved, 22 of 22 quoted claims found in source, 21 of 23 on topic, none off topic. Used from the report: PMID:20434205 (the 83-patient cohort that defines the clonality criteria and shows the clonal-but-not-SM group is three of eighty-three), PMID:34298172 (KIT prevalence and diagnostic relevance in MCAS), PMID:35623575 (ECNM-AIM ICD-10-CM classification and the delineation between confirmed MCAS and MCAD not fulfilling MCAS criteria), PMID:28740494 (anaphylaxis prevalence in clonal mast cell disease, underdiagnosis without skin lesions, bone marrow study requirement), PMID:34545185 (2021 refined bone marrow mastocytosis criteria, cited on the moving boundary), PMID:30948489 (D816V-KIT constitutive signalling through STAT5 and PI3K via JAK2 and MEK/ERK1/2, and persistent IL-6 production), and PMID:21035176 (the 2010 MCAS criteria proposal, cited both for the lowered-threshold hypothesis and for its own statement that MCAS is not generally accepted as a distinct entity). Evidence-grading decisions. Five items are graded directness INDIRECT with the reason stated in each explanation, mostly because the source cohort is systemic mastocytosis or bone marrow mastocytosis rather than MMAS and the shared driver or the adjacent threshold is what is being relied on. Three items use evidence_source OTHER because the 2010 criteria paper is expert consensus reasoning about a then-hypothetical disease class rather than a data report; the lowered-threshold node in particular is supported only by that consensus-conference conjecture, and both the node description and a KNOWLEDGE_GAP discussion say so. One evidence item was dropped rather than defended: a quote from PMID:28262030 that was the paper's title and would not verify because I had not fetched that reference. The claim it supported (assay sensitivity matters because the clone is small) follows from the definition, and the treatment description now says that rather than citing a source for it. Schema note: I initially wrote a pathophysiology downstream target pointing at 'Elevated total serum tryptase', which is a biochemical entry name and not a pathograph node, so check-causal-targets flagged it as a dangling target. Replaced with the documented pattern: a readouts block on the biochemical entry with target Systemic Mast Cell Mediator Release and relationship READOUT_OF. Three knowledge gaps recorded, all about the stability of the concept rather than about its mechanism: how much of MMAS the 2021 refined bone marrow mastocytosis criteria have absorbed and whether a distinct population remains; whether a lowered mast cell activation threshold has ever been measured in these patients rather than inferred, with the competing account that mediator output simply scales with clone size; and whether MCAS as a category is established enough for its monoclonal subset to be a stable disease concept. prevalence_class is UNKNOWN deliberately: only a proportion within a highly selected referred cohort exists, and it predates the criteria change. Validation: just validate passes; 24/24 snippets verified. check-duplicate-keys, check-entity-refs, check-causal-targets, check-qualifier-terms, check-enum-values, check-snippet-length, check-snippet-grading and check-title-snippets all pass.
Disease: Monoclonal mast cell activation syndrome (MMAS) MONDO ID: MONDO:0033954 Template category: Listed as "Mendelian," but see the important caveat in the Summary — the driver lesion is somatic, not germline.
Monoclonal mast cell activation syndrome (MMAS) is a rare, adult-onset, primary (clonal) mast cell activation syndrome (MCAS) in which patients experience recurrent, often severe, mast-cell mediator-release symptoms — most characteristically hypotensive anaphylaxis — together with laboratory demonstration of mast-cell clonality, but who do not fulfill the full World Health Organization (WHO) criteria for systemic mastocytosis (SM). Clonality is established by detection of the somatic KIT D816V gain-of-function mutation and/or aberrant mast-cell expression of CD25 (±CD2/CD30), while the patient carries fewer than the required number of minor SM criteria and lacks the major criterion (multifocal dense mast-cell aggregates in bone marrow) PMID: 28262030, PMID: 34298172.
Mechanistically, MMAS sits on the same biological continuum as SM but at the lowest clonal-burden end. An acquired KIT D816V lesion constitutively activates downstream STAT5, PI3K and MAPK signaling, driving persistent pro-inflammatory output (e.g., IL-6) and, critically, lowering the mast-cell activation threshold so that ordinary triggers — above all Hymenoptera (bee/wasp) venom — precipitate severe, cardiovascular-dominant, characteristically urticaria-poor anaphylaxis, predominantly in males PMID: 30948489, PMID: 20434205, PMID: 42542541. Because KIT D816V alone is insufficient for full neoplastic transformation, MMAS clones remain sub-threshold and rarely progress PMID: 34424959.
An important classification caveat: despite the "Mendelian" template label, MMAS is not an inherited Mendelian disease. The driver KIT D816V is a somatic (acquired) mutation restricted to the hematopoietic/mast-cell lineage. The one bona fide germline genetic contributor is hereditary alpha-tryptasemia (HAT) — increased TPSAB1 copy number — which is an autosomal-dominant modifier that amplifies anaphylaxis severity but does not cause MMAS PMID: 37818990, PMID: 41932753. Prognosis is favorable, with recurrent life-threatening anaphylaxis (rather than clonal progression) constituting the principal morbidity; management centers on anti-mediator therapy, anaphylaxis prevention (epinephrine plus venom immunotherapy), and osteoporosis surveillance, with selective KIT inhibitors reserved for refractory cases PMID: 40274818, PMID: 39187156.
Overview. MMAS is a subtype of primary (clonal) MCAS. MCAS as a whole is defined by episodic, multisystem mast-cell mediator-release symptoms, an objective transient rise in a validated mast-cell mediator (typically serum tryptase), and symptomatic response to mediator-targeting therapy. MCAS is divided into primary (monoclonal/clonal), secondary, and idiopathic forms. MMAS is the primary/monoclonal form in which clonal mast cells are demonstrable but do not meet the diagnostic bar for systemic mastocytosis PMID: 28262030, PMID: 34298172.
"These MC activation syndromes (MCAS) can be divided into primary (monoclonal) MCAS (MMAS) vs. secondary and idiopathic MCAS." — PMID: 28262030
"In contrast to clonal MCAS in which MCA is associated with a primary MC disorder (ie, primary MCAS) such as mastocytosis or monoclonal MCAS, nonclonal MCAS can be secondary to known or unidentified triggers." — PMID: 34298172
Key identifiers. - Mondo: MONDO:0033954 - ICD-10-CM: Mast cell activation disorders, including MMAS, have been assigned ICD-10-CM codes under the ECNM–AIM consortium global classification PMID: 35623575. - OMIM / Orphanet: No dedicated Mendelian OMIM phenotype entry exists for MMAS because the driver is somatic; the related entity systemic mastocytosis is catalogued separately. (Not applicable as an inherited-disease OMIM phenotype.) - MeSH: Best mapped under "Mastocytosis" / "Mast Cell Activation Syndrome" concepts.
"some of these conditions have recently been assigned to an International Classification of Diseases-10-Clinical Modification code (ICD-10-CM)." — PMID: 35623575
Synonyms / alternative names. Monoclonal MCAS; mono(clonal) mast cell activation syndrome; primary MCAS (non-mastocytosis clonal subtype); clonal MCAS without SM. In older literature it overlaps with "other clonal mast cell activation disorders (c-MCAD)" that do not meet WHO SM criteria PMID: 20434205.
Information source. Knowledge is derived from aggregated disease-level clinical cohorts and reference-center case series (e.g., REMA, ECNM registries; diagnostic work-up cohorts) rather than a single EHR or a Mendelian gene–disease catalogue.
Primary causal factor (genetic, somatic). MMAS is caused by an acquired, somatic gain-of-function point mutation in KIT, most commonly D816V (a substitution in codon 816 of exon 17), arising in the hematopoietic/mast-cell lineage. This is a driver of mast-cell clonality, not an inherited variant PMID: 28262030, PMID: 34298172.
Genetic risk / modifier factors. - Hereditary alpha-tryptasemia (HAT) — germline increased copy number of TPSAB1 (α-tryptase). HAT is autosomal dominant, present in ~4–6% of the general population, and is enriched among clonal and non-clonal MCAS and mastocytosis patients; it independently amplifies anaphylaxis severity PMID: 37818990, PMID: 41932753.
Environmental / triggering factors. MMAS itself is not caused by environmental exposures, but mediator-release episodes are triggered by: - Hymenoptera venom (bee/wasp stings) — the single most characteristic trigger PMID: 40641447, PMID: 39187156 - Idiopathic/allergen-induced triggers, drugs, physical stimuli, and other IgE-independent activators.
Demographic risk factors. Male sex and adult onset are associated with the clonal phenotype PMID: 20434205.
Protective factors. No specific genetic or environmental protective factors have been established for MMAS. (Data not available.)
Gene–environment interaction. The core gene–environment interaction is that the KIT D816V clone lowers the mast-cell activation threshold, so that an environmental trigger (venom) that would be benign in a normal individual produces severe anaphylaxis. Co-inherited HAT (germline TPSAB1 duplication) further potentiates this interaction PMID: 42542541, PMID: 41932753.
MMAS produces episodic, multisystem mediator-release symptoms. The full MCAS symptom spectrum spans skin, gastrointestinal, cardiovascular, respiratory and neurologic systems PMID: 25944644:
"episodic symptoms with mast cell mediators affecting two or more organ systems with urticaria, angioedema, flushing, nausea, vomiting, diarrhea, abdominal cramping, hypotensive syncope, tachycardia, wheezing, conjunctival injection, pruritus, nasal stuffiness." — PMID: 25944644
Distinctive MMAS phenotype. Clonal (monoclonal) MCAS characteristically skews toward isolated hypotensive/cardiovascular anaphylaxis and, importantly, lacks the mucocutaneous signs (urticaria/angioedema) that dominate idiopathic MCAS. In a 703-patient cohort, mucocutaneous symptoms were significantly less prevalent in clonal MCAS (P = .015) PMID: 38056692.
"these symptoms were less prevalent in patients with clonal MCAS (P = .015)." — PMID: 38056692
| Phenotype | Type | HPO suggestion | Characteristics in MMAS |
|---|---|---|---|
| Anaphylaxis (recurrent, severe) | Clinical sign / event | HP:0100845 (Anaphylaxis) | Adult-onset; episodic; often severe/life-threatening; principal morbidity |
| Hypotension / syncope / presyncope | Clinical sign | HP:0002615; HP:0001279 | Cardiovascular-dominant; predictive of clonality |
| Flushing | Symptom | HP:0031284 | Episodic |
| Absence of urticaria/angioedema | Distinguishing feature | (absence of HP:0200025 / HP:0100665) | Characteristic of clonal vs idiopathic MCAS |
| GI symptoms (nausea, vomiting, diarrhea, cramping) | Symptom | HP:0002018; HP:0002014 | Variable, episodic |
| Elevated basal serum tryptase | Laboratory abnormality | Abnormal circulating tryptase | Higher in clonal than non-clonal MCAD |
| Osteoporosis | Physical manifestation | HP:0000939 | Comorbidity requiring surveillance |
Onset / severity / progression / frequency. Adult-onset; severity ranges from moderate to life-threatening; course is episodic/fluctuating (attacks separated by relatively asymptomatic intervals); frequency of the cardiovascular/insect-trigger phenotype is enriched in clonal patients versus non-clonal MCAD PMID: 20434205.
Quality-of-life impact. Recurrent unpredictable anaphylaxis imposes substantial anxiety, activity restriction, and burden; disease-specific PROMs and general instruments (SF-12, SF-36) capture mediator-symptom burden in clonal mast-cell disease, and mediator symptoms improve with effective therapy PMID: 32437738. (Direct MMAS-specific QoL datasets are limited.)
Causal gene. KIT (HGNC:6342; NCBI Gene 3815; UniProt P10721), encoding the type-III receptor tyrosine kinase / stem cell factor receptor (CD117).
Pathogenic variant. - KIT D816V — activating missense point mutation in codon 816 of exon 17. This is the canonical minor SM criterion and the molecular hallmark of clonality in MMAS. In MMAS the mutant allele burden is very low (e.g., 0.007–9% mutated cells in one series), so highly sensitive detection (allele-specific PCR on purified mast cells) is mandatory PMID: 28262030. - Variant classification: Pathogenic (activating, gain-of-function). - Variant type: Missense (single-nucleotide substitution). - Somatic vs germline: Somatic (acquired in the mast-cell/hematopoietic lineage). This is why MMAS is not inherited despite the template's "Mendelian" label PMID: 34298172. - Functional consequence: Gain of function — constitutive, ligand-independent kinase activation.
"the KIT D816V mutation was detected in all SM patients but in only 2 patients with MMAS." — PMID: 28262030
Aberrant surface phenotype (second clonality marker). Aberrant expression of CD25 (±CD2/CD30) on bone-marrow mast cells is a minor SM criterion and was present in all SM and MMAS patients in a diagnostic work-up cohort PMID: 28262030.
"Flow cytometric analysis of bone marrow showed CD25 expression of MCs in all patients with SM and MMAS." — PMID: 28262030
Germline modifier gene. TPSAB1 — increased α-tryptase copy number causes hereditary alpha-tryptasemia (HAT), an autosomal-dominant trait that raises basal tryptase and amplifies anaphylaxis severity PMID: 37818990, PMID: 41932753.
Additional somatic mutations. Multi-mutated disease (e.g., SRSF2, ASXL1, RUNX1, NRAS) characterizes advanced SM and confers poor prognosis; these are generally absent in low-burden MMAS, consistent with its indolent behavior PMID: 34424959, PMID: 38142424.
Epigenetic / chromosomal abnormalities. No MMAS-specific epigenetic signature or recurrent chromosomal abnormality is established. (Data not available.)
"The clinical presentation of anaphylaxis after stinging -cardiovascular symptoms and absence of cutaneous- may point to a clonal mast cell disease." — PMID: 40641447
"aberrant KIT activity and signaling are critical for the induction of IL-6 and involve STAT5 and PI3K pathways but not STAT3 or STAT4." — PMID: 30948489
"mast cell lines expressing D816V-KIT, but not those expressing normal KIT or other KIT variants, produced constitutively high IL-6 amounts at the message and protein levels." — PMID: 30948489
"clonal mast cell disorders-including systemic mastocytosis and monoclonal mast cell activation syndrome-are now recognized as major risk amplifiers for severe and fatal anaphylaxis, particularly following Hymenoptera venom exposure, reinforcing the role of the KIT D816V mutation in lowering the mast cell activation threshold." — PMID: 42542541
"attempts to demonstrate its oncogenic effect alone have repeatedly failed, suggesting that additional pathways are involved in MC transformation." — PMID: 34424959
Upstream vs downstream. Upstream = somatic KIT D816V. Intermediate = STAT5/PI3K/MAPK signaling, IL-6, lowered activation threshold, MRGPRX2-mediated releasability. Downstream = mediator release → anaphylaxis. HAT is a parallel germline amplifier.
Ontology suggestions. - GO biological processes: mast cell activation (GO:0045576); mast cell degranulation (GO:0043303); transmembrane receptor protein tyrosine kinase signaling pathway (GO:0007169); STAT cascade / JAK-STAT (GO:0007259); positive regulation of inflammatory response (GO:0050729). - CL cell types: mast cell (CL:0000097); connective tissue / mucosal mast cell subsets; hematopoietic stem cell (CL:0000037) as the mutation-origin compartment. - CHEBI mediators: histamine (CHEBI:18295); prostaglandin D2 (CHEBI:15555); platelet-activating factor (CHEBI:52450).
Molecular profiling. Human mast-cell line and xenotransplant studies (see Model Organisms) provide the transcriptomic/signaling evidence (constitutive IL-6, STAT5/PI3K dependence; TNF/survivin-driven clonal dominance). No dedicated MMAS-specific transcriptomic/proteomic/metabolomic cohort exists.
"The overall prevalence of iMCAS was 4.4% in the entire cohort." — PMID: 38056692
"HAT was detected in 15/346 (4%) HD versus 43/149 (29%) non-clonal MCAS and 84/464 (18%) mastocytosis cases." — PMID: 37818990
Diagnostic framework (two-step). MMAS is diagnosed when the three consensus MCAS criteria are met AND bone-marrow study demonstrates mast-cell clonality without fulfilling full WHO SM criteria PMID: 21035176, PMID: 23179866, PMID: 20434205, PMID: 28262030.
Consensus MCAS criteria (Akin/Valent/Metcalfe): 1. Typical episodic mediator-release symptoms in ≥2 organ systems. 2. Objective transient rise in a validated mast-cell mediator — serum tryptase increasing by ≥20% above baseline + 2 ng/mL during an event. 3. Symptomatic response to mast-cell mediator-targeting therapy.
"an increase of the marker above the patient's baseline value during symptomatic periods on more than two occasions, or baseline serum tryptase levels that are persistently above 15 ng/ml." — PMID: 23179866
Clonality demonstration (bone marrow): - KIT D816V detection by highly sensitive allele-specific PCR on purified mast cells (essential given very low clonal burden) PMID: 28262030. - Flow cytometry for aberrant CD25 (±CD2/CD30) on bone-marrow mast cells PMID: 28262030. - Bone-marrow histology/immunohistochemistry to confirm the absence of the major SM criterion (multifocal dense aggregates) and insufficient minor criteria.
Biomarkers. Serum baseline tryptase (higher in clonal than non-clonal MCAD); transient event-related tryptase rise. Baseline tryptase interpretation must account for HAT (TPSAB1 duplication) PMID: 20434205, PMID: 37818990.
Risk stratification to decide on bone-marrow biopsy — the REMA score. Uses sex, absence of urticaria/pruritus, presyncope/syncope, and baseline serum tryptase to predict clonality and indicate when bone-marrow study is warranted PMID: 39187156, PMID: 20434205.
"followed by the Red Española de Mastocitosis score, which is calculated using anaphylaxis clinical features, BST, and the patient's sex." — PMID: 39187156
Genetic testing. Somatic KIT D816V on peripheral blood (high-sensitivity ddPCR/ASO-PCR) and/or purified bone-marrow mast cells; germline TPSAB1 copy-number analysis for HAT. Myeloid NGS panels can be used to exclude advanced-disease mutations.
Differential diagnosis. Systemic mastocytosis (esp. indolent SM without skin lesions / bone-marrow mastocytosis — distinguished by meeting full WHO criteria); idiopathic MCAS (clonality-negative, urticaria-predominant); secondary MCAS (IgE allergy); HAT alone; non-mast-cell causes of flushing/hypotension.
"The prognosis of cutaneous mastocytosis and non-advanced SM is mostly favourable." — PMID: 40274818
"The estimated 10-year progression-free survival of BMM and typical ISM was 95.9% and 92.6%, respectively." — PMID: 34545185
Management mirrors that of non-advanced clonal mast-cell disease: anti-mediator therapy, anaphylaxis prevention, and comorbidity surveillance PMID: 40274818, PMID: 39187156.
| Therapy | Agent/approach | Role in MMAS | NCIT suggestion |
|---|---|---|---|
| H1 antihistamines | e.g., cetirizine, fexofenadine | First-line anti-mediator | NCIT:C265 (Antihistamine) |
| H2 antihistamines | e.g., famotidine | GI mediator symptoms | — |
| Mast-cell stabilizer | Cromolyn sodium | GI/systemic symptom control | NCIT:C61762 (Cromolyn) |
| Leukotriene antagonist | Montelukast | Adjunct anti-mediator | NCIT:C1876 (Montelukast) |
| Anti-IgE mAb | Omalizumab | Refractory anaphylaxis/mediator symptoms | NCIT:C2075 (Omalizumab) |
| Emergency | Epinephrine autoinjectors (≥3) | Anaphylaxis rescue — essential | NCIT:C692 (Epinephrine) |
| Venom immunotherapy (VIT) | Hymenoptera venom | Lifelong (>5 yr/indefinite) for venom-triggered clonal disease | NCIT:C15321 (Immunotherapy) |
| Osteoporosis therapy | Bisphosphonates, Ca/vitamin D | Comorbidity prevention | — |
| Selective KIT inhibitor | Avapritinib (KIT D816V inhibitor) | Reserved for refractory cases; reduces tryptase, MC burden, symptoms in non-advanced SM | NCIT:C123834 (Avapritinib) |
| Multikinase inhibitor | Midostaurin | Advanced disease (not standard for MMAS) | NCIT:C1439 (Midostaurin) |
"Management of mastocytosis consists of symptomatic therapy, including anti-mast cell mediator drugs, and cytoreductive agents for patients with advanced disease and selected individuals with non-advanced disease, as well as recognition and prevention of comorbidities such as osteoporosis and anaphylaxis." — PMID: 40274818
"it is recommended to continue immunotherapy for more than 5 years or indefinitely and to carry at least three epinephrine autoinjectors." — PMID: 39187156
KIT inhibitors — evidence. Selective KIT D816V inhibition with avapritinib reduces serum tryptase, mast-cell burden and mediator symptoms in non-advanced SM (including at low 25 mg dosing), supporting its candidacy for refractory clonal disease including MMAS PMID: 40963125, PMID: 40274818. Midostaurin improves QoL and mediator symptoms in advanced SM PMID: 32437738, but cytoreduction is generally unnecessary in low-burden MMAS.
Pharmacogenomics / personalized medicine. The KIT D816V genotype directly guides selection of D816V-active inhibitors (avapritinib, midostaurin) over D816V-resistant agents (imatinib, which is effective only for rare non-D816V/imatinib-sensitive KIT variants) PMID: 37309222.
No dedicated MMAS-specific animal model exists; the biology is studied through KIT D816V mast-cell models shared with systemic mastocytosis PMID: 37025992, PMID: 38142424, PMID: 34424959.
| Model | Type | Use / findings | Reference |
|---|---|---|---|
| HMC-1.2 human mast-cell line; CRISPR/Cas9 single-D816V-KIT derivative | In vitro human cell line | Principal preclinical model for D816V-KIT biology and drug testing | PMID: 37025992 |
| Murine xenotransplantation of neoplastic mast cells | Mammalian (mouse) | KIT D816V-driven, TNF/survivin (BIRC5)-mediated clonal dominance; TNF knockout prolonged survival | PMID: 38142424 |
| GCPS / Gli3-haploinsufficient mouse | Mammalian (mouse) | Demonstrated KIT + Hedgehog synergy in mastocytosis onset | PMID: 34424959 |
"CRISPR/Cas9-engineering of HMC-1.2 cells renders a human mast cell line with a single D816V-KIT mutation: An improved preclinical model for research on mastocytosis." — PMID: 37025992
"knockout of TNF in neoplastic MC prolonged survival and reduced myelosuppression in a murine xenotransplantation model." — PMID: 38142424
Phenotype recapitulation / limitations. These models faithfully reproduce KIT D816V signaling and mediator biology but model the high-burden neoplastic (SM/advanced) end of the spectrum rather than the defining feature of MMAS — a sub-threshold, low-burden clone with a lowered activation threshold and anaphylaxis phenotype. No model captures the clinical anaphylaxis-dominant, urticaria-poor presentation of human MMAS.
SOMATIC EVENT (acquired, non-germline)
|
KIT D816V gain-of-function ── (GO:0007169 RTK signaling)
|
Constitutive STAT5 / PI3K / MEK-ERK activation
|
+--------+----------+
| |
Pro-inflammatory LOWERED MAST-CELL
output (IL-6) ACTIVATION THRESHOLD
| |
(limited clonal + germline HAT (TPSAB1 dup) -> amplifies severity
expansion; KIT |
D816V alone TRIGGER (Hymenoptera venom /
insufficient -> idiopathic / MRGPRX2 IgE-independent)
stays BELOW SM |
threshold = MMAS) Explosive degranulation:
| tryptase, histamine, PAF, PGD2
| |
Favorable CARDIOVASCULAR-DOMINANT,
prognosis; URTICARIA-POOR ANAPHYLAXIS
low progression (hypotension, syncope; male-predominant)
MMAS is best understood as systemic mastocytosis' "shadow": the same somatic KIT D816V engine and the same aberrant CD25+ clonal phenotype, but with a clone too small to satisfy WHO SM criteria. The pathological consequence is not tissue infiltration/organ damage (as in advanced SM) but a hair-trigger anaphylaxis diathesis. The two clonality markers (KIT D816V; CD25) define the entity; the low burden defines its separation from SM; and the lowered activation threshold defines its danger. HAT is a distinct, germline, additive severity amplifier — a genuine gene–environment interaction node.
| PMID | Contribution | Supports |
|---|---|---|
| 28262030 | Defines MMAS vs SM; documents low KIT D816V burden and CD25 in MMAS (4/23 monoclonal disorders were MMAS) | F001, F002, F010 |
| 34298172 | Places monoclonal MCAS within primary/clonal MCAS; KIT diagnostic relevance | F001, F004 |
| 30948489 | D816V-KIT → STAT5/PI3K → constitutive IL-6 | F003 |
| 20434205 | Clinical/molecular features of clonal MCAD; male sex, cardiovascular, insect-trigger, higher tryptase; predictive model for clonality | F004, F005, F009 |
| 39187156 | REMA score; VIT + epinephrine recommendations | F004, F006 |
| 40641447 | Post-sting hypotensive, non-cutaneous anaphylaxis points to clonal MC disease | F004 |
| 21035176 | Proposes consensus MCAS diagnostic criteria | F005 |
| 23179866 | Tryptase mediator criterion detail | F005 |
| 40274818 | Management framework; favorable prognosis of non-advanced disease | F006, F007 |
| 34545185 | 95.9% 10-yr PFS for bone-marrow mastocytosis (MMAS analogue) | F007 |
| 37818990 | HAT enrichment in MCAS/mastocytosis (REMA, n=959) | F008, F010 |
| 41932753 | HAT as independent severity modifier | F008 |
| 38056692 | Clonal MCAS has fewer mucocutaneous symptoms (P=.015); iMCAS prevalence 4.4% | F009, F010 |
| 25944644 | Multisystem MCAS symptom spectrum | F009 |
| 35623575 | ICD-10-CM coding of MCA disorders (ECNM-AIM) | F010 |
| 37025992 | Engineered single-D816V HMC-1.2 model | F011 |
| 38142424 | Murine xenotransplant; TNF/survivin clonal dominance | F011, F012 |
| 34424959 | KIT D816V alone insufficient; Hedgehog synergy | F012 |
| 42542541 | KIT D816V lowers activation threshold; MRGPRX2/non-IgE mechanisms | F012 |
| 40963125 | Avapritinib reduces tryptase/MC burden/symptoms | F006 |
| 32437738 | Midostaurin improves QoL/mediator symptoms (advanced SM) | Contextual (treatment) |
| 37309222 | SM diagnosis/risk/management; genotype-guided TKI choice | Contextual (pharmacogenomics) |
Report compiled from 12 confirmed findings across 5 investigation iterations and 52 reviewed papers. Evidence sources are predominantly human clinical cohorts and reference-center series, supplemented by in vitro human mast-cell line and murine model studies for mechanism.
Checked with linkml-reference-validator 0.2.1.
| Outcome | Count |
|---|---|
| References checked | 23 |
| Resolved | 23 |
| Unresolved (possible confabulation) | 0 |
| Unverifiable | 0 |
| Quoted claims checked | 22 |
| Quoted claims found in source | 22 |
| Quoted claims not found in source | 0 |
| References weighed for topical relevance | 23 |
| On topic | 21 |
| Off topic | 0 |
All extracted references resolved successfully.