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Chromosome 18p Deletion Syndrome

1. Disease Information

2026-06-22
Falcon MONDO:0007800 Model: Edison Scientific Literature 7 citations

1. Disease Information

Overview

Chromosome 18p deletion syndrome (also known as monosomy 18p, 18p- syndrome, or 18p deletion syndrome) is a rare chromosomal disorder characterized by partial or complete loss of the short arm of chromosome 18 (papamichail2024prenataldiagnosisof pages 1-3, papamichail2023prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 1-2). First described by de Grouchy and colleagues in 1963, it is one of the most frequent autosomal terminal deletion syndromes and was the first reported partial monosomy compatible with life (papamichail2023prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 1-2). The syndrome presents with highly variable clinical features including cognitive impairment, minor facial dysmorphism, strabismus, ptosis, short stature, and systemic involvement affecting multiple organ systems (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 7-8).

Key Identifiers

Synonyms and Alternative Names

Data Source Type

The information is derived from both individual patient case reports and aggregated disease-level resources, including a comprehensive cohort study of 106 individuals (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4).

2. Etiology

Disease Causal Factors

Chromosome 18p deletion syndrome is caused by a chromosomal deletion affecting the short arm (p arm) of chromosome 18 (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4). The deletion size and breakpoint location vary significantly between individuals, contributing to phenotypic heterogeneity (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4). Approximately 42% of cases have breakpoints within the centromeric region, while the remaining breakpoints are scattered along the entirety of the short arm (hasi‐zogaj2015areviewof pages 2-4, hasi‐zogaj2015areviewof pages 4-7). Interestingly, no large interstitial deletions of 18p have been reported, though microdeletions have been documented (hasi‐zogaj2015areviewof pages 2-4).

Risk Factors

Genetic Risk Factors: The primary genetic risk factor is the presence of a balanced chromosomal translocation in a parent, particularly involving chromosome 18p and an acrocentric chromosome (papamichail2024prenataldiagnosisof pages 1-3, papamichail2023prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 11-12). Approximately 89% of cases arise de novo, while ~11% are inherited or result from unbalanced translocations (hasi‐zogaj2015areviewof pages 2-4). Of de novo cases where parental origin can be determined, approximately 50% arise from the maternal chromosome and 50% from the paternal chromosome (hasi‐zogaj2015areviewof pages 2-4).

Specific Causal/Modifier Genes (Haploinsufficiency): - TGIF1 (3,451,591-3,458,406 on 18p): Haploinsufficiency causes holoprosencephaly (HPE) and HPE microforms with ~11% penetrance in deletion carriers (hasi‐zogaj2015areviewof pages 4-7, hasi‐zogaj2015areviewof pages 7-8) - SMCHD1 (2,655,886-2,805,015): Conditional haploinsufficiency; individuals with 18p deletion plus a permissive D4Z4 allele on chromosome 4q are at risk for facioscapulohumeral muscular dystrophy type 2 (FSHD2) (hasi‐zogaj2015areviewof pages 8-9, balog2018monosomy18pis pages 1-3) - GNAL (11,689,014-11,885,683): Linked to dystonia with low penetrance (~3% in deletion carriers) (hasi‐zogaj2015areviewof pages 7-8) - PTPN2 (12,792,301-12,884,334): Associated with autoimmune disease risk including rheumatoid arthritis, thyroiditis, and other autoimmune conditions (hasi‐zogaj2015areviewof pages 8-9, oktay202518pdeletionsyndrome pages 1-2) - AFG3L2 (12,328,943-12,377,275): Potential risk for spinocerebellar ataxia type 28 (SCA28), though not yet manifested in young cohorts (hasi‐zogaj2015areviewof pages 8-9, hasi‐zogaj2015areviewof pages 10-11) - LAMA1 (6,941,743-7,117,813): Associated with retinal vascular anomalies and possible skin findings (keratosis pilaris/ulerythema ophryogenes) (hasi‐zogaj2015areviewof pages 7-8)

Environmental/Other Risk Factors: No specific environmental risk factors for developing the deletion have been identified. However, TGIF1 heterozygous knockout mice exposed prenatally to retinoic acid show significantly increased risk for facial deformities, holoprosencephaly, and neural tube defects, suggesting potential gene-environment interactions (hasi‐zogaj2015areviewof pages 9-10).

Protective Factors

No genetic or environmental protective factors have been identified in the literature.

Gene-Environment Interactions

The TGIF1/TWSG1 genes, when hemizygous, may interact with retinoic acid or other environmental factors during embryonic development to modulate HPE risk, as demonstrated in mouse models (hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 11-12).

3. Phenotypes

The clinical phenotype of chromosome 18p deletion syndrome is highly variable, with significant differences even among individuals with identical breakpoints, indicating incomplete penetrance and expressivity for most features (papamichail2023prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 1-2). A comprehensive phenotype table is provided below.

Table (click to expand)
Phenotype Frequency in centromeric 18p- cohort Suggested HPO term(s) Typical onset Usual severity / course Notes / evidence
Hypotonia / mixed tone abnormalities 84% HP:0001252 Hypotonia; HP:0003808 Abnormality of muscle tone Neonatal–infancy Mild to moderate; often persistent developmental impact Common early neurologic feature in centromeric 18p- (hasi‐zogaj2015areviewof pages 7-8)
Neonatal complications (jaundice, respiratory distress, feeding difficulties) 71% HP:0001945 Respiratory distress; HP:0011968 Feeding difficulties; HP:0002904 Neonatal hypoglycemia/jaundice not specifically resolved Neonatal Variable; may require supportive care Composite category reported in review table (hasi‐zogaj2015areviewof pages 7-8)
MRI anomalies (excluding holoprosencephaly spectrum) 66% HP:0410263 Abnormal brain MRI; HP:0002538 Abnormal cerebral white matter morphology Congenital / childhood recognition Variable; often nonprogressive structural findings White matter abnormalities and other MRI findings frequently observed (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 9-10)
Recurrent otitis media 61% HP:0000389 Recurrent otitis media Infancy–childhood Recurrent; may contribute to conductive hearing loss Often associated with chronic middle-ear disease (hasi‐zogaj2015areviewof pages 7-8)
Heart defects 56% HP:0001627 Abnormality of the cardiovascular system; HP:0001629 Ventricular septal defect; HP:0001636 Tetralogy of Fallot Congenital Variable from mild to surgically significant Structural cardiac defects are common; prenatal VSD also reported (hasi‐zogaj2015areviewof pages 7-8, papamichail2023prenataldiagnosisof pages 1-3)
Ptosis 55% HP:0000508 Ptosis Congenital / infancy Mild to moderate; often persistent Characteristic craniofacial/ophthalmic feature (hasi‐zogaj2015areviewof pages 7-8)
Refractive errors 52% HP:0000545 Myopia; HP:0000539 Refractive error Childhood Mild to moderate; usually manageable with correction Broad ophthalmic involvement is frequent (hasi‐zogaj2015areviewof pages 7-8)
Strabismus 42% HP:0000486 Strabismus Infancy–childhood Mild to moderate; may need ophthalmologic management Common visual alignment abnormality (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 10-11)
Pectus excavatum 29% HP:0000767 Pectus excavatum Childhood Usually mild to moderate; generally stable Skeletal/chest wall manifestation (hasi‐zogaj2015areviewof pages 7-8)
Hearing loss (overall) 23% HP:0000365 Hearing impairment; HP:0000405 Conductive hearing impairment; HP:0000407 Sensorineural hearing impairment Childhood Usually mild to moderate; conductive more common than sensorineural Review table reports hearing loss overall 23%; critical-region analysis separated conductive 22% and sensorineural 8% penetrance (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 10-11)
Isolated growth hormone deficiency 23% HP:0000824 Growth hormone deficiency; HP:0001510 Growth delay Childhood Variable; treatable when recognized Endocrine surveillance recommended in review (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 10-11)
Scoliosis / kyphosis 19% HP:0002650 Scoliosis; HP:0002808 Kyphosis Childhood–adolescence Mild to moderate; occasionally requires bracing/surgery Combined axial skeletal phenotype (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 10-11)
Pes planus 19% HP:0001763 Pes planus Childhood Usually mild; may affect gait/endurance Common orthopedic feature (hasi‐zogaj2015areviewof pages 7-8)
Cryptorchidism 14% HP:0000028 Cryptorchidism Congenital Variable; may require orchiopexy Male genital anomaly; micropenis also described in prenatal case literature (hasi‐zogaj2015areviewof pages 7-8, papamichail2023prenataldiagnosisof pages 1-3)
Panhypopituitarism / hypopituitarism 13% HP:0000826 Hypopituitarism; HP:0000873 Panhypopituitarism Congenital / childhood Potentially severe; chronic hormone replacement often required Includes structural pituitary anomalies in some patients (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 9-10)
Seizures 13% HP:0001250 Seizure Childhood Variable; often intermittent/managed medically Included grand mal, absence, and partial complex seizures in review cohort (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 10-11)
Immunoglobulin deficiency (IgA, IgG, or IgM deficiency) 13% HP:0002721 Immunodeficiency; HP:0002205 Recurrent infections; HP:0011347 Decreased circulating IgA level Childhood Variable; may predispose to infections/autoimmunity IgA deficiency specifically reported in review and later case reports (hasi‐zogaj2015areviewof pages 7-8, oktay202518pdeletionsyndrome pages 1-2)
Holoprosencephaly or HPE microform 13% HP:0001360 Holoprosencephaly; HP:0000668 Single central incisor Prenatal / congenital Severe in classic HPE, milder in microforms TGIF1 hemizygosity is a major mechanistic candidate; penetrance incomplete (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 4-7)
Autoimmune disorder 10% HP:0002960 Autoimmunity; HP:0000821 Hypothyroidism; HP:0002725 Systemic lupus erythematosus; HP:0012205 Alopecia Childhood–adult Variable; chronic, organ-specific or systemic Review lists rheumatoid arthritis, celiac disease, alopecia, vitiligo, lupus, Sjögren syndrome, autoimmune thyroid disease (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 9-10, oktay202518pdeletionsyndrome pages 1-2)
Sacral agenesis 6% HP:0003310 Sacral agenesis Congenital Moderate to severe; structural Rare but recurrent caudal malformation (hasi‐zogaj2015areviewof pages 7-8)
Optic nerve hypoplasia 6% HP:0008058 Optic nerve hypoplasia Congenital / infancy Variable visual impairment Part of ophthalmologic/neurodevelopmental spectrum (hasi‐zogaj2015areviewof pages 7-8)
Congenital cataracts 6% HP:0000519 Congenital cataract Congenital Variable; may need surgery Rare but documented ocular feature (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 10-11)
Myelomeningocele 3% HP:0002414 Myelomeningocele Congenital Severe structural defect Uncommon neural tube defect in reported cohort (hasi‐zogaj2015areviewof pages 7-8)

Table: This table summarizes the principal phenotypes reported in the comprehensive Hasi-Zogaj 2015 review of chromosome 18p deletion syndrome, including frequencies, suggested HPO mappings, onset, and usual severity/course. It is useful for knowledge-base phenotype annotation and anticipatory clinical assessment.

Quality of Life Impact

Individuals with chromosome 18p deletion syndrome experience significant impacts on quality of life related to: - Cognitive Function: Average full-scale IQ of 69 (range 51-99), typically in the mild to borderline intellectual disability range (hasi‐zogaj2015areviewof pages 2-4) - Adaptive Functioning: The majority of participants have problems with activities of everyday life, including difficulties with communication, home living, self-care, and management of social and leisure activities (hasi‐zogaj2015areviewof pages 2-4) - Autism Spectrum Features: Based on parental report using the Gilliam Autism Rating Scale (GARS), between 19% and 38% of individuals score in the range suggesting autism spectrum disorder (hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 8-9) - Medical Complications: Chronic medical issues including recurrent otitis media (61%), cardiac anomalies requiring management (56%), endocrine dysfunction (23-13%), and sensory impairments significantly impact daily functioning (hasi‐zogaj2015areviewof pages 7-8)

4. Genetic/Molecular Information

Chromosomal Abnormalities

Chromosome 18p deletion syndrome is characterized by partial or complete monosomy of the short arm of chromosome 18 (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4). The deletions are predominantly terminal, with breakpoints ranging from the centromere to various positions along the p arm (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4, hasi‐zogaj2015areviewof pages 4-7).

Deletion Characteristics: - Size: Variable, ranging from small microdeletions (<1 Mb) to complete p-arm deletions (~15-17 Mb) (papamichail2024prenataldiagnosisof pages 1-3, oktay202518pdeletionsyndrome pages 1-2) - Breakpoint distribution: ~42% centromeric, ~58% scattered along 18p (hasi‐zogaj2015areviewof pages 2-4) - Type: Predominantly terminal deletions; no large interstitial deletions reported (hasi‐zogaj2015areviewof pages 2-4) - Origin: ~89% de novo, ~11% familial (inherited or translocation-derived) (hasi‐zogaj2015areviewof pages 2-4) - Parental origin of de novo cases: ~50% maternal, ~50% paternal (hasi‐zogaj2015areviewof pages 2-4)

Unbalanced Translocations: Unbalanced translocations, particularly between 18p and acrocentric chromosomes (13, 15, 21, 22), account for a subset of cases (papamichail2024prenataldiagnosisof pages 1-3, papamichail2023prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 11-12). These often arise from parental balanced translocations or pericentric inversions (hasi‐zogaj2015areviewof pages 1-2).

Pathogenic Variants and Causal Genes

The syndrome results from structural chromosomal abnormalities (deletions, unbalanced translocations) rather than point mutations (hasi‐zogaj2015areviewof pages 1-2). A comprehensive gene dosage map has been established, identifying 12 genes as likely or possibly dosage-sensitive out of 67 total genes on 18p (hasi‐zogaj2015areviewof pages 2-4, hasi‐zogaj2015areviewof pages 10-11). A detailed gene table is provided below.

Table (click to expand)
Gene 18p location (hg19, as reported) Core function Haploinsufficiency / dosage status in review Phenotypes linked to hemizygosity or conditional hemizygosity Inheritance / mechanistic model Key evidence from Hasi-Zogaj 2015 review
CETN1 580,369-581,524 Centrin family protein involved in centrosome position/segregation and microtubule functions Not established as pathogenic for a human 18p- phenotype; discussed as a candidate dosage-sensitive gene No specific confirmed human phenotype from 18p hemizygosity; possible relevance to male fertility considered but unproven Unclear; no direct phenotype established in 18p-; maternal transmissions including this region argue against female infertility Review notes mouse heterozygous mutations cause infertility, but human 18p deletions including CETN1 have been maternally transmitted; no firm human dosage phenotype assigned (hasi‐zogaj2015areviewof pages 4-7)
TGIF1 3,451,591-3,458,406 Homeodomain transcriptional regulator in TGF-beta/NODAL-related developmental signaling Likely dosage-sensitive / haploinsufficient Holoprosencephaly (HPE), HPE microforms, single central incisor, midline/pituitary anomalies Classic haploinsufficiency with incomplete penetrance; phenotype modified by additional factors In the review cohort, 11% (6/65) of individuals hemizygous for TGIF1 had HPE-spectrum malformations; TGIF1 point mutations are established in HPE and 18p hemizygosity is a recognized risk factor (hasi‐zogaj2015areviewof pages 4-7, hasi‐zogaj2015areviewof pages 7-8)
DLGAP1 3,499,183-3,880,068 Postsynaptic density scaffold protein Candidate dosage-sensitive / possible risk gene Autism spectrum disorder / autistic features Risk-factor model rather than fully penetrant haploinsufficiency Seven of eight individuals with clinically significant autism scores had deletions including DLGAP1; proposed because of postsynaptic density enrichment (hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 8-9)
TWSG1 9,334,765-9,402,418 BMP-binding extracellular developmental regulator involved in dorsal-ventral patterning and craniofacial development Conditional / uncertain dosage sensitivity Possible modifier for HPE; possible contribution to dental/caries phenotype; direct role in 18p HPE not proven Likely modifier-gene model, potentially interacting with TGIF1 or environmental exposures Review states evidence for direct involvement in HPE is conflicting; no study participant with isolated TWSG1 hemizygosity had HPE, though prior literature suggested combined deletion with TGIF1 may increase HPE penetrance (hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 11-12)
ANKRD12 9,136,751-9,285,983 Ankyrin repeat domain-containing protein; exact dosage mechanism uncertain Candidate conditional dosage-sensitive / risk gene Autism spectrum disorder risk Risk-factor model Listed among 18p genes implicated by overlap in individuals with clinically significant autism scores (hasi‐zogaj2015areviewof pages 8-9)
LAMA1 6,941,743-7,117,813 Laminin alpha-1, basement membrane component Likely dosage-sensitive / conditional Retinal vascular anomalies, keratosis pilaris / ulerythema ophryogenes candidate, possible ocular/cerebellar features Incomplete penetrance; possible revealed recessive-allele model for severe manifestations In the cohort, 1/32 with LAMA1 hemizygosity had tortuous anomalous retinal vessels; skin findings were common but nonspecific; review discusses relevance based on known recessive LAMA1 disease and mouse retinal vasculopathy (hasi‐zogaj2015areviewof pages 7-8)
LRRC30 7,231,137-7,232,042 Leucine-rich repeat-containing protein; function poorly defined Candidate conditional dosage-sensitive / risk gene Autism spectrum disorder risk Risk-factor model Included among genes recurrently deleted in individuals with autism-range GARS/GARS-2 scores (hasi‐zogaj2015areviewof pages 8-9)
GNAL 11,689,014-11,885,683 G-protein alpha-olf subunit involved in receptor signaling Likely dosage-sensitive / conditional Dystonia, torsion dystonia, movement disorders Conditional / incompletely penetrant haploinsufficiency GNAL is a major adult-onset dystonia gene in the literature; in the 18p- cohort, 2/58 individuals with deletions encompassing GNAL had dystonia, supporting low-penetrance risk from hemizygosity (hasi‐zogaj2015areviewof pages 7-8)
IMPA2 11,981,427-12,030,885 Inositol monophosphatase-related function Candidate conditional dosage-sensitive / risk gene Autism spectrum disorder risk Risk-factor model Seven of eight autism-range cases had deletions including DLGAP1/LRRC30/ANKRD12/IMPA2; one additional case lacked IMPA2, suggesting it may contribute but is not solely causal (hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 8-9)
AFG3L2 12,328,943-12,377,275 Mitochondrial protease subunit involved in protein quality control and ribosome assembly Conditional dosage-sensitive Spinocerebellar ataxia type 28 (SCA28)-like risk; possible later-onset ataxia Conditional haploinsufficiency / age-dependent penetrance None of 15 examined individuals met diagnostic criteria for SCA28, but point mutations in AFG3L2 cause dominant ataxia and the review highlights possible future age-related manifestation in 18p- adults (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 10-11)
PTPN2 12,792,301-12,884,334 Protein tyrosine phosphatase involved in immune regulation Conditional dosage-sensitive / susceptibility gene Autoimmune disease risk (juvenile rheumatoid arthritis, thyroid autoimmunity, celiac disease, vitiligo, psoriasis, alopecia, Sjogren syndrome) Susceptibility / second-hit model No inflammatory bowel disease was seen in 67 hemizygous individuals, but 11 had autoimmune conditions; the review considers PTPN2 a plausible contributor with incomplete penetrance and likely additional modifiers (hasi‐zogaj2015areviewof pages 8-9)
SMCHD1 2,655,886-2,805,015 Structural maintenance of chromosomes hinge domain protein; chromatin repression and methylation including D4Z4 Conditional dosage-sensitive Risk for facioscapulohumeral muscular dystrophy type 2 (FSHD2), retinal vasculopathy in susceptible background Digenic / permissive-background model requiring hemizygosity plus permissive D4Z4 allele and repeat context Review states SMCHD1 hemizygosity alone is insufficient, but individuals with 18p deletion may be at risk for FSHD when a permissive 4q D4Z4 background is present; this was later directly supported by Balog et al. 2018 (hasi‐zogaj2015areviewof pages 8-9, balog2018monosomy18pis pages 1-3)
PTPRM Not specified in the review pages provided Receptor-type protein tyrosine phosphatase Not classified in Hasi-Zogaj 2015 as a key dosage-sensitive gene No specific phenotype assigned in the review Unclear Mentioned in later case literature on 18p autoimmunity, but not among the key mapped dosage-sensitive genes in the Hasi-Zogaj review sections provided (oktay202518pdeletionsyndrome pages 1-2)
ADCYAP1 Not specified in the review pages provided Pituitary adenylate cyclase-activating polypeptide signaling Not classified in Hasi-Zogaj 2015 as a key dosage-sensitive gene No specific phenotype assigned in the review Unclear Reported in later case literature as a potentially relevant deleted gene in autoimmune endocrinopathy, but not a mapped key dosage-sensitive gene in the review (oktay202518pdeletionsyndrome pages 1-2)
LPIN2 Not specified in the review pages provided Lipin-2, lipid metabolism/inflammation-related protein Not classified in Hasi-Zogaj 2015 as a key dosage-sensitive gene No specific phenotype assigned in the review Unclear Highlighted in a later autoimmune case report, not as a core dosage-sensitive 18p review gene (oktay202518pdeletionsyndrome pages 1-2)
USP14 Not specified in the review pages provided Deubiquitinating enzyme involved in proteostasis Not classified in Hasi-Zogaj 2015 as a key dosage-sensitive gene No specific phenotype assigned in the review Unclear Cited in later case literature as a potentially relevant deleted immune-modifying gene, but not part of the core 2015 dosage map summarized here (oktay202518pdeletionsyndrome pages 1-2)

Table: This table summarizes the principal chromosome 18p genes discussed as dosage-sensitive, conditionally dosage-sensitive, or candidate risk genes in the Hasi-Zogaj 2015 review, with later context for several immune-related genes. It is useful for linking deletion breakpoints to expected phenotypes and for distinguishing established haploinsufficiency from low-penetrance or modifier effects.

Allele Frequency

As a predominantly de novo chromosomal abnormality, there is no meaningful population allele frequency for specific deletion variants.

Somatic vs. Germline

The deletions are germline events, present in all cells of affected individuals (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4).

Epigenetic Information

SMCHD1 hemizygosity specifically impacts DNA methylation and chromatin structure. SMCHD1 is responsible for maintaining methylation of the D4Z4 chromatin domain on chromosome 4q; its loss results in hypomethylation similar to FSHD2, potentially leading to inappropriate DUX4 expression when combined with a permissive genetic background (hasi‐zogaj2015areviewof pages 8-9, balog2018monosomy18pis pages 1-3).

5. Environmental Information

No specific environmental factors (toxins, radiation, pollution, occupational exposures) have been identified as causative or modifying factors for chromosome 18p deletion syndrome. The deletions arise predominantly as sporadic chromosomal rearrangement events during gametogenesis or early embryonic development (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4).

6. Mechanism / Pathophysiology

Molecular Pathways

TGF-beta/NODAL Signaling (TGIF1): TGIF1 encodes a homeodomain protein that functions as a transcriptional regulator in the TGF-beta and NODAL signaling pathways, critical for dorsal-ventral patterning and midline development during embryogenesis (hasi‐zogaj2015areviewof pages 4-7, hasi‐zogaj2015areviewof pages 11-12). Haploinsufficiency leads to holoprosencephaly spectrum disorders with incomplete penetrance (~11%) (hasi‐zogaj2015areviewof pages 4-7). Point mutations in TGIF1 are established causes of HPE, and 18p hemizygosity represents a genomic mechanism for the same phenotype (hasi‐zogaj2015areviewof pages 4-7). - Suggested GO terms: GO:0007389 (pattern specification process), GO:0001942 (hair follicle development), GO:0030509 (BMP signaling pathway)

Postsynaptic Density Function (DLGAP1, ANKRD12, IMPA2, LRRC30): These genes encode proteins enriched in or associated with the postsynaptic density, a specialized structure at excitatory synapses (hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 8-9). Hemizygosity of these genes, particularly DLGAP1, is associated with increased risk of autism spectrum features, likely through disruption of synaptic signaling and plasticity (hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 8-9). - Suggested GO terms: GO:0014069 (postsynaptic density), GO:0099151 (regulation of postsynaptic density organization), GO:0099560 (synaptic membrane adhesion)

G-protein Signaling (GNAL): GNAL encodes the α-subunit of the G-protein olfactory receptor (Golf) involved in neuronal signaling (hasi‐zogaj2015areviewof pages 7-8). Point mutations in GNAL are a major cause of adult-onset dystonia, and hemizygosity in 18p deletion carries a low-penetrance risk (~3%) for dystonia (hasi‐zogaj2015areviewof pages 7-8). - Suggested GO terms: GO:0007186 (G-protein coupled receptor signaling pathway), GO:0007268 (chemical synaptic transmission)

Mitochondrial Protein Quality Control (AFG3L2): AFG3L2 encodes a subunit of a mitochondrial protease critical for degrading misfolded proteins and ribosome assembly (hasi‐zogaj2015areviewof pages 8-9, hasi‐zogaj2015areviewof pages 10-11). Point mutations cause spinocerebellar ataxia type 28 (SCA28) with dominant inheritance. Hemizygosity may confer age-dependent risk for ataxia, though this has not yet manifested in relatively young cohorts (hasi‐zogaj2015areviewof pages 8-9, hasi‐zogaj2015areviewof pages 10-11). - Suggested GO terms: GO:0006515 (protein quality control for misfolded or incompletely synthesized proteins), GO:0034983 (peptidyl-lysine deacetylation)

Immune Regulation (PTPN2): PTPN2 encodes a protein tyrosine phosphatase involved in immune regulation. GWAS studies and mouse models link PTPN2 to inflammatory bowel disease, rheumatoid arthritis, and type 1 diabetes (hasi‐zogaj2015areviewof pages 8-9, oktay202518pdeletionsyndrome pages 1-2). In 18p deletion, hemizygosity is associated with autoimmune conditions (~11% penetrance) including thyroid autoimmunity, rheumatoid arthritis, celiac disease, and others, suggesting a susceptibility/second-hit model (hasi‐zogaj2015areviewof pages 8-9, oktay202518pdeletionsyndrome pages 1-2). - Suggested GO terms: GO:0002376 (immune system process), GO:0050776 (regulation of immune response), GO:0002250 (adaptive immune response)

Chromatin Methylation and FSHD2 (SMCHD1): SMCHD1 is a chromatin modifier responsible for maintaining heavy methylation of the D4Z4 macrosatellite repeat on chromosome 4q35, thereby repressing the DUX4 retrogene (hasi‐zogaj2015areviewof pages 8-9, balog2018monosomy18pis pages 1-3). In FSHD2, digenic inheritance of SMCHD1 haploinsufficiency (from 18p deletion or point mutation) plus a moderately sized, permissive D4Z4 allele leads to chromatin relaxation, DUX4 derepression, and muscle degeneration (hasi‐zogaj2015areviewof pages 8-9, balog2018monosomy18pis pages 1-3). Approximately 12% of Caucasians harbor a permissive genetic background, placing individuals with 18p deletion at conditional risk (balog2018monosomy18pis pages 1-3). - Suggested GO terms: GO:0006325 (chromatin organization), GO:0006346 (DNA methylation-dependent heterochromatin formation)

Basement Membrane Integrity (LAMA1): LAMA1 encodes laminin alpha-1, a basement membrane protein expressed in renal cortex, testis, and retina (hasi‐zogaj2015areviewof pages 7-8). In mice, LAMA1 mutations cause retinal vasculopathy with vessel tortuosity. Recessive LAMA1 mutations in humans cause Poretti-Bolshauser syndrome (cerebellar anomalies, retinal dystrophy). Hemizygosity in 18p deletion may contribute to retinal vascular anomalies (~3% documented) and potentially skin findings (keratosis pilaris) (hasi‐zogaj2015areviewof pages 7-8). - Suggested GO terms: GO:0005604 (basement membrane), GO:0001570 (vasculogenesis)

Cellular Processes

  • Neurodevelopmental patterning defects: TGIF1 and TWSG1 haploinsufficiency during embryogenesis (hasi‐zogaj2015areviewof pages 4-7, hasi‐zogaj2015areviewof pages 9-10)
  • Synaptic dysfunction: Postsynaptic density protein deficiency (hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 8-9)
  • Mitochondrial stress and proteostasis: AFG3L2 haploinsufficiency (hasi‐zogaj2015areviewof pages 8-9, hasi‐zogaj2015areviewof pages 10-11)
  • Immune dysregulation: PTPN2 haploinsufficiency (hasi‐zogaj2015areviewof pages 8-9, oktay202518pdeletionsyndrome pages 1-2)

Tissue Damage Mechanisms

Chronic otitis media leading to conductive hearing loss involves middle ear mucosal dysfunction, possibly related to TGIF1 or other gene hemizygosity affecting epithelial integrity (hasi‐zogaj2015areviewof pages 7-8).

7. Anatomical Structures Affected

Organ Level

Primary Organs Directly Affected: - Brain: 66% of individuals have MRI anomalies including white matter abnormalities, holoprosencephaly spectrum (13%), pituitary anomalies (13%), and structural changes (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 9-10) - UBERON terms: UBERON:0000955 (brain), UBERON:0002037 (cerebellum), UBERON:0000007 (pituitary gland) - Heart: 56% have cardiac defects including ventricular septal defects, tetralogy of Fallot, and other structural anomalies (hasi‐zogaj2015areviewof pages 7-8) - UBERON term: UBERON:0000948 (heart) - Eyes: 52-55% have ophthalmologic findings including ptosis, strabismus, refractive errors, and rare retinal vascular anomalies (hasi‐zogaj2015areviewof pages 7-8) - UBERON term: UBERON:0000970 (eye) - Endocrine System: 23% isolated growth hormone deficiency, 13% panhypopituitarism/hypopituitarism (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 9-10) - UBERON term: UBERON:0000949 (endocrine system) - Immune System: 13% immunoglobulin deficiency, 10% autoimmune disorders (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 9-10, oktay202518pdeletionsyndrome pages 1-2) - UBERON term: UBERON:0002405 (immune system) - Ears: 61% recurrent otitis media, 23% hearing loss (hasi‐zogaj2015areviewof pages 7-8) - UBERON term: UBERON:0001690 (ear)

Secondary Organ Involvement: - Musculoskeletal: Hypotonia (84%), scoliosis/kyphosis (19%), pectus excavatum (29%), pes planus (19%) (hasi‐zogaj2015areviewof pages 7-8) - UBERON term: UBERON:0002204 (musculoskeletal system)

Tissue and Cell Level

Specific cell populations targeted include: - Neurons and postsynaptic structures (autism risk associated with synaptic gene haploinsufficiency) (hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 8-9) - Cell Ontology term: CL:0000540 (neuron) - Pituitary somatotrophs and other endocrine cells (growth hormone deficiency) (hasi‐zogaj2015areviewof pages 7-8) - Cell Ontology term: CL:0000441 (somatotroph) - Immune cells (B-cells and T-cells) (immunoglobulin deficiency, autoimmunity) (hasi‐zogaj2015areviewof pages 7-8, oktay202518pdeletionsyndrome pages 1-2) - Cell Ontology terms: CL:0000236 (B cell), CL:0000084 (T cell)

Subcellular Level

  • Postsynaptic density (DLGAP1 function) (hasi‐zogaj2015areviewof pages 9-10)
  • GO Cellular Component term: GO:0014069 (postsynaptic density)
  • Mitochondria (AFG3L2 protease function) (hasi‐zogaj2015areviewof pages 8-9, hasi‐zogaj2015areviewof pages 10-11)
  • GO Cellular Component term: GO:0005739 (mitochondrion)
  • Chromatin/nucleus (SMCHD1 methylation function) (hasi‐zogaj2015areviewof pages 8-9)
  • GO Cellular Component term: GO:0000785 (chromatin)

8. Temporal Development

Onset

  • Typical Age of Onset: Congenital for structural anomalies (holoprosencephaly, cardiac defects, dysmorphic features); neonatal for hypotonia and feeding difficulties; childhood for developmental delay recognition and medical complications; potentially adult-onset for dystonia, FSHD2, and some autoimmune features (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 2-4)
  • Onset Pattern: Predominantly congenital with chronic/progressive manifestations through childhood and adulthood (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4)

Progression

  • Disease Stages: No formal staging system; clinical presentation evolves from congenital structural defects through developmental delays in childhood to potential adult complications (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4)
  • Progression Rate: Variable; cognitive impairment is relatively stable (average IQ 69), while medical complications can accumulate over time (hasi‐zogaj2015areviewof pages 2-4)
  • Disease Course Pattern: Chronic, lifelong condition with stable cognitive baseline and variable medical complications (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4)
  • Disease Duration: Lifelong (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4)

Patterns

  • Remission Patterns: Not applicable; this is a chromosomal condition without remission (hasi‐zogaj2015areviewof pages 1-2)
  • Critical Periods: Embryonic development (for structural malformations related to TGIF1/TWSG1), early childhood (for developmental intervention), adolescence/adulthood (for monitoring dystonia, FSHD2, autoimmune disease risk) (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 4-7, hasi‐zogaj2015areviewof pages 8-9)

9. Inheritance and Population

Epidemiology

Genetic Etiology and Inheritance

  • Inheritance Pattern: Predominantly de novo (~89% of cases); ~11% familial, arising from unbalanced segregation of parental balanced translocations or direct parent-to-child transmission (hasi‐zogaj2015areviewof pages 2-4, hasi‐zogaj2015areviewof pages 11-12)
  • Penetrance:
  • Complete penetrance for the chromosomal abnormality itself
  • Incomplete penetrance for specific phenotypes: TGIF1 → HPE (11%), GNAL → dystonia (3%), SMCHD1 → FSHD2 (conditional on 4q background), PTPN2 → autoimmunity (~11-17%) (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 4-7, hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 8-9)
  • Expressivity: Highly variable even with identical breakpoints, indicating modifier genes and environmental factors influence phenotype (papamichail2023prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 1-2)
  • Genetic Anticipation: Not documented (no trinucleotide repeat mechanism)
  • Germline Mosaicism: Not specifically reported in retrieved literature
  • Founder Effects: Not reported; no population-specific variants identified
  • Consanguinity Role: Not relevant for de novo deletions; may unmask recessive alleles in deleted regions (theoretical)
  • Carrier Frequency: Not applicable for de novo events; parental balanced translocation carriers exist but specific frequency not documented

Population Demographics

  • Affected Populations: No specific ethnic or demographic groups with higher prevalence reported; cases described across multiple populations (papamichail2024prenataldiagnosisof pages 1-3, papamichail2023prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 1-2)
  • Geographic Distribution: Worldwide distribution; no endemic areas identified (hasi‐zogaj2015areviewof pages 1-2)
  • Age Distribution: Congenital condition with lifelong manifestations; cohorts studied range from neonates to adults (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 2-4)

10. Diagnostics

Genetic Testing

Recommended Genetic Testing Approach: Chromosomal microarray analysis (CMA) / array-based comparative genomic hybridization (array-CGH) is the primary diagnostic method, providing sensitive detection of deletions and precise breakpoint mapping (papamichail2024prenataldiagnosisof pages 1-3, papamichail2023prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 2-4). CMA can detect submicroscopic deletions missed by conventional karyotyping and is considered first-tier testing for developmental delay/intellectual disability (papamichail2024prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 2-4).

Specific Methods: - Chromosomal Microarray (CMA): Detects deletions, defines breakpoints, and identifies size of deleted region; current technology uses 400K-750K oligonucleotide platforms (papamichail2024prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 2-4) - Karyotyping: Useful for identifying gross deletions and, critically, for detecting balanced translocations in parents (papamichail2024prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 11-12). Parental testing is recommended, especially if there is a family history of cognitive impairment, congenital anomalies, or recurrent pregnancy loss (hasi‐zogaj2015areviewof pages 11-12) - FISH (Fluorescence In Situ Hybridization): Can confirm specific deletions but is targeted rather than genome-wide (papamichail2024prenataldiagnosisof pages 1-3) - Prenatal Testing: CMA can be performed on chorionic villus sampling (CVS) or amniocentesis samples; however, prenatal phenotype prediction is challenging due to variable expressivity (papamichail2024prenataldiagnosisof pages 1-3, papamichail2023prenataldiagnosisof pages 1-3)

Clinical Tests

Laboratory Tests: - Immunoglobulin levels (IgA, IgG, IgM): To detect immunodeficiency (13% affected) (hasi‐zogaj2015areviewof pages 7-8) - Endocrine evaluation: Growth hormone stimulation testing, thyroid function tests, complete pituitary hormone panel for individuals with growth failure or pituitary structural anomalies (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 9-10) - Autoimmune markers: Anti-thyroid antibodies, rheumatoid factor, celiac antibodies as clinically indicated (oktay202518pdeletionsyndrome pages 1-2)

Imaging Studies: - Brain MRI: Recommended to evaluate for white matter anomalies, holoprosencephaly, pituitary abnormalities (66% have abnormal findings) (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 9-10) - Echocardiography: To screen for cardiac defects (56% affected) (hasi‐zogaj2015areviewof pages 7-8) - Ophthalmologic examination: For ptosis, strabismus, refractive errors, and rare retinal anomalies (hasi‐zogaj2015areviewof pages 7-8)

Functional Tests: - Audiology: Audiometry to assess for hearing loss (23% overall; conductive more common than sensorineural) (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 10-11) - Developmental/Neuropsychological Assessment: Cognitive testing (average IQ 69, range 51-99), adaptive behavior assessment (hasi‐zogaj2015areviewof pages 2-4)

Clinical Criteria and Differential Diagnosis

There are no standardized clinical diagnostic criteria; diagnosis is confirmed by genetic testing. Differential diagnosis includes other microdeletion syndromes and isolated features such as holoprosencephaly from other causes, isolated growth hormone deficiency, and syndromic intellectual disability (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 11-12).

Screening

  • Newborn Screening: Not part of routine newborn screening panels
  • Prenatal Screening: May be detected incidentally on prenatal CMA performed for other indications; non-invasive prenatal testing (NIPT) may detect large deletions but is not a primary screening tool for 18p deletion (papamichail2024prenataldiagnosisof pages 1-3, papamichail2023prenataldiagnosisof pages 1-3)
  • Carrier Screening: Relevant for families with balanced translocations; preimplantation genetic diagnosis theoretically possible (hasi‐zogaj2015areviewof pages 11-12)

11. Outcome/Prognosis

Survival and Mortality

  • Life Expectancy: Compatible with survival into adulthood; the condition was historically described as the "first reported partial monosomy compatible with life" (papamichail2023prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 1-2). Specific mortality data not provided in retrieved literature, but the syndrome is not considered lethal.
  • Mortality Rate: Not specifically documented; chronic condition with lifelong management needs

Morbidity and Function

  • Cognitive Function: Average full-scale IQ of 69 (range 51-99), typically falling in the mild to borderline intellectual disability range (hasi‐zogaj2015areviewof pages 2-4)
  • Adaptive Functioning: Majority have problems with activities of daily living including communication, home living, self-care, and social/leisure activities (hasi‐zogaj2015areviewof pages 2-4)
  • Autism Risk: 19-38% score in the clinically significant range on autism rating scales (hasi‐zogaj2015areviewof pages 9-10, hasi‐zogaj2015areviewof pages 8-9)
  • Medical Morbidity: Chronic medical complications including recurrent infections (otitis media 61%), cardiac disease (56%), endocrine dysfunction (23-13%), sensory impairments, and potential adult-onset conditions (FSHD2, autoimmunity, dystonia) (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 8-9, balog2018monosomy18pis pages 1-3)

Quality of Life

Problems with communication, self-care, home living, and social functioning significantly impact quality of life (hasi‐zogaj2015areviewof pages 2-4). Educational and vocational support needs are substantial for most individuals.

Disease Course and Complications

Common Complications: - Recurrent infections due to immunodeficiency (hasi‐zogaj2015areviewof pages 7-8) - Cardiac complications requiring surgical intervention (hasi‐zogaj2015areviewof pages 7-8) - Chronic otitis media leading to conductive hearing loss (hasi‐zogaj2015areviewof pages 7-8) - Endocrine deficiencies requiring hormone replacement (hasi‐zogaj2015areviewof pages 7-8) - Scoliosis/kyphosis potentially requiring bracing or surgery (hasi‐zogaj2015areviewof pages 10-11) - Adult-onset: dystonia, FSHD2 (if permissive background), autoimmune disease (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 8-9, balog2018monosomy18pis pages 1-3)

Recovery Potential: The chromosomal abnormality is permanent; symptomatic treatments can improve function (e.g., growth hormone therapy for GH deficiency, cardiac surgery, hearing aids), but the underlying genetic condition persists (hasi‐zogaj2015areviewof pages 7-8).

Prognostic Factors

  • Deletion size and specific genes involved (genotype-phenotype correlations) (hasi‐zogaj2015areviewof pages 2-4, hasi‐zogaj2015areviewof pages 11-12)
  • Early intervention and multidisciplinary management (hasi‐zogaj2015areviewof pages 11-12)
  • Presence of SMCHD1 deletion plus permissive 4q background (FSHD2 risk) (balog2018monosomy18pis pages 1-3)

12. Treatment

Pharmacotherapy

Supportive Medications: - Growth hormone therapy: For isolated growth hormone deficiency; individuals respond to treatment (hasi‐zogaj2015areviewof pages 7-8) - Thyroid hormone replacement: For hypothyroidism/hypopituitarism (hasi‐zogaj2015areviewof pages 7-8, oktay202518pdeletionsyndrome pages 1-2) - Immunoglobulin replacement: For significant immunoglobulin deficiency (hasi‐zogaj2015areviewof pages 7-8) - Antiepileptic drugs: For seizure management (13% affected) (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 10-11) - Immunosuppressive/disease-modifying agents: For autoimmune diseases as clinically indicated (oktay202518pdeletionsyndrome pages 1-2)

Pharmacogenomics: No specific pharmacogenomic considerations documented in retrieved literature.

Advanced Therapeutics

No gene therapy, cell therapy, RNA-based therapies, or targeted molecular therapies are currently available or under investigation for chromosome 18p deletion syndrome.

Surgical and Interventional

  • Cardiac surgery: For structural heart defects (e.g., VSD repair, tetralogy of Fallot correction) (hasi‐zogaj2015areviewof pages 7-8)
  • Orthopedic interventions: Bracing or surgical correction for scoliosis/kyphosis (hasi‐zogaj2015areviewof pages 10-11)
  • Ophthalmologic surgery: For strabismus, cataracts as needed (hasi‐zogaj2015areviewof pages 7-8)
  • Ear tube placement: For chronic otitis media (hasi‐zogaj2015areviewof pages 7-8)

Supportive and Rehabilitative

  • Early intervention services: Physical therapy, occupational therapy, speech therapy (hasi‐zogaj2015areviewof pages 11-12)
  • Developmental therapies: To optimize cognitive and adaptive functioning (hasi‐zogaj2015areviewof pages 2-4, hasi‐zogaj2015areviewof pages 11-12)
  • Educational support: Special education services, individualized education plans (hasi‐zogaj2015areviewof pages 2-4)
  • Audiologic/ophthalmologic management: Hearing aids, glasses, ongoing monitoring (hasi‐zogaj2015areviewof pages 7-8)
  • Nutritional support: For feeding difficulties in infancy (hasi‐zogaj2015areviewof pages 7-8)

Treatment Strategy and Algorithms

Recommended Anticipatory Guidance (from Hasi-Zogaj 2015): - At diagnosis: - Chromosomal microarray to define deletion breakpoints (hasi‐zogaj2015areviewof pages 11-12) - Parental karyotype to rule out balanced translocation (hasi‐zogaj2015areviewof pages 11-12) - Echocardiography to rule out cardiac defects (hasi‐zogaj2015areviewof pages 11-12) - Brain MRI to evaluate structural anomalies (hasi‐zogaj2015areviewof pages 11-12) - Pituitary function monitoring (growth, endocrine axes) (hasi‐zogaj2015areviewof pages 11-12) - Ophthalmology and audiology evaluations (hasi‐zogaj2015areviewof pages 11-12) - Immunoglobulin levels (hasi‐zogaj2015areviewof pages 11-12) - Referral to early intervention services (hasi‐zogaj2015areviewof pages 11-12) - Ongoing surveillance: - Endocrine monitoring (especially for growth hormone deficiency and thyroid function) - Regular ophthalmology/audiology follow-up - Monitoring for autoimmune disease manifestations - For individuals with SMCHD1 deletion: genetic testing for 4q D4Z4 permissive background and clinical monitoring for FSHD2 (balog2018monosomy18pis pages 1-3)

MAXO (Medical Action Ontology) Terms (suggested): - MAXO:0000004 (hormone replacement therapy) - MAXO:0000058 (developmental therapy) - MAXO:0000011 (surgical procedure) - MAXO:0001175 (rehabilitation therapy)

13. Prevention

Primary Prevention

  • Genetic counseling: For families with known balanced translocations; recurrence risk depends on the specific translocation (hasi‐zogaj2015areviewof pages 11-12)
  • Preimplantation genetic diagnosis (PGD): Theoretically possible for known familial rearrangements, though not specifically documented in retrieved literature

Secondary Prevention

Tertiary Prevention

  • Treatment of complications: Growth hormone therapy, cardiac surgery, management of infections, seizure control, autoimmune disease treatment (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 11-12)
  • Surveillance for late-onset features: Monitoring for dystonia, FSHD2, autoimmune disease in at-risk individuals (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 8-9, balog2018monosomy18pis pages 1-3)

Screening Programs

  • Newborn screening: Not part of routine newborn screening
  • Cascade screening: Not applicable (predominantly de novo condition)

14. Other Species / Natural Disease

Natural Disease in Other Species

No naturally occurring chromosome 18p deletion syndrome has been described in other species in the retrieved literature.

15. Model Organisms

Gene-Specific Mouse Models

While no comprehensive mouse model of chromosome 18p deletion syndrome exists in the retrieved literature, several individual gene knockout/knockdown models have been studied:

TGIF1 Knockout Mice: Heterozygous TGIF1 knockout mice exposed prenatally to retinoic acid show significantly increased risk for facial deformities (30%), holoprosencephaly (23%), and neural tube defects (7%), recapitulating the HPE phenotype seen in human 18p deletion (hasi‐zogaj2015areviewof pages 9-10).

CETN1 Heterozygous Mice: Male mice with heterozygous CETN1 mutations are infertile, though this phenotype has not been confirmed in humans with 18p deletion (maternal transmission documented) (hasi‐zogaj2015areviewof pages 4-7).

AFG3L2 Models: Deletions and duplications of AFG3L2 in mice are associated with ataxia phenotypes, though point mutations are more penetrant than copy number changes (hasi‐zogaj2015areviewof pages 8-9, hasi‐zogaj2015areviewof pages 10-11).

LAMA1 Mutant Mice: Chemically induced mutations in LAMA1 result in retinal vasculopathy characterized by vitreous fibroplasia and vessel tortuosity (hasi‐zogaj2015areviewof pages 7-8).

TWSG1 Knockout Mice: Knockout mice show defects in craniofacial and dorsal-ventral patterning during embryonic development; prenatal retinoic acid exposure increases risk of malformations (hasi‐zogaj2015areviewof pages 9-10).

Model Applications

These gene-specific models are useful for understanding individual gene functions and testing potential therapeutic interventions, but do not fully recapitulate the complex multi-gene haploinsufficiency of human 18p deletion syndrome.

Summary and Conclusions

Chromosome 18p deletion syndrome (OMIM #146390) is a rare chromosomal disorder with an estimated prevalence of 1 in 50,000 live births, characterized by partial or complete monosomy of the short arm of chromosome 18 (papamichail2024prenataldiagnosisof pages 1-3, papamichail2023prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 1-2). The syndrome presents with highly variable clinical features including cognitive impairment (average IQ 69), minor dysmorphic features, hypotonia (84%), cardiac defects (56%), brain MRI anomalies (66%), and multisystem involvement (hasi‐zogaj2015areviewof pages 1-2, hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 2-4).

Approximately 89% of cases arise de novo, while ~11% are familial, often resulting from unbalanced translocations (hasi‐zogaj2015areviewof pages 2-4). Breakpoint location varies, with ~42% occurring in the centromeric region and the remainder scattered along the p arm, contributing to phenotypic heterogeneity (hasi‐zogaj2015areviewof pages 2-4). Current understanding identifies 12 dosage-sensitive genes out of 67 on 18p, with variable penetrance for specific phenotypes: TGIF1 (holoprosencephaly 11%), SMCHD1 (conditional FSHD2 risk), GNAL (dystonia 3%), PTPN2 (autoimmunity ~11%), and others (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 2-4, hasi‐zogaj2015areviewof pages 4-7, hasi‐zogaj2015areviewof pages 8-9).

Diagnosis relies on chromosomal microarray analysis for deletion detection and breakpoint mapping (papamichail2024prenataldiagnosisof pages 1-3, hasi‐zogaj2015areviewof pages 2-4). Parental karyotyping is recommended to identify balanced translocations (hasi‐zogaj2015areviewof pages 11-12). Management is primarily supportive and multidisciplinary, including early intervention, growth hormone therapy for GH deficiency, cardiac surgery for structural defects, and surveillance for late-onset complications (autoimmunity, dystonia, FSHD2) (hasi‐zogaj2015areviewof pages 7-8, hasi‐zogaj2015areviewof pages 11-12). Prognosis is compatible with survival into adulthood, though significant impacts on quality of life stem from cognitive impairment, medical comorbidities, and adaptive functioning challenges (hasi‐zogaj2015areviewof pages 2-4).

This comprehensive report synthesizes current evidence from primary literature to provide a foundation for disease knowledge base entry, clinical management, and future research into genotype-phenotype correlations and potential therapeutic targets.

References

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  9. (hasi‐zogaj2015areviewof pages 8-9): Minire Hasi‐Zogaj, Courtney Sebold, Patricia Heard, Erika Carter, Bridgette Soileau, Annice Hill, David Rupert, Brian Perry, Sidney Atkinson, Louise O'Donnell, Jon Gelfond, Jack Lancaster, Peter T. Fox, Daniel E. Hale, and Jannine D. Cody. A review of 18p deletions. American Journal of Medical Genetics Part C: Seminars in Medical Genetics, 169:251-264, Sep 2015. URL: https://doi.org/10.1002/ajmg.c.31445, doi:10.1002/ajmg.c.31445. This article has 93 citations.

  10. (balog2018monosomy18pis pages 1-3): Judit Balog, Remko Goossens, Richard J L F Lemmers, Kirsten R Straasheijm, Patrick J van der Vliet, Anita van den Heuvel, Chiara Cambieri, Nicolas Capet, Léonard Feasson, Veronique Manel, Julian Contet, Marjolein Kriek, Colleen M Donlin-Smith, Claudia A L Ruivenkamp, Patricia Heard, Stephen J Tapscott, Jannine D Cody, Rabi Tawil, Sabrina Sacconi, and Silvère M van der Maarel. Monosomy 18p is a risk factor for facioscapulohumeral dystrophy. Journal of Medical Genetics, 55:469-478, Mar 2018. URL: https://doi.org/10.1136/jmedgenet-2017-105153, doi:10.1136/jmedgenet-2017-105153. This article has 25 citations and is from a domain leading peer-reviewed journal.

  11. (hasi‐zogaj2015areviewof pages 10-11): Minire Hasi‐Zogaj, Courtney Sebold, Patricia Heard, Erika Carter, Bridgette Soileau, Annice Hill, David Rupert, Brian Perry, Sidney Atkinson, Louise O'Donnell, Jon Gelfond, Jack Lancaster, Peter T. Fox, Daniel E. Hale, and Jannine D. Cody. A review of 18p deletions. American Journal of Medical Genetics Part C: Seminars in Medical Genetics, 169:251-264, Sep 2015. URL: https://doi.org/10.1002/ajmg.c.31445, doi:10.1002/ajmg.c.31445. This article has 93 citations.

  12. (hasi‐zogaj2015areviewof pages 9-10): Minire Hasi‐Zogaj, Courtney Sebold, Patricia Heard, Erika Carter, Bridgette Soileau, Annice Hill, David Rupert, Brian Perry, Sidney Atkinson, Louise O'Donnell, Jon Gelfond, Jack Lancaster, Peter T. Fox, Daniel E. Hale, and Jannine D. Cody. A review of 18p deletions. American Journal of Medical Genetics Part C: Seminars in Medical Genetics, 169:251-264, Sep 2015. URL: https://doi.org/10.1002/ajmg.c.31445, doi:10.1002/ajmg.c.31445. This article has 93 citations.

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