Heme Oxygenase-1 Deficiency: Disease Characteristics Report

Executive summary and evidence limits

Heme oxygenase-1 deficiency is an ultra-rare, usually severe autosomal-recessive disorder caused by biallelic pathogenic variants in HMOX1. Loss of inducible HO-1-mediated heme degradation produces a distinctive combination of Coombs-negative intravascular hemolysis, paradoxically low/normal bilirubin, very high LDH and ferritin, leukocytosis, thrombocytosis, systemic inflammation, endothelial injury, nephropathy, hepatic iron deposition, and absent or dysfunctional spleen. Pulmonary fibrosis, hemophagocytic flares, pericardial disease, and AA amyloidosis expand the recognized spectrum. The strongest systematic clinical evidence remains a 2021 review of nine independent patients; therefore, percentages below are case-series proportions, not population estimates. Publications in 2023–2024 added renal-amyloidosis and pulmonary/variant reports, but no cohort, guideline, approved disease-modifying treatment, or disease-specific clinical trial was identified.

Evidence classes: human clinical denotes affected patients; model denotes knockout animals; in vitro denotes patient or engineered cells. Broader associations involving common HMOX1 promoter polymorphisms are not equivalent to Mendelian HO-1 deficiency.

1. Disease information

Definition and nomenclature

HO-1 deficiency is an inherited failure of the inducible heme-degrading enzyme HO-1. HO-1 normally catalyzes the rate-limiting conversion of heme to biliverdin, carbon monoxide (CO), and ferrous iron; biliverdin is subsequently reduced to bilirubin. The disease is consequently both an enzyme deficiency and a disorder of heme detoxification, iron recycling, redox defense, and inflammatory restraint. The first molecularly defined patient was reported in 1999 by Yachie et al.; the primary paper is J Clin Invest 103:129–135, DOI 10.1172/JCI4165, PMID 9927502. The later review was published 3 February 2021, DOI 10.3390/ijms22041514. (yachie2021hemeoxygenase1deficiency pages 1-3, yachie2021hemeoxygenase1deficiency pages 15-16)

Preferred name: heme oxygenase-1 deficiency. Synonyms: HO-1 deficiency; HMOX1 deficiency; human heme oxygenase-1 deficiency; heme oxygenase 1 deficiency. “Heme oxygenase deficiency” is imprecise because HMOX2 encodes the constitutive HO-2 isozyme.

Identifiers:

• Causal gene: HMOX1; OMIM gene 141250 (often displayed as 141250); HGNC 5013; NCBI Gene 3162*.
• Disease OMIM/MONDO: a confidently verified separate phenotype number or MONDO identifier was not available in the retrieved evidence. OMIM 141250 is the gene entry*, not necessarily a disease-phenotype identifier, and should not be entered as such without direct database verification.
• Orphanet: no verified dedicated identifier found.
• MeSH: no disease-specific descriptor found; use “Heme Oxygenase-1” plus appropriate manifestations.
• ICD-10/ICD-11: no specific code identified. Practical coding would require an “other specified disorder of metabolism/hematologic disorder” code plus manifestations, varying by jurisdiction.

The evidence is principally aggregated disease-level literature reconstructed from individual published cases, not EHR-derived population data. The 2021 synthesis explicitly states that only nine independent cases had been described. (yachie2021hemeoxygenase1deficiency pages 7-8, yachie2021hemeoxygenase1deficiency pages 1-3)

2. Etiology, risk, protection, and gene–environment interaction

Causal factor

The primary cause is biallelic germline HMOX1 dysfunction. Reported genotypes include a compound exon-2 deletion/exon-3 2-bp deletion, homozygous p.Arg44Ter (R44X), homozygous p.Lys204Ter (K204X), homozygous p.Gly139Val (G139V), and compound c.264_269delCTGG (p.Leu89SerfsTer24) plus c.636+2T>A. Most are null alleles; p.G139V retains abnormal protein with reduced HO activity and acquired peroxidase behavior. Variants are germline, not somatic. (yachie2021hemeoxygenase1deficiency pages 7-8, yachie2021hemeoxygenase1deficiency pages 6-7, yachie2021hemeoxygenase1deficiency pages 4-6)

Population allele frequencies and current ClinVar ACMG classifications were not recoverable from the retrieved documents. Given severe recessive disease and the tiny number of families, the causal alleles are expected to be very rare, but “absent from gnomAD” should not be asserted without direct version-specific lookup.

Risk factors

• Genetic: biallelic pathogenic HMOX1 variants; consanguinity was documented in the Iranian and Turkish families. The five Indian patients shared homozygous p.R44X, suggesting a founder allele. Family history may include fetal loss: the first patient’s mother had two intrauterine deaths, and the Iranian family had a spontaneous fetal loss, although causality was not genetically established. (yachie2021hemeoxygenase1deficiency pages 3-4, yachie2021hemeoxygenase1deficiency pages 6-7, yachie2021hemeoxygenase1deficiency pages 4-6)
• Environmental/physiologic triggers: infections, hemolysis, transfusion, hypoxemia, and other oxidative insults plausibly precipitate inflammatory deterioration because HO-1 is normally stress inducible. The Indian patients could remain well for years and then deteriorate rapidly after inflammation began. The p.G139V patient had paradoxical inflammation after red-cell transfusion. These are trigger interactions, not causes of the Mendelian disorder. (yachie2021hemeoxygenase1deficiency pages 6-7, yachie2021hemeoxygenase1deficiency pages 1-3, yachie2021hemeoxygenase1deficiency pages 4-6)
• Age/sex/lifestyle: no established sex, diet, smoking, occupation, toxin, or lifestyle risk effect. Onset ranged from 3 months to 15 years, and both sexes were affected. (yachie2021hemeoxygenase1deficiency pages 7-8)

Protective factors

No validated human protective allele, diet, lifestyle, or prophylactic drug is known. Avoiding unnecessary oxidative stress, promptly treating infection, and cautious transfusion practice are biologically reasonable but untested. HO-1 induction cannot restore an absent/null enzyme. CO donors, bilirubin/biliverdin, haptoglobin–CD163 enhancement, and wild-type macrophage replacement remain experimental concepts. The review’s conclusion—“Avoidance of exogenous stress along with appropriate treatment may prevent early death”—is expert opinion rather than trial evidence. (yachie2021hemeoxygenase1deficiency pages 13-15)

3. Phenotypes

Across the nine historical patients, fever and hemolytic anemia occurred in 9/9; jaundice occurred in 0/9 despite hemolysis. Hematuria/proteinuria occurred in all six Japanese/Indian cases and in the US case, but were unreported in two others. Absent/hypoplastic spleen affected seven of nine; one had splenomegaly and one a normal-sized spleen. These fractions are descriptive only. (yachie2021hemeoxygenase1deficiency pages 7-8, yachie2021hemeoxygenase1deficiency pages 8-10)

Table: This table compacts the currently gathered human evidence for HMOX1 deficiency into case groups, highlighting genotype, hallmark phenotype/laboratory patterns, and outcomes. It is useful for quickly comparing the recurrent diagnostic denominators and notable phenotype expansions across the 9 reported cases.

Knowledge-base phenotype mapping

• Recurrent fever—infancy through adolescence; episodic then potentially persistent/severe; 9/9. HPO: HP:0001954 Recurrent fever.
• Coombs-negative hemolytic anemia, often severe and fragmented-cell/microangiopathic—9/9; HPO: HP:0001878 Hemolytic anemia, HP:0001937 Microangiopathic hemolytic anemia, HP:0001892 Abnormal bleeding where present.
• Low or normal bilirubin despite hemolysis—9/9 historical cases; a highly discriminating laboratory abnormality. HPO: HP:0002905 Hypob​​ilirubinemia if locally supported; otherwise record quantitative laboratory phenotype rather than force an HPO term. (yachie2021hemeoxygenase1deficiency pages 7-8, yachie2021hemeoxygenase1deficiency pages 8-10)
• Extreme LDH/hyperferritinemia—LDH approximately 4,000–21,400 IU/L and ferritin 780–27,425 ng/mL in the tabulated cases. HPO: HP:0031964 Elevated circulating ferritin concentration, HP:0025435 Elevated circulating lactate dehydrogenase concentration. (yachie2021hemeoxygenase1deficiency pages 7-8)
• Leukocytosis and thrombocytosis—nearly invariant and useful in distinguishing the disorder from classic HLH-associated cytopenias. HPO: HP:0001974 Leukocytosis, HP:0001894 Thrombocytosis. (yachie2021hemeoxygenase1deficiency pages 8-10)
• Asplenia/hyposplenia or evolving splenic dysfunction—usually congenital/early but variable; HPO: HP:0001746 Asplenia, HP:0001870 Acquired abnormality of spleen, HP:0031417 Hyposplenism, and HP:0001744 Splenomegaly for the alternate trajectory.
• Renal disease—hematuria, proteinuria, glomerular endothelial swelling/detachment, mesangial proliferation, tubular atrophy, iron deposition, and later amyloidosis. HPO: HP:0000790 Hematuria, HP:0000093 Proteinuria, HP:0000099 Glomerulonephritis, HP:0000077 Abnormality of the kidney. (yachie2021hemeoxygenase1deficiency pages 10-12, yachie2021hemeoxygenase1deficiency pages 4-6)
• Hepatomegaly/hepatic siderosis—common; HPO: HP:0002240 Hepatomegaly, HP:0001392 Abnormality of the liver, HP:0003233 Abnormality of iron homeostasis.
• Hyperlipidemia—prominent in the first case; HPO: HP:0003077 Hyperlipidemia. Serum triglycerides were 638 mg/dL and cholesterol 552 mg/dL. (yachie2021hemeoxygenase1deficiency pages 3-4)
• Systemic hyperinflammation/MAS-HLH-like episodes—variable, particularly cases 7–9; HPO: HP:0001945 Fever, HP:0410133 Hemophagocytosis, HP:0002910 Elevated hepatic transaminase.
• Vascular/coagulation disease—hypertension, endothelial injury, extreme coagulation/fibrinolysis activation, occasional intracranial hemorrhage. HPO: HP:0000822 Hypertension, HP:0002167 Neurological hemorrhage, HP:0001928 Abnormality of coagulation. (yachie2021hemeoxygenase1deficiency pages 3-4, yachie2021hemeoxygenase1deficiency pages 12-13)
• Pulmonary disease—progressive fibrotic nonspecific interstitial pneumonia, hypoxemia, and cholesterol granulomas in the US patient; HPO: HP:0002206 Pulmonary fibrosis, HP:0002091 Restrictive lung disease, HP:0012418 Hypoxemia. (yachie2021hemeoxygenase1deficiency pages 6-7)
• Growth delay and craniofacial appearance—growth restriction was common but not universal; frontal bossing/prominent forehead, flat nasal bridge, and eyelid edema occurred in several patients. HPO: HP:0001510 Growth delay, HP:0002007 Frontal bossing, HP:0005280 Depressed nasal bridge, HP:0000280 Coarse facial features only if clinically documented.

Formal EQ-5D, SF-36, PROMIS, neurobehavioral, or disease-specific quality-of-life data do not exist. Severe anemia, recurrent hospitalization, transfusion dependence, organ failure, growth impairment, and respiratory limitation imply profound functional burden, but this has not been quantified.

4. Genetic and molecular information

HMOX1 encodes the 288-amino-acid inducible heme oxygenase-1, an endoplasmic-reticulum-associated microsomal enzyme. Suggested annotations include GO:0004392 heme oxygenase (decyclizing) activity, GO:0042167 heme catabolic process, GO:0006788 heme oxidation, GO:0055114 oxidation–reduction process, and GO:0005783 endoplasmic reticulum.

The exon deletions, frameshift, nonsense, and canonical splice-donor variants are predicted loss-of-function. Patient cells carrying null variants failed to produce inducible HO-1 after cadmium, sodium arsenite, or cobalt protoporphyrin stimulation. p.K204Ter truncates the protein at 203 rather than 288 residues. p.G139V is mechanistically unusual: constitutive mutant protein was present, stress induction was defective, catalytic HO activity was reduced, and abnormal peroxidase activity and inflammatory cytokine production increased. (yachie2021hemeoxygenase1deficiency pages 3-4, yachie2021hemeoxygenase1deficiency pages 6-7)

No validated modifier gene, anticipation, germline mosaicism, pathogenic epigenetic signature, recurrent copy-number syndrome, aneuploidy, translocation, or inversion is known. Common HMOX1 promoter (GT)n polymorphisms regulate expression in other diseases but are susceptibility modifiers, not established causes of this recessive deficiency.

5. Environmental and infectious information

No toxin, radiation, pollutant, pathogen, or lifestyle exposure independently causes the disease. Infection can trigger systemic inflammation and is a major treatment hazard: one Indian patient died with fungal sepsis after immunosuppression. Hemolysis itself supplies excess extracellular heme; transfusion adds heme burden and provoked inflammation in the p.G139V patient. Hypoxia and vascular shear are additional endogenous stresses to HO-1-dependent cells. No zoonotic or transmissible component exists. (yachie2021hemeoxygenase1deficiency pages 6-7, yachie2021hemeoxygenase1deficiency pages 10-12, yachie2021hemeoxygenase1deficiency pages 4-6)

6. Mechanism and pathophysiology

Causal chain

1. Upstream genetic lesion: biallelic HMOX1 loss or catalytic dysfunction.
2. Primary biochemical defect: impaired inducible conversion of heme to biliverdin, CO, and Fe²⁺, followed by reduced bilirubin production. This explains severe hemolysis accompanied by unexpectedly low bilirubin. (yachie2021hemeoxygenase1deficiency pages 1-3, yachie2021hemeoxygenase1deficiency pages 8-10)
3. Heme/Hb accumulation and failed recycling: extracellular oxyhemoglobin/methemoglobin and Hb–haptoglobin complexes accumulate. In the first patient, serum heme reached 490 µM versus normal <1 µM, while haptoglobin paradoxically reached 800–1,200 mg/dL. (yachie2021hemeoxygenase1deficiency pages 3-4)
4. Redox and metabolic injury: unresolved heme catalyzes oxidative macromolecular and membrane damage; iron is misdistributed into renal tubular and hepatic cells rather than efficiently recycled. Loss of bilirubin/biliverdin antioxidant action and CO signaling further reduces stress tolerance.
5. Monocyte/macrophage failure: erythrophagocytic Kupffer and splenic macrophages cannot safely process heme, lose CD163/scavenging competence, die or become abnormally activated, and release TNF-α, IL-1β, and IL-6. Suggested GO terms: GO:0006954 inflammatory response, GO:0006909 phagocytosis, GO:0030217 T-cell differentiation only for downstream immune studies, GO:0071345 cellular response to cytokine stimulus. Cell Ontology: CL:0000235 macrophage, CL:0000091 Kupffer cell, CL:0000576 monocyte. (yachie2021hemeoxygenase1deficiency pages 12-13, yachie2021hemeoxygenase1deficiency pages 10-12)
6. Endothelial dysfunction: heme/ROS and inflammatory signaling activate NF-κB/MAPK and tissue factor, producing coagulation/fibrinolysis dysregulation, hypertension, thrombosis/bleeding, glomerular microvascular injury, and occasional cerebral hemorrhage. Cell Ontology: CL:0000115 endothelial cell; GO: GO:0007596 blood coagulation, GO:0006979 response to oxidative stress. (yachie2021hemeoxygenase1deficiency pages 3-4, yachie2021hemeoxygenase1deficiency pages 12-13)
7. Downstream organ injury: chronic inflammation, ischemia, fibrosis, iron deposition, and AA amyloid damage kidney, liver, spleen, lung, and cardiovascular tissues.

The primary patient-study abstract states that HO-1 deficiency causes “enhanced endothelial cell injury”; the 2021 synthesis concludes that impaired HO-1 causes “progressive monocyte dysfunction, unregulated macrophage activation and endothelial cell dysfunction.” These are authoritative mechanistic interpretations supported by patient tissue and cells, not merely computational inference. (yachie2021hemeoxygenase1deficiency pages 1-3, yachie2021hemeoxygenase1deficiency pages 15-16)

Molecular profiling and advanced technology

There is no disease-specific human single-cell atlas, spatial transcriptomic study, lipidomic signature, integrated multi-omics cohort, or CRISPR screen. Whole-exome sequencing diagnosed post-mortem cases. Model metabolomics has connected HO-1 loss to impaired HIF-1α stabilization and ischemic metabolic adaptation, but this has not been validated as a diagnostic signature in affected humans. Epigenetic findings concern HMOX1 regulation generally, not the Mendelian disease.

7. Anatomy

Primary organs: blood/bone marrow, spleen, liver, kidneys, vascular endothelium, and monocyte–macrophage system. Secondary/variable: lung, heart/pericardium, brain vasculature, placenta, and growth tissues.

Suggested UBERON mappings include UBERON:0000178 blood, UBERON:0001987 placental blood, UBERON:0002106 spleen, UBERON:0002107 liver, UBERON:0002113 kidney, UBERON:0002048 lung, UBERON:0000948 heart, UBERON:0001981 blood vessel, UBERON:0001225 renal tubule, and UBERON:0001285 glomerular capillary. Relevant cells are erythrocytes (CL:0000232), erythroid progenitors (CL:0000038), monocytes, macrophages, Kupffer cells, renal tubular epithelial cells, hepatocytes (CL:0000182), podocytes (CL:0000653), and endothelial cells. Relevant compartments are ER membrane (GO:0005789), cytosol (GO:0005829), lysosome/phagolysosome (GO:0005764/GO:0032010), and extracellular blood space. No lateralization is expected.

8. Temporal development

Onset is usually pediatric but highly variable: 3 months–15 years among the nine historical patients. Some children had congenital/early asplenia or growth disturbance; others remained apparently well until an inflammatory trigger. Once clinically active, disease may become rapidly progressive, with recurrent fever, worsening hemolysis, renal/endothelial injury, and multiorgan failure. The US pulmonary phenotype followed a chronic progressive course from age four to respiratory death at ten. There is no validated staging system or predictable remission pattern. (yachie2021hemeoxygenase1deficiency pages 7-8, yachie2021hemeoxygenase1deficiency pages 6-7, yachie2021hemeoxygenase1deficiency pages 4-6)

Potential critical periods include fetal splenic/placental development, infancy with high erythrocyte turnover, and acute infections. Mouse data support fetal loss and placental vascular vulnerability, but prenatal human penetrance is unknown. A 2024 study found that HO-1 knockdown impaired trophoblast-spheroid attachment and that CO reversed this in vitro; Hmox1-null uterus showed altered angiogenesis/stress expression. This supports developmental biology but does not establish a human prenatal therapy. DOI 10.3390/cells13050376, published February 2024. (zenclussen2024absenceofheme pages 11-13)

9. Inheritance and population

Inheritance is autosomal recessive. Both sexes are affected; among the nine tabulated cases, six were male and three female, a sample too small to infer sex bias. Cases arose in Japan, India, Iran, Turkey, and the United States. Consanguinity contributed in two families, while the shared Indian p.R44X allele suggests a founder effect. Carrier parents are clinically unaffected, consistent with recessive transmission. (yachie2021hemeoxygenase1deficiency pages 7-8, yachie2021hemeoxygenase1deficiency pages 6-7)

No incidence, prevalence per 100,000, carrier frequency, penetrance estimate, or population registry exists. “Nine cases by 2021” is a reported-case count, not prevalence. Embryonic lethality in mice and fetal losses in two families raise the possibility of prenatal under-ascertainment, but this remains an expert hypothesis. (yachie2021hemeoxygenase1deficiency pages 3-4, yachie2021hemeoxygenase1deficiency pages 7-8, yachie2021hemeoxygenase1deficiency pages 8-10)

10. Diagnostics

When to suspect the disorder

The highest-yield pattern is:

1. Coombs-negative fragmented-cell hemolytic anemia;
2. strikingly low/normal bilirubin despite hemolysis;
3. LDH and ferritin often in the thousands to tens of thousands;
4. leukocytosis plus thrombocytosis rather than HLH-like cytopenias;
5. asplenia/hyposplenia without congenital heart disease, or unexplained splenomegaly;
6. fever/systemic inflammation, hepatomegaly, proteinuria/hematuria, hypertension, or interstitial lung disease. (yachie2021hemeoxygenase1deficiency pages 3-4, yachie2021hemeoxygenase1deficiency pages 8-10)

Tests

• CBC/smear: anemia, schistocytes/fragmented erythrocytes, nucleated RBCs, Howell–Jolly bodies, leukocytosis, thrombocytosis.
• Hemolysis/heme: LDH, bilirubin fractions, haptoglobin, plasma-free Hb/heme, methemoglobin, hemopexin, reticulocytes, urinalysis; direct antiglobulin testing is usually negative.
• Inflammation/organ assessment: ferritin, CRP/ESR, triglycerides, AST/ALT, renal function, urine protein/creatinine, coagulation/fibrinolysis markers, cytokines where available.
• Imaging: abdominal ultrasound/CT for spleen and liver; echocardiography for pericardial/cardiac disease; high-resolution chest CT and pulmonary function testing for respiratory disease; brain imaging when hypertensive or neurologically symptomatic.
• Pathology: kidney may show endothelial swelling, mesangial proliferation and tubular atrophy; liver may show Kupffer-cell/parenchymal iron, extramedullary hematopoiesis, hemophagocytosis or AA amyloid; lung may show fibrotic NSIP and cholesterol granulomas. (yachie2021hemeoxygenase1deficiency pages 6-7, yachie2021hemeoxygenase1deficiency pages 10-12, yachie2021hemeoxygenase1deficiency pages 4-6)
• Functional assay: absent or noninducible HO-1 protein/activity in stimulated PBMCs, monocytes, fibroblasts, or lymphoblastoid cells can support pathogenicity, especially for missense variants.

Genetic testing strategy

Sequence HMOX1 with deletion/duplication and splice analysis. A heme-metabolism/hemolytic-anemia/autoinflammatory panel can be used, but the laboratory must include HMOX1 and copy-number calling. WES/WGS is appropriate when the phenotype is atypical or first-line testing is negative; several cases were diagnosed by post-mortem WES. RNA sequencing may demonstrate aberrant splicing for c.636+2T>A or other splice variants. CMA, karyotype, FISH, mitochondrial testing, and repeat-expansion assays are not first-line unless another diagnosis is suspected. Cascade parental testing establishes phase. (yachie2021hemeoxygenase1deficiency pages 7-8, yachie2021hemeoxygenase1deficiency pages 4-6)

Differential diagnosis

Exclude thrombotic microangiopathy/HUS, autoimmune hemolysis, hereditary red-cell membrane/enzyme disorders, congenital asplenia syndromes, familial HLH/MAS, systemic juvenile idiopathic arthritis, CAPS/NOMID, vasculitis, infection, malignancy, Wilson disease, aceruloplasminemia, and other iron-recycling disorders. Low bilirubin with very high LDH/ferritin and thrombocytosis plus asplenia is especially discriminating from conventional hemolysis and HLH.

There are no standardized diagnostic criteria, newborn screen, approved enzyme assay, or population-screening program.

11. Outcome and prognosis

Prognosis is frequently poor but cannot be represented by five- or ten-year survival statistics. Several reported children died soon after onset or diagnosis from fungal sepsis, intracranial hemorrhage, heart failure/bleeding, or respiratory failure. The Indian p.R44X series showed prolonged asymptomatic periods followed by rapid decline; the US patient died at ten after six years of progressive lung disease. (yachie2021hemeoxygenase1deficiency pages 7-8, yachie2021hemeoxygenase1deficiency pages 4-6)

Major morbidity includes transfusion-dependent anemia, recurrent inflammatory hospitalization, chronic kidney disease/proteinuria, hypertension, cerebral bleeding, hepatic damage/amyloidosis, splenic dysfunction and infection risk, growth failure, pulmonary fibrosis, and heart failure. Prognostic factors are unvalidated; plausible adverse markers include very early onset, sustained hyperinflammation, pulmonary fibrosis, renal amyloid, severe endothelial dysfunction, and inability to control triggers. No validated prognostic biomarker or quality-of-life instrument exists.

12. Treatment

Current clinical management

There is no approved disease-specific therapy or consensus algorithm. Management is multidisciplinary and supportive:

• stabilize severe anemia, but transfuse cautiously and monitor inflammation/heme burden;
• promptly identify and treat infection;
• manage hypertension, renal disease/proteinuria, respiratory failure, coagulopathy, heart failure, and nutrition/growth;
• apply asplenia precautions: vaccination against encapsulated bacteria, fever action plan, and jurisdiction-appropriate antibiotic prophylaxis;
• avoid iron supplementation unless iron deficiency is objectively established—the p.G139V patient’s microcytic anemia was iron-unresponsive.

Suggested NCIt concepts include Blood Transfusion, Corticosteroid Therapy, Immunosuppressive Therapy, Anti-inflammatory Therapy, Hematopoietic Stem Cell Transplantation, Gene Therapy, and Supportive Care; exact NCIt codes should be resolved against the current thesaurus release.

Reported therapies and outcomes

Corticosteroids were ineffective in the Iranian patient. The Turkish patient received HLH-2004 immunochemotherapy, achieving remission of HLH-like signs but persistent biochemical inflammation. In the US case, corticosteroids, IL-1 receptor blockade, IL-6 blockade, and cyclosporine provided minimal benefit; death followed from respiratory failure. Broad immunosuppression can increase infection risk and does not correct failed heme catabolism. (yachie2021hemeoxygenase1deficiency pages 7-8, yachie2021hemeoxygenase1deficiency pages 6-7, yachie2021hemeoxygenase1deficiency pages 4-6)

Experimental strategies

• Macrophage/cell replacement: wild-type macrophages reversed disease in Hmox1-null mice, supporting replacement of the erythrophagocytic compartment. This is compelling model evidence, not proven human therapy. Primary report: Blood 2014;124:1522–1530, DOI 10.1182/blood-2014-02-554162. (yachie2021hemeoxygenase1deficiency pages 15-16)
• CO or CO-releasing molecules: CO can restore anti-inflammatory/endothelial signaling and rescued implantation or vascular phenotypes in models. Safety, dosing, and efficacy in congenital deficiency are unestablished. (yachie2021hemeoxygenase1deficiency pages 13-15, yachie2021hemeoxygenase1deficiency pages 12-13, zenclussen2024absenceofheme pages 11-13)
• Gene replacement/editing or autologous corrected hematopoietic stem cells: mechanistically attractive because macrophages are central, but no human trial or reported implementation was identified.
• Haptoglobin/hemopexin/CD163 pathway augmentation, biliverdin/bilirubin replacement, antioxidants, and iron redistribution therapy: preclinical concepts only.

The ClinicalTrials.gov tool search found no relevant disease-specific interventional study and no NCT identifier. There are no established response rates, pharmacogenomic recommendations, RNA therapy, surgery, or approved targeted biologic.

13. Prevention

Primary prevention is limited to genetic counseling. For a carrier couple, each pregnancy has the standard autosomal-recessive probabilities: 25% affected, 50% carrier, and 25% unaffected/noncarrier, assuming both parental variants are confirmed. Targeted prenatal diagnosis and preimplantation genetic testing are technically feasible. Cascade testing should be offered to adult relatives.

Secondary prevention consists of early recognition in siblings or children with the diagnostic laboratory pattern, followed by HMOX1 testing before irreversible kidney, vascular, or lung injury. There is no population or newborn screening. Tertiary prevention includes asplenia vaccination/prophylaxis, infection control, blood-pressure and renal monitoring, cautious transfusion, respiratory surveillance, and avoidance of unnecessary oxidative/toxic exposures. No vaccine prevents the genetic disease itself.

14. Other species and natural disease

No naturally occurring veterinary HMOX1-deficiency syndrome, breed association, or zoonotic transmission was identified. Relevant taxa are Homo sapiens (NCBI Taxon 9606), Mus musculus (10090), Rattus norvegicus (10116), and Danio rerio (7955). Orthologs are Hmox1 in mouse/rat and hmox1a/hmox1b paralogs in zebrafish. Comparative evidence demonstrates strong evolutionary conservation of heme detoxification, iron recycling, macrophage survival, and vascular protection, but induced knockout phenotypes should not be mislabeled as natural animal disease.

15. Model organisms and experimental systems

Mouse

Global Hmox1-null mice reproduce anemia, defective iron reutilization, renal/hepatic iron deposition, chronic inflammation, growth delay, oxidative-stress hypersensitivity, and splenic pathology. Depending on genetic background and age, spleens enlarge or progress from enlargement to fibrosis, atrophy, and functional hyposplenism. Null embryonic fibroblasts are hypersensitive to hemin, hydrogen peroxide, paraquat, and heavy metals. These are the highest-fidelity models for systemic disease, although mouse splenic development and survival vary by strain. Primary reports: Poss & Tonegawa, PNAS 1997;94:10919–10924 and 10925–10930. (yachie2021hemeoxygenase1deficiency pages 8-10, yachie2021hemeoxygenase1deficiency pages 15-16)

Macrophage-focused studies show erythrophagocytic macrophage death, reduced CD163, and altered tissue iron distribution. Infusion of wild-type macrophages can reverse key disease features, identifying macrophages as both a pathogenic hub and therapeutic target. Mouse studies also demonstrate abnormal erythroblastic islands, microcytic anemia, oxidative RBC stress, endothelial thrombosis, ischemic necrosis, inflammasome activation, and developmental/placental defects. Limitations are substantial embryonic loss, strain-dependent spleen phenotypes, and incomplete reproduction of human pulmonary or amyloid disease.

Rat

HO-1-depleted Sprague–Dawley rats develop hemolytic anemia, poikilocytes/target cells/acanthocytes, leukocytosis, growth impairment, splenomegaly, proteinuria, mesangial expansion, focal segmental sclerosis, and podocyte edema; most died by six months. Unlike humans and mice, renal tubular/hepatic iron deposition was not prominent, illustrating species-specific iron handling. (yachie2021hemeoxygenase1deficiency pages 8-10, yachie2021hemeoxygenase1deficiency pages 10-12)

Zebrafish and cellular systems

Zebrafish hmox1a disruption affects development and macrophage migration and is useful for imaging innate immune behavior, but duplicated genes and aquatic physiology limit direct clinical translation. Patient PBMCs, monocytes, lymphoblastoid cells, fibroblasts, HUVECs, trophoblast spheroids, and Hmox1-null embryonic fibroblasts support functional variant testing and investigation of oxidative stress, cytokines, coagulation, hypoxia, and CO rescue.

Recent developments and expert interpretation

The most disease-relevant recent clinical developments are: (1) a 2023 report expanding the renal spectrum to AA-type renal amyloidosis (Clinical Rheumatology 42:597–606; online 2022, issue 2023; DOI 10.1007/s10067-022-06465-9); (2) a 2024 clinical/molecular report of a novel variant with inflammation, heme-metabolism abnormalities, and pulmonary disease (Molecular Genetics and Metabolism Reports 38:101038; DOI 10.1016/j.ymgmr.2023.101038); and (3) a 2024 case report/review (Clinical Case Reports, DOI 10.1002/ccr3.8986). These publications indicate continuing phenotype expansion, not a change in standard care.

Current expert analysis increasingly emphasizes heme-detoxifying macrophages rather than treating HO-1 merely as a generic antioxidant. A 2024 review describes macrophage HMOX1 as essential for limiting oxidative damage in hemolytic disorders and for balancing inflammation and ferroptosis (published March 2024, DOI 10.3389/fimmu.2024.1379967). Another 2024 review stresses that erythrophagocyte HO-1 converts heme into CO, biliverdin and Fe²⁺ while controlling apoptosis, inflammation, and oxidative injury (published October 2024, DOI 10.3389/fimmu.2024.1433113). This supports macrophage replacement or corrected hematopoietic-cell strategies, but human efficacy remains wholly unproven.

Data-quality conclusions

The disease signature is strong and internally consistent, but evidence quality is constrained by single cases, retrospective descriptions, publication bias, and genotype/age heterogeneity. Frequencies should therefore be stored with denominators and an evidence tag such as “9-case literature series, through 2021”. Variant-level ClinVar/gnomAD status, current MONDO/Orphanet identifiers, exact incidence, standardized diagnostic criteria, natural-history survival, patient-reported outcomes, and treatment-response rates remain unavailable or require direct database validation. Mechanistic confidence is highest for defective heme catabolism, macrophage/endothelial dysfunction, oxidative injury, and iron misdistribution; confidence is lower for proposed CO, macrophage, stem-cell, or gene therapies because these are supported predominantly by models rather than treated patients.

References

1. (yachie2021hemeoxygenase1deficiency pages 1-3): Akihiro Yachie. Heme oxygenase-1 deficiency and oxidative stress: a review of 9 independent human cases and animal models. International Journal of Molecular Sciences, 22:1514, Feb 2021. URL: https://doi.org/10.3390/ijms22041514, doi:10.3390/ijms22041514. This article has 104 citations.

2. (yachie2021hemeoxygenase1deficiency pages 15-16): Akihiro Yachie. Heme oxygenase-1 deficiency and oxidative stress: a review of 9 independent human cases and animal models. International Journal of Molecular Sciences, 22:1514, Feb 2021. URL: https://doi.org/10.3390/ijms22041514, doi:10.3390/ijms22041514. This article has 104 citations.

3. (yachie2021hemeoxygenase1deficiency pages 7-8): Akihiro Yachie. Heme oxygenase-1 deficiency and oxidative stress: a review of 9 independent human cases and animal models. International Journal of Molecular Sciences, 22:1514, Feb 2021. URL: https://doi.org/10.3390/ijms22041514, doi:10.3390/ijms22041514. This article has 104 citations.

4. (yachie2021hemeoxygenase1deficiency pages 6-7): Akihiro Yachie. Heme oxygenase-1 deficiency and oxidative stress: a review of 9 independent human cases and animal models. International Journal of Molecular Sciences, 22:1514, Feb 2021. URL: https://doi.org/10.3390/ijms22041514, doi:10.3390/ijms22041514. This article has 104 citations.

5. (yachie2021hemeoxygenase1deficiency pages 4-6): Akihiro Yachie. Heme oxygenase-1 deficiency and oxidative stress: a review of 9 independent human cases and animal models. International Journal of Molecular Sciences, 22:1514, Feb 2021. URL: https://doi.org/10.3390/ijms22041514, doi:10.3390/ijms22041514. This article has 104 citations.

6. (yachie2021hemeoxygenase1deficiency pages 3-4): Akihiro Yachie. Heme oxygenase-1 deficiency and oxidative stress: a review of 9 independent human cases and animal models. International Journal of Molecular Sciences, 22:1514, Feb 2021. URL: https://doi.org/10.3390/ijms22041514, doi:10.3390/ijms22041514. This article has 104 citations.

7. (yachie2021hemeoxygenase1deficiency pages 13-15): Akihiro Yachie. Heme oxygenase-1 deficiency and oxidative stress: a review of 9 independent human cases and animal models. International Journal of Molecular Sciences, 22:1514, Feb 2021. URL: https://doi.org/10.3390/ijms22041514, doi:10.3390/ijms22041514. This article has 104 citations.

8. (yachie2021hemeoxygenase1deficiency pages 8-10): Akihiro Yachie. Heme oxygenase-1 deficiency and oxidative stress: a review of 9 independent human cases and animal models. International Journal of Molecular Sciences, 22:1514, Feb 2021. URL: https://doi.org/10.3390/ijms22041514, doi:10.3390/ijms22041514. This article has 104 citations.

9. (yachie2021hemeoxygenase1deficiency pages 10-12): Akihiro Yachie. Heme oxygenase-1 deficiency and oxidative stress: a review of 9 independent human cases and animal models. International Journal of Molecular Sciences, 22:1514, Feb 2021. URL: https://doi.org/10.3390/ijms22041514, doi:10.3390/ijms22041514. This article has 104 citations.

10. (yachie2021hemeoxygenase1deficiency pages 12-13): Akihiro Yachie. Heme oxygenase-1 deficiency and oxidative stress: a review of 9 independent human cases and animal models. International Journal of Molecular Sciences, 22:1514, Feb 2021. URL: https://doi.org/10.3390/ijms22041514, doi:10.3390/ijms22041514. This article has 104 citations.

11. (zenclussen2024absenceofheme pages 11-13): Maria Laura Zenclussen, Sina Ulrich, Mario Bauer, Beate Fink, Ana Claudia Zenclussen, Anne Schumacher, and Nicole Meyer. Absence of heme oxygenase-1 affects trophoblastic spheroid implantation and provokes dysregulation of stress and angiogenesis gene expression in the uterus. Cells, 13(5):376, Feb 2024. URL: https://doi.org/10.3390/cells13050376, doi:10.3390/cells13050376. This article has 5 citations.

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