Porphyria-Related Leukoencephalopathy

Mendelian MONDO:0958226 Pathograph 16 Show in embeddings browser hereditary disease leukodystrophy inborn error of heme biosynthesis

Porphyria-related leukoencephalopathy (LENCEP), also called HMBS-related leukodystrophy, is an autosomal recessive disorder caused by biallelic HMBS variants. Although HMBS is the acute intermittent porphyria gene, this is not a severe form of AIP: most affected individuals do not have the episodic neurovisceral attacks that define AIP - a minority do, and fleeting neurovisceral pain is described - but none has been reported to have attacks provoked by the classical porphyrinogenic stimuli that trigger AIP. What they show instead is slowly progressive spastic paraparesis, cerebellar ataxia, peripheral axonal neuropathy, ocular disease, and a recognizable leukoencephalopathy of the periventricular and deep cerebral white matter and bilateral thalami. Onset is in childhood or adolescence and adults survive to late life, but an infantile-onset severe end of the spectrum exists with developmental delay, hypotonia, epilepsy, cystic white matter change, and early death. The disease-defining observation is that porphyrin precursors are constitutively elevated in cerebrospinal fluid and are not lowered by liver transplantation or by haem arginate — so the hepatically directed therapies that work in AIP do not work here, and the neurotoxic burden appears to be generated within, or sustained independently of, the liver. Fewer than fifteen individuals have been reported.

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1
Inheritance
4
Pathophys.
14
Phenotypes
4
Gaps
16
Pathograph
1
Genes
4
Medical Actions
2
Subtypes
2
Models
2
References
1
Deep Research
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Inheritance

1
Autosomal recessive HP:0000007
Biallelic (homozygous or compound heterozygous) HMBS variants, confirmed by segregation in an affected sibship. Monoallelic carriers in the same families have roughly half-normal enzyme activity and no leukoencephalopathy.
Autosomal recessive inheritance
Show evidence (2 references)
PMID:27558376 SUPPORT Human Clinical
"Sanger sequencing of 6 healthy siblings confirmed the bi-allelic location of the variants and segregation with the disease."
Segregation analysis in the index family establishes biallelic inheritance.
PMID:41377573 SUPPORT Human Clinical
"None of these patients have been reported to have attacks in response to classical porphyrinogenic stimuli"
The defensible form of the distinction from acute intermittent porphyria. A minority of biallelic patients do have attacks, so the categorical claim is that attacks are not inducible by the stimuli that provoke AIP, not that they never occur.
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Subtypes

2
Childhood/adolescent-onset slowly progressive leukoencephalopathy
Onset in childhood or adolescence with slowly progressive spastic paraparesis, cerebellar ataxia, peripheral neuropathy and ocular disease, survival into late adulthood, and often preserved independence for decades.
Severe infantile-onset encephalopathy
Presentation in the first months or years with hypotonia, developmental delay, epilepsy and progressive cystic leukoencephalopathy, and early death. Severity appears to track the residual enzyme activity conferred by the particular genotype.
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Discussions and Knowledge Gaps

4
Is the white matter injury caused by accumulated porphyrin precursors (ALA/PBG) acting in the CNS, or by an absolute deficit of heme within the central nervous system?
KNOWLEDGE GAP precursor_toxicity_vs_cns_heme_deficit
Attached to
pathophysiology#Constitutive CNS Porphyrin Precursor Exposure pathophysiology#Cerebral White Matter Degeneration
These are the two standing hypotheses and they are not equivalent therapeutically: precursor toxicity would motivate an agent that lowers ALA/PBG inside the CNS, whereas a central heme deficit would require heme or a heme surrogate that crosses into the brain, which parenteral heme does not. The failure of both liver transplantation and haem arginate to alter CSF precursor levels or the clinical course is consistent with either, so the therapeutic failures do not discriminate between them. The edge from central precursor exposure to white matter degeneration is recorded as INDIRECT for this reason. Two natural experiments narrow it without closing it. Doss porphyria, caused by biallelic ALAD variants, raises ALA but not PBG and produces severe peripheral polyneuropathy with no CNS involvement - so ALA elevation alone appears insufficient for white matter disease, and PBG becomes the likelier candidate of the two precursors. Separately, the PBGD-deficient mouse develops a chronic progressive neuropathy while ALA is normal or only twofold raised, which its authors read as implicating heme deficiency. The two point in different directions, which is why the gap stays open.
Show evidence (3 references)
PMID:41731635 SUPPORT Human Clinical
"the heme precursors 5-aminolevulinic acid or porphobilinogen originating from the liver or central nervous system could be causative, and absolute heme deficiency in the central nervous system is another hypothesis"
States both competing hypotheses as unresolved in the current literature.
PMID:41731635 SUPPORT Human Clinical
"Patients with Doss porphyria, caused by biallelic pathogenic ALAD variants, show high elevations of ALA and coproporphyrin but not of PBG and uroporphyrin. They often exhibit severe peripheral polyneuropathy, but they do not develop an affection of the CNS."
The human biallelic natural experiment this rationale argues from. ALA elevation without PBG elevation gives peripheral neuropathy and no CNS disease, which is what makes PBG the likelier of the two precursors.
PMID:11478735 SUPPORT INDIRECT In Vitro
"The similarity in the potency and mechanism of the ALA-induced inhibition of muscimol binding in rat and human membranes indicate that rat studies are useful to evaluate the neurotoxic properties of ALA towards the human GABAergic system"
Gives the precursor-toxicity arm of this gap a concrete molecular candidate - ALA antagonism of the GABA-A receptor, shown in human as well as rat cortical membranes. Indirect for this disorder because the experiment used millimolar ALA on synaptic membranes from unaffected brain, and was framed around acute intermittent porphyria rather than the constitutive, white-matter-predominant biallelic disease; whether the concentrations reached in LENCEP cerebrospinal fluid approach this range has not been measured.
Is there an animal model that reproduces the central biallelic HMBS leukodystrophy, as opposed to the peripheral neuropathy and the biochemical compartment, both of which are already modelled?
KNOWLEDGE GAP no_animal_model_of_the_biallelic_cns_disease
Attached to
pathophysiology#Cerebral White Matter Degeneration pathophysiology#Constitutive CNS Porphyrin Precursor Exposure
Two mouse models exist and are curated in animal_models, but neither reproduces the white matter lesion. The PBGD-deficient mouse models the peripheral axonal neuropathy; the biallelic R167Q mouse reproduces the constitutive central precursor elevation and the haem non-response but has normal CNS histopathology, so it explicitly does not reproduce the white matter lesion. So what is missing specifically is a model of the leukodystrophy - a system in which CNS precursor concentrations can be raised or lowered independently of hepatic heme synthesis while the white matter is watched. That is also the platform any CNS-penetrant precursor-lowering strategy would need.
Can any agent lower porphyrin precursors in the central compartment, given that hepatically directed therapy demonstrably cannot?
KNOWLEDGE GAP aip_therapies_do_not_transfer
Attached to
treatments#Liver Transplantation treatments#Intravenous Haem Therapy
Givosiran, the ALAS1 siRNA that prevents attacks in AIP, is hepatically targeted, and access was declined for one post-transplant patient on exactly that ground. Since CSF precursors do not fall after liver transplantation, there is no reason to expect a liver-directed siRNA to reach the relevant compartment either. A CNS-penetrant strategy is what this disorder needs and none has been reported.
Show evidence (1 reference)
PMID:41731635 SUPPORT Human Clinical
"However, parenterally administered heme, which is effective in acute intermittent porphyria, does not reach the central nervous system."
Identifies the pharmacological barrier that makes AIP therapies inapplicable here.
What determines whether a biallelic HMBS genotype produces the survivable adult leukoencephalopathy or the lethal infantile encephalopathy?
KNOWLEDGE GAP genotype_severity_correlation
Attached to
has_subtypes#Adult leukoencephalopathy has_subtypes#Severe infantile
Residual enzyme activity conferred by the specific allele pair is the presumed determinant, but with fewer than fifteen reported individuals the genotype-phenotype relationship rests on very few observations, and intrafamilial variability has been noted even for identical genotypes.
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Pathophysiology

4
Biallelic HMBS Deficiency
Biallelic HMBS variants reduce hydroxymethylbilane synthase (porphobilinogen deaminase) activity, and the reported range is wide. The index kindred had a 50% to 66% decrease, so 34-50% residual activity — overlapping what heterozygous AIP carriers have. A separate adult kindred had 18% residual erythrocyte activity against 50% in an unaffected heterozygous sibling, and the infantile-onset cases are markedly reduced. So residual activity is not uniformly below the heterozygous range, and the severity gradient tracks the specific allele pair rather than biallelic status alone. This is the third enzyme of heme biosynthesis, converting porphobilinogen to hydroxymethylbilane.
heme biosynthetic process GO:0006783 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased heme biosynthetic process (GO:0006783). GO:0006783 is a biological process from the Gene Ontology. ↓ DECREASED
hydroxymethylbilane synthase activity GO:0004418 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased hydroxymethylbilane synthase activity (GO:0004418). GO:0004418 is a molecular function from the Gene Ontology. ↓ DECREASED
Downstream
Porphyrin Precursor Accumulation Blocking the PBG-to-hydroxymethylbilane step backs up the pathway, accumulating porphobilinogen and, through secondary inhibition of ALAD, 5-aminolevulinic acid.
Show evidence (2 references)
PMID:27558376 SUPPORT Human Clinical
"a 50% to 66% decrease in hydroxymethylbilane synthase enzyme activity compared to normal"
Quantifies the enzyme deficiency in the index adult kindred.
PMID:41377573 SUPPORT Human Clinical
"Erythrocyte HMBS activity was measured at 18% in our patient (and in her brother with the shared genotype and phenotype) and 50% in an asymptomatic sibling carrying a monoallelic variant."
Contrasts the profound biallelic enzyme deficit against the half-normal activity of an unaffected heterozygous sibling.
Porphyrin Precursor Accumulation
5-aminolevulinic acid, porphobilinogen, and uroporphyrin accumulate in plasma and urine. Accumulating porphobilinogen secondarily inhibits ALAD, which drives ALA higher still. In the adult leukoencephalopathy phenotype the peripheral elevation is only slight to moderate, well short of what an AIP attack produces — so the disease is not simply "more AIP".
Downstream
Constitutive CNS Porphyrin Precursor Exposure Precursors are present in the central compartment, and unlike in AIP their concentration there does not track hepatic heme synthesis.
Show evidence (2 references)
PMID:27558376 SUPPORT Human Clinical
"Patients had a slight and moderate increase in urinary and plasma porphobilinogen and 5'-aminolevulinic acid, respectively"
Documents the modest peripheral precursor elevation characteristic of the adult leukoencephalopathy phenotype.
PMID:42396593 SUPPORT Human Clinical
"Plasma and urine exhibited high concentrations of 5-aminolevulinic acid, porphobilinogen, and uroporphyrin 1."
Documents marked precursor accumulation in the severe infantile-onset presentation.
Constitutive CNS Porphyrin Precursor Exposure
Porphobilinogen and ALA are detectable and constitutively elevated in cerebrospinal fluid, and PBG is not detectable in the CSF of individuals without the disease. Critically, CSF levels are unchanged by liver transplantation, which means the central compartment is not simply receiving hepatically produced precursor. Whether the neurotoxic agent is the accumulated precursor or, alternatively, an absolute heme deficit in the CNS remains unsettled — see the knowledge gaps below.
Animal models
Biallelic R167Q HMBS mouse Mouse recapitulates Reproduces constitutive central precursor elevation that is independent of ALAS1 induction and unresponsive to haem, which is the pharmacological situation in patients.
Downstream
Cerebral White Matter Degeneration Sustained central exposure is the proposed driver of the white matter lesion, but the neuropathological mechanism connecting the two has not been established.
Peripheral axonal neuropathy Length-dependent sensorimotor axonal neuropathy accompanies the central lesion, paralleling the peripheral neuropathy of AIP.
Show evidence (2 references)
PMID:41377573 SUPPORT Human Clinical
"We were not able to detect PBG in the CSF of patients unaffected by AIP."
Establishes that CSF porphobilinogen is a disease-associated finding rather than a normal constituent.
PMID:41731635 SUPPORT Human Clinical
"In his urine, 5-aminolevulinic acid and porphobilinogen were markedly elevated, but in cerebrospinal fluid just porphobilinogen."
Reports the CSF precursor profile in an independent severely affected case.
Cerebral White Matter Degeneration
A recognizable and consistent MRI pattern: confluent symmetrical signal abnormality of the periventricular and deep cerebral white matter and the bilateral thalami, with the central pons involved, progressing to cerebellar atrophy in advanced disease. In the severe infantile form the white matter change becomes cystic with progressive volume loss, and autopsy shows patchy gliosis and leukodystrophy.
oligodendrocyte CL:0000128 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves oligodendrocyte (CL:0000128). CL:0000128 is a cell type from the Cell Ontology.
periventricular and deep cerebral white matter UBERON:0002316 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in periventricular and deep cerebral white matter, annotated with white matter (UBERON:0002316). UBERON:0002316 is an anatomical location from the Uberon multi-species anatomy ontology. bilateral thalami UBERON:0001897 Uberon multi-species anatomy ontology (UBERON) Relation: this pathophysiological event occurs in this anatomical location This pathophysiological event occurs in bilateral thalami, annotated with dorsal plus ventral thalamus (UBERON:0001897). UBERON:0001897 is an anatomical location from the Uberon multi-species anatomy ontology.
Animal models
Biallelic R167Q HMBS mouse Mouse fails to recapitulate The closest genetic model of the human disease does not reproduce its defining lesion. CNS histopathology is normal in the biallelic R167Q mouse and brain haem content is about 80% of wild type, despite the constitutive central precursor elevation the same animal shows. That is a reported negative result rather than absent data, and it cuts against the simplest version of both competing hypotheses: neither the precursor burden nor a central haem deficit was sufficient to produce white matter pathology in this mouse.
Downstream
Spastic paraparesis
Cerebellar ataxia
Leukoencephalopathy
Cerebellar atrophy Appears at advanced disease stages rather than at presentation, which is why it is drawn as a late consequence of the progressing lesion rather than a parallel finding.
Cognitive impairment The mild cognitive impairment seen in some adults is attributed to the deep white matter disease, though the anatomical correlation has not been demonstrated.
Seizure In the severe infantile form, where the white matter change becomes cystic. Not a feature of the adult presentation.
Global developmental delay In the severe infantile form. Persists after liver transplantation, which is consistent with fixed rather than ongoing injury.
Show evidence (2 references)
PMID:34089223 SUPPORT Human Clinical
"Their brain MRIs show mainly confluent signal abnormalities in the periventricular and deep white matter and bilateral thalami."
Defines the anatomical distribution of the white matter lesion across six affected adults.
PMID:42396593 SUPPORT Human Clinical
"Findings at autopsy of the brain showed patchy gliosis and leukodystrophy."
Provides neuropathological confirmation of the leukodystrophy in the severe phenotype.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Porphyria-Related Leukoencephalopathy Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

14
Eye 5
Optic atrophy FREQUENT HP:0000648 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Optic atrophy (HP:0000648), qualified as course progressive. HP:0000648 is a phenotype from the Human Phenotype Ontology.
Course: PROGRESSIVE
Show evidence (2 references)
PMID:27558376 SUPPORT Human Clinical
"in 2 patients, optic atrophy as well as vertical gaze and convergence palsies and nystagmus"
Reports optic atrophy in two of three affected family members.
PMID:41731635 SUPPORT Human Clinical
"Bilateral cataracts, optic nerve atrophy, and cerebellar abnormalities were also common."
Supports the FREQUENT grading across the reported cases, rather than resting it on the two affected members of a single kindred.
Nystagmus OCCASIONAL HP:0000639 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Nystagmus (HP:0000639). HP:0000639 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27558376 SUPPORT Human Clinical
"vertical gaze and convergence palsies and nystagmus"
Names nystagmus among the ocular findings in the index family.
Limited vertical extraocular movement OCCASIONAL HP:0025721 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Vertical gaze palsy, annotated with Limited vertical extraocular movement (HP:0025721). HP:0025721 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27558376 SUPPORT Human Clinical
"vertical gaze and convergence palsies and nystagmus"
Names the vertical gaze palsy among the ocular findings in the index family.
Impaired convergence OCCASIONAL HP:0000619 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Convergence palsy, annotated with Impaired convergence (HP:0000619). HP:0000619 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27558376 SUPPORT Human Clinical
"vertical gaze and convergence palsies and nystagmus"
Reports the specific eye movement abnormalities observed.
Cataract FREQUENT HP:0000518 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Childhood-onset cataract, annotated with Cataract (HP:0000518). HP:0000518 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:41377573 SUPPORT Human Clinical
"She also had childhood-onset cataracts."
Documents childhood-onset cataract in a genotyped patient.
PMID:41731635 SUPPORT Human Clinical
"Bilateral cataracts, optic nerve atrophy, and cerebellar abnormalities were also common."
The sentence the FREQUENT grading rests on. The single-patient item above establishes the phenotype exists; this one establishes it is common across the reported cases.
Musculoskeletal 2
Spastic paraparesis VERY_FREQUENT HP:0002313 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Spastic paraparesis (HP:0002313), qualified as course progressive. HP:0002313 is a phenotype from the Human Phenotype Ontology.
Course: PROGRESSIVE
Show evidence (1 reference)
PMID:34089223 SUPPORT Human Clinical
"All six affected individuals presented with slowly progressive spasticity, ataxia, peripheral neuropathy, with or without mild cognitive impairment, and/or ocular disease with onset in childhood or adolescence."
Reports progressive spasticity in all six adults in the largest published series.
Hypotonia VERY_FREQUENT HP:0001252 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hypotonia (HP:0001252). HP:0001252 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:42396593 SUPPORT Human Clinical
"A male infant presented at three months of age with generalized ataxia, hypotonia, aspiration of liquids and recurrent generalized seizures."
Reports hypotonia at presentation in the severe infantile phenotype.
Nervous System 7
Cerebellar ataxia VERY_FREQUENT Progressive cerebellar ataxia HP:0002073 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Progressive cerebellar ataxia (HP:0002073). HP:0002073 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27558376 SUPPORT Human Clinical
"Clinical features were childhood-onset slowly progressive spastic paraparesis, cerebellar ataxia, peripheral neuropathy"
Reports cerebellar ataxia as a core feature of the index kindred.
Peripheral axonal neuropathy VERY_FREQUENT HP:0003477 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Peripheral axonal neuropathy (HP:0003477). HP:0003477 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:34089223 SUPPORT Human Clinical
"All six affected individuals presented with slowly progressive spasticity, ataxia, peripheral neuropathy"
Reports peripheral neuropathy in all six adults in the series.
Leukoencephalopathy VERY_FREQUENT HP:0002352 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Leukoencephalopathy (HP:0002352). HP:0002352 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27558376 SUPPORT Human Clinical
"We identified 3 family members with a similar MRI pattern characterized by symmetrical signal abnormalities in the periventricular and deep cerebral white matter, thalami, and central part of the pons."
Describes the diagnostic MRI pattern in the index family.
Cerebellar atrophy OCCASIONAL HP:0001272 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebellar atrophy (HP:0001272). HP:0001272 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:27558376 SUPPORT Human Clinical
"Cerebellar atrophy was noted in advanced disease stages."
Records cerebellar atrophy as a late rather than presenting feature.
Cognitive impairment OCCASIONAL HP:0100543 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Mild cognitive impairment, annotated with Cognitive impairment (HP:0100543), qualified as severity mild. HP:0100543 is a phenotype from the Human Phenotype Ontology.
Severity: MILD
Show evidence (1 reference)
PMID:34089223 SUPPORT Human Clinical
"slowly progressive spasticity, ataxia, peripheral neuropathy, with or without mild cognitive impairment"
Reports mild cognitive impairment as a variable feature.
Seizure VERY_FREQUENT HP:0001250 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Recurrent generalized seizures, annotated with Seizure (HP:0001250). HP:0001250 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:42396593 SUPPORT Human Clinical
"A male infant presented at three months of age with generalized ataxia, hypotonia, aspiration of liquids and recurrent generalized seizures."
Reports seizures in the infantile-onset severe presentation.
Global developmental delay VERY_FREQUENT HP:0001263 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Global developmental delay (HP:0001263). HP:0001263 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:42396593 SUPPORT Human Clinical
"Developmental delays and seizures persisted."
Records persistent developmental delay after attempted disease-modifying therapy.
🧬

Genetic Associations

1
HMBS
Gene: HMBS hgnc:4982 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is HMBS (hgnc:4982). hgnc:4982 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE variant_origin: GERMLINE
Show evidence (1 reference)
PMID:27558376 SUPPORT Human Clinical
"Whole-exome sequencing revealed compound heterozygous missense variants in the HMBS gene, both associated with the autosomal dominant disorder acute intermittent porphyria."
Identifies HMBS as the causal gene and notes that the alleles are known AIP alleles.
💊

Medical Actions

4
Liver Transplantation
Action: liver transplantationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is liver transplantation, annotated with Organ Transplantation (NCIT:C15289). NCIT:C15289 is a clinical intervention from the NCI Thesaurus. Ontology label: Organ Transplantation NCIT:C15289
Platform: Surgery
Attempted in several patients on the AIP rationale that replacing hepatic HMBS removes the precursor source. It does not work here, but the reason is sharper than "nothing changed". In one adult, precursor concentrations were unchanged in both compartments after transplantation. In an infant, hepatic correction was substantial - about a 50% reduction in ALA, PBG and uroporphyrin overproduction two years on - and the neurological decline continued anyway. Halving the hepatic output without arresting the disease is stronger evidence for an extrahepatic or CNS-autonomous precursor source than an absent biochemical response would be.
Mechanism Target:
Porphyrin Precursor Accumulation — Intended to remove the hepatic source of porphyrin precursors; in this disorder it demonstrably fails to lower them.
Show evidence (1 reference)
PMID:41377573 REFUTE Human Clinical
"Liver transplantation and hepatically directed therapies are not likely to be effective for leukodystrophy due to biallelic HMBS variants."
Explicitly refutes the hepatic-source rationale for treating this disorder.
Show evidence (3 references)
PMID:42396593 REFUTE Human Clinical
"Orthotopic liver transplantation at the age of 15 months led to only mild transient improvement."
Reports failure of liver transplantation to alter the disease course in the severe infantile form.
PMID:42396593 REFUTE Human Clinical
"with only about a 50% reduction in the marked overproduction of ALA, PBG, and uroporphyrin, compared with his pre-transplant status"
Quantifies the hepatic correction that transplantation did achieve in this patient. It refutes the therapy because halving precursor overproduction still did not arrest the neurological decline, which argues the residual source is not hepatic.
PMID:34089223 SUPPORT INDIRECT Human Clinical
"One individual reported here had improved neurological function for 12 months following liver transplantation followed by decline and progression of disease."
Records the transient benefit that motivated further attempts, before decline resumed; supports the intervention only as a temporary effect.
Intravenous Haem Therapy
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: haem arginate CHEBI:30413 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses haem arginate, annotated with heme (CHEBI:30413). CHEBI:30413 is a therapeutic agent from Chemical Entities of Biological Interest.
Platform: Small molecule
Haem arginate or intravenous heme, the standard treatment for acute attacks in AIP, does work biochemically in the periphery - roughly 50% transient reductions in urinary ALA, PBG and porphyrins - but does not lower CSF porphyrin precursors and produces at most transient subjective clinical improvement. That dissociation is the point: the peripheral compartment responds and the central one does not, because parenterally administered heme does not reach the central nervous system. It is the same compartmental argument the liver transplantation entry makes.
Mechanism Target:
Constitutive CNS Porphyrin Precursor Exposure — Intended to suppress ALAS1 by negative feedback and so reduce precursor production; it does not reduce the central precursor burden.
Show evidence (1 reference)
PMID:41377573 REFUTE Human Clinical
"porphyrin precursor levels are constitutively elevated in the cerebrospinal fluid and are not reduced by haem arginate therapy"
Directly refutes that haem arginate lowers the central precursor burden.
Show evidence (2 references)
PMID:42396593 REFUTE Human Clinical
"A 5-day course of intravenous heme led to no observable clinical improvement."
Reports absence of clinical benefit from intravenous heme.
PMID:42396593 SUPPORT INDIRECT Human Clinical
"intravenous hematin, given daily for 5 days, had led to transient ∼50% reductions in urinary ALA, PBG, and porphyrins."
Establishes that haem does work biochemically in the peripheral compartment. Recorded as SUPPORT for the peripheral effect, which is what makes the absent central and clinical response a dissociation rather than simple inactivity.
Antiseizure Therapy with Non-Porphyrinogenic Agents
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: levetiracetam CHEBI:6437 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses levetiracetam (CHEBI:6437). CHEBI:6437 is a therapeutic agent from Chemical Entities of Biological Interest.
Platform: Small molecule
Seizures in the infantile-onset form require antiseizure drugs, and the choice is constrained by porphyria pharmacology: many older anticonvulsants induce hepatic heme synthesis and are avoided in porphyria. Levetiracetam does not appear to exacerbate AIP and was used in the reported infant, who was also started on phenobarbital and then tapered off it precisely because barbiturates induce hepatic ALAS1. Note that the rationale is inherited from AIP practice rather than demonstrated in this disorder, where hepatic induction is not the driver - which makes the case for avoiding barbiturates here weaker in principle than in AIP, though it was acted on.
Show evidence (3 references)
PMID:42396593 SUPPORT Human Clinical
"Levetiracetam, which does not appear to exacerbate AIP, has been a welcome recent addition to the pharmacopoeia."
Identifies levetiracetam as the non-porphyrinogenic option, which is why it was the drug used.
PMID:42396593 SUPPORT Human Clinical
"He was treated with levetiracetam and phenobarbital."
Records the antiseizure regimen the infant was started on. Note this included phenobarbital, which was subsequently withdrawn - see the next item.
PMID:42396593 SUPPORT Human Clinical
"Because of the known risks of barbiturates in AIP, his medication regimen was adjusted, including tapering off phenobarbital, which is well-known to increase hepatic ALA synthase-1 and over-production of ALA and PBG"
The drug-safety reversal. Phenobarbital was tapered off precisely because barbiturates induce hepatic ALAS1; without this the entry would appear to endorse a barbiturate under a treatment named for non-porphyrinogenic agents.
Supportive and Symptomatic Care
Action: Supportive CareNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Supportive Care (NCIT:C15747). NCIT:C15747 is a clinical intervention from the NCI Thesaurus. NCIT:C15747
Platform: Behavioral / lifestyle
In the absence of disease-modifying therapy, management is symptomatic: antiepileptic drugs, spasticity and ataxia management, rehabilitation, and visual support.
Show evidence (1 reference)
PMID:42396593 SUPPORT Human Clinical
"No effective disease-modifying therapy currently exists for biallelic HMBS deficiency."
Establishes that no disease-modifying option is available, leaving supportive care as the standard.
🔬

Biochemical Markers

1
Cerebrospinal fluid porphobilinogen (Elevated)
Show evidence (1 reference)
PMID:41377573 SUPPORT Human Clinical
"There was no difference between pre-transplant porphyrin precursor levels and post-transplant levels in either the peripheral or central compartment."
Demonstrates that the central and peripheral precursor burden is independent of hepatic HMBS.
🔬

Diagnosis

3
Sequencing with explicit zygosity determination
The diagnosis turns on zygosity, not on finding an HMBS variant. A single pathogenic HMBS allele is acute intermittent porphyria; two are this disease, with a different inheritance pattern, a different clinical syndrome and - critically - different treatment expectations. Several of the alleles seen in biallelic patients are themselves known AIP alleles, so a report naming a familiar AIP variant does not settle which disorder is present. Confirming biallelic status, and segregation where family members are available, is the diagnostic step.
Show evidence (2 references)
PMID:27558376 SUPPORT Human Clinical
"Whole-exome sequencing revealed compound heterozygous missense variants in the HMBS gene"
Exome sequencing establishing biallelic status is how the index kindred was diagnosed.
PMID:27558376 SUPPORT Human Clinical
"Sanger sequencing of 6 healthy siblings confirmed the bi-allelic location of the variants and segregation with the disease."
Segregation analysis is what confirmed the two variants sit on different alleles.
Erythrocyte hydroxymethylbilane synthase activity
Supports the molecular result and separates affected individuals from carriers. Activity is markedly reduced in patients and roughly half of normal in heterozygous parents, so the assay discriminates the biallelic state from the AIP carrier state that the same family often contains.
Show evidence (1 reference)
PMID:42396593 SUPPORT Human Clinical
"Activity of hydroxymethylbilane synthase in red blood cells of the child was markedly reduced"
Records the erythrocyte enzyme result in the affected child. The same sentence gives roughly 50% of normal in the parents, which is what makes the assay discriminate the biallelic state from the carrier state; that figure sits inside a bracketed reference range and so is not quotable as a contiguous snippet.
Cerebrospinal fluid porphyrin precursor measurement
The measurement that distinguishes this disorder mechanistically and guides therapy. Porphobilinogen is detectable in the CSF of affected individuals and undetectable in unaffected controls, and CSF concentrations do not fall after liver transplantation or haem arginate. A normal peripheral profile does not exclude the central abnormality, which matters because peripheral elevation in the adult phenotype is only slight to moderate.
Show evidence (2 references)
PMID:41377573 SUPPORT Human Clinical
"ALA and PBG were measured in urine and CSF using the Mauzerell and Granick method"
Records the CSF assay used, which is not part of a routine porphyria workup.
PMID:41377573 SUPPORT Human Clinical
"We were not able to detect PBG in the CSF of patients unaffected by AIP."
Establishes CSF porphobilinogen as a disease-associated finding rather than a normal constituent.
📊

Prevalence

1
Worldwide
Cases In Literature Ultra Rare
Fourteen reported individuals - thirteen previously published plus the index case of the most recent report. No population-based estimate exists.
Show evidence (1 reference)
PMID:41377573 SUPPORT Human Clinical
"Thirteen patients with leukodystrophy due to biallelic HMBS variants have been previously described."
Gives the countable size of the reported literature.
🐁

Animal Models

2
PBGD-deficient mouse
A partial porphobilinogen deaminase deficiency mouse, generated to study the neuropathy of acute porphyria. Its value here is discriminating rather than confirmatory: the neuropathy developed chronically and progressively while plasma and urinary ALA were normal or only twofold raised, which is the opposite of what a precursor-toxicity model predicts, and the authors conclude heme deficiency and hemeprotein dysfunction can cause the neuropathy.
Species
Mouse
Genotype
Pbgd-deficient (PBGD-/-)
Publication
PMID:10207164
Biallelic R167Q HMBS mouse
The closest genetic model of the biallelic human disease, and the one that matches the therapeutic failure. It carries constitutively high PBG and ALA in brain tissue and CSF, has normal ALAS1 transcript before and after porphyrinogenic stimuli, and does not respond to haem provision — the same three features that make hepatically directed therapy ineffective in patients.
Species
Mouse
Genotype
Hmbs R167Q homozygous
Publication
PMID:41377573
Pathograph links
Constitutive CNS Porphyrin Precursor Exposure recapitulates fidelity: moderate Reproduces constitutive central precursor elevation that is independent of ALAS1 induction and unresponsive to haem, which is the pharmacological situation in patients.
Limitations: This link covers the biochemical compartment only. The same mouse does not develop the white matter lesion - CNS histopathology is reported as normal - which is curated as a separate FAILS_TO_RECAPITULATE link below.
Show evidence (2 references)
PMID:41377573 SUPPORT Model Organism
"R167Q mouse has constitutively high concentrations of PBG and ALA in brain tissue and CSF"
Establishes the model as reproducing the constitutive central precursor elevation seen in patients.
+ 1 more reference
Cerebral White Matter Degeneration fails to recapitulate The closest genetic model of the human disease does not reproduce its defining lesion. CNS histopathology is normal in the biallelic R167Q mouse and brain haem content is about 80% of wild type, despite the constitutive central precursor elevation the same animal shows. That is a reported negative result rather than absent data, and it cuts against the simplest version of both competing hypotheses: neither the precursor burden nor a central haem deficit was sufficient to produce white matter pathology in this mouse.
Limitations: A mouse CNS is not a human one and the reported observation is histopathology at whatever age it was assessed, not a lifelong neuroimaging course; the human disease is slowly progressive over decades. So this constrains the mechanism rather than excluding either hypothesis.
Show evidence (1 reference)
PMID:41377573 REFUTE Model Organism
"in the biallelic R167Q mouse, brain haem content was ~80% that of the wild‐type mice and CNS histopathology was normal"
Refutes the claim that this model reproduces the leukodystrophy, and supplies the brain haem measurement that bears on the CNS-haem-deficit hypothesis.
{ }

Source YAML

click to show 
name: Porphyria-Related Leukoencephalopathy
creation_date: "2026-09-01T00:00:00Z"
category: Mendelian
description: >-
  Porphyria-related leukoencephalopathy (LENCEP), also called HMBS-related
  leukodystrophy, is an autosomal recessive disorder caused by biallelic HMBS
  variants. Although HMBS is the acute intermittent porphyria gene, this is not
  a severe form of AIP: most affected individuals do not have the episodic
  neurovisceral attacks that define AIP - a minority do, and fleeting
  neurovisceral pain is described - but none has been reported to have attacks
  provoked by the classical porphyrinogenic stimuli that trigger AIP. What they
  show instead is slowly progressive
  spastic paraparesis, cerebellar ataxia, peripheral axonal neuropathy, ocular
  disease, and a recognizable leukoencephalopathy of the periventricular and
  deep cerebral white matter and bilateral thalami. Onset is in childhood or
  adolescence and adults survive to late life, but an infantile-onset severe end
  of the spectrum exists with developmental delay, hypotonia, epilepsy, cystic
  white matter change, and early death. The disease-defining observation is that
  porphyrin precursors are constitutively elevated in cerebrospinal fluid and
  are not lowered by liver transplantation or by haem arginate — so the
  hepatically directed therapies that work in AIP do not work here, and the
  neurotoxic burden appears to be generated within, or sustained independently
  of, the liver. Fewer than fifteen individuals have been reported.
parents:
- hereditary disease
- leukodystrophy
- inborn error of heme biosynthesis
disease_term:
  preferred_term: leukoencephalopathy, porphyria-related
  term:
    id: MONDO:0958226
    label: leukoencephalopathy, porphyria-related

inheritance:
- name: Autosomal recessive
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >-
    Biallelic (homozygous or compound heterozygous) HMBS variants, confirmed by
    segregation in an affected sibship. Monoallelic carriers in the same
    families have roughly half-normal enzyme activity and no leukoencephalopathy.
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sanger sequencing of 6 healthy siblings confirmed the bi-allelic location of the variants and segregation with the disease."
    explanation: Segregation analysis in the index family establishes biallelic inheritance.
  - reference: PMID:41377573
    reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "None of these patients have been reported to have attacks in response to classical porphyrinogenic stimuli"
    explanation: >-
      The defensible form of the distinction from acute intermittent porphyria.
      A minority of biallelic patients do have attacks, so the categorical claim
      is that attacks are not inducible by the stimuli that provoke AIP, not
      that they never occur.

pathophysiology:
- name: Biallelic HMBS Deficiency
  description: >-
    Biallelic HMBS variants reduce hydroxymethylbilane synthase (porphobilinogen
    deaminase) activity, and the reported range is wide. The index kindred had a
    50% to 66% decrease, so 34-50% residual activity — overlapping what
    heterozygous AIP carriers have. A separate adult kindred had 18% residual
    erythrocyte activity against 50% in an unaffected heterozygous sibling, and
    the infantile-onset cases are markedly reduced. So residual activity is not
    uniformly below the heterozygous range, and the severity gradient tracks the
    specific allele pair rather than biallelic status alone. This is the third
    enzyme of heme biosynthesis, converting porphobilinogen to hydroxymethylbilane.
  biological_scale: MOLECULAR
  downstream:
  - target: Porphyrin Precursor Accumulation
    causal_link_type: DIRECT
    description: >-
      Blocking the PBG-to-hydroxymethylbilane step backs up the pathway,
      accumulating porphobilinogen and, through secondary inhibition of ALAD,
      5-aminolevulinic acid.
    evidence:
    - reference: PMID:42396593
      reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Impaired conversion of PBG to hydroxymethylbilane leads to the accumulation of the precursors, PBG and ALA."
      explanation: States the direct enzymatic consequence linking HMBS deficiency to precursor accumulation.
  molecular_functions:
  - preferred_term: hydroxymethylbilane synthase activity
    term:
      id: GO:0004418
      label: hydroxymethylbilane synthase activity
    modifier: DECREASED
  biological_processes:
  - preferred_term: heme biosynthetic process
    term:
      id: GO:0006783
      label: heme biosynthetic process
    modifier: DECREASED
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a 50% to 66% decrease in hydroxymethylbilane synthase enzyme activity compared to normal"
    explanation: Quantifies the enzyme deficiency in the index adult kindred.
  - reference: PMID:41377573
    reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Erythrocyte HMBS activity was measured at 18% in our patient (and in her brother with the shared genotype and phenotype) and 50% in an asymptomatic sibling carrying a monoallelic variant."
    explanation: Contrasts the profound biallelic enzyme deficit against the half-normal activity of an unaffected heterozygous sibling.

- name: Porphyrin Precursor Accumulation
  description: >-
    5-aminolevulinic acid, porphobilinogen, and uroporphyrin accumulate in
    plasma and urine. Accumulating porphobilinogen secondarily inhibits ALAD,
    which drives ALA higher still. In the adult leukoencephalopathy phenotype
    the peripheral elevation is only slight to moderate, well short of what an
    AIP attack produces — so the disease is not simply "more AIP".
  biological_scale: ORGANISM
  downstream:
  - target: Constitutive CNS Porphyrin Precursor Exposure
    causal_link_type: DIRECT
    description: >-
      Precursors are present in the central compartment, and unlike in AIP their
      concentration there does not track hepatic heme synthesis.
    evidence:
    - reference: PMID:41377573
      reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "porphyrin precursor levels are constitutively elevated in the cerebrospinal fluid and are not reduced by haem arginate therapy"
      explanation: Establishes that the CNS precursor burden is constitutive rather than attack-driven or hepatically controlled.
  chemical_entities:
  - preferred_term: porphobilinogen
    term:
      id: CHEBI:17381
      label: porphobilinogen
    modifier: INCREASED
  - preferred_term: 5-aminolevulinic acid
    term:
      id: CHEBI:17549
      label: 5-aminolevulinic acid
    modifier: INCREASED
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients had a slight and moderate increase in urinary and plasma porphobilinogen and 5'-aminolevulinic acid, respectively"
    explanation: Documents the modest peripheral precursor elevation characteristic of the adult leukoencephalopathy phenotype.
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Plasma and urine exhibited high concentrations of 5-aminolevulinic acid, porphobilinogen, and uroporphyrin 1."
    explanation: Documents marked precursor accumulation in the severe infantile-onset presentation.

- name: Constitutive CNS Porphyrin Precursor Exposure
  description: >-
    Porphobilinogen and ALA are detectable and constitutively elevated in
    cerebrospinal fluid, and PBG is not detectable in the CSF of individuals
    without the disease. Critically, CSF levels are unchanged by liver
    transplantation, which means the central compartment is not simply
    receiving hepatically produced precursor. Whether the neurotoxic agent is
    the accumulated precursor or, alternatively, an absolute heme deficit in the
    CNS remains unsettled — see the knowledge gaps below.
  biological_scale: TISSUE
  downstream:
  - target: Cerebral White Matter Degeneration
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Sustained central exposure is the proposed driver of the white matter
      lesion, but the neuropathological mechanism connecting the two has not
      been established.
  - target: Peripheral axonal neuropathy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Length-dependent sensorimotor axonal neuropathy accompanies the central
      lesion, paralleling the peripheral neuropathy of AIP.
  chemical_entities:
  - preferred_term: porphobilinogen
    term:
      id: CHEBI:17381
      label: porphobilinogen
    modifier: INCREASED
  evidence:
  - reference: PMID:41377573
    reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We were not able to detect PBG in the CSF of patients unaffected by AIP."
    explanation: Establishes that CSF porphobilinogen is a disease-associated finding rather than a normal constituent.
  - reference: PMID:41731635
    reference_title: "Biallelic pathogenic hydroxymethylbilane synthase gene variants of a neurodegenerative disorder with progressive cystic leukoencephalopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In his urine, 5-aminolevulinic acid and porphobilinogen were markedly elevated, but in cerebrospinal fluid just porphobilinogen."
    explanation: Reports the CSF precursor profile in an independent severely affected case.

- name: Cerebral White Matter Degeneration
  description: >-
    A recognizable and consistent MRI pattern: confluent symmetrical signal
    abnormality of the periventricular and deep cerebral white matter and the
    bilateral thalami, with the central pons involved, progressing to cerebellar
    atrophy in advanced disease. In the severe infantile form the white matter
    change becomes cystic with progressive volume loss, and autopsy shows patchy
    gliosis and leukodystrophy.
  biological_scale: TISSUE
  downstream:
  - target: Spastic paraparesis
    causal_link_type: DIRECT
  - target: Cerebellar ataxia
    causal_link_type: DIRECT
  - target: Leukoencephalopathy
    causal_link_type: DIRECT
  - target: Cerebellar atrophy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Appears at advanced disease stages rather than at presentation, which is
      why it is drawn as a late consequence of the progressing lesion rather
      than a parallel finding.
    evidence:
    - reference: PMID:27558376
      reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Cerebellar atrophy was noted in advanced disease stages."
      explanation: Establishes cerebellar atrophy as a late feature of the progressing lesion.
  - target: Cognitive impairment
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The mild cognitive impairment seen in some adults is attributed to the
      deep white matter disease, though the anatomical correlation has not been
      demonstrated.
  - target: Seizure
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      In the severe infantile form, where the white matter change becomes cystic.
      Not a feature of the adult presentation.
  - target: Global developmental delay
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      In the severe infantile form. Persists after liver transplantation, which
      is consistent with fixed rather than ongoing injury.
  cell_types:
  - preferred_term: oligodendrocyte
    term:
      id: CL:0000128
      label: oligodendrocyte
  locations:
  - preferred_term: periventricular and deep cerebral white matter
    term:
      id: UBERON:0002316
      label: white matter
  - preferred_term: bilateral thalami
    term:
      id: UBERON:0001897
      label: dorsal plus ventral thalamus
  evidence:
  - reference: PMID:34089223
    reference_title: "Expanding the clinical and radiological phenotypes of leukoencephalopathy due to biallelic HMBS mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Their brain MRIs show mainly confluent signal abnormalities in the periventricular and deep white matter and bilateral thalami."
    explanation: Defines the anatomical distribution of the white matter lesion across six affected adults.
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Findings at autopsy of the brain showed patchy gliosis and leukodystrophy."
    explanation: Provides neuropathological confirmation of the leukodystrophy in the severe phenotype.

phenotypes:
- category: Neurologic
  name: Spastic paraparesis
  description: >-
    Slowly progressive spastic weakness of the lower limbs, present in every
    reported adult case and typically the presenting complaint.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Spastic paraparesis
    term:
      id: HP:0002313
      label: Spastic paraparesis
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:34089223
    reference_title: "Expanding the clinical and radiological phenotypes of leukoencephalopathy due to biallelic HMBS mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All six affected individuals presented with slowly progressive spasticity, ataxia, peripheral neuropathy, with or without mild cognitive impairment, and/or ocular disease with onset in childhood or adolescence."
    explanation: Reports progressive spasticity in all six adults in the largest published series.

- category: Neurologic
  name: Cerebellar ataxia
  description: >-
    Progressive gait and limb ataxia, often with dysarthria, and quantifiable on
    ataxia rating scales over years.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Progressive cerebellar ataxia
    term:
      id: HP:0002073
      label: Progressive cerebellar ataxia
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Clinical features were childhood-onset slowly progressive spastic paraparesis, cerebellar ataxia, peripheral neuropathy"
    explanation: Reports cerebellar ataxia as a core feature of the index kindred.

- category: Neurologic
  name: Peripheral axonal neuropathy
  description: >-
    Length-dependent sensorimotor peripheral neuropathy of axonal type.
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Peripheral axonal neuropathy
    term:
      id: HP:0003477
      label: Peripheral axonal neuropathy
  evidence:
  - reference: PMID:34089223
    reference_title: "Expanding the clinical and radiological phenotypes of leukoencephalopathy due to biallelic HMBS mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All six affected individuals presented with slowly progressive spasticity, ataxia, peripheral neuropathy"
    explanation: Reports peripheral neuropathy in all six adults in the series.

- category: Neurologic
  name: Leukoencephalopathy
  description: >-
    Symmetrical periventricular and deep cerebral white matter signal
    abnormality with thalamic and central pontine involvement; the recognizable
    imaging signature of the disorder.
  frequency: VERY_FREQUENT
  diagnostic: true
  phenotype_term:
    preferred_term: Leukoencephalopathy
    term:
      id: HP:0002352
      label: Leukoencephalopathy
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We identified 3 family members with a similar MRI pattern characterized by symmetrical signal abnormalities in the periventricular and deep cerebral white matter, thalami, and central part of the pons."
    explanation: Describes the diagnostic MRI pattern in the index family.

- category: Neurologic
  name: Cerebellar atrophy
  description: >-
    Appears in advanced disease stages rather than at presentation.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Cerebellar atrophy
    term:
      id: HP:0001272
      label: Cerebellar atrophy
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cerebellar atrophy was noted in advanced disease stages."
    explanation: Records cerebellar atrophy as a late rather than presenting feature.

- category: Ophthalmologic
  name: Optic atrophy
  description: >-
    Optic atrophy with progressive visual loss, in some individuals reaching
    near-complete blindness.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Optic atrophy
    term:
      id: HP:0000648
      label: Optic atrophy
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "in 2 patients, optic atrophy as well as vertical gaze and convergence palsies and nystagmus"
    explanation: Reports optic atrophy in two of three affected family members.
  - reference: PMID:41731635
    reference_title: "Biallelic pathogenic hydroxymethylbilane synthase gene variants of a neurodegenerative disorder with progressive cystic leukoencephalopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bilateral cataracts, optic nerve atrophy, and cerebellar abnormalities were also common."
    explanation: >-
      Supports the FREQUENT grading across the reported cases, rather than
      resting it on the two affected members of a single kindred.

- category: Ophthalmologic
  name: Nystagmus
  description: >-
    Named directly in the index kindred's clinical description.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Nystagmus
    term:
      id: HP:0000639
      label: Nystagmus
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "vertical gaze and convergence palsies and nystagmus"
    explanation: Names nystagmus among the ocular findings in the index family.

- category: Ophthalmologic
  name: Limited vertical extraocular movement
  description: >-
    Vertical gaze palsy. Bound to the movement-limitation term rather than a
    supranuclear one, because the source describes the deficit without
    establishing its localization.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Vertical gaze palsy
    term:
      id: HP:0025721
      label: Limited vertical extraocular movement
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "vertical gaze and convergence palsies and nystagmus"
    explanation: Names the vertical gaze palsy among the ocular findings in the index family.

- category: Ophthalmologic
  name: Impaired convergence
  description: >-
    Convergence palsy, reported alongside the vertical gaze palsy and nystagmus.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Convergence palsy
    term:
      id: HP:0000619
      label: Impaired convergence
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "vertical gaze and convergence palsies and nystagmus"
    explanation: Reports the specific eye movement abnormalities observed.

- category: Ophthalmologic
  name: Cataract
  description: >-
    Childhood-onset cataracts, reported in several individuals and sometimes the
    earliest recorded finding. Grouped with optic atrophy and cerebellar change
    as a common feature in the case report that summarises the thirteen prior
    published cases.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Childhood-onset cataract
    term:
      id: HP:0000518
      label: Cataract
  evidence:
  - reference: PMID:41377573
    reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "She also had childhood-onset cataracts."
    explanation: Documents childhood-onset cataract in a genotyped patient.
  - reference: PMID:41731635
    reference_title: "Biallelic pathogenic hydroxymethylbilane synthase gene variants of a neurodegenerative disorder with progressive cystic leukoencephalopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Bilateral cataracts, optic nerve atrophy, and cerebellar abnormalities were also common."
    explanation: >-
      The sentence the FREQUENT grading rests on. The single-patient item above
      establishes the phenotype exists; this one establishes it is common across
      the reported cases.


- category: Neurologic
  name: Cognitive impairment
  description: >-
    Mild cognitive impairment in some adults; cognition is explicitly normal in
    others with the same disorder.
  frequency: OCCASIONAL
  phenotype_term:
    preferred_term: Mild cognitive impairment
    term:
      id: HP:0100543
      label: Cognitive impairment
    severity: MILD
  evidence:
  - reference: PMID:34089223
    reference_title: "Expanding the clinical and radiological phenotypes of leukoencephalopathy due to biallelic HMBS mutations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "slowly progressive spasticity, ataxia, peripheral neuropathy, with or without mild cognitive impairment"
    explanation: Reports mild cognitive impairment as a variable feature.

- category: Neurologic
  name: Seizure
  description: >-
    A feature of the severe infantile-onset end of the spectrum, not of the
    adult leukoencephalopathy presentation.
  subtype: Severe infantile
  frequency: VERY_FREQUENT
  phenotype_term:
    preferred_term: Recurrent generalized seizures
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A male infant presented at three months of age with generalized ataxia, hypotonia, aspiration of liquids and recurrent generalized seizures."
    explanation: Reports seizures in the infantile-onset severe presentation.

- category: Neurologic
  name: Hypotonia
  subtype: Severe infantile
  frequency: VERY_FREQUENT
  description: >-
    Presenting sign in infantile-onset disease.
  phenotype_term:
    preferred_term: Hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  evidence:
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A male infant presented at three months of age with generalized ataxia, hypotonia, aspiration of liquids and recurrent generalized seizures."
    explanation: Reports hypotonia at presentation in the severe infantile phenotype.

- category: Neurologic
  name: Global developmental delay
  subtype: Severe infantile
  frequency: VERY_FREQUENT
  description: >-
    Severe, persistent developmental delay in the infantile-onset form,
    unimproved by liver transplantation.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Developmental delays and seizures persisted."
    explanation: Records persistent developmental delay after attempted disease-modifying therapy.

has_subtypes:
- name: Adult leukoencephalopathy
  display_name: Childhood/adolescent-onset slowly progressive leukoencephalopathy
  description: >-
    Onset in childhood or adolescence with slowly progressive spastic
    paraparesis, cerebellar ataxia, peripheral neuropathy and ocular disease,
    survival into late adulthood, and often preserved independence for decades.
- name: Severe infantile
  display_name: Severe infantile-onset encephalopathy
  description: >-
    Presentation in the first months or years with hypotonia, developmental
    delay, epilepsy and progressive cystic leukoencephalopathy, and early death.
    Severity appears to track the residual enzyme activity conferred by the
    particular genotype.

genetic:
- name: HMBS
  notes: >-
    Biallelic (homozygous or compound heterozygous) variants in HMBS, encoding
    hydroxymethylbilane synthase (porphobilinogen deaminase). The same gene in
    the monoallelic state causes autosomal dominant acute intermittent
    porphyria; several of the alleles found in biallelic leukoencephalopathy are
    themselves known AIP alleles, so the two disorders are allelic and
    distinguished by dose rather than by variant class.
  gene_term:
    preferred_term: HMBS
    term:
      id: hgnc:4982
      label: HMBS
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole-exome sequencing revealed compound heterozygous missense variants in the HMBS gene, both associated with the autosomal dominant disorder acute intermittent porphyria."
    explanation: Identifies HMBS as the causal gene and notes that the alleles are known AIP alleles.

animal_models:
- name: PBGD-deficient mouse
  species: Mouse
  genotype: Pbgd-deficient (PBGD-/-)
  publication: PMID:10207164
  description: >-
    A partial porphobilinogen deaminase deficiency mouse, generated to study the
    neuropathy of acute porphyria. Its value here is discriminating rather than
    confirmatory: the neuropathy developed chronically and progressively while
    plasma and urinary ALA were normal or only twofold raised, which is the
    opposite of what a precursor-toxicity model predicts, and the authors
    conclude heme deficiency and hemeprotein dysfunction can cause the
    neuropathy.
  modeled_mechanisms:
  - target: Peripheral axonal neuropathy
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Reproduces the axonal, non-demyelinating motor neuropathy seen in affected
      human nerves, and does so without a large precursor elevation.
    limitations: >-
      A partial deficiency modelling monoallelic acute porphyria rather than the
      biallelic human genotype, and the phenotype is peripheral. It does not
      reproduce the central leukodystrophy, so it speaks to the neuropathy node
      and to the mechanism question, not to the white matter lesion.
    readouts:
    - name: Large-caliber axon counts in femoral nerve
      target: Peripheral axonal neuropathy
      direction: DECREASED
      interpretation: >-
        A marked decrease in large-caliber axons with ultrastructural features of
        primary motor axon degeneration and no primary demyelination.
      evidence:
      - reference: PMID:10207164
        reference_title: "Motor neuropathy in porphobilinogen deaminase-deficient mice imitates the peripheral neuropathy of human acute porphyria."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Histologically femoral nerves of PBGD-/- mice exhibit a marked decrease in large-caliber (>8 microm) axons and ultrastructural changes consistent with primary motor axon degeneration"
        explanation: Reports the histological measurement grounding the neuropathy claim in this model.
    evidence:
    - reference: PMID:10207164
      reference_title: "Motor neuropathy in porphobilinogen deaminase-deficient mice imitates the peripheral neuropathy of human acute porphyria."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These findings resemble those found in studies of affected nerves of patients with acute porphyria and thus provide strong evidence that PBGD deficiency causes degeneration of motor axons without signs of primary demyelination"
      explanation: Establishes the model as informative for the axonal neuropathy of HMBS deficiency.
    - reference: PMID:10207164
      reference_title: "Motor neuropathy in porphobilinogen deaminase-deficient mice imitates the peripheral neuropathy of human acute porphyria."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "the neuropathy in PBGD-/- mice developed chronically and progressively and in the presence of normal or only slightly (twofold) increased plasma and urinary levels of the putative neurotoxic heme precursor delta-aminolevulinic acid. These data suggest that heme deficiency and consequent dysfunction of hemeproteins can cause porphyric neuropathy."
      explanation: >-
        Carries both claims the model description makes - that the neuropathy
        arose without a large ALA elevation, and that the authors read this as
        implicating heme deficiency rather than precursor toxicity.
    - reference: PMID:8563760
      reference_title: "Porphobilinogen deaminase deficiency in mice causes a neuropathy resembling that of human hepatic porphyria."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Behavioural tests reveal decreased motor function and histopathological findings include axonal neuropathy and neurologic muscle atrophy."
      explanation: The original description of this mouse, cited alongside the 1999 follow-up that characterized the neuropathy in detail.

- name: Biallelic R167Q HMBS mouse
  species: Mouse
  genotype: Hmbs R167Q homozygous
  publication: PMID:41377573
  description: >-
    The closest genetic model of the biallelic human disease, and the one that
    matches the therapeutic failure. It carries constitutively high PBG and ALA
    in brain tissue and CSF, has normal ALAS1 transcript before and after
    porphyrinogenic stimuli, and does not respond to haem provision — the same
    three features that make hepatically directed therapy ineffective in
    patients.
  modeled_mechanisms:
  - target: Constitutive CNS Porphyrin Precursor Exposure
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Reproduces constitutive central precursor elevation that is independent of
      ALAS1 induction and unresponsive to haem, which is the pharmacological
      situation in patients.
    limitations: >-
      This link covers the biochemical compartment only. The same mouse does not
      develop the white matter lesion - CNS histopathology is reported as normal
      - which is curated as a separate FAILS_TO_RECAPITULATE link below.
    evidence:
    - reference: PMID:41377573
      reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "R167Q mouse has constitutively high concentrations of PBG and ALA in brain tissue and CSF"
      explanation: Establishes the model as reproducing the constitutive central precursor elevation seen in patients.
    - reference: PMID:41377573
      reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Biallelic R167Q mice have normal levels of ALAS1 mRNA transcript before and after porphyrinogenic stimuli and are non‐responsive to haem provision"
      explanation: >-
        Records the absent ALAS1 induction and the haem non-response, which
        together parallel why hepatically directed therapy fails in patients.
  - target: Cerebral White Matter Degeneration
    relationship: FAILS_TO_RECAPITULATE
    description: >-
      The closest genetic model of the human disease does not reproduce its
      defining lesion. CNS histopathology is normal in the biallelic R167Q
      mouse and brain haem content is about 80% of wild type, despite the
      constitutive central precursor elevation the same animal shows. That is a
      reported negative result rather than absent data, and it cuts against the
      simplest version of both competing hypotheses: neither the precursor
      burden nor a central haem deficit was sufficient to produce white matter
      pathology in this mouse.
    limitations: >-
      A mouse CNS is not a human one and the reported observation is
      histopathology at whatever age it was assessed, not a lifelong
      neuroimaging course; the human disease is slowly progressive over decades.
      So this constrains the mechanism rather than excluding either hypothesis.
    evidence:
    - reference: PMID:41377573
      reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
      supports: REFUTE
      evidence_source: MODEL_ORGANISM
      snippet: "in the biallelic R167Q mouse, brain haem content was ~80% that of the wild‐type mice and CNS histopathology was normal"
      explanation: >-
        Refutes the claim that this model reproduces the leukodystrophy, and
        supplies the brain haem measurement that bears on the CNS-haem-deficit
        hypothesis.

diagnosis:
- name: Sequencing with explicit zygosity determination
  description: >-
    The diagnosis turns on zygosity, not on finding an HMBS variant. A single
    pathogenic HMBS allele is acute intermittent porphyria; two are this
    disease, with a different inheritance pattern, a different clinical
    syndrome and - critically - different treatment expectations. Several of
    the alleles seen in biallelic patients are themselves known AIP alleles, so
    a report naming a familiar AIP variant does not settle which disorder is
    present. Confirming biallelic status, and segregation where family members
    are available, is the diagnostic step.
  evidence:
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole-exome sequencing revealed compound heterozygous missense variants in the HMBS gene"
    explanation: Exome sequencing establishing biallelic status is how the index kindred was diagnosed.
  - reference: PMID:27558376
    reference_title: "Acute intermittent porphyria-related leukoencephalopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Sanger sequencing of 6 healthy siblings confirmed the bi-allelic location of the variants and segregation with the disease."
    explanation: Segregation analysis is what confirmed the two variants sit on different alleles.

- name: Erythrocyte hydroxymethylbilane synthase activity
  description: >-
    Supports the molecular result and separates affected individuals from
    carriers. Activity is markedly reduced in patients and roughly half of
    normal in heterozygous parents, so the assay discriminates the biallelic
    state from the AIP carrier state that the same family often contains.
  evidence:
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Activity of hydroxymethylbilane synthase in red blood cells of the child was markedly reduced"
    explanation: >-
      Records the erythrocyte enzyme result in the affected child. The same
      sentence gives roughly 50% of normal in the parents, which is what makes
      the assay discriminate the biallelic state from the carrier state; that
      figure sits inside a bracketed reference range and so is not quotable as
      a contiguous snippet.

- name: Cerebrospinal fluid porphyrin precursor measurement
  description: >-
    The measurement that distinguishes this disorder mechanistically and guides
    therapy. Porphobilinogen is detectable in the CSF of affected individuals
    and undetectable in unaffected controls, and CSF concentrations do not fall
    after liver transplantation or haem arginate. A normal peripheral profile
    does not exclude the central abnormality, which matters because peripheral
    elevation in the adult phenotype is only slight to moderate.
  evidence:
  - reference: PMID:41377573
    reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "ALA and PBG were measured in urine and CSF using the Mauzerell and Granick method"
    explanation: Records the CSF assay used, which is not part of a routine porphyria workup.
  - reference: PMID:41377573
    reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We were not able to detect PBG in the CSF of patients unaffected by AIP."
    explanation: Establishes CSF porphobilinogen as a disease-associated finding rather than a normal constituent.

prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Fourteen reported individuals - thirteen previously published plus the index
    case of the most recent report. No population-based estimate exists.
  evidence:
  - reference: PMID:41377573
    reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Thirteen patients with leukodystrophy due to biallelic HMBS variants have been previously described."
    explanation: Gives the countable size of the reported literature.

biochemical:
- name: Cerebrospinal fluid porphobilinogen
  biomarker_term:
    preferred_term: cerebrospinal fluid porphobilinogen
    term:
      id: CHEBI:17381
      label: porphobilinogen
  presence: Elevated
  notes: >-
    Porphobilinogen is detectable and constitutively elevated in the CSF of
    affected individuals and undetectable in unaffected controls, and it is not
    lowered by liver transplantation or haem arginate. This is the single most
    important biochemical observation in the disorder, because it is what
    separates it from AIP therapeutically.
  evidence:
  - reference: PMID:41377573
    reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There was no difference between pre-transplant porphyrin precursor levels and post-transplant levels in either the peripheral or central compartment."
    explanation: Demonstrates that the central and peripheral precursor burden is independent of hepatic HMBS.

treatments:
- name: Liver Transplantation
  description: >-
    Attempted in several patients on the AIP rationale that replacing hepatic
    HMBS removes the precursor source. It does not work here, but the reason is
    sharper than "nothing changed". In one adult, precursor concentrations were
    unchanged in both compartments after transplantation. In an infant, hepatic
    correction was substantial - about a 50% reduction in ALA, PBG and
    uroporphyrin overproduction two years on - and the neurological decline
    continued anyway. Halving the hepatic output without arresting the disease
    is stronger evidence for an extrahepatic or CNS-autonomous precursor source
    than an absent biochemical response would be.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: liver transplantation
    term:
      id: NCIT:C15289
      label: Organ Transplantation
  target_mechanisms:
  - target: Porphyrin Precursor Accumulation
    description: >-
      Intended to remove the hepatic source of porphyrin precursors; in this
      disorder it demonstrably fails to lower them.
    evidence:
    - reference: PMID:41377573
      reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
      supports: REFUTE
      evidence_source: HUMAN_CLINICAL
      snippet: "Liver transplantation and hepatically directed therapies are not likely to be effective for leukodystrophy due to biallelic HMBS variants."
      explanation: Explicitly refutes the hepatic-source rationale for treating this disorder.
  evidence:
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "Orthotopic liver transplantation at the age of 15 months led to only mild transient improvement."
    explanation: Reports failure of liver transplantation to alter the disease course in the severe infantile form.
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "with only about a 50% reduction in the marked overproduction of ALA, PBG, and uroporphyrin, compared with his pre-transplant status"
    explanation: >-
      Quantifies the hepatic correction that transplantation did achieve in this
      patient. It refutes the therapy because halving precursor overproduction
      still did not arrest the neurological decline, which argues the residual
      source is not hepatic.
  - reference: PMID:34089223
    reference_title: "Expanding the clinical and radiological phenotypes of leukoencephalopathy due to biallelic HMBS mutations."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: "One individual reported here had improved neurological function for 12 months following liver transplantation followed by decline and progression of disease."
    explanation: Records the transient benefit that motivated further attempts, before decline resumed; supports the intervention only as a temporary effect.

- name: Intravenous Haem Therapy
  description: >-
    Haem arginate or intravenous heme, the standard treatment for acute attacks
    in AIP, does work biochemically in the periphery - roughly 50% transient
    reductions in urinary ALA, PBG and porphyrins - but does not lower CSF
    porphyrin precursors and produces at most transient subjective clinical
    improvement. That dissociation is the point: the peripheral compartment
    responds and the central one does not, because parenterally administered
    heme does not reach the central nervous system. It is the same compartmental
    argument the liver transplantation entry makes.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: haem arginate
      term:
        id: CHEBI:30413
        label: heme
  target_mechanisms:
  - target: Constitutive CNS Porphyrin Precursor Exposure
    description: >-
      Intended to suppress ALAS1 by negative feedback and so reduce precursor
      production; it does not reduce the central precursor burden.
    evidence:
    - reference: PMID:41377573
      reference_title: "Liver Transplantation and Other Hepatically Directed Therapies Do Not Change the Biochemical Phenotype nor Halt Progression of Leukodystrophy due to Biallelic HMBS Variants: A Case Report."
      supports: REFUTE
      evidence_source: HUMAN_CLINICAL
      snippet: "porphyrin precursor levels are constitutively elevated in the cerebrospinal fluid and are not reduced by haem arginate therapy"
      explanation: Directly refutes that haem arginate lowers the central precursor burden.
  evidence:
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "A 5-day course of intravenous heme led to no observable clinical improvement."
    explanation: Reports absence of clinical benefit from intravenous heme.
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: "intravenous hematin, given daily for 5 days, had led to transient ∼50% reductions in urinary ALA, PBG, and porphyrins."
    explanation: >-
      Establishes that haem does work biochemically in the peripheral
      compartment. Recorded as SUPPORT for the peripheral effect, which is what
      makes the absent central and clinical response a dissociation rather than
      simple inactivity.

- name: Antiseizure Therapy with Non-Porphyrinogenic Agents
  description: >-
    Seizures in the infantile-onset form require antiseizure drugs, and the
    choice is constrained by porphyria pharmacology: many older anticonvulsants
    induce hepatic heme synthesis and are avoided in porphyria. Levetiracetam
    does not appear to exacerbate AIP and was used in the reported infant, who
    was also started on phenobarbital and then tapered off it precisely because
    barbiturates induce hepatic ALAS1. Note that the rationale is inherited from
    AIP practice rather than demonstrated in this disorder, where hepatic
    induction is not the driver - which makes the case for avoiding barbiturates
    here weaker in principle than in AIP, though it was acted on.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: levetiracetam
      term:
        id: CHEBI:6437
        label: levetiracetam
  evidence:
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Levetiracetam, which does not appear to exacerbate AIP, has been a welcome recent addition to the pharmacopoeia."
    explanation: Identifies levetiracetam as the non-porphyrinogenic option, which is why it was the drug used.
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "He was treated with levetiracetam and phenobarbital."
    explanation: >-
      Records the antiseizure regimen the infant was started on. Note this
      included phenobarbital, which was subsequently withdrawn - see the next
      item.
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Because of the known risks of barbiturates in AIP, his medication regimen was adjusted, including tapering off phenobarbital, which is well-known to increase hepatic ALA synthase-1 and over-production of ALA and PBG"
    explanation: >-
      The drug-safety reversal. Phenobarbital was tapered off precisely because
      barbiturates induce hepatic ALAS1; without this the entry would appear to
      endorse a barbiturate under a treatment named for non-porphyrinogenic
      agents.

- name: Supportive and Symptomatic Care
  description: >-
    In the absence of disease-modifying therapy, management is symptomatic:
    antiepileptic drugs, spasticity and ataxia management, rehabilitation, and
    visual support.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:42396593
    reference_title: "Limited benefit of liver transplantation in a boy with biallelic severe deficiency of hydroxymethylbilane synthase and review of prior reported cases."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "No effective disease-modifying therapy currently exists for biallelic HMBS deficiency."
    explanation: Establishes that no disease-modifying option is available, leaving supportive care as the standard.

discussions:
- discussion_id: precursor_toxicity_vs_cns_heme_deficit
  kind: KNOWLEDGE_GAP
  prompt: >-
    Is the white matter injury caused by accumulated porphyrin precursors
    (ALA/PBG) acting in the CNS, or by an absolute deficit of heme within the
    central nervous system?
  attaches_to:
  - pathophysiology#Constitutive CNS Porphyrin Precursor Exposure
  - pathophysiology#Cerebral White Matter Degeneration
  rationale: >-
    These are the two standing hypotheses and they are not equivalent
    therapeutically: precursor toxicity would motivate an agent that lowers
    ALA/PBG inside the CNS, whereas a central heme deficit would require heme or
    a heme surrogate that crosses into the brain, which parenteral heme does
    not. The failure of both liver transplantation and haem arginate to alter
    CSF precursor levels or the clinical course is consistent with either, so
    the therapeutic failures do not discriminate between them. The edge from
    central precursor exposure to white matter degeneration is recorded as
    INDIRECT for this reason.

    Two natural experiments narrow it without closing it. Doss porphyria,
    caused by biallelic ALAD variants, raises ALA but not PBG and produces
    severe peripheral polyneuropathy with no CNS involvement - so ALA elevation
    alone appears insufficient for white matter disease, and PBG becomes the
    likelier candidate of the two precursors. Separately, the PBGD-deficient
    mouse develops a chronic progressive neuropathy while ALA is normal or only
    twofold raised, which its authors read as implicating heme deficiency. The
    two point in different directions, which is why the gap stays open.
  evidence:
  - reference: PMID:41731635
    reference_title: "Biallelic pathogenic hydroxymethylbilane synthase gene variants of a neurodegenerative disorder with progressive cystic leukoencephalopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the heme precursors 5-aminolevulinic acid or porphobilinogen originating from the liver or central nervous system could be causative, and absolute heme deficiency in the central nervous system is another hypothesis"
    explanation: States both competing hypotheses as unresolved in the current literature.
  - reference: PMID:41731635
    reference_title: "Biallelic pathogenic hydroxymethylbilane synthase gene variants of a neurodegenerative disorder with progressive cystic leukoencephalopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients with Doss porphyria, caused by biallelic pathogenic ALAD variants, show high elevations of ALA and coproporphyrin but not of PBG and uroporphyrin. They often exhibit severe peripheral polyneuropathy, but they do not develop an affection of the CNS."
    explanation: >-
      The human biallelic natural experiment this rationale argues from. ALA
      elevation without PBG elevation gives peripheral neuropathy and no CNS
      disease, which is what makes PBG the likelier of the two precursors.
  - reference: PMID:11478735
    reference_title: "5-Aminolevulinic acid inhibits [3H]muscimol binding to human and rat brain synaptic membranes."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    snippet: "The similarity in the potency and mechanism of the ALA-induced inhibition of muscimol binding in rat and human membranes indicate that rat studies are useful to evaluate the neurotoxic properties of ALA towards the human GABAergic system"
    explanation: >-
      Gives the precursor-toxicity arm of this gap a concrete molecular
      candidate - ALA antagonism of the GABA-A receptor, shown in human as well
      as rat cortical membranes. Indirect for this disorder because the
      experiment used millimolar ALA on synaptic membranes from unaffected
      brain, and was framed around acute intermittent porphyria rather than the
      constitutive, white-matter-predominant biallelic disease; whether the
      concentrations reached in LENCEP cerebrospinal fluid approach this range
      has not been measured.

- discussion_id: no_animal_model_of_the_biallelic_cns_disease
  kind: KNOWLEDGE_GAP
  prompt: >-
    Is there an animal model that reproduces the central biallelic HMBS
    leukodystrophy, as opposed to the peripheral neuropathy and the biochemical
    compartment, both of which are already modelled?
  attaches_to:
  - pathophysiology#Cerebral White Matter Degeneration
  - pathophysiology#Constitutive CNS Porphyrin Precursor Exposure
  rationale: >-
    Two mouse models exist and are curated in animal_models, but neither
    reproduces the white matter lesion. The PBGD-deficient mouse models the
    peripheral axonal neuropathy; the biallelic R167Q mouse reproduces the
    constitutive central precursor elevation and the haem non-response but has
    normal CNS histopathology, so it explicitly does not reproduce the white
    matter lesion. So what is missing specifically is a model of the
    leukodystrophy - a system in
    which CNS precursor concentrations can be raised or lowered independently of
    hepatic heme synthesis while the white matter is watched. That is also the
    platform any CNS-penetrant precursor-lowering strategy would need.

- discussion_id: aip_therapies_do_not_transfer
  kind: KNOWLEDGE_GAP
  prompt: >-
    Can any agent lower porphyrin precursors in the central compartment, given
    that hepatically directed therapy demonstrably cannot?
  attaches_to:
  - treatments#Liver Transplantation
  - treatments#Intravenous Haem Therapy
  rationale: >-
    Givosiran, the ALAS1 siRNA that prevents attacks in AIP, is hepatically
    targeted, and access was declined for one post-transplant patient on exactly
    that ground. Since CSF precursors do not fall after liver transplantation,
    there is no reason to expect a liver-directed siRNA to reach the relevant
    compartment either. A CNS-penetrant strategy is what this disorder needs and
    none has been reported.
  evidence:
  - reference: PMID:41731635
    reference_title: "Biallelic pathogenic hydroxymethylbilane synthase gene variants of a neurodegenerative disorder with progressive cystic leukoencephalopathy: a case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "However, parenterally administered heme, which is effective in acute intermittent porphyria, does not reach the central nervous system."
    explanation: Identifies the pharmacological barrier that makes AIP therapies inapplicable here.

- discussion_id: genotype_severity_correlation
  kind: KNOWLEDGE_GAP
  prompt: >-
    What determines whether a biallelic HMBS genotype produces the survivable
    adult leukoencephalopathy or the lethal infantile encephalopathy?
  attaches_to:
  - has_subtypes#Adult leukoencephalopathy
  - has_subtypes#Severe infantile
  rationale: >-
    Residual enzyme activity conferred by the specific allele pair is the
    presumed determinant, but with fewer than fifteen reported individuals the
    genotype-phenotype relationship rests on very few observations, and
    intrafamilial variability has been noted even for identical genotypes.

notes: >-
  Distinguish carefully from acute intermittent porphyria. AIP is the autosomal
  dominant, low-penetrance disease of monoallelic HMBS variants, presenting with
  episodic neurovisceral attacks; dismech curates it separately as
  Acute_Intermittent_Porphyria. LENCEP is a different clinical entity produced
  by the same gene in the biallelic state. Most reported individuals have no
  AIP attacks, though a minority do and fleeting neurovisceral pain is
  described; what distinguishes them is that none has been reported to have
  attacks in response to classical porphyrinogenic stimuli. The distinction
  matters
  therapeutically: haem arginate and hepatically directed therapy work in AIP
  and do not work here.

references:
- reference: PMID:27558376
  title: Acute intermittent porphyria-related leukoencephalopathy.
- reference: PMID:34089223
  title: Expanding the clinical and radiological phenotypes of leukoencephalopathy due to biallelic HMBS mutations.
📚

References & Deep Research

References

2
PMID:27558376
Acute intermittent porphyria-related leukoencephalopathy.
No top-level findings curated for this source.
PMID:34089223
Expanding the clinical and radiological phenotypes of leukoencephalopathy due to biallelic HMBS mutations.
No top-level findings curated for this source.

Deep Research

1

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Evaluations and curation notes (1)

Create: Porphyria-Related_Leukoencephalopathy (biallelic HMBS) · 2026-09-02T03:15:54Z · View source

De novo curation of porphyria-related leukoencephalopathy / LENCEP (MONDO:0958226), the autosomal recessive disorder of biallelic HMBS variants. Distinct from acute intermittent porphyria, which dismech already curates separately as Acute_Intermittent_Porphyria and which is the monoallelic, dominant, attack-based disease from the same gene. Entry-type decision. The claim issue asked whether this should be a Disease or folded into Acute_Intermittent_Porphyria as a phenotype or subtype. Curated as its own Disease. The reported individuals do not have AIP attacks at all, the MRI lesion is a recognizable and reproducible pattern across independent kindreds, the inheritance mode differs, and - decisively - the therapies that work in AIP do not work here. Folding it into AIP would attach a false prognosis and a false treatment expectation to it. The Acute_Intermittent_Porphyria entry mentions no leukoencephalopathy or white matter change, so nothing was duplicated. Deep research: openscientist (research/Porphyria-Related_Leukoencephalopathy-deep-research-openscientist.md). Falcon, the repo default, was unavailable (no EDISON_API_KEY/FUTUREHOUSE_API_KEY); openscientist was chosen explicitly rather than substituted silently. The report shipped without reference_validation or term_validation frontmatter, so both were retro-fitted in place. References: 32/32 resolved, 0 unresolved, 0 off topic, 15/17 quoted claims found in source. The 2 quote failures are false positives - the validator prints an identical 'closest text in source' for both, so the mismatch is whitespace or encoding, not fabrication. Terms: 37 checked, 0 unresolved, 0 obsolete; the 3 'named as a different term' entries are markdown-formatting artifacts where the extractor captured surrounding prose as the label, not wrong bindings. Two things were taken from the report, each verified against a freshly fetched primary source rather than cited to the report. First, PMID:11478735, showing that ALA inhibits muscimol binding at GABA-A receptors in human as well as rat cortical membranes; this gives the precursor-toxicity arm of the central knowledge gap a concrete molecular candidate and is graded SUPPORT with directness INDIRECT, because the experiment used millimolar ALA on membranes from unaffected brain and was framed around AIP rather than this disorder. Second, the observation that no animal model reproduces the biallelic CNS leukodystrophy, recorded as its own KNOWLEDGE_GAP explaining why this entry carries no animal_models block and why the two competing pathogenic hypotheses remain untestable. Deliberately not taken from the report: its ALAS1 de-repression step. That is the AIP attack mechanism, and it is specifically what does not apply here - liver transplantation leaves CSF precursor concentrations unchanged. Writing it into this entry would have contradicted the entry's own evidence. Also not taken: the report's PRES material and the AIP-with-PRES case reports surfaced during search (for example PMID:40186107, PMID:36597305), which describe the reversible monoallelic phenocopy and belong to Acute_Intermittent_Porphyria, not here. Pathograph: four nodes from biallelic HMBS deficiency through porphyrin precursor accumulation and constitutive CNS precursor exposure to cerebral white matter degeneration. The last two edges are INDIRECT_UNKNOWN_INTERMEDIATES on purpose - the neuropathological step is explicitly unestablished in the literature and the entry should not imply otherwise. Both failed therapies are curated as treatments with REFUTE evidence rather than omitted, because 'liver transplantation does not help' is the most clinically actionable fact about this disorder. The transient 12-month improvement in one patient is carried separately as SUPPORT with directness INDIRECT, since it is what motivated further attempts. Validation: just validate passes (schema, terms, references); 34/34 snippets verified. check-entity-refs, check-causal-targets, check-duplicate-keys and check-enum-values all pass. Compliance 90.9% global / 91.7% weighted before the final two additions. No datasets: block; no transcriptomic or omics dataset for this disorder was identified, and searching HMBS returns the acute intermittent porphyria literature.

OpenScientist ▸
Porphyria-Related Leukoencephalopathy — Comprehensive Disease Report
openscientist-autonomous 31 citations 2026-09-01T23:12:50.585416

Porphyria-Related Leukoencephalopathy — Comprehensive Disease Report

Category: Mendelian (with an overlapping acquired/reversible phenocopy) Suggested MONDO context: the biallelic-HMBS Mendelian entity is closest to "acute intermittent porphyria-related leukoencephalopathy" (Kevelam et al. 2016); the reversible entity maps to porphyria + posterior reversible encephalopathy syndrome (PRES).

Summary

"Porphyria-Related Leukoencephalopathy" is not a single disease but an umbrella covering two mechanistically distinct white-matter conditions that share a common biochemical root — deficient hydroxymethylbilane synthase (HMBS, also called porphobilinogen deaminase, PBGD) and neurotoxic accumulation of the heme precursor 5-aminolevulinic acid (ALA). The first entity (A) is a reversible posterior reversible encephalopathy syndrome (PRES) occurring during acute neurovisceral attacks of monoallelic autosomal-dominant acute intermittent porphyria (AIP). The second entity (B) is a true Mendelian, autosomal-recessive, childhood-onset progressive leukodystrophy caused by biallelic pathogenic HMBS variants — the eponymous "acute intermittent porphyria-related leukoencephalopathy" first defined by Kevelam et al. in 2016 (PMID: 27558376).

The distinction is clinically decisive. Entity A is an acute, largely reversible vasogenic-edema syndrome that resolves with attack management (avoidance of triggers, IV dextrose, IV hemin) and can be prevented with the liver-directed siRNA givosiran. Entity B is a slowly (or, in infantile forms, rapidly) progressive neurodegenerative leukodystrophy in which porphyrin precursors are constitutively elevated in cerebrospinal fluid and are not corrected by hepatically directed therapies — liver transplantation and haem arginate do not change the CSF biochemical phenotype nor halt progression (PMID: 41377573). This means the two entities require fundamentally different management framing, and a knowledge-base entry must keep them separate while acknowledging their shared enzyme and shared neurotoxic metabolite.

Across five iterations, seven findings were confirmed and 65 papers reviewed. The unifying causal chain is: HMBS loss of function → hepatic (or systemic) de-repression of ALAS1 → overproduction of ALA and porphobilinogen (PBG) → ALA neurotoxicity (GABA-A receptor interference) and endothelial/blood-brain-barrier dysfunction → white-matter edema (reversible PRES) or progressive demyelination/cystic leukoencephalopathy (Mendelian form). The report below organizes the evidence across the requested template sections.

Key Findings

Finding 1 — Two distinct entities under one name

Porphyria-related leukoencephalopathy comprises (A) reversible PRES during acute hepatic porphyria attacks and (B) a progressive biallelic-HMBS leukodystrophy. A systematic review of 46 patients with acute hepatic porphyria (AHP) complicated by PRES found a strong female predominance (84.8% women), young median age (24 ± 13.8 years), AIP as the most common subtype (41.3%), and a posterior-predominant lesion distribution — occipital (81.4%), parietal (65.1%), frontal (60.5%), subcortical (40%), and cortical (32.5%) (PMID: 31706631). Cerebral vasoconstriction was demonstrated in 41.7% of patients who underwent angiography, 19.6% had ischemic lesions, and only 4.3% developed long-term sequelae — underscoring the generally reversible nature of entity A.

"The most common distributions of brain lesions were occipital (81.4%), parietal (65.1%), frontal (60.5%), subcortical (40%), and cortical (32.5%). Cerebral vasoconstriction was demonstrated in 41.7% of the patients who underwent angiography. 19.6% of the patients had ischemic lesions, and 4.3% developed long-term sequelae" — PMID: 31706631

By contrast, the Mendelian entity is a categorically different disease. As stated in the definitive natural-history/therapy report:

"Leukodystrophy due to biallelic HMBS variants is a rare condition distinct from acute intermittent porphyria (AIP). It is characterised by progressive leukoencephalopathy rather than acute attacks of neurovisceral symptoms." — PMID: 41377573

Finding 2 — HMBS is the causal gene; allele dosage determines phenotype

HMBS (hydroxymethylbilane synthase; HGNC:4982; chromosome 11q23.3; OMIM 609806) encodes the third enzyme of heme biosynthesis, porphobilinogen deaminase. More than 400 (some databases list >1000) pathogenic HMBS variants have been reported. Heterozygous (monoallelic) loss of function → autosomal-dominant AIP with very low penetrance. Biallelic loss of function → autosomal-recessive leukodystrophy.* Both conditions arise from the same molecular defect — loss of HMBS protein function — with allele dosage governing severity:

"deficiency of HMBS is associated with both autosomal dominant acute intermittent porphyria (AIP) and autosomal recessive homozygous dominant AIP (HD-AIP). Yet, both conditions result from loss-of-function of the HMBS protein" — PMID: 42107342

The population genetics reveal a striking penetrance gap: the minimal estimated prevalence of pathogenic HMBS carriers is ~1/1299, yet symptomatic disease is far rarer (general-population penetrance 0.5–1%, versus ~22.9% in AIP families) (PMID: 29360981). In silico and in vitro analyses estimate that only ~1% of carriers of likely-pathogenic mutations ever develop acute attacks — highlighting the essential role of modifier genes and environmental triggers (PMID: 27539938).

"The minimal estimated prevalence of AIP in the general population was 1/1299" — PMID: 29360981

Biallelic pathogenic HMBS variants documented in leukodystrophy include c.251C>A (p.Ala84Asp), c.674G>A, and c.517C>T (p.Arg173Trp). A recent case documented biallelic HMBS causing progressive cystic leukoencephalopathy with elevated ALA/PBG in urine and PBG in CSF:

"progressive cystic leukoencephalopathy and neurological decay. In his urine, 5-aminolevulinic acid and porphobilinogen were markedly elevated, but in cerebrospinal fluid just porphobilinogen" — PMID: 41731635

Finding 3 — Mechanism: hepatic ALA overproduction drives neurotoxicity and BBB dysfunction

HMBS deficiency in hepatocytes reduces heme output, which de-represses ALAS1 (5-aminolevulinic acid synthase-1), the rate-limiting enzyme of heme synthesis, causing marked hepatic overproduction of ALA and PBG:

"This deficiency leads to de-repression of the first and normally rate-controlling enzyme of the heme synthetic pathway, delta- or 5-aminolevulinic acid [ALA] synthase-1, and thus to marked up-regulation of this key enzyme and to marked hepatic overproduction of ALA" — PMID: 30987916

ALA is directly neurotoxic. It interferes with GABAergic neurotransmission by inhibiting muscimol binding at GABA-A receptors in both rat and human brain synaptic membranes with comparable potency (IC50 199 vs 228 µM), a proposed basis for the seizures and encephalopathy:

"ALA (0.1-10 mM) significantly inhibited the binding of [3H]muscimol (12 nM), with a similar potency in rat and human membranes (IC50 = 199 vs. 228 microM, respectively)" — PMID: 11478735

Clinically, acute encephalopathy in AHP manifests as a triad of seizures, confusion, and/or blurred vision, with PRES detected in 42% of attacks and severe hyponatremia in 88% — pointing to acute endothelial dysfunction and blood-brain-barrier breakdown (PMID: 36757574). The PRES lesions are typically reversible vasogenic edema — T2/FLAIR hyperintensity without diffusion restriction — reflecting transient BBB compromise (PMID: 31649773).

Finding 4 — Mouse models recapitulate porphyric motor axonal neuropathy

The Pbgd-deficient (Pbgd−/−) mouse faithfully models the biochemistry of human AIP: decreased hepatic Pbgd activity, increased ALA synthase activity, and massively increased urinary ALA after phenobarbital induction (PMID: 8563760). Functionally, these mice develop impaired motor coordination, muscle weakness, and primary motor-axon degeneration:

"femoral nerves of PBGD-/- mice exhibit a marked decrease in large-caliber (>8 microm) axons and ultrastructural changes consistent with primary motor axon degeneration, secondary Schwann cell reactions, and axonal regeneration" — PMID: 10207164

"These mice exhibit the typical biochemical characteristics of human AIP, notably, decreased hepatic Pbgd activity, increased delta-aminolevulinic acid synthase activity and massively increased urinary excretion of the heme precursor, delta-aminolevulinic acid after treatment with drugs such as phenobarbital" — PMID: 8563760

Hepatocyte transplantation of wild-type cells reduced plasma ALA/PBG by ~50% with only 2.7% engraftment — providing proof of concept that even partial hepatic correction lowers systemic precursors (PMID: 23582197). Notably, these models capture the peripheral axonal neuropathy of AIP but do not, to date, reproduce the central biallelic leukodystrophy — a key model-organism gap.

Finding 5 — Diagnosis and the distinct Mendelian phenotype/MRI pattern; NfL biomarker

Acute attacks are diagnosed by markedly elevated urinary PBG and ALA, screened with rapid bedside tests (Hoesch/Watson-Schwartz) and confirmed quantitatively (PMID: 41704990, PMID: 41069899). Serum neurofilament light chain (NfL) is an emerging biomarker of axonal damage, elevated ~68-fold during acute attacks and correlating strongly with ALA/PBG:

"During acute attacks, serum NfL levels were 68 times higher compared to normal controls and disclosed a strong correlation with ALA and PBG levels" — PMID: 38715693

The Mendelian biallelic-HMBS leukodystrophy has a recognizable clinical and radiological signature. In a series of 6 adults:

"All six affected individuals presented with slowly progressive spasticity, ataxia, peripheral neuropathy, with or without mild cognitive impairment, and/or ocular disease with onset in childhood or adolescence. Their brain MRIs show mainly confluent signal abnormalities in the periventricular and deep white matter and bilateral thalami" — PMID: 34089223

Severe infantile biallelic cases present even earlier — with ataxia, hypotonia, and seizures from ~3 months of age, and substantial irreversible injury already present at diagnosis (PMID: 42396593).

Finding 6 — Treatment: hepatic-directed therapies help attacks but NOT the Mendelian leukodystrophy

For acute attacks (entity A), the AGA Clinical Practice Update lists the cornerstones of management:

"The cornerstones of management include discontinuation of porphyrinogenic drugs and chemicals, administration of oral or intravenous dextrose and intravenous hemin, and use of analgesics and antiemetics" — PMID: 36642627

Givosiran (Givlaari), an FDA-approved liver-directed siRNA targeting ALAS1, produces durable normalization of ALA and significantly reduces attack rates and hemin need; approved for adults and adolescents ≥12 years:

"Givosiran is a novel siRNA-based therapy targeted specifically to hepatocytes to inhibit ALA synthase 1, the first and rate-limiting step in heme biosynthesis. Patients with frequent recurrent attacks treated with givosiran had durable normalization of ALA and significantly reduced numbers of acute attacks" — PMID: 33769375

Liver transplantation remains the only curative option for refractory AIP (PMID: 41287633). Critically, none of these hepatic-directed strategies works for the biallelic leukodystrophy, because the CSF precursor accumulation is generated behind/within the blood-brain barrier and is constitutively elevated:

"porphyrin precursor levels are constitutively elevated in the cerebrospinal fluid and are not reduced by haem arginate therapy. Liver transplantation and hepatically directed therapies are not likely to be effective for leukodystrophy due to biallelic" — PMID: 41377573

Givosiran improved a biallelic case biochemically, but neurological injury was largely irreversible (PMID: 41731635, PMID: 42396593). Seizures during attacks require non-porphyrinogenic anticonvulsants — levetiracetam is preferred; enzyme-inducing agents (phenytoin, valproate, carbamazepine, barbiturates) can precipitate attacks (PMID: 31649773).

Finding 7 — The eponymous entity was defined by Kevelam et al. 2016

The named disease "AIP-related leukoencephalopathy" was established in a single family with 3 affected members sharing a distinct MRI pattern:

"We identified 3 family members with a similar MRI pattern characterized by symmetrical signal abnormalities in the periventricular and deep cerebral white matter, thalami, and central part of the pons. Cerebellar atrophy was noted in advanced disease stages." — PMID: 27558376

Its inheritance is autosomal recessive by biallelic HMBS variants:

"Whole-exome sequencing revealed compound heterozygous missense variants in the HMBS gene, both associated with the autosomal dominant disorder acute intermittent porphyria. Sanger sequencing of 6 healthy siblings confirmed the bi-allelic location of the variants and segregation with the disease." — PMID: 27558376

And its biochemistry is distinctively mild relative to florid AIP, with only slight-to-moderate precursor elevation and 50–66% residual enzyme activity:

"Patients had a slight and moderate increase in urinary and plasma porphobilinogen and 5'-aminolevulinic acid, respectively, and a 50% to 66% decrease in hydroxymethylbilane synthase enzyme activity compared to normal." — PMID: 27558376

Section-by-Section Report

1. Disease Information

  • Overview: Two entities. (A) Porphyria-associated PRES — a reversible posterior-predominant vasogenic-edema encephalopathy occurring during acute hepatic porphyria attacks (most often AIP). (B) Biallelic-HMBS leukodystrophy — a Mendelian, autosomal-recessive, childhood/adolescent-onset progressive white-matter disease (the eponymous "acute intermittent porphyria-related leukoencephalopathy," Kevelam 2016).
  • Key identifiers: Gene HMBS (OMIM *609806). AIP: OMIM #176000; Orphanet ORPHA:79276; ICD-10 E80.21; ICD-11 5C58.10; MeSH D017118 (Porphyria, Acute Intermittent). PRES: MeSH D054038 (Posterior Leukoencephalopathy Syndrome); ICD-11 8D43. The biallelic leukodystrophy does not yet have a widely adopted standalone OMIM number and is described under HMBS-related disease. Suggested MONDO: map entity B to the biallelic HMBS leukodystrophy concept; entity A to porphyria + PRES.
  • Synonyms/alternatives: AIP-related leukoencephalopathy; HMBS-related leukoencephalopathy/leukodystrophy; biallelic HMBS leukodystrophy; homozygous-dominant AIP (HD-AIP) with CNS disease; porphyria-associated PRES; reversible posterior leukoencephalopathy in porphyria.
  • Information source: Predominantly aggregated disease-level resources plus individual case reports and small case series (the Mendelian form is described in <20 individuals worldwide); the PRES association derives from a 46-patient systematic review and numerous single-patient reports.

2. Etiology

  • Causal factors: Genetic — loss-of-function variants in HMBS. Monoallelic → dominant AIP (entity A substrate); biallelic → recessive leukodystrophy (entity B). Entity A additionally requires an environmental/physiological trigger to precipitate an attack.
  • Genetic risk factors: Pathogenic HMBS variants (>400 reported). Modifier genes influence penetrance — CYP2D64 and 5 defective alleles may be protective against attacks (CYP2D6 as a penetrance-modifying gene) (PMID: 30808393); oligogenic/environmental modifiers shift inheritance from a purely dominant to an oligogenic model (PMID: 29360981).
  • Environmental/trigger risk factors: Porphyrinogenic drugs (barbiturates, sulfonamides, enzyme-inducing anticonvulsants, some hormonal agents e.g., levonorgestrel), fasting/low-carbohydrate states, alcohol, endocrine/hormonal fluctuations (premenstrual), stress, infection (including COVID-19), and surgery/anesthesia. Female sex and reproductive age are strong demographic risk factors for attacks.
  • Protective factors: Adequate carbohydrate intake; avoidance of porphyrinogenic drugs; possibly CYP2D6 loss-of-function alleles (genetic).
  • Gene–environment interactions: The ~1% penetrance of likely-pathogenic HMBS variants shows attacks are contingent on interactions between the causal allele, modifier genotypes (CYP2D6), and environmental triggers that induce ALAS1 (PMID: 27539938).

3. Phenotypes

Entity A (porphyria/PRES attack): | Phenotype | HPO term (suggested) | Frequency / notes | |---|---|---| | Seizures | HP:0001250 | Common in AE; PRES in 42% of AE attacks | | Encephalopathy/confusion | HP:0001298 | Part of AE triad | | Visual disturbance/cortical blindness | HP:0000618 | Occipital-predominant edema | | Abdominal pain (neurovisceral) | HP:0002027 | Hallmark of attacks | | Hyponatremia (SIADH) | HP:0002902 | 88% of severe AE attacks | | Peripheral motor neuropathy | HP:0007141 | Axonal, can cause paralysis | | Autonomic dysfunction (tachycardia, hypertension) | HP:0002571 | Frequent |

Entity B (biallelic leukodystrophy): | Phenotype | HPO term (suggested) | Onset/course | |---|---|---| | Spastic paraparesis | HP:0002061 | Childhood/adolescent, slowly progressive | | Cerebellar ataxia | HP:0001251 | Progressive | | Peripheral neuropathy | HP:0009830 | Progressive | | Cognitive impairment (mild) | HP:0100543 | Variable | | Optic atrophy | HP:0000648 | In subset | | Nystagmus / gaze palsy | HP:0000639 / HP:0000496 | In subset | | Leukoencephalopathy | HP:0002352 | Defining feature | | Infantile hypotonia/seizures | HP:0001290 / HP:0001250 | Severe infantile form (~3 months) |

  • QoL impact: Entity A attacks are severely disabling acutely but usually reversible (4.3% long-term sequelae; PMID: 31706631); rare severe cases lead to lasting disability (PMID: 38274883). Entity B causes chronic, accruing disability (mobility, vision, cognition), lifelong.

4. Genetic / Molecular Information

  • Causal gene: HMBS (HGNC:4982), OMIM *609806, chromosome 11q23.3. Encodes porphobilinogen deaminase (EC 2.5.1.61), the 3rd enzyme of heme biosynthesis; substrate porphobilinogen (CHEBI:17381) → hydroxymethylbilane.
  • Variant classification/types: >400 reported pathogenic/likely-pathogenic variants — missense, nonsense, frameshift, splice-site, and small indels. Documented leukodystrophy variants: c.251C>A (p.Ala84Asp), c.674G>A, c.517C>T (p.Arg173Trp); example AIP variants include c.669_698del30 (Spanish founder), c.1005dupC, c.405_406delAA. Functional consequence: loss of function in both dominant and recessive forms (PMID: 42107342).
  • Allele frequency: Combined likely-pathogenic HMBS allele frequency in Caucasians ~0.00056; carrier prevalence ~1/1299 (PMID: 27539938, PMID: 29360981).
  • Origin: Germline (both entities).
  • Modifier genes: CYP2D6 (penetrance modifier; 4/5 possibly protective) (PMID: 30808393); additional oligogenic modifiers proposed (PMID: 29360981).
  • Epigenetics / chromosomal abnormalities: Not established as contributors; no recurrent large-scale rearrangements described for these entities.

5. Environmental Information

  • Environmental factors / toxins: Porphyrinogenic drugs and chemicals that induce hepatic ALAS1 (barbiturates, sulfonamides, rifampin, many anticonvulsants, certain steroids).
  • Lifestyle factors: Caloric restriction/fasting, alcohol, smoking; hormonal contraceptives can trigger attacks.
  • Infectious agents: No causal pathogen; however, infections (including COVID-19) can trigger/unmask attacks and precipitate PRES (PMID: 40186107). Entity B (Mendelian leukodystrophy) is not trigger-dependent.

6. Mechanism / Pathophysiology

Ordered causal chain:

  1. HMBS loss-of-function variant (monoallelic or biallelic) leads to reduced porphobilinogen deaminase activity (50–66% residual in biallelic leukodystrophy; more severe deficits or trigger-dependent decompensation in AIP).
  2. Reduced HMBS activity results in diminished heme output in hepatocytes (and, inferred, in CNS-resident cells for the biallelic form).
  3. Low heme de-represses ALAS1, the rate-limiting enzyme (demonstrated hepatic mechanism; PMID: 30987916).
  4. ALAS1 up-regulation causes overproduction and accumulation of ALA and PBG (systemic in AIP; constitutively elevated in CSF in biallelic leukodystrophy — PMID: 41377573).
  5. Branch A (attack/PRES): Circulating ALA leads to (i) GABA-A receptor interference (demonstrated in vitro; PMID: 11478735) and neuronal hyperexcitability, and (ii) endothelial/blood-brain-barrier dysfunction (inferred) causing reversible vasogenic edema in posterior white matter (PRES), often with cerebral vasoconstriction and hyponatremia.
  6. Branch A resolution: Removal of trigger + lowering hepatic ALA (dextrose, hemin, givosiran) results in normalization and radiological/clinical reversal (4.3% sequelae).
  7. Branch B (Mendelian leukodystrophy): Chronic CNS precursor accumulation leads to progressive oligodendrocyte/white-matter injury, demyelination, and (in some) cystic change and thalamic/pontine involvement — largely irreversible; hepatic-directed therapy does not correct CSF precursors (demonstrated; PMID: 41377573).

  8. In both branches, ALA/PBG neurotoxicity causes axonal degeneration (peripheral motor axonopathy; modeled in Pbgd−/− mice — PMID: 10207164; reflected by ~68-fold NfL rise — PMID: 38715693).

  9. Molecular pathway: Heme biosynthesis (KEGG hsa00860; Reactome R-HSA-189451). Suggested GO: heme biosynthetic process (GO:0006783); porphyrin-containing compound metabolic process (GO:0006778).

  10. Cellular processes: Neuronal excitotoxicity, endothelial dysfunction, demyelination, axonal degeneration, oxidative stress (ALA autoxidation generates reactive oxygen species — inferred contributor).
  11. Protein dysfunction: PBGD is a morpheein — an equilibrium of octamer/hexamer/dimer assemblies; destabilizing variants reduce active-octamer function (PMID: 31952692). UniProt P08397.
  12. Metabolic changes: Accumulation of ALA (CHEBI:17549) and PBG (CHEBI:17381); relative heme deficiency.
  13. Cell types (suggested CL): neuron (CL:0000540), oligodendrocyte (CL:0000128), brain microvascular endothelial cell (CL:1001568/CL:0002139), hepatocyte (CL:0000182), motor neuron/Schwann cell (CL:0002573).
  14. Subcellular (GO CC): mitochondrion (GO:0005739; ALAS1/ALA synthesis), cytosol (GO:0005829; HMBS reaction).

7. Anatomical Structures Affected

  • Organ level: Primary — brain/CNS (UBERON:0000955), particularly cerebral white matter (UBERON:0002316), and peripheral nerves (UBERON:0001021). Secondary/systemic — liver (UBERON:0002107; source of precursors), autonomic nervous system.
  • Regional (entity A / PRES): Posterior-predominant — occipital (UBERON:0002021, 81.4%), parietal (65.1%), frontal (60.5%) cortex/subcortex; bilateral, often symmetric (PMID: 31706631).
  • Regional (entity B): Periventricular and deep cerebral white matter, bilateral thalami (UBERON:0001897), central pons (UBERON:0000988), with cerebellar atrophy in advanced disease; bilateral/symmetric (PMID: 27558376, PMID: 34089223).
  • Tissue/cell: White matter (myelin/oligodendrocytes), large-caliber motor axons, cerebral microvascular endothelium.

8. Temporal Development

  • Entity A onset: Acute/subacute during attacks; typically young adults (median 24 y), 84.8% female. Course: episodic, reversible; recurrent PRES possible (PMID: 31153599).
  • Entity B onset: Childhood/adolescent (slowly progressive form) or infantile (~3 months, severe form). Course: chronic, progressive, largely irreversible; critical window for intervention is likely very early, before fixed white-matter injury (PMID: 42396593).
  • Remission: Entity A remits with treatment/trigger removal; pregnancy can transiently remit AHP with postpartum escalation (PMID: 42367478). Entity B does not remit.

9. Inheritance and Population

  • Epidemiology: AIP minimal prevalence ~1/1299 carriers; symptomatic AIP far rarer (~5.4/million symptomatic Europeans; regional founder populations higher, e.g., 17.7/million in Murcia, Spain). Biallelic leukodystrophy is ultra-rare (<20 reported individuals).
  • Inheritance: Entity A — autosomal dominant, markedly reduced penetrance (~1% of likely-pathogenic carriers; ~22.9% in AIP families). Entity B — autosomal recessive (biallelic HMBS).
  • Penetrance/expressivity: Incomplete, modifier- and trigger-dependent (A); more consistent but variable severity (B).
  • Founder effects/consanguinity: Spanish founder mutation c.669_698del30 (PMID: 30808393); recessive entity B favored by consanguinity/compound heterozygosity.
  • Sex ratio: Attacks strongly female-predominant (~85% women); entity B without strong sex bias.
  • Anticipation/mosaicism: Not characteristic (no repeat expansion).

10. Diagnostics

  • Biochemistry (first-line): Markedly elevated urinary PBG and ALA during attacks; rapid screening with Hoesch/Watson-Schwartz tests (PMID: 41704990, PMID: 41069899). In biallelic leukodystrophy, precursor elevations may be mild but CSF PBG/ALA are constitutively elevated (PMID: 41731635, PMID: 41377573).
  • Biomarkers: Serum NfL (axonal damage; ~68-fold in attacks) (PMID: 38715693); hyponatremia and elevated CK as clues; transaminase elevation common.
  • Imaging: MRI — entity A shows T2/FLAIR posterior hyperintensity without diffusion restriction (reversible vasogenic edema/PRES) (PMID: 31649773); entity B shows confluent periventricular/deep white matter + bilateral thalamic ± pontine signal change, cerebellar atrophy late (PMID: 27558376).
  • Genetic testing: HMBS single-gene sequencing or 4-gene AHP panel; zygosity determination is essential (mono- vs biallelic). WES/WGS useful for the leukodystrophy (compound heterozygosity often found on reanalysis; note WGS can be negative when the clinicobiochemical phenotype is compelling — PMID: 42367478). Enzyme assay: erythrocyte/leukocyte HMBS activity (50–66% residual in biallelic leukodystrophy).
  • Differential diagnosis: Hypertensive/eclamptic PRES, other leukodystrophies, Guillain-Barré (peripheral neuropathy mimicry), cyclic vomiting syndrome, POTS (screening generally low-yield — PMID: 40856938), heavy-metal toxicity, mitochondrial disease.

11. Outcome / Prognosis

  • Entity A: Generally favorable — 4.3% long-term sequelae, 19.6% ischemic lesions in the PRES cohort (PMID: 31706631); mortality low with prompt treatment, but severe attacks can cause lasting disability or death. Chronic AHP complications include hepatocellular carcinoma, chronic kidney disease, and hypertension (PMID: 41287633).
  • Entity B: Poor — progressive disability; infantile form leaves irreversible injury by diagnosis with no effective disease-modifying therapy (PMID: 42396593).
  • Prognostic markers: NfL for axonal damage; ALA/PBG burden; residual HMBS activity; age of onset (earlier = worse in entity B).

12. Treatment

Intervention Entity A (attacks/PRES) Entity B (leukodystrophy) NCIT (suggested)
Trigger removal + IV/oral dextrose First-line, effective Not applicable C1948 (glucose)
IV hemin / haem arginate Effective for attacks Ineffective (no CSF change) C29027 (hemin)
Givosiran (siRNA vs ALAS1) Prophylaxis; reduces attacks Biochemical improvement only; injury irreversible —
Liver transplantation Curative for refractory AIP Ineffective C15329
Levetiracetam (seizures) Preferred (non-porphyrinogenic) Symptomatic C61814
Analgesics/antiemetics, Na+ correction Supportive Supportive —

Avoid enzyme-inducing anticonvulsants (phenytoin, valproate, carbamazepine, barbiturates) — they can precipitate attacks (PMID: 31649773). Rehabilitation (PT/OT) for chronic deficits. No disease-modifying therapy currently exists for biallelic HMBS deficiency (PMID: 42396593). Pharmacogenomics: CYP2D6 genotype may inform attack risk (PMID: 30808393).

13. Prevention

  • Primary: Avoid porphyrinogenic drugs, fasting, and alcohol; maintain carbohydrate intake; trigger education (entity A). Not applicable to genetically determined onset of entity B.
  • Secondary/prophylaxis: Givosiran or off-label prophylactic hemin for recurrent attackers; hormone suppression for menstrual-cycle-linked attacks (PMID: 39313028).
  • Tertiary: Monitor chronic AHP complications (HCC, CKD, hypertension) (PMID: 41287633).
  • Genetic counseling / screening: Cascade family testing; carrier screening; determine zygosity for recessive-risk counseling. No routine newborn screening.

14. Other Species / Natural Disease

  • Orthology: Hmbs is conserved (mouse Hmbs, NCBI Gene 15288; rat ortholog present). No well-documented naturally occurring porphyria-related leukoencephalopathy in companion animals/wildlife identified in this investigation. Comparative relevance derives chiefly from engineered rodent models (below).

15. Model Organisms

  • Primary model: Pbgd/Hmbs-deficient (Pbgd−/−) mouse — biochemically faithful to human AIP (↓hepatic Pbgd, ↑ALAS activity, massive urinary ALA after phenobarbital) and reproduces porphyric motor axonal neuropathy (loss of large-caliber femoral-nerve axons, primary motor axon degeneration) (PMID: 8563760, PMID: 10207164).
  • Therapeutic-model applications: Hepatocyte transplantation lowered plasma ALA/PBG ~50% with 2.7% engraftment (PMID: 23582197); non-viral PBGD gene delivery tested with limited hepatic expression (PMID: 15110317).
  • Limitations: Existing models capture peripheral neuropathy and hepatic biochemistry but do not reproduce the biallelic central leukodystrophy — a major gap. Resources: MGI for murine alleles.

Mechanistic Model / Interpretation

    HMBS loss-of-function variant
                │
  (monoallelic) │ (biallelic, ~34–66% residual activity)
┌───────────────┴────────────────────┐
▼                                     ▼
   Latent AIP carrier                Constitutive CNS + systemic
   (needs a trigger)                 precursor accumulation
│                                     │
   + trigger (drug/fast/                       │
     hormone/infection)                        │
▼                                     ▼
  ↓ hepatic heme → ALAS1 de-repression   ↑ CSF ALA/PBG (behind BBB;
│                                 hepatic therapy cannot correct)
▼                                     │
  ↑↑ ALA / PBG (systemic)                      │
│                                     │
   ┌────┴─────┐                                │
   ▼          ▼                                ▼
GABA-A     endothelial/BBB              chronic oligodendrocyte /
inhibition dysfunction                 white-matter injury
   │          │                                │
   ▼          ▼                                ▼
seizures,  vasogenic edema             progressive spasticity,
encephalop. (posterior → PRES)         ataxia, neuropathy,
   │          │                        cystic leukoencephalopathy
   └────┬─────┘                                │
▼                                     ▼
  REVERSIBLE (Entity A)              LARGELY IRREVERSIBLE (Entity B)
  responds to dextrose/hemin/        no disease-modifying therapy;
  givosiran/trigger removal          hepatic-directed Rx ineffective

The two entities are best understood as the same biochemical lesion expressed at two doses and two timescales. Monoallelic disease produces intermittent, trigger-dependent, systemic precursor surges that transiently poison the posterior cerebral vasculature and GABAergic neurons — reversible if caught. Biallelic disease produces a lower-grade but constant, compartmentalized precursor excess within the CNS that the liver-centric therapeutic toolkit cannot reach, yielding cumulative, fixed white-matter damage.

Evidence Base

PMID Role Contribution
27558376 Founding Defines the eponymous biallelic-HMBS leukodystrophy, MRI signature, AR inheritance, mild biochemistry
34089223 Confirmatory Expands phenotype/MRI in 6 adults
41377573 Pivotal Shows hepatic-directed therapy fails to correct CSF precursors or halt progression
41731635 Case Biallelic cystic leukoencephalopathy; CSF precursor accumulation
42396593 Case Severe infantile form; irreversible injury; liver transplant of limited benefit
31706631 Systematic review PRES lesion distribution, vasoconstriction, outcomes (n=46)
42107342 Genetics HMBS LOF underlies both dominant and recessive disease
29360981 Epidemiology Prevalence/penetrance; oligogenic model
27539938 Genetics ~1% penetrance of likely-pathogenic variants
30987916 Mechanism ALAS1 de-repression / hepatic ALA overproduction
11478735 Mechanism ALA inhibits GABA-A receptor binding
36757574 Clinical AE triad; PRES 42%, hyponatremia 88%
8563760 / 10207164 Model Pbgd−/− mouse biochemistry and motor axonopathy
38715693 Biomarker NfL ~68× in attacks, correlates with ALA/PBG
36642627 Guideline AGA acute management
33769375 / 35067977 Therapy Givosiran mechanism/efficacy
30808393 Modifier CYP2D6 as penetrance modifier / protective alleles
31649773 Clinical Reversible vasogenic edema; anticonvulsant choice

Limitations and Knowledge Gaps

  1. Small sample size for the Mendelian entity. The biallelic-HMBS leukodystrophy is described in fewer than ~20 individuals worldwide; phenotype frequencies, natural history, and genotype–phenotype correlations are provisional.
  2. Nosological ambiguity. "Porphyria-related leukoencephalopathy" conflates a reversible acquired phenocopy (PRES during attacks) with a true Mendelian disease. Knowledge-base curation must keep them separate to avoid propagating errors in prognosis and treatment.
  3. Mechanism partly inferred. The endothelial/BBB-dysfunction step of PRES and the exact CNS cell-type target of biallelic precursor toxicity are not fully demonstrated; direct human tissue and single-cell data are lacking.
  4. Model gap. No animal model reproduces the central biallelic leukodystrophy, limiting mechanistic and therapeutic study of entity B.
  5. No CNS-penetrant therapy. Because hepatic-directed drugs do not reach CNS precursor pools, there is currently no disease-modifying option for entity B.
  6. Genetic testing pitfalls. WGS can be negative when the clinicobiochemical phenotype is compelling; zygosity interpretation and reanalysis are critical.

Proposed Follow-up Experiments / Actions

  1. Develop a biallelic-Hmbs CNS model (e.g., conditional/neural-restricted Hmbs knockout or humanized knock-in of leukodystrophy variants) to test whether constitutive CNS ALA/PBG drives demyelination and to serve as a therapeutic platform.
  2. Test CNS-penetrant precursor-lowering strategies — CNS-directed ALAS1 knockdown (intrathecal siRNA/ASO), AAV-mediated HMBS gene replacement to CNS, or small molecules that stabilize the PBGD octamer — since liver-directed therapy is proven ineffective for entity B.
  3. Prospective natural-history registry for biallelic-HMBS individuals with serial MRI, CSF ALA/PBG, and serum NfL to define progression rate and identify a treatment window.
  4. Validate NfL and CSF precursor ratios as prognostic/monitoring biomarkers across both entities.
  5. Characterize the CNS cell-type target via single-cell/spatial transcriptomics of affected white matter (postmortem or organoid), testing oligodendrocyte and microvascular-endothelial vulnerability.
  6. Curation action: create/align MONDO entries that explicitly separate (A) porphyria-associated PRES from (B) biallelic-HMBS leukodystrophy, cross-referencing HMBS (HGNC:4982), OMIM *609806, and the Kevelam 2016 definition.

Report compiled from 5 discovery iterations, 7 confirmed findings, and 65 reviewed papers. Evidence types span human clinical case series/systematic reviews, mouse models, and in vitro biochemistry, as annotated above.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome Count
References checked 32
Resolved 32
Unresolved (possible confabulation) 0
Unverifiable 0
Quoted claims checked 17
Quoted claims found in source 15
Quoted claims not found in source 2
References weighed for topical relevance 32
On topic 13
Off topic 1

Quotes not found in the cited source

Searched the abstract, any retrieved full text, and the title. A quote drawn from a part of the paper that was not retrieved will appear here too, so check before treating one as invented:

Every one of these was searched against an abstract alone, with no full text retrieved - marked abstract only below. Where full text can be fetched, re-running with it will settle them; where the source publishes only a summary to PubMed, as GeneReviews chapters do, it will not, and the quote has to be checked by hand against the chapter itself.

  • PMID:30987916 (abstract only): "This deficiency leads to de-repression of the first and normally rate-controlling enzyme of the heme synthetic pathway, delta- or 5-aminolevulinic acid [ALA] synthase-1, and thus to marked up-regulation of this key enzyme and to marked hepatic overproduction of ALA"
  • closest text in source: "This deficiency leads to de-repression of the first and normally rate-controlling enzyme of the heme synthetic pathway, delta- or 5-aminolevulinic acid [ALA] synthase-1, and thus to marked up-regulation of this key enzyme and to marked hepatic overproduction of ALA"
  • PMID:11478735 (abstract only): "ALA (0.1-10 mM) significantly inhibited the binding of [3H]muscimol (12 nM), with a similar potency in rat and human membranes (IC50 = 199 vs. 228 microM, respectively)"
  • closest text in source: "ALA (0.1-10 mM) significantly inhibited the binding of [3H]muscimol (12 nM), with a similar potency in rat and human membranes (IC50 = 199 vs. 228 microM, respectively)"

References that may not be about this subject

These identifiers resolve, so they are not fabrications, but the records they resolve to share almost none of this report's vocabulary. That is a clue and not a verdict - a paper can be relevant in ways its title and abstract do not spell out - so read them before deciding:

  • PMID:31952692 (3 mentions) - Porphobilinogen synthase: An equilibrium of different assemblies in human health.
  • shared terms: variant, hepatic

Weighed against this report's own most characteristic terms: attack, leukodystrophy, entity, biallelic, hmbs, pres, disease, aip, ala, reversible, acute, pbg, precursor, leukoencephalopathy, variant, hepatic, progressive, mendelian, gene, alas1.

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Outcome Count
Terms checked 37
Resolved 35
Unresolved (possible confabulation) 0
Obsolete 0
Unverifiable 2
Terms whose name was checked 15
Terms named correctly 4
Terms named as a different term 3
Terms whose name is worth a second look 8

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

  • HP:0002571 (1 mention) - the report calls it "Autonomic dysfunction (tachycardia, hypertension)"; HP calls it Achalasia
  • CHEBI:17549 (1 mention) - the report calls it "Metabolic changes: Accumulation of ALA"; CHEBI calls it 5-aminolevulinic acid**
  • UBERON:0000955 (1 mention) - the report calls it "Organ level: Primary — brain/CNS"; UBERON calls it brain**

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

  • HP:0001298 (1 mention) - the report calls it "Encephalopathy/confusion"; HP calls it Encephalopathy
  • HP:0000618 (1 mention) - the report calls it "Visual disturbance/cortical blindness"; HP calls it Blindness, and lists "Legal blindness" among its other names
  • HP:0002027 (1 mention) - the report calls it "Abdominal pain (neurovisceral)"; HP calls it Abdominal pain
  • HP:0002902 (1 mention) - the report calls it "Hyponatremia (SIADH)"; HP calls it Hyponatremia
  • HP:0007141 (1 mention) - the report calls it "Peripheral motor neuropathy"; HP calls it Sensorimotor neuropathy
  • HP:0002061 (1 mention) - the report calls it "Spastic paraparesis"; HP calls it Lower limb spasticity, and lists "Spastic lower extremity" among its other names
  • HP:0001251 (1 mention) - the report calls it "Cerebellar ataxia"; HP calls it Ataxia, and lists "Cerebellar ataxia" among its other names
  • HP:0100543 (1 mention) - the report calls it "Cognitive impairment (mild)"; HP calls it Cognitive impairment

Terms named inconsistently

The report gives these identifiers more than one name of its own:

  • HGNC:4982 - called "HMBS", "Causal gene: HMBS"

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: ORPHA.