PACS2-Related Developmental and Epileptic Encephalopathy (DEE66): Comprehensive Research Report

1. Disease Information

Overview. PACS2-Related Developmental and Epileptic Encephalopathy — officially catalogued as Developmental and Epileptic Encephalopathy 66 (DEE66), and previously termed Early Infantile Epileptic Encephalopathy 66 (EIEE66) — is an autosomal dominant neurodevelopmental disorder caused by a heterozygous, almost always de novo, missense variant in PACS2 (phosphofurin acidic cluster sorting protein 2), located at chromosome 14q32.33. It is characterized by neonatal- or early-infantile-onset epilepsy, global developmental delay/intellectual disability, hypotonia, characteristic facial dysmorphism, and cerebellar dysgenesis on brain MRI, often accompanied by extraneurologic features (ophthalmologic, cardiac, limb, hematologic) (OMIM #618067; Olson et al. 2018, PMID:29656858).

Key identifiers: - OMIM: #618067 (DEE66, phenotype); 610423 (PACS2, gene) - Gene: PACS2, HGNC:23794, chromosome 14q32.33 - NIH GTR condition: C4748070 ("Developmental and epileptic encephalopathy, 66") - Disease Ontology:* DOID:0080446 - Likely MONDO term for this entity corresponds to DEE66 (curators should verify exact MONDO CURIE via OMIM/Mondo cross-reference at curation time — not independently confirmed in this research pass)

Synonyms: Early Infantile Epileptic Encephalopathy 66 (EIEE66); PACS2 syndrome; DEE66; PACS2-related neurodevelopmental disorder.

Evidence basis: Nearly all published knowledge derives from aggregated case series and systematic reviews of individually reported and cohort patients (not large-cohort EHR/registry data) — the disease was first delineated in 2018 in 14 unrelated patients (PMID:29656858) and subsequent literature has grown the total to roughly ~50–100 reported individuals worldwide as of 2022–2024 (PACS2 Research Foundation; Genetics in Medicine Open 2024).

2. Etiology

Disease causal factor: A single, essentially monogenic mechanism — heterozygous missense variation in PACS2, almost always the recurrent c.625G>A, p.(Glu209Lys) [E209K] variant, arising de novo in the vast majority of cases (PMID:29656858; PMC10968252 — 30/30 reviewed patients carried this variant).

Genetic risk factors: - The E209K substitution affects a highly conserved glutamic acid residue within a domain of PACS2 involved in phosphorylation-dependent regulatory interactions (Ser207/208/213 cluster) ([Mammalian Genome 2024, doi:10.1007/s00335-024-10098-5]). - A second, rarer recurrent variant, E211K, has also been reported and studied mechanistically alongside E209K, both impairing mitochondria-associated membrane (MAM) integrity via disturbance of PACS2 phosphorylation at the Ser207/208/213 cluster. - Additional distinct missense variants have been reported in isolated cases with atypical/milder or expanded phenotypes (e.g., malformations of cortical development, migrating focal seizures of infancy), suggesting some allelic heterogeneity beyond the two hotspot residues (Checri et al. 2024, Epileptic Disorders, doi:10.1002/epd2.20184; ScienceDirect migrating focal seizures paper). - No inherited/parental transmission has been documented in the majority of families — parents are typically wild-type at the variant position, confirming de novo origin (e.g., the Saudi family study, PMC10963950). - PACS2's paralog PACS1 causes the related Schuurs-Hoeijmakers syndrome (PACS1 neurodevelopmental disorder, recurrent p.Arg203Trp), with substantial phenotypic overlap (developmental delay 100%, dysmorphism 100%, seizures 63% in PACS1) — evidence of a shared pathway mechanism (Karger Molecular Syndromology 2024, PACS2/PACS1/VACTERL overlap).

Environmental risk factors: None established; this is a purely genetic, non-environmentally-triggered disorder based on current literature.

Protective factors: None reported in the literature (genetic or environmental). No modifier alleles have been characterized.

Gene-environment interactions: Not applicable / not studied — no evidence of environmental modulation of phenotype severity.

3. Phenotypes

Seizures / epilepsy (universal, ~100%)

Onset: Neonatal to early infantile; in the largest systematic review (n=30), onset ranged from day 1 of life to 10 months, with 76.7% (23/30) presenting within the first 2 weeks of life (PMC10968252).
Seizure semiology: Focal motor seizures, tonic seizures (often affecting upper limbs), autonomic manifestations (apnea, cyanosis), abnormal eye movements/eye rolling, myoclonic seizures, and generalized tonic-clonic seizures (often febrile-triggered); focal-onset with secondary generalization predominates in the neonatal period (PMC10137075).
Course: Seizures are typically difficult to control in infancy/early childhood, requiring multiple anti-seizure medication trials, but frequently become easier to control — or resolve — with advancing age; 29% of patients ≥5 years old discontinued anti-seizure medication in one cohort.
HPO suggestions: HP:0032796 (Seizure — DEE), HP:0032810 (focal-onset seizure), HP:0002123 (generalized tonic-clonic seizure), HP:0002121 (generalized non-motor seizure), HP:0011097 (epileptic spasm as needed), HP:0007359 (focal-onset seizure), HP:0011182 (electroencephalographic abnormality), HP:0002133 (status epilepticus if applicable).

Global developmental delay / intellectual disability (~80%, universal in most series)

Delayed motor and speech milestones (delayed global development 80% [24/30]; speech delay 63% [19/30]).
Intellectual disability ranges mild to severe; progressive cognitive decline reported in adult cases.
HPO: HP:0001263 (Global developmental delay), HP:0001249 (Intellectual disability), HP:0000750 (Delayed speech and language development).

Hypotonia (~57%, 17/30)

HPO: HP:0001252 (Hypotonia).

Behavioral abnormalities (~50%, 15/30) including autism spectrum features

ASD reported in ~18% (4/22 with data) in one cohort.
HPO: HP:0000708 (Behavioral abnormality), HP:0000717 (Autism).

Movement/neurological signs

Nystagmus (13%, 4/30), wide-based gait (13%), pyramidal syndrome (13%).
HPO: HP:0000639 (Nystagmus), HP:0002136 (Broad-based gait), HP:0007256 (Progressive spasticity/pyramidal signs).

Facial dysmorphism (common but variable/subtle)

Hypertelorism, broad/wide nasal root, thin upper lip, highly arched eyebrows, long eyelashes, wide-spaced teeth, down-turned corners of the mouth, down-slanting palpebral fissures.
HPO: HP:0000316 (Hypertelorism), HP:0000414 (Broad nasal tip/root — HP:0000455), HP:0000219 (Thin upper lip vermilion), HP:0004585 (Highly arched eyebrow), HP:0000582 (Downslanted palpebral fissures), HP:0000160 (Narrow mouth or wide mouth per variant description).

Ophthalmologic features (~37%, 11/30)

Strabismus, nystagmus, hypermetropia, astigmatism, myopia, coloboma.
HPO: HP:0000486 (Strabismus), HP:0000540 (Hypermetropia), HP:0000544 (Coloboma).

Cardiac (septal defects; also tetralogy of Fallot reported in one case expanding the phenotype)

Atrial/ventricular septal defects: 4/30 cases (~13%); complex congenital heart disease (tetralogy of Fallot) reported in a novel case with VACTERL-like overlap.
HPO: HP:0001631 (ASD), HP:0001629 (VSD), HP:0001636 (Tetralogy of Fallot).

Other systemic features

Cryptorchidism (5 cases), distal limb malformations (12 cases), hematologic disturbances (5 cases), hydronephrosis (2 cases); anal atresia and vertebral anomalies reported in one expanded VACTERL-overlap case.
HPO: HP:0000028 (Cryptorchidism), HP:0002830 (Limb undergrowth/distal anomaly per specific finding), HP:0004320 (Ectopic anus/anal atresia — HP:0002023), HP:0000924 (Abnormality of the skeletal system for vertebral anomalies).

Brain imaging (MRI) findings

Cerebellar foliar dysgenesis (50%, 15/30) — the most characteristic neuroimaging finding.
Mega cisterna magna (43%, 13/30).
Inferior vermis hypoplasia (27%, 8/30).
Reduced white matter (20%, 6/30).
Lateral ventricle enlargement (13%).
Hypothalamic fusion anomalies (10%).
Negative/normal neuroimaging in ~20% (6/30).
Progressive findings in adults: severe cerebral/cerebellar atrophy, demyelinating lesions.
HPO: HP:0007033 (Cerebellar dysplasia/dysgenesis), HP:0002324 (Cerebellar vermis hypoplasia), HP:0002534 (Cisterna magna malformation — mega cisterna magna HP:0006955), HP:0002500 (delayed CNS myelination/HP:0002500 or reduced white matter HP:0002119), HP:0002119 (Ventriculomegaly).

Quality of life impact: Not systematically studied via validated instruments (EQ-5D/SF-36) in the literature reviewed; qualitative reports describe substantial burden from refractory neonatal/infantile seizures, limited verbal communication, poor social functioning, and — in adults — progressive neurological decline including new-onset facial hemispasm, ataxia, and tetraparesis in the oldest reported cases (PMC10968252).

4. Genetic/Molecular Information

Causal gene: PACS2 (HGNC:23794; OMIM *610423), chromosome 14q32.33.

Recurrent pathogenic variant: - c.625G>A, p.Glu209Lys (E209K) — the dominant, recurrent, de novo variant found in the overwhelming majority (~30/30 in the largest systematic review) of published DEE66 patients (PMID:29656858; PMC10968252). - c.631G>A, p.Glu211Lys (E211K) — a second, rarer recurrent hotspot variant, mechanistically studied alongside E209K. - Additional rare missense variants reported in association with somewhat expanded/atypical phenotypes (cortical malformation, migrating focal seizures of infancy, complex congenital heart disease/VACTERL overlap).

Variant classification: Pathogenic/likely pathogenic per ACMG criteria for the recurrent hotspot variants (de novo, absent from population databases, functionally validated). Curators should confirm current ClinVar star-rating and classification directly.

Variant type/class: Missense, heterozygous, gain-of-function/dominant-negative-like mechanism (see below) rather than simple loss-of-function.

Population frequency: The E209K and E211K variants are not present in gnomAD population databases (consistent with a highly penetrant de novo dominant disorder); specific PACS2 gene-level constraint metrics (pLI/LOEUF) were not confirmed in this research pass and should be pulled directly from the gnomAD browser at curation time.

Origin: Predominantly germline de novo; no confirmed cases of parental mosaicism or inherited transmission were found in this search, though the possibility of parental gonadal mosaicism has not been excluded in the literature.

Functional consequence — molecular mechanism: - PACS2 is a multifunctional sorting protein and a key regulator of mitochondria-associated membranes (MAMs) — physical tethering sites between the endoplasmic reticulum (ER) and mitochondria — controlling ER–mitochondria calcium signaling, lipid synthesis, mitophagy, ER homeostasis, and apoptosis (PACS-2: A key regulator of MAMs, PMID:32673704; PMC6627983). - The E209K (and E211K) mutations disturb PACS2 phosphorylation at the Ser207/208/213 cluster, impairing MAM integrity (Mammalian Genome 2024, doi:10.1007/s00335-024-10098-5). - Functional studies (HCT116 cell model) show E209K PACS-2 has slower protein turnover relative to wild-type upon cycloheximide treatment, and increased association with 14-3-3ε by co-immunoprecipitation. Upon apoptotic stress (staurosporine), E209K-expressing cells show markedly increased apoptosis (~80% apoptotic cells vs. ~41% for wild-type PACS-2), whereas wild-type PACS-2 is protective against stress-induced cell death (PMC9520720). The authors state: "increased levels of apoptosis agree with DEE66 patient phenotypes involving epilepsy and cerebellar dysgenesis." - This suggests a gain-of-function/altered-function mechanism (aberrant 14-3-3 client recruitment promoting apoptosis) rather than simple haploinsufficiency — consistent with the recurrent, hotspot nature of the variant and absence of reported loss-of-function (truncating) alleles causing the same phenotype. - Downstream cellular consequences implicated: disrupted MAM formation, impaired ER–mitochondria Ca²⁺ flux, inhibited mitophagy, impaired energy metabolism, and increased apoptotic susceptibility in neurons/cerebellar cells, plausibly explaining the progressive cerebellar dysgenesis/atrophy phenotype. - Suggested GO terms: GO:0032865 (regulation of mitochondrial outer membrane permeabilization), GO:0032469 (endoplasmic reticulum calcium ion homeostasis), GO:0044233 (ER-mitochondrion membrane contact site organization), GO:0006915 (apoptotic process), GO:0000422 (mitophagy).

Epigenetics/chromosomal abnormalities: No epigenetic mechanism or chromosomal-scale abnormality has been implicated; disease is driven by point missense variants.

5. Environmental Information

No environmental factors, lifestyle exposures, or infectious triggers have been implicated as causal or modifying for PACS2-related DEE66 in the literature reviewed. Febrile illness has been noted as a trigger for individual generalized tonic-clonic seizure episodes in some patients (a symptomatic exacerbant, not a disease cause) (PMC10137075).

6. Mechanism / Pathophysiology

Causal chain (proposed): 1. Trigger: De novo heterozygous PACS2 missense variant (predominantly E209K) alters a conserved residue near the Ser207/208/213 phosphorylation cluster. 2. Molecular consequence: Altered PACS2 phosphorylation/turnover kinetics and increased binding to 14-3-3ε, altering the normal pool of PACS2 client protein interactions. 3. MAM/organelle consequence: Impaired mitochondria-associated membrane (ER-mitochondria contact site) integrity; disrupted ER-mitochondria Ca²⁺ transfer; disrupted mitophagy and mitochondrial energy metabolism. 4. Cellular consequence: Increased susceptibility to apoptosis under cellular stress, particularly relevant to cerebellar granule/Purkinje neuron populations and cortical neurons during critical developmental windows. 5. Tissue/organ consequence: Cerebellar foliar dysgenesis, vermis hypoplasia, progressive cerebral/cerebellar atrophy (especially notable in the few reported adult cases), and cortical network dysfunction predisposing to neonatal-onset epilepsy. 6. Clinical manifestation: Neonatal/infantile-onset, often refractory, focal-onset epilepsy; global developmental delay/ID; hypotonia; behavioral/autistic features; and variable dysmorphic/extraneurologic features.

Upstream vs. downstream: The PACS2 phosphorylation/14-3-3 interaction defect is the most upstream molecular lesion identified; MAM/Ca²⁺-handling disruption and apoptotic susceptibility are intermediate; cerebellar dysgenesis/atrophy and cortical hyperexcitability are downstream tissue-level consequences producing the clinical phenotype.

Cell types implicated: Cerebellar Purkinje and granule neurons (dysgenesis/atrophy phenotype), cortical neurons (epileptogenesis), and more broadly any cell type dependent on ER-mitochondria MAM signaling (vascular smooth muscle cells and cardiomyocytes have also been separately studied for PACS2's MAM role, relevant to the cardiac phenotype overlap with PACS1/VACTERL).

Molecular pathway/GO terms: Suggested — GO:0044233 (ER-mitochondrion membrane contact site organization), GO:0051560 (mitochondrial calcium ion homeostasis), GO:0000422 (mitophagy), GO:0006915 (apoptotic process), GO:0035556 (intracellular signal transduction, for 14-3-3-mediated signaling).

Cell Ontology suggestions: CL:0000121 (Purkinje cell), CL:0000120 (granule cell), CL:0000540 (neuron), CL:0002319 (neural cell, generic).

Advanced/omics data: No large-scale transcriptomic, proteomic, or single-cell datasets specific to PACS2 patient tissue were identified in this search; iPSC-neuron models are in active development (see Model Organisms, section 15) but published multi-omic datasets from these models were not found as of this research date.

7. Anatomical Structures Affected

Organ level: - Primary: Central nervous system — cerebrum (cortex), cerebellum (vermis and foliae). - Secondary/associated: Cardiovascular system (septal defects, tetralogy of Fallot), ophthalmologic structures (globe/refraction, strabismus), genitourinary (cryptorchidism, hydronephrosis), gastrointestinal (anal atresia in expanded phenotype), hematologic system, and craniofacial skeleton (dysmorphic features). - Body systems: Nervous, cardiovascular, ophthalmologic, musculoskeletal, genitourinary.

UBERON suggestions: UBERON:0002037 (cerebellum), UBERON:0002038 (cerebellar vermis), UBERON:0000955 (brain), UBERON:0001017 (central nervous system), UBERON:0007100 (primary circulatory organ/heart region for septal defects), UBERON:0000970 (eye).

Tissue/cell level: Cerebellar cortex (Purkinje/granule cell layers), cerebral cortical neurons; craniofacial soft tissue (dysmorphism).

Subcellular level: Mitochondria-associated ER membranes (MAMs), mitochondrial outer membrane, endoplasmic reticulum. GO Cellular Component: GO:0044233 (ER-mitochondrion membrane contact site), GO:0005741 (mitochondrial outer membrane), GO:0005783 (endoplasmic reticulum).

Localization: Bilateral/symmetric cerebellar involvement typically; no reported lateralization pattern.

8. Temporal Development

Onset: Congenital/neonatal-to-infantile for the core neurological phenotype; seizure onset from day 1 of life to 10 months, with the large majority (76.7%) within the first two weeks of life. Rare later-onset (childhood) presentations reported for milder allelic variants.
Onset pattern: Typically acute/abrupt seizure onset in the neonatal period.
Progression: Epilepsy often initially difficult to control (multiple ASM trials/combinations needed in infancy/childhood), frequently improving or resolving with age (many patients seizure-free or medication-free by later childhood/adolescence). In contrast, the cognitive/neurodegenerative trajectory in the very limited adult cohort (only 4 adults reported as of the 2024 systematic review) shows progressive cerebral/cerebellar atrophy, new-onset movement abnormalities (facial hemispasm, ataxia), and cognitive regression — suggesting a biphasic course (early epileptic encephalopathy, later progressive neurodegeneration).
Disease course pattern: Chronic, lifelong; not classically relapsing-remitting but with an early severe epileptic phase that may partially remit, followed by potential later-life progressive decline.
Critical periods: Neonatal/early infantile period represents the critical window of epileptogenic vulnerability; timely genetic diagnosis in this window is emphasized in the literature as important for guiding management (e.g., trial of pyridoxal phosphate) and counseling.

9. Inheritance and Population

Epidemiology: True population prevalence/incidence is unknown (ultra-rare disease). Approximately ~50 cases were known by December 2022, growing to roughly ~100 individuals described worldwide by 2024 (PACS2 Research Foundation; PMC10968252).

Inheritance pattern: Autosomal dominant, with the overwhelming majority of cases arising de novo. No confirmed familial recurrence via germline transmission was found in this search; recurrence risk beyond the general de novo/gonadal mosaicism risk (~1%) has not been formally established in the literature.

Penetrance: Appears to be high/complete for the core epilepsy/developmental phenotype among E209K carriers reported to date, though one case of a "developmentally typical" patient with the classic c.625G>A variant and a milder phenotype (responsive to carbamazepine) has been reported, raising the possibility of variable expressivity (mosaicism was considered but excluded by deep sequencing coverage in that case).

Expressivity: Variable — phenotype severity ranges from classic severe DEE66 with refractory neonatal seizures and marked cerebellar dysgenesis to milder presentations with typical development.

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

Germline mosaicism: Not formally documented but biologically plausible given the pattern of exclusively de novo occurrence; not excluded as an explanation for rare phenotypic variability.

Founder effects: None reported; cases have been described across diverse populations (including a Saudi family), consistent with recurrent de novo mutation at a mutational hotspot rather than a founder allele.

Consanguinity: Not implicated as a risk factor (autosomal dominant de novo mechanism).

Sex ratio: Roughly equal in the largest reviewed cohort (16 male, 14 female, n=30) — no strong sex bias reported.

Geographic distribution: Global; cases reported from North America, Europe, Middle East (Saudi Arabia), and elsewhere with no evident geographic clustering.

10. Diagnostics

Genetic testing (primary diagnostic modality): - Whole exome sequencing (WES) or whole genome sequencing (WGS), typically trio-based (proband + parents), is the standard diagnostic approach given the phenotype's genetic heterogeneity overlap with other early-infantile DEEs; this is how essentially all reported cases have been ascertained. - Epilepsy/DEE gene panels including PACS2 are also used clinically (e.g., listed on Genomics England PanelApp "Early onset or syndromic epilepsy" panel). - Single-gene Sanger confirmation of the recurrent E209K hotspot can be pursued when clinical suspicion is high (classic facial/cerebellar/seizure phenotype). - Genetic diagnosis timing in the reviewed cohort ranged widely (4 months to 37 years of age), reflecting historically low disease awareness, especially in adults.

Neuroimaging: Brain MRI is a key diagnostic-supportive test — cerebellar foliar dysgenesis, mega cisterna magna, and vermis hypoplasia are characteristic (though absent in ~20% of cases, so a normal MRI does not exclude the diagnosis).

EEG: Variable findings — focal/multifocal interictal discharges (often centrotemporal/frontal), burst suppression, or hypsarrhythmia in some neonatal presentations; some normalize over time.

Differential diagnosis: Other genetic DEEs presenting in the neonatal/early infantile period (e.g., KCNQ2-, SCN2A-, STXBP1-, CDKL5-related DEEs), pyridoxine-dependent epilepsy (given some PACS2 patients show transient pyridoxal-phosphate responsiveness), other cerebellar dysgenesis syndromes, and — given phenotypic overlap — PACS1-related neurodevelopmental disorder (Schuurs-Hoeijmakers syndrome).

Screening: No population or newborn screening program exists (ultra-rare, not detected by standard newborn metabolic screening); diagnosis relies entirely on clinical suspicion triggering genetic testing.

Standardized diagnostic criteria: No formal consensus diagnostic criteria; diagnosis is molecular (identification of a pathogenic PACS2 variant) in the appropriate clinical context per ILAE epilepsy classification framework for developmental and epileptic encephalopathies.

11. Outcome / Prognosis

Survival/mortality: No systematic mortality data identified in this search; the disorder is not classically described as life-limiting in early reports, though severity varies widely.

Seizure outcome: Often improves with age — many patients achieve better seizure control or seizure freedom in later childhood, with ~29% of patients ≥5 years old able to discontinue anti-seizure medication in one series.

Developmental/cognitive outcome: Ranges from low-average intelligence to severe developmental impairment; most patients have some degree of intellectual disability, speech delay, and behavioral disturbance (including autism spectrum features in ~18%).

Adult/long-term course: Limited data (only 4 adult cases reported by 2024) suggest a concerning pattern of progressive neurodegeneration in adulthood — accelerating cerebral/cerebellar atrophy, demyelinating changes, cognitive regression, and new motor symptoms (facial hemispasm, ataxia, tetraparesis) — highlighting a need for longitudinal natural history studies as the cohort ages.

Prognostic factors: Specific genotype-phenotype correlations beyond "E209K = classic/most common phenotype" are not well established; the rarer E211K and other missense variants may correlate with atypical presentations (milder or with additional features like cortical malformation or complex congenital heart disease), but sample sizes are too small for robust conclusions.

12. Treatment

Anti-seizure pharmacotherapy (symptomatic, not disease-modifying): - Valproic acid and levetiracetam — reported effective in roughly half of patients. - Phenobarbital — improvement in about one-third. - Carbamazepine/oxcarbazepine — effective in about one-quarter; notably one patient with typical development responded well to carbamazepine. - Vigabatrin — reported effective in some cases. - Pyridoxal phosphate (vitamin B6, active form) — showed promising, though sometimes transient, effectiveness in some cases; seizure recurrence occurred when switched to standard oral B6 (pyridoxine) in at least one report, suggesting a possible pyridoxal-phosphate-specific responsiveness worth trialing early.

NCIT suggestion: NCIT:C15986 (Pharmacotherapy) as the treatment_term, with therapeutic_agent bound to CHEBI terms for valproic acid (CHEBI:39867), levetiracetam (CHEBI:6437), carbamazepine (CHEBI:3387), vigabatrin (CHEBI:9944), phenobarbital (CHEBI:8069).

Supportive/rehabilitative care: Physical therapy, occupational therapy, speech therapy for hypotonia, motor delay, and speech delay; behavioral/developmental interventions for autism spectrum features. NCIT: NCIT:C15302 (Physical Therapy), NCIT:C159273 (Speech Therapy), NCIT:C121351 (Occupational Therapy).

Surgical/interventional: Cardiac surgical repair for structural congenital heart defects (septal defect closure, tetralogy of Fallot repair) in affected individuals. NCIT:C15329 (Surgical Procedure).

Experimental / emerging precision therapies (active development, not yet clinically available): - Antisense oligonucleotide (ASO) therapy: An allele-specific ASO strategy targeting the mutant E209K transcript while sparing wild-type PACS2 expression is in development in partnership with the n-Lorem Foundation (a nonprofit providing individualized ASO therapies for ultra-rare diseases), modeled on the analogous, more advanced PACS1 syndrome ASO program (see PACS1 mouse model RNA-targeted therapy work, PMC9901029; Nature Communications 2023). - Drug repurposing via high-throughput Cell Painting screening: The PACS2 Research Foundation, in partnership with Charles River Laboratories (Leiden), screened the Broad Institute's Drug Repurposing Hub (~6,808 compounds) against patient-derived fibroblasts using morphological (Cell Painting) profiling compared to an unaffected twin sibling's cells, identifying several candidate compounds that shifted the diseased-cell morphological signature toward the healthy phenotype; potency/dose-optimization screening is ongoing as of the most recent reporting (Charles River Eureka blog). - PROTAC (targeted protein degradation) approaches to selectively degrade mutant PACS2 protein are proposed as an emerging strategy, pending further mechanistic clarification (PACS2 Cure Roadmap, Perlara).

Genetic counseling: Recommended for all newly diagnosed families given the near-universal de novo occurrence, low but non-zero recurrence risk (germline mosaicism), and autosomal dominant inheritance pattern once established. NCIT:C15240 (Genetic Counseling).

Clinical trials: No PACS2-specific registered interventional clinical trials (NCT identifiers) were identified in this search; the disease remains in the preclinical/translational research stage for targeted therapies.

13. Prevention

No primary, secondary, or tertiary prevention strategies exist for this de novo genetic disorder — there are no known modifiable risk factors. The main "prevention" mechanism available is prenatal/preimplantation genetic diagnosis for families with a previously affected child (relevant given theoretical germline mosaicism recurrence risk), and genetic counseling for recurrence risk discussion. No immunization, screening program, or public health intervention is applicable. NCIT:C15240 (Genetic Counseling) remains the most relevant preventive/counseling intervention captured in ontology terms.

14. Other Species / Natural Disease

No naturally occurring PACS2-associated disease has been reported in non-human species (companion animals or wildlife) in the literature reviewed — this is a human-only reported condition to date, consistent with its very recent (2018) delineation and ultra-rare status. PACS2 is evolutionarily conserved (the E209 residue is conserved down to zebrafish, per the PACS2 Cure Roadmap document), supporting cross-species relevance for engineered models (see below) but no evidence of spontaneous veterinary cases.

15. Model Organisms

This is an area of very active, ongoing translational development, primarily driven by the PACS2 Research Foundation and academic/industry collaborators (Perlara PBC "cure roadmap"; Jackson Laboratory; Charles River Laboratories):

Yeast: Not a viable model — PACS2 has no ortholog in yeast.
Drosophila / C. elegans: Deprioritized — the critical glutamate at position 209 is not conserved in fly or worm PACS orthologs, limiting translational relevance.
Zebrafish: E209 is conserved in zebrafish PACS2, making a heterozygous E209K knock-in zebrafish model biologically feasible; proposed for future generation (deprioritized pending cellular-assay proof of concept as of the most recent roadmap update). No published zebrafish PACS2 model was identified as of this search.
Mouse: A Pacs2^E209K/+ mouse model reportedly exists (noted as "approved in Poland" per the PACS2 Research Foundation) but requires further phenotypic characterization; a humanized Pacs2 E209K/+ mouse model at The Jackson Laboratory has been proposed specifically to support preclinical ASO testing. No peer-reviewed publication describing detailed phenotype recapitulation in this mouse model was identified in this search — this represents a knowledge gap in the current literature (model exists per foundation reporting, but published characterization is not yet available).
Patient-derived iPSCs: Multiple iPSC lines have been generated from PACS2 patients (e.g., "Lena's" fibroblast-derived iPSCs) and are being differentiated into neurons in collaboration with academic labs (e.g., Dr. A. Guemez-Gamboa) to study E209K effects on neuronal mitochondrial function, protein interactions, and developmental phenotypes, alongside CRISPR-corrected isogenic controls. No published dataset from these iPSC-neuron models was identified as of this research pass.
Cell line models (non-neuronal): HCT116 human colorectal carcinoma cells transfected with PACS-2 WT vs. E209K have been used to directly demonstrate increased apoptotic susceptibility and altered 14-3-3ε binding (PMC9520720). Patient-derived dermal fibroblasts have been used for the Cell Painting drug-repurposing screen described above.

Model limitations: No model to date has published in vivo confirmation of the characteristic cerebellar dysgenesis or epilepsy phenotype seen in human patients — this is an important translational gap. Curators modeling this disease should flag a HUMAN_MODEL_MISMATCH-type caveat: current functional/mechanistic data (apoptosis susceptibility, MAM/14-3-3 interaction) derive from heterologous cell-line overexpression systems (HCT116) rather than neurons or an in vivo nervous system model, so translational validity of the apoptosis-centric mechanism to human cerebellar/cortical pathology in situ remains to be directly confirmed.

Summary of Key Ontology Term Suggestions

Table (click to expand)

Domain	Suggested terms
Disease	OMIM:618067; DOID:0080446; (MONDO CURIE to be confirmed)
Gene	HGNC:23794 (PACS2); OMIM:*610423
Phenotypes (HP)	HP:0032796/HP:0032810 (seizures), HP:0001263 (global DD), HP:0001249 (ID), HP:0001252 (hypotonia), HP:0000717 (autism), HP:0007033 (cerebellar dysgenesis), HP:0002324 (vermis hypoplasia), HP:0006955 (mega cisterna magna), HP:0000582 (downslanted palpebral fissures), HP:0000316 (hypertelorism), HP:0001629/HP:0001631 (septal defects), HP:0000486 (strabismus)
GO (biological process)	GO:0044233 (ER-mitochondrion MCS organization), GO:0051560 (mitochondrial Ca²⁺ homeostasis), GO:0000422 (mitophagy), GO:0006915 (apoptotic process)
CL (cell type)	CL:0000121 (Purkinje cell), CL:0000120 (cerebellar granule cell), CL:0000540 (neuron)
UBERON	UBERON:0002037 (cerebellum), UBERON:0002038 (cerebellar vermis), UBERON:0000955 (brain)
CHEBI (drugs)	CHEBI:39867 (valproic acid), CHEBI:6437 (levetiracetam), CHEBI:3387 (carbamazepine), CHEBI:9944 (vigabatrin), CHEBI:8069 (phenobarbital)
NCIT (treatment)	NCIT:C15986 (Pharmacotherapy), NCIT:C15302 (Physical Therapy), NCIT:C15240 (Genetic Counseling), NCIT:C15329 (Surgical Procedure)

Sources

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Table (click to expand)

Outcome	Count
References checked	12
Resolved	10
Unresolved (possible confabulation)	2
Unverifiable	0
References weighed for topical relevance	10
On topic	5
Off topic	2

Unresolved references

These identifiers did not resolve to a record and may be fabricated. A lookup that failed for transport reasons is indistinguishable from one that failed because the record does not exist, so spot-check before acting on them:

DOI:10.1002/epd2.20184](https://onlinelibrary.wiley.com/doi/abs/10.1002/epd2.20184 (1 mention) - Identifier did not resolve to a record
DOI:10.1159/000539473/911172/PACS2-PACS1-and-VACTERL-A-Clinical-Overlap (3 mentions) - Identifier did not resolve to a record

References that may not be about this subject

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

PMID:32673704 (4 mentions) - PACS-2: A key regulator of mitochondria-associated membranes (MAMs).
shared terms: disease
PMC:PMC6627983 (4 mentions) - The Multifunctional Sorting Protein PACS-2 Controls Mitophagosome Formation in Human Vascular Smooth Muscle Cells through Mitochondria-ER Contact Sites.
shared terms: cell, model

Weighed against this report's own most characteristic terms: phenotype, pacs2, seizure, disease, cerebellar, patient, developmental, variant, genetic, e209k, hpo, cell, dysgenesis, epilepsy, novo, model, development, disorder, neonatal, feature.