Hyperprolinemia Type 2

Metabolic Disorder MONDO:0009401 Pathograph 12 Show in embeddings browser Inborn Error of Metabolism Hyperprolinemia Disorder of Amino Acid Metabolism

Hyperprolinemia type 2 (HPII) is an autosomal recessive disorder of proline degradation caused by biallelic ALDH4A1 pathogenic variants. ALDH4A1 encodes the mitochondrial enzyme delta-1-pyrroline-5-carboxylate dehydrogenase (P5CDH), which oxidizes glutamate-gamma-semialdehyde to glutamate in the second step of proline catabolism. Loss of P5CDH activity causes accumulation of its substrate delta-1-pyrroline-5-carboxylate (P5C) together with marked hyperprolinemia and prolinuria. Unlike hyperprolinemia type 1 (PRODH/proline oxidase deficiency), which blocks the pathway one step earlier and does not generate P5C, the accumulating P5C in HPII is chemically reactive: it condenses with the aldehyde carbon of pyridoxal 5'-phosphate (PLP) and inactivates it, producing a secondary vitamin B6 deficiency. This "endogenous vitamin antagonist" mechanism is the leading explanation for the seizures and metabolic encephalopathy that distinguish HPII from the largely benign type 1 disorder, and it is the rationale for high-dose pyridoxine supplementation. Presentation is variable — many individuals are asymptomatic or mildly affected, while others have recurrent or refractory seizures, often precipitated by febrile intercurrent illness, and rare late-onset presentations with status epilepticus and lactic acidosis are described.

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1
Inheritance
7
Pathophys.
13
Phenotypes
2
Gaps
12
Pathograph
1
Genes
1
Medical Actions
1
Deep Research
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Classifications

ICIMD (Inherited Metabolic Disorders)
orn pro and hyp
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Inheritance

1
Autosomal recessive inheritance HP:0000007
HPII is caused by biallelic loss-of-function ALDH4A1 variants. The G521fs(+1) frameshift allele segregates in the large Irish Traveller pedigree in which the HPII phenotype was originally defined.
Autosomal recessive inheritance
Show evidence (2 references)
PMID:34037900 SUPPORT Human Clinical
"Hyperprolinemia type II (HPII) is a rare autosomal recessive disorder of proline degradation pathway due to deficiency of delta-1-pyrroline-5-carboxylate dehydrogenase."
States the autosomal recessive mode of inheritance of HPII.
PMID:24931297 SUPPORT Human Clinical
"Each type is caused by an autosomal recessive inborn error of the proline metabolic pathway."
Confirms autosomal recessive inheritance for both hereditary hyperprolinemia types.
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Discussions and Knowledge Gaps

2
Is P5C-mediated inactivation of pyridoxal 5'-phosphate actually the cause of seizures in hyperprolinemia type 2, and which PLP-dependent enzymes (e.g. glutamate decarboxylase and the GABA synthetic pathway) are rate-limiting in vivo? Do all symptomatic HPII patients have measurable vitamin B6 depletion, and does pyridoxine responsiveness track with it?
KNOWLEDGE GAP OPEN gap_hpii_plp_seizure_causality
Attached to
pathophysiology#Secondary Vitamin B6 Deficiency pathophysiology#Seizures and Metabolic Encephalopathy
The PLP-inactivation mechanism is supported by in vitro chemistry, in vivo recovery of analogous P5C adducts, and single-case observations of low serum B6 with B6-responsive seizures — but it remains explicitly framed in the primary literature as what "is thought to be" the pathophysiological mechanism. The intervening neurotransmitter steps have not been measured in HPII patients, so the link from cofactor depletion to seizure generation is inferred from general vitamin B6 biochemistry. The one cohort study to address it partially answers the pyridoxine-responsiveness half of the question in the negative: all four patients had low-normal B6 and the clinical course was independent of both B6 concentration and B6 therapy. That result does not settle the mechanism — the cohort was not designed as a treatment trial and n=4 — but it means the gap is now one of conflicting evidence rather than absent evidence.
Show evidence (2 references)
PMID:24173411 SUPPORT Human Clinical
"The clinical course was non-progressive and independent from the B6 concentration and B6 therapy."
Partially addresses this gap: clinical course was independent of B6 concentration and B6 therapy in the only HPII cohort, arguing against a simple B6-depletion-drives-seizures model.
PMID:24173411 SUPPORT Human Clinical
"We found four patients with increased urinary P5C diagnosed with hyperprolinemia type II, and only one patient had hyperprolinemia type I. All children with hyperprolinemia type II had low normal B6 concentration, and three of the patients had biochemical markers suggesting mitochondrial dysfunction."
All four HPII patients had low-normal rather than frankly deficient B6, which bears on whether measurable depletion is present in every symptomatic patient.
Why do only some individuals with biallelic ALDH4A1 loss-of-function variants develop seizures or neurodevelopmental impairment while others remain asymptomatic, and does HPII act principally as a threshold-lowering susceptibility factor unmasked by febrile intercurrent illness rather than as a fully penetrant epilepsy syndrome?
KNOWLEDGE GAP OPEN gap_hpii_penetrance_and_phenotype
Attached to
pathophysiology#Seizures and Metabolic Encephalopathy pathophysiology#Neurodevelopmental and Behavioral Impairment
The clinical spectrum of HPII is described in the literature as unclear, with ascertainment dominated by small case reports. The one Japanese case was ascertained during influenza-associated encephalopathy, and the reported infantile case decompensated after an upper respiratory infection, suggesting a stress-unmasked susceptibility model that has never been tested systematically. A systematic review of 35 studies found no correlation between biochemical and clinical phenotype, so severity is not predicted by proline level and the modifiers of penetrance remain entirely unidentified.
Show evidence (1 reference)
PMID:34302426 SUPPORT Human Clinical
"No evidence for a biochemical phenotype-clinical phenotype correlation was found; that is, no association between higher proline levels and specific psychiatric phenotypes was observed."
Establishes that no biochemical-clinical phenotype correlation has been demonstrated, and specifies what that means: higher proline levels are not associated with specific phenotypes, so metabolite burden does not explain the variable presentation. The gap is genuinely open.
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Pathophysiology

7
ALDH4A1 P5C Dehydrogenase Deficiency
Biallelic loss-of-function ALDH4A1 variants abolish mitochondrial delta-1-pyrroline-5-carboxylate dehydrogenase (P5CDH) activity, the second enzymatic step of proline catabolism, which oxidizes glutamate-gamma-semialdehyde to glutamate. Structural work on the human enzyme shows how the recurrent S352L missense allele inactivates catalysis by rearranging the catalytic loop and eliminating NAD+ binding.
ALDH4A1 hgnc:406 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves ALDH4A1 (hgnc:406). hgnc:406 is a gene from the HUGO Gene Nomenclature Committee.
L-proline catabolic process GO:0006562 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased L-proline catabolic process (GO:0006562). GO:0006562 is a biological process from the Gene Ontology. ↓ DECREASED
L-glutamate gamma-semialdehyde dehydrogenase activity GO:0003842 Gene Ontology (GO) Relation: this pathophysiological event involves this molecular function This pathophysiological event involves decreased L-glutamate gamma-semialdehyde dehydrogenase activity (GO:0003842). GO:0003842 is a molecular function from the Gene Ontology. ↓ DECREASED
mitochondrion GO:0005739 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves mitochondrion (GO:0005739). GO:0005739 is a cellular component from the Gene Ontology.
Downstream
Pyrroline-5-Carboxylate and Proline Accumulation Loss of the P5CDH step causes its substrate P5C, and the upstream amino acid proline, to accumulate.
Mitochondrial Dysfunction Because ALDH4A1 is a mitochondrial matrix enzyme, its loss was hypothesized to compromise mitochondrial function; the steps between the missing dehydrogenase and impaired oxidative phosphorylation have not been established, so this edge records an observed association rather than a worked-out route.
Show evidence (4 references)
PMID:9700195 SUPPORT Human Clinical
"We surveyed Delta1-pyrroline 5-carboxylate dehydrogenase genes from four patients with hyperprolinemia type II using RT-PCR amplification, genomic PCR amplification and direct sequencing."
The founding molecular study surveyed the P5C dehydrogenase gene in HPII patients and identified the causal mutant alleles.
PMID:9700195 SUPPORT In Vitro
"In contrast to wild-type human P5CDh, yeast expressing S352L and G521fs(+1) failed to grow on proline and had no detectable P5CDh activity."
Yeast complementation shows the patient S352L and G521fs(+1) alleles produce enzymatically dead P5CDh, establishing loss of function.
PMID:22516612 SUPPORT In Vitro
"Type II hyperprolinemia is an autosomal recessive disorder caused by a deficiency in Δ(1)-pyrroline-5-carboxylate dehydrogenase (P5CDH; also known as ALDH4A1), the aldehyde dehydrogenase that catalyzes the oxidation of glutamate semialdehyde to glutamate."
Defines HPII as a deficiency of P5CDH/ALDH4A1, the aldehyde dehydrogenase oxidizing glutamate semialdehyde to glutamate.
+ 1 more reference
Pyrroline-5-Carboxylate and Proline Accumulation
The metabolic block causes accumulation of delta-1-pyrroline-5-carboxylate (P5C), the reactive intermediate of proline catabolism, together with markedly elevated proline in plasma, urine, and cerebrospinal fluid. The presence of accumulating P5C is the biochemical feature that separates HPII from hyperprolinemia type 1, in which the earlier proline-oxidase block prevents P5C formation.
Downstream
Pyridoxal 5'-Phosphate Inactivation by P5C Accumulated P5C condenses with pyridoxal 5'-phosphate at the activated C-4 carbon of the pyrroline ring, forming stable adducts.
Neurodevelopmental and Behavioral Impairment Whether the metabolic abnormality causes the neurodevelopmental and psychiatric features is explicitly unsettled: the cohort literature exempts only the secondary B6 deficiency from that doubt, and a systematic review found no correlation between proline levels and psychiatric phenotype.
Show evidence (2 references)
PMID:36131087 SUPPORT Human Clinical
"We show that these biomarkers can differentiate between HPI, caused by a deficiency of proline oxidase activity, and HPII."
P5C-derived adducts distinguish HPII from HPI, confirming that P5C accumulation is specific to the ALDH4A1 block.
PMID:31884946 SUPPORT Human Clinical
"A diagnostic work-up revealed up to 400-times increased prolin-level in urine CSF and blood."
Documents up to 400-fold elevation of proline in urine, CSF, and blood in an ALDH4A1-confirmed patient.
Pyridoxal 5'-Phosphate Inactivation by P5C
P5C acts as an endogenous vitamin antagonist. In vitro NMR and mass spectrometry show that the activated C-4 carbon of the pyrroline ring undergoes a Claisen (Knoevenagel-type) condensation with the aldehyde carbon of pyridoxal 5'-phosphate, the active vitamin B6 coenzyme, generating stable adducts and removing PLP from the functional cofactor pool. Analogous P5C adducts with acetoacetic acid and malonic acid have been recovered from patient urine and plasma, establishing that the reaction occurs in vivo.
Downstream
Secondary Vitamin B6 Deficiency Consumption of pyridoxal 5'-phosphate by adduct formation depletes the functional vitamin B6 pool.
Show evidence (4 references)
PMID:11134058 SUPPORT In Vitro
"These were the result of a Claisen condensation (or Knoevenagel type of reaction) of the activated C-4 carbon of the pyrroline ring with the aldehyde carbon of pyridoxal phosphate."
Identifies the Claisen condensation between P5C and pyridoxal phosphate as the chemical basis of cofactor inactivation.
PMID:11134058 SUPPORT In Vitro
"Pyrroline-5-carboxylic acid is thus found to be a unique endogenous vitamin antagonist."
Characterizes P5C as an endogenous antagonist of vitamin B6.
PMID:36131087 SUPPORT Human Clinical
"The accumulating P5C spontaneously reacts with, and inactivates, pyridoxal 5'-phosphate, a crucial cofactor for many enzymatic processes, which is thought to be the pathophysiological mechanism for HPII."
States that PLP inactivation by P5C is the presumed pathophysiological mechanism of HPII, and marks it as a proposed rather than proven step.
+ 1 more reference
Secondary Vitamin B6 Deficiency
Ongoing P5C-driven consumption of pyridoxal 5'-phosphate produces a secondary, acquired vitamin B6 deficiency that is not explained by diet or medication. Measured serum vitamin B6 is low in affected patients. Because PLP is the coenzyme for many decarboxylases and transaminases, including those of neurotransmitter metabolism, this deficiency is the proposed proximate cause of the seizure phenotype; the specific PLP-dependent enzymes limiting in vivo have not been established in HPII patients.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
Downstream
Seizures and Metabolic Encephalopathy Functional vitamin B6 depletion lowers the seizure threshold; the intervening PLP-dependent neurotransmitter steps are inferred from general B6 biochemistry rather than measured in HPII patients.
Show evidence (2 references)
PMID:31884946 SUPPORT Human Clinical
"Furthermore, a low vitamin-B6 serum value was found, consistent with a HPII causing secondary pyridoxine deficiency and seizures."
Documents a low serum vitamin B6 in an ALDH4A1-confirmed patient, interpreted as HPII-induced secondary pyridoxine deficiency.
PMID:11134058 SUPPORT In Vitro
"We hypothesized that pyridoxal phosphate (vitamin B6 coenzyme) was de-activated by L-Delta(1)-pyrroline-5-carboxylic acid, the major intermediate that accumulates endogenously in hyperprolinemia type II."
States the mechanism by which the accumulating HPII intermediate de-activates the vitamin B6 coenzyme.
Seizures and Metabolic Encephalopathy
The clinical endpoint of HPII is a metabolic epilepsy. Reported presentations range from infantile seizures refractory to multiple antiepileptic drugs to a late-onset adult presentation with status epilepticus and severe lactic acidosis. Intercurrent febrile illness is a recurrent precipitant: reported episodes followed upper respiratory infection, and the single Japanese HPII case was ascertained during influenza-associated encephalopathy, prompting the proposal that HPII lowers the convulsive threshold.
neuron CL:0000540 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves neuron (CL:0000540). CL:0000540 is a cell type from the Cell Ontology.
Show evidence (4 references)
PMID:34037900 SUPPORT Human Clinical
"We here describe an 11-month-old infant with recurrent seizures refractory to multiple antiepileptic drugs."
Reports drug-refractory infantile seizures in a genetically confirmed HPII patient.
PMID:34037900 SUPPORT Human Clinical
"She was hospitalized in view of acute-onset encephalopathy, exacerbation of generalized seizures following an upper respiratory infection."
Documents febrile intercurrent illness (upper respiratory infection) as a precipitant of encephalopathy and seizure exacerbation.
PMID:24931297 SUPPORT Human Clinical
"This suggests that HPII might reduce the threshold for convulsions, thereby increasing the sensitivity of individuals with influenza-associated encephalopathy."
Proposes that HPII lowers the convulsive threshold, based on the Japanese case ascertained during influenza-associated encephalopathy.
+ 1 more reference
Mitochondrial Dysfunction
A mild mitochondrial arm accompanies HPII. In the only cohort to look for it, three of four metabolically confirmed patients had biochemical markers suggesting mitochondrial dysfunction, and the finding was confirmed on muscle biopsy in one; mitochondrial enzyme complex activity and fibroblast oxygen consumption were measured because ALDH4A1 is itself a mitochondrial matrix enzyme. The arm is modeled as provisional because no mechanistic route from the enzyme deficiency to impaired oxidative phosphorylation has been demonstrated. It is retained because it is the only candidate explanation on offer for the severe lactic acidosis reported during HPII crisis, which otherwise has no mechanistic anchor in this pathograph.
oxidative phosphorylation GO:0006119 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased oxidative phosphorylation (GO:0006119). GO:0006119 is a biological process from the Gene Ontology. ↓ DECREASED
mitochondrion GO:0005739 Gene Ontology (GO) Relation: this pathophysiological event involves this cellular component This pathophysiological event involves mitochondrion (GO:0005739). GO:0005739 is a cellular component from the Gene Ontology.
Show evidence (4 references)
PMID:24173411 SUPPORT Human Clinical
"All children with hyperprolinemia type II had low normal B6 concentration, and three of the patients had biochemical markers suggesting mitochondrial dysfunction."
Three of the four confirmed HPII patients carried biochemical markers of mitochondrial dysfunction.
PMID:24173411 SUPPORT Human Clinical
"Mitochondrial dysfunction was confirmed in a muscle biopsy in one case."
Muscle biopsy confirmed mitochondrial dysfunction in one patient, raising the finding above a purely biochemical inference.
PMID:21168532 SUPPORT Model Organism
"Heterozygous (CG7145(f04633)/TM3) individuals developed normally, while homozygous (CG7145(f04633)/CG7145(f04633)) individuals displayed proline levels twice that of normal, swollen mitochondria, and ultimately larval and pupal lethality."
A Drosophila P5CDh-null model independently reproduces the mitochondrial arm: homozygotes show doubled proline together with swollen mitochondria. Cross-system corroboration from a different organism is what raises this node above a single small human cohort.
+ 1 more reference
Neurodevelopmental and Behavioral Impairment
Beyond seizures, HPII patients show intellectual disability and prominent behavioral and psychiatric features. In the four-patient cohort, all had significant behavioral problems including anxiety and hallucinations, and two adolescents had intellectual disability; intellectual deficit and mild developmental delay are also part of the authoritative Orphanet disease definition. This node is marked HYPOTHETICAL rather than established because no mechanistic route links the metabolic block to these features: the primary literature exempts only the secondary B6 deficiency from that doubt, and a systematic review of 35 studies found no biochemical-clinical phenotype correlation.
Show evidence (4 references)
PMID:24173411 SUPPORT Human Clinical
"Intellectual disability was found in two adolescent patients."
Documents intellectual disability in two of the four confirmed HPII patients.
PMID:24173411 SUPPORT Human Clinical
"All patients showed seizures and significant behavioral problems, including anxiety and hallucinations."
All four confirmed HPII patients had significant behavioral problems alongside their seizures.
ORPHA:79101 SUPPORT Other
"The condition is often benign but clinical signs may include seizures, intellectual deficit and mild developmental delay."
The Orphanet definition of the MONDO term this entry binds lists intellectual deficit and developmental delay alongside seizures.
+ 1 more reference
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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 Hyperprolinemia Type 2 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

13
Metabolism 1
Lactic acidosis HP:0003128 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Lactic acidosis (HP:0003128). HP:0003128 is a phenotype from the Human Phenotype Ontology.
Reported in a single late-onset case; frequency deliberately omitted because no cohort data support a frequency band. Linked as a readout of the provisional Mitochondrial Dysfunction node rather than left orphaned, but the connection is inferred: the cohort that documented mitochondrial dysfunction did not report lactic acidosis, and the case that reported lactic acidosis did not assess mitochondrial function.
Show evidence (1 reference)
PMID:31884946 SUPPORT Human Clinical
"Ten years later she was admitted into the neurological intensive-care-unit with acute abdominal pain, multiple generalized epileptic seizures, a vertical gaze palsy accompanied by extensive lactic acidosis in serum 26.0 mmol/l (reference: 0.55-2.2 mmol/l) and CSF 12.01 mmol/l (reference:..."
Documents extreme serum lactic acidosis during the HPII crisis.
Nervous System 8
Seizure HP:0001250 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Seizure (HP:0001250). HP:0001250 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:34037900 SUPPORT Human Clinical
"We here describe an 11-month-old infant with recurrent seizures refractory to multiple antiepileptic drugs."
Reports drug-refractory seizures as the presenting feature of HPII.
Context-specific annotations (1)
Onset: INFANTILE
Onset is most often neonatal, in early infancy, or in early childhood, but an adult (age 64) first presentation has been reported.
Show evidence (1 reference)
PMID:31884946 SUPPORT Human Clinical
"The manifestation is mostly in neonatal age, in early infancy or early childhood."
States that HPII manifestation is mostly neonatal, early infantile, or in early childhood; the same report documents the late-onset exception.
Encephalopathy HP:0001298 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Encephalopathy (HP:0001298). HP:0001298 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:34037900 SUPPORT Human Clinical
"She was hospitalized in view of acute-onset encephalopathy, exacerbation of generalized seizures following an upper respiratory infection."
The infant was hospitalized for acute-onset encephalopathy with seizure exacerbation following an upper respiratory infection.
Intellectual disability HP:0001249 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Intellectual disability (HP:0001249). HP:0001249 is a phenotype from the Human Phenotype Ontology.
Two of four is not curated as a frequency band: the entire denominator is a single four-patient cohort, which per docs/frequency-evidence-guidelines.md is too small to support a FrequencyEnum value.
Show evidence (2 references)
PMID:24173411 SUPPORT Human Clinical
"Intellectual disability was found in two adolescent patients."
Two of four confirmed HPII patients had intellectual disability.
ORPHA:79101 SUPPORT Other
"The condition is often benign but clinical signs may include seizures, intellectual deficit and mild developmental delay."
Orphanet lists intellectual deficit among the clinical signs of hyperprolinemia type 2.
Global developmental delay 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.
The systematic review pools HPI and HPII and cannot be used to assign an HPII-specific frequency; the Orphanet definition and PMID:41602883 are the HPII-specific anchors. Note that the genotype in PMID:41602883 is a homozygous ALDH4A1 variant of uncertain significance rather than a established pathogenic allele, so it corroborates rather than proves the association.
Show evidence (3 references)
ORPHA:79101 SUPPORT Other
"The condition is often benign but clinical signs may include seizures, intellectual deficit and mild developmental delay."
Orphanet lists mild developmental delay among the clinical signs of hyperprolinemia type 2.
PMID:34302426 SUPPORT INDIRECT Human Clinical
"From these studies, a common psychiatric phenotype is observed with a high prevalence of developmental delay, intellectual disability, autism spectrum disorders, and psychosis spectrum disorders."
Systematic review of 35 studies reports a high prevalence of developmental delay. Graded INDIRECT because the review pools hyperprolinemia types I and II, so the prevalence is not reported for HPII on its own.
PMID:41602883 SUPPORT Human Clinical
"Herein, we report a case of a preschool-aged Saudi girl who was born from consanguineous parents and presented with global developmental delay."
An HPII-specific case report in which global developmental delay was the presenting feature.
Autistic behavior HP:0000729 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Autistic behavior (HP:0000729). HP:0000729 is a phenotype from the Human Phenotype Ontology.
Deliberately curated with no frequency band. The single HPII-specific report rests on a variant of uncertain significance, and the systematic review does not separate HPII from HPI, so neither source alone establishes the association.
Show evidence (2 references)
PMID:41602883 SUPPORT Human Clinical
"Herein, we report a case of a preschool-aged Saudi girl who was born from consanguineous parents and presented with global developmental delay. The patient was clinically diagnosed with autism spectrum disorder with associated disruptive behaviors."
A child with biochemically confirmed hyperprolinemia and a homozygous ALDH4A1 variant was clinically diagnosed with autism spectrum disorder. The variant is of uncertain significance, so the genotype-phenotype link rests on the biochemical confirmation rather than on the variant call.
PMID:34302426 SUPPORT INDIRECT Human Clinical
"From these studies, a common psychiatric phenotype is observed with a high prevalence of developmental delay, intellectual disability, autism spectrum disorders, and psychosis spectrum disorders."
Systematic review reports autism spectrum disorders in hyperprolinemia. Graded INDIRECT because it pools types I and II rather than reporting HPII separately.
Atypical behavior HP:0000708 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Behavioral abnormality, annotated with Atypical behavior (HP:0000708). HP:0000708 is a phenotype from the Human Phenotype Ontology.
4/4 in a single four-patient cohort; frequency band omitted because the denominator is too small to support one.
Show evidence (2 references)
PMID:24173411 SUPPORT Human Clinical
"All patients showed seizures and significant behavioral problems, including anxiety and hallucinations."
All four confirmed HPII patients had significant behavioral problems.
PMID:24173411 SUPPORT Human Clinical
"Individuals with hyperprolinemia should be monitored closely due to their frequent behavioral problems."
The cohort's closing recommendation is close monitoring specifically because of the frequent behavioral problems.
Anxiety HP:0000739 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Anxiety (HP:0000739). HP:0000739 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:24173411 SUPPORT Human Clinical
"All patients showed seizures and significant behavioral problems, including anxiety and hallucinations."
Anxiety is named as one of the behavioral problems in the confirmed HPII patients.
Hallucinations HP:0000738 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hallucinations (HP:0000738). HP:0000738 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:24173411 SUPPORT Human Clinical
"All patients showed seizures and significant behavioral problems, including anxiety and hallucinations."
Hallucinations are named among the behavioral problems in the confirmed HPII patients.
PMID:34302426 SUPPORT INDIRECT Human Clinical
"From these studies, a common psychiatric phenotype is observed with a high prevalence of developmental delay, intellectual disability, autism spectrum disorders, and psychosis spectrum disorders."
Systematic review reports psychosis spectrum disorders in hyperprolinemia. Graded INDIRECT because it pools types I and II rather than reporting HPII separately.
Other 4
Hyperprolinemia HP:0008358 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hyperprolinemia (HP:0008358). HP:0008358 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:34037900 SUPPORT Human Clinical
"Laboratory investigation revealed significantly elevated proline levels in dried blood spots."
Documents markedly elevated blood proline in a genetically confirmed HPII patient.
PMID:24931297 SUPPORT Human Clinical
"The current study presents diagnostic criteria for HPI and HPII, based on plasma proline level, with or without measurements of urinary P5C."
Plasma proline level is the basis of the published HPII diagnostic criteria.
Prolinuria HP:0003137 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Prolinuria (HP:0003137). HP:0003137 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31884946 SUPPORT Human Clinical
"A diagnostic work-up revealed up to 400-times increased prolin-level in urine CSF and blood."
Reports up to 400-fold elevated proline in urine as well as CSF and blood.
Reduced circulating vitamin B6 Reduced circulating vitamin B6 circulating HP:0008326 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Reduced circulating vitamin B6, annotated with Reduced circulating vitamin B6 circulating (HP:0008326). HP:0008326 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:31884946 SUPPORT Human Clinical
"Furthermore, a low vitamin-B6 serum value was found, consistent with a HPII causing secondary pyridoxine deficiency and seizures."
Documents a low serum vitamin B6 value in an ALDH4A1-confirmed HPII patient.
PMID:11134058 SUPPORT Human Clinical
"We previously identified vitamin B6 deficiency in a child presenting with seizures whose primary diagnosis was the inherited disorder hyperprolinemia type II. This is an unrecognized association, which was not explained by diet or medication."
Establishes that the vitamin B6 deficiency in HPII is not attributable to dietary or medication causes.
Status epilepticus HP:0002133 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Status epilepticus (HP:0002133). HP:0002133 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:31884946 SUPPORT Human Clinical
"Due to repeated epileptic seizures and secondary complications a long-term sedation with a ventilation therapy over 20 days was administered."
Describes repeated epileptic seizures necessitating 20 days of sedation and ventilation, i.e. refractory status epilepticus.
🧬

Genetic Associations

1
ALDH4A1
Gene: ALDH4A1 hgnc:406 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is ALDH4A1 (hgnc:406). hgnc:406 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE
Show evidence (4 references)
PMID:9700195 SUPPORT Human Clinical
"We found four mutant alleles, two with frameshift mutations ... and two with missense mutations (S352L and P16L)."
Reports the four causal alleles (two frameshift, two missense) identified in the founding molecular study.
PMID:9700195 SUPPORT In Vitro
"The P16L allele, however, produced fully functional P5CDh and subsequent analysis suggests that it is polymorphic in the relevant (Spanish) population."
The P16L allele retained full activity and is polymorphic, cautioning against assuming pathogenicity of ALDH4A1 missense changes.
PMID:9700195 SUPPORT Human Clinical
"Interestingly, the G521fs(+1) allele segregates in the large Irish Traveller pedigree used to define the HPII phenotype."
Identifies the founder allele in the pedigree that defined the HPII phenotype.
+ 1 more reference
💊

Medical Actions

1
High-Dose Pyridoxine (Vitamin B6) Supplementation
Action: nutritional supplementationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is nutritional supplementation, annotated with Nutritional Support (NCIT:C15433). NCIT:C15433 is a clinical intervention from the NCI Thesaurus. Ontology label: Nutritional Support NCIT:C15433
Agent: pyridoxine CHEBI:16709 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses pyridoxine (CHEBI:16709). CHEBI:16709 is a therapeutic agent from Chemical Entities of Biological Interest.
Because the proposed pathomechanism is consumption of pyridoxal 5'-phosphate by accumulating P5C, replacing the depleted cofactor is the mechanistically targeted intervention. High-dose vitamin B6 abolished further seizures in the reported late-onset case, and long-term supplementation was proposed as preventive when the P5C-PLP condensation was first characterized. Evidence is limited to case-level observation; no controlled trial exists.
Mechanism Target:
RESTORES Secondary Vitamin B6 Deficiency — Pyridoxine supplementation replenishes the vitamin B6 pool depleted by P5C adduct formation, counteracting the secondary deficiency.
Show evidence (3 references)
PMID:31884946 SUPPORT Human Clinical
"Under high-dose vitamin-B6 therapy no further seizures occurred."
High-dose vitamin B6 stopped the seizures in the reported ALDH4A1-confirmed patient.
PMID:11134058 SUPPORT In Vitro
"Vitamin B6 de-activation may contribute to seizures in hyperprolinemia type II, which are so far unexplained, but they may be preventable with long term vitamin B6 supplementation."
Proposes long-term vitamin B6 supplementation as preventive; this is a mechanistic proposal from an in vitro study, not clinical trial evidence.
PMID:24173411 REFUTE Human Clinical
"The clinical course was non-progressive and independent from the B6 concentration and B6 therapy."
Counter-evidence: in the only published HPII cohort, the clinical course was non-progressive and independent of both B6 concentration and B6 therapy. This is the largest series in the literature and directly opposes the single-case observation of B6-responsive seizures, so the benefit of pyridoxine in HPII should be regarded as unresolved rather than established.
🔬

Biochemical Markers

2
Plasma and urinary proline elevation (INCREASED)
Pathograph Readouts
Readout Of Pyrroline-5-Carboxylate and Proline Accumulation Positive Diagnostic
Elevated proline reports the block in proline catabolism and is the primary diagnostic screen for hyperprolinemia.
Show evidence (1 reference)
PMID:24931297 SUPPORT Human Clinical
"The current study presents diagnostic criteria for HPI and HPII, based on plasma proline level, with or without measurements of urinary P5C."
Plasma proline is the analyte underpinning the HPI/HPII diagnostic criteria.
Show evidence (1 reference)
PMID:34037900 SUPPORT Human Clinical
"Laboratory investigation revealed significantly elevated proline levels in dried blood spots."
Documents significantly elevated proline on dried blood spot testing.
P5C-derived adduct biomarkers (INCREASED)
Pathograph Readouts
Readout Of Pyridoxal 5'-Phosphate Inactivation by P5C Positive Diagnostic
Detection of P5C condensation adducts in vivo supports the same condensation chemistry that inactivates pyridoxal 5'-phosphate.
Show evidence (1 reference)
PMID:12691858 SUPPORT Human Clinical
"Evidence suggests it formed in vivo."
The authors conclude the P5C-acetoacetate adduct recovered from patient urine formed in vivo.
Show evidence (1 reference)
PMID:36131087 SUPPORT Human Clinical
"Here, we describe the use of a combination of LC-QTOF untargeted metabolomics, NMR spectroscopy and infrared ion spectroscopy (IRIS) to identify and characterize biomarkers for HPII that result of the spontaneous reaction of P5C with malonic acid and acetoacetic acid."
Identifies and structurally characterizes the P5C-derived malonate and acetoacetate adducts as HPII biomarkers.
🔬

Diagnosis

2
Plasma proline with urinary P5C measurement
The published diagnostic criteria for hyperprolinemia are based on plasma proline level, with or without measurement of urinary delta-1-pyrroline-5-carboxylate. Urinary P5C is the discriminating analyte: it is present in HPII and absent in HPI, because the earlier proline-oxidase block in HPI prevents P5C from forming at all.
biochemical diagnostic testing NCIT:C18020 NCI Thesaurus (NCIT)
Markers: plasma proline; urinary delta-1-pyrroline-5-carboxylate (P5C)
Results: Markedly elevated plasma proline with detectable urinary P5C supports HPII; elevated proline without urinary P5C indicates HPI.
Show evidence (2 references)
PMID:24931297 SUPPORT Human Clinical
"The current study presents diagnostic criteria for HPI and HPII, based on plasma proline level, with or without measurements of urinary P5C."
Establishes plasma proline as the basis of the published diagnostic criteria for HPI and HPII.
PMID:24931297 SUPPORT Human Clinical
"The current study presents diagnostic criteria for HPI and HPII, based on plasma proline level, with or without measurements of urinary P5C."
Urinary P5C measurement is the adjunct that distinguishes HPII within the published criteria.
P5C-derived adduct biomarker assay
Mass-spectrometric and NMR identification of the stable adducts P5C forms with malonic acid and acetoacetic acid provides an HPII-specific biomarker panel that separates HPII from HPI. The same chemistry that produces these adducts is the chemistry proposed to inactivate pyridoxal 5'-phosphate, so the assay is a direct readout of the mechanism modeled in this entry.
biochemical diagnostic testing NCIT:C18020 NCI Thesaurus (NCIT)
Markers: urinary and plasma P5C-malonic acid and P5C-acetoacetic acid adducts
Results: Detection of P5C-derived adducts distinguishes HPII from HPI.
Show evidence (2 references)
PMID:36131087 SUPPORT Human Clinical
"Here, we describe the use of a combination of LC-QTOF untargeted metabolomics, NMR spectroscopy and infrared ion spectroscopy (IRIS) to identify and characterize biomarkers for HPII that result of the spontaneous reaction of P5C with malonic acid and acetoacetic acid."
Identifies the P5C-derived adducts as HPII biomarkers.
PMID:36131087 SUPPORT Human Clinical
"We show that these biomarkers can differentiate between HPI, caused by a deficiency of proline oxidase activity, and HPII."
The biomarkers discriminate HPII from HPI, which is the diagnostic question that plasma proline alone cannot answer.
📊

Prevalence

1
Worldwide
Unknown Ultra Rare
The precise incidence of HPII is unknown. A Japanese national questionnaire survey combined with a literature review identified only a single HPII case in Japan, consistent with earlier Western reports of extreme rarity.
Show evidence (2 references)
PMID:24931297 SUPPORT Human Clinical
"In addition, the precise incidences of HPI and HPII are unknown."
States directly that HPII incidence is not established.
PMID:24931297 SUPPORT Human Clinical
"This suggests that hyperprolinemia is a very rare disease in Japan, consistent with earlier reports in Western countries."
A national survey found one HPII case in Japan, supporting an ultra-rare classification consistent with Western reports.
{ }

Source YAML

click to show 
name: Hyperprolinemia Type 2
creation_date: "2026-07-31T00:00:00Z"
description: >-
  Hyperprolinemia type 2 (HPII) is an autosomal recessive disorder of proline
  degradation caused by biallelic ALDH4A1 pathogenic variants. ALDH4A1 encodes
  the mitochondrial enzyme delta-1-pyrroline-5-carboxylate dehydrogenase
  (P5CDH), which oxidizes glutamate-gamma-semialdehyde to glutamate in the
  second step of proline catabolism. Loss of P5CDH activity causes accumulation
  of its substrate delta-1-pyrroline-5-carboxylate (P5C) together with marked
  hyperprolinemia and prolinuria. Unlike hyperprolinemia type 1 (PRODH/proline
  oxidase deficiency), which blocks the pathway one step earlier and does not
  generate P5C, the accumulating P5C in HPII is chemically reactive: it
  condenses with the aldehyde carbon of pyridoxal 5'-phosphate (PLP) and
  inactivates it, producing a secondary vitamin B6 deficiency. This
  "endogenous vitamin antagonist" mechanism is the leading explanation for the
  seizures and metabolic encephalopathy that distinguish HPII from the largely
  benign type 1 disorder, and it is the rationale for high-dose pyridoxine
  supplementation. Presentation is variable — many individuals are
  asymptomatic or mildly affected, while others have recurrent or refractory
  seizures, often precipitated by febrile intercurrent illness, and rare
  late-onset presentations with status epilepticus and lactic acidosis are
  described.
category: Metabolic Disorder
parents:
- Inborn Error of Metabolism
- Hyperprolinemia
- Disorder of Amino Acid Metabolism
synonyms:
- HPII
- Hyperprolinemia, type II
- Delta-1-pyrroline-5-carboxylate dehydrogenase deficiency
- P5CDH deficiency
- ALDH4A1 hyperprolinemia
classifications:
  icimd_category:
  - classification_value: orn_pro_and_hyp
    notes: >-
      ICIMD sub-branch "Disorders of ornithine, proline and hydroxyproline
      metabolism". HPII is the ALDH4A1 / P5C dehydrogenase defect at the second
      step of the proline degradation pathway.
disease_term:
  preferred_term: hyperprolinemia type 2
  term:
    id: MONDO:0009401
    label: hyperprolinemia type 2
inheritance:
- name: Autosomal recessive inheritance
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >-
    HPII is caused by biallelic loss-of-function ALDH4A1 variants. The
    G521fs(+1) frameshift allele segregates in the large Irish Traveller
    pedigree in which the HPII phenotype was originally defined.
  evidence:
  - reference: PMID:34037900
    reference_title: Metabolic epilepsy in hyperprolinemia type II due to a novel nonsense ALDH4A1 gene variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hyperprolinemia type II (HPII) is a rare autosomal recessive disorder of proline degradation pathway due to deficiency of delta-1-pyrroline-5-carboxylate dehydrogenase."
    explanation: States the autosomal recessive mode of inheritance of HPII.
  - reference: PMID:24931297
    reference_title: Biochemical and clinical features of hereditary hyperprolinemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Each type is caused by an autosomal recessive inborn error of the proline metabolic pathway."
    explanation: Confirms autosomal recessive inheritance for both hereditary hyperprolinemia types.
pathophysiology:
- name: ALDH4A1 P5C Dehydrogenase Deficiency
  biological_scale: MOLECULAR
  description: >-
    Biallelic loss-of-function ALDH4A1 variants abolish mitochondrial
    delta-1-pyrroline-5-carboxylate dehydrogenase (P5CDH) activity, the second
    enzymatic step of proline catabolism, which oxidizes
    glutamate-gamma-semialdehyde to glutamate. Structural work on the human
    enzyme shows how the recurrent S352L missense allele inactivates catalysis
    by rearranging the catalytic loop and eliminating NAD+ binding.
  role: trigger
  genes:
  - preferred_term: ALDH4A1
    term:
      id: hgnc:406
      label: ALDH4A1
  molecular_functions:
  - preferred_term: L-glutamate gamma-semialdehyde dehydrogenase activity
    term:
      id: GO:0003842
      label: L-glutamate gamma-semialdehyde dehydrogenase activity
    modifier: DECREASED
  biological_processes:
  - preferred_term: L-proline catabolic process
    term:
      id: GO:0006562
      label: L-proline catabolic process
    modifier: DECREASED
  cellular_components:
  - preferred_term: mitochondrion
    term:
      id: GO:0005739
      label: mitochondrion
  evidence:
  - reference: PMID:9700195
    reference_title: Mutations in the Delta1-pyrroline 5-carboxylate dehydrogenase gene cause type II hyperprolinemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We surveyed Delta1-pyrroline 5-carboxylate dehydrogenase genes from four patients with hyperprolinemia type II using RT-PCR amplification, genomic PCR amplification and direct sequencing."
    explanation: >-
      The founding molecular study surveyed the P5C dehydrogenase gene in HPII
      patients and identified the causal mutant alleles.
  - reference: PMID:9700195
    reference_title: Mutations in the Delta1-pyrroline 5-carboxylate dehydrogenase gene cause type II hyperprolinemia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In contrast to wild-type human P5CDh, yeast expressing S352L and G521fs(+1) failed to grow on proline and had no detectable P5CDh activity."
    explanation: >-
      Yeast complementation shows the patient S352L and G521fs(+1) alleles
      produce enzymatically dead P5CDh, establishing loss of function.
  - reference: PMID:22516612
    reference_title: The three-dimensional structural basis of type II hyperprolinemia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Type II hyperprolinemia is an autosomal recessive disorder caused by a deficiency in Δ(1)-pyrroline-5-carboxylate dehydrogenase (P5CDH; also known as ALDH4A1), the aldehyde dehydrogenase that catalyzes the oxidation of glutamate semialdehyde to glutamate."
    explanation: >-
      Defines HPII as a deficiency of P5CDH/ALDH4A1, the aldehyde dehydrogenase
      oxidizing glutamate semialdehyde to glutamate.
  - reference: PMID:22516612
    reference_title: The three-dimensional structural basis of type II hyperprolinemia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Mutation of Ser352 to Leu is shown to abolish catalytic activity and eliminate NAD(+) binding."
    explanation: >-
      Crystal structures show the disease-associated S352L substitution abolishes
      catalytic activity and NAD+ binding.
  downstream:
  - target: Pyrroline-5-Carboxylate and Proline Accumulation
    causal_link_type: DIRECT
    description: >-
      Loss of the P5CDH step causes its substrate P5C, and the upstream amino
      acid proline, to accumulate.
    evidence:
    - reference: PMID:36131087
      reference_title: Identification of Δ-1-pyrroline-5-carboxylate derived biomarkers for hyperprolinemia type II.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "This leads to an accumulation of toxic levels of P5C, an intermediate in proline catabolism."
      explanation: Directly links the ALDH4A1/P5CDH deficiency to accumulation of P5C.
  - target: Mitochondrial Dysfunction
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Because ALDH4A1 is a mitochondrial matrix enzyme, its loss was
      hypothesized to compromise mitochondrial function; the steps between the
      missing dehydrogenase and impaired oxidative phosphorylation have not been
      established, so this edge records an observed association rather than a
      worked-out route.
    evidence:
    - reference: PMID:24173411
      reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Due to the mitochondrial localization of both ALDH4A1 and PRODH mitochondrial enzyme complex activity was evaluated and oxygen consumption was measured to assess ATP production in patient-fibroblasts."
      explanation: >-
        The mitochondrial localization of ALDH4A1 is the stated reason the
        cohort study evaluated mitochondrial enzyme complex activity at all.
- name: Pyrroline-5-Carboxylate and Proline Accumulation
  biological_scale: MOLECULAR
  description: >-
    The metabolic block causes accumulation of delta-1-pyrroline-5-carboxylate
    (P5C), the reactive intermediate of proline catabolism, together with
    markedly elevated proline in plasma, urine, and cerebrospinal fluid. The
    presence of accumulating P5C is the biochemical feature that separates HPII
    from hyperprolinemia type 1, in which the earlier proline-oxidase block
    prevents P5C formation.
  role: amplifier
  chemical_entities:
  - preferred_term: 1-pyrroline-5-carboxylate
    term:
      id: CHEBI:15893
      label: 1-pyrroline-5-carboxylate
    modifier: INCREASED
  - preferred_term: L-proline
    term:
      id: CHEBI:17203
      label: L-proline
    modifier: INCREASED
  evidence:
  - reference: PMID:36131087
    reference_title: Identification of Δ-1-pyrroline-5-carboxylate derived biomarkers for hyperprolinemia type II.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We show that these biomarkers can differentiate between HPI, caused by a deficiency of proline oxidase activity, and HPII."
    explanation: >-
      P5C-derived adducts distinguish HPII from HPI, confirming that P5C
      accumulation is specific to the ALDH4A1 block.
  - reference: PMID:31884946
    reference_title: "Novel variants in a patient with late-onset hyperprolinemia type II: diagnostic key for status epilepticus and lactic acidosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A diagnostic work-up revealed up to 400-times increased prolin-level in urine CSF and blood."
    explanation: >-
      Documents up to 400-fold elevation of proline in urine, CSF, and blood in
      an ALDH4A1-confirmed patient.
  downstream:
  - target: Pyridoxal 5'-Phosphate Inactivation by P5C
    causal_link_type: DIRECT
    description: >-
      Accumulated P5C condenses with pyridoxal 5'-phosphate at the activated C-4
      carbon of the pyrroline ring, forming stable adducts.
    evidence:
    - reference: PMID:36131087
      reference_title: Identification of Δ-1-pyrroline-5-carboxylate derived biomarkers for hyperprolinemia type II.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The accumulating P5C spontaneously reacts with, and inactivates, pyridoxal 5'-phosphate, a crucial cofactor for many enzymatic processes, which is thought to be the pathophysiological mechanism for HPII."
      explanation: Links P5C accumulation directly to the reaction that inactivates pyridoxal 5'-phosphate.
  - target: Neurodevelopmental and Behavioral Impairment
    causal_link_type: UNKNOWN
    description: >-
      Whether the metabolic abnormality causes the neurodevelopmental and
      psychiatric features is explicitly unsettled: the cohort literature
      exempts only the secondary B6 deficiency from that doubt, and a systematic
      review found no correlation between proline levels and psychiatric
      phenotype.
    evidence:
    - reference: PMID:24173411
      reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Although hyperprolinemia type-II has a discriminative metabolic phenotype and is frequently associated with neurological system involvement, the casual relation between the metabolic abnormalities and the clinical features, except for those of the secondary B6 deficiency, has been frequently debated."
      explanation: >-
        States that the causal relation between the metabolic abnormalities and
        the clinical features — other than those of the secondary B6 deficiency
        — has been frequently debated, which is why this edge is left UNKNOWN.
- name: Pyridoxal 5'-Phosphate Inactivation by P5C
  biological_scale: MOLECULAR
  description: >-
    P5C acts as an endogenous vitamin antagonist. In vitro NMR and mass
    spectrometry show that the activated C-4 carbon of the pyrroline ring
    undergoes a Claisen (Knoevenagel-type) condensation with the aldehyde carbon
    of pyridoxal 5'-phosphate, the active vitamin B6 coenzyme, generating stable
    adducts and removing PLP from the functional cofactor pool. Analogous P5C
    adducts with acetoacetic acid and malonic acid have been recovered from
    patient urine and plasma, establishing that the reaction occurs in vivo.
  role: central_effector
  chemical_entities:
  - preferred_term: pyridoxal 5'-phosphate
    term:
      id: CHEBI:18405
      label: pyridoxal 5'-phosphate
    modifier: DECREASED
  - preferred_term: 1-pyrroline-5-carboxylate
    term:
      id: CHEBI:15893
      label: 1-pyrroline-5-carboxylate
    modifier: INCREASED
  evidence:
  - reference: PMID:11134058
    reference_title: "Pyridoxal phosphate de-activation by pyrroline-5-carboxylic acid. Increased risk of vitamin B6 deficiency and seizures in hyperprolinemia type II."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "These were the result of a Claisen condensation (or Knoevenagel type of reaction) of the activated C-4 carbon of the pyrroline ring with the aldehyde carbon of pyridoxal phosphate."
    explanation: >-
      Identifies the Claisen condensation between P5C and pyridoxal phosphate as
      the chemical basis of cofactor inactivation.
  - reference: PMID:11134058
    reference_title: "Pyridoxal phosphate de-activation by pyrroline-5-carboxylic acid. Increased risk of vitamin B6 deficiency and seizures in hyperprolinemia type II."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Pyrroline-5-carboxylic acid is thus found to be a unique endogenous vitamin antagonist."
    explanation: Characterizes P5C as an endogenous antagonist of vitamin B6.
  - reference: PMID:36131087
    reference_title: Identification of Δ-1-pyrroline-5-carboxylate derived biomarkers for hyperprolinemia type II.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The accumulating P5C spontaneously reacts with, and inactivates, pyridoxal 5'-phosphate, a crucial cofactor for many enzymatic processes, which is thought to be the pathophysiological mechanism for HPII."
    explanation: >-
      States that PLP inactivation by P5C is the presumed pathophysiological
      mechanism of HPII, and marks it as a proposed rather than proven step.
  - reference: PMID:12691858
    reference_title: A novel pyrroline-5-carboxylic acid and acetoacetic acid adduct in hyperprolinaemia type II.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The adduct was demonstrated unequivocally in the child's acute urine and traces in plasma."
    explanation: >-
      Recovery of the P5C-acetoacetate adduct from patient urine supports that
      the P5C condensation chemistry occurs in vivo, not only in vitro.
  downstream:
  - target: Secondary Vitamin B6 Deficiency
    causal_link_type: DIRECT
    description: >-
      Consumption of pyridoxal 5'-phosphate by adduct formation depletes the
      functional vitamin B6 pool.
    evidence:
    - reference: PMID:11134058
      reference_title: "Pyridoxal phosphate de-activation by pyrroline-5-carboxylic acid. Increased risk of vitamin B6 deficiency and seizures in hyperprolinemia type II."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "We previously identified vitamin B6 deficiency in a child presenting with seizures whose primary diagnosis was the inherited disorder hyperprolinemia type II."
      explanation: >-
        The hypothesis originated from an observed, otherwise unexplained vitamin
        B6 deficiency in an HPII patient with seizures.
- name: Secondary Vitamin B6 Deficiency
  biological_scale: ORGANISM
  description: >-
    Ongoing P5C-driven consumption of pyridoxal 5'-phosphate produces a
    secondary, acquired vitamin B6 deficiency that is not explained by diet or
    medication. Measured serum vitamin B6 is low in affected patients. Because
    PLP is the coenzyme for many decarboxylases and transaminases, including
    those of neurotransmitter metabolism, this deficiency is the proposed
    proximate cause of the seizure phenotype; the specific PLP-dependent enzymes
    limiting in vivo have not been established in HPII patients.
  role: amplifier
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  chemical_entities:
  - preferred_term: pyridoxal 5'-phosphate
    term:
      id: CHEBI:18405
      label: pyridoxal 5'-phosphate
    modifier: DECREASED
  evidence:
  - reference: PMID:31884946
    reference_title: "Novel variants in a patient with late-onset hyperprolinemia type II: diagnostic key for status epilepticus and lactic acidosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Furthermore, a low vitamin-B6 serum value was found, consistent with a HPII causing secondary pyridoxine deficiency and seizures."
    explanation: >-
      Documents a low serum vitamin B6 in an ALDH4A1-confirmed patient,
      interpreted as HPII-induced secondary pyridoxine deficiency.
  - reference: PMID:11134058
    reference_title: "Pyridoxal phosphate de-activation by pyrroline-5-carboxylic acid. Increased risk of vitamin B6 deficiency and seizures in hyperprolinemia type II."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We hypothesized that pyridoxal phosphate (vitamin B6 coenzyme) was de-activated by L-Delta(1)-pyrroline-5-carboxylic acid, the major intermediate that accumulates endogenously in hyperprolinemia type II."
    explanation: >-
      States the mechanism by which the accumulating HPII intermediate
      de-activates the vitamin B6 coenzyme.
  downstream:
  - target: Seizures and Metabolic Encephalopathy
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      Functional vitamin B6 depletion lowers the seizure threshold; the
      intervening PLP-dependent neurotransmitter steps are inferred from general
      B6 biochemistry rather than measured in HPII patients.
    evidence:
    - reference: PMID:11134058
      reference_title: "Pyridoxal phosphate de-activation by pyrroline-5-carboxylic acid. Increased risk of vitamin B6 deficiency and seizures in hyperprolinemia type II."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Vitamin B6 de-activation may contribute to seizures in hyperprolinemia type II, which are so far unexplained, but they may be preventable with long term vitamin B6 supplementation."
      explanation: >-
        Proposes vitamin B6 de-activation as a contributor to the otherwise
        unexplained HPII seizures.
- name: Seizures and Metabolic Encephalopathy
  biological_scale: ORGANISM
  conforms_to: "epilepsy_excitation_inhibition_imbalance#Seizure Generation and Epileptogenesis"
  description: >-
    The clinical endpoint of HPII is a metabolic epilepsy. Reported
    presentations range from infantile seizures refractory to multiple
    antiepileptic drugs to a late-onset adult presentation with status
    epilepticus and severe lactic acidosis. Intercurrent febrile illness is a
    recurrent precipitant: reported episodes followed upper respiratory
    infection, and the single Japanese HPII case was ascertained during
    influenza-associated encephalopathy, prompting the proposal that HPII lowers
    the convulsive threshold.
  role: consequence
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  evidence:
  - reference: PMID:34037900
    reference_title: Metabolic epilepsy in hyperprolinemia type II due to a novel nonsense ALDH4A1 gene variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We here describe an 11-month-old infant with recurrent seizures refractory to multiple antiepileptic drugs."
    explanation: >-
      Reports drug-refractory infantile seizures in a genetically confirmed HPII
      patient.
  - reference: PMID:34037900
    reference_title: Metabolic epilepsy in hyperprolinemia type II due to a novel nonsense ALDH4A1 gene variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "She was hospitalized in view of acute-onset encephalopathy, exacerbation of generalized seizures following an upper respiratory infection."
    explanation: >-
      Documents febrile intercurrent illness (upper respiratory infection) as a
      precipitant of encephalopathy and seizure exacerbation.
  - reference: PMID:24931297
    reference_title: Biochemical and clinical features of hereditary hyperprolinemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This suggests that HPII might reduce the threshold for convulsions, thereby increasing the sensitivity of individuals with influenza-associated encephalopathy."
    explanation: >-
      Proposes that HPII lowers the convulsive threshold, based on the Japanese
      case ascertained during influenza-associated encephalopathy.
  - reference: PMID:31884946
    reference_title: "Novel variants in a patient with late-onset hyperprolinemia type II: diagnostic key for status epilepticus and lactic acidosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ten years later she was admitted into the neurological intensive-care-unit with acute abdominal pain, multiple generalized epileptic seizures, a vertical gaze palsy accompanied by extensive lactic acidosis in serum 26.0 mmol/l (reference: 0.55-2.2 mmol/l) and CSF 12.01 mmol/l (reference: 1.12-2.47 mmol/l)."
    explanation: >-
      Describes the severe late-onset HPII presentation with repeated generalized
      seizures and lactic acidosis.
- name: Mitochondrial Dysfunction
  biological_scale: CELLULAR
  mechanism_confidence: PROVISIONAL
  description: >-
    A mild mitochondrial arm accompanies HPII. In the only cohort to look for
    it, three of four metabolically confirmed patients had biochemical markers
    suggesting mitochondrial dysfunction, and the finding was confirmed on
    muscle biopsy in one; mitochondrial enzyme complex activity and fibroblast
    oxygen consumption were measured because ALDH4A1 is itself a mitochondrial
    matrix enzyme. The arm is modeled as provisional because no mechanistic
    route from the enzyme deficiency to impaired oxidative phosphorylation has
    been demonstrated. It is retained because it is the only candidate
    explanation on offer for the severe lactic acidosis reported during HPII
    crisis, which otherwise has no mechanistic anchor in this pathograph.
  role: amplifier
  cellular_components:
  - preferred_term: mitochondrion
    term:
      id: GO:0005739
      label: mitochondrion
  biological_processes:
  - preferred_term: oxidative phosphorylation
    term:
      id: GO:0006119
      label: oxidative phosphorylation
    modifier: DECREASED
  evidence:
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All children with hyperprolinemia type II had low normal B6 concentration, and three of the patients had biochemical markers suggesting mitochondrial dysfunction."
    explanation: >-
      Three of the four confirmed HPII patients carried biochemical markers of
      mitochondrial dysfunction.
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mitochondrial dysfunction was confirmed in a muscle biopsy in one case."
    explanation: >-
      Muscle biopsy confirmed mitochondrial dysfunction in one patient, raising
      the finding above a purely biochemical inference.
  - reference: PMID:21168532
    reference_title: "Drosophila delta-1-pyrroline-5-carboxylate dehydrogenase (P5CDh) is required for proline breakdown and mitochondrial integrity-Establishing a fly model for human type II hyperprolinemia."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Heterozygous (CG7145(f04633)/TM3) individuals developed normally, while homozygous (CG7145(f04633)/CG7145(f04633)) individuals displayed proline levels twice that of normal, swollen mitochondria, and ultimately larval and pupal lethality."
    explanation: >-
      A Drosophila P5CDh-null model independently reproduces the mitochondrial
      arm: homozygotes show doubled proline together with swollen mitochondria.
      Cross-system corroboration from a different organism is what raises this
      node above a single small human cohort.
  - reference: PMID:21168532
    reference_title: "Drosophila delta-1-pyrroline-5-carboxylate dehydrogenase (P5CDh) is required for proline breakdown and mitochondrial integrity-Establishing a fly model for human type II hyperprolinemia."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We believe this is the first correlation between the loss of P5CDh and morphological defects in mitochondria."
    explanation: >-
      The authors frame the P5CDh-mitochondrial morphology link as a
      correlation, not a demonstrated mechanism, which is consistent with
      keeping this node PROVISIONAL.
  notes: >-
    Scope caution: the source cohort studied hyperprolinemia broadly and
    evaluated mitochondrial function because both ALDH4A1 (HPII) and PRODH
    (HPI) are mitochondrial. The three affected patients are explicitly HPII
    patients, so the finding is HPII-specific, but n=4 and the mechanism is
    unresolved.
- name: Neurodevelopmental and Behavioral Impairment
  biological_scale: ORGANISM
  mechanism_confidence: HYPOTHETICAL
  description: >-
    Beyond seizures, HPII patients show intellectual disability and prominent
    behavioral and psychiatric features. In the four-patient cohort, all had
    significant behavioral problems including anxiety and hallucinations, and
    two adolescents had intellectual disability; intellectual deficit and mild
    developmental delay are also part of the authoritative Orphanet disease
    definition. This node is marked HYPOTHETICAL rather than established
    because no mechanistic route links the metabolic block to these features:
    the primary literature exempts only the secondary B6 deficiency from that
    doubt, and a systematic review of 35 studies found no biochemical-clinical
    phenotype correlation.
  role: consequence
  evidence:
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Intellectual disability was found in two adolescent patients."
    explanation: >-
      Documents intellectual disability in two of the four confirmed HPII
      patients.
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients showed seizures and significant behavioral problems, including anxiety and hallucinations."
    explanation: >-
      All four confirmed HPII patients had significant behavioral problems
      alongside their seizures.
  - reference: ORPHA:79101
    reference_title: "Hyperprolinemia type 2"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The condition is often benign but clinical signs may include seizures, intellectual deficit and mild developmental delay."
    explanation: >-
      The Orphanet definition of the MONDO term this entry binds lists
      intellectual deficit and developmental delay alongside seizures.
  - reference: PMID:34302426
    reference_title: "Psychiatric phenotypes associated with hyperprolinemia: A systematic review."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: "No evidence for a biochemical phenotype-clinical phenotype correlation was found; that is, no association between higher proline levels and specific psychiatric phenotypes was observed."
    explanation: >-
      A systematic review found no correlation between proline levels and
      psychiatric phenotype, which is why this node is marked HYPOTHETICAL
      rather than established. Graded INDIRECT because the review pools
      hyperprolinemia types I and II, so the finding reaches HPII through the
      pooled cohort rather than from HPII patients alone.
phenotypes:
- category: Biochemical
  name: Hyperprolinemia
  description: >-
    Marked elevation of plasma proline is the defining biochemical abnormality
    and the basis of the diagnostic criteria for HPII.
  phenotype_term:
    preferred_term: Hyperprolinemia
    term:
      id: HP:0008358
      label: Hyperprolinemia
  evidence:
  - reference: PMID:34037900
    reference_title: Metabolic epilepsy in hyperprolinemia type II due to a novel nonsense ALDH4A1 gene variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Laboratory investigation revealed significantly elevated proline levels in dried blood spots."
    explanation: Documents markedly elevated blood proline in a genetically confirmed HPII patient.
  - reference: PMID:24931297
    reference_title: Biochemical and clinical features of hereditary hyperprolinemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The current study presents diagnostic criteria for HPI and HPII, based on plasma proline level, with or without measurements of urinary P5C."
    explanation: Plasma proline level is the basis of the published HPII diagnostic criteria.
- category: Biochemical
  name: Prolinuria
  description: Proline is also markedly elevated in urine and cerebrospinal fluid.
  phenotype_term:
    preferred_term: Prolinuria
    term:
      id: HP:0003137
      label: Prolinuria
  evidence:
  - reference: PMID:31884946
    reference_title: "Novel variants in a patient with late-onset hyperprolinemia type II: diagnostic key for status epilepticus and lactic acidosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A diagnostic work-up revealed up to 400-times increased prolin-level in urine CSF and blood."
    explanation: Reports up to 400-fold elevated proline in urine as well as CSF and blood.
- category: Biochemical
  name: Reduced circulating vitamin B6
  description: >-
    Secondary vitamin B6 deficiency, unexplained by diet or medication, has been
    documented in HPII patients with seizures.
  phenotype_term:
    preferred_term: Reduced circulating vitamin B6
    term:
      id: HP:0008326
      label: Reduced circulating vitamin B6 circulating
  evidence:
  - reference: PMID:31884946
    reference_title: "Novel variants in a patient with late-onset hyperprolinemia type II: diagnostic key for status epilepticus and lactic acidosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Furthermore, a low vitamin-B6 serum value was found, consistent with a HPII causing secondary pyridoxine deficiency and seizures."
    explanation: Documents a low serum vitamin B6 value in an ALDH4A1-confirmed HPII patient.
  - reference: PMID:11134058
    reference_title: "Pyridoxal phosphate de-activation by pyrroline-5-carboxylic acid. Increased risk of vitamin B6 deficiency and seizures in hyperprolinemia type II."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We previously identified vitamin B6 deficiency in a child presenting with seizures whose primary diagnosis was the inherited disorder hyperprolinemia type II. This is an unrecognized association, which was not explained by diet or medication."
    explanation: >-
      Establishes that the vitamin B6 deficiency in HPII is not attributable to
      dietary or medication causes.
- category: Clinical
  name: Seizure
  description: >-
    Seizures are the principal clinical manifestation of HPII, ranging from
    drug-refractory infantile seizures to adult-onset status epilepticus. They
    are frequently precipitated by intercurrent febrile illness.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:34037900
    reference_title: Metabolic epilepsy in hyperprolinemia type II due to a novel nonsense ALDH4A1 gene variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We here describe an 11-month-old infant with recurrent seizures refractory to multiple antiepileptic drugs."
    explanation: Reports drug-refractory seizures as the presenting feature of HPII.
  phenotype_contexts:
  - onset:
      onset_category: INFANTILE
    notes: >-
      Onset is most often neonatal, in early infancy, or in early childhood, but
      an adult (age 64) first presentation has been reported.
    evidence:
    - reference: PMID:31884946
      reference_title: "Novel variants in a patient with late-onset hyperprolinemia type II: diagnostic key for status epilepticus and lactic acidosis."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The manifestation is mostly in neonatal age, in early infancy or early childhood."
      explanation: >-
        States that HPII manifestation is mostly neonatal, early infantile, or in
        early childhood; the same report documents the late-onset exception.
- category: Clinical
  name: Status epilepticus
  description: >-
    Status epilepticus requiring prolonged sedation and ventilation has been
    reported in late-onset HPII.
  phenotype_term:
    preferred_term: Status epilepticus
    term:
      id: HP:0002133
      label: Status epilepticus
  evidence:
  - reference: PMID:31884946
    reference_title: "Novel variants in a patient with late-onset hyperprolinemia type II: diagnostic key for status epilepticus and lactic acidosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Due to repeated epileptic seizures and secondary complications a long-term sedation with a ventilation therapy over 20 days was administered."
    explanation: >-
      Describes repeated epileptic seizures necessitating 20 days of sedation and
      ventilation, i.e. refractory status epilepticus.
- category: Clinical
  name: Encephalopathy
  description: >-
    Acute encephalopathy, typically in the setting of intercurrent infection,
    has been described in HPII.
  phenotype_term:
    preferred_term: Encephalopathy
    term:
      id: HP:0001298
      label: Encephalopathy
  evidence:
  - reference: PMID:34037900
    reference_title: Metabolic epilepsy in hyperprolinemia type II due to a novel nonsense ALDH4A1 gene variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "She was hospitalized in view of acute-onset encephalopathy, exacerbation of generalized seizures following an upper respiratory infection."
    explanation: >-
      The infant was hospitalized for acute-onset encephalopathy with seizure
      exacerbation following an upper respiratory infection.
- category: Clinical
  name: Lactic acidosis
  description: >-
    Severe lactic acidosis in serum and CSF accompanied the reported late-onset
    HPII crisis; it is not an established feature of typical HPII.
  phenotype_term:
    preferred_term: Lactic acidosis
    term:
      id: HP:0003128
      label: Lactic acidosis
  evidence:
  - reference: PMID:31884946
    reference_title: "Novel variants in a patient with late-onset hyperprolinemia type II: diagnostic key for status epilepticus and lactic acidosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ten years later she was admitted into the neurological intensive-care-unit with acute abdominal pain, multiple generalized epileptic seizures, a vertical gaze palsy accompanied by extensive lactic acidosis in serum 26.0 mmol/l (reference: 0.55-2.2 mmol/l) and CSF 12.01 mmol/l (reference: 1.12-2.47 mmol/l)."
    explanation: Documents extreme serum lactic acidosis during the HPII crisis.
  reports_on:
  - target: Mitochondrial Dysfunction
    relationship: READOUT_OF
    direction: POSITIVE
    interpretation: >-
      Lactic acidosis is the expected laboratory readout of impaired oxidative
      phosphorylation, and the mild mitochondrial dysfunction documented in the
      HPII cohort is the only candidate mechanism for it in this pathograph.
      This is an observational readout link, not a claim that the mitochondrial
      arm has been shown to cause the reported crisis-level lactate.
    evidence:
    - reference: PMID:24173411
      reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Mitochondrial dysfunction was confirmed in a muscle biopsy in one case."
      explanation: >-
        Establishes that biopsy-confirmed mitochondrial dysfunction occurs in
        HPII; the cohort itself did not report lactic acidosis, so the link to
        the late-onset case's lactate is inferential.
  notes: >-
    Reported in a single late-onset case; frequency deliberately omitted because
    no cohort data support a frequency band. Linked as a readout of the
    provisional Mitochondrial Dysfunction node rather than left orphaned, but
    the connection is inferred: the cohort that documented mitochondrial
    dysfunction did not report lactic acidosis, and the case that reported
    lactic acidosis did not assess mitochondrial function.
- category: Clinical
  name: Intellectual disability
  description: >-
    Intellectual disability was present in two of the four metabolically
    confirmed HPII patients in the only published cohort, and intellectual
    deficit is one of the three clinical signs named in the Orphanet definition
    of the disorder.
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  evidence:
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Intellectual disability was found in two adolescent patients."
    explanation: >-
      Two of four confirmed HPII patients had intellectual disability.
  - reference: ORPHA:79101
    reference_title: "Hyperprolinemia type 2"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The condition is often benign but clinical signs may include seizures, intellectual deficit and mild developmental delay."
    explanation: >-
      Orphanet lists intellectual deficit among the clinical signs of
      hyperprolinemia type 2.
  notes: >-
    Two of four is not curated as a frequency band: the entire denominator is a
    single four-patient cohort, which per docs/frequency-evidence-guidelines.md
    is too small to support a FrequencyEnum value.
- category: Clinical
  name: Global developmental delay
  description: >-
    Mild developmental delay is named in the Orphanet definition of HPII, and a
    systematic review of hyperprolinemia reported a high prevalence of
    developmental delay across the disorder.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: ORPHA:79101
    reference_title: "Hyperprolinemia type 2"
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "The condition is often benign but clinical signs may include seizures, intellectual deficit and mild developmental delay."
    explanation: >-
      Orphanet lists mild developmental delay among the clinical signs of
      hyperprolinemia type 2.
  - reference: PMID:34302426
    reference_title: "Psychiatric phenotypes associated with hyperprolinemia: A systematic review."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: "From these studies, a common psychiatric phenotype is observed with a high prevalence of developmental delay, intellectual disability, autism spectrum disorders, and psychosis spectrum disorders."
    explanation: >-
      Systematic review of 35 studies reports a high prevalence of
      developmental delay. Graded INDIRECT because the review pools
      hyperprolinemia types I and II, so the prevalence is not reported for
      HPII on its own.
  - reference: PMID:41602883
    reference_title: "Case Report: Hyperprolinemia type II in a child with autism spectrum disorder and ALDH4A1 gene variant in a consanguineous family."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Herein, we report a case of a preschool-aged Saudi girl who was born from consanguineous parents and presented with global developmental delay."
    explanation: >-
      An HPII-specific case report in which global developmental delay was the
      presenting feature.
  notes: >-
    The systematic review pools HPI and HPII and cannot be used to assign an
    HPII-specific frequency; the Orphanet definition and PMID:41602883 are the
    HPII-specific anchors. Note that the genotype in PMID:41602883 is a
    homozygous ALDH4A1 variant of uncertain significance rather than a
    established pathogenic allele, so it corroborates rather than proves the
    association.
- category: Clinical
  name: Autistic behavior
  description: >-
    Autism spectrum presentation has been reported in HPII and is among the
    phenotypes associated with hyperprolinemia in systematic review. The
    association is based on limited and partly indirect evidence and should not
    be read as an established feature of the disorder.
  phenotype_term:
    preferred_term: Autistic behavior
    term:
      id: HP:0000729
      label: Autistic behavior
  evidence:
  - reference: PMID:41602883
    reference_title: "Case Report: Hyperprolinemia type II in a child with autism spectrum disorder and ALDH4A1 gene variant in a consanguineous family."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Herein, we report a case of a preschool-aged Saudi girl who was born from consanguineous parents and presented with global developmental delay. The patient was clinically diagnosed with autism spectrum disorder with associated disruptive behaviors."
    explanation: >-
      A child with biochemically confirmed hyperprolinemia and a homozygous
      ALDH4A1 variant was clinically diagnosed with autism spectrum disorder.
      The variant is of uncertain significance, so the genotype-phenotype link
      rests on the biochemical confirmation rather than on the variant call.
  - reference: PMID:34302426
    reference_title: "Psychiatric phenotypes associated with hyperprolinemia: A systematic review."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: "From these studies, a common psychiatric phenotype is observed with a high prevalence of developmental delay, intellectual disability, autism spectrum disorders, and psychosis spectrum disorders."
    explanation: >-
      Systematic review reports autism spectrum disorders in hyperprolinemia.
      Graded INDIRECT because it pools types I and II rather than reporting
      HPII separately.
  notes: >-
    Deliberately curated with no frequency band. The single HPII-specific report
    rests on a variant of uncertain significance, and the systematic review does
    not separate HPII from HPI, so neither source alone establishes the
    association.
- category: Clinical
  name: Atypical behavior
  description: >-
    Significant behavioral problems were present in every patient in the only
    published HPII cohort, and are the feature the authors single out as the
    reason for close monitoring.
  phenotype_term:
    preferred_term: Behavioral abnormality
    term:
      id: HP:0000708
      label: Atypical behavior
  evidence:
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients showed seizures and significant behavioral problems, including anxiety and hallucinations."
    explanation: >-
      All four confirmed HPII patients had significant behavioral problems.
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Individuals with hyperprolinemia should be monitored closely due to their frequent behavioral problems."
    explanation: >-
      The cohort's closing recommendation is close monitoring specifically
      because of the frequent behavioral problems.
  notes: >-
    4/4 in a single four-patient cohort; frequency band omitted because the
    denominator is too small to support one.
- category: Clinical
  name: Anxiety
  description: >-
    Anxiety was one of the two named behavioral manifestations in the published
    HPII cohort.
  phenotype_term:
    preferred_term: Anxiety
    term:
      id: HP:0000739
      label: Anxiety
  evidence:
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients showed seizures and significant behavioral problems, including anxiety and hallucinations."
    explanation: >-
      Anxiety is named as one of the behavioral problems in the confirmed HPII
      patients.
- category: Clinical
  name: Hallucinations
  description: >-
    Hallucinations were reported alongside anxiety in the published HPII
    cohort, and psychosis spectrum disorders are among the phenotypes
    associated with hyperprolinemia in systematic review.
  phenotype_term:
    preferred_term: Hallucinations
    term:
      id: HP:0000738
      label: Hallucinations
  evidence:
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients showed seizures and significant behavioral problems, including anxiety and hallucinations."
    explanation: >-
      Hallucinations are named among the behavioral problems in the confirmed
      HPII patients.
  - reference: PMID:34302426
    reference_title: "Psychiatric phenotypes associated with hyperprolinemia: A systematic review."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: HUMAN_CLINICAL
    snippet: "From these studies, a common psychiatric phenotype is observed with a high prevalence of developmental delay, intellectual disability, autism spectrum disorders, and psychosis spectrum disorders."
    explanation: >-
      Systematic review reports psychosis spectrum disorders in
      hyperprolinemia. Graded INDIRECT because it pools types I and II rather
      than reporting HPII separately.
biochemical:
- name: Plasma and urinary proline elevation
  presence: INCREASED
  notes: >-
    Elevated plasma proline, with or without measurement of urinary P5C, is the
    basis of the published diagnostic criteria distinguishing HPI from HPII.
  biomarker_term:
    preferred_term: L-proline
    term:
      id: CHEBI:17203
      label: L-proline
  readouts:
  - target: Pyrroline-5-Carboxylate and Proline Accumulation
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: >-
      Elevated proline reports the block in proline catabolism and is the primary
      diagnostic screen for hyperprolinemia.
    evidence:
    - reference: PMID:24931297
      reference_title: Biochemical and clinical features of hereditary hyperprolinemia.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The current study presents diagnostic criteria for HPI and HPII, based on plasma proline level, with or without measurements of urinary P5C."
      explanation: Plasma proline is the analyte underpinning the HPI/HPII diagnostic criteria.
  evidence:
  - reference: PMID:34037900
    reference_title: Metabolic epilepsy in hyperprolinemia type II due to a novel nonsense ALDH4A1 gene variant.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Laboratory investigation revealed significantly elevated proline levels in dried blood spots."
    explanation: Documents significantly elevated proline on dried blood spot testing.
- name: P5C-derived adduct biomarkers
  presence: INCREASED
  notes: >-
    Adducts formed by spontaneous condensation of accumulated P5C with malonic
    acid and acetoacetic acid are detectable in patient body fluids and
    discriminate HPII from HPI, providing direct biochemical evidence that P5C
    accumulates and reacts in vivo.
  biomarker_term:
    preferred_term: 1-pyrroline-5-carboxylate
    term:
      id: CHEBI:15893
      label: 1-pyrroline-5-carboxylate
  readouts:
  - target: Pyridoxal 5'-Phosphate Inactivation by P5C
    relationship: READOUT_OF
    direction: POSITIVE
    endpoint_context: DIAGNOSTIC
    interpretation: >-
      Detection of P5C condensation adducts in vivo supports the same
      condensation chemistry that inactivates pyridoxal 5'-phosphate.
    evidence:
    - reference: PMID:12691858
      reference_title: A novel pyrroline-5-carboxylic acid and acetoacetic acid adduct in hyperprolinaemia type II.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Evidence suggests it formed in vivo."
      explanation: >-
        The authors conclude the P5C-acetoacetate adduct recovered from patient
        urine formed in vivo.
  evidence:
  - reference: PMID:36131087
    reference_title: Identification of Δ-1-pyrroline-5-carboxylate derived biomarkers for hyperprolinemia type II.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we describe the use of a combination of LC-QTOF untargeted metabolomics, NMR spectroscopy and infrared ion spectroscopy (IRIS) to identify and characterize biomarkers for HPII that result of the spontaneous reaction of P5C with malonic acid and acetoacetic acid."
    explanation: >-
      Identifies and structurally characterizes the P5C-derived malonate and
      acetoacetate adducts as HPII biomarkers.
genetic:
- name: ALDH4A1
  gene_term:
    preferred_term: ALDH4A1
    term:
      id: hgnc:406
      label: ALDH4A1
  relationship_type: CAUSATIVE
  notes: >-
    Biallelic ALDH4A1 loss-of-function variants cause HPII. The founding survey
    identified two frameshift alleles [A7fs(-1) and G521fs(+1)] and the S352L
    missense allele as functionally null in a yeast complementation assay, while
    P16L proved to be a benign polymorphism in the Spanish population — a useful
    caution against over-calling ALDH4A1 missense variants. The G521fs(+1) allele
    segregates in the Irish Traveller pedigree that defined the HPII phenotype.
    Relatively few pathogenic variants were known as of 2019, and a nonsense
    variant in the last exon (predicted to escape nonsense-mediated decay) has
    been proposed as a candidate target for readthrough therapy.
  evidence:
  - reference: PMID:9700195
    reference_title: Mutations in the Delta1-pyrroline 5-carboxylate dehydrogenase gene cause type II hyperprolinemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We found four mutant alleles, two with frameshift mutations ... and two with missense mutations (S352L and P16L)."
    explanation: >-
      Reports the four causal alleles (two frameshift, two missense) identified in
      the founding molecular study.
  - reference: PMID:9700195
    reference_title: Mutations in the Delta1-pyrroline 5-carboxylate dehydrogenase gene cause type II hyperprolinemia.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The P16L allele, however, produced fully functional P5CDh and subsequent analysis suggests that it is polymorphic in the relevant (Spanish) population."
    explanation: >-
      The P16L allele retained full activity and is polymorphic, cautioning
      against assuming pathogenicity of ALDH4A1 missense changes.
  - reference: PMID:9700195
    reference_title: Mutations in the Delta1-pyrroline 5-carboxylate dehydrogenase gene cause type II hyperprolinemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Interestingly, the G521fs(+1) allele segregates in the large Irish Traveller pedigree used to define the HPII phenotype."
    explanation: Identifies the founder allele in the pedigree that defined the HPII phenotype.
  - reference: PMID:31884946
    reference_title: "Novel variants in a patient with late-onset hyperprolinemia type II: diagnostic key for status epilepticus and lactic acidosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "So far only four different pathogenic mutations are known."
    explanation: >-
      Notes that only four pathogenic ALDH4A1 variants were known at the time of
      the 2019 report, before the compound heterozygous variants it describes.
prevalence:
- population: Worldwide
  measure_type: UNKNOWN
  prevalence_class: ULTRA_RARE
  notes: >-
    The precise incidence of HPII is unknown. A Japanese national questionnaire
    survey combined with a literature review identified only a single HPII case
    in Japan, consistent with earlier Western reports of extreme rarity.
  evidence:
  - reference: PMID:24931297
    reference_title: Biochemical and clinical features of hereditary hyperprolinemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In addition, the precise incidences of HPI and HPII are unknown."
    explanation: States directly that HPII incidence is not established.
  - reference: PMID:24931297
    reference_title: Biochemical and clinical features of hereditary hyperprolinemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This suggests that hyperprolinemia is a very rare disease in Japan, consistent with earlier reports in Western countries."
    explanation: >-
      A national survey found one HPII case in Japan, supporting an ultra-rare
      classification consistent with Western reports.
diagnosis:
- name: Plasma proline with urinary P5C measurement
  description: >-
    The published diagnostic criteria for hyperprolinemia are based on plasma
    proline level, with or without measurement of urinary
    delta-1-pyrroline-5-carboxylate. Urinary P5C is the discriminating analyte:
    it is present in HPII and absent in HPI, because the earlier proline-oxidase
    block in HPI prevents P5C from forming at all.
  diagnosis_term:
    preferred_term: biochemical diagnostic testing
    term:
      id: NCIT:C18020
      label: Diagnostic Procedure
  markers: plasma proline; urinary delta-1-pyrroline-5-carboxylate (P5C)
  results: >-
    Markedly elevated plasma proline with detectable urinary P5C supports HPII;
    elevated proline without urinary P5C indicates HPI.
  evidence:
  - reference: PMID:24931297
    reference_title: Biochemical and clinical features of hereditary hyperprolinemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The current study presents diagnostic criteria for HPI and HPII, based on plasma proline level, with or without measurements of urinary P5C."
    explanation: >-
      Establishes plasma proline as the basis of the published diagnostic
      criteria for HPI and HPII.
  - reference: PMID:24931297
    reference_title: Biochemical and clinical features of hereditary hyperprolinemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The current study presents diagnostic criteria for HPI and HPII, based on plasma proline level, with or without measurements of urinary P5C."
    explanation: >-
      Urinary P5C measurement is the adjunct that distinguishes HPII within the
      published criteria.
- name: P5C-derived adduct biomarker assay
  description: >-
    Mass-spectrometric and NMR identification of the stable adducts P5C forms
    with malonic acid and acetoacetic acid provides an HPII-specific biomarker
    panel that separates HPII from HPI. The same chemistry that produces these
    adducts is the chemistry proposed to inactivate pyridoxal 5'-phosphate, so
    the assay is a direct readout of the mechanism modeled in this entry.
  diagnosis_term:
    preferred_term: biochemical diagnostic testing
    term:
      id: NCIT:C18020
      label: Diagnostic Procedure
  markers: urinary and plasma P5C-malonic acid and P5C-acetoacetic acid adducts
  results: Detection of P5C-derived adducts distinguishes HPII from HPI.
  evidence:
  - reference: PMID:36131087
    reference_title: Identification of Δ-1-pyrroline-5-carboxylate derived biomarkers for hyperprolinemia type II.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Here, we describe the use of a combination of LC-QTOF untargeted metabolomics, NMR spectroscopy and infrared ion spectroscopy (IRIS) to identify and characterize biomarkers for HPII that result of the spontaneous reaction of P5C with malonic acid and acetoacetic acid."
    explanation: >-
      Identifies the P5C-derived adducts as HPII biomarkers.
  - reference: PMID:36131087
    reference_title: Identification of Δ-1-pyrroline-5-carboxylate derived biomarkers for hyperprolinemia type II.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We show that these biomarkers can differentiate between HPI, caused by a deficiency of proline oxidase activity, and HPII."
    explanation: >-
      The biomarkers discriminate HPII from HPI, which is the diagnostic
      question that plasma proline alone cannot answer.
treatments:
- name: High-Dose Pyridoxine (Vitamin B6) Supplementation
  description: >-
    Because the proposed pathomechanism is consumption of pyridoxal
    5'-phosphate by accumulating P5C, replacing the depleted cofactor is the
    mechanistically targeted intervention. High-dose vitamin B6 abolished
    further seizures in the reported late-onset case, and long-term
    supplementation was proposed as preventive when the P5C-PLP condensation was
    first characterized. Evidence is limited to case-level observation; no
    controlled trial exists.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: nutritional supplementation
    term:
      id: NCIT:C15433
      label: Nutritional Support
    therapeutic_agent:
    - preferred_term: pyridoxine
      term:
        id: CHEBI:16709
        label: pyridoxine
  target_mechanisms:
  - target: Secondary Vitamin B6 Deficiency
    treatment_effect: RESTORES
    description: >-
      Pyridoxine supplementation replenishes the vitamin B6 pool depleted by P5C
      adduct formation, counteracting the secondary deficiency.
  evidence:
  - reference: PMID:31884946
    reference_title: "Novel variants in a patient with late-onset hyperprolinemia type II: diagnostic key for status epilepticus and lactic acidosis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Under high-dose vitamin-B6 therapy no further seizures occurred."
    explanation: >-
      High-dose vitamin B6 stopped the seizures in the reported ALDH4A1-confirmed
      patient.
  - reference: PMID:11134058
    reference_title: "Pyridoxal phosphate de-activation by pyrroline-5-carboxylic acid. Increased risk of vitamin B6 deficiency and seizures in hyperprolinemia type II."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Vitamin B6 de-activation may contribute to seizures in hyperprolinemia type II, which are so far unexplained, but they may be preventable with long term vitamin B6 supplementation."
    explanation: >-
      Proposes long-term vitamin B6 supplementation as preventive; this is a
      mechanistic proposal from an in vitro study, not clinical trial evidence.
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical course was non-progressive and independent from the B6 concentration and B6 therapy."
    explanation: >-
      Counter-evidence: in the only published HPII cohort, the clinical course
      was non-progressive and independent of both B6 concentration and B6
      therapy. This is the largest series in the literature and directly
      opposes the single-case observation of B6-responsive seizures, so the
      benefit of pyridoxine in HPII should be regarded as unresolved rather
      than established.
  notes: >-
    The evidence for this treatment is genuinely conflicting and is curated as
    such: one case report describes seizures stopping under high-dose vitamin
    B6, while the only cohort study reports a course independent of B6 therapy.
    No controlled trial exists. The conflict is a live question rather than a
    settled contraindication, and is tracked in
    gap_hpii_plp_seizure_causality.
discussions:
- discussion_id: gap_hpii_plp_seizure_causality
  prompt: >-
    Is P5C-mediated inactivation of pyridoxal 5'-phosphate actually the cause of
    seizures in hyperprolinemia type 2, and which PLP-dependent enzymes (e.g.
    glutamate decarboxylase and the GABA synthetic pathway) are rate-limiting in
    vivo? Do all symptomatic HPII patients have measurable vitamin B6 depletion,
    and does pyridoxine responsiveness track with it?
  kind: KNOWLEDGE_GAP
  status: OPEN
  rationale: >-
    The PLP-inactivation mechanism is supported by in vitro chemistry, in vivo
    recovery of analogous P5C adducts, and single-case observations of low serum
    B6 with B6-responsive seizures — but it remains explicitly framed in the
    primary literature as what "is thought to be" the pathophysiological
    mechanism. The intervening neurotransmitter steps have not been measured in
    HPII patients, so the link from cofactor depletion to seizure generation is
    inferred from general vitamin B6 biochemistry. The one cohort study to
    address it partially answers the pyridoxine-responsiveness half of the
    question in the negative: all four patients had low-normal B6 and the
    clinical course was independent of both B6 concentration and B6 therapy.
    That result does not settle the mechanism — the cohort was not designed as
    a treatment trial and n=4 — but it means the gap is now one of conflicting
    evidence rather than absent evidence.
  attaches_to:
  - pathophysiology#Secondary Vitamin B6 Deficiency
  - pathophysiology#Seizures and Metabolic Encephalopathy
  evidence:
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical course was non-progressive and independent from the B6 concentration and B6 therapy."
    explanation: >-
      Partially addresses this gap: clinical course was independent of B6
      concentration and B6 therapy in the only HPII cohort, arguing against a
      simple B6-depletion-drives-seizures model.
  - reference: PMID:24173411
    reference_title: "Long-term clinical outcome, therapy and mild mitochondrial dysfunction in hyperprolinemia."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We found four patients with increased urinary P5C diagnosed with hyperprolinemia type II, and only one patient had hyperprolinemia type I. All children with hyperprolinemia type II had low normal B6 concentration, and three of the patients had biochemical markers suggesting mitochondrial dysfunction."
    explanation: >-
      All four HPII patients had low-normal rather than frankly deficient B6,
      which bears on whether measurable depletion is present in every
      symptomatic patient.
- discussion_id: gap_hpii_penetrance_and_phenotype
  prompt: >-
    Why do only some individuals with biallelic ALDH4A1 loss-of-function
    variants develop seizures or neurodevelopmental impairment while others
    remain asymptomatic, and does HPII act principally as a threshold-lowering
    susceptibility factor unmasked by febrile intercurrent illness rather than
    as a fully penetrant epilepsy syndrome?
  kind: KNOWLEDGE_GAP
  status: OPEN
  rationale: >-
    The clinical spectrum of HPII is described in the literature as unclear, with
    ascertainment dominated by small case reports. The one Japanese case was
    ascertained during influenza-associated encephalopathy, and the reported
    infantile case decompensated after an upper respiratory infection, suggesting
    a stress-unmasked susceptibility model that has never been tested
    systematically. A systematic review of 35 studies found no correlation
    between biochemical and clinical phenotype, so severity is not predicted by
    proline level and the modifiers of penetrance remain entirely unidentified.
  attaches_to:
  - pathophysiology#Seizures and Metabolic Encephalopathy
  - pathophysiology#Neurodevelopmental and Behavioral Impairment
  evidence:
  - reference: PMID:34302426
    reference_title: "Psychiatric phenotypes associated with hyperprolinemia: A systematic review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "No evidence for a biochemical phenotype-clinical phenotype correlation was found; that is, no association between higher proline levels and specific psychiatric phenotypes was observed."
    explanation: >-
      Establishes that no biochemical-clinical phenotype correlation has been
      demonstrated, and specifies what that means: higher proline levels are not
      associated with specific phenotypes, so metabolite burden does not explain
      the variable presentation. The gap is genuinely open.
notes: >-
  IEMbase package seed WP-003 (issue #5558), classification code 1.7.06.01
  (ALDH4A1-related pyrroline-5-carboxylate dehydrogenase deficiency);
  OMIM:239510; ORPHA:79101. MONDO provides an exact disease term
  (MONDO:0009401, gene HGNC:406 ALDH4A1, OMIM xref 239510), so HPII is curated
  as a standalone Disease entry rather than a subtype. Hyperprolinemia type 1
  (PRODH, MONDO:0009400, WP-003 row 1.7.05.01) is a mechanistically distinct
  entry that remains to be curated: it blocks the pathway one step earlier and
  does not generate P5C, which is why it lacks the PLP-inactivation arm modeled
  here. The entry deliberately does not declare conformance to
  metabolic_intoxication_decompensation: although febrile illness precipitates
  HPII crises, the toxic species acts as a cofactor antagonist rather than
  through the module's acidosis/hyperammonemia/hypoglycemia decompensation
  chain.
📚

References & Deep Research

Deep Research

1
Hyperprolinemia Type 2 (ALDH4A1, MONDO:0009401) — Claude Code literature sweep ▸

Hyperprolinemia Type 2 (ALDH4A1, MONDO:0009401) — Claude Code literature sweep

Date: 2026-07-31 Tool: Claude Code (curation scanner, high_effort tier) Target entry: kb/disorders/Hyperprolinemia_Type_2.yaml Context: review follow-up on PR #7338 (issue #5558, IEMbase WP-003 row 1.7.06.01)

This artifact records a systematic literature sweep run after the initial entry was drafted, in response to review finding #1 on PR #7338 ("no deep-research artifact ships with this new entry, and findings 2–4 are all things a literature sweep would have surfaced"). It documents what was searched, what was found, and — importantly — which candidate papers were inspected and deliberately not used.

Method

PubMed E-utilities esearch, relevance-sorted, retmax=40, three complementary queries chosen to catch gene-anchored, disease-name-anchored, and enzyme-anchored literature respectively:

Query Hits Retrieved
ALDH4A1 89 40
hyperprolinemia type II 61 40
pyrroline-5-carboxylate dehydrogenase deficiency 30 30

Titles for every PMID appearing in the union that was not already cited by the entry were retrieved via esummary and triaged. Every paper selected for use was then fetched with just fetch-reference and each snippet verified as an exact substring of the cached abstract via just validate-references.

Baseline check

GeneReviews: none exists. Confirmed independently in the PR review — both hyperprolinemia[TI] AND GeneReviews[Book] and (hyperprolinemia OR ALDH4A1 OR "pyrroline-5-carboxylate dehydrogenase") AND GeneReviews[Book] return zero results against a working control. No GeneReviews baseline requirement applies to this entry.

Orphanet: ORPHA:79101 cached via just structured-rebuild-orphanet --id 79101. The record carries a definition and cross-references but no HPO phenotype table, so it cannot supply frequency annotations for this disorder. Its definition sentence is used as an evidence snippet for the intellectual-deficit / developmental-delay phenotypes.

Papers newly incorporated

PMID:24173411 — van de Ven 2014, J Inherit Metab Dis

The largest HPII series in the literature: 4 metabolically confirmed patients ascertained from 20,991 urinary organic acid profiles. Supplies four things the entry lacked:

  1. Counter-evidence to the pyridoxine claim — "The clinical course was non-progressive and independent from the B6 concentration and B6 therapy." Curated as supports: REFUTE on the treatment, against the single supporting case report. The treatment is now presented as genuinely conflicting rather than established.
  2. Intellectual disability — 2 of 4 patients.
  3. Behavioral phenotype — 4 of 4 with "significant behavioral problems, including anxiety and hallucinations."
  4. The mitochondrial arm — 3 of 4 with biochemical markers of mitochondrial dysfunction, biopsy-confirmed in one.

PMID:34302426 — Namavar 2021, Am J Med Genet B

PRISMA systematic review, 1753 studies screened, 35 included. Reports a common psychiatric phenotype (developmental delay, intellectual disability, ASD, psychosis spectrum) and — critically — no biochemical–clinical phenotype correlation. Used both as phenotype support and as the reason the neurodevelopmental pathophysiology node is marked HYPOTHETICAL.

Scope caveat, applied throughout: this review pools hyperprolinemia types I and II. Every citation of it in the entry is therefore supports: PARTIAL, and the HPII-specific anchors (PMID:24173411, ORPHA:79101, PMID:41602883) carry the SUPPORT weight.

PMID:21168532 — He & DiMario 2011, Mitochondrion

Not flagged in the PR review; surfaced by the ALDH4A1 and enzyme-anchored queries. A Drosophila P5CDh-null model ("establishing a fly model for human type II hyperprolinemia") showing doubled proline together with swollen mitochondria and larval/pupal lethality. This is independent, cross-species corroboration of the mitochondrial arm from a completely different system, which is what justifies modeling that arm at all rather than treating the human n=4 finding as noise. Tagged MODEL_ORGANISM. The authors' own "first correlation between the loss of P5CDh and morphological defects in mitochondria" framing is quoted to keep the node PROVISIONAL.

PMID:41602883 — AlQurashi 2025, Front Pediatr

Also not flagged in the review; the most recent HPII case report. A consanguineous Saudi child presenting with global developmental delay and clinically diagnosed ASD, with a homozygous ALDH4A1 variant. Supplies the only HPII-specific primary citation for developmental delay (previously resting on the Orphanet definition alone).

Caveat recorded in the entry: the variant is of uncertain significance, not an established pathogenic allele. The ASD phenotype is therefore curated as two PARTIAL items with no frequency band, and the developmental-delay note states the VUS status explicitly.

Candidates inspected and NOT used

Recording these so a future curator does not re-triage them:

PMID Title Why not used
37141741 PYCR2 deficiency causes hereditary spastic paraplegia Different gene/disease (PYCR2, not ALDH4A1). Relevant to the deferred PYCR1/PYCR2 scoping question on #5558, not to this entry.
23462603 PRODH mutations in Korean neonates with type I hyperprolinemia HPI, not HPII.
24842239 Long-term neuropsychiatric follow-up in hyperprolinemia type I HPI, not HPII. Belongs to the future HPI entry.
21643764 Behavioral and neurochemical effects of proline Proline-loading pharmacology, not HPII disease biology.
28712849 Structure, function, mechanism of proline utilization A (PutA) Bacterial enzymology; entry already cites the human structural paper (PMID:22516612).
18806117 Inborn errors of proline metabolism Review; adds no claim not already primary-sourced.
26693506 SAXS fingerprints of aldehyde dehydrogenase oligomers Biophysics of the ALDH family generally, not disease-relevant.
18062169 / 36980111 / 30930802 Vitamin B6-related / B6-dependent epilepsies Cover the primary B6-dependent epilepsies (ALDH7A1, PNPO). HPII causes a secondary B6 deficiency by a different route; citing these would blur a distinction the entry deliberately makes.
25391710 Schizophrenia/first-episode psychosis in children General psychiatry; hyperprolinemia not the subject.
9590014 [Hyperprolinemia type II] Japanese-language 1998 review; superseded by PMID:24931297, already cited.

Named Entity Confusion (NEC) preflight

Re-verified for this sweep, since "hyperprolinemia type II" is a numbered-series label of exactly the kind flagged in research/nec_risk_disease_classes.md:

uv run runoak -i sqlite:obo:mondo info MONDO:0009401 -O obo
  relationship: RO:0004003 HGNC:406 ! ALDH4A1
  xref: OMIM:239510
  xref: Orphanet:79101

Gene, OMIM, and ORPHA all match WP-003 row 1.7.06.01. Every paper incorporated above was checked to be about ALDH4A1/HPII specifically, with the HPI-pooling caveat on PMID:34302426 handled by downgrading it to PARTIAL. PASS.

Residual gaps after this sweep

Both are tracked as KNOWLEDGE_GAP discussions in the entry rather than left implicit:

  • gap_hpii_plp_seizure_causality — the PLP-depletion-causes-seizures model is now a matter of conflicting evidence, not absent evidence: one case reports B6-responsive seizures, the only cohort reports a course independent of B6 therapy. No controlled trial exists and none is likely at this prevalence.
  • gap_hpii_penetrance_and_phenotype — no biochemical–clinical correlation has been demonstrated, so neither penetrance nor severity is predicted by proline level, and the modifiers remain unidentified.

A third gap is noted here but not curated as a discussion because it is a literature-coverage limitation rather than a mechanistic one: every clinical statement about HPII rests on a total of roughly a dozen patients worldwide. No frequency bands are curated anywhere in the entry for this reason.