🏷

Classifications

Harrison's Part

ICIMD (Inherited Metabolic Disorders)

ISDS Skeletal Nosology

👪

Inheritance

2

X-linked dominant inheritance with male lethality HP:0001423

CHILD syndrome is transmitted as an X-linked dominant trait. Hemizygous male conceptuses generally do not survive gestation, so the disorder is seen almost exclusively in heterozygous females. Because affected males abort or resorb, the expected live-born distribution from a heterozygous mother departs from the usual X-linked ratios, and recurrent male pregnancy loss is a family-history clue.

X-linked dominant inheritance

Show evidence (4 references)

PMID:10710235 SUPPORT Human Clinical

"We report for the first time that CHILD syndrome (MIM 308050), an X-linked dominant, male-lethal trait characterized by an inflammatory nevus with striking lateralization and strict midline demarcation, as well as ipsilateral hypoplasia of the body is caused by mutations in the gene NSDHL located at Xq28"

The gene-discovery paper states the X-linked dominant, male-lethal mode of inheritance alongside the causal locus.

PMID:21290788 SUPPORT Human Clinical

"CHILD syndrome is usually male lethal during gestation and thus predominantly affects females."

GeneReviews states the male-lethal consequence and the resulting female predominance.

PMID:21290788 SUPPORT Human Clinical

"However, since studies suggest that male conceptuses with an NSDHL pathogenic variant generally abort or resorb spontaneously, the expected live-born distribution is: 33% heterozygous (typically) affected females; 33% unaffected females; and 33% unaffected males."

Supplies the quantitative recurrence-risk figures that this description would otherwise only paraphrase, including the reason the live-born ratios depart from the usual X-linked expectation.

+ 1 more reference

Functional mosaicism from random X inactivation HP:0001442

NSDHL lies at Xq28 and is therefore subject to X inactivation, so a heterozygous female is a functional mosaic of cells expressing either the mutant or the wild-type allele. This is not somatic mosaicism at the DNA level, which is why sequencing genomic DNA from lesional and non-lesional keratinocytes recovers both alleles equally; the mosaicism is only visible in expressed RNA. Lesional keratinocytes express the mutant allele exclusively, whereas clinically normal contralateral keratinocytes express only the wild-type allele. The X-inactivation pattern, rather than the particular allele, appears to set the distribution of skin involvement, which is why an exceptional patient can have an almost symmetric bilateral CHILD nevus.

Typified by somatic mosaicism

Show evidence (2 references)

PMID:21753784 SUPPORT Human Clinical

"The unusual lateralization of abnormalities in CHILD syndrome reflects selective clearance of keratinocytes and fibroblasts that express the mutant allele from the unaffected side."

Directly ties the mosaic distribution of mutant-allele-expressing cells to the lateralized clinical pattern.

PMID:11907515 SUPPORT Human Clinical

"Apparently, the effect of random X-inactivation is responsible for different patterns of cutaneous involvement in female carriers of NSDHL mutations."

Attributes the variable cutaneous distribution, including the exceptional bilateral case, to the X-inactivation pattern rather than to allele identity.

?

Discussions and Knowledge Gaps

3

What mechanism converts random X inactivation into a body-half boundary in CHILD syndrome, rather than the finer Blaschko-line mosaicism seen in other X-linked genodermatoses?

KNOWLEDGE GAP OPEN lateralization_mechanism_unknown

Attached to

pathophysiology#X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells

The observation that lesional keratinocytes express only the mutant allele and contralateral keratinocytes only the wild-type allele explains why affected skin is affected, but it does not explain the geometry. Negative selection of mutant-expressing cells accounts for their depletion, not for its confinement to one side of the midline. The authors who established the clearance model describe the lateralization as poorly understood, and no available animal system reproduces it: both mouse models pattern as patches or stripes, and the canine model follows Blaschko lines. This disease would be an unusually good subject for single-cell or spatial methods, which have not been applied to it, because the mosaic boundary is sharp and the selecting phenotype is cell-autonomous.

Proposed experiments

Lineage and allele-expression mapping across the midline in CHILD skin

lateralization_lineage_mapping

Single-cell or spatial transcriptomic mapping of NSDHL allele expression across the midline boundary in affected individuals, paired with clonal lineage inference, to determine whether the boundary reflects an early embryonic left-right lineage restriction, a later selective clearance front, or an unrecognized signalling boundary.

Show evidence (1 reference)

PMID:21753784 SUPPORT Human Clinical

"We also evaluated the basis for the poorly understood, unique lateralization of the cutaneous and bone malformations of CHILD syndrome by analyzing gene activation in abnormal and unaffected skin."

The authors describe the lateralization as poorly understood at the outset of the study that produced the current best account of it.

Does the placental labyrinth mechanism established for male lethality in Nsdhl mutant mice hold for human male conceptuses with NSDHL loss of function?

HUMAN MODEL MISMATCH OPEN male_lethality_placental_mechanism_human

Attached to

pathophysiology#Placental Labyrinth Failure and Male Lethality

The placental account of male lethality rests entirely on mouse work, in which affected male embryos die in midgestation with a thin, poorly vascularized labyrinth while showing no consistent embryonic anomaly. Extrapolating to humans is not straightforward: X inactivation in most rodent placental lineages is imprinted with preferential paternal inactivation, which is not the human arrangement, and the reasoning that heterozygous females should phenocopy affected males in extraembryonic tissue depends on that difference. Human material is not wholly unavailable: a maternally inherited NSDHL variant has been identified by exome sequencing in a male conceptus lost at ten weeks, confirming that the lethality is real and NSDHL-linked in humans. What that report does not supply is any placental histology or Hedgehog readout, so it establishes the outcome while leaving the murine mechanism for it untested in humans.

Proposed experiments

NSDHL-null human trophoblast organoid vascularization assay

human_nsdhl_trophoblast_model

Generate NSDHL loss-of-function human trophoblast stem cell or placental organoid models and assay Hedgehog pathway activity, trophoblast proliferation, and vascular network formation, to test whether the murine labyrinth phenotype has a human counterpart independent of rodent-specific placental X inactivation.

Show evidence (2 references)

PMID:15639195 SUPPORT Model Organism

"X-inactivation is non-random in females in most lineages of the rodent placenta with preferential inactivation of the paternal X chromosome."

States the rodent-specific placental X-inactivation arrangement that the mouse inference depends on and that does not transfer to humans.

PMID:36504312 SUPPORT Human Clinical

"A 33-year-old pregnant woman with recurrent spontaneous abortion was experiencing her third pregnancy with a male embryo. In this pregnancy, a miscarriage occurred at a gestational age of 10+6 weeks with no copy number variants."

Establishes the human male loss and its timing, which is the outcome side of the mismatch. PARTIAL because the report supplies no placental histology or Hedgehog readout, so it cannot confirm or refute the murine labyrinth mechanism that is the actual open question.

Is "CHILD syndrome" one disease with two loci, or is the EBP-related case a CDPX2 variant that was clinically misassigned?

KNOWLEDGE GAP OPEN nsdhl_versus_ebp_nosological_boundary

Attached to

genetic#EBP

Two papers published in the same journal issue in 2000 reach opposite-facing conclusions. One identified NSDHL variants in six CHILD patients and reported EBP unaffected in all of them, concluding the disorders are not allelic. The other measured plasma sterols in a CHILD patient, found the CDPX2 signature, identified an EBP nonsense variant, and speculated that at least some CHILD cases are allelic with CDPX2. Both observations can stand; what is unresolved is whether a CHILD phenotype arising from EBP should be curated as CHILD syndrome with locus heterogeneity or as an unusually lateralized CDPX2. This is a lump-versus-split decision affecting how the entity boundary is drawn, and it has not been settled by any subsequent cohort. The practical consequence is diagnostic: plasma sterol analysis separates the two routes even though it is uninformative within NSDHL-related disease.

Proposed experiments

Genotype and sterol reassessment of the assembled CHILD case literature

child_locus_reassessment_cohort

Assemble the published CHILD syndrome cases with molecular data, and where material allows, determine NSDHL versus EBP genotype together with plasma sterol profile and lesion distribution, to establish whether EBP-related cases form a clinically separable subgroup or are distributed through the CHILD phenotype.

Show evidence (1 reference)

PMID:10710233 SUPPORT Human Clinical

"We speculate that at least some cases of CHILD syndrome are allelic with CDPX2 caused by 3beta-hydroxysteroid-delta8,delta7-isomerase deficiency."

The authors' own hedged phrasing is the open question this discussion records.

⚙

Pathophysiology

6

NSDHL Loss of Function

Pathogenic NSDHL variants abolish or severely reduce the activity of the NAD(P)H steroid dehydrogenase-like protein. Missense alleles do not simply produce a catalytically dead but abundant enzyme: across a survey of thirteen disease-causing missense changes and one deletion, protein expression levels were uniformly low, and some mutants could be partially rescued by lowering the temperature or by adding a chemical chaperone, implicating a folding and stability defect rather than pure catalytic loss.

NSDHL hgnc:13398

sterol-4-alpha-carboxylate 3-dehydrogenase (NSDHL) activity GO:0000252 ⇓ LOSS OF FUNCTION

Experimental models

Heterologous expression of NSDHL missense mutants CELL LINE Quantifies how disease alleles reduce NSDHL protein abundance and identifies a rescuable folding component.

Animal models

Bare patches and striated mice (Nsdhl mutant) Mouse recapitulates Independent Bpa and Str alleles are mutations in Nsdhl, and tissue sterol analysis confirms the biosynthetic role.

Downstream

Blocked C4-Demethylation with Cholesterol Deficiency and Methylsterol Accumulation Loss of the C4-demethylating enzyme interrupts the post-squalene pathway, simultaneously reducing cholesterol output and causing intermediates above the block to accumulate.

Show evidence (2 references)

PMID:40222685 SUPPORT In Vitro

"Here we show that protein expression levels were low for all mutants, but some could be rescued by a lower temperature (30°C vs. 37°C) and/or the chemical chaperone glycerol."

Characterizes the molecular consequence of disease-causing missense alleles as reduced protein expression with a partially rescuable folding component.

PMID:10710235 SUPPORT Human Clinical

"(NAD(P)H steroid dehydrogenase-like protein) encoding a 3beta-hydroxysteroid dehydrogenase functioning in the cholesterol biosynthetic pathway"

Identifies the enzymatic function that is lost.

Blocked C4-Demethylation with Cholesterol Deficiency and Methylsterol Accumulation

The enzymatic block has two simultaneous consequences that are usually described as one. Downstream, flux to cholesterol falls, so affected cells are deprived of the pathway end product. Upstream, methylsterol intermediates accumulate; these are not inert, but insert into membranes and perturb membrane function, oxysterol generation, Hedgehog signalling, and HMG-CoA reductase turnover. In cultured cerebellar granule cell precursors from a conditional Nsdhl mouse, the two arms are experimentally separable: exogenous cholesterol almost completely rescued the proliferation defect, while methylsterol accumulation above the block was associated instead with increased cell death.

cholesterol biosynthetic process GO:0006695 ↓ DECREASED

Downstream

Impaired Epidermal Lamellar Body Secretion and Barrier Formation Cholesterol is a required constituent of the stratum corneum lipid lamellae; its depletion, together with toxic intermediate accumulation, disorganizes the epidermal lipid secretory system.

Impaired Hedgehog Signal Transduction Cholesterol is required for Hedgehog signal transduction, and accumulated sterol intermediates further perturb it.

X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells The metabolic lesion is also what makes a mutant-expressing cell less fit than its wild-type neighbour, which is the proposed driver of the progressive negative selection.

Stippled epiphyseal calcification Punctate calcification of cartilage is the radiographic finding CHILD syndrome shares with CDPX2, and what the two disorders have in common is a block in the same post-squalene sterol pathway rather than any shared gene. That shared biochemistry is the basis for placing the stippling downstream of the metabolic block; the chondro-osseous mechanism connecting them is not established.

Show evidence (2 references)

PMID:24060582 SUPPORT Human Clinical

"CHILD syndrome results from loss of function mutations in the NSDHL gene, which leads to inhibition of cholesterol synthesis and accumulation of toxic metabolic intermediates in affected tissues."

States both arms of the dual biochemical lesion explicitly.

PMID:25652406 SUPPORT Model Organism

"Furthermore, this defect is almost completely rescued by supplementation of the culture media with exogenous cholesterol, while methylsterol accumulation above the enzymatic block appears to be associated with increased cell death."

Experimentally dissociates the end-product-deficiency arm from the toxic-intermediate arm within one model system.

X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells

Because NSDHL is X-linked, a heterozygous female is a functional mosaic of cells expressing either allele. Lesional and contralateral clinically normal skin carry equal amounts of mutant and wild-type DNA, but differ completely in what they express: cultured keratinocytes from lesional skin expressed only the mutant allele, and keratinocytes from the unaffected side expressed only the wild-type allele. Fibroblasts showed the same asymmetry less completely. Skewed X inactivation in peripheral blood is consistent with overall depletion of mutant-expressing cells. The mouse work adds the temporal dimension that makes this a process rather than a snapshot: NSDHL-negative cells survive and differentiate during embryogenesis but are progressively outcompeted afterwards, with the mutant fraction in liver falling from about half at postnatal day 6 to about a fifth by one year. That selection is the likely reason skin lesions tend to improve with age, and why the disease is fundamentally a developmental one, doing its damage in the window while mutant clones are still abundant. The model also predicts the clinical observation that a full-thickness graft taken from the unaffected side stays normal after transfer to the affected side. What it does not explain is why the boundary is a sharp body midline rather than the finer Blaschko striping typical of X-linked mosaic skin disease.

keratinocyte CL:0000312 fibroblast CL:0000057

random X inactivation GO:0060816

Animal models

Bare patches and striated mice (Nsdhl mutant) Mouse recapitulates Heterozygous Bpa(1H) females are mosaic for NSDHL expression through normal random X inactivation, and their mutant clones are progressively depleted after birth, giving the human clearance model a longitudinal grounding it cannot get from single human biopsies.

Labrador Retriever NSDHL deletion Dog partially recapitulates Reproduces the X-linked mosaic cutaneous patterning in a heterozygous female carrier of an NSDHL loss-of-function allele.

Downstream

Impaired Epidermal Lamellar Body Secretion and Barrier Formation The epidermal phenotype is confined to the territory populated by mutant-allele-expressing keratinocytes, which is what makes the skin disease unilateral.

Strict midline demarcation of skin lesions The distribution of surviving mutant-expressing clones is what the clinical lateralization is a map of.

Ipsilateral body hypoplasia The hemidysplasia follows the same mosaic territory as the skin, so one side of the body grows out of a cell population that is largely mutant-expressing. Why that produces hypoplasia rather than a purely cutaneous phenotype is not worked out.

Cardiac malformation Visceral anomalies in CHILD syndrome are ipsilateral, following the same body territory as the skin and limb involvement, which places them downstream of the mosaic distribution of mutant-expressing cells. How NSDHL deficiency in that territory produces a given cardiac malformation is not worked out.

Kidney malformation As for the cardiac malformations, renal anomalies fall on the affected side, placing them downstream of the mosaic territory rather than of a separate systemic mechanism.

Lung malformation The lung belongs to the same set of ipsilaterally affected internal organs as the heart and kidney, so it sits downstream of the mosaic territory on the same footing as they do.

Joint contracture Contractures follow the one-sided skeletal and soft-tissue involvement, alongside the scoliosis, rather than arising from a separate mechanism.

Scoliosis Involvement of skeletal structures confined to one side of the body produces asymmetric axial growth, which is the standing explanation for the scoliosis. The edge is at the level of the lateralized skeletal involvement rather than a specific vertebral mechanism.

Show evidence (2 references)

PMID:21753784 SUPPORT Human Clinical

"The unusual lateralization of abnormalities in CHILD syndrome reflects selective clearance of keratinocytes and fibroblasts that express the mutant allele from the unaffected side."

States the cell-level result that clinically normal skin is normal because the mutant-expressing cells are gone from it.

PMID:19631568 SUPPORT Model Organism

"Our results suggest that while NSDHL-deficient cells in the mosaic Bpa(1H) female are able to survive and differentiate during embryonic development, they are subject to negative selection over the life of the animal."

Supplies the longitudinal evidence that clearance of mutant-expressing cells is a postnatal, progressive process rather than an embryonic one.

Impaired Epidermal Lamellar Body Secretion and Barrier Formation

In lesional epidermis the lipid secretory apparatus is structurally disorganized. Lamellar bodies contain few internal lamellae, fuse prematurely into multivesicular bodies, and are incompletely secreted, so organelle contents remain entombed within corneocytes and the stratum corneum extracellular space is left with sparse lamellar material interspersed with non-lamellar vesicles. Histologically this appears as marked hyperkeratosis and acanthosis with loss of the granular layer and dermal foam cells. The severity of this secretory disruption exceeds what topical statin-induced cholesterol deficiency alone produces in normal skin, which is the ultrastructural argument that toxic intermediates and not simply cholesterol depletion drive the ichthyosiform phenotype.

keratinocyte CL:0000312

establishment of skin barrier GO:0061436 ↓ DECREASED keratinocyte differentiation GO:0030216 ⚠ ABNORMAL

Downstream

Ichthyosiform nevus The disordered lipid barrier and compensatory hyperkeratotic response produce the waxy, scaling, inflammatory plaques of the CHILD nevus.

Onychodystrophy and periungual hyperkeratosis The nail unit is a keratinized epidermal appendage, so the same defective cornification is the natural explanation for the nail changes. The intermediates are not established: this edge is inferred from shared keratinization biology and from the observation that nail changes persist under a therapy that clears the skin, not from any measurement made in the nail matrix.

Show evidence (3 references)

PMID:21753784 SUPPORT Human Clinical

"Ultrastructural analysis of affected skin showed evidence of both cholesterol depletion and toxic metabolic accumulation."

Reports the ultrastructural finding in lesional epidermis that grounds this node.

PMID:32886633 SUPPORT Human Clinical

"Congenital ichthyoses are a heterogeneous group of genetic skin disorders characterized by defects in the critical barrier function of the skin."

Places CHILD syndrome within the barrier-defect class of congenital ichthyoses. This supports the barrier framing only, not the specific lamellar body lesion, hence PARTIAL.

PMID:16776722 SUPPORT Human Clinical

"Electron microscopy revealed vesicular structures in the intercellular spaces of the stratum corneum and vacuoles or vesicular structures in upper prickle cell layer. Some of them can be recognized as abnormal lamellar granules. Within the foamy cells in the papillary dermis, large vacuoles were found."

Independent ultrastructural confirmation of the abnormal lamellar granules and of the dermal foam cells this description asserts, from a different patient and laboratory than the treatment study.

Impaired Hedgehog Signal Transduction

Cholesterol is required for Hedgehog signal transduction, so the distal biosynthetic block reaches developmental patterning as well as membrane lipid supply. The cleanest experimental demonstration is in cerebellar granule cell precursors from a conditional Nsdhl mouse, where the proliferation defect was shown to result from defective signalling by SHH and was rescued by exogenous cholesterol. The same dependency is invoked for the persistent failure of hair growth in treated CHILD skin, where the ichthyosis resolves but hair does not return.

smoothened signaling pathway GO:0007224 ↓ DECREASED

Animal models

Conditional Nsdhl ablation in radial glia Mouse recapitulates Establishes the causal chain from NSDHL loss to defective SHH signalling and shows that cholesterol supply is the limiting factor for it.

Downstream

Ipsilateral limb hypoplasia or aplasia Hedgehog-dependent limb patterning is disrupted in the territory of mutant-expressing cells, contributing to the ipsilateral skeletal defects.

Alopecia of affected skin Hair follicle development is Hedgehog-dependent, which is the standing explanation for hair failing to return in skin that has otherwise been cleared by topical therapy.

Placental Labyrinth Failure and Male Lethality Hedgehog signalling is required for development of a functional labyrinth, and its failure is implicated in the placental pathology of NSDHL deficiency.

Show evidence (2 references)

PMID:25652406 SUPPORT Model Organism

"We replicated the granule cell precursor proliferation defect in vitro and demonstrate that it results from defective signaling by SHH."

Directly demonstrates that NSDHL loss impairs Hedgehog signal transduction in a cholesterol-dependent manner.

PMID:17028112 SUPPORT Model Organism

"These data strongly implicate the hedgehog signaling pathway in the pathogenesis of the placental defects in NSDHL deficiency and provide evidence for a role for the hedgehog pathway in the development of a functional mammalian placenta."

Links the Hedgehog defect specifically to the placental arm of NSDHL deficiency.

Placental Labyrinth Failure and Male Lethality

The prenatal male lethality that shapes the inheritance pattern appears to be placental rather than embryonic. In Nsdhl mutant mice, affected male embryos die in midgestation with no consistent anomaly in the embryo itself, but with a labyrinth layer that is always thinner, with fewer fetal vessels and decreased trophoblast proliferation. Heterozygous female placentas show the defect non-cell-autonomously, and Hedgehog reporter activity is markedly reduced or absent in affected male placentas, with failure of Indian hedgehog-expressing cells to migrate into the developing placenta. The mechanism is inferred from mouse; it has not been demonstrated in human conceptuses.

trophoblast cell CL:0000351

placenta development GO:0001890 ⚠ ABNORMAL

Animal models

Bare patches and striated mice (Nsdhl mutant) Mouse recapitulates Affected male embryos die in midgestation with a thin, poorly vascularized placental labyrinth and reduced trophoblast proliferation, and with loss of Hedgehog reporter activity in the affected placenta.

Show evidence (2 references)

PMID:15639195 SUPPORT Model Organism

"Although no consistent anomalies were identified in affected male embryos themselves, the labyrinth layer of the fetal placenta was always thinner, with fewer fetal vessels and decreased proliferation of labyrinth trophoblast cells."

Locates the lethal lesion in the placenta rather than the embryo.

PMID:17028112 SUPPORT Model Organism

"Further, affected placentas from mutant Nsdhl(Bpa-8H) male embryos demonstrate markedly decreased or no Ptch1-lacZ staining and no migration of Ihh expressing cells into the developing placenta."

Supplies the signalling readout showing Hedgehog pathway failure in the affected placenta.

✶

Histopathology

3

Psoriasiform hyperkeratosis and acanthosis with loss of the granular layer

Lesional skin shows marked hyperkeratosis and acanthosis with absence of the granular layer, together with foam cells in the dermis. This is the routine light-microscopic picture, and it reverses on effective pathogenesis-based therapy: after three months of topical cholesterol plus lovastatin the epidermis was largely normal with a prominent granular layer and orthokeratotic scale.

Show evidence (1 reference)

PMID:21753784 SUPPORT Human Clinical

"Baseline biopsy sections of skin lesions from both patients prior to treatment showed marked hyperkeratosis and acanthosis with absence of the granular layer and dermal foam cells"

Reports the light-microscopic findings in lesional skin from both patients, including the dermal foam cells.

Verruciform xanthoma-like change in the CHILD nevus

The CHILD nevus is not histologically generic. Alongside the psoriasiform changes it shows the microscopic features of verruciform xanthoma, which together with the diffuse lateralization and ptychotropism (a predilection for body folds) is what distinguishes it from the other epidermal nevus syndromes. This is a lesion that can grow, not only a microscopic descriptor: one of the two patients in the topical therapy report developed a frank verruciform xanthoma of the vulva and inner thigh that required debulking.

Show evidence (2 references)

PMID:7640190 SUPPORT Human Clinical

"The associated CHILD nevus shows unique features such as a diffuse form of lateralization, ptychotropism, and microscopic changes of verruciform xanthoma."

Names the histological triad that identifies the CHILD nevus among the epidermal nevus syndromes, which is what makes this finding diagnostic.

PMID:16776722 SUPPORT Human Clinical

"The histology shared many features with verruciform xanthoma."

Independent case confirmation of the verruciform xanthoma resemblance.

Abnormal lamellar granules on electron microscopy

Ultrastructurally the lesion is a lipid secretory failure. Vesicular structures appear in the intercellular spaces of the stratum corneum and vacuoles in the upper prickle cell layer, some recognizable as abnormal lamellar granules, with large vacuoles inside the foamy cells of the papillary dermis. This is the electron-microscopic counterpart of the lamellar body secretory defect modelled in the pathophysiology, observed in a separate patient and laboratory from the treatment study.

Show evidence (1 reference)

PMID:16776722 SUPPORT Human Clinical

"These findings suggested that abnormal lipid metabolism involving lamellar granules may be responsible to the skin lesion of CHILD syndrome."

States the authors' interpretation linking the ultrastructural finding to the lipid-metabolic lesion, preserving the hedged form they used.

⬡

Pathograph

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

●

Phenotypes

13

Cardiovascular 1

Cardiac malformation Abnormal heart morphology HP:0001627

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"Heart, lung, and kidney malformations can also occur."

GeneReviews states the visceral malformations. No frequency band is asserted, because "can also occur" is weaker than any of the qualitative terms the frequency SOP maps to a band.

Integument 3

Ichthyosiform nevus Epidermal nevus HP:0010816

Bound to HP:0010816 (Epidermal nevus) rather than HP:0008064 (Ichthyosis). The lesion is genuinely ichthyosiform, but it is a localized, mosaic, unilateral nevus, not a generalized disorder of cornification, and the nevoid character is what discriminates it; the classic delineation places the CHILD nevus among the epidermal nevus syndromes. HP:0034275 (Verrucous epidermal nevus) was considered and not used, since the verrucous quality is reported in some lesions rather than being a general property of the CHILD nevus. The ichthyosiform quality is carried by the preferred_term. Separately, no frequency band is asserted. The lesion is effectively definitional, and grading a defining feature as VERY_FREQUENT on the strength of the phrase "is characterized by" would be circular rather than evidenced. There is no published CHILD syndrome cohort with a denominator from which a real band could be derived.

Show evidence (2 references)

PMID:21290788 SUPPORT Human Clinical

"CHILD syndrome is characterized by unilateral distribution of ichthyosiform skin lesions and ipsilateral limb defects that range from shortening of the metacarpals and phalanges to absence of the entire limb."

GeneReviews names the unilateral ichthyosiform lesion as a defining feature.

PMID:24060582 SUPPORT Human Clinical

"The CHILD syndrome skin is characterized by plaques topped by waxy scales and a variety of developmental defects in extracutaneous tissues, particularly limb hypoplasia or aplasia."

Describes the characteristic morphology of the plaques.

Onychodystrophy and periungual hyperkeratosis FREQUENT Nail dystrophy HP:0008404

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"Onychodystrophy and periungual hyperkeratosis are common."

GeneReviews states both the finding and a qualitative frequency. "Common" is the literature term the curation SOP maps to the FREQUENT band.

Alopecia of affected skin HP:0001596

Show evidence (1 reference)

PMID:21753784 SUPPORT Human Clinical

"By 6 months, treated skin largely normalized, but did not grow hair"

Reports the persistence of alopecia despite otherwise successful treatment.

Musculoskeletal 2

Scoliosis HP:0002650

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"Scoliosis and joint contractures are treated with braces and/or corrective surgery."

A management statement, so it presupposes rather than reports the occurrence of scoliosis in CHILD syndrome. PARTIAL for that reason.

Joint contracture HP:0034392

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"monitor for new cutaneous lesions, musculoskeletal deformities such as scoliosis and joint contractures, and neurologic, cardiac, and/or kidney manifestations annually or as needed"

GeneReviews names joint contractures among the musculoskeletal deformities that warrant ongoing monitoring in CHILD syndrome.

Nervous System 1

Normal intellect Intellectual disability HP:0001249

Show evidence (1 reference)

PMID:21290788 REFUTE Human Clinical

"absence of the entire limb. Intellect is usually normal."

GeneReviews states that intellect is usually normal in CHILD syndrome, which refutes intellectual disability as a feature of this entity.

Growth 1

Ipsilateral body hypoplasia Asymmetric growth HP:0100555

Show evidence (2 references)

PMID:10710235 SUPPORT Human Clinical

"an inflammatory nevus with striking lateralization and strict midline demarcation, as well as ipsilateral hypoplasia of the body"

States the ipsilateral hypoplasia of the body, which is the asymmetric growth this term describes.

PMID:7408908 SUPPORT Human Clinical

"In addition, ipsilateral hypoplasia of other parts of the skeleton, as well as defects of the brain and the viscera are found."

Extends the ipsilateral hypoplasia beyond the limbs to the rest of the skeleton and the viscera.

Other 5

Strict midline demarcation of skin lesions

No ontology term is bound, deliberately. HPO has no term for strict midline demarcation of a cutaneous eruption. HP:0100555 (Asymmetric growth) was used here previously and has been removed: it describes asymmetry of body growth, not the distribution of a skin lesion, so it is a cross-category proxy rather than a merely broad parent, and no term is better than a misleading one. That term now sits on the separate `Ipsilateral body hypoplasia` phenotype, which it genuinely fits. HP:0025293 (Distributed along Blaschko lines) is explicitly not a substitute, since the whole point of this phenotype is that the demarcation is sharper than Blaschko mosaicism.

Show evidence (3 references)

PMID:10710235 SUPPORT Human Clinical

"an inflammatory nevus with striking lateralization and strict midline demarcation, as well as ipsilateral hypoplasia of the body"

States the midline demarcation of the nevus, which is the claim this phenotype makes.

PMID:7408908 SUPPORT Human Clinical

"The syndrome is characterized by unilateral erythema and scaling, with a distinct demarcation in the middle of the trunk."

The original delineation locates the demarcation anatomically, at the middle of the trunk.

PMID:11907515 SUPPORT Human Clinical

"A symmetric distribution of this nevus can exceptionally be seen in patients with CHILD syndrome, and this bilateral involvement should not mislead the clinician to any other diagnosis."

Qualifies rather than supports the lateralization claim, documenting the exception that makes midline demarcation typical but not obligatory.

Ipsilateral limb hypoplasia or aplasia Aplasia/hypoplasia involving bones of the extremities HP:0045060

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"unilateral distribution of ichthyosiform skin lesions and ipsilateral limb defects that range from shortening of the metacarpals and phalanges to absence of the entire limb"

GeneReviews states the ipsilateral limb defect and its severity range as a defining characteristic.

Stippled epiphyseal calcification Stippled chondral calcification HP:0002764

Show evidence (1 reference)

PMID:10710233 SUPPORT Human Clinical

"CHILD (congenital hemidysplasia, ichthyosis, and limb defects) syndrome is a rare, usually sporadic disorder associated with unilateral distribution of ichthyosiform skin lesions, limb defects, punctate calcifications of cartilaginous structures, and visceral anomalies."

Names punctate calcification of cartilaginous structures among the characteristic features.

Kidney malformation Abnormal renal morphology HP:0012210

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"Heart, lung, and kidney malformations can also occur."

GeneReviews states kidney malformation among the visceral anomalies.

Lung malformation Abnormal lung morphology HP:0002088

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"Heart, lung, and kidney malformations can also occur."

GeneReviews names the lung among the organs that may be malformed, on the same footing as the heart and kidney.

🧬

Genetic Associations

2

NSDHL

Gene: NSDHL hgnc:13398 relationship_type: CAUSATIVE

Show evidence (3 references)

PMID:10710235 SUPPORT Human Clinical

"SSCA and genomic sequence analysis of NSDHL identified in 6 patients with CHILD syndrome, including one boy as well as a mother and her daughter, mutations potentially impairing protein function."

Establishes NSDHL variants as the molecular cause in a patient series.

PMID:21290788 SUPPORT Human Clinical

"The diagnosis of CHILD syndrome is established in a female proband with a heterozygous NSDHL pathogenic variant identified by molecular genetic testing that results in loss of functional decarboxylating sterol-4-alpha-carboxylate 3-dehydrogenase, the protein encoded by NSDHL."

GeneReviews states the diagnostic criterion and the loss-of-function consequence at the protein level.

PMID:25652406 SUPPORT Model Organism

"Rare, hypomorphic NSDHL mutations are also associated with X-linked intellectual disability in males with CK syndrome."

Supports the allelic relationship in which residual enzyme activity yields CK syndrome rather than CHILD syndrome.

EBP

Gene: EBP hgnc:3133 relationship_type: CAUSATIVE

Show evidence (1 reference)

PMID:10710233 SUPPORT Human Clinical

"Subsequently, we identified a nonsense mutation in exon 3 of the patient's 3beta-hydroxysteroid-delta8,delta7-isomerase gene. We speculate that at least some cases of CHILD syndrome are allelic with CDPX2 caused by 3beta-hydroxysteroid-delta8,delta7-isomerase deficiency."

Reports a single patient with an EBP nonsense variant and the matching sterol signature. PARTIAL because the authors themselves generalize only speculatively, from one patient, to "at least some cases".

💊

Medical Actions

6

Topical Cholesterol plus Statin (Pathogenesis-Based Therapy)

Action: Pharmacotherapy NCIT:C15986

Agent: cholesterol CHEBI:16113 lovastatin CHEBI:40303 simvastatin CHEBI:9150

Twice-daily topical application of a compounded lotion containing cholesterol together with an HMG-CoA reductase inhibitor (lovastatin or simvastatin) is the pathogenesis-directed treatment of the cutaneous disease. It addresses both arms of the biochemical lesion at once: cholesterol replaces the missing end product, and the statin shuts off flux into the pathway so the toxic methylsterol intermediates are not generated. The design is validated by the negative control within the same study, since topical cholesterol alone produced no improvement. Lesions clear over weeks to months, with histological and ultrastructural normalization, and the effect is maintained on reduced-frequency dosing. Hair growth and onychodystrophy do not recover. Note the evidence ceiling: every report is an uncontrolled case series of one or two patients, with the publication bias that implies, and there has been no trial. Systemic absorption and long-term safety of compounded topical statins applied to large body-surface areas in infants have not been studied.

Mechanism Target:

INHIBITS Blocked C4-Demethylation with Cholesterol Deficiency and Methylsterol Accumulation — The statin component suppresses flux through HMG-CoA reductase so that intermediates above the NSDHL block are not generated, while the cholesterol component supplies the missing end product. The combination therefore acts on both arms of this node.

Show evidence (1 reference)

PMID:21753784 SUPPORT Human Clinical

"On the basis of the putative pathogenic role of both pathway-product deficiency of cholesterol and accumulation of toxic metabolic intermediates, we assessed the efficacy of combined therapy with lovastatin and cholesterol."

States the mechanistic rationale linking the two drug components to the two arms of the biochemical block.

Show evidence (3 references)

PMID:21753784 SUPPORT Human Clinical

"Topical treatment with lovastatin/cholesterol (but not cholesterol alone) virtually cleared skin lesions by 3 months, accompanied by histological and ultrastructural normalization of epidermal structure and lipid secretion."

Reports the therapeutic result together with the cholesterol-alone negative control that establishes the combination is required.

PMID:25845514 SUPPORT Human Clinical

"We report the case of a 10-year-old girl treated with topical simvastatin and cholesterol ointment, after which her skin lesions significantly improved within the first 30 days of treatment."

Independent case replication using simvastatin in place of lovastatin.

PMID:31365666 SUPPORT Human Clinical

"The authors report a 2-month-old patient presenting with typical features of CHILD syndrome that was treated with a topical solution containing cholesterol and lovastatin, with complete clearance of her CHILD nevus."

Further independent replication, and at the youngest reported age of treatment.

Symptomatic Topical Care

Action: Pharmacotherapy NCIT:C15986

Agent: ketoconazole CHEBI:47519

Emollients, keratolytics such as lactic acid creams for itching, urea creams for dryness, topical corticosteroids, and oral or topical ketoconazole have all been used. GeneReviews is explicit that no single therapy ameliorates the cutaneous findings in every affected individual, and the two patients in the pathogenesis-based therapy report had each failed years of emollients, keratolytics, retinoids, corticosteroids, calcineurin inhibitors, or cryotherapy before responding to cholesterol plus statin. This entry is retained as the historical standard of care rather than as a recommended first choice.

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"In CHILD syndrome, no one therapy described to date appears to ameliorate the cutaneous findings for every reported individual. Oral and topical ketoconazole may reduce lesions."

Documents the symptomatic options while stating the limitation that none works universally, hence PARTIAL.

Skin Grafting from the Unaffected Side

Action: Skin Transplantation NCIT:C15325

Full-thickness grafting of skin taken from the contralateral unaffected region onto an inflammatory nevus has been successful. The mosaic account of the disease predicts why: donor skin from the unaffected side is populated by keratinocytes and fibroblasts expressing only the wild-type allele, so the graft is genetically competent to build a normal barrier in its new location.

Mechanism Target:

BYPASSES X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells — Grafting replaces mutant-expressing epidermis with wild-type-expressing epidermis, bypassing rather than correcting the mosaic distribution.

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"Treatment of an inflammatory nevus by grafting skin obtained from a contralateral unaffected region has been successful."

Reports the intervention and its dependence on an unaffected-side donor site, which is what makes it a mosaic-bypassing strategy.

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"Treatment of an inflammatory nevus by grafting skin obtained from a contralateral unaffected region has been successful."

GeneReviews records grafting as an effective option for a localized inflammatory nevus.

Orthopedic and Rehabilitative Management of the Limb and Spine Defects

Action: Orthopedic Surgical Procedure NCIT:C16186

The skeletal side of the disease is managed mechanically, not pharmacologically: scoliosis and joint contractures with bracing or corrective surgery, and severe limb hypoplasia with amputation of a rudimentary limb where that permits prosthetic fitting. This arm of care is entirely untouched by the topical therapy that clears the skin, which is worth stating plainly because the pathogenesis-based treatment is sometimes described as if it addressed the syndrome rather than its cutaneous component.

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"Scoliosis and joint contractures are treated with braces and/or corrective surgery."

GeneReviews states the orthopedic management of the axial and joint deformities.

Long-Term Surveillance

Action: Supportive Care NCIT:C15747

Because the disease is multisystem and lesions can appear after infancy, management includes scheduled monitoring rather than one-time assessment: new cutaneous lesions, musculoskeletal deformity, and neurologic, cardiac, and kidney manifestations are reviewed annually or as needed. This is the care pathway through which the visceral phenotypes curated above are actually detected.

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"In CHILD syndrome, monitor for new cutaneous lesions, musculoskeletal deformities such as scoliosis and joint contractures, and neurologic, cardiac, and/or kidney manifestations annually or as needed."

GeneReviews states the surveillance schedule and the organ systems it covers.

Genetic Counseling

Action: Genetic Counseling NCIT:C15240

Counseling addresses the X-linked transmission and the distorted live-born ratios that follow from male lethality. Once the familial NSDHL variant is known, prenatal and preimplantation genetic testing are possible, and heterozygous female relatives can be identified.

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"Once the NSDHL pathogenic variant has been identified in a family member with an NSDHL-related disorder, prenatal and preimplantation genetic testing are possible."

States the reproductive testing options that counseling is built around.

🔬

Biochemical Markers

2

Methylsterol accumulation in affected tissue (INCREASED)

Show evidence (2 references)

PMID:24060582 SUPPORT Human Clinical

"which leads to inhibition of cholesterol synthesis and accumulation of toxic metabolic intermediates in affected tissues"

States the accumulation of intermediates and localizes it to affected tissues rather than the circulation.

PMID:10369263 SUPPORT Model Organism

"Quantitative analysis of sterols from tissues of affected Bpa mice support a role for Nsdhl in cholesterol biosynthesis."

Reports the tissue sterol measurements in the mouse counterpart of the disorder.

🔬

Diagnosis

2

NSDHL molecular genetic testing

The diagnosis is established in a female proband by identifying a heterozygous NSDHL pathogenic variant. This is the definitive test, and it is also what makes carrier identification and prenatal or preimplantation testing possible in the family. Sequence analysis alone is not a sufficient strategy, though: multi-exon deletions and microdeletions are described as not rare in CHILD syndrome and are invisible to classical Sanger sequencing, so gene-targeted deletion and duplication analysis belongs alongside it. A negative sequencing result therefore means "not detected by this assay" rather than "not CHILD syndrome". Reported cases include a deletion of NSDHL exons 3 and 4, and structural variants at Xq28 spanning exon-containing regions have been resolved by optical genome mapping in patients whose conventional genetic work-up was inconclusive.

Genetic Testing NCIT:C15709

Show evidence (3 references)

PMID:21290788 SUPPORT Human Clinical

"The diagnosis of CHILD syndrome is established in a female proband with a heterozygous NSDHL pathogenic variant identified by molecular genetic testing that results in loss of functional decarboxylating sterol-4-alpha-carboxylate 3-dehydrogenase, the protein encoded by NSDHL."

GeneReviews states the diagnostic criterion.

PMID:29341259 SUPPORT Human Clinical

"Multiple exons deletions or microdeletion was not rare in CHILD syndrome. Classical Sanger sequencing may not be useful enough to find all kinds of mutations. Next-generation sequencing may be more effective."

States the assay blind spot directly, and the same report supplies the worked example, a large deletion of NSDHL exons 3 and 4.

PMID:41625319 SUPPORT Human Clinical

"The identified structural variants consisted of deletions of varying sizes in the Xq28 cytoband, encompassing regions that contain exons."

Second source for the same gap, from the other end. The cohort was pediatric patients with a compatible phenotype whose conventional genetic studies were inconclusive, and this is what optical genome mapping then found in them: exon-containing deletions at Xq28, the NSDHL locus. Quoted from the reported result rather than from the study's stated aim.

Sterol profiling of skin flakes and plasma

Sterol analysis is the biochemical confirmatory test, and the tissue it is run on matters. In a reported case, analysis of skin flakes showed a mono 4-alpha methyl sterol also detectable in plasma, plus 4-alpha-carboxy-4-methyl-cholest-8(9)-en-3beta-ol and several keto-sterols normally below the limit of detection, a pattern consistent with a defective 4-alpha-methylsterol-4-demethylase complex. Its practical value is demonstrated by that same case: the pattern was what allowed an NSDHL variant of uncertain significance to be reclassified as likely pathogenic. Note the limits recorded in the biochemical section, though, since plasma sterols are frequently near-normal because functional mosaicism leaves most of the body expressing the wild-type allele.

Diagnostic Procedure NCIT:C18020

Show evidence (2 references)

PMID:33139364 SUPPORT Human Clinical

"Sterol analysis from skin flakes revealed increased levels of a mono 4-alpha methyl sterol also seen in plasma as well as the presence of 4-alpha-carboxy-4-methyl-cholest-8(9)-en-3beta-ol and several keto-sterols, which are usually below the limit of detection. This sterol pattern is consistent..."

Reports the sterol pattern and its interpretation as evidence of the enzymatic block.

PMID:33139364 SUPPORT Human Clinical

"On this basis, this variant has been reclassified as likely pathogenic."

Documents the concrete diagnostic use of the sterol result, resolving a variant of uncertain significance.

📈

Progression

1

Postnatal improvement of cutaneous lesions

Skin lesions tend to become less inflammatory over childhood. The mechanistic account for this is the progressive negative selection of mutant-expressing cells demonstrated in the heterozygous Bpa(1H) mouse, in which the mutant fraction in liver fell from roughly half at postnatal day 6 to roughly a fifth by one year. The corresponding longitudinal measurement has not been made in human CHILD syndrome skin, so the human natural history is described clinically while the mechanism behind it is murine.

Show evidence (1 reference)

PMID:19631568 SUPPORT Model Organism

"In the liver, the proportion of NSDHL negative cells dropped from approximately 50% at postnatal day 6 to approximately 20% at one year of age."

Quantifies the postnatal depletion of mutant-expressing cells in the mouse. PARTIAL because the measurement is murine hepatic, not human cutaneous, and so supports the mechanism rather than the clinical course directly.

📊

Prevalence

1

Worldwide, published case literature

Cases In Literature Ultra Rare

No cohort, registry, or population study exists, so no rate can be given. The original 1980 delineation assembled twenty observations in total, two new plus eighteen reviewed, and the published total has since grown into the low hundreds. The figure of one in 100,000 that circulates in review literature has no traceable denominator and is inconsistent with that case count, so it is deliberately not recorded here. The same series gives the female-to-male ratio as 19:1, which is a consequence of male lethality rather than an independent epidemiological finding.

Show evidence (1 reference)

PMID:7408908 SUPPORT Human Clinical

"Two further cases of this syndrome are reported, and a review of 18 previous observations is presented. The ratio of females to males is 19 : 1."

Supplies the literature case count behind the CASES_IN_LITERATURE measure and the sex ratio, which is the only quantitative epidemiology available.

🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from CHILD Syndrome:

X-linked chondrodysplasia punctata 2 (Conradi-Hunermann-Happle syndrome) MONDO:0020603

Overlapping Features The closest mechanistic neighbour and the most important differential. CDPX2 is caused by variants in EBP, a delta8-delta7 sterol isomerase acting downstream of NSDHL in the same post-squalene pathway, and it is likewise an X-linked dominant, male-lethal disorder with patterned ichthyotic skin and bone malformations. The relationship between the two entities is not fully settled: the NSDHL gene-discovery paper found EBP unaffected in its CHILD cohort and concluded they are not allelic, while a paper published alongside it identified an EBP nonsense variant in a patient carrying a clinical CHILD diagnosis.

Distinguishing Features

CDPX2 skin lesions and skeletal defects are bilateral and asymmetric, without the strict lateralization and midline demarcation of CHILD syndrome
CDPX2 is caused by EBP variants acting one step downstream of NSDHL in the same pathway
CDPX2 carries a plasma sterol signature of raised 8-dehydrocholesterol and 8(9)-cholestenol that NSDHL-related CHILD syndrome does not

Show evidence (2 references)

PMID:10710235 SUPPORT Human Clinical

"EBP was unaffected in the patients analyzed by us demonstrating that CHILD syndrome and CDPX2 are not caused by allelic mutations."

Establishes that in this cohort the two are genetically distinct despite their shared pathway and overlapping clinical picture.

PMID:10710233 SUPPORT Human Clinical

"although the skeletal defects and skin lesions in CDPX2 are bilateral and asymmetric"

States the clinical distinction in lesion distribution that separates the two on examination.

Inflammatory linear verrucous epidermal nevus (ILVEN) Not Yet Curated MONDO:0019318

Overlapping Features The dermatologic mimic, as opposed to the mechanistic neighbours above. The CHILD nevus is an inflammatory verrucous lesion with psoriasiform hyperkeratosis and loss of the granular layer, and ILVEN looks like that too, so the differential is made on the systemic findings rather than on the skin. It is a real trap in both directions: a boy with widespread right-sided inflammatory lesions was worked up as possible CHILD syndrome before being confirmed as severe ILVEN, and separately a genuine CHILD patient was described as mimicking verrucous nevus. The hard part is that the ILVEN case also had an ipsilateral limb contracture and short stature, so even the extracutaneous findings are not cleanly discriminating.

Distinguishing Features

ILVEN has no NSDHL variant and no sterol pathway biochemical signature, which is the definitive separator
ILVEN lesions follow Blaschko lines rather than showing the strict midline demarcation typical of the CHILD nevus
ILVEN lacks the ipsilateral skeletal hypoplasia or aplasia of CHILD syndrome, though an ipsilateral limb contracture has been reported in ILVEN, so limb involvement alone does not settle it
Male sex argues against CHILD syndrome, which is usually male lethal, and the reported ILVEN mimic was a boy

Show evidence (2 references)

PMID:39953436 SUPPORT Human Clinical

"Initially, we suspected this may be a rare case of CHILD syndrome in a boy. However, after further investigation, we confirmed the diagnosis as a severe form of ILVEN."

A worked case of the differential actually being made, with CHILD syndrome as the initial working diagnosis and ILVEN as the resolution.

PMID:39953436 SUPPORT Human Clinical

"He also had an associated contracture in the ipsilateral limb and short stature."

Documents that the ILVEN case carried ipsilateral limb findings too, which is why extracutaneous involvement does not by itself distinguish the two.

CK syndrome

Overlapping Features The allelic NSDHL disorder in males. CK syndrome arises from hypomorphic NSDHL variants retaining partial enzyme activity, and presents with cognitive impairment, behavioural problems, infantile-onset seizures, cerebral cortical malformation, and microcephaly rather than with the cutaneous and limb phenotype of CHILD syndrome. Same gene, opposite sex distribution, and an essentially non-overlapping clinical picture.

Distinguishing Features

CK syndrome affects males hemizygous for a hypomorphic allele; CHILD syndrome affects heterozygous females and is male lethal
CK syndrome is dominated by cognitive impairment, seizures, and cortical malformation, whereas intellect is usually normal in CHILD syndrome
CK syndrome lacks the lateralized ichthyosiform nevus and ipsilateral limb defects

Show evidence (1 reference)

PMID:21290788 SUPPORT Human Clinical

"The diagnosis of CK syndrome is established in a male proband with a hemizygous NSDHL hypomorphic pathogenic variant identified by molecular genetic testing that results in partial loss of functional decarboxylating sterol-4-alpha-carboxylate 3-dehydrogenase."

States the allelic relationship and the partial-loss-of-function basis that separates CK syndrome from CHILD syndrome.

🧫

Experimental Models

1

Heterologous expression of NSDHL missense mutants CELL_LINE

Thirteen disease-causing NSDHL missense mutations and one deletion mutation were expressed and assayed for protein level. All showed low expression; some were partially rescued by culture at 30°C rather than 37°C or by the chemical chaperone glycerol, and optimal expression required heat shock proteins 70 and 90. This reframes NSDHL missense disease as substantially a protein stability and folding problem, which is what makes chaperone-directed approaches conceivable.

Publication

PMID:40222685

Pathograph links

NSDHL Loss of Function measures fidelity: moderate Quantifies how disease alleles reduce NSDHL protein abundance and identifies a rescuable folding component.

Limitations: Heterologous overexpression in cultured cells does not report on endogenous expression in keratinocytes or on the mosaic tissue context, and a temperature or glycerol rescue in culture is not evidence that any clinically usable chaperone therapy exists.

Readouts

NSDHL mutant protein expression level

Direction: DECREASED

Interpretation: Low steady-state protein for every disease allele tested indicates loss of function operates substantially through protein instability.

Show evidence (1 reference)

PMID:40222685 SUPPORT In Vitro

"Here we show that protein expression levels were low for all mutants, but some could be rescued by a lower temperature (30°C vs. 37°C) and/or the chemical chaperone glycerol."

Reports the expression measurement behind this readout.

Show evidence (1 reference)

PMID:40222685 SUPPORT In Vitro

"Additionally, heat shock proteins 70 and 90 are needed for optimal NSDHL protein expression suggesting that disease mutations in NSDHL may interfere with this interaction"

Supports treating this system as informative about the molecular basis of NSDHL loss of function.

Show evidence (1 reference)

PMID:40222685 SUPPORT In Vitro

"We examined 13 missense mutations and one deletion mutation in the cholesterol synthesis enzyme NSDHL (NAD(P) Dependent Steroid Dehydrogenase-Like), known to cause the X-linked developmental disorders CHILD (congenital hemidysplasia with ichthyosiform erythroderma and limb defects) syndrome and..."

States that the alleles assayed in this system are the ones that cause CHILD syndrome, which is the model-level relevance claim.

🐁

Animal Models

3

Bare patches and striated mice (Nsdhl mutant)

Bare patches (Bpa) and striated (Str) are allelic X-linked dominant, male-lethal mouse mutations in Nsdhl, and were the first mammalian locus associated with such a phenotype. They reproduce the male lethality, the cholesterol biosynthetic defect, and the mosaic negative selection of mutant-expressing cells. They do not reproduce the lateralization of human CHILD syndrome, which is the single most distinctive human feature.

Species

Mouse

Genotype

Nsdhl mutant alleles Bpa and Str, X-linked dominant male-lethal

Genes

NSDHL hgnc:13398

Publication

PMID:10369263

Pathograph links

NSDHL Loss of Function recapitulates fidelity: high Independent Bpa and Str alleles are mutations in Nsdhl, and tissue sterol analysis confirms the biosynthetic role.

Limitations: The mouse skin phenotype is patterned as patches and stripes rather than the lateralized, midline-demarcated distribution seen in humans, so the model is informative for the enzymatic lesion but not for the lateralization.

Readouts

Tissue sterol profile in affected Bpa mice

Direction: ALTERED

Interpretation: Quantitative tissue sterol analysis is the biochemical correlate of the enzymatic block in this model.

Show evidence (1 reference)

PMID:10369263 SUPPORT Model Organism

"Quantitative analysis of sterols from tissues of affected Bpa mice support a role for Nsdhl in cholesterol biosynthesis."

Reports the sterol measurement underlying this readout.

Show evidence (1 reference)

PMID:10369263 SUPPORT Model Organism

"Here we report mutations in one of these genes, Nsdhl, encoding an NAD(P)H steroid dehydrogenase-like protein, in two independent Bpa and three independent Str alleles."

Establishes that these mouse mutants carry lesions in the orthologous gene.

X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells recapitulates fidelity: moderate Heterozygous Bpa(1H) females are mosaic for NSDHL expression through normal random X inactivation, and their mutant clones are progressively depleted after birth, giving the human clearance model a longitudinal grounding it cannot get from single human biopsies.

Limitations: The clonal patterning is patches and stripes rather than a body-half boundary, so the model supports the selection process but not the geometry. The quantitative depletion was measured in liver and brain rather than skin.

Readouts

Fraction of NSDHL-negative cells in liver over the first year

Direction: DECREASED

Interpretation: A falling mutant fraction with age is the direct measurement of negative selection against NSDHL-deficient cells.

Show evidence (1 reference)

PMID:19631568 SUPPORT Model Organism

"In the liver, the proportion of NSDHL negative cells dropped from approximately 50% at postnatal day 6 to approximately 20% at one year of age."

Reports the quantitative measurement behind this readout.

Show evidence (1 reference)

PMID:19631568 SUPPORT Model Organism

"Heterozygous Bpa(1H) females are mosaic for NSDHL expression due to normal random X-inactivation."

Establishes that the heterozygous mouse reproduces the functional mosaicism that this node describes.

Placental Labyrinth Failure and Male Lethality recapitulates fidelity: moderate Affected male embryos die in midgestation with a thin, poorly vascularized placental labyrinth and reduced trophoblast proliferation, and with loss of Hedgehog reporter activity in the affected placenta.

Limitations: Human male conceptuses are not available for study, so the placental mechanism of male lethality is inferred from mouse and has not been confirmed in human material. Rodent placental X inactivation is imprinted in most lineages rather than random, which is not the human arrangement, so the lineage logic behind the heterozygous female placental phenotype may not transfer.

Readouts

Placental labyrinth thickness and fetal vessel density

Direction: DECREASED

Interpretation: Structural correlate of the placental failure that accompanies male lethality.

Show evidence (1 reference)

PMID:15639195 SUPPORT Model Organism

"the labyrinth layer of the fetal placenta was always thinner, with fewer fetal vessels and decreased proliferation of labyrinth trophoblast cells"

Reports the histological measurement behind this readout.

Show evidence (1 reference)

PMID:15639195 SUPPORT Model Organism

"To begin to examine the pathogenesis of these disorders, we have determined that affected male embryos for several Nsdhl alleles die in midgestation, between E10.5 and 13.5"

Supports treating this model as informative for the timing and nature of male lethality.

Show evidence (1 reference)

PMID:10710235 SUPPORT Human Clinical

"They provide animal models for the study of CHILD syndrome, a further human condition due to mutations in a gene of the cholesterol synthesis pathway."

The human gene-discovery paper explicitly nominates these mouse mutants as models for CHILD syndrome, which is the model-level claim.

Conditional Nsdhl ablation in radial glia

Because hemizygous Nsdhl mutant males die by midgestation, a conditional allele was needed to reach postnatal biology. Ablating Nsdhl in radial glia produced live-born males that developed ataxia by postnatal day 8 to 10, with loss of cortical and hippocampal neurons and defective proliferation and migration of cerebellar granule precursors followed by massive apoptosis. The proliferation defect was traced to defective SHH signalling and was almost completely rescued by exogenous cholesterol.

Species

Mouse

Genotype

Nsdhl(tm1.1Hrm) conditional allele, GFAP-cre driver

Genes

NSDHL hgnc:13398

Publication

PMID:25652406

Pathograph links

Impaired Hedgehog Signal Transduction recapitulates fidelity: moderate Establishes the causal chain from NSDHL loss to defective SHH signalling and shows that cholesterol supply is the limiting factor for it.

Limitations: The tissue is CNS rather than skin or limb, and the phenotype it produces is the CK syndrome-like neurological picture rather than the CHILD phenotype, so the Hedgehog result transfers as a mechanism while the phenotype does not. Conditional ablation in one lineage is also not equivalent to the mosaic heterozygous state of human CHILD syndrome.

Readouts

Cerebellar granule cell precursor proliferation

Direction: RESTORED

Interpretation: Rescue by exogenous cholesterol identifies cholesterol availability, rather than a separate consequence of the mutation, as the proximate cause of the Hedgehog signalling failure.

Show evidence (1 reference)

PMID:25652406 SUPPORT Model Organism

"Furthermore, this defect is almost completely rescued by supplementation of the culture media with exogenous cholesterol"

Reports the rescue experiment behind this readout.

Show evidence (1 reference)

PMID:25652406 SUPPORT Model Organism

"We replicated the granule cell precursor proliferation defect in vitro and demonstrate that it results from defective signaling by SHH."

Supports treating this model as informative for the Hedgehog node.

Show evidence (1 reference)

PMID:25652406 SUPPORT Model Organism

"Since hemizygous male mice with Nsdhl mutations die by midgestation, we generated a conditional targeted Nsdhl mutation (Nsdhl(tm1.1Hrm)) to investigate the essential role of cholesterol in the early postnatal CNS."

States why this conditional model exists and what it is a model of, which is the model-level claim.

Labrador Retriever NSDHL deletion

A naturally occurring canine counterpart. A female Labrador Retriever and her crossbred daughter carried a large heterozygous NSDHL deletion and showed cornification-disorder lesions following Blaschko lines; both male littermates of the affected daughter died at birth, consistent with X-linked semidominant male lethality. The value of this model is that it is spontaneous and shows the mosaic cutaneous patterning in a large outbred mammal.

Species

Dog

Genotype

Heterozygous deletion removing the last three exons of NSDHL

Genes

NSDHL hgnc:13398

Publication

PMID:28739597

Pathograph links

X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells partially recapitulates fidelity: moderate Reproduces the X-linked mosaic cutaneous patterning in a heterozygous female carrier of an NSDHL loss-of-function allele.

Limitations: The canine lesions follow Blaschko lines rather than showing the strict midline demarcation that defines human CHILD syndrome, so the model captures functional X-inactivation mosaicism but not the lateralization that is the open question. It also carries no longitudinal measurement of clone depletion, so it does not speak to the negative-selection arm of this node. Causality was inferred from genome sequencing and the known role of NSDHL rather than demonstrated functionally.

Readouts

Blaschko-linear cutaneous lesion distribution

Direction: ALTERED

Interpretation: The patterned lesion distribution is the visible readout of functional X-inactivation mosaicism in the epidermis.

Show evidence (1 reference)

PMID:28739597 SUPPORT Model Organism

"We investigated a female Labrador Retriever and her crossbred daughter, which both showed similar clinical lesions that followed Blaschko's lines."

Reports the lesion distribution behind this readout.

Show evidence (1 reference)

PMID:28739597 SUPPORT Model Organism

"Taken together, our genetic data and the known role of NSDHL in X-linked skin disorders strongly suggest that the identified structural variant in the NSDHL gene is causative for the phenotype in the two affected dogs."

Supports treating the canine phenotype as an NSDHL-driven model, with the authors' own hedged causal language preserved.

Show evidence (1 reference)

PMID:28739597 SUPPORT Model Organism

"Variants in the NSDHL gene cause CHILD syndrome in humans, and the bare patches (Bpa) and striated (Str) phenotypes in mice."

The authors situate their canine finding against the human disease and the established mouse models, which is what makes this dog a model for CHILD syndrome rather than an isolated veterinary case.

{ }

Source YAML

click to show

name: CHILD Syndrome
creation_date: "2026-08-13T19:20:00Z"
category: Mendelian
description: >-
  CHILD syndrome (Congenital Hemidysplasia with Ichthyosiform nevus and Limb
  Defects) is an X-linked dominant, usually male-lethal disorder of the distal
  (post-squalene) cholesterol biosynthetic pathway, caused by loss-of-function
  variants in NSDHL at Xq28. NSDHL encodes the NAD(P)H steroid
  dehydrogenase-like protein, the 3-beta-hydroxysteroid dehydrogenase and
  C4-decarboxylase that removes the C-4 methyl groups from lanosterol-derived
  intermediates. Its mechanistic interest is that the lesion is dual rather than
  single: affected cells are simultaneously starved of the pathway end product,
  cholesterol, and poisoned by the methylsterol intermediates that pile up above
  the enzymatic block. Neither arm alone explains the disease, and the
  therapeutic proof of this came from a topical trial in which cholesterol
  replacement alone failed while cholesterol combined with a statin, which shuts
  off the flux generating the toxic intermediates, cleared the skin. The
  syndrome's other signature is its striking lateralization: because NSDHL is
  X-linked and subject to random X inactivation, a heterozygous female is a
  functional mosaic, and lesional skin is populated by keratinocytes expressing
  the mutant allele while contralateral clinically normal skin expresses only
  the wild-type allele. Mutant-expressing cells are under continuing negative
  selection, which accounts both for that asymmetry and for the tendency of skin
  lesions to improve with age; what remains unexplained is why the boundary is a
  sharp body midline rather than the finer Blaschko striping seen in most
  X-linked mosaic genodermatoses. Cholesterol is also required for Hedgehog
  signal transduction, which links NSDHL deficiency to the ipsilateral limb
  defects, to the hair-follicle failure that persists even after the skin
  clears, and to the placental labyrinth defect that appears to underlie male
  lethality. Hypomorphic NSDHL alleles cause a clinically distinct allelic
  disorder in males, CK syndrome.
synonyms:
- CHILD syndrome
- congenital hemidysplasia with ichthyosiform nevus and limb defects
- congenital hemidysplasia with ichthyosiform erythroderma and limb defects
- CHILD nevus
disease_term:
  preferred_term: CHILD syndrome
  term:
    id: MONDO:0010621
    label: CHILD syndrome
classifications:
  harrisons_chapter:
  - classification_value: GENETICS_ENVIRONMENT_DISEASE
    notes: >-
      An X-linked Mendelian disorder whose clinical logic - mosaic expression
      under random X inactivation, male lethality - is genetic rather than
      organ-system based.
  - classification_value: DERMATOLOGY
    notes: >-
      The presenting and most persistent manifestation is a cutaneous one, the
      unilateral ichthyosiform nevus, and the disorder's therapeutic proof of
      mechanism (topical cholesterol plus statin) is a dermatologic
      intervention.
  - classification_value: ENDOCRINOLOGY_METABOLISM
    notes: >-
      Also an inborn error of metabolism: an enzymatic block in post-squalene
      cholesterol biosynthesis, confirmed by the sterol profile.
  icimd_category:
  - classification_value: sterol_metabolism
    notes: >-
      ICIMD (Ferreira et al. 2021, PMID:33340416): group "Disorders of sterol
      metabolism" under category "Disorders of lipid metabolism". NSDHL is the
      C4-demethylation-complex sterol dehydrogenase/decarboxylase of the distal
      cholesterol pathway.
  isds_skeletal_category:
  - classification_value: chondrodysplasia_punctata
    notes: >-
      ISDS Nosology of Genetic Skeletal Disorders, 2023 revision (Unger et al.,
      PMID:36779427), group 23 "Chondrodysplasia punctata (CDP) group", row
      NOS 23-0030 "Congenital hemidysplasia, ichthyosis, limb defects (CHILD)
      syndrome, NSDHL-related" (MIM 308050). The placement is the nosology's,
      not an inference from the skeletal phenotype: CHILD syndrome is grouped
      with the other CDPs because affected infants show punctate epiphyseal
      calcification, and it sits alongside the other post-squalene
      cholesterol-pathway member of the group, CDPX2 (EBP). Note the group is
      radiographically rather than mechanistically defined, so CHILD's nearest
      mechanistic relatives in dismech - Smith-Lemli-Opitz syndrome and the other
      sterol-biosynthesis disorders - are not group-23 entities.

parents:
- sterol biosynthesis disorder
- X-linked ichthyosis syndrome
- multiple congenital anomalies/dysmorphic syndrome without intellectual disability

references:
- reference: PMID:21290788
  title: "NSDHL-Related Disorders."
  tags:
  - GeneReviews
- reference: PMID:24060582
  title: "The role of abnormalities in the distal pathway of cholesterol synthesis in the Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects (CHILD) syndrome."
- reference: PMID:21753784
  title: "Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism."

inheritance:
- name: X-linked dominant inheritance with male lethality
  inheritance_term:
    preferred_term: X-linked dominant inheritance
    term:
      id: HP:0001423
      label: X-linked dominant inheritance
  description: >-
    CHILD syndrome is transmitted as an X-linked dominant trait. Hemizygous male
    conceptuses generally do not survive gestation, so the disorder is seen
    almost exclusively in heterozygous females. Because affected males abort or
    resorb, the expected live-born distribution from a heterozygous mother
    departs from the usual X-linked ratios, and recurrent male pregnancy loss is
    a family-history clue.
  evidence:
  - reference: PMID:10710235
    reference_title: "Mutations in the NSDHL gene, encoding a 3beta-hydroxysteroid dehydrogenase, cause CHILD syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report for the first time that CHILD syndrome (MIM 308050), an X-linked
      dominant, male-lethal trait characterized by an inflammatory nevus with
      striking lateralization and strict midline demarcation, as well as
      ipsilateral hypoplasia of the body is caused by mutations in the gene NSDHL
      located at Xq28
    explanation: >-
      The gene-discovery paper states the X-linked dominant, male-lethal mode of
      inheritance alongside the causal locus.
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      CHILD syndrome is usually male lethal during gestation and thus
      predominantly affects females.
    explanation: >-
      GeneReviews states the male-lethal consequence and the resulting female
      predominance.
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      However, since studies suggest that male conceptuses with an NSDHL
      pathogenic variant generally abort or resorb spontaneously, the expected
      live-born distribution is: 33% heterozygous (typically) affected females;
      33% unaffected females; and 33% unaffected males.
    explanation: >-
      Supplies the quantitative recurrence-risk figures that this description
      would otherwise only paraphrase, including the reason the live-born ratios
      depart from the usual X-linked expectation.
  - reference: PMID:36504312
    reference_title: "Etiological identification of recurrent male fatality due to a novel NSDHL gene mutation using trio whole-exome sequencing: A rare case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In this study, we detected c.790-6C>T, a novel variant in the NSDHL gene
      that results in recurrent miscarriage in males.
    explanation: >-
      Human evidence that the male lethality is real and observable as recurrent
      male pregnancy loss in a carrier mother, rather than being inferred only
      from the skewed sex ratio among liveborn patients.
- name: Functional mosaicism from random X inactivation
  inheritance_term:
    preferred_term: Typified by somatic mosaicism
    term:
      id: HP:0001442
      label: Typified by somatic mosaicism
  description: >-
    NSDHL lies at Xq28 and is therefore subject to X inactivation, so a
    heterozygous female is a functional mosaic of cells expressing either the
    mutant or the wild-type allele. This is not somatic mosaicism at the DNA
    level, which is why sequencing genomic DNA from lesional and non-lesional
    keratinocytes recovers both alleles equally; the mosaicism is only visible in
    expressed RNA. Lesional keratinocytes express the mutant allele exclusively,
    whereas clinically normal contralateral keratinocytes express only the
    wild-type allele. The X-inactivation pattern, rather than the particular
    allele, appears to set the distribution of skin involvement, which is why an
    exceptional patient can have an almost symmetric bilateral CHILD nevus.
  evidence:
  - reference: PMID:21753784
    reference_title: "Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The unusual lateralization of abnormalities in CHILD syndrome reflects
      selective clearance of keratinocytes and fibroblasts that express the mutant
      allele from the unaffected side.
    explanation: >-
      Directly ties the mosaic distribution of mutant-allele-expressing cells to
      the lateralized clinical pattern.
  - reference: PMID:11907515
    reference_title: "A novel missense mutation of NSDHL in an unusual case of CHILD syndrome showing bilateral, almost symmetric involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Apparently, the effect of random X-inactivation is responsible for
      different patterns of cutaneous involvement in female carriers of NSDHL
      mutations.
    explanation: >-
      Attributes the variable cutaneous distribution, including the exceptional
      bilateral case, to the X-inactivation pattern rather than to allele
      identity.

genetic:
- name: NSDHL
  gene_term:
    preferred_term: NSDHL
    term:
      id: hgnc:13398
      label: NSDHL
  relationship_type: CAUSATIVE
  notes: >-
    Loss-of-function variants in NSDHL, encoding NAD(P)H steroid
    dehydrogenase-like protein, cause CHILD syndrome. The gene maps to Xq28. The
    encoded enzyme is a 3-beta-hydroxysteroid dehydrogenase and C4-decarboxylase
    that removes C-4 methyl groups in the post-squalene segment of cholesterol
    biosynthesis. Reported alleles include missense changes such as p.Gly83Asp
    and premature termination changes such as p.Ser106X. CHILD syndrome and CK
    syndrome are allelic: CK syndrome arises from hypomorphic alleles retaining
    partial enzyme function and affects males.
  evidence:
  - reference: PMID:10710235
    reference_title: "Mutations in the NSDHL gene, encoding a 3beta-hydroxysteroid dehydrogenase, cause CHILD syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      SSCA and genomic sequence analysis of NSDHL identified in 6 patients with
      CHILD syndrome, including one boy as well as a mother and her daughter,
      mutations potentially impairing protein function.
    explanation: >-
      Establishes NSDHL variants as the molecular cause in a patient series.
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The diagnosis of CHILD syndrome is established in a female proband with a
      heterozygous NSDHL pathogenic variant identified by molecular genetic
      testing that results in loss of functional decarboxylating
      sterol-4-alpha-carboxylate 3-dehydrogenase, the protein encoded by NSDHL.
    explanation: >-
      GeneReviews states the diagnostic criterion and the loss-of-function
      consequence at the protein level.
  - reference: PMID:25652406
    reference_title: "Analysis of hedgehog signaling in cerebellar granule cell precursors in a conditional Nsdhl allele demonstrates an essential role for cholesterol in postnatal CNS development."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Rare, hypomorphic NSDHL mutations are also associated with X-linked
      intellectual disability in males with CK syndrome.
    explanation: >-
      Supports the allelic relationship in which residual enzyme activity yields
      CK syndrome rather than CHILD syndrome.

- name: EBP
  gene_term:
    preferred_term: EBP
    term:
      id: hgnc:3133
      label: EBP
  relationship_type: CAUSATIVE
  notes: >-
    A minority route to a CHILD-syndrome phenotype runs through EBP, the
    delta8-delta7 sterol isomerase acting one step downstream of NSDHL and the
    gene of CDPX2. A patient with a clinical diagnosis of CHILD syndrome was
    found to have the CDPX2 plasma sterol signature (raised 8-dehydrocholesterol
    and 8(9)-cholestenol) and a nonsense variant in EBP. This is a genuine
    locus-heterogeneity finding, but it sits in unresolved tension with the
    NSDHL gene-discovery paper published alongside it, which found EBP
    unaffected in its own CHILD cohort and concluded the two disorders are not
    allelic. Both can be true if "CHILD syndrome" as clinically diagnosed spans
    two loci; which patients belong where is a nosological question this entry
    does not settle. See the corresponding discussion. EBP is deliberately left
    unwired to any pathophysiology node: it catalyses delta8-delta7
    isomerization, a different step from the C4-demethylation that this
    pathograph models, so attaching it to the existing metabolic block node
    would misstate the enzymology. Wiring it properly would mean curating a
    second, EBP-specific block, which is not warranted by one hedged case
    report.
  evidence:
  - reference: PMID:10710233
    reference_title: "CHILD syndrome caused by deficiency of 3beta-hydroxysteroid-delta8, delta7-isomerase."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Subsequently, we identified a nonsense mutation in exon 3 of the patient's
      3beta-hydroxysteroid-delta8,delta7-isomerase gene. We speculate that at
      least some cases of CHILD syndrome are allelic with CDPX2 caused by
      3beta-hydroxysteroid-delta8,delta7-isomerase deficiency.
    explanation: >-
      Reports a single patient with an EBP nonsense variant and the matching
      sterol signature. PARTIAL because the authors themselves generalize only
      speculatively, from one patient, to "at least some cases".

pathophysiology:
- name: NSDHL Loss of Function
  biological_scale: MOLECULAR
  description: >-
    Pathogenic NSDHL variants abolish or severely reduce the activity of the
    NAD(P)H steroid dehydrogenase-like protein. Missense alleles do not simply
    produce a catalytically dead but abundant enzyme: across a survey of thirteen
    disease-causing missense changes and one deletion, protein expression levels
    were uniformly low, and some mutants could be partially rescued by lowering
    the temperature or by adding a chemical chaperone, implicating a folding and
    stability defect rather than pure catalytic loss.
  molecular_functions:
  - preferred_term: sterol-4-alpha-carboxylate 3-dehydrogenase (NSDHL) activity
    modifier: LOSS_OF_FUNCTION
    term:
      id: GO:0000252
      label: 3-beta-hydroxysteroid dehydrogenase [NAD(P)+]/C4-decarboxylase activity
  genes:
  - preferred_term: NSDHL
    term:
      id: hgnc:13398
      label: NSDHL
  downstream:
  - target: Blocked C4-Demethylation with Cholesterol Deficiency and Methylsterol Accumulation
    description: >-
      Loss of the C4-demethylating enzyme interrupts the post-squalene pathway,
      simultaneously reducing cholesterol output and causing intermediates above
      the block to accumulate.
  evidence:
  - reference: PMID:40222685
    reference_title: "Comprehensive survey of disease-causing missense mutations of the cholesterol synthesis enzyme NSDHL: Low temperature and a chemical chaperone rescue low protein expression of select mutants."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Here we show that protein expression levels were low for all mutants, but
      some could be rescued by a lower temperature (30°C vs. 37°C) and/or the
      chemical chaperone glycerol.
    explanation: >-
      Characterizes the molecular consequence of disease-causing missense alleles
      as reduced protein expression with a partially rescuable folding component.
  - reference: PMID:10710235
    reference_title: "Mutations in the NSDHL gene, encoding a 3beta-hydroxysteroid dehydrogenase, cause CHILD syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      (NAD(P)H steroid dehydrogenase-like protein) encoding a 3beta-hydroxysteroid
      dehydrogenase functioning in the cholesterol biosynthetic pathway
    explanation: >-
      Identifies the enzymatic function that is lost.

- name: Blocked C4-Demethylation with Cholesterol Deficiency and Methylsterol Accumulation
  biological_scale: MOLECULAR
  description: >-
    The enzymatic block has two simultaneous consequences that are usually
    described as one. Downstream, flux to cholesterol falls, so affected cells
    are deprived of the pathway end product. Upstream, methylsterol intermediates
    accumulate; these are not inert, but insert into membranes and perturb
    membrane function, oxysterol generation, Hedgehog signalling, and HMG-CoA
    reductase turnover. In cultured cerebellar granule cell precursors from a
    conditional Nsdhl mouse, the two arms are experimentally separable: exogenous
    cholesterol almost completely rescued the proliferation defect, while
    methylsterol accumulation above the block was associated instead with
    increased cell death.
  biological_processes:
  - preferred_term: cholesterol biosynthetic process
    modifier: DECREASED
    term:
      id: GO:0006695
      label: cholesterol biosynthetic process
  chemical_entities:
  - preferred_term: cholesterol
    modifier: DECREASED
    term:
      id: CHEBI:16113
      label: cholesterol
  downstream:
  - target: Impaired Epidermal Lamellar Body Secretion and Barrier Formation
    description: >-
      Cholesterol is a required constituent of the stratum corneum lipid
      lamellae; its depletion, together with toxic intermediate accumulation,
      disorganizes the epidermal lipid secretory system.
  - target: Impaired Hedgehog Signal Transduction
    description: >-
      Cholesterol is required for Hedgehog signal transduction, and accumulated
      sterol intermediates further perturb it.
  - target: X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells
    description: >-
      The metabolic lesion is also what makes a mutant-expressing cell less fit
      than its wild-type neighbour, which is the proposed driver of the
      progressive negative selection.
  - target: Stippled epiphyseal calcification
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Punctate calcification of cartilage is the radiographic finding CHILD
      syndrome shares with CDPX2, and what the two disorders have in common is a
      block in the same post-squalene sterol pathway rather than any shared gene.
      That shared biochemistry is the basis for placing the stippling downstream
      of the metabolic block; the chondro-osseous mechanism connecting them is
      not established.
    evidence:
    - reference: PMID:10710235
      reference_title: "Mutations in the NSDHL gene, encoding a 3beta-hydroxysteroid dehydrogenase, cause CHILD syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        This phenotype is distinct from, but shares various clinical and
        biochemical findings with chondrodysplasia punctata (CDPX2, MIM 302960).
      explanation: >-
        Supports the shared clinical and biochemical ground between the two sterol
        pathway disorders, which is the argument for this edge, but does not
        itself demonstrate the causal step. Hence PARTIAL.
  evidence:
  - reference: PMID:24060582
    reference_title: "The role of abnormalities in the distal pathway of cholesterol synthesis in the Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects (CHILD) syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      CHILD syndrome results from loss of function mutations in the NSDHL gene,
      which leads to inhibition of cholesterol synthesis and accumulation of toxic
      metabolic intermediates in affected tissues.
    explanation: >-
      States both arms of the dual biochemical lesion explicitly.
  - reference: PMID:25652406
    reference_title: "Analysis of hedgehog signaling in cerebellar granule cell precursors in a conditional Nsdhl allele demonstrates an essential role for cholesterol in postnatal CNS development."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Furthermore, this defect is almost completely rescued by supplementation of
      the culture media with exogenous cholesterol, while methylsterol
      accumulation above the enzymatic block appears to be associated with
      increased cell death.
    explanation: >-
      Experimentally dissociates the end-product-deficiency arm from the
      toxic-intermediate arm within one model system.

- name: X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells
  biological_scale: CELLULAR
  description: >-
    Because NSDHL is X-linked, a heterozygous female is a functional mosaic of
    cells expressing either allele. Lesional and contralateral clinically normal
    skin carry equal amounts of mutant and wild-type DNA, but differ completely
    in what they express: cultured keratinocytes from lesional skin expressed
    only the mutant allele, and keratinocytes from the unaffected side expressed
    only the wild-type allele. Fibroblasts showed the same asymmetry less
    completely. Skewed X inactivation in peripheral blood is consistent with
    overall depletion of mutant-expressing cells. The mouse work adds the
    temporal dimension that makes this a process rather than a snapshot:
    NSDHL-negative cells survive and differentiate during embryogenesis but are
    progressively outcompeted afterwards, with the mutant fraction in liver
    falling from about half at postnatal day 6 to about a fifth by one year.
    That selection is the likely reason skin lesions tend to improve with age,
    and why the disease is fundamentally a developmental one, doing its damage in
    the window while mutant clones are still abundant. The model also predicts
    the clinical observation that a full-thickness graft taken from the
    unaffected side stays normal after transfer to the affected side. What it
    does not explain is why the boundary is a sharp body midline rather than the
    finer Blaschko striping typical of X-linked mosaic skin disease.
  cell_types:
  - preferred_term: keratinocyte
    term:
      id: CL:0000312
      label: keratinocyte
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  biological_processes:
  - preferred_term: random X inactivation
    term:
      id: GO:0060816
      label: random inactivation of X chromosome
  downstream:
  - target: Impaired Epidermal Lamellar Body Secretion and Barrier Formation
    description: >-
      The epidermal phenotype is confined to the territory populated by
      mutant-allele-expressing keratinocytes, which is what makes the skin
      disease unilateral.
  - target: Strict midline demarcation of skin lesions
    description: >-
      The distribution of surviving mutant-expressing clones is what the clinical
      lateralization is a map of.
  - target: Ipsilateral body hypoplasia
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The hemidysplasia follows the same mosaic territory as the skin, so one
      side of the body grows out of a cell population that is largely
      mutant-expressing. Why that produces hypoplasia rather than a purely
      cutaneous phenotype is not worked out.
  - target: Cardiac malformation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Visceral anomalies in CHILD syndrome are ipsilateral, following the same
      body territory as the skin and limb involvement, which places them
      downstream of the mosaic distribution of mutant-expressing cells. How
      NSDHL deficiency in that territory produces a given cardiac malformation
      is not worked out.
    evidence:
    - reference: PMID:11907515
      reference_title: "A novel missense mutation of NSDHL in an unusual case of CHILD syndrome showing bilateral, almost symmetric involvement."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Ipsilateral defects involve all skeletal structures and internal organs
        such as the brain, the lung, the heart, or the kidney.
      explanation: >-
        Establishes that the visceral anomalies follow the same lateralized
        territory as the cutaneous disease, which is what licenses this edge.
  - target: Kidney malformation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      As for the cardiac malformations, renal anomalies fall on the affected
      side, placing them downstream of the mosaic territory rather than of a
      separate systemic mechanism.
    evidence:
    - reference: PMID:11907515
      reference_title: "A novel missense mutation of NSDHL in an unusual case of CHILD syndrome showing bilateral, almost symmetric involvement."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Ipsilateral defects involve all skeletal structures and internal organs
        such as the brain, the lung, the heart, or the kidney.
      explanation: >-
        Names the kidney among the ipsilaterally affected internal organs.
  - target: Lung malformation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The lung belongs to the same set of ipsilaterally affected internal organs
      as the heart and kidney, so it sits downstream of the mosaic territory on
      the same footing as they do.
    evidence:
    - reference: PMID:11907515
      reference_title: "A novel missense mutation of NSDHL in an unusual case of CHILD syndrome showing bilateral, almost symmetric involvement."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Ipsilateral defects involve all skeletal structures and internal organs
        such as the brain, the lung, the heart, or the kidney.
      explanation: >-
        Names the lung explicitly among the ipsilaterally affected internal
        organs.
  - target: Joint contracture
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Contractures follow the one-sided skeletal and soft-tissue involvement,
      alongside the scoliosis, rather than arising from a separate mechanism.
    evidence:
    - reference: PMID:11907515
      reference_title: "A novel missense mutation of NSDHL in an unusual case of CHILD syndrome showing bilateral, almost symmetric involvement."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Ipsilateral defects involve all skeletal structures and internal organs
        such as the brain, the lung, the heart, or the kidney.
      explanation: >-
        Supports the one-sided skeletal involvement but does not name joint
        contractures specifically, hence PARTIAL.
  - target: Scoliosis
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Involvement of skeletal structures confined to one side of the body
      produces asymmetric axial growth, which is the standing explanation for
      the scoliosis. The edge is at the level of the lateralized skeletal
      involvement rather than a specific vertebral mechanism.
    evidence:
    - reference: PMID:11907515
      reference_title: "A novel missense mutation of NSDHL in an unusual case of CHILD syndrome showing bilateral, almost symmetric involvement."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Ipsilateral defects involve all skeletal structures and internal organs
        such as the brain, the lung, the heart, or the kidney.
      explanation: >-
        Supports the one-sided skeletal involvement but says nothing about
        scoliosis specifically, hence PARTIAL.
  evidence:
  - reference: PMID:21753784
    reference_title: "Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The unusual lateralization of abnormalities in CHILD syndrome reflects
      selective clearance of keratinocytes and fibroblasts that express the mutant
      allele from the unaffected side.
    explanation: >-
      States the cell-level result that clinically normal skin is normal because
      the mutant-expressing cells are gone from it.
  - reference: PMID:19631568
    reference_title: "Developmental expression pattern of the cholesterogenic enzyme NSDHL and negative selection of NSDHL-deficient cells in the heterozygous Bpa(1H)/+ mouse."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Our results suggest that while NSDHL-deficient cells in the mosaic Bpa(1H)
      female are able to survive and differentiate during embryonic development,
      they are subject to negative selection over the life of the animal.
    explanation: >-
      Supplies the longitudinal evidence that clearance of mutant-expressing cells
      is a postnatal, progressive process rather than an embryonic one.

- name: Impaired Epidermal Lamellar Body Secretion and Barrier Formation
  biological_scale: TISSUE
  description: >-
    In lesional epidermis the lipid secretory apparatus is structurally
    disorganized. Lamellar bodies contain few internal lamellae, fuse prematurely
    into multivesicular bodies, and are incompletely secreted, so organelle
    contents remain entombed within corneocytes and the stratum corneum
    extracellular space is left with sparse lamellar material interspersed with
    non-lamellar vesicles. Histologically this appears as marked hyperkeratosis
    and acanthosis with loss of the granular layer and dermal foam cells. The
    severity of this secretory disruption exceeds what topical statin-induced
    cholesterol deficiency alone produces in normal skin, which is the
    ultrastructural argument that toxic intermediates and not simply cholesterol
    depletion drive the ichthyosiform phenotype.
  cell_types:
  - preferred_term: keratinocyte
    term:
      id: CL:0000312
      label: keratinocyte
  biological_processes:
  - preferred_term: establishment of skin barrier
    modifier: DECREASED
    term:
      id: GO:0061436
      label: establishment of skin barrier
  - preferred_term: keratinocyte differentiation
    modifier: ABNORMAL
    term:
      id: GO:0030216
      label: keratinocyte differentiation
  downstream:
  - target: Ichthyosiform nevus
    description: >-
      The disordered lipid barrier and compensatory hyperkeratotic response
      produce the waxy, scaling, inflammatory plaques of the CHILD nevus.
  - target: Onychodystrophy and periungual hyperkeratosis
    causal_link_type: UNKNOWN
    description: >-
      The nail unit is a keratinized epidermal appendage, so the same defective
      cornification is the natural explanation for the nail changes. The
      intermediates are not established: this edge is inferred from shared
      keratinization biology and from the observation that nail changes persist
      under a therapy that clears the skin, not from any measurement made in the
      nail matrix.
  evidence:
  - reference: PMID:21753784
    reference_title: "Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Ultrastructural analysis of affected skin showed evidence of both
      cholesterol depletion and toxic metabolic accumulation.
    explanation: >-
      Reports the ultrastructural finding in lesional epidermis that grounds this
      node.
  - reference: PMID:32886633
    reference_title: "Topical Cholesterol/Simvastatin Gel for the Treatment of CHILD Syndrome in an Adolescent."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Congenital ichthyoses are a heterogeneous group of genetic skin disorders
      characterized by defects in the critical barrier function of the skin.
    explanation: >-
      Places CHILD syndrome within the barrier-defect class of congenital
      ichthyoses. This supports the barrier framing only, not the specific
      lamellar body lesion, hence PARTIAL.
  - reference: PMID:16776722
    reference_title: "Abnormal lamellar granules in a case of CHILD syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Electron microscopy revealed vesicular structures in the intercellular
      spaces of the stratum corneum and vacuoles or vesicular structures in
      upper prickle cell layer. Some of them can be recognized as abnormal
      lamellar granules. Within the foamy cells in the papillary dermis, large
      vacuoles were found.
    explanation: >-
      Independent ultrastructural confirmation of the abnormal lamellar granules
      and of the dermal foam cells this description asserts, from a different
      patient and laboratory than the treatment study.

- name: Impaired Hedgehog Signal Transduction
  biological_scale: MOLECULAR
  description: >-
    Cholesterol is required for Hedgehog signal transduction, so the distal
    biosynthetic block reaches developmental patterning as well as membrane
    lipid supply. The cleanest experimental demonstration is in cerebellar
    granule cell precursors from a conditional Nsdhl mouse, where the
    proliferation defect was shown to result from defective signalling by SHH and
    was rescued by exogenous cholesterol. The same dependency is invoked for the
    persistent failure of hair growth in treated CHILD skin, where the ichthyosis
    resolves but hair does not return.
  biological_processes:
  - preferred_term: smoothened signaling pathway
    modifier: DECREASED
    term:
      id: GO:0007224
      label: smoothened signaling pathway
  downstream:
  - target: Ipsilateral limb hypoplasia or aplasia
    description: >-
      Hedgehog-dependent limb patterning is disrupted in the territory of
      mutant-expressing cells, contributing to the ipsilateral skeletal defects.
  - target: Alopecia of affected skin
    description: >-
      Hair follicle development is Hedgehog-dependent, which is the standing
      explanation for hair failing to return in skin that has otherwise been
      cleared by topical therapy.
  - target: Placental Labyrinth Failure and Male Lethality
    description: >-
      Hedgehog signalling is required for development of a functional
      labyrinth, and its failure is implicated in the placental pathology of
      NSDHL deficiency.
  evidence:
  - reference: PMID:25652406
    reference_title: "Analysis of hedgehog signaling in cerebellar granule cell precursors in a conditional Nsdhl allele demonstrates an essential role for cholesterol in postnatal CNS development."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      We replicated the granule cell precursor proliferation defect in vitro and
      demonstrate that it results from defective signaling by SHH.
    explanation: >-
      Directly demonstrates that NSDHL loss impairs Hedgehog signal transduction
      in a cholesterol-dependent manner.
  - reference: PMID:17028112
    reference_title: "Analysis of Nsdhl-deficient embryos reveals a role for Hedgehog signaling in early placental development."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      These data strongly implicate the hedgehog signaling pathway in the
      pathogenesis of the placental defects in NSDHL deficiency and provide
      evidence for a role for the hedgehog pathway in the development of a
      functional mammalian placenta.
    explanation: >-
      Links the Hedgehog defect specifically to the placental arm of NSDHL
      deficiency.
  notes: >-
    This is a sterol-dependent Hedgehog defect and is deliberately NOT declared
    as conforming to the Hedgehog node of the ciliopathy_dysfunction module,
    whose chain runs from a basal body, transition zone, or intraflagellar
    transport lesion. CHILD syndrome has no primary ciliary assembly defect; the
    two routes converge on the same pathway from different upstream causes. Note
    also that the direct experimental evidence here is murine and comes from CNS
    tissue; the limb and hair consequences are inferences from the established
    cholesterol dependence of Hedgehog signalling rather than measurements made
    in CHILD syndrome tissue.

- name: Placental Labyrinth Failure and Male Lethality
  biological_scale: ORGANISM
  description: >-
    The prenatal male lethality that shapes the inheritance pattern appears to be
    placental rather than embryonic. In Nsdhl mutant mice, affected male embryos
    die in midgestation with no consistent anomaly in the embryo itself, but with
    a labyrinth layer that is always thinner, with fewer fetal vessels and
    decreased trophoblast proliferation. Heterozygous female placentas show the
    defect non-cell-autonomously, and Hedgehog reporter activity is markedly
    reduced or absent in affected male placentas, with failure of Indian
    hedgehog-expressing cells to migrate into the developing placenta. The
    mechanism is inferred from mouse; it has not been demonstrated in human
    conceptuses.
  cell_types:
  - preferred_term: trophoblast cell
    term:
      id: CL:0000351
      label: trophoblast cell
  biological_processes:
  - preferred_term: placenta development
    modifier: ABNORMAL
    term:
      id: GO:0001890
      label: placenta development
  evidence:
  - reference: PMID:15639195
    reference_title: "Placental defects are associated with male lethality in bare patches and striated embryos deficient in the NAD(P)H Steroid Dehydrogenase-like (NSDHL) Enzyme."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Although no consistent anomalies were identified in affected male embryos
      themselves, the labyrinth layer of the fetal placenta was always thinner,
      with fewer fetal vessels and decreased proliferation of labyrinth
      trophoblast cells.
    explanation: >-
      Locates the lethal lesion in the placenta rather than the embryo.
  - reference: PMID:17028112
    reference_title: "Analysis of Nsdhl-deficient embryos reveals a role for Hedgehog signaling in early placental development."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Further, affected placentas from mutant Nsdhl(Bpa-8H) male embryos
      demonstrate markedly decreased or no Ptch1-lacZ staining and no migration of
      Ihh expressing cells into the developing placenta.
    explanation: >-
      Supplies the signalling readout showing Hedgehog pathway failure in the
      affected placenta.

phenotypes:
- category: Cutaneous
  name: Ichthyosiform nevus
  description: >-
    Unilateral inflammatory, erythematous plaques topped by waxy yellow scale,
    present at birth or in the first weeks of life. New lesions can appear later
    in life. More limited lesions may follow one or more lines of Blaschko.
  phenotype_term:
    preferred_term: Ichthyosiform nevus
    term:
      id: HP:0010816
      label: Epidermal nevus
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      CHILD syndrome is characterized by unilateral distribution of ichthyosiform
      skin lesions and ipsilateral limb defects that range from shortening of the
      metacarpals and phalanges to absence of the entire limb.
    explanation: >-
      GeneReviews names the unilateral ichthyosiform lesion as a defining feature.
  - reference: PMID:24060582
    reference_title: "The role of abnormalities in the distal pathway of cholesterol synthesis in the Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects (CHILD) syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The CHILD syndrome skin is characterized by plaques topped by waxy scales
      and a variety of developmental defects in extracutaneous tissues,
      particularly limb hypoplasia or aplasia.
    explanation: >-
      Describes the characteristic morphology of the plaques.
  notes: >-
    Bound to HP:0010816 (Epidermal nevus) rather than HP:0008064 (Ichthyosis).
    The lesion is genuinely ichthyosiform, but it is a localized, mosaic,
    unilateral nevus, not a generalized disorder of cornification, and the
    nevoid character is what discriminates it; the classic delineation places
    the CHILD nevus among the epidermal nevus syndromes. HP:0034275 (Verrucous
    epidermal nevus) was considered and not used, since the verrucous quality is
    reported in some lesions rather than being a general property of the CHILD
    nevus. The ichthyosiform quality is carried by the preferred_term.
    Separately, no frequency band is asserted. The lesion is effectively
    definitional, and grading a defining feature as VERY_FREQUENT on the
    strength of the phrase "is characterized by" would be circular rather than
    evidenced. There is no published CHILD syndrome cohort with a denominator
    from which a real band could be derived.

- category: Cutaneous
  name: Strict midline demarcation of skin lesions
  description: >-
    The cutaneous lesions typically stop abruptly at the body midline, a
    lateralization far sharper than the Blaschko-line mosaicism seen in other
    X-linked genodermatoses, and it is the single most distinctive clinical sign
    of the syndrome. It is a strong tendency rather than a rule: an exceptional
    patient with a molecularly confirmed NSDHL variant had an almost symmetric
    bilateral CHILD nevus, and bilateral involvement should not be taken to
    exclude the diagnosis.
  phenotype_term:
    preferred_term: strict midline demarcation of the cutaneous eruption
  evidence:
  - reference: PMID:10710235
    reference_title: "Mutations in the NSDHL gene, encoding a 3beta-hydroxysteroid dehydrogenase, cause CHILD syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      an inflammatory nevus with striking lateralization and strict midline
      demarcation, as well as ipsilateral hypoplasia of the body
    explanation: >-
      States the midline demarcation of the nevus, which is the claim this
      phenotype makes.
  - reference: PMID:7408908
    reference_title: "The CHILD syndrome. Congenital hemidysplasia with ichthyosiform erythroderma and limb defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The syndrome is characterized by unilateral erythema and scaling, with a
      distinct demarcation in the middle of the trunk.
    explanation: >-
      The original delineation locates the demarcation anatomically, at the
      middle of the trunk.
  - reference: PMID:11907515
    reference_title: "A novel missense mutation of NSDHL in an unusual case of CHILD syndrome showing bilateral, almost symmetric involvement."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A symmetric distribution of this nevus can exceptionally be seen in
      patients with CHILD syndrome, and this bilateral involvement should not
      mislead the clinician to any other diagnosis.
    explanation: >-
      Qualifies rather than supports the lateralization claim, documenting the
      exception that makes midline demarcation typical but not obligatory.
  notes: >-
    No ontology term is bound, deliberately. HPO has no term for strict midline
    demarcation of a cutaneous eruption. HP:0100555 (Asymmetric growth) was used
    here previously and has been removed: it describes asymmetry of body growth,
    not the distribution of a skin lesion, so it is a cross-category proxy
    rather than a merely broad parent, and no term is better than a misleading
    one. That term now sits on the separate `Ipsilateral body hypoplasia`
    phenotype, which it genuinely fits. HP:0025293 (Distributed along Blaschko
    lines) is explicitly not a substitute, since the whole point of this
    phenotype is that the demarcation is sharper than Blaschko mosaicism.

- category: Skeletal
  name: Ipsilateral body hypoplasia
  description: >-
    Hemidysplasia, the H in the acronym: hypoplasia of one side of the body,
    involving skeletal structures and internal organs on the same side as the
    cutaneous eruption. This is the body-growth asymmetry, distinct from both
    the distribution of the skin lesion and the limb-specific defect below.
  phenotype_term:
    preferred_term: Asymmetric growth
    term:
      id: HP:0100555
      label: Asymmetric growth
  evidence:
  - reference: PMID:10710235
    reference_title: "Mutations in the NSDHL gene, encoding a 3beta-hydroxysteroid dehydrogenase, cause CHILD syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      an inflammatory nevus with striking lateralization and strict midline
      demarcation, as well as ipsilateral hypoplasia of the body
    explanation: >-
      States the ipsilateral hypoplasia of the body, which is the asymmetric
      growth this term describes.
  - reference: PMID:7408908
    reference_title: "The CHILD syndrome. Congenital hemidysplasia with ichthyosiform erythroderma and limb defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In addition, ipsilateral hypoplasia of other parts of the skeleton, as well
      as defects of the brain and the viscera are found.
    explanation: >-
      Extends the ipsilateral hypoplasia beyond the limbs to the rest of the
      skeleton and the viscera.

- category: Skeletal
  name: Ipsilateral limb hypoplasia or aplasia
  description: >-
    Limb defects on the same side as the skin lesions, ranging in severity from
    shortening of metacarpals and phalanges through to absence of the entire
    limb. Severe cases have required amputation of a rudimentary limb in early
    childhood to permit prosthetic fitting.
  phenotype_term:
    preferred_term: Aplasia/hypoplasia involving bones of the extremities
    term:
      id: HP:0045060
      label: Aplasia/hypoplasia involving bones of the extremities
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      unilateral distribution of ichthyosiform skin lesions and ipsilateral limb
      defects that range from shortening of the metacarpals and phalanges to
      absence of the entire limb
    explanation: >-
      GeneReviews states the ipsilateral limb defect and its severity range as a
      defining characteristic.

- category: Cutaneous
  name: Onychodystrophy and periungual hyperkeratosis
  description: >-
    Nail dystrophy with hyperkeratosis of the periungual skin is a common
    accompaniment of the cutaneous phenotype. Nails respond poorly to topical
    pathogenesis-based therapy, plausibly because the compounded lotion cannot
    penetrate the nail plate.
  phenotype_term:
    preferred_term: Nail dystrophy
    term:
      id: HP:0008404
      label: Nail dystrophy
  frequency: FREQUENT
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Onychodystrophy and periungual hyperkeratosis are common.
    explanation: >-
      GeneReviews states both the finding and a qualitative frequency. "Common"
      is the literature term the curation SOP maps to the FREQUENT band.

- category: Cutaneous
  name: Alopecia of affected skin
  description: >-
    Hair does not grow in affected territory, and notably does not return even
    after topical cholesterol and statin therapy has otherwise normalized the
    skin. This dissociation is read as evidence that hair follicle development
    requires cholesterol-dependent Hedgehog signalling, a requirement that
    restoring the barrier does not retrospectively satisfy.
  phenotype_term:
    preferred_term: Alopecia
    term:
      id: HP:0001596
      label: Alopecia
  evidence:
  - reference: PMID:21753784
    reference_title: "Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      By 6 months, treated skin largely normalized, but did not grow hair
    explanation: >-
      Reports the persistence of alopecia despite otherwise successful treatment.

- category: Skeletal
  name: Stippled epiphyseal calcification
  description: >-
    Punctate calcification of cartilaginous structures may be seen in infancy, a
    radiographic finding shared with the other X-linked sterol biosynthesis
    disorder CDPX2 and the reason CHILD syndrome is classified under
    chondrodysplasia punctata in MONDO.
  phenotype_term:
    preferred_term: Stippled chondral calcification
    term:
      id: HP:0002764
      label: Stippled chondral calcification
  evidence:
  - reference: PMID:10710233
    reference_title: "CHILD syndrome caused by deficiency of 3beta-hydroxysteroid-delta8, delta7-isomerase."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      CHILD (congenital hemidysplasia, ichthyosis, and limb defects) syndrome is
      a rare, usually sporadic disorder associated with unilateral distribution of
      ichthyosiform skin lesions, limb defects, punctate calcifications of
      cartilaginous structures, and visceral anomalies.
    explanation: >-
      Names punctate calcification of cartilaginous structures among the
      characteristic features.

- category: Skeletal
  name: Scoliosis
  description: >-
    Scoliosis and joint contractures occur and are managed with bracing or
    corrective surgery. Severe lumbar scoliosis was documented in one of the two
    patients in the pathogenesis-based therapy report.
  phenotype_term:
    preferred_term: Scoliosis
    term:
      id: HP:0002650
      label: Scoliosis
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Scoliosis and joint contractures are treated with braces and/or corrective
      surgery.
    explanation: >-
      A management statement, so it presupposes rather than reports the
      occurrence of scoliosis in CHILD syndrome. PARTIAL for that reason.

- category: Cardiovascular
  name: Cardiac malformation
  description: >-
    Structural heart malformations occur as part of the visceral involvement and
    warrant surveillance.
  phenotype_term:
    preferred_term: Abnormal heart morphology
    term:
      id: HP:0001627
      label: Abnormal heart morphology
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Heart, lung, and kidney malformations can also occur.
    explanation: >-
      GeneReviews states the visceral malformations. No frequency band is
      asserted, because "can also occur" is weaker than any of the qualitative
      terms the frequency SOP maps to a band.

- category: Renal
  name: Kidney malformation
  description: >-
    Renal malformations are part of the visceral spectrum and are monitored on
    surveillance.
  phenotype_term:
    preferred_term: Abnormal kidney morphology
    term:
      id: HP:0012210
      label: Abnormal renal morphology
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Heart, lung, and kidney malformations can also occur.
    explanation: >-
      GeneReviews states kidney malformation among the visceral anomalies.

- category: Respiratory
  name: Lung malformation
  description: >-
    Pulmonary malformations belong to the same ipsilateral visceral spectrum as
    the cardiac and renal anomalies, and GeneReviews names the lung alongside
    them. The original clinical delineation likewise recorded ipsilateral
    defects of the viscera generally rather than enumerating pulmonary lesions,
    so the anatomy is recorded at the level the sources actually support.
  phenotype_term:
    preferred_term: Abnormal lung morphology
    term:
      id: HP:0002088
      label: Abnormal lung morphology
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Heart, lung, and kidney malformations can also occur.
    explanation: >-
      GeneReviews names the lung among the organs that may be malformed, on the
      same footing as the heart and kidney.

- category: Musculoskeletal
  name: Joint contracture
  description: >-
    Joint contractures occur with the skeletal involvement and, like scoliosis,
    are managed with bracing or corrective surgery and tracked on surveillance.
  phenotype_term:
    preferred_term: Joint contracture
    term:
      id: HP:0034392
      label: Joint contracture
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      monitor for new cutaneous lesions, musculoskeletal deformities such as
      scoliosis and joint contractures, and neurologic, cardiac, and/or kidney
      manifestations annually or as needed
    explanation: >-
      GeneReviews names joint contractures among the musculoskeletal deformities
      that warrant ongoing monitoring in CHILD syndrome.

- category: Neurological
  name: Normal intellect
  description: >-
    Intellect is usually normal in CHILD syndrome. This is a deliberate negative
    characterization and is the sharpest clinical separator from the allelic
    NSDHL disorder in males, CK syndrome, which is defined by cognitive
    impairment, seizures, and cortical malformation.
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      absence of the entire limb. Intellect is usually normal.
    explanation: >-
      GeneReviews states that intellect is usually normal in CHILD syndrome, which
      refutes intellectual disability as a feature of this entity.

prevalence:
- population: Worldwide, published case literature
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    No cohort, registry, or population study exists, so no rate can be given.
    The original 1980 delineation assembled twenty observations in total, two
    new plus eighteen reviewed, and the published total has since grown into the
    low hundreds. The figure of one in 100,000 that circulates in review
    literature has no traceable denominator and is inconsistent with that case
    count, so it is deliberately not recorded here. The same series gives the
    female-to-male ratio as 19:1, which is a consequence of male lethality
    rather than an independent epidemiological finding.
  evidence:
  - reference: PMID:7408908
    reference_title: "The CHILD syndrome. Congenital hemidysplasia with ichthyosiform erythroderma and limb defects."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Two further cases of this syndrome are reported, and a review of 18
      previous observations is presented. The ratio of females to males is 19 :
      1.
    explanation: >-
      Supplies the literature case count behind the CASES_IN_LITERATURE measure
      and the sex ratio, which is the only quantitative epidemiology available.

histopathology:
- name: Psoriasiform hyperkeratosis and acanthosis with loss of the granular layer
  description: >-
    Lesional skin shows marked hyperkeratosis and acanthosis with absence of the
    granular layer, together with foam cells in the dermis. This is the routine
    light-microscopic picture, and it reverses on effective pathogenesis-based
    therapy: after three months of topical cholesterol plus lovastatin the
    epidermis was largely normal with a prominent granular layer and
    orthokeratotic scale.
  finding_term:
    preferred_term: hyperkeratosis with acanthosis
    term:
      id: NCIT:C35541
      label: Hyperkeratosis
  evidence:
  - reference: PMID:21753784
    reference_title: "Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Baseline biopsy sections of skin lesions from both patients prior to
      treatment showed marked hyperkeratosis and acanthosis with absence of the
      granular layer and dermal foam cells
    explanation: >-
      Reports the light-microscopic findings in lesional skin from both patients,
      including the dermal foam cells.

- name: Verruciform xanthoma-like change in the CHILD nevus
  description: >-
    The CHILD nevus is not histologically generic. Alongside the psoriasiform
    changes it shows the microscopic features of verruciform xanthoma, which
    together with the diffuse lateralization and ptychotropism (a predilection
    for body folds) is what distinguishes it from the other epidermal nevus
    syndromes. This is a lesion that can grow, not only a microscopic
    descriptor: one of the two patients in the topical therapy report developed
    a frank verruciform xanthoma of the vulva and inner thigh that required
    debulking.
  diagnostic: true
  finding_term:
    preferred_term: verruciform xanthoma-like change
  evidence:
  - reference: PMID:7640190
    reference_title: "Epidermal nevus syndromes."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The associated CHILD nevus shows unique features such as a diffuse form of
      lateralization, ptychotropism, and microscopic changes of verruciform
      xanthoma.
    explanation: >-
      Names the histological triad that identifies the CHILD nevus among the
      epidermal nevus syndromes, which is what makes this finding diagnostic.
  - reference: PMID:16776722
    reference_title: "Abnormal lamellar granules in a case of CHILD syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The histology shared many features with verruciform xanthoma.
    explanation: >-
      Independent case confirmation of the verruciform xanthoma resemblance.
  notes: >-
    No ontology term is bound. HP:0000991 (Xanthomatosis) is not a member of the
    HistopathologyFindingTerm enum, and the nearest NCIT candidate,
    NCIT:C221710 Cutaneous Xanthomatous Alteration, is not reachable from
    NCIT:C83490 (Histopathology Result) either. Binding instead to the generic
    hyperkeratosis or acanthosis terms would attach the label to the least
    distinctive part of the finding, so the free-text preferred_term carries it.

- name: Abnormal lamellar granules on electron microscopy
  description: >-
    Ultrastructurally the lesion is a lipid secretory failure. Vesicular
    structures appear in the intercellular spaces of the stratum corneum and
    vacuoles in the upper prickle cell layer, some recognizable as abnormal
    lamellar granules, with large vacuoles inside the foamy cells of the
    papillary dermis. This is the electron-microscopic counterpart of the
    lamellar body secretory defect modelled in the pathophysiology, observed in
    a separate patient and laboratory from the treatment study.
  finding_term:
    preferred_term: abnormal epidermal lamellar granules
  evidence:
  - reference: PMID:16776722
    reference_title: "Abnormal lamellar granules in a case of CHILD syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      These findings suggested that abnormal lipid metabolism involving lamellar
      granules may be responsible to the skin lesion of CHILD syndrome.
    explanation: >-
      States the authors' interpretation linking the ultrastructural finding to
      the lipid-metabolic lesion, preserving the hedged form they used.
  notes: >-
    No ontology term is bound. There is no NCIT ultrastructural-finding term for
    abnormal epidermal lamellar granules, and the available broader terms would
    lose the organelle-level specificity that is the entire content of the
    finding.

progression:
- phase: Postnatal improvement of cutaneous lesions
  notes: >-
    Skin lesions tend to become less inflammatory over childhood. The mechanistic
    account for this is the progressive negative selection of mutant-expressing
    cells demonstrated in the heterozygous Bpa(1H) mouse, in which the mutant
    fraction in liver fell from roughly half at postnatal day 6 to roughly a
    fifth by one year. The corresponding longitudinal measurement has not been
    made in human CHILD syndrome skin, so the human natural history is described
    clinically while the mechanism behind it is murine.
  evidence:
  - reference: PMID:19631568
    reference_title: "Developmental expression pattern of the cholesterogenic enzyme NSDHL and negative selection of NSDHL-deficient cells in the heterozygous Bpa(1H)/+ mouse."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      In the liver, the proportion of NSDHL negative cells dropped from
      approximately 50% at postnatal day 6 to approximately 20% at one year of
      age.
    explanation: >-
      Quantifies the postnatal depletion of mutant-expressing cells in the mouse.
      PARTIAL because the measurement is murine hepatic, not human cutaneous, and
      so supports the mechanism rather than the clinical course directly.

biochemical:
- name: Methylsterol accumulation in affected tissue
  presence: INCREASED
  notes: >-
    Sterol intermediates immediately above the enzymatic block accumulate in
    affected tissue. An important negative is that this is a tissue rather than a
    plasma finding: serum sterol profiles in CHILD syndrome and in the mouse
    models are near-normal, with only minimal elevation of intermediates, because
    functional mosaicism and skewed X inactivation mean most of the body outside
    the lesions is expressing the normal allele. Plasma sterol analysis therefore
    does not reliably diagnose or exclude the condition. The one exception is
    diagnostically useful in the opposite direction: a CHILD-like patient with
    raised plasma 8-dehydrocholesterol and 8(9)-cholestenol has the EBP sterol
    signature, not the NSDHL one, and should be investigated for a CDPX2-locus
    variant.
  evidence:
  - reference: PMID:24060582
    reference_title: "The role of abnormalities in the distal pathway of cholesterol synthesis in the Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects (CHILD) syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      which leads to inhibition of cholesterol synthesis and accumulation of toxic
      metabolic intermediates in affected tissues
    explanation: >-
      States the accumulation of intermediates and localizes it to affected
      tissues rather than the circulation.
  - reference: PMID:10369263
    reference_title: "The gene mutated in bare patches and striated mice encodes a novel 3beta-hydroxysteroid dehydrogenase."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Quantitative analysis of sterols from tissues of affected Bpa mice support a
      role for Nsdhl in cholesterol biosynthesis.
    explanation: >-
      Reports the tissue sterol measurements in the mouse counterpart of the
      disorder.

- name: Plasma 8-dehydrocholesterol and 8(9)-cholestenol elevation in the EBP-related minority
  presence: INCREASED
  notes: >-
    This is the EBP (CDPX2-locus) biochemical signature, not the NSDHL one, and
    it is recorded here because it distinguishes the two routes to a CHILD
    phenotype at the bench. A patient with a clinical CHILD diagnosis and this
    plasma pattern was found to carry an EBP nonsense variant. Do not treat these
    analytes as a screening test for CHILD syndrome in general; NSDHL-related
    disease does not produce them.
  evidence:
  - reference: PMID:10710233
    reference_title: "CHILD syndrome caused by deficiency of 3beta-hydroxysteroid-delta8, delta7-isomerase."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      we measured plasma sterols in a patient with CHILD syndrome and found
      levels of 8-dehydrocholesterol and 8(9)-cholestenol increased to the same
      degree as in CDPX2 patients
    explanation: >-
      Reports the plasma sterol measurement in the EBP-related patient.

diagnosis:
- name: NSDHL molecular genetic testing
  description: >-
    The diagnosis is established in a female proband by identifying a
    heterozygous NSDHL pathogenic variant. This is the definitive test, and it
    is also what makes carrier identification and prenatal or preimplantation
    testing possible in the family. Sequence analysis alone is not a sufficient
    strategy, though: multi-exon deletions and microdeletions are described as
    not rare in CHILD syndrome and are invisible to classical Sanger sequencing,
    so gene-targeted deletion and duplication analysis belongs alongside it.
    A negative sequencing result therefore means "not detected by this assay"
    rather than "not CHILD syndrome". Reported cases include a deletion of NSDHL
    exons 3 and 4, and structural variants at Xq28 spanning exon-containing
    regions have been resolved by optical genome mapping in patients whose
    conventional genetic work-up was inconclusive.
  diagnosis_term:
    preferred_term: Genetic Testing
    term:
      id: NCIT:C15709
      label: Genetic Testing
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The diagnosis of CHILD syndrome is established in a female proband with a
      heterozygous NSDHL pathogenic variant identified by molecular genetic
      testing that results in loss of functional decarboxylating
      sterol-4-alpha-carboxylate 3-dehydrogenase, the protein encoded by NSDHL.
    explanation: >-
      GeneReviews states the diagnostic criterion.
  - reference: PMID:29341259
    reference_title: "CHILD syndrome mimicking verrucous nevus in a Chinese patient responded well to the topical therapy of compound of simvastatin and cholesterol."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Multiple exons deletions or microdeletion was not rare in CHILD syndrome.
      Classical Sanger sequencing may not be useful enough to find all kinds of
      mutations. Next-generation sequencing may be more effective.
    explanation: >-
      States the assay blind spot directly, and the same report supplies the
      worked example, a large deletion of NSDHL exons 3 and 4.
  - reference: PMID:41625319
    reference_title: "Diagnostic Utility of Optical Genome Mapping in X-Linked Dominant Genodermatoses: Incontinentia Pigmenti and CHILD Syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The identified structural variants consisted of deletions of varying sizes
      in the Xq28 cytoband, encompassing regions that contain exons.
    explanation: >-
      Second source for the same gap, from the other end. The cohort was
      pediatric patients with a compatible phenotype whose conventional genetic
      studies were inconclusive, and this is what optical genome mapping then
      found in them: exon-containing deletions at Xq28, the NSDHL locus. Quoted
      from the reported result rather than from the study's stated aim.

- name: Sterol profiling of skin flakes and plasma
  description: >-
    Sterol analysis is the biochemical confirmatory test, and the tissue it is
    run on matters. In a reported case, analysis of skin flakes showed a
    mono 4-alpha methyl sterol also detectable in plasma, plus
    4-alpha-carboxy-4-methyl-cholest-8(9)-en-3beta-ol and several keto-sterols
    normally below the limit of detection, a pattern consistent with a defective
    4-alpha-methylsterol-4-demethylase complex. Its practical value is
    demonstrated by that same case: the pattern was what allowed an NSDHL
    variant of uncertain significance to be reclassified as likely pathogenic.
    Note the limits recorded in the biochemical section, though, since plasma
    sterols are frequently near-normal because functional mosaicism leaves most
    of the body expressing the wild-type allele.
  diagnosis_term:
    preferred_term: Diagnostic Procedure
    term:
      id: NCIT:C18020
      label: Diagnostic Procedure
  evidence:
  - reference: PMID:33139364
    reference_title: "Novel NSDHL gene variant for congenital hemidysplasia with ichthyosiform erythroderma and limb defects (CHILD) syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Sterol analysis from skin flakes revealed increased levels of a mono
      4-alpha methyl sterol also seen in plasma as well as the presence of
      4-alpha-carboxy-4-methyl-cholest-8(9)-en-3beta-ol and several keto-sterols,
      which are usually below the limit of detection. This sterol pattern is
      consistent with abnormal function of the 4-alpha-methylsterol-4-demethylase
      complex.
    explanation: >-
      Reports the sterol pattern and its interpretation as evidence of the
      enzymatic block.
  - reference: PMID:33139364
    reference_title: "Novel NSDHL gene variant for congenital hemidysplasia with ichthyosiform erythroderma and limb defects (CHILD) syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      On this basis, this variant has been reclassified as likely pathogenic.
    explanation: >-
      Documents the concrete diagnostic use of the sterol result, resolving a
      variant of uncertain significance.

treatments:
- name: Topical Cholesterol plus Statin (Pathogenesis-Based Therapy)
  description: >-
    Twice-daily topical application of a compounded lotion containing cholesterol
    together with an HMG-CoA reductase inhibitor (lovastatin or simvastatin) is
    the pathogenesis-directed treatment of the cutaneous disease. It addresses
    both arms of the biochemical lesion at once: cholesterol replaces the missing
    end product, and the statin shuts off flux into the pathway so the toxic
    methylsterol intermediates are not generated. The design is validated by the
    negative control within the same study, since topical cholesterol alone
    produced no improvement. Lesions clear over weeks to months, with histological
    and ultrastructural normalization, and the effect is maintained on
    reduced-frequency dosing. Hair growth and onychodystrophy do not recover.
    Note the evidence ceiling: every report is an uncontrolled case series of one
    or two patients, with the publication bias that implies, and there has been
    no trial. Systemic absorption and long-term safety of compounded topical
    statins applied to large body-surface areas in infants have not been studied.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: cholesterol
      term:
        id: CHEBI:16113
        label: cholesterol
    - preferred_term: lovastatin
      term:
        id: CHEBI:40303
        label: lovastatin
    - preferred_term: simvastatin
      term:
        id: CHEBI:9150
        label: simvastatin
  target_mechanisms:
  - target: Blocked C4-Demethylation with Cholesterol Deficiency and Methylsterol Accumulation
    treatment_effect: INHIBITS
    description: >-
      The statin component suppresses flux through HMG-CoA reductase so that
      intermediates above the NSDHL block are not generated, while the
      cholesterol component supplies the missing end product. The combination
      therefore acts on both arms of this node.
    evidence:
    - reference: PMID:21753784
      reference_title: "Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        On the basis of the putative pathogenic role of both pathway-product
        deficiency of cholesterol and accumulation of toxic metabolic
        intermediates, we assessed the efficacy of combined therapy with lovastatin
        and cholesterol.
      explanation: >-
        States the mechanistic rationale linking the two drug components to the two
        arms of the biochemical block.
  evidence:
  - reference: PMID:21753784
    reference_title: "Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Topical treatment with lovastatin/cholesterol (but not cholesterol alone)
      virtually cleared skin lesions by 3 months, accompanied by histological and
      ultrastructural normalization of epidermal structure and lipid secretion.
    explanation: >-
      Reports the therapeutic result together with the cholesterol-alone negative
      control that establishes the combination is required.
  - reference: PMID:25845514
    reference_title: "CHILD Syndrome: Successful Treatment of Skin Lesions with Topical Simvastatin/Cholesterol Ointment--A Case Report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We report the case of a 10-year-old girl treated with topical simvastatin
      and cholesterol ointment, after which her skin lesions significantly improved
      within the first 30 days of treatment.
    explanation: >-
      Independent case replication using simvastatin in place of lovastatin.
  - reference: PMID:31365666
    reference_title: "CHILD syndrome: successful treatment of skin lesions with topical lovastatin and cholesterol lotion."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The authors report a 2-month-old patient presenting with typical features of
      CHILD syndrome that was treated with a topical solution containing
      cholesterol and lovastatin, with complete clearance of her CHILD nevus.
    explanation: >-
      Further independent replication, and at the youngest reported age of
      treatment.

- name: Symptomatic Topical Care
  description: >-
    Emollients, keratolytics such as lactic acid creams for itching, urea creams
    for dryness, topical corticosteroids, and oral or topical ketoconazole have
    all been used. GeneReviews is explicit that no single therapy ameliorates the
    cutaneous findings in every affected individual, and the two patients in the
    pathogenesis-based therapy report had each failed years of emollients,
    keratolytics, retinoids, corticosteroids, calcineurin inhibitors, or
    cryotherapy before responding to cholesterol plus statin. This entry is
    retained as the historical standard of care rather than as a recommended
    first choice.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: ketoconazole
      term:
        id: CHEBI:47519
        label: ketoconazole
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In CHILD syndrome, no one therapy described to date appears to ameliorate
      the cutaneous findings for every reported individual. Oral and topical
      ketoconazole may reduce lesions.
    explanation: >-
      Documents the symptomatic options while stating the limitation that none
      works universally, hence PARTIAL.

- name: Skin Grafting from the Unaffected Side
  description: >-
    Full-thickness grafting of skin taken from the contralateral unaffected
    region onto an inflammatory nevus has been successful. The mosaic account of
    the disease predicts why: donor skin from the unaffected side is populated by
    keratinocytes and fibroblasts expressing only the wild-type allele, so the
    graft is genetically competent to build a normal barrier in its new location.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: Skin Transplantation
    term:
      id: NCIT:C15325
      label: Skin Transplantation
  target_mechanisms:
  - target: X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells
    treatment_effect: BYPASSES
    description: >-
      Grafting replaces mutant-expressing epidermis with wild-type-expressing
      epidermis, bypassing rather than correcting the mosaic distribution.
    evidence:
    - reference: PMID:21290788
      reference_title: "NSDHL-Related Disorders."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        Treatment of an inflammatory nevus by grafting skin obtained from a
        contralateral unaffected region has been successful.
      explanation: >-
        Reports the intervention and its dependence on an unaffected-side donor
        site, which is what makes it a mosaic-bypassing strategy.
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Treatment of an inflammatory nevus by grafting skin obtained from a
      contralateral unaffected region has been successful.
    explanation: >-
      GeneReviews records grafting as an effective option for a localized
      inflammatory nevus.

- name: Orthopedic and Rehabilitative Management of the Limb and Spine Defects
  description: >-
    The skeletal side of the disease is managed mechanically, not
    pharmacologically: scoliosis and joint contractures with bracing or
    corrective surgery, and severe limb hypoplasia with amputation of a
    rudimentary limb where that permits prosthetic fitting. This arm of care is
    entirely untouched by the topical therapy that clears the skin, which is
    worth stating plainly because the pathogenesis-based treatment is sometimes
    described as if it addressed the syndrome rather than its cutaneous
    component.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: Orthopedic Surgical Procedure
    term:
      id: NCIT:C16186
      label: Orthopedic Surgical Procedure
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Scoliosis and joint contractures are treated with braces and/or corrective
      surgery.
    explanation: >-
      GeneReviews states the orthopedic management of the axial and joint
      deformities.

- name: Long-Term Surveillance
  description: >-
    Because the disease is multisystem and lesions can appear after infancy,
    management includes scheduled monitoring rather than one-time assessment:
    new cutaneous lesions, musculoskeletal deformity, and neurologic, cardiac,
    and kidney manifestations are reviewed annually or as needed. This is the
    care pathway through which the visceral phenotypes curated above are
    actually detected.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Supportive Care
    term:
      id: NCIT:C15747
      label: Supportive Care
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In CHILD syndrome, monitor for new cutaneous lesions, musculoskeletal
      deformities such as scoliosis and joint contractures, and neurologic,
      cardiac, and/or kidney manifestations annually or as needed.
    explanation: >-
      GeneReviews states the surveillance schedule and the organ systems it
      covers.

- name: Genetic Counseling
  description: >-
    Counseling addresses the X-linked transmission and the distorted live-born
    ratios that follow from male lethality. Once the familial NSDHL variant is
    known, prenatal and preimplantation genetic testing are possible, and
    heterozygous female relatives can be identified.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: Genetic Counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Once the NSDHL pathogenic variant has been identified in a family member
      with an NSDHL-related disorder, prenatal and preimplantation genetic testing
      are possible.
    explanation: >-
      States the reproductive testing options that counseling is built around.

animal_models:
- name: Bare patches and striated mice (Nsdhl mutant)
  species: Mouse
  genotype: Nsdhl mutant alleles Bpa and Str, X-linked dominant male-lethal
  publication: PMID:10369263
  description: >-
    Bare patches (Bpa) and striated (Str) are allelic X-linked dominant,
    male-lethal mouse mutations in Nsdhl, and were the first mammalian locus
    associated with such a phenotype. They reproduce the male lethality, the
    cholesterol biosynthetic defect, and the mosaic negative selection of
    mutant-expressing cells. They do not reproduce the lateralization of human
    CHILD syndrome, which is the single most distinctive human feature.
  genes:
  - preferred_term: NSDHL
    term:
      id: hgnc:13398
      label: NSDHL
  evidence:
  - reference: PMID:10710235
    reference_title: "Mutations in the NSDHL gene, encoding a 3beta-hydroxysteroid dehydrogenase, cause CHILD syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      They provide animal models for the study of CHILD syndrome, a further human
      condition due to mutations in a gene of the cholesterol synthesis pathway.
    explanation: >-
      The human gene-discovery paper explicitly nominates these mouse mutants as
      models for CHILD syndrome, which is the model-level claim.
  modeled_mechanisms:
  - target: NSDHL Loss of Function
    relationship: RECAPITULATES
    fidelity: HIGH
    description: >-
      Independent Bpa and Str alleles are mutations in Nsdhl, and tissue sterol
      analysis confirms the biosynthetic role.
    limitations: >-
      The mouse skin phenotype is patterned as patches and stripes rather than the
      lateralized, midline-demarcated distribution seen in humans, so the model is
      informative for the enzymatic lesion but not for the lateralization.
    readouts:
    - name: Tissue sterol profile in affected Bpa mice
      target: NSDHL Loss of Function
      direction: ALTERED
      interpretation: >-
        Quantitative tissue sterol analysis is the biochemical correlate of the
        enzymatic block in this model.
      evidence:
      - reference: PMID:10369263
        reference_title: "The gene mutated in bare patches and striated mice encodes a novel 3beta-hydroxysteroid dehydrogenase."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Quantitative analysis of sterols from tissues of affected Bpa mice
          support a role for Nsdhl in cholesterol biosynthesis.
        explanation: >-
          Reports the sterol measurement underlying this readout.
    evidence:
    - reference: PMID:10369263
      reference_title: "The gene mutated in bare patches and striated mice encodes a novel 3beta-hydroxysteroid dehydrogenase."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Here we report mutations in one of these genes, Nsdhl, encoding an NAD(P)H
        steroid dehydrogenase-like protein, in two independent Bpa and three
        independent Str alleles.
      explanation: >-
        Establishes that these mouse mutants carry lesions in the orthologous gene.
  - target: X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Heterozygous Bpa(1H) females are mosaic for NSDHL expression through normal
      random X inactivation, and their mutant clones are progressively depleted
      after birth, giving the human clearance model a longitudinal grounding it
      cannot get from single human biopsies.
    limitations: >-
      The clonal patterning is patches and stripes rather than a body-half
      boundary, so the model supports the selection process but not the geometry.
      The quantitative depletion was measured in liver and brain rather than skin.
    readouts:
    - name: Fraction of NSDHL-negative cells in liver over the first year
      target: X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells
      direction: DECREASED
      interpretation: >-
        A falling mutant fraction with age is the direct measurement of negative
        selection against NSDHL-deficient cells.
      evidence:
      - reference: PMID:19631568
        reference_title: "Developmental expression pattern of the cholesterogenic enzyme NSDHL and negative selection of NSDHL-deficient cells in the heterozygous Bpa(1H)/+ mouse."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          In the liver, the proportion of NSDHL negative cells dropped from
          approximately 50% at postnatal day 6 to approximately 20% at one year of
          age.
        explanation: >-
          Reports the quantitative measurement behind this readout.
    evidence:
    - reference: PMID:19631568
      reference_title: "Developmental expression pattern of the cholesterogenic enzyme NSDHL and negative selection of NSDHL-deficient cells in the heterozygous Bpa(1H)/+ mouse."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Heterozygous Bpa(1H) females are mosaic for NSDHL expression due to normal
        random X-inactivation.
      explanation: >-
        Establishes that the heterozygous mouse reproduces the functional
        mosaicism that this node describes.
  - target: Placental Labyrinth Failure and Male Lethality
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Affected male embryos die in midgestation with a thin, poorly vascularized
      placental labyrinth and reduced trophoblast proliferation, and with loss of
      Hedgehog reporter activity in the affected placenta.
    limitations: >-
      Human male conceptuses are not available for study, so the placental
      mechanism of male lethality is inferred from mouse and has not been
      confirmed in human material. Rodent placental X inactivation is imprinted in
      most lineages rather than random, which is not the human arrangement, so the
      lineage logic behind the heterozygous female placental phenotype may not
      transfer.
    readouts:
    - name: Placental labyrinth thickness and fetal vessel density
      target: Placental Labyrinth Failure and Male Lethality
      direction: DECREASED
      interpretation: >-
        Structural correlate of the placental failure that accompanies male
        lethality.
      evidence:
      - reference: PMID:15639195
        reference_title: "Placental defects are associated with male lethality in bare patches and striated embryos deficient in the NAD(P)H Steroid Dehydrogenase-like (NSDHL) Enzyme."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          the labyrinth layer of the fetal placenta was always thinner, with fewer
          fetal vessels and decreased proliferation of labyrinth trophoblast cells
        explanation: >-
          Reports the histological measurement behind this readout.
    evidence:
    - reference: PMID:15639195
      reference_title: "Placental defects are associated with male lethality in bare patches and striated embryos deficient in the NAD(P)H Steroid Dehydrogenase-like (NSDHL) Enzyme."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        To begin to examine the pathogenesis of these disorders, we have determined
        that affected male embryos for several Nsdhl alleles die in midgestation,
        between E10.5 and 13.5
      explanation: >-
        Supports treating this model as informative for the timing and nature of
        male lethality.

- name: Conditional Nsdhl ablation in radial glia
  species: Mouse
  genotype: Nsdhl(tm1.1Hrm) conditional allele, GFAP-cre driver
  publication: PMID:25652406
  description: >-
    Because hemizygous Nsdhl mutant males die by midgestation, a conditional
    allele was needed to reach postnatal biology. Ablating Nsdhl in radial glia
    produced live-born males that developed ataxia by postnatal day 8 to 10, with
    loss of cortical and hippocampal neurons and defective proliferation and
    migration of cerebellar granule precursors followed by massive apoptosis. The
    proliferation defect was traced to defective SHH signalling and was almost
    completely rescued by exogenous cholesterol.
  genes:
  - preferred_term: NSDHL
    term:
      id: hgnc:13398
      label: NSDHL
  evidence:
  - reference: PMID:25652406
    reference_title: "Analysis of hedgehog signaling in cerebellar granule cell precursors in a conditional Nsdhl allele demonstrates an essential role for cholesterol in postnatal CNS development."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Since hemizygous male mice with Nsdhl mutations die by midgestation, we
      generated a conditional targeted Nsdhl mutation (Nsdhl(tm1.1Hrm)) to
      investigate the essential role of cholesterol in the early postnatal CNS.
    explanation: >-
      States why this conditional model exists and what it is a model of, which is
      the model-level claim.
  modeled_mechanisms:
  - target: Impaired Hedgehog Signal Transduction
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Establishes the causal chain from NSDHL loss to defective SHH signalling and
      shows that cholesterol supply is the limiting factor for it.
    limitations: >-
      The tissue is CNS rather than skin or limb, and the phenotype it produces is
      the CK syndrome-like neurological picture rather than the CHILD phenotype, so
      the Hedgehog result transfers as a mechanism while the phenotype does not.
      Conditional ablation in one lineage is also not equivalent to the mosaic
      heterozygous state of human CHILD syndrome.
    readouts:
    - name: Cerebellar granule cell precursor proliferation
      target: Impaired Hedgehog Signal Transduction
      direction: RESTORED
      interpretation: >-
        Rescue by exogenous cholesterol identifies cholesterol availability, rather
        than a separate consequence of the mutation, as the proximate cause of the
        Hedgehog signalling failure.
      evidence:
      - reference: PMID:25652406
        reference_title: "Analysis of hedgehog signaling in cerebellar granule cell precursors in a conditional Nsdhl allele demonstrates an essential role for cholesterol in postnatal CNS development."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          Furthermore, this defect is almost completely rescued by supplementation
          of the culture media with exogenous cholesterol
        explanation: >-
          Reports the rescue experiment behind this readout.
    evidence:
    - reference: PMID:25652406
      reference_title: "Analysis of hedgehog signaling in cerebellar granule cell precursors in a conditional Nsdhl allele demonstrates an essential role for cholesterol in postnatal CNS development."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        We replicated the granule cell precursor proliferation defect in vitro and
        demonstrate that it results from defective signaling by SHH.
      explanation: >-
        Supports treating this model as informative for the Hedgehog node.

- name: Labrador Retriever NSDHL deletion
  species: Dog
  genotype: Heterozygous deletion removing the last three exons of NSDHL
  publication: PMID:28739597
  description: >-
    A naturally occurring canine counterpart. A female Labrador Retriever and her
    crossbred daughter carried a large heterozygous NSDHL deletion and showed
    cornification-disorder lesions following Blaschko lines; both male littermates
    of the affected daughter died at birth, consistent with X-linked semidominant
    male lethality. The value of this model is that it is spontaneous and shows
    the mosaic cutaneous patterning in a large outbred mammal.
  genes:
  - preferred_term: NSDHL
    term:
      id: hgnc:13398
      label: NSDHL
  evidence:
  - reference: PMID:28739597
    reference_title: "A Large Deletion in the NSDHL Gene in Labrador Retrievers with a Congenital Cornification Disorder."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Variants in the NSDHL gene cause CHILD syndrome in humans, and the bare
      patches (Bpa) and striated (Str) phenotypes in mice.
    explanation: >-
      The authors situate their canine finding against the human disease and the
      established mouse models, which is what makes this dog a model for CHILD
      syndrome rather than an isolated veterinary case.
  modeled_mechanisms:
  - target: X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Reproduces the X-linked mosaic cutaneous patterning in a heterozygous female
      carrier of an NSDHL loss-of-function allele.
    limitations: >-
      The canine lesions follow Blaschko lines rather than showing the strict
      midline demarcation that defines human CHILD syndrome, so the model captures
      functional X-inactivation mosaicism but not the lateralization that is the
      open question. It also carries no longitudinal measurement of clone
      depletion, so it does not speak to the negative-selection arm of this node.
      Causality was inferred from genome sequencing and the known role of NSDHL
      rather than demonstrated functionally.
    readouts:
    - name: Blaschko-linear cutaneous lesion distribution
      target: X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells
      direction: ALTERED
      interpretation: >-
        The patterned lesion distribution is the visible readout of functional
        X-inactivation mosaicism in the epidermis.
      evidence:
      - reference: PMID:28739597
        reference_title: "A Large Deletion in the NSDHL Gene in Labrador Retrievers with a Congenital Cornification Disorder."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: >-
          We investigated a female Labrador Retriever and her crossbred daughter,
          which both showed similar clinical lesions that followed Blaschko's lines.
        explanation: >-
          Reports the lesion distribution behind this readout.
    evidence:
    - reference: PMID:28739597
      reference_title: "A Large Deletion in the NSDHL Gene in Labrador Retrievers with a Congenital Cornification Disorder."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Taken together, our genetic data and the known role of NSDHL in X-linked
        skin disorders strongly suggest that the identified structural variant in
        the NSDHL gene is causative for the phenotype in the two affected dogs.
      explanation: >-
        Supports treating the canine phenotype as an NSDHL-driven model, with the
        authors' own hedged causal language preserved.

experimental_models:
- name: Heterologous expression of NSDHL missense mutants
  experimental_model_type: CELL_LINE
  publication: PMID:40222685
  description: >-
    Thirteen disease-causing NSDHL missense mutations and one deletion mutation
    were expressed and assayed for protein level. All showed low expression;
    some were partially rescued by culture at 30°C rather than 37°C or by the
    chemical chaperone glycerol, and optimal expression required heat shock
    proteins 70 and 90. This reframes NSDHL missense disease as substantially a
    protein stability and folding problem, which is what makes chaperone-directed
    approaches conceivable.
  evidence:
  - reference: PMID:40222685
    reference_title: "Comprehensive survey of disease-causing missense mutations of the cholesterol synthesis enzyme NSDHL: Low temperature and a chemical chaperone rescue low protein expression of select mutants."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We examined 13 missense mutations and one deletion mutation in the
      cholesterol synthesis enzyme NSDHL (NAD(P) Dependent Steroid
      Dehydrogenase-Like), known to cause the X-linked developmental disorders
      CHILD (congenital hemidysplasia with ichthyosiform erythroderma and limb
      defects) syndrome and CK syndrome.
    explanation: >-
      States that the alleles assayed in this system are the ones that cause CHILD
      syndrome, which is the model-level relevance claim.
  modeled_mechanisms:
  - target: NSDHL Loss of Function
    relationship: MEASURES
    fidelity: MODERATE
    description: >-
      Quantifies how disease alleles reduce NSDHL protein abundance and identifies
      a rescuable folding component.
    limitations: >-
      Heterologous overexpression in cultured cells does not report on endogenous
      expression in keratinocytes or on the mosaic tissue context, and a
      temperature or glycerol rescue in culture is not evidence that any
      clinically usable chaperone therapy exists.
    readouts:
    - name: NSDHL mutant protein expression level
      target: NSDHL Loss of Function
      direction: DECREASED
      interpretation: >-
        Low steady-state protein for every disease allele tested indicates loss of
        function operates substantially through protein instability.
      evidence:
      - reference: PMID:40222685
        reference_title: "Comprehensive survey of disease-causing missense mutations of the cholesterol synthesis enzyme NSDHL: Low temperature and a chemical chaperone rescue low protein expression of select mutants."
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: >-
          Here we show that protein expression levels were low for all mutants, but
          some could be rescued by a lower temperature (30°C vs. 37°C) and/or the
          chemical chaperone glycerol.
        explanation: >-
          Reports the expression measurement behind this readout.
    evidence:
    - reference: PMID:40222685
      reference_title: "Comprehensive survey of disease-causing missense mutations of the cholesterol synthesis enzyme NSDHL: Low temperature and a chemical chaperone rescue low protein expression of select mutants."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: >-
        Additionally, heat shock proteins 70 and 90 are needed for optimal NSDHL
        protein expression suggesting that disease mutations in NSDHL may interfere
        with this interaction
      explanation: >-
        Supports treating this system as informative about the molecular basis of
        NSDHL loss of function.

differential_diagnoses:
- name: X-linked chondrodysplasia punctata 2 (Conradi-Hunermann-Happle syndrome)
  disease_term:
    preferred_term: X-linked chondrodysplasia punctata 2
    term:
      id: MONDO:0020603
      label: X-linked chondrodysplasia punctata 2
  description: >-
    The closest mechanistic neighbour and the most important differential. CDPX2
    is caused by variants in EBP, a delta8-delta7 sterol isomerase acting
    downstream of NSDHL in the same post-squalene pathway, and it is likewise an
    X-linked dominant, male-lethal disorder with patterned ichthyotic skin and
    bone malformations. The relationship between the two entities is not fully
    settled: the NSDHL gene-discovery paper found EBP unaffected in its CHILD
    cohort and concluded they are not allelic, while a paper published alongside
    it identified an EBP nonsense variant in a patient carrying a clinical CHILD
    diagnosis.
  distinguishing_features:
  - CDPX2 skin lesions and skeletal defects are bilateral and asymmetric, without the strict lateralization and midline demarcation of CHILD syndrome
  - CDPX2 is caused by EBP variants acting one step downstream of NSDHL in the same pathway
  - CDPX2 carries a plasma sterol signature of raised 8-dehydrocholesterol and 8(9)-cholestenol that NSDHL-related CHILD syndrome does not
  evidence:
  - reference: PMID:10710235
    reference_title: "Mutations in the NSDHL gene, encoding a 3beta-hydroxysteroid dehydrogenase, cause CHILD syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      EBP was unaffected in the patients analyzed by us demonstrating that CHILD
      syndrome and CDPX2 are not caused by allelic mutations.
    explanation: >-
      Establishes that in this cohort the two are genetically distinct despite
      their shared pathway and overlapping clinical picture.
  - reference: PMID:10710233
    reference_title: "CHILD syndrome caused by deficiency of 3beta-hydroxysteroid-delta8, delta7-isomerase."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      although the skeletal defects and skin lesions in CDPX2 are bilateral and
      asymmetric
    explanation: >-
      States the clinical distinction in lesion distribution that separates the
      two on examination.

- name: Inflammatory linear verrucous epidermal nevus (ILVEN)
  disease_term:
    preferred_term: inflammatory linear verrucous epidermal nevus
    term:
      id: MONDO:0019318
      label: inflammatory linear verrucous epidermal nevus
  description: >-
    The dermatologic mimic, as opposed to the mechanistic neighbours above. The
    CHILD nevus is an inflammatory verrucous lesion with psoriasiform
    hyperkeratosis and loss of the granular layer, and ILVEN looks like that
    too, so the differential is made on the systemic findings rather than on the
    skin. It is a real trap in both directions: a boy with widespread
    right-sided inflammatory lesions was worked up as possible CHILD syndrome
    before being confirmed as severe ILVEN, and separately a genuine CHILD
    patient was described as mimicking verrucous nevus. The hard part is that
    the ILVEN case also had an ipsilateral limb contracture and short stature,
    so even the extracutaneous findings are not cleanly discriminating.
  distinguishing_features:
  - ILVEN has no NSDHL variant and no sterol pathway biochemical signature, which is the definitive separator
  - ILVEN lesions follow Blaschko lines rather than showing the strict midline demarcation typical of the CHILD nevus
  - ILVEN lacks the ipsilateral skeletal hypoplasia or aplasia of CHILD syndrome, though an ipsilateral limb contracture has been reported in ILVEN, so limb involvement alone does not settle it
  - Male sex argues against CHILD syndrome, which is usually male lethal, and the reported ILVEN mimic was a boy
  evidence:
  - reference: PMID:39953436
    reference_title: "Unilateral widespread inflammatory linear verrucous epidermal nevus with ipsilateral limb contracture along the lines of Blaschko: A case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Initially, we suspected this may be a rare case of CHILD syndrome in a boy.
      However, after further investigation, we confirmed the diagnosis as a
      severe form of ILVEN.
    explanation: >-
      A worked case of the differential actually being made, with CHILD syndrome
      as the initial working diagnosis and ILVEN as the resolution.
  - reference: PMID:39953436
    reference_title: "Unilateral widespread inflammatory linear verrucous epidermal nevus with ipsilateral limb contracture along the lines of Blaschko: A case report."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      He also had an associated contracture in the ipsilateral limb and short
      stature.
    explanation: >-
      Documents that the ILVEN case carried ipsilateral limb findings too, which
      is why extracutaneous involvement does not by itself distinguish the two.

- name: CK syndrome
  description: >-
    The allelic NSDHL disorder in males. CK syndrome arises from hypomorphic
    NSDHL variants retaining partial enzyme activity, and presents with cognitive
    impairment, behavioural problems, infantile-onset seizures, cerebral cortical
    malformation, and microcephaly rather than with the cutaneous and limb
    phenotype of CHILD syndrome. Same gene, opposite sex distribution, and an
    essentially non-overlapping clinical picture.
  distinguishing_features:
  - CK syndrome affects males hemizygous for a hypomorphic allele; CHILD syndrome affects heterozygous females and is male lethal
  - CK syndrome is dominated by cognitive impairment, seizures, and cortical malformation, whereas intellect is usually normal in CHILD syndrome
  - CK syndrome lacks the lateralized ichthyosiform nevus and ipsilateral limb defects
  evidence:
  - reference: PMID:21290788
    reference_title: "NSDHL-Related Disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The diagnosis of CK syndrome is established in a male proband with a
      hemizygous NSDHL hypomorphic pathogenic variant identified by molecular
      genetic testing that results in partial loss of functional decarboxylating
      sterol-4-alpha-carboxylate 3-dehydrogenase.
    explanation: >-
      States the allelic relationship and the partial-loss-of-function basis that
      separates CK syndrome from CHILD syndrome.

discussions:
- discussion_id: lateralization_mechanism_unknown
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - pathophysiology#X-Inactivation Mosaicism and Negative Selection of Mutant-Expressing Cells
  prompt: >-
    What mechanism converts random X inactivation into a body-half boundary in
    CHILD syndrome, rather than the finer Blaschko-line mosaicism seen in other
    X-linked genodermatoses?
  rationale: >-
    The observation that lesional keratinocytes express only the mutant allele and
    contralateral keratinocytes only the wild-type allele explains why affected
    skin is affected, but it does not explain the geometry. Negative selection of
    mutant-expressing cells accounts for their depletion, not for its confinement
    to one side of the midline. The authors who established the clearance model
    describe the lateralization as poorly understood, and no available animal
    system reproduces it: both mouse models pattern as patches or stripes, and
    the canine model follows Blaschko lines. This disease would be an unusually
    good subject for single-cell or spatial methods, which have not been applied
    to it, because the mosaic boundary is sharp and the selecting phenotype is
    cell-autonomous.
  proposed_experiments:
  - experiment_id: lateralization_lineage_mapping
    name: Lineage and allele-expression mapping across the midline in CHILD skin
    description: >-
      Single-cell or spatial transcriptomic mapping of NSDHL allele expression
      across the midline boundary in affected individuals, paired with clonal
      lineage inference, to determine whether the boundary reflects an early
      embryonic left-right lineage restriction, a later selective clearance front,
      or an unrecognized signalling boundary.
  evidence:
  - reference: PMID:21753784
    reference_title: "Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We also evaluated the basis for the poorly understood, unique lateralization
      of the cutaneous and bone malformations of CHILD syndrome by analyzing gene
      activation in abnormal and unaffected skin.
    explanation: >-
      The authors describe the lateralization as poorly understood at the outset of
      the study that produced the current best account of it.

- discussion_id: male_lethality_placental_mechanism_human
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Placental Labyrinth Failure and Male Lethality
  prompt: >-
    Does the placental labyrinth mechanism established for male lethality in
    Nsdhl mutant mice hold for human male conceptuses with NSDHL loss of function?
  rationale: >-
    The placental account of male lethality rests entirely on mouse work, in which
    affected male embryos die in midgestation with a thin, poorly vascularized
    labyrinth while showing no consistent embryonic anomaly. Extrapolating to
    humans is not straightforward: X inactivation in most rodent placental
    lineages is imprinted with preferential paternal inactivation, which is not the
    human arrangement, and the reasoning that heterozygous females should
    phenocopy affected males in extraembryonic tissue depends on that difference.
    Human material is not wholly unavailable: a maternally inherited NSDHL
    variant has been identified by exome sequencing in a male conceptus lost at
    ten weeks, confirming that the lethality is real and NSDHL-linked in humans.
    What that report does not supply is any placental histology or Hedgehog
    readout, so it establishes the outcome while leaving the murine mechanism
    for it untested in humans.
  proposed_experiments:
  - experiment_id: human_nsdhl_trophoblast_model
    name: NSDHL-null human trophoblast organoid vascularization assay
    description: >-
      Generate NSDHL loss-of-function human trophoblast stem cell or placental
      organoid models and assay Hedgehog pathway activity, trophoblast
      proliferation, and vascular network formation, to test whether the murine
      labyrinth phenotype has a human counterpart independent of rodent-specific
      placental X inactivation.
  evidence:
  - reference: PMID:15639195
    reference_title: "Placental defects are associated with male lethality in bare patches and striated embryos deficient in the NAD(P)H Steroid Dehydrogenase-like (NSDHL) Enzyme."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      X-inactivation is non-random in females in most lineages of the rodent
      placenta with preferential inactivation of the paternal X chromosome.
    explanation: >-
      States the rodent-specific placental X-inactivation arrangement that the
      mouse inference depends on and that does not transfer to humans.
  - reference: PMID:36504312
    reference_title: "Etiological identification of recurrent male fatality due to a novel NSDHL gene mutation using trio whole-exome sequencing: A rare case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A 33-year-old pregnant woman with recurrent spontaneous abortion was
      experiencing her third pregnancy with a male embryo. In this pregnancy, a
      miscarriage occurred at a gestational age of 10+6  weeks with no copy
      number variants.
    explanation: >-
      Establishes the human male loss and its timing, which is the outcome side
      of the mismatch. PARTIAL because the report supplies no placental
      histology or Hedgehog readout, so it cannot confirm or refute the murine
      labyrinth mechanism that is the actual open question.

- discussion_id: nsdhl_versus_ebp_nosological_boundary
  kind: KNOWLEDGE_GAP
  status: OPEN
  attaches_to:
  - genetic#EBP
  prompt: >-
    Is "CHILD syndrome" one disease with two loci, or is the EBP-related case a
    CDPX2 variant that was clinically misassigned?
  rationale: >-
    Two papers published in the same journal issue in 2000 reach opposite-facing
    conclusions. One identified NSDHL variants in six CHILD patients and reported
    EBP unaffected in all of them, concluding the disorders are not allelic. The
    other measured plasma sterols in a CHILD patient, found the CDPX2 signature,
    identified an EBP nonsense variant, and speculated that at least some CHILD
    cases are allelic with CDPX2. Both observations can stand; what is unresolved
    is whether a CHILD phenotype arising from EBP should be curated as CHILD
    syndrome with locus heterogeneity or as an unusually lateralized CDPX2. This
    is a lump-versus-split decision affecting how the entity boundary is drawn,
    and it has not been settled by any subsequent cohort. The practical
    consequence is diagnostic: plasma sterol analysis separates the two routes
    even though it is uninformative within NSDHL-related disease.
  proposed_experiments:
  - experiment_id: child_locus_reassessment_cohort
    name: Genotype and sterol reassessment of the assembled CHILD case literature
    description: >-
      Assemble the published CHILD syndrome cases with molecular data, and where
      material allows, determine NSDHL versus EBP genotype together with plasma
      sterol profile and lesion distribution, to establish whether EBP-related
      cases form a clinically separable subgroup or are distributed through the
      CHILD phenotype.
  evidence:
  - reference: PMID:10710233
    reference_title: "CHILD syndrome caused by deficiency of 3beta-hydroxysteroid-delta8, delta7-isomerase."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We speculate that at least some cases of CHILD syndrome are allelic with
      CDPX2 caused by 3beta-hydroxysteroid-delta8,delta7-isomerase deficiency.
    explanation: >-
      The authors' own hedged phrasing is the open question this discussion
      records.

notes: >-
  Four scope decisions. First, this entry deliberately does not declare
  conformance to the Hedgehog node of the ciliopathy_dysfunction module: that
  module's chain runs from a basal body, transition zone, or intraflagellar
  transport lesion, and CHILD syndrome reaches the same pathway by a sterol
  route with no primary ciliary defect. Second, CK syndrome is modeled as a
  differential diagnosis rather than a subtype. Although CK syndrome is allelic,
  its clinical picture is essentially non-overlapping with CHILD syndrome and it
  affects the opposite sex, so treating it as a subtype of this entry would
  merge two distinct disease entities under one pathograph. Third, prevalence is
  recorded only as a CASES_IN_LITERATURE / ULTRA_RARE band with no rate. The
  figure of one in 100,000 circulates in review literature but is inconsistent
  with a total published case count in the low hundreds, and there is no cohort,
  registry, or natural-history study from which a defensible rate could be
  derived; for the same reason most phenotypes here carry no frequency value.
  Fourth, no datasets are
  curated. GEO discovery returned twelve candidates, all GENE_ONLY and all
  spurious: every one matched the string EBP against the unrelated CCAAT
  enhancer-binding protein (C/EBP) family rather than the sterol isomerase, and
  none concerned NSDHL or CHILD syndrome. Recording the absence here so the
  search is not repeated and so the near-miss is not mistaken for coverage.
  Fifth, four further lines from the research report were considered and
  declined, recorded here so they are not repeatedly re-proposed.
  Gastrointestinal and colonic xanthomas (PMID:39466221, PMID:40517742) are a
  real and newly recognized part of the adult phenotype, but both cache with no
  retrievable content, so there is no quotable snippet and curating them would
  mean asserting a phenotype on an uncheckable citation. Greenberg dysplasia
  (PMID:32304187) is a different sterol-pathway disorder (LBR) whose overlap
  with CHILD syndrome is at the level of the pathway rather than the
  presentation, so it does not earn a differential entry alongside CDPX2 and
  ILVEN. The additional topical statin case reports (PMID:34787337,
  PMID:39431262, PMID:40464756, PMID:42083494) are redundant with the three
  already cited on that treatment and would add citation count without adding
  evidential weight.

📚

References & Deep Research

References

3

PMID:21290788

NSDHL-Related Disorders.

No top-level findings curated for this source.

PMID:24060582

The role of abnormalities in the distal pathway of cholesterol synthesis in the Congenital Hemidysplasia with Ichthyosiform erythroderma and Limb Defects (CHILD) syndrome.

No top-level findings curated for this source.

PMID:21753784

Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism.

No top-level findings curated for this source.

Deep Research

1

Claude Code ▸

CHILD Syndrome — Comprehensive Research Report

claude-haiku-4-5-20251001, claude-opus-5[1m] 38 citations 2026-08-13T21:43:09.533274

CHILD Syndrome — Comprehensive Research Report

Congenital Hemidysplasia with Ichthyosiform erythroderma/nevus and Limb Defects

Prepared 13 August 2026. Evidence sources labeled as HUMAN_CLINICAL / MODEL_ORGANISM / IN_VITRO / COMPUTATIONAL throughout. Quoted text marked with quotation marks is verbatim from the cited abstract or resource; unquoted text is synthesis.

1. Disease Information

Overview

CHILD syndrome is a rare X-linked dominant, male-lethal inborn error of the distal (post-squalene) cholesterol biosynthesis pathway, caused by loss-of-function variants in NSDHL at Xq28. Its defining feature is not simply a skin disease but a strikingly lateralized developmental field defect: an inflammatory ichthyosiform nevus with sharp midline demarcation on one side of the body, accompanied by ipsilateral hypoplasia of limbs, skeleton, and viscera. The lateralization is a direct visible readout of random X-chromosome inactivation combined with negative selection against mutant-expressing cells — a rare instance where a mosaic pattern is essentially the disease definition.

The original clinical delineation (HUMAN_CLINICAL):

"The term CHILD syndrome is proposed as an acronym for congenital hemidysplasia with ichthyosiform erythroderma and limb defects. The syndrome is characterized by unilateral erythema and scaling, with a distinct demarcation in the middle of the trunk. The dermatosis is either present at birth or develops during the first weeks of life. Ipsilateral limb defects may vary from hypoplasia of some fingers to complete absence of an extremity." — Happle R, Koch H, Lenz W. Eur J Pediatr. 1980;134(1):27-33. PMID:7408908

Orphanet's definition (aggregated disease-level resource):

"A rare developmental defect during embryogenesis characterized by unilateral inflammatory and scaling skin lesions with ipsilateral visceral and limb anomalies." — Orphanet ORPHA:139, definition retrieved 2026-07-02 via the Orphanet API (https://api.orphacode.org)

Key identifiers

Resource	Identifier	Notes
MONDO	MONDO:0010621	label "CHILD syndrome"; verified via EBI OLS4
OMIM	308050	CONGENITAL HEMIDYSPLASIA WITH ICHTHYOSIFORM ERYTHRODERMA AND LIMB DEFECTS
Orphanet	ORPHA:139
MeSH	C562515	supplementary concept record
UMLS	C0265267
MedGen	82697
SNOMED CT	17608003
DOID	DOID:0111822
GARD	0006039
NORD	1284
ICD-9-CM	759.89	MONDO `relatedTo` mapping
ICD-10	Commonly coded Q82.8 ("other specified congenital malformations of skin")	⚠️ Not verifiable from an authoritative API in this session — verify before curation
ICD-11	Not confirmed	⚠️ No authoritative mapping retrieved — do not curate an ICD-11 code without checking the WHO browser
Gene: HGNC	HGNC:13398 (`hgnc:13398` in dismech lowercase convention)	Symbol NSDHL; "NAD(P) dependent 3-beta-hydroxysteroid dehydrogenase NSDHL"
Gene: OMIM	300275	NSDHL
Gene: NCBI Gene	50814
Gene: Ensembl	ENSG00000147383
Protein: UniProt	Q15738	"Sterol-4-alpha-carboxylate 3-dehydrogenase, decarboxylating"; EC 1.1.1.170
RefSeq transcript	NM_015922

Source for MONDO xrefs, HGNC record, and UniProt fields: EBI OLS4 API, genenames.org REST API, UniProt REST API (all queried 2026-08-13).

Synonyms and alternative names

CHILD syndrome (the standard clinical name)
Congenital hemidysplasia with ichthyosiform nevus and limb defects — this is the preferred modern expansion; GeneReviews and much of the recent literature has moved away from "erythroderma," since the lesion is a nevus rather than a true erythroderma
Congenital hemidysplasia with ichthyosiform erythroderma and limb defects (the original 1980 expansion, retained by OMIM)
CHILD syndrome, X-linked dominant
CHILD nevus (referring to the cutaneous lesion specifically)
Ichthyosis, CHILD syndrome
Unilateral ichthyosiform erythroderma with ipsilateral malformations (historical descriptive term)

Synonym list from MONDO:0010621 (OLS4).

Nomenclature caveat worth curating: a 2026 report explicitly argues the name is misleading, describing "CHILD Syndrome without Limb Defects in a 1-year-old: revised nomenclature and successful treatment with Topical 2% Simvastatin/2% Cholesterol" (Wyer J, Moss C, Poudel P, Ibbs S. Clin Exp Dermatol. 2026 May 4:llag193. PMID:42082353). Limb defects are common but not obligate.

Data provenance

Essentially all knowledge of CHILD syndrome derives from aggregated case reports and small case series, not from EHR cohorts or registries. GeneReviews states plainly: "More than 60 individuals have been reported to date." (Kurban M, El Feghaly J, Hamie L. NSDHL-Related Disorders. GeneReviews®, initially published 1 Feb 2011, last update 5 Sep 2024, Bookshelf NBK51754, PMID:21290788). There is no natural-history study, no patient registry with published outcomes, and no population-based prevalence estimate that I could verify. Every frequency figure below should be treated as case-report-derived and subject to ascertainment bias — the severe end of the spectrum is over-represented, and the recent discovery of mild adult phenotypes presenting only as gastrointestinal xanthomas (PMID:39466221, PMID:40517742) suggests substantial under-ascertainment of mild disease.

2. Etiology

Primary causal factor

Germline (or, rarely, post-zygotic somatic) loss-of-function variants in NSDHL (Xq28), encoding decarboxylating sterol-4-alpha-carboxylate 3-dehydrogenase, an enzyme of the C4-demethylation complex in post-squalene cholesterol biosynthesis.

The landmark gene-identification study (HUMAN_CLINICAL):

"We report for the first time that CHILD syndrome (MIM 308050), an X-linked dominant, male-lethal trait characterized by an inflammatory nevus with striking lateralization and strict midline demarcation, as well as ipsilateral hypoplasia of the body is caused by mutations in the gene NSDHL located at Xq28 (NAD(P)H steroid dehydrogenase-like protein) encoding a 3beta-hydroxysteroid dehydrogenase functioning in the cholesterol biosynthetic pathway." — König A, Happle R, Bornholdt D, Engel H, Grzeschik KH. Am J Med Genet. 2000;90(4):339-46. PMID:10710235

GeneReviews specifies the mechanism as loss of enzyme function: the diagnosis rests on "a heterozygous NSDHL pathogenic variant identified by molecular genetic testing that results in loss of functional decarboxylating sterol-4-alpha-carboxylate 3-dehydrogenase" (PMID:21290788).

A genetically distinct minority: EBP-related CHILD-like phenotype

A small subset of clinically diagnosed CHILD syndrome is not NSDHL-related but caused by deficiency of the downstream enzyme 3β-hydroxysteroid-Δ8,Δ7-isomerase (EBP, the CDPX2 gene) (HUMAN_CLINICAL):

"Because CDPX2 patients have abnormal 8-dehydrosterol metabolism caused by mutations in 3beta-hydroxysteroid-delta8,delta7-isomerase, we measured plasma sterols in a patient with CHILD syndrome and found levels of 8-dehydrocholesterol and 8(9)-cholestenol increased to the same degree as in CDPX2 patients. Subsequently, we identified a nonsense mutation in exon 3 of the patient's 3beta-hydroxysteroid-delta8,delta7-isomerase gene. We speculate that at least some cases of CHILD syndrome are allelic with CDPX2 caused by 3beta-hydroxysteroid-delta8,delta7-isomerase deficiency." — Grange DK, Kratz LE, Braverman NE, Kelley RI. Am J Med Genet. 2000;90(4):328-35. PMID:10710233

This is a genuine locus heterogeneity finding and should be curated as such (the HPO annotation set for OMIM:308050 includes HP:0003462 Elevated 8-dehydrocholesterol and HP:0003465 Elevated 8(9)-cholestenol, which are the EBP biochemical signature rather than the NSDHL one — a subtlety worth noting when curating biomarker blocks).

Risk factors

Genetic risk factors - The causal NSDHL variant is essentially the whole risk story. There are no reported susceptibility loci, modifier genes, or polygenic contributions to CHILD syndrome. This is a fully penetrant Mendelian disorder in heterozygous females (see §9). - De novo occurrence is the norm. Most cases are sporadic; GeneReviews notes familial transmission is documented (König 2000 reported a mother–daughter pair, PMID:10710235) but is uncommon. - Maternal carriage is the one true "family-history" risk factor: a heterozygous mother has a 50% transmission risk per pregnancy, with recurrent male miscarriage as a family-history clue (see §9 and PMID:36504312). - The degree/pattern of X-inactivation is the closest thing to a modifier — it determines laterality, extent, and (in the exceptional bilateral cases) symmetry. König et al. 2002 concluded: "Apparently, the effect of random X-inactivation is responsible for different patterns of cutaneous involvement in female carriers of NSDHL mutations." (PMID:11907515)

Environmental risk factors - None identified. No toxin, drug, infection, maternal exposure, occupational hazard, or lifestyle factor has been reported as contributing to CHILD syndrome onset. Age, sex (see §9 — sex is a determinant of viability, not of risk), and family history are the only relevant demographic variables. - ⚠️ Do not curate any environmental factor for disease causation. There is, however, an interesting temperature angle under gene–environment interaction, below.

Protective factors

No protective variants or alleles have been reported.
The one biologically meaningful "protective" phenomenon is cell-intrinsic selection: in the mosaic heterozygote, wild-type-expressing cells progressively outcompete and replace mutant-expressing cells in many tissues, effectively protecting the contralateral side and improving the skin over time (see §6 and PMID:19631568, PMID:21753784).
No dietary, nutritional, or lifestyle protective factor has been demonstrated. Notably, dietary cholesterol supplementation is not effective for the skin — the defect is in local cutaneous synthesis behind the epidermal permeability barrier, and serum cholesterol is typically normal.

Gene–environment interactions

Two are worth curating, both from the mechanistic literature rather than epidemiology:

Temperature sensitivity of mutant NSDHL protein (IN_VITRO). Several disease-causing missense variants are conformationally unstable and their expression is temperature-dependent:

"Here we show that protein expression levels were low for all mutants, but some could be rescued by a lower temperature (30°C vs. 37°C) and/or the chemical chaperone glycerol. Additionally, heat shock proteins 70 and 90 are needed for optimal NSDHL protein expression suggesting that disease mutations in NSDHL may interfere with this interaction, perhaps during translation resulting in lower protein synthesis." — Fenton NM, Sharpe LJ, Fitzsimmons DM, Capell-Hattam IM, Brown AJ. J Steroid Biochem Mol Biol. 2025;251:106758. PMID:40222685

The allelic CK syndrome variants were likewise shown to "alter protein folding, show temperature-sensitive protein stability" (PMID:21129721). This creates a genuine, if speculative, gene–environment axis (febrile illness, skin surface temperature) and — more importantly — a pharmacological chaperone therapeutic hypothesis.

Drug–pathway interaction as therapy. Topical HMG-CoA reductase inhibitors (statins) and topical ketoconazole (a CYP51A1/lanosterol 14α-demethylase inhibitor) deliberately manipulate the pathway upstream of the block — a therapeutic gene–environment interaction (see §12).

3. Phenotypes

Frequency caveat (important for KB curation): CHILD syndrome has no published cohort with denominators. Frequency estimates below come from StatPearls' synthesis of the case literature, HPO's curated annotation set for OMIM:308050 (which carries no frequency modifiers), and GeneReviews' qualitative language ("common," "usually," "can occur"). Per the dismech frequency-evidence SOP, most of these should be curated without a frequency: value, or with only the coarse qualitative bands the source actually supports.

3.1 Cutaneous phenotypes (the defining domain)

Feature	HPO suggestion	Onset	Course	Frequency evidence
Congenital ichthyosiform erythroderma (the CHILD nevus)	HP:0007431 Congenital ichthyosiform erythroderma	Birth to first weeks–months of life	Erythema decreases and hyperkeratosis increases through infancy; often improves with age; new lesions can arise in puberty/adulthood	Essentially obligate (definitional)
Epidermal nevus / nevus	HP:0010816 Epidermal nevus; HP:0003764 Nevus	Congenital	Persistent	Definitional
Hyperkeratosis	HP:0000962 Hyperkeratosis	Congenital/infancy	Increases relative to erythema over infancy	"Hyperkeratosis is seen in 30% to 79% of patients" (StatPearls, NBK507813)
Parakeratosis	HP:0001036 Parakeratosis	—	—	Histologic, characteristic
Orthokeratosis (alternating with parakeratosis)	HP:0040162 Orthokeratosis	—	—	Histologic
Epidermal acanthosis	HP:0025092 Epidermal acanthosis	—	—	Histologic
Erythema	HP:0010783 Erythema	Congenital	Wanes with age	Common
Nail dystrophy / onychodystrophy, onychorrhexis, periungual hyperkeratosis	HP:0008404 Nail dystrophy; HP:0001792 Small nail	Congenital/infancy	Persistent	GeneReviews: "Onychodystrophy, onychorrhexis, and periungual hyperkeratosis are common" (PMID:21290788)
Scarring alopecia (ipsilateral scalp)	HP:0001596 Alopecia	Congenital	Persistent, non-regrowing	Reported, frequency unknown

Distinctive morphological features of the CHILD nevus (these are diagnostically the most useful and worth curating as descriptors rather than as separate HP terms):

Strict midline demarcation with unilateral (lateralized) distribution — "diffuse" lateralization rather than Blaschko-linear
Ptychotropism — a striking predilection for body folds and creases (axillary, inguinal, intergluteal, popliteal). Happle coined the term:

"The associated CHILD nevus shows unique features such as a diffuse form of lateralization, ptychotropism, and microscopic changes of verruciform xanthoma." — Happle R. Semin Dermatol. 1995;14(2):111-21. PMID:7640190 (HUMAN_CLINICAL / expert review)
Waxy, yellowish adherent scale (the yellow color reflects lipid/foam-cell content)
Face is usually spared but can be involved (GeneReviews, PMID:21290788)
Right side affected roughly twice as often as left (StatPearls, NBK507813) — an unexplained and epidemiologically odd finding that would be worth flagging as a knowledge gap

Bilateral/symmetric exception. Rare but genuine:

"As an exception to this rule, in some cases the CHILD nevus may occur in a more or less bilateral distribution... A symmetric distribution of this nevus can exceptionally be seen in patients with CHILD syndrome, and this bilateral involvement should not mislead the clinician to any other diagnosis." — König A et al. J Am Acad Dermatol. 2002;46(4):594-6. PMID:11907515 (HUMAN_CLINICAL)

Confirmed again in 2026 with a novel variant: a 7-year-old female "with bilateral involvement and a novel c.449 T>C (p.Phe150Ser) mutation in the NSDHL gene whose lesions cleared completely with topical cholesterol-lovastatin cream" (Zeyrek M, Balan K, Ersoy-Evans S. Pediatr Dermatol. 2026 May 5. PMID:42083494).

Quality-of-life impact (cutaneous): pruritus, malodor and maceration in intertriginous lesions (ptychotropism means the worst lesions sit in flexures), visible disfigurement with attendant psychosocial burden, and secondary bacterial/fungal infection risk. ⚠️ No formal QoL instrument (EQ-5D, SF-36, DLQI, CDLQI, PROMIS) has been applied to a CHILD syndrome cohort — this is a genuine, citable knowledge gap.

3.2 Musculoskeletal / limb phenotypes

Feature	HPO suggestion	Notes
Ipsilateral limb reduction defect — spectrum from digit shortening to complete amelia	HP:0009827 Amelia (severe end); HP:0009812 / HP:0009818 (upper/lower limb amelia)	GeneReviews: limb defects "range from shortening of the metacarpals and phalanges to absence of the entire limb" (PMID:21290788)
Aplasia of distal phalanges (2nd, 3rd fingers)	HP:0009565, HP:0009429	HPO-annotated for OMIM:308050
Absent middle phalanx (2nd, 3rd fingers)	HP:0009576, HP:0009438
Absent toe	HP:0010760
Syndactyly (finger, toe, cutaneous 2-5, 4-5 toe)	HP:0001159, HP:0006101, HP:0005650, HP:0004692	A 2020 case report specifically links a novel NSDHL variant with syndactyly (BMC Med Genet, PMC7439548)
Epiphyseal stippling (chondrodysplasia punctata)	HP:0010655 Epiphyseal stippling	"Radiographic epiphyseal stippling is a common diagnostic feature of CHILD syndrome, reported in 80% to 99% of cases" and "typically resolves by late childhood" (StatPearls, NBK507813). GeneReviews: "usually seen in the affected limb or body part"
Flexion contracture	HP:0001371	Progressive; a target for orthopedic management
Scoliosis	HP:0002650	Progressive; braces/surgery
Hemiatrophy / ipsilateral body hypoplasia	HP:0100556 Hemiatrophy (+ HP:0100558, HP:0100557 for limb-specific)	The "hemidysplasia" of the acronym
Vertebral hypoplasia	HP:0008417	Ipsilateral
Hypoplastic scapulae	HP:0000882
Short clavicles	HP:0000894
Short ribs	HP:0000773
Hypoplastic pelvis	HP:0008839
Congenital hip dislocation	HP:0001374
Single transverse palmar crease	HP:0000954
Short stature	HP:0004322

Course: limb defects are congenital, static, and non-progressive in their structural form; the functional consequences (contracture, scoliosis, gait) are progressive without intervention. Epiphyseal stippling is the exception — it resolves spontaneously in late childhood, making it a time-limited diagnostic window (a critical practical point: a normal skeletal survey in a teenager does not exclude CHILD syndrome).

Quality-of-life impact: severe. Limb hypoplasia/amelia drives lifelong mobility and dexterity limitation, prosthetic dependence, and educational/occupational impact. StatPearls notes complications of "contractures, immobility, poor dexterity in severe limb hypoplasia."

3.3 Visceral phenotypes (all characteristically ipsilateral)

System	Feature	HPO suggestion
Cardiovascular	Abnormal cardiac septum morphology (ASD/VSD)	HP:0001671
Cardiovascular	Single ventricle	HP:0001750
Cardiovascular	Single coronary artery, unilateral ventricle (StatPearls)	—
Respiratory	Pulmonary hypoplasia (unilateral)	HP:0002089
Renal	Unilateral renal agenesis	HP:0000122
Renal	Hydronephrosis	HP:0000126
CNS	Aplasia/hypoplasia involving the CNS; ipsilateral brain, cranial nerve, and spinal cord hypoplasia; meningomyelocele	HP:0002977
CNS	Mild intellectual disability (minority)	HP:0001256
Endocrine	Thyroid hypoplasia	HP:0005990
Endocrine	Adrenal hypoplasia	HP:0000835
Reproductive	Ovarian and fallopian tube agenesis (StatPearls)	—
Craniofacial	Micrognathia; depressed nasal bridge; cleft upper lip	HP:0000347, HP:0005280, HP:0000204
Sensory	Hearing impairment	HP:0000365
Connective tissue	Umbilical hernia	HP:0001537
Growth	Mild intrauterine growth retardation	HP:0008883
GI (newly recognized)	Gastrointestinal/colonic xanthomas	⚠️ No precise HP term identified — consider curating descriptively; see PMID:39466221, PMID:40517742

Source: HPO disease-annotation set for OMIM:308050, retrieved via https://ontology.jax.org/api/network/annotation/OMIM:308050 (2026-08-13); supplemented by StatPearls NBK507813 and GeneReviews PMID:21290788.

Cognition is a key negative finding. GeneReviews states: "Intellect is usually normal." (PMID:21290788). Intellectual disability is the exception, not the rule, and this cleanly separates CHILD syndrome from the allelic CK syndrome.

Left-sided disease appears worse. StatPearls: "Left-sided involvement carries poorer prognosis due to higher visceral abnormality prevalence" (NBK507813) — plausibly because the heart is a left-sided structure. ⚠️ This is a synthesis claim in a review; it should be curated with appropriate epistemic hedging.

3.4 Laboratory abnormalities

Finding	HPO / LOINC	Comment
Elevated 4α-methyl and 4α-carboxy sterol intermediates in plasma and skin scales	— (no precise HP term found)	The direct biochemical signature of NSDHL deficiency
Elevated 8-dehydrocholesterol	HP:0003462	The EBP-related subset (PMID:10710233)
Elevated 8(9)-cholestenol	HP:0003465	Ditto
Serum total cholesterol: normal	—	Critical negative finding — do not expect hypocholesterolemia
Dyslipidemia (reported in an adult with haploinsufficiency)	—	PMID:40517742

Sterol profiling in a real patient (HUMAN_CLINICAL):

"Sterol analysis from skin flakes revealed increased levels of a mono 4-alpha methyl sterol also seen in plasma as well as the presence of 4-alpha-carboxy-4-methyl-cholest-8(9)-en-3beta-ol and several keto-sterols, which are usually below the limit of detection. This sterol pattern is consistent with abnormal function of the 4-alpha-methylsterol-4-demethylase complex." — Maceda EBG, Kratz LE, Ramos VME, Abacan MAR. BMJ Case Rep. 2020;13(11):e236859. PMID:33139364

3.5 Clinical course descriptors

HP:0003577 Congenital onset
HP:0003826 Stillbirth (reflecting male in-utero lethality)
HP:0001423 X-linked dominant inheritance

4. Genetic / Molecular Information

Causal gene

NSDHL — NAD(P) dependent 3-beta-hydroxysteroid dehydrogenase, hgnc:13398, OMIM 300275, Xq28, NCBI Gene 50814, Ensembl ENSG00000147383, RefSeq NM_015922, UniProt Q15738 (373 aa). Aliases: XAP104, H105e3, SDR31E1.

Enzyme: decarboxylating sterol-4-alpha-carboxylate 3-dehydrogenase, EC 1.1.1.170.

UniProt-annotated function (Q15738):

"Catalyzes the NAD(P)(+)-dependent oxidative decarboxylation of the C4 methyl groups of 4-alpha-carboxysterols in post-squalene cholesterol biosynthesis (By similarity). Also plays a role in the regulation of the endocytic trafficking of EGFR (By similarity)"

Representative catalytic reactions (UniProt/Rhea): 4α-carboxyzymosterol + NADP⁺ → zymosterone + CO₂ + NADPH (RHEA:33455); 4β-methylzymosterol-4α-carboxylate + NADP⁺ → 3-dehydro-4-methylzymosterol + CO₂ + NADPH (RHEA:33447).

Subcellular localization: endoplasmic reticulum membrane (single-pass), evidence ECO:0000269 PubMed:21129721; also lipid droplet. UniProt note: "Trafficking through the Golgi is necessary for ER membrane localization."

GeneReviews frames the enzyme's role compactly: it "functions as a C4 demethylase in post-squalene cholesterol biosynthesis" (PMID:21290788).

Pathogenic variants

Variant classes and detection yield (GeneReviews, NBK51754 / PMID:21290788): - ~90% detected by sequence analysis - ~10% detected by gene-targeted deletion/duplication analysis

This ~10% CNV fraction is clinically important. It has been demonstrated repeatedly:

"The gene mutation is a large deletion of exon 3 and 4 of the NSDHL gene, which was discovered and reported for the first time in CHILD syndrome... Multiple exons deletions or microdeletion was not rare in CHILD syndrome. Classical Sanger sequencing may not be useful enough to find all kinds of mutations. Next-generation sequencing may be more effective." — Yu X et al. J Eur Acad Dermatol Venereol. 2018;32(7):1209-1213. PMID:29341259 (HUMAN_CLINICAL)

And, most recently, via optical genome mapping (HUMAN_CLINICAL):

"This study examines 3 pediatric patients exhibiting a compatible phenotype with inconclusive genetic studies, aiming to evaluate the diagnostic utility of optical genome mapping (OGM)... The identified structural variants consisted of deletions of varying sizes in the Xq28 cytoband, encompassing regions that contain exons." — Vergara A et al. Mol Syndromol. 2025 Dec 19. PMID:41625319

Variant types reported: missense, nonsense, frameshift (small indels), splice-site, and multi-exon deletions. The foundational spectrum survey is Bornholdt D et al., "Mutational spectrum of NSDHL in CHILD syndrome," J Med Genet. 2005;42(2):e17, PMID:15689440 (⚠️ this paper has no PubMed abstract; I could not retrieve its full text in this session. Do not curate specific variant counts from it without reading the PDF — PMC1735983).

Specific variants with published support (all HUMAN_CLINICAL unless noted):

Variant	Consequence	Context	Source
c.262C>T (p.Arg88Ter)	Nonsense	Somatic mosaicism in a surviving male with CHILD syndrome	GeneReviews PMID:21290788
c.130G>A (p.Gly44Ser)	Missense; initially VUS, reclassified likely pathogenic after de novo confirmation; absent from ExAC	Female with classic CHILD + confirmatory sterol profile	PMID:33139364
c.123delA (p.Val42Ter)	Frameshift/nonsense	14-year-old Chinese girl, CHILD without limb defects, co-occurring linear porokeratosis	PMID:40464756
c.449T>C (p.Phe150Ser)	Missense, novel	7-year-old female with bilateral involvement	PMID:42083494
Deletion of exons 3 and 4	Multi-exon deletion	Chinese patient mimicking verrucous nevus	PMID:29341259
c.790-6C>T	Intronic/splice-region, novel	Male fetus, recurrent miscarriage; maternally inherited	PMID:36504312
Xq28 deletions (variable size, exon-containing)	Structural	Detected only by OGM after inconclusive standard testing	PMID:41625319
c.455G>A; c.696_698delGAA (p.Lys232del); c.1098dupT (p.Arg367SerfsTer33)	Hypomorphic — cause CK syndrome, not CHILD	Only three CK-causing variants known	GeneReviews PMID:21290788; PMID:21129721

ACMG/AMP classification landscape. ClinVar holds 545 variant records for NSDHL, of which 203 carry a pathogenic clinical-significance property (NCBI E-utilities esearch against ClinVar, queried 2026-08-13). ⚠️ These counts include CK syndrome and non-disease submissions; treat as an order-of-magnitude figure, not a curated CHILD-specific count.

Allele frequency. Pathogenic NSDHL alleles are absent from population databases — expected for an X-linked male-lethal condition under strong purifying selection. The Maceda variant is explicitly noted as "not included in population databases (ExAC no frequency)" (PMID:33139364). ⚠️ I did not query gnomAD directly in this session; a curator should pull gnomAD v4 constraint metrics (pLI / LOEUF) for NSDHL before making a constraint claim.

Somatic vs germline. Predominantly germline (de novo or maternally inherited). Post-zygotic somatic mosaicism is the accepted explanation for rare surviving affected males (GeneReviews, PMID:21290788). COSMIC/TCGA somatic NSDHL mutation in cancer is a separate topic — see the EGFR/oncology link in §6.

Functional consequences of variants

CHILD syndrome: complete or near-complete loss of function → "loss of functional decarboxylating sterol-4-alpha-carboxylate 3-dehydrogenase" (GeneReviews).
CK syndrome (allelic, X-linked, affects males): partial loss of function via hypomorphic, conformationally unstable alleles. This dosage relationship is the cleanest genotype–phenotype axis in the NSDHL literature: near-null → female-limited CHILD with male lethality; hypomorphic → male-viable CK syndrome with neurodevelopmental rather than dermatologic dominance.

"These two mutations, which alter protein folding, show temperature-sensitive protein stability and complementation in Erg26-deficient yeast." — McLarren KW et al. Am J Hum Genet. 2010;87(6):905-14. PMID:21129721 (HUMAN_CLINICAL + IN_VITRO)
Missense variants act largely by destabilizing the protein (IN_VITRO): "protein expression levels were low for all mutants" across 13 missense and one deletion variant (PMID:40222685).

⚠️ Curation note on the GOF/LOF slot: for variant consequence use GeneticContext.functional_impact_category: LOSS_OF_FUNCTION (CHILD) / PARTIAL_LOSS_OF_FUNCTION (CK). For the pathway state node ("cholesterol biosynthesis"), use Descriptor.modifier: DECREASED — this is a quantitative reduction in a normally-regulated pathway, not escape from regulatory control.

Modifier genes

None established. The principal phenotype modifier is stochastic X-inactivation ratio, not a second locus. A single case reported co-occurring compound heterozygous PMVK variants (c.88C>T, p.Gln30Ter) alongside the NSDHL variant, in a patient with both CHILD syndrome and linear porokeratosis (PMID:40464756) — but this is a coincidental second mendelian disorder (porokeratosis is PMVK-related), not a modifier, and should be curated as such.

Epigenetic information

X-chromosome inactivation is the central epigenetic mechanism of this disease — arguably CHILD syndrome is the textbook dermatologic demonstration of functional X mosaicism. GO term suggestion for the mechanism: consider curating XCI descriptively; no disease-specific DNA-methylation or histone-modification study of CHILD syndrome tissue exists in ENCODE/Roadmap/DiseaseMeth as far as I could determine. ⚠️ No CHILD-specific methylome or epigenome dataset identified.

Chromosomal abnormalities

No aneuploidy, translocation, or inversion is characteristic. The relevant structural lesions are submicroscopic Xq28 deletions encompassing NSDHL exons — below karyotype resolution, at or below chromosomal-microarray resolution in some cases, and detectable by NGS-based CNV calling or optical genome mapping (PMID:41625319, PMID:29341259).

5. Environmental Information

Environmental factors: none causally implicated. No CTD/TOXNET/EPA-relevant exposure has been associated with CHILD syndrome.
Lifestyle factors: none implicated in causation. Relevant only to symptom management (emollient use, avoidance of skin trauma/maceration in flexures).
Infectious agents: none causal. Secondary bacterial and fungal infection of the eroded, macerated ptychotropic lesions is a recognized complication (StatPearls, NBK507813) — curate as a downstream consequence, not an etiology.

⚠️ For a dismech entry, the environmental: block should be absent or empty for etiology. If curated at all, restrict it to therapeutic/topical exposures with influences_mechanisms targeting the cutaneous nodes.

6. Mechanism / Pathophysiology

The causal chain, from lesion to clinic

NSDHL loss-of-function variant (Xq28, heterozygous female)
  │
  ├─► random X-chromosome inactivation
  │     └─► mosaic patches of NSDHL-null vs NSDHL-competent cells
  │
  ▼
Loss of decarboxylating sterol-4α-carboxylate 3-dehydrogenase (EC 1.1.1.170)
  │
  ├─► BLOCK in C4-demethylation step of post-squalene cholesterol biosynthesis
  │
  ├──────────────► (A) DEFICIENCY of end-product cholesterol in situ
  │                      │
  │                      ├─► impaired lamellar-body lipid processing
  │                      ├─► defective epidermal permeability barrier
  │                      ├─► compensatory keratinocyte hyperproliferation
  │                      │      → psoriasiform acanthosis, hyperkeratosis,
  │                      │        parakeratosis  → the CHILD nevus
  │                      └─► impaired SHH signaling (cholesterol is required
  │                             both for SHH autoprocessing/lipid modification
  │                             and for Smoothened function)
  │                             → disrupted limb/organ patterning
  │
  └──────────────► (B) ACCUMULATION of toxic 4α-methyl / 4α-carboxy sterol
          intermediates
          │
          ├─► direct cytotoxicity → cell death
          ├─► foam-cell formation in papillary dermis
          │      (lipid-laden macrophages, CD68+/CD163+)
          │      → verruciform-xanthoma-like histology
          ├─► deranged EGFR/ERBB endocytic trafficking
          │      and growth-factor-receptor signaling
          └─► NEGATIVE SELECTION of mutant-expressing cells
                 │
                 ▼
       Progressive clearance of NSDHL-null keratinocytes and
       fibroblasts from the unaffected side
                 │
                 ▼
       STRICT LATERALIZATION with midline demarcation

The two-hit metabolic logic (deficiency and toxicity)

This is the single most important mechanistic concept for CHILD syndrome, because it directly dictates therapy. Paller et al. established both arms and used them to design treatment (HUMAN_CLINICAL + ultrastructure):

"On the basis of the putative pathogenic role of both pathway-product deficiency of cholesterol and accumulation of toxic metabolic intermediates, we assessed the efficacy of combined therapy with lovastatin and cholesterol... Ultrastructural analysis of affected skin showed evidence of both cholesterol depletion and toxic metabolic accumulation. Topical treatment with lovastatin/cholesterol (but not cholesterol alone) virtually cleared skin lesions by 3 months, accompanied by histological and ultrastructural normalization of epidermal structure and lipid secretion." — Paller AS, van Steensel MA, Rodriguez-Martín M, Sorrell J, Heath C, Crumrine D, van Geel M, Cabrera AN, Elias PM. J Invest Dermatol. 2011;131(11):2242-8. PMID:21753784

The "but not cholesterol alone" clause is the decisive experimental evidence that toxic intermediate accumulation, not cholesterol deficiency alone, drives the cutaneous phenotype. This is a rare instance of a therapeutic result functioning as a mechanistic proof.

The same principle was independently argued for the allelic disorder: "We hypothesize that methyl sterol accumulation, not only cholesterol deficiency, causes CKS, given that cerebrospinal fluid cholesterol, plasma cholesterol, and plasma 24S-hydroxycholesterol levels are normal in males with CKS." (PMID:21129721)

The lateralization mechanism — cell-autonomous negative selection

Paller et al. resolved a 30-year mystery:

"The unusual lateralization of abnormalities in CHILD syndrome reflects selective clearance of keratinocytes and fibroblasts that express the mutant allele from the unaffected side." — PMID:21753784

The mouse work supports the same logic and adds a striking temporal dimension (MODEL_ORGANISM):

"Clonal populations of mutant cells were visible in the brain, skin and liver of Bpa(1H) pups. In the liver, the proportion of NSDHL negative cells dropped from approximately 50% at postnatal day 6 to approximately 20% at one year of age. In the brain... the proportion of NSDHL negative cells also dropped dramatically over the first year of life. Our results suggest that while NSDHL-deficient cells in the mosaic Bpa(1H) female are able to survive and differentiate during embryonic development, they are subject to negative selection over the life of the animal." — Cunningham D, Spychala K, McLarren KW, Garza LA, Boerkoel CF, Herman GE. Mol Genet Metab. 2009;98(4):356-66. PMID:19631568

This directly explains the natural history: skin lesions improve with age because mutant clones are progressively outcompeted. It also explains why the disease is a developmental one — the damage is done during the window when mutant cells are still abundant.

Sonic hedgehog signaling — the link to malformation

Cholesterol is mechanistically required for Hedgehog signaling at two levels: SHH undergoes autocatalytic cleavage with covalent cholesterol modification of its N-terminal signaling domain, and Smoothened activity is sterol-regulated. The most direct experimental demonstration in an Nsdhl model (MODEL_ORGANISM + IN_VITRO):

"Histological abnormalities include progressive loss of cortical and hippocampal neurons, as well as deficits in the proliferation and migration of cerebellar granule precursors and subsequent massive apoptosis of the cerebellar cortex. We replicated the granule cell precursor proliferation defect in vitro and demonstrate that it results from defective signaling by SHH. Furthermore, this defect is almost completely rescued by supplementation of the culture media with exogenous cholesterol, while methylsterol accumulation above the enzymatic block appears to be associated with increased cell death." — Cunningham D, DeBarber AE, Bir N, Binkley L, Merkens LS, Steiner RD, Herman GE. Hum Mol Genet. 2015;24(10):2808-25. PMID:25652406

Note the elegance: both arms of the two-hit model are separately demonstrated in one experiment — cholesterol rescue fixes the SHH proliferation defect, while methylsterol accumulation independently causes cell death.

StatPearls summarizes the developmental consequence: deficient enzyme activity "disrupts sonic hedgehog (SHH) protein signaling during embryonic limb development and organogenesis, explaining the characteristic unilateral presentation" (NBK507813).

GO suggestion: GO:0007224 smoothened signaling pathway (modifier: DECREASED).

Protein dysfunction

Mutant NSDHL is predominantly a protein-stability problem rather than a catalytic-site problem, and it is chaperone-dependent (IN_VITRO, PMID:40222685): all 13 missense mutants surveyed showed low expression; some were rescued by 30°C or by glycerol; HSP70 and HSP90 are needed for optimal wild-type expression. Combined with the temperature-sensitive CK alleles (PMID:21129721), this establishes misfolding/instability as the dominant molecular mechanism for missense alleles — and opens a pharmacological chaperone therapeutic direction.

GO/CC suggestions: GO:0005789 endoplasmic reticulum membrane; GO:0005811 lipid droplet (⚠️ verify this ID against OLS before curating — I verified GO:0042599 lamellar body but did not verify lipid droplet). GO/MF suggestion: GO:0000252 — 3-beta-hydroxysteroid dehydrogenase [NAD(P)+]/C4-decarboxylase activity (verified via OLS4), modifier: LOSS_OF_FUNCTION. GO/BP suggestions: GO:0006695 cholesterol biosynthetic process (modifier: DECREASED); GO:0007224 smoothened signaling pathway (DECREASED); GO:0061436 establishment of skin barrier (DECREASED); GO:0042599 lamellar body (CC, abnormal).

Metabolic changes

Block at the C4-demethylation step of post-squalene cholesterol synthesis (lanosterol → zymosterol segment).
Accumulation of 4α-methyl and 4α-carboxy sterols, plus keto-sterols normally below detection (PMID:33139364).
Systemic cholesterol is preserved — dietary uptake and hepatic synthesis in wild-type-expressing cells compensate. The lesion is compartmental: skin (behind the permeability barrier) and brain (behind the blood-brain barrier) cannot import cholesterol and therefore bear the phenotype. Cunningham et al. make this argument explicitly: "These data support the absolute requirement for cholesterol synthesis in situ once the blood-brain-barrier forms and cholesterol transport to the fetus is abolished." (PMID:25652406)

CHEBI suggestions: CHEBI:16113 cholesterol; CHEBI:16521 lanosterol (all verified via OLS4).

Tissue damage mechanisms

Epidermal barrier failure from abnormal lamellar-granule (lamellar body) content and secretion (IN_VITRO/ultrastructural, HUMAN_CLINICAL tissue):

"Electron microscopy revealed vesicular structures in the intercellular spaces of the stratum corneum and vacuoles or vesicular structures in upper prickle cell layer. Some of them can be recognized as abnormal lamellar granules. Within the foamy cells in the papillary dermis, large vacuoles were found... These findings suggested that abnormal lipid metabolism involving lamellar granules may be responsible to the skin lesion of CHILD syndrome." — Ishibashi M, Matsuda F, Oka H, Ishiko A. J Cutan Pathol. 2006;33(6):447-53. PMID:16776722
Foam-cell (xanthomatous) infiltration of the papillary dermis — lipid-laden macrophages accumulating undegradable sterol intermediates. StatPearls: "Dermal foam cells express macrophage markers (CD68, CD163) but lack epithelial markers (AE1/AE3, S100)."
Apoptotic cell death driven by methylsterol accumulation (PMID:25652406).
No evidence for oxidative-stress-, ischemia-, or fibrosis-driven damage as a primary mechanism. Note, however, that the 2026 review of post-lanosterol disorders lists "oxidative stress" among the shared pathogenic themes for this disease class (PMID:42589509) — curate that as class-level, not CHILD-specific.

Immune system involvement

Not an immune-mediated disease. The lesion is called "inflammatory" descriptively (erythema, psoriasiform histology), and macrophage-derived foam cells are prominent, but there is no autoimmunity, no immunodeficiency, and no evidence of a primary immune driver. The inflammation is secondary to barrier failure and lipid accumulation. ⚠️ Do not curate autoimmune or immunodeficiency mechanisms.

The unexpected oncology connection (NSDHL–EGFR trafficking)

A mechanistically important finding that also explains why ketoconazole works (IN_VITRO + MODEL_ORGANISM):

"We established that inactivation of 2 sterol biosynthesis pathway genes, SC4MOL (sterol C4-methyl oxidase-like) and its partner, NSDHL (NADP-dependent steroid dehydrogenase-like), sensitized tumor cells to EGFR inhibitors... an unexpected role for SC4MOL and NSDHL in controlling the signaling, vesicular trafficking, and degradation of EGFR and its dimerization partners, ERBB2 and ERBB3. Metabolic block upstream of SC4MOL with ketoconazole or CYP51A1 siRNA rescued cancer cell viability and EGFR degradation... Analysis of Nsdhl-deficient Bpa(1H/+) mice confirmed dramatic and selective loss of internalized platelet-derived growth factor receptor in fibroblasts, and reduced activation of EGFR and its effectors in regions of skin lacking NSDHL." — Sukhanova A et al. Cancer Discov. 2013;3(1):96-111. PMID:23125191

Two things fall out of this for CHILD syndrome specifically: (i) reduced EGFR signaling in NSDHL-null skin is a plausible contributor to the abnormal keratinocyte phenotype; and (ii) ketoconazole's therapeutic benefit has a mechanistic rationale — blocking CYP51A1 upstream prevents accumulation of the toxic C4-methylsterols and restores receptor trafficking.

Molecular profiling

Transcriptomics: Paller et al. analyzed "gene activation in abnormal and unaffected skin" (PMID:21753784) — the only such analysis I identified. ⚠️ No GEO/ArrayExpress accession verified in this session.
Proteomics / metabolomics / lipidomics: ⚠️ No dedicated CHILD syndrome dataset identified in PRIDE, MetaboLights, or Metabolomics Workbench. Clinical sterol profiling by GC-MS (Kennedy Krieger Biochemical Genetics Laboratory is the reference lab in published cases) serves the metabolomic function in practice.
Single-cell / spatial transcriptomics: ⚠️ None published. This is a conspicuous gap: a mosaic disease with a sharp midline boundary is close to an ideal spatial-transcriptomics subject, and the "which cells are cleared, and when" question is directly addressable by single-cell work.
Functional genomics screens: the SC4MOL/NSDHL EGFR-sensitization work (PMID:23125191) used siRNA-based approaches; DepMap contains NSDHL dependency data for cancer lines (⚠️ not queried in this session).

Cell types and biological processes for KB curation

Cell type	CL term (verified via OLS4)	Role
Keratinocyte	CL:0000312	Primary affected cell; site of the barrier defect and of clonal clearance
Foam cell	CL:0000891	Dermal lipid-laden cells; verruciform-xanthoma histology
Macrophage-derived foam cell	CL:0000517	CD68⁺/CD163⁺ per StatPearls
Fibroblast	CL:0000057	Second cell type shown to undergo selective clearance (PMID:21753784)
Macrophage	⚠️ verify `CL:0000235` before use
Chondrocyte	CL:0000138	Epiphyseal stippling / chondrodysplasia punctata

7. Anatomical Structures Affected

Organ level

Primary: - Skin — UBERON:0002097 skin of body; UBERON:0001003 skin epidermis (verified via OLS4). Stratum corneum UBERON:0002027, stratum spinosum UBERON:0002026, stratum granulosum UBERON:0002069, stratum basale UBERON:0002025. - Papillary dermis — UBERON:0001992 papillary layer of dermis (foam-cell infiltrate). - Limbs — UBERON:0002101 limb. - Epiphyses / cartilaginous structures — UBERON:0001437 epiphysis (stippling). - Nails — nail unit (onychodystrophy, periungual hyperkeratosis).

Secondary / variable ipsilateral involvement: - Heart (septa, coronary artery, ventricle) — cardiovascular system - Lung (unilateral hypoplasia) — respiratory system - Kidney (agenesis, hydronephrosis) — genitourinary system - Brain, cranial nerves, spinal cord (ipsilateral hypoplasia, meningomyelocele) — nervous system - Thyroid and adrenal glands — endocrine system - Ovary, fallopian tube — reproductive system - Vertebrae, ribs, scapula, clavicle, pelvis — skeletal system - Colon/GI tract (xanthomas — newly recognized, PMID:39466221, PMID:40517742) — digestive system

Tissue and cell level

Epithelial tissue — epidermal keratinocytes (CL:0000312), the dominant affected population
Connective tissue — dermal fibroblasts (CL:0000057); cartilage chondrocytes (CL:0000138)
Immune/myeloid — dermal foam cells (CL:0000891), macrophage-derived (CL:0000517)

Subcellular level

Endoplasmic reticulum membrane — GO:0005789 — NSDHL's site of action (UniProt evidence ECO:0000269 PubMed:21129721)
Lipid droplet — secondary NSDHL localization
Lamellar body (lamellar granule) — GO:0042599 (verified via OLS4) — structurally abnormal in CHILD skin (PMID:16776722); the proximate cause of barrier failure
Stratum corneum intercellular lipid lamellae — abnormal vesicular structures on EM (PMID:16776722)

Localization and lateralization

This is the single most distinctive anatomical fact about CHILD syndrome and deserves explicit structured curation: - Strictly unilateral in the overwhelming majority, with sharp midline demarcation on the trunk - Right side ≈ 2× more often than left (StatPearls, NBK507813) - Ipsilateral concordance across skin, skeleton, and viscera — the skin lesion predicts which side the visceral anomalies will be on - Bilateral/near-symmetric distribution is a rare but real exception (PMID:11907515, PMID:42083494) and must not redirect the diagnosis - Face usually spared - Ptychotropic — preferentially in flexural creases - Not Blaschko-linear in the classic narrow-banded sense — the lateralization is "diffuse" (PMID:7640190), which distinguishes the CHILD nevus from ILVEN and most epidermal nevi

8. Temporal Development

Onset

Congenital (HP:0003577). Limb and visceral malformations are established in embryogenesis.
Skin: "The ichthyosiform skin lesions are usually present at birth or in the first weeks of life" (GeneReviews, PMID:21290788); Happle 1980: "The dermatosis is either present at birth or develops during the first weeks of life" (PMID:7408908). GeneReviews extends the window to "the first weeks to few months of life."
Late-onset presentation exists: StatPearls notes symptoms "may appear as late as age 9 years" (NBK507813).
Onset pattern: congenital/insidious for the skin; the malformations are simply present from birth.

Progression

No formal disease staging system exists. CHILD syndrome is not staged.
Skeletal malformations: static/non-progressive in structure.
Skin: dynamic and, on balance, improving. Erythema decreases while hyperkeratosis increases during infancy; overall "Dermatologic symptoms improve with age" (StatPearls). The mechanistic basis is progressive negative selection against mutant clones (PMID:19631568, PMID:21753784).
Important counter-current: "new lesions can develop in later life" and GeneReviews' surveillance guidance specifically warns that "new lesions may occur in puberty or early adulthood" (PMID:21290788). So the course is best described as improving with episodic new lesion formation, not monotonically remitting.
Epiphyseal stippling resolves by late childhood (StatPearls) — a genuinely time-limited sign.
Secondary progressive complications: joint contractures and scoliosis worsen without orthopedic management.
Duration: chronic and lifelong.

Patterns

Remission: partial spontaneous improvement of skin with age (natural, clone-selection-driven). Treatment-induced near-complete remission of skin lesions is achievable with topical statin/cholesterol — "virtually cleared skin lesions by 3 months" (PMID:21753784); "cleared completely" (PMID:42083494); improvement within 4 weeks (PMID:40464756).
Critical periods:
Embryogenesis — the only window in which the limb/visceral malformations could theoretically be prevented; no intervention exists.
First weeks to months of life — optimal window to initiate skin therapy and to complete imaging/organ evaluation. StatPearls: imaging "detect[s] skeletal and visceral anomalies, enabling early intervention," and "Early detection and timely management improve patient outcomes substantially."
Early childhood — the diagnostic window during which epiphyseal stippling is still radiographically visible.
Puberty/early adulthood — surveillance window for new cutaneous lesions.
Neonatal period — highest mortality risk if severe cardiac or pulmonary anomalies are present.

9. Inheritance and Population

Epidemiology

Reported cases: GeneReviews (2024 update): "More than 60 individuals have been reported to date." (PMID:21290788). StatPearls: "fewer than 100 CHILD syndrome cases have been reported in the literature."
Incidence: StatPearls cites "approximately 1 in 100,000 live births" (NBK507813). ⚠️ This figure is internally inconsistent with "fewer than 100 reported cases" — at 1/100,000 live births, world annual births alone would generate ~1,300 cases per year. Treat the 1/100,000 figure as unreliable/unsourced. Orphanet does not publish a numeric prevalence for ORPHA:139 that I could retrieve (the Orphanet API endpoints for epidemiological data returned 404 in this session).
Recommended dismech prevalence curation: measure_type: CASES_IN_LITERATURE, prevalence_class: ULTRA_RARE, with notes recording the ">60 reported individuals" figure and its GeneReviews source. Do not curate rate_per_100000: 1.0 on the strength of the StatPearls sentence.
Prevalence/incidence by geography: no registry data. Cases are reported worldwide (Germany, Japan, China, Philippines, Turkey, USA, Saudi Arabia, Brazil, Spain, UK) with no evident geographic clustering.

Genetic epidemiology

Inheritance pattern: X-linked dominant, male-lethal (HP:0001423). Happle's original inference from the sex ratio: "Arguments are presented in favor of the hypothesis that the conditions is due to an X-linked dominant gene lethal in hemizygous males." (PMID:7408908)
Sex ratio: Happle 1980, from 20 cases: "The ratio of females to males is 19 : 1." (PMID:7408908) That single male is now understood to reflect somatic mosaicism.
Recurrence risk (GeneReviews, PMID:21290788):

"If the mother of a proband has an NSDHL pathogenic variant, the chance of transmitting it in each pregnancy is 50%. However, since studies suggest that male conceptuses with an NSDHL pathogenic variant generally abort or resorb spontaneously, the expected live-born distribution is: 33% heterozygous (typically) affected females; 33% unaffected females; and 33% unaffected males."
Male lethality — direct molecular evidence (HUMAN_CLINICAL):

"A 33-year-old pregnant woman with recurrent spontaneous abortion was experiencing her third pregnancy with a male embryo. In this pregnancy, a miscarriage occurred at a gestational age of 10+6 weeks with no copy number variants. However, a novel mutation c.790-6C>T in the NSDHL gene was observed in the fetus through whole-exome sequencing (WES). Parental verification indicated that the NSDHL gene variant was inherited from the mother." — Zhuang J et al. Mol Genet Genomic Med. 2023;11(3):e2121. PMID:36504312

This makes recurrent male miscarriage a recognizable presenting feature of a maternal NSDHL variant — clinically actionable and worth curating explicitly. - Surviving affected males: explained by post-zygotic somatic mosaicism; the documented example is a male "mosaic for NSDHL pathogenic variant c.262C>T (p.Arg88Ter)" (GeneReviews, PMID:21290788). König et al. also reported "one boy" among their six patients (PMID:10710235). - Penetrance: effectively complete in heterozygous females. No unaffected obligate female carriers are described. - Expressivity: highly variable — from complete amelia with multi-organ malformation to a mild adult presenting with GI xanthomas and ichthyosis only (PMID:39466221). The variability is driven principally by X-inactivation pattern, not by allele. - Genetic anticipation: not applicable (no repeat expansion). - Germline mosaicism: not specifically documented for NSDHL, but somatic mosaicism is established; germline mosaicism cannot be excluded and should be mentioned in counseling. - Founder effects: none reported. - Consanguinity: not relevant — this is an X-linked dominant, not a recessive, disorder. - Carrier frequency: not applicable in the usual sense. Heterozygous females are affected; there is no asymptomatic carrier state.

Population demographics

Sex: overwhelmingly female (~95%+). Affected males are exceptional and mosaic.
Ethnicity/ancestry: no predilection reported; cases published across East Asian, South Asian, European, Middle Eastern, Southeast Asian, and Latin American populations.
Age distribution: diagnosis is typically neonatal/infantile; a growing tail of adult diagnoses is emerging as mild phenotypes are recognized (PMID:39565229 — adult treated with ketoconazole; PMID:39466221, PMID:40517742 — adults presenting through gastroenterology).
Geographic distribution of variants: none — variants are private/de novo. No recurrent hotspot has been established beyond the three known CK-syndrome alleles.

The allelic disorder, for contrast

CK syndrome (OMIM 300831), caused by hypomorphic NSDHL alleles, is X-linked recessive and affects males. GeneReviews: "To date, 25 affected males from three unrelated families have been reported." Features: "mild-to-severe intellectual disability," seizures in infancy in all affected males, cortical malformations (most consistent with polymicrogyria), microcephaly "greater than 2-3 standard deviations below the mean," thin habitus with long thin digits, strabismus, optic atrophy, scoliosis/kyphosis, and behavioral problems (aggression, ADHD, irritability). Heterozygous females "may have a range of behavioral problems including irritability and aggression" but have "normal physical features, intellect, and brain imaging."

10. Diagnostics

Clinical tests

Laboratory / biochemical - Plasma and skin-scale sterol profiling by GC-MS — the disease-specific biochemical test. Elevated C4-methylated and C4-carboxylated sterol intermediates; see the verbatim sterol pattern in PMID:33139364. Reference laboratory in published cases: Kennedy Krieger Institute Biochemical Genetics Laboratory. - Serum total cholesterol — typically normal; a normal value does not exclude the diagnosis. ⚠️ This is a common source of diagnostic error. - For the EBP-related subset: elevated 8-dehydrocholesterol (HP:0003462) and 8(9)-cholestenol (HP:0003465) (PMID:10710233). - LOINC: standard cholesterol panels (e.g., LOINC:2093-3 total cholesterol) ⚠️ not verified in this session. There is no standard LOINC code for the diagnostic methylsterol panel — it is a specialized send-out.

Imaging - Radiographs / skeletal survey — for limb reduction defects and epiphyseal stippling (HP:0010655). Must be done in early childhood before stippling resolves. - Echocardiography — septal defects, single ventricle, coronary anomalies. - Renal ultrasound — agenesis, hydronephrosis. - Chest imaging — pulmonary hypoplasia. - Brain MRI — ipsilateral cerebral hypoplasia, cranial nerve/spinal cord anomalies, meningomyelocele. - Colonoscopy — newly relevant for xanthomas in mild adult phenotypes (PMID:40517742).

Biopsy / histopathology — a strong diagnostic anchor: - Psoriasiform epidermal hyperplasia with hyperkeratosis, alternating orthokeratosis and parakeratosis, acanthosis, papillomatosis (StatPearls, NBK507813) - Foam cells in the papillary dermis, CD68⁺/CD163⁺, negative for AE1/AE3 and S100 (StatPearls) - Verruciform xanthoma-like features — "The histology shared many features with verruciform xanthoma" (PMID:16776722); Happle described "microscopic changes of verruciform xanthoma" as a defining feature of the CHILD nevus (PMID:7640190) - Electron microscopy: abnormal lamellar granules; intercellular vesicular structures in stratum corneum; large vacuoles in dermal foam cells (PMID:16776722)

Functional tests / electrophysiology — not disease-specific; ordered according to organ involvement (echo, ECG if cardiac; audiometry given HP:0000365; ophthalmology assessment).

Genetic testing

Recommended approach: 1. Single-gene NSDHL sequencing (or targeted gene panel including NSDHL and EBP) — first-line given the highly recognizable phenotype. Yield ~90% by sequence analysis (GeneReviews). 2. Gene-targeted deletion/duplication analysis — mandatory second step; ~10% of pathogenic variants are CNVs (GeneReviews). Sanger-only testing is explicitly inadequate: "Classical Sanger sequencing may not be useful enough to find all kinds of mutations" (PMID:29341259). 3. Whole-exome sequencing (WES) — useful when the phenotype is atypical, or in the prenatal/recurrent-miscarriage setting (PMID:36504312). 4. Whole-genome sequencing (WGS) — used in at least one report to resolve both NSDHL and a second locus (PMID:40464756). 5. Optical genome mapping (OGM) — a genuinely new option for cases with compatible phenotype but inconclusive standard testing (PMID:41625319). 6. Chromosomal microarray (CMA) — can detect larger Xq28 deletions but was negative in the fetal case where WES found a splice variant (PMID:36504312). 7. Karyotyping / FISH — not useful; the lesions are submicroscopic. 8. Mitochondrial DNA testing / repeat expansion testing — not applicable. 9. Ichthyosis gene panels typically include NSDHL — a practical route when the presenting specialty is dermatology.

GTR: "NSDHL-Related Disorders" clinical genetic test, GTR test ID 317493.

Biochemical testing as an adjunct to VUS resolution: in PMID:33139364, a sterol profile consistent with 4α-methylsterol-4-demethylase dysfunction plus de novo status upgraded a VUS (c.130G>A) to likely pathogenic. This is a model workflow worth curating: biochemistry supplies the functional evidence line (PS3-adjacent) that variant-level data alone cannot.

Omics-based diagnostics

RNA sequencing: could clarify splice-region variants such as c.790-6C>T (PMID:36504312) — no published diagnostic RNA-seq series for NSDHL.
Proteomics / epigenomics / liquid biopsy: ⚠️ not established, not applicable.
Targeted metabolomics (sterol GC-MS) is, in practice, the operative "omics" diagnostic.

Clinical criteria

No formal consensus diagnostic criteria (no DSM/ICD-style checklist, no society guideline) exist. GeneReviews frames it as suggestive findings + molecular confirmation:

Suggestive findings for CHILD syndrome (GeneReviews, PMID:21290788): - Unilateral ichthyosiform nevus - Ipsilateral limb defects - Punctate cartilage calcifications - CNS and visceral anomalies

Diagnosis established in a female proband with a heterozygous NSDHL pathogenic (or likely pathogenic) variant identified by molecular genetic testing.

Happle's point (PMID:11907515) that "a diagnosis of CHILD syndrome can be based on clinical features such as the highly characteristic morphology of the CHILD nevus" remains the practical clinical reality.

Differential diagnosis

Condition	Gene / MONDO	Distinguishing features
X-linked dominant chondrodysplasia punctata (CDPX2 / Conradi-Hünermann-Happle)	EBP	GeneReviews: "Absence of strict midline demarcation & lack of unilaterality seen in CHILD syndrome"; "Skin findings fade over time"; "Ocular anomalies are prominent" (cataracts). Grange 2000 notes "the skeletal defects and skin lesions in CDPX2 are bilateral and asymmetric" (PMID:10710233). ⚠️ *But some CHILD phenotypes are EBP-caused* — the boundary is genuinely blurred
Inflammatory linear verrucous epidermal nevus (ILVEN)	Mosaic, various	Blaschko-linear narrow bands rather than diffuse lateralization; no ptychotropism; no limb reduction defect; no xanthomatous foam cells. A 2022 paper argues ILVEN "encompasses a spectrum of inflammatory mosaic disorders" (PMID:35853659), and a 2025 case reports unilateral ILVEN with ipsilateral limb contracture (PMID:39953436) — so this differential is harder than it looks
Epidermal nevus / epidermal nevus syndromes	FGFR3, PIK3CA, HRAS, KRAS mosaic	No xanthomatous histology, no cholesterol pathway abnormality, no ipsilateral hemidysplasia
Nevus sebaceus / Schimmelpenning syndrome	HRAS/KRAS mosaic	Sebaceous differentiation; cerebral anomalies, coloboma, conjunctival lipodermoid (PMID:7640190)
Phacomatosis pigmentokeratotica	HRAS mosaic	Combined organoid nevus + speckled lentiginous nevus
Incontinentia pigmenti	IKBKG	GeneReviews: "Cutaneous lesions evolve through multiple stages" — vesicular → verrucous → hyperpigmented → hypopigmented, along Blaschko lines
Congenital ichthyosiform erythroderma (non-syndromic)	TGM1, ALOX12B, ALOXE3, etc.	Generalized and bilateral; no lateralization, no limb defects
Linear porokeratosis	MVK, PMVK, MVD, FDPS (mevalonate pathway)	Cornoid lamella on histology. Note the reported co-occurrence of both diseases in one patient (PMID:40464756)
Hailey-Hailey disease	ATP2C1	A 2026 report documents CHILD syndrome mimicking Hailey-Hailey — a flexural-distribution trap (Wu/Cheng et al., J Am Acad Dermatol. 2026 Aug 10, PMID:42575321)
Verrucous nevus / verrucous carcinoma	—	PMID:29341259 describes CHILD syndrome mimicking verrucous nevus; a 2024 German article is titled "[Only a wart?—Characteristic skin changes in CHILD syndrome]" (PMID:39278872)
CK syndrome	NSDHL (allelic!)	Males, ID + seizures + microcephaly + polymicrogyria, no lateralized nevus
Greenberg dysplasia / dappled diaphyseal dysplasia	LBR	Another sterol-pathway skeletal dysplasia (PMID:32304187)

Screening

Newborn screening: CHILD syndrome is not on any newborn screening panel. The diagnosis is made clinically at birth from the skin lesion.
Carrier screening: not applicable in the classical sense (heterozygous females are affected). Testing of a proband's mother is cascade testing for affected-status determination, not carrier screening.
Cascade testing: appropriate for at-risk female relatives once a familial variant is known.
Prenatal testing: "Once the NSDHL pathogenic variant has been identified in a family member with an NSDHL-related disorder, prenatal and preimplantation genetic testing are possible." (GeneReviews, PMID:21290788). Prenatal ultrasound may detect limb reduction defects.
Consider NSDHL testing in recurrent male pregnancy loss — the actionable insight from PMID:36504312.

11. Outcome / Prognosis

Survival and mortality

Affected females: normal or near-normal life expectancy when severe visceral malformations are absent. Multiple adult patients are reported at ages 14, 32, 33, and older (PMID:40464756, PMID:16776722, StatPearls, PMID:39466221).
Affected males: prenatal lethality is the rule — "male conceptuses with an NSDHL pathogenic variant generally abort or resorb spontaneously" (GeneReviews, PMID:21290788); documented at 10+6 weeks' gestation (PMID:36504312). Surviving males are somatic mosaics.
Early mortality in affected females occurs with severe cardiac or pulmonary malformation. StatPearls: cardiac disease is "potentially fatal in early weeks if severe."
⚠️ No 5-year or 10-year survival figures, no mortality rate, and no disease-specific mortality data exist. There is no registry, no SEER-equivalent, no GBD entry. Do not curate numeric survival statistics.

Morbidity and function

Dermatologic: chronic pruritus, scaling, malodor, secondary infection, disfigurement. Substantially modifiable with modern topical therapy.
Musculoskeletal: the dominant long-term disability driver — contractures, immobility, poor dexterity, scoliosis. StatPearls lists these explicitly.
Neurologic: hearing loss, visual impairment, and cognitive impairment in the minority with CNS involvement — StatPearls lists "hearing loss, blindness, cognitive impairment" among complications. But recall: "Intellect is usually normal."
Quality of life: ⚠️ No EQ-5D, SF-36, PROMIS, DLQI, or disease-specific instrument has been applied. This is a real gap for a visibly disfiguring lifelong condition and is worth curating as a KNOWLEDGE_GAP.

Disease course and complications

Squamous cell carcinoma arising in a CHILD nevus — documented in a 33-year-old female (StatPearls, NBK507813). ⚠️ A single case; do not curate as an established cancer risk, but do note it as a surveillance rationale.
Secondary skin infection from the eroded/macerated lesions.
Cardiac failure / reduced exercise tolerance depending on severity.
Progressive scoliosis and contracture without orthopedic management.
Gastrointestinal xanthomas — a newly appreciated late/adult manifestation (PMID:39466221, PMID:40517742).

Recovery potential: skeletal malformations are permanent. Skin lesions are now largely reversible with pathogenesis-based therapy — the most important prognostic change of the last 15 years.

Prognostic factors

Extent and severity of visceral (especially cardiac and pulmonary) involvement — the dominant determinant of early survival.
Laterality: "Left-sided involvement carries poorer prognosis due to higher visceral abnormality prevalence" (StatPearls). ⚠️ Review-level synthesis, no cohort backing.
Severity of limb reduction — determines lifelong function.
CNS involvement — determines cognitive/neurological outcome.
Age — skin improves with time.
Access to pathogenesis-based topical therapy.
Prognostic biomarkers: ⚠️ none identified. Sterol levels are diagnostic, not prognostic. No molecular marker predicts disease course.

12. Treatment

The headline: CHILD syndrome is a flagship success story for pathogenesis-based topical therapy

The core logic — supply the missing end product, and simultaneously shut down flux into the toxic intermediates — is what makes this treatment work, and it is why cholesterol alone fails.

"Topical treatment with lovastatin/cholesterol (but not cholesterol alone) virtually cleared skin lesions by 3 months, accompanied by histological and ultrastructural normalization of epidermal structure and lipid secretion... These findings validate pathogenesis-based therapy that provides the deficient end product and prevents accumulation of toxic metabolites, an approach of potential utility for other syndromic lipid metabolic disorders." — Paller AS et al. J Invest Dermatol. 2011;131(11):2242-8. PMID:21753784 (HUMAN_CLINICAL, n=2)

Pharmacotherapy — topical (skin-directed)

Therapy	Reported effect	Evidence
Topical lovastatin 2% + cholesterol 2%	"Complete healing in a few persons" (GeneReviews); "virtually cleared skin lesions by 3 months"; complete clearance in bilateral disease	PMID:21753784 (n=2); PMID:42083494 (n=1, bilateral); PMID:40464756 (n=1, 4 weeks, no adverse events)
Topical simvastatin 2% + cholesterol 2%	"Remarkable improvement"; verruciform and VX-like lesions "improved obviously"	PMID:29341259 (n=1); PMID:42082353 (n=1)
Topical simvastatin monotherapy	"Pathogenesis-based therapy: Cutaneous abnormalities of CHILD syndrome successfully treated with topical simvastatin monotherapy"	Bajawi SM et al. JAAD Case Rep. 2018;4(3):232-234. PMID:29687057 ⚠️ title only — no abstract in PubMed
Topical simvastatin 5% ointment	Reported effective in a 4-year-old	Acta Med Philipp 2024. PMID:39431262
Topical ketoconazole 2%	GeneReviews: "90% reduction of lesions after 10 days" (oral/topical ketoconazole)	GeneReviews PMID:21290788; "Improvement of Skin Lesions in an Adult with CHILD Syndrome Treated with 2% Ketoconazole Cream," Omi M et al. Acta Derm Venereol. 2024;104:adv41929. PMID:39565229 ⚠️ title only
Glycolic acid (added to statin/cholesterol)	"Improved penetrance into thick skin scales" (GeneReviews)	PMID:21290788
Topical tretinoin	Localized benefit	StatPearls NBK507813
Lactic acid 12%	"can reduce itching"	GeneReviews PMID:21290788
Urea creams	"can reduce dryness"	GeneReviews PMID:21290788
Topical corticosteroids, emollients	Adjunctive; "limited evidence"	StatPearls NBK507813

Mechanism of each agent (for KB target_mechanisms curation): - Cholesterol (topical) — ACTIVATES/restores the deficient end product node ("Cholesterol Deficiency in Epidermis"). CHEBI:16113. - Lovastatin / simvastatin (topical) — INHIBITS HMG-CoA reductase, cutting flux into the pathway and thereby the node "Toxic Methylsterol Intermediate Accumulation." CHEBI:40303 (lovastatin), CHEBI:9150 (simvastatin). - Ketoconazole — INHIBITS CYP51A1 (lanosterol 14α-demethylase), blocking the pathway upstream of the NSDHL step and thus also reducing accumulation of C4-methylsterols. Mechanistic support: "Metabolic block upstream of SC4MOL with ketoconazole or CYP51A1 siRNA rescued cancer cell viability and EGFR degradation" (PMID:23125191). CHEBI:47519.

⚠️ Critical curation note: statin monotherapy and statin+cholesterol combination target different nodes and are not interchangeable. Paller's negative result — cholesterol alone was ineffective — is a REFUTE/PARTIAL evidence item worth curating explicitly, because it is what establishes the toxicity arm of the mechanism.

Systemic pharmacotherapy

Systemic retinoids — for widespread involvement (StatPearls). Long-term skeletal toxicity is a concern in a population that already has skeletal disease.
Oral ketoconazole — GeneReviews mentions oral or topical; hepatotoxicity and drug-interaction burden argue against routine oral use now that topical works.
Oral/dietary cholesterol supplementation — not effective for skin (barrier-compartment problem). Do not curate as a skin therapy.

Pharmacogenomics

⚠️ No pharmacogenomic data for CHILD syndrome. No PharmGKB/CPIC guidance. Standard CYP3A4/SLCO1B1 considerations for systemic statins are irrelevant at topical doses; ketoconazole is itself a potent CYP3A4 inhibitor, which is a drug-interaction consideration for oral use.

Advanced therapeutics

Gene therapy: ⚠️ none in development. Skin is theoretically an attractive gene-therapy target (cf. beremagene geperpavec for dystrophic EB), and the mosaic nature means only mutant clones need correction — but nothing is published.
Cell therapy: the closest published analogue is skin grafting: "Treatment of an inflammatory nevus by grafting skin obtained from a contralateral unaffected region has been successful" (GeneReviews) — "Successful in 1 person." This is an elegant exploitation of the mosaicism: the contralateral side carries wild-type-expressing cells.
RNA-based therapies: ⚠️ none. Splice-modulating ASOs could conceivably address splice-region variants like c.790-6C>T, but nothing exists.
Pharmacological chaperones — the most promising emerging direction. The Fenton 2025 finding that glycerol and lower temperature rescue mutant NSDHL expression (PMID:40222685) is an explicit therapeutic lead: the authors state their findings "can help inform future treatments for CHILD and CK syndrome." IN_VITRO only.
Targeted therapies / immunotherapies: not applicable.

Surgical and interventional

Orthopedic surgery — corrective surgery for limb deformity, contracture release, scoliosis correction. NCIT:C16186 Orthopedic Surgical Procedure.
Cardiac surgery — for septal defects and complex cardiac anomalies. NCIT:C15329 Surgical Procedure.
Dermatologic surgery / skin grafting — from contralateral unaffected skin (GeneReviews).
Bracing — "Scoliosis and joint contractures are treated with braces and/or corrective surgery" (GeneReviews).

Supportive and rehabilitative

Emollients, keratolytics, itch control.
Physical therapy (NCIT:C15302), occupational therapy, prosthetics/orthotics for limb deficiency.
Rehabilitation (NCIT:C15315), supportive care (NCIT:C15747).
Genetic counseling (NCIT:C15240) — essential given the 33/33/33 live-born recurrence distribution.
Multidisciplinary team — StatPearls: "orthopedic surgeons for limb defects, pediatric cardiologists for cardiac abnormalities, neurologists for neurologic symptoms, dermatologists for skin disease."

Experimental treatments / clinical trials

⚠️ No clinical trials of any intervention for CHILD syndrome were identified. All therapeutic evidence is from case reports and small series (n = 1–2). There is no NCT identifier to curate. This is expected for a disorder with fewer than 100 reported patients, but it means every treatment claim in this section rests on uncontrolled, unblinded, single-patient evidence with obvious publication bias toward successes.

Treatment outcomes and adverse events

Response rates: uniformly reported as good-to-complete for topical statin/cholesterol — but from a published-cases denominator that is certainly biased. GeneReviews' honest framing: "In CHILD syndrome, no one therapy described to date appears to ameliorate the cutaneous findings for every reported individual." (PMID:21290788)
Adverse events: minimal reported. PMID:40464756: "we didn't observe any adverse events." Systemic absorption of topical statins over large body-surface areas in infants is a theoretical concern that has not been systematically studied. ⚠️ No FAERS signal or systematic safety data exists for compounded topical statin/cholesterol in this population.

Treatment strategy / algorithm

There is no published guideline. The de facto algorithm from the literature:

Confirm diagnosis (clinical morphology → NSDHL sequencing + del/dup → sterol profiling if variant is a VUS).
Comprehensive organ evaluation at diagnosis — echo, renal US, chest imaging, skeletal survey (early, while stippling is visible), brain MRI, audiology, ophthalmology.
Initiate topical pathogenesis-based therapy — compounded 2% statin (lovastatin or simvastatin) + 2% cholesterol, twice daily; add glycolic acid for thick scale. Ketoconazole 2% cream is a reasonable alternative or adjunct.
Symptomatic skin care — 12% lactic acid for pruritus, urea for xerosis, emollients.
Organ-specific management by the relevant subspecialty.
Orthopedic management of contracture and scoliosis, with prosthetics/rehabilitation.
Genetic counseling for the family, including discussion of recurrent male miscarriage risk and prenatal/PGT options.
Lifelong surveillance (see below).

Surveillance (GeneReviews, PMID:21290788), annually or as needed: - "Examine for new cutaneous manifestations; new lesions may occur in puberty or early adulthood" - Clinical assessment for joint contractures and scoliosis - Assessment for neurologic, cardiac, or kidney manifestations with imaging

Personalized medicine: the only genotype-guided consideration is EBP- vs NSDHL-related disease, and even that does not currently change therapy (both are downstream cholesterol-pathway blocks amenable to the same topical strategy). ⚠️ Note that a statin would also be mechanistically rational for EBP-related disease, but this has not been tested.

NCIT term suggestions

Treatment	`treatment_term` (NCIT)	`therapeutic_agent` (CHEBI, verified)
Topical lovastatin/cholesterol	NCIT:C15986 Pharmacotherapy	CHEBI:40303 lovastatin + CHEBI:16113 cholesterol
Topical simvastatin/cholesterol	NCIT:C15986 Pharmacotherapy	CHEBI:9150 simvastatin + CHEBI:16113 cholesterol
Topical ketoconazole	NCIT:C15986 Pharmacotherapy	CHEBI:47519 ketoconazole
Skin grafting from contralateral side	NCIT:C15329 Surgical Procedure	—
Orthopedic correction	NCIT:C16186 Orthopedic Surgical Procedure	—
Physical therapy	NCIT:C15302 Physical Therapy	—
Rehabilitation	NCIT:C15315 Rehabilitation	—
Supportive care / emollients	NCIT:C15747 Supportive Care	—
Genetic counseling	NCIT:C15240 Genetic Counseling	—

therapeutic_modality suggestions: SMALL_MOLECULE for all pharmacotherapy; SURGERY for grafting/orthopedic; BEHAVIORAL for physical/occupational therapy.

13. Prevention

Prevention levels

Primary prevention: not possible. The disease is a de novo or inherited germline event. There is no modifiable exposure, no vaccination, no behavioral intervention that reduces incidence. The only "primary prevention" available is reproductive: prenatal diagnosis or preimplantation genetic testing in families with a known NSDHL variant.
Secondary prevention (early detection + early treatment): highly relevant and effective. The distinctive neonatal skin lesion makes early recognition realistic; StatPearls emphasizes that imaging "detect[s] skeletal and visceral anomalies, enabling early intervention," and that "Early detection and timely management improve patient outcomes substantially." Early initiation of topical pathogenesis-based therapy prevents years of morbidity.
Tertiary prevention (preventing complications): the core of long-term care — orthopedic bracing/surgery to prevent fixed contracture and progressive scoliosis; emollients and prompt antimicrobial treatment to prevent secondary skin infection; annual surveillance for new lesions and for evolving cardiac/renal/neurologic manifestations (GeneReviews surveillance table, PMID:21290788).

Immunization

Not applicable — no infectious etiology, no vaccine strategy. Routine childhood immunizations should proceed normally.

Screening and early detection

Population/newborn screening: not performed and not recommended — no biochemical marker suitable for dried-blood-spot screening (serum cholesterol is normal), and the phenotype is visible at birth anyway.
Genetic screening:
Cascade testing of at-risk female relatives once a familial variant is identified.
Prenatal testing (CVS/amniocentesis) and preimplantation genetic testing (PGT-M) — both explicitly available per GeneReviews: "Once the NSDHL pathogenic variant has been identified in a family member with an NSDHL-related disorder, prenatal and preimplantation genetic testing are possible."
Prenatal ultrasound for limb reduction defects in at-risk pregnancies.
Consider NSDHL testing in couples with recurrent male pregnancy loss (PMID:36504312) — a genuinely new, actionable screening indication.
Risk stratification: the only stratifier is maternal carrier status. Within an affected individual, the presence of cardiac/pulmonary anomalies stratifies early-mortality risk.

Behavioral interventions

Not preventive of disease. Relevant only to symptom management: gentle skin care, avoidance of flexural maceration and friction, sun protection over affected skin (given the single reported SCC), and consistent emollient use.

Counseling

Genetic counseling is a core, non-optional component of care. Key content: - X-linked dominant, male-lethal inheritance. - The 33% / 33% / 33% expected live-born distribution for a heterozygous mother. - Male conceptuses with the variant "generally abort or resorb spontaneously" — so recurrent male miscarriage is expected and should be anticipated rather than treated as an unexplained obstetric problem. - Possibility of somatic (and, theoretically, germline) mosaicism. - Availability of prenatal testing and PGT-M. - Highly variable expressivity means an affected daughter's severity cannot be predicted from the mother's.

Source: GeneReviews NSDHL-Related Disorders, PMID:21290788.

Public health and environmental interventions

Not applicable. No sanitation, vector-control, health-education, or environmental-remediation measure is relevant.

Prophylaxis

No preventive medication or procedure exists. The nearest analogue is continuous maintenance topical therapy to prevent lesion recurrence — but this is maintenance treatment, not prophylaxis.

14. Other Species / Natural Disease

Taxonomy

Mus musculus — NCBITaxon:10090 — the only species with a well-characterized Nsdhl disease phenotype.
Homo sapiens — NCBITaxon:9606.
Saccharomyces cerevisiae — NCBITaxon:4932 — the ERG26 ortholog, used for complementation assays (PMID:21129721).
Arabidopsis thaliana — NCBITaxon:3702 — 3β-hydroxysteroid dehydrogenase/C4-decarboxylase orthologs are "essential for the pollen and embryonic development" (Int J Mol Sci. 2023 Oct 25. PMID:37958553), a striking demonstration of how deeply conserved the requirement for C4-demethylation is.

Breed

Not applicable — no VBO-codable breed predisposition; no naturally occurring companion-animal or livestock form of this disease has been reported.

Orthologous genes

Mouse Nsdhl — UniProt Q9R1J0 (the ortholog UniProt uses as the evidence source for most human NSDHL functional annotations, ECO:0000250 "by similarity"). NCBI Gene ID: ⚠️ not verified in this session.
Yeast ERG26 — functionally complements human NSDHL, established experimentally (PMID:21129721).
Arabidopsis 3βHSD/C4-decarboxylases (PMID:37958553).

Natural disease in other species

The mouse mutants bare patches (Bpa) and striated (Str) are naturally arising (X-irradiation-induced) X-linked dominant male-lethal mutations that turned out to be allelic Nsdhl mutations — and they were characterized before the human gene was found, then used to find it. This is one of the cleanest cases of a mouse mutant leading to a human disease gene.

"The bare patches (Bpa) and striated (Str) mouse mutations were originally identified in female offspring of X-irradiated males... Here we report mutations in one of these genes, Nsdhl, encoding an NAD(P)H steroid dehydrogenase-like protein, in two independent Bpa and three independent Str alleles. Quantitative analysis of sterols from tissues of affected Bpa mice support a role for Nsdhl in cholesterol biosynthesis. Our results demonstrate that Bpa and Str are allelic mutations and identify the first mammalian locus associated with an X-linked dominant, male-lethal phenotype." — Liu XY, Dangel AW, Kelley RI, Zhao W, Denny P, Botcherby M, Cattanach B, Peters J, Hunsicker PR, Mallon AM, Strivens MA, Bate R, Miller W, Rhodes M, Brown SD, Herman GE. Nat Genet. 1999;22(2):182-7. PMID:10369263 (MODEL_ORGANISM)

König et al. explicitly recognized their value: "Two mouse X-linked dominant male-lethal traits, bare patches (Bpa) and striated (Str) had previously been associated with mutations in Nsdhl. They provide animal models for the study of CHILD syndrome" (PMID:10710235).

Veterinary relevance: none. There is no naturally occurring NSDHL disease of veterinary importance; these are laboratory mutants.

Comparative biology

Comparative pathology: Bpa/Str heterozygous females recapitulate the core human features — patchy/striped skin lesions along Blaschko-equivalent lines from X-inactivation mosaicism, skeletal abnormalities, and male hemizygous lethality. Human CHILD's diffuse lateralization vs mouse striping likely reflects differences in the geometry of clonal expansion in the two species' skin.
Evolutionary conservation: the C4-demethylation step is conserved from yeast (ERG26) through plants (Arabidopsis) to mammals, and the human enzyme functionally complements the yeast mutant (PMID:21129721) — strong evidence of deep conservation of both structure and function.

Transmission

Not applicable — no zoonotic potential, no cross-species susceptibility. This is a heritable metabolic disorder.

15. Model Organisms

Mouse — the workhorse

Spontaneous/induced X-linked dominant male-lethal alleles:

Model	Type	Key features	Reference
bare patches (Bpa) — multiple independent alleles including Bpa^1H^	X-irradiation-induced point/structural mutations in Nsdhl	X-linked dominant, male-lethal; heterozygous females show patchy skin lesions, skeletal abnormalities, and mosaic NSDHL expression. Hemizygous males "die by midgestation"	PMID:10369263; PMID:19631568; PMID:25652406
striated (Str) — three independent alleles	Allelic to Bpa	Striped coat/skin phenotype from X-inactivation mosaicism	PMID:10369263
*Nsdhl^tm1.1Hrm^* — conditional (floxed) allele	Engineered, Cre-conditional	Built precisely because "hemizygous male mice with Nsdhl mutations die by midgestation"; crossed to GFAP-cre to ablate Nsdhl in radial glia	PMID:25652406

Phenotype recapitulation:

Strengths — - Male lethality is faithfully reproduced ("hemizygous male mice with Nsdhl mutations die by midgestation," PMID:25652406). - Mosaic skin phenotype from X-inactivation is reproduced. - The negative-selection mechanism is reproduced and quantifiable — Bpa^1H^/+ liver dropped from ~50% NSDHL-negative cells at P6 to ~20% at one year (PMID:19631568). This mouse result predicted and explains the human lateralization finding. - Sterol biochemistry is reproduced — "Quantitative analysis of sterols from tissues of affected Bpa mice support a role for Nsdhl in cholesterol biosynthesis" (PMID:10369263). - Growth-factor-receptor trafficking defect is reproduced in vivo — "Analysis of Nsdhl-deficient Bpa(1H/+) mice confirmed dramatic and selective loss of internalized platelet-derived growth factor receptor in fibroblasts, and reduced activation of EGFR and its effectors in regions of skin lacking NSDHL" (PMID:23125191). - Developmental expression mapping — highest embryonic Nsdhl expression in "liver, dorsal root ganglia, central nervous system, retina, adrenal gland and testis" (PMID:19631568), which usefully predicts which human organs are at risk.

Limitations — - Skin lesion morphology differs: mouse shows striping/patches; human shows diffuse lateralization with strict midline demarcation and ptychotropism. The most diagnostically characteristic human features are not reproduced. - The limb reduction defects central to the human acronym are not the mouse model's dominant feature. - Hemizygous male embryonic lethality prevents study of the null state in vivo without conditional alleles — the reason Nsdhl^tm1.1Hrm^ was made. - The conditional CNS model is a CK-syndrome/neurodevelopmental model, not a CHILD model: GFAP-cre; Nsdhl^fl^ males "develop overt ataxia by postnatal day 8-10 and die shortly thereafter" (PMID:25652406) — informative about cholesterol in brain development, but not about the lateralized cutaneous disease. - ⚠️ Curate CNS phenotypes from this model with a HUMAN_MODEL_MISMATCH discussion, since human CHILD syndrome usually has normal intellect.

Yeast

Saccharomyces cerevisiae Erg26-deficient complementation assay (IN_VITRO) — used as a functional readout for human NSDHL variants: the CK-syndrome alleles "show temperature-sensitive protein stability and complementation in Erg26-deficient yeast" (PMID:21129721). This is a validated functional assay for VUS resolution and is directly relevant to ACMG PS3-level evidence.

Cell-based and in vitro systems

Patient-derived fibroblasts — the standard system for sterol profiling and functional confirmation.
Heterologous expression systems for mutant NSDHL stability — the 13-missense-variant survey with temperature and chemical-chaperone rescue (PMID:40222685).
Cerebellar granule cell precursor cultures — used to demonstrate the SHH signaling defect and cholesterol rescue (PMID:25652406).
Cancer cell lines + A431 xenografts — for the NSDHL/SC4MOL–EGFR trafficking work (PMID:23125191).
iPSC-derived and CRISPR-edited systems — an area of active development but limited CHILD-specific progress. The 2026 review is the authoritative survey:

"This review provides a comprehensive overview of currently available experimental models used to investigate inherited cholesterol biosynthesis disorders, including genetically engineered animal models, patient-derived fibroblasts, immortalized and CRISPR/Cas9-edited cell lines, and induced pluripotent stem cell-based systems. Particular emphasis is placed on Smith-Lemli-Opitz syndrome, the most extensively studied disorder within this group, while recent advances in modeling desmosterolosis, lathosterolosis, and congenital hemidysplasia with ichthyosiform nevus and limb defects syndrome are also critically discussed." — Akhmetzyanova E, Nasybullina E, Rizvanov A, Mukhamedshina Y. Int J Mol Sci. 2026;27(15):6853. PMID:42589509

Note the framing: CHILD syndrome is explicitly the less well-modeled member of its disease class. This is the single best citation for a knowledge-gap statement about CHILD syndrome modeling.

Not available

Zebrafish, Drosophila, C. elegans, rat — ⚠️ no published NSDHL/CHILD model identified in any of these.
Organoids / organ-on-chip — ⚠️ no skin organoid or epidermal-equivalent CHILD model published. Given that this is fundamentally an epidermal barrier disease with a topical therapy, a 3D human epidermal equivalent carrying an NSDHL knockout would be an obvious and currently missing tool.

Model databases

MGI (Mouse Genome Informatics) — Nsdhl alleles: Bpa series, Str series, Nsdhl^tm1.1Hrm^. ⚠️ Specific MGI allele accessions not retrieved in this session; look them up before curating identifiers.
IMPC / KOMP / IMSR / MMRRC / EMMA — ⚠️ availability of Nsdhl lines not verified.
Alliance of Genome Resources — for ortholog and phenotype integration.
SGD — ERG26.

Research applications the models support: sterol biochemistry of the C4-demethylation block; X-inactivation mosaicism and clonal selection dynamics; SHH signaling dependence on cholesterol; EGFR/ERBB and PDGFR endocytic trafficking; testing of pharmacological chaperones; preclinical testing of topical pathogenesis-based formulations.

Summary of Key Knowledge Gaps (for `discussions` / `KNOWLEDGE_GAP` curation)

No reliable prevalence estimate. The widely-repeated "1 in 100,000" figure is inconsistent with the ">60 reported individuals" case count and should not be curated as a rate.
No natural-history study, registry, or longitudinal cohort. All frequency figures are case-report-derived.
No quality-of-life measurement of any kind in a visibly disfiguring, lifelong condition.
No clinical trial of any therapy. All treatment evidence is n = 1–2 uncontrolled case reports with obvious positive-publication bias, and GeneReviews' own caveat stands: no therapy works for everyone.
The right > left laterality bias (~2:1) is reported but entirely unexplained mechanistically.
The claim that left-sided disease has worse prognosis is review-level synthesis without cohort support.
No single-cell or spatial transcriptomics — despite the disease being an almost ideal subject (sharp mosaic boundary, cell-autonomous selection).
No skin organoid / epidermal-equivalent model, despite the disease being an epidermal barrier disorder with a topical therapy.
Systemic absorption and long-term safety of compounded topical statins in neonates and infants over large body-surface areas is unstudied.
The EBP- vs NSDHL-caused boundary of "CHILD syndrome" remains nosologically unresolved (PMID:10710233) — a genuine lump/split question for the KB.
Mild adult phenotypes (GI xanthomas, ichthyosis without limb defects) are only just being recognized (PMID:39466221, PMID:40517742, PMID:42082353), implying substantial historical under-ascertainment and probable under-estimation of prevalence.
ICD-10 and ICD-11 mappings could not be authoritatively verified in this session.

Sources

Primary literature (PubMed): - Happle R, Koch H, Lenz W. The CHILD syndrome. Eur J Pediatr. 1980. PMID:7408908 - Happle R. Epidermal nevus syndromes. Semin Dermatol. 1995. PMID:7640190 - Liu XY et al. The gene mutated in bare patches and striated mice encodes a novel 3beta-hydroxysteroid dehydrogenase. Nat Genet. 1999. PMID:10369263 - König A et al. Mutations in the NSDHL gene... cause CHILD syndrome. Am J Med Genet. 2000. PMID:10710235 - Grange DK et al. CHILD syndrome caused by deficiency of 3beta-hydroxysteroid-delta8,delta7-isomerase. Am J Med Genet. 2000. PMID:10710233 - König A et al. A novel missense mutation of NSDHL in an unusual case of CHILD syndrome showing bilateral, almost symmetric involvement. J Am Acad Dermatol. 2002. PMID:11907515 - Bornholdt D et al. Mutational spectrum of NSDHL in CHILD syndrome. J Med Genet. 2005. PMID:15689440 - Ishibashi M et al. Abnormal lamellar granules in a case of CHILD syndrome. J Cutan Pathol. 2006. PMID:16776722 - Cunningham D et al. Developmental expression pattern of the cholesterogenic enzyme NSDHL and negative selection of NSDHL-deficient cells in the heterozygous Bpa(1H)/+ mouse. Mol Genet Metab. 2009. PMID:19631568 - McLarren KW et al. Hypomorphic temperature-sensitive alleles of NSDHL cause CK syndrome. Am J Hum Genet. 2010. PMID:21129721 - Paller AS et al. Pathogenesis-based therapy reverses cutaneous abnormalities in an inherited disorder of distal cholesterol metabolism. J Invest Dermatol. 2011. PMID:21753784 - Sukhanova A et al. Targeting C4-demethylating genes in the cholesterol pathway sensitizes cancer cells to EGF receptor inhibitors. Cancer Discov. 2013. PMID:23125191 - Cunningham D et al. Analysis of hedgehog signaling in cerebellar granule cell precursors in a conditional Nsdhl allele. Hum Mol Genet. 2015. PMID:25652406 - Yu X et al. CHILD syndrome mimicking verrucous nevus... simvastatin and cholesterol. J Eur Acad Dermatol Venereol. 2018. PMID:29341259 - Bajawi SM et al. Pathogenesis-based therapy: ...topical simvastatin monotherapy. JAAD Case Rep. 2018. PMID:29687057 - Maceda EBG et al. Novel NSDHL gene variant for CHILD syndrome. BMJ Case Rep. 2020. PMID:33139364 - Zhuang J et al. Etiological identification of recurrent male fatality due to a novel NSDHL gene mutation. Mol Genet Genomic Med. 2023. PMID:36504312 - Omi M et al. Improvement of Skin Lesions in an Adult with CHILD Syndrome Treated with 2% Ketoconazole Cream. Acta Derm Venereol. 2024. PMID:39565229 - Kim DH et al. Gastrointestinal Xanthomas and Ichthyosis: A Mild Phenotype of CHILD Syndrome. Am J Gastroenterol. 2025. PMID:39466221 - Fenton NM et al. Comprehensive survey of disease-causing missense mutations of NSDHL. J Steroid Biochem Mol Biol. 2025. PMID:40222685 - Chen K et al. CHILD syndrome combined linear porokeratosis... topical lovastatin/cholesterol ointment. J Dermatolog Treat. 2025. PMID:40464756 - Olarewaju BA et al. Colonic xanthomas in an adult... NSDHL haploinsufficiency. Mol Genet Metab. 2025. PMID:40517742 - Vergara A et al. Diagnostic Utility of Optical Genome Mapping in X-Linked Dominant Genodermatoses. Mol Syndromol. 2025. PMID:41625319 - Wyer J et al. CHILD Syndrome without Limb Defects in a 1-year-old: revised nomenclature. Clin Exp Dermatol. 2026. PMID:42082353 - Zeyrek M et al. Bilateral Involvement in CHILD Syndrome Successfully Treated With Cholesterol-Lovastatin Combination. Pediatr Dermatol. 2026. PMID:42083494 - Akhmetzyanova E et al. Modeling Inherited Disorders of Post-Lanosterol Cholesterol Biosynthesis. Int J Mol Sci. 2026. PMID:42589509

Reference resources: - GeneReviews®: NSDHL-Related Disorders (NBK51754), last update 5 Sep 2024. PMID:21290788 - StatPearls: CHILD Syndrome (NBK507813) - OMIM #308050 — CHILD syndrome (⚠️ returned HTTP 403 to automated fetch; identifiers taken from MONDO/HPO cross-references) - HPO disease annotations for OMIM:308050 - MONDO:0010621 via EBI OLS4 - Orphanet ORPHA:139 (definition via https://api.orphacode.org) - UniProt Q15738 — NSDHL / sterol-4-alpha-carboxylate 3-dehydrogenase - HGNC:13398 — NSDHL - ClinVar — NSDHL variants - GTR test 317493 — NSDHL-Related Disorders - FIRST — Foundation for Ichthyosis & Related Skin Types: CHILD Syndrome

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome	Count
References checked	36
Resolved	36
Unresolved (possible confabulation)	0
Unverifiable	0
Quoted claims checked	33
Quoted claims found in source	27

Quotes not found in the cited source

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

PMID:21290788: "Onychodystrophy, onychorrhexis, and periungual hyperkeratosis are common"
closest text in source: "Onychodystrophy and periungual hyperkeratosis are common"
PMID:21290788: "functions as a C4 demethylase in post-squalene cholesterol biosynthesis"
closest text in source: "Topical statin treatment alone or combined with cholesterol and/or glycolic acid can be beneficial"
PMID:21290788: "new lesions may occur in puberty or early adulthood"
closest text in source: "The ichthyosiform skin lesions are usually present at birth or in the first weeks of life; new lesions can develop in later life"
PMID:21290788: "mosaic for NSDHL pathogenic variant c.262C>T (p.Arg88Ter)"
closest text in source: "CK syndrome: If the mother of a proband is heterozygous for an NSDHL pathogenic variant, the expected chance of transmitting it in each pregnancy is 50%: males who inherit the pathogenic variant will be affected; females who inherit the pathogenic variant will be heterozygous and may have a range of behavioral problems"
PMID:35853659: "encompasses a spectrum of inflammatory mosaic disorders"
closest text in source: "CONCLUSION: ILVEN is a clinical descriptor for a heterogenous group of mosaic inflammatory disorders"
PMID:37958553: "essential for the pollen and embryonic development"
closest text in source: "Pollen-specific expression of HSD2 in the heterozygous hsd1 hsd2/+ mutant not only rescued the pollen lethality but also revealed the critical roles of the two HSD genes in embryogenesis"

Ask OpenScientist

OpenScientist Report

Classifications

Inheritance

Discussions and Knowledge Gaps

Pathophysiology

Histopathology

Pathograph

Phenotypes

Genetic Associations

Medical Actions

Biochemical Markers

Diagnosis

Progression

Prevalence

Differential Diagnoses

Experimental Models

Animal Models

Source YAML

References & Deep Research

References

Deep Research

CHILD Syndrome — Comprehensive Research Report

1. Disease Information

Overview

Key identifiers

Synonyms and alternative names

Data provenance

2. Etiology

Primary causal factor

A genetically distinct minority: EBP-related CHILD-like phenotype

Risk factors

Protective factors

Gene–environment interactions

3. Phenotypes

3.1 Cutaneous phenotypes (the defining domain)

3.2 Musculoskeletal / limb phenotypes

3.3 Visceral phenotypes (all characteristically ipsilateral)

3.4 Laboratory abnormalities

3.5 Clinical course descriptors

4. Genetic / Molecular Information

Causal gene

Pathogenic variants

Functional consequences of variants

Modifier genes

Epigenetic information

Chromosomal abnormalities

5. Environmental Information

6. Mechanism / Pathophysiology

The causal chain, from lesion to clinic

The two-hit metabolic logic (deficiency and toxicity)

The lateralization mechanism — cell-autonomous negative selection

Sonic hedgehog signaling — the link to malformation

Protein dysfunction

Metabolic changes

Tissue damage mechanisms

Immune system involvement

The unexpected oncology connection (NSDHL–EGFR trafficking)

Molecular profiling

Cell types and biological processes for KB curation

7. Anatomical Structures Affected

Organ level

Tissue and cell level

Subcellular level

Localization and lateralization

8. Temporal Development

Onset

Progression

Patterns

9. Inheritance and Population

Epidemiology

Genetic epidemiology

Population demographics

The allelic disorder, for contrast

10. Diagnostics

Clinical tests

Genetic testing

Omics-based diagnostics

Clinical criteria

Differential diagnosis

Summary of Key Knowledge Gaps (for `discussions` / `KNOWLEDGE_GAP` curation)