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Classifications

Harrison's Part

Mechanistic Nosology

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Inheritance

2

Mitochondrial inheritance HP:0001427

OMIM 500000 assigns histiocytoid cardiomyopathy to mitochondrial (maternal) inheritance on the basis of the heteroplasmic MT-CYB m.15498G>A variant reported by Andreu et al. A separate proband in the histiocytoid cardiomyopathy registry exome study inherited a mitochondrial mutation from a heteroplasmic mother, as did an affected brother.

Mitochondrial inheritance

Show evidence (2 references)

PMID:10960495 SUPPORT Human Clinical

"We describe a pathogenic mutation in the mitochondrial cytochrome b gene in a patient with a multisystem disorder presenting as histiocytoid cardiomyopathy in whom a defect of ubiquinol cytochrome c oxidoreductase of the electron transport chain had been documented biochemically."

Documents the mitochondrial-DNA (MT-CYB) lesion that underlies the mitochondrial-inheritance assignment of OMIM 500000.

PMID:25921236 SUPPORT Human Clinical

"In a fourth case, the proband with Histiocytoid CM inherited a mitochondrial mutation from her heteroplasmic mother, as did her brother who presented with cardiac arrhythmia."

Maternal transmission of a heteroplasmic mtDNA variant to two affected siblings is the signature of mitochondrial inheritance.

X-linked dominant inheritance HP:0001423

The marked female excess (approximately 3:1) prompted the long-standing hypothesis that histiocytoid cardiomyopathy is an X-linked dominant disorder with prenatal lethality in hemizygous males, originally mapped to Xp22. The subsequent identification of de novo truncating variants in the X-linked nuclear gene NDUFB11 (Xp11.23) in affected girls is consistent with this model, although the mechanistically related NDUFB11 hypertrophic cardiomyopathy reports include surviving hemizygous males, so strict male lethality is not universal.

X-linked dominant inheritance

Show evidence (3 references)

PMID:10463272 SUPPORT Human Clinical

"It was recently proposed that this disorder is X-linked dominant with the associated gene located in the region of Xp22."

States the X-linked dominant hypothesis for histiocytoid cardiomyopathy.

PMID:21585276 SUPPORT Human Clinical

"Because of the female predominance, it was suggested that HC is an X-linked genetic disorder with prenatal lethality in males."

Links the observed female predominance to the X-linked, male-prenatal-lethal model.

PMID:25921236 SUPPORT Human Clinical

"Two probands had de novo non-sense mutations in the second exon of the X-linked nuclear gene NDUFB11."

De novo truncating variants in an X-linked nuclear gene in affected probands are consistent with, but do not by themselves prove, X-linked dominant inheritance with male lethality.

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Mechanistic Hypotheses

3

Mitochondrial respiratory-chain cardiomyopathy

mitochondrial_respiratory_chain_model CANONICAL

Evidence balance 1 support

Histiocytoid cardiomyopathy is fundamentally a mitochondrial cardiomyopathy: a respiratory-chain subunit defect (MT-CYB/complex III or NDUFB11/complex I) causes bioenergetic failure and compensatory mitochondrial proliferation in cardiomyocytes, with the histiocytoid morphology as the cellular readout. This is the dominant contemporary model and is supported by direct enzymology in patient heart mitochondria and by molecular genetics.

Show evidence (1 reference)

PMID:18473377 SUPPORT Human Clinical

"HICMP should be regarded as mitochondrial cardiomyopathy."

Explicit review-level endorsement of the mitochondrial model.

Purkinje-cell/oncocyte hamartoma rather than diffuse mitochondrial cardiomyopathy

purkinje_hamartoma_model ALTERNATIVE

Evidence balance 2 support

The competing pathology-derived model holds that the lesion is a hamartoma-like focal aggregation of oncocyte-like cardiac myocytes and explicitly rejects a diffuse mitochondrial cardiomyopathy, a multifocal Purkinje-cell tumour, and a developmental arrest, proposing instead a prenatal myocardial or systemic (possibly viral) injury. The WHO 5th edition adoption of the name "conduction system hamartoma" keeps this framing alive. The two models are not fully reconciled: the focal, conduction-system-restricted distribution of the lesions is the main observation the purely mitochondrial model does not yet explain.

Show evidence (2 references)

PMID:7942464 SUPPORT Human Clinical

"Evidence is presented to exclude the possibilities that the disorder represents a developmental anomaly of the atrioventricular conduction system, a multifocal tumor of Purkinje cells, a developmental arrest of cardiac myocytes, and a diffuse type of mitochondrial cardiomyopathy."

States the alternative model's explicit exclusion of a diffuse mitochondrial cardiomyopathy.

PMID:7942464 SUPPORT Human Clinical

"This syndrome is likely caused by prenatal myocardial or systemic (viral?) injury."

States the alternative model's proposed prenatal-injury aetiology.

Downregulated IL-33/IL1RL1-p38-MAPK/S100A axis

il33_s100a_axis_model EMERGING

Evidence balance 1 support

Whole-genome expression profiling of registry hearts identified coordinate downregulation of two chromosomally clustered gene sets (S100A8/S100A9/ S100A12 at 1q21.3 and IL1RL1/IL18R1/IL18RAP at 2q12.1) together with reduced interleukin 33, with confirmed copy-number loss of the S100A cluster. This suggests an inflammatory-signalling axis contributing to susceptibility. It has not been replicated, no causal variant has been demonstrated in these loci, and the finding could be a consequence rather than a cause of the myocardial lesion.

Show evidence (1 reference)

PMID:21585276 SUPPORT Human Clinical

"These data suggest a model in which the interleukin 33-IL1RL1/p38-MAPK/ S100A8-S100A9 axis is downregulated in HC cardiac tissue and provide several candidate genes on 1q21.3c and 2q12.1a for inherited mutations that may predispose individuals to HC."

Statement of the emerging expression-based model and its candidate loci.

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Discussions and Knowledge Gaps

3

What explains the majority of histiocytoid cardiomyopathy cases that carry no variant in NDUFB11, MT-CYB, or another complex I gene?

KNOWLEDGE GAP OPEN hc_causal_gene_unsolved

Attached to

pathophysiology#Mitochondrial Respiratory Chain Subunit Defect

Trio exome sequencing of five registry probands identified a plausible nuclear mitochondrial-protein variant in only three, and no strong recessive or compound-heterozygous candidate in two. MT-CYB, the first reported locus, was not confirmed in 27 further registry cases. The disease therefore remains molecularly unexplained in a large fraction of patients, which blocks both prenatal/cascade testing and any mechanism-directed therapy.

Proposed experiments

Registry whole-genome sequencing plus lesional-myocardium multi-omics

observational molecular profiling study

exp_hc_registry_wgs_and_lesional_omics

Re-analyse the histiocytoid cardiomyopathy registry trios by whole-genome rather than exome sequencing, with structural-variant calling and deep mitochondrial-heteroplasmy detection, and pair this with RNA sequencing, respiratory-chain enzymology, and blue-native supercomplex analysis on banked lesional myocardium. Add single-nucleus transcriptomics of histiocytoid versus adjacent normal myocardium from the same hearts to test whether the lesional cells occupy a distinct somatic or epigenetic state.

Show evidence (1 reference)

PMID:25921236 SUPPORT Human Clinical

"Strong candidate recessive or compound heterozygous variants were not found for this individual or for the fifth case."

Documents the unsolved fraction even in a systematic trio exome study.

Why does a germline respiratory-chain defect produce focal, hamartoma-like nests of oncocytic Purkinje-like cells rather than uniform cardiomyocyte dysfunction?

OPEN QUESTION OPEN hc_focal_distribution_unexplained

Attached to

pathophysiology#Hamartomatous Nodule Formation in the Conduction System

This is the central unresolved tension between the canonical mitochondrial model and the alternative Purkinje-hamartoma model. A germline mtDNA or X-linked nuclear variant is present in every cardiomyocyte, yet the lesions are discrete and conduction-system-biased. Candidate explanations include mitochondrial heteroplasmy segregation, X-inactivation mosaicism in the NDUFB11 arm, a cell-intrinsic vulnerability of Purkinje-lineage myocytes with high oxidative demand, or a superimposed somatic/developmental event. None has been tested.

Proposed experiments

Lesional versus adjacent-normal myocyte mosaicism assay

comparative single-cell molecular pathology study

exp_hc_lesional_vs_normal_single_cell_mosaicism

Within single histiocytoid cardiomyopathy hearts, compare lesional histiocytoid cells with adjacent morphologically normal myocardium for mitochondrial heteroplasmy level and X-inactivation ratio, and use laser-capture microdissection with targeted deep sequencing to search for a lesion-restricted second somatic hit. Complement this with a lineage-resolved comparison of Purkinje versus working cardiomyocyte respiratory capacity and vulnerability to complex I/III inhibition.

Show evidence (1 reference)

PMID:25921236 SUPPORT Human Clinical

"Unlike other mitochondrial diseases, however, Histiocytoid CM does not affect all cardiomyocytes equally."

States the observation that the open question is about.

Does the zebrafish ndufb11 morpholino knockdown actually model histiocytoid cardiomyopathy, given that it reproduces cardiomegaly and arrhythmia but has not been shown to produce the defining histiocytoid/oncocytic Purkinje-like cell?

HUMAN MODEL MISMATCH OPEN hc_zebrafish_model_fidelity

Attached to

pathophysiology#Oncocytic Transformation of Purkinje-like Cardiomyocytes

The zebrafish result is the only experimental support that NDUFB11 loss can cause cardiac disease, and it is cited as such. But the reported readouts are cardiomegaly, looping defects, and arrhythmia - non-specific outputs obtainable from many cardiac perturbations - not the oncocytic transformation that defines the human entity. Morpholino knockdown also differs mechanistically from a de novo human nonsense allele acting in a mosaic X-inactivation context, and the zebrafish conduction system is not homologous in detail to the mammalian His-Purkinje system. Treating the model as validating the human cellular mechanism would overstate it.

Proposed experiments

Cross-model test for oncocytic Purkinje-like transformation on NDUFB11 loss

cross-species model validation study

exp_ndufb11_model_ladder_for_oncocytic_transformation

Examine ndufb11-deficient zebrafish - ideally a stable genetic mutant rather than a morphant - histologically and ultrastructurally for oncocytic myocyte transformation rather than gross cardiac phenotypes alone. In parallel, generate a conditional cardiomyocyte- or conduction-cell-specific Ndufb11 mouse and assess it for histiocytoid lesions and arrhythmia, and assay NDUFB11-null human iPSC-derived cardiomyocytes and conduction-like cells for mitochondrial accumulation, myofibrillar loss, and triggered activity.

Show evidence (1 reference)

PMID:25921236 SUPPORT Model Organism

"morpholino-mediated knockdown of ndufb11 in zebrafish embryos generated defective cardiac tissue with cardiomegaly, looping defects, and arrhythmia"

The reported model phenotypes are non-specific cardiac abnormalities; the defining histiocytoid morphology is not among them.

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Pathophysiology

6

Mitochondrial Respiratory Chain Subunit Defect

The proximal lesion is a defect in a structural subunit of the mitochondrial respiratory chain. Two arms are established. In the mitochondrial-DNA arm a heteroplasmic MT-CYB missense variant (m.15498G>A, p.Gly251Asp) impairs ubiquinol-cytochrome c oxidoreductase (complex III). In the nuclear X-linked arm, de novo truncating variants in NDUFB11, an accessory subunit of complex I, abolish or reduce NDUFB11 protein and impair complex I assembly and activity. A minority of probands carry rare variants in other complex I components (NDUFAF2, NDUFB9), so the disease is genetically heterogeneous and remains unsolved in many historical cases.

MT-CYB hgnc:7427 NDUFB11 hgnc:20372

mitochondrial respiratory chain complex I GO:0045271 mitochondrial respiratory chain complex III GO:0045275

mitochondrion GO:0005739

Downstream

Oxidative Phosphorylation Failure in Cardiomyocytes Loss of a complex I or complex III subunit directly reduces electron transfer through the respiratory chain in cardiac mitochondria.

Show evidence (4 references)

PMID:10960495 SUPPORT Human Clinical

"The mutation, a G to A transition at nucleotide 15498, results in the substitution of glycine with aspartic acid at amino acid position 251."

Defines the MT-CYB (complex III) variant in the mitochondrial arm.

PMID:28050600 SUPPORT Human Clinical

"Variants in NDUFB11, which encodes a structural component of complex I of the mitochondrial respiratory chain (MRC), were recently independently reported to cause histiocytoid cardiomyopathy (histiocytoid CM) and microphthalmia with linear skin defects syndrome (MLS syndrome)."

Establishes NDUFB11, a complex I structural component, as a histiocytoid cardiomyopathy gene.

PMID:25921236 SUPPORT Human Clinical

"A third proband was doubly heterozygous for inherited rare variants in additional components of complex I, NDUFAF2 and NDUFB9, confirming that Histiocytoid CM is genetically heterogeneous."

Documents the genetic heterogeneity of the subunit-defect node.

+ 1 more reference

Oxidative Phosphorylation Failure in Cardiomyocytes

Respiratory-chain enzymology in histiocytoid cardiomyopathy hearts shows markedly decreased succinate-cytochrome c reductase and rotenone-sensitive NADH-cytochrome c reductase activity, i.e. a segmental block of electron transport at complex III (and, in the NDUFB11 arm, at complex I) with preserved cytochrome c oxidase. The resulting failure of oxidative ATP synthesis in a high-demand tissue is the energetic lesion that the downstream cellular phenotype is built on.

cardiac muscle cell CL:0000746

mitochondrial electron transport, ubiquinol to cytochrome c GO:0006122 ↓ DECREASED mitochondrial electron transport, NADH to ubiquinone GO:0006120 ↓ DECREASED mitochondrial ATP synthesis coupled electron transport GO:0042775 ↓ DECREASED

myocardium UBERON:0002349

Downstream

Oncocytic Transformation of Purkinje-like Cardiomyocytes Chronic energy failure is followed by compensatory mitochondrial proliferation, lipid microvacuolation, and loss of contractile apparatus in affected cells. The steps linking the bioenergetic defect to the focal, conduction-system-biased distribution of the transformed cells are not established.

Show evidence (4 references)

PMID:6093033 SUPPORT Human Clinical

"Biochemical studies showed markedly decreased succinate-cytochrome c reductase and rotenone-sensitive NADH-cytochrome c reductase activities, while other mitochondrial enzymes were normal."

Quantifies the segmental respiratory-chain block in a histiocytoid cardiomyopathy heart.

PMID:6093033 SUPPORT Human Clinical

"Histiocytoid cardiomyopathy appears to be due to a defect of complex III (reduced coenzyme Q-cytochrome c reductase) in the respiratory chain of heart mitochondria."

States the complex III localisation of the defect.

PMID:18473377 SUPPORT Human Clinical

"Since HICMP is frequently found in patients with mitochondrial deoxyribonucleic acid (DNA) mutations, HICMP cardiomyocytes carry an increased number of normal or abnormal mitochondria, and may show markedly decreased succinate-cytochrome c reductase or NADH-cytochrome c reductase activity; HICMP..."

Review-level statement tying the enzymology to the classification of HC as a mitochondrial cardiomyopathy.

+ 1 more reference

Oncocytic Transformation of Purkinje-like Cardiomyocytes

The cellular hallmark of the disease. Affected cells are enlarged, polygonal, and pale, with abundant faintly eosinophilic foamy granular cytoplasm, coarse granules and microvacuoles, few or no myofibrils, and poorly developed or absent intercellular junctions. Ultrastructurally they are filled with excessive and aberrantly shaped mitochondria plus scattered lipid droplets. They are not histiocytes: immunophenotype and distribution identify them as modified cardiomyocytes with Purkinje/conduction-cell features, which is why the WHO 5th edition renamed the lesion "conduction system hamartoma".

Purkinje myocyte CL:0002068 ⚠ ABNORMAL

mitochondrion GO:0005739

cardiac Purkinje fiber UBERON:0002354

Downstream

Hamartomatous Nodule Formation in the Conduction System Transformed cells accumulate as multifocal, sharply demarcated clusters and nodules rather than diffusely, which is what distinguishes histiocytoid cardiomyopathy from a generalised mitochondrial cardiomyopathy.

Show evidence (4 references)

PMID:10463272 SUPPORT Human Clinical

"Microscopic examination in all three cases revealed multiple, scattered clusters of histiocytoid myocytes which on ultrastructural examination were filled with abnormal mitochondria, scattered lipid droplets, and scanty myofibrils."

Defines the ultrastructural signature of the transformed cell.

PMID:7942464 SUPPORT Human Clinical

"These cells were remarkably similar in all patients and had poorly developed or absent intercellular junctions, few or no contractile elements, and markedly increased numbers of mitochondria, which imparted a granular or vacuolated appearance to the cytoplasm."

Establishes the loss of junctions and contractile elements alongside the mitochondrial excess across a 53-patient review.

PMID:22758650 SUPPORT Human Clinical

"Histiocytoid cardiomyopathy is a rare arrhythmogenic disorder characterized by aggregates of oncocytic cells involving predominantly the subendocardium. These cells are thought to be abnormal Purkinje cells."

Identifies the transformed cells as abnormal Purkinje cells.

+ 1 more reference

Hamartomatous Nodule Formation in the Conduction System

Transformed cells form multifocal, sharply demarcated clusters, sheets, or grossly visible yellow-tan nodules that are biased to the subendocardium, epicardium, and valves and cluster in the vicinity of the Purkinje fibres and nodal conduction tissue. This focal, conduction-system-weighted distribution - rather than uniform involvement of all cardiomyocytes - is the structural reason the disease is arrhythmogenic out of proportion to its contractile burden.

endocardium UBERON:0002165 myocardium UBERON:0002349

Downstream

Arrhythmogenic Substrate and Triggered Activity Nests of electrically abnormal, poorly coupled Purkinje-like cells within and adjacent to the conduction system generate ectopic impulses and triggered activity.

Show evidence (3 references)

PMID:18473377 SUPPORT Human Clinical

"Pathoanatomically, HICMP is characterized by subendocardial, epicardial, or valvular yellow-tan nodules, which are histologically built up of abnormal Purkinje fibers and multiple, scattered clusters of histiocytoid myocytes, which are filled with an increased number of normal or abnormal mitochondria."

Describes the anatomic distribution of the nodular lesions.

PMID:25921236 SUPPORT Human Clinical

"Unlike other mitochondrial diseases, however, Histiocytoid CM does not affect all cardiomyocytes equally."

Contrasts the focal distribution of histiocytoid cardiomyopathy with generalised mitochondrial cardiomyopathy.

PMID:25921236 SUPPORT Human Clinical

"The observation that lesions tend to be clustered in the vicinity of the Purkinje fibers has also prompted the argument that the disease may be related to defective activity of this sub-type of cardiomyocyte that coordinate the cardiac action potential"

States the conduction-system bias of lesion distribution and its proposed electrophysiological consequence.

Arrhythmogenic Substrate and Triggered Activity

The lesional cells behave as autonomous arrhythmogenic foci. Clinically this produces incessant ventricular tachycardia and short-coupled premature ventricular complexes that initiate torsade de pointes, typically refractory to standard antiarrhythmic drugs. Catheter mapping localises discrete triggering foci with recordable pre-potentials, and successful ablation or surgical excision of the foci can abolish the arrhythmia, directly implicating the lesions as the substrate. The response of otherwise-refractory arrhythmia to high-dose carvedilol has been proposed to implicate ryanodine-receptor store-overload-induced calcium release, but this rests on a single case.

Purkinje myocyte CL:0002068

Purkinje myocyte action potential GO:0086017 ⚠ ABNORMAL cardiac muscle cell action potential GO:0086001 ⚠ ABNORMAL

Downstream

Circulatory Failure and Sudden Cardiac Death Incessant ventricular tachyarrhythmia degenerates into torsade de pointes, ventricular fibrillation, cardiac arrest, or tachycardia-related cardiogenic shock.

Show evidence (3 references)

PMID:10463272 SUPPORT Human Clinical

"Histiocytoid cardiomyopathy (HC), a rare arrhythmogenic disorder, presents as difficult-to-control arrhythmias or sudden death in infants and children, particularly girls."

Establishes the refractory-arrhythmia clinical signature.

PMID:31449642 SUPPORT Human Clinical

"The PVCs triggering TdP were successfully ablated in a 4-month-old girl with histiocytoid cardiomyopathy."

Abolition of the arrhythmia by ablating a mapped focus is direct evidence that the lesions form the arrhythmogenic substrate.

PMID:38089118 SUPPORT Human Clinical

"This clinical course suggested that the arrhythmias in histiocytoid cardiomyopathy might be related with SOICR."

Single-case pharmacological inference that store-overload-induced calcium release contributes to the triggered activity; hypothesis-generating only.

Circulatory Failure and Sudden Cardiac Death

The clinical endpoint. Untreated disease is fatal in the great majority, usually within the first two years of life, either as unheralded sudden death - frequently first diagnosed at autopsy and easily miscertified as sudden infant death syndrome - or as progressive heart failure from cardiomegaly, dilated/hypertrophic/noncompaction remodelling, and tachycardia-mediated dysfunction.

heart UBERON:0000948

Show evidence (2 references)

PMID:37831311 SUPPORT Human Clinical

"The disease is fatal in the vast majority and diagnosis is nearly always established at autopsy, but this is only possible with adequate myocardial sampling."

States the case fatality and the autopsy-dependence of diagnosis, which is the source of SIDS misclassification.

PMID:25921236 SUPPORT Human Clinical

"Histiocytoid cardiomyopathy (Histiocytoid CM) is a rare form of cardiomyopathy observed predominantly in newborn females that is fatal unless treated early in life."

States that the natural history is fatal without early treatment.

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Histopathology

3

Clusters of Histiocytoid (Oncocytic) Myocytes

The diagnostic lesion: discrete, sharply demarcated groups of enlarged polygonal cells with abundant faintly eosinophilic foamy granular cytoplasm, coarse granules, microvacuoles, and irregular round nuclei, predominantly immediately deep to the endocardium. They resemble histiocytes by light microscopy but are modified cardiomyocytes.

Show evidence (3 references)

PMID:37831311 SUPPORT Human Clinical

"The disease is characterised histologically by discrete groups of enlarged, polygonal histiocyte-like cells with distinct margins and abundant faintly eosinophilic foamy cytoplasm."

Contemporary description of the diagnostic light-microscopic lesion.

PMID:37831311 SUPPORT Human Clinical

"Cells often contain coarse granules, microvacuoles and irregular, round nuclei."

Cytological detail of the lesional cell.

PMID:39225698 SUPPORT Human Clinical

"Microscopy revealed sharply demarcated cell groups or layers with histiocyte-like cells with foamy cytoplasm."

Independent autopsy-series confirmation of the same lesion.

Yellow-Tan Endocardial, Epicardial and Valvular Nodules

Grossly visible yellow-tan or yellowish-white nodules and multifocal uneven yellowish endocardial thickening. In a 53-patient review, 16 had such nodules while the remaining 37 had only microscopic foci of abnormal myocytes scattered through the myocardium, so absence of gross nodules does not exclude the diagnosis.

Show evidence (2 references)

PMID:7942464 SUPPORT Human Clinical

"Sixteen patients had yellowish nodules on the endocardium, epicardium, and/or valves; the other 37 had foci of abnormal myocytes throughout the myocardium."

Quantifies how often gross nodules are present versus microscopic-only disease.

PMID:28045696 SUPPORT Human Clinical

"The most common grossly change is multifocal uneven thickening of the endocardium, presenting a yellowish color, with some area forming nodular appearance, and histologically featured by scattered clusters of histiocytoid myocytes under the endocardium."

Describes the characteristic gross appearance.

Mitochondrial Hyperplasia with Myofibrillar Depletion on Electron Microscopy

Ultrastructural examination shows the lesional cells packed with excessive and abnormally shaped mitochondria plus scattered lipid droplets, with scanty or absent myofibrils and poorly developed intercellular junctions. This is the ultrastructural basis of both the foamy cytoplasm and the mitochondrial classification of the disease.

Show evidence (2 references)

PMID:10463272 SUPPORT Human Clinical

"Microscopic examination in all three cases revealed multiple, scattered clusters of histiocytoid myocytes which on ultrastructural examination were filled with abnormal mitochondria, scattered lipid droplets, and scanty myofibrils."

Primary ultrastructural description.

PMID:8563940 SUPPORT Human Clinical

"Electron micrographs showed mitochondrial hyperplasia in these cells."

Independent electron-microscopic confirmation of mitochondrial hyperplasia.

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Pathograph

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Phenotypes

15

Cardiovascular 8

Ventricular Tachycardia VERY_FREQUENT HP:0004756

Temporal: RECURRENT

Show evidence (2 references)

PMID:21585276 SUPPORT Human Clinical

"Histiocytoid cardiomyopathy (HC) is a rare but distinctive arrhythmogenic disorder characterized by incessant ventricular tachycardia, cardiomegaly, and often sudden death by age 2 years."

Names incessant ventricular tachycardia as a defining feature of the disorder, supporting a VERY_FREQUENT band.

PMID:28045696 SUPPORT Human Clinical

"It may manifest as ventricular tachycardia or dilated cardiomyopathy and frequently causes sudden death."

Independent review statement of the ventricular tachycardia presentation.

Cardiomegaly HP:0001640

Show evidence (3 references)

PMID:21585276 SUPPORT Human Clinical

"Histiocytoid cardiomyopathy (HC) is a rare but distinctive arrhythmogenic disorder characterized by incessant ventricular tachycardia, cardiomegaly, and often sudden death by age 2 years."

Lists cardiomegaly among the characteristic features.

PMID:6093033 SUPPORT Human Clinical

"A 3-week-old girl with failure to thrive and cardiomegaly died of cardiac arrest at age 4 weeks."

Case-level documentation of cardiomegaly.

PMID:39225698 REFUTE Human Clinical

"heart weight was found within the normal range when evaluated according to age and gender"

Counter-observation: all four cases in a contemporary forensic autopsy series of conduction system hamartoma had age- and sex-normal heart weights. Cardiomegaly is therefore not obligate, which is why no frequency band is asserted for it.

Sudden Cardiac Death HP:0001645

Show evidence (2 references)

PMID:37831311 SUPPORT Human Clinical

"The disease is fatal in the vast majority and diagnosis is nearly always established at autopsy, but this is only possible with adequate myocardial sampling."

Establishes that the disease is usually fatal and usually diagnosed post mortem. No frequency band is asserted, because overall case fatality is not the same claim as the fraction of deaths that are sudden and arrhythmic: this entry separately curates progressive heart failure as a competing fatal endpoint, and no source partitions the two.

PMID:10463272 SUPPORT Human Clinical

"Histiocytoid cardiomyopathy (HC), a rare arrhythmogenic disorder, presents as difficult-to-control arrhythmias or sudden death in infants and children, particularly girls."

Names sudden death as a presenting mode.

Cardiac Arrest HP:0001695

Show evidence (2 references)

PMID:8563940 SUPPORT Human Clinical

"A 15 month old female, who had suffered from ventricular tachycardia from the prenatal period, experienced cardiac arrest at home."

Case-level documentation of cardiac arrest.

PMID:38089118 SUPPORT Human Clinical

"A 4-month-old girl presented with asystole, and recurrent ventricular tachycardias."

Documents asystolic arrest as a presenting event.

Dilated Cardiomyopathy HP:0001644

Show evidence (2 references)

PMID:18473377 SUPPORT Human Clinical

"Histiocytoid cardiomyopathy (HICMP) is a rare, genetic, cardiac disorder of infancy or childhood, predominantly affecting girls, and clinically manifesting as severe cardiac arrhythmias or dilated cardiomyopathy."

Names dilated cardiomyopathy as one of the two clinical presentations.

PMID:28045696 SUPPORT Human Clinical

"It may manifest as ventricular tachycardia or dilated cardiomyopathy and frequently causes sudden death."

Independent review statement of the dilated cardiomyopathy presentation.

Congestive Heart Failure HP:0001635

Course: PROGRESSIVE

Show evidence (2 references)

PMID:28045696 SUPPORT Human Clinical

"Here, we present a 19-month-old male infant who died of heart failure, and an autopsy was performed and confirmed the diagnosis of HC."

Death from heart failure in an autopsy-confirmed case.

PMID:31449642 SUPPORT Human Clinical

"However, she died of uncontrolled heart failure at 6 months of age."

Heart failure remained the fatal endpoint even after successful arrhythmia ablation.

Ventricular Pre-excitation Wolff-Parkinson-White syndrome HP:0001716

Show evidence (1 reference)

PMID:31449642 SUPPORT Human Clinical

"A neonate was referred to our hospital on the day of birth for Wolff-Parkinson-White syndrome, repeated episodes of supraventricular tachycardia, and a left ventricular non-compaction."

Documents WPW and supraventricular tachycardia as the presenting electrophysiological abnormality in a case later proven histologically.

Hypertrophic Cardiomyopathy HP:0001639

Show evidence (1 reference)

PMID:39132302 SUPPORT Human Clinical

"NDUFB11 is an X-linked mitochondrial Complex I protein and is known to cause histiocytoid cardiomyopathy but has not been described in female infants with hypertrophic cardiomyopathy."

Explicitly links the NDUFB11 hypertrophic phenotype to the same gene that causes histiocytoid cardiomyopathy, while noting they are distinct presentations.

Metabolism 1

Lactic Acidosis HP:0003128

Show evidence (2 references)

PMID:39132302 SUPPORT Human Clinical

"She developed lactic acidosis after birth and whole-genome sequencing identified a de novo variant in the mitochondrial Complex I gene, NDUFB11 (c.391G>A, p.Glu131Lys)."

Lactic acidosis in an NDUFB11 complex I neonate. Partial because this proband had hypertrophic rather than pathology-proven histiocytoid cardiomyopathy.

PMID:36675256 SUPPORT Human Clinical

"We report a neonatal patient with hypertrophic cardiomyopathy (HCM), lactic acidosis and isolated complex I deficiency."

Second NDUFB11 neonate with lactic acidosis and complex I deficiency; same caveat about cardiomyopathy morphology.

Musculoskeletal 1

Generalized Hypotonia HP:0001290

Show evidence (1 reference)

PMID:8563940 SUPPORT Human Clinical

"Interestingly, the present case showed hypotonia. Her muscle biopsy revealed decreased activity of cytochrome c oxidase, suggesting that histiocytoid cardiomyopathy is related to mitochondrial cytopathy."

Documents hypotonia with an independently abnormal skeletal-muscle respiratory-chain assay.

Nervous System 1

Extracardiac Anomalies of the Nervous System and Eye Abnormality of the nervous system HP:0000707

Show evidence (1 reference)

PMID:7942464 SUPPORT Human Clinical

"There was a high prevalence of anomalies involving the nervous system and eyes and of oncocytic cells in various glands."

Source statement of the extracardiac anomaly burden.

Growth 1

Failure to Thrive HP:0001508

Show evidence (1 reference)

PMID:6093033 SUPPORT Human Clinical

"A 3-week-old girl with failure to thrive and cardiomegaly died of cardiac arrest at age 4 weeks."

Case-level documentation of failure to thrive.

Other 3

Torsade de Pointes HP:0001664

Temporal: RECURRENT

Show evidence (1 reference)

PMID:31449642 SUPPORT Human Clinical

"On the 5th day after ablation, she had recurrent TdP episodes resistant to various antiarrhythmic drugs and received extracorporeal membrane oxygenation at 86 days of age."

Documents drug-resistant recurrent torsade de pointes in a proven case.

Left Ventricular Noncompaction Left ventricular noncompaction cardiomyopathy HP:0011664

Show evidence (2 references)

PMID:22758650 SUPPORT Human Clinical

"We report an association between ventricular noncompaction and histiocytoid cardiomyopathy."

Directly reports the noncompaction association.

PMID:35439193 SUPPORT Human Clinical

"We report a 13 month old girl with left ventricular noncompaction and preserved systolic function who presented in cardiogenic shock secondary to incessant ventricular arrhythmias."

Independent case of histiocytoid cardiomyopathy with LV noncompaction.

Supraventricular Tachycardia HP:0004755

Temporal: RECURRENT

Show evidence (1 reference)

PMID:31449642 SUPPORT Human Clinical

"repeated episodes of supraventricular tachycardia"

Documents recurrent supraventricular tachycardia in a proven case.

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Genetic Associations

6

MT-CYB

Gene: MT-CYB hgnc:7427 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (3 references)

PMID:10960495 SUPPORT Human Clinical

"This is the first molecular defect associated with histiocytoid cardiomyopathy."

Establishes MT-CYB as the first reported causal locus.

PMID:10960495 SUPPORT Human Clinical

"The mutation, which is heteroplasmic and fulfills all accepted criteria for pathogenicity, is likely to impair the function of the holoenzyme as deduced from its effects on the crystal structure of ubiquinol cytochrome c oxidoreductase."

States heteroplasmy and the pathogenicity argument for the variant.

PMID:25921236 REFUTE Human Clinical

"However, analysis of additional 27 cases from the Histiocytoid CM registry through collaborative work with two institutions failed to verify this gene as a causal gene for Histiocytoid CM."

Refutes MT-CYB as a general cause of histiocytoid cardiomyopathy; it remains a single-case association.

NDUFB11

Gene: NDUFB11 hgnc:20372 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (5 references)

PMID:25921236 SUPPORT Human Clinical

"Two probands had de novo non-sense mutations in the second exon of the X-linked nuclear gene NDUFB11."

Original identification of de novo NDUFB11 nonsense variants in HC probands.

PMID:28050600 SUPPORT Human Clinical

"Here we report an additional case of histiocytoid CM, which carries a de novo nonsense variant in NDUFB11"

Independent replication of a de novo NDUFB11 nonsense variant in a further histiocytoid cardiomyopathy proband.

PMID:28050600 SUPPORT Human Clinical

"An identical variant has been previously reported in association with MLS syndrome."

Documents that the same allele recurs in the allelic MLS phenotype, the basis for the p.Arg88* recurrence statement.

+ 2 more references

NDUFAF2

Gene: NDUFAF2 hgnc:28086 relationship_type: UNKNOWN variant_origin: GERMLINE

Show evidence (1 reference)

PMID:25921236 SUPPORT Human Clinical

"A third proband was doubly heterozygous for inherited rare variants in additional components of complex I, NDUFAF2 and NDUFB9, confirming that Histiocytoid CM is genetically heterogeneous."

Single-proband, doubly-heterozygous finding; supports heterogeneity but not a validated NDUFAF2 gene-disease relationship.

NDUFB9

Gene: NDUFB9 hgnc:7704 relationship_type: UNKNOWN variant_origin: GERMLINE

Show evidence (1 reference)

PMID:25921236 SUPPORT Human Clinical

"A third proband was doubly heterozygous for inherited rare variants in additional components of complex I, NDUFAF2 and NDUFB9, confirming that Histiocytoid CM is genetically heterogeneous."

Same single-proband observation; candidate-level evidence only.

HCCS

Gene: HCCS hgnc:4837 relationship_type: DISPUTED

Show evidence (1 reference)

PMID:28050600 REFUTE Human Clinical

"However, a systematic review of WES data from previously published histiocytoid CM cases, alongside four additional cases presented here for the first time, did not identify any variants in these genes."

Explicitly excludes HCCS and COX7B as histiocytoid cardiomyopathy genes in the available exome data.

COX7B

Gene: COX7B hgnc:2291 relationship_type: DISPUTED

Show evidence (2 references)

PMID:28050600 REFUTE Human Clinical

"However, a systematic review of WES data from previously published histiocytoid CM cases, alongside four additional cases presented here for the first time, did not identify any variants in these genes."

The "these genes" of this sentence are HCCS and COX7B, named in the immediately preceding sentence; the systematic exome review found no variants in either, which is the direct basis for the DISPUTED relationship type.

PMID:28050600 SUPPORT Human Clinical

"Heterozygous variants in HCCS (which encodes an important mitochondrially targeted protein) and COX7B, which, like NDUFB11, encodes a protein of the MRC, have also previously been identified in MLS syndrome including a case with features of both MLS syndrome and histiocytoid CM."

Context for why COX7B was tested at all: it is an MLS gene, and one reported patient had features of both MLS syndrome and histiocytoid cardiomyopathy. This weakly associates COX7B with the overlap phenotype and is the claim the preceding exclusion sentence refutes for histiocytoid cardiomyopathy proper.

💊

Medical Actions

5

High-Dose Carvedilol

Category: Therapeutic Action: Pharmacotherapy NCIT:C15986

Agent: carvedilol CHEBI:3441

In a 4-month-old girl with histiocytoid cardiomyopathy whose ventricular arrhythmias were refractory to bisoprolol, landiolol, verapamil, and cardiac sympathetic denervation, high-dose carvedilol achieved control. Carvedilol is distinctive among beta blockers in acting directly on the ryanodine receptor to inhibit store-overload-induced calcium release, which is the proposed basis of the response. This rests on a single case and should not be generalised.

Mechanism Target:

INHIBITS Arrhythmogenic Substrate and Triggered Activity — Proposed to suppress ryanodine-receptor store-overload-induced calcium release in the abnormal myocardium, reducing triggered activity.

Show evidence (3 references)

PMID:38089118 SUPPORT Human Clinical

"This is the first report of infantile histiocytoid cardiomyopathy whose refractory ventricular arrhythmias were successfully controlled by high-dose carvedilol."

Single-case report of arrhythmia control with high-dose carvedilol.

PMID:38089118 SUPPORT Human Clinical

"Carvedilol is the only beta blocker that directly acts on the ryanodine receptor (RyR2) and inhibits store-overload-induced Ca2+ release (SOICR) in myocardium at high dosage."

States the proposed molecular rationale for the drug choice.

PMID:38089118 SUPPORT Human Clinical

"There is no specific treatment for these arrhythmias."

Establishes that no disease-specific antiarrhythmic therapy is established, framing carvedilol as investigational.

Catheter Ablation of Arrhythmogenic Foci

Category: Therapeutic Action: cardiac ablation Ontology label: Cardiac Ablation NCIT:C100068

Radiofrequency catheter ablation of mapped premature-ventricular-complex or ventricular-tachycardia foci can abolish otherwise refractory arrhythmia, and has been performed successfully in an infant weighing 3.4 kg. Durability is limited by the multifocal nature of the disease: in the reported case the arrhythmia was abolished but the child died of heart failure at 6 months.

Mechanism Target:

INHIBITS Arrhythmogenic Substrate and Triggered Activity — Destroys the mapped triggering focus, removing the arrhythmogenic substrate at that site.

Show evidence (2 references)

PMID:31449642 SUPPORT Human Clinical

"Two types of PVCs triggering TdP were successfully ablated, which originated from the right ventricle (RV). Pre-potentials were recorded at the earliest ventricular activation sites of the targeted PVCs."

Documents successful ablation of mapped trigger foci.

PMID:31449642 SUPPORT Human Clinical

"After the ablation, she had no TdP episodes and the cardiac assist device was removed. However, she died of uncontrolled heart failure at 6 months of age."

Shows that arrhythmia control does not necessarily prevent death from heart failure.

Surgical Excision of Histiocytoid Nodules

Category: Therapeutic Action: surgical procedure Ontology label: Surgical Procedure NCIT:C15329

Where a discrete arrhythmogenic nodule can be localised, surgical excision can produce clinical remission. Together with ablation this is the main lesion-directed strategy documented in the histiocytoid cardiomyopathy registry cohort of early-diagnosed, treated survivors.

Mechanism Target:

INHIBITS Hamartomatous Nodule Formation in the Conduction System — Physically removes the hamartomatous nodule that constitutes the arrhythmogenic lesion.

Show evidence (1 reference)

PMID:7942464 SUPPORT Human Clinical

"Surgical excision of nodules of histiocytoid cells can result in clinical remission."

States the efficacy of lesion-directed surgical excision.

Mechanical Circulatory Support (ECMO and Ventricular Assist Device)

Category: Therapeutic Action: ventricular assist device placement Ontology label: Ventricular Assist Device Placement NCIT:C80452

Extracorporeal membrane oxygenation and biventricular assist device support have been used as rescue and bridge-to-transplant strategies for cardiogenic shock from incessant ventricular arrhythmia. In the reported BiVAD case an atrial rather than apical left inflow cannula was needed because preserved systolic function, noncompaction, and frequent rhythm changes caused mechanical inflow obstruction.

Show evidence (3 references)

PMID:35439193 SUPPORT Human Clinical

"BiVAD can support pediatric patients with hemodynamically significant arrhythmias to transplantation."

States the demonstrated role of BiVAD as a bridge to transplantation.

PMID:35439193 SUPPORT Human Clinical

"Atrial cannulation strategy may be preferred in cases of preserved systolic function, ventricular noncompaction, and frequent rhythm changes."

Records the disease-specific technical caveat for device implantation in histiocytoid cardiomyopathy.

PMID:31449642 SUPPORT Human Clinical

"she had recurrent TdP episodes resistant to various antiarrhythmic drugs and received extracorporeal membrane oxygenation at 86 days of age"

Documents ECMO as rescue support for refractory arrhythmia.

Heart Transplantation

Category: Therapeutic Action: heart transplantation Ontology label: Heart Transplantation NCIT:C15246

Orthotopic cardiac transplantation is the definitive option for extensive disease not amenable to lesion-directed treatment, and has been performed in registry cases with widespread myocardial involvement.

Show evidence (2 references)

PMID:25921236 SUPPORT Human Clinical

"In cases where there is extensive involvement of Histiocytoid CM a cardiac transplant was performed."

Documents transplantation for extensive disease in the registry cohort.

PMID:35439193 SUPPORT Human Clinical

"Successful heart transplantation for HICMP has been described"

Independent confirmation that transplantation has been successful in HC.

🔬

Diagnosis

4

Autopsy or Endomyocardial Tissue Examination

Histopathology remains the definitive diagnostic test. Because the lesions are focal and subendocardial, the diagnosis is missed unless the heart is adequately sampled; this is the main reason cases are miscertified as sudden infant death syndrome, sudden unexplained death in childhood, or inflicted injury.

endomyocardial biopsy NCIT:C51674

Show evidence (2 references)

PMID:37831311 SUPPORT Human Clinical

"Meticulous gross and histological examination of the heart in collaboration with a cardiovascular-trained pathologist maximises the opportunity to make specific diagnoses (and therefore rule out the differentials of SIDS, SUDC and child abuse), guide genetic testing, and inform potentially..."

States the requirement for meticulous cardiac histology and its implications for family members.

PMID:39225698 SUPPORT Human Clinical

"Autopsy findings should be evaluated by taking extra samples from the heart if necessary, especially in suspected cases below 2 years of age."

Operational guidance on adequate myocardial sampling at autopsy.

Rhythm Monitoring and Electrophysiological Mapping

Twelve-lead ECG and continuous rhythm monitoring establish the arrhythmia phenotype; invasive electrophysiological mapping localises the triggering foci, at which pre-potentials can be recorded, and guides ablation.

electrocardiography NCIT:C38053

Show evidence (1 reference)

PMID:31449642 SUPPORT Human Clinical

"Pre-potentials were recorded at the earliest ventricular activation sites of the targeted PVCs."

Demonstrates the electrophysiological signature used to localise the arrhythmogenic foci.

Echocardiography

Transthoracic echocardiography characterises the cardiomyopathy morphology that accompanies the arrhythmia - ventricular noncompaction, chamber dilation or hypertrophy, and systolic function. It does not visualise the histiocytoid lesions themselves, so a structurally near-normal echo with preserved ejection fraction does not exclude the diagnosis.

echocardiography NCIT:C16525

Show evidence (1 reference)

PMID:31449642 SUPPORT Human Clinical

"Echocardiography revealed a left ventricular non-compaction with a left ventricular ejection fraction (LVEF) of 59% and no other abnormalities."

Shows both what echocardiography contributes (detection of noncompaction) and its limitation in this disease: ejection fraction was preserved and no other abnormality was seen despite histologically extensive biventricular histiocytoid disease at autopsy.

Genomic Testing Including NDUFB11 and mtDNA

Trio exome or rapid whole-genome sequencing with mitochondrial-genome analysis is the preferred genetic approach. Targeted cardiomyopathy panels are insufficient: a pathogenic NDUFB11 variant has been reported that commercial cardiomyopathy panels did not cover.

whole genome sequencing NCIT:C101294

Show evidence (2 references)

PMID:39132302 SUPPORT Human Clinical

"This variant was not identifiable by commercial cardiomyopathy panels."

Direct evidence that panel-based testing can miss the causal variant, justifying WGS/WES.

PMID:25921236 SUPPORT Human Clinical

"We have performed whole exome sequencing on five parent-proband trios and identified nuclear-encoded mitochondrial protein mutations in three cases."

Establishes trio exome sequencing as the diagnostic yield-bearing approach.

📈

Progression

4

Prenatal to neonatal onset

Age: fetal life to first weeks

Arrhythmia may begin before birth. One autopsy-confirmed patient had ventricular tachycardia from the prenatal period, and severe NDUFB11 complex I disease can present within hours of birth with lactic acidosis and heart failure.

Show evidence (1 reference)

PMID:8563940 SUPPORT Human Clinical

"A 15 month old female, who had suffered from ventricular tachycardia from the prenatal period, experienced cardiac arrest at home."

Documents prenatal onset of the arrhythmia.

Infantile arrhythmic phase

Age: first two years of life

The great majority of recognised patients present in the first two years with incessant ventricular tachycardia, cardiomegaly, or sudden death. A contemporary autopsy series of conduction system hamartoma had a mean age of 5.8 months.

Show evidence (2 references)

PMID:21585276 SUPPORT Human Clinical

"It primarily occurs in the first two years of life and has a female to male ratio of 3:1."

Defines the age window of presentation.

PMID:39225698 SUPPORT Human Clinical

"The mean age of the cases was 5.8 months."

Gives the mean age at death in a contemporary autopsy series.

Fatal outcome without early treatment

Age: usually before 2 years

Untreated disease is fatal in the vast majority, typically by age two, either as sudden arrhythmic death or as progressive heart failure. Diagnosis is very often made only at autopsy, and requires adequate myocardial sampling.

Show evidence (1 reference)

PMID:25921236 SUPPORT Human Clinical

"Infantile histiocytoid cardiomyopathy (Histiocytoid CM, MIM 500000) is a rare, but distinctive arrhythmogenic disorder characterized by incessant ventricular tachycardia, cardiomegaly, and sudden death within the first two years of life if left untreated."

States the untreated natural history and its time course.

Survival after early intervention

Age: beyond 2 years

Children diagnosed early and treated by surgical excision or ablation of the arrhythmogenic foci, or by cardiac transplantation for extensive disease, can survive beyond the usual fatal window.

Show evidence (1 reference)

PMID:25921236 SUPPORT Human Clinical

"over 20 cases collected from children who were diagnosed early and treated either with surgical excision or ablation of the arrhythmogenic foci. In cases where there is extensive involvement of Histiocytoid CM a cardiac transplant was performed."

Documents the treated cohort in the registry and the interventions that permitted survival.

📊

Prevalence

1

Worldwide

Cases In Literature Ultra Rare

No population prevalence or incidence estimate exists. The published literature amounts to roughly 100 reported cases plus the material in the Emory histiocytoid cardiomyopathy registry. The true occurrence is probably higher than the publication count because an unknown fraction of cases is certified as sudden infant death syndrome when the heart is not adequately sampled at autopsy.

Show evidence (2 references)

PMID:21585276 SUPPORT Human Clinical

"Although approximately 100 HC cases have been reported in the literature, the prevalence of this disease may be higher than the reported cases would suggest, as some cases are mistaken for Sudden Infant Death Syndrome (SIDS)"

Gives the literature case count and states the SIDS-misclassification reason why it understates true occurrence.

PMID:25921236 SUPPORT Human Clinical

"Approximately 100 Histiocytoid CM cases have been reported in the literature"

Independent statement of the same literature case count.

🌍

Epidemiology

3

Female predominance

Histiocytoid cardiomyopathy affects girls roughly three times as often as boys, and this sex skew is the principal observation behind the X-linked male-prenatal-lethality hypothesis. The predominance is reproduced in an independent contemporary forensic autopsy series.

Show evidence (2 references)

PMID:21585276 SUPPORT Human Clinical

"It primarily occurs in the first two years of life and has a female to male ratio of 3:1."

Quantifies the female-to-male ratio and the age window.

PMID:39225698 SUPPORT Human Clinical

"The female-to-male ratio was 3/1."

A small independent 2012-2022 forensic autopsy series (n = 4; three girls, one boy) shows the same direction of sex skew. With four cases this is directionally consistent rather than statistical replication of the registry ratio.

Ancestry distribution

Reported cases from the histiocytoid cardiomyopathy registry are predominantly of European ancestry. This distribution reflects the ascertainment of a registry assembled largely from North American autopsy referrals and should not be read as a validated population risk estimate.

Show evidence (1 reference)

PMID:21585276 SUPPORT Human Clinical

"It predominantly affects Caucasians (80%) followed by African-American (15%) and Latin-American infants (3%); it is rare in Oriental infants"

Source statement of the reported ancestry distribution in the registry material.

Sibling recurrence

The histiocytoid cardiomyopathy registry investigators estimate a sibling recurrence risk of about 5%, which they note is probably an underestimate because of recurrent miscarriage in some families - itself consistent with prenatal lethality of the more severe genotypes.

Show evidence (1 reference)

PMID:25921236 SUPPORT Human Clinical

"We estimate the sibling recurrence rate to be approximately 5%, but note that this is likely to be downward biased due to the incidence of recurrent miscarriages in some families."

Direct statement of the registry sibling recurrence estimate and its bias.

🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from Histiocytoid Cardiomyopathy:

Sudden infant death syndrome

Overlapping Features The most consequential differential. Histiocytoid cardiomyopathy causes unheralded infant death and is only distinguishable by adequate myocardial sampling at autopsy, so an unknown fraction of cases is certified as SIDS.

Show evidence (1 reference)

PMID:21585276 SUPPORT Human Clinical

"the prevalence of this disease may be higher than the reported cases would suggest, as some cases are mistaken for Sudden Infant Death Syndrome (SIDS)"

States the SIDS misclassification directly.

Microphthalmia with linear skin defects (MLS) syndrome

Overlapping Features Allelic at NDUFB11, with the identical p.Arg88* variant reported in both. MLS is distinguished by microphthalmia and linear skin defects; a histiocytoid cardiomyopathy proband carrying the same variant lacked those diagnostic features, although detailed comparison showed phenotypic overlap.

Show evidence (1 reference)

PMID:28050600 SUPPORT Human Clinical

"The case we describe here lacked the diagnostic features of MLS syndrome, but a detailed clinical comparison of the two cases revealed significant phenotypic overlap."

Establishes both the distinction and the overlap between the two allelic phenotypes.

Cardiac rhabdomyoma

Overlapping Features Histiocytoid cardiomyopathy was originally confused with rhabdomyoma, a benign myocardial tumour, and was not recognised as a separate entity until 1962.

Show evidence (1 reference)

PMID:25921236 SUPPORT Human Clinical

"The disease was first confused with rhabdomyoma, benign tumors of the myocardium, and was not recognized as a separate pathologic entity until 1962 by Voth"

Documents the historical confusion with rhabdomyoma.

🐁

Animal Models

2

ndufb11 morpholino knockdown Danio rerio Morpholino knockdown

Morpholino-mediated knockdown of ndufb11 in zebrafish embryos produced defective cardiac tissue with cardiomegaly, looping defects, and arrhythmia. This is the only reported experimental model addressing the NDUFB11 arm of the disease. It reproduces cardiomegaly and arrhythmia but has not been shown to reproduce the defining histiocytoid/oncocytic cell morphology.

Species

Danio rerio

Genotype

ndufb11 morpholino knockdown

Show evidence (1 reference)

PMID:25921236 SUPPORT Model Organism

"morpholino-mediated knockdown of ndufb11 in zebrafish embryos generated defective cardiac tissue with cardiomegaly, looping defects, and arrhythmia which suggests the role of NDUFB11 in the pathogenesis of this abnormal cardiac pathology"

Describes the zebrafish knockdown phenotype and its interpretation.

naturally occurring, genotype unknown Felis catus Naturally occurring veterinary case

A 2-month-old female Savannah kitten died suddenly with cardiac lesions described as identical to human histiocytoid cardiomyopathy - large, angular, tightly packed Purkinje fibres with few disorganised myofibrils in a rarified cytoplasm - together with biventricular noncompaction. This is a spontaneous comparative-pathology observation, not an engineered model, and no causal genotype was established.

Species

Felis catus

Genotype

naturally occurring, genotype unknown

Show evidence (2 references)

PMID:19276060 SUPPORT Model Organism

"These findings are identical to those that occur in children with histiocytoid cardiomyopathy, a fatal genetic mitochondrial disorder of Purkinje fibers."

States the equivalence of the feline lesion to the human disease. Tagged MODEL_ORGANISM per the dismech rule that veterinary observations in non-human mammals are model-organism evidence.

PMID:19276060 SUPPORT Model Organism

"The Purkinje fibers were large, angular, and tightly packed. They contained few disorganized myofibrils among a rarified cytoplasm."

Describes the feline Purkinje-fibre lesion in detail.

{ }

Source YAML

click to show

name: Histiocytoid Cardiomyopathy
creation_date: "2026-08-01T13:20:00Z"
category: Mendelian
description: >-
  Histiocytoid cardiomyopathy (HC; OMIM 500000) is an ultra-rare, strongly
  female-predominant arrhythmogenic cardiomyopathy of infancy and early
  childhood. Its defining lesion is multifocal aggregation of enlarged,
  polygonal, pale "histiocyte-like" (oncocytic) cells that are not histiocytes
  at all but modified Purkinje-like cardiomyocytes: they are packed with
  abnormal mitochondria, contain lipid microvacuoles giving a foamy granular
  cytoplasm, have few or no myofibrils, and lack normal intercellular
  junctions. The aggregates sit preferentially in the subendocardium and in the
  vicinity of the Purkinje/conduction system, and may form grossly visible
  yellow-tan nodules on endocardium, epicardium, or valves. Clinically the
  disease presents in the first two years of life with incessant ventricular
  tachycardia, cardiomegaly, refractory arrhythmia, and sudden death; some
  children instead present with dilated cardiomyopathy or ventricular
  noncompaction morphology (a hypertrophic presentation is documented in the
  allelic NDUFB11 complex I spectrum rather than in pathology-proven
  histiocytoid disease). HC is genetically heterogeneous and, in most
  historical cases, molecularly unsolved. Two mitochondrial respiratory-chain
  arms are established: a mitochondrial-DNA arm (the OMIM 500000 mitochondrial
  inheritance entry, anchored on a heteroplasmic MT-CYB missense variant with
  documented complex III deficiency) and a nuclear X-linked arm (de novo
  truncating variants in NDUFB11, a complex I accessory subunit, which is
  allelic with microphthalmia with linear skin defects syndrome). Rarer
  candidate complex I alleles (NDUFAF2, NDUFB9) have been reported. The 5th
  edition WHO classification of cardiac tumours now names the lesion
  "conduction system hamartoma".
disease_term:
  preferred_term: histiocytoid cardiomyopathy
  term:
    id: MONDO:0010771
    label: histiocytoid cardiomyopathy
parents:
- Mitochondrial Disease
- Cardiomyopathy
synonyms:
- infantile histiocytoid cardiomyopathy
- infantile xanthomatous cardiomyopathy
- oncocytic cardiomyopathy
- foamy myocardial transformation of infancy
- Purkinje cell hamartoma
- conduction system hamartoma
- arachnocytosis of the myocardium
- infantile cardiomyopathy with histiocytoid change
- focal lipid cardiomyopathy
- isolated cardiac lipidosis
notes: >-
  No GeneReviews chapter exists for histiocytoid cardiomyopathy - a PubMed
  search for "histiocytoid cardiomyopathy GeneReviews[All Fields]" returned zero
  results on 2026-08-01 - so the usual GeneReviews phenotype baseline is
  unavailable and this entry is anchored directly on primary case reports, the
  histiocytoid cardiomyopathy registry publications, and forensic/autopsy
  series. Scope decisions: (1) microphthalmia and linear skin defects are NOT
  curated as HC phenotypes even though NDUFB11 is allelic with MLS syndrome,
  because the index NDUFB11 histiocytoid case explicitly "lacked the diagnostic
  features of MLS syndrome" (PMID:28050600); the allelic relationship is
  recorded under `genetic` and `differential_diagnoses` instead. The scope line
  drawn is deliberately CARDIAC: phenotypes of the NDUFB11 complex I spectrum
  that are cardiac or that are direct biochemical readouts of the
  respiratory-chain lesion (hypertrophic cardiomyopathy, lactic acidosis) are
  curated but marked `supports: PARTIAL` with an explicit caveat, whereas
  extracardiac NDUFB11/MLS features that are not consequences of the cardiac
  lesion (microphthalmia, linear skin defects, sideroblastic anemia) are
  excluded entirely. (2) Frequency bands are asserted only where a source gives
  a count or an explicit qualitative statement about that specific phenotype;
  the literature is a ~100-case aggregate of published reports with heavy
  autopsy ascertainment bias, so only Ventricular Tachycardia carries a
  `frequency:`. In particular, no band is asserted for Sudden cardiac death
  (overall case fatality is not the same claim as the sudden-arrhythmic
  fraction) or for Cardiomegaly (a contemporary autopsy series found normal
  age- and sex-adjusted heart weights in all four cases, curated as REFUTE
  evidence on that phenotype). (3) Sudden death is curated both as a phenotype
  (HP:0001645 Sudden cardiac death, which is inside the Phenotypic abnormality
  subtree) and under `progression` as natural history. (4) Two NDUFB11 reports used here
  (PMID:36675256, PMID:39132302) describe neonatal hypertrophic cardiomyopathy
  with complex I deficiency rather than pathology-proven histiocytoid change;
  they are cited for the NDUFB11/complex I molecular mechanism and for the
  X-inactivation modifier, and their evidence items are marked PARTIAL and say
  so explicitly rather than implying histiocytoid morphology. (5) The
  Edison/falcon deep-research report used as a lead source asserted that no
  disease-specific OMIM identifier could be verified; that is incorrect (OMIM
  500000 is xrefed by MONDO:0010771) and the claim was not carried over. Two of
  its quantitative claims sourced to the 2015 WHO cardiac-tumour review
  (">1/3 with additional anomalies", "fewer than 150 reported cases") are not
  present in that paper's abstract (PMID:26725181) and were dropped rather than
  cited.
references:
- reference: PMID:18473377
  title: "Histiocytoid cardiomyopathy: a mitochondrial disorder."
- reference: PMID:10463272
  title: "Histiocytoid cardiomyopathy: three new cases and a review of the literature."
- reference: PMID:7942464
  title: "Infantile histiocytoid cardiomyopathy: three cases and literature review."
- reference: PMID:6093033
  title: "Histiocytoid cardiomyopathy of infancy: deficiency of reducible cytochrome b in heart mitochondria."
- reference: PMID:10960495
  title: "A missense mutation in the mitochondrial cytochrome b gene in a revisited case with histiocytoid cardiomyopathy."
- reference: PMID:25921236
  title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
- reference: PMID:28050600
  title: "Histiocytoid cardiomyopathy and microphthalmia with linear skin defects syndrome: phenotypes linked by truncating variants in NDUFB11."
- reference: PMID:21585276
  title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
- reference: PMID:37831311
  title: "Histiocytoid cardiomyopathy presenting as sudden death in an 18-month-old infant."
- reference: PMID:39225698
  title: "Conduction System Hamartoma: Autopsy Case Series."
- reference: PMID:38089118
  title: "Case report: high-dose carvedilol as a potential key drug for arrhythmias in histiocytoid cardiomyopathy."
- reference: PMID:35439193
  title: "Biventricular Assist Device Support for Intractable Arrhythmias From Histiocytoid Cardiomyopathy."
- reference: PMID:31449642
  title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
- reference: PMID:36675256
  title: "A Novel Mutation Associated with Neonatal Lethal Cardiomyopathy Leads to an Alternative Transcript Expression in the X-Linked Complex I NDUFB11 Gene."
- reference: PMID:39132302
  title: "Case report: severe hypertrophic cardiomyopathy in a female neonate caused by de novo variant in NDUFB11."
- reference: PMID:28045696
  title: "Sudden Death in a Male Infant Due to Histiocytoid Cardiomyopathy: An Autopsy Case and Review of the Literature."
- reference: PMID:22758650
  title: "Association of ventricular noncompaction and histiocytoid cardiomyopathy: case report and review of the literature."
- reference: PMID:8563940
  title: "Histiocytoid cardiomyopathy with hypotonia in an infant."
- reference: PMID:19276060
  title: "Purkinje fiber dysplasia (histiocytoid cardiomyopathy) with ventricular noncompaction in a savannah kitten."
classifications:
  harrisons_chapter:
  - classification_value: CARDIOVASCULAR
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
  mechanistic_category:
  - classification_value: mitochondrial disease
    notes: >-
      Both established molecular arms are mitochondrial respiratory-chain
      defects: a mitochondrial-DNA MT-CYB (complex III) arm and a nuclear
      X-linked NDUFB11 (complex I) arm.
inheritance:
- name: Mitochondrial inheritance
  description: >-
    OMIM 500000 assigns histiocytoid cardiomyopathy to mitochondrial
    (maternal) inheritance on the basis of the heteroplasmic MT-CYB
    m.15498G>A variant reported by Andreu et al. A separate proband in the
    histiocytoid cardiomyopathy registry exome study inherited a mitochondrial
    mutation from a heteroplasmic mother, as did an affected brother.
  inheritance_term:
    preferred_term: Mitochondrial inheritance
    term:
      id: HP:0001427
      label: Mitochondrial inheritance
  evidence:
  - reference: PMID:10960495
    reference_title: "A missense mutation in the mitochondrial cytochrome b gene in a revisited case with histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We describe a pathogenic mutation in the mitochondrial cytochrome b gene in a patient with a multisystem disorder presenting as histiocytoid cardiomyopathy in whom a defect of ubiquinol cytochrome c oxidoreductase of the electron transport chain had been documented biochemically."
    explanation: >-
      Documents the mitochondrial-DNA (MT-CYB) lesion that underlies the
      mitochondrial-inheritance assignment of OMIM 500000.
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In a fourth case, the proband with Histiocytoid CM inherited a mitochondrial mutation from her heteroplasmic mother, as did her brother who presented with cardiac arrhythmia."
    explanation: >-
      Maternal transmission of a heteroplasmic mtDNA variant to two affected
      siblings is the signature of mitochondrial inheritance.
- name: X-linked dominant inheritance
  description: >-
    The marked female excess (approximately 3:1) prompted the long-standing
    hypothesis that histiocytoid cardiomyopathy is an X-linked dominant
    disorder with prenatal lethality in hemizygous males, originally mapped to
    Xp22. The subsequent identification of de novo truncating variants in the
    X-linked nuclear gene NDUFB11 (Xp11.23) in affected girls is consistent
    with this model, although the mechanistically related NDUFB11 hypertrophic
    cardiomyopathy reports include surviving hemizygous males, so strict male
    lethality is not universal.
  inheritance_term:
    preferred_term: X-linked dominant inheritance
    term:
      id: HP:0001423
      label: X-linked dominant inheritance
  evidence:
  - reference: PMID:10463272
    reference_title: "Histiocytoid cardiomyopathy: three new cases and a review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It was recently proposed that this disorder is X-linked dominant with the associated gene located in the region of Xp22."
    explanation: States the X-linked dominant hypothesis for histiocytoid cardiomyopathy.
  - reference: PMID:21585276
    reference_title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Because of the female predominance, it was suggested that HC is an X-linked genetic disorder with prenatal lethality in males."
    explanation: >-
      Links the observed female predominance to the X-linked, male-prenatal-lethal
      model.
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two probands had de novo non-sense mutations in the second exon of the X-linked nuclear gene NDUFB11."
    explanation: >-
      De novo truncating variants in an X-linked nuclear gene in affected
      probands are consistent with, but do not by themselves prove, X-linked
      dominant inheritance with male lethality.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    No population prevalence or incidence estimate exists. The published
    literature amounts to roughly 100 reported cases plus the material in the
    Emory histiocytoid cardiomyopathy registry. The true occurrence is probably
    higher than the publication count because an unknown fraction of cases is
    certified as sudden infant death syndrome when the heart is not
    adequately sampled at autopsy.
  evidence:
  - reference: PMID:21585276
    reference_title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Although approximately 100 HC cases have been reported in the literature, the prevalence of this disease may be higher than the reported cases would suggest, as some cases are mistaken for Sudden Infant Death Syndrome (SIDS)"
    explanation: >-
      Gives the literature case count and states the SIDS-misclassification
      reason why it understates true occurrence.
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Approximately 100 Histiocytoid CM cases have been reported in the literature"
    explanation: Independent statement of the same literature case count.
epidemiology:
- name: Female predominance
  description: >-
    Histiocytoid cardiomyopathy affects girls roughly three times as often as
    boys, and this sex skew is the principal observation behind the X-linked
    male-prenatal-lethality hypothesis. The predominance is reproduced in an
    independent contemporary forensic autopsy series.
  evidence:
  - reference: PMID:21585276
    reference_title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It primarily occurs in the first two years of life and has a female to male ratio of 3:1."
    explanation: Quantifies the female-to-male ratio and the age window.
  - reference: PMID:39225698
    reference_title: "Conduction System Hamartoma: Autopsy Case Series."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The female-to-male ratio was 3/1."
    explanation: >-
      A small independent 2012-2022 forensic autopsy series (n = 4; three girls,
      one boy) shows the same direction of sex skew. With four cases this is
      directionally consistent rather than statistical replication of the
      registry ratio.
- name: Ancestry distribution
  description: >-
    Reported cases from the histiocytoid cardiomyopathy registry are
    predominantly of European ancestry. This distribution reflects the
    ascertainment of a registry assembled largely from North American autopsy
    referrals and should not be read as a validated population risk estimate.
  evidence:
  - reference: PMID:21585276
    reference_title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It predominantly affects Caucasians (80%) followed by African-American (15%) and Latin-American infants (3%); it is rare in Oriental infants"
    explanation: >-
      Source statement of the reported ancestry distribution in the registry
      material.
- name: Sibling recurrence
  description: >-
    The histiocytoid cardiomyopathy registry investigators estimate a sibling
    recurrence risk of about 5%, which they note is probably an underestimate
    because of recurrent miscarriage in some families - itself consistent with
    prenatal lethality of the more severe genotypes.
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We estimate the sibling recurrence rate to be approximately 5%, but note that this is likely to be downward biased due to the incidence of recurrent miscarriages in some families."
    explanation: Direct statement of the registry sibling recurrence estimate and its bias.
pathophysiology:
- name: Mitochondrial Respiratory Chain Subunit Defect
  description: >-
    The proximal lesion is a defect in a structural subunit of the
    mitochondrial respiratory chain. Two arms are established. In the
    mitochondrial-DNA arm a heteroplasmic MT-CYB missense variant
    (m.15498G>A, p.Gly251Asp) impairs ubiquinol-cytochrome c oxidoreductase
    (complex III). In the nuclear X-linked arm, de novo truncating variants in
    NDUFB11, an accessory subunit of complex I, abolish or reduce NDUFB11
    protein and impair complex I assembly and activity. A minority of probands
    carry rare variants in other complex I components (NDUFAF2, NDUFB9), so
    the disease is genetically heterogeneous and remains unsolved in many
    historical cases.
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  protein_complexes:
  - preferred_term: mitochondrial respiratory chain complex I
    term:
      id: GO:0045271
      label: respiratory chain complex I
    modifier: DECREASED
  - preferred_term: mitochondrial respiratory chain complex III
    term:
      id: GO:0045275
      label: respiratory chain complex III
    modifier: DECREASED
  cellular_components:
  - preferred_term: mitochondrion
    term:
      id: GO:0005739
      label: mitochondrion
  genes:
  - preferred_term: MT-CYB
    term:
      id: hgnc:7427
      label: MT-CYB
  - preferred_term: NDUFB11
    term:
      id: hgnc:20372
      label: NDUFB11
  evidence:
  - reference: PMID:10960495
    reference_title: "A missense mutation in the mitochondrial cytochrome b gene in a revisited case with histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mutation, a G to A transition at nucleotide 15498, results in the substitution of glycine with aspartic acid at amino acid position 251."
    explanation: Defines the MT-CYB (complex III) variant in the mitochondrial arm.
  - reference: PMID:28050600
    reference_title: "Histiocytoid cardiomyopathy and microphthalmia with linear skin defects syndrome: phenotypes linked by truncating variants in NDUFB11."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Variants in NDUFB11, which encodes a structural component of complex I of the mitochondrial respiratory chain (MRC), were recently independently reported to cause histiocytoid cardiomyopathy (histiocytoid CM) and microphthalmia with linear skin defects syndrome (MLS syndrome)."
    explanation: >-
      Establishes NDUFB11, a complex I structural component, as a histiocytoid
      cardiomyopathy gene.
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A third proband was doubly heterozygous for inherited rare variants in additional components of complex I, NDUFAF2 and NDUFB9, confirming that Histiocytoid CM is genetically heterogeneous."
    explanation: Documents the genetic heterogeneity of the subunit-defect node.
  - reference: PMID:36675256
    reference_title: "A Novel Mutation Associated with Neonatal Lethal Cardiomyopathy Leads to an Alternative Transcript Expression in the X-Linked Complex I NDUFB11 Gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "transcript is required for the proper NDUFB11 protein synthesis, which is essential for optimal CI assembly and activity"
    explanation: >-
      Patient-tissue functional work showing that loss of the canonical NDUFB11
      transcript removes the protein and thereby impairs complex I assembly and
      activity - the molecular step this node asserts. Partial because the
      proband studied had hypertrophic rather than pathology-proven
      histiocytoid cardiomyopathy, so this evidences the NDUFB11 mechanism, not
      the histiocytoid morphology.
  downstream:
  - target: Oxidative Phosphorylation Failure in Cardiomyocytes
    causal_link_type: DIRECT
    description: >-
      Loss of a complex I or complex III subunit directly reduces electron
      transfer through the respiratory chain in cardiac mitochondria.
    evidence:
    - reference: PMID:6093033
      reference_title: "Histiocytoid cardiomyopathy of infancy: deficiency of reducible cytochrome b in heart mitochondria."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "In isolated mitochondria, cytochrome spectra showed a severe defect of reducible cytochrome b and a less marked defect of cytochrome cc1, while the content of cytochrome aa3 (cytochrome c oxidase) was normal."
      explanation: >-
        Directly demonstrates the cytochrome b defect in heart mitochondria of a
        histiocytoid cardiomyopathy patient.
- name: Oxidative Phosphorylation Failure in Cardiomyocytes
  description: >-
    Respiratory-chain enzymology in histiocytoid cardiomyopathy hearts shows
    markedly decreased succinate-cytochrome c reductase and rotenone-sensitive
    NADH-cytochrome c reductase activity, i.e. a segmental block of electron
    transport at complex III (and, in the NDUFB11 arm, at complex I) with
    preserved cytochrome c oxidase. The resulting failure of oxidative ATP
    synthesis in a high-demand tissue is the energetic lesion that the
    downstream cellular phenotype is built on.
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  cell_types:
  - preferred_term: cardiac muscle cell
    term:
      id: CL:0000746
      label: cardiac muscle cell
  biological_processes:
  - preferred_term: mitochondrial electron transport, ubiquinol to cytochrome c
    term:
      id: GO:0006122
      label: mitochondrial electron transport, ubiquinol to cytochrome c
    modifier: DECREASED
  - preferred_term: mitochondrial electron transport, NADH to ubiquinone
    term:
      id: GO:0006120
      label: mitochondrial electron transport, NADH to ubiquinone
    modifier: DECREASED
  - preferred_term: mitochondrial ATP synthesis coupled electron transport
    term:
      id: GO:0042775
      label: mitochondrial ATP synthesis coupled electron transport
    modifier: DECREASED
  locations:
  - preferred_term: myocardium
    term:
      id: UBERON:0002349
      label: myocardium
  evidence:
  - reference: PMID:6093033
    reference_title: "Histiocytoid cardiomyopathy of infancy: deficiency of reducible cytochrome b in heart mitochondria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Biochemical studies showed markedly decreased succinate-cytochrome c reductase and rotenone-sensitive NADH-cytochrome c reductase activities, while other mitochondrial enzymes were normal."
    explanation: >-
      Quantifies the segmental respiratory-chain block in a histiocytoid
      cardiomyopathy heart.
  - reference: PMID:6093033
    reference_title: "Histiocytoid cardiomyopathy of infancy: deficiency of reducible cytochrome b in heart mitochondria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histiocytoid cardiomyopathy appears to be due to a defect of complex III (reduced coenzyme Q-cytochrome c reductase) in the respiratory chain of heart mitochondria."
    explanation: States the complex III localisation of the defect.
  - reference: PMID:18473377
    reference_title: "Histiocytoid cardiomyopathy: a mitochondrial disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since HICMP is frequently found in patients with mitochondrial deoxyribonucleic acid (DNA) mutations, HICMP cardiomyocytes carry an increased number of normal or abnormal mitochondria, and may show markedly decreased succinate-cytochrome c reductase or NADH-cytochrome c reductase activity; HICMP should be regarded as mitochondrial cardiomyopathy."
    explanation: >-
      Review-level statement tying the enzymology to the classification of HC as
      a mitochondrial cardiomyopathy.
  - reference: PMID:36675256
    reference_title: "A Novel Mutation Associated with Neonatal Lethal Cardiomyopathy Leads to an Alternative Transcript Expression in the X-Linked Complex I NDUFB11 Gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report a neonatal patient with hypertrophic cardiomyopathy (HCM), lactic acidosis and isolated complex I deficiency."
    explanation: >-
      Demonstrates isolated complex I deficiency in an NDUFB11 neonate. Partial
      because this proband had hypertrophic, not pathology-proven histiocytoid,
      cardiomyopathy; it supports the NDUFB11 complex I arm of the mechanism
      rather than the histiocytoid morphology.
  downstream:
  - target: Oncocytic Transformation of Purkinje-like Cardiomyocytes
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Chronic energy failure is followed by compensatory mitochondrial
      proliferation, lipid microvacuolation, and loss of contractile apparatus
      in affected cells. The steps linking the bioenergetic defect to the
      focal, conduction-system-biased distribution of the transformed cells are
      not established.
    evidence:
    - reference: PMID:25921236
      reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Lipids in small vacuoles are commonly observed to fill the intracellular space, generating a foamy cytoplasm that has been considered to result from failure of energy generation."
      explanation: >-
        Explicitly attributes the foamy cytoplasm of the histiocytoid cell to
        failure of energy generation.
- name: Oncocytic Transformation of Purkinje-like Cardiomyocytes
  description: >-
    The cellular hallmark of the disease. Affected cells are enlarged,
    polygonal, and pale, with abundant faintly eosinophilic foamy granular
    cytoplasm, coarse granules and microvacuoles, few or no myofibrils, and
    poorly developed or absent intercellular junctions. Ultrastructurally they
    are filled with excessive and aberrantly shaped mitochondria plus scattered
    lipid droplets. They are not histiocytes: immunophenotype and distribution
    identify them as modified cardiomyocytes with Purkinje/conduction-cell
    features, which is why the WHO 5th edition renamed the lesion "conduction
    system hamartoma".
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  cell_types:
  - preferred_term: Purkinje myocyte
    term:
      id: CL:0002068
      label: Purkinje myocyte
    modifier: ABNORMAL
  cellular_components:
  - preferred_term: mitochondrion
    term:
      id: GO:0005739
      label: mitochondrion
    modifier: INCREASED
  locations:
  - preferred_term: cardiac Purkinje fiber
    term:
      id: UBERON:0002354
      label: cardiac Purkinje fiber
  evidence:
  - reference: PMID:10463272
    reference_title: "Histiocytoid cardiomyopathy: three new cases and a review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Microscopic examination in all three cases revealed multiple, scattered clusters of histiocytoid myocytes which on ultrastructural examination were filled with abnormal mitochondria, scattered lipid droplets, and scanty myofibrils."
    explanation: Defines the ultrastructural signature of the transformed cell.
  - reference: PMID:7942464
    reference_title: "Infantile histiocytoid cardiomyopathy: three cases and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These cells were remarkably similar in all patients and had poorly developed or absent intercellular junctions, few or no contractile elements, and markedly increased numbers of mitochondria, which imparted a granular or vacuolated appearance to the cytoplasm."
    explanation: >-
      Establishes the loss of junctions and contractile elements alongside the
      mitochondrial excess across a 53-patient review.
  - reference: PMID:22758650
    reference_title: "Association of ventricular noncompaction and histiocytoid cardiomyopathy: case report and review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histiocytoid cardiomyopathy is a rare arrhythmogenic disorder characterized by aggregates of oncocytic cells involving predominantly the subendocardium. These cells are thought to be abnormal Purkinje cells."
    explanation: Identifies the transformed cells as abnormal Purkinje cells.
  - reference: PMID:39225698
    reference_title: "Conduction System Hamartoma: Autopsy Case Series."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Conduction system hamartoma is a benign hamartomatous lesion arising from Purkinje and Purkinje-like cells of the heart."
    explanation: >-
      Current WHO-aligned statement of the Purkinje/Purkinje-like origin of the
      lesional cells.
  downstream:
  - target: Hamartomatous Nodule Formation in the Conduction System
    causal_link_type: DIRECT
    description: >-
      Transformed cells accumulate as multifocal, sharply demarcated clusters
      and nodules rather than diffusely, which is what distinguishes
      histiocytoid cardiomyopathy from a generalised mitochondrial
      cardiomyopathy.
    evidence:
    - reference: PMID:7942464
      reference_title: "Infantile histiocytoid cardiomyopathy: three cases and literature review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Histiocytoid cardiomyopathy is considered to be the result of hamartoma-like aggregations of cardiac myocytes with features similar to those of oncocytes."
      explanation: States the hamartoma-like aggregation model.
- name: Hamartomatous Nodule Formation in the Conduction System
  description: >-
    Transformed cells form multifocal, sharply demarcated clusters, sheets, or
    grossly visible yellow-tan nodules that are biased to the subendocardium,
    epicardium, and valves and cluster in the vicinity of the Purkinje fibres
    and nodal conduction tissue. This focal, conduction-system-weighted
    distribution - rather than uniform involvement of all cardiomyocytes - is
    the structural reason the disease is arrhythmogenic out of proportion to
    its contractile burden.
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  locations:
  - preferred_term: endocardium
    term:
      id: UBERON:0002165
      label: endocardium
  - preferred_term: myocardium
    term:
      id: UBERON:0002349
      label: myocardium
  evidence:
  - reference: PMID:18473377
    reference_title: "Histiocytoid cardiomyopathy: a mitochondrial disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Pathoanatomically, HICMP is characterized by subendocardial, epicardial, or valvular yellow-tan nodules, which are histologically built up of abnormal Purkinje fibers and multiple, scattered clusters of histiocytoid myocytes, which are filled with an increased number of normal or abnormal mitochondria."
    explanation: Describes the anatomic distribution of the nodular lesions.
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Unlike other mitochondrial diseases, however, Histiocytoid CM does not affect all cardiomyocytes equally."
    explanation: >-
      Contrasts the focal distribution of histiocytoid cardiomyopathy with
      generalised mitochondrial cardiomyopathy.
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The observation that lesions tend to be clustered in the vicinity of the Purkinje fibers has also prompted the argument that the disease may be related to defective activity of this sub-type of cardiomyocyte that coordinate the cardiac action potential"
    explanation: >-
      States the conduction-system bias of lesion distribution and its proposed
      electrophysiological consequence.
  downstream:
  - target: Arrhythmogenic Substrate and Triggered Activity
    causal_link_type: DIRECT
    description: >-
      Nests of electrically abnormal, poorly coupled Purkinje-like cells within
      and adjacent to the conduction system generate ectopic impulses and
      triggered activity.
    evidence:
    - reference: PMID:31449642
      reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The PVCs were likely caused by triggered activity and associated with abnormal Purkinje cells."
      explanation: >-
        Electrophysiological mapping in a histiocytoid cardiomyopathy infant
        attributes the trigger beats to abnormal Purkinje cells. Partial because
        the authors hedge ("likely caused by", "associated with") and this is a
        single case; the mechanism is inferred from mapping, not proven.
- name: Arrhythmogenic Substrate and Triggered Activity
  description: >-
    The lesional cells behave as autonomous arrhythmogenic foci. Clinically
    this produces incessant ventricular tachycardia and short-coupled premature
    ventricular complexes that initiate torsade de pointes, typically
    refractory to standard antiarrhythmic drugs. Catheter mapping localises
    discrete triggering foci with recordable pre-potentials, and successful
    ablation or surgical excision of the foci can abolish the arrhythmia,
    directly implicating the lesions as the substrate. The response of
    otherwise-refractory arrhythmia to high-dose carvedilol has been proposed
    to implicate ryanodine-receptor store-overload-induced calcium release, but
    this rests on a single case.
  biological_scale: TISSUE
  mechanism_confidence: ESTABLISHED
  cell_types:
  - preferred_term: Purkinje myocyte
    term:
      id: CL:0002068
      label: Purkinje myocyte
  biological_processes:
  - preferred_term: Purkinje myocyte action potential
    term:
      id: GO:0086017
      label: Purkinje myocyte action potential
    modifier: ABNORMAL
  - preferred_term: cardiac muscle cell action potential
    term:
      id: GO:0086001
      label: cardiac muscle cell action potential
    modifier: ABNORMAL
  evidence:
  - reference: PMID:10463272
    reference_title: "Histiocytoid cardiomyopathy: three new cases and a review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histiocytoid cardiomyopathy (HC), a rare arrhythmogenic disorder, presents as difficult-to-control arrhythmias or sudden death in infants and children, particularly girls."
    explanation: Establishes the refractory-arrhythmia clinical signature.
  - reference: PMID:31449642
    reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The PVCs triggering TdP were successfully ablated in a 4-month-old girl with histiocytoid cardiomyopathy."
    explanation: >-
      Abolition of the arrhythmia by ablating a mapped focus is direct evidence
      that the lesions form the arrhythmogenic substrate.
  - reference: PMID:38089118
    reference_title: "Case report: high-dose carvedilol as a potential key drug for arrhythmias in histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This clinical course suggested that the arrhythmias in histiocytoid cardiomyopathy might be related with SOICR."
    explanation: >-
      Single-case pharmacological inference that store-overload-induced calcium
      release contributes to the triggered activity; hypothesis-generating only.
  downstream:
  - target: Circulatory Failure and Sudden Cardiac Death
    causal_link_type: DIRECT
    description: >-
      Incessant ventricular tachyarrhythmia degenerates into torsade de
      pointes, ventricular fibrillation, cardiac arrest, or tachycardia-related
      cardiogenic shock.
    evidence:
    - reference: PMID:21585276
      reference_title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Histiocytoid cardiomyopathy (HC) is a rare but distinctive arrhythmogenic disorder characterized by incessant ventricular tachycardia, cardiomegaly, and often sudden death by age 2 years."
      explanation: >-
        Links the incessant ventricular tachycardia directly to death by age
        2 years.
- name: Circulatory Failure and Sudden Cardiac Death
  description: >-
    The clinical endpoint. Untreated disease is fatal in the great majority,
    usually within the first two years of life, either as unheralded sudden
    death - frequently first diagnosed at autopsy and easily miscertified as
    sudden infant death syndrome - or as progressive heart failure from
    cardiomegaly, dilated/hypertrophic/noncompaction remodelling, and
    tachycardia-mediated dysfunction.
  biological_scale: ORGANISM
  mechanism_confidence: ESTABLISHED
  locations:
  - preferred_term: heart
    term:
      id: UBERON:0000948
      label: heart
  evidence:
  - reference: PMID:37831311
    reference_title: "Histiocytoid cardiomyopathy presenting as sudden death in an 18-month-old infant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disease is fatal in the vast majority and diagnosis is nearly always established at autopsy, but this is only possible with adequate myocardial sampling."
    explanation: >-
      States the case fatality and the autopsy-dependence of diagnosis, which is
      the source of SIDS misclassification.
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histiocytoid cardiomyopathy (Histiocytoid CM) is a rare form of cardiomyopathy observed predominantly in newborn females that is fatal unless treated early in life."
    explanation: States that the natural history is fatal without early treatment.
mechanistic_hypotheses:
- hypothesis_group_id: mitochondrial_respiratory_chain_model
  hypothesis_label: Mitochondrial respiratory-chain cardiomyopathy
  status: CANONICAL
  description: >-
    Histiocytoid cardiomyopathy is fundamentally a mitochondrial
    cardiomyopathy: a respiratory-chain subunit defect (MT-CYB/complex III or
    NDUFB11/complex I) causes bioenergetic failure and compensatory
    mitochondrial proliferation in cardiomyocytes, with the histiocytoid
    morphology as the cellular readout. This is the dominant contemporary
    model and is supported by direct enzymology in patient heart mitochondria
    and by molecular genetics.
  evidence:
  - reference: PMID:18473377
    reference_title: "Histiocytoid cardiomyopathy: a mitochondrial disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "HICMP should be regarded as mitochondrial cardiomyopathy."
    explanation: Explicit review-level endorsement of the mitochondrial model.
- hypothesis_group_id: purkinje_hamartoma_model
  hypothesis_label: Purkinje-cell/oncocyte hamartoma rather than diffuse mitochondrial cardiomyopathy
  status: ALTERNATIVE
  description: >-
    The competing pathology-derived model holds that the lesion is a
    hamartoma-like focal aggregation of oncocyte-like cardiac myocytes and
    explicitly rejects a diffuse mitochondrial cardiomyopathy, a multifocal
    Purkinje-cell tumour, and a developmental arrest, proposing instead a
    prenatal myocardial or systemic (possibly viral) injury. The WHO 5th
    edition adoption of the name "conduction system hamartoma" keeps this
    framing alive. The two models are not fully reconciled: the focal,
    conduction-system-restricted distribution of the lesions is the main
    observation the purely mitochondrial model does not yet explain.
  evidence:
  - reference: PMID:7942464
    reference_title: "Infantile histiocytoid cardiomyopathy: three cases and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Evidence is presented to exclude the possibilities that the disorder represents a developmental anomaly of the atrioventricular conduction system, a multifocal tumor of Purkinje cells, a developmental arrest of cardiac myocytes, and a diffuse type of mitochondrial cardiomyopathy."
    explanation: >-
      States the alternative model's explicit exclusion of a diffuse
      mitochondrial cardiomyopathy.
  - reference: PMID:7942464
    reference_title: "Infantile histiocytoid cardiomyopathy: three cases and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This syndrome is likely caused by prenatal myocardial or systemic (viral?) injury."
    explanation: States the alternative model's proposed prenatal-injury aetiology.
- hypothesis_group_id: il33_s100a_axis_model
  hypothesis_label: Downregulated IL-33/IL1RL1-p38-MAPK/S100A axis
  status: EMERGING
  description: >-
    Whole-genome expression profiling of registry hearts identified coordinate
    downregulation of two chromosomally clustered gene sets (S100A8/S100A9/
    S100A12 at 1q21.3 and IL1RL1/IL18R1/IL18RAP at 2q12.1) together with
    reduced interleukin 33, with confirmed copy-number loss of the S100A
    cluster. This suggests an inflammatory-signalling axis contributing to
    susceptibility. It has not been replicated, no causal variant has been
    demonstrated in these loci, and the finding could be a consequence rather
    than a cause of the myocardial lesion.
  evidence:
  - reference: PMID:21585276
    reference_title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These data suggest a model in which the interleukin 33-IL1RL1/p38-MAPK/ S100A8-S100A9 axis is downregulated in HC cardiac tissue and provide several candidate genes on 1q21.3c and 2q12.1a for inherited mutations that may predispose individuals to HC."
    explanation: Statement of the emerging expression-based model and its candidate loci.
phenotypes:
- category: Cardiovascular
  name: Ventricular Tachycardia
  description: >-
    Incessant, drug-refractory ventricular tachycardia is the cardinal clinical
    manifestation and often the presenting event. Short-coupled premature
    ventricular complexes triggering torsade de pointes are a documented
    variant.
  phenotype_term:
    preferred_term: Ventricular tachycardia
    term:
      id: HP:0004756
      label: Ventricular tachycardia
    temporality: RECURRENT
  frequency: VERY_FREQUENT
  diagnostic: true
  evidence:
  - reference: PMID:21585276
    reference_title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histiocytoid cardiomyopathy (HC) is a rare but distinctive arrhythmogenic disorder characterized by incessant ventricular tachycardia, cardiomegaly, and often sudden death by age 2 years."
    explanation: >-
      Names incessant ventricular tachycardia as a defining feature of the
      disorder, supporting a VERY_FREQUENT band.
  - reference: PMID:28045696
    reference_title: "Sudden Death in a Male Infant Due to Histiocytoid Cardiomyopathy: An Autopsy Case and Review of the Literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It may manifest as ventricular tachycardia or dilated cardiomyopathy and frequently causes sudden death."
    explanation: Independent review statement of the ventricular tachycardia presentation.
- category: Cardiovascular
  name: Cardiomegaly
  description: >-
    Enlargement of the heart is part of the classical triad with incessant
    ventricular tachycardia and sudden death.
  phenotype_term:
    preferred_term: Cardiomegaly
    term:
      id: HP:0001640
      label: Cardiomegaly
  evidence:
  - reference: PMID:21585276
    reference_title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histiocytoid cardiomyopathy (HC) is a rare but distinctive arrhythmogenic disorder characterized by incessant ventricular tachycardia, cardiomegaly, and often sudden death by age 2 years."
    explanation: Lists cardiomegaly among the characteristic features.
  - reference: PMID:6093033
    reference_title: "Histiocytoid cardiomyopathy of infancy: deficiency of reducible cytochrome b in heart mitochondria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A 3-week-old girl with failure to thrive and cardiomegaly died of cardiac arrest at age 4 weeks."
    explanation: Case-level documentation of cardiomegaly.
  - reference: PMID:39225698
    reference_title: "Conduction System Hamartoma: Autopsy Case Series."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "heart weight was found within the normal range when evaluated according to age and gender"
    explanation: >-
      Counter-observation: all four cases in a contemporary forensic autopsy
      series of conduction system hamartoma had age- and sex-normal heart
      weights. Cardiomegaly is therefore not obligate, which is why no
      frequency band is asserted for it.
- category: Cardiovascular
  name: Sudden Cardiac Death
  description: >-
    Unheralded sudden death, often without prodrome and often as the first
    manifestation of the disease, typically before the age of two years. It is
    a recognised mimic of sudden infant death syndrome and of sudden unexplained
    death in childhood.
  phenotype_term:
    preferred_term: Sudden cardiac death
    term:
      id: HP:0001645
      label: Sudden cardiac death
  diagnostic: true
  evidence:
  - reference: PMID:37831311
    reference_title: "Histiocytoid cardiomyopathy presenting as sudden death in an 18-month-old infant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disease is fatal in the vast majority and diagnosis is nearly always established at autopsy, but this is only possible with adequate myocardial sampling."
    explanation: >-
      Establishes that the disease is usually fatal and usually diagnosed post
      mortem. No frequency band is asserted, because overall case fatality is
      not the same claim as the fraction of deaths that are sudden and
      arrhythmic: this entry separately curates progressive heart failure as a
      competing fatal endpoint, and no source partitions the two.
  - reference: PMID:10463272
    reference_title: "Histiocytoid cardiomyopathy: three new cases and a review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histiocytoid cardiomyopathy (HC), a rare arrhythmogenic disorder, presents as difficult-to-control arrhythmias or sudden death in infants and children, particularly girls."
    explanation: Names sudden death as a presenting mode.
- category: Cardiovascular
  name: Cardiac Arrest
  description: >-
    Aborted or fatal cardiac arrest secondary to ventricular tachyarrhythmia,
    reported both at home and in hospital, sometimes with survival and
    recurrent arrhythmia afterwards.
  phenotype_term:
    preferred_term: Cardiac arrest
    term:
      id: HP:0001695
      label: Cardiac arrest
  evidence:
  - reference: PMID:8563940
    reference_title: "Histiocytoid cardiomyopathy with hypotonia in an infant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A 15 month old female, who had suffered from ventricular tachycardia from the prenatal period, experienced cardiac arrest at home."
    explanation: Case-level documentation of cardiac arrest.
  - reference: PMID:38089118
    reference_title: "Case report: high-dose carvedilol as a potential key drug for arrhythmias in histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A 4-month-old girl presented with asystole, and recurrent ventricular tachycardias."
    explanation: Documents asystolic arrest as a presenting event.
- category: Cardiovascular
  name: Torsade de Pointes
  description: >-
    Polymorphic ventricular tachycardia of the short-coupled torsade de pointes
    type, initiated by premature ventricular complexes arising from the
    abnormal Purkinje tissue and typically resistant to antiarrhythmic drugs.
  phenotype_term:
    preferred_term: Torsade de pointes
    term:
      id: HP:0001664
      label: Torsade de pointes
    temporality: RECURRENT
  evidence:
  - reference: PMID:31449642
    reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "On the 5th day after ablation, she had recurrent TdP episodes resistant to various antiarrhythmic drugs and received extracorporeal membrane oxygenation at 86 days of age."
    explanation: Documents drug-resistant recurrent torsade de pointes in a proven case.
- category: Cardiovascular
  name: Dilated Cardiomyopathy
  description: >-
    A subset of children present with dilated cardiomyopathy morphology and
    systolic dysfunction rather than with isolated arrhythmia.
  phenotype_term:
    preferred_term: Dilated cardiomyopathy
    term:
      id: HP:0001644
      label: Dilated cardiomyopathy
  evidence:
  - reference: PMID:18473377
    reference_title: "Histiocytoid cardiomyopathy: a mitochondrial disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Histiocytoid cardiomyopathy (HICMP) is a rare, genetic, cardiac disorder of infancy or childhood, predominantly affecting girls, and clinically manifesting as severe cardiac arrhythmias or dilated cardiomyopathy."
    explanation: Names dilated cardiomyopathy as one of the two clinical presentations.
  - reference: PMID:28045696
    reference_title: "Sudden Death in a Male Infant Due to Histiocytoid Cardiomyopathy: An Autopsy Case and Review of the Literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It may manifest as ventricular tachycardia or dilated cardiomyopathy and frequently causes sudden death."
    explanation: Independent review statement of the dilated cardiomyopathy presentation.
- category: Cardiovascular
  name: Left Ventricular Noncompaction
  description: >-
    Ventricular noncompaction co-occurs with histiocytoid cardiomyopathy more
    often than chance would suggest and has been reported in infants and in the
    naturally occurring feline counterpart. Whether the two lesions share a
    developmental origin or merely co-segregate is unresolved.
  phenotype_term:
    preferred_term: Left ventricular noncompaction
    term:
      id: HP:0011664
      label: Left ventricular noncompaction cardiomyopathy
  evidence:
  - reference: PMID:22758650
    reference_title: "Association of ventricular noncompaction and histiocytoid cardiomyopathy: case report and review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report an association between ventricular noncompaction and histiocytoid cardiomyopathy."
    explanation: Directly reports the noncompaction association.
  - reference: PMID:35439193
    reference_title: "Biventricular Assist Device Support for Intractable Arrhythmias From Histiocytoid Cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report a 13 month old girl with left ventricular noncompaction and preserved systolic function who presented in cardiogenic shock secondary to incessant ventricular arrhythmias."
    explanation: Independent case of histiocytoid cardiomyopathy with LV noncompaction.
- category: Cardiovascular
  name: Congestive Heart Failure
  description: >-
    Progressive heart failure occurs both as tachycardia-mediated dysfunction
    and as a consequence of the underlying cardiomyopathy, and is a common
    proximate cause of death in children who survive the initial arrhythmic
    presentation.
  phenotype_term:
    preferred_term: Congestive heart failure
    term:
      id: HP:0001635
      label: Congestive heart failure
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:28045696
    reference_title: "Sudden Death in a Male Infant Due to Histiocytoid Cardiomyopathy: An Autopsy Case and Review of the Literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we present a 19-month-old male infant who died of heart failure, and an autopsy was performed and confirmed the diagnosis of HC."
    explanation: Death from heart failure in an autopsy-confirmed case.
  - reference: PMID:31449642
    reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "However, she died of uncontrolled heart failure at 6 months of age."
    explanation: >-
      Heart failure remained the fatal endpoint even after successful arrhythmia
      ablation.
- category: Cardiovascular
  name: Ventricular Pre-excitation
  description: >-
    Wolff-Parkinson-White syndrome with supraventricular tachycardia has been
    reported as the presenting rhythm disturbance in histiocytoid
    cardiomyopathy, consistent with the conduction-system localisation of the
    lesions.
  phenotype_term:
    preferred_term: Wolff-Parkinson-White syndrome
    term:
      id: HP:0001716
      label: Wolff-Parkinson-White syndrome
  evidence:
  - reference: PMID:31449642
    reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A neonate was referred to our hospital on the day of birth for Wolff-Parkinson-White syndrome, repeated episodes of supraventricular tachycardia, and a left ventricular non-compaction."
    explanation: >-
      Documents WPW and supraventricular tachycardia as the presenting
      electrophysiological abnormality in a case later proven histologically.
- category: Cardiovascular
  name: Supraventricular Tachycardia
  description: >-
    Recurrent supraventricular tachycardia mediated by an accessory pathway,
    reported alongside the pre-excitation phenotype.
  phenotype_term:
    preferred_term: Supraventricular tachycardia
    term:
      id: HP:0004755
      label: Supraventricular tachycardia
    temporality: RECURRENT
  evidence:
  - reference: PMID:31449642
    reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "repeated episodes of supraventricular tachycardia"
    explanation: Documents recurrent supraventricular tachycardia in a proven case.
- category: Growth
  name: Failure to Thrive
  description: >-
    Poor weight gain in early infancy, reported in the index biochemically
    characterised case.
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:6093033
    reference_title: "Histiocytoid cardiomyopathy of infancy: deficiency of reducible cytochrome b in heart mitochondria."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A 3-week-old girl with failure to thrive and cardiomegaly died of cardiac arrest at age 4 weeks."
    explanation: Case-level documentation of failure to thrive.
- category: Neuromuscular
  name: Generalized Hypotonia
  description: >-
    Muscular hypotonia with biochemically documented skeletal-muscle
    respiratory-chain deficiency has been reported, providing extracardiac
    evidence that the mitochondrial defect is not confined to the heart. It is
    not a universal feature.
  phenotype_term:
    preferred_term: Generalized hypotonia
    term:
      id: HP:0001290
      label: Generalized hypotonia
  evidence:
  - reference: PMID:8563940
    reference_title: "Histiocytoid cardiomyopathy with hypotonia in an infant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Interestingly, the present case showed hypotonia. Her muscle biopsy revealed decreased activity of cytochrome c oxidase, suggesting that histiocytoid cardiomyopathy is related to mitochondrial cytopathy."
    explanation: >-
      Documents hypotonia with an independently abnormal skeletal-muscle
      respiratory-chain assay.
- category: Metabolic
  name: Lactic Acidosis
  description: >-
    Elevated blood lactate reflecting respiratory-chain failure. Documented in
    NDUFB11 complex I neonates; it is a mitochondrial-disease marker rather
    than a specific histiocytoid cardiomyopathy finding, and normal lactate
    does not exclude the diagnosis.
  phenotype_term:
    preferred_term: Lactic acidosis
    term:
      id: HP:0003128
      label: Lactic acidosis
  evidence:
  - reference: PMID:39132302
    reference_title: "Case report: severe hypertrophic cardiomyopathy in a female neonate caused by de novo variant in NDUFB11."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "She developed lactic acidosis after birth and whole-genome sequencing identified a de novo variant in the mitochondrial Complex I gene, NDUFB11 (c.391G>A, p.Glu131Lys)."
    explanation: >-
      Lactic acidosis in an NDUFB11 complex I neonate. Partial because this
      proband had hypertrophic rather than pathology-proven histiocytoid
      cardiomyopathy.
  - reference: PMID:36675256
    reference_title: "A Novel Mutation Associated with Neonatal Lethal Cardiomyopathy Leads to an Alternative Transcript Expression in the X-Linked Complex I NDUFB11 Gene."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report a neonatal patient with hypertrophic cardiomyopathy (HCM), lactic acidosis and isolated complex I deficiency."
    explanation: >-
      Second NDUFB11 neonate with lactic acidosis and complex I deficiency; same
      caveat about cardiomyopathy morphology.
- category: Cardiovascular
  name: Hypertrophic Cardiomyopathy
  description: >-
    Hypertrophic, sometimes obstructive, cardiomyopathy is part of the NDUFB11
    complex I disease spectrum that is allelic and mechanistically continuous
    with histiocytoid cardiomyopathy, and is included here for that reason
    rather than as a classical histiocytoid presentation.
  phenotype_term:
    preferred_term: Hypertrophic cardiomyopathy
    term:
      id: HP:0001639
      label: Hypertrophic cardiomyopathy
  evidence:
  - reference: PMID:39132302
    reference_title: "Case report: severe hypertrophic cardiomyopathy in a female neonate caused by de novo variant in NDUFB11."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "NDUFB11 is an X-linked mitochondrial Complex I protein and is known to cause histiocytoid cardiomyopathy but has not been described in female infants with hypertrophic cardiomyopathy."
    explanation: >-
      Explicitly links the NDUFB11 hypertrophic phenotype to the same gene that
      causes histiocytoid cardiomyopathy, while noting they are distinct
      presentations.
- category: Multisystem
  name: Extracardiac Anomalies of the Nervous System and Eye
  description: >-
    A review of 53 patients noted a high prevalence of nervous-system and
    ocular anomalies and of oncocytic cells in various glands, indicating that
    histiocytoid cardiomyopathy is not always a heart-limited disease. The
    anomaly spectrum is heterogeneous and has never been systematically
    catalogued, so no specific HPO annotations beyond the broad organ-system
    term are asserted.
  phenotype_term:
    preferred_term: Abnormality of the nervous system
    term:
      id: HP:0000707
      label: Abnormality of the nervous system
  evidence:
  - reference: PMID:7942464
    reference_title: "Infantile histiocytoid cardiomyopathy: three cases and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There was a high prevalence of anomalies involving the nervous system and eyes and of oncocytic cells in various glands."
    explanation: Source statement of the extracardiac anomaly burden.
histopathology:
- name: Clusters of Histiocytoid (Oncocytic) Myocytes
  finding_term:
    preferred_term: histiocytoid (oncocytic) cardiomyocyte clusters
    term:
      id: HP:0031331
      label: Abnormal cardiomyocyte morphology
  description: >-
    The diagnostic lesion: discrete, sharply demarcated groups of enlarged
    polygonal cells with abundant faintly eosinophilic foamy granular
    cytoplasm, coarse granules, microvacuoles, and irregular round nuclei,
    predominantly immediately deep to the endocardium. They resemble
    histiocytes by light microscopy but are modified cardiomyocytes.
  diagnostic: true
  evidence:
  - reference: PMID:37831311
    reference_title: "Histiocytoid cardiomyopathy presenting as sudden death in an 18-month-old infant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disease is characterised histologically by discrete groups of enlarged, polygonal histiocyte-like cells with distinct margins and abundant faintly eosinophilic foamy cytoplasm."
    explanation: Contemporary description of the diagnostic light-microscopic lesion.
  - reference: PMID:37831311
    reference_title: "Histiocytoid cardiomyopathy presenting as sudden death in an 18-month-old infant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cells often contain coarse granules, microvacuoles and irregular, round nuclei."
    explanation: Cytological detail of the lesional cell.
  - reference: PMID:39225698
    reference_title: "Conduction System Hamartoma: Autopsy Case Series."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Microscopy revealed sharply demarcated cell groups or layers with histiocyte-like cells with foamy cytoplasm."
    explanation: Independent autopsy-series confirmation of the same lesion.
- name: Yellow-Tan Endocardial, Epicardial and Valvular Nodules
  finding_term:
    preferred_term: multifocal yellow-tan myocardial nodules
    term:
      id: NCIT:C35900
      label: Multinodular Pattern
  description: >-
    Grossly visible yellow-tan or yellowish-white nodules and multifocal uneven
    yellowish endocardial thickening. In a 53-patient review, 16 had such
    nodules while the remaining 37 had only microscopic foci of abnormal
    myocytes scattered through the myocardium, so absence of gross nodules does
    not exclude the diagnosis.
  evidence:
  - reference: PMID:7942464
    reference_title: "Infantile histiocytoid cardiomyopathy: three cases and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sixteen patients had yellowish nodules on the endocardium, epicardium, and/or valves; the other 37 had foci of abnormal myocytes throughout the myocardium."
    explanation: >-
      Quantifies how often gross nodules are present versus microscopic-only
      disease.
  - reference: PMID:28045696
    reference_title: "Sudden Death in a Male Infant Due to Histiocytoid Cardiomyopathy: An Autopsy Case and Review of the Literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The most common grossly change is multifocal uneven thickening of the endocardium, presenting a yellowish color, with some area forming nodular appearance, and histologically featured by scattered clusters of histiocytoid myocytes under the endocardium."
    explanation: Describes the characteristic gross appearance.
- name: Mitochondrial Hyperplasia with Myofibrillar Depletion on Electron Microscopy
  finding_term:
    preferred_term: cardiomyocyte mitochondrial proliferation
    term:
      id: HP:0031320
      label: Cardiomyocyte mitochondrial proliferation
  description: >-
    Ultrastructural examination shows the lesional cells packed with excessive
    and abnormally shaped mitochondria plus scattered lipid droplets, with
    scanty or absent myofibrils and poorly developed intercellular junctions.
    This is the ultrastructural basis of both the foamy cytoplasm and the
    mitochondrial classification of the disease.
  diagnostic: true
  evidence:
  - reference: PMID:10463272
    reference_title: "Histiocytoid cardiomyopathy: three new cases and a review of the literature."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Microscopic examination in all three cases revealed multiple, scattered clusters of histiocytoid myocytes which on ultrastructural examination were filled with abnormal mitochondria, scattered lipid droplets, and scanty myofibrils."
    explanation: Primary ultrastructural description.
  - reference: PMID:8563940
    reference_title: "Histiocytoid cardiomyopathy with hypotonia in an infant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Electron micrographs showed mitochondrial hyperplasia in these cells."
    explanation: Independent electron-microscopic confirmation of mitochondrial hyperplasia.
genetic:
- name: MT-CYB
  gene_term:
    preferred_term: MT-CYB
    term:
      id: hgnc:7427
      label: MT-CYB
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  presence: reported in a single molecularly characterised patient
  features: >-
    Heteroplasmic, maternally transmissible m.15498G>A (p.Gly251Asp) missense
    variant in the mitochondrially encoded cytochrome b subunit of complex III,
    in a patient with biochemically documented ubiquinol-cytochrome c
    oxidoreductase deficiency. This was the first molecular defect identified
    in histiocytoid cardiomyopathy and is the basis of the OMIM 500000
    mitochondrial-inheritance assignment. Its generality is disputed: screening
    27 further registry cases failed to confirm MT-CYB as a recurrent cause.
  evidence:
  - reference: PMID:10960495
    reference_title: "A missense mutation in the mitochondrial cytochrome b gene in a revisited case with histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This is the first molecular defect associated with histiocytoid cardiomyopathy."
    explanation: Establishes MT-CYB as the first reported causal locus.
  - reference: PMID:10960495
    reference_title: "A missense mutation in the mitochondrial cytochrome b gene in a revisited case with histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mutation, which is heteroplasmic and fulfills all accepted criteria for pathogenicity, is likely to impair the function of the holoenzyme as deduced from its effects on the crystal structure of ubiquinol cytochrome c oxidoreductase."
    explanation: States heteroplasmy and the pathogenicity argument for the variant.
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "However, analysis of additional 27 cases from the Histiocytoid CM registry through collaborative work with two institutions failed to verify this gene as a causal gene for Histiocytoid CM."
    explanation: >-
      Refutes MT-CYB as a general cause of histiocytoid cardiomyopathy; it
      remains a single-case association.
- name: NDUFB11
  gene_term:
    preferred_term: NDUFB11
    term:
      id: hgnc:20372
      label: NDUFB11
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  presence: recurrent; the best-supported histiocytoid cardiomyopathy gene
  features: >-
    De novo nonsense variants in exon 2 of the X-linked complex I accessory
    subunit NDUFB11 (Xp11.23) have been identified in multiple unrelated
    probands with histiocytoid cardiomyopathy, including the recurrent
    c.262C>T p.Arg88* allele. NDUFB11 is allelic with microphthalmia with
    linear skin defects (MLS) syndrome: the identical p.Arg88* change has been
    reported in MLS, and the two disorders are therefore genetically related
    even though an individual histiocytoid case may lack any MLS feature.
    Splice-disrupting and missense NDUFB11 alleles cause a broader neonatal
    complex I cardiomyopathy spectrum in which severity in females tracks with
    skewed X-inactivation.
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two probands had de novo non-sense mutations in the second exon of the X-linked nuclear gene NDUFB11."
    explanation: Original identification of de novo NDUFB11 nonsense variants in HC probands.
  - reference: PMID:28050600
    reference_title: "Histiocytoid cardiomyopathy and microphthalmia with linear skin defects syndrome: phenotypes linked by truncating variants in NDUFB11."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here we report an additional case of histiocytoid CM, which carries a de novo nonsense variant in NDUFB11"
    explanation: >-
      Independent replication of a de novo NDUFB11 nonsense variant in a further
      histiocytoid cardiomyopathy proband.
  - reference: PMID:28050600
    reference_title: "Histiocytoid cardiomyopathy and microphthalmia with linear skin defects syndrome: phenotypes linked by truncating variants in NDUFB11."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "An identical variant has been previously reported in association with MLS syndrome."
    explanation: >-
      Documents that the same allele recurs in the allelic MLS phenotype, the
      basis for the p.Arg88* recurrence statement.
  - reference: PMID:28050600
    reference_title: "Histiocytoid cardiomyopathy and microphthalmia with linear skin defects syndrome: phenotypes linked by truncating variants in NDUFB11."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We conclude that NDUFB11 variants play a role in the pathogenesis of both histiocytoid CM and MLS and that these disorders are allelic (genetically related)."
    explanation: Establishes the allelic relationship with MLS syndrome.
  - reference: PMID:39132302
    reference_title: "Case report: severe hypertrophic cardiomyopathy in a female neonate caused by de novo variant in NDUFB11."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hemizygous variants in NDUFB11 have been associated with hypertrophic cardiomyopathy in male infants previously, and skewed X-linked inactivation likely resulted in the presentation described here in a female infant."
    explanation: >-
      Documents skewed X-inactivation as the modifier of female severity in the
      NDUFB11 spectrum; partial because the phenotype described is hypertrophic
      rather than histiocytoid cardiomyopathy.
- name: NDUFAF2
  gene_term:
    preferred_term: NDUFAF2
    term:
      id: hgnc:28086
      label: NDUFAF2
  relationship_type: UNKNOWN
  variant_origin: GERMLINE
  presence: single doubly-heterozygous proband; candidate only
  features: >-
    Inherited rare variant in the complex I assembly factor NDUFAF2, found
    together with a rare NDUFB9 variant in one histiocytoid cardiomyopathy
    proband from the registry exome study. Reported as evidence of genetic
    heterogeneity within complex I; not independently replicated, so the
    gene-disease relationship is unresolved.
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A third proband was doubly heterozygous for inherited rare variants in additional components of complex I, NDUFAF2 and NDUFB9, confirming that Histiocytoid CM is genetically heterogeneous."
    explanation: >-
      Single-proband, doubly-heterozygous finding; supports heterogeneity but
      not a validated NDUFAF2 gene-disease relationship.
- name: NDUFB9
  gene_term:
    preferred_term: NDUFB9
    term:
      id: hgnc:7704
      label: NDUFB9
  relationship_type: UNKNOWN
  variant_origin: GERMLINE
  presence: single doubly-heterozygous proband; candidate only
  features: >-
    Inherited rare variant in the complex I accessory subunit NDUFB9, found in
    the same proband as the NDUFAF2 variant. Candidate-level evidence only.
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A third proband was doubly heterozygous for inherited rare variants in additional components of complex I, NDUFAF2 and NDUFB9, confirming that Histiocytoid CM is genetically heterogeneous."
    explanation: Same single-proband observation; candidate-level evidence only.
- name: HCCS
  gene_term:
    preferred_term: HCCS
    term:
      id: hgnc:4837
      label: HCCS
  relationship_type: DISPUTED
  presence: systematically excluded in published histiocytoid cases
  features: >-
    HCCS (with COX7B) is an established MLS syndrome gene, and one reported
    patient had features of both MLS syndrome and histiocytoid cardiomyopathy.
    A systematic review of exome data from previously published histiocytoid
    cardiomyopathy cases plus four new cases found no variants in these genes,
    so neither is currently a histiocytoid cardiomyopathy gene.
  evidence:
  - reference: PMID:28050600
    reference_title: "Histiocytoid cardiomyopathy and microphthalmia with linear skin defects syndrome: phenotypes linked by truncating variants in NDUFB11."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "However, a systematic review of WES data from previously published histiocytoid CM cases, alongside four additional cases presented here for the first time, did not identify any variants in these genes."
    explanation: >-
      Explicitly excludes HCCS and COX7B as histiocytoid cardiomyopathy genes in
      the available exome data.
- name: COX7B
  gene_term:
    preferred_term: COX7B
    term:
      id: hgnc:2291
      label: COX7B
  relationship_type: DISPUTED
  presence: systematically excluded in published histiocytoid cases
  features: >-
    COX7B encodes a mitochondrial respiratory-chain protein and is an
    established MLS syndrome gene. As with HCCS, systematic review of
    histiocytoid cardiomyopathy exome data identified no COX7B variants.
  evidence:
  - reference: PMID:28050600
    reference_title: "Histiocytoid cardiomyopathy and microphthalmia with linear skin defects syndrome: phenotypes linked by truncating variants in NDUFB11."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "However, a systematic review of WES data from previously published histiocytoid CM cases, alongside four additional cases presented here for the first time, did not identify any variants in these genes."
    explanation: >-
      The "these genes" of this sentence are HCCS and COX7B, named in the
      immediately preceding sentence; the systematic exome review found no
      variants in either, which is the direct basis for the DISPUTED
      relationship type.
  - reference: PMID:28050600
    reference_title: "Histiocytoid cardiomyopathy and microphthalmia with linear skin defects syndrome: phenotypes linked by truncating variants in NDUFB11."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Heterozygous variants in HCCS (which encodes an important mitochondrially targeted protein) and COX7B, which, like NDUFB11, encodes a protein of the MRC, have also previously been identified in MLS syndrome including a case with features of both MLS syndrome and histiocytoid CM."
    explanation: >-
      Context for why COX7B was tested at all: it is an MLS gene, and one
      reported patient had features of both MLS syndrome and histiocytoid
      cardiomyopathy. This weakly associates COX7B with the overlap phenotype
      and is the claim the preceding exclusion sentence refutes for
      histiocytoid cardiomyopathy proper.
progression:
- phase: Prenatal to neonatal onset
  age_range: fetal life to first weeks
  notes: >-
    Arrhythmia may begin before birth. One autopsy-confirmed patient had
    ventricular tachycardia from the prenatal period, and severe NDUFB11
    complex I disease can present within hours of birth with lactic acidosis
    and heart failure.
  evidence:
  - reference: PMID:8563940
    reference_title: "Histiocytoid cardiomyopathy with hypotonia in an infant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A 15 month old female, who had suffered from ventricular tachycardia from the prenatal period, experienced cardiac arrest at home."
    explanation: Documents prenatal onset of the arrhythmia.
- phase: Infantile arrhythmic phase
  age_range: first two years of life
  notes: >-
    The great majority of recognised patients present in the first two years
    with incessant ventricular tachycardia, cardiomegaly, or sudden death. A
    contemporary autopsy series of conduction system hamartoma had a mean age
    of 5.8 months.
  evidence:
  - reference: PMID:21585276
    reference_title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "It primarily occurs in the first two years of life and has a female to male ratio of 3:1."
    explanation: Defines the age window of presentation.
  - reference: PMID:39225698
    reference_title: "Conduction System Hamartoma: Autopsy Case Series."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The mean age of the cases was 5.8 months."
    explanation: Gives the mean age at death in a contemporary autopsy series.
- phase: Fatal outcome without early treatment
  age_range: usually before 2 years
  notes: >-
    Untreated disease is fatal in the vast majority, typically by age two,
    either as sudden arrhythmic death or as progressive heart failure.
    Diagnosis is very often made only at autopsy, and requires adequate
    myocardial sampling.
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Infantile histiocytoid cardiomyopathy (Histiocytoid CM, MIM 500000) is a rare, but distinctive arrhythmogenic disorder characterized by incessant ventricular tachycardia, cardiomegaly, and sudden death within the first two years of life if left untreated."
    explanation: States the untreated natural history and its time course.
- phase: Survival after early intervention
  age_range: beyond 2 years
  notes: >-
    Children diagnosed early and treated by surgical excision or ablation of
    the arrhythmogenic foci, or by cardiac transplantation for extensive
    disease, can survive beyond the usual fatal window.
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "over 20 cases collected from children who were diagnosed early and treated either with surgical excision or ablation of the arrhythmogenic foci. In cases where there is extensive involvement of Histiocytoid CM a cardiac transplant was performed."
    explanation: >-
      Documents the treated cohort in the registry and the interventions that
      permitted survival.
diagnosis:
- name: Autopsy or Endomyocardial Tissue Examination
  description: >-
    Histopathology remains the definitive diagnostic test. Because the lesions
    are focal and subendocardial, the diagnosis is missed unless the heart is
    adequately sampled; this is the main reason cases are miscertified as
    sudden infant death syndrome, sudden unexplained death in childhood, or
    inflicted injury.
  diagnosis_term:
    preferred_term: endomyocardial biopsy
    term:
      id: NCIT:C51674
      label: Endomyocardial Biopsy
  evidence:
  - reference: PMID:37831311
    reference_title: "Histiocytoid cardiomyopathy presenting as sudden death in an 18-month-old infant."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Meticulous gross and histological examination of the heart in collaboration with a cardiovascular-trained pathologist maximises the opportunity to make specific diagnoses (and therefore rule out the differentials of SIDS, SUDC and child abuse), guide genetic testing, and inform potentially life-saving medical interventions for blood relations."
    explanation: >-
      States the requirement for meticulous cardiac histology and its
      implications for family members.
  - reference: PMID:39225698
    reference_title: "Conduction System Hamartoma: Autopsy Case Series."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Autopsy findings should be evaluated by taking extra samples from the heart if necessary, especially in suspected cases below 2 years of age."
    explanation: Operational guidance on adequate myocardial sampling at autopsy.
- name: Rhythm Monitoring and Electrophysiological Mapping
  description: >-
    Twelve-lead ECG and continuous rhythm monitoring establish the arrhythmia
    phenotype; invasive electrophysiological mapping localises the triggering
    foci, at which pre-potentials can be recorded, and guides ablation.
  diagnosis_term:
    preferred_term: electrocardiography
    term:
      id: NCIT:C38053
      label: Electrocardiography
  evidence:
  - reference: PMID:31449642
    reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Pre-potentials were recorded at the earliest ventricular activation sites of the targeted PVCs."
    explanation: >-
      Demonstrates the electrophysiological signature used to localise the
      arrhythmogenic foci.
- name: Echocardiography
  description: >-
    Transthoracic echocardiography characterises the cardiomyopathy morphology
    that accompanies the arrhythmia - ventricular noncompaction, chamber
    dilation or hypertrophy, and systolic function. It does not visualise the
    histiocytoid lesions themselves, so a structurally near-normal echo with
    preserved ejection fraction does not exclude the diagnosis.
  diagnosis_term:
    preferred_term: echocardiography
    term:
      id: NCIT:C16525
      label: Echocardiography Test
  evidence:
  - reference: PMID:31449642
    reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Echocardiography revealed a left ventricular non-compaction with a left ventricular ejection fraction (LVEF) of 59% and no other abnormalities."
    explanation: >-
      Shows both what echocardiography contributes (detection of noncompaction)
      and its limitation in this disease: ejection fraction was preserved and no
      other abnormality was seen despite histologically extensive biventricular
      histiocytoid disease at autopsy.
- name: Genomic Testing Including NDUFB11 and mtDNA
  description: >-
    Trio exome or rapid whole-genome sequencing with mitochondrial-genome
    analysis is the preferred genetic approach. Targeted cardiomyopathy panels
    are insufficient: a pathogenic NDUFB11 variant has been reported that
    commercial cardiomyopathy panels did not cover.
  diagnosis_term:
    preferred_term: whole genome sequencing
    term:
      id: NCIT:C101294
      label: Whole Genome Sequencing
  evidence:
  - reference: PMID:39132302
    reference_title: "Case report: severe hypertrophic cardiomyopathy in a female neonate caused by de novo variant in NDUFB11."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This variant was not identifiable by commercial cardiomyopathy panels."
    explanation: >-
      Direct evidence that panel-based testing can miss the causal variant,
      justifying WGS/WES.
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We have performed whole exome sequencing on five parent-proband trios and identified nuclear-encoded mitochondrial protein mutations in three cases."
    explanation: Establishes trio exome sequencing as the diagnostic yield-bearing approach.
differential_diagnoses:
- name: Sudden infant death syndrome
  description: >-
    The most consequential differential. Histiocytoid cardiomyopathy causes
    unheralded infant death and is only distinguishable by adequate myocardial
    sampling at autopsy, so an unknown fraction of cases is certified as SIDS.
  evidence:
  - reference: PMID:21585276
    reference_title: "Identification of candidate genes for histiocytoid cardiomyopathy (HC) using whole genome expression analysis: analyzing material from the HC registry."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the prevalence of this disease may be higher than the reported cases would suggest, as some cases are mistaken for Sudden Infant Death Syndrome (SIDS)"
    explanation: States the SIDS misclassification directly.
- name: Microphthalmia with linear skin defects (MLS) syndrome
  description: >-
    Allelic at NDUFB11, with the identical p.Arg88* variant reported in both.
    MLS is distinguished by microphthalmia and linear skin defects; a
    histiocytoid cardiomyopathy proband carrying the same variant lacked those
    diagnostic features, although detailed comparison showed phenotypic
    overlap.
  evidence:
  - reference: PMID:28050600
    reference_title: "Histiocytoid cardiomyopathy and microphthalmia with linear skin defects syndrome: phenotypes linked by truncating variants in NDUFB11."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The case we describe here lacked the diagnostic features of MLS syndrome, but a detailed clinical comparison of the two cases revealed significant phenotypic overlap."
    explanation: >-
      Establishes both the distinction and the overlap between the two allelic
      phenotypes.
- name: Cardiac rhabdomyoma
  description: >-
    Histiocytoid cardiomyopathy was originally confused with rhabdomyoma, a
    benign myocardial tumour, and was not recognised as a separate entity until
    1962.
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The disease was first confused with rhabdomyoma, benign tumors of the myocardium, and was not recognized as a separate pathologic entity until 1962 by Voth"
    explanation: Documents the historical confusion with rhabdomyoma.
treatments:
- name: High-Dose Carvedilol
  description: >-
    In a 4-month-old girl with histiocytoid cardiomyopathy whose ventricular
    arrhythmias were refractory to bisoprolol, landiolol, verapamil, and
    cardiac sympathetic denervation, high-dose carvedilol achieved control.
    Carvedilol is distinctive among beta blockers in acting directly on the
    ryanodine receptor to inhibit store-overload-induced calcium release, which
    is the proposed basis of the response. This rests on a single case and
    should not be generalised.
  action_category: THERAPEUTIC
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: carvedilol
      term:
        id: CHEBI:3441
        label: carvedilol
  target_mechanisms:
  - target: Arrhythmogenic Substrate and Triggered Activity
    treatment_effect: INHIBITS
    description: >-
      Proposed to suppress ryanodine-receptor store-overload-induced calcium
      release in the abnormal myocardium, reducing triggered activity.
  evidence:
  - reference: PMID:38089118
    reference_title: "Case report: high-dose carvedilol as a potential key drug for arrhythmias in histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This is the first report of infantile histiocytoid cardiomyopathy whose refractory ventricular arrhythmias were successfully controlled by high-dose carvedilol."
    explanation: Single-case report of arrhythmia control with high-dose carvedilol.
  - reference: PMID:38089118
    reference_title: "Case report: high-dose carvedilol as a potential key drug for arrhythmias in histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Carvedilol is the only beta blocker that directly acts on the ryanodine receptor (RyR2) and inhibits store-overload-induced Ca2+ release (SOICR) in myocardium at high dosage."
    explanation: States the proposed molecular rationale for the drug choice.
  - reference: PMID:38089118
    reference_title: "Case report: high-dose carvedilol as a potential key drug for arrhythmias in histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There is no specific treatment for these arrhythmias."
    explanation: >-
      Establishes that no disease-specific antiarrhythmic therapy is
      established, framing carvedilol as investigational.
- name: Catheter Ablation of Arrhythmogenic Foci
  description: >-
    Radiofrequency catheter ablation of mapped premature-ventricular-complex or
    ventricular-tachycardia foci can abolish otherwise refractory arrhythmia,
    and has been performed successfully in an infant weighing 3.4 kg. Durability
    is limited by the multifocal nature of the disease: in the reported case the
    arrhythmia was abolished but the child died of heart failure at 6 months.
  action_category: THERAPEUTIC
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: cardiac ablation
    term:
      id: NCIT:C100068
      label: Cardiac Ablation
  target_mechanisms:
  - target: Arrhythmogenic Substrate and Triggered Activity
    treatment_effect: INHIBITS
    description: >-
      Destroys the mapped triggering focus, removing the arrhythmogenic
      substrate at that site.
  evidence:
  - reference: PMID:31449642
    reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two types of PVCs triggering TdP were successfully ablated, which originated from the right ventricle (RV). Pre-potentials were recorded at the earliest ventricular activation sites of the targeted PVCs."
    explanation: Documents successful ablation of mapped trigger foci.
  - reference: PMID:31449642
    reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "After the ablation, she had no TdP episodes and the cardiac assist device was removed. However, she died of uncontrolled heart failure at 6 months of age."
    explanation: >-
      Shows that arrhythmia control does not necessarily prevent death from
      heart failure.
- name: Surgical Excision of Histiocytoid Nodules
  description: >-
    Where a discrete arrhythmogenic nodule can be localised, surgical excision
    can produce clinical remission. Together with ablation this is the main
    lesion-directed strategy documented in the histiocytoid cardiomyopathy
    registry cohort of early-diagnosed, treated survivors.
  action_category: THERAPEUTIC
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: surgical procedure
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  target_mechanisms:
  - target: Hamartomatous Nodule Formation in the Conduction System
    treatment_effect: INHIBITS
    description: >-
      Physically removes the hamartomatous nodule that constitutes the
      arrhythmogenic lesion.
  evidence:
  - reference: PMID:7942464
    reference_title: "Infantile histiocytoid cardiomyopathy: three cases and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Surgical excision of nodules of histiocytoid cells can result in clinical remission."
    explanation: States the efficacy of lesion-directed surgical excision.
- name: Mechanical Circulatory Support (ECMO and Ventricular Assist Device)
  description: >-
    Extracorporeal membrane oxygenation and biventricular assist device support
    have been used as rescue and bridge-to-transplant strategies for
    cardiogenic shock from incessant ventricular arrhythmia. In the reported
    BiVAD case an atrial rather than apical left inflow cannula was needed
    because preserved systolic function, noncompaction, and frequent rhythm
    changes caused mechanical inflow obstruction.
  action_category: THERAPEUTIC
  therapeutic_modality: DEVICE
  treatment_term:
    preferred_term: ventricular assist device placement
    term:
      id: NCIT:C80452
      label: Ventricular Assist Device Placement
  evidence:
  - reference: PMID:35439193
    reference_title: "Biventricular Assist Device Support for Intractable Arrhythmias From Histiocytoid Cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "BiVAD can support pediatric patients with hemodynamically significant arrhythmias to transplantation."
    explanation: States the demonstrated role of BiVAD as a bridge to transplantation.
  - reference: PMID:35439193
    reference_title: "Biventricular Assist Device Support for Intractable Arrhythmias From Histiocytoid Cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Atrial cannulation strategy may be preferred in cases of preserved systolic function, ventricular noncompaction, and frequent rhythm changes."
    explanation: >-
      Records the disease-specific technical caveat for device implantation in
      histiocytoid cardiomyopathy.
  - reference: PMID:31449642
    reference_title: "Successful catheter ablation of premature ventricular contractions triggering torsade de pointes in a small infant with histiocytoid cardiomyopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "she had recurrent TdP episodes resistant to various antiarrhythmic drugs and received extracorporeal membrane oxygenation at 86 days of age"
    explanation: Documents ECMO as rescue support for refractory arrhythmia.
- name: Heart Transplantation
  description: >-
    Orthotopic cardiac transplantation is the definitive option for extensive
    disease not amenable to lesion-directed treatment, and has been performed
    in registry cases with widespread myocardial involvement.
  action_category: THERAPEUTIC
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: heart transplantation
    term:
      id: NCIT:C15246
      label: Heart Transplantation
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In cases where there is extensive involvement of Histiocytoid CM a cardiac transplant was performed."
    explanation: Documents transplantation for extensive disease in the registry cohort.
  - reference: PMID:35439193
    reference_title: "Biventricular Assist Device Support for Intractable Arrhythmias From Histiocytoid Cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Successful heart transplantation for HICMP has been described"
    explanation: Independent confirmation that transplantation has been successful in HC.
animal_models:
- species: Danio rerio
  genotype: ndufb11 morpholino knockdown
  category: Morpholino knockdown
  description: >-
    Morpholino-mediated knockdown of ndufb11 in zebrafish embryos produced
    defective cardiac tissue with cardiomegaly, looping defects, and
    arrhythmia. This is the only reported experimental model addressing the
    NDUFB11 arm of the disease. It reproduces cardiomegaly and arrhythmia but
    has not been shown to reproduce the defining histiocytoid/oncocytic cell
    morphology.
  associated_phenotypes:
  - Cardiomegaly
  - Cardiac looping defect
  - Arrhythmia
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "morpholino-mediated knockdown of ndufb11 in zebrafish embryos generated defective cardiac tissue with cardiomegaly, looping defects, and arrhythmia which suggests the role of NDUFB11 in the pathogenesis of this abnormal cardiac pathology"
    explanation: Describes the zebrafish knockdown phenotype and its interpretation.
- species: Felis catus
  genotype: naturally occurring, genotype unknown
  category: Naturally occurring veterinary case
  description: >-
    A 2-month-old female Savannah kitten died suddenly with cardiac lesions
    described as identical to human histiocytoid cardiomyopathy - large,
    angular, tightly packed Purkinje fibres with few disorganised myofibrils in
    a rarified cytoplasm - together with biventricular noncompaction. This is a
    spontaneous comparative-pathology observation, not an engineered model, and
    no causal genotype was established.
  associated_phenotypes:
  - Abnormal Purkinje fibers
  - Ventricular noncompaction
  - Sudden death
  evidence:
  - reference: PMID:19276060
    reference_title: "Purkinje fiber dysplasia (histiocytoid cardiomyopathy) with ventricular noncompaction in a savannah kitten."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "These findings are identical to those that occur in children with histiocytoid cardiomyopathy, a fatal genetic mitochondrial disorder of Purkinje fibers."
    explanation: >-
      States the equivalence of the feline lesion to the human disease. Tagged
      MODEL_ORGANISM per the dismech rule that veterinary observations in
      non-human mammals are model-organism evidence.
  - reference: PMID:19276060
    reference_title: "Purkinje fiber dysplasia (histiocytoid cardiomyopathy) with ventricular noncompaction in a savannah kitten."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "The Purkinje fibers were large, angular, and tightly packed. They contained few disorganized myofibrils among a rarified cytoplasm."
    explanation: Describes the feline Purkinje-fibre lesion in detail.
discussions:
- discussion_id: hc_causal_gene_unsolved
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    What explains the majority of histiocytoid cardiomyopathy cases that carry
    no variant in NDUFB11, MT-CYB, or another complex I gene?
  attaches_to:
  - pathophysiology#Mitochondrial Respiratory Chain Subunit Defect
  rationale: >-
    Trio exome sequencing of five registry probands identified a plausible
    nuclear mitochondrial-protein variant in only three, and no strong recessive
    or compound-heterozygous candidate in two. MT-CYB, the first reported locus,
    was not confirmed in 27 further registry cases. The disease therefore
    remains molecularly unexplained in a large fraction of patients, which
    blocks both prenatal/cascade testing and any mechanism-directed therapy.
  proposed_experiments:
  - experiment_id: exp_hc_registry_wgs_and_lesional_omics
    name: Registry whole-genome sequencing plus lesional-myocardium multi-omics
    description: >-
      Re-analyse the histiocytoid cardiomyopathy registry trios by whole-genome
      rather than exome sequencing, with structural-variant calling and deep
      mitochondrial-heteroplasmy detection, and pair this with RNA sequencing,
      respiratory-chain enzymology, and blue-native supercomplex analysis on
      banked lesional myocardium. Add single-nucleus transcriptomics of
      histiocytoid versus adjacent normal myocardium from the same hearts to
      test whether the lesional cells occupy a distinct somatic or epigenetic
      state.
    experiment_type:
      preferred_term: observational molecular profiling study
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Strong candidate recessive or compound heterozygous variants were not found for this individual or for the fifth case."
    explanation: Documents the unsolved fraction even in a systematic trio exome study.
- discussion_id: hc_focal_distribution_unexplained
  kind: OPEN_QUESTION
  status: OPEN
  prompt: >-
    Why does a germline respiratory-chain defect produce focal, hamartoma-like
    nests of oncocytic Purkinje-like cells rather than uniform cardiomyocyte
    dysfunction?
  attaches_to:
  - pathophysiology#Hamartomatous Nodule Formation in the Conduction System
  rationale: >-
    This is the central unresolved tension between the canonical mitochondrial
    model and the alternative Purkinje-hamartoma model. A germline mtDNA or
    X-linked nuclear variant is present in every cardiomyocyte, yet the lesions
    are discrete and conduction-system-biased. Candidate explanations include
    mitochondrial heteroplasmy segregation, X-inactivation mosaicism in the
    NDUFB11 arm, a cell-intrinsic vulnerability of Purkinje-lineage myocytes
    with high oxidative demand, or a superimposed somatic/developmental event.
    None has been tested.
  proposed_experiments:
  - experiment_id: exp_hc_lesional_vs_normal_single_cell_mosaicism
    name: Lesional versus adjacent-normal myocyte mosaicism assay
    description: >-
      Within single histiocytoid cardiomyopathy hearts, compare lesional
      histiocytoid cells with adjacent morphologically normal myocardium for
      mitochondrial heteroplasmy level and X-inactivation ratio, and use
      laser-capture microdissection with targeted deep sequencing to search for
      a lesion-restricted second somatic hit. Complement this with a
      lineage-resolved comparison of Purkinje versus working cardiomyocyte
      respiratory capacity and vulnerability to complex I/III inhibition.
    experiment_type:
      preferred_term: comparative single-cell molecular pathology study
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Unlike other mitochondrial diseases, however, Histiocytoid CM does not affect all cardiomyocytes equally."
    explanation: States the observation that the open question is about.
- discussion_id: hc_zebrafish_model_fidelity
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    Does the zebrafish ndufb11 morpholino knockdown actually model histiocytoid
    cardiomyopathy, given that it reproduces cardiomegaly and arrhythmia but has
    not been shown to produce the defining histiocytoid/oncocytic Purkinje-like
    cell?
  attaches_to:
  - pathophysiology#Oncocytic Transformation of Purkinje-like Cardiomyocytes
  rationale: >-
    The zebrafish result is the only experimental support that NDUFB11 loss can
    cause cardiac disease, and it is cited as such. But the reported readouts
    are cardiomegaly, looping defects, and arrhythmia - non-specific outputs
    obtainable from many cardiac perturbations - not the oncocytic
    transformation that defines the human entity. Morpholino knockdown also
    differs mechanistically from a de novo human nonsense allele acting in a
    mosaic X-inactivation context, and the zebrafish conduction system is not
    homologous in detail to the mammalian His-Purkinje system. Treating the
    model as validating the human cellular mechanism would overstate it.
  proposed_experiments:
  - experiment_id: exp_ndufb11_model_ladder_for_oncocytic_transformation
    name: Cross-model test for oncocytic Purkinje-like transformation on NDUFB11 loss
    description: >-
      Examine ndufb11-deficient zebrafish - ideally a stable genetic mutant
      rather than a morphant - histologically and ultrastructurally for
      oncocytic myocyte transformation rather than gross cardiac phenotypes
      alone. In parallel, generate a conditional cardiomyocyte- or
      conduction-cell-specific Ndufb11 mouse and assess it for histiocytoid
      lesions and arrhythmia, and assay NDUFB11-null human iPSC-derived
      cardiomyocytes and conduction-like cells for mitochondrial accumulation,
      myofibrillar loss, and triggered activity.
    experiment_type:
      preferred_term: cross-species model validation study
  evidence:
  - reference: PMID:25921236
    reference_title: "Exome sequencing of patients with histiocytoid cardiomyopathy reveals a de novo NDUFB11 mutation that plays a role in the pathogenesis of histiocytoid cardiomyopathy."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "morpholino-mediated knockdown of ndufb11 in zebrafish embryos generated defective cardiac tissue with cardiomegaly, looping defects, and arrhythmia"
    explanation: >-
      The reported model phenotypes are non-specific cardiac abnormalities; the
      defining histiocytoid morphology is not among them.

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References & Deep Research