Platelet-type Bleeding Disorder 15

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Classifications

Harrison's Part

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Inheritance

1

Autosomal dominant inheritance HP:0000006

Heterozygous ACTN1 missense variants segregate with macrothrombocytopenia in dominant pedigrees, demonstrated first in six Japanese families and then in a six-generation French kindred with 26 affected subjects. Penetrance is recorded as incomplete because the two components of the phenotype behave differently: in the largest assembled cohort platelet macrocytosis was present in every affected subject while thrombocytopenia was present in fewer than 80%, so a carrier with a normal platelet count is expected rather than anomalous. A homozygous variant has been reported in two sisters whose haematological phenotype was more severe than that of their heterozygous relatives, which makes the inheritance semidominant with a gene-dosage effect rather than strictly dominant, but the heterozygous state is itself fully disease-causing.

Autosomal dominant inheritance Penetrance: INCOMPLETE

Show evidence (3 references)

PMID:23434115 SUPPORT Human Clinical

"In 13 Japanese CMTP-affected pedigrees, we identified six (46%) affected by ACTN1 variants cosegregating with CMTP."

Cosegregation of heterozygous ACTN1 variants with disease in dominantly transmitting pedigrees.

PMID:24069336 SUPPORT Human Clinical

"Candidate gene analysis complemented by targeted next-generation sequencing identified a missense mutation (c.137GA; p.Arg46Gln) in the alpha-actinin 1 gene (ACTN1) that segregated with macrothrombocytopenia in this large pedigree."

Independent segregation of a single heterozygous allele through a six-generation dominant pedigree.

PMID:30351444 SUPPORT Human Clinical

"We concluded that ACTN1-RT is the fourth most frequent form of IT worldwide and it is characterized by platelet macrocytosis in all affected subjects and mild thrombocytopenia in less than 80% of cases."

Supports incomplete penetrance of the thrombocytopenia component specifically - fewer than 80% of carriers have a low platelet count - while the macrocytosis component was universal in this series.

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Mechanistic Hypotheses

2

Disease alleles increase alpha-actinin-1 association with actin

actn1_increased_actin_association CANONICAL

Evidence balance 1 support

The account with direct biochemical support. Actin-binding-domain substitutions raise the affinity of alpha-actinin-1 for F-actin, and calmodulin-like and rod-domain substitutions raise its filament-bundling activity, so alleles scattered across three structurally distinct domains converge on a single measurable change in the same direction. Under this model the megakaryocyte actin network is over-cross-linked rather than under-cross-linked, and the proplatelet defect follows from a cytoskeleton too rigidly bundled to remodel normally.

Pathograph links

ACTN1 Missense Variant Altering Alpha-Actinin-1 -> Increased Alpha-Actinin-1 Actin Cross-Linking Activity DIRECT The substitution changes the protein's interaction with F-actin. Shown biochemically for nine calmodulin-like and rod-domain mutants and stated for actin-binding-domain mutants.

Show evidence (1 reference)

PMID:31365757 SUPPORT In Vitro

"Thus, CMTP-causing actinin-1 mutations outside the actin-binding domain also increase actin association, suggesting a common molecular mechanism underlying actinin-1 related CMTP."

The primary statement of the hypothesis by the group that measured it.

Disease alleles act by loss of alpha-actinin-1 function or dominant-negative interference

actn1_loss_of_function ALTERNATIVE

Evidence balance 1 support 1 refute

The reading that the phenotype invites and that much secondary literature still repeats: if cross-linking is lost, or if mutant subunits poison wild-type dimers, the megakaryocyte cytoskeleton fails and platelet production suffers. It is not baseless - megakaryocyte-specific deletion of Actn1 in the mouse also produces a low platelet count through defective thrombocytopoiesis, so simply removing the protein reproduces part of the human picture. What it does not explain is why the human alleles increase rather than decrease actin association in vitro, and the knockout mouse additionally shows a platelet-function defect and impaired haemostasis that ACTN1-RT patients do not have. Kept as ALTERNATIVE rather than deprecated because no experiment has yet tested the two models against each other in a megakaryocyte carrying a patient allele.

Show evidence (2 references)

PMID:39813624 SUPPORT Model Organism

"Using megakaryocyte (MK)-specific α-actinin-1 knockout (KO; PF4-Actn1-/-) mice, we demonstrated that PF4-Actn1-/- mice exhibited reduced platelet counts."

Shows that absence of the protein is sufficient for thrombocytopenia, which is the strongest evidence available for a loss-of-function reading.

PMID:39813624 REFUTE Model Organism

"Platelet spreading, clot retraction, aggregation, integrin αIIbβ3 activation, and CD62P exposure in response to various agonists were decreased in PF4-Actn1-/- platelets."

Cuts against the loss-of-function model as an account of the human disease: losing the protein produces a broad platelet-function defect, whereas ACTN1-RT platelets function normally.

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Discussions and Knowledge Gaps

3

Does ACTN1-related thrombocytopenia arise because mutant alpha-actinin-1 binds and bundles actin too strongly, or because it fails to cross-link actin normally?

KNOWLEDGE GAP OPEN bdplt15_gain_versus_loss_of_actin_association

Attached to

pathophysiology#Increased Alpha-Actinin-1 Actin Cross-Linking Activity mechanistic_hypotheses#actn1_increased_actin_association

Every biochemical measurement points one way and much of the descriptive literature points the other. Purified mutant proteins bind F-actin more avidly and bundle filaments more effectively, across substitutions in three different domains, and the authors of that work explicitly propose increased actin association as the common mechanism. Yet the disease is routinely summarised as a loss of actin cross-linking or a dominant-negative effect, including by the deep-research report that seeded this entry, presumably because a cytoskeleton that fails to make platelets reads intuitively as a cytoskeleton that has lost a function. The distinction is not cosmetic: an over-bundled network and an under-bundled network make opposite predictions about what a heterozygous knock-in should show relative to a heterozygous knockout, and would call for opposite interventions if any were ever wanted. No published experiment compares them in a megakaryocyte, and the one in vivo model is a complete knockout, which by construction cannot distinguish them.

Proposed experiments

Heterozygous knock-in versus heterozygous knockout megakaryocytes

exp_bdplt15_knockin_versus_knockout_megakaryocytes

Generate isogenic human induced pluripotent stem cell lines carrying a patient ACTN1 missense allele heterozygously and, separately, a heterozygous ACTN1 null allele, differentiate both to megakaryocytes, and compare actin network architecture, proplatelet tip number and size, and released platelet size. A knock-in phenotype the null does not reproduce would establish that the allele acts by more than reduced dosage.

Would support

mechanistic_hypotheses#actn1_increased_actin_association

Would refute

mechanistic_hypotheses#actn1_increased_actin_association

Supporting outcome

The missense knock-in shows thickened, over-bundled actin and fewer, larger proplatelet tips while the heterozygous null is close to wild type.

Refuting outcome

The heterozygous null reproduces the knock-in phenotype, indicating that reduced functional alpha-actinin-1 dosage is sufficient.

Show evidence (1 reference)

PMID:31365757 SUPPORT In Vitro

"Actinin-1 mutations cause dominantly inherited congenital macrothrombocytopenia (CMTP), with mutations in the actin-binding domain increasing actinin's affinity for F-actin."

The measured direction of the biochemical change, which is what puts it at odds with the loss-of-function reading this gap is about.

Can a megakaryocyte-specific Actn1 knockout mouse be used to reason about ACTN1-related thrombocytopenia, when it produces a platelet-function defect and impaired haemostasis that patients do not have?

HUMAN MODEL MISMATCH OPEN bdplt15_knockout_mouse_versus_human_disease

Attached to

animal_models#Megakaryocyte-specific alpha-actinin-1 knockout mouse (PF4-Actn1-/-) pathophysiology#Reduced Platelet Output with Increased Platelet Size

The knockout mouse is the only in vivo system available, and it does reproduce the low platelet count and locate the defect in thrombocytopoiesis. But it also shows impaired platelet spreading, clot retraction, aggregation, integrin alphaIIbbeta3 activation and granule release, plus impaired haemostasis and thrombosis and a mitochondrial phenotype - none of which is described in ACTN1-RT, whose defining clinical feature is that platelet function is normal and bleeding negligible. Two readings are possible and they have different consequences. Either the mismatch follows straightforwardly from the genetic difference, complete absence of protein versus one altered allele, in which case the mouse tells us about the gene's requirements and nothing about patients' bleeding risk. Or alpha-actinin-1 also has a platelet-function role that is subtly impaired in patients and has not been looked for with sensitive enough assays, in which case the received statement that platelet function is normal in ACTN1-RT rests on aggregometry that would not detect it. Nobody has phenotyped patients' platelets with the assays used in the mouse.

Proposed experiments

Deep platelet phenotyping in ACTN1-RT carriers

exp_bdplt15_patient_platelet_deep_phenotyping

Apply the assays that were abnormal in the knockout mouse - spreading on fibrinogen, clot retraction, integrin alphaIIbbeta3 activation and P-selectin exposure by flow cytometry, and mitochondrial membrane potential - to platelets from genotyped ACTN1-RT carriers and matched controls, rather than relying on light transmission aggregometry alone.

Would support

pathophysiology#Reduced Platelet Output with Increased Platelet Size

Supporting outcome

Carrier platelets are indistinguishable from controls across the sensitive assays, confirming that the human defect is purely one of platelet production.

Refuting outcome

Carrier platelets show measurable spreading, integrin activation or granule release defects, indicating that ACTN1-RT has a subclinical qualitative component that routine testing misses.

Show evidence (1 reference)

PMID:39813624 SUPPORT Model Organism

"Platelet spreading, clot retraction, aggregation, integrin αIIbβ3 activation, and CD62P exposure in response to various agonists were decreased in PF4-Actn1-/- platelets."

The model finding that human patients are not reported to share, which is the mismatch this discussion is about.

Do ACTN1 variants, in the homozygous state, cause heart valve defects, or is the reported association coincidental?

KNOWLEDGE GAP OPEN bdplt15_cardiac_valve_association

Attached to

phenotypes#Mild Heart Valve Defects in Homozygous Carriers pathophysiology#Biallelic ACTN1 Variant Dosage Effect

ACTN1-RT is described in the largest series as having no associated defects, congenital or acquired. Against that, two homozygous sisters had mild heart valve defects with no alternative explanation on exome sequencing, and the reporting authors hypothesised an ACTN1 contribution. The evidence is two related individuals and a negative-exome argument, which cannot distinguish causation from coincidence within one family, and mild valve abnormalities are common enough in the general population that two cases are not a signal. The question is worth keeping open rather than dismissing because alpha-actinin-1 is expressed well beyond the megakaryocyte, and a dosage-dependent extra-haematological phenotype appearing only in homozygotes is what a mild gain-of-binding allele would be expected to produce if it produced one at all.

Proposed experiments

Systematic echocardiography in ACTN1 variant carriers

exp_bdplt15_echocardiography_in_carriers

Echocardiograph an unselected cohort of heterozygous ACTN1-RT carriers and any further homozygotes identified, with age- and sex-matched controls, and compare valve abnormality rates against population estimates.

Would support

phenotypes#Mild Heart Valve Defects in Homozygous Carriers

Supporting outcome

Valve abnormalities are more frequent in carriers than in controls, with a dose relationship between the homozygous and heterozygous states.

Refuting outcome

Valve abnormality rates in carriers match population rates, indicating the reported association was coincidental.

Show evidence (1 reference)

PMID:38594875 SUPPORT Human Clinical

"Moreover, we hypothesize that some ACTN1 variants, especially when present in the homozygous state, may also contribute to the cardiac abnormalities."

The authors state the cardiac claim as a hypothesis, which is exactly the uncertainty recorded here.

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Pathophysiology

6

ACTN1 Missense Variant Altering Alpha-Actinin-1

Mechanism confidence: Established

The initiating lesion is a heterozygous germline missense substitution in ACTN1. Alpha-actinin-1 is an actin-crosslinking protein of the spectrin superfamily that organises the cytoskeleton, and it works as an antiparallel dimer whose two actin-binding domains hold neighbouring filaments together. Substitutions in the actin-binding domain, in the spectrin-like rod repeats and in the calmodulin-like domain all cause the same disease, which is the first clue that they share a downstream effect rather than each disabling a separate function.

Disease alleles increase alpha-actinin-1 association with actin

Megakaryocyte CL:0000556

ACTN1 hgnc:163

Genetic context variant_origin: GERMLINE functional_impact_category: GAIN_OF_FUNCTION

Heterozygous germline missense substitutions. Recorded as a gain of function because the measured biochemical effect of the disease alleles is an increase - higher affinity for F-actin for actin-binding-domain substitutions, and greater filament-bundling activity for calmodulin-like and rod-domain substitutions. This runs against the loss-of-crosslinking reading that the phenotype superficially suggests, and the disagreement is carried explicitly as a mechanistic hypothesis and an open discussion in this entry rather than settled here.

Downstream

Increased Alpha-Actinin-1 Actin Cross-Linking Activity The substitution changes the protein's interaction with F-actin. Shown biochemically for nine calmodulin-like and rod-domain mutants and stated for actin-binding-domain mutants.

Show evidence (2 references)

PMID:23434115 SUPPORT Human Clinical

"ACTN1 encodes α-actinin-1, a member of the actin-crosslinking protein superfamily that participates in the organization of the cytoskeleton."

Identifies the gene product and the molecular function the variants perturb.

PMID:31237726 SUPPORT Human Clinical

"Nine variants were located in the α-actinin-1 (ACTN1) rod domain and were predicted to hinder dimer formation."

Documents the rod-domain arm of the allelic spectrum and its predicted structural consequence.

Increased Alpha-Actinin-1 Actin Cross-Linking Activity

Mechanism confidence: Established

Mutant alpha-actinin-1 binds and bundles actin filaments more avidly than wild type. Nine calmodulin-like and rod-domain mutants were shown to increase filament bundling in vitro to varying degrees, two of them measurably increased the cytoskeletal association of actinin inside cells, and the actin-binding-domain mutants raise affinity for F-actin directly. The authors of that work read the convergence as a common molecular mechanism for ACTN1-linked macrothrombocytopenia, which is why this node sits on the main chain rather than being treated as one of several parallel biochemical consequences.

actin crosslink formation GO:0051764 ↑ INCREASED actin filament bundle assembly GO:0051017 ↑ INCREASED

actin filament binding GO:0051015 ↑ INCREASED

Experimental models

In vitro actin bundling assays with recombinant mutant alpha-actinin-1 OTHER Quantifies filament bundling by nine calmodulin-like and rod-domain mutants and the cytoskeletal association of mutant protein in cells.

Downstream

Disorganized Megakaryocyte Actin Cytoskeleton Excessive cross-linking distorts the actin network that mutant-expressing cells assemble.

Show evidence (3 references)

PMID:31365757 SUPPORT In Vitro

"In this study, we examined nine CMTP-causing mutations in the calmodulin-like and rod domains of actinin-1. These mutations increase, to varying degrees, actinin's ability to bundle actin filaments in vitro."

Direct biochemical measurement of increased bundling activity for disease alleles outside the actin-binding domain.

PMID:31365757 SUPPORT In Vitro

"Thus, CMTP-causing actinin-1 mutations outside the actin-binding domain also increase actin association, suggesting a common molecular mechanism underlying actinin-1 related CMTP."

The authors' own statement that increased actin association is the shared mechanism across domains.

PMID:41153762 SUPPORT Other

"The pathophysiological mechanisms by which α-actinin-1 mutations lead to macrothrombocytopenia have been attributed to alterations in actin organization, increased binding affinity of α-actinin-1 to actin filaments, and modulation of integrin αIIbβ3 signaling."

A review's summary of the field, cited to show that increased actin binding is an accepted account rather than a single group's result. Graded OTHER because the source is a narrative review, not primary data.

Disorganized Megakaryocyte Actin Cytoskeleton

Mechanism confidence: Established

Expression of a disease-associated ACTN1 allele disorganises the actin cytoskeleton. The finding is unusually well replicated for a rare disorder: it has been reproduced in Chinese hamster ovary cells for actin-binding and calmodulin-domain alleles, in the same system for a rod-domain allele, in COS-7 cells and cultured megakaryocytes for the French p.Arg46Gln allele, and in the expression work accompanying the largest variant series, where the network was disorganised and the fibres themselves were thicker. Thickened fibres are the morphological counterpart of the increased bundling measured biochemically upstream.

Megakaryocyte CL:0000556

actin cytoskeleton organization GO:0030036 ⚠ ABNORMAL

bone marrow UBERON:0002371

Experimental models

Chinese hamster ovary cells expressing mutant ACTN1 CELL LINE Reproduces the actin disorganisation caused by disease alleles, and is the assay by which most reported variants have been called deleterious.

Downstream

Impaired Proplatelet Formation Proplatelet extension and branching are actin-dependent, so a disorganised circumferential actin network changes the geometry of the proplatelets a megakaryocyte can build.

Show evidence (4 references)

PMID:23434115 SUPPORT In Vitro

"In vitro transfection experiments in Chinese hamster ovary cells demonstrated that altered α-actinin-1 disrupted the normal actin-based cytoskeletal structure."

The original demonstration that the mutant protein disrupts actin architecture.

PMID:31237726 SUPPORT In Vitro

"In vitro expression of the new ACTN1 variants induced actin network disorganization and led to increased thickness of actin fibers."

Replicates the disorganisation for eleven previously unreported variants and adds fibre thickening, the morphological signature of excess bundling.

PMID:26453073 SUPPORT In Vitro

"Consistent with the previous reports of mutations that reside in the ABD or the CaM domain, immunofluorescence examination revealed disorganization of the actin cytoskeleton in Gln395 mutant-transduced Chinese hamster ovary cells."

Extends the same cellular phenotype to a spectrin-like-repeat allele, supporting convergence across domains.

+ 1 more reference

Impaired Proplatelet Formation

Mechanism confidence: Established

Megakaryocytes expressing a disease allele extend fewer proplatelet processes, and the tips those processes end in are abnormally large. Because each tip is released as a platelet, that geometry predicts the clinical picture directly: fewer platelets, each bigger. Human data agree that the lesion is late rather than early in megakaryopoiesis - patients have low reticulated platelet counts with only slightly raised thrombopoietin, which is the profile of impaired platelet release rather than of failed megakaryocyte production or of peripheral platelet destruction.

Megakaryocyte CL:0000556

platelet formation GO:0030220 ↓ DECREASED megakaryocyte development GO:0035855 ⚠ ABNORMAL

bone marrow UBERON:0002371

Experimental models

Mouse fetal liver-derived megakaryocytes transduced with disease-associated ACTN1 variants PRIMARY CELL CULTURE Reproduces both halves of the predicted output defect in a megakaryocyte - fewer proplatelet tips, and tips that are abnormally large.

Animal models

Megakaryocyte-specific alpha-actinin-1 knockout mouse (PF4-Actn1-/-) Mouse partially recapitulates Reproduces the low platelet count and traces it to defective thrombocytopoiesis, including a reduced proportion of megakaryocytes bearing proplatelets - the same cellular step this node names.

Downstream

Reduced Platelet Output with Increased Platelet Size Fewer tips means fewer platelets; larger tips means larger platelets. The inference from tip geometry to circulating platelet indices is the authors' own and has not been measured directly in a patient's marrow.

Show evidence (3 references)

PMID:23434115 SUPPORT Model Organism

"Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."

The direct observation of fewer and larger proplatelet tips in megakaryocytes carrying a disease allele.

PMID:25361813 SUPPORT Human Clinical

"Low reticulated platelet counts and only slightly increased serum thrombopoietin levels indicated that the latest phases of megakaryopoiesis were affected."

Human laboratory evidence locating the defect in the final steps of megakaryopoiesis, consistent with a proplatelet-release problem rather than a production or destruction problem.

PMID:41153762 SUPPORT Other

"As a pivotal scaffold protein, α-actinin-1 interacts with a complex network of partners, including integrin αIIbβ3, and actin filaments, to modulate cytoskeletal dynamics, megakaryocyte maturation, and proplatelet formation."

Establishes that proplatelet formation is among the normal functions alpha-actinin-1 supports, which is what makes it the plausible step to fail. Graded OTHER as a narrative review.

Reduced Platelet Output with Increased Platelet Size

Mechanism confidence: Established

The circulating result: a platelet population reduced in number, enlarged, and unusually variable in size. What does not follow is a functional defect. Aggregation and secretion responses are preserved in ACTN1-RT, so the mechanism ends at a quantitative and morphological abnormality, and the small bleeding tendency that follows is proportionate to the platelet count rather than amplified by a platelet-function defect. That is the single feature that most distinguishes this disorder from the rest of the OMIM platelet-type bleeding disorder series.

Platelet CL:0000233

hemostasis GO:0007599 ↓ DECREASED

blood UBERON:0000178

Animal models

Megakaryocyte-specific alpha-actinin-1 knockout mouse (PF4-Actn1-/-) Mouse fails to recapitulate The node asserts reduced platelet output with preserved platelet function. The knockout mouse has the reduced output but not the preserved function: spreading, clot retraction, aggregation, integrin activation and granule release are all impaired, and haemostasis is impaired in vivo. Human ACTN1-RT is defined partly by the absence of exactly those abnormalities.

Downstream

Thrombocytopenia

Platelet Macrocytosis

Macrothrombocytopenia

Platelet Anisocytosis

Mild Mucocutaneous Bleeding Tendency

Show evidence (2 references)

PMID:23434115 SUPPORT Human Clinical

"Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency."

Pairs the circulating platelet abnormality with the mildness of its clinical consequence.

PMID:25361813 SUPPORT Human Clinical

"The clinical and laboratory findings of 31 affected individuals confirmed that ACTN1-RT is a mild macrothrombocytopenia with low risk for bleeding."

Confirms the same combination in an independent cohort of 31 affected individuals.

Biallelic ACTN1 Variant Dosage Effect

Mechanism confidence: Provisional

Two sisters homozygous for ACTN1 c.982G>A had moderate thrombocytopenia and marked platelet macrocytosis with giant platelets, a more severe haematological picture than their heterozygous relatives in the same family. Carrying twice the dose of mutant protein therefore worsens the phenotype quantitatively without changing its character, which is what a gain-of-actin-association mechanism predicts. It is curated as a PROVISIONAL modifier rather than a step in the chain because it rests on a single family with two homozygotes.

ACTN1 hgnc:163

Genetic context variant_origin: GERMLINE

A single reported homozygous missense allele, c.982G>A. Homozygosity for an ACTN1 disease allele is otherwise unreported, so nothing is known about whether the dosage effect generalises across alleles.

Downstream

Reduced Platelet Output with Increased Platelet Size A larger dose of mutant protein produces a quantitatively larger version of the same output defect.

Giant Platelets in Homozygous Carriers

Mild Heart Valve Defects in Homozygous Carriers Advanced by the reporting authors as a hypothesis, not a demonstrated link - the association rests on two related patients in whom exome sequencing found no alternative explanation.

Show evidence (1 reference)

PMID:38594875 SUPPORT Human Clinical

"Within the reported family, the homozygous sisters have moderate thrombocytopenia and marked platelet macrocytosis with giant platelets, revealing a more severe haematological phenotype compared to their heterozygous relatives and highlighting a significant effect of allelic burden on platelet size."

Within-family comparison of homozygotes against heterozygotes, which controls for genetic background better than a cross-cohort comparison would.

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

7

Blood 6

Thrombocytopenia FREQUENT HP:0001873

Course: STABLE

Show evidence (2 references)

PMID:30351444 SUPPORT Human Clinical

"We concluded that ACTN1-RT is the fourth most frequent form of IT worldwide and it is characterized by platelet macrocytosis in all affected subjects and mild thrombocytopenia in less than 80% of cases."

Supports the FREQUENT band directly: present in fewer than 80% of the 49 carriers in the largest series, which falls in the 30-79% range.

PMID:25361813 SUPPORT Human Clinical

"The clinical and laboratory findings of 31 affected individuals confirmed that ACTN1-RT is a mild macrothrombocytopenia with low risk for bleeding."

Independent confirmation that the thrombocytopenia is mild in an unselected cohort of 31 affected individuals.

Platelet Macrocytosis VERY_FREQUENT Increased mean platelet volume HP:0011877

Show evidence (3 references)

PMID:30351444 SUPPORT Human Clinical

"We concluded that ACTN1-RT is the fourth most frequent form of IT worldwide and it is characterized by platelet macrocytosis in all affected subjects and mild thrombocytopenia in less than 80% of cases."

Macrocytosis was present in every affected subject in the 49-carrier series. The band is set at VERY_FREQUENT rather than OBLIGATE because the refuting item below shows it is not universal across all variant classes.

PMID:31237726 REFUTE Human Clinical

"The rod domain, like other ACTN1 functional domains, may be mutated resulting in actin disorganization in vitro and thrombocytopenia with normal platelet size in most cases."

Contradicts platelet macrocytosis being an obligate feature: carriers of rod-domain variants mostly have normal platelet size. This is why the frequency band is not OBLIGATE.

PMID:26453073 SUPPORT Human Clinical

"They showed a mild form of thrombocytopenia without severe bleeding, accompanied by an elevated mean platelet volume."

A measured raised mean platelet volume in an affected mother and son, which is the parameter this phenotype is bound to.

Macrothrombocytopenia VERY_FREQUENT HP:0040185

Course: STABLE

Show evidence (2 references)

PMID:23434115 SUPPORT Human Clinical

"Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency."

States the presenting composite phenotype in the discovery cohort.

PMID:24069336 SUPPORT Human Clinical

"Of the 55 subjects available for analysis, 26 were diagnosed with isolated macrothrombocytopenia."

Documents the same composite phenotype in 26 members of one pedigree, and the word isolated carries the additional point that it occurs without syndromic features.

Platelet Anisocytosis HP:0032438

Show evidence (1 reference)

PMID:23434115 SUPPORT Human Clinical

"Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency."

Records anisocytosis as a feature of the presenting blood picture.

Giant Platelets in Homozygous Carriers VERY_RARE HP:0001902

Show evidence (1 reference)

PMID:38594875 SUPPORT Human Clinical

"Within the reported family, the homozygous sisters have moderate thrombocytopenia and marked platelet macrocytosis with giant platelets, revealing a more severe haematological phenotype compared to their heterozygous relatives and highlighting a significant effect of allelic burden on platelet size."

The only report of giant platelets in this disorder, and it is confined to the homozygous state.

Mild Mucocutaneous Bleeding Tendency OCCASIONAL Abnormal bleeding HP:0001892

Show evidence (3 references)

PMID:31237726 SUPPORT Human Clinical

"Twenty-eight of 32 cases with monoallelic ACTN1 variants had mild to no bleeding complications."

The quantitative basis for the OCCASIONAL band - 4 of 32 carriers had more than mild bleeding, and the remainder had mild bleeding or none.

PMID:30351444 SUPPORT Human Clinical

"The risk of bleeding, either spontaneous or upon haemostatic challenge, is negligible and there are no other associated defects, either congenital or acquired."

The strongest statement of mildness available, and it also excludes associated congenital or acquired defects in the 49-carrier series.

PMID:23434115 SUPPORT Human Clinical

"Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency."

Independent statement of the same mildness in the discovery cohort.

Cardiovascular 1

Mild Heart Valve Defects in Homozygous Carriers VERY_RARE Abnormal heart valve morphology HP:0001654

Show evidence (2 references)

PMID:38594875 SUPPORT Human Clinical

"We describe for the first time two patients affected with ACTN1-RT caused by a homozygous variant in ACTN1 (c.982G>A) with mild heart valve defects unexplained by any other genetic variants investigated by WES."

The observation itself, including its limits - two patients, and a negative-exome argument for attribution.

PMID:30351444 REFUTE Human Clinical

"The risk of bleeding, either spontaneous or upon haemostatic challenge, is negligible and there are no other associated defects, either congenital or acquired."

The 49-carrier series found no associated defects of any kind, which is evidence against valve disease being a feature of the disorder in the heterozygous state.

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Genetic Associations

1

ACTN1

Gene: ACTN1 hgnc:163 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (5 references)

PMID:25361813 SUPPORT Human Clinical

"To better characterize this disease, we screened ACTN1 in 128 probands and found 10 (8 novel) missense heterozygous variants in 11 families."

Establishes the heterozygous missense allelic spectrum in an unselected referral cohort.

PMID:25361813 SUPPORT In Vitro

"Combining bioinformatics, segregation, and functional studies, we demonstrated that all but 1 amino acid substitution had deleterious effects."

The functional arm of the same study, split into its own item so the evidence_source describes the assay rather than the cohort. One of the ten substitutions was not shown to be deleterious.

PMID:31237726 SUPPORT Human Clinical

"We identified 15 rare, monoallelic, nonsynonymous and likely pathogenic ACTN1 variants in 20 index cases from 20 unrelated families."

The largest single sequencing series, confirming monoallelic nonsynonymous variants as the disease allele class.

+ 2 more references

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External Assertions

1

OMIM bleeding disorder, platelet-type, 15 record

OMIM disease record OMIM:615193

The OMIM phenotype record that MONDO:0014078 xrefs, and the source of the entry name used here. Recorded structurally rather than in prose because the clinical literature almost never uses the OMIM name - papers say ACTN1-related thrombocytopenia - so the identity of the concept curated here should be auditable against the source identifier. Note this is not mappings.omim_mappings: the DiseaseMappings class carries only ICD-10-CM, ICD-11, MONDO and NCIT slots.

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Medical Actions

2

Genetic diagnosis and counselling

Action: Genetic counseling Ontology label: Genetic Counseling NCIT:C15240

The main intervention this disorder needs, and it is informational. Making the molecular diagnosis converts an unexplained low platelet count into a benign trait with a known prognosis, stops immunosuppression and splenectomy being considered, and lets first-degree relatives with the same blood picture be explained rather than investigated repeatedly. Autosomal dominant transmission means a 50% recurrence risk per pregnancy for a heterozygous carrier, which is what counselling covers.

Target Phenotypes: Macrothrombocytopenia HP:0040185

Show evidence (2 references)

PMID:30351444 SUPPORT Human Clinical

"Therefore, ACTN1-RT is a benign form of IT, whose diagnosis provides affected individuals and their families with a good prognosis."

States the benefit of diagnosis in exactly these terms - prognostic information for the individual and the family.

PMID:25361813 SUPPORT Human Clinical

"Given its relatively high frequency in our cohort (4.2%), ACTN1-RT has to be taken into consideration in the differential diagnosis of ITs."

Supports testing for the disorder as part of the diagnostic pathway this intervention depends on.

Observation without disease-specific therapy

Action: Supportive care Ontology label: Supportive Care NCIT:C15747

No treatment targets the mechanism, and in the great majority of carriers none is needed. The largest series found the bleeding risk negligible both spontaneously and under haemostatic challenge, so the default management is observation with advice about antiplatelet drugs and a plan for surgery rather than any ongoing therapy. Haemostatic support around procedures is discussed in this entry's notes; it is standard practice for inherited thrombocytopenia rather than an ACTN1-specific recommendation, and no published series reports its use in this disorder.

Target Phenotypes: Mild mucocutaneous bleeding HP:0001892

Show evidence (2 references)

PMID:30351444 SUPPORT Human Clinical

"The risk of bleeding, either spontaneous or upon haemostatic challenge, is negligible and there are no other associated defects, either congenital or acquired."

The evidential basis for not treating: negligible bleeding risk even under haemostatic challenge, and no other defects to monitor for.

PMID:25361813 SUPPORT Human Clinical

"The clinical and laboratory findings of 31 affected individuals confirmed that ACTN1-RT is a mild macrothrombocytopenia with low risk for bleeding."

Independent cohort evidence that the low bleeding risk is a property of the disorder rather than of one series.

🔬

Diagnosis

3

Full blood count with peripheral blood film

The test that finds the disease. A persistently low platelet count with a raised mean platelet volume, enlarged platelets and marked size variation on the film, in someone without systemic illness, is the trigger for considering an inherited thrombocytopenia. A film matters as well as the count, because automated counters can under-report platelets large enough to fall outside the platelet gate.

Show evidence (1 reference)

PMID:23434115 SUPPORT Human Clinical

"Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency."

Describes the blood-count and film picture that leads to the diagnosis, and the absence of symptoms to prompt it.

ACTN1 sequencing

The confirmatory test, done as a targeted gene panel for inherited thrombocytopenia, as single-gene sequencing, or as exome sequencing when the picture is unexplained. Because disease alleles are largely private missense variants scattered across the whole coding sequence rather than recurrent hotspots, the entire gene is sequenced. The yield in a referral population is high enough to justify including ACTN1 in any inherited-thrombocytopenia panel.

Show evidence (2 references)

PMID:31237726 SUPPORT Human Clinical

"To decipher the spectrum of variants and phenotype of ACTN1-related thrombocytopenia, we sequenced the ACTN1 gene in 272 cases of unexplained chronic or familial thrombocytopenia."

Establishes targeted ACTN1 sequencing of unexplained thrombocytopenia as the diagnostic approach in practice.

PMID:23434115 SUPPORT Human Clinical

"We herein performed whole-exome sequencing and targeted Sanger sequencing to identify mutations that cause CMTP, in which a dominant mode of transmission had been suspected but for which no known responsible mutations have been documented."

The exome route, which is what is used when the gene is not yet suspected.

Reticulated platelet count and serum thrombopoietin

Not required for diagnosis, but the pair of measurements that distinguishes a production defect from peripheral destruction. In ACTN1-RT the reticulated platelet count is low and thrombopoietin only slightly raised - the profile of impaired late platelet release, and the opposite of what immune thrombocytopenia would give.

Show evidence (1 reference)

PMID:25361813 SUPPORT Human Clinical

"Low reticulated platelet counts and only slightly increased serum thrombopoietin levels indicated that the latest phases of megakaryopoiesis were affected."

The measurements and the inference the authors drew from them.

📊

Prevalence

3

Japanese pedigrees with congenital macrothrombocytopenia

Unknown Unknown

Diagnostic yield within a disease-class cohort, not a population rate. ACTN1 variants accounted for 5.5% of dominant congenital macrothrombocytopenia in the Japanese cohort of the discovery study and ranked as the fourth most common cause there. Recorded with measure_type UNKNOWN because none of the available measure types describes a share of a cohort of patients with a related disease, which is the only quantitative epidemiology this disorder has.

Show evidence (1 reference)

PMID:23434115 SUPPORT Human Clinical

"In the entire cohort, ACNT1 variants accounted for 5.5% of the dominant forms of CMTP cases and represented the fourth most common cause in Japanese individuals."

The quoted figure is the share of dominant congenital macrothrombocytopenia attributable to ACTN1 in that cohort. The gene symbol is misspelled ACNT1 in the published abstract and is quoted here as printed.

Italian probands screened for inherited thrombocytopenia of unknown origin

Unknown Unknown

ACTN1 variants explained 4.2% of a 128-proband inherited-thrombocytopenia referral series. Again a diagnostic yield rather than a prevalence, and subject to ascertainment: the cohort was pre-selected for unexplained thrombocytopenia.

Show evidence (1 reference)

PMID:25361813 SUPPORT Human Clinical

"Given its relatively high frequency in our cohort (4.2%), ACTN1-RT has to be taken into consideration in the differential diagnosis of ITs."

States the frequency of the disorder within the screened inherited-thrombocytopenia cohort.

Worldwide

Unknown Unknown

No population prevalence has been published for ACTN1-RT. The only worldwide statement in the literature is a rank - fourth most frequent inherited thrombocytopenia - which cannot be converted into cases per 100,000, so rate_per_100000 is deliberately left empty rather than estimated.

Show evidence (1 reference)

PMID:30351444 SUPPORT Human Clinical

"We concluded that ACTN1-RT is the fourth most frequent form of IT worldwide and it is characterized by platelet macrocytosis in all affected subjects and mild thrombocytopenia in less than 80% of cases."

The only worldwide frequency claim available, and it is ordinal rather than a rate.

🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from Platelet-type Bleeding Disorder 15:

Immune thrombocytopenia

Overlapping Features The misdiagnosis that matters, because it is treated. A lifelong stable macrothrombocytopenia mistaken for immune thrombocytopenia attracts corticosteroids, immunosuppression and occasionally splenectomy, none of which can help a defect of platelet production. Two findings separate them: a family history with the same blood picture, and a low reticulated platelet count with near-normal thrombopoietin, which is a production profile rather than a destruction profile.

Distinguishing Features

Lifelong stable course with an affected first-degree relative
Low reticulated platelet count and only slightly increased serum thrombopoietin
Enlarged platelets with anisocytosis rather than a normal platelet size distribution

Show evidence (2 references)

PMID:30471777 SUPPORT INDIRECT Human Clinical

"We describe the case of a 27-year-old Caucasian woman, diagnosed initially with idiopathic thrombocytopenic purpura."

Documents that an inherited macrothrombocytopenia is misdiagnosed as immune thrombocytopenia in practice. Marked INDIRECT because the reported patient had MYH9-related disease, not ACTN1-RT, so the quote supports the general diagnostic trap rather than a case of this disorder.

PMID:25361813 SUPPORT Human Clinical

"Low reticulated platelet counts and only slightly increased serum thrombopoietin levels indicated that the latest phases of megakaryopoiesis were affected."

Provides the laboratory discriminator - a production defect, where immune thrombocytopenia is a destruction process.

Other inherited thrombocytopenias

Overlapping Features ACTN1-RT sits inside a differential of dozens of inherited thrombocytopenias, and its practical significance is that it is common enough within that group to be worth testing for early. The others that most need excluding are Bernard-Soulier syndrome, which is recessive and bleeds far more severely, and ANKRD26- and ETV6-related thrombocytopenia, which carry a risk of myeloid malignancy that ACTN1-RT does not.

Distinguishing Features

Severe mucocutaneous bleeding and absent ristocetin-induced agglutination in Bernard-Soulier syndrome
Predisposition to myeloid malignancy in ANKRD26- and ETV6-related thrombocytopenia
Normal platelet function testing in ACTN1-RT

Show evidence (1 reference)

PMID:25361813 SUPPORT Human Clinical

"Given its relatively high frequency in our cohort (4.2%), ACTN1-RT has to be taken into consideration in the differential diagnosis of ITs."

States the disorder's place in the inherited-thrombocytopenia differential and the frequency argument for testing it early.

🧫

Experimental Models

3

Mouse fetal liver-derived megakaryocytes transduced with disease-associated ACTN1 variants PRIMARY_CELL_CULTURE

Primary megakaryocytes cultured from mouse fetal liver and transduced with human disease-associated ACTN1 alleles. It remains the only system in which the disease alleles have been shown to change proplatelet architecture, and it is the source of the mechanistic claim at the centre of this entry.

Organism

house mouse NCBITaxon:10090

Publication

PMID:23434115

Pathograph links

Impaired Proplatelet Formation recapitulates fidelity: moderate observes: cellular Reproduces both halves of the predicted output defect in a megakaryocyte - fewer proplatelet tips, and tips that are abnormally large.

Limitations: Mouse rather than human megakaryocytes, from fetal liver rather than adult marrow, and the human allele is introduced by transduction on top of two normal mouse Actn1 alleles, so the stoichiometry is not that of a heterozygous patient. Proplatelet tips are counted in culture without the marrow sinusoidal shear that shapes platelet release in vivo, and the size of the platelets actually shed was not measured.

Readouts

Proplatelet tip number

Direction: DECREASED

Interpretation: Fewer release sites per megakaryocyte, the cellular correlate of the low platelet count.

Show evidence (1 reference)

PMID:23434115 SUPPORT Model Organism

"Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."

The measurement of reduced tip number.

Proplatelet tip size

Direction: INCREASED

Interpretation: Larger release sites, the cellular correlate of the enlarged platelets.

Show evidence (1 reference)

PMID:23434115 SUPPORT Model Organism

"Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."

The same sentence reports tip size; it is the only published measurement of this readout.

Show evidence (1 reference)

PMID:23434115 SUPPORT Model Organism

"Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."

Establishes the model as informative for the proplatelet node - it carries a disease allele and reports the proplatelet phenotype.

Show evidence (1 reference)

PMID:23434115 SUPPORT Model Organism

"Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."

Establishes that this culture system exists and was used to express disease-associated ACTN1 alleles in megakaryocytes.

Chinese hamster ovary cells expressing mutant ACTN1 CELL_LINE

The workhorse assay of this field. Mutant alpha-actinin-1 is expressed in Chinese hamster ovary cells and the actin cytoskeleton is imaged. Its value is throughput and reproducibility across laboratories - the same readout has been applied to actin-binding-domain, calmodulin-domain and spectrin-repeat alleles and gives the same answer - rather than physiological fidelity.

Publication

PMID:23434115

Pathograph links

Disorganized Megakaryocyte Actin Cytoskeleton partially recapitulates fidelity: low observes: cellular Reproduces the actin disorganisation caused by disease alleles, and is the assay by which most reported variants have been called deleterious.

Limitations: Chinese hamster ovary cells are non-haematopoietic fibroblast-like cells with no megakaryocyte biology at all - no polyploidisation, no demarcation membrane system, no proplatelets - so the model shows that the mutant protein can disorganise an actin network, not that it disorganises the specific circumferential network a megakaryocyte uses to make platelets. Expression is heterologous and typically above endogenous levels.

Readouts

Actin cytoskeletal organisation on immunofluorescence

Direction: ALTERED

Interpretation: Disorganised actin filament architecture in cells expressing a disease allele.

Show evidence (1 reference)

PMID:26453073 SUPPORT In Vitro

"Consistent with the previous reports of mutations that reside in the ABD or the CaM domain, immunofluorescence examination revealed disorganization of the actin cytoskeleton in Gln395 mutant-transduced Chinese hamster ovary cells."

The readout as applied to a rod-domain allele, and its agreement with earlier alleles.

Show evidence (1 reference)

PMID:23434115 SUPPORT In Vitro

"In vitro transfection experiments in Chinese hamster ovary cells demonstrated that altered α-actinin-1 disrupted the normal actin-based cytoskeletal structure."

Establishes the assay as informative for the actin-disorganisation node.

Show evidence (1 reference)

PMID:23434115 SUPPORT In Vitro

"In vitro transfection experiments in Chinese hamster ovary cells demonstrated that altered α-actinin-1 disrupted the normal actin-based cytoskeletal structure."

Establishes the assay system and the result that made it the standard test for a candidate ACTN1 variant.

In vitro actin bundling assays with recombinant mutant alpha-actinin-1 OTHER

Purified recombinant alpha-actinin-1 carrying disease substitutions, assayed for filament bundling and for actin association, plus imaging of mutant protein in cultured HeLa cells. This is the only system that measures the direction of the biochemical change rather than inferring it from morphology, and it is why this entry curates the lesion as increased rather than decreased actin association.

Publication

PMID:31365757

Pathograph links

Increased Alpha-Actinin-1 Actin Cross-Linking Activity measures fidelity: moderate observes: molecular Quantifies filament bundling by nine calmodulin-like and rod-domain mutants and the cytoskeletal association of mutant protein in cells.

Limitations: A cell-free system with purified protein and F-actin, so it reports the intrinsic property of the mutant protein rather than its behaviour in a megakaryocyte, where actin dynamics are shaped by many competing cross-linkers and by calcium signalling. The cellular arm uses HeLa cells, which are as remote from megakaryocytes as Chinese hamster ovary cells are.

Readouts

Actin filament bundling activity

Direction: INCREASED

Interpretation: Disease alleles bundle actin filaments more effectively than wild-type protein.

Show evidence (1 reference)

PMID:31365757 SUPPORT In Vitro

"In this study, we examined nine CMTP-causing mutations in the calmodulin-like and rod domains of actinin-1. These mutations increase, to varying degrees, actinin's ability to bundle actin filaments in vitro."

The bundling measurement itself, across nine disease alleles.

Cytoskeletal association of mutant actinin in cells

Direction: INCREASED

Interpretation: Two mutants show increased association with the cellular cytoskeleton, extending the cell-free result into cells.

Show evidence (1 reference)

PMID:31365757 SUPPORT In Vitro

"The G764S and E769K mutations increase cytoskeletal association of actinin in cells, and all mutant proteins colocalize with F-actin in cultured HeLa cells."

Names the two alleles for which the increase was demonstrated in cells; the remaining seven were shown only to colocalise with F-actin.

Show evidence (1 reference)

PMID:31365757 SUPPORT In Vitro

"Thus, CMTP-causing actinin-1 mutations outside the actin-binding domain also increase actin association, suggesting a common molecular mechanism underlying actinin-1 related CMTP."

Establishes the assay as informative for the cross-linking node and states the conclusion drawn from it.

Show evidence (1 reference)

PMID:31365757 SUPPORT In Vitro

"Thus, CMTP-causing actinin-1 mutations outside the actin-binding domain also increase actin association, suggesting a common molecular mechanism underlying actinin-1 related CMTP."

Establishes the cell-free assay system and the conclusion the authors drew from it.

🐁

Animal Models

1

Megakaryocyte-specific alpha-actinin-1 knockout mouse (PF4-Actn1-/-)

The only in vivo model of altered megakaryocyte alpha-actinin-1. It deletes the protein in the megakaryocyte lineage, which is a different genetic lesion from the human disease: patients are heterozygous for missense alleles whose measured effect is increased actin association, not absence of protein. The model is therefore informative about what alpha-actinin-1 is needed for in thrombocytopoiesis, and only partly informative about ACTN1-RT.

Species

Mouse

Genotype

Actn1 conditional knockout crossed to PF4-Cre, giving megakaryocyte-lineage deletion

Genes

ACTN1 hgnc:163

Publication

PMID:39813624

Pathograph links

Impaired Proplatelet Formation partially recapitulates fidelity: low observes: organism Reproduces the low platelet count and traces it to defective thrombocytopoiesis, including a reduced proportion of megakaryocytes bearing proplatelets - the same cellular step this node names.

Limitations: A knockout, not a knock-in of a patient allele, so it models absence of alpha-actinin-1 rather than the gain of actin association the human alleles produce, and the two predict opposite effects on cross-linking. The knockout mice also have reduced megakaryocyte numbers and shifted megakaryocyte ploidy, which patients are not reported to have.

Divergences from the human mechanism

cause unrepresented qualifying

The human lesion is a heterozygous missense substitution that increases alpha-actinin-1 association with F-actin. The model removes the protein from the lineage entirely, so the specific molecular cause curated upstream in this entry is not present in the model at all.

species mismatch qualifying

Mouse megakaryopoiesis differs from human in platelet size distribution and marrow kinetics, and mouse platelets are smaller, so a platelet-size phenotype measured in the mouse is not directly comparable with human mean platelet volume.

Readouts

Proportion of megakaryocytes forming proplatelets

Direction: DECREASED

Interpretation: Fewer megakaryocytes extend proplatelets when alpha-actinin-1 is absent.

Show evidence (1 reference)

PMID:39813624 SUPPORT Model Organism

"Colony-forming unit-MK colony formation, the ratio of proplatelet formation-bearing MKs, and MK migration in response to stromal cell-derived factor-1 signaling were inhibited in PF4-Actn1-/- mice."

The measurement of reduced proplatelet-forming megakaryocytes.

Peripheral platelet count

Direction: DECREASED

Interpretation: The organism-level consequence of the thrombocytopoiesis defect.

Show evidence (1 reference)

PMID:39813624 SUPPORT Model Organism

"The decreased platelet number in PF4-Actn1-/- mice was due to defects in thrombocytopoiesis."

States both the platelet-count phenotype and its attribution to thrombocytopoiesis rather than to platelet destruction.

Show evidence (1 reference)

PMID:39813624 SUPPORT Model Organism

"Using megakaryocyte (MK)-specific α-actinin-1 knockout (KO; PF4-Actn1-/-) mice, we demonstrated that PF4-Actn1-/- mice exhibited reduced platelet counts."

Establishes the model and the platelet phenotype that makes it relevant to this node.

Reduced Platelet Output with Increased Platelet Size fails to recapitulate fidelity: low observes: organism The node asserts reduced platelet output with preserved platelet function. The knockout mouse has the reduced output but not the preserved function: spreading, clot retraction, aggregation, integrin activation and granule release are all impaired, and haemostasis is impaired in vivo. Human ACTN1-RT is defined partly by the absence of exactly those abnormalities.

Limitations: Complete absence of alpha-actinin-1 from the lineage removes a scaffold the mature platelet needs for integrin signalling and spreading, which a heterozygous missense allele does not. The discrepancy is therefore expected from the genetics and does not impugn the model; it means the model cannot be used to argue about bleeding risk or platelet function in patients.

Divergences from the human mechanism

cause unrepresented invalidating

Knockout of the gene versus a heterozygous gain-of-actin-association missense allele. For the claim that platelet function is preserved, this difference is not a caveat but a reversal - the model reports the opposite of the human finding, and cannot be cited for it.

Show evidence (2 references)

PMID:39813624 REFUTE Model Organism

"Platelet spreading, clot retraction, aggregation, integrin αIIbβ3 activation, and CD62P exposure in response to various agonists were decreased in PF4-Actn1-/- platelets."

The platelet-function defect that the human disorder lacks, which is what makes this a failure to recapitulate rather than a partial success.

+ 1 more reference

Show evidence (1 reference)

PMID:39813624 SUPPORT Model Organism

"Using megakaryocyte (MK)-specific α-actinin-1 knockout (KO; PF4-Actn1-/-) mice, we demonstrated that PF4-Actn1-/- mice exhibited reduced platelet counts."

Establishes that this mouse line exists and states its principal haematological phenotype.

{ }

Source YAML

click to show

name: Platelet-type Bleeding Disorder 15
creation_date: "2026-09-05T06:37:27Z"
category: Mendelian
description: >-
  Platelet-type bleeding disorder 15 (BDPLT15), usually called ACTN1-related
  thrombocytopenia in the haematology literature, is an autosomal dominant
  inherited thrombocytopenia caused by heterozygous missense variants in ACTN1,
  the gene for the non-muscle actin cross-linking protein alpha-actinin-1.
  Alpha-actinin-1 bundles actin filaments in the megakaryocyte, and the
  disease-associated substitutions - distributed across the N-terminal
  actin-binding domain, the spectrin-like rod repeats and the C-terminal
  calmodulin-like domain - converge on a single biochemical effect: they
  increase the protein's association with F-actin and its ability to bundle
  filaments. The megakaryocyte actin cytoskeleton is disorganised as a result,
  proplatelet formation is distorted so that fewer and abnormally large
  proplatelet tips are produced, and the circulating consequence is a reduced
  number of enlarged platelets with marked size variation. Unlike most entries
  in the OMIM platelet-type bleeding disorder series, the defect is quantitative
  and morphological rather than functional: platelet activation, secretion and
  aggregation are preserved, and the bleeding tendency is correspondingly mild
  or absent. Its clinical importance is diagnostic rather than therapeutic -
  recognising it prevents a benign lifelong trait being managed as immune
  thrombocytopenia.
parents:
- hereditary disease
synonyms:
- BDPLT15
- ACTN1-related thrombocytopenia
- ACTN1-related macrothrombocytopenia
- ACTN1-RT
- autosomal dominant macrothrombocytopenia ACTN1-related
- bleeding disorder, platelet-type, 15
disease_term:
  preferred_term: Platelet-type bleeding disorder 15
  term:
    id: MONDO:0014078
    label: platelet-type bleeding disorder 15
classifications:
  harrisons_chapter:
  - classification_value: ONCOLOGY_HEMATOLOGY
    evidence:
    - reference: PMID:30351444
      reference_title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "We concluded that ACTN1-RT is the fourth most frequent form of IT worldwide and it is characterized by platelet macrocytosis in all affected subjects and mild thrombocytopenia in less than 80% of cases."
      explanation: Places the disorder among the inherited thrombocytopenias, a haematological disease class.
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    evidence:
    - reference: PMID:23434115
      reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "In 13 Japanese CMTP-affected pedigrees, we identified six (46%) affected by ACTN1 variants cosegregating with CMTP."
      explanation: Establishes a single-gene Mendelian basis with cosegregation in pedigrees.
external_assertions:
- name: OMIM bleeding disorder, platelet-type, 15 record
  source: OMIM
  assertion_type: disease_record
  external_id: OMIM:615193
  url: https://omim.org/entry/615193
  description: >-
    The OMIM phenotype record that MONDO:0014078 xrefs, and the source of the
    entry name used here. Recorded structurally rather than in prose because the
    clinical literature almost never uses the OMIM name - papers say
    ACTN1-related thrombocytopenia - so the identity of the concept curated here
    should be auditable against the source identifier. Note this is not
    mappings.omim_mappings: the DiseaseMappings class carries only ICD-10-CM,
    ICD-11, MONDO and NCIT slots.
references:
- reference: PMID:23434115
  title: ACTN1 mutations cause congenital macrothrombocytopenia.
- reference: PMID:24069336
  title: A missense mutation in the alpha-actinin 1 gene (ACTN1) is the cause of autosomal dominant macrothrombocytopenia in a large French family.
- reference: PMID:25361813
  title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
- reference: PMID:26453073
  title: ACTN1 rod domain mutation associated with congenital macrothrombocytopenia.
- reference: PMID:30351444
  title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
- reference: PMID:30471777
  title: MYH9 Associated nephropathy.
- reference: PMID:31237726
  title: Novel ACTN1 variants in cases of thrombocytopenia.
- reference: PMID:31365757
  title: Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia.
- reference: PMID:38594875
  title: "ACTN1-related thrombocytopenia: Homozygosity for an ACTN1 variant results in a more severe phenotype."
- reference: PMID:39813624
  title: α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment.
- reference: PMID:41153762
  title: "α-Actinin-1 in Megakaryocytes: Its Structure, Interacting Proteins and Implications for Thrombopoiesis."
inheritance:
- name: Autosomal dominant inheritance
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  penetrance: INCOMPLETE
  description: >-
    Heterozygous ACTN1 missense variants segregate with macrothrombocytopenia in
    dominant pedigrees, demonstrated first in six Japanese families and then in a
    six-generation French kindred with 26 affected subjects. Penetrance is
    recorded as incomplete because the two components of the phenotype behave
    differently: in the largest assembled cohort platelet macrocytosis was
    present in every affected subject while thrombocytopenia was present in fewer
    than 80%, so a carrier with a normal platelet count is expected rather than
    anomalous. A homozygous variant has been reported in two sisters whose
    haematological phenotype was more severe than that of their heterozygous
    relatives, which makes the inheritance semidominant with a gene-dosage effect
    rather than strictly dominant, but the heterozygous state is itself fully
    disease-causing.
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In 13 Japanese CMTP-affected pedigrees, we identified six (46%) affected by ACTN1 variants cosegregating with CMTP."
    explanation: Cosegregation of heterozygous ACTN1 variants with disease in dominantly transmitting pedigrees.
  - reference: PMID:24069336
    reference_title: A missense mutation in the alpha-actinin 1 gene (ACTN1) is the cause of autosomal dominant macrothrombocytopenia in a large French family.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Candidate gene analysis complemented by targeted next-generation sequencing identified a missense mutation (c.137GA; p.Arg46Gln) in the alpha-actinin 1 gene (ACTN1) that segregated with macrothrombocytopenia in this large pedigree."
    explanation: Independent segregation of a single heterozygous allele through a six-generation dominant pedigree.
  - reference: PMID:30351444
    reference_title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We concluded that ACTN1-RT is the fourth most frequent form of IT worldwide and it is characterized by platelet macrocytosis in all affected subjects and mild thrombocytopenia in less than 80% of cases."
    explanation: >-
      Supports incomplete penetrance of the thrombocytopenia component
      specifically - fewer than 80% of carriers have a low platelet count - while
      the macrocytosis component was universal in this series.
prevalence:
- population: Japanese pedigrees with congenital macrothrombocytopenia
  measure_type: UNKNOWN
  prevalence_class: UNKNOWN
  notes: >-
    Diagnostic yield within a disease-class cohort, not a population rate. ACTN1
    variants accounted for 5.5% of dominant congenital macrothrombocytopenia in
    the Japanese cohort of the discovery study and ranked as the fourth most
    common cause there. Recorded with measure_type UNKNOWN because none of the
    available measure types describes a share of a cohort of patients with a
    related disease, which is the only quantitative epidemiology this disorder
    has.
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In the entire cohort, ACNT1 variants accounted for 5.5% of the dominant forms of CMTP cases and represented the fourth most common cause in Japanese individuals."
    explanation: >-
      The quoted figure is the share of dominant congenital macrothrombocytopenia
      attributable to ACTN1 in that cohort. The gene symbol is misspelled ACNT1 in
      the published abstract and is quoted here as printed.
- population: Italian probands screened for inherited thrombocytopenia of unknown origin
  measure_type: UNKNOWN
  prevalence_class: UNKNOWN
  notes: >-
    ACTN1 variants explained 4.2% of a 128-proband inherited-thrombocytopenia
    referral series. Again a diagnostic yield rather than a prevalence, and
    subject to ascertainment: the cohort was pre-selected for unexplained
    thrombocytopenia.
  evidence:
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Given its relatively high frequency in our cohort (4.2%), ACTN1-RT has to be taken into consideration in the differential diagnosis of ITs."
    explanation: States the frequency of the disorder within the screened inherited-thrombocytopenia cohort.
- population: Worldwide
  measure_type: UNKNOWN
  prevalence_class: UNKNOWN
  notes: >-
    No population prevalence has been published for ACTN1-RT. The only worldwide
    statement in the literature is a rank - fourth most frequent inherited
    thrombocytopenia - which cannot be converted into cases per 100,000, so
    rate_per_100000 is deliberately left empty rather than estimated.
  evidence:
  - reference: PMID:30351444
    reference_title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We concluded that ACTN1-RT is the fourth most frequent form of IT worldwide and it is characterized by platelet macrocytosis in all affected subjects and mild thrombocytopenia in less than 80% of cases."
    explanation: The only worldwide frequency claim available, and it is ordinal rather than a rate.
genetic:
- name: ACTN1
  gene_term:
    preferred_term: ACTN1
    term:
      id: hgnc:163
      label: ACTN1
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  presence: PRESENT
  notes: >-
    ACTN1 (14q24.1) encodes alpha-actinin-1, one of the two non-muscle
    alpha-actinin isoforms, built from an N-terminal actin-binding domain, four
    spectrin-like repeats forming the antiparallel dimerisation rod, and a
    C-terminal calmodulin-like domain. Disease alleles are heterozygous missense
    substitutions and have now been found in all three regions: p.Arg46Gln in the
    actin-binding domain, p.Leu395Gln in spectrin-like repeat 2, and
    calmodulin-domain substitutions in the original Japanese series. Reported
    variants are largely private to families rather than recurrent, so there is
    no hotspot to screen and the whole coding sequence is sequenced. One
    homozygous variant, c.982G>A, has been described.
  evidence:
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "To better characterize this disease, we screened ACTN1 in 128 probands and found 10 (8 novel) missense heterozygous variants in 11 families."
    explanation: Establishes the heterozygous missense allelic spectrum in an unselected referral cohort.
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Combining bioinformatics, segregation, and functional studies, we demonstrated that all but 1 amino acid substitution had deleterious effects."
    explanation: >-
      The functional arm of the same study, split into its own item so the
      evidence_source describes the assay rather than the cohort. One of the ten
      substitutions was not shown to be deleterious.
  - reference: PMID:31237726
    reference_title: Novel ACTN1 variants in cases of thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We identified 15 rare, monoallelic, nonsynonymous and likely pathogenic ACTN1 variants in 20 index cases from 20 unrelated families."
    explanation: The largest single sequencing series, confirming monoallelic nonsynonymous variants as the disease allele class.
  - reference: PMID:24069336
    reference_title: A missense mutation in the alpha-actinin 1 gene (ACTN1) is the cause of autosomal dominant macrothrombocytopenia in a large French family.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The missense mutation occurred within actin-binding domain of alpha-actinin 1, a functionally critical domain that crosslinks actin filaments into bundles."
    explanation: Locates the classic p.Arg46Gln allele in the actin-binding domain and names the function it perturbs.
  - reference: PMID:26453073
    reference_title: ACTN1 rod domain mutation associated with congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In this report, we describe a mutation in SLR2 of α-actinin-1 (p.Leu395Gln) associated with familial macrothrombocytopenia."
    explanation: Extends the allelic spectrum into the spectrin-like rod, which earlier reports had not implicated.
pathophysiology:
- name: ACTN1 Missense Variant Altering Alpha-Actinin-1
  biological_scale: MOLECULAR
  role: trigger
  mechanism_confidence: ESTABLISHED
  description: >-
    The initiating lesion is a heterozygous germline missense substitution in
    ACTN1. Alpha-actinin-1 is an actin-crosslinking protein of the spectrin
    superfamily that organises the cytoskeleton, and it works as an antiparallel
    dimer whose two actin-binding domains hold neighbouring filaments together.
    Substitutions in the actin-binding domain, in the spectrin-like rod repeats
    and in the calmodulin-like domain all cause the same disease, which is the
    first clue that they share a downstream effect rather than each disabling a
    separate function.
  genes:
  - preferred_term: ACTN1
    term:
      id: hgnc:163
      label: ACTN1
  genetic_context:
    functional_impact_category: GAIN_OF_FUNCTION
    variant_origin: GERMLINE
    description: >-
      Heterozygous germline missense substitutions. Recorded as a gain of
      function because the measured biochemical effect of the disease alleles is
      an increase - higher affinity for F-actin for actin-binding-domain
      substitutions, and greater filament-bundling activity for calmodulin-like
      and rod-domain substitutions. This runs against the loss-of-crosslinking
      reading that the phenotype superficially suggests, and the disagreement is
      carried explicitly as a mechanistic hypothesis and an open discussion in
      this entry rather than settled here.
  cell_types:
  - preferred_term: Megakaryocyte
    term:
      id: CL:0000556
      label: megakaryocyte
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "ACTN1 encodes α-actinin-1, a member of the actin-crosslinking protein superfamily that participates in the organization of the cytoskeleton."
    explanation: Identifies the gene product and the molecular function the variants perturb.
  - reference: PMID:31237726
    reference_title: Novel ACTN1 variants in cases of thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Nine variants were located in the α-actinin-1 (ACTN1) rod domain and were predicted to hinder dimer formation."
    explanation: Documents the rod-domain arm of the allelic spectrum and its predicted structural consequence.
  downstream:
  - target: Increased Alpha-Actinin-1 Actin Cross-Linking Activity
    description: >-
      The substitution changes the protein's interaction with F-actin. Shown
      biochemically for nine calmodulin-like and rod-domain mutants and stated
      for actin-binding-domain mutants.
    causal_link_type: DIRECT
    hypothesis_groups:
    - actn1_increased_actin_association
    evidence:
    - reference: PMID:31365757
      reference_title: Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Actinin-1 mutations cause dominantly inherited congenital macrothrombocytopenia (CMTP), with mutations in the actin-binding domain increasing actinin's affinity for F-actin."
      explanation: States the direct consequence of an actin-binding-domain substitution on actin affinity.
- name: Increased Alpha-Actinin-1 Actin Cross-Linking Activity
  biological_scale: MOLECULAR
  role: central_effector
  mechanism_confidence: ESTABLISHED
  description: >-
    Mutant alpha-actinin-1 binds and bundles actin filaments more avidly than
    wild type. Nine calmodulin-like and rod-domain mutants were shown to increase
    filament bundling in vitro to varying degrees, two of them measurably
    increased the cytoskeletal association of actinin inside cells, and the
    actin-binding-domain mutants raise affinity for F-actin directly. The authors
    of that work read the convergence as a common molecular mechanism for
    ACTN1-linked macrothrombocytopenia, which is why this node sits on the main
    chain rather than being treated as one of several parallel biochemical
    consequences.
  molecular_functions:
  - preferred_term: actin filament binding
    modifier: INCREASED
    term:
      id: GO:0051015
      label: actin filament binding
  biological_processes:
  - preferred_term: actin crosslink formation
    modifier: INCREASED
    term:
      id: GO:0051764
      label: actin crosslink formation
  - preferred_term: actin filament bundle assembly
    modifier: INCREASED
    term:
      id: GO:0051017
      label: actin filament bundle assembly
  evidence:
  - reference: PMID:31365757
    reference_title: Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In this study, we examined nine CMTP-causing mutations in the calmodulin-like and rod domains of actinin-1. These mutations increase, to varying degrees, actinin's ability to bundle actin filaments in vitro."
    explanation: Direct biochemical measurement of increased bundling activity for disease alleles outside the actin-binding domain.
  - reference: PMID:31365757
    reference_title: Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Thus, CMTP-causing actinin-1 mutations outside the actin-binding domain also increase actin association, suggesting a common molecular mechanism underlying actinin-1 related CMTP."
    explanation: The authors' own statement that increased actin association is the shared mechanism across domains.
  - reference: PMID:41153762
    reference_title: "α-Actinin-1 in Megakaryocytes: Its Structure, Interacting Proteins and Implications for Thrombopoiesis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The pathophysiological mechanisms by which α-actinin-1 mutations lead to macrothrombocytopenia have been attributed to alterations in actin organization, increased binding affinity of α-actinin-1 to actin filaments, and modulation of integrin αIIbβ3 signaling."
    explanation: >-
      A review's summary of the field, cited to show that increased actin binding
      is an accepted account rather than a single group's result. Graded OTHER
      because the source is a narrative review, not primary data.
  downstream:
  - target: Disorganized Megakaryocyte Actin Cytoskeleton
    description: >-
      Excessive cross-linking distorts the actin network that mutant-expressing
      cells assemble.
    causal_link_type: DIRECT
- name: Disorganized Megakaryocyte Actin Cytoskeleton
  biological_scale: CELLULAR
  role: central_effector
  mechanism_confidence: ESTABLISHED
  description: >-
    Expression of a disease-associated ACTN1 allele disorganises the actin
    cytoskeleton. The finding is unusually well replicated for a rare disorder:
    it has been reproduced in Chinese hamster ovary cells for actin-binding and
    calmodulin-domain alleles, in the same system for a rod-domain allele, in
    COS-7 cells and cultured megakaryocytes for the French p.Arg46Gln allele, and
    in the expression work accompanying the largest variant series, where the
    network was disorganised and the fibres themselves were thicker. Thickened
    fibres are the morphological counterpart of the increased bundling measured
    biochemically upstream.
  cell_types:
  - preferred_term: Megakaryocyte
    term:
      id: CL:0000556
      label: megakaryocyte
  biological_processes:
  - preferred_term: actin cytoskeleton organization
    modifier: ABNORMAL
    term:
      id: GO:0030036
      label: actin cytoskeleton organization
  locations:
  - preferred_term: bone marrow
    term:
      id: UBERON:0002371
      label: bone marrow
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In vitro transfection experiments in Chinese hamster ovary cells demonstrated that altered α-actinin-1 disrupted the normal actin-based cytoskeletal structure."
    explanation: The original demonstration that the mutant protein disrupts actin architecture.
  - reference: PMID:31237726
    reference_title: Novel ACTN1 variants in cases of thrombocytopenia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In vitro expression of the new ACTN1 variants induced actin network disorganization and led to increased thickness of actin fibers."
    explanation: >-
      Replicates the disorganisation for eleven previously unreported variants and
      adds fibre thickening, the morphological signature of excess bundling.
  - reference: PMID:26453073
    reference_title: ACTN1 rod domain mutation associated with congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Consistent with the previous reports of mutations that reside in the ABD or the CaM domain, immunofluorescence examination revealed disorganization of the actin cytoskeleton in Gln395 mutant-transduced Chinese hamster ovary cells."
    explanation: Extends the same cellular phenotype to a spectrin-like-repeat allele, supporting convergence across domains.
  - reference: PMID:24069336
    reference_title: A missense mutation in the alpha-actinin 1 gene (ACTN1) is the cause of autosomal dominant macrothrombocytopenia in a large French family.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The evaluation of cultured mutation-harboring megakaryocytes by electron microscopy and the immunofluorescence examination of transfected COS-7 cells suggested that the mutation causes disorganization of the cellular cytoplasm."
    explanation: >-
      The only observation made in megakaryocytes carrying a patient's own allele
      rather than in a transduced heterologous cell; the authors state it as
      suggestive rather than demonstrated.
  downstream:
  - target: Impaired Proplatelet Formation
    description: >-
      Proplatelet extension and branching are actin-dependent, so a disorganised
      circumferential actin network changes the geometry of the proplatelets a
      megakaryocyte can build.
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:23434115
      reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."
      explanation: >-
        The single experiment that carries this edge: within one megakaryocyte
        preparation, the disorganised cytoskeleton and the altered proplatelet tips
        are reported as cause and effect.
- name: Impaired Proplatelet Formation
  biological_scale: CELLULAR
  role: central_effector
  mechanism_confidence: ESTABLISHED
  description: >-
    Megakaryocytes expressing a disease allele extend fewer proplatelet
    processes, and the tips those processes end in are abnormally large. Because
    each tip is released as a platelet, that geometry predicts the clinical
    picture directly: fewer platelets, each bigger. Human data agree that the
    lesion is late rather than early in megakaryopoiesis - patients have low
    reticulated platelet counts with only slightly raised thrombopoietin, which
    is the profile of impaired platelet release rather than of failed
    megakaryocyte production or of peripheral platelet destruction.
  cell_types:
  - preferred_term: Megakaryocyte
    term:
      id: CL:0000556
      label: megakaryocyte
  biological_processes:
  - preferred_term: platelet formation
    modifier: DECREASED
    term:
      id: GO:0030220
      label: platelet formation
  - preferred_term: megakaryocyte development
    modifier: ABNORMAL
    term:
      id: GO:0035855
      label: megakaryocyte development
  locations:
  - preferred_term: bone marrow
    term:
      id: UBERON:0002371
      label: bone marrow
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."
    explanation: The direct observation of fewer and larger proplatelet tips in megakaryocytes carrying a disease allele.
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Low reticulated platelet counts and only slightly increased serum thrombopoietin levels indicated that the latest phases of megakaryopoiesis were affected."
    explanation: >-
      Human laboratory evidence locating the defect in the final steps of
      megakaryopoiesis, consistent with a proplatelet-release problem rather than a
      production or destruction problem.
  - reference: PMID:41153762
    reference_title: "α-Actinin-1 in Megakaryocytes: Its Structure, Interacting Proteins and Implications for Thrombopoiesis."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "As a pivotal scaffold protein, α-actinin-1 interacts with a complex network of partners, including integrin αIIbβ3, and actin filaments, to modulate cytoskeletal dynamics, megakaryocyte maturation, and proplatelet formation."
    explanation: >-
      Establishes that proplatelet formation is among the normal functions
      alpha-actinin-1 supports, which is what makes it the plausible step to fail.
      Graded OTHER as a narrative review.
  downstream:
  - target: Reduced Platelet Output with Increased Platelet Size
    description: >-
      Fewer tips means fewer platelets; larger tips means larger platelets. The
      inference from tip geometry to circulating platelet indices is the authors'
      own and has not been measured directly in a patient's marrow.
    causal_link_type: DIRECT
- name: Reduced Platelet Output with Increased Platelet Size
  biological_scale: ORGANISM
  role: consequence
  mechanism_confidence: ESTABLISHED
  description: >-
    The circulating result: a platelet population reduced in number, enlarged,
    and unusually variable in size. What does not follow is a functional defect.
    Aggregation and secretion responses are preserved in ACTN1-RT, so the
    mechanism ends at a quantitative and morphological abnormality, and the small
    bleeding tendency that follows is proportionate to the platelet count rather
    than amplified by a platelet-function defect. That is the single feature that
    most distinguishes this disorder from the rest of the OMIM platelet-type
    bleeding disorder series.
  cell_types:
  - preferred_term: Platelet
    term:
      id: CL:0000233
      label: platelet
  biological_processes:
  - preferred_term: hemostasis
    modifier: DECREASED
    term:
      id: GO:0007599
      label: hemostasis
  locations:
  - preferred_term: blood
    term:
      id: UBERON:0000178
      label: blood
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency."
    explanation: Pairs the circulating platelet abnormality with the mildness of its clinical consequence.
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical and laboratory findings of 31 affected individuals confirmed that ACTN1-RT is a mild macrothrombocytopenia with low risk for bleeding."
    explanation: Confirms the same combination in an independent cohort of 31 affected individuals.
  downstream:
  - target: Thrombocytopenia
  - target: Platelet Macrocytosis
  - target: Macrothrombocytopenia
  - target: Platelet Anisocytosis
  - target: Mild Mucocutaneous Bleeding Tendency
- name: Biallelic ACTN1 Variant Dosage Effect
  biological_scale: MOLECULAR
  role: modifier
  mechanism_confidence: PROVISIONAL
  description: >-
    Two sisters homozygous for ACTN1 c.982G>A had moderate thrombocytopenia and
    marked platelet macrocytosis with giant platelets, a more severe
    haematological picture than their heterozygous relatives in the same family.
    Carrying twice the dose of mutant protein therefore worsens the phenotype
    quantitatively without changing its character, which is what a
    gain-of-actin-association mechanism predicts. It is curated as a PROVISIONAL
    modifier rather than a step in the chain because it rests on a single family
    with two homozygotes.
  genes:
  - preferred_term: ACTN1
    term:
      id: hgnc:163
      label: ACTN1
  genetic_context:
    variant_origin: GERMLINE
    description: >-
      A single reported homozygous missense allele, c.982G>A. Homozygosity for an
      ACTN1 disease allele is otherwise unreported, so nothing is known about
      whether the dosage effect generalises across alleles.
  evidence:
  - reference: PMID:38594875
    reference_title: "ACTN1-related thrombocytopenia: Homozygosity for an ACTN1 variant results in a more severe phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Within the reported family, the homozygous sisters have moderate thrombocytopenia and marked platelet macrocytosis with giant platelets, revealing a more severe haematological phenotype compared to their heterozygous relatives and highlighting a significant effect of allelic burden on platelet size."
    explanation: >-
      Within-family comparison of homozygotes against heterozygotes, which
      controls for genetic background better than a cross-cohort comparison would.
  downstream:
  - target: Reduced Platelet Output with Increased Platelet Size
    description: >-
      A larger dose of mutant protein produces a quantitatively larger version of
      the same output defect.
    causal_link_type: DIRECT
  - target: Giant Platelets in Homozygous Carriers
    causal_link_type: DIRECT
  - target: Mild Heart Valve Defects in Homozygous Carriers
    description: >-
      Advanced by the reporting authors as a hypothesis, not a demonstrated link -
      the association rests on two related patients in whom exome sequencing found
      no alternative explanation.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    evidence:
    - reference: PMID:38594875
      reference_title: "ACTN1-related thrombocytopenia: Homozygosity for an ACTN1 variant results in a more severe phenotype."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Moreover, we hypothesize that some ACTN1 variants, especially when present in the homozygous state, may also contribute to the cardiac abnormalities."
      explanation: >-
        The authors frame the cardiac link as a hypothesis; the edge is curated at
        exactly that strength.
mechanistic_hypotheses:
- hypothesis_group_id: actn1_increased_actin_association
  hypothesis_label: Disease alleles increase alpha-actinin-1 association with actin
  status: CANONICAL
  description: >-
    The account with direct biochemical support. Actin-binding-domain
    substitutions raise the affinity of alpha-actinin-1 for F-actin, and
    calmodulin-like and rod-domain substitutions raise its filament-bundling
    activity, so alleles scattered across three structurally distinct domains
    converge on a single measurable change in the same direction. Under this
    model the megakaryocyte actin network is over-cross-linked rather than
    under-cross-linked, and the proplatelet defect follows from a cytoskeleton
    too rigidly bundled to remodel normally.
  evidence:
  - reference: PMID:31365757
    reference_title: Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Thus, CMTP-causing actinin-1 mutations outside the actin-binding domain also increase actin association, suggesting a common molecular mechanism underlying actinin-1 related CMTP."
    explanation: The primary statement of the hypothesis by the group that measured it.
- hypothesis_group_id: actn1_loss_of_function
  hypothesis_label: Disease alleles act by loss of alpha-actinin-1 function or dominant-negative interference
  status: ALTERNATIVE
  description: >-
    The reading that the phenotype invites and that much secondary literature
    still repeats: if cross-linking is lost, or if mutant subunits poison
    wild-type dimers, the megakaryocyte cytoskeleton fails and platelet
    production suffers. It is not baseless - megakaryocyte-specific deletion of
    Actn1 in the mouse also produces a low platelet count through defective
    thrombocytopoiesis, so simply removing the protein reproduces part of the
    human picture. What it does not explain is why the human alleles increase
    rather than decrease actin association in vitro, and the knockout mouse
    additionally shows a platelet-function defect and impaired haemostasis that
    ACTN1-RT patients do not have. Kept as ALTERNATIVE rather than deprecated
    because no experiment has yet tested the two models against each other in a
    megakaryocyte carrying a patient allele.
  evidence:
  - reference: PMID:39813624
    reference_title: α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Using megakaryocyte (MK)-specific α-actinin-1 knockout (KO; PF4-Actn1-/-) mice, we demonstrated that PF4-Actn1-/- mice exhibited reduced platelet counts."
    explanation: >-
      Shows that absence of the protein is sufficient for thrombocytopenia, which
      is the strongest evidence available for a loss-of-function reading.
  - reference: PMID:39813624
    reference_title: α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment.
    supports: REFUTE
    evidence_source: MODEL_ORGANISM
    snippet: "Platelet spreading, clot retraction, aggregation, integrin αIIbβ3 activation, and CD62P exposure in response to various agonists were decreased in PF4-Actn1-/- platelets."
    explanation: >-
      Cuts against the loss-of-function model as an account of the human disease:
      losing the protein produces a broad platelet-function defect, whereas
      ACTN1-RT platelets function normally.
phenotypes:
- category: Hematologic
  name: Thrombocytopenia
  description: >-
    Mild in nearly all reported carriers and moderate in the two reported
    homozygotes. It is the less penetrant of the two laboratory abnormalities:
    fewer than 80% of affected subjects in the largest assembled series had a low
    platelet count, so a normal count does not exclude the diagnosis in a family
    where the variant segregates.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Thrombocytopenia
    term:
      id: HP:0001873
      label: Thrombocytopenia
    clinical_course: STABLE
  evidence:
  - reference: PMID:30351444
    reference_title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We concluded that ACTN1-RT is the fourth most frequent form of IT worldwide and it is characterized by platelet macrocytosis in all affected subjects and mild thrombocytopenia in less than 80% of cases."
    explanation: >-
      Supports the FREQUENT band directly: present in fewer than 80% of the 49
      carriers in the largest series, which falls in the 30-79% range.
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical and laboratory findings of 31 affected individuals confirmed that ACTN1-RT is a mild macrothrombocytopenia with low risk for bleeding."
    explanation: Independent confirmation that the thrombocytopenia is mild in an unselected cohort of 31 affected individuals.
- category: Hematologic
  name: Platelet Macrocytosis
  description: >-
    Enlarged platelets with a raised mean platelet volume. This is the more
    penetrant abnormality and in some families the only one, which is why the
    disorder has been described as a benign platelet macrocytosis that is not
    always accompanied by thrombocytopenia. It is not universal across the whole
    allelic spectrum: rod-domain variants raise platelet size less than variants
    elsewhere in the gene, and most carriers of rod-domain variants have normal
    platelet size.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Increased mean platelet volume
    term:
      id: HP:0011877
      label: Increased mean platelet volume
  evidence:
  - reference: PMID:30351444
    reference_title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We concluded that ACTN1-RT is the fourth most frequent form of IT worldwide and it is characterized by platelet macrocytosis in all affected subjects and mild thrombocytopenia in less than 80% of cases."
    explanation: >-
      Macrocytosis was present in every affected subject in the 49-carrier series.
      The band is set at VERY_FREQUENT rather than OBLIGATE because the refuting
      item below shows it is not universal across all variant classes.
  - reference: PMID:31237726
    reference_title: Novel ACTN1 variants in cases of thrombocytopenia.
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "The rod domain, like other ACTN1 functional domains, may be mutated resulting in actin disorganization in vitro and thrombocytopenia with normal platelet size in most cases."
    explanation: >-
      Contradicts platelet macrocytosis being an obligate feature: carriers of
      rod-domain variants mostly have normal platelet size. This is why the
      frequency band is not OBLIGATE.
  - reference: PMID:26453073
    reference_title: ACTN1 rod domain mutation associated with congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "They showed a mild form of thrombocytopenia without severe bleeding, accompanied by an elevated mean platelet volume."
    explanation: A measured raised mean platelet volume in an affected mother and son, which is the parameter this phenotype is bound to.
- category: Hematologic
  name: Macrothrombocytopenia
  description: >-
    The composite finding by which the disorder is recognised on a full blood
    count and film - a reduced platelet count together with enlarged platelets.
    It is present from birth and stable across life; carriers are commonly
    identified incidentally or during family study rather than through bleeding.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Macrothrombocytopenia
    term:
      id: HP:0040185
      label: Macrothrombocytopenia
    clinical_course: STABLE
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency."
    explanation: States the presenting composite phenotype in the discovery cohort.
  - reference: PMID:24069336
    reference_title: A missense mutation in the alpha-actinin 1 gene (ACTN1) is the cause of autosomal dominant macrothrombocytopenia in a large French family.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Of the 55 subjects available for analysis, 26 were diagnosed with isolated macrothrombocytopenia."
    explanation: >-
      Documents the same composite phenotype in 26 members of one pedigree, and
      the word isolated carries the additional point that it occurs without
      syndromic features.
- category: Hematologic
  name: Platelet Anisocytosis
  description: >-
    Marked variation in platelet size on the blood film, reported alongside the
    macrothrombocytopenia in the discovery series. It reflects the underlying
    lesion fairly directly: if proplatelet tips vary abnormally in size, so will
    the platelets shed from them.
  phenotype_term:
    preferred_term: Platelet anisocytosis
    term:
      id: HP:0032438
      label: Platelet anisocytosis
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency."
    explanation: Records anisocytosis as a feature of the presenting blood picture.
- category: Hematologic
  name: Giant Platelets in Homozygous Carriers
  description: >-
    Reported only in the two homozygous sisters, whose platelets were not merely
    large but giant, alongside moderate rather than mild thrombocytopenia. Kept
    as a separate phenotype from the ordinary macrocytosis because it is a
    dosage-dependent finding restricted to the biallelic state.
  frequency: VERY_RARE
  phenotype_term:
    preferred_term: Giant platelets
    term:
      id: HP:0001902
      label: Giant platelets
  evidence:
  - reference: PMID:38594875
    reference_title: "ACTN1-related thrombocytopenia: Homozygosity for an ACTN1 variant results in a more severe phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Within the reported family, the homozygous sisters have moderate thrombocytopenia and marked platelet macrocytosis with giant platelets, revealing a more severe haematological phenotype compared to their heterozygous relatives and highlighting a significant effect of allelic burden on platelet size."
    explanation: The only report of giant platelets in this disorder, and it is confined to the homozygous state.
- category: Hematologic
  name: Mild Mucocutaneous Bleeding Tendency
  description: >-
    Most carriers bleed no more than anyone else. Where symptoms occur they are
    mild and mucocutaneous, and the largest series concluded that the risk of
    bleeding, spontaneous or on haemostatic challenge, is negligible. This is the
    clinically operative fact about the disorder: it drives the advice given to
    patients and the decision not to treat.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Mild mucocutaneous bleeding
    term:
      id: HP:0001892
      label: Abnormal bleeding
  evidence:
  - reference: PMID:31237726
    reference_title: Novel ACTN1 variants in cases of thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Twenty-eight of 32 cases with monoallelic ACTN1 variants had mild to no bleeding complications."
    explanation: >-
      The quantitative basis for the OCCASIONAL band - 4 of 32 carriers had more
      than mild bleeding, and the remainder had mild bleeding or none.
  - reference: PMID:30351444
    reference_title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The risk of bleeding, either spontaneous or upon haemostatic challenge, is negligible and there are no other associated defects, either congenital or acquired."
    explanation: >-
      The strongest statement of mildness available, and it also excludes
      associated congenital or acquired defects in the 49-carrier series.
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency."
    explanation: Independent statement of the same mildness in the discovery cohort.
- category: Cardiovascular
  name: Mild Heart Valve Defects in Homozygous Carriers
  description: >-
    Two homozygous sisters had mild heart valve defects for which exome
    sequencing found no other genetic explanation, and the reporting authors
    raised the possibility that ACTN1 variants in the homozygous state contribute
    to them. This is curated because it is the only report of an
    extra-haematological feature in a disorder repeatedly described as having
    none, and it should be read as an open question rather than an established
    part of the phenotype - two related patients, an
    absence-of-alternative-explanation argument, and an explicitly hypothetical
    claim by the authors.
  frequency: VERY_RARE
  phenotype_term:
    preferred_term: Mild heart valve defect
    term:
      id: HP:0001654
      label: Abnormal heart valve morphology
  evidence:
  - reference: PMID:38594875
    reference_title: "ACTN1-related thrombocytopenia: Homozygosity for an ACTN1 variant results in a more severe phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We describe for the first time two patients affected with ACTN1-RT caused by a homozygous variant in ACTN1 (c.982G>A) with mild heart valve defects unexplained by any other genetic variants investigated by WES."
    explanation: The observation itself, including its limits - two patients, and a negative-exome argument for attribution.
  - reference: PMID:30351444
    reference_title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "The risk of bleeding, either spontaneous or upon haemostatic challenge, is negligible and there are no other associated defects, either congenital or acquired."
    explanation: >-
      The 49-carrier series found no associated defects of any kind, which is
      evidence against valve disease being a feature of the disorder in the
      heterozygous state.
diagnosis:
- name: Full blood count with peripheral blood film
  description: >-
    The test that finds the disease. A persistently low platelet count with a
    raised mean platelet volume, enlarged platelets and marked size variation on
    the film, in someone without systemic illness, is the trigger for considering
    an inherited thrombocytopenia. A film matters as well as the count, because
    automated counters can under-report platelets large enough to fall outside
    the platelet gate.
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals with ACTN1 variants presented with moderate macrothrombocytopenia with anisocytosis but were either asymptomatic or had only a modest bleeding tendency."
    explanation: Describes the blood-count and film picture that leads to the diagnosis, and the absence of symptoms to prompt it.
- name: ACTN1 sequencing
  description: >-
    The confirmatory test, done as a targeted gene panel for inherited
    thrombocytopenia, as single-gene sequencing, or as exome sequencing when the
    picture is unexplained. Because disease alleles are largely private missense
    variants scattered across the whole coding sequence rather than recurrent
    hotspots, the entire gene is sequenced. The yield in a referral population is
    high enough to justify including ACTN1 in any inherited-thrombocytopenia
    panel.
  evidence:
  - reference: PMID:31237726
    reference_title: Novel ACTN1 variants in cases of thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "To decipher the spectrum of variants and phenotype of ACTN1-related thrombocytopenia, we sequenced the ACTN1 gene in 272 cases of unexplained chronic or familial thrombocytopenia."
    explanation: Establishes targeted ACTN1 sequencing of unexplained thrombocytopenia as the diagnostic approach in practice.
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We herein performed whole-exome sequencing and targeted Sanger sequencing to identify mutations that cause CMTP, in which a dominant mode of transmission had been suspected but for which no known responsible mutations have been documented."
    explanation: The exome route, which is what is used when the gene is not yet suspected.
- name: Reticulated platelet count and serum thrombopoietin
  description: >-
    Not required for diagnosis, but the pair of measurements that distinguishes a
    production defect from peripheral destruction. In ACTN1-RT the reticulated
    platelet count is low and thrombopoietin only slightly raised - the profile
    of impaired late platelet release, and the opposite of what immune
    thrombocytopenia would give.
  evidence:
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Low reticulated platelet counts and only slightly increased serum thrombopoietin levels indicated that the latest phases of megakaryopoiesis were affected."
    explanation: The measurements and the inference the authors drew from them.
differential_diagnoses:
- name: MYH9-related disease
  description: >-
    The macrothrombocytopenia most often confused with this one, and the reason a
    film report of large platelets is not by itself a diagnosis. MYH9-related
    disease is also autosomal dominant and also presents with large platelets,
    but it carries leukocyte inclusions and a lifetime risk of nephropathy,
    sensorineural hearing loss and early-onset cataract - none of which occurs in
    ACTN1-RT. The distinction determines whether the patient needs lifelong renal
    and audiological surveillance or reassurance.
  distinguishing_features:
  - Leukocyte cytoplasmic inclusions on the blood film
  - Progressive nephropathy, sensorineural hearing loss and early-onset cataract
  - Absence of any extra-haematological feature in heterozygous ACTN1-RT
  evidence:
  - reference: PMID:30471777
    reference_title: MYH9 Associated nephropathy.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disorder is characterized by the presence of macrothrombocytopenia, leukocyte inclusions and a variable risk of developing renal failure, hearing loss and early-onset cataracts."
    explanation: Names both the shared feature that creates the confusion and the features that resolve it.
  - reference: PMID:30351444
    reference_title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The risk of bleeding, either spontaneous or upon haemostatic challenge, is negligible and there are no other associated defects, either congenital or acquired."
    explanation: The other half of the contrast - no associated defects in ACTN1-RT, where MYH9 disease accumulates them.
- name: Immune thrombocytopenia
  description: >-
    The misdiagnosis that matters, because it is treated. A lifelong stable
    macrothrombocytopenia mistaken for immune thrombocytopenia attracts
    corticosteroids, immunosuppression and occasionally splenectomy, none of
    which can help a defect of platelet production. Two findings separate them: a
    family history with the same blood picture, and a low reticulated platelet
    count with near-normal thrombopoietin, which is a production profile rather
    than a destruction profile.
  distinguishing_features:
  - Lifelong stable course with an affected first-degree relative
  - Low reticulated platelet count and only slightly increased serum thrombopoietin
  - Enlarged platelets with anisocytosis rather than a normal platelet size distribution
  evidence:
  - reference: PMID:30471777
    reference_title: MYH9 Associated nephropathy.
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: "We describe the case of a 27-year-old Caucasian woman, diagnosed initially with idiopathic thrombocytopenic purpura."
    explanation: >-
      Documents that an inherited macrothrombocytopenia is misdiagnosed as immune
      thrombocytopenia in practice. Marked INDIRECT because the reported patient
      had MYH9-related disease, not ACTN1-RT, so the quote supports the general
      diagnostic trap rather than a case of this disorder.
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Low reticulated platelet counts and only slightly increased serum thrombopoietin levels indicated that the latest phases of megakaryopoiesis were affected."
    explanation: Provides the laboratory discriminator - a production defect, where immune thrombocytopenia is a destruction process.
- name: Other inherited thrombocytopenias
  description: >-
    ACTN1-RT sits inside a differential of dozens of inherited thrombocytopenias,
    and its practical significance is that it is common enough within that group
    to be worth testing for early. The others that most need excluding are
    Bernard-Soulier syndrome, which is recessive and bleeds far more severely,
    and ANKRD26- and ETV6-related thrombocytopenia, which carry a risk of myeloid
    malignancy that ACTN1-RT does not.
  distinguishing_features:
  - Severe mucocutaneous bleeding and absent ristocetin-induced agglutination in Bernard-Soulier syndrome
  - Predisposition to myeloid malignancy in ANKRD26- and ETV6-related thrombocytopenia
  - Normal platelet function testing in ACTN1-RT
  evidence:
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Given its relatively high frequency in our cohort (4.2%), ACTN1-RT has to be taken into consideration in the differential diagnosis of ITs."
    explanation: >-
      States the disorder's place in the inherited-thrombocytopenia differential
      and the frequency argument for testing it early.
treatments:
- name: Genetic diagnosis and counselling
  description: >-
    The main intervention this disorder needs, and it is informational. Making
    the molecular diagnosis converts an unexplained low platelet count into a
    benign trait with a known prognosis, stops immunosuppression and splenectomy
    being considered, and lets first-degree relatives with the same blood picture
    be explained rather than investigated repeatedly. Autosomal dominant
    transmission means a 50% recurrence risk per pregnancy for a heterozygous
    carrier, which is what counselling covers.
  treatment_term:
    preferred_term: Genetic counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  target_phenotypes:
  - preferred_term: Macrothrombocytopenia
    term:
      id: HP:0040185
      label: Macrothrombocytopenia
  evidence:
  - reference: PMID:30351444
    reference_title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Therefore, ACTN1-RT is a benign form of IT, whose diagnosis provides affected individuals and their families with a good prognosis."
    explanation: >-
      States the benefit of diagnosis in exactly these terms - prognostic
      information for the individual and the family.
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Given its relatively high frequency in our cohort (4.2%), ACTN1-RT has to be taken into consideration in the differential diagnosis of ITs."
    explanation: Supports testing for the disorder as part of the diagnostic pathway this intervention depends on.
- name: Observation without disease-specific therapy
  description: >-
    No treatment targets the mechanism, and in the great majority of carriers
    none is needed. The largest series found the bleeding risk negligible both
    spontaneously and under haemostatic challenge, so the default management is
    observation with advice about antiplatelet drugs and a plan for surgery
    rather than any ongoing therapy. Haemostatic support around procedures is
    discussed in this entry's notes; it is standard practice for inherited
    thrombocytopenia rather than an ACTN1-specific recommendation, and no
    published series reports its use in this disorder.
  treatment_term:
    preferred_term: Supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  target_phenotypes:
  - preferred_term: Mild mucocutaneous bleeding
    term:
      id: HP:0001892
      label: Abnormal bleeding
  evidence:
  - reference: PMID:30351444
    reference_title: ACTN1 mutations lead to a benign form of platelet macrocytosis not always associated with thrombocytopenia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The risk of bleeding, either spontaneous or upon haemostatic challenge, is negligible and there are no other associated defects, either congenital or acquired."
    explanation: >-
      The evidential basis for not treating: negligible bleeding risk even under
      haemostatic challenge, and no other defects to monitor for.
  - reference: PMID:25361813
    reference_title: "ACTN1-related thrombocytopenia: identification of novel families for phenotypic characterization."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical and laboratory findings of 31 affected individuals confirmed that ACTN1-RT is a mild macrothrombocytopenia with low risk for bleeding."
    explanation: Independent cohort evidence that the low bleeding risk is a property of the disorder rather than of one series.
experimental_models:
- name: Mouse fetal liver-derived megakaryocytes transduced with disease-associated ACTN1 variants
  experimental_model_type: PRIMARY_CELL_CULTURE
  publication: PMID:23434115
  organism:
    preferred_term: house mouse
    term:
      id: NCBITaxon:10090
      label: Mus musculus
  cell_types:
  - preferred_term: Megakaryocyte
    term:
      id: CL:0000556
      label: megakaryocyte
  description: >-
    Primary megakaryocytes cultured from mouse fetal liver and transduced with
    human disease-associated ACTN1 alleles. It remains the only system in which
    the disease alleles have been shown to change proplatelet architecture, and
    it is the source of the mechanistic claim at the centre of this entry.
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."
    explanation: Establishes that this culture system exists and was used to express disease-associated ACTN1 alleles in megakaryocytes.
  modeled_mechanisms:
  - target: Impaired Proplatelet Formation
    relationship: RECAPITULATES
    fidelity: MODERATE
    model_scale: CELLULAR
    description: >-
      Reproduces both halves of the predicted output defect in a megakaryocyte -
      fewer proplatelet tips, and tips that are abnormally large.
    limitations: >-
      Mouse rather than human megakaryocytes, from fetal liver rather than adult
      marrow, and the human allele is introduced by transduction on top of two
      normal mouse Actn1 alleles, so the stoichiometry is not that of a
      heterozygous patient. Proplatelet tips are counted in culture without the
      marrow sinusoidal shear that shapes platelet release in vivo, and the size
      of the platelets actually shed was not measured.
    readouts:
    - name: Proplatelet tip number
      target: Impaired Proplatelet Formation
      direction: DECREASED
      interpretation: Fewer release sites per megakaryocyte, the cellular correlate of the low platelet count.
      evidence:
      - reference: PMID:23434115
        reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."
        explanation: The measurement of reduced tip number.
    - name: Proplatelet tip size
      target: Impaired Proplatelet Formation
      direction: INCREASED
      interpretation: Larger release sites, the cellular correlate of the enlarged platelets.
      evidence:
      - reference: PMID:23434115
        reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."
        explanation: The same sentence reports tip size; it is the only published measurement of this readout.
    evidence:
    - reference: PMID:23434115
      reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Moreover, transduction of mouse fetal liver-derived megakaryocytes with disease-associated ACTN1 variants caused a disorganized actin-based cytoskeleton in megakaryocytes, resulting in the production of abnormally large proplatelet tips, which were reduced in number."
      explanation: Establishes the model as informative for the proplatelet node - it carries a disease allele and reports the proplatelet phenotype.
- name: Chinese hamster ovary cells expressing mutant ACTN1
  experimental_model_type: CELL_LINE
  publication: PMID:23434115
  description: >-
    The workhorse assay of this field. Mutant alpha-actinin-1 is expressed in
    Chinese hamster ovary cells and the actin cytoskeleton is imaged. Its value
    is throughput and reproducibility across laboratories - the same readout has
    been applied to actin-binding-domain, calmodulin-domain and spectrin-repeat
    alleles and gives the same answer - rather than physiological fidelity.
  evidence:
  - reference: PMID:23434115
    reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In vitro transfection experiments in Chinese hamster ovary cells demonstrated that altered α-actinin-1 disrupted the normal actin-based cytoskeletal structure."
    explanation: Establishes the assay system and the result that made it the standard test for a candidate ACTN1 variant.
  modeled_mechanisms:
  - target: Disorganized Megakaryocyte Actin Cytoskeleton
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    model_scale: CELLULAR
    description: >-
      Reproduces the actin disorganisation caused by disease alleles, and is the
      assay by which most reported variants have been called deleterious.
    limitations: >-
      Chinese hamster ovary cells are non-haematopoietic fibroblast-like cells
      with no megakaryocyte biology at all - no polyploidisation, no demarcation
      membrane system, no proplatelets - so the model shows that the mutant
      protein can disorganise an actin network, not that it disorganises the
      specific circumferential network a megakaryocyte uses to make platelets.
      Expression is heterologous and typically above endogenous levels.
    readouts:
    - name: Actin cytoskeletal organisation on immunofluorescence
      target: Disorganized Megakaryocyte Actin Cytoskeleton
      direction: ALTERED
      interpretation: Disorganised actin filament architecture in cells expressing a disease allele.
      evidence:
      - reference: PMID:26453073
        reference_title: ACTN1 rod domain mutation associated with congenital macrothrombocytopenia.
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: "Consistent with the previous reports of mutations that reside in the ABD or the CaM domain, immunofluorescence examination revealed disorganization of the actin cytoskeleton in Gln395 mutant-transduced Chinese hamster ovary cells."
        explanation: The readout as applied to a rod-domain allele, and its agreement with earlier alleles.
    evidence:
    - reference: PMID:23434115
      reference_title: ACTN1 mutations cause congenital macrothrombocytopenia.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "In vitro transfection experiments in Chinese hamster ovary cells demonstrated that altered α-actinin-1 disrupted the normal actin-based cytoskeletal structure."
      explanation: Establishes the assay as informative for the actin-disorganisation node.
- name: In vitro actin bundling assays with recombinant mutant alpha-actinin-1
  experimental_model_type: OTHER
  publication: PMID:31365757
  description: >-
    Purified recombinant alpha-actinin-1 carrying disease substitutions, assayed
    for filament bundling and for actin association, plus imaging of mutant
    protein in cultured HeLa cells. This is the only system that measures the
    direction of the biochemical change rather than inferring it from morphology,
    and it is why this entry curates the lesion as increased rather than
    decreased actin association.
  evidence:
  - reference: PMID:31365757
    reference_title: Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Thus, CMTP-causing actinin-1 mutations outside the actin-binding domain also increase actin association, suggesting a common molecular mechanism underlying actinin-1 related CMTP."
    explanation: Establishes the cell-free assay system and the conclusion the authors drew from it.
  modeled_mechanisms:
  - target: Increased Alpha-Actinin-1 Actin Cross-Linking Activity
    relationship: MEASURES
    fidelity: MODERATE
    model_scale: MOLECULAR
    description: >-
      Quantifies filament bundling by nine calmodulin-like and rod-domain mutants
      and the cytoskeletal association of mutant protein in cells.
    limitations: >-
      A cell-free system with purified protein and F-actin, so it reports the
      intrinsic property of the mutant protein rather than its behaviour in a
      megakaryocyte, where actin dynamics are shaped by many competing
      cross-linkers and by calcium signalling. The cellular arm uses HeLa cells,
      which are as remote from megakaryocytes as Chinese hamster ovary cells are.
    readouts:
    - name: Actin filament bundling activity
      target: Increased Alpha-Actinin-1 Actin Cross-Linking Activity
      direction: INCREASED
      interpretation: Disease alleles bundle actin filaments more effectively than wild-type protein.
      evidence:
      - reference: PMID:31365757
        reference_title: Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia.
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: "In this study, we examined nine CMTP-causing mutations in the calmodulin-like and rod domains of actinin-1. These mutations increase, to varying degrees, actinin's ability to bundle actin filaments in vitro."
        explanation: The bundling measurement itself, across nine disease alleles.
    - name: Cytoskeletal association of mutant actinin in cells
      target: Increased Alpha-Actinin-1 Actin Cross-Linking Activity
      direction: INCREASED
      interpretation: Two mutants show increased association with the cellular cytoskeleton, extending the cell-free result into cells.
      evidence:
      - reference: PMID:31365757
        reference_title: Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia.
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: "The G764S and E769K mutations increase cytoskeletal association of actinin in cells, and all mutant proteins colocalize with F-actin in cultured HeLa cells."
        explanation: >-
          Names the two alleles for which the increase was demonstrated in cells;
          the remaining seven were shown only to colocalise with F-actin.
    evidence:
    - reference: PMID:31365757
      reference_title: Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Thus, CMTP-causing actinin-1 mutations outside the actin-binding domain also increase actin association, suggesting a common molecular mechanism underlying actinin-1 related CMTP."
      explanation: Establishes the assay as informative for the cross-linking node and states the conclusion drawn from it.
animal_models:
- name: Megakaryocyte-specific alpha-actinin-1 knockout mouse (PF4-Actn1-/-)
  species: Mouse
  genotype: Actn1 conditional knockout crossed to PF4-Cre, giving megakaryocyte-lineage deletion
  publication: PMID:39813624
  genes:
  - preferred_term: ACTN1
    term:
      id: hgnc:163
      label: ACTN1
  description: >-
    The only in vivo model of altered megakaryocyte alpha-actinin-1. It deletes
    the protein in the megakaryocyte lineage, which is a different genetic lesion
    from the human disease: patients are heterozygous for missense alleles whose
    measured effect is increased actin association, not absence of protein. The
    model is therefore informative about what alpha-actinin-1 is needed for in
    thrombocytopoiesis, and only partly informative about ACTN1-RT.
  evidence:
  - reference: PMID:39813624
    reference_title: α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Using megakaryocyte (MK)-specific α-actinin-1 knockout (KO; PF4-Actn1-/-) mice, we demonstrated that PF4-Actn1-/- mice exhibited reduced platelet counts."
    explanation: Establishes that this mouse line exists and states its principal haematological phenotype.
  modeled_mechanisms:
  - target: Impaired Proplatelet Formation
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    model_scale: ORGANISM
    description: >-
      Reproduces the low platelet count and traces it to defective
      thrombocytopoiesis, including a reduced proportion of megakaryocytes
      bearing proplatelets - the same cellular step this node names.
    limitations: >-
      A knockout, not a knock-in of a patient allele, so it models absence of
      alpha-actinin-1 rather than the gain of actin association the human alleles
      produce, and the two predict opposite effects on cross-linking. The
      knockout mice also have reduced megakaryocyte numbers and shifted
      megakaryocyte ploidy, which patients are not reported to have.
    divergences:
    - divergence_type: CAUSE_UNREPRESENTED
      materiality: QUALIFYING
      description: >-
        The human lesion is a heterozygous missense substitution that increases
        alpha-actinin-1 association with F-actin. The model removes the protein
        from the lineage entirely, so the specific molecular cause curated
        upstream in this entry is not present in the model at all.
    - divergence_type: SPECIES_MISMATCH
      materiality: QUALIFYING
      description: >-
        Mouse megakaryopoiesis differs from human in platelet size distribution
        and marrow kinetics, and mouse platelets are smaller, so a platelet-size
        phenotype measured in the mouse is not directly comparable with human
        mean platelet volume.
    readouts:
    - name: Proportion of megakaryocytes forming proplatelets
      target: Impaired Proplatelet Formation
      direction: DECREASED
      interpretation: Fewer megakaryocytes extend proplatelets when alpha-actinin-1 is absent.
      evidence:
      - reference: PMID:39813624
        reference_title: α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Colony-forming unit-MK colony formation, the ratio of proplatelet formation-bearing MKs, and MK migration in response to stromal cell-derived factor-1 signaling were inhibited in PF4-Actn1-/- mice."
        explanation: The measurement of reduced proplatelet-forming megakaryocytes.
    - name: Peripheral platelet count
      target: Impaired Proplatelet Formation
      direction: DECREASED
      interpretation: The organism-level consequence of the thrombocytopoiesis defect.
      evidence:
      - reference: PMID:39813624
        reference_title: α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "The decreased platelet number in PF4-Actn1-/- mice was due to defects in thrombocytopoiesis."
        explanation: States both the platelet-count phenotype and its attribution to thrombocytopoiesis rather than to platelet destruction.
    evidence:
    - reference: PMID:39813624
      reference_title: α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Using megakaryocyte (MK)-specific α-actinin-1 knockout (KO; PF4-Actn1-/-) mice, we demonstrated that PF4-Actn1-/- mice exhibited reduced platelet counts."
      explanation: Establishes the model and the platelet phenotype that makes it relevant to this node.
  - target: Reduced Platelet Output with Increased Platelet Size
    relationship: FAILS_TO_RECAPITULATE
    fidelity: LOW
    model_scale: ORGANISM
    description: >-
      The node asserts reduced platelet output with preserved platelet function.
      The knockout mouse has the reduced output but not the preserved function:
      spreading, clot retraction, aggregation, integrin activation and granule
      release are all impaired, and haemostasis is impaired in vivo. Human
      ACTN1-RT is defined partly by the absence of exactly those abnormalities.
    limitations: >-
      Complete absence of alpha-actinin-1 from the lineage removes a scaffold the
      mature platelet needs for integrin signalling and spreading, which a
      heterozygous missense allele does not. The discrepancy is therefore
      expected from the genetics and does not impugn the model; it means the
      model cannot be used to argue about bleeding risk or platelet function in
      patients.
    divergences:
    - divergence_type: CAUSE_UNREPRESENTED
      materiality: INVALIDATING
      description: >-
        Knockout of the gene versus a heterozygous gain-of-actin-association
        missense allele. For the claim that platelet function is preserved, this
        difference is not a caveat but a reversal - the model reports the
        opposite of the human finding, and cannot be cited for it.
    evidence:
    - reference: PMID:39813624
      reference_title: α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment.
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      snippet: "Platelet spreading, clot retraction, aggregation, integrin αIIbβ3 activation, and CD62P exposure in response to various agonists were decreased in PF4-Actn1-/- platelets."
      explanation: >-
        The platelet-function defect that the human disorder lacks, which is what
        makes this a failure to recapitulate rather than a partial success.
    - reference: PMID:39813624
      reference_title: α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment.
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      snippet: "Furthermore, the PF4-Actn1-/- mice exhibited impaired hemostasis and thrombosis."
      explanation: >-
        The whole-animal consequence of that function defect, again absent from
        the human disorder, where bleeding risk is negligible.
discussions:
- discussion_id: bdplt15_gain_versus_loss_of_actin_association
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Does ACTN1-related thrombocytopenia arise because mutant alpha-actinin-1
    binds and bundles actin too strongly, or because it fails to cross-link actin
    normally?
  attaches_to:
  - "pathophysiology#Increased Alpha-Actinin-1 Actin Cross-Linking Activity"
  - "mechanistic_hypotheses#actn1_increased_actin_association"
  rationale: >-
    Every biochemical measurement points one way and much of the descriptive
    literature points the other. Purified mutant proteins bind F-actin more
    avidly and bundle filaments more effectively, across substitutions in three
    different domains, and the authors of that work explicitly propose increased
    actin association as the common mechanism. Yet the disease is routinely
    summarised as a loss of actin cross-linking or a dominant-negative effect,
    including by the deep-research report that seeded this entry, presumably
    because a cytoskeleton that fails to make platelets reads intuitively as a
    cytoskeleton that has lost a function. The distinction is not cosmetic: an
    over-bundled network and an under-bundled network make opposite predictions
    about what a heterozygous knock-in should show relative to a heterozygous
    knockout, and would call for opposite interventions if any were ever wanted.
    No published experiment compares them in a megakaryocyte, and the one in vivo
    model is a complete knockout, which by construction cannot distinguish them.
  evidence:
  - reference: PMID:31365757
    reference_title: Investigation of calmodulin-like and rod domain mutations suggests common molecular mechanism for α-actinin-1-linked congenital macrothrombocytopenia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Actinin-1 mutations cause dominantly inherited congenital macrothrombocytopenia (CMTP), with mutations in the actin-binding domain increasing actinin's affinity for F-actin."
    explanation: The measured direction of the biochemical change, which is what puts it at odds with the loss-of-function reading this gap is about.
  proposed_experiments:
  - experiment_id: exp_bdplt15_knockin_versus_knockout_megakaryocytes
    name: Heterozygous knock-in versus heterozygous knockout megakaryocytes
    description: >-
      Generate isogenic human induced pluripotent stem cell lines carrying a
      patient ACTN1 missense allele heterozygously and, separately, a
      heterozygous ACTN1 null allele, differentiate both to megakaryocytes, and
      compare actin network architecture, proplatelet tip number and size, and
      released platelet size. A knock-in phenotype the null does not reproduce
      would establish that the allele acts by more than reduced dosage.
    would_support:
    - "mechanistic_hypotheses#actn1_increased_actin_association"
    supporting_outcome:
    - >-
        The missense knock-in shows thickened, over-bundled actin and fewer, larger
        proplatelet tips while the heterozygous null is close to wild type.
    would_refute:
    - "mechanistic_hypotheses#actn1_increased_actin_association"
    refuting_outcome:
    - >-
        The heterozygous null reproduces the knock-in phenotype, indicating that
        reduced functional alpha-actinin-1 dosage is sufficient.
- discussion_id: bdplt15_knockout_mouse_versus_human_disease
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    Can a megakaryocyte-specific Actn1 knockout mouse be used to reason about
    ACTN1-related thrombocytopenia, when it produces a platelet-function defect
    and impaired haemostasis that patients do not have?
  attaches_to:
  - "animal_models#Megakaryocyte-specific alpha-actinin-1 knockout mouse (PF4-Actn1-/-)"
  - "pathophysiology#Reduced Platelet Output with Increased Platelet Size"
  rationale: >-
    The knockout mouse is the only in vivo system available, and it does
    reproduce the low platelet count and locate the defect in thrombocytopoiesis.
    But it also shows impaired platelet spreading, clot retraction, aggregation,
    integrin alphaIIbbeta3 activation and granule release, plus impaired
    haemostasis and thrombosis and a mitochondrial phenotype - none of which is
    described in ACTN1-RT, whose defining clinical feature is that platelet
    function is normal and bleeding negligible. Two readings are possible and
    they have different consequences. Either the mismatch follows
    straightforwardly from the genetic difference, complete absence of protein
    versus one altered allele, in which case the mouse tells us about the gene's
    requirements and nothing about patients' bleeding risk. Or alpha-actinin-1
    also has a platelet-function role that is subtly impaired in patients and has
    not been looked for with sensitive enough assays, in which case the received
    statement that platelet function is normal in ACTN1-RT rests on aggregometry
    that would not detect it. Nobody has phenotyped patients' platelets with the
    assays used in the mouse.
  evidence:
  - reference: PMID:39813624
    reference_title: α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Platelet spreading, clot retraction, aggregation, integrin αIIbβ3 activation, and CD62P exposure in response to various agonists were decreased in PF4-Actn1-/- platelets."
    explanation: The model finding that human patients are not reported to share, which is the mismatch this discussion is about.
  proposed_experiments:
  - experiment_id: exp_bdplt15_patient_platelet_deep_phenotyping
    name: Deep platelet phenotyping in ACTN1-RT carriers
    description: >-
      Apply the assays that were abnormal in the knockout mouse - spreading on
      fibrinogen, clot retraction, integrin alphaIIbbeta3 activation and
      P-selectin exposure by flow cytometry, and mitochondrial membrane
      potential - to platelets from genotyped ACTN1-RT carriers and matched
      controls, rather than relying on light transmission aggregometry alone.
    would_support:
    - "pathophysiology#Reduced Platelet Output with Increased Platelet Size"
    supporting_outcome:
    - >-
        Carrier platelets are indistinguishable from controls across the sensitive
        assays, confirming that the human defect is purely one of platelet
        production.
    refuting_outcome:
    - >-
        Carrier platelets show measurable spreading, integrin activation or granule
        release defects, indicating that ACTN1-RT has a subclinical qualitative
        component that routine testing misses.
- discussion_id: bdplt15_cardiac_valve_association
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Do ACTN1 variants, in the homozygous state, cause heart valve defects, or is
    the reported association coincidental?
  attaches_to:
  - "phenotypes#Mild Heart Valve Defects in Homozygous Carriers"
  - "pathophysiology#Biallelic ACTN1 Variant Dosage Effect"
  rationale: >-
    ACTN1-RT is described in the largest series as having no associated defects,
    congenital or acquired. Against that, two homozygous sisters had mild heart
    valve defects with no alternative explanation on exome sequencing, and the
    reporting authors hypothesised an ACTN1 contribution. The evidence is two
    related individuals and a negative-exome argument, which cannot distinguish
    causation from coincidence within one family, and mild valve abnormalities
    are common enough in the general population that two cases are not a signal.
    The question is worth keeping open rather than dismissing because
    alpha-actinin-1 is expressed well beyond the megakaryocyte, and a
    dosage-dependent extra-haematological phenotype appearing only in homozygotes
    is what a mild gain-of-binding allele would be expected to produce if it
    produced one at all.
  evidence:
  - reference: PMID:38594875
    reference_title: "ACTN1-related thrombocytopenia: Homozygosity for an ACTN1 variant results in a more severe phenotype."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Moreover, we hypothesize that some ACTN1 variants, especially when present in the homozygous state, may also contribute to the cardiac abnormalities."
    explanation: The authors state the cardiac claim as a hypothesis, which is exactly the uncertainty recorded here.
  proposed_experiments:
  - experiment_id: exp_bdplt15_echocardiography_in_carriers
    name: Systematic echocardiography in ACTN1 variant carriers
    description: >-
      Echocardiograph an unselected cohort of heterozygous ACTN1-RT carriers and
      any further homozygotes identified, with age- and sex-matched controls, and
      compare valve abnormality rates against population estimates.
    would_support:
    - "phenotypes#Mild Heart Valve Defects in Homozygous Carriers"
    supporting_outcome:
    - >-
        Valve abnormalities are more frequent in carriers than in controls, with a
        dose relationship between the homozygous and heterozygous states.
    refuting_outcome:
    - >-
        Valve abnormality rates in carriers match population rates, indicating the
        reported association was coincidental.
notes: >-
  Lump/split. One Disease entry covering ACTN1-related thrombocytopenia in full.
  MONDO:0014078, OMIM 615193 and the clinical literature's ACTN1-related
  thrombocytopenia all name the same concept, and there is no basis for
  splitting: disease alleles in the actin-binding, rod and calmodulin-like
  domains produce the same phenotype through the same demonstrated cellular
  mechanism, and the domain-level differences that do exist - rod-domain variants
  raise platelet size less - are quantitative gradations within one entity rather
  than separate diseases. No has_subtypes block is curated for the same reason.
  The homozygous state is modelled as a dosage modifier in the pathograph, not as
  a subtype, because the two reported homozygotes have a more marked version of
  the same phenotype rather than a different one, and because a subtype backed by
  a single family would overstate what is known.

  Not conformed to primary_hemostatic_plug_failure, deliberately.
  kb/groupings/Inherited_Platelet_Function_Disorders.yaml lists ACTN1 (BDPLT15)
  among the entries it expects to gain as they are curated, and that grouping's
  membership criterion is a node conforming to that module. This entry does not
  declare one. The module models loss or dysfunction of a component of the
  platelet primary-haemostatic apparatus - an adhesion receptor, the aggregation
  receptor, the secretion machinery, the procoagulant membrane response - and
  ACTN1-RT has none of those. Its platelets are fewer and larger but functionally
  normal, the lesion is in megakaryocyte proplatelet formation rather than in the
  circulating platelet's haemostatic apparatus, and the mild bleeding is
  proportionate to the platelet count. Forcing a conformance would assert a
  platelet function defect the literature specifically denies. The right fix is a
  megakaryopoiesis and proplatelet-formation module, which does not yet exist in
  kb/modules/; other production-defect thrombocytopenias such as MYH9-related
  disease, TUBB1, DIAPH1 and TPM4 would conform to it alongside this entry. Until
  then the grouping's expectation is left unmet on purpose, and this paragraph is
  the reason.

  Direction of the molecular lesion. This entry curates the ACTN1 variants as
  increasing alpha-actinin-1 association with actin, which is what the
  biochemistry measures, and not as a loss of cross-linking, which is the
  intuitive reading and the one the seeding deep-research report asserted. Both
  are carried as mechanistic hypotheses with the canonical one marked, and an
  open discussion states what would separate them. A curator revising this entry
  should not quietly flip functional_impact_category back to LOSS_OF_FUNCTION
  without new data.

  Treatment evidence, and what is not curated. No trial, cohort or case series
  reports a treatment given for ACTN1-RT, because in the published series nobody
  needed one. Standard practice for inherited thrombocytopenia around surgery or
  major bleeding - tranexamic acid or another antifibrinolytic, and platelet
  transfusion if the count is low enough to matter - is what a haematologist
  would apply, but it is generic to the disease class and no source found in this
  curation states it for this disorder, so it is recorded here rather than as a
  treatments entry with a borrowed citation. Splenectomy and corticosteroids are
  actively inappropriate: the platelets are not being destroyed. There are no
  clinical trials to curate and no NCT identifiers.

  Phenotypes deliberately omitted. Epistaxis, easy bruising and menorrhagia are
  listed for this disorder by OMIM and repeated by the seeding report, and they
  are plausible - they are what mild mucocutaneous bleeding consists of. They are
  not curated as separate phenotypes because none of the primary sources read in
  this curation names them, and OMIM is not a citable reference type in this
  repository. They sit inside Mild Mucocutaneous Bleeding Tendency, which is
  evidenced. The report also proposed HP:0000425 for epistaxis, which does not
  exist in HPO, and HP:0000138 for menorrhagia, which HPO calls Ovarian cyst;
  neither was used.

  Evidence base. Five clinical series (a Japanese discovery cohort, a
  six-generation French pedigree, an Italian 128-proband screen, a French
  272-case screen, and a 49-carrier assembled cohort), one homozygous family, one
  biochemical study of purified mutant protein, one megakaryocyte-specific
  knockout mouse, and one narrative review. All human evidence is observational.
  The mouse is a knockout rather than a knock-in, which is the entry's main model
  limitation and is recorded as a HUMAN_MODEL_MISMATCH discussion rather than
  smoothed over.

📚

References & Deep Research