Peroxisome Biogenesis Disorder 4B

1. Disease Information

Overview. Peroxisome biogenesis disorder 4B (PBD4B) is an autosomal recessive peroxisomal biogenesis disorder caused by biallelic (or, rarely, a specific monoallelic allelic-expression-imbalance mechanism — see §4) pathogenic variants in PEX6. It sits within the Zellweger spectrum disorder (ZSD) continuum — a single phenotypic spectrum historically split into three named entities (Zellweger syndrome > neonatal adrenoleukodystrophy [NALD] > infantile Refsum disease [IRD], severe→mild) that are now understood to reflect residual peroxisomal function rather than distinct diseases (GeneReviews, NBK1448). PEX6 defects are the second most common cause of ZSD after PEX1, accounting for roughly 10–14.5% of ZSD cases (GeneReviews, NBK1448; Ebberink et al. 2010, PMID:19877282). PEX6-caused ZSD spans an unusually wide severity range — from classic lethal neonatal Zellweger syndrome (PBD4A, OMIM:614862) through NALD/IRD-type PBD4B, to an ataxia–deafness–blindness (SCAR3/SCABD) presentation recognized in later childhood/adulthood, to the mildest end, Heimler syndrome 2 (hearing loss + amelogenesis imperfecta ± mild/late retinal disease, OMIM:616617).

Key identifiers: | Resource | ID | |---|---| | OMIM (phenotype, milder/NALD-IRD end) | #614863 PEROXISOME BIOGENESIS DISORDER 4B; PBD4B | | OMIM (phenotype, classic Zellweger end, same gene) | #614862 PBD4A | | OMIM (gene) | *601498 PEX6 | | OMIM (mildest allelic end) | #616617 Heimler syndrome 2 (HMLR2) | | MONDO | MONDO:0013931 | | Orphanet (gene page) | ORPHA PEX6 gene entry; component disorders NALD ORPHA:44, Infantile Refsum disease ORPHA:772, Zellweger syndrome ORPHA:912, umbrella "Peroxisome biogenesis disorder, Zellweger syndrome spectrum" ORPHA:79189 | | MeSH | Zellweger Syndrome (D015211) | | ICD-11 | 5C56.0 Zellweger spectrum | | HGNC | PEX6, HGNC:8858 |

Synonyms: PBD4B; NALD (PEX6-caused); IRD/Infantile Refsum disease (PEX6-caused); SCAR3; SCABD; Autosomal recessive spinocerebellar ataxia-blindness-hearing loss syndrome; Autosomal recessive cerebellar ataxia-blindness-deafness syndrome (Monarch Initiative MONDO:0013931).

Evidence basis of this report: predominantly aggregated disease-level resources (OMIM, GeneReviews consensus chapter, Orphanet, systematic mutation surveys of 75–77 PEX6 patients) supplemented by individual case reports/small case series (Mixteco founder cluster n=3; late-onset PEX6 case n=1; French-Canadian founder cohort) rather than large single-cohort EHR data — consistent with an ultra-rare monogenic disease.

2. Etiology

Disease causal factor: Exclusively genetic/mechanistic — biallelic (or the specific monoallelic AEI mechanism, see §4) pathogenic loss-of-function or hypomorphic variants in PEX6 (6p21.1), encoding an AAA-ATPase peroxin required for peroxisomal matrix-protein import. There is no known environmental, infectious, or acquired cause; PBD4B is a Mendelian disorder in the strict sense.

Genetic risk factors: - Biallelic PEX6 pathogenic variants (missense, nonsense, frameshift, splice-site, large deletion) — causal. - The specific c.2578C>T (p.Arg860Trp) variant acting in the heterozygous state when in cis with a 3′UTR polyadenylation-site variant (rs144286892, c.442_445delTAAA) that causes allelic expression imbalance (AEI), effectively producing dominant-like disease from one overexpressed hypomorphic allele (Falkenberg et al. 2017, AJHG, PMC/ResearchGate; identified in 7 unrelated ZSD patients + 1 affected half-sibling). - Population-specific founder alleles increase local risk: a French-Canadian PEX6 founder mutation raising ZSD incidence in the Saguenay–Lac-Saint-Jean region of Quebec toward ~1/12,000 (vs. ~1/50,000 general North American incidence) (PMC3483250); a Mixteco-population founder variant c.1409G>C (p.Gly470Ala)* identified in 2/3 related neonatal cases from Central California (Slaton et al. 2023, Cureus, PMID:37842507). - Consanguinity increases risk in any AR disorder; not PEX6-specific but relevant to case ascertainment in the Mixteco and other founder clusters.

Protective factors: None specifically documented for PEX6-ZSD; general "protective" modifiers are hypomorphic (residual-function) missense alleles rather than null alleles — i.e., allelic severity itself is the modifying axis (genotype–phenotype correlation, §4), not an independent protective factor.

Gene–environment interactions: None established; this is a cell-autonomous biosynthetic/organelle-biogenesis defect not modulated by known exposures. (No CTD or GWAS-catalog environmental signal for PEX6-ZSD was found in this search; the disease is fully explained by the biallelic genetic lesion.)

Suggested terms: HP:0010984 (Digenic inheritance) is not applicable — PEX6-ZSD is monogenic AR, with the AEI mechanism being an unusual cis-regulatory dosage effect on a single locus rather than true digenic inheritance.

3. Phenotypes

Phenotype burden and severity track the ZSD continuum; PBD4B (PEX6, NALD/IRD-range) sits milder than classic Zellweger (PBD4A) but generally more severe than PEX1 p.Gly843Asp-type mild ZSD, though PEX6 alleles span an unusually broad range down to Heimler syndrome.

Quality-of-life impact: A dedicated caregiver-report QoL instrument for ZSD exists — "Proxy-Reported Symptoms and Quality of Life Survey in Zellweger Spectrum Disorders" (ClinicalTrials.gov NCT03440905) — but disease-specific EQ-5D/SF-36 published results were not surfaced in this search; QoL is dominated by combined sensory (hearing+vision) loss, motor/ataxia disability, and — where present — cognitive impairment and hepatic disease burden.

Severity/course as a class: A 2022 scoping review/meta-analysis, "Characterization of Severity in Zellweger Spectrum Disorder by Clinical Findings" (MDPI, Cells), formally stratifies ZSD severity by clinical-finding clusters and is a good source for quantitative frequency data across the whole ZSD population (PEX-gene-agnostic; PEX6 subgroup extractable).

4. Genetic/Molecular Information

Causal gene: PEX6 (HGNC:8858; NCBI Gene 5190; OMIM 601498), chromosome 6p21.1, 17 exons, encoding a AAA-ATPase family peroxin* with two tandem AAA-ATPase cassettes (Ebberink et al. 2010, PMID:19877282).

Variant landscape. A systematic screen of 75 PEX6-complementation-group patients identified 77 distinct mutations, 47 of them novel at the time, spanning missense, nonsense, frameshift, and splice-site classes (Ebberink et al. 2010, PMID:19877282: "Analysis of 75 patients assigned to the PEX6 complementation group revealed a total of 77 distinct mutations, with 47 being previously unreported and 14 representing polymorphic variants."). Loss-of-function alleles (nonsense/frameshift/large deletion) generally cluster with the severe (classic Zellweger, PBD4A) end; missense/hypomorphic alleles retaining partial function produce the PBD4B/NALD-IRD, SCAR3/SCABD, or Heimler-syndrome milder phenotypes (genotype–phenotype principle summarized in GeneReviews, NBK1448).

Notable specific variants: - c.2578C>T (p.Arg860Trp) — the unique monoallelic-sufficient PEX6 variant, pathogenic only when in cis with the 3′UTR AEI-driving variant rs144286892 (Falkenberg et al. 2017, Am J Hum Genet). This is a rare, mechanistically distinct example of dosage-driven "dominant" disease at a canonically AR peroxin locus. - c.1409G>C (p.Gly470Ala) — founder allele in the Mixteco population of Central California/Oaxaca-origin families, identified in 2 of 3 related neonatal PBD cases with severe (classic-range) presentation (Slaton et al. 2023, PMID:37842507). - A distinct French-Canadian founder PEX6 mutation elevates regional incidence in the Saguenay–Lac-Saint-Jean population of Quebec (PMC3483250).

Classification (ACMG/ClinVar): Multiple PEX6 variants are curated in ClinVar with pathogenic/likely-pathogenic classifications across "multiple conditions" (ZSD spectrum + Heimler syndrome), e.g., NM_000287.4(PEX6):c.2626C>T (p.Arg876Trp) reported for multiple conditions in ClinVar.

Population/allele frequency: Formal gnomAD-based carrier-frequency figures specific to PEX6 were not retrievable via this search pass (recommend a direct gnomAD v4 query for the curation step); one older ExAC data point noted a PEX6 c.1082G>A allele at ~0.41% in the European population (context/source secondary — verify directly before citing).

Somatic vs. germline: Germline only; PEX6-ZSD is not associated with somatic mosaicism reports in this search, though germline mosaicism cannot be excluded generically for an AR condition (no PEX6-specific report surfaced).

Functional consequence: Loss- or reduced-function of the PEX1/PEX6 AAA-ATPase heterohexameric motor (see Mechanism, §6) — impaired peroxisomal matrix-protein import, not a gain-of-function or dominant-negative mechanism in the classical biallelic-null cases; the AEI allele is a dosage/expression-level, not structural gain-of-function, mechanism.

Modifier genes: None specifically documented for PEX6 beyond the cis-acting 3′UTR AEI variant itself, which functions as its own allele-specific modifier.

Epigenetic/chromosomal information: No PEX6-specific DNA-methylation or chromosomal-rearrangement etiology was identified in this search; disease arises from coding/splice/UTR-regulatory sequence variants, not large chromosomal abnormalities.

Suggested gene/molecular terms: hgnc:8858 (PEX6); GO Molecular Function GO:0016887 (ATP hydrolysis activity) and GO:0043495 (protein-membrane adaptor activity, for the PEX1-PEX6-PEX26 anchoring complex) — verify exact GO ID via OAK before curation use.

5. Environmental Information

No environmental, lifestyle, or infectious contributing factors are described for PEX6-related PBD4B in the literature surveyed — it is a fully genetically determined organelle-biogenesis disorder. This section is not applicable beyond the population-genetic "environment" of founder effects in isolated/consanguineous communities (Mixteco, French-Canadian Saguenay–Lac-Saint-Jean) documented above, which are demographic/genetic rather than exposure-based risk factors.

6. Mechanism / Pathophysiology

Causal chain (upstream → downstream):

1. Molecular lesion: Biallelic PEX6 pathogenic variants (or the monoallelic AEI mechanism) reduce/abolish functional PEX6 protein.
2. Complex disruption: PEX6 normally heterohexamerizes with PEX1 to form the peroxisomal receptor export module (REM), anchored to the peroxisomal membrane via PEX26 (mammalian ortholog of yeast Pex15/plant APEM9) (Nature Communications 2023, s41467-023-41640-9; PMC5762779; PMC9265785 review "Insights into the Structure and Function of the Pex1/Pex6 AAA-ATPase in Peroxisome Homeostasis"). "Pex1 and Pex6 form a heterohexameric motor essential for peroxisome biogenesis and function, and mutations in these AAA-ATPases cause most peroxisome-biogenesis disorders in humans."
3. Failure of PEX5 receptor recycling: The PTS1-import receptor PEX5, after delivering matrix (PTS1-tagged) enzymes into the peroxisomal lumen, is mono-ubiquitinated at a conserved cysteine and must be extracted back to the cytosol by the PEX1/PEX6 AAA-ATPase, which processively threads and unfolds ubiquitinated PEX5 through its central pore in an ATP-hydrolysis-dependent manner (PMC5762779, "The peroxisomal AAA-ATPase Pex1/Pex6 unfolds substrates by processive threading"). Loss of PEX6 function stalls this receptor-recycling/export step.
4. Peroxisomal matrix protein import failure: With PEX5 recycling blocked, both PTS1- and PTS2-mediated matrix protein import are impaired — demonstrated directly in PEX6-knockout cells by immunofluorescence, alongside a reduction in peroxisome number; overexpression of wild-type PEX6 restored import, confirming causality and highlighting a genetic-therapy rationale (search synthesis of PEX6 knockout/complementation studies).
5. Metabolic consequences (biochemical abnormalities): Failure to import peroxisomal beta-oxidation and ether-lipid-synthesis enzymes produces the ZSD biochemical signature: elevated very-long-chain fatty acids (VLCFA, C26:0/C26:1), decreased plasmalogens (C16/C18 erythrocyte membrane), elevated pipecolic acid, elevated C27 bile-acid intermediates (THCA/DHCA), and elevated C26:0-lysophosphatidylcholine (C26:0-LPC) on dried blood spot (GeneReviews NBK1448; JIMD 2017, PMID:28677031, sensitivity 89.2% for C26:0-LPC).
6. Downstream tissue injury:
7. Hepatotoxicity from accumulated C27 bile-acid intermediates (THCA/DHCA) — rationale for cholic-acid replacement therapy (§12).
8. Neural/white-matter injury from VLCFA/plasmalogen deficiency contributing to demyelination (leukodystrophy) and, in the SCAR3/SCABD presentation, cerebellar white-matter changes without atrophy plus demyelinating peripheral motor neuropathy (MONDO:0013931 definition).
9. Sensorineural hearing loss and retinal dystrophy, thought to reflect the combined effect of impaired ether-phospholipid (plasmalogen) content — essential in myelin and photoreceptor/cochlear membranes — and VLCFA accumulation.
10. Renal, skeletal (osteopenia), adrenal, and dental (amelogenesis imperfecta) involvement at the milder end of the spectrum.

Cell types/tissues implicated: hepatocytes, cochlear hair cells/spiral ganglion, retinal photoreceptors/RPE, cerebellar Purkinje neurons and oligodendrocytes (white matter), peripheral Schwann cells (demyelinating neuropathy), adrenal cortex, renal tubular epithelium, ameloblasts (dental enamel).

Zebrafish/mouse mechanistic model data (§15) reinforce this chain: zebrafish pex1/pex2 loss-of-function recapitulates "increased tissue levels of VLCFA and branched chain fatty acids as well as a reduction in ether phospholipids," with gene-expression changes in "crystallin (lens), troponin, parvalbumin (muscle contraction), and fatty acid metabolic genes," directly linking the biochemical lesion to the cataract/lens and myopathic phenotypic themes seen clinically.

Suggested GO Biological Process terms (verify via OAK before use): GO:0016558 (protein import into peroxisome matrix), GO:0007031 (peroxisome organization), GO:0006635 (fatty acid beta-oxidation), GO:0008611 (ether lipid biosynthetic process / plasmalogen synthesis). Suggested CL terms: CL:0000182 (hepatocyte), CL:0000540 (neuron; refine to Purkinje cell / photoreceptor / cochlear hair cell as appropriate), CL:0002573 (Schwann cell), CL:0000064 (ciliated columnar cell — not applicable, remove) — refine per node.

7. Anatomical Structures Affected

Organ level (primary): liver, central nervous system (cerebrum white matter, cerebellum), peripheral nervous system, inner ear (cochlea), eye (retina, lens), adrenal gland, kidney, skeleton, teeth. Body systems: hepatobiliary, nervous (central and peripheral), special sensory (audiovestibular, visual), endocrine (adrenal), renal, skeletal, dental/craniofacial. Tissue/cell level: hepatocytes and biliary epithelium; cerebellar cortex (Purkinje cells) and cerebral/cerebellar white matter (oligodendrocytes/myelin); peripheral motor nerve myelin (Schwann cells); cochlear hair cells and spiral ganglion neurons; retinal photoreceptors and RPE; lens epithelium (cataract); adrenal cortical cells; renal tubular epithelium; ameloblasts/odontoblasts (enamel/dentin). Subcellular level: the peroxisome itself (matrix and membrane), with GO Cellular Component anchors GO:0005777 (peroxisome), GO:0005778 (peroxisomal membrane); secondary organelle stress in mitochondria (shared fission machinery/metabolic crosstalk) is plausible but not directly documented in this search. Localization/laterality: bilateral/symmetric in essentially all reported manifestations (symmetric leukodystrophy on MRI per PMID:25079577; bilateral sensorineural hearing loss; bilateral retinal dystrophy) — consistent with a systemic, non-lateralized metabolic mechanism.

Suggested UBERON terms (verify before use): UBERON:0002107 (liver), UBERON:0002037 (cerebellum), UBERON:0001851 (cortex), UBERON:0001846 (cochlea... verify exact ID), UBERON:0000970 (eye), UBERON:0000029 (lymph node — not relevant, omit), UBERON:0002369 (adrenal gland), UBERON:0002113 (kidney).

8. Temporal Development

Onset: Ranges continuously across the PEX6 allelic series: - Neonatal (severe/classic end, PBD4A): hypotonia, dysmorphism, seizures at birth. - Infantile (NALD/IRD-type, PBD4B core): developmental delay, hepatic and sensory (hearing/vision) involvement emerging in infancy–early childhood. - Later childhood/school-age (SCAR3/SCABD presentation): can present first as an isolated finding on a school hearing screen at age 6.5–7 years, with ataxia and leukodystrophy following (PMID:25079577). - Very mild/Heimler end: hearing loss + dental enamel defects recognized in childhood, sometimes with only late or subtle retinal findings.

Onset pattern: insidious/progressive in the milder forms; acute-appearing decompensation (diplopia, coordination loss, cognitive decline) can punctuate an otherwise stable course, as in the PMID:25079577 case ("acute-onset diplopia, coordination difficulties, and cognitive decline at age 7" after years of normal development).

Progression / disease course pattern: predominantly progressive (leukodystrophy, sensorineural loss, hepatic fibrosis, osteopenia) but with a subgroup showing a non-progressive, stable course after an initial insult — GeneReviews notes "children who survive the first year and who have a non-progressive course have a 77% probability of reaching school age." Course is therefore bimodal: progressive-demyelinating (worse prognosis) vs. stable/non-progressive (better prognosis).

Duration: classic/severe end is typically fatal in infancy ("usually die during the first year of life"); milder NALD/IRD/SCAR3/Heimler-range disease is chronic and lifelong, with IRD-range patients reported reaching adulthood.

Remission: Not applicable — this is a fixed genetic enzymatic/organelle defect without spontaneous remission; "remission" concepts apply only to individual complications (e.g., seizure control) via symptomatic treatment.

Critical periods: Neonatal/early-infantile window is critical for diagnosis (newborn-screening C26:0-LPC assays) and initiation of nutritional/hepatic supportive care before irreversible white-matter or hepatic injury accrues; there is no known disease-modifying intervention that alters the peroxisomal defect itself once diagnosed (§12).

9. Inheritance and Population

Inheritance pattern: Autosomal recessive (biallelic PEX6 pathogenic variants), with the well-documented exception of the p.Arg860Trp allelic-expression-imbalance mechanism, which produces disease from a single (over-expressed) mutant allele in cis with a specific 3′UTR variant (Falkenberg et al. 2017) — described by GeneReviews as "One PEX6 variant, p.Arg860Trp, has been associated with ZSD in the heterozygous state due to allelic expression imbalance dependent on allelic background." Asymptomatic parents heterozygous for the same coding variant but lacking the 3′UTR AEI variant do not manifest disease, confirming the cis-regulatory (not simple dominant) mechanism.

Penetrance: Effectively complete for biallelic null/severe genotypes; variable expressivity governs the resulting phenotype (severe vs. milder ZSD vs. Heimler) rather than penetrance per se.

Expressivity: Markedly variable, correlating with residual peroxisomal-import function — this is the central genotype–phenotype axis for PEX6, spanning classic Zellweger through NALD/IRD, SCAR3/SCABD, and Heimler syndrome from different combinations of PEX6 alleles.

Genetic anticipation: Not applicable/not reported (not a repeat-expansion disorder).

Germline mosaicism: Not specifically reported for PEX6 in this search.

Founder effects: - French-Canadian (Saguenay–Lac-Saint-Jean, Quebec) founder PEX6 mutation, associated with regional ZSD incidence approaching ~1/12,000 vs. ~1/50,000 North American baseline (PMC3483250). - Mixteco population (Central California, Oaxaca-origin) founder variant c.1409G>C (p.Gly470Ala), identified in a 2023 cluster of 3 related neonatal cases (Slaton et al., PMID:37842507), with authors recommending targeted community screening/awareness.

Consanguinity: Relevant risk-amplifier in founder/isolated populations (implied in both founder reports) though not separately quantified in this search.

Carrier frequency: Formal PEX6-specific gnomAD carrier-frequency figures were not directly retrieved in this pass (recommend direct gnomAD v4 lookup for curation); general ZSD (all-PEX-gene) carrier frequency is consistent with the ~1/50,000–1/100,000 birth-incidence estimates below.

Epidemiology (birth prevalence/incidence, whole-ZSD, PEX-gene-agnostic since PEX6-specific figures are not separately tabulated): - North America/US: ~1/50,000 births (classic estimate); newborn-screening-based C26:0-LPC data from New York gave 1:133,000 births (GeneReviews) — the discrepancy is attributed to biochemical assays underestimating mild/atypical ZSD. - Quebec (Saguenay–Lac-Saint-Jean), Canada: highest reported regional incidence, ~1/12,000, driven by the PEX6 founder allele. - Japan: ~1/500,000, reflecting absence of the common European PEX1 founder alleles (p.Ile700Tyrfs42, p.Gly843Asp); PEX6 relative contribution in Japan not separately reported here. - Within ZSD, PEX6 accounts for ~10–14.5%* of genetically solved cases (second only to PEX1's ~60%) (GeneReviews NBK1448; Ebberink et al. 2010).

Sex ratio: No sex bias reported (autosomal recessive; consistent with equal male:female representation in described cohorts, e.g., the Mixteco case series and the late-onset PEX6 case being male — anecdotal, not indicative of a true sex bias).

Geographic/ethnic distribution: Elevated in French-Canadian (Saguenay–Lac-Saint-Jean) and Mixteco (Central California/Oaxaca) founder populations specifically for PEX6; broader ZSD (all genes) shows the North-America-vs.-Japan contrast above driven mainly by PEX1 founder-allele presence/absence.

10. Diagnostics

Biochemical screening (first-line): - Plasma VLCFA (C26:0, C26:1; ratios) — elevated; caution re: false positives in non-fasting samples. - Erythrocyte membrane plasmalogens (C16, C18) — decreased; may be normal in mild disease. - Plasma/urine pipecolic acid — elevated (urine more sensitive in neonates, plasma in older children). - Plasma/urine C27 bile-acid intermediates (THCA, DHCA) — elevated. - C26:0-lysophosphatidylcholine (C26:0-LPC) on dried blood spot — newborn-screening-compatible marker; sensitivity 89.2% (86/91 DBS samples, 33/37 patients) in a dedicated evaluation study (JIMD 2017, PMID:28677031). GeneReviews explicitly cautions: "Some individuals with ZSD do not have abnormalities of these screening assays," mandating molecular confirmation for atypical/mild cases.

Genetic testing (confirmatory, required for diagnosis per GeneReviews): - Multigene PEX panel (13 known PEX genes) is the preferred first-tier molecular test for a suggestive phenotype. - Exome/genome sequencing for atypical presentations (e.g., isolated hearing loss/ataxia without classic biochemical signature, as in the Usher-mimic and late-onset leukodystrophy cases). - Single-gene PEX6 testing is not generally recommended as a first step (panel/exome preferred), per GeneReviews sequence-detection-rate table (PEX6 ~100% detection rate, 77/77 alleles, once the complementation group is known).

Other modalities: - Brain MRI: symmetric leukodystrophy (white-matter change, non-enhancing) in the milder/late-onset presentations; cerebellar white-matter change without atrophy in the SCAR3/SCABD presentation. - Audiology: baseline and annual sensorineural hearing loss assessment. - Ophthalmology: annual assessment for retinal dystrophy/pigmentary retinopathy and cataract. - Liver panel / imaging: LFTs, coagulation factors, hepatic ultrasound/fibroscan for fibrosis surveillance. - Fibroblast complementation/functional studies: historically used to assign PEX6 complementation group and confirm impaired PTS1/PTS2 import (as in the original PEX6-defective family report, PMID:11873320). - Dental exam: enamel/dentin abnormality assessment, especially relevant at the Heimler end.

Differential diagnosis: - Other PEX-gene ZSD (PEX1 especially — clinically indistinguishable without molecular testing). - X-linked adrenoleukodystrophy (X-ALD, ABCD1) — elevated VLCFA but normal other peroxisomal markers; explicitly the key differential in the late-onset PEX6 case (PMID:25079577), where ABCD1 sequencing/dosage was normal, prompting the correct PEX6 diagnosis. - D-bifunctional protein deficiency, acyl-CoA oxidase deficiency (single peroxisomal enzyme defects mimicking ZSD biochemically) — GeneReviews notes ~15% of ZSD-like/VLCFA-elevated cases are actually single-enzyme defects. - Usher syndrome — the PEX6 "Usher-syndrome mimic" phenomenon (deafness + retinitis pigmentosa) led to a negative Usher panel before compound-heterozygous PEX6 variants were found in a 12-year-old boy (ScienceDirect/PMC PEX6-Usher-mimic report). - Other syndromic hearing-loss/retinal-dystrophy conditions; other leukodystrophies and hypotonia syndromes (myotonic dystrophy, SMA, Prader-Willi) at the neonatal-severe end.

Screening: Newborn screening for ZSD via C26:0-LPC on dried blood spot is implemented in some US states/programs (e.g., 9 California NBS-positive infants 2016–2022, 7 confirmed ZSD by biallelic PEX-gene variants); carrier/targeted screening is recommended in the Mixteco founder population per Slaton et al. 2023.

Suggested LOINC/marker anchors for curation: VLCFA panel, plasmalogen assay, pipecolic acid, THCA/DHCA, C26:0-LPC — verify specific LOINC codes at curation time.

11. Outcome/Prognosis

Severe end (classic Zellweger, PBD4A-range PEX6 genotypes): poor prognosis; "usually die during the first year of life, usually having made no developmental progress," typically from progressive apnea or respiratory infection (GeneReviews NBK1448).

Milder end (PBD4B/NALD-IRD, SCAR3/SCABD, Heimler): - Survivors past year one with a non-progressive course have a 77% probability of reaching school age (GeneReviews). - A subset develops progressive demyelinating leukodystrophy, causing skill loss and eventually death — the key prognostic bifurcation within the milder group. - Progressive sensory deficits (hearing, vision) are common even in stable/non-progressive courses. - Some individuals retain normal intellectual function. - Adults are rarely diagnosed (historically under-recognized) and typically present with predominantly sensory (hearing/vision) deficits and otherwise normal neurologic development — consistent with the IRD/SCAR3-type adult survivors.

Complications driving morbidity: hepatic fibrosis/failure and esophageal varices, adrenal insufficiency, osteopenia/fracture risk, renal oxalate stones, combined sensory (dual hearing-vision) impairment, seizures.

Prognostic factors: genotype (null/severe vs. hypomorphic/missense allele combination — the dominant driver, §4/§9), progressive vs. non-progressive leukoencephalopathy course, age at diagnosis/intervention, degree of residual peroxisomal import function.

Formal severity-stratification resource: the 2022 MDPI Cells scoping review/meta-analysis/chart review on "Characterization of Severity in Zellweger Spectrum Disorder by Clinical Findings" is a good source for quantitative clinical-finding-based severity/prognostic staging across ZSD (verify PMID/exact figures directly for curation-grade quotes).

12. Treatment

There is no disease-modifying/curative therapy for the underlying peroxisomal defect; management is symptomatic/supportive, organized around annual multisystem surveillance (GeneReviews NBK1448):

Cholic-acid pharmacotherapy detail: rationale is suppression of hepatotoxic C27 bile-acid intermediate (THCA/DHCA) synthesis via restored feedback inhibition. A 19-patient open-label pretest–posttest trial (9 months) found cholic acid "can suppress bile acid synthesis in ZSD patients and, thereby, decrease plasma levels of toxic C27-bile acid intermediates. However, no effect on clinically relevant outcome measures could be observed after 9 months of CA treatment" (Cholic acid therapy in ZSD, PMC5065608 / PMID:27469511), with an important safety caveat that cholic acid can worsen liver disease in individuals with pre-existing fibrosis/advanced liver disease — necessitating careful patient selection. Long-term case reports (Karger Case Reports in Gastroenterology, PMC6062720) describe extended cholic-acid treatment courses.

Experimental/investigational: No PEX6-specific gene therapy, ASO, or targeted molecular therapy in active late-stage development was identified in this search; a US patent ("Compositions and methods for the treatment of Zellweger spectrum disorder," USPTO 11065247) indicates active IP/early-stage interest but no confirmed clinical-trial-stage disease-modifying agent. The PEX6-overexpression rescue of matrix-protein import in PEX6-knockout cells (fibroblast complementation data, §6) provides in vitro proof-of-concept for a gene-supplementation therapeutic strategy, but this remains preclinical.

Ongoing trials: NCT03440905 (Proxy-Reported Symptoms and Quality of Life Survey in ZSD) is a natural-history/outcomes-measure study rather than an interventional trial — useful for future trial-readiness and outcome-measure development, not itself a treatment.

Treatment strategy: Management follows an annual/biannual multidisciplinary surveillance algorithm (audiology, ophthalmology, hepatology labs+imaging, adrenal function, urine oxalate, dental exam every 6 months post-secondary-dentition eruption, head MRI as needed for new neurologic decline, growth/nutrition and developmental monitoring at every visit) rather than a linear treatment algorithm, since no curative option exists (GeneReviews NBK1448).

13. Prevention

Primary prevention: Not applicable in the classic sense (monogenic disorder, no modifiable environmental cause); the closest analog is carrier screening and genetic counseling in at-risk/founder populations.

Secondary prevention (early detection): - Newborn screening via C26:0-LPC dried-blood-spot assay is implemented in some jurisdictions and enables presymptomatic identification and earlier supportive-care initiation. - Targeted community screening recommended for the Mixteco population given the identified c.1409G>C founder allele (Slaton et al. 2023).

Genetic counseling / reproductive options: - Standard AR recurrence-risk counseling: 25% recurrence risk per pregnancy for carrier couples. - Carrier screening, preimplantation genetic diagnosis, and prenatal testing are appropriate once a familial PEX6 genotype is known — standard GTR/ACMG-consistent recommendations for a well-characterized AR gene; no PEX6-specific prenatal-screening program beyond general peroxisomal-disorder prenatal biochemical/molecular testing was identified in this search. - In founder populations (French-Canadian Saguenay–Lac-Saint-Jean, Mixteco), population-targeted carrier screening is the most actionable, evidence-supported prevention lever documented.

Tertiary prevention: the entire supportive-care/surveillance regimen in §12 functions as tertiary prevention (preventing/mitigating complications — hepatic decompensation, fracture, renal stone complications, missed sensory-loss-related developmental impact) in individuals already diagnosed.

Immunization: Annual influenza and RSV vaccination per standard pediatric schedules is explicitly recommended as part of ZSD management (GeneReviews) to reduce respiratory-infection mortality risk, particularly relevant given respiratory infection is a leading proximate cause of death in severe ZSD.

14. Other Species / Natural Disease

No naturally occurring PEX6-associated disease in companion animals or wildlife (OMIA-type veterinary entity) was identified in this search — peroxisome biogenesis disorders are not documented as a recognized spontaneous veterinary disease class for PEX6 specifically. PEX6 is broadly conserved across vertebrates (ortholog present in mouse, zebrafish — see §15) and lower eukaryotes (yeast Pex6p performs the analogous AAA-ATPase/Pex15p-anchored receptor-export function, underscoring deep evolutionary conservation of the PEX1/PEX6/PEX26(Pex15) module described in §6). No zoonotic or transmission relevance — this is a non-infectious inherited metabolic/organelle-biogenesis disorder.

15. Model Organisms

Mouse: - Murine Pex6 (MGI:2385054) knockout/complementation studies show, consistent with human pathophysiology: fewer peroxisomes, impaired PTS1/PTS2-mediated matrix protein import (immunofluorescence-confirmed), and rescue of import upon PEX6 overexpression — supporting a gene-supplementation therapeutic rationale (search-synthesized from PEX6-knockout literature; IMPC/MGI hold the formal allele/phenotype records). - Related PEX1 mouse models (e.g., the PEX1-p.Gly844Asp knock-in) have been used to study RPE structural/lipid changes relevant to the retinal phenotype in milder ZSD (bioRxiv 2024.09.05.611330) — directly informative for the PEX6-associated retinal dystrophy phenotype by extension of the shared PEX1/PEX6 complex biology. - Classic Pex1-null and other Pex-null mice are frequently early embryonic/perinatal lethal, limiting study of postnatal disease progression — a key model limitation relative to human milder ZSD.

Zebrafish (increasingly favored for peroxisomal-disorder modeling because postnatal lethality is circumvented): - pex2 zebrafish mutants: locomotive defects, feeding disability, liver abnormalities, early death — recapitulating classic-ZSD-like severity (Takashima et al. 2021, cited in "Modelling Peroxisomal Disorders in Zebrafish," PMC11764017/MDPI 2073-4409/14/2/147). - pex1 loss-of-function zebrafish: viable (unlike mouse), enabling study of ZSD pathophysiology beyond early development; recapitulates hallmark biochemical features — increased VLCFA and branched-chain fatty acids, reduced ether phospholipids (plasmalogens) — plus organ-specific fatty-acid-species accumulation and broad transcriptomic changes including reduced crystallin (lens), troponin, and parvalbumin (muscle) gene expression (bioRxiv 2021.01.03.425169; Frontiers in Molecular Neuroscience 2025, "Pex1 loss-of-function in zebrafish is viable and recapitulates hallmarks of Zellweger spectrum disorders"). - No PEX6-specific zebrafish line was identified by name in this search, but given the shared PEX1/PEX6 heterohexameric complex mechanism, the pex1 zebrafish model is considered broadly informative for PEX6-mediated disease and is the most translationally active current small-vertebrate ZSD model.

Cellular models: Patient-derived fibroblasts are the primary human cellular model, used historically to assign PEX6 complementation-group status and to demonstrate the PEX6-overexpression rescue of peroxisomal import described above — directly bridging molecular mechanism (§6) to therapeutic hypothesis-generation.

Model limitations (general, applicable to PEX6-ZSD): mouse null models are often too severe/lethal to model the milder NALD/IRD/SCAR3/Heimler end of the human PEX6 allelic spectrum; zebrafish, while viable and biochemically faithful, differ from humans in CNS complexity (limiting direct modeling of the cerebellar ataxia/leukodystrophy phenotype) and audiovestibular/retinal anatomy (limiting precise recapitulation of the sensorineural-hearing-loss and retinal-dystrophy phenotypes that dominate the milder PEX6 clinical picture) — a human-model-mismatch consideration worth flagging explicitly if this is curated into a dismech HUMAN_MODEL_MISMATCH discussion node, particularly for the SCAR3/SCABD ataxia-deafness-blindness presentation, which has not yet been shown to be faithfully reproduced in any existing PEX6 animal model in the literature surveyed.

Key PMIDs / Citations Compiled

Data gaps flagged for curation: (1) precise gnomAD v4 PEX6 carrier-frequency figures — needs direct database query; (2) quantitative phenotype-frequency percentages specific to the PEX6 subgroup (vs. all-ZSD) — the 2022 Cells severity-characterization meta-analysis is the best lead; (3) confirmation of whether any PEX6-specific (as opposed to pan-PEX1/pan-ZSD) animal model exists in current MGI/ZFIN records; (4) exact current OMIM clinical-synopsis field values for #614863 (OMIM.org blocked direct fetch in this session — recommend direct OMIM API/manual lookup before finalizing a KB entry).

Sources: - Entry - #614863 - PEROXISOME BIOGENESIS DISORDER 4B; PBD4B - OMIM - Entry - #614862 - PEROXISOME BIOGENESIS DISORDER 4A (ZELLWEGER); PBD4A - OMIM - Entry - *601498 - PEROXISOME BIOGENESIS FACTOR 6; PEX6 - OMIM - Zellweger Spectrum Disorder - GeneReviews - NCBI Bookshelf (NBK1448) - Spectrum of PEX6 mutations in Zellweger syndrome spectrum patients - PubMed (PMID:19877282) - Late-onset Zellweger spectrum disorder caused by PEX6 mutations mimicking X-linked adrenoleukodystrophy - PubMed (PMID:25079577) - Zellweger's Syndrome With PEX6 Gene Mutation in Mixteco Neonates Due to Possible Founder Effect - PMC (PMID:37842507) - A founder mutation in the PEX6 gene is responsible for increased incidence of Zellweger syndrome in a French Canadian population - PMC - Allelic Expression Imbalance Promoting a Mutant PEX6 Allele Causes Zellweger Spectrum Disorder - Cell.com AJHG - PEX6 Mutations in Peroxisomal Biogenesis Disorders: An Usher Syndrome Mimic - ScienceDirect - Heimler Syndrome Is Caused by Hypomorphic Mutations in the Peroxisome-Biogenesis Genes PEX1 and PEX6 - PMC - Spectrum of PEX1 and PEX6 variants in Heimler syndrome - EJHG - Structure of the peroxisomal Pex1/Pex6 ATPase complex bound to a substrate - Nature Communications - The peroxisomal AAA-ATPase Pex1/Pex6 unfolds substrates by processive threading - PMC - Insights into the Structure and Function of the Pex1/Pex6 AAA-ATPase in Peroxisome Homeostasis - PMC - Peroxisomal monoubiquitinated PEX5 interacts with the AAA ATPases PEX1 and PEX6 - ScienceDirect - Evaluation of C26:0-lysophosphatidylcholine and C26:0-carnitine as diagnostic markers for Zellweger spectrum disorders - PubMed - Cholic acid therapy in Zellweger spectrum disorders - PMC - Long-Term Cholic Acid Therapy in Zellweger Spectrum Disorders - PMC - Braverman et al., Peroxisome biogenesis disorders in the Zellweger spectrum overview - Mol Genet Metab (PMID:26750748) - Zellweger spectrum disorders: clinical overview and management approach - PubMed/Orphanet J Rare Dis - Characterization of Severity in Zellweger Spectrum Disorder by Clinical Findings - MDPI Cells - Modelling Peroxisomal Disorders in Zebrafish - PMC - Pex1 loss-of-function in zebrafish is viable and recapitulates hallmarks of Zellweger spectrum disorders - Frontiers - Zebrafish model of human Zellweger syndrome reveals organ specific accumulation of distinct fatty acid species - bioRxiv - Pex6 MGI Mouse Gene Detail - MGI:2385054 - peroxisome biogenesis disorder 4B MONDO:0013931 - Monarch Initiative - Peroxisome biogenesis disorder 4B - NIH Genetic Testing Registry (GTR) - Orphanet: PEX6-peroxisomal biogenesis factor 6 - Orphanet: Infantile Refsum disease - Orphanet: Neonatal adrenoleukodystrophy - Proxy-Reported Symptoms and Quality of Life Survey in Zellweger Spectrum Disorders - ClinicalTrials.gov NCT03440905