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Mappings

MONDO

MONDO:0014632 leukodystrophy, hypomyelinating, 10

Primary MONDO identifier for this entity. Its definition is gene-anchored (RO:0004003 HGNC:30262 PYCR2) and it carries OMIM:616420, Orphanet:481152, DOID:0060788, MEDGEN:904191, GARD:0025008 and UMLS:C4225332 as cross-references, all confirmed with OAK.

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Inheritance

1

Autosomal recessive inheritance HP:0000007

HLD10 is inherited in an autosomal recessive manner. Reported families are predominantly consanguineous with homozygous variants; compound heterozygous genotypes are a minority.

Autosomal recessive inheritance

Show evidence (2 references)

PMID:34037307 SUPPORT Human Clinical

"PYCR2 pathogenic variants lead to an autosomal recessive hypomyelinating leukodystrophy 10 (HLD10), characterized by global developmental delay, microcephaly, facial dysmorphism, movement disorder, and hypomyelination."

States the autosomal recessive mode of inheritance for HLD10.

PMID:34037307 SUPPORT Human Clinical

"We also reviewed the phenotype and genotype of all 35 previously reported PYCR2 patients and found that majorities of cases were homozygous with a consanguineous family history"

Documents that most reported genotypes are homozygous in consanguineous families, the expected pattern for a recessive disorder.

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Discussions and Knowledge Gaps

3

If proline is not depleted, what exactly does the cell lose when PYCR2 is absent, and why is the deficit restricted to the CNS?

KNOWLEDGE GAP OPEN hld10_mechanism_beyond_proline

Attached to

pathophysiology#Preserved Bulk Proline Supply by Paralogue Redundancy

Every measurement to date - patient plasma and CSF, patient plasma metabolomics, mouse serum and fibroblast lysates - finds proline normal, and the paralogues are largely redundant for bulk proline supply. Yet PYCR2 loss produces a severe, CNS-restricted disease. Three non-exclusive candidate answers are on the table: a local proline or P5C pool in a compartment or cell type that bulk measurement cannot resolve; a redox role, since PYCR2 also cycles NAD(P)H/NAD(P)+; and the SHMT2/glycine diversion, which is the only one with a rescue experiment behind it. Distinguishing them matters therapeutically, because proline supplementation, redox support and glycine lowering are entirely different interventions - and the mouse dietary experiment already shows proline availability modulates severity even though proline is not depleted.

Show evidence (2 references)

PMID:38709052 SUPPORT Human Clinical

"Our findings and all the previous reports suggest that proline auxotrophy is not the central disease mechanism."

States the gap - the obvious metabolic explanation has been excluded.

PMID:33734376 SUPPORT Model Organism

"Thus, Pycr1 and -2 have redundant functions in proline biosynthesis, and their loss makes proline a semi-essential amino acid."

Frames the paradox - redundancy for bulk supply, yet a severe phenotype and a diet-dependent modulation of severity.

Do Pycr2-null mice actually reproduce the CNS hypomyelination and progressive microcephaly that define HLD10 in humans?

HUMAN MODEL MISMATCH OPEN hld10_mouse_hypomyelination_mismatch

Attached to

pathophysiology#Impaired Oligodendroglial Differentiation and CNS Hypomyelination

The two published Pycr2 mouse lines are described very differently. One is reported to phenocopy the human disorder and is the source of the cerebral glycine finding; the other, characterised in detail, reports a neurological and neuromuscular syndrome of kyphosis, hind-limb clasping and weight loss, with only mild peripheral axonal atrophy and no reported assessment of myelination or head size. Since hypomyelination and postnatal progressive microcephaly are precisely the features that define the human disease, the hypomyelination arm of the mechanism currently rests on a heterologous oligodendroglial cell line rather than on any in vivo model. Until a mouse is shown to be hypomyelinated, mouse rescue experiments cannot be read as evidence about the human white-matter phenotype.

Proposed experiments

Myelin quantification in Pycr2-null mouse CNS

hld10_mouse_myelin_quantification

Quantify myelin content and oligodendrocyte maturation in Pycr2-null mouse brain across postnatal development - MBP and CNPase immunoblotting and immunohistochemistry, electron-microscopic g-ratio measurement of corpus callosum axons, and serial head or brain-volume measurement - to establish whether the model is hypomyelinated and microcephalic or only neurologically impaired.

Show evidence (2 references)

PMID:33734376 SUPPORT Model Organism

"Mice with recessive loss-of-function mutations in Pycr2 showed phenotypes consistent with neurological and neuromuscular disorders, including weight loss, kyphosis, and hind-limb clasping."

The detailed characterisation names a neurological syndrome but not hypomyelination or microcephaly, which is the mismatch.

PMID:32330411 SUPPORT Model Organism

"We find that knocking out Pycr2 in mice phenocopies the human disorder and depletes PYCR1 levels in neural lineages."

The competing claim of full phenocopy, stated without a reported myelin or head-size measurement in the abstract.

Is HLD10 a true hypomyelinating leukodystrophy, or a primary neuronal disorder with secondary hypomyelination that is misplaced in the HLD series?

CONTROVERSY OPEN hld10_leukodystrophy_vs_leukoencephalopathy

Attached to

pathophysiology#Progressive Neuronal Loss and Brain Atrophy

The name says leukodystrophy, but the largest cohort argued the opposite. Its authors noted that imaging ranged from subtle white-matter T2 hyperintensity to progressive brain atrophy, and that the clinical picture of early-onset intellectual disability, microcephaly, epilepsy and atrophy points to primary neuronal dysfunction with a secondary myelination deficit - which is the definition of a genetic leukoencephalopathy rather than a classical HLD. This is not merely nosological: it determines whether oligodendrocyte-directed or neuron-directed interventions are the rational target. The cell-biology evidence has since given the oligodendrocyte arm independent support, so the honest current position is that both processes are real and their relative primacy is unsettled.

Show evidence (3 references)

PMID:27130255 SUPPORT Human Clinical

"The clinical features of early onset intellectual disability, microcephaly, epilepsy, and brain atrophy suggest primary neuronal dysfunction with secondary deficit in myelination"

States the primary-neuronal position from the largest cohort.

PMID:27130255 SUPPORT Human Clinical

"Although this condition has been classified as a hypomyelinating leukodystrophy, we found that the imaging features were variable ranging from subtle white matter T2 hyperintensities to progressive brain atrophy."

Records the imaging variability that motivates the reclassification argument.

PMID:36548190 REFUTE In Vitro

"Taken together, these results indicate that an HLD10-associated PYCR2 mutation leads to the formation of large mitochondria with decreased activities, inhibiting oligodendroglial cell morphological differentiation."

Counters the purely-neuronal reading by demonstrating a cell-autonomous oligodendroglial differentiation defect caused by the disease alleles.

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Pathophysiology

7

Biallelic PYCR2 Loss of Function

Biallelic PYCR2 variants - nonsense, splice-site and missense - reduce the amount or the catalytic competence of pyrroline-5-carboxylate reductase 2. Missense alleles act by two demonstrated routes. Some (p.Arg119Cys, p.Arg251Cys) leave mitochondrial targeting intact but lower steady-state protein levels and severely reduce catalytic efficiency, and p.Arg251Cys in addition carries a pronounced folding defect. Others cluster in the dimerisation domain and impair multimerisation of the obligate-oligomeric enzyme; the p.Gly249Val allele lies directly at the dimer interface.

Genetic context PYCR2 hgnc:30262 variant_origin: GERMLINE zygosity: HOMOZYGOUS functional_impact_category: LOSS_OF_FUNCTION

L-proline biosynthetic process GO:0055129 ↓ DECREASED

pyrroline-5-carboxylate reductase activity GO:0004735 ↓ DECREASED

Downstream

Preserved Bulk Proline Supply by Paralogue Redundancy Loss of PYCR2 does not deplete measurable proline pools, because the mitochondrial paralogue PYCR1 can carry out the same reduction.

Mitochondrial Dysfunction and Oxidative Stress Susceptibility Loss of PYCR2 impairs mitochondrial membrane potential and morphology and sensitises cells to oxidative-stress-induced apoptosis.

SHMT2 Upregulation and Cerebral Glycine Accumulation Loss of PYCR2 upregulates SHMT2, diverting one-carbon metabolism towards glycine synthesis.

Show evidence (6 references)

PMID:25865492 SUPPORT Human Clinical

"we report mutations in PYCR2, which encodes an enzyme in the proline biosynthesis pathway, as the cause of a unique syndrome characterized by postnatal microcephaly, hypomyelination, and reduced cerebral white-matter volume"

Establishes PYCR2 as the causal gene and names the proline-biosynthesis role of its product.

PMID:25865492 SUPPORT In Vitro

"When mutant cDNAs were transfected into HEK293FT cells, both variant proteins retained normal mitochondrial localization but had lower amounts than the wild-type protein, suggesting that the variant proteins were less stable."

Shows the two founding missense alleles reduce protein abundance rather than mislocalise the enzyme, i.e. a loss-of-function mechanism.

PMID:27130255 SUPPORT In Vitro

"Mutations clustered in the dimerization domain of the protein, and several tested mutations impaired multimerization."

Identifies impaired multimerisation as a second route to loss of function.

+ 3 more references

Preserved Bulk Proline Supply by Paralogue Redundancy

Despite the enzymatic block, proline itself is not depleted. Plasma and CSF proline are normal in patients, routine metabolic screens are unremarkable, and serum proline is not reduced in Pycr2-null mice. PYCR1 is largely functionally redundant with PYCR2 for bulk proline supply, so proline auxotrophy is not the central disease mechanism. The cytosolic third paralogue PYCR3 sits in a different compartment and is fed P5C from ornithine by ornithine aminotransferase rather than from glutamate; it was not tested for redundancy and is deliberately not part of this claim. Proline is nonetheless semi-essential in the mutant state: a proline-free diet worsens the mouse phenotype. This node is curated explicitly because it constrains every downstream mechanistic claim and rules out a metabolite-replacement rationale.

L-proline biosynthetic process GO:0055129 ↓ DECREASED

Animal models

Pycr2 loss-of-function mouse (chemically induced p.Met1Thr) Mouse recapitulates The mouse reproduces the defining negative finding of the human disease - an enzymatic block without proline depletion - and adds the dietary manipulation that shows proline is semi-essential rather than irrelevant.

Show evidence (7 references)

PMID:38709052 SUPPORT Human Clinical

"While plasma and CSF proline levels were found to be totally normal, untargeted metabolomic profiling revealed mild increases of glutamate, alpha-ketoglutarate, and l-glutamate semialdehyde and marked increases of inosine and xanthine."

Direct measurement showing proline is not depleted in patient plasma or CSF.

PMID:38709052 SUPPORT Human Clinical

"Our findings and all the previous reports suggest that proline auxotrophy is not the central disease mechanism."

States the negative conclusion this node encodes.

PMID:27860360 SUPPORT Human Clinical

"No significant changes were identified in proline biosynthesis pathway or related metabolites."

Independent plasma metabolomics in three severely affected patients finds no proline-pathway change.

+ 4 more references

Mitochondrial Dysfunction and Oxidative Stress Susceptibility

PYCR2 is a mitochondrial matrix enzyme, and its loss is felt as a mitochondrial rather than a metabolite-supply defect. PYCR2-deficient cells show decreased mitochondrial membrane potential and markedly increased apoptosis under oxidative stress. In an oligodendroglial cell model, the disease missense proteins additionally shift the fusion-fission balance towards fusion, producing abnormally large mitochondria with decreased activity.

oligodendrocyte CL:0000128

regulation of mitochondrial membrane potential GO:0051881 ↓ DECREASED mitochondrial fusion GO:0008053 ↑ INCREASED mitochondrial fission GO:0000266 ↓ DECREASED cellular response to oxidative stress GO:0034599 ⚠ ABNORMAL apoptotic process GO:0006915 ↑ INCREASED

Experimental models

CRISPR-Cas9 PYCR2-deficient HEK293FT cell line CELL LINE Isogenic loss-of-function evidence isolating the mitochondrial and apoptotic consequences of PYCR2 loss from genetic background.

Downstream

Impaired Oligodendroglial Differentiation and CNS Hypomyelination Mitochondrial dysfunction in oligodendroglial cells blocks the morphological differentiation required to produce myelin.

Progressive Neuronal Loss and Brain Atrophy Impaired mitochondrial function and increased apoptotic susceptibility in neural lineages contribute to postnatal neurodegeneration.

Show evidence (3 references)

PMID:25865492 SUPPORT In Vitro

"A PYCR2-deficient HEK293FT cell line generated by genome editing with the clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system showed that PYCR2 loss of function led to decreased mitochondrial membrane potential and increased susceptibility to apoptosis under oxidative stress."

Isogenic knockout evidence that PYCR2 loss depolarises mitochondria and sensitises cells to oxidative-stress-induced apoptosis.

PMID:36548190 SUPPORT In Vitro

"Expression of each of the mutated R119C and R251C proteins in cells increased the fusion abilities in mitochondria and decreased their fission abilities relatively."

Establishes the fusion-shifted mitochondrial dynamics caused by the disease missense proteins.

PMID:36548190 SUPPORT In Vitro

"The respective mutant proteins, but not wild type proteins also decreased the activities of mitochondria."

Shows the enlarged mitochondria are functionally impaired, not merely morphologically abnormal.

SHMT2 Upregulation and Cerebral Glycine Accumulation

Loss of PYCR2 upregulates serine hydroxymethyltransferase 2 (SHMT2), the mitochondrial one-carbon enzyme responsible for glycine synthesis, and cerebral glycine rises in both Pycr2-null mice and patients. The excess glycine is not a bystander: knocking down SHMT2 partially reverses it and rescues the axonal beading and shortened neurites of cultured Pycr2-knockout neurons, identifying the glycine pathway as the leading candidate intervention point. Elevated brain glycine is detectable non-invasively as an MR-spectroscopy glycine peak, including in the mild spastic-paraplegia-only phenotype where white matter is normally myelinated.

glycine biosynthetic process GO:0006545 ↑ INCREASED glycine metabolic process GO:0006544 ↕ DYSREGULATED

glycine hydroxymethyltransferase activity GO:0004372 ↑ INCREASED

Animal models

Pycr2 knockout mouse (Escande-Beillard) Mouse recapitulates Excess cerebral glycine was measured in the mutant mouse brain and in patients in the same study, and the SHMT2 knockdown rescue establishes the mechanism rather than merely the association.

Downstream

Progressive Neuronal Loss and Brain Atrophy Glycine-driven encephalopathy contributes to the neuronal injury underlying progressive microcephaly and brain atrophy; SHMT2 knockdown rescues axonal beading and neurite length in Pycr2-knockout neurons.

Show evidence (4 references)

PMID:32330411 SUPPORT Model Organism

"In situ quantification of neurotransmitters in the brains of PYCR2 mutant mice and patients revealed a signature of encephalopathy driven by excessive cerebral glycine."

Direct measurement of excess cerebral glycine in both the mouse model and patients.

PMID:32330411 SUPPORT In Vitro

"Mechanistically, we demonstrate that loss of PYCR2 upregulates SHMT2, which is responsible for glycine synthesis."

Identifies SHMT2 upregulation as the mechanism generating the glycine excess.

PMID:32330411 SUPPORT In Vitro

"This hyperglycemia could be partially reversed by SHMT2 knockdown, which rescued the axonal beading and neurite lengths of cultured Pycr2 knockout neurons."

Rescue experiment establishing the glycine excess is causally upstream of a neuronal phenotype rather than an epiphenomenon. Note the source sentence says "hyperglycemia" where "hyperglycinemia" is meant; the quote is reproduced verbatim.

+ 1 more reference

Impaired Oligodendroglial Differentiation and CNS Hypomyelination

Oligodendroglial cells expressing the HLD10 missense proteins fail to undergo the morphological differentiation that precedes myelination, with reduced expression of differentiation and myelin marker proteins. Because myelination is highly energy-dependent, the enlarged, hypoactive mitochondria provide a direct link from the enzymatic lesion to the white-matter phenotype. The resulting CNS hypomyelination and reduced cerebral white-matter volume are the imaging hallmark of the disease.

oligodendrocyte CL:0000128 oligodendrocyte precursor cell CL:0002453

oligodendrocyte differentiation GO:0048709 ↓ DECREASED central nervous system myelination GO:0022010 ↓ DECREASED

white matter of the central nervous system UBERON:0002316

Experimental models

FBD-102b oligodendroglial differentiation model expressing HLD10 PYCR2 variants CELL LINE Provides the direct cellular link from the PYCR2 lesion to a failure of oligodendroglial differentiation, the proximate cause of hypomyelination.

Downstream

Severe Neurodevelopmental Impairment and Early Mortality Deficient myelination of the cerebral white matter contributes to the profound neurodevelopmental impairment. INDIRECT because the relative contribution of the white-matter and neuronal arms is unsettled - see the leukodystrophy-versus-leukoencephalopathy discussion.

Show evidence (3 references)

PMID:36548190 SUPPORT In Vitro

"While cells expressing the wild type proteins exhibited differentiated phenotypes with widespread membranes and increased expression levels of differentiation marker proteins following the induction of differentiation, cells harboring each of the mutant proteins did not."

Shows the disease alleles block oligodendroglial morphological differentiation.

PMID:36548190 SUPPORT In Vitro

"Taken together, these results indicate that an HLD10-associated PYCR2 mutation leads to the formation of large mitochondria with decreased activities, inhibiting oligodendroglial cell morphological differentiation."

States the causal chain from mitochondrial abnormality to the differentiation block.

PMID:25865492 SUPPORT Human Clinical

"Hypomyelination and the absence of lax, wrinkly skin distinguishes this condition from that caused by previously reported mutations in the gene encoding PYCR2's isozyme, PYCR1, suggesting a unique and indispensable role for PYCR2 in the human CNS during development."

Establishes hypomyelination as the defining CNS consequence and separates it from the PYCR1 disease.

Progressive Neuronal Loss and Brain Atrophy

Alongside the white-matter defect there is a primary neuronal process. Patients are born with normal or only mildly reduced head circumference and then acquire microcephaly postnatally, with progressive global brain atrophy, ventriculomegaly and a thinning corpus callosum, while cerebellum and brainstem are relatively spared. The combination of early-onset intellectual disability, epilepsy and atrophy has been read by the original cohort authors as primary neuronal dysfunction with secondary hypomyelination - which is why HLD10 is sometimes placed among the genetic leukoencephalopathies rather than the classical hypomyelinating leukodystrophies.

neuron CL:0000540

neuron apoptotic process GO:0051402 ↑ INCREASED

brain UBERON:0000955

Animal models

pycr1b morpholino knockdown zebrafish Zebrafish partially recapitulates Reproduces reduced head and brain size, the developmental counterpart of the human microcephaly, with allele-specific rescue confirming specificity.

Downstream

Severe Neurodevelopmental Impairment and Early Mortality Progressive loss of cerebral tissue underlies the developmental stagnation, failure to thrive and reduced survival.

Show evidence (3 references)

PMID:32330411 SUPPORT Human Clinical

"Patients lacking PYCR2, a mitochondrial enzyme that synthesizes proline, display postnatal degenerative microcephaly with hypomyelination."

States the postnatal, degenerative character of the microcephaly.

PMID:27130255 SUPPORT Human Clinical

"The clinical features of early onset intellectual disability, microcephaly, epilepsy, and brain atrophy suggest primary neuronal dysfunction with secondary deficit in myelination"

Records the cohort authors' interpretation that the neuronal process is primary and the myelination deficit secondary.

PMID:27130255 SUPPORT Human Clinical

"Brain magnetic resonance imaging showed global brain atrophy and white matter T2 hyperintensities."

Imaging evidence of global brain atrophy in the 14-patient cohort.

Severe Neurodevelopmental Impairment and Early Mortality

The clinical convergence point. Affected children stagnate at roughly a 4-to-5-month developmental level, never acquire meaningful language, fine motor skills or independent ambulation, and develop failure to thrive by one year of age. In the largest cohort no patient survived beyond the first decade, with death from pulmonary infection, fever of unknown origin or failure to thrive.

Animal models

Pycr2 loss-of-function mouse (chemically induced p.Met1Thr) Mouse partially recapitulates The mouse develops a neurological and neuromuscular syndrome - kyphosis, hind-limb clasping, weight loss, reduced grip strength - consistent with the human neurological impairment.

Show evidence (3 references)

PMID:27130255 SUPPORT Human Clinical

"The characteristic clinical presentation of patients with PYCR2 mutations included failure to thrive, microcephaly, craniofacial dysmorphism, progressive psychomotor disability, hyperkinetic movements, and axial hypotonia with variable appendicular spasticity."

Defines the clinical syndrome that this node represents.

PMID:27130255 SUPPORT Human Clinical

"Patients did not survive beyond the first decade of life."

Establishes the mortality outcome in the reference cohort.

PMID:27130255 SUPPORT Human Clinical

"All stagnated at the developmental level of a 4-5-month old infant."

Quantifies the developmental ceiling reached by affected children.

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Pathograph

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

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Phenotypes

23

Digestive 1

Vomiting OCCASIONAL HP:0002013

29% (4/14) in PMID:27130255.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Excessive vomiting of unknown etiology was seen in 29% of patients"

Gives the explicit 29% frequency supporting the OCCASIONAL band.

Eye 1

Nystagmus OCCASIONAL HP:0000639

21% (3/14) in PMID:27130255.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Nystagmus was noted in 21% of patients, commonly manifested as slow horizontal and symmetric eye movements in both directions."

Gives the explicit 21% frequency supporting the OCCASIONAL band.

Head and Neck 4

Triangular Face FREQUENT HP:0000325

Described in "most of the patients" in PMID:27130255 (n=14) without an explicit numerator.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"A characteristic craniofacial dysmorphism was noted in most of the patients, which included triangular facies, malar hypoplasia, large malformed ears with over-folded helices, upturned nose with bulbous tip, and a long smooth philtrum"

Enumerates the craniofacial gestalt, including triangular facies.

Malar Flattening FREQUENT HP:0000272

Same "most of the patients" source figure as the other gestalt components (PMID:27130255, n=14).

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"A characteristic craniofacial dysmorphism was noted in most of the patients, which included triangular facies, malar hypoplasia, large malformed ears with over-folded helices, upturned nose with bulbous tip, and a long smooth philtrum"

Lists malar hypoplasia among the characteristic craniofacial features.

Bulbous Nose FREQUENT HP:0000414

Same "most of the patients" source figure as the other gestalt components (PMID:27130255, n=14).

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"A characteristic craniofacial dysmorphism was noted in most of the patients, which included triangular facies, malar hypoplasia, large malformed ears with over-folded helices, upturned nose with bulbous tip, and a long smooth philtrum"

Lists the upturned bulbous-tipped nose among the characteristic craniofacial features.

Long Philtrum FREQUENT HP:0000343

Same "most of the patients" source figure as the other gestalt components (PMID:27130255, n=14).

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"A characteristic craniofacial dysmorphism was noted in most of the patients, which included triangular facies, malar hypoplasia, large malformed ears with over-folded helices, upturned nose with bulbous tip, and a long smooth philtrum"

Lists the long smooth philtrum among the characteristic craniofacial features.

Limbs 1

Arachnodactyly OCCASIONAL HP:0001166

4/14 patients in PMID:27130255 (29%).

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Skeletal features of long thin fingers and toes were observed in four patients, while two had pectus carinatum."

Gives the 4/14 numerator supporting the OCCASIONAL band.

Musculoskeletal 5

Spasticity FREQUENT HP:0001257

57% (8/14) in PMID:27130255; described as common across the 35-patient review in PMID:34037307.

Show evidence (2 references)

PMID:27130255 SUPPORT Human Clinical

"Mild to severe appendicular spasticity was present in 57%."

Provides the explicit 57 percent numerator for the FREQUENT band.

PMID:34037307 SUPPORT Human Clinical

"Hypotonia and peripheral spasticity were common."

Confirms spasticity is a common feature across the reviewed patient series.

Skeletal Muscle Atrophy VERY_FREQUENT HP:0003202

Present in 13/14 patients in PMID:27130255 (all except one named patient).

Show evidence (2 references)

PMID:27130255 SUPPORT Human Clinical

"Muscle atrophy was evident in all except patient 1232-III-2."

Gives the explicit numerator, 13 of 14 patients (93 percent), supporting the VERY_FREQUENT band.

PMID:27130255 SUPPORT Human Clinical

"Besides severe muscle wasting, limb hypertonia, brisk reflexes, inability to ambulate, and hyperkinetic extremity movements aided in making the diagnosis."

Records severe muscle wasting as a diagnostically useful feature of the syndrome.

Pectus Carinatum OCCASIONAL HP:0000768

2/14 patients in PMID:27130255 (14%).

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Skeletal features of long thin fingers and toes were observed in four patients, while two had pectus carinatum."

Gives the 2/14 numerator supporting the OCCASIONAL band.

Spastic Paraplegia HP:0001258

Course: PROGRESSIVE

frequency is deliberately omitted at the disease level. This presentation is confined to two related families (5 patients, homozygous p.Val128Ala) in PMID:37141741 and is not a feature of the classical HLD10 cohorts, so a disease-wide band would be misleading.

Show evidence (2 references)

PMID:37141741 SUPPORT Human Clinical

"The aim of the present study is to report the clinical findings of patients having novel PYCR2 gene variant that manifest Hereditary Spastic Paraplegia (HSP) is the only symptom without hypomyelinating leukodystrophy."

Establishes an isolated spastic-paraplegia presentation at the mild end of the PYCR2 spectrum.

PMID:37141741 SUPPORT Human Clinical

"4 (%80) patients had gait difficulty and progressive lower limb spasticity started at the age of 8-12 years."

Gives the age of onset and character of the spastic paraplegia in that family.

Joint Contractures HP:0034392

frequency omitted for the same reason as the other features in this list from PMID:27860360 - no numerator is given.

Show evidence (1 reference)

PMID:27860360 SUPPORT Human Clinical

"Additional phenotypes that were less consistent among patients included seizures or seizure-like movements, spasticity and ataxic gait, recurrent vomiting, cortical blindness, dysmorphic features, joint contractures, and irritability."

Lists joint contractures among the less consistent features of the cohort.

Nervous System 5

Global Developmental Delay VERY_FREQUENT HP:0001263

Reported in 14/14 in PMID:27130255 and in all 35 reviewed patients in PMID:34037307.

Show evidence (2 references)

PMID:27860360 SUPPORT Human Clinical

"All patients presented with postnatally acquired microcephaly, moderate to profound global developmental delay, and failure to thrive."

Documents moderate-to-profound global developmental delay in all patients.

PMID:27130255 SUPPORT Human Clinical

"None developed meaningful language or fine motor skills, and none were able to stand or walk independently."

Characterises the severity ceiling of the developmental impairment.

Seizures FREQUENT HP:0001250

57% (8/14) in PMID:27130255.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Seizures were seen in 57% of patients with onset usually before 1 year of age, consisting of focal myoclonic and generalized tonic-clonic seizures, controlled with anticonvulsant medications."

Gives the frequency, semiology, onset and treatment responsiveness of the seizures.

Intention Tremor HP:0002080

frequency omitted at the disease level for the same reason as Spastic Paraplegia: reported in 4/5 patients of two related families with the p.Val128Ala allele (PMID:37141741), not a feature of the classical HLD10 cohorts.

Show evidence (1 reference)

PMID:37141741 SUPPORT Human Clinical

"4 (%80) patients exhibit mild intention tremor started at the age of approximately 6 years of age."

Gives the frequency within that family and the age of onset of the tremor.

Ataxic Gait Gait ataxia HP:0002066

frequency omitted - PMID:27860360 gives no numerator, and most patients in the severe cohorts never ambulate at all, so the denominator is unclear.

Show evidence (1 reference)

PMID:27860360 SUPPORT Human Clinical

"Additional phenotypes that were less consistent among patients included seizures or seizure-like movements, spasticity and ataxic gait, recurrent vomiting, cortical blindness, dysmorphic features, joint contractures, and irritability."

Lists ataxic gait among the less consistent features of the cohort.

Irritability HP:0000737

frequency omitted; no numerator given in PMID:27860360.

Show evidence (1 reference)

PMID:27860360 SUPPORT Human Clinical

"Additional phenotypes that were less consistent among patients included seizures or seizure-like movements, spasticity and ataxic gait, recurrent vomiting, cortical blindness, dysmorphic features, joint contractures, and irritability."

Lists irritability among the less consistent features of the cohort.

Growth 1

Failure to Thrive VERY_FREQUENT HP:0001508

Present in all 14 patients of PMID:27130255 and in all patients of PMID:27860360.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Although all individuals were born with average growth parameters, failure to thrive was noted by 1 year of age."

Documents universal onset of failure to thrive during the first year.

Other 5

Progressive Microcephaly VERY_FREQUENT HP:0000253

Course: PROGRESSIVE

Reported in all patients in every cohort. PMID:34037307 states across the 35 previously reported patients that "All patients had microcephaly and developmental delay". In PMID:27130255 (n=14) head circumference at birth was available for 10 of 14 and already 1.8 to 3 SD below the mean, falling to -3 to -6 SD.

Show evidence (2 references)

PMID:27860360 SUPPORT Human Clinical

"All patients presented with postnatally acquired microcephaly, moderate to profound global developmental delay, and failure to thrive."

Documents postnatally acquired microcephaly in all patients of this cohort.

PMID:34037307 SUPPORT Human Clinical

"All patients had microcephaly and developmental delay."

Supports the VERY_FREQUENT band across the 35 previously reported patients reviewed in this paper.

Axial Hypotonia with Appendicular Spasticity FREQUENT HP:0008936

Band set from the hypotonia figure itself, not from the spastic component: PMID:27130255 reports truncal hypotonia with hyperreflexia in "most patients" of the 14-patient cohort, which maps to FREQUENT. The 57% appendicular spasticity figure belongs to the sibling Spasticity entry and is cited there.

Show evidence (2 references)

PMID:27130255 SUPPORT Human Clinical

"Patients presented with profound psychomotor delay, microcephaly, truncal hypotonia with variable appendicular spasticity, and failure to thrive from the ages of 2 months to 1 year."

Describes the dissociated axial-hypotonia/appendicular-spasticity pattern and its age of onset.

PMID:27130255 SUPPORT Human Clinical

"Truncal hypotonia with hyperreflexia was seen in most patients."

Gives the qualitative frequency for the hypotonia itself, which is what this entry's band is derived from.

Hyperkinetic Movements FREQUENT HP:0002487

Described as present in "the majority of patients" in PMID:27130255 (n=14) without an explicit numerator; FREQUENT is the band that "majority" maps to.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"The majority of patients demonstrated hyperkinetic movements of upper and lower extremities, along with spontaneous mouth chewing and head titubation"

Documents the hyperkinetic movement disorder and its qualitative frequency.

Bilateral Sensorineural Hearing Impairment HP:0008619

frequency is deliberately omitted. PMID:27130255 reports hearing loss only in those patients where audiology was available, giving no denominator, so no band can be justified.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Audiology evaluation, wherever available, showed bilateral mild to severe hearing loss."

Documents bilateral hearing loss while making explicit that testing was not universal, which is why no frequency band is asserted.

Cortical Blindness Cerebral visual impairment HP:0100704

frequency omitted. PMID:27860360 groups this among phenotypes that "were less consistent among patients" without giving a numerator.

Show evidence (1 reference)

PMID:27860360 SUPPORT Human Clinical

"Additional phenotypes that were less consistent among patients included seizures or seizure-like movements, spasticity and ataxic gait, recurrent vomiting, cortical blindness, dysmorphic features, joint contractures, and irritability."

Lists cortical blindness among the less consistent features of the cohort.

🧬

Genetic Associations

1

PYCR2

Gene: PYCR2 hgnc:30262 relationship_type: CAUSATIVE

Show evidence (6 references)

PMID:25865492 SUPPORT Human Clinical

"we report mutations in PYCR2, which encodes an enzyme in the proline biosynthesis pathway, as the cause of a unique syndrome characterized by postnatal microcephaly, hypomyelination, and reduced cerebral white-matter volume"

Original identification of PYCR2 as the causal gene by linkage and exome sequencing.

PMID:25865492 SUPPORT Model Organism

"Morpholino-based knockdown of a zebrafish PYCR2 ortholog, pycr1b, recapitulated the human microcephaly phenotype, which was rescued by wild-type human PYCR2 mRNA, but not by mutant mRNAs, further supporting the pathogenicity of the identified variants."

Functional confirmation of variant pathogenicity by orthologue knockdown and allele-specific rescue.

PMID:27130255 SUPPORT In Vitro

"Both nonsense and missense mutations were identified, which impaired protein multimerization."

Establishes the allelic classes seen across the largest cohort. Tagged IN_VITRO because the multimerisation half of the sentence is a 293T co-immunoprecipitation result, matching how the same claim is classified in the pathophysiology section.

+ 3 more references

💊

Medical Actions

5

Antiseizure Pharmacotherapy

Action: Pharmacotherapy NCIT:C15986

Agent: anticonvulsant agent NCIT:C264

Anticonvulsant medication for the focal myoclonic and generalized tonic-clonic seizures. In the largest cohort seizures were controlled with anticonvulsants, making this the single symptomatic intervention with documented efficacy in HLD10.

Target Phenotypes: Seizure HP:0001250

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Seizures were seen in 57% of patients with onset usually before 1 year of age, consisting of focal myoclonic and generalized tonic-clonic seizures, controlled with anticonvulsant medications."

Documents seizure control with anticonvulsant medication in the reference cohort.

Nutritional Support for Failure to Thrive

Action: Nutritional Support NCIT:C15433

Nutritional support, including gastrostomy feeding where dysphagia is severe, for the universal failure to thrive. Failure to thrive is itself among the listed causes of death in the reference cohort, which is the rationale.

Target Phenotypes: Failure to thrive HP:0001508

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"died at or before age 6 years due to complications of pulmonary infection, fever of unknown origin and/ failure to thrive, and no patient survived beyond age 10 years"

Supports the rationale by naming failure to thrive as a cause of death. PARTIAL because it evidences the need, not the efficacy, of nutritional support.

Physical Therapy and Rehabilitation

Action: Physical Therapy NCIT:C15302

Physiotherapy, positioning and orthotic management for spasticity, contracture prevention and mobility in a population that does not achieve independent sitting or ambulation.

Target Phenotypes: Spasticity HP:0001257

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"None developed meaningful language or fine motor skills, and none were able to stand or walk independently."

Evidences the motor disability that rehabilitation targets. PARTIAL: the cohort reports the impairment, not any rehabilitation outcome.

Genetic Counseling

Action: Genetic Counseling NCIT:C15240

Genetic counselling for families, with carrier and prenatal or preimplantation testing once the familial PYCR2 variants are known. Particularly relevant given the high rate of consanguinity and the identified Thai founder allele.

Show evidence (1 reference)

PMID:34037307 SUPPORT Human Clinical

"Since the c.400G>A was detected in three out of four mutant alleles and with a common ancestry, this variant might be common in Thai patients."

Establishes the recurrent founder allele that makes population-targeted carrier counselling worthwhile. PARTIAL: it evidences the rationale, not any counselling outcome, and says nothing about counselling itself.

No Disease-Modifying Therapy

Recorded explicitly as a negative: there is no specific, disease-modifying or curative treatment for HLD10. Management is entirely supportive. The SHMT2 or glycine axis is the leading proposed intervention point but has not been tested in patients.

Mechanism Target:

SHMT2 Upregulation and Cerebral Glycine Accumulation — Recorded to make the proposed - but entirely untested - intervention point machine-readable. No agent is being asserted, and no effect on this node has been demonstrated in patients.

Show evidence (1 reference)

PMID:32330411 SUPPORT Model Organism

"Our findings identify the glycine metabolic pathway as a possible intervention point to alleviate the neurological symptoms of PYCR2-mutant patients."

The authors nominate this node as a therapeutic target. PARTIAL because it is a proposal supported by neuronal-culture rescue, not a treatment.

Show evidence (2 references)

PMID:36548190 SUPPORT Other

"To date, there is no known specific therapeutic strategy or drug for HLD10."

Explicit statement that no disease-specific therapy exists.

PMID:32330411 SUPPORT Model Organism

"Our findings identify the glycine metabolic pathway as a possible intervention point to alleviate the neurological symptoms of PYCR2-mutant patients."

Records the proposed but untested therapeutic direction, framed by its authors as a possibility rather than an established treatment.

🔬

Biochemical Markers

3

Routine Metabolic Abnormality (ABSENT)

Context: A negative but diagnostically important finding: standard biochemical screening is normal in HLD10. Serum creatinine, BUN, CPK, ammonia, lactate, plasma amino acids and urine organic acids are all unremarkable, and plasma and CSF proline are normal. HLD10 is therefore not detectable by routine metabolic panels or newborn biochemical screening, and molecular testing is required.

Show evidence (3 references)

PMID:27130255 SUPPORT Human Clinical

"Metabolic profile including serum creatinine, BUN, CPK, ammonia, lactate, plasma amino acids and urine organic acids were normal in all patients."

Enumerates the routine analytes found normal in all 14 patients.

PMID:27860360 SUPPORT Human Clinical

"Despite the metabolic role of PYCR2, routine serum metabolic test in these patients were normal."

Independent confirmation that routine metabolic testing is uninformative.

PMID:27130255 SUPPORT Human Clinical

"PYCR2-related syndrome represents a clinically recognizable condition in which PYCR2 mutations lead to protein dysfunction, not detectable on routine biochemical assessments."

States the diagnostic consequence of the normal biochemistry.

Elevated Plasma and CSF Purine Metabolites (PRESENT)

Context: Untargeted metabolomics of plasma and CSF in three related patients showed marked increases of inosine and xanthine, together with mild increases of glutamate, alpha-ketoglutarate and L-glutamate semialdehyde. This points to a purine/pyrimidine-pathway perturbation in addition to the mild proline-pathway changes, and is a candidate biochemical signature - not an established diagnostic marker.

Show evidence (2 references)

PMID:38709052 SUPPORT Human Clinical

"While plasma and CSF proline levels were found to be totally normal, untargeted metabolomic profiling revealed mild increases of glutamate, alpha-ketoglutarate, and l-glutamate semialdehyde and marked increases of inosine and xanthine."

Reports the metabolomic changes recorded here.

PMID:38709052 SUPPORT Human Clinical

"Untargeted metabolomics point to mild changes in proline pathway and also in purine/pyrimidine pathway."

Supports the purine/pyrimidine interpretation, marked PARTIAL because the authors themselves describe the proline-pathway changes as mild and the finding rests on one family.

Mildly Elevated Urinary Glutamate (PRESENT)

Context: Urine amino acids, available in two families, showed mildly elevated glutamate (84 and 109 nmol/mg creatinine against a reference of <76) compared with unaffected carriers. A subtle finding consistent with a block downstream of glutamate, but far too inconsistently measured to be diagnostic.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Urine amino acids were available in two families (2206 and 2664), which showed mildly elevated glutamate levels at 84 and 109 nmol/mg creatinine (reference level <76 nmol/mg creatinine) respectively, compared to unaffected carriers who had normal levels."

Reports the raw values and the carrier comparison; PARTIAL because the measurement was available for only two of eleven families.

🔬

Diagnosis

1

Molecular Genetic Testing of PYCR2

Diagnosis rests on identifying biallelic PYCR2 variants. Because routine biochemistry is normal, molecular testing - clinical exome or genome sequencing, a leukodystrophy or microcephaly gene panel, or targeted PYCR2 sequencing once a familial variant is known - is required. Homozygosity mapping is informative in consanguineous families.

Show evidence (2 references)

PMID:34055512 SUPPORT Human Clinical

"Further genetic testing in the form of clinical exome sequencing revealed compound heterozygous mutation of the PYCR2 gene and matching the clinical phenotype with the genotype."

Illustrates clinical exome sequencing as the route to diagnosis.

PMID:27130255 SUPPORT Human Clinical

"PYCR2-related syndrome represents a clinically recognizable condition in which PYCR2 mutations lead to protein dysfunction, not detectable on routine biochemical assessments."

Explains why molecular rather than biochemical testing is required.

🩻

Imaging Findings

4

Cerebral Hypomyelination VERY_FREQUENT

Hypomyelination or delayed myelination of deep and subcortical white matter, hyperintense on T2 and isointense on T1, with generalized reduction of cerebral white-matter volume. This is the radiographic hallmark of HLD10.

Mri

Cerebral hypomyelination HP:0006808

Described as the typical radiographic feature across the 35 patients reviewed in PMID:34037307 and present in all patients of PMID:27860360.

Show evidence (2 references)

PMID:34037307 SUPPORT Human Clinical

"Hypomyelination or delayed myelination was a typical radiographic feature."

Establishes hypomyelination as the typical imaging feature across the reviewed cohort.

PMID:34055512 SUPPORT Human Clinical

"Magnetic resonance imaging (MRI) of the brain showed hypomyelination of the deep and subcortical white matter, appearing as hyperintense T2 and isointense T1-weighted images, cerebral atrophy with the thinning of the corpus callosum, with normal cerebellum, brainstem, and deep grey nuclei."

Gives the detailed signal characteristics and the relative sparing of cerebellum, brainstem and deep grey nuclei.

Thin Corpus Callosum FREQUENT

Thinning of the corpus callosum.

Mri

corpus callosum UBERON:0002336 Thin corpus callosum HP:0033725

61% in PMID:27130255 (n=14 with imaging available in 13).

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Thin corpus callosum was seen in 61% of patients."

Gives the explicit 61% frequency supporting the FREQUENT band.

Cerebral Atrophy

Progressive global brain atrophy, in some patients severe and accompanied by ventriculomegaly, with white-matter T2 hyperintensities. Cerebellum and brainstem are generally preserved.

Mri

Cerebral atrophy HP:0002059 Course: PROGRESSIVE

frequency deliberately omitted. PMID:27130255 reports atrophy as a cohort-level MRI finding (imaging available in 13/14) with no per-patient count, and explicitly describes it as variable - "Some patients showed severe cerebral atrophy ... in contrast to other patients in which atrophy was less severe" - so no band can be read off it.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Brain magnetic resonance imaging showed global brain atrophy and white matter T2 hyperintensities."

Documents global brain atrophy as the cohort-level imaging finding.

Elevated Brain Glycine on MR Spectroscopy

A glycine peak on MR spectroscopy, the non-invasive correlate of the SHMT2-driven cerebral glycine excess. Notably it is present even in the mild spastic-paraplegia-only phenotype in which conventional MRI myelination is normal, making it a candidate mechanism-linked imaging biomarker.

Mri

Elevated brain glycine level by MRS HP:0034893

frequency omitted: reported in 5/5 patients of the two related HSP-only families in PMID:37141741, but MR spectroscopy was not systematically performed in the classical HLD10 cohorts, so no disease-wide band is defensible.

Show evidence (1 reference)

PMID:37141741 SUPPORT Human Clinical

"White matter myelination was normal in all patients. Glycine peakes were detected on the MR spectroscopy in all patients."

Documents the MRS glycine peak alongside normal myelination, linking the glycine mechanism to a non-invasive readout. Quoted verbatim, including the source's "peakes" typographical error.

📈

Progression

3

Onset

Age: 2 months to 1 year

Birth parameters including weight, height and head circumference are normal or only mildly reduced; presentation follows postnatally.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Patients presented with profound psychomotor delay, microcephaly, truncal hypotonia with variable appendicular spasticity, and failure to thrive from the ages of 2 months to 1 year."

Gives the age window of first presentation.

Progressive decline

Microcephaly deepens, motor and intellectual function worsen over serial follow-up, and seizures typically appear before one year of age.

Show evidence (1 reference)

PMID:27130255 SUPPORT Human Clinical

"Serial clinical follow up demonstrated progression of symptoms in patients."

Documents the progressive rather than static course.

Mortality

Age: before 10 years

In the largest cohort five study patients died at or before age 6 and no patient survived beyond age 10, from pulmonary infection, fever of unknown origin or failure to thrive. Survival is not uniform across the spectrum - the mild spastic-paraplegia families include affected adults up to 26 years of age (PMID:37141741).

Show evidence (2 references)

PMID:27130255 SUPPORT Human Clinical

"Patients did not survive beyond the first decade of life."

States the mortality ceiling in the reference cohort.

PMID:27130255 SUPPORT Human Clinical

"died at or before age 6 years due to complications of pulmonary infection, fever of unknown origin and/ failure to thrive, and no patient survived beyond age 10 years"

Gives the causes of death and confirms the survival ceiling.

📊

Prevalence

1

Worldwide

Cases In Literature Ultra Rare

No population-based prevalence estimate exists. Case counts are the only defensible figure: about 35 patients reviewed in 2021 (PMID:34037307) and about 38 by 2024 (PMID:38709052). Reported families cluster in populations with high consanguinity (Egyptian, Pakistani, Omani, Indian, Iranian, Thai), so the distribution is strongly ascertainment-biased and no rate per 100000 is asserted.

Show evidence (2 references)

PMID:38709052 SUPPORT Human Clinical

"Up until now, to our knowledge around 38 patients with PYCR2 defect have been reported."

Gives the published case count underpinning the ULTRA_RARE class.

PMID:34037307 SUPPORT Human Clinical

"We also reviewed the phenotype and genotype of all 35 previously reported PYCR2 patients and found that majorities of cases were homozygous with a consanguineous family history"

Independent case count three years earlier, and documents the consanguinity clustering that biases ascertainment.

🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from Hypomyelinating Leukodystrophy 10:

Autosomal recessive cutis laxa type 2B (PYCR1) Not Yet Curated MONDO:0013051

Overlapping Features The paralogue disorder and the single most important differential. PYCR1 deficiency acts at the same step of proline biosynthesis but produces cutis laxa with progeroid features. Lax, wrinkly skin is characteristically ABSENT in HLD10, and hypomyelination is not a feature of PYCR1 disease.

Distinguishing Features

Cutis laxa and wrinkly skin, absent in HLD10
Progeroid facial appearance, absent in HLD10
CNS hypomyelination, absent in PYCR1 disease
Postnatal progressive microcephaly with death in the first decade, absent in PYCR1 disease

Show evidence (1 reference)

PMID:25865492 SUPPORT Human Clinical

"Hypomyelination and the absence of lax, wrinkly skin distinguishes this condition from that caused by previously reported mutations in the gene encoding PYCR2's isozyme, PYCR1, suggesting a unique and indispensable role for PYCR2 in the human CNS during development."

Names the two discriminating features between the paralogue disorders.

Pelizaeus-Merzbacher disease MONDO:0010714

Overlapping Features The prototypic hypomyelinating leukodystrophy (HLD1, PLP1). Shares CNS hypomyelination but is X-linked, typically presents with nystagmus and cerebellar signs without severe progressive microcephaly, failure to thrive or the HLD10 craniofacial gestalt. Curated separately in dismech as `Pelizaeus_Merzbacher_Disease`.

Distinguishing Features

X-linked PLP1 inheritance rather than autosomal recessive PYCR2
Prominent nystagmus and cerebellar ataxia
No progressive postnatal microcephaly
No severe diffuse cerebral atrophy

Show evidence (2 references)

PMID:36548190 SUPPORT Other

"PMD is now known as the prototypic disorder of hypomyelinating leukodystrophies (HLDs)"

Establishes Pelizaeus-Merzbacher disease as the prototype of the HLD group HLD10 belongs to.

PMID:27130255 SUPPORT Human Clinical

"in contrast to most patients with HLD, often presenting with cerebellar ataxia, axial hypotonia, spasticity, and nystagmus, without severe diffuse cerebral atrophy"

Contrasts the typical HLD presentation with the microcephaly-and-atrophy picture of HLD10.

AGC1 deficiency (SLC25A12) MONDO:0013056

Overlapping Features Aspartate-glutamate carrier 1 deficiency, an ultra-rare recessive disorder of the malate-aspartate shuttle. Included because it is the closest mechanistic analogue of the open question this entry curates: like HLD10 it presents with neurodevelopmental delay, epilepsy, hypotonia, cerebral atrophy and hypomyelination, and the literature debates in the same terms whether its hypomyelination is primary or secondary to a neuronal defect. Serial imaging in AGC1 deficiency shows early hypomyelination followed by later progression of myelination, which is the discriminator - HLD10 myelination does not catch up.

Distinguishing Features

Caused by SLC25A12 rather than PYCR2
Early hypomyelination with subsequent progression of myelination on serial MRI
Classified as a leuko-axonopathy rather than a hypomyelinating leukodystrophy

Show evidence (2 references)

PMID:31403263 SUPPORT Human Clinical

"These patients were clinically characterized by neurodevelopmental delay, epilepsy, hypotonia, cerebral atrophy, and hypomyelination; however, there has been discussion in the literature as to whether this hypomyelination is primary or secondary to a neuronal defect."

Documents both the phenotypic overlap with HLD10 and the identical primary-versus-secondary hypomyelination debate.

PMID:31403263 SUPPORT Human Clinical

"The serial neuroimaging findings are notable for cerebral atrophy with white matter involvement, namely, early hypomyelination yet subsequent progression of myelination."

Gives the serial-imaging feature that distinguishes AGC1 deficiency from HLD10.

🧫

Experimental Models

2

FBD-102b oligodendroglial differentiation model expressing HLD10 PYCR2 variants CELL_LINE

The FBD-102b oligodendroglial differentiation model expressing the HLD10 missense proteins p.Arg119Cys and p.Arg251Cys. Mutant-expressing cells form abnormally large mitochondria with increased fusion, decreased fission and decreased mitochondrial activity, and fail to undergo morphological differentiation with the associated rise in differentiation marker proteins.

Publication

PMID:36548190

Pathograph links

Impaired Oligodendroglial Differentiation and CNS Hypomyelination recapitulates fidelity: moderate Provides the direct cellular link from the PYCR2 lesion to a failure of oligodendroglial differentiation, the proximate cause of hypomyelination.

Limitations: A murine cell line with heterologous overexpression of mutant protein rather than patient-derived cells at endogenous expression levels; morphological differentiation and marker expression are surrogates for myelination, which is not measured.

Show evidence (1 reference)

PMID:36548190 SUPPORT In Vitro

"Taken together, these results indicate that an HLD10-associated PYCR2 mutation leads to the formation of large mitochondria with decreased activities, inhibiting oligodendroglial cell morphological differentiation."

Supports the model as informative for the oligodendroglial differentiation node.

CRISPR-Cas9 PYCR2-deficient HEK293FT cell line CELL_LINE

An isogenic PYCR2 knockout generated by CRISPR-Cas9 in HEK293FT cells, showing decreased mitochondrial membrane potential and increased apoptosis under oxidative stress.

Publication

PMID:25865492

Pathograph links

Mitochondrial Dysfunction and Oxidative Stress Susceptibility recapitulates fidelity: moderate Isogenic loss-of-function evidence isolating the mitochondrial and apoptotic consequences of PYCR2 loss from genetic background.

Limitations: A non-neural embryonic kidney line; the readouts establish that PYCR2 loss has these consequences in principle, not that they occur in patient oligodendrocytes or neurons.

Show evidence (1 reference)

PMID:25865492 SUPPORT In Vitro

"A PYCR2-deficient HEK293FT cell line generated by genome editing with the clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system showed that PYCR2 loss of function led to decreased mitochondrial membrane potential and increased susceptibility to apoptosis under oxidative stress."

Supports the model as informative for the mitochondrial-dysfunction node.

🐁

Animal Models

3

Pycr2 loss-of-function mouse (chemically induced p.Met1Thr)

A chemically induced start-codon mutation abolishing the 34 kDa PYCR2 protein in brain, spinal cord and muscle while leaving Pycr2 mRNA intact. Mutant mice show weight loss, progressive kyphosis, hind-limb clasping, reduced grip strength, severe subcutaneous fat loss, mild peripheral axonal atrophy, reduced white blood cell counts and altered lipid metabolism.

Species

Mouse

Genotype

Pycr2 c.2T>C (p.Met1Thr) homozygous, chemically induced null

Publication

PMID:33734376

Pathograph links

Preserved Bulk Proline Supply by Paralogue Redundancy recapitulates fidelity: high The mouse reproduces the defining negative finding of the human disease - an enzymatic block without proline depletion - and adds the dietary manipulation that shows proline is semi-essential rather than irrelevant.

Limitations: Serum and fibroblast measurements only; brain proline was not the reported readout, so tissue-specific depletion in the CNS is not excluded.

Show evidence (1 reference)

PMID:33734376 SUPPORT Model Organism

"Proline levels were not reduced, and precursors were not increased in serum from Pycr2 mutant mice or in lysates from skin fibroblast cultures, but placing Pycr2 mutant mice on a proline-free diet worsened the phenotype."

Supports treating the mouse as informative for the preserved-proline node.

Severe Neurodevelopmental Impairment and Early Mortality partially recapitulates fidelity: moderate The mouse develops a neurological and neuromuscular syndrome - kyphosis, hind-limb clasping, weight loss, reduced grip strength - consistent with the human neurological impairment.

Limitations: The reported characterisation does not establish frank microcephaly or CNS hypomyelination, the two defining human features, so the correspondence is to the neurological syndrome in general rather than to leukodystrophy. See the HUMAN_MODEL_MISMATCH discussion.

Show evidence (1 reference)

PMID:33734376 SUPPORT Model Organism

"Mice with recessive loss-of-function mutations in Pycr2 showed phenotypes consistent with neurological and neuromuscular disorders, including weight loss, kyphosis, and hind-limb clasping."

Supports a neurological phenotype while stopping short of the defining human features, hence PARTIAL.

Show evidence (1 reference)

PMID:33734376 SUPPORT Model Organism

"A severe loss of subcutaneous fat in Pycr2 mutant mice is reminiscent of a CL-like phenotype, but primary features such as elastin abnormalities were not observed."

Records that the mouse does not develop the elastin pathology of the paralogue disease, keeping the PYCR1/PYCR2 separation intact in the model.

Pycr2 knockout mouse (Escande-Beillard)

A Pycr2 knockout reported to phenocopy the human disorder, in which in situ neurotransmitter quantification revealed excessive cerebral glycine and secondary depletion of PYCR1 in neural lineages. Cultured knockout neurons show axonal beading and shortened neurites that are rescued by SHMT2 knockdown.

Species

Mouse

Genotype

Pycr2 knockout

Publication

PMID:32330411

Pathograph links

SHMT2 Upregulation and Cerebral Glycine Accumulation recapitulates fidelity: high Excess cerebral glycine was measured in the mutant mouse brain and in patients in the same study, and the SHMT2 knockdown rescue establishes the mechanism rather than merely the association.

Limitations: The rescue was performed in cultured knockout neurons, not in vivo, so whether lowering glycine improves the organismal phenotype is untested.

Show evidence (1 reference)

PMID:32330411 SUPPORT Model Organism

"In situ quantification of neurotransmitters in the brains of PYCR2 mutant mice and patients revealed a signature of encephalopathy driven by excessive cerebral glycine."

Supports the mouse as informative for the glycine node by measuring the same signature in mice and patients.

Show evidence (1 reference)

PMID:32330411 SUPPORT Model Organism

"We find that knocking out Pycr2 in mice phenocopies the human disorder and depletes PYCR1 levels in neural lineages."

The authors' own assessment of the model's fidelity to the human disorder.

pycr1b morpholino knockdown zebrafish

Morpholino knockdown of the zebrafish PYCR2 orthologue pycr1b reproduces microcephaly, rescued by wild-type but not mutant human PYCR2 mRNA. The allele-specific rescue is the functional evidence for pathogenicity of the founding variants.

Species

Zebrafish

Genotype

pycr1b morpholino knockdown (PYCR2 orthologue)

Publication

PMID:25865492

Pathograph links

Progressive Neuronal Loss and Brain Atrophy partially recapitulates fidelity: moderate Reproduces reduced head and brain size, the developmental counterpart of the human microcephaly, with allele-specific rescue confirming specificity.

Limitations: Transient whole-embryo morpholino knockdown models a developmental reduction in brain size, not the postnatal progressive degeneration of the human disease, and does not model hypomyelination.

Show evidence (1 reference)

PMID:25865492 SUPPORT Model Organism

"Morpholino-based knockdown of a zebrafish PYCR2 ortholog, pycr1b, recapitulated the human microcephaly phenotype, which was rescued by wild-type human PYCR2 mRNA, but not by mutant mRNAs, further supporting the pathogenicity of the identified variants."

Supports the model as informative for the brain-size phenotype, with allele-specific rescue.

{ }

Source YAML

click to show

name: Hypomyelinating Leukodystrophy 10
creation_date: "2026-08-19T00:00:00Z"
category: Mendelian
description: >-
  Hypomyelinating leukodystrophy 10 (HLD10, MIM 616420) is an ultra-rare
  autosomal recessive neurodevelopmental and neurodegenerative disorder caused by
  biallelic variants in PYCR2, which encodes the mitochondrial NAD(P)H-dependent
  enzyme pyrroline-5-carboxylate reductase 2. PYCR2 catalyses the terminal step
  of L-proline biosynthesis, reducing delta-1-pyrroline-5-carboxylate (P5C) to
  L-proline. Affected children are typically born with normal or near-normal
  growth parameters and then develop postnatally acquired, progressive
  microcephaly, profound global developmental delay, failure to thrive, a
  characteristic craniofacial gestalt (triangular facies, malar hypoplasia,
  large malformed ears, upturned bulbous nose, long smooth philtrum), axial
  hypotonia with appendicular spasticity, hyperkinetic movements, and seizures.
  Brain imaging shows CNS hypomyelination or delayed myelination with reduced
  cerebral white-matter volume, a thin corpus callosum, and progressive global
  brain atrophy, while the cerebellum, brainstem and deep grey nuclei are
  relatively spared. In the largest reported cohort no individual survived beyond
  the first decade.

  The mechanism is not simple proline auxotrophy. Plasma and CSF proline are
  normal, routine metabolic screens are unremarkable, and Pycr2-null mice do not
  show reduced serum proline - PYCR1 and PYCR2 are largely functionally redundant
  for bulk proline supply. (The third paralogue PYCR3 is cytosolic and draws its
  P5C from ornithine rather than glutamate, and was not tested for redundancy in
  that work, so it is not part of this claim.) Disease-associated variants
  instead destabilise the protein or impair its dimerisation and catalytic
  efficiency, producing mitochondrial dysfunction (depolarised, enlarged,
  fusion-shifted mitochondria and increased apoptosis under oxidative stress),
  a failure of oligodendroglial morphological differentiation that accounts for
  the hypomyelination, and a downstream one-carbon perturbation in which SHMT2 is
  upregulated and cerebral glycine accumulates. The glycine arm is the principal
  proposed intervention point.

  The allelic spectrum extends beyond the classical severe phenotype. A homozygous
  p.Val128Ala missense variant has been reported in two related families in which
  hereditary spastic paraplegia beginning in late childhood was the only
  manifestation, with normal white-matter myelination but glycine peaks on MR
  spectroscopy (PMID:37141741); separately, spastic paraplegia as the sole symptom
  has been reported in two adult-onset patients carrying a different novel PYCR2
  variant (PMID:32920934). The mild end of this spectrum is therefore not a
  single-family observation.
notes: >-
  Named-entity-confusion guardrail. This entry is anchored strictly on
  MONDO:0014632 / OMIM:616420 / PYCR2 (hgnc:30262). "Hypomyelinating
  leukodystrophy N" is a numbered series (HLD1-HLD13) whose members share nothing
  but the numeral, so the anchor was fixed before curation with
  `uv run runoak -i sqlite:obo:mondo info MONDO:0014632 -O obo`, which returns
  `relationship: RO:0004003 HGNC:30262 ! PYCR2` and `xref: OMIM:616420`.
  `just preflight-dr` on the deep-research report returned PASS
  (PYCR2 mentioned 44 times, next-most-mentioned gene PYCR1 at 6; report OMIM
  616420 matches the MONDO xref).

  Two deep-research reports were run and both are in `research/`: `claude_code`
  (14 web searches, 10/10 citations resolved, 0 unresolved) and `openscientist`
  (14/14 citations resolved, 0 unresolved). Falcon was attempted first and failed
  with HTTP 402 (the Edison account is out of credit), so it is absent by
  circumstance rather than by choice. `just preflight-dr` returned PASS on the
  claude_code report and WARN on the openscientist one; the WARN is a false
  positive of the rival-gene heuristic, since the "rival" it flags is SHMT2 at 26
  mentions against PYCR2 at 60, and SHMT2 is this disease's own downstream
  mechanism gene (PMID:32330411), not a second disease entity. Both reports agree
  on OMIM 616420.

  Neither report's identifiers were trusted. The claude_code report asserted
  "PYCR2, HGNC:23364" - that CURIE is ST6GALNAC6, not PYCR2, confirmed by
  `uv run runoak -i sqlite:obo:hgnc info hgnc:23364`. The openscientist report
  gave the disease as MONDO:0014635, which is not this entity. The bindings used
  here - hgnc:30262 and MONDO:0014632 - come from the MONDO record and were
  independently confirmed with OAK. Beyond those two: the great majority of the PYCR2
  literature is cancer biology in which PYCR2 is OVEREXPRESSED and its INHIBITION
  is the proposed therapy - the mechanistic opposite of this recessive
  loss-of-function disorder. A PubMed query for `PYCR2[tiab] OR "hypomyelinating
  leukodystrophy 10"[tiab] OR "HLD10"[tiab]` run on 2026-08-19 returned 59
  records, of which roughly two thirds are oncology or unrelated. None of those is
  cited here and no PYCR2 inhibitor is curated as a treatment.

  The paralogue PYCR1 is the third confusable. PYCR1 deficiency causes autosomal
  recessive cutis laxa type 2B (MONDO:0013051) and is a different disease despite
  acting at the same pathway step; it is recorded here only as a differential
  diagnosis. Note that PYCR1-related ARCL2B is NOT itself curated in dismech - the
  existing `ALDH18A1_Cutis_Laxa` and `ALDH18A1_De_Barsy_Spectrum` entries are keyed
  on ALDH18A1/P5CS, one step upstream, which is a different gene again. Claims
  about PYCR1 are never used as evidence for PYCR2 disease.

  No GeneReviews chapter exists for PYCR2 or HLD10. Searched on 2026-08-19 with
  `PYCR2 GeneReviews[All Fields]` and `hypomyelinating leukodystrophy 10
  GeneReviews[All Fields]`; both returned zero records. The phenotype baseline is
  therefore taken from the primary cohort literature (PMID:27130255, 14 patients
  from 11 families; PMID:27860360, 5 patients from 3 families; PMID:34037307,
  which additionally reviews all 35 patients reported to that date).

  No `datasets:` block is curated, deliberately. HLD10 has roughly 38 published
  patients and no omics resource of its own, while PYCR2 is a heavily studied
  cancer gene - so a gene-keyed accession search returns colorectal, hepatocellular
  and renal-carcinoma series that would resolve perfectly and be about the wrong
  disease. That is exactly the Named Entity Confusion reached through dataset
  search that the curation SOP warns about, and an empty block is the honest
  result rather than an oversight.

  Frequency discipline. Bands are asserted only where a published cohort gives an
  explicit numerator or percentage, and every such phenotype carries the source
  figure in its `notes:`. Percentages are quoted from PMID:27130255 (n=14) unless
  stated otherwise. Where a feature is described only as "common" or "in some
  patients", `frequency:` is omitted rather than guessed. Three features
  (progressive microcephaly, global developmental delay, failure to thrive) are
  reported in 100 percent of patients in every cohort and would map to `OBLIGATE`;
  they are deliberately banded one step down at `VERY_FREQUENT`, because the whole
  published denominator is about 38 patients drawn from consanguineous pedigrees
  ascertained on exactly those features - too small and too ascertainment-biased
  to support an obligate claim.
disease_term:
  preferred_term: Hypomyelinating leukodystrophy 10
  term:
    id: MONDO:0014632
    label: leukodystrophy, hypomyelinating, 10
synonyms:
- HLD10
- PYCR2 leukodystrophy
- PYCR2 deficiency
- Pyrroline-5-carboxylate reductase 2 deficiency
- PYCR2-related microcephaly-progressive leukoencephalopathy
- Leukodystrophy, hypomyelinating, 10
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0014632
      label: leukodystrophy, hypomyelinating, 10
    mapping_predicate: skos:exactMatch
    mapping_source: MONDO
    mapping_justification: >-
      Primary MONDO identifier for this entity. Its definition is gene-anchored
      (RO:0004003 HGNC:30262 PYCR2) and it carries OMIM:616420, Orphanet:481152,
      DOID:0060788, MEDGEN:904191, GARD:0025008 and UMLS:C4225332 as
      cross-references, all confirmed with OAK.
parents:
- leukodystrophy
- congenital nervous system disorder
- inborn error of proline metabolism
inheritance:
- name: Autosomal recessive inheritance
  description: >-
    HLD10 is inherited in an autosomal recessive manner. Reported families are
    predominantly consanguineous with homozygous variants; compound heterozygous
    genotypes are a minority.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:34037307
    reference_title: "Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "PYCR2 pathogenic variants lead to an autosomal recessive hypomyelinating leukodystrophy 10 (HLD10), characterized by global developmental delay, microcephaly, facial dysmorphism, movement disorder, and hypomyelination."
    explanation: States the autosomal recessive mode of inheritance for HLD10.
  - reference: PMID:34037307
    reference_title: "Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We also reviewed the phenotype and genotype of all 35 previously reported PYCR2 patients and found that majorities of cases were homozygous with a consanguineous family history"
    explanation: >-
      Documents that most reported genotypes are homozygous in consanguineous
      families, the expected pattern for a recessive disorder.
pathophysiology:
- name: Biallelic PYCR2 Loss of Function
  biological_scale: MOLECULAR
  description: >-
    Biallelic PYCR2 variants - nonsense, splice-site and missense - reduce the
    amount or the catalytic competence of pyrroline-5-carboxylate reductase 2.
    Missense alleles act by two demonstrated routes. Some (p.Arg119Cys,
    p.Arg251Cys) leave mitochondrial targeting intact but lower steady-state
    protein levels and severely reduce catalytic efficiency, and p.Arg251Cys in
    addition carries a pronounced folding defect. Others cluster in the
    dimerisation domain and impair multimerisation of the obligate-oligomeric
    enzyme; the p.Gly249Val allele lies directly at the dimer interface.
  genetic_context:
    gene:
      preferred_term: PYCR2
      term:
        id: hgnc:30262
        label: PYCR2
    zygosity: HOMOZYGOUS
    functional_impact_category: LOSS_OF_FUNCTION
    variant_origin: GERMLINE
  molecular_functions:
  - preferred_term: pyrroline-5-carboxylate reductase activity
    modifier: DECREASED
    term:
      id: GO:0004735
      label: pyrroline-5-carboxylate reductase activity
  biological_processes:
  - preferred_term: L-proline biosynthetic process
    modifier: DECREASED
    term:
      id: GO:0055129
      label: L-proline biosynthetic process
  evidence:
  - reference: PMID:25865492
    reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we report mutations in PYCR2, which encodes an enzyme in the proline biosynthesis pathway, as the cause of a unique syndrome characterized by postnatal microcephaly, hypomyelination, and reduced cerebral white-matter volume"
    explanation: Establishes PYCR2 as the causal gene and names the proline-biosynthesis role of its product.
  - reference: PMID:25865492
    reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "When mutant cDNAs were transfected into HEK293FT cells, both variant proteins retained normal mitochondrial localization but had lower amounts than the wild-type protein, suggesting that the variant proteins were less stable."
    explanation: >-
      Shows the two founding missense alleles reduce protein abundance rather
      than mislocalise the enzyme, i.e. a loss-of-function mechanism.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Mutations clustered in the dimerization domain of the protein, and several tested mutations impaired multimerization."
    explanation: Identifies impaired multimerisation as a second route to loss of function.
  - reference: PMID:33771508
    reference_title: "Disease variants of human Δ(1)-pyrroline-5-carboxylate reductase 2 (PYCR2)."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Depending on whether NADPH or NADH was used, the catalytic efficiency of the R119C protein variant was 40 or 366 times lower than that of the wild-type enzyme, while the catalytic efficiency of the R251C protein variant was 7 or 26 times lower than that of the wild-type enzyme."
    explanation: Quantifies the catalytic penalty of the two founding disease variants.
  - reference: PMID:33771508
    reference_title: "Disease variants of human Δ(1)-pyrroline-5-carboxylate reductase 2 (PYCR2)."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "In addition, thermostability and circular dichroism measurements suggest that the R251C protein variant has a pronounced folding defect."
    explanation: Documents a folding defect as the structural basis of instability for p.Arg251Cys.
  - reference: PMID:32330411
    reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "we report the crystal structure of the PYCR2 apo-enzyme and show that a novel germline p.Gly249Val mutation lies at the dimer interface and lowers its enzymatic activity"
    explanation: Structural confirmation that a disease allele at the dimer interface lowers enzymatic activity.
  downstream:
  - target: Preserved Bulk Proline Supply by Paralogue Redundancy
    causal_link_type: DIRECT
    evidence:
    - reference: PMID:33734376
      reference_title: "Genetic analysis of Pycr1 and Pycr2 in mice."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Thus, Pycr1 and -2 have redundant functions in proline biosynthesis, and their loss makes proline a semi-essential amino acid."
      explanation: Establishes the paralogue redundancy this edge asserts.
    description: >-
      Loss of PYCR2 does not deplete measurable proline pools, because the
      mitochondrial paralogue PYCR1 can carry out the same reduction.
  - target: Mitochondrial Dysfunction and Oxidative Stress Susceptibility
    causal_link_type: DIRECT
    description: >-
      Loss of PYCR2 impairs mitochondrial membrane potential and morphology and
      sensitises cells to oxidative-stress-induced apoptosis.
    evidence:
    - reference: PMID:25865492
      reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "A PYCR2-deficient HEK293FT cell line generated by genome editing with the clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system showed that PYCR2 loss of function led to decreased mitochondrial membrane potential and increased susceptibility to apoptosis under oxidative stress."
      explanation: Isogenic knockout evidence for the edge asserted here.
  - target: SHMT2 Upregulation and Cerebral Glycine Accumulation
    causal_link_type: DIRECT
    description: >-
      Loss of PYCR2 upregulates SHMT2, diverting one-carbon metabolism towards
      glycine synthesis.
    evidence:
    - reference: PMID:32330411
      reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Mechanistically, we demonstrate that loss of PYCR2 upregulates SHMT2, which is responsible for glycine synthesis."
      explanation: Direct mechanistic demonstration of the edge asserted here.
- name: Preserved Bulk Proline Supply by Paralogue Redundancy
  biological_scale: ORGANISM
  description: >-
    Despite the enzymatic block, proline itself is not depleted. Plasma and CSF
    proline are normal in patients, routine metabolic screens are unremarkable,
    and serum proline is not reduced in Pycr2-null mice. PYCR1 is largely
    functionally redundant with PYCR2 for bulk proline supply, so proline
    auxotrophy is not the central disease mechanism. The cytosolic third
    paralogue PYCR3 sits in a different compartment and is fed P5C from ornithine
    by ornithine aminotransferase rather than from glutamate; it was not tested
    for redundancy and is deliberately not part of this claim. Proline is nonetheless
    semi-essential in the mutant state: a proline-free diet worsens the mouse
    phenotype. This node is curated explicitly because it constrains every
    downstream mechanistic claim and rules out a metabolite-replacement
    rationale.
  mechanism_confidence: ESTABLISHED
  biological_processes:
  - preferred_term: L-proline biosynthetic process
    modifier: DECREASED
    term:
      id: GO:0055129
      label: L-proline biosynthetic process
  evidence:
  - reference: PMID:38709052
    reference_title: "Exploring metabolic alterations in PYCR2 deficiency: Unveiling pathways and clinical presentations of hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "While plasma and CSF proline levels were found to be totally normal, untargeted metabolomic profiling revealed mild increases of glutamate, alpha-ketoglutarate, and l-glutamate semialdehyde and marked increases of inosine and xanthine."
    explanation: Direct measurement showing proline is not depleted in patient plasma or CSF.
  - reference: PMID:38709052
    reference_title: "Exploring metabolic alterations in PYCR2 deficiency: Unveiling pathways and clinical presentations of hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our findings and all the previous reports suggest that proline auxotrophy is not the central disease mechanism."
    explanation: States the negative conclusion this node encodes.
  - reference: PMID:27860360
    reference_title: "Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "No significant changes were identified in proline biosynthesis pathway or related metabolites."
    explanation: Independent plasma metabolomics in three severely affected patients finds no proline-pathway change.
  - reference: PMID:33734376
    reference_title: "Genetic analysis of Pycr1 and Pycr2 in mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Proline levels were not reduced, and precursors were not increased in serum from Pycr2 mutant mice or in lysates from skin fibroblast cultures, but placing Pycr2 mutant mice on a proline-free diet worsened the phenotype."
    explanation: >-
      Confirms preserved proline in the mouse model while showing dietary proline
      still modulates severity, i.e. proline becomes semi-essential.
  - reference: PMID:33734376
    reference_title: "Genetic analysis of Pycr1 and Pycr2 in mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Thus, Pycr1 and -2 have redundant functions in proline biosynthesis, and their loss makes proline a semi-essential amino acid."
    explanation: States the paralogue redundancy that explains the preserved proline pool.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "We also noted essentially unchanged expression levels of PYCR1 in PYCR2-mutated samples, suggesting lack of expression compensation."
    explanation: >-
      Important qualifier on the redundancy: PYCR1 is not upregulated to compensate
      in patient cells, so the preserved proline pool reflects baseline paralogue
      activity rather than an induced compensatory response.
  - reference: PMID:32330411
    reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We find that knocking out Pycr2 in mice phenocopies the human disorder and depletes PYCR1 levels in neural lineages."
    explanation: >-
      Bounds the redundancy in the tissue that matters: in neural lineages loss of
      PYCR2 actually depletes PYCR1, so paralogue cover is weakest in the CNS.
      PARTIAL because it qualifies rather than simply supports the claim.
- name: Mitochondrial Dysfunction and Oxidative Stress Susceptibility
  biological_scale: CELLULAR
  description: >-
    PYCR2 is a mitochondrial matrix enzyme, and its loss is felt as a
    mitochondrial rather than a metabolite-supply defect. PYCR2-deficient cells
    show decreased mitochondrial membrane potential and markedly increased
    apoptosis under oxidative stress. In an oligodendroglial cell model, the
    disease missense proteins additionally shift the fusion-fission balance
    towards fusion, producing abnormally large mitochondria with decreased
    activity.
  cell_types:
  - preferred_term: oligodendrocyte
    term:
      id: CL:0000128
      label: oligodendrocyte
  biological_processes:
  - preferred_term: regulation of mitochondrial membrane potential
    modifier: DECREASED
    term:
      id: GO:0051881
      label: regulation of mitochondrial membrane potential
  - preferred_term: mitochondrial fusion
    modifier: INCREASED
    term:
      id: GO:0008053
      label: mitochondrial fusion
  - preferred_term: mitochondrial fission
    modifier: DECREASED
    term:
      id: GO:0000266
      label: mitochondrial fission
  - preferred_term: cellular response to oxidative stress
    modifier: ABNORMAL
    term:
      id: GO:0034599
      label: cellular response to oxidative stress
  - preferred_term: apoptotic process
    modifier: INCREASED
    term:
      id: GO:0006915
      label: apoptotic process
  evidence:
  - reference: PMID:25865492
    reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "A PYCR2-deficient HEK293FT cell line generated by genome editing with the clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system showed that PYCR2 loss of function led to decreased mitochondrial membrane potential and increased susceptibility to apoptosis under oxidative stress."
    explanation: >-
      Isogenic knockout evidence that PYCR2 loss depolarises mitochondria and
      sensitises cells to oxidative-stress-induced apoptosis.
  - reference: PMID:36548190
    reference_title: "Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Expression of each of the mutated R119C and R251C proteins in cells increased the fusion abilities in mitochondria and decreased their fission abilities relatively."
    explanation: Establishes the fusion-shifted mitochondrial dynamics caused by the disease missense proteins.
  - reference: PMID:36548190
    reference_title: "Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The respective mutant proteins, but not wild type proteins also decreased the activities of mitochondria."
    explanation: Shows the enlarged mitochondria are functionally impaired, not merely morphologically abnormal.
  downstream:
  - target: Impaired Oligodendroglial Differentiation and CNS Hypomyelination
    causal_link_type: DIRECT
    description: >-
      Mitochondrial dysfunction in oligodendroglial cells blocks the
      morphological differentiation required to produce myelin.
    evidence:
    - reference: PMID:36548190
      reference_title: "Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Taken together, these results indicate that an HLD10-associated PYCR2 mutation leads to the formation of large mitochondria with decreased activities, inhibiting oligodendroglial cell morphological differentiation."
      explanation: States exactly the mitochondria-to-differentiation edge asserted here.
  - target: Progressive Neuronal Loss and Brain Atrophy
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      Impaired mitochondrial function and increased apoptotic susceptibility in
      neural lineages contribute to postnatal neurodegeneration.
    evidence:
    - reference: PMID:25865492
      reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "A PYCR2-deficient HEK293FT cell line generated by genome editing with the clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system showed that PYCR2 loss of function led to decreased mitochondrial membrane potential and increased susceptibility to apoptosis under oxidative stress."
      explanation: >-
        Supports the apoptotic-susceptibility step. PARTIAL: shown in a non-neural
        cell line, so the link to neurodegeneration in vivo is inferred.
- name: SHMT2 Upregulation and Cerebral Glycine Accumulation
  biological_scale: MOLECULAR
  description: >-
    Loss of PYCR2 upregulates serine hydroxymethyltransferase 2 (SHMT2), the
    mitochondrial one-carbon enzyme responsible for glycine synthesis, and
    cerebral glycine rises in both Pycr2-null mice and patients. The excess
    glycine is not a bystander: knocking down SHMT2 partially reverses it and
    rescues the axonal beading and shortened neurites of cultured Pycr2-knockout
    neurons, identifying the glycine pathway as the leading candidate
    intervention point. Elevated brain glycine is detectable non-invasively as
    an MR-spectroscopy glycine peak, including in the mild
    spastic-paraplegia-only phenotype where white matter is normally myelinated.
  biological_processes:
  - preferred_term: glycine biosynthetic process
    modifier: INCREASED
    term:
      id: GO:0006545
      label: glycine biosynthetic process
  - preferred_term: glycine metabolic process
    modifier: DYSREGULATED
    term:
      id: GO:0006544
      label: glycine metabolic process
  molecular_functions:
  - preferred_term: glycine hydroxymethyltransferase activity
    modifier: INCREASED
    term:
      id: GO:0004372
      label: glycine hydroxymethyltransferase activity
  chemical_entities:
  - preferred_term: glycine
    modifier: INCREASED
    term:
      id: CHEBI:15428
      label: glycine
  evidence:
  - reference: PMID:32330411
    reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "In situ quantification of neurotransmitters in the brains of PYCR2 mutant mice and patients revealed a signature of encephalopathy driven by excessive cerebral glycine."
    explanation: Direct measurement of excess cerebral glycine in both the mouse model and patients.
  - reference: PMID:32330411
    reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Mechanistically, we demonstrate that loss of PYCR2 upregulates SHMT2, which is responsible for glycine synthesis."
    explanation: Identifies SHMT2 upregulation as the mechanism generating the glycine excess.
  - reference: PMID:32330411
    reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "This hyperglycemia could be partially reversed by SHMT2 knockdown, which rescued the axonal beading and neurite lengths of cultured Pycr2 knockout neurons."
    explanation: >-
      Rescue experiment establishing the glycine excess is causally upstream of a
      neuronal phenotype rather than an epiphenomenon. Note the source sentence
      says "hyperglycemia" where "hyperglycinemia" is meant; the quote is
      reproduced verbatim.
  - reference: PMID:32645307
    reference_title: "PYRC2-Related Hypomyelinating Leukodystrophy: More to This Than Meets the Eye."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "reveal that PYRC2 is a key enzyme for proper brain development and a regulator of glycine homeostasis, uncovering hyperglycinemia as a driver of HLD10 pathogenesis"
    explanation: >-
      Expert commentary framing hyperglycinemia as a driver of HLD10 pathogenesis.
      This is a Preview of PMID:32330411 in the same journal issue, so it is
      secondary interpretation of that dataset rather than independent
      corroboration; note also that it misspells the gene as PYRC2 throughout.
  downstream:
  - target: Progressive Neuronal Loss and Brain Atrophy
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      Glycine-driven encephalopathy contributes to the neuronal injury underlying
      progressive microcephaly and brain atrophy; SHMT2 knockdown rescues axonal
      beading and neurite length in Pycr2-knockout neurons.
    evidence:
    - reference: PMID:32330411
      reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "This hyperglycemia could be partially reversed by SHMT2 knockdown, which rescued the axonal beading and neurite lengths of cultured Pycr2 knockout neurons."
      explanation: >-
        Rescue evidence that the glycine excess is causally upstream of the neuronal
        phenotype, which is what makes this edge more than an association.
- name: Impaired Oligodendroglial Differentiation and CNS Hypomyelination
  conforms_to: "cns_myelin_failure#Deficient or Unstable CNS Myelin Sheath"
  biological_scale: TISSUE
  description: >-
    Oligodendroglial cells expressing the HLD10 missense proteins fail to undergo
    the morphological differentiation that precedes myelination, with reduced
    expression of differentiation and myelin marker proteins. Because myelination
    is highly energy-dependent, the enlarged, hypoactive mitochondria provide a
    direct link from the enzymatic lesion to the white-matter phenotype. The
    resulting CNS hypomyelination and reduced cerebral white-matter volume are
    the imaging hallmark of the disease.
  cell_types:
  - preferred_term: oligodendrocyte
    term:
      id: CL:0000128
      label: oligodendrocyte
  - preferred_term: oligodendrocyte precursor cell
    term:
      id: CL:0002453
      label: oligodendrocyte precursor cell
  biological_processes:
  - preferred_term: oligodendrocyte differentiation
    modifier: DECREASED
    term:
      id: GO:0048709
      label: oligodendrocyte differentiation
  - preferred_term: central nervous system myelination
    modifier: DECREASED
    term:
      id: GO:0022010
      label: central nervous system myelination
  locations:
  - preferred_term: white matter of the central nervous system
    term:
      id: UBERON:0002316
      label: white matter
  evidence:
  - reference: PMID:36548190
    reference_title: "Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "While cells expressing the wild type proteins exhibited differentiated phenotypes with widespread membranes and increased expression levels of differentiation marker proteins following the induction of differentiation, cells harboring each of the mutant proteins did not."
    explanation: Shows the disease alleles block oligodendroglial morphological differentiation.
  - reference: PMID:36548190
    reference_title: "Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Taken together, these results indicate that an HLD10-associated PYCR2 mutation leads to the formation of large mitochondria with decreased activities, inhibiting oligodendroglial cell morphological differentiation."
    explanation: States the causal chain from mitochondrial abnormality to the differentiation block.
  - reference: PMID:25865492
    reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hypomyelination and the absence of lax, wrinkly skin distinguishes this condition from that caused by previously reported mutations in the gene encoding PYCR2's isozyme, PYCR1, suggesting a unique and indispensable role for PYCR2 in the human CNS during development."
    explanation: >-
      Establishes hypomyelination as the defining CNS consequence and separates it
      from the PYCR1 disease.
  downstream:
  - target: Severe Neurodevelopmental Impairment and Early Mortality
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      Deficient myelination of the cerebral white matter contributes to the
      profound neurodevelopmental impairment. INDIRECT because the relative
      contribution of the white-matter and neuronal arms is unsettled - see the
      leukodystrophy-versus-leukoencephalopathy discussion.
    evidence:
    - reference: PMID:34037307
      reference_title: "Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "PYCR2 pathogenic variants lead to an autosomal recessive hypomyelinating leukodystrophy 10 (HLD10), characterized by global developmental delay, microcephaly, facial dysmorphism, movement disorder, and hypomyelination."
      explanation: >-
        Places hypomyelination and the neurodevelopmental impairment together as
        features of the same disorder, which is what this edge asserts.
- name: Progressive Neuronal Loss and Brain Atrophy
  biological_scale: TISSUE
  description: >-
    Alongside the white-matter defect there is a primary neuronal process.
    Patients are born with normal or only mildly reduced head circumference and
    then acquire microcephaly postnatally, with progressive global brain atrophy,
    ventriculomegaly and a thinning corpus callosum, while cerebellum and
    brainstem are relatively spared. The combination of early-onset intellectual
    disability, epilepsy and atrophy has been read by the original cohort authors
    as primary neuronal dysfunction with secondary hypomyelination - which is why
    HLD10 is sometimes placed among the genetic leukoencephalopathies rather than
    the classical hypomyelinating leukodystrophies.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  biological_processes:
  - preferred_term: neuron apoptotic process
    modifier: INCREASED
    term:
      id: GO:0051402
      label: neuron apoptotic process
  locations:
  - preferred_term: brain
    term:
      id: UBERON:0000955
      label: brain
  evidence:
  - reference: PMID:32330411
    reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients lacking PYCR2, a mitochondrial enzyme that synthesizes proline, display postnatal degenerative microcephaly with hypomyelination."
    explanation: States the postnatal, degenerative character of the microcephaly.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical features of early onset intellectual disability, microcephaly, epilepsy, and brain atrophy suggest primary neuronal dysfunction with secondary deficit in myelination"
    explanation: >-
      Records the cohort authors' interpretation that the neuronal process is
      primary and the myelination deficit secondary.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brain magnetic resonance imaging showed global brain atrophy and white matter T2 hyperintensities."
    explanation: Imaging evidence of global brain atrophy in the 14-patient cohort.
  downstream:
  - target: Severe Neurodevelopmental Impairment and Early Mortality
    causal_link_type: DIRECT
    description: >-
      Progressive loss of cerebral tissue underlies the developmental stagnation,
      failure to thrive and reduced survival.
    evidence:
    - reference: PMID:32330411
      reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Patients lacking PYCR2, a mitochondrial enzyme that synthesizes proline, display postnatal degenerative microcephaly with hypomyelination."
      explanation: Ties the degenerative brain process directly to the patients' presentation.
- name: Severe Neurodevelopmental Impairment and Early Mortality
  biological_scale: ORGANISM
  description: >-
    The clinical convergence point. Affected children stagnate at roughly a
    4-to-5-month developmental level, never acquire meaningful language, fine
    motor skills or independent ambulation, and develop failure to thrive by one
    year of age. In the largest cohort no patient survived beyond the first
    decade, with death from pulmonary infection, fever of unknown origin or
    failure to thrive.
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The characteristic clinical presentation of patients with PYCR2 mutations included failure to thrive, microcephaly, craniofacial dysmorphism, progressive psychomotor disability, hyperkinetic movements, and axial hypotonia with variable appendicular spasticity."
    explanation: Defines the clinical syndrome that this node represents.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients did not survive beyond the first decade of life."
    explanation: Establishes the mortality outcome in the reference cohort.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All stagnated at the developmental level of a 4-5-month old infant."
    explanation: Quantifies the developmental ceiling reached by affected children.
phenotypes:
- category: Neurologic
  name: Progressive Microcephaly
  description: >-
    Head circumference at birth is already mildly reduced (-1.8 to -3 SD in 10 of
    14 patients with a birth measurement) and falls progressively thereafter to -3
    to -6 SD. The literature's framing is "postnatally acquired", meaning the bulk
    of the deficit accrues after birth, which discriminates HLD10 from the primary
    (congenital) microcephalies - but this is a matter of trajectory, not of a
    normal starting head size.
  frequency: VERY_FREQUENT
  notes: >-
    Reported in all patients in every cohort. PMID:34037307 states across the
    35 previously reported patients that "All patients had microcephaly and
    developmental delay". In PMID:27130255 (n=14) head circumference at birth was
    available for 10 of 14 and already 1.8 to 3 SD below the mean, falling to
    -3 to -6 SD.
  phenotype_term:
    preferred_term: Progressive microcephaly
    term:
      id: HP:0000253
      label: Progressive microcephaly
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:27860360
    reference_title: "Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients presented with postnatally acquired microcephaly, moderate to profound global developmental delay, and failure to thrive."
    explanation: Documents postnatally acquired microcephaly in all patients of this cohort.
  - reference: PMID:34037307
    reference_title: "Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients had microcephaly and developmental delay."
    explanation: >-
      Supports the VERY_FREQUENT band across the 35 previously reported patients
      reviewed in this paper.
- category: Neurologic
  name: Global Developmental Delay
  description: >-
    Profound global developmental delay with developmental stagnation. Affected
    children achieve some head control and visual tracking but no meaningful
    language, fine motor skills, independent sitting or ambulation.
  frequency: VERY_FREQUENT
  notes: Reported in 14/14 in PMID:27130255 and in all 35 reviewed patients in PMID:34037307.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:27860360
    reference_title: "Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients presented with postnatally acquired microcephaly, moderate to profound global developmental delay, and failure to thrive."
    explanation: Documents moderate-to-profound global developmental delay in all patients.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "None developed meaningful language or fine motor skills, and none were able to stand or walk independently."
    explanation: Characterises the severity ceiling of the developmental impairment.
- category: Growth
  name: Failure to Thrive
  description: >-
    Growth parameters are normal at birth, with failure to thrive established by
    one year of age; weight falls to -4 to -9 SD and height to -2 to -5 SD.
  frequency: VERY_FREQUENT
  notes: Present in all 14 patients of PMID:27130255 and in all patients of PMID:27860360.
  phenotype_term:
    preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Although all individuals were born with average growth parameters, failure to thrive was noted by 1 year of age."
    explanation: Documents universal onset of failure to thrive during the first year.
- category: Neurologic
  name: Axial Hypotonia with Appendicular Spasticity
  description: >-
    A characteristic dissociated tone pattern: truncal (axial) hypotonia together
    with hyperreflexia and mild-to-severe spasticity of the limbs.
  frequency: FREQUENT
  notes: >-
    Band set from the hypotonia figure itself, not from the spastic component:
    PMID:27130255 reports truncal hypotonia with hyperreflexia in "most patients"
    of the 14-patient cohort, which maps to FREQUENT. The 57% appendicular
    spasticity figure belongs to the sibling Spasticity entry and is cited there.
  phenotype_term:
    preferred_term: Axial hypotonia
    term:
      id: HP:0008936
      label: Axial hypotonia
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients presented with profound psychomotor delay, microcephaly, truncal hypotonia with variable appendicular spasticity, and failure to thrive from the ages of 2 months to 1 year."
    explanation: Describes the dissociated axial-hypotonia/appendicular-spasticity pattern and its age of onset.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Truncal hypotonia with hyperreflexia was seen in most patients."
    explanation: >-
      Gives the qualitative frequency for the hypotonia itself, which is what this
      entry's band is derived from.
- category: Neurologic
  name: Spasticity
  description: >-
    Limb spasticity with hyperreflexia. Spasticity is also the presenting and, in
    the mildest reported families, the only feature.
  frequency: FREQUENT
  notes: 57% (8/14) in PMID:27130255; described as common across the 35-patient review in PMID:34037307.
  phenotype_term:
    preferred_term: Spasticity
    term:
      id: HP:0001257
      label: Spasticity
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Mild to severe appendicular spasticity was present in 57%."
    explanation: Provides the explicit 57 percent numerator for the FREQUENT band.
  - reference: PMID:34037307
    reference_title: "Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hypotonia and peripheral spasticity were common."
    explanation: Confirms spasticity is a common feature across the reviewed patient series.
- category: Neurologic
  name: Seizures
  description: >-
    Focal myoclonic and generalized tonic-clonic seizures, with onset usually
    before one year of age and generally controllable with anticonvulsants.
  frequency: FREQUENT
  notes: 57% (8/14) in PMID:27130255.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Seizures were seen in 57% of patients with onset usually before 1 year of age, consisting of focal myoclonic and generalized tonic-clonic seizures, controlled with anticonvulsant medications."
    explanation: Gives the frequency, semiology, onset and treatment responsiveness of the seizures.
- category: Neurologic
  name: Hyperkinetic Movements
  description: >-
    Hyperkinetic movements of the upper and lower extremities, with spontaneous
    mouth chewing and head titubation.
  frequency: FREQUENT
  notes: >-
    Described as present in "the majority of patients" in PMID:27130255 (n=14)
    without an explicit numerator; FREQUENT is the band that "majority" maps to.
  phenotype_term:
    preferred_term: Hyperkinetic movements
    term:
      id: HP:0002487
      label: Hyperkinetic movements
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The majority of patients demonstrated hyperkinetic movements of upper and lower extremities, along with spontaneous mouth chewing and head titubation"
    explanation: Documents the hyperkinetic movement disorder and its qualitative frequency.
- category: Musculoskeletal
  name: Skeletal Muscle Atrophy
  description: Severe muscle wasting accompanying the neurological impairment.
  frequency: VERY_FREQUENT
  notes: Present in 13/14 patients in PMID:27130255 (all except one named patient).
  phenotype_term:
    preferred_term: Skeletal muscle atrophy
    term:
      id: HP:0003202
      label: Skeletal muscle atrophy
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Muscle atrophy was evident in all except patient 1232-III-2."
    explanation: >-
      Gives the explicit numerator, 13 of 14 patients (93 percent), supporting the
      VERY_FREQUENT band.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Besides severe muscle wasting, limb hypertonia, brisk reflexes, inability to ambulate, and hyperkinetic extremity movements aided in making the diagnosis."
    explanation: Records severe muscle wasting as a diagnostically useful feature of the syndrome.
- category: Craniofacial
  name: Triangular Face
  description: >-
    Part of the characteristic HLD10 craniofacial gestalt of triangular facies,
    malar hypoplasia, large malformed ears with over-folded helices, upturned
    nose with bulbous tip and a long smooth philtrum.
  frequency: FREQUENT
  notes: >-
    Described in "most of the patients" in PMID:27130255 (n=14) without an
    explicit numerator.
  phenotype_term:
    preferred_term: Triangular face
    term:
      id: HP:0000325
      label: Triangular face
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A characteristic craniofacial dysmorphism was noted in most of the patients, which included triangular facies, malar hypoplasia, large malformed ears with over-folded helices, upturned nose with bulbous tip, and a long smooth philtrum"
    explanation: Enumerates the craniofacial gestalt, including triangular facies.
- category: Craniofacial
  name: Malar Flattening
  description: Malar (zygomatic) hypoplasia, part of the characteristic facial gestalt.
  frequency: FREQUENT
  notes: Same "most of the patients" source figure as the other gestalt components (PMID:27130255, n=14).
  phenotype_term:
    preferred_term: Malar flattening
    term:
      id: HP:0000272
      label: Malar flattening
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A characteristic craniofacial dysmorphism was noted in most of the patients, which included triangular facies, malar hypoplasia, large malformed ears with over-folded helices, upturned nose with bulbous tip, and a long smooth philtrum"
    explanation: Lists malar hypoplasia among the characteristic craniofacial features.
- category: Craniofacial
  name: Bulbous Nose
  description: Upturned nose with a bulbous tip.
  frequency: FREQUENT
  notes: Same "most of the patients" source figure as the other gestalt components (PMID:27130255, n=14).
  phenotype_term:
    preferred_term: Bulbous nose
    term:
      id: HP:0000414
      label: Bulbous nose
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A characteristic craniofacial dysmorphism was noted in most of the patients, which included triangular facies, malar hypoplasia, large malformed ears with over-folded helices, upturned nose with bulbous tip, and a long smooth philtrum"
    explanation: Lists the upturned bulbous-tipped nose among the characteristic craniofacial features.
- category: Craniofacial
  name: Long Philtrum
  description: A long, smooth philtrum.
  frequency: FREQUENT
  notes: Same "most of the patients" source figure as the other gestalt components (PMID:27130255, n=14).
  phenotype_term:
    preferred_term: Long philtrum
    term:
      id: HP:0000343
      label: Long philtrum
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A characteristic craniofacial dysmorphism was noted in most of the patients, which included triangular facies, malar hypoplasia, large malformed ears with over-folded helices, upturned nose with bulbous tip, and a long smooth philtrum"
    explanation: Lists the long smooth philtrum among the characteristic craniofacial features.
- category: Ophthalmologic
  name: Nystagmus
  description: >-
    Slow, horizontal and symmetric nystagmus in both directions of gaze.
  frequency: OCCASIONAL
  notes: 21% (3/14) in PMID:27130255.
  phenotype_term:
    preferred_term: Nystagmus
    term:
      id: HP:0000639
      label: Nystagmus
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Nystagmus was noted in 21% of patients, commonly manifested as slow horizontal and symmetric eye movements in both directions."
    explanation: Gives the explicit 21% frequency supporting the OCCASIONAL band.
- category: Gastrointestinal
  name: Vomiting
  description: Excessive vomiting of unknown aetiology, with onset at about 1-2 months of age.
  frequency: OCCASIONAL
  notes: 29% (4/14) in PMID:27130255.
  phenotype_term:
    preferred_term: Vomiting
    term:
      id: HP:0002013
      label: Vomiting
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Excessive vomiting of unknown etiology was seen in 29% of patients"
    explanation: Gives the explicit 29% frequency supporting the OCCASIONAL band.
- category: Auditory
  name: Bilateral Sensorineural Hearing Impairment
  description: Bilateral mild-to-severe hearing loss on audiological assessment.
  notes: >-
    frequency is deliberately omitted. PMID:27130255 reports hearing loss only in
    those patients where audiology was available, giving no denominator, so no
    band can be justified.
  phenotype_term:
    preferred_term: Bilateral sensorineural hearing impairment
    term:
      id: HP:0008619
      label: Bilateral sensorineural hearing impairment
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Audiology evaluation, wherever available, showed bilateral mild to severe hearing loss."
    explanation: >-
      Documents bilateral hearing loss while making explicit that testing was not
      universal, which is why no frequency band is asserted.
- category: Musculoskeletal
  name: Arachnodactyly
  description: Long thin fingers and toes.
  frequency: OCCASIONAL
  notes: 4/14 patients in PMID:27130255 (29%).
  phenotype_term:
    preferred_term: Arachnodactyly
    term:
      id: HP:0001166
      label: Arachnodactyly
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Skeletal features of long thin fingers and toes were observed in four patients, while two had pectus carinatum."
    explanation: Gives the 4/14 numerator supporting the OCCASIONAL band.
- category: Musculoskeletal
  name: Pectus Carinatum
  description: Anterior protrusion of the sternum.
  frequency: OCCASIONAL
  notes: 2/14 patients in PMID:27130255 (14%).
  phenotype_term:
    preferred_term: Pectus carinatum
    term:
      id: HP:0000768
      label: Pectus carinatum
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Skeletal features of long thin fingers and toes were observed in four patients, while two had pectus carinatum."
    explanation: Gives the 2/14 numerator supporting the OCCASIONAL band.
- category: Neurologic
  name: Spastic Paraplegia
  description: >-
    In the mildest end of the allelic spectrum, progressive lower-limb spasticity
    and gait difficulty beginning at 8-12 years of age is the presenting and only
    manifestation, with normal early development and normal white-matter
    myelination on MRI.
  notes: >-
    frequency is deliberately omitted at the disease level. This presentation is
    confined to two related families (5 patients, homozygous p.Val128Ala) in
    PMID:37141741 and is not a feature of the classical HLD10 cohorts, so a
    disease-wide band would be misleading.
  phenotype_term:
    preferred_term: Spastic paraplegia
    term:
      id: HP:0001258
      label: Spastic paraplegia
    clinical_course: PROGRESSIVE
  evidence:
  - reference: PMID:37141741
    reference_title: "Pyrroline-5-carboxylate reductase 2 (PYCR2) deficiency causes hereditary spastic paraplaegia in late childhood."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The aim of the present study is to report the clinical findings of patients having novel PYCR2 gene variant that manifest Hereditary Spastic Paraplegia (HSP) is the only symptom without hypomyelinating leukodystrophy."
    explanation: Establishes an isolated spastic-paraplegia presentation at the mild end of the PYCR2 spectrum.
  - reference: PMID:37141741
    reference_title: "Pyrroline-5-carboxylate reductase 2 (PYCR2) deficiency causes hereditary spastic paraplaegia in late childhood."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "4 (%80) patients had gait difficulty and progressive lower limb spasticity started at the age of 8-12 years."
    explanation: Gives the age of onset and character of the spastic paraplegia in that family.
- category: Neurologic
  name: Intention Tremor
  description: >-
    Mild intention tremor beginning at about 6 years of age, reported in the mild
    spastic-paraplegia-only families and preceding the gait difficulty.
  notes: >-
    frequency omitted at the disease level for the same reason as Spastic
    Paraplegia: reported in 4/5 patients of two related families with the
    p.Val128Ala allele (PMID:37141741), not a feature of the classical HLD10
    cohorts.
  phenotype_term:
    preferred_term: Intention tremor
    term:
      id: HP:0002080
      label: Intention tremor
  evidence:
  - reference: PMID:37141741
    reference_title: "Pyrroline-5-carboxylate reductase 2 (PYCR2) deficiency causes hereditary spastic paraplaegia in late childhood."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "4 (%80) patients exhibit mild intention tremor started at the age of approximately 6 years of age."
    explanation: Gives the frequency within that family and the age of onset of the tremor.
- category: Ophthalmologic
  name: Cortical Blindness
  description: Cortical visual impairment, reported as a less consistent feature.
  notes: >-
    frequency omitted. PMID:27860360 groups this among phenotypes that "were less
    consistent among patients" without giving a numerator.
  phenotype_term:
    preferred_term: Cortical blindness
    term:
      id: HP:0100704
      label: Cerebral visual impairment
  evidence:
  - reference: PMID:27860360
    reference_title: "Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additional phenotypes that were less consistent among patients included seizures or seizure-like movements, spasticity and ataxic gait, recurrent vomiting, cortical blindness, dysmorphic features, joint contractures, and irritability."
    explanation: Lists cortical blindness among the less consistent features of the cohort.
- category: Musculoskeletal
  name: Joint Contractures
  description: Joint contractures, reported as a less consistent feature.
  notes: >-
    frequency omitted for the same reason as the other features in this list from
    PMID:27860360 - no numerator is given.
  phenotype_term:
    preferred_term: Joint contracture
    term:
      id: HP:0034392
      label: Joint contracture
  evidence:
  - reference: PMID:27860360
    reference_title: "Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additional phenotypes that were less consistent among patients included seizures or seizure-like movements, spasticity and ataxic gait, recurrent vomiting, cortical blindness, dysmorphic features, joint contractures, and irritability."
    explanation: Lists joint contractures among the less consistent features of the cohort.
- category: Neurologic
  name: Ataxic Gait
  description: >-
    Ataxic gait, reported as a less consistent feature in those patients who
    achieve any ambulation.
  notes: >-
    frequency omitted - PMID:27860360 gives no numerator, and most patients in the
    severe cohorts never ambulate at all, so the denominator is unclear.
  phenotype_term:
    preferred_term: Ataxic gait
    term:
      id: HP:0002066
      label: Gait ataxia
  evidence:
  - reference: PMID:27860360
    reference_title: "Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additional phenotypes that were less consistent among patients included seizures or seizure-like movements, spasticity and ataxic gait, recurrent vomiting, cortical blindness, dysmorphic features, joint contractures, and irritability."
    explanation: Lists ataxic gait among the less consistent features of the cohort.
- category: Behavioral
  name: Irritability
  description: Irritability, reported as a less consistent feature.
  notes: >-
    frequency omitted; no numerator given in PMID:27860360.
  phenotype_term:
    preferred_term: Irritability
    term:
      id: HP:0000737
      label: Irritability
  evidence:
  - reference: PMID:27860360
    reference_title: "Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Additional phenotypes that were less consistent among patients included seizures or seizure-like movements, spasticity and ataxic gait, recurrent vomiting, cortical blindness, dysmorphic features, joint contractures, and irritability."
    explanation: Lists irritability among the less consistent features of the cohort.
imaging_findings:
- name: Cerebral Hypomyelination
  modality: MRI
  description: >-
    Hypomyelination or delayed myelination of deep and subcortical white matter,
    hyperintense on T2 and isointense on T1, with generalized reduction of
    cerebral white-matter volume. This is the radiographic hallmark of HLD10.
  frequency: VERY_FREQUENT
  notes: >-
    Described as the typical radiographic feature across the 35 patients
    reviewed in PMID:34037307 and present in all patients of PMID:27860360.
  phenotype_term:
    preferred_term: Cerebral hypomyelination
    term:
      id: HP:0006808
      label: Cerebral hypomyelination
  evidence:
  - reference: PMID:34037307
    reference_title: "Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hypomyelination or delayed myelination was a typical radiographic feature."
    explanation: Establishes hypomyelination as the typical imaging feature across the reviewed cohort.
  - reference: PMID:34055512
    reference_title: "PYCR2 Mutation Causing Hypomyelination and Microcephaly in an Indian Child."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Magnetic resonance imaging (MRI) of the brain showed hypomyelination of the deep and subcortical white matter, appearing as hyperintense T2 and isointense T1-weighted images, cerebral atrophy with the thinning of the corpus callosum, with normal cerebellum, brainstem, and deep grey nuclei."
    explanation: >-
      Gives the detailed signal characteristics and the relative sparing of
      cerebellum, brainstem and deep grey nuclei.
- name: Thin Corpus Callosum
  modality: MRI
  description: Thinning of the corpus callosum.
  frequency: FREQUENT
  notes: 61% in PMID:27130255 (n=14 with imaging available in 13).
  phenotype_term:
    preferred_term: Thin corpus callosum
    term:
      id: HP:0033725
      label: Thin corpus callosum
  located_in:
    preferred_term: corpus callosum
    term:
      id: UBERON:0002336
      label: corpus callosum
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Thin corpus callosum was seen in 61% of patients."
    explanation: Gives the explicit 61% frequency supporting the FREQUENT band.
- name: Cerebral Atrophy
  modality: MRI
  description: >-
    Progressive global brain atrophy, in some patients severe and accompanied by
    ventriculomegaly, with white-matter T2 hyperintensities. Cerebellum and
    brainstem are generally preserved.
  phenotype_term:
    preferred_term: Cerebral atrophy
    term:
      id: HP:0002059
      label: Cerebral atrophy
    clinical_course: PROGRESSIVE
  notes: >-
    frequency deliberately omitted. PMID:27130255 reports atrophy as a cohort-level
    MRI finding (imaging available in 13/14) with no per-patient count, and
    explicitly describes it as variable - "Some patients showed severe cerebral
    atrophy ... in contrast to other patients in which atrophy was less severe" -
    so no band can be read off it.
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Brain magnetic resonance imaging showed global brain atrophy and white matter T2 hyperintensities."
    explanation: Documents global brain atrophy as the cohort-level imaging finding.
- name: Elevated Brain Glycine on MR Spectroscopy
  modality: MRI
  description: >-
    A glycine peak on MR spectroscopy, the non-invasive correlate of the SHMT2-driven
    cerebral glycine excess. Notably it is present even in the mild
    spastic-paraplegia-only phenotype in which conventional MRI myelination is
    normal, making it a candidate mechanism-linked imaging biomarker.
  phenotype_term:
    preferred_term: Elevated brain glycine level by MRS
    term:
      id: HP:0034893
      label: Elevated brain glycine level by MRS
  notes: >-
    frequency omitted: reported in 5/5 patients of the two related HSP-only
    families in PMID:37141741, but MR spectroscopy was not systematically
    performed in the classical HLD10 cohorts, so no disease-wide band is
    defensible.
  evidence:
  - reference: PMID:37141741
    reference_title: "Pyrroline-5-carboxylate reductase 2 (PYCR2) deficiency causes hereditary spastic paraplaegia in late childhood."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "White matter myelination was normal in all patients. Glycine peakes were detected on the MR spectroscopy in all patients."
    explanation: >-
      Documents the MRS glycine peak alongside normal myelination, linking the
      glycine mechanism to a non-invasive readout. Quoted verbatim, including the
      source's "peakes" typographical error.
biochemical:
- name: Routine Metabolic Abnormality
  context: >-
    A negative but diagnostically important finding: standard biochemical
    screening is normal in HLD10. Serum creatinine, BUN, CPK, ammonia, lactate,
    plasma amino acids and urine organic acids are all unremarkable, and plasma
    and CSF proline are normal. HLD10 is therefore not detectable by routine
    metabolic panels or newborn biochemical screening, and molecular testing is
    required.
  presence: ABSENT
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Metabolic profile including serum creatinine, BUN, CPK, ammonia, lactate, plasma amino acids and urine organic acids were normal in all patients."
    explanation: Enumerates the routine analytes found normal in all 14 patients.
  - reference: PMID:27860360
    reference_title: "Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Despite the metabolic role of PYCR2, routine serum metabolic test in these patients were normal."
    explanation: Independent confirmation that routine metabolic testing is uninformative.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "PYCR2-related syndrome represents a clinically recognizable condition in which PYCR2 mutations lead to protein dysfunction, not detectable on routine biochemical assessments."
    explanation: States the diagnostic consequence of the normal biochemistry.
- name: Elevated Plasma and CSF Purine Metabolites
  context: >-
    Untargeted metabolomics of plasma and CSF in three related patients showed
    marked increases of inosine and xanthine, together with mild increases of
    glutamate, alpha-ketoglutarate and L-glutamate semialdehyde. This points to a
    purine/pyrimidine-pathway perturbation in addition to the mild proline-pathway
    changes, and is a candidate biochemical signature - not an established
    diagnostic marker.
  presence: PRESENT
  notes: >-
    Based on three genetically related patients from a single family
    (PMID:38709052); has not been replicated in an independent cohort.
  evidence:
  - reference: PMID:38709052
    reference_title: "Exploring metabolic alterations in PYCR2 deficiency: Unveiling pathways and clinical presentations of hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "While plasma and CSF proline levels were found to be totally normal, untargeted metabolomic profiling revealed mild increases of glutamate, alpha-ketoglutarate, and l-glutamate semialdehyde and marked increases of inosine and xanthine."
    explanation: Reports the metabolomic changes recorded here.
  - reference: PMID:38709052
    reference_title: "Exploring metabolic alterations in PYCR2 deficiency: Unveiling pathways and clinical presentations of hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Untargeted metabolomics point to mild changes in proline pathway and also in purine/pyrimidine pathway."
    explanation: >-
      Supports the purine/pyrimidine interpretation, marked PARTIAL because the
      authors themselves describe the proline-pathway changes as mild and the
      finding rests on one family.
- name: Mildly Elevated Urinary Glutamate
  context: >-
    Urine amino acids, available in two families, showed mildly elevated glutamate
    (84 and 109 nmol/mg creatinine against a reference of <76) compared with
    unaffected carriers. A subtle finding consistent with a block downstream of
    glutamate, but far too inconsistently measured to be diagnostic.
  presence: PRESENT
  notes: Measured in only 2 of 11 families in PMID:27130255.
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Urine amino acids were available in two families (2206 and 2664), which showed mildly elevated glutamate levels at 84 and 109 nmol/mg creatinine (reference level <76 nmol/mg creatinine) respectively, compared to unaffected carriers who had normal levels."
    explanation: >-
      Reports the raw values and the carrier comparison; PARTIAL because the
      measurement was available for only two of eleven families.
genetic:
- name: PYCR2
  notes: >-
    PYCR2 (1q42.12) encodes pyrroline-5-carboxylate reductase 2, a mitochondrial
    NAD(P)H-dependent oxidoreductase that catalyses the terminal step of
    L-proline biosynthesis. Biallelic variants cause HLD10. Reported pathogenic
    alleles include the founding missense pair p.Arg119Cys and p.Arg251Cys; the
    recurrent nonsense allele p.Arg266Ter (shared by five of eleven families in
    the largest cohort); missense alleles p.Cys232Gly, p.Arg199Trp, p.Val184Ala,
    p.Gly159Arg, p.Val193Met, p.Gly249Val, p.Val86Gly, p.Val134Met and the
    mild-phenotype allele p.Val128Ala; the additional nonsense allele p.Glu10Ter;
    and a 3-prime splice-site variant of intron 2. The Thai c.400G>A
    (p.Val134Met) allele is a founder variant on a shared 2.3 Mb haplotype
    estimated at about 1450 years old.
  relationship_type: CAUSATIVE
  gene_term:
    preferred_term: PYCR2
    term:
      id: hgnc:30262
      label: PYCR2
  evidence:
  - reference: PMID:25865492
    reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we report mutations in PYCR2, which encodes an enzyme in the proline biosynthesis pathway, as the cause of a unique syndrome characterized by postnatal microcephaly, hypomyelination, and reduced cerebral white-matter volume"
    explanation: Original identification of PYCR2 as the causal gene by linkage and exome sequencing.
  - reference: PMID:25865492
    reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Morpholino-based knockdown of a zebrafish PYCR2 ortholog, pycr1b, recapitulated the human microcephaly phenotype, which was rescued by wild-type human PYCR2 mRNA, but not by mutant mRNAs, further supporting the pathogenicity of the identified variants."
    explanation: Functional confirmation of variant pathogenicity by orthologue knockdown and allele-specific rescue.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Both nonsense and missense mutations were identified, which impaired protein multimerization."
    explanation: >-
      Establishes the allelic classes seen across the largest cohort. Tagged
      IN_VITRO because the multimerisation half of the sentence is a 293T
      co-immunoprecipitation result, matching how the same claim is classified in
      the pathophysiology section.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This is an autosomal recessive disorder mapped to chromosome 1q42.12 due to mutations in the PYCR2 gene, encoding an enzyme involved in proline synthesis in mitochondria."
    explanation: Gives the chromosomal locus and the mitochondrial proline-synthesis role of the gene product.
  - reference: PMID:34037307
    reference_title: "Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Haplotype analysis revealed that the two families' members shared a 2.3 Mb region covering the c.400G>A variant, indicating a common ancestry. The variant was estimated to age 1450 years ago."
    explanation: Documents the Thai founder haplotype and its estimated age.
  - reference: PMID:27860360
    reference_title: "Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Exome sequencing identified homozygous variants in PYCR2 in the proband from each family: c.28C>T (p.(Glu10Ter)), c.796C>T (p.(Arg266Ter)), and c.577G>A (p.(Val193Met))."
    explanation: Names three additional homozygous pathogenic alleles identified by clinical exome sequencing.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    No population-based prevalence estimate exists. Case counts are the only
    defensible figure: about 35 patients reviewed in 2021 (PMID:34037307) and
    about 38 by 2024 (PMID:38709052). Reported families cluster in populations
    with high consanguinity (Egyptian, Pakistani, Omani, Indian, Iranian, Thai),
    so the distribution is strongly ascertainment-biased and no rate per 100000
    is asserted.
  evidence:
  - reference: PMID:38709052
    reference_title: "Exploring metabolic alterations in PYCR2 deficiency: Unveiling pathways and clinical presentations of hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Up until now, to our knowledge around 38 patients with PYCR2 defect have been reported."
    explanation: Gives the published case count underpinning the ULTRA_RARE class.
  - reference: PMID:34037307
    reference_title: "Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We also reviewed the phenotype and genotype of all 35 previously reported PYCR2 patients and found that majorities of cases were homozygous with a consanguineous family history"
    explanation: >-
      Independent case count three years earlier, and documents the consanguinity
      clustering that biases ascertainment.
progression:
- phase: Onset
  age_range: 2 months to 1 year
  notes: >-
    Birth parameters including weight, height and head circumference are normal
    or only mildly reduced; presentation follows postnatally.
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients presented with profound psychomotor delay, microcephaly, truncal hypotonia with variable appendicular spasticity, and failure to thrive from the ages of 2 months to 1 year."
    explanation: Gives the age window of first presentation.
- phase: Progressive decline
  notes: >-
    Microcephaly deepens, motor and intellectual function worsen over serial
    follow-up, and seizures typically appear before one year of age.
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Serial clinical follow up demonstrated progression of symptoms in patients."
    explanation: Documents the progressive rather than static course.
- phase: Mortality
  age_range: before 10 years
  notes: >-
    In the largest cohort five study patients died at or before age 6 and no
    patient survived beyond age 10, from pulmonary infection, fever of unknown
    origin or failure to thrive. Survival is not uniform across the spectrum -
    the mild spastic-paraplegia families include affected adults up to 26 years
    of age (PMID:37141741).
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Patients did not survive beyond the first decade of life."
    explanation: States the mortality ceiling in the reference cohort.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "died at or before age 6 years due to complications of pulmonary infection, fever of unknown origin and/ failure to thrive, and no patient survived beyond age 10 years"
    explanation: Gives the causes of death and confirms the survival ceiling.
diagnosis:
- name: Molecular Genetic Testing of PYCR2
  description: >-
    Diagnosis rests on identifying biallelic PYCR2 variants. Because routine
    biochemistry is normal, molecular testing - clinical exome or genome
    sequencing, a leukodystrophy or microcephaly gene panel, or targeted PYCR2
    sequencing once a familial variant is known - is required. Homozygosity
    mapping is informative in consanguineous families.
  evidence:
  - reference: PMID:34055512
    reference_title: "PYCR2 Mutation Causing Hypomyelination and Microcephaly in an Indian Child."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Further genetic testing in the form of clinical exome sequencing revealed compound heterozygous mutation of the PYCR2 gene and matching the clinical phenotype with the genotype."
    explanation: Illustrates clinical exome sequencing as the route to diagnosis.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "PYCR2-related syndrome represents a clinically recognizable condition in which PYCR2 mutations lead to protein dysfunction, not detectable on routine biochemical assessments."
    explanation: Explains why molecular rather than biochemical testing is required.
differential_diagnoses:
- name: Autosomal recessive cutis laxa type 2B (PYCR1)
  description: >-
    The paralogue disorder and the single most important differential.
    PYCR1 deficiency acts at the same step of proline biosynthesis but produces
    cutis laxa with progeroid features. Lax, wrinkly skin is characteristically
    ABSENT in HLD10, and hypomyelination is not a feature of PYCR1 disease.
  disease_term:
    preferred_term: Autosomal recessive cutis laxa type 2B (PYCR1)
    term:
      id: MONDO:0013051
      label: autosomal recessive cutis laxa type 2B
  distinguishing_features:
  - Cutis laxa and wrinkly skin, absent in HLD10
  - Progeroid facial appearance, absent in HLD10
  - CNS hypomyelination, absent in PYCR1 disease
  - Postnatal progressive microcephaly with death in the first decade, absent in PYCR1 disease
  evidence:
  - reference: PMID:25865492
    reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Hypomyelination and the absence of lax, wrinkly skin distinguishes this condition from that caused by previously reported mutations in the gene encoding PYCR2's isozyme, PYCR1, suggesting a unique and indispensable role for PYCR2 in the human CNS during development."
    explanation: Names the two discriminating features between the paralogue disorders.
- name: Pelizaeus-Merzbacher disease
  description: >-
    The prototypic hypomyelinating leukodystrophy (HLD1, PLP1). Shares CNS
    hypomyelination but is X-linked, typically presents with nystagmus and
    cerebellar signs without severe progressive microcephaly, failure to thrive
    or the HLD10 craniofacial gestalt. Curated separately in dismech as
    `Pelizaeus_Merzbacher_Disease`.
  disease_term:
    preferred_term: Pelizaeus-Merzbacher disease
    term:
      id: MONDO:0010714
      label: Pelizaeus-Merzbacher spectrum disorder
  distinguishing_features:
  - X-linked PLP1 inheritance rather than autosomal recessive PYCR2
  - Prominent nystagmus and cerebellar ataxia
  - No progressive postnatal microcephaly
  - No severe diffuse cerebral atrophy
  evidence:
  - reference: PMID:36548190
    reference_title: "Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "PMD is now known as the prototypic disorder of hypomyelinating leukodystrophies (HLDs)"
    explanation: Establishes Pelizaeus-Merzbacher disease as the prototype of the HLD group HLD10 belongs to.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "in contrast to most patients with HLD, often presenting with cerebellar ataxia, axial hypotonia, spasticity, and nystagmus, without severe diffuse cerebral atrophy"
    explanation: >-
      Contrasts the typical HLD presentation with the microcephaly-and-atrophy
      picture of HLD10.
- name: AGC1 deficiency (SLC25A12)
  description: >-
    Aspartate-glutamate carrier 1 deficiency, an ultra-rare recessive disorder of
    the malate-aspartate shuttle. Included because it is the closest mechanistic
    analogue of the open question this entry curates: like HLD10 it presents with
    neurodevelopmental delay, epilepsy, hypotonia, cerebral atrophy and
    hypomyelination, and the literature debates in the same terms whether its
    hypomyelination is primary or secondary to a neuronal defect. Serial imaging in
    AGC1 deficiency shows early hypomyelination followed by later progression of
    myelination, which is the discriminator - HLD10 myelination does not catch up.
  disease_term:
    preferred_term: AGC1 deficiency
    term:
      id: MONDO:0013056
      label: developmental and epileptic encephalopathy, 39
  distinguishing_features:
  - Caused by SLC25A12 rather than PYCR2
  - Early hypomyelination with subsequent progression of myelination on serial MRI
  - Classified as a leuko-axonopathy rather than a hypomyelinating leukodystrophy
  evidence:
  - reference: PMID:31403263
    reference_title: "Longitudinal MRI findings in patient with SLC25A12 pathogenic variants inform disease progression and classification."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These patients were clinically characterized by neurodevelopmental delay, epilepsy, hypotonia, cerebral atrophy, and hypomyelination; however, there has been discussion in the literature as to whether this hypomyelination is primary or secondary to a neuronal defect."
    explanation: >-
      Documents both the phenotypic overlap with HLD10 and the identical
      primary-versus-secondary hypomyelination debate.
  - reference: PMID:31403263
    reference_title: "Longitudinal MRI findings in patient with SLC25A12 pathogenic variants inform disease progression and classification."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The serial neuroimaging findings are notable for cerebral atrophy with white matter involvement, namely, early hypomyelination yet subsequent progression of myelination."
    explanation: Gives the serial-imaging feature that distinguishes AGC1 deficiency from HLD10.
treatments:
- name: Antiseizure Pharmacotherapy
  description: >-
    Anticonvulsant medication for the focal myoclonic and generalized tonic-clonic
    seizures. In the largest cohort seizures were controlled with anticonvulsants,
    making this the single symptomatic intervention with documented efficacy in
    HLD10.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: anticonvulsant agent
      term:
        id: NCIT:C264
        label: Anticonvulsant Agent
  target_phenotypes:
  - preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Seizures were seen in 57% of patients with onset usually before 1 year of age, consisting of focal myoclonic and generalized tonic-clonic seizures, controlled with anticonvulsant medications."
    explanation: Documents seizure control with anticonvulsant medication in the reference cohort.
- name: Nutritional Support for Failure to Thrive
  description: >-
    Nutritional support, including gastrostomy feeding where dysphagia is severe,
    for the universal failure to thrive. Failure to thrive is itself among the
    listed causes of death in the reference cohort, which is the rationale.
  treatment_term:
    preferred_term: Nutritional Support
    term:
      id: NCIT:C15433
      label: Nutritional Support
  target_phenotypes:
  - preferred_term: Failure to thrive
    term:
      id: HP:0001508
      label: Failure to thrive
  notes: >-
    Curated as standard supportive care extrapolated from the documented severity
    and mortality contribution of failure to thrive; no HLD10-specific trial or
    outcome study of nutritional support exists.
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "died at or before age 6 years due to complications of pulmonary infection, fever of unknown origin and/ failure to thrive, and no patient survived beyond age 10 years"
    explanation: >-
      Supports the rationale by naming failure to thrive as a cause of death.
      PARTIAL because it evidences the need, not the efficacy, of nutritional
      support.
- name: Physical Therapy and Rehabilitation
  description: >-
    Physiotherapy, positioning and orthotic management for spasticity, contracture
    prevention and mobility in a population that does not achieve independent
    sitting or ambulation.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Physical Therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  target_phenotypes:
  - preferred_term: Spasticity
    term:
      id: HP:0001257
      label: Spasticity
  notes: >-
    Generic supportive care for hypomyelinating leukodystrophies; no HLD10-specific
    efficacy data exist. Included because it is the standard of care, not because
    it is evidenced in this disease.
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "None developed meaningful language or fine motor skills, and none were able to stand or walk independently."
    explanation: >-
      Evidences the motor disability that rehabilitation targets. PARTIAL: the
      cohort reports the impairment, not any rehabilitation outcome.
- name: Genetic Counseling
  description: >-
    Genetic counselling for families, with carrier and prenatal or preimplantation
    testing once the familial PYCR2 variants are known. Particularly relevant given
    the high rate of consanguinity and the identified Thai founder allele.
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:34037307
    reference_title: "Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Since the c.400G>A was detected in three out of four mutant alleles and with a common ancestry, this variant might be common in Thai patients."
    explanation: >-
      Establishes the recurrent founder allele that makes population-targeted
      carrier counselling worthwhile. PARTIAL: it evidences the rationale, not any
      counselling outcome, and says nothing about counselling itself.
- name: No Disease-Modifying Therapy
  target_mechanisms:
  - target: SHMT2 Upregulation and Cerebral Glycine Accumulation
    description: >-
      Recorded to make the proposed - but entirely untested - intervention point
      machine-readable. No agent is being asserted, and no effect on this node has
      been demonstrated in patients.
    evidence:
    - reference: PMID:32330411
      reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Our findings identify the glycine metabolic pathway as a possible intervention point to alleviate the neurological symptoms of PYCR2-mutant patients."
      explanation: >-
        The authors nominate this node as a therapeutic target. PARTIAL because it
        is a proposal supported by neuronal-culture rescue, not a treatment.
  description: >-
    Recorded explicitly as a negative: there is no specific, disease-modifying or
    curative treatment for HLD10. Management is entirely supportive. The SHMT2
    or glycine axis is the leading proposed intervention point but has not been
    tested in patients.
  evidence:
  - reference: PMID:36548190
    reference_title: "Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: "To date, there is no known specific therapeutic strategy or drug for HLD10."
    explanation: Explicit statement that no disease-specific therapy exists.
  - reference: PMID:32330411
    reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Our findings identify the glycine metabolic pathway as a possible intervention point to alleviate the neurological symptoms of PYCR2-mutant patients."
    explanation: >-
      Records the proposed but untested therapeutic direction, framed by its
      authors as a possibility rather than an established treatment.
animal_models:
- name: Pycr2 loss-of-function mouse (chemically induced p.Met1Thr)
  species: Mouse
  genotype: Pycr2 c.2T>C (p.Met1Thr) homozygous, chemically induced null
  publication: PMID:33734376
  description: >-
    A chemically induced start-codon mutation abolishing the 34 kDa PYCR2 protein
    in brain, spinal cord and muscle while leaving Pycr2 mRNA intact. Mutant mice
    show weight loss, progressive kyphosis, hind-limb clasping, reduced grip
    strength, severe subcutaneous fat loss, mild peripheral axonal atrophy,
    reduced white blood cell counts and altered lipid metabolism.
  modeled_mechanisms:
  - target: Preserved Bulk Proline Supply by Paralogue Redundancy
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      The mouse reproduces the defining negative finding of the human disease -
      an enzymatic block without proline depletion - and adds the dietary
      manipulation that shows proline is semi-essential rather than irrelevant.
    limitations: >-
      Serum and fibroblast measurements only; brain proline was not the reported
      readout, so tissue-specific depletion in the CNS is not excluded.
    evidence:
    - reference: PMID:33734376
      reference_title: "Genetic analysis of Pycr1 and Pycr2 in mice."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Proline levels were not reduced, and precursors were not increased in serum from Pycr2 mutant mice or in lysates from skin fibroblast cultures, but placing Pycr2 mutant mice on a proline-free diet worsened the phenotype."
      explanation: Supports treating the mouse as informative for the preserved-proline node.
  - target: Severe Neurodevelopmental Impairment and Early Mortality
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      The mouse develops a neurological and neuromuscular syndrome - kyphosis,
      hind-limb clasping, weight loss, reduced grip strength - consistent with
      the human neurological impairment.
    limitations: >-
      The reported characterisation does not establish frank microcephaly or CNS
      hypomyelination, the two defining human features, so the correspondence is
      to the neurological syndrome in general rather than to leukodystrophy. See
      the HUMAN_MODEL_MISMATCH discussion.
    evidence:
    - reference: PMID:33734376
      reference_title: "Genetic analysis of Pycr1 and Pycr2 in mice."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Mice with recessive loss-of-function mutations in Pycr2 showed phenotypes consistent with neurological and neuromuscular disorders, including weight loss, kyphosis, and hind-limb clasping."
      explanation: >-
        Supports a neurological phenotype while stopping short of the defining
        human features, hence PARTIAL.
  evidence:
  - reference: PMID:33734376
    reference_title: "Genetic analysis of Pycr1 and Pycr2 in mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "A severe loss of subcutaneous fat in Pycr2 mutant mice is reminiscent of a CL-like phenotype, but primary features such as elastin abnormalities were not observed."
    explanation: >-
      Records that the mouse does not develop the elastin pathology of the
      paralogue disease, keeping the PYCR1/PYCR2 separation intact in the model.
- name: Pycr2 knockout mouse (Escande-Beillard)
  species: Mouse
  genotype: Pycr2 knockout
  publication: PMID:32330411
  description: >-
    A Pycr2 knockout reported to phenocopy the human disorder, in which in situ
    neurotransmitter quantification revealed excessive cerebral glycine and
    secondary depletion of PYCR1 in neural lineages. Cultured knockout neurons
    show axonal beading and shortened neurites that are rescued by SHMT2
    knockdown.
  modeled_mechanisms:
  - target: SHMT2 Upregulation and Cerebral Glycine Accumulation
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Excess cerebral glycine was measured in the mutant mouse brain and in
      patients in the same study, and the SHMT2 knockdown rescue establishes the
      mechanism rather than merely the association.
    limitations: >-
      The rescue was performed in cultured knockout neurons, not in vivo, so
      whether lowering glycine improves the organismal phenotype is untested.
    evidence:
    - reference: PMID:32330411
      reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "In situ quantification of neurotransmitters in the brains of PYCR2 mutant mice and patients revealed a signature of encephalopathy driven by excessive cerebral glycine."
      explanation: >-
        Supports the mouse as informative for the glycine node by measuring the
        same signature in mice and patients.
  evidence:
  - reference: PMID:32330411
    reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We find that knocking out Pycr2 in mice phenocopies the human disorder and depletes PYCR1 levels in neural lineages."
    explanation: The authors' own assessment of the model's fidelity to the human disorder.
- name: pycr1b morpholino knockdown zebrafish
  species: Zebrafish
  genotype: pycr1b morpholino knockdown (PYCR2 orthologue)
  publication: PMID:25865492
  description: >-
    Morpholino knockdown of the zebrafish PYCR2 orthologue pycr1b reproduces
    microcephaly, rescued by wild-type but not mutant human PYCR2 mRNA. The
    allele-specific rescue is the functional evidence for pathogenicity of the
    founding variants.
  modeled_mechanisms:
  - target: Progressive Neuronal Loss and Brain Atrophy
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Reproduces reduced head and brain size, the developmental counterpart of the
      human microcephaly, with allele-specific rescue confirming specificity.
    limitations: >-
      Transient whole-embryo morpholino knockdown models a developmental
      reduction in brain size, not the postnatal progressive degeneration of the
      human disease, and does not model hypomyelination.
    evidence:
    - reference: PMID:25865492
      reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "Morpholino-based knockdown of a zebrafish PYCR2 ortholog, pycr1b, recapitulated the human microcephaly phenotype, which was rescued by wild-type human PYCR2 mRNA, but not by mutant mRNAs, further supporting the pathogenicity of the identified variants."
      explanation: Supports the model as informative for the brain-size phenotype, with allele-specific rescue.
experimental_models:
- name: FBD-102b oligodendroglial differentiation model expressing HLD10 PYCR2 variants
  experimental_model_type: CELL_LINE
  publication: PMID:36548190
  description: >-
    The FBD-102b oligodendroglial differentiation model expressing the HLD10
    missense proteins p.Arg119Cys and p.Arg251Cys. Mutant-expressing cells form
    abnormally large mitochondria with increased fusion, decreased fission and
    decreased mitochondrial activity, and fail to undergo morphological
    differentiation with the associated rise in differentiation marker proteins.
  cell_types:
  - preferred_term: oligodendrocyte
    term:
      id: CL:0000128
      label: oligodendrocyte
  modeled_mechanisms:
  - target: Impaired Oligodendroglial Differentiation and CNS Hypomyelination
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Provides the direct cellular link from the PYCR2 lesion to a failure of
      oligodendroglial differentiation, the proximate cause of hypomyelination.
    limitations: >-
      A murine cell line with heterologous overexpression of mutant protein
      rather than patient-derived cells at endogenous expression levels;
      morphological differentiation and marker expression are surrogates for
      myelination, which is not measured.
    evidence:
    - reference: PMID:36548190
      reference_title: "Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "Taken together, these results indicate that an HLD10-associated PYCR2 mutation leads to the formation of large mitochondria with decreased activities, inhibiting oligodendroglial cell morphological differentiation."
      explanation: Supports the model as informative for the oligodendroglial differentiation node.
- name: CRISPR-Cas9 PYCR2-deficient HEK293FT cell line
  experimental_model_type: CELL_LINE
  publication: PMID:25865492
  description: >-
    An isogenic PYCR2 knockout generated by CRISPR-Cas9 in HEK293FT cells,
    showing decreased mitochondrial membrane potential and increased apoptosis
    under oxidative stress.
  modeled_mechanisms:
  - target: Mitochondrial Dysfunction and Oxidative Stress Susceptibility
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Isogenic loss-of-function evidence isolating the mitochondrial and
      apoptotic consequences of PYCR2 loss from genetic background.
    limitations: >-
      A non-neural embryonic kidney line; the readouts establish that PYCR2 loss
      has these consequences in principle, not that they occur in patient
      oligodendrocytes or neurons.
    evidence:
    - reference: PMID:25865492
      reference_title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "A PYCR2-deficient HEK293FT cell line generated by genome editing with the clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system showed that PYCR2 loss of function led to decreased mitochondrial membrane potential and increased susceptibility to apoptosis under oxidative stress."
      explanation: Supports the model as informative for the mitochondrial-dysfunction node.
discussions:
- discussion_id: hld10_mechanism_beyond_proline
  kind: KNOWLEDGE_GAP
  attaches_to:
  - pathophysiology#Preserved Bulk Proline Supply by Paralogue Redundancy
  status: OPEN
  prompt: >-
    If proline is not depleted, what exactly does the cell lose when PYCR2 is
    absent, and why is the deficit restricted to the CNS?
  rationale: >-
    Every measurement to date - patient plasma and CSF, patient plasma
    metabolomics, mouse serum and fibroblast lysates - finds proline normal, and
    the paralogues are largely redundant for bulk proline supply. Yet PYCR2 loss
    produces a severe, CNS-restricted disease. Three non-exclusive candidate
    answers are on the table: a local proline or P5C pool in a compartment or cell
    type that bulk measurement cannot resolve; a redox role, since PYCR2 also
    cycles NAD(P)H/NAD(P)+; and the SHMT2/glycine diversion, which is the only one
    with a rescue experiment behind it. Distinguishing them matters
    therapeutically, because proline supplementation, redox support and glycine
    lowering are entirely different interventions - and the mouse dietary
    experiment already shows proline availability modulates severity even though
    proline is not depleted.
  evidence:
  - reference: PMID:38709052
    reference_title: "Exploring metabolic alterations in PYCR2 deficiency: Unveiling pathways and clinical presentations of hypomyelinating leukodystrophy 10."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our findings and all the previous reports suggest that proline auxotrophy is not the central disease mechanism."
    explanation: "States the gap - the obvious metabolic explanation has been excluded."
  - reference: PMID:33734376
    reference_title: "Genetic analysis of Pycr1 and Pycr2 in mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Thus, Pycr1 and -2 have redundant functions in proline biosynthesis, and their loss makes proline a semi-essential amino acid."
    explanation: >-
      Frames the paradox - redundancy for bulk supply, yet a severe phenotype and
      a diet-dependent modulation of severity.
- discussion_id: hld10_mouse_hypomyelination_mismatch
  kind: HUMAN_MODEL_MISMATCH
  attaches_to:
  - pathophysiology#Impaired Oligodendroglial Differentiation and CNS Hypomyelination
  status: OPEN
  prompt: >-
    Do Pycr2-null mice actually reproduce the CNS hypomyelination and progressive
    microcephaly that define HLD10 in humans?
  rationale: >-
    The two published Pycr2 mouse lines are described very differently. One is
    reported to phenocopy the human disorder and is the source of the cerebral
    glycine finding; the other, characterised in detail, reports a neurological
    and neuromuscular syndrome of kyphosis, hind-limb clasping and weight loss,
    with only mild peripheral axonal atrophy and no reported assessment of
    myelination or head size. Since hypomyelination and postnatal progressive
    microcephaly are precisely the features that define the human disease, the
    hypomyelination arm of the mechanism currently rests on a heterologous
    oligodendroglial cell line rather than on any in vivo model. Until a mouse is
    shown to be hypomyelinated, mouse rescue experiments cannot be read as
    evidence about the human white-matter phenotype.
  proposed_experiments:
  - experiment_id: hld10_mouse_myelin_quantification
    name: Myelin quantification in Pycr2-null mouse CNS
    description: >-
      Quantify myelin content and oligodendrocyte maturation in Pycr2-null mouse
      brain across postnatal development - MBP and CNPase immunoblotting and
      immunohistochemistry, electron-microscopic g-ratio measurement of corpus
      callosum axons, and serial head or brain-volume measurement - to establish
      whether the model is hypomyelinated and microcephalic or only neurologically
      impaired.
  evidence:
  - reference: PMID:33734376
    reference_title: "Genetic analysis of Pycr1 and Pycr2 in mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Mice with recessive loss-of-function mutations in Pycr2 showed phenotypes consistent with neurological and neuromuscular disorders, including weight loss, kyphosis, and hind-limb clasping."
    explanation: >-
      The detailed characterisation names a neurological syndrome but not
      hypomyelination or microcephaly, which is the mismatch.
  - reference: PMID:32330411
    reference_title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "We find that knocking out Pycr2 in mice phenocopies the human disorder and depletes PYCR1 levels in neural lineages."
    explanation: >-
      The competing claim of full phenocopy, stated without a reported myelin or
      head-size measurement in the abstract.
- discussion_id: hld10_leukodystrophy_vs_leukoencephalopathy
  kind: CONTROVERSY
  attaches_to:
  - pathophysiology#Progressive Neuronal Loss and Brain Atrophy
  status: OPEN
  prompt: >-
    Is HLD10 a true hypomyelinating leukodystrophy, or a primary neuronal
    disorder with secondary hypomyelination that is misplaced in the HLD series?
  rationale: >-
    The name says leukodystrophy, but the largest cohort argued the opposite. Its
    authors noted that imaging ranged from subtle white-matter T2 hyperintensity
    to progressive brain atrophy, and that the clinical picture of early-onset
    intellectual disability, microcephaly, epilepsy and atrophy points to primary
    neuronal dysfunction with a secondary myelination deficit - which is the
    definition of a genetic leukoencephalopathy rather than a classical HLD. This
    is not merely nosological: it determines whether oligodendrocyte-directed or
    neuron-directed interventions are the rational target. The cell-biology
    evidence has since given the oligodendrocyte arm independent support, so the
    honest current position is that both processes are real and their relative
    primacy is unsettled.
  evidence:
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The clinical features of early onset intellectual disability, microcephaly, epilepsy, and brain atrophy suggest primary neuronal dysfunction with secondary deficit in myelination"
    explanation: States the primary-neuronal position from the largest cohort.
  - reference: PMID:27130255
    reference_title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Although this condition has been classified as a hypomyelinating leukodystrophy, we found that the imaging features were variable ranging from subtle white matter T2 hyperintensities to progressive brain atrophy."
    explanation: Records the imaging variability that motivates the reclassification argument.
  - reference: PMID:36548190
    reference_title: "Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation."
    supports: REFUTE
    evidence_source: IN_VITRO
    snippet: "Taken together, these results indicate that an HLD10-associated PYCR2 mutation leads to the formation of large mitochondria with decreased activities, inhibiting oligodendroglial cell morphological differentiation."
    explanation: >-
      Counters the purely-neuronal reading by demonstrating a cell-autonomous
      oligodendroglial differentiation defect caused by the disease alleles.
references:
- reference: PMID:25865492
  title: "Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination."
- reference: PMID:27130255
  title: "PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive."
- reference: PMID:32330411
  title: "Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2."
- reference: PMID:27860360
  title: "Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy."
- reference: PMID:34037307
  title: "Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10."
- reference: PMID:36548190
  title: "Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation."
- reference: PMID:33734376
  title: "Genetic analysis of Pycr1 and Pycr2 in mice."
- reference: PMID:37141741
  title: "Pyrroline-5-carboxylate reductase 2 (PYCR2) deficiency causes hereditary spastic paraplaegia in late childhood."

📚

References & Deep Research

References

8

PMID:25865492

Mutations in PYCR2, Encoding Pyrroline-5-Carboxylate Reductase 2, Cause Microcephaly and Hypomyelination.

No top-level findings curated for this source.

PMID:27130255

PYCR2 Mutations cause a lethal syndrome of microcephaly and failure to thrive.

No top-level findings curated for this source.

PMID:32330411

Loss of PYCR2 Causes Neurodegeneration by Increasing Cerebral Glycine Levels via SHMT2.

No top-level findings curated for this source.

PMID:27860360

Homozygous variants in pyrroline-5-carboxylate reductase 2 (PYCR2) in patients with progressive microcephaly and hypomyelinating leukodystrophy.

No top-level findings curated for this source.

PMID:34037307

Expanding the genotypic spectrum of PYCR2 and a common ancestry in Thai patients with hypomyelinating leukodystrophy 10.

No top-level findings curated for this source.

PMID:36548190

Hypomyelinating Leukodystrophy 10 (HLD10)-Associated Mutations of PYCR2 Form Large Size Mitochondria, Inhibiting Oligodendroglial Cell Morphological Differentiation.

No top-level findings curated for this source.

PMID:33734376

Genetic analysis of Pycr1 and Pycr2 in mice.

No top-level findings curated for this source.

PMID:37141741

Pyrroline-5-carboxylate reductase 2 (PYCR2) deficiency causes hereditary spastic paraplaegia in late childhood.

No top-level findings curated for this source.

Deep Research

2

Claude Code ▸

Hypomyelinating Leukodystrophy 10 (HLD10) — Comprehensive Research Report

claude-haiku-4-5-20251001, claude-sonnet-5 12 citations 2026-08-19T09:24:32.173489

Hypomyelinating Leukodystrophy 10 (HLD10) — Comprehensive Research Report

1. Disease Information

Overview: Hypomyelinating leukodystrophy 10 (HLD10) is a rare, autosomal recessive neurodevelopmental disorder characterized by postnatally acquired progressive microcephaly, severe global developmental delay, failure to thrive, and hypomyelination with reduced cerebral white-matter volume on brain MRI. It is caused by biallelic loss-of-function variants in PYCR2 (pyrroline-5-carboxylate reductase 2), a mitochondrial enzyme catalyzing the terminal step of proline biosynthesis. The disease spans a phenotypic spectrum from a "milder" hypomyelinating-leukodystrophy presentation to a severe, sometimes lethal syndrome with childhood mortality (Zaki et al., 2016, PMID:27130255).

Key identifiers: - OMIM: #616420 (LEUKODYSTROPHY, HYPOMYELINATING, 10; HLD10) — gene entry PYCR2 616406 - Gene: PYCR2, HGNC:23364, chromosome 1q42.12 - MONDO: MONDO:0014632 - Orphanet: ORPHA:481152 ("PYCR2-related microcephaly-progressive leukoencephalopathy") - Disease Ontology: DOID:0060788 - Inheritance:* Autosomal recessive

Synonyms/alternative names: HLD10; PYCR2-related hypomyelinating leukodystrophy; PYCR2-related microcephaly–progressive leukoencephalopathy; microcephaly, autosomal recessive, PYCR2-related; PYCR2 deficiency; pyrroline-5-carboxylate reductase 2 deficiency.

Evidence basis: Information is drawn from aggregated case-series/cohort publications (11 consanguineous families, 14 patients in Zaki et al. 2016; additional families in Nakayama/Reversade 2015, Meng et al. 2017, and a Thai cohort in 2021) plus individual case reports (Indian, Iranian patients), rather than large-scale EHR/registry data — consistent with an ultra-rare Mendelian disorder (~35 patients reported in the literature as of the 2021 Thai-cohort review).

Sources: - OMIM #616420 - OMIM *616406 PYCR2 - Orphanet ORPHA:481152 - NORD/MONDO summary

2. Etiology

Disease causal factor: HLD10 is caused exclusively by biallelic (homozygous or compound heterozygous) pathogenic variants in PYCR2, encoding a mitochondrial NAD(P)H-dependent oxidoreductase that catalyzes reduction of Δ1-pyrroline-5-carboxylate (P5C) to L-proline — the final step of proline biosynthesis (Nakayama et al., 2015, PMID:25865492).

Genetic risk factors: - Homozygous or compound heterozygous PYCR2 loss-of-function or hypomorphic missense variants. - Reported pathogenic variants include: p.Arg119Cys, p.Arg251Cys (Nakayama 2015); p.Arg266 (most common — found in 5 of 11 Egyptian families), p.Cys232Gly, p.Arg199Trp, p.Val184Ala, p.Gly159Arg, and a 3′ splice-site mutation of intron 2 (Zaki 2016, PMID:27130255); p.Arg119His plus a start-loss variant p.Met1? (compound heterozygous, Indian patient — Srivastava et al. 2021, PMC8143271); p.Val86Gly and p.Val134Met (Thai cohort, with c.400G>A/p.Val134Met found on 3 of 4 mutant alleles studied and estimated to have arisen ~1,450 years ago on a shared 2.3 Mb haplotype, indicating a founder effect in the Thai population) (PMID:34037307). - Consanguinity* is a major risk factor: the majority of reported families are consanguineous (Egyptian, Pakistani, Omani, Palestinian, Iranian pedigrees).

Protective factors: None specifically established. Mouse studies suggest dietary proline supplementation does not rescue and a proline-free diet worsens the phenotype in Pycr2-null mice, implying proline availability modulates (but does not fully explain) severity (Stum et al., 2021, PMID:33734376).

Gene–environment interactions: No established environmental modifiers in humans; the mouse model dietary-proline finding is the only reported gene–diet interaction signal.

Sources: - Nakayama et al. 2015, AJHG (PMC4570282) - Zaki et al. 2016, Ann Neurol (PMC4938747) - Meng et al. 2017, AJMG-A - Thai cohort 2021 (PMID:34037307) - Stum et al. 2021, Genetics (PMID:33734376)

3. Phenotypes

Clinical signs/symptoms (with suggested HPO terms)

Phenotype	Frequency	Suggested HPO
Postnatal progressive microcephaly (OFC −3 to −7.7 SD)	~100%	HP:0000253 (Progressive microcephaly) / HP:0000252
Failure to thrive	~100%	HP:0001508
Global developmental delay / profound intellectual disability	~100%	HP:0001263 / HP:0001249
Triangular facies, malar hypoplasia, bulbous/upturned nose, prominent low-set ears (facial dysmorphism)	Common, majority	HP:0000322 (triangular face), HP:0000637 (malar flattening), HP:0000414 (bulbous nose)
Axial (truncal) hypotonia with appendicular hypertonia/spasticity	~57–93%	HP:0008936 (axial hypotonia), HP:0002510 (spasticity)
Muscle atrophy/wasting	93%	HP:0003202
Seizures (focal myoclonic, generalized tonic-clonic), onset typically <1 yr	~50–57%	HP:0001250
Ataxia / absent independent gait	Common	HP:0001251
Hyperkinetic movement disorder	Common	HP:0002378
Nystagmus	21%	HP:0000639
Cortical/cerebral atrophy	100% (imaged)	HP:0002120
Thin corpus callosum	61%	HP:0033725
Hypomyelination / delayed myelination on MRI	~100% (defining feature)	HP:0003429
Reduced cerebral white-matter volume	~100%	HP:0034295 (approx.)
Hearing loss	Reported in some cohorts	HP:0000365
Joint hyperlaxity (in one family)	Uncommon	HP:0001382
Cortical blindness	Less consistent	HP:0100704
Regression of milestones	Some patients	HP:0002376

Onset/course: Onset is typically in the first year of life (2 months–1 year), following apparently normal early development in some cases (e.g., the Indian case regressed starting at 9 months). The course is progressive: microcephaly worsens postnatally, motor function deteriorates, and in the more severe cohort (Zaki 2016) patients "did not survive beyond the first decade of life," with 5 study patients and 4 deceased siblings dying by age 8, most commonly from pulmonary infections, fever of unknown origin, or failure to thrive — contrasting with the original 2015/milder reports in which "none of the patients died and the longest living survivor was 11 years 6 months old." This establishes a recognized severity spectrum.

QoL impact: Profound — patients typically function at GMFCS level V (inability to sit or stand independently), require full-time care, gastrostomy feeding, and have no expressive language.

Sources: as above (Nakayama 2015, Zaki 2016, Meng 2017, Srivastava 2021/PMC8143271).

4. Genetic/Molecular Information

Causal gene: PYCR2 (HGNC:23364), OMIM *616406, chr1q42.12.
Protein function: PYCR2 is a mitochondrial matrix enzyme; together with paralogs PYCR1 and PYCR3, it catalyzes NAD(P)H-dependent reduction of P5C to L-proline, the final and rate-limiting step of proline biosynthesis. PYCR1 loss causes autosomal recessive cutis laxa type IIB (distinct disease), while PYCR2 has "a unique and indispensable role... in the human CNS during development" (Nakayama 2015).
Variant classification/type: Missense (e.g., p.Arg119Cys, p.Arg251Cys, p.Arg199Trp, p.Val184Ala, p.Gly159Arg, p.Val86Gly, p.Val134Met, p.Arg119His), nonsense (p.Arg266*), and splice-site (3′ splice site, intron 2) variants reported; ClinVar entry VCV000254247 documents at least one classified variant.
Functional consequences: Missense variants (e.g., R119C, R251C, R199W, R266X) reduce protein stability/abundance without necessarily abolishing mitochondrial localization; disease mechanism is predominantly loss-of-function, though newer structural work (Torii et al. 2022, PMID:36548190/PMC9787162) suggests R119C and R251C also promote aberrant dimeric/trimeric protein complexes (vs normal monomer), implying a possible gain-of-function/dominant-negative structural component in addition to loss of catalytic activity.
Allele frequency: No specific gnomAD population allele-frequency figure was recovered in this search; PYCR2 pathogenic variants are extremely rare/private, consistent with an ultra-rare autosomal recessive disease (prevalence estimated by some registries at 1 per 200,000–500,000).
Somatic vs germline: Germline only (constitutional, inherited).
Founder effect: c.400G>A (p.Val134Met) identified as a likely Thai founder variant, on a shared 2.3 Mb haplotype dated to ~1,450 years ago (PMID:34037307).
Modifier genes: None specifically established in humans. In Pycr2-null mice, loss of PYCR2 causes secondary loss of PYCR1 protein in brain (and vice versa), indicating the paralogs do not compensate for one another despite similar biochemical activity, and loss of PYCR2 elevates serine hydroxymethyltransferase 2 (SHMT2) and brain glycine levels in both patients and mice — a downstream one-carbon/glycine metabolism perturbation.
Chromosomal abnormalities: Not applicable — HLD10 is caused by sequence-level PYCR2 variants, not large chromosomal rearrangements.

Suggested ontology terms: HGNC:23364 (PYCR2), GO:0004735 (pyrroline-5-carboxylate reductase activity), GO:0006561 (proline biosynthetic process), GO:0005759 (mitochondrial matrix).

Sources: - Nakayama 2015 (PMC4570282) - Torii et al. 2022, Neurology International (PMC9787162) - ClinVar VCV000254247

5. Environmental Information

No established environmental, lifestyle, or infectious triggers of HLD10 itself (it is a purely Mendelian genetic disorder). The only environmental modulator identified is dietary proline in the Pycr2-null mouse model: a proline-free diet worsened the mutant phenotype, suggesting nutritional proline may partially buffer disease severity, although "proline levels were not reduced, and precursors were not increased in serum" in the mutant mice, so proline auxotrophy per se is not the central mechanism (Stum et al. 2021, PMID:33734376). No infectious agents are implicated in etiology; secondary infections (pulmonary) are a reported cause of death in severely affected children.

6. Mechanism / Pathophysiology

Causal chain (initiating lesion → clinical manifestation):

Molecular lesion: Biallelic PYCR2 variants → reduced PYCR2 protein stability/abundance or altered oligomerization (monomer → aberrant dimer/trimer for R119C/R251C) (GO:0004735 pyrroline-5-carboxylate reductase activity).
Mitochondrial dysfunction: Loss/dysfunction of PYCR2 → decreased mitochondrial membrane potential (demonstrated by MitoTracker/JC-1 assays) and formation of enlarged mitochondria with increased fusion and decreased fission capacity in patient-mutant-expressing cells (Torii et al. 2022). PYCR2-deficient cells show reduced energy-production capacity.
Increased susceptibility to oxidative-stress-induced apoptosis: CRISPR-engineered PYCR2-deficient cells showed significantly more TUNEL-positive apoptotic cells after H2O2 exposure than wild-type, establishing a cytoprotective/anti-apoptotic role for PYCR2 under oxidative stress (Nakayama 2015).
Neuronal and oligodendroglial consequences:
Primary neuronal dysfunction is thought to drive microcephaly, intellectual disability, epilepsy, and brain atrophy (Zaki 2016 interpretation).
Separately, mutant PYCR2 (R119C, R251C) fails to support oligodendroglial cell morphological differentiation, with reduced expression of the myelin markers MBP (myelin basic protein) and CNPase (2′,3′-cyclic-nucleotide 3′-phosphodiesterase) compared to wild type — directly linking mitochondrial dysfunction to the hypomyelination phenotype, since myelination is a highly energy-dependent process (Torii et al. 2022; also emphasized in the Srivastava case report, PMC8143271).
Secondary metabolic perturbation: Loss of PYCR2 upregulates SHMT2 and elevates brain glycine in both patients and Pycr2−/− mice, indicating a downstream one-carbon/glycine metabolic disturbance (Stum et al. 2021).
Clinical convergence: The combination of primary neuronal vulnerability + impaired oligodendrocyte differentiation/energy failure → progressive microcephaly, developmental regression, seizures, spasticity, and hypomyelination/white-matter volume loss on MRI.

Cell types involved: neurons (CL:0000540), oligodendrocytes/oligodendrocyte precursor cells (CL:0000128 / CL:0002453), fibroblasts (used in functional studies, CL:0000057).

Zebrafish model (developmental confirmation): Morpholino knockdown of pycr1b (zebrafish PYCR2 ortholog) recapitulated microcephaly — reduced head width by 4 dpf and smaller forebrain/midbrain/hindbrain on histology — and was rescued by co-injection of wild-type human PYCR2 mRNA but not by mutant mRNA, confirming variant pathogenicity (Nakayama 2015).

Mouse model (systemic/metabolic confirmation): Pycr2-null mice show weight loss (41–58% less than controls), progressive kyphosis, hind-limb clasping (CNS-attributable), 33% reduced grip strength, 53% loss of total body fat, mild peripheral axonal atrophy, reduced white blood cell counts, and altered lipid metabolism — a broad neurological/neuromuscular and systemic metabolic phenotype, without primary elastin/cutis-laxa-type skin pathology (Stum et al. 2021, PMID:33734376).

Suggested GO terms: GO:0006561 (proline biosynthetic process), GO:0055114 (oxidation-reduction process), GO:0007005 (mitochondrion organization), GO:0006915 (apoptotic process), GO:0022010 (central nervous system myelination), GO:0048709 (oligodendrocyte differentiation).

Sources: - Torii et al. 2022 (PMC9787162) - Nakayama 2015 (PMC4570282) - Stum et al. 2021 (PMID:33734376)

7. Anatomical Structures Affected

Organ level (primary): Central nervous system — cerebral white matter (UBERON:0002316 white matter), corpus callosum (UBERON:0002336), cerebral cortex (UBERON:0000956), brainstem (UBERON:0002298; "mildly thin" in Nakayama 2015 cohort).
Secondary/systemic: Musculoskeletal system (muscle atrophy, spasticity, joint contractures); growth/nutrition (failure to thrive); in mouse models, adipose tissue (subcutaneous fat loss) and hematologic system (reduced WBC counts).
Tissue/cell level: Myelin/white matter tracts; oligodendrocytes (CL:0000128) — impaired differentiation; neurons (CL:0000540) — primary dysfunction; peripheral nerve (mild axonal atrophy in mouse model).
Subcellular level: Mitochondria (GO:0005739 cellular component; PYCR2 is mitochondrial-matrix resident, GO:0005759) — enlarged, dysmorphic mitochondria with abnormal fusion/fission balance and reduced membrane potential.
Localization: Bilateral, diffuse (not lateralized) — supratentorial white matter predominantly affected; cerebellum, brainstem, and deep gray nuclei reported as relatively spared/normal in at least one case report (Srivastava 2021).

Suggested UBERON terms: UBERON:0002316 (white matter of central nervous system), UBERON:0002336 (corpus callosum), UBERON:0000955 (brain), UBERON:0001851 (cortex).

8. Temporal Development

Onset: Congenital-to-early-infantile; head circumference typically normal or near-normal at birth with postnatally acquired progressive microcephaly emerging over the first months of life (onset of clinical presentation generally 2 months to 1 year of age).
Onset pattern: Insidious/progressive rather than acute.
Progression: Chronic and progressive — microcephaly deepens, developmental regression may occur (documented from ~9 months in one case), spasticity and seizures often emerge within the first year.
Disease course: Progressive, non-remitting; no spontaneous or treatment-induced remission reported. Severity is variable — a "milder"/longer-survival phenotype (original 2015 report; longest survivor 11 years 6 months) versus a more severe/lethal phenotype (Zaki 2016 Egyptian/Pakistani cohort; deaths by age 8, no survivors beyond age 10).
Duration: Chronic, lifelong for survivors; in the more severe cohort, life-limiting (death typically before age 10, from pulmonary infection, fever of unknown origin, or failure to thrive).
Critical periods: Early infancy appears to be the critical window for both diagnosis (before irreversible regression) and any future intervention, given the "postnatal" (rather than prenatal) onset of microcephaly.

9. Inheritance and Population

Inheritance pattern: Autosomal recessive (HP:0000007).
Penetrance: Appears fully penetrant among biallelic carriers in reported families (all homozygous/compound-heterozygous individuals are affected), though disease severity is variable.
Expressivity: Variable — ranges from a "hypomyelinating leukodystrophy" phenotype with longer survival to a "lethal syndrome of microcephaly and failure to thrive" with early childhood death, even among patients with the same genotype in some instances (e.g., differing outcomes noted between the 2015 and 2016 cohorts).
Genetic anticipation: Not reported/not applicable (not a repeat-expansion disorder).
Consanguinity role: Prominent — most reported families are consanguineous (Egyptian, Pakistani, Omani, Palestinian).
Founder effect: c.400G>A (p.Val134Met) — Thai founder variant, ~1,450 years old, shared 2.3 Mb haplotype (PMID:34037307).
Epidemiology: Ultra-rare; approximately 35 patients had been reported in the literature as of the 2021 Thai-cohort review. One rare-disease registry estimates prevalence at roughly 1 in 200,000–500,000, though this figure was not independently traced to a primary epidemiological source in this search and should be treated as approximate.
Population demographics: Reported cases cluster in populations with high consanguinity rates — Egyptian (largest single cohort, Zaki 2016), Pakistani, Omani, Palestinian, Iranian, Indian, and Thai (with an identified founder variant) patients. No clear sex predilection (Zaki 2016 cohort: 7 females, 7 males). Geographic distribution appears broad but ascertainment-biased toward regions/populations with consanguineous marriage practices and access to exome sequencing.

10. Diagnostics

Genetic testing (primary diagnostic modality): Molecular confirmation via single-gene PYCR2 sequencing, multi-gene leukodystrophy/microcephaly panels, clinical exome sequencing (WES), or whole genome sequencing (WGS) — WES/WGS is typically how cases are identified given the phenotypic overlap with other microcephaly-hypomyelination syndromes. Homozygosity mapping/linkage analysis has been used in consanguineous families (original discovery, Nakayama 2015, LOD score 3.72 on chr1q).
Chromosomal microarray: Used to exclude copy-number causes of microcephaly as part of a standard microcephaly diagnostic workup, though HLD10 itself is not caused by CNVs.
Neuroimaging (brain MRI): Central to diagnosis — shows hypomyelination (T2 hyperintense/T1 isointense deep and subcortical white matter), thin corpus callosum, generalized cerebral/cortical atrophy, and a mildly thin brainstem; cerebellum and deep gray nuclei may be relatively spared.
Laboratory/biochemical tests: Generally unremarkable — "routine serum metabolic profiles were unremarkable," with normal plasma amino acids and urine organic acids in most patients; slightly elevated urinary glutamate noted in 2 families (Zaki 2016). This indicates HLD10 is not reliably detectable by standard metabolic newborn screening or biochemical panels — genetic testing is required.
Differential diagnosis: Other hypomyelinating leukodystrophies (HLD1–HLD8+, e.g., Pelizaeus-Merzbacher disease/PLP1, GJC2-related HLD2, TUBB4A-related HLD6, POLR3A/B-related HLD7/8), other genetic microcephaly-with-brain-atrophy syndromes, and PYCR1-related autosomal recessive cutis laxa type IIB (which shares proline-pathway biology but is clinically distinguished by cutis laxa/wrinkly skin, which is characteristically absent in HLD10).
Screening: No population-level or newborn screening program exists for HLD10 (ultra-rare, no biochemical marker); carrier screening/prenatal diagnosis is feasible via targeted PYCR2 variant testing once a familial pathogenic variant is known, particularly relevant given the consanguinity and founder-effect patterns described.

11. Outcome / Prognosis

Survival/mortality: Highly variable by cohort/severity. In the original 2015 discovery cohort, no reported deaths (longest survivor 11 years 6 months). In the larger 2016 Egyptian/Pakistani cohort (14 patients, 11 families), "patients did not survive beyond the first decade of life" — 5 study patients plus 4 deceased siblings died by age 8; causes of death included pulmonary infections, fever of unknown origin, and failure to thrive.
Morbidity/function: Profound and lifelong — patients typically achieve, at best, GMFCS level V functional status (unable to sit or stand independently), with global developmental delay/intellectual disability, absent or minimal expressive language, and dependence on caregivers for all activities of daily living.
Complications: Recurrent pulmonary infections (aspiration-related, given dysphagia/hypotonia), seizures, failure to thrive/malnutrition, spasticity-related joint contractures.
Recovery potential: None described — the disease course is progressive/degenerative rather than static; no reports of developmental catch-up.
Prognostic factors: Genotype-severity correlation is not cleanly established, but truncating (nonsense/splice) variants and the specific cohort/ethnic background (Egyptian severe cohort vs. original milder cohort) have been associated with differing survival outcomes in the literature to date; this remains an area of uncertainty given small sample sizes.

12. Treatment

No disease-modifying or curative therapy exists for HLD10. Management is entirely supportive/symptomatic, consistent with hypomyelinating leukodystrophies generally:

Pharmacotherapy for symptoms:
Anti-seizure medications for epilepsy (NCIT:C15986 Pharmacotherapy; specific agent depends on seizure semiology).
Anti-spasticity agents (e.g., baclofen, tizanidine — general class) for spasticity management.
Surgical/interventional:
Gastrostomy tube placement (NCIT:C15329 Surgical Procedure) for severe dysphagia/failure to thrive.
Orthopedic surgery for joint contractures/scoliosis (NCIT:C16186 Orthopedic Surgical Procedure).
Rehabilitative/supportive care:
Physical therapy (NCIT:C15302) for spasticity/mobility and orthotic bracing.
Occupational and speech/communication therapy, assistive communication devices.
Special education services.
Nutritional support (NCIT:C15433 or NCIT:C15447 Dietary Intervention) for failure to thrive.
Wheelchair seating/positioning to manage scoliosis risk.
Monitoring: Serial brain MRI to track hypomyelination/atrophy progression; ongoing surveillance for neurologic complications (per general hypomyelinating-leukodystrophy management literature).
Experimental/investigational: No PYCR2-specific gene therapy, enzyme replacement, or targeted molecular therapy has reached clinical trials as of this search. General hypomyelinating-leukodystrophy therapeutic development (e.g., intrathecal approaches in other HLDs such as metachromatic leukodystrophy) is not yet applicable to HLD10 specifically. No ClinicalTrials.gov-registered HLD10/PYCR2-specific interventional trial was identified in this search.
Genetic counseling: Recommended for families given autosomal recessive inheritance, high consanguinity prevalence, and availability of carrier/prenatal testing once a familial variant is identified.

Sources: - Alex TLC — Hypomyelinating Leukodystrophies overview - General HLD management literature (Pouwels et al. 2014, Ann Neurol, translational review)

13. Prevention

Primary prevention: Genetic counseling and carrier screening in populations/families with known consanguinity or a previously identified familial PYCR2 variant; preimplantation genetic diagnosis (PGD) or prenatal diagnosis is technically feasible once the familial variant is known, though not specifically reported as routinely offered for HLD10 in the literature reviewed.
Secondary prevention: Early genetic diagnosis in at-risk families (e.g., after an index case) allows earlier initiation of supportive care and family planning counseling.
Tertiary prevention: Proactive management of complications — nutritional support/gastrostomy to prevent aspiration and failure-to-thrive complications, seizure control, and infection surveillance/prophylaxis (given pulmonary infection as a leading cause of death in the severe cohort).
Immunization: No disease-specific vaccine strategy; standard childhood immunizations remain important given increased vulnerability to pulmonary infection.
Public health/environmental interventions: Not applicable — no environmental modifiable risk factor identified.

14. Other Species / Natural Disease

No naturally occurring PYCR2-associated disease has been reported in companion animals or wildlife in the sources reviewed. All animal data derive from engineered/induced models (see Model Organisms, below), not spontaneous veterinary disease. PYCR2 orthologs are broadly conserved across vertebrates (zebrafish pycr1b functions as the PYCR2 ortholog; mouse Pycr2 is a direct ortholog).

15. Model Organisms

Zebrafish (induced, morpholino knockdown)

Model: Morpholino-based knockdown of pycr1b (the zebrafish PYCR2 ortholog).
Phenotype recapitulation: Small head size evident by 1 day post-fertilization; significantly reduced maximum head width by 4 dpf; histologically smaller forebrain, midbrain, and hindbrain — recapitulating the human microcephaly phenotype.
Rescue experiment: Co-injection of wild-type human PYCR2 mRNA rescued the microcephaly phenotype, whereas mutant (patient-variant) mRNAs showed absent or only partial rescue — providing strong functional confirmation of variant pathogenicity (Nakayama et al. 2015, PMID:25865492).
Limitations: Morpholino knockdown models transient, whole-embryo loss of function rather than the postnatal, progressive, CNS-restricted human phenotype; does not model hypomyelination, myelin markers, or longer-term neurodevelopmental/behavioral outcomes.

Mouse (Pycr2 knockout, ENU/targeted null allele)

Model: Recessive loss-of-function Pycr2 mutant mice (Stum et al. 2021, PMID:33734376), studied alongside a Pycr1-null model.
Phenotype recapitulation: Weight loss (41–58% less than controls at 3 and 9 months), progressive kyphosis, hind-limb clasping (attributed to CNS rather than peripheral dysfunction), 33% reduced grip strength, 53% total-body-fat loss, mild peripheral axonal atrophy without denervation, reduced white blood cell counts, and altered lipid metabolism — broadly recapitulating the neurological/neuromuscular and systemic-metabolic character of human HLD10, though not modeling frank microcephaly or hypomyelination directly in the reported characterization.
Mechanistic insights: Serum proline levels were not reduced and proline precursors were not increased despite enzyme loss, arguing against simple proline auxotrophy as the core mechanism; a proline-free diet worsened the phenotype. Brain loss of PYCR2 also caused secondary loss of PYCR1 protein (and vice versa), showing the paralogs do not compensate for one another in vivo. Elevated brain glycine and increased SHMT2 were observed in both patients and mutant mice, pointing to a shared downstream one-carbon/glycine-metabolism perturbation.
Limitations: No elastin/cutis-laxa-type skin phenotype was observed despite subcutaneous fat loss, distinguishing the mouse Pycr2-null phenotype from human PYCR1-related cutis laxa; the degree to which the mouse model reproduces the specific hypomyelination and severe microcephaly of human HLD10 was not fully characterized in the source reviewed — flagged here as a candidate HUMAN_MODEL_MISMATCH consideration for curation (murine CNS phenotype centers on kyphosis/clasping/weight loss rather than confirmed hypomyelination).

Cellular/in vitro models

CRISPR-engineered PYCR2-deficient human cell lines: decreased mitochondrial membrane potential, increased apoptosis under oxidative stress (H2O2/TUNEL assay) (Nakayama 2015).
Patient-variant (R119C, R251C) expression studies: enlarged mitochondria with altered fusion/fission dynamics, reduced membrane potential, and failure of oligodendroglial morphological differentiation (reduced MBP/CNPase) — directly modeling the hypomyelination mechanism at the cellular level (Torii et al. 2022, PMID:36548190/PMC9787162).

Resources: MGI (Pycr2 allele records), ZFIN (zebrafish pycr1b), Alliance of Genome Resources.

Summary of Key Evidence Citations (PMIDs)

PMID	First author, year, journal	Contribution
25865492	Nakayama et al., 2015, Am J Hum Genet	Original disease-gene discovery (PYCR2); functional/zebrafish studies
27130255	Zaki et al., 2016, Ann Neurol	Severe/lethal phenotype expansion, 14 patients/11 families, mortality data
27860360	Meng et al., 2017 (pub. AJMG-A 2016/2017)	5 additional patients, 3 families, clinical/MRI characterization
34037307	(Thai cohort), 2021	Genotypic spectrum expansion, Thai founder variant c.400G>A
36548190	Torii et al., 2022, Neurology International	Mitochondrial mechanism, oligodendroglial differentiation defect
33734376	Stum et al., 2021, Genetics	Pycr2 mouse knockout model, systemic/metabolic phenotype
(PMC8143271)	Srivastava et al., 2021, Cureus	Indian case, first compound-heterozygous report

Note on gaps: This report is compiled from web-accessible abstracts and secondary summaries (PubMed/PMC/OMIM/Orphanet listings); several primary full-text sources (OMIM #616420 full clinical synopsis, original PubMed abstract pages) returned bot-protection errors during retrieval and could not be quoted directly — figures and quotes above are drawn from PMC full-text mirrors and search-engine-extracted summaries where direct fetch failed. A precise gnomAD population allele-frequency figure for specific PYCR2 pathogenic variants was not located in this search and should be independently verified (e.g., directly via gnomad.broadinstitute.org) before being cited as a hard prevalence/carrier-frequency figure in a knowledge-base entry.

Reference Validation

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OpenScientist ▸

Key Findings

openscientist-autonomous 12 citations 2026-08-19T09:33:44.753786

Key Findings

Finding 1 — HLD10 is an autosomal recessive disorder caused by biallelic PYCR2 variants

HLD10 (OMIM #616420) maps to chromosome 1q42.12 and is caused by biallelic (homozygous or compound heterozygous) loss-of-function and missense variants in PYCR2, encoding pyrroline-5-carboxylate reductase 2, a mitochondrial enzyme catalyzing the final step of proline biosynthesis. Zaki et al. identified 11 consanguineous families, establishing autosomal recessive inheritance. The disorder's molecular architecture is consistent across cohorts: the majority of reported cases are homozygous with consanguineous family histories, with only rare compound-heterozygous exceptions.

"This is an autosomal recessive disorder mapped to chromosome 1q42.12 due to mutations in the PYCR2 gene, encoding an enzyme involved in proline synthesis in mitochondria." — PMID: 27130255

"PYCR2 pathogenic variants lead to an autosomal recessive hypomyelinating leukodystrophy 10 (HLD10), characterized by global developmental delay, microcephaly, facial dysmorphism, movement disorder, and hypomyelination." — PMID: 34037307

Finding 2 — Core clinical phenotype: postnatal microcephaly, developmental delay, hypomyelination, poor survival

The characteristic presentation includes postnatally acquired (progressive) microcephaly, moderate-to-profound global developmental delay, failure to thrive, craniofacial dysmorphism, hyperkinetic movements, axial hypotonia with variable appendicular spasticity, and seizures. Brain MRI shows hypomyelination/delayed myelination, thin corpus callosum, global brain/white-matter atrophy, and T2 white-matter hyperintensities. Severely affected patients do not survive beyond the first decade. A crucial diagnostic clue is that routine serum metabolic profiles are unremarkable/normal, distinguishing HLD10 from classical inborn errors of metabolism with peripheral biochemical signatures.

"The characteristic clinical presentation of patients with PYCR2 mutations included failure to thrive, microcephaly, craniofacial dysmorphism, progressive psychomotor disability, hyperkinetic movements, and axial hypotonia with variable appendicular spasticity. Patients did not survive beyond the first decade of life." — PMID: 27130255

"All patients presented with postnatally acquired microcephaly, moderate to profound global developmental delay, and failure to thrive. Brain MRI in these patients showed thin corpus callosum, delayed myelination, and generalized white-matter volume loss." — PMID: 27860360

Finding 3 — Mechanism: PYCR2 loss raises cerebral glycine via SHMT2 upregulation, driving neurodegeneration

Escande-Beillard et al. (2020) solved the PYCR2 apo-enzyme crystal structure, showed that a p.Gly249Val mutation at the dimer interface lowers enzymatic activity, and demonstrated that Pycr2-knockout mice phenocopy the human disorder and deplete PYCR1 in neural lineages. In situ neurotransmitter quantification in mutant mouse and patient brains revealed encephalopathy driven by excessive cerebral glycine caused by SHMT2 upregulation; SHMT2 knockdown partially reversed axonal beading and rescued neurite length in Pycr2-KO neurons. Zaki et al. independently showed that both missense and nonsense mutations impair PYCR2 protein multimerization, the biophysical basis of loss of function.

"loss of PYCR2 upregulates SHMT2, which is responsible for glycine synthesis. This hyperglycemia could be partially reversed by SHMT2 knockdown, which rescued the axonal beading and neurite lengths of cultured Pycr2 knockout neurons." — PMID: 32330411

"knocking out Pycr2 in mice phenocopies the human disorder and depletes PYCR1 levels in neural lineages" — PMID: 32330411

"Both nonsense and missense mutations were identified, which impaired protein multimerization." — PMID: 27130255

Finding 4 — Gene identification (2015): variants destabilize the protein and sensitize cells to oxidative-stress apoptosis

Nakayama et al. (2015, Am J Hum Genet) identified biallelic PYCR2 mutations as the cause of postnatal microcephaly with hypomyelination through linkage mapping plus whole-exome sequencing in two consanguineous families (homozygous c.355C>T p.Arg119Cys and c.751C>T p.Arg251Cys). Patient lymphoblastoid cells showed strongly reduced PYCR2; transfected variant proteins retained normal mitochondrial localization but were present at lower amounts, indicating reduced protein stability as the loss-of-function mechanism. A CRISPR-Cas9 PYCR2-knockout HEK293FT line showed decreased mitochondrial membrane potential and increased susceptibility to apoptosis under oxidative stress, linking PYCR2 loss to mitochondrial dysfunction.

"A PYCR2-deficient HEK293FT cell line generated by genome editing with the clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system showed that PYCR2 loss of function led to decreased mitochondrial membrane potential and increased susceptibility to apoptosis under oxidative stress." — PMID: 25865492

"both variant proteins retained normal mitochondrial localization but had lower amounts than the wild-type protein, suggesting that the variant proteins were less stable" — PMID: 25865492

Finding 5 — Zebrafish pycr1b knockdown recapitulates microcephaly, rescued by wild-type human PYCR2 mRNA

Nakayama et al. (2015) performed morpholino-based knockdown of the zebrafish PYCR2 ortholog pycr1b, which recapitulated the human microcephaly phenotype. The phenotype was rescued by wild-type human PYCR2 mRNA but not by mutant (p.Arg119Cys / p.Arg251Cys) mRNAs, confirming both the pathogenicity of the specific variants and the functional conservation of the gene across vertebrates.

"Morpholino-based knockdown of a zebrafish PYCR2 ortholog, pycr1b, recapitulated the human microcephaly phenotype, which was rescued by wild-type human PYCR2 mRNA, but not by mutant mRNAs" — PMID: 25865492

Finding 6 — Phenotypic spectrum extends to a milder late-childhood hereditary spastic paraplegia

Sager et al. (2023) reported a novel homozygous missense PYCR2 variant (NM_013328 c.383T>C, p.Val128Ala) in 5 male patients from 2 related families presenting as hereditary spastic paraplegia (HSP) in late childhood WITHOUT hypomyelinating leukodystrophy. Developmental milestones were normal without dysmorphic features; ~80% had mild intention tremor from ~6 years and ~80% had progressive lower-limb spasticity/gait difficulty from age 8–12 years; ages ranged 6–26 years. This is the first report of PYCR2 variants causing HSP and considerably widens the recognized clinical spectrum, with implications for genetic diagnosis of milder cases.

"manifest Hereditary Spastic Paraplegia (HSP) is the only symptom without hypomyelinating leukodystrophy. This is the first study that report the PYCR2 gene variants as a cause of HSP in late childhood." — PMID: 37141741

"A novel homozygous missense (NM_013328: c.383T > C, p.V128A) variant in the PYCR2 gene is detected in 5 patient from 2 related families." — PMID: 37141741

Finding 7 — Founder effects and consanguinity-driven population genetics (Thai c.400G>A founder allele)

Manaspon et al. (2021) reviewed all 35 previously reported PYCR2 patients: the majority were homozygous with consanguineous family history (except two compound-heterozygous cases); all had microcephaly and developmental delay; hypotonia and peripheral spasticity were common; hypomyelination/delayed myelination was the typical radiographic feature. In two unrelated Thai families, the c.400G>A (p.Val134Met) variant was found on a shared 2.3 Mb haplotype (estimated allele age ~1450 years), indicating a common ancestor/founder effect; it accounted for 3 of 4 mutant alleles in Thai patients.

"Haplotype analysis revealed that the two families' members shared a 2.3 Mb region covering the c.400G>A variant, indicating a common ancestry. The variant was estimated to age 1450 years ago." — PMID: 34037307

"majorities of cases were homozygous with a consanguineous family history, except patient 1 and another reported case who were compound heterozygous. All patients had microcephaly and developmental delay. Hypotonia and peripheral spasticity were common." — PMID: 34037307

Finding 8 — No disease-specific therapy; SHMT2/glycine-lowering is the leading mechanism-based target

No curative or disease-modifying therapy is approved for HLD10; care is supportive (anticonvulsants; spasticity, nutritional, and rehabilitation management). Because routine serum metabolic profiles are normal, peripheral proline supplementation is not clearly rational. The strongest mechanism-based lead is lowering cerebral glycine: Escande-Beillard et al. showed SHMT2 knockdown partially reversed axonal beading and rescued neurite lengths in Pycr2-knockout neurons, identifying the glycine metabolic pathway as a possible intervention point. No HLD10-specific clinical trials are currently registered.

"Our findings identify the glycine metabolic pathway as a possible intervention point to alleviate the neurological symptoms of PYCR2-mutant patients." — PMID: 32330411

Section-by-Section Report

1. Disease Information

HLD10 is a rare autosomal recessive hypomyelinating leukodystrophy — a genetic white-matter disorder characterized by MRI evidence of absent or near-absent myelin development combined with postnatal (progressive) microcephaly, severe neurodevelopmental impairment, and failure to thrive. It is one of a numbered series of hypomyelinating leukodystrophies (HLD1–HLD~24+), each defined by a distinct causal gene; HLD10 is the PYCR2-associated entity.

Key identifiers:

Resource	Identifier
OMIM	#616420
MONDO	MONDO:0014635
Gene (HGNC)	PYCR2
Locus	1q42.12
MeSH	Hereditary Central Nervous System Demyelinating Diseases (closest); Leukodystrophy
Orphanet	Within genetic hypomyelinating leukodystrophy group

Synonyms / alternative names: Hypomyelinating leukodystrophy 10 with microcephaly; PYCR2-related microcephaly with hypomyelination; postnatal microcephaly with hypomyelination and failure to thrive; PYCR2 deficiency.

Source of information: Aggregated disease-level resources (OMIM, published case series/cohorts) and individual patient case reports; no large EHR-derived registry exists given the disorder's rarity.

2. Etiology

Primary cause: Biallelic (homozygous or compound heterozygous) pathogenic variants in PYCR2 (genetic, Mendelian). The etiology is monogenic; there is no evidence of infectious, environmental, or acquired causation.

Genetic risk factors: The only established genetic risk factor is inheriting two pathogenic PYCR2 alleles. Consanguinity is the dominant epidemiologic driver — most reported families are consanguineous, and homozygosity for founder or private variants predominates (Findings 1, 7).

Environmental risk factors: None identified. HLD10 is fully genetically determined. No toxin, exposure, dietary, or lifestyle factor has been implicated.

Protective factors: None specifically identified. In principle, heterozygous carriers are unaffected (recessive), and outbreeding in consanguineous populations reduces incidence.

Gene–environment interactions: No documented GxE interaction. Because peripheral metabolism is normal and the disorder is highly penetrant when biallelic, environmental modulation appears minimal; phenotypic variability is primarily allele-driven (e.g., the milder p.Val128Ala HSP allele — Finding 6).

3. Phenotypes

Phenotype	Type	Onset	Severity/Progression	Frequency	Suggested HPO
Progressive (postnatal) microcephaly	Physical/sign	Postnatal (acquired)	Severe, progressive	Nearly universal	HP:0005484 (Postnatal microcephaly)
Global developmental delay	Sign	Infantile	Moderate–profound	Universal	HP:0001263
Failure to thrive	Sign	Infantile	Severe	Very common	HP:0001508
Hypomyelination / delayed myelination (MRI)	Imaging/lab	Infantile	Progressive	Universal	HP:0006808 (Hypomyelination)
Thin corpus callosum	Imaging	Congenital/infantile	Static/progressive	Common	HP:0033725
Cerebral/white-matter atrophy	Imaging	Infantile	Progressive	Common	HP:0002283 / HP:0012762
Axial hypotonia	Sign	Infantile	Variable	Common	HP:0008936
Appendicular / lower-limb spasticity	Sign	Infantile–childhood	Variable	Common	HP:0001257 / HP:0002061
Hyperkinetic movements	Sign	Infantile	Variable	Common	HP:0002487
Seizures	Sign	Infantile	Variable	Frequent	HP:0001250
Craniofacial dysmorphism	Physical	Congenital	Variable	Common	HP:0001999
Intention tremor (mild allele)	Sign	~6 yr	Mild	~80% of HSP-phenotype patients	HP:0002080
Hereditary spastic paraplegia (mild allele)	Sign	Late childhood (8–12 yr)	Progressive, milder	Mild-allele families	HP:0001258

Quality of life impact: In the severe (classic) form, profound global disability, non-ambulatory/non-verbal status, feeding difficulty, and death within the first decade impose maximal burden on affected children and caregivers. In the milder HSP form, quality of life is affected principally by progressive gait impairment with preserved cognition (Finding 6). Formal QoL instrument data (EQ-5D, SF-36) are not available for this ultra-rare disease.

4. Genetic / Molecular Information

Causal gene: PYCR2 (HGNC), 1q42.12; encodes pyrroline-5-carboxylate reductase 2, a mitochondrial enzyme catalyzing the final (NAD(P)H-dependent) step of L-proline biosynthesis (reduction of Δ¹-pyrroline-5-carboxylate to proline).

Pathogenic variants (representative):

Variant (cDNA)	Protein	Type	Phenotype	Population/Note	Source
c.355C>T	p.Arg119Cys	Missense (destabilizing)	Classic HLD10	Consanguineous; original 2015 report	PMID: 25865492
c.751C>T	p.Arg251Cys	Missense (destabilizing)	Classic HLD10	Consanguineous; original 2015 report	PMID: 25865492
c.400G>A	p.Val134Met	Missense	Classic HLD10	Thai founder allele (~1450 yr)	PMID: 34037307
(Gly249Val)	p.Gly249Val	Missense at dimer interface (↓activity)	HLD10	Functional/structural study	PMID: 32330411
c.383T>C	p.Val128Ala	Missense (mild)	HSP without leukodystrophy	2 related families, 5 males	PMID: 37141741

Variant classification: Reported disease alleles are classified pathogenic/likely pathogenic under ACMG/AMP, supported by functional evidence (reduced protein stability, impaired multimerization, decreased enzymatic activity, zebrafish rescue failure).

Variant types: Both missense (reduced stability / impaired multimerization / reduced activity) and nonsense variants have been reported; all converge on loss of function (Findings 3, 4).

Allele frequency: Pathogenic alleles are rare in population databases (gnomAD), consistent with a recessive, largely founder/consanguinity-driven disorder.

Origin: Germline (constitutional, biallelic). No somatic contribution.

Functional consequence: Loss of function — missense variants act principally by destabilizing the protein and impairing multimerization; nonsense variants truncate the protein. Downstream, PYCR1 is depleted in neural lineages (Finding 3).

Modifier genes: None formally established. SHMT2 is a mechanistic effector (its upregulation drives glycine toxicity) rather than a classic modifier; PYCR1 depletion is a downstream consequence. Phenotype severity tracks primarily with the specific PYCR2 allele.

Epigenetic information: No disease-specific DNA-methylation or histone-modification signature has been reported for HLD10 (not available).

Chromosomal abnormalities: None; HLD10 is a single-gene disorder without recurrent structural/copy-number changes (not applicable).

5. Environmental Information

No environmental, lifestyle, or infectious factors contribute to HLD10. It is a purely genetic Mendelian disorder. Consanguinity (a social/demographic rather than environmental exposure) increases the probability of homozygosity for pathogenic alleles but is not an environmental cause of the molecular defect. Infectious agents are not applicable.

6. Mechanism / Pathophysiology

Molecular pathway: Proline biosynthesis and one-carbon/serine–glycine metabolism. PYCR2 catalyzes the terminal reduction of pyrroline-5-carboxylate (P5C) to L-proline in mitochondria. Loss of PYCR2 activity perturbs this node and, critically, triggers compensatory upregulation of SHMT2 (serine hydroxymethyltransferase 2), which synthesizes glycine — producing pathological cerebral glycine excess (Finding 3).

Causal chain (upstream → downstream):

Biallelic PYCR2 LoF variants (missense destabilizing / nonsense)
│
▼
Reduced PYCR2 protein amount + impaired multimerization  → loss of enzyme activity
│
├─► Depletion of PYCR1 in neural lineages
│
├─► ↓ Mitochondrial membrane potential → ↑ apoptosis under oxidative stress
│
└─► ↑ SHMT2 expression → ↑ cerebral glycine
     │
     ▼
    Axonal beading, reduced neurite length, neurodegeneration
     │
     ▼
     Hypomyelination + progressive microcephaly + white-matter atrophy
     │
     ▼
   Global developmental delay, movement disorder, seizures, failure to thrive

Cellular processes: Apoptosis (increased under oxidative stress), mitochondrial dysfunction (decreased membrane potential), and neuronal/axonal degeneration (axonal beading, neurite shortening). The hypomyelination appears to be at least partly secondary to a primary neuronal/axonal defect (a "leuko-axonopathy"-type mechanism), consistent with the neurotransmitter/glycine-driven neurodegeneration.

Protein dysfunction: Missense variants retain correct mitochondrial localization but are present at lower amounts (reduced stability) and impair multimerization; the p.Gly249Val substitution at the dimer interface lowers catalytic activity — the apo-enzyme crystal structure was solved to demonstrate this (Findings 3, 4).

Metabolic changes: Elevated cerebral glycine (via SHMT2); perturbed proline biosynthesis. Importantly, peripheral/serum metabolic profiles are normal, indicating a CNS-compartmentalized metabolic derangement (Finding 2).

Immune involvement / tissue-damage mechanisms: No autoimmune or inflammatory driver; oxidative stress-sensitized apoptosis and glycine excitotoxic-type neurodegeneration are the operative injury mechanisms.

Suggested ontology terms: - GO biological process: proline biosynthetic process (GO:0006561); glycine biosynthetic process (GO:0006545); myelination (GO:0042552); apoptotic process (GO:0006915); neuron projection development (GO:0031175). - GO cellular component: mitochondrion (GO:0005739); mitochondrial matrix (GO:0005759). - CL cell types: oligodendrocyte (CL:0000128); neuron (CL:0000540); central nervous system neuron. - CHEBI: L-proline (CHEBI:17203); glycine (CHEBI:15428); L-1-pyrroline-5-carboxylate (CHEBI:17388).

7. Anatomical Structures Affected

Primary organ / body system: Central nervous system / brain (nervous system). UBERON: brain (UBERON:0000955); white matter (UBERON:0002316); corpus callosum (UBERON:0002336); cerebral hemisphere.
Tissue level: Cerebral white matter (myelin) with generalized volume loss; thin corpus callosum; cerebral atrophy.
Cell level: Oligodendrocytes (myelinating cells; CL:0000128) and neurons/axons (CL:0000540) are affected; PYCR1 depletion occurs in neural lineages.
Subcellular level: Mitochondria (GO:0005739) — reduced membrane potential and increased oxidative-stress apoptosis.
Localization / lateralization: Diffuse and bilateral / symmetric white-matter involvement, typical of hypomyelinating leukodystrophies.

8. Temporal Development

Onset: Congenital-to-infantile in the classic form; microcephaly is postnatally acquired and progressive (head circumference normal at birth then decelerating). The milder HSP allele has late-childhood onset (spasticity 8–12 yr; tremor from ~6 yr).
Onset pattern: Insidious/chronic and progressive.
Progression: Progressive neurodegeneration with failure to thrive; severe cases are fatal within the first decade. The mild HSP phenotype progresses more slowly, with survival into adulthood (ages reported up to 26 years).
Course: Chronic, progressive, lifelong; no remission.
Critical periods: Early postnatal myelination window is the period of maximal vulnerability and the theoretical window for any myelination-directed or glycine-lowering intervention.

9. Inheritance and Population

Inheritance: Autosomal recessive.
Penetrance: High/complete for the biallelic classic phenotype; variable expressivity exists across alleles (severe leukodystrophy vs. milder HSP).
Genetic anticipation: Not applicable (not a repeat-expansion disorder).
Founder effects: Documented — the Thai c.400G>A (p.Val134Met) founder allele on a shared 2.3 Mb haplotype, estimated age ~1450 years, accounting for 3 of 4 mutant alleles in Thai patients (Finding 7).
Consanguinity: Central to the epidemiology; most families are consanguineous and patients homozygous.
Carrier frequency / prevalence / incidence: Not precisely established; the disorder is ultra-rare with ~35+ patients reported in the literature as of the 2021 review. Prevalence in outbred populations is very low; locally elevated where founder alleles and consanguinity coincide.
Population demographics: Reported across consanguineous populations (Middle Eastern, South/Southeast Asian including Thai and Indian patients). No strong sex bias in the classic form; the reported HSP-allele families comprised affected males (small sample). Age distribution skews pediatric owing to early mortality in the severe form.

10. Diagnostics

Recommended approach: Molecular genetic diagnosis is definitive. Because routine serum/urine metabolic tests are normal, biochemical screening does not establish the diagnosis and can mislead.
Genetic testing: Whole-exome sequencing (WES) is the highest-yield test and was the discovery method; whole-genome sequencing (WGS) or leukodystrophy/hypomyelination gene panels including PYCR2 are appropriate. Single-gene testing is reasonable in populations with known founder alleles (e.g., Thai c.400G>A). Chromosomal microarray/karyotype/FISH are not indicated (single-gene disorder).
Imaging: Brain MRI is the key phenotyping modality — hypomyelination/delayed myelination (delayed T2 hypointensity, often T1 hyperintensity), thin corpus callosum, cerebral and white-matter atrophy. Serial MRI helps distinguish primary hypomyelination from progressive atrophy.
Biomarkers: No validated peripheral biomarker; cerebral glycine elevation is a mechanistic finding (MR spectroscopy could theoretically detect elevated glycine, but this is not an established clinical biomarker).
Differential diagnosis: Other hypomyelinating leukodystrophies and microcephaly syndromes — e.g., PMD/PLP1 (HLD1), HIKESHI-related HLD (with febrile-illness crises, Ashkenazi founder), SLC25A12/AGC1-related leuko-axonopathy, and KIF1C-related spastic-ataxia/HLD. PYCR2 disease is distinguished by progressive postnatal microcephaly, failure to thrive, normal peripheral metabolics, and biallelic PYCR2 variants. The milder allele overlaps clinically with hereditary spastic paraplegias.
Screening: Cascade/carrier testing in affected consanguineous families; targeted founder-allele carrier screening is feasible where relevant (e.g., Thai c.400G>A).

11. Outcome / Prognosis

Survival/mortality: Severe (classic) HLD10 is fatal within the first decade of life. The milder HSP-phenotype patients survive into adulthood.
Morbidity/function: Profound, lifelong disability in the classic form (non-ambulatory, non-verbal, feeding-dependent, seizures). The mild form causes progressive gait impairment with relatively preserved cognition.
Complications: Failure to thrive, feeding difficulty, seizures, aspiration/respiratory complications, and consequences of severe neurodisability.
Prognostic factors: Genotype is the principal determinant — null/severely destabilizing biallelic variants predict the severe lethal phenotype, whereas partial-function alleles (e.g., p.Val128Ala) predict the milder HSP course.
Recovery potential: None; the disorder is progressive and neurodegenerative.

12. Treatment

Disease-specific therapy: None approved. Management is supportive/symptomatic — anticonvulsants for seizures, spasticity management (physiotherapy, antispasticity agents), nutritional support for failure to thrive, and multidisciplinary rehabilitation (physical, occupational, speech therapy).
Mechanism-based experimental direction: Glycine-lowering / SHMT2 inhibition is the leading strategy, supported by the demonstration that SHMT2 knockdown partially rescues axonal beading and neurite length in Pycr2-KO neurons (Finding 8). Dietary/pharmacologic glycine reduction and SHMT2-targeted approaches are conceptually motivated but unproven clinically.
Rational cautions: Because peripheral proline metabolism is normal, systemic proline supplementation lacks clear rationale.
Clinical trials: No HLD10-specific registered trials.
Suggested NCIT terms: Supportive Care; Anticonvulsant Agent; Physical Therapy; Nutritional Support (used generically; no disease-specific intervention exists).

13. Prevention

Primary prevention: Genetic counseling for consanguineous couples and affected families; carrier testing and reproductive options (preimplantation genetic testing, prenatal diagnosis) are the principal preventive measures given the recessive, high-penetrance nature.
Screening: Cascade carrier testing within families; founder-allele carrier screening where population-relevant (e.g., Thai c.400G>A).
Behavioral/public-health: Awareness of consanguinity-associated recessive disease risk; no vaccine or environmental intervention is applicable.
Counseling: 25% recurrence risk for carrier–carrier couples; genetic counseling is central.

14. Other Species / Natural Disease

Orthologs / model species: Mouse Pycr2; zebrafish ortholog pycr1b. No naturally occurring animal disease has been documented (OMIA); model organisms are engineered/experimental.
Comparative biology: The gene and its function are evolutionarily conserved — zebrafish pycr1b knockdown reproduces microcephaly rescued by human PYCR2 mRNA (Finding 5), and Pycr2-KO mice phenocopy the human disorder (Finding 3), demonstrating conserved requirement for PYCR2 in neurodevelopment.
Zoonotic/transmission: Not applicable (genetic disorder).

15. Model Organisms

Model	Type	Key features	Recapitulation	Reference
Pycr2 knockout mouse	Mammalian, genetic KO	Phenocopies human disorder; depletes PYCR1 in neural lineages; elevated cerebral glycine via SHMT2; SHMT2 knockdown rescues axonal beading/neurite length	High — reproduces neurodegeneration and the core mechanism	PMID: 32330411
Zebrafish pycr1b morphant	Vertebrate, morpholino knockdown	Recapitulates microcephaly; rescued by WT human PYCR2 mRNA but not mutant mRNAs	Good for microcephaly; validates variant pathogenicity	PMID: 25865492
PYCR2-KO HEK293FT (CRISPR-Cas9)	Cellular, in vitro	↓ Mitochondrial membrane potential; ↑ apoptosis under oxidative stress	Models mitochondrial/apoptotic mechanism	PMID: 25865492
Patient lymphoblastoid cells; transfected variant proteins	In vitro	Reduced PYCR2; variants normally localized but less stable	Models loss-of-function via reduced stability	PMID: 25865492
Recombinant PYCR2 (crystal structure)	Structural/biochemical	Apo-enzyme structure; p.Gly249Val at dimer interface lowers activity	Structural basis of pathogenicity	PMID: 32330411

Model applications: Dissecting the SHMT2/glycine mechanism, testing glycine-lowering interventions, validating variant pathogenicity, and studying mitochondrial dysfunction. Limitations: Morpholino knockdown is transient and can carry off-target effects; the milder human HSP phenotype has not been separately modeled; therapeutic rescue to date is partial and in vitro/animal only.

Mechanistic Model / Interpretation

The evidence converges on a coherent model in which PYCR2 is a mitochondrial enzyme whose loss produces a compartmentalized cerebral metabolic and mitochondrial crisis. Two mechanistic arms operate downstream of the same biallelic loss-of-function lesion:

A mitochondrial/apoptotic arm (established 2015): destabilized or truncated PYCR2 → reduced enzyme → decreased mitochondrial membrane potential → heightened apoptosis under oxidative stress, with PYCR1 co-depletion in neural lineages.
A glycine-excess arm (established 2020): PYCR2 loss → SHMT2 upregulation → excess cerebral glycine → axonal beading, neurite shortening, and neurodegeneration.

The second arm is the more actionable one because it is reversible in models — SHMT2 knockdown partially rescues neuronal morphology. Both arms terminate in the same clinical endpoint: progressive postnatal microcephaly, hypomyelination/white-matter atrophy, and severe neurodevelopmental disability. The observation that peripheral metabolics are normal despite cerebral glycine elevation underscores that this is a brain-restricted metabolic disease, which also explains why classical biochemical newborn screening does not detect it and why genetic testing is essential.

Allelic severity maps onto phenotype: severe destabilizing/null biallelic genotypes produce the lethal infantile leukodystrophy, while partial-function alleles (p.Val128Ala) produce a milder, later-onset hereditary spastic paraplegia without overt leukodystrophy — a genotype–phenotype gradient rather than two separate diseases.

Evidence Base

PMID	Study	Contribution	Evidence type
25865492	Nakayama et al. 2015, Am J Hum Genet	Gene identification; reduced protein stability; CRISPR-KO mitochondrial/apoptosis phenotype; zebrafish pycr1b rescue	Human genetics + in vitro + model organism
27130255	Zaki et al. — PYCR2 mutations cause a lethal syndrome	11 consanguineous families; AR locus 1q42.12; impaired multimerization; core phenotype and lethal prognosis	Human clinical/genetics
27860360	Homozygous PYCR2 variants, progressive microcephaly & hypomyelination	Confirms postnatal microcephaly, MRI features, normal metabolics	Human clinical
32330411	Escande-Beillard et al. 2020 — Loss of PYCR2 causes neurodegeneration via SHMT2	Crystal structure; Pycr2-KO mouse; SHMT2/glycine mechanism and rescue; therapeutic target	Structural + model organism + mechanistic
34037307	Manaspon et al. 2021 — Thai cohort	Review of 35 patients; consanguinity dominance; Thai c.400G>A founder allele (~1450 yr)	Human genetics/population
37141741	Sager et al. 2023 — PYCR2 causes HSP in late childhood	First HSP phenotype; p.Val128Ala; spectrum expansion	Human clinical/genetics
34055512	Indian child case report	Compound-heterozygous HLD10; normal metabolics; MRI hypomyelination	Human clinical (case)
33771508	Disease variants of human Δ¹-pyrroline-5-carboxylate reductase	Biochemistry of PYCR enzymology	In vitro/biochemical

Supporting/contextual literature on the hypomyelinating leukodystrophy landscape and differentials includes reviews of hypomyelinating disorders and MRI approaches (PMID: 26477299, PMID: 27235001), the expanded genetic white-matter disorder gene catalog (PMID: 32704519), and comparators such as HIKESHI-related HLD (PMID: 34111619), SLC25A12/AGC1 leuko-axonopathy (PMID: 31403263), and KIF1C-related classification ambiguity (PMID: 40794111).

Limitations and Knowledge Gaps

Small evidence base: Only ~35+ patients reported; prevalence, incidence, carrier frequency, and sex ratio are not precisely quantified.
Genotype–phenotype correlations are still coarse; the full determinants of the severe-vs-mild spectrum are not systematically mapped.
Biomarkers: No validated peripheral or imaging biomarker (e.g., MRS glycine) is clinically established for diagnosis or monitoring.
Therapeutics: Glycine-lowering/SHMT2 inhibition rescue is partial and demonstrated only in vitro/animal; no human therapeutic data or registered trials exist.
Two mechanistic arms (mitochondrial-apoptotic vs glycine-excess) are not fully integrated — their relative contributions to hypomyelination versus neuronal loss remain to be resolved.
Epigenetics, immune involvement, and structural genomics are not characterized (not applicable/unknown).
Model gaps: The milder HSP phenotype lacks a dedicated model; morpholino data carry inherent caveats.

Proposed Follow-up Experiments / Actions

Test glycine-lowering interventions in vivo: Evaluate dietary glycine restriction and/or SHMT2 pharmacologic inhibition in Pycr2-KO mice for effects on myelination, brain growth, and survival — the most direct translation of the mechanistic finding.
Genotype–phenotype registry: Aggregate all reported and new PYCR2 patients with standardized allele annotation, MRI phenotyping, and outcomes to define severity predictors and refine the severe-vs-HSP spectrum.
MR spectroscopy for cerebral glycine: Prospectively test whether MRS-detectable brain glycine elevation can serve as a diagnostic/monitoring biomarker.
iPSC-derived oligodendrocyte/neuron models from patients (severe and mild alleles) to dissect whether hypomyelination is primary (oligodendrocyte-autonomous) or secondary to axonal/neuronal glycine toxicity.
Structure-guided variant functional classification: Use the apo-enzyme structure to model additional missense VUS (stability, dimer-interface, activity) and improve ACMG classification.
Population carrier screening for founder alleles (e.g., Thai c.400G>A) in high-consanguinity communities, paired with genetic counseling programs.
Integrate the two mechanistic arms: Experiments manipulating oxidative-stress/mitochondrial function and glycine levels independently to determine their relative causal weight for the hypomyelination endpoint.

Report compiled from an autonomous multi-iteration literature investigation (8 confirmed findings, 17 papers reviewed). Evidence types span human clinical/genetics, model organism (mouse, zebrafish), in vitro/cellular, and structural/biochemical studies.

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Discussions and Knowledge Gaps

Pathophysiology

Pathograph

Phenotypes

Genetic Associations

Medical Actions

Biochemical Markers

Diagnosis

Imaging Findings

Progression

Prevalence

Differential Diagnoses

Experimental Models

Animal Models

Source YAML

References & Deep Research

References

Deep Research

Hypomyelinating Leukodystrophy 10 (HLD10) — Comprehensive Research Report

1. Disease Information

2. Etiology

3. Phenotypes

Clinical signs/symptoms (with suggested HPO terms)

4. Genetic/Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome / Prognosis

12. Treatment

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Zebrafish (induced, morpholino knockdown)

Mouse (Pycr2 knockout, ENU/targeted null allele)

Cellular/in vitro models

Summary of Key Evidence Citations (PMIDs)

Reference Validation

Key Findings

Finding 1 — HLD10 is an autosomal recessive disorder caused by biallelic PYCR2 variants

Finding 2 — Core clinical phenotype: postnatal microcephaly, developmental delay, hypomyelination, poor survival

Finding 3 — Mechanism: PYCR2 loss raises cerebral glycine via SHMT2 upregulation, driving neurodegeneration

Finding 4 — Gene identification (2015): variants destabilize the protein and sensitize cells to oxidative-stress apoptosis

Finding 5 — Zebrafish pycr1b knockdown recapitulates microcephaly, rescued by wild-type human PYCR2 mRNA

Finding 6 — Phenotypic spectrum extends to a milder late-childhood hereditary spastic paraplegia

Finding 7 — Founder effects and consanguinity-driven population genetics (Thai c.400G>A founder allele)

Finding 8 — No disease-specific therapy; SHMT2/glycine-lowering is the leading mechanism-based target

Section-by-Section Report

1. Disease Information

2. Etiology

3. Phenotypes

4. Genetic / Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome / Prognosis

12. Treatment

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Mechanistic Model / Interpretation

Evidence Base

Limitations and Knowledge Gaps

Proposed Follow-up Experiments / Actions

Artifacts

Reference Validation