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Inheritance

1

Autosomal dominant, predominantly de novo HP:0000006

AD-NSID is transmitted as an autosomal dominant trait at the level of the individual lesion - a single heterozygous loss-of-function allele is sufficient to cause disease - but affected individuals are almost always sporadic. Reduced reproductive fitness means the causal variant is usually de novo in the proband rather than inherited from an affected parent.

Autosomal dominant inheritance Penetrance: COMPLETE Expressivity: VARIABLE

Show evidence (4 references)

PMID:19196676 SUPPORT Human Clinical

"These results indicate that SYNGAP1 disruption is a cause of autosomal dominant nonsyndromic mental retardation."

First demonstration that de novo truncating variants in a single autosomal gene cause non-syndromic ID by an autosomal dominant mechanism.

PMID:25356899 SUPPORT Human Clinical

"Interestingly, no case was explained by inherited mutations."

In a trio exome screen of moderate-to-severe ID, every solved case was accounted for by a de novo rather than an inherited variant, supporting the sporadic-dominant architecture.

PMID:19196676 SUPPORT Other

"Moreover, autosomal dominant genes have yet to be identified, mainly because mental retardation results in lower reproductive fitness, which in turn decreases the likelihood of identifying families that are amenable to linkage analysis."

Explains why autosomal dominant non-syndromic ID genes could not be found by pedigree-based linkage and required a de novo, trio-sequencing approach.

+ 1 more reference

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Mechanistic Hypotheses

1

The non-syndromic/syndromic boundary reflects depth of phenotyping rather than a distinct mechanism

nonsyndromic_boundary_is_ascertainment EMERGING

Evidence balance 3 support

Under this model AD-NSID is not a mechanistically separate disease class but the leading edge of gene discovery: a gene-disease pair is labelled "non-syndromic" while the case count is small and the phenotyping shallow, and is reclassified as syndromic once enough patients are described to reveal a gestalt. The prediction is that MRD-series entities will migrate into named syndromes over time (as several already have), and that the residual truly non-syndromic set will be enriched for genes with brain-restricted expression. The competing model - that a real subclass of brain-restricted genes produces isolated cognitive impairment - is not mutually exclusive and is supported by SYNGAP1's expression pattern.

Show evidence (3 references)

PMID:23020937 SUPPORT Human Clinical

"Several patients did not meet the expected syndromic manifestation, suggesting a strong bias in present clinical syndrome descriptions."

Explicitly attributes syndromic/non-syndromic misclassification to bias in existing clinical syndrome descriptions.

PMID:23020937 SUPPORT Human Clinical

"The large number of de-novo variants in known intellectual disability genes is only partially attributable to known non-specific phenotypes."

Notes that patients ascertained as non-syndromic frequently carry variants in genes catalogued as syndromic.

PMID:26989088 SUPPORT Human Clinical

"SYNGAP1 encephalopathy is characterised by early neurodevelopmental delay typically preceding the onset of a relatively recognisable epilepsy comprising generalised seizures (absences, myoclonic jerks) and frequent triggers."

Worked example of the predicted migration: SYNGAP1, first reported as a cause of NON-syndromic ID, is redescribed as a recognisable developmental and epileptic encephalopathy once a larger series is phenotyped.

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Discussions and Knowledge Gaps

4

Which of the remaining Open Targets gene associations for MONDO:0015802 warrant curation as full genetic records in this entry, and which are better placed on gene-specific syndromic entries?

KNOWLEDGE GAP OPEN adnsid_opentargets_gene_backlog

Attached to

pathophysiology#Haploinsufficiency of a Dosage-Sensitive Neurodevelopmental Gene

Open Targets associates MONDO:0015802 with TLK2, ASH1L, STAG1, CLTC, CAMK2A, CIC, NAA15, TRIP12, KMT5B, TRIO, SET and DLG4. This entry curates four of those (CAMK2A, DLG4, KMT5B, TRIO) alongside six genes drawn from the general de novo ID literature (SYNGAP1, STXBP1, ZBTB18, GRIN2B, GATAD2B, MED13L). The gene list is deliberately illustrative of the major functional modules rather than exhaustive, so the remaining eight associations - TLK2, ASH1L, STAG1, CLTC, CIC, NAA15, TRIP12, SET - are recorded here as an explicit backlog rather than left as a silent omission. Triage is not purely mechanical: several of these are already described with recognisable syndromic features, so for each the curation decision is entangled with the syndromic/non-syndromic boundary question tracked in the adnsid_entity_boundary discussion. Note also that Open Targets associations are themselves reassessed over time, so the target list is a moving one.

Proposed experiments

Deep-phenotype triage of the remaining Open Targets associations

exp_adnsid_opentargets_gene_triage

For each of TLK2, ASH1L, STAG1, CLTC, CIC, NAA15, TRIP12 and SET, systematically review the published case series for recurrent dysmorphic, malformative, growth or neuroimaging features, and classify the gene-disease pair as (a) genuinely non-syndromic and therefore a curation target for this entry, or (b) syndromic and therefore better served by a dedicated gene-specific entry cross-referenced from here. The classification should record which specific feature drove any syndromic call, so the boundary decision is auditable rather than asserted.

How many autosomal dominant non-syndromic intellectual disability genes remain undiscovered, and why has discovery not plateaued?

KNOWLEDGE GAP OPEN adnsid_gene_list_incomplete

Attached to

pathophysiology#Haploinsufficiency of a Dosage-Sensitive Neurodevelopmental Gene

Modelling of the residual de novo protein-truncating burden in 31,058 trios implies roughly 1,000 further haploinsufficient developmental disorder genes, at a fold-enrichment about three times lower than for currently known genes. Lower enrichment implies reduced penetrance and/or pre/perinatal lethality, meaning the undiscovered tail is systematically different from the known genes and may not behave as cleanly monogenic. This is a first-order limit on the completeness of any AD-NSID gene list, including this entry's.

Proposed experiments

Scale trio sequencing to the calculated discovery-power threshold

exp_adnsid_scale_trio_sequencing

Expand pooled clinical and research parent-offspring trio sequencing toward the ~350,000-trio sample size calculated to give 80% power to detect a 10-fold de novo protein-truncating-variant enrichment for an average gene, and re-run gene-level enrichment testing.

Biobank penetrance test for low-enrichment candidate genes

exp_adnsid_penetrance_biobank

For candidate genes showing lower-than-expected de novo PTV enrichment, measure PTV frequency in unselected adult population biobanks to distinguish reduced penetrance from limited statistical power.

Show evidence (2 references)

PMID:33057194 SUPPORT Computational

"Modelling suggests that more than 1,000 genes associated with developmental disorders have not yet been described, many of which are likely to be less penetrant than the currently known genes."

Quantifies the size and expected properties of the undiscovered gene set.

PMID:33057194 SUPPORT Computational

"We calculated that a sample size of ~350,000 parent-offspring trios would be needed to have 80% power to detect a 10-fold enrichment of de novo PTVs for an average gene."

Sets the concrete sample-size target for completing the gene list.

Does the mouse critical-window result - that repairing the causal lesion in adulthood fails to rescue cognition - hold in humans, and does it genuinely foreclose postnatal molecular therapy for AD-NSID?

HUMAN MODEL MISMATCH OPEN adnsid_critical_window_translational_validity

Attached to

pathophysiology#Impaired Cortical Circuit Formation During Critical Developmental Windows

The critical-window claim rests on conditional Syngap1 mouse genetics, and it is doing very heavy lifting: it is the main mechanistic argument that AD-NSID is therapeutically closed after early childhood. But the human cortical developmental timetable is far longer than the mouse's, plasticity windows differ by circuit, and the experiment has been performed for one gene in the synaptic module only - it has not been tested for the chromatin/transcription module at all. Treating a single-gene mouse result as a general prohibition on postnatal therapy across a class of hundreds of genes is an extrapolation the evidence does not license. Resolving this matters directly to whether ASO or gene-dosage therapies are worth developing for this class.

Proposed experiments

Adult-rescue test in chromatin-module AD-NSID mouse models

exp_adnsid_adult_rescue_chromatin_module

Repeat the conditional adult-restoration experiment in mouse models of chromatin-module AD-NSID genes (e.g. Gatad2b, Zbtb18) to test whether the critical-window property generalises beyond synaptic genes or is specific to the SYNGAP1 spine-maturation mechanism.

Human cortical organoid staged-correction window mapping

exp_adnsid_organoid_window_mapping

Use human iPSC-derived cortical organoids carrying an AD-NSID variant with an inducible correction system, applying correction at staged timepoints, to map the equivalent plasticity window on a human rather than murine developmental timetable.

Partial-rescue endpoint analysis after window closure

exp_adnsid_partial_rescue_endpoints

In existing adult-rescue paradigms, assess whether adaptive behaviour and seizure burden improve after window closure even when IQ-equivalent cognitive measures do not, to test whether "no rescue" is endpoint-specific rather than absolute.

Show evidence (1 reference)

PMID:23141534 SUPPORT Model Organism

"Inducing SYNGAP1 mutations after critical developmental windows closed had minimal impact on spine synapse function, whereas repairing these pathogenic mutations in adulthood did not improve behavior and cognition."

The mouse result whose human translational validity is the open question.

Is autosomal dominant non-syndromic intellectual disability a coherent disease entity, or an ascertainment bin that should be dissolved into per-gene entries?

CONTROVERSY OPEN adnsid_entity_boundary

Attached to

pathophysiology#Static Global Cognitive and Adaptive Impairment Without Syndromic Features

The class is defined by a negative (no recognised gestalt) plus an inheritance mode, not by a shared mechanism - its members span synaptic, chromatin, and cytoskeletal biology. Several flagship members (SYNGAP1, MED13L) have already been redescribed as syndromic, and the ZBTB18 case literature shows the label persists mainly where case counts are small. Against dissolution: the class does capture a genuinely shared architecture (de novo, monoallelic, constraint-intolerant gene, static developmental course) that is clinically actionable - it is the group for which trio sequencing is the correct first test. For dismech, the open question is whether this should remain a Disease entry or be remodelled as a Grouping over per-gene MRD entries once enough of those exist.

Show evidence (2 references)

PMID:26989088 SUPPORT Human Clinical

"SYNGAP1 encephalopathy is characterised by early neurodevelopmental delay typically preceding the onset of a relatively recognisable epilepsy comprising generalised seizures (absences, myoclonic jerks) and frequent triggers."

The flagship member of the class redescribed as a recognisable syndrome, the core argument for dissolving the category.

PMID:23020937 SUPPORT Human Clinical

"After exclusion of copy-number variants, de-novo point mutations and small indels are associated with severe, sporadic non-syndromic intellectual disability, accounting for 45-55% of patients with high locus heterogeneity."

The counter-argument: the class does describe a real, quantified, shared genetic architecture even if its members are mechanistically diverse.

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Pathophysiology

9

De Novo Heterozygous Variant in a Constraint-Intolerant Neurodevelopmental Gene

The initiating lesion is a single new (de novo) protein-truncating or damaging missense variant arising in the parental germline and present heterozygously in the proband. Because such variants are not transmitted through families, this class of disease was undetectable by linkage and was only exposed by parent-offspring trio exome sequencing. The affected genes are drawn overwhelmingly from the minority of the genome under strong selective constraint against heterozygous protein-truncating variants, i.e. genes whose normal function requires two working copies. An emerging, mechanistically distinct minority arm of the same trigger is the NON-CODING de novo mutation: whole-genome sequencing of ID trios finds regulatory DNMs selectively enriched in fetal-brain-specific enhancers, including recurrently mutated enhancer clusters regulating nervous-system development genes. This arm is captured here as part of the trigger rather than as a separate node because the downstream consequence - dosage perturbation of a constraint-intolerant neurodevelopmental gene - is the same; only the lesion type differs. Evidence comes from small, mixed developmental-disorder cohorts rather than AD-NSID-specific series, so it is an emerging rather than established contributor for this class.

nervous system development GO:0007399

brain UBERON:0000955

Downstream

Haploinsufficiency of a Dosage-Sensitive Neurodevelopmental Gene A protein-truncating allele triggers nonsense-mediated decay or produces a non-functional protein, leaving a single functional copy.

Show evidence (8 references)

PMID:36854624 SUPPORT Human Clinical

"We found that regulatory DNMs were selectively enriched in fetal brain-specific enhancers as compared with adult brain enhancers."

Establishes the non-coding regulatory arm of the de novo trigger: enhancer DNMs are enriched specifically in fetal-brain enhancers, matching the critical-window logic modelled downstream.

PMID:36854624 SUPPORT Human Clinical

"we identified recurrently mutated enhancer clusters that regulate genes involved in nervous system development (CSMD1, OLFM1, and POU3F3)."

Names the recurrently mutated enhancer clusters and their nervous-system-development target genes, showing the regulatory arm converges on the same gene classes as the coding arm.

PMID:36854624 SUPPORT In Vitro

"Using CRISPR-mediated mutation and editing of epigenomic marks, we show that DNMs at regulatory elements affect the expression of putative target genes."

Functional confirmation that the regulatory lesions change target-gene expression. PARTIAL because the cohort is small and mixed developmental-disorder rather than AD-NSID-specific, so clinical penetrance for individual regulatory variants is not established.

+ 5 more references

Haploinsufficiency of a Dosage-Sensitive Neurodevelopmental Gene

Loss of one functional allele reduces gene product to roughly 50% of normal. For the genes implicated in AD-NSID this is below the threshold required for normal neurodevelopment, which is why they are among the most constraint-intolerant genes in the genome. Haploinsufficiency is the predominant - though not the only - mechanism: a growing minority of AD-NSID genes act instead through altered-function (dominant-negative or gain-of-function) missense alleles, and these two mechanisms are not interchangeable when interpreting a new variant.

nervous system development GO:0007399

Downstream

Disrupted Synaptic Transmission and Plasticity For the synaptic module of AD-NSID genes, reduced gene dosage directly perturbs postsynaptic signalling complexes and receptor trafficking.

Disrupted Chromatin and Transcriptional Regulation For the chromatin/transcription module, reduced dosage of a chromatin-modifying or transcription-regulating factor dysregulates neurodevelopmental gene expression programmes.

Disrupted Neuronal Cytoskeletal Dynamics and Rho GTPase Signalling For the cytoskeletal module, reduced dosage of a cytoskeletal regulator or Rho GTPase pathway component impairs actin and microtubule dynamics in developing neurons.

Show evidence (4 references)

PMID:23020937 SUPPORT Human Clinical

"More participants in the case group had loss-of-function variants than in the control group (20/51 vs 2/20; p=0·022), suggesting their contribution to disease development."

Demonstrates statistical excess of heterozygous loss-of-function alleles in non-syndromic ID cases versus controls.

PMID:33057194 SUPPORT Human Clinical

"This residual burden of protein-truncating DNMs is greatest in genes that are intolerant of PTVs in the general population"

Confirms the haploinsufficiency mechanism concentrates in constraint-intolerant genes and that many such disorders remain undiscovered.

PMID:33057194 SUPPORT Human Clinical

"Consequently, we expect that a greater proportion of the novel genes will act via altered-function mechanisms (e.g. dominant negative or gain-of-function)."

Qualifies the haploinsufficiency model: newly discovered dominant developmental-disorder genes increasingly act by altered function rather than pure dosage loss.

+ 1 more reference

Disrupted Synaptic Transmission and Plasticity

A large fraction of AD-NSID genes encode components of the excitatory postsynaptic density, the NMDA-receptor-associated signalling complex, or the presynaptic release machinery. SYNGAP1, a brain-specific Ras GTPase-activating protein within the NMDA-receptor complex, is the paradigm: it normally restrains Ras-ERK signalling and thereby limits AMPA receptor insertion at the postsynaptic membrane. Presynaptic examples include STXBP1/Munc18-1, a regulator of SNARE-mediated neurotransmitter release. Reduced dosage of these proteins shifts excitatory synaptic strength and impairs activity-dependent plasticity, the cellular substrate of learning and memory.

glutamatergic (excitatory) neuron CL:0000679 hippocampal pyramidal neuron CL:0002608

chemical synaptic transmission GO:0007268 ⚠ ABNORMAL regulation of synaptic plasticity GO:0048167 ↓ DECREASED regulation of postsynaptic AMPA receptor levels GO:0099072 ⚠ ABNORMAL learning or memory GO:0007611 ↓ DECREASED

Ras GTPase-activating protein activity (SYNGAP1) GO:0005096 ↓ DECREASED

Downstream

Premature Dendritic Spine Maturation In the SYNGAP1 model, de-repressed excitatory synaptic signalling accelerates spine maturation and raises circuit excitability.

Show evidence (5 references)

PMID:23141534 SUPPORT Other

"Mutations that cause intellectual disability (ID) and autism spectrum disorder (ASD) are commonly found in genes that encode for synaptic proteins."

States the synaptic-gene convergence that defines this pathophysiology module.

PMID:19196676 SUPPORT Human Clinical

"We sequenced the autosomal gene SYNGAP1, which encodes a ras GTPase-activating protein that is critical for cognition and synapse function, in 94 patients with nonsyndromic mental retardation."

Identifies the molecular function (Ras GTPase-activating protein) whose loss underlies the paradigmatic AD-NSID gene.

PMID:19196676 SUPPORT Other

"SYNGAP1, a GTPase-activating protein that is selectively expressed in the brain and is a component of the NMDA-receptor complex, acts downstream of the receptor, blocking the insertion of the AMPA receptor at the postsynaptic membrane"

Describes the specific postsynaptic signalling step (restraint of AMPA receptor insertion via inhibition of RAS-ERK) that haploinsufficiency de-represses.

+ 2 more references

Disrupted Chromatin and Transcriptional Regulation

The second major functional module among AD-NSID genes comprises chromatin modifiers, chromatin-remodelling complex subunits, and sequence-specific transcriptional regulators (e.g. ARID1B, CHD2, GATAD2B, MBD5, TCF4, MED13L, ZBTB18). Halving the dose of these factors dysregulates the coordinated gene-expression programmes that drive neuronal differentiation, migration, and synaptogenesis. Systematic phenomics analysis shows chromatin-related genes form one of the biologically coherent modules into which ID genes decompose.

chromatin organization GO:0006325 ⚠ ABNORMAL regulation of transcription by RNA polymerase II GO:0006357 ⚠ ABNORMAL

Downstream

Impaired Cortical Circuit Formation During Critical Developmental Windows Dysregulated transcriptional programmes alter neuronal differentiation and connectivity during cortical development.

Premature Dendritic Spine Maturation Chromatin modifiers such as ARID1B, KANSL1, and WDR5 converge on dendritic spine morphogenesis and synaptic function, so the chromatin arm reaches the same synaptic endpoint as the synaptic arm.

Show evidence (5 references)

PMID:26748517 SUPPORT Computational

"Systematic identification of highly enriched functional themes and phenotypes revealed typical phenotype combinations characterizing process-defined groups of ID disorders, such as chromatin-related disorders and deficiencies in DNA repair."

Establishes chromatin regulation as a distinct, statistically coherent functional module among ID-associated genes.

PMID:26748517 SUPPORT Computational

"we show that ID-AGs are substantially enriched with co-expression, protein-protein interactions, and specific biological functions."

Demonstrates that the genetically heterogeneous ID gene set is functionally convergent rather than random.

PMID:25356899 SUPPORT Human Clinical

"A total of 12 likely pathogenic DNMs were identified in genes previously associated with ID (ARID1B, CHD2, FOXG1, GABRB3, GATAD2B, GRIN2B, MBD5, MED13L, SETBP1, TBR1, TCF4, WDR45), resulting in a diagnostic yield of ∼29%."

The recovered gene list is dominated by chromatin and transcriptional regulators, illustrating this module in a real ID cohort.

+ 2 more references

Disrupted Neuronal Cytoskeletal Dynamics and Rho GTPase Signalling

The third functional module comprises regulators of the neuronal actin and microtubule cytoskeleton, whose control converges on Rho GTPase transduction (e.g. TRIO, and the wider set of Rho-pathway ID genes). Cytoskeletal disruption acts earlier in neurodevelopment than the synaptic module: it impairs neuronal migration and neuritogenesis as well as synaptic plasticity, so the same molecular lesion propagates upward through cell processes, circuits, and finally cognition. Bibliomic analysis of the >1,000 known ID genes shows this convergence onto a small number of modules is the reason a genetically unrelated gene set produces a common cognitive phenotype.

neuron CL:0000540 pyramidal neuron CL:0000598

actin cytoskeleton organization GO:0030036 ⚠ ABNORMAL neuron migration GO:0001764 ⚠ ABNORMAL neuron projection development GO:0031175 ⚠ ABNORMAL

Downstream

Impaired Cortical Circuit Formation During Critical Developmental Windows Impaired migration, neuritogenesis, and plasticity prevent establishment of an effective cortical network.

Show evidence (3 references)

PMID:34200511 SUPPORT Computational

"Bibliomic analysis reveals that ID genes converge onto a few biological modules, including cytoskeleton dynamics, whose regulation depends on Rho GTPases transduction."

Establishes cytoskeletal dynamics under Rho GTPase control as one of the small number of convergent biological modules among ID genes.

PMID:34200511 SUPPORT Other

"Thus, cytoskeleton alterations that have an impact on cell processes such as neuronal migration, neuritogenesis, and synaptic plasticity rebound on the overall establishment of an effective network and consequently on the cognitive phenotype."

Specifies the cellular processes disrupted and the propagation from cytoskeletal lesion through network formation to cognition.

PMID:34200511 SUPPORT Other

"More than 1000 genes have been found mutated in ID patients pointing out that, despite the common phenotype, the genetic bases are highly heterogeneous and apparently unrelated."

Quantifies the locus heterogeneity that makes module-level convergence the only tractable way to describe the class mechanistically.

Premature Dendritic Spine Maturation

In the SYNGAP1 mouse model, loss of one Syngap1 allele accelerates the normal developmental timetable of dendritic spine synapse maturation during the early postnatal period. Because the pace of spine maturation - not merely its endpoint - determines the outcome, the critical variable is developmental timing rather than steady-state synapse number.

hippocampal pyramidal neuron CL:0002608

dendritic spine development GO:0060996 ⚠ ABNORMAL

hippocampal formation UBERON:0002421

Downstream

Cortical Hyperexcitability Prematurely mature spines drive excessive excitability in the developing hippocampus.

Show evidence (1 reference)

PMID:23141534 SUPPORT Model Organism

"In the SYNGAP1 mouse model of ID/ASD, we found that dendritic spine synapses develop prematurely during the early postnatal period."

Directly demonstrates accelerated dendritic spine synapse maturation in a genetic mouse model of AD-NSID.

Cortical Hyperexcitability

Loss of the SynGAP developmental brake on neural excitability leaves developing hippocampal and cortical circuits excessively excitable. This is the node that antiseizure medication acts on; it is downstream of, and not equivalent to, the spine-maturation defect, which no available drug reverses.

hippocampal pyramidal neuron CL:0002608

excitatory postsynaptic potential GO:0060079 ↑ INCREASED

cerebral cortex UBERON:0000956 hippocampal formation UBERON:0002421

Downstream

Impaired Cortical Circuit Formation During Critical Developmental Windows Mistimed spine maturation and the resulting hyperexcitability corrupt activity-dependent circuit refinement during the critical period.

Show evidence (1 reference)

PMID:23141534 SUPPORT Model Organism

"These data demonstrate that SynGAP protein acts as a critical developmental repressor of neural excitability that promotes the development of life-long cognitive abilities."

Frames the gene product's normal role as a developmental brake on excitability whose loss produces the hyperexcitable state.

Impaired Cortical Circuit Formation During Critical Developmental Windows

The convergent consequence of both the synaptic and chromatin modules is maldevelopment of cortical and hippocampal circuits during time-limited windows of activity-dependent refinement. The window property is mechanistically important and therapeutically sobering: in the SYNGAP1 model, introducing the mutation after the critical window has closed has little effect, and correcting it in adulthood does not rescue cognition. This is the principal reason AD-NSID is a static (non-progressive) developmental condition rather than a neurodegenerative one, and it sets a hard early-intervention constraint on any future molecular therapy.

neuron CL:0000540

cerebral cortex development GO:0021987 ⚠ ABNORMAL modulation of chemical synaptic transmission GO:0050804 ⚠ ABNORMAL

cerebral cortex UBERON:0000956 hippocampal formation UBERON:0002421

Downstream

Static Global Cognitive and Adaptive Impairment Without Syndromic Features Miswired cortical circuits manifest as a fixed deficit in intellectual and adaptive functioning.

Show evidence (2 references)

PMID:23141534 SUPPORT Model Organism

"Inducing SYNGAP1 mutations after critical developmental windows closed had minimal impact on spine synapse function, whereas repairing these pathogenic mutations in adulthood did not improve behavior and cognition."

Establishes the critical-window constraint - both the pathogenic and the therapeutic direction - in a genetic mouse model.

PMID:23141534 SUPPORT Model Organism

"We propose that the pace of dendritic spine synapse maturation in early life is a critical determinant of normal intellectual development."

States the developmental-timing model linking synaptic maturation pace to intellectual outcome.

Static Global Cognitive and Adaptive Impairment Without Syndromic Features

The clinical endpoint is significant, non-progressive impairment of intellectual functioning and adaptive behaviour with onset in the developmental period. What makes the presentation "non-syndromic" is the absence - on current examination - of a distinctive constellation of dysmorphic, growth, malformative, or neuroimaging features. This absence is partly a real biological property (several causal genes, such as SYNGAP1, are expressed almost exclusively in brain, so no somatic phenotype is expected) and partly an artefact of limited phenotyping and small case series: patients with variants in newly discovered dominant genes are, measurably, less phenotypically similar to one another than patients with variants in long-established genes.

Show evidence (3 references)

PMID:19196676 SUPPORT Human Clinical

"The absence of specific dysmorphic features and growth abnormalities in these patients is consistent with the fact that SYNGAP1 is exclusively expressed in the brain."

Gives the biological rationale for a genuinely brain-restricted, non-syndromic presentation.

PMID:19196676 SUPPORT Other

"Most patients have the nonsyndromic form of the disorder, which is characterized by the absence of associated morphologic, radiologic, and metabolic features."

Defines the non-syndromic category operationally.

PMID:33057194 SUPPORT Human Clinical

"patients with DNMs in the same novel DD-associated gene were less phenotypically similar to each other, on average, than patients with DNMs in a consensus gene"

Supports the ascertainment component of "non-syndromic": recently discovered dominant genes produce less distinctive, less consistent phenotypes.

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Pathograph

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Phenotypes

7

Musculoskeletal 1

Hypotonia FREQUENT HP:0001252

Show evidence (1 reference)

PMID:26989088 SUPPORT Human Clinical

"Hypotonia and unstable gait were frequent associated neurological features."

Reports hypotonia as a frequent feature in a 17-patient SYNGAP1 series. Frequency basis: author wording "frequent" maps to the FREQUENT band (30-79%) under the DisMech qualitative mapping. Derived from a single-gene series, so it is an anchor for the class rather than a measured class-level rate.

Nervous System 5

Intellectual Disability OBLIGATE HP:0001249

Show evidence (2 references)

PMID:23033978 SUPPORT Human Clinical

"We then sequenced the coding regions of more than 21,000 genes obtained from 100 patients with an IQ below 50 and their unaffected parents."

Defines the core cognitive phenotype and the severity band of the cohorts in which the disease class was characterised.

PMID:19196676 SUPPORT Human Clinical

"Scores on the Mullen Scales of Early Learning and the Vineland Adaptive Behavior Scales showed profiles that were consistent with moderate-to-severe mental retardation in all patients."

Instrumented confirmation of the intellectual and adaptive-behaviour deficit in molecularly confirmed AD-NSID patients. The OBLIGATE band is definitional rather than measured: intellectual disability is the defining feature of this disease class, so by construction it is present in 100% of cases. No cohort frequency is cited because none is meaningful for a defining criterion.

Global Developmental Delay HP:0001263

Show evidence (2 references)

PMID:26989088 SUPPORT Human Clinical

"Developmental delay was the first manifestation of SYNGAP1-related encephalopathy; intellectual disability became progressively obvious and was associated with autistic behaviours in eight patients."

Documents developmental delay as the presenting manifestation preceding an established ID diagnosis. Frequency is intentionally omitted: this snippet establishes the temporal ordering of the phenotype, not a cohort proportion, and no class-level frequency estimate exists.

PMID:19196676 SUPPORT Human Clinical

"Their early development was characterized by global delay and hypotonia, with the onset of walking at the age of 2 years."

Direct description of global developmental delay in molecularly confirmed AD-NSID.

Delayed Speech and Language Development FREQUENT HP:0000750

Show evidence (3 references)

PMID:19196676 SUPPORT Human Clinical

"The disruption of SYNGAP1 appears to be associated with a homogeneous clinical phenotype that is characterized by moderate-to-severe mental retardation accompanied by severe language impairment."

Identifies severe language impairment as a prominent feature of the paradigmatic AD-NSID entity.

PMID:38215144 SUPPORT Human Clinical

"He struggles with expressing themselves and forming complete sentences, relying mostly on gestures and pointing."

Expressive language impairment in a molecularly confirmed MRD22 case.

PMID:39528574 SUPPORT Human Clinical

"Other prevalent clinical features included speech delay (59/173, 34.1%)"

Frequency basis: 59/173 = 34.1%, which falls in the FREQUENT band (30-79%). PARTIAL because the denominator is a mixed DD/ID cohort rather than a molecularly confirmed AD-NSID cohort, so this is the closest available quantitative anchor, not a class-specific rate.

Seizures HP:0001250

Show evidence (3 references)

PMID:19196676 SUPPORT Human Clinical

"Two of the patients were mildly epileptic."

Epilepsy present in 2 of the 3 patients in the original non-syndromic SYNGAP1 series, and mild/controllable in both.

PMID:19557857 SUPPORT Human Clinical

"We identified de novo mutations in STXBP1 (nonsense, p.R388X; splicing, c.169+1G>A) in two patients with severe mental retardation and nonsyndromic epilepsy."

Documents epilepsy co-occurring with non-syndromic ID under a de novo autosomal dominant lesion.

PMID:38215144 SUPPORT Human Clinical

"However, he did not have growth retardation, microcephaly, corpus callosum hypoplasia, epilepsy, or dysmorphic facial features."

Counter-example: a molecularly confirmed MRD22 patient without epilepsy, showing the feature is not obligate across the class. Frequency is intentionally omitted because the available sources are irreconcilable at class level: 2/3 in the index SYNGAP1 series, 17/17 in a larger SYNGAP1 cohort, and 0/1 here. Any single band would contradict at least one cited source.

Autistic Behaviour Autism HP:0000717

Show evidence (3 references)

PMID:26989088 SUPPORT Human Clinical

"intellectual disability became progressively obvious and was associated with autistic behaviours in eight patients"

Autistic behaviour in 8 of 17 patients in a molecularly defined AD-NSID series.

PMID:21237447 SUPPORT Human Clinical

"We provide evidence that truncating mutations in SYNGAP1 are common in NSID and can be also associated with autism."

Establishes the conditional ("can be also associated") rather than obligate relationship between AD-NSID and autism.

PMID:19196676 SUPPORT Human Clinical

"Nonverbal social interactions were within the normal range. In particular, evaluation of Patient 3 with the Autism Diagnostic Observation Schedule was negative."

Counter-evidence that autism is not obligate: structured assessment was negative in the index non-syndromic series. Frequency is intentionally omitted: the two cited series disagree directly (8/17 = 47% in one, 0/3 on structured assessment in the other), and no class-level estimate exists.

Other 1

Delayed Ability to Walk HP:0031936

Show evidence (1 reference)

PMID:19196676 SUPPORT Human Clinical

"Their early development was characterized by global delay and hypotonia, with the onset of walking at the age of 2 years."

Documents delayed independent ambulation in the index AD-NSID series. Frequency is intentionally omitted: the source describes three patients and reports no proportion.

🧬

Genetic Associations

10

SYNGAP1

Gene: SYNGAP1 hgnc:11497 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (2 references)

PMID:19196676 SUPPORT Human Clinical

"We identified de novo truncating mutations (K138X, R579X, and L813RfsX22) in three of these patients."

Three independent de novo protein-truncating variants in 94 patients with non-syndromic ID.

PMID:21237447 SUPPORT Human Clinical

"Recently, we reported de novo truncating mutations in the SYNGAP1 gene of 3 of 94 NSID cases, suggesting that its disruption represents a common cause of autosomal dominant NSID."

Replication cohort confirming SYNGAP1 as a common cause of autosomal dominant non-syndromic ID.

STXBP1

Gene: STXBP1 hgnc:11444 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (2 references)

PMID:19557857 SUPPORT Human Clinical

"These results suggest that STXBP1 disruption is associated with autosomal dominant mental retardation and nonsyndromic epilepsy."

Establishes STXBP1 as an autosomal dominant non-syndromic ID gene.

PMID:23020937 SUPPORT Human Clinical

"16 patients carried de-novo variants in known intellectual disability genes with three recurrently mutated genes (STXBP1, SYNGAP1, and SCN2A)."

Independent replication of STXBP1 recurrence in a severe sporadic non-syndromic ID cohort.

ZBTB18

Gene: ZBTB18 hgnc:13030 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (2 references)

PMID:38215144 SUPPORT Human Clinical

"Clinical whole exome sequencing revealed a de novo pathogenic variant in the ZBTB18 gene (c.1207delC, p. Arg403Alafs*60), which is a previously unreported site."

De novo frameshift variant in a patient diagnosed with MRD22.

PMID:38215144 SUPPORT Human Clinical

"It is noteworthy that only 31 cases of this disorder have been reported thus far."

Quantifies the small case count that keeps this MRD-series entity in the non-syndromic category.

GRIN2B

Gene: GRIN2B hgnc:4586 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (1 reference)

PMID:25356899 SUPPORT Human Clinical

"A total of 12 likely pathogenic DNMs were identified in genes previously associated with ID (ARID1B, CHD2, FOXG1, GABRB3, GATAD2B, GRIN2B, MBD5, MED13L, SETBP1, TBR1, TCF4, WDR45), resulting in a diagnostic yield of ∼29%."

GRIN2B recovered as a de novo cause in a moderate-to-severe ID trio cohort.

GATAD2B

Gene: GATAD2B hgnc:30778 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (1 reference)

PMID:25356899 SUPPORT Human Clinical

"A total of 12 likely pathogenic DNMs were identified in genes previously associated with ID (ARID1B, CHD2, FOXG1, GABRB3, GATAD2B, GRIN2B, MBD5, MED13L, SETBP1, TBR1, TCF4, WDR45), resulting in a diagnostic yield of ∼29%."

GATAD2B recovered as a de novo cause in a moderate-to-severe ID trio cohort.

MED13L

Gene: MED13L hgnc:22962 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (1 reference)

PMID:25356899 SUPPORT Human Clinical

"A total of 12 likely pathogenic DNMs were identified in genes previously associated with ID (ARID1B, CHD2, FOXG1, GABRB3, GATAD2B, GRIN2B, MBD5, MED13L, SETBP1, TBR1, TCF4, WDR45), resulting in a diagnostic yield of ∼29%."

MED13L recovered as a de novo cause in a moderate-to-severe ID trio cohort. PARTIAL because this supports MED13L as a de novo dominant ID gene, not as a NON-SYNDROMIC one; the source cohort was ascertained on ID severity, not on absence of syndromic features.

TRIO

Gene: TRIO hgnc:12303 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (3 references)

PMID:27418539 SUPPORT Human Clinical

"Trio is a Dbl family guanine nucleotide exchange factor (GEF) and a major regulator of neuronal development, controlling actin cytoskeleton dynamics by activating the GTPase Rac1."

Places TRIO in the cytoskeletal/Rho GTPase pathophysiology module.

PMID:27418539 SUPPORT Human Clinical

"We identified a frameshift deletion in TRIO that segregated autosomal dominantly. By scrutinising data from DDD, we further identified three unrelated children with a similar phenotype who harboured de novo missense mutations in TRIO."

Documents both an inherited autosomal dominant lesion and de novo lesions in the same gene, supporting the caveat that AD-NSID is not exclusively a de novo class.

PMID:27418539 SUPPORT Human Clinical

"Additionally, we identify pathogenic de novo missense mutations in TRIO associated with the same consistent phenotype, intellectual disability, microcephaly and dysmorphism with striking digital features."

Only PARTIAL support for inclusion in a NON-syndromic class: the reported phenotype includes microcephaly and dysmorphism, so this gene-disease pair is arguably syndromic - the boundary problem this entry documents.

CAMK2A

Gene: CAMK2A hgnc:1460 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (2 references)

PMID:29100089 SUPPORT Human Clinical

"Through a multi-center collaborative study based on a whole-exome sequencing approach, we identified 19 exceedingly rare de novo CAMK2A or CAMK2B variants in 24 unrelated individuals with intellectual disability."

Establishes CAMK2A as a de novo heterozygous cause of intellectual disability in a multi-centre trio-sequencing cohort.

PMID:29100089 SUPPORT In Vitro

"We further found that all mutations affecting auto-phosphorylation also affected neuronal migration, highlighting the importance of tightly regulated CAMK2 auto-phosphorylation in neuronal function and neurodevelopment."

Functional assays tie the variant class to a specific cellular mechanism, supporting the synaptic/plasticity pathophysiology module.

DLG4

Gene: DLG4 hgnc:2903 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (2 references)

PMID:33597769 SUPPORT Human Clinical

"The clinical picture was predominated by early onset global developmental delay, intellectual disability, autism spectrum disorder, and attention deficit-hyperactivity disorder, all of which point to a brain disorder."

A 53-patient series showing a brain-restricted, neurodevelopmental-only phenotype - the expression pattern the competing brain-restricted model of AD-NSID predicts.

PMID:33597769 SUPPORT Human Clinical

"despite some overlapping features, a distinct facial dysmorphism could not be established"

Explicit negative finding for a recognisable dysmorphic gestalt, supporting non-syndromic classification on positive grounds rather than by default.

KMT5B

Gene: KMT5B hgnc:24283 relationship_type: CAUSATIVE variant_origin: DE_NOVO

Show evidence (2 references)

PMID:35433545 SUPPORT Human Clinical

"Recently, deleterious heterozygous variants in KMT5B were implicated in individuals with intellectual disability (ID) and/or autism spectrum disorder."

Establishes heterozygous KMT5B variants as a cause of intellectual disability, placing the gene in the chromatin/transcription module.

PMID:35433545 SUPPORT Human Clinical

"suggest that this gene should be considered in the differential diagnosis of neurodevelopmental disorders accompanied by macrocephaly and/or overgrowth"

Only PARTIAL for a NON-syndromic class: the recurring macrocephaly and overgrowth constitute an emerging recognisable gestalt, the same boundary instability documented for SYNGAP1, TRIO and MED13L.

💊

Medical Actions

5

Genetic Counselling and Recurrence-Risk Assessment

Action: genetic counseling Ontology label: Genetic Counseling NCIT:C15240

After a de novo variant is confirmed, sibling recurrence risk is low but not negligible, because a proportion of apparently de novo variants arise from undetected low-level parental gonadal or somatic mosaicism. Counsel on this residual risk rather than quoting a zero recurrence risk; parental mosaicism detectable in blood raises recurrence risk substantially more than mosaicism confined to the germline.

Show evidence (4 references)

PMID:25087610 SUPPORT Human Clinical

"Here, we show that somatic mosaicism for transmitted mutations among parents of children with simplex genetic disease is more common than currently appreciated."

Establishes that apparently de novo variants in simplex cases can arise from underdetected parental mosaicism, the basis for a non-zero recurrence-risk counselling message.

PMID:25087610 SUPPORT Computational

"Integrated probabilistic modeling of gametogenesis developed in response to our observations predicts that mutations in parental blood increase recurrence risk substantially more than parental mutations confined to the germline."

Provides the quantitative rationale distinguishing blood-detectable parental mosaicism from germline-confined mosaicism in recurrence-risk counselling.

PMID:34697084 SUPPORT Human Clinical

"Here we present 12 cases of apparent mosaicism identified in the CAUSES cohort: nine cases of parental mosaicism for a disease-causing variant found in a child and three cases of mosaicism in the proband for a de novo variant."

Quantifies how often mosaicism is found in an unselected trio sequencing cohort of exactly the kind used to diagnose AD-NSID.

+ 1 more reference

Early Intervention, Rehabilitation, and Special Education

Action: early intervention and special education Ontology label: Rehabilitation NCIT:C15315

There is no disease-modifying therapy. Management is developmental and supportive: individualised special education, early intervention programmes, rehabilitative training, and structured behavioural support. The critical-window biology gives an explicit mechanistic argument for intervening as early as possible, though it should not be over-read as evidence that any specific educational programme alters the underlying circuit defect.

Show evidence (1 reference)

PMID:38215144 SUPPORT Human Clinical

"INTERVENTIONS: Rehabilitation training."

Rehabilitative/developmental training was the entirety of the intervention offered in a molecularly confirmed MRD22 case, reflecting the absence of disease-specific therapy.

Speech and Language Therapy

Action: speech therapy Ontology label: Speech Language Therapy NCIT:C159273

Targeted therapy for the expressive language impairment that is disproportionately prominent in several AD-NSID entities, including augmentative and alternative communication for minimally verbal children.

Target Phenotypes: Delayed speech and language development HP:0000750

Show evidence (1 reference)

PMID:19196676 SUPPORT Human Clinical

"The disruption of SYNGAP1 appears to be associated with a homogeneous clinical phenotype that is characterized by moderate-to-severe mental retardation accompanied by severe language impairment."

Establishes the language impairment that this therapy targets; the citation supports the indication, not the efficacy of the intervention.

Physical and Occupational Therapy

Action: physical therapy Ontology label: Physical Therapy NCIT:C15302

Addresses the hypotonia, delayed gross motor milestones, and gait instability that commonly accompany AD-NSID, and supports acquisition of adaptive daily-living skills.

Target Phenotypes: Hypotonia HP:0001252 Delayed ability to walk HP:0031936

Show evidence (1 reference)

PMID:26989088 SUPPORT Human Clinical

"Hypotonia and unstable gait were frequent associated neurological features."

Documents the motor impairments that motivate physiotherapy; supports the indication rather than the efficacy.

Antiseizure Pharmacotherapy for Comorbid Epilepsy

Action: Pharmacotherapy NCIT:C15986

When epilepsy is present, standard antiseizure medication is used. Response is variable and gene-dependent: seizures were well controlled by topiramate or valproate in the original SYNGAP1 series, whereas roughly half the patients in a larger SYNGAP1 cohort were pharmacoresistant. There is a mechanistic rationale for AMPA-directed agents in SYNGAP1 haploinsufficiency, but it is a hypothesis generated from mouse work and small case observations, not a validated precision-therapy indication.

Mechanism Target:

INHIBITS Cortical Hyperexcitability — Topiramate and valproate reduce AMPA-receptor-mediated excitatory transmission, the pathway de-repressed by SYNGAP1 haploinsufficiency. The drug edge attaches to the hyperexcitability node only: no available antiseizure medication reverses the upstream spine-maturation defect, which is why seizure control does not translate into cognitive rescue.

Show evidence (1 reference)

PMID:19196676 SUPPORT Other

"Indeed, topiramate directly inhibits AMPA-receptor activity, whereas valproate reduces the level of GluR1 AMPA-receptor subunit at hippocampal synapses and therefore indirectly reduces AMPA-receptor activity."

Mechanistic rationale for the drug acting on the hyperexcitability node; authors' interpretation rather than a controlled trial.

Target Phenotypes: Seizure HP:0001250

Show evidence (3 references)

PMID:19196676 SUPPORT Human Clinical

"Patient 1 had brief, generalized tonic–clonic seizures and was seizure-free while taking topiramate, whereas Patient 2 had some myoclonic and absence seizures, which were well controlled with valproate."

Documents effective seizure control with standard antiseizure medication in molecularly confirmed AD-NSID.

PMID:19196676 SUPPORT Other

"Indeed, topiramate directly inhibits AMPA-receptor activity, whereas valproate reduces the level of GluR1 AMPA-receptor subunit at hippocampal synapses and therefore indirectly reduces AMPA-receptor activity."

Gives the mechanistic rationale linking drug choice to the AMPA-receptor-mediated hyperexcitability node; this is authors' interpretation, not a controlled comparison.

PMID:26989088 SUPPORT Human Clinical

"Seizures were pharmacoresistant in half of the patients."

Qualifies rather than endorses the efficacy claim: a larger series found half of patients pharmacoresistant, so antiseizure medication controls seizures in only about half of this group.

🔬

Diagnosis

2

Trio Exome or Genome Sequencing

Parent-offspring trio exome (or genome) sequencing is the definitive diagnostic modality and is now recommended as a first-tier test for unexplained neurodevelopmental disorders, ahead of chromosomal microarray. Trio design is essential rather than merely convenient: de novo status is the primary evidence of pathogenicity for a novel heterozygous variant in a constraint-intolerant gene, and cannot be established from a proband-only test.

molecular genetic testing NCIT:C19770

Show evidence (5 references)

PMID:31182824 SUPPORT Human Clinical

"Yield of ES was 36% overall, 31% for isolated NDD, and 53% for the NDD plus associated conditions."

Quantifies exome yield, and specifically the ISOLATED (i.e. non-syndromic) NDD yield of 31% - directly relevant to this disease class.

PMID:31182824 SUPPORT Human Clinical

"Our review demonstrates that ES consistently outperforms CMA for evaluation of unexplained NDDs. We propose a diagnostic algorithm placing ES at the beginning of the evaluation of unexplained NDDs."

Consensus recommendation placing exome sequencing first-tier, displacing chromosomal microarray.

PMID:23033978 SUPPORT Human Clinical

"De novo mutations represent an important cause of intellectual disability; exome sequencing was used as an effective diagnostic strategy for their detection."

Establishes trio exome sequencing as the effective diagnostic strategy for the de novo dominant class.

+ 2 more references

Exclusion of Recognised Syndromic and Metabolic Causes

A non-syndromic designation requires prior exclusion of a recognised aetiology. Historically this meant karyotype, subtelomeric FISH or comparative genomic hybridisation, FMR1 repeat testing, and brain imaging; in current practice chromosomal microarray and targeted metabolic testing play this role, with exome sequencing increasingly performed in parallel rather than afterwards.

disease screening NCIT:C15419

Show evidence (2 references)

PMID:19196676 SUPPORT Human Clinical

"Mental retardation was unexplained in these patients despite standard investigations, including karyotyping, subtelomeric fluorescence in situ hybridization analysis, or comparative genomic hybridization targeting regions associated with known syndromes, molecular testing for the common..."

Specifies the exclusion workup that operationally defines the non-syndromic, unexplained category.

PMID:19196676 SUPPORT Human Clinical

"All patients were examined by at least one experienced clinical geneticist, who ruled out the presence of specific dysmorphic features."

Expert dysmorphology assessment is the clinical step that assigns a case to the non-syndromic category.

📊

Prevalence

2

Worldwide (all-cause intellectual disability, the denominator class)

Point Prevalence 1037.0 per 100,000 >1 in 1,000

Prevalence of intellectual disability from all causes, not of the autosomal dominant non-syndromic subset specifically. AD-NSID is a large but unquantified fraction of this total; because it is defined by a heterogeneous and still-expanding gene list, no direct population prevalence estimate exists.

Show evidence (1 reference)

PMID:21236634 SUPPORT Human Clinical

"The prevalence of intellectual disability across all 52 studies included in the meta-analysis was 10.37/1000 population."

Provides the all-cause ID denominator; supports the order of magnitude but not an AD-NSID-specific rate.

Live births, developmental disorders attributable to de novo mutations

Birth Prevalence 320.0 per 100,000 (223.0–469.0) >1 in 1,000

Birth prevalence of de-novo-mutation-caused developmental disorders (1 in 213 to 1 in 448 births, parental-age dependent). AD-NSID is the subset of this group that presents without recognised syndromic features, so this is an upper bound rather than a direct estimate.

Show evidence (1 reference)

PMID:28135719 SUPPORT Human Clinical

"We estimate that developmental disorders caused by DNMs have an average prevalence of 1 in 213 to 1 in 448 births, depending on parental age."

Quantifies the birth prevalence of the de-novo-dominant developmental disorder class of which AD-NSID is the non-syndromic part.

🔀

Differential Diagnoses

4

Conditions with similar clinical presentations that must be differentiated from Autosomal Dominant Non-Syndromic Intellectual Disability:

Syndromic Intellectual Disability

Overlapping Features The primary differential. Any recognisable pattern of dysmorphic, growth, malformative, or neuroimaging features moves a case out of this class. The distinction is unstable in practice, since patients ascertained as non-syndromic frequently carry variants in genes catalogued as syndromic.

Distinguishing Features

Presence of a recognisable dysmorphic gestalt or major malformation
Growth abnormality, microcephaly, or structural brain anomaly on imaging

Show evidence (1 reference)

PMID:23020937 SUPPORT Human Clinical

"Several patients did not meet the expected syndromic manifestation, suggesting a strong bias in present clinical syndrome descriptions."

Documents the instability of the syndromic/non-syndromic distinction in real cohorts.

X-linked Intellectual Disability

Overlapping Features X-linked non-syndromic ID was historically the best-characterised non-syndromic ID class and remains a major differential, particularly in affected males with a maternal family history. Trio sequencing distinguishes the two by demonstrating a maternally inherited hemizygous variant rather than an autosomal de novo one.

Distinguishing Features

Male predominance with affected maternal male relatives
Maternally inherited hemizygous X-chromosome variant rather than an autosomal de novo variant

Show evidence (1 reference)

PMID:23033978 SUPPORT Human Clinical

"A total of 10 de novo mutations and 3 X-linked (maternally inherited) mutations that had been previously predicted to compromise the function of known intellectual-disability genes were found in 13 patients."

Shows both classes co-occurring in the same diagnostic cohort and distinguished by inheritance mode on trio analysis.

Autosomal Recessive Non-Syndromic Intellectual Disability

Overlapping Features Biallelic recessive causes are the major alternative genetic architecture, prominent in consanguineous populations but contributing little in outbred populations - a key reason the de novo dominant model became dominant in European cohorts.

Distinguishing Features

Consanguinity or an endogamous founder population
Biallelic (homozygous or compound heterozygous) variants inherited from unaffected carrier parents

Show evidence (1 reference)

PMID:23020937 SUPPORT Human Clinical

"Autosomal recessive inheritance seems to contribute little in the outbred population investigated."

Quantifies the limited recessive contribution in an outbred cohort, supporting the dominant/de novo model for that setting.

Copy Number Variant Intellectual Disability

Overlapping Features Pathogenic de novo copy number variants were the first recognised class of de novo dominant lesions in ID and remain a major cause. They are detected by chromosomal microarray and are excluded before, or in parallel with, sequence-level analysis.

Distinguishing Features

Detected by chromosomal microarray rather than sequencing
Contiguous multi-gene deletion or duplication rather than a single-gene variant

Show evidence (1 reference)

PMID:21076407 SUPPORT Human Clinical

"Together with de novo copy number variation, de novo point mutations of large effect could explain the majority of all mental retardation cases in the population."

Places de novo CNVs alongside de novo point mutations as the two arms of the de novo dominant architecture.

{ }

Source YAML

click to show

name: Autosomal Dominant Non-Syndromic Intellectual Disability
creation_date: "2026-07-31T00:00:00Z"
description: >-
  Autosomal dominant non-syndromic intellectual disability (AD-NSID; the OMIM
  "mental retardation, autosomal dominant" / MRD series) is a genetically
  extremely heterogeneous class of intellectual disability in which
  significantly impaired intellectual functioning and adaptive behaviour occur
  in the absence of a consistent pattern of associated dysmorphic,
  malformative, metabolic, or neuroimaging features that would define a
  clinically recognisable syndrome. Almost all affected individuals are
  simplex cases: the causal lesion is typically a heterozygous de novo
  loss-of-function variant that halves the dose of a strongly
  constraint-intolerant neurodevelopmental gene, so autosomal dominance is
  inferred from the monoallelic, fully penetrant lesion rather than observed
  as vertical family transmission (reduced reproductive fitness makes
  multigenerational pedigrees rare). The class was essentially invisible to
  linkage mapping and was defined only after parent-offspring trio exome
  sequencing became feasible, beginning with the "de novo paradigm" studies of
  2010-2012 and continuing through the Deciphering Developmental Disorders
  study and its successors. Causal genes converge on a small number of
  biological modules - synaptic transmission and plasticity, chromatin and
  transcriptional regulation, and cytoskeletal/signalling control of neuronal
  morphogenesis - with SYNGAP1 (MRD5) the paradigmatic example. The
  syndromic/non-syndromic boundary is unstable: deeper phenotyping repeatedly
  reclassifies "non-syndromic" gene-disease pairs as syndromic, which has led
  to the proposal (see `mechanistic_hypotheses`) that AD-NSID is at least
  partly an ascertainment category defined by the current absence of a
  recognised phenotypic gestalt rather than a mechanistically distinct entity.
  A competing, not mutually exclusive model holds that a subset of causal genes
  is genuinely brain-restricted in expression and therefore produces isolated
  cognitive impairment; the boundary question remains open.
category: Genetic
disease_term:
  preferred_term: autosomal dominant non-syndromic intellectual disability
  term:
    id: MONDO:0015802
    label: autosomal dominant non-syndromic intellectual disability
parents:
- Non-Syndromic Intellectual Disability
- Intellectual Disability, Autosomal Dominant
synonyms:
- non-syndromic intellectual disability, autosomal dominant
- autosomal dominant mental retardation
- mental retardation, autosomal dominant (MRD)
- autosomal dominant nonsyndromic mental retardation
- AD-NSID

references:
- reference: PMID:26503795
  title: "Genetic studies in intellectual disability and related disorders."
- reference: PMID:26748517
  title: "Systematic Phenomics Analysis Deconvolutes Genes Mutated in Intellectual Disability into Biologically Coherent Modules."

inheritance:
- name: Autosomal dominant, predominantly de novo
  description: >-
    AD-NSID is transmitted as an autosomal dominant trait at the level of the
    individual lesion - a single heterozygous loss-of-function allele is
    sufficient to cause disease - but affected individuals are almost always
    sporadic. Reduced reproductive fitness means the causal variant is usually
    de novo in the proband rather than inherited from an affected parent.
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  penetrance: COMPLETE
  expressivity: VARIABLE
  evidence:
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      These results indicate that SYNGAP1 disruption is a cause of autosomal
      dominant nonsyndromic mental retardation.
    explanation: >-
      First demonstration that de novo truncating variants in a single
      autosomal gene cause non-syndromic ID by an autosomal dominant
      mechanism.
  - reference: PMID:25356899
    reference_title: "De novo mutations in moderate or severe intellectual disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Interestingly, no case was explained by inherited mutations."
    explanation: >-
      In a trio exome screen of moderate-to-severe ID, every solved case was
      accounted for by a de novo rather than an inherited variant, supporting
      the sporadic-dominant architecture.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Moreover, autosomal dominant genes have yet to be identified, mainly
      because mental retardation results in lower reproductive fitness, which
      in turn decreases the likelihood of identifying families that are
      amenable to linkage analysis.
    explanation: >-
      Explains why autosomal dominant non-syndromic ID genes could not be
      found by pedigree-based linkage and required a de novo, trio-sequencing
      approach.
  - reference: PMID:39528574
    reference_title: "Trio-whole exome sequencing reveals the importance of de novo variants in children with intellectual disability and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      95.4% (165/173) had no documented family history of epilepsy or DD/ID
    explanation: >-
      Quantifies how uninformative family history is in this class: a negative
      pedigree is the norm and does not argue against a dominant diagnosis.

prevalence:
- population: Worldwide (all-cause intellectual disability, the denominator class)
  measure_type: POINT_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 1037.0
  notes: >-
    Prevalence of intellectual disability from all causes, not of the autosomal
    dominant non-syndromic subset specifically. AD-NSID is a large but
    unquantified fraction of this total; because it is defined by a
    heterogeneous and still-expanding gene list, no direct population
    prevalence estimate exists.
  evidence:
  - reference: PMID:21236634
    reference_title: "Prevalence of intellectual disability: a meta-analysis of population-based studies."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The prevalence of intellectual disability across all 52 studies included
      in the meta-analysis was 10.37/1000 population.
    explanation: >-
      Provides the all-cause ID denominator; supports the order of magnitude
      but not an AD-NSID-specific rate.
- population: Live births, developmental disorders attributable to de novo mutations
  measure_type: BIRTH_PREVALENCE
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 320.0
  rate_low: 223.0
  rate_high: 469.0
  notes: >-
    Birth prevalence of de-novo-mutation-caused developmental disorders (1 in
    213 to 1 in 448 births, parental-age dependent). AD-NSID is the subset of
    this group that presents without recognised syndromic features, so this is
    an upper bound rather than a direct estimate.
  evidence:
  - reference: PMID:28135719
    reference_title: "Prevalence and architecture of de novo mutations in developmental disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We estimate that developmental disorders caused by DNMs have an average
      prevalence of 1 in 213 to 1 in 448 births, depending on parental age.
    explanation: >-
      Quantifies the birth prevalence of the de-novo-dominant developmental
      disorder class of which AD-NSID is the non-syndromic part.

pathophysiology:
- name: De Novo Heterozygous Variant in a Constraint-Intolerant Neurodevelopmental Gene
  biological_scale: MOLECULAR
  description: >-
    The initiating lesion is a single new (de novo) protein-truncating or
    damaging missense variant arising in the parental germline and present
    heterozygously in the proband. Because such variants are not transmitted
    through families, this class of disease was undetectable by linkage and
    was only exposed by parent-offspring trio exome sequencing. The affected
    genes are drawn overwhelmingly from the minority of the genome under
    strong selective constraint against heterozygous protein-truncating
    variants, i.e. genes whose normal function requires two working copies.
    An emerging, mechanistically distinct minority arm of the same trigger is
    the NON-CODING de novo mutation: whole-genome sequencing of ID trios finds
    regulatory DNMs selectively enriched in fetal-brain-specific enhancers,
    including recurrently mutated enhancer clusters regulating nervous-system
    development genes. This arm is captured here as part of the trigger rather
    than as a separate node because the downstream consequence - dosage
    perturbation of a constraint-intolerant neurodevelopmental gene - is the
    same; only the lesion type differs. Evidence comes from small,
    mixed developmental-disorder cohorts rather than AD-NSID-specific series,
    so it is an emerging rather than established contributor for this class.
  role: trigger
  locations:
  - preferred_term: brain
    term:
      id: UBERON:0000955
      label: brain
  biological_processes:
  - preferred_term: nervous system development
    term:
      id: GO:0007399
      label: nervous system development
  evidence:
  - reference: PMID:36854624
    reference_title: "Regulatory de novo mutations underlying intellectual disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We found that regulatory DNMs were selectively enriched in fetal
      brain-specific enhancers as compared with adult brain enhancers.
    explanation: >-
      Establishes the non-coding regulatory arm of the de novo trigger:
      enhancer DNMs are enriched specifically in fetal-brain enhancers,
      matching the critical-window logic modelled downstream.
  - reference: PMID:36854624
    reference_title: "Regulatory de novo mutations underlying intellectual disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      we identified recurrently mutated enhancer clusters that regulate genes
      involved in nervous system development (CSMD1, OLFM1, and POU3F3).
    explanation: >-
      Names the recurrently mutated enhancer clusters and their
      nervous-system-development target genes, showing the regulatory arm
      converges on the same gene classes as the coding arm.
  - reference: PMID:36854624
    reference_title: "Regulatory de novo mutations underlying intellectual disability."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      Using CRISPR-mediated mutation and editing of epigenomic marks, we show
      that DNMs at regulatory elements affect the expression of putative
      target genes.
    explanation: >-
      Functional confirmation that the regulatory lesions change target-gene
      expression. PARTIAL because the cohort is small and mixed
      developmental-disorder rather than AD-NSID-specific, so clinical
      penetrance for individual regulatory variants is not established.
  - reference: PMID:21076407
    reference_title: "A de novo paradigm for mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here we used a family based exome sequencing approach to test this de
      novo mutation hypothesis in ten individuals with unexplained mental
      retardation. We identified and validated unique non-synonymous de novo
      mutations in nine genes.
    explanation: >-
      The founding trio-exome study establishing de novo point mutation as the
      causal mechanism in unexplained (non-syndromic) ID.
  - reference: PMID:21076407
    reference_title: "A de novo paradigm for mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Together with de novo copy number variation, de novo point mutations of
      large effect could explain the majority of all mental retardation cases
      in the population.
    explanation: >-
      States the central architectural claim - large-effect de novo lesions,
      not inherited variation, dominate the aetiology.
  - reference: PMID:23020937
    reference_title: "Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      After exclusion of copy-number variants, de-novo point mutations and
      small indels are associated with severe, sporadic non-syndromic
      intellectual disability, accounting for 45-55% of patients with high
      locus heterogeneity.
    explanation: >-
      Directly quantifies the contribution of de novo point mutations to
      severe sporadic NON-SYNDROMIC ID - the exact disease class curated here.
  - reference: PMID:26503795
    reference_title: "Genetic studies in intellectual disability and related disorders."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Genetic factors play a major part in intellectual disability (ID), but
      genetic studies have been complicated for a long time by the extreme
      clinical and genetic heterogeneity.
    explanation: >-
      Review statement of the extreme locus heterogeneity that defines this
      class and explains why it resisted pre-sequencing gene discovery.
  - reference: PMID:33057194
    reference_title: "Evidence for 28 genetic disorders discovered by combining healthcare and research data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Known dominant DD-associated genes are strongly enriched in the minority
      of genes that exhibit strong selective constraint on heterozygous PTVs
    explanation: >-
      Establishes that the target gene set is defined by intolerance to
      heterozygous protein-truncating variants, i.e. dosage sensitivity.
  downstream:
  - target: Haploinsufficiency of a Dosage-Sensitive Neurodevelopmental Gene
    causal_link_type: DIRECT
    description: >-
      A protein-truncating allele triggers nonsense-mediated decay or produces
      a non-functional protein, leaving a single functional copy.
    evidence:
    - reference: PMID:33057194
      reference_title: "Evidence for 28 genetic disorders discovered by combining healthcare and research data."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        This residual burden of protein-truncating DNMs is greatest in genes
        that are intolerant of PTVs in the general population
      explanation: >-
        Links the de novo protein-truncating lesion specifically to
        dosage-intolerant genes, i.e. to a haploinsufficiency outcome.

- name: Haploinsufficiency of a Dosage-Sensitive Neurodevelopmental Gene
  biological_scale: MOLECULAR
  description: >-
    Loss of one functional allele reduces gene product to roughly 50% of
    normal. For the genes implicated in AD-NSID this is below the threshold
    required for normal neurodevelopment, which is why they are among the most
    constraint-intolerant genes in the genome. Haploinsufficiency is the
    predominant - though not the only - mechanism: a growing minority of
    AD-NSID genes act instead through altered-function (dominant-negative or
    gain-of-function) missense alleles, and these two mechanisms are not
    interchangeable when interpreting a new variant.
  biological_processes:
  - preferred_term: nervous system development
    term:
      id: GO:0007399
      label: nervous system development
  evidence:
  - reference: PMID:23020937
    reference_title: "Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      More participants in the case group had loss-of-function variants than
      in the control group (20/51 vs 2/20; p=0·022), suggesting their
      contribution to disease development.
    explanation: >-
      Demonstrates statistical excess of heterozygous loss-of-function alleles
      in non-syndromic ID cases versus controls.
  - reference: PMID:33057194
    reference_title: "Evidence for 28 genetic disorders discovered by combining healthcare and research data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      This residual burden of protein-truncating DNMs is greatest in genes
      that are intolerant of PTVs in the general population
    explanation: >-
      Confirms the haploinsufficiency mechanism concentrates in
      constraint-intolerant genes and that many such disorders remain
      undiscovered.
  - reference: PMID:33057194
    reference_title: "Evidence for 28 genetic disorders discovered by combining healthcare and research data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Consequently, we expect that a greater proportion of the novel genes
      will act via altered-function mechanisms (e.g. dominant negative or
      gain-of-function).
    explanation: >-
      Qualifies the haploinsufficiency model: newly discovered dominant
      developmental-disorder genes increasingly act by altered function rather
      than pure dosage loss.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Second, mice that are heterozygous for null alleles of Syngap1 have
      impaired synaptic plasticity and learning, which suggests that the
      disruption of a single SYNGAP1 allele is sufficient to cause cognitive
      dysfunction in humans.
    explanation: >-
      Mouse heterozygote data establishing that a single-allele dose reduction
      is sufficient for the cognitive phenotype.
  downstream:
  - target: Disrupted Synaptic Transmission and Plasticity
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      For the synaptic module of AD-NSID genes, reduced gene dosage directly
      perturbs postsynaptic signalling complexes and receptor trafficking.
  - target: Disrupted Chromatin and Transcriptional Regulation
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      For the chromatin/transcription module, reduced dosage of a
      chromatin-modifying or transcription-regulating factor dysregulates
      neurodevelopmental gene expression programmes.
  - target: Disrupted Neuronal Cytoskeletal Dynamics and Rho GTPase Signalling
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      For the cytoskeletal module, reduced dosage of a cytoskeletal regulator
      or Rho GTPase pathway component impairs actin and microtubule dynamics
      in developing neurons.

- name: Disrupted Synaptic Transmission and Plasticity
  biological_scale: CELLULAR
  description: >-
    A large fraction of AD-NSID genes encode components of the excitatory
    postsynaptic density, the NMDA-receptor-associated signalling complex, or
    the presynaptic release machinery. SYNGAP1, a brain-specific Ras
    GTPase-activating protein within the NMDA-receptor complex, is the
    paradigm: it normally restrains Ras-ERK signalling and thereby limits AMPA
    receptor insertion at the postsynaptic membrane. Presynaptic examples
    include STXBP1/Munc18-1, a regulator of SNARE-mediated neurotransmitter
    release. Reduced dosage of these proteins shifts excitatory synaptic
    strength and impairs activity-dependent plasticity, the cellular substrate
    of learning and memory.
  cell_types:
  - preferred_term: glutamatergic (excitatory) neuron
    term:
      id: CL:0000679
      label: glutamatergic neuron
  - preferred_term: hippocampal pyramidal neuron
    term:
      id: CL:0002608
      label: hippocampal neuron
  biological_processes:
  - preferred_term: chemical synaptic transmission
    term:
      id: GO:0007268
      label: chemical synaptic transmission
    modifier: ABNORMAL
  - preferred_term: regulation of synaptic plasticity
    term:
      id: GO:0048167
      label: regulation of synaptic plasticity
    modifier: DECREASED
  - preferred_term: regulation of postsynaptic AMPA receptor levels
    term:
      id: GO:0099072
      label: regulation of postsynaptic membrane neurotransmitter receptor levels
    modifier: ABNORMAL
  - preferred_term: learning or memory
    term:
      id: GO:0007611
      label: learning or memory
    modifier: DECREASED
  molecular_functions:
  - preferred_term: Ras GTPase-activating protein activity (SYNGAP1)
    term:
      id: GO:0005096
      label: GTPase activator activity
    modifier: DECREASED
  evidence:
  - reference: PMID:23141534
    reference_title: "Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Mutations that cause intellectual disability (ID) and autism spectrum
      disorder (ASD) are commonly found in genes that encode for synaptic
      proteins.
    explanation: >-
      States the synaptic-gene convergence that defines this pathophysiology
      module.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We sequenced the autosomal gene SYNGAP1, which encodes a ras
      GTPase-activating protein that is critical for cognition and synapse
      function, in 94 patients with nonsyndromic mental retardation.
    explanation: >-
      Identifies the molecular function (Ras GTPase-activating protein) whose
      loss underlies the paradigmatic AD-NSID gene.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      SYNGAP1, a GTPase-activating protein that is selectively expressed in
      the brain and is a component of the NMDA-receptor complex, acts
      downstream of the receptor, blocking the insertion of the AMPA receptor
      at the postsynaptic membrane
    explanation: >-
      Describes the specific postsynaptic signalling step (restraint of AMPA
      receptor insertion via inhibition of RAS-ERK) that haploinsufficiency
      de-represses.
  - reference: PMID:19557857
    reference_title: "De novo STXBP1 mutations in mental retardation and nonsyndromic epilepsy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We sequenced genes coding for components of the SNARE complex (STX1A,
      VAMP2, SNAP25) and their regulatory proteins (STXBP1/Munc18-1, SYT1),
      which are essential for neurotransmission, in 95 patients with
      idiopathic mental retardation.
    explanation: >-
      Extends the synaptic module to the presynaptic release machinery,
      identifying STXBP1 as an AD-NSID gene.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      The classic observation that various forms of mental retardation are
      associated with abnormalities in the morphology of dendritic spines
      suggests that disruption of pathways involved in synaptic plasticity may
      be a common mechanism of this disorder.
    explanation: >-
      States the long-standing convergence of ID pathophysiology on dendritic
      spine morphology and synaptic plasticity.
  downstream:
  - target: Premature Dendritic Spine Maturation
    causal_link_type: DIRECT
    description: >-
      In the SYNGAP1 model, de-repressed excitatory synaptic signalling
      accelerates spine maturation and raises circuit excitability.
    evidence:
    - reference: PMID:23141534
      reference_title: "Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        In the SYNGAP1 mouse model of ID/ASD, we found that dendritic spine
        synapses develop prematurely during the early postnatal period.
        Premature spine maturation dramatically enhanced excitability in the
        developing hippocampus, which corresponded with the emergence of
        behavioral abnormalities.
      explanation: >-
        Demonstrates the causal step from synaptic-gene haploinsufficiency to
        accelerated spine maturation and hyperexcitability.

- name: Disrupted Chromatin and Transcriptional Regulation
  biological_scale: MOLECULAR
  description: >-
    The second major functional module among AD-NSID genes comprises
    chromatin modifiers, chromatin-remodelling complex subunits, and
    sequence-specific transcriptional regulators (e.g. ARID1B, CHD2, GATAD2B,
    MBD5, TCF4, MED13L, ZBTB18). Halving the dose of these factors
    dysregulates the coordinated gene-expression programmes that drive
    neuronal differentiation, migration, and synaptogenesis. Systematic
    phenomics analysis shows chromatin-related genes form one of the
    biologically coherent modules into which ID genes decompose.
  biological_processes:
  - preferred_term: chromatin organization
    term:
      id: GO:0006325
      label: chromatin organization
    modifier: ABNORMAL
  - preferred_term: regulation of transcription by RNA polymerase II
    term:
      id: GO:0006357
      label: regulation of transcription by RNA polymerase II
    modifier: ABNORMAL
  evidence:
  - reference: PMID:26748517
    reference_title: "Systematic Phenomics Analysis Deconvolutes Genes Mutated in Intellectual Disability into Biologically Coherent Modules."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: >-
      Systematic identification of highly enriched functional themes and
      phenotypes revealed typical phenotype combinations characterizing
      process-defined groups of ID disorders, such as chromatin-related
      disorders and deficiencies in DNA repair.
    explanation: >-
      Establishes chromatin regulation as a distinct, statistically coherent
      functional module among ID-associated genes.
  - reference: PMID:26748517
    reference_title: "Systematic Phenomics Analysis Deconvolutes Genes Mutated in Intellectual Disability into Biologically Coherent Modules."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: >-
      we show that ID-AGs are substantially enriched with co-expression,
      protein-protein interactions, and specific biological functions.
    explanation: >-
      Demonstrates that the genetically heterogeneous ID gene set is
      functionally convergent rather than random.
  - reference: PMID:25356899
    reference_title: "De novo mutations in moderate or severe intellectual disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A total of 12 likely pathogenic DNMs were identified in genes previously
      associated with ID (ARID1B, CHD2, FOXG1, GABRB3, GATAD2B, GRIN2B, MBD5,
      MED13L, SETBP1, TBR1, TCF4, WDR45), resulting in a diagnostic yield of
      ∼29%.
    explanation: >-
      The recovered gene list is dominated by chromatin and transcriptional
      regulators, illustrating this module in a real ID cohort.
  - reference: PMID:38215144
    reference_title: "A de novo variant in ZBTB18 gene caused autosomal dominant non-syndromic intellectual disability 22 syndrome: A case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Autosomal dominant non-syndromic intellectual disability 22 is a rare
      genetic disorder caused by the ZBTB18 gene.
    explanation: >-
      A named MRD-series entity (MRD22) caused by haploinsufficiency of a zinc
      finger transcriptional repressor.
  - reference: PMID:36743289
    reference_title: "Dendritic spine and synapse pathology in chromatin modifier-associated autism spectrum disorders and intellectual disability."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Emerging evidence highlights the critical status of these chromatin
      remodeling molecules in dendritic spine morphogenesis and synaptic
      functions.
    explanation: >-
      Links the chromatin module to the same dendritic-spine and synaptic
      endpoint as the synaptic module, supporting the convergence of the two
      arms rather than treating them as parallel unrelated pathways.
  downstream:
  - target: Impaired Cortical Circuit Formation During Critical Developmental Windows
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Dysregulated transcriptional programmes alter neuronal differentiation
      and connectivity during cortical development.
  - target: Premature Dendritic Spine Maturation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Chromatin modifiers such as ARID1B, KANSL1, and WDR5 converge on
      dendritic spine morphogenesis and synaptic function, so the chromatin
      arm reaches the same synaptic endpoint as the synaptic arm.
    evidence:
    - reference: PMID:36743289
      reference_title: "Dendritic spine and synapse pathology in chromatin modifier-associated autism spectrum disorders and intellectual disability."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Emerging evidence highlights the critical status of these chromatin
        remodeling molecules in dendritic spine morphogenesis and synaptic
        functions.
      explanation: >-
        Supports the causal edge from the chromatin module to the dendritic
        spine/synaptic endpoint.

- name: Disrupted Neuronal Cytoskeletal Dynamics and Rho GTPase Signalling
  biological_scale: CELLULAR
  description: >-
    The third functional module comprises regulators of the neuronal actin and
    microtubule cytoskeleton, whose control converges on Rho GTPase
    transduction (e.g. TRIO, and the wider set of Rho-pathway ID genes).
    Cytoskeletal disruption acts earlier in neurodevelopment than the synaptic
    module: it impairs neuronal migration and neuritogenesis as well as
    synaptic plasticity, so the same molecular lesion propagates upward
    through cell processes, circuits, and finally cognition. Bibliomic
    analysis of the >1,000 known ID genes shows this convergence onto a small
    number of modules is the reason a genetically unrelated gene set produces
    a common cognitive phenotype.
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  - preferred_term: pyramidal neuron
    term:
      id: CL:0000598
      label: pyramidal neuron
  biological_processes:
  - preferred_term: actin cytoskeleton organization
    term:
      id: GO:0030036
      label: actin cytoskeleton organization
    modifier: ABNORMAL
  - preferred_term: neuron migration
    term:
      id: GO:0001764
      label: neuron migration
    modifier: ABNORMAL
  - preferred_term: neuron projection development
    term:
      id: GO:0031175
      label: neuron projection development
    modifier: ABNORMAL
  evidence:
  - reference: PMID:34200511
    reference_title: "Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: >-
      Bibliomic analysis reveals that ID genes converge onto a few biological
      modules, including cytoskeleton dynamics, whose regulation depends on
      Rho GTPases transduction.
    explanation: >-
      Establishes cytoskeletal dynamics under Rho GTPase control as one of the
      small number of convergent biological modules among ID genes.
  - reference: PMID:34200511
    reference_title: "Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Thus, cytoskeleton alterations that have an impact on cell processes
      such as neuronal migration, neuritogenesis, and synaptic plasticity
      rebound on the overall establishment of an effective network and
      consequently on the cognitive phenotype.
    explanation: >-
      Specifies the cellular processes disrupted and the propagation from
      cytoskeletal lesion through network formation to cognition.
  - reference: PMID:34200511
    reference_title: "Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      More than 1000 genes have been found mutated in ID patients pointing out
      that, despite the common phenotype, the genetic bases are highly
      heterogeneous and apparently unrelated.
    explanation: >-
      Quantifies the locus heterogeneity that makes module-level convergence
      the only tractable way to describe the class mechanistically.
  downstream:
  - target: Impaired Cortical Circuit Formation During Critical Developmental Windows
    causal_link_type: INDIRECT_KNOWN_INTERMEDIATES
    description: >-
      Impaired migration, neuritogenesis, and plasticity prevent establishment
      of an effective cortical network.
    evidence:
    - reference: PMID:34200511
      reference_title: "Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Thus, cytoskeleton alterations that have an impact on cell processes
        such as neuronal migration, neuritogenesis, and synaptic plasticity
        rebound on the overall establishment of an effective network and
        consequently on the cognitive phenotype.
      explanation: >-
        Explicitly states the causal chain from cytoskeletal alteration to
        failed network establishment.

- name: Premature Dendritic Spine Maturation
  biological_scale: CELLULAR
  description: >-
    In the SYNGAP1 mouse model, loss of one Syngap1 allele accelerates the
    normal developmental timetable of dendritic spine synapse maturation
    during the early postnatal period. Because the pace of spine maturation -
    not merely its endpoint - determines the outcome, the critical variable is
    developmental timing rather than steady-state synapse number.
  locations:
  - preferred_term: hippocampal formation
    term:
      id: UBERON:0002421
      label: hippocampal formation
  cell_types:
  - preferred_term: hippocampal pyramidal neuron
    term:
      id: CL:0002608
      label: hippocampal neuron
  biological_processes:
  - preferred_term: dendritic spine development
    term:
      id: GO:0060996
      label: dendritic spine development
    modifier: ABNORMAL
  mechanism_confidence: PROVISIONAL
  evidence:
  - reference: PMID:23141534
    reference_title: "Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      In the SYNGAP1 mouse model of ID/ASD, we found that dendritic spine
      synapses develop prematurely during the early postnatal period.
    explanation: >-
      Directly demonstrates accelerated dendritic spine synapse maturation in
      a genetic mouse model of AD-NSID.
  downstream:
  - target: Cortical Hyperexcitability
    causal_link_type: DIRECT
    description: >-
      Prematurely mature spines drive excessive excitability in the developing
      hippocampus.
    evidence:
    - reference: PMID:23141534
      reference_title: "Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        Premature spine maturation dramatically enhanced excitability in the
        developing hippocampus, which corresponded with the emergence of
        behavioral abnormalities.
      explanation: >-
        States the causal step from mistimed spine maturation to
        hyperexcitability in the same model.

- name: Cortical Hyperexcitability
  biological_scale: CELLULAR
  description: >-
    Loss of the SynGAP developmental brake on neural excitability leaves
    developing hippocampal and cortical circuits excessively excitable. This
    is the node that antiseizure medication acts on; it is downstream of, and
    not equivalent to, the spine-maturation defect, which no available drug
    reverses.
  locations:
  - preferred_term: cerebral cortex
    term:
      id: UBERON:0000956
      label: cerebral cortex
  - preferred_term: hippocampal formation
    term:
      id: UBERON:0002421
      label: hippocampal formation
  cell_types:
  - preferred_term: hippocampal pyramidal neuron
    term:
      id: CL:0002608
      label: hippocampal neuron
  biological_processes:
  - preferred_term: excitatory postsynaptic potential
    term:
      id: GO:0060079
      label: excitatory postsynaptic potential
    modifier: INCREASED
  mechanism_confidence: PROVISIONAL
  evidence:
  - reference: PMID:23141534
    reference_title: "Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      These data demonstrate that SynGAP protein acts as a critical
      developmental repressor of neural excitability that promotes the
      development of life-long cognitive abilities.
    explanation: >-
      Frames the gene product's normal role as a developmental brake on
      excitability whose loss produces the hyperexcitable state.
  downstream:
  - target: Impaired Cortical Circuit Formation During Critical Developmental Windows
    causal_link_type: DIRECT
    description: >-
      Mistimed spine maturation and the resulting hyperexcitability corrupt
      activity-dependent circuit refinement during the critical period.
    evidence:
    - reference: PMID:23141534
      reference_title: "Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: >-
        We propose that the pace of dendritic spine synapse maturation in early
        life is a critical determinant of normal intellectual development.
      explanation: >-
        States the causal dependence of intellectual development on the timing
        of spine maturation.

- name: Impaired Cortical Circuit Formation During Critical Developmental Windows
  biological_scale: TISSUE
  description: >-
    The convergent consequence of both the synaptic and chromatin modules is
    maldevelopment of cortical and hippocampal circuits during time-limited
    windows of activity-dependent refinement. The window property is
    mechanistically important and therapeutically sobering: in the SYNGAP1
    model, introducing the mutation after the critical window has closed has
    little effect, and correcting it in adulthood does not rescue cognition.
    This is the principal reason AD-NSID is a static (non-progressive)
    developmental condition rather than a neurodegenerative one, and it sets a
    hard early-intervention constraint on any future molecular therapy.
  biological_processes:
  - preferred_term: cerebral cortex development
    term:
      id: GO:0021987
      label: cerebral cortex development
    modifier: ABNORMAL
  - preferred_term: modulation of chemical synaptic transmission
    term:
      id: GO:0050804
      label: modulation of chemical synaptic transmission
    modifier: ABNORMAL
  locations:
  - preferred_term: cerebral cortex
    term:
      id: UBERON:0000956
      label: cerebral cortex
  - preferred_term: hippocampal formation
    term:
      id: UBERON:0002421
      label: hippocampal formation
  cell_types:
  - preferred_term: neuron
    term:
      id: CL:0000540
      label: neuron
  mechanism_confidence: PROVISIONAL
  evidence:
  - reference: PMID:23141534
    reference_title: "Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Inducing SYNGAP1 mutations after critical developmental windows closed
      had minimal impact on spine synapse function, whereas repairing these
      pathogenic mutations in adulthood did not improve behavior and
      cognition.
    explanation: >-
      Establishes the critical-window constraint - both the pathogenic and the
      therapeutic direction - in a genetic mouse model.
  - reference: PMID:23141534
    reference_title: "Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      We propose that the pace of dendritic spine synapse maturation in early
      life is a critical determinant of normal intellectual development.
    explanation: >-
      States the developmental-timing model linking synaptic maturation pace
      to intellectual outcome.
  downstream:
  - target: Static Global Cognitive and Adaptive Impairment Without Syndromic Features
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Miswired cortical circuits manifest as a fixed deficit in intellectual
      and adaptive functioning.
    evidence:
    - reference: PMID:34200511
      reference_title: "Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Intellectual disability (ID) is a pathological condition characterized
        by limited intellectual functioning and adaptive behaviors.
      explanation: >-
        Defines the clinical endpoint that failed network establishment
        produces.

- name: Static Global Cognitive and Adaptive Impairment Without Syndromic Features
  biological_scale: ORGANISM
  description: >-
    The clinical endpoint is significant, non-progressive impairment of
    intellectual functioning and adaptive behaviour with onset in the
    developmental period. What makes the presentation "non-syndromic" is the
    absence - on current examination - of a distinctive constellation of
    dysmorphic, growth, malformative, or neuroimaging features. This absence
    is partly a real biological property (several causal genes, such as
    SYNGAP1, are expressed almost exclusively in brain, so no somatic
    phenotype is expected) and partly an artefact of limited phenotyping and
    small case series: patients with variants in newly discovered dominant
    genes are, measurably, less phenotypically similar to one another than
    patients with variants in long-established genes.
  evidence:
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The absence of specific dysmorphic features and growth abnormalities in
      these patients is consistent with the fact that SYNGAP1 is exclusively
      expressed in the brain.
    explanation: >-
      Gives the biological rationale for a genuinely brain-restricted,
      non-syndromic presentation.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Most patients have the nonsyndromic form of the disorder, which is
      characterized by the absence of associated morphologic, radiologic, and
      metabolic features.
    explanation: >-
      Defines the non-syndromic category operationally.
  - reference: PMID:33057194
    reference_title: "Evidence for 28 genetic disorders discovered by combining healthcare and research data."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      patients with DNMs in the same novel DD-associated gene were less
      phenotypically similar to each other, on average, than patients with
      DNMs in a consensus gene
    explanation: >-
      Supports the ascertainment component of "non-syndromic": recently
      discovered dominant genes produce less distinctive, less consistent
      phenotypes.

mechanistic_hypotheses:
- hypothesis_group_id: nonsyndromic_boundary_is_ascertainment
  hypothesis_label: >-
    The non-syndromic/syndromic boundary reflects depth of phenotyping rather
    than a distinct mechanism
  status: EMERGING
  description: >-
    Under this model AD-NSID is not a mechanistically separate disease class
    but the leading edge of gene discovery: a gene-disease pair is labelled
    "non-syndromic" while the case count is small and the phenotyping shallow,
    and is reclassified as syndromic once enough patients are described to
    reveal a gestalt. The prediction is that MRD-series entities will migrate
    into named syndromes over time (as several already have), and that the
    residual truly non-syndromic set will be enriched for genes with
    brain-restricted expression. The competing model - that a real subclass of
    brain-restricted genes produces isolated cognitive impairment - is not
    mutually exclusive and is supported by SYNGAP1's expression pattern.
  evidence:
  - reference: PMID:23020937
    reference_title: "Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Several patients did not meet the expected syndromic manifestation,
      suggesting a strong bias in present clinical syndrome descriptions.
    explanation: >-
      Explicitly attributes syndromic/non-syndromic misclassification to bias
      in existing clinical syndrome descriptions.
  - reference: PMID:23020937
    reference_title: "Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The large number of de-novo variants in known intellectual disability
      genes is only partially attributable to known non-specific phenotypes.
    explanation: >-
      Notes that patients ascertained as non-syndromic frequently carry
      variants in genes catalogued as syndromic.
  - reference: PMID:26989088
    reference_title: "Genetic and neurodevelopmental spectrum of SYNGAP1-associated intellectual disability and epilepsy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      SYNGAP1 encephalopathy is characterised by early neurodevelopmental
      delay typically preceding the onset of a relatively recognisable
      epilepsy comprising generalised seizures (absences, myoclonic jerks) and
      frequent triggers.
    explanation: >-
      Worked example of the predicted migration: SYNGAP1, first reported as a
      cause of NON-syndromic ID, is redescribed as a recognisable
      developmental and epileptic encephalopathy once a larger series is
      phenotyped.

phenotypes:
- category: Neurologic
  name: Intellectual Disability
  description: >-
    Significantly impaired intellectual functioning and adaptive behaviour
    with onset in the developmental period. Severity across the AD-NSID class
    spans mild to severe; cohorts ascertained for trio exome sequencing have
    been enriched for moderate-to-severe disability (e.g. IQ below 50).
  phenotype_term:
    preferred_term: Intellectual disability
    term:
      id: HP:0001249
      label: Intellectual disability
  frequency: OBLIGATE
  diagnostic: true
  evidence:
  - reference: PMID:23033978
    reference_title: "Diagnostic exome sequencing in persons with severe intellectual disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We then sequenced the coding regions of more than 21,000 genes obtained
      from 100 patients with an IQ below 50 and their unaffected parents.
    explanation: >-
      Defines the core cognitive phenotype and the severity band of the
      cohorts in which the disease class was characterised.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Scores on the Mullen Scales of Early Learning and the Vineland Adaptive
      Behavior Scales showed profiles that were consistent with
      moderate-to-severe mental retardation in all patients.
    explanation: >-
      Instrumented confirmation of the intellectual and adaptive-behaviour
      deficit in molecularly confirmed AD-NSID patients. The OBLIGATE band is
      definitional rather than measured: intellectual disability is the
      defining feature of this disease class, so by construction it is present
      in 100% of cases. No cohort frequency is cited because none is
      meaningful for a defining criterion.

- category: Neurologic
  name: Global Developmental Delay
  description: >-
    Delay across multiple developmental streams is the presenting feature in
    infancy and early childhood, before an IQ-based diagnosis of intellectual
    disability can be established.
  phenotype_term:
    preferred_term: Global developmental delay
    term:
      id: HP:0001263
      label: Global developmental delay
  evidence:
  - reference: PMID:26989088
    reference_title: "Genetic and neurodevelopmental spectrum of SYNGAP1-associated intellectual disability and epilepsy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Developmental delay was the first manifestation of SYNGAP1-related
      encephalopathy; intellectual disability became progressively obvious and
      was associated with autistic behaviours in eight patients.
    explanation: >-
      Documents developmental delay as the presenting manifestation preceding
      an established ID diagnosis. Frequency is intentionally omitted: this
      snippet establishes the temporal ordering of the phenotype, not a
      cohort proportion, and no class-level frequency estimate exists.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Their early development was characterized by global delay and hypotonia,
      with the onset of walking at the age of 2 years.
    explanation: >-
      Direct description of global developmental delay in molecularly
      confirmed AD-NSID.

- category: Neurologic
  name: Delayed Speech and Language Development
  description: >-
    Expressive language is disproportionately affected in several AD-NSID
    entities; severe language impairment was a defining feature of the
    original SYNGAP1 series and remains a common presenting complaint.
  phenotype_term:
    preferred_term: Delayed speech and language development
    term:
      id: HP:0000750
      label: Delayed speech and language development
  frequency: FREQUENT
  evidence:
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The disruption of SYNGAP1 appears to be associated with a homogeneous
      clinical phenotype that is characterized by moderate-to-severe mental
      retardation accompanied by severe language impairment.
    explanation: >-
      Identifies severe language impairment as a prominent feature of the
      paradigmatic AD-NSID entity.
  - reference: PMID:38215144
    reference_title: "A de novo variant in ZBTB18 gene caused autosomal dominant non-syndromic intellectual disability 22 syndrome: A case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      He struggles with expressing themselves and forming complete sentences,
      relying mostly on gestures and pointing.
    explanation: >-
      Expressive language impairment in a molecularly confirmed MRD22 case.
  - reference: PMID:39528574
    reference_title: "Trio-whole exome sequencing reveals the importance of de novo variants in children with intellectual disability and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Other prevalent clinical features included speech delay (59/173, 34.1%)
    explanation: >-
      Frequency basis: 59/173 = 34.1%, which falls in the FREQUENT band
      (30-79%). PARTIAL because the denominator is a mixed DD/ID cohort rather
      than a molecularly confirmed AD-NSID cohort, so this is the closest
      available quantitative anchor, not a class-specific rate.

- category: Neurologic
  name: Hypotonia
  description: >-
    Central hypotonia in infancy is a frequent non-specific accompaniment and
    does not by itself make the presentation syndromic.
  phenotype_term:
    preferred_term: Hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  frequency: FREQUENT
  evidence:
  - reference: PMID:26989088
    reference_title: "Genetic and neurodevelopmental spectrum of SYNGAP1-associated intellectual disability and epilepsy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Hypotonia and unstable gait were frequent associated neurological
      features.
    explanation: >-
      Reports hypotonia as a frequent feature in a 17-patient SYNGAP1 series.
      Frequency basis: author wording "frequent" maps to the FREQUENT band
      (30-79%) under the DisMech qualitative mapping. Derived from a
      single-gene series, so it is an anchor for the class rather than a
      measured class-level rate.

- category: Neurologic
  name: Seizures
  description: >-
    Epilepsy is a common comorbidity in several AD-NSID entities, ranging from
    well-controlled generalised seizures to a pharmacoresistant developmental
    and epileptic encephalopathy. Its presence is one of the features that
    most often triggers reclassification of a nominally non-syndromic
    gene-disease pair as a recognisable syndrome; conversely, some MRD-series
    entities (e.g. ZBTB18/MRD22) present without epilepsy.
  phenotype_term:
    preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  evidence:
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two of the patients were mildly epileptic."
    explanation: >-
      Epilepsy present in 2 of the 3 patients in the original non-syndromic
      SYNGAP1 series, and mild/controllable in both.
  - reference: PMID:19557857
    reference_title: "De novo STXBP1 mutations in mental retardation and nonsyndromic epilepsy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We identified de novo mutations in STXBP1 (nonsense, p.R388X; splicing,
      c.169+1G>A) in two patients with severe mental retardation and
      nonsyndromic epilepsy.
    explanation: >-
      Documents epilepsy co-occurring with non-syndromic ID under a de novo
      autosomal dominant lesion.
  - reference: PMID:38215144
    reference_title: "A de novo variant in ZBTB18 gene caused autosomal dominant non-syndromic intellectual disability 22 syndrome: A case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      However, he did not have growth retardation, microcephaly, corpus
      callosum hypoplasia, epilepsy, or dysmorphic facial features.
    explanation: >-
      Counter-example: a molecularly confirmed MRD22 patient without epilepsy,
      showing the feature is not obligate across the class. Frequency is
      intentionally omitted because the available sources are irreconcilable
      at class level: 2/3 in the index SYNGAP1 series, 17/17 in a larger
      SYNGAP1 cohort, and 0/1 here. Any single band would contradict at least
      one cited source.

- category: Behavioral
  name: Autistic Behaviour
  description: >-
    Autism spectrum features co-occur with AD-NSID at higher than population
    frequency but are not universal; the original SYNGAP1 series was
    explicitly negative for autism on structured assessment, whereas larger
    later series report autistic behaviours in roughly half of patients.
  phenotype_term:
    preferred_term: Autism
    term:
      id: HP:0000717
      label: Autism
  evidence:
  - reference: PMID:26989088
    reference_title: "Genetic and neurodevelopmental spectrum of SYNGAP1-associated intellectual disability and epilepsy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      intellectual disability became progressively obvious and was associated
      with autistic behaviours in eight patients
    explanation: >-
      Autistic behaviour in 8 of 17 patients in a molecularly defined AD-NSID
      series.
  - reference: PMID:21237447
    reference_title: "De novo SYNGAP1 mutations in nonsyndromic intellectual disability and autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We provide evidence that truncating mutations in SYNGAP1 are common in
      NSID and can be also associated with autism.
    explanation: >-
      Establishes the conditional ("can be also associated") rather than
      obligate relationship between AD-NSID and autism.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Nonverbal social interactions were within the normal range. In
      particular, evaluation of Patient 3 with the Autism Diagnostic
      Observation Schedule was negative.
    explanation: >-
      Counter-evidence that autism is not obligate: structured assessment was
      negative in the index non-syndromic series. Frequency is intentionally
      omitted: the two cited series disagree directly (8/17 = 47% in one,
      0/3 on structured assessment in the other), and no class-level estimate
      exists.

- category: Neurologic
  name: Delayed Ability to Walk
  description: >-
    Gross motor milestones are commonly delayed, with independent walking
    frequently achieved around or after the second birthday.
  phenotype_term:
    preferred_term: Delayed ability to walk
    term:
      id: HP:0031936
      label: Delayed ability to walk
  evidence:
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Their early development was characterized by global delay and hypotonia,
      with the onset of walking at the age of 2 years.
    explanation: >-
      Documents delayed independent ambulation in the index AD-NSID series.
      Frequency is intentionally omitted: the source describes three patients
      and reports no proportion.

genetic:
- name: SYNGAP1
  gene_term:
    preferred_term: SYNGAP1
    term:
      id: hgnc:11497
      label: SYNGAP1
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  frequency: >-
    Approximately 3% of unselected non-syndromic ID series in the index
    report; one of the most frequently recurrent AD-NSID genes.
  notes: >-
    MRD5. The paradigmatic AD-NSID gene: a brain-restricted Ras
    GTPase-activating protein in the NMDA receptor complex. Note that larger
    later series redescribe SYNGAP1 disease as a recognisable developmental
    and epileptic encephalopathy, making it the clearest worked example of the
    non-syndromic-to-syndromic migration.
  evidence:
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We identified de novo truncating mutations (K138X, R579X, and
      L813RfsX22) in three of these patients.
    explanation: >-
      Three independent de novo protein-truncating variants in 94 patients
      with non-syndromic ID.
  - reference: PMID:21237447
    reference_title: "De novo SYNGAP1 mutations in nonsyndromic intellectual disability and autism."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Recently, we reported de novo truncating mutations in the SYNGAP1 gene
      of 3 of 94 NSID cases, suggesting that its disruption represents a
      common cause of autosomal dominant NSID.
    explanation: >-
      Replication cohort confirming SYNGAP1 as a common cause of autosomal
      dominant non-syndromic ID.

- name: STXBP1
  gene_term:
    preferred_term: STXBP1
    term:
      id: hgnc:11444
      label: STXBP1
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  notes: >-
    Encodes Munc18-1, an essential regulator of SNARE-mediated synaptic
    vesicle fusion. One of the three recurrently mutated genes in the Rauch
    severe sporadic non-syndromic ID cohort.
  evidence:
  - reference: PMID:19557857
    reference_title: "De novo STXBP1 mutations in mental retardation and nonsyndromic epilepsy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      These results suggest that STXBP1 disruption is associated with
      autosomal dominant mental retardation and nonsyndromic epilepsy.
    explanation: >-
      Establishes STXBP1 as an autosomal dominant non-syndromic ID gene.
  - reference: PMID:23020937
    reference_title: "Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      16 patients carried de-novo variants in known intellectual disability
      genes with three recurrently mutated genes (STXBP1, SYNGAP1, and SCN2A).
    explanation: >-
      Independent replication of STXBP1 recurrence in a severe sporadic
      non-syndromic ID cohort.

- name: ZBTB18
  gene_term:
    preferred_term: ZBTB18
    term:
      id: hgnc:13030
      label: ZBTB18
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  notes: >-
    MRD22. A zinc finger transcriptional repressor; illustrates the
    chromatin/transcription module of AD-NSID. Fewer than ~35 cases reported,
    which is itself the reason the phenotype has not consolidated into a
    recognised gestalt.
  evidence:
  - reference: PMID:38215144
    reference_title: "A de novo variant in ZBTB18 gene caused autosomal dominant non-syndromic intellectual disability 22 syndrome: A case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Clinical whole exome sequencing revealed a de novo pathogenic variant in
      the ZBTB18 gene (c.1207delC, p. Arg403Alafs*60), which is a previously
      unreported site.
    explanation: >-
      De novo frameshift variant in a patient diagnosed with MRD22.
  - reference: PMID:38215144
    reference_title: "A de novo variant in ZBTB18 gene caused autosomal dominant non-syndromic intellectual disability 22 syndrome: A case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      It is noteworthy that only 31 cases of this disorder have been reported
      thus far.
    explanation: >-
      Quantifies the small case count that keeps this MRD-series entity in the
      non-syndromic category.

- name: GRIN2B
  gene_term:
    preferred_term: GRIN2B
    term:
      id: hgnc:4586
      label: GRIN2B
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  notes: >-
    MRD6. Encodes the GluN2B subunit of the NMDA receptor - the receptor
    complex that SYNGAP1 also acts within - placing it squarely in the
    synaptic module.
  evidence:
  - reference: PMID:25356899
    reference_title: "De novo mutations in moderate or severe intellectual disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A total of 12 likely pathogenic DNMs were identified in genes previously
      associated with ID (ARID1B, CHD2, FOXG1, GABRB3, GATAD2B, GRIN2B, MBD5,
      MED13L, SETBP1, TBR1, TCF4, WDR45), resulting in a diagnostic yield of
      ∼29%.
    explanation: >-
      GRIN2B recovered as a de novo cause in a moderate-to-severe ID trio
      cohort.

- name: GATAD2B
  gene_term:
    preferred_term: GATAD2B
    term:
      id: hgnc:30778
      label: GATAD2B
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  notes: >-
    A subunit of the NuRD chromatin-remodelling complex; a member of the
    chromatin/transcription module.
  evidence:
  - reference: PMID:25356899
    reference_title: "De novo mutations in moderate or severe intellectual disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A total of 12 likely pathogenic DNMs were identified in genes previously
      associated with ID (ARID1B, CHD2, FOXG1, GABRB3, GATAD2B, GRIN2B, MBD5,
      MED13L, SETBP1, TBR1, TCF4, WDR45), resulting in a diagnostic yield of
      ∼29%.
    explanation: >-
      GATAD2B recovered as a de novo cause in a moderate-to-severe ID trio
      cohort.

- name: MED13L
  gene_term:
    preferred_term: MED13L
    term:
      id: hgnc:22962
      label: MED13L
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  notes: >-
    A subunit of the Mediator transcriptional coactivator complex. CAUSATIVE
    is asserted for the gene-disease relationship in the broad de novo
    dominant ID sense, NOT for continuing membership of the non-syndromic
    class: MED13L is now widely regarded as syndromic (MED13L syndrome), so
    it is causative for that recognised disorder rather than for AD-NSID as
    curated here. Retained deliberately as a worked example of the
    reclassification the mechanistic hypothesis predicts; the supporting
    evidence is typed PARTIAL for the same reason.
  evidence:
  - reference: PMID:25356899
    reference_title: "De novo mutations in moderate or severe intellectual disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A total of 12 likely pathogenic DNMs were identified in genes previously
      associated with ID (ARID1B, CHD2, FOXG1, GABRB3, GATAD2B, GRIN2B, MBD5,
      MED13L, SETBP1, TBR1, TCF4, WDR45), resulting in a diagnostic yield of
      ∼29%.
    explanation: >-
      MED13L recovered as a de novo cause in a moderate-to-severe ID trio
      cohort. PARTIAL because this supports MED13L as a de novo dominant ID
      gene, not as a NON-SYNDROMIC one; the source cohort was ascertained on
      ID severity, not on absence of syndromic features.

- name: TRIO
  gene_term:
    preferred_term: TRIO
    term:
      id: hgnc:12303
      label: TRIO
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  notes: >-
    Illustrates the cytoskeletal/Rho GTPase module: TRIO is a Dbl-family
    guanine nucleotide exchange factor that activates Rac1 and controls actin
    dynamics in developing neurons. Included here with two explicit caveats
    that make it instructive rather than clean. First, the best-characterised
    TRIO series reports microcephaly and dysmorphism alongside the ID, so this
    gene-disease pair sits on the syndromic side of an unstable boundary.
    Second, that series includes an autosomal dominant frameshift that
    SEGREGATED through a family as well as de novo missense variants,
    demonstrating that the class is not exclusively de novo. Both caveats
    argue against treating "autosomal dominant" and "de novo" as synonyms.
    A third caveat is that the head-size feature is not directionally fixed:
    TRIO-related disease includes both MRD44 with microcephaly and MRD63 with
    macrocephaly. The dedicated TRIO-Related Neurodevelopmental Disorder entry
    now models that domain-resolved, RAC1-direction-dependent split and is the
    canonical home for TRIO variants and mechanisms. TRIO is retained here only
    as a boundary exemplar, not as a clean non-syndromic gene; the cephalic and
    dysmorphic findings illustrate why umbrella AD-NSID membership is partial
    (see GitHub issue 6041).
  evidence:
  - reference: PMID:27418539
    reference_title: "Mutations specific to the Rac-GEF domain of TRIO cause intellectual disability and microcephaly."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Trio is a Dbl family guanine nucleotide exchange factor (GEF) and a
      major regulator of neuronal development, controlling actin cytoskeleton
      dynamics by activating the GTPase Rac1.
    explanation: >-
      Places TRIO in the cytoskeletal/Rho GTPase pathophysiology module.
  - reference: PMID:27418539
    reference_title: "Mutations specific to the Rac-GEF domain of TRIO cause intellectual disability and microcephaly."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      We identified a frameshift deletion in TRIO that segregated autosomal
      dominantly. By scrutinising data from DDD, we further identified three
      unrelated children with a similar phenotype who harboured de novo
      missense mutations in TRIO.
    explanation: >-
      Documents both an inherited autosomal dominant lesion and de novo
      lesions in the same gene, supporting the caveat that AD-NSID is not
      exclusively a de novo class.
  - reference: PMID:27418539
    reference_title: "Mutations specific to the Rac-GEF domain of TRIO cause intellectual disability and microcephaly."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Additionally, we identify pathogenic de novo missense mutations in TRIO
      associated with the same consistent phenotype, intellectual disability,
      microcephaly and dysmorphism with striking digital features.
    explanation: >-
      Only PARTIAL support for inclusion in a NON-syndromic class: the
      reported phenotype includes microcephaly and dysmorphism, so this
      gene-disease pair is arguably syndromic - the boundary problem this
      entry documents.
- name: CAMK2A
  gene_term:
    preferred_term: CAMK2A
    term:
      id: hgnc:1460
      label: CAMK2A
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  notes: >-
    Included because Open Targets lists CAMK2A among the genes associated with
    MONDO:0015802 specifically, anchoring this entry to the curated umbrella
    entity rather than to the general de novo ID literature. The alpha subunit
    of calcium/calmodulin-dependent protein kinase II - a synaptic-module gene
    whose autophosphorylation state governs plasticity and neuronal migration.
  evidence:
  - reference: PMID:29100089
    reference_title: "De Novo Mutations in Protein Kinase Genes CAMK2A and CAMK2B Cause Intellectual Disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Through a multi-center collaborative study based on a whole-exome
      sequencing approach, we identified 19 exceedingly rare de novo CAMK2A or
      CAMK2B variants in 24 unrelated individuals with intellectual
      disability.
    explanation: >-
      Establishes CAMK2A as a de novo heterozygous cause of intellectual
      disability in a multi-centre trio-sequencing cohort.
  - reference: PMID:29100089
    reference_title: "De Novo Mutations in Protein Kinase Genes CAMK2A and CAMK2B Cause Intellectual Disability."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: >-
      We further found that all mutations affecting auto-phosphorylation also
      affected neuronal migration, highlighting the importance of tightly
      regulated CAMK2 auto-phosphorylation in neuronal function and
      neurodevelopment.
    explanation: >-
      Functional assays tie the variant class to a specific cellular
      mechanism, supporting the synaptic/plasticity pathophysiology module.

- name: DLG4
  gene_term:
    preferred_term: DLG4
    term:
      id: hgnc:2903
      label: DLG4
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  notes: >-
    Encodes PSD-95, the core postsynaptic density scaffold of the same NMDA
    receptor complex in which SYNGAP1 acts, making it the closest mechanistic
    neighbour of the paradigmatic gene. Listed by Open Targets against
    MONDO:0015802. Notably, the largest series could NOT establish a distinct
    facial gestalt, which is the positive form of the non-syndromic
    designation rather than a mere absence of reported features.
  evidence:
  - reference: PMID:33597769
    reference_title: "DLG4-related synaptopathy: a new rare brain disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The clinical picture was predominated by early onset global
      developmental delay, intellectual disability, autism spectrum disorder,
      and attention deficit-hyperactivity disorder, all of which point to a
      brain disorder.
    explanation: >-
      A 53-patient series showing a brain-restricted, neurodevelopmental-only
      phenotype - the expression pattern the competing brain-restricted model
      of AD-NSID predicts.
  - reference: PMID:33597769
    reference_title: "DLG4-related synaptopathy: a new rare brain disorder."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      despite some overlapping features, a distinct facial dysmorphism could
      not be established
    explanation: >-
      Explicit negative finding for a recognisable dysmorphic gestalt,
      supporting non-syndromic classification on positive grounds rather than
      by default.

- name: KMT5B
  gene_term:
    preferred_term: KMT5B
    term:
      id: hgnc:24283
      label: KMT5B
  relationship_type: CAUSATIVE
  variant_origin: DE_NOVO
  notes: >-
    A histone H4K20 methyltransferase; a chromatin/transcription-module gene
    listed by Open Targets against MONDO:0015802. Included with the caveat
    that the associated macrocephaly/overgrowth is itself a recognisable
    feature, so KMT5B is a further candidate for migration out of the
    non-syndromic class.
  evidence:
  - reference: PMID:35433545
    reference_title: "Refining the Phenotypic Spectrum of KMT5B-Associated Developmental Delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Recently, deleterious heterozygous variants in KMT5B were implicated in
      individuals with intellectual disability (ID) and/or autism spectrum
      disorder.
    explanation: >-
      Establishes heterozygous KMT5B variants as a cause of intellectual
      disability, placing the gene in the chromatin/transcription module.
  - reference: PMID:35433545
    reference_title: "Refining the Phenotypic Spectrum of KMT5B-Associated Developmental Delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      suggest that this gene should be considered in the differential
      diagnosis of neurodevelopmental disorders accompanied by macrocephaly
      and/or overgrowth
    explanation: >-
      Only PARTIAL for a NON-syndromic class: the recurring macrocephaly and
      overgrowth constitute an emerging recognisable gestalt, the same
      boundary instability documented for SYNGAP1, TRIO and MED13L.

diagnosis:
- name: Trio Exome or Genome Sequencing
  description: >-
    Parent-offspring trio exome (or genome) sequencing is the definitive
    diagnostic modality and is now recommended as a first-tier test for
    unexplained neurodevelopmental disorders, ahead of chromosomal microarray.
    Trio design is essential rather than merely convenient: de novo status is
    the primary evidence of pathogenicity for a novel heterozygous variant in
    a constraint-intolerant gene, and cannot be established from a
    proband-only test.
  diagnosis_term:
    preferred_term: molecular genetic testing
    term:
      id: NCIT:C19770
      label: Molecular Analysis
  evidence:
  - reference: PMID:31182824
    reference_title: "Meta-analysis and multidisciplinary consensus statement: exome sequencing is a first-tier clinical diagnostic test for individuals with neurodevelopmental disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Yield of ES was 36% overall, 31% for isolated NDD, and 53% for the NDD
      plus associated conditions.
    explanation: >-
      Quantifies exome yield, and specifically the ISOLATED (i.e.
      non-syndromic) NDD yield of 31% - directly relevant to this disease
      class.
  - reference: PMID:31182824
    reference_title: "Meta-analysis and multidisciplinary consensus statement: exome sequencing is a first-tier clinical diagnostic test for individuals with neurodevelopmental disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Our review demonstrates that ES consistently outperforms CMA for
      evaluation of unexplained NDDs. We propose a diagnostic algorithm
      placing ES at the beginning of the evaluation of unexplained NDDs.
    explanation: >-
      Consensus recommendation placing exome sequencing first-tier, displacing
      chromosomal microarray.
  - reference: PMID:23033978
    reference_title: "Diagnostic exome sequencing in persons with severe intellectual disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      De novo mutations represent an important cause of intellectual
      disability; exome sequencing was used as an effective diagnostic
      strategy for their detection.
    explanation: >-
      Establishes trio exome sequencing as the effective diagnostic strategy
      for the de novo dominant class.
  - reference: PMID:39528574
    reference_title: "Trio-whole exome sequencing reveals the importance of de novo variants in children with intellectual disability and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      De novo variants in autosomal dominant genes are significant
      contributors to DD/ID, particularly in non-consanguineous families.
    explanation: >-
      Contemporary trio-WES cohort confirming the de novo autosomal dominant
      architecture, with the non-consanguineous qualifier that separates this
      class from recessive ID.
  - reference: PMID:39528574
    reference_title: "Trio-whole exome sequencing reveals the importance of de novo variants in children with intellectual disability and developmental delay."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The diagnostic yield in the syndromic DD/ID group was higher than that
      in the non-syndromic DD/ID group (57.8% vs. 47.2%, P < 0.001).
    explanation: >-
      Important caveat for this specific class: yield is significantly lower
      in non-syndromic than syndromic DD/ID, so a negative trio exome is more
      common here than headline all-comers yields suggest.

- name: Exclusion of Recognised Syndromic and Metabolic Causes
  description: >-
    A non-syndromic designation requires prior exclusion of a recognised
    aetiology. Historically this meant karyotype, subtelomeric FISH or
    comparative genomic hybridisation, FMR1 repeat testing, and brain imaging;
    in current practice chromosomal microarray and targeted metabolic testing
    play this role, with exome sequencing increasingly performed in parallel
    rather than afterwards.
  diagnosis_term:
    preferred_term: disease screening
    term:
      id: NCIT:C15419
      label: Disease Screening
  evidence:
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Mental retardation was unexplained in these patients despite standard
      investigations, including karyotyping, subtelomeric fluorescence in situ
      hybridization analysis, or comparative genomic hybridization targeting
      regions associated with known syndromes, molecular testing for the
      common expansion mutation in FMR1, and computed tomography or magnetic
      resonance imaging of the brain.
    explanation: >-
      Specifies the exclusion workup that operationally defines the
      non-syndromic, unexplained category.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      All patients were examined by at least one experienced clinical
      geneticist, who ruled out the presence of specific dysmorphic features.
    explanation: >-
      Expert dysmorphology assessment is the clinical step that assigns a case
      to the non-syndromic category.

treatments:
- name: Genetic Counselling and Recurrence-Risk Assessment
  description: >-
    After a de novo variant is confirmed, sibling recurrence risk is low but
    not negligible, because a proportion of apparently de novo variants arise
    from undetected low-level parental gonadal or somatic mosaicism. Counsel
    on this residual risk rather than quoting a zero recurrence risk; parental
    mosaicism detectable in blood raises recurrence risk substantially more
    than mosaicism confined to the germline.
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:25087610
    reference_title: "Parental somatic mosaicism is underrecognized and influences recurrence risk of genomic disorders."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here, we show that somatic mosaicism for transmitted mutations among
      parents of children with simplex genetic disease is more common than
      currently appreciated.
    explanation: >-
      Establishes that apparently de novo variants in simplex cases can arise
      from underdetected parental mosaicism, the basis for a non-zero
      recurrence-risk counselling message.
  - reference: PMID:25087610
    reference_title: "Parental somatic mosaicism is underrecognized and influences recurrence risk of genomic disorders."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: >-
      Integrated probabilistic modeling of gametogenesis developed in response
      to our observations predicts that mutations in parental blood increase
      recurrence risk substantially more than parental mutations confined to
      the germline.
    explanation: >-
      Provides the quantitative rationale distinguishing blood-detectable
      parental mosaicism from germline-confined mosaicism in recurrence-risk
      counselling.
  - reference: PMID:34697084
    reference_title: "Somatic mosaicism detected by genome-wide sequencing in 500 parent-child trios with suspected genetic disease: clinical and genetic counseling implications."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Here we present 12 cases of apparent mosaicism identified in the CAUSES
      cohort: nine cases of parental mosaicism for a disease-causing variant
      found in a child and three cases of mosaicism in the proband for a de
      novo variant.
    explanation: >-
      Quantifies how often mosaicism is found in an unselected trio
      sequencing cohort of exactly the kind used to diagnose AD-NSID.
  - reference: PMID:34697084
    reference_title: "Somatic mosaicism detected by genome-wide sequencing in 500 parent-child trios with suspected genetic disease: clinical and genetic counseling implications."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      In six of these cases, there was no evidence of mosaicism on Sanger
      sequencing-the variant was not detected on Sanger sequencing in three
      cases, and it appeared to be heterozygous in three others.
    explanation: >-
      Shows that confirmatory Sanger sequencing misses half of mosaic cases,
      which is why a "de novo" call from standard confirmation does not
      justify quoting a zero recurrence risk.

- name: Early Intervention, Rehabilitation, and Special Education
  description: >-
    There is no disease-modifying therapy. Management is developmental and
    supportive: individualised special education, early intervention
    programmes, rehabilitative training, and structured behavioural support.
    The critical-window biology gives an explicit mechanistic argument for
    intervening as early as possible, though it should not be over-read as
    evidence that any specific educational programme alters the underlying
    circuit defect.
  treatment_term:
    preferred_term: early intervention and special education
    term:
      id: NCIT:C15315
      label: Rehabilitation
  therapeutic_modality: BEHAVIORAL
  evidence:
  - reference: PMID:38215144
    reference_title: "A de novo variant in ZBTB18 gene caused autosomal dominant non-syndromic intellectual disability 22 syndrome: A case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "INTERVENTIONS: Rehabilitation training."
    explanation: >-
      Rehabilitative/developmental training was the entirety of the
      intervention offered in a molecularly confirmed MRD22 case, reflecting
      the absence of disease-specific therapy.

- name: Speech and Language Therapy
  description: >-
    Targeted therapy for the expressive language impairment that is
    disproportionately prominent in several AD-NSID entities, including
    augmentative and alternative communication for minimally verbal children.
  treatment_term:
    preferred_term: speech therapy
    term:
      id: NCIT:C159273
      label: Speech Language Therapy
  therapeutic_modality: BEHAVIORAL
  target_phenotypes:
  - preferred_term: Delayed speech and language development
    term:
      id: HP:0000750
      label: Delayed speech and language development
  evidence:
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The disruption of SYNGAP1 appears to be associated with a homogeneous
      clinical phenotype that is characterized by moderate-to-severe mental
      retardation accompanied by severe language impairment.
    explanation: >-
      Establishes the language impairment that this therapy targets; the
      citation supports the indication, not the efficacy of the intervention.

- name: Physical and Occupational Therapy
  description: >-
    Addresses the hypotonia, delayed gross motor milestones, and gait
    instability that commonly accompany AD-NSID, and supports acquisition of
    adaptive daily-living skills.
  treatment_term:
    preferred_term: physical therapy
    term:
      id: NCIT:C15302
      label: Physical Therapy
  therapeutic_modality: BEHAVIORAL
  target_phenotypes:
  - preferred_term: Hypotonia
    term:
      id: HP:0001252
      label: Hypotonia
  - preferred_term: Delayed ability to walk
    term:
      id: HP:0031936
      label: Delayed ability to walk
  evidence:
  - reference: PMID:26989088
    reference_title: "Genetic and neurodevelopmental spectrum of SYNGAP1-associated intellectual disability and epilepsy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Hypotonia and unstable gait were frequent associated neurological
      features.
    explanation: >-
      Documents the motor impairments that motivate physiotherapy; supports
      the indication rather than the efficacy.

- name: Antiseizure Pharmacotherapy for Comorbid Epilepsy
  description: >-
    When epilepsy is present, standard antiseizure medication is used.
    Response is variable and gene-dependent: seizures were well controlled by
    topiramate or valproate in the original SYNGAP1 series, whereas roughly
    half the patients in a larger SYNGAP1 cohort were pharmacoresistant. There
    is a mechanistic rationale for AMPA-directed agents in SYNGAP1
    haploinsufficiency, but it is a hypothesis generated from mouse work and
    small case observations, not a validated precision-therapy indication.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  therapeutic_modality: SMALL_MOLECULE
  target_phenotypes:
  - preferred_term: Seizure
    term:
      id: HP:0001250
      label: Seizure
  target_mechanisms:
  - target: Cortical Hyperexcitability
    treatment_effect: INHIBITS
    description: >-
      Topiramate and valproate reduce AMPA-receptor-mediated excitatory
      transmission, the pathway de-repressed by SYNGAP1 haploinsufficiency.
      The drug edge attaches to the hyperexcitability node only: no available
      antiseizure medication reverses the upstream spine-maturation defect,
      which is why seizure control does not translate into cognitive rescue.
    evidence:
    - reference: PMID:19196676
      reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
      supports: SUPPORT
      evidence_source: OTHER
      snippet: >-
        Indeed, topiramate directly inhibits AMPA-receptor activity, whereas
        valproate reduces the level of GluR1 AMPA-receptor subunit at
        hippocampal synapses and therefore indirectly reduces AMPA-receptor
        activity.
      explanation: >-
        Mechanistic rationale for the drug acting on the hyperexcitability
        node; authors' interpretation rather than a controlled trial.
  evidence:
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Patient 1 had brief, generalized tonic–clonic seizures and was
      seizure-free while taking topiramate, whereas Patient 2 had some
      myoclonic and absence seizures, which were well controlled with
      valproate.
    explanation: >-
      Documents effective seizure control with standard antiseizure medication
      in molecularly confirmed AD-NSID.
  - reference: PMID:19196676
    reference_title: "Mutations in SYNGAP1 in autosomal nonsyndromic mental retardation."
    supports: SUPPORT
    evidence_source: OTHER
    snippet: >-
      Indeed, topiramate directly inhibits AMPA-receptor activity, whereas
      valproate reduces the level of GluR1 AMPA-receptor subunit at
      hippocampal synapses and therefore indirectly reduces AMPA-receptor
      activity.
    explanation: >-
      Gives the mechanistic rationale linking drug choice to the
      AMPA-receptor-mediated hyperexcitability node; this is authors'
      interpretation, not a controlled comparison.
  - reference: PMID:26989088
    reference_title: "Genetic and neurodevelopmental spectrum of SYNGAP1-associated intellectual disability and epilepsy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Seizures were pharmacoresistant in half of the patients."
    explanation: >-
      Qualifies rather than endorses the efficacy claim: a larger series found
      half of patients pharmacoresistant, so antiseizure medication controls
      seizures in only about half of this group.

differential_diagnoses:
- name: Syndromic Intellectual Disability
  description: >-
    The primary differential. Any recognisable pattern of dysmorphic, growth,
    malformative, or neuroimaging features moves a case out of this class. The
    distinction is unstable in practice, since patients ascertained as
    non-syndromic frequently carry variants in genes catalogued as syndromic.
  distinguishing_features:
  - Presence of a recognisable dysmorphic gestalt or major malformation
  - Growth abnormality, microcephaly, or structural brain anomaly on imaging
  evidence:
  - reference: PMID:23020937
    reference_title: "Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Several patients did not meet the expected syndromic manifestation,
      suggesting a strong bias in present clinical syndrome descriptions.
    explanation: >-
      Documents the instability of the syndromic/non-syndromic distinction in
      real cohorts.

- name: X-linked Intellectual Disability
  description: >-
    X-linked non-syndromic ID was historically the best-characterised
    non-syndromic ID class and remains a major differential, particularly in
    affected males with a maternal family history. Trio sequencing
    distinguishes the two by demonstrating a maternally inherited hemizygous
    variant rather than an autosomal de novo one.
  distinguishing_features:
  - Male predominance with affected maternal male relatives
  - Maternally inherited hemizygous X-chromosome variant rather than an autosomal de novo variant
  evidence:
  - reference: PMID:23033978
    reference_title: "Diagnostic exome sequencing in persons with severe intellectual disability."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A total of 10 de novo mutations and 3 X-linked (maternally inherited)
      mutations that had been previously predicted to compromise the function
      of known intellectual-disability genes were found in 13 patients.
    explanation: >-
      Shows both classes co-occurring in the same diagnostic cohort and
      distinguished by inheritance mode on trio analysis.

- name: Autosomal Recessive Non-Syndromic Intellectual Disability
  description: >-
    Biallelic recessive causes are the major alternative genetic architecture,
    prominent in consanguineous populations but contributing little in outbred
    populations - a key reason the de novo dominant model became dominant in
    European cohorts.
  distinguishing_features:
  - Consanguinity or an endogamous founder population
  - Biallelic (homozygous or compound heterozygous) variants inherited from unaffected carrier parents
  evidence:
  - reference: PMID:23020937
    reference_title: "Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Autosomal recessive inheritance seems to contribute little in the
      outbred population investigated.
    explanation: >-
      Quantifies the limited recessive contribution in an outbred cohort,
      supporting the dominant/de novo model for that setting.

- name: Copy Number Variant Intellectual Disability
  description: >-
    Pathogenic de novo copy number variants were the first recognised class of
    de novo dominant lesions in ID and remain a major cause. They are detected
    by chromosomal microarray and are excluded before, or in parallel with,
    sequence-level analysis.
  distinguishing_features:
  - Detected by chromosomal microarray rather than sequencing
  - Contiguous multi-gene deletion or duplication rather than a single-gene variant
  evidence:
  - reference: PMID:21076407
    reference_title: "A de novo paradigm for mental retardation."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Together with de novo copy number variation, de novo point mutations of
      large effect could explain the majority of all mental retardation cases
      in the population.
    explanation: >-
      Places de novo CNVs alongside de novo point mutations as the two arms of
      the de novo dominant architecture.

discussions:
- discussion_id: adnsid_opentargets_gene_backlog
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Which of the remaining Open Targets gene associations for MONDO:0015802
    warrant curation as full genetic records in this entry, and which are
    better placed on gene-specific syndromic entries?
  attaches_to:
  - pathophysiology#Haploinsufficiency of a Dosage-Sensitive Neurodevelopmental Gene
  rationale: >-
    Open Targets associates MONDO:0015802 with TLK2, ASH1L, STAG1, CLTC,
    CAMK2A, CIC, NAA15, TRIP12, KMT5B, TRIO, SET and DLG4. This entry curates
    four of those (CAMK2A, DLG4, KMT5B, TRIO) alongside six genes drawn from
    the general de novo ID literature (SYNGAP1, STXBP1, ZBTB18, GRIN2B,
    GATAD2B, MED13L). The gene list is deliberately illustrative of the major
    functional modules rather than exhaustive, so the remaining eight
    associations - TLK2, ASH1L, STAG1, CLTC, CIC, NAA15, TRIP12, SET - are
    recorded here as an explicit backlog rather than left as a silent
    omission. Triage is not purely mechanical: several of these are already
    described with recognisable syndromic features, so for each the curation
    decision is entangled with the syndromic/non-syndromic boundary question
    tracked in the adnsid_entity_boundary discussion. Note also that Open
    Targets associations are themselves reassessed over time, so the target
    list is a moving one.
  proposed_experiments:
  - experiment_id: exp_adnsid_opentargets_gene_triage
    name: Deep-phenotype triage of the remaining Open Targets associations
    description: >-
      For each of TLK2, ASH1L, STAG1, CLTC, CIC, NAA15, TRIP12 and SET,
      systematically review the published case series for recurrent
      dysmorphic, malformative, growth or neuroimaging features, and classify
      the gene-disease pair as (a) genuinely non-syndromic and therefore a
      curation target for this entry, or (b) syndromic and therefore better
      served by a dedicated gene-specific entry cross-referenced from here.
      The classification should record which specific feature drove any
      syndromic call, so the boundary decision is auditable rather than
      asserted.

- discussion_id: adnsid_gene_list_incomplete
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    How many autosomal dominant non-syndromic intellectual disability genes
    remain undiscovered, and why has discovery not plateaued?
  attaches_to:
  - pathophysiology#Haploinsufficiency of a Dosage-Sensitive Neurodevelopmental Gene
  rationale: >-
    Modelling of the residual de novo protein-truncating burden in 31,058
    trios implies roughly 1,000 further haploinsufficient developmental
    disorder genes, at a fold-enrichment about three times lower than for
    currently known genes. Lower enrichment implies reduced penetrance and/or
    pre/perinatal lethality, meaning the undiscovered tail is systematically
    different from the known genes and may not behave as cleanly monogenic.
    This is a first-order limit on the completeness of any AD-NSID gene list,
    including this entry's.
  proposed_experiments:
  - experiment_id: exp_adnsid_scale_trio_sequencing
    name: Scale trio sequencing to the calculated discovery-power threshold
    description: >-
      Expand pooled clinical and research parent-offspring trio sequencing
      toward the ~350,000-trio sample size calculated to give 80% power to
      detect a 10-fold de novo protein-truncating-variant enrichment for an
      average gene, and re-run gene-level enrichment testing.
  - experiment_id: exp_adnsid_penetrance_biobank
    name: Biobank penetrance test for low-enrichment candidate genes
    description: >-
      For candidate genes showing lower-than-expected de novo PTV enrichment,
      measure PTV frequency in unselected adult population biobanks to
      distinguish reduced penetrance from limited statistical power.
  evidence:
  - reference: PMID:33057194
    reference_title: "Evidence for 28 genetic disorders discovered by combining healthcare and research data."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: >-
      Modelling suggests that more than 1,000 genes associated with
      developmental disorders have not yet been described, many of which are
      likely to be less penetrant than the currently known genes.
    explanation: >-
      Quantifies the size and expected properties of the undiscovered gene
      set.
  - reference: PMID:33057194
    reference_title: "Evidence for 28 genetic disorders discovered by combining healthcare and research data."
    supports: SUPPORT
    evidence_source: COMPUTATIONAL
    snippet: >-
      We calculated that a sample size of ~350,000 parent-offspring trios
      would be needed to have 80% power to detect a 10-fold enrichment of de
      novo PTVs for an average gene.
    explanation: >-
      Sets the concrete sample-size target for completing the gene list.

- discussion_id: adnsid_critical_window_translational_validity
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    Does the mouse critical-window result - that repairing the causal lesion
    in adulthood fails to rescue cognition - hold in humans, and does it
    genuinely foreclose postnatal molecular therapy for AD-NSID?
  attaches_to:
  - pathophysiology#Impaired Cortical Circuit Formation During Critical Developmental Windows
  rationale: >-
    The critical-window claim rests on conditional Syngap1 mouse genetics, and
    it is doing very heavy lifting: it is the main mechanistic argument that
    AD-NSID is therapeutically closed after early childhood. But the human
    cortical developmental timetable is far longer than the mouse's,
    plasticity windows differ by circuit, and the experiment has been
    performed for one gene in the synaptic module only - it has not been
    tested for the chromatin/transcription module at all. Treating a
    single-gene mouse result as a general prohibition on postnatal therapy
    across a class of hundreds of genes is an extrapolation the evidence does
    not license. Resolving this matters directly to whether ASO or gene-dosage
    therapies are worth developing for this class.
  proposed_experiments:
  - experiment_id: exp_adnsid_adult_rescue_chromatin_module
    name: Adult-rescue test in chromatin-module AD-NSID mouse models
    description: >-
      Repeat the conditional adult-restoration experiment in mouse models of
      chromatin-module AD-NSID genes (e.g. Gatad2b, Zbtb18) to test whether
      the critical-window property generalises beyond synaptic genes or is
      specific to the SYNGAP1 spine-maturation mechanism.
  - experiment_id: exp_adnsid_organoid_window_mapping
    name: Human cortical organoid staged-correction window mapping
    description: >-
      Use human iPSC-derived cortical organoids carrying an AD-NSID variant
      with an inducible correction system, applying correction at staged
      timepoints, to map the equivalent plasticity window on a human rather
      than murine developmental timetable.
  - experiment_id: exp_adnsid_partial_rescue_endpoints
    name: Partial-rescue endpoint analysis after window closure
    description: >-
      In existing adult-rescue paradigms, assess whether adaptive behaviour
      and seizure burden improve after window closure even when IQ-equivalent
      cognitive measures do not, to test whether "no rescue" is
      endpoint-specific rather than absolute.
  evidence:
  - reference: PMID:23141534
    reference_title: "Pathogenic SYNGAP1 mutations impair cognitive development by disrupting maturation of dendritic spine synapses."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: >-
      Inducing SYNGAP1 mutations after critical developmental windows closed
      had minimal impact on spine synapse function, whereas repairing these
      pathogenic mutations in adulthood did not improve behavior and
      cognition.
    explanation: >-
      The mouse result whose human translational validity is the open
      question.

- discussion_id: adnsid_entity_boundary
  kind: CONTROVERSY
  status: OPEN
  prompt: >-
    Is autosomal dominant non-syndromic intellectual disability a coherent
    disease entity, or an ascertainment bin that should be dissolved into
    per-gene entries?
  attaches_to:
  - pathophysiology#Static Global Cognitive and Adaptive Impairment Without Syndromic Features
  rationale: >-
    The class is defined by a negative (no recognised gestalt) plus an
    inheritance mode, not by a shared mechanism - its members span synaptic,
    chromatin, and cytoskeletal biology. Several flagship members (SYNGAP1,
    MED13L) have already been redescribed as syndromic, and the ZBTB18 case
    literature shows the label persists mainly where case counts are small.
    Against dissolution: the class does capture a genuinely shared
    architecture (de novo, monoallelic, constraint-intolerant gene, static
    developmental course) that is clinically actionable - it is the group for
    which trio sequencing is the correct first test. For dismech, the open
    question is whether this should remain a Disease entry or be remodelled as
    a Grouping over per-gene MRD entries once enough of those exist.
  evidence:
  - reference: PMID:26989088
    reference_title: "Genetic and neurodevelopmental spectrum of SYNGAP1-associated intellectual disability and epilepsy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      SYNGAP1 encephalopathy is characterised by early neurodevelopmental
      delay typically preceding the onset of a relatively recognisable
      epilepsy comprising generalised seizures (absences, myoclonic jerks) and
      frequent triggers.
    explanation: >-
      The flagship member of the class redescribed as a recognisable syndrome,
      the core argument for dissolving the category.
  - reference: PMID:23020937
    reference_title: "Range of genetic mutations associated with severe non-syndromic sporadic intellectual disability: an exome sequencing study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      After exclusion of copy-number variants, de-novo point mutations and
      small indels are associated with severe, sporadic non-syndromic
      intellectual disability, accounting for 45-55% of patients with high
      locus heterogeneity.
    explanation: >-
      The counter-argument: the class does describe a real, quantified, shared
      genetic architecture even if its members are mechanistically diverse.

notes: >-
  Lump/split rationale: this entry is deliberately curated as a single
  class-level Disease rather than as a set of per-gene MRD entries, because
  the shared, clinically actionable content (de novo dominant architecture,
  trio-sequencing diagnostic strategy, static developmental course, absence of
  disease-modifying therapy, recurrence-risk counselling) sits at the class
  level, while gene-specific detail belongs in per-gene entries. If and when
  dismech accumulates enough individual MRD-series Disease entries, this should
  be reconsidered as a Grouping - see the adnsid_entity_boundary discussion.
  The genetic list is illustrative of the two major functional modules, not
  exhaustive: several hundred genes have been implicated, and modelling
  suggests over a thousand remain undescribed.

📚

References & Deep Research

References

2

PMID:26503795

Genetic studies in intellectual disability and related disorders.

No top-level findings curated for this source.

PMID:26748517

Systematic Phenomics Analysis Deconvolutes Genes Mutated in Intellectual Disability into Biologically Coherent Modules.

No top-level findings curated for this source.

Deep Research

1

Falcon ▸

Autosomal Dominant Non-Syndromic Intellectual Disability: Disease Characteristics Report

Edison Scientific Literature 37 citations 2026-07-31T00:25:40.556864

Autosomal Dominant Non-Syndromic Intellectual Disability: Disease Characteristics Report

Executive summary and scope

Autosomal dominant non-syndromic intellectual disability (AD-NSID) is not one uniform disorder. It is an umbrella for genetically heterogeneous neurodevelopmental disorders in which a heterozygous pathogenic variant causes intellectual and adaptive-function impairment as the principal phenotype, without a consistently recognizable congenital-malformation or multisystem syndrome. The preferred umbrella identifier is MONDO:0015802. It should be kept distinct from broader autosomal dominant intellectual disability (MONDO:0100172) and from numbered, gene-specific OMIM/MONDO entities. “Non-syndromic,” “nonspecific,” “isolated,” and “autosomal dominant mental retardation” are historical or alternative terms; however, “non-syndromic” is provisional because deeper phenotyping often identifies speech, behavioral, epileptic, growth, or subtle dysmorphic features. Open Targets currently associates MONDO:0015802 with genes including TLK2, ASH1L, STAG1, CLTC, CAMK2A, CIC, NAA15, TRIP12, KMT5B, TRIO, SET, and DLG4. This is a representative—not exhaustive—list and changes as gene–disease validity is reassessed. (OpenTargets Search: autosomal dominant intellectual disability)

Most evidence is aggregated from disease databases, gene-discovery studies, and mixed DD/ID sequencing cohorts, not from a single disease-specific EHR registry. Consequently, epidemiologic and phenotype frequencies reported for general ID/DD must not be interpreted as exact frequencies for MONDO:0015802.

Domain	Current evidence	Suggested ontology/identifier
Scope / definition	Autosomal dominant non-syndromic intellectual disability is best treated as a heterogeneous umbrella disorder rather than a single uniform entity. Open Targets maps autosomal dominant non-syndromic intellectual disability to MONDO:0015802 and lists representative associated genes including TLK2, ASH1L, STAG1, CLTC, CAMK2A, CIC, NAA15, TRIP12, KMT5B, TRIO, SET, DLG4; broader autosomal dominant ID is separated as MONDO:0100172. NSID denotes ID as the principal/isolated phenotype, though many reported “non-syndromic” cases may later accrue additional neurodevelopmental features (OpenTargets Search: autosomal dominant intellectual disability, maia2021intellectualdisabilitygenomics pages 1-2).	MONDO:0015802; related umbrella: MONDO:0100172
Disease level vs patient level evidence	Available evidence is mainly aggregated from disease/gene discovery studies, sequencing cohorts, and reviews; this is not primarily an EHR-derived single-disease registry. Most current knowledge comes from heterogeneous DD/ID/NDD cohorts in which AD de novo etiologies are overrepresented (li2024triowholeexomesequencing pages 1-2, ko2023genomewidesequencingmodalities pages 2-4, maia2021intellectualdisabilitygenomics pages 1-2).	Evidence type: aggregated disease/gene-level resource
Cardinal phenotype: intellectual disability	ID is defined by impaired intellectual functioning and adaptive behavior; overall ID prevalence is commonly cited at 1–3% worldwide, with GDD/ID together affecting nearly 2% of the pediatric population. Severity can range mild to profound; severe/profound forms are more often monogenic (maia2021intellectualdisabilitygenomics pages 1-2, ko2023genomewidesequencingmodalities pages 1-2, liaci2021neuronalcytoskeletonin pages 1-2).	HPO: Intellectual disability HP:0001249; Global developmental delay HP:0001263
Common associated neurodevelopmental phenotypes	Across DD/ID cohorts and mechanism reviews, common co-phenotypes include developmental delay, speech/language delay, seizures/epilepsy, autism/behavioral abnormalities, microcephaly, and dysmorphic features; these often increase diagnostic yield even when the target label is “non-syndromic” (li2024triowholeexomesequencing pages 1-2, ko2023genomewidesequencingmodalities pages 2-4, wayhelova2024exomesequencingimproves pages 1-2, kim2024diagnosticyieldof pages 1-2).	HPO: Developmental delay HP:0001263; Speech delay HP:0000750; Seizure HP:0001250; Autism HP:0000717; Behavioral abnormality HP:0000708; Microcephaly HP:0000252; Facial dysmorphism HP:0001999
Representative causal genes: synaptic / excitability	Representative AD genes linked in umbrella resources or cohort literature include GRIN1, DLG4, CAMK2A, TRIO, NRXN1, STXBP1, DYRK1A, KIF1A, GNB1; these converge on synapse formation/function, neuronal signaling, and plasticity (OpenTargets Search: autosomal dominant intellectual disability, vas2023regulatorydenovo pages 1-2, ford2023dendriticspineand pages 1-2).	HGNC gene symbols; GO: synapse organization GO:0050808; regulation of synaptic plasticity GO:0048167
Representative causal genes: chromatin / transcription / cohesin	Representative AD genes include ASH1L, KMT5B, STAG1, CIC, TLK2, ARID1B, KANSL1, WDR5 and related chromatin modifiers highlighted as strong ASD/ID susceptibility factors affecting dendritic spine and synapse biology (OpenTargets Search: autosomal dominant intellectual disability, vas2023regulatorydenovo pages 1-2, ford2023dendriticspineand pages 1-2).	HGNC gene symbols; GO: chromatin organization GO:0006325; regulation of transcription by RNA polymerase II GO:0006357
Representative causal genes: cytoskeleton / trafficking / neuronal morphogenesis	Reviews emphasize convergence of many ID genes on cytoskeleton dynamics and Rho GTPase-regulated neuronal migration, neuritogenesis, and synaptic plasticity; examples include TRIO, KIF1A, CLTC and Rho-pathway-related genes in broader ID biology (liaci2021neuronalcytoskeletonin pages 1-2).	GO: actin cytoskeleton organization GO:0030036; microtubule cytoskeleton organization GO:0000226; neuron projection development GO:0031175
Variant classes / molecular lesion types	Relevant pathogenic mechanisms include coding missense, nonsense, frameshift, splice, CNVs, small structural variants, and increasingly regulatory de novo mutations in fetal-brain enhancers. Vas et al. showed regulatory DNMs enriched in fetal brain-specific enhancers, with recurrent enhancer clusters linked to CSMD1, OLFM1, POU3F3 (vas2023regulatorydenovo pages 1-2, kim2024diagnosticyieldof pages 1-2).	Variant classes per ACMG/AMP; regulatory regions / enhancers
Inheritance / de novo contribution	AD ID is often caused by de novo variants, especially in moderate/severe disease. In a Finnish ID cohort, 75% of variants in known ID genes were de novo/suspected de novo, including 64% autosomal dominant and 11% X-linked; only 25% were inherited (jarvela2021exomesequencingreveals pages 1-2). Trio-WES in unexplained DD/ID also concluded that de novo variants in AD genes are major contributors, especially in non-consanguineous families (li2024triowholeexomesequencing pages 1-2).	Inheritance: autosomal dominant; de novo germline origin
Mosaicism / recurrence risk	Apparent de novo AD findings do not guarantee negligible recurrence risk. In a trio genome-wide study of 500 families, 12 cases of apparent mosaicism were identified, including 9 parental and 3 proband mosaic cases; empirical recurrence for a child with a de novo dominant condition is often cited as ~1%, but parental gonadal mosaicism can raise risk substantially (cook2021somaticmosaicismdetected pages 1-2).	Counseling concept: parental/gonadal mosaicism; somatic mosaicism
Anatomy / organ systems	Primary affected organ/system is the central nervous system, especially cortical and hippocampal circuits supporting cognition, learning, and adaptive behavior. Reviews also note involvement of excitation/inhibition balance and, in some genetic subgroups, broader multisystem findings despite a nominally nonsyndromic label (liaci2021neuronalcytoskeletonin pages 1-2, ford2023dendriticspineand pages 1-2).	UBERON: brain UBERON:0000955; cerebral cortex UBERON:0000956; hippocampus UBERON:0002421
Cells / subcellular processes	Mechanistic evidence highlights neurons, especially pyramidal neurons, dendrites, dendritic spines, and synapses; abnormal neuronal migration, neuritogenesis, and synaptic plasticity are recurrent themes. Subcellular emphasis includes chromatin, actin/microtubule cytoskeleton, and synaptic compartments (liaci2021neuronalcytoskeletonin pages 1-2, ford2023dendriticspineand pages 1-2).	CL: neuron CL:0000540; pyramidal neuron CL:0000598; GO CC: dendrite GO:0030425; dendritic spine GO:0043197; synapse GO:0045202; nucleus GO:0005634
Pathophysiology / causal chain	Upstream lesions include de novo coding or regulatory variants affecting chromatin regulation, transcription, enhancer activity, cytoskeleton dynamics, trafficking, and synaptic proteins. Downstream consequences include altered neuronal migration/neuritogenesis, abnormal dendritic spine and synapse morphogenesis, impaired network formation, excitation/inhibition imbalance, and finally impaired cognition/adaptive behavior (liaci2021neuronalcytoskeletonin pages 1-2, vas2023regulatorydenovo pages 1-2, ford2023dendriticspineand pages 1-2).	GO: neuron migration GO:0001764; axonogenesis GO:0007409; dendrite morphogenesis GO:0048813; learning or memory GO:0007611
Diagnostic workflow	Current evidence supports trio exome sequencing as first-tier for unexplained DD/ID/NDD, with genome sequencing used early or after nondiagnostic CMA/exome, particularly where structural, intronic, or regulatory variants are suspected. Standard pre-genomic workup often includes history/exam, CMA, Fragile X or targeted testing when indicated, and phenotype-guided metabolic testing (ko2023genomewidesequencingmodalities pages 1-2, wayhelova2024exomesequencingimproves pages 1-2, wigby2024evidencereviewand pages 1-2, kim2024diagnosticyieldof pages 1-2).	ACMG-aligned genome-wide testing strategy
Recent diagnostic yield data (2023–2024 priority)	Recent cohorts report: trio-WES 49.7% (86/173) in unexplained DD/ID; syndromic 57.8% vs non-syndromic 47.2% (li2024triowholeexomesequencing pages 1-2). Trio-ES in severe pediatric NDDs 48.9% (44/90), with 53.2% of causative variants novel (wayhelova2024exomesequencingimproves pages 1-2). Singleton WES in undiagnosed rare pediatric disease 43% (25/58) and clinical utility/actionability 76% (slaba2024diagnosticefficacyand pages 1-2). Trio-WGS after prior chromosome analysis/CMA/CES gave an additional 19.2% (10/52) diagnoses, 8/10 due to AD de novo variants (kim2024diagnosticyieldof pages 1-2). First-line GS review found mean diagnostic yield 45% for first-line GS and management changes 24–100% depending on cohort (wigby2024evidencereviewand pages 1-2).	Diagnostic modalities: WES, WGS, CMA
Real-world implementation	Real-world implementation now centers on clinical genetics and pediatric neurology pathways using trio-based ES/GS, ACMG-guided variant classification, and phenotype-driven reanalysis. ES/GS findings influence surveillance, reproductive counseling, and personalized management even when no disease-specific drug exists (wayhelova2024exomesequencingimproves pages 1-2, wigby2024evidencereviewand pages 1-2, slaba2024diagnosticefficacyand pages 1-2).	ACMG/AMP variant classification framework
Management / treatment	There is no single disease-specific pharmacotherapy for the umbrella disorder. Care is supportive and phenotype-directed: developmental therapies, speech/language therapy, educational support, behavioral management, seizure treatment when present, surveillance based on the molecular diagnosis, and genetic counseling. Reviews note that no pharmacological therapies are currently available for ID broadly, although pathway-based therapeutic opportunities are being explored (liaci2021neuronalcytoskeletonin pages 1-2, slaba2024diagnosticefficacyand pages 1-2).	MAXO: genetic counseling; developmental therapy; speech therapy; educational intervention; seizure management
Prevention / family planning	Primary prevention is generally not available for de novo monogenic disease; practical prevention focuses on genetic counseling, discussion of recurrence including mosaicism, and reproductive options after molecular diagnosis. Earlier genomic diagnosis can shorten the diagnostic odyssey and inform family planning (kim2024diagnosticyieldof pages 1-2, slaba2024diagnosticefficacyand pages 1-2, cook2021somaticmosaicismdetected pages 1-2).	MAXO: genetic counseling; reproductive counseling
Model systems / mechanistic platforms	Evidence includes animal and neuronal model studies summarized in reviews, especially mouse models of chromatin modifiers (ARID1B, KANSL1, WDR5, ZMIZ1) and systems-biology approaches emphasizing convergent neuronal network defects. These models are useful for studying dendritic spine, synapse, and developmental circuitry abnormalities (liaci2021neuronalcytoskeletonin pages 1-2, ford2023dendriticspineand pages 1-2).	Model categories: mouse models; neuronal cultures; systems biology
Evidence limitations	Disease boundaries are porous: “non-syndromic” cases may later show additional features; many data come from mixed DD/ID/NDD cohorts rather than pure MONDO:0015802 cohorts; gene lists change rapidly; penetrance/expressivity are often incompletely quantified; and few disorder-specific natural history or treatment trials exist (maia2021intellectualdisabilitygenomics pages 1-2, wayhelova2024exomesequencingimproves pages 1-2, slaba2024diagnosticefficacyand pages 1-2).	Curation note: maintain umbrella-vs-gene-specific distinction

Table: This table compacts the most actionable current evidence for autosomal dominant non-syndromic intellectual disability into a knowledge-base-ready format. It emphasizes scope, representative mechanisms and genes, ontology suggestions, modern diagnostic practice, and key evidence limitations.

1. Disease information

ID is defined by significant limitations in both intellectual functioning and adaptive behavior—conceptual, social, and practical skills—with onset during the developmental period. Global developmental delay (GDD) is used in young children, generally under five years, when standardized intellectual assessment is not yet reliable. The older IQ-only definition is insufficient; adaptive impairment is essential. ID overall affects approximately 1–3% of the population, while GDD/ID together affect nearly 2% of children. These figures cover all etiologies, not specifically AD-NSID. Mild ID represents approximately 85% of ID, moderate 10%, severe 3.5%, and profound 1.5% in commonly cited aggregate classifications. (maia2021intellectualdisabilitygenomics pages 1-2, ko2023genomewidesequencingmodalities pages 1-2, liaci2021neuronalcytoskeletonin pages 1-2)

Key identifiers and terminology

MONDO: MONDO:0015802, autosomal dominant non-syndromic intellectual disability.
Related MONDO: MONDO:0100172, intellectual disability, autosomal dominant.
MeSH: Intellectual Disability is the applicable broad concept; there is no uniquely specific MeSH descriptor for the AD-NSID umbrella.
ICD-10: F70–F79 classify intellectual disabilities by severity; they do not encode AD-NSID etiology.
ICD-11: 6A00, disorders of intellectual development, with severity extensions; again, inheritance is not encoded.
OMIM/Orphanet: individual gene-defined disorders generally have separate entries; no single OMIM number adequately represents the full umbrella.
Synonyms: autosomal dominant nonsyndromic ID; autosomal dominant non-syndromic mental retardation; isolated autosomal dominant intellectual disability; nonspecific autosomal dominant ID.

2. Etiology, risk, protection, and gene–environment interaction

Genetic causes

The defining cause is a germline heterozygous pathogenic variant. Moderate-to-severe cases are frequently de novo; familial transmission is more readily observed in mild disease because affected adults may reproduce. A 2024 review estimated that more than 400 genes have been associated with autosomal-dominant ID, while broader ID databases contain well over 1,500 established genes. (hou2024a10yearreview pages 5-7, wayhelova2024exomesequencingimproves pages 1-2)

Variant classes include missense, nonsense, frameshift, splice-altering and in-frame variants; exon-level or larger CNVs; balanced/complex structural rearrangements; and regulatory variants. Mechanisms differ by gene and allele: haploinsufficiency/loss of function, dominant-negative effects, and gain of function are all possible. Therefore, a gene cannot be assigned one universal molecular mechanism without allele-specific evidence.

The Finnish exome study of 39 ID families found pathogenic/likely pathogenic variants in 25/39 (64%). Among variants in known ID genes, 75% were de novo or suspected de novo—64% autosomal dominant and 11% X-linked—whereas 25% were inherited. This mixed syndromic/non-syndromic cohort supports the dominant de novo contribution but is not a prevalence study of AD-NSID. The abstract states: “de novo variants were the most common variants underlying ID in the studied Finnish population.” Published March 12, 2021; DOI: https://doi.org/10.1007/s00439-021-02268-1. (jarvela2021exomesequencingreveals pages 1-2)

Regulatory and chromosomal causes

De Vas et al. sequenced 21 ID trios and analyzed another 30 exome-negative probands. Regulatory de novo variants were enriched in fetal-brain-specific enhancers, including recurrent enhancer clusters linked to CSMD1, OLFM1, and POU3F3. Luciferase assays showed allele-specific activity for most tested variants, and CRISPR mutation/epigenome editing altered putative target-gene expression. Their exact conclusion was that these results “provide new evidence to indicate that DNMs in fetal brain-specific enhancers play an essential role in the aetiology of ID.” This is primary genomic, in-vitro reporter, and CRISPR functional evidence, but the small cohort does not establish clinical penetrance for each regulatory variant. Published February 28, 2023; DOI: https://doi.org/10.26508/lsa.202201843. (vas2023regulatorydenovo pages 1-2)

Large CNVs explain approximately 10–14% of ID overall, mostly de novo, but many produce syndromic phenotypes and should not automatically be labeled AD-NSID. (maia2021intellectualdisabilitygenomics pages 1-2)

Environmental, infectious, and lifestyle factors

No toxin, pathogen, diet, exercise pattern, smoking exposure, or occupational factor is established as a cause of a genetically confirmed AD-NSID disorder. Prenatal infection, hypoxic–ischemic injury, prematurity complications, neonatal meningitis, toxins, severe malnutrition, and other acquired insults are important alternative or additive causes of DD/ID, not causes of Mendelian inheritance. They may worsen developmental outcome in an affected child, but quantitative AD-NSID-specific gene–environment interaction data are unavailable. (liaci2021neuronalcytoskeletonin pages 1-2)

No validated genetic or environmental protective factor prevents expression after a highly penetrant pathogenic variant. Favorable modifiers may exist, but none is sufficiently validated for clinical annotation. Early enriched education, communication support, and rehabilitation improve function rather than biologically preventing the genotype.

3. Phenotypes and quality-of-life effects

The cardinal phenotype is developmental-onset impairment of cognition and adaptive functioning. Severity ranges from mild learning disability to profound dependence. Onset is congenital/developmental but recognition usually occurs in infancy or childhood through delayed milestones, speech delay, school difficulty, or impaired adaptive skills. The disorder is generally chronic and lifelong, not relapsing-remitting. Skills may continue to develop slowly; true neurodegeneration or loss of milestones should trigger evaluation for a different or additional diagnosis.

Phenotype	Typical characteristics	Suggested HPO term
Intellectual disability	Universal by definition; mild to profound; lifelong	HP:0001249
Global developmental delay	Common presenting label in children under five	HP:0001263
Delayed speech/language	Frequent across AD-ID genes; severity variable	HP:0000750
Motor developmental delay	Variable; often less severe than language/cognitive delay	HP:0001270
Impaired adaptive functioning	Conceptual, social, and practical limitations	HP:0011446, Abnormality of higher mental function, plus domain-specific terms
Behavioral abnormality	Autism, attention problems, anxiety, aggression, or stereotypy may occur but are not defining	HP:0000708; HP:0000717 for autism
Seizures/epilepsy	Gene-dependent rather than universal; may shift classification toward a syndromic/developmental epileptic encephalopathy	HP:0001250
Hypotonia	Variable, often early childhood	HP:0001252
Microcephaly or macrocephaly	Gene-dependent; not required	HP:0000252 / HP:0000256
Subtle facial dysmorphism	May emerge with systematic examination and challenges a strict “non-syndromic” designation	HP:0001999

Quality-of-life impact is dominated by communication limitations, educational needs, reduced independence, social participation barriers, caregiver burden, and need for lifelong support. Gene-specific QoL, EQ-5D, SF-36, survival, and phenotype-frequency datasets are generally absent. A 2024 consensus analysis found that phenotype descriptions in novel Mendelian disorder reports were superficial or deficient in more than 87% of publications across prioritized domains, illustrating why exact frequencies, adulthood outcomes, sleep, pain, and QoL remain uncertain.

4. Genetic and molecular information

Representative gene architecture

The following mechanistic groups are useful for curation; they are not an exhaustive diagnostic panel:

Chromatin, transcription, and cohesin: ASH1L, KMT5B, STAG1, CIC, SET, TLK2, NAA15, TRIP12; broader AD-ID genes include ARID1B and other chromatin regulators.
Synapse and excitability: CAMK2A, DLG4, GRIN1, GNB1 and gene-specific disorders involving STXBP1 or NRXN1.
Rho-GTPase/cytoskeletal development: TRIO and related neuronal morphogenesis pathways.
Vesicle trafficking/transport: CLTC, KIF1A, DYNC1H1.

Open Targets supports the association of the first set with MONDO:0015802 but does not, by itself, establish allele-level pathogenicity. Each variant requires ClinGen/ClinVar review, ACMG/AMP classification, segregation analysis, population frequency evaluation, and mechanism compatibility. (OpenTargets Search: autosomal dominant intellectual disability)

Pathogenic variants and classification

Pathogenic/likely pathogenic variants are ordinarily absent or extremely rare in population databases such as gnomAD, although gene-specific reduced penetrance can complicate interpretation. A 2024 trio-WES study filtered variants at minor allele frequency ≤0.01, but the threshold for a severe de novo dominant disorder is usually much lower and should be set using disease prevalence, penetrance, and allelic heterogeneity. A VUS does not establish diagnosis and must not guide predictive testing or irreversible reproductive decisions. (li2024triowholeexomesequencing pages 2-3)

Most causal variants are constitutional germline variants. Postzygotic mosaicism in the proband can attenuate severity; parental somatic/gonadal mosaicism can make an apparently de novo variant recurrent. In 500 genome-wide-sequenced families, Cook et al. identified 12 apparent mosaic cases: nine parental and three proband; six were not recognized as mosaic by Sanger sequencing. The authors concluded that genome-wide detection “can permit more accurate genetic counseling.” Published 2021; DOI: https://doi.org/10.1101/mcs.a006125. (cook2021somaticmosaicismdetected pages 1-2)

No consistently validated modifier gene, protective allele, or umbrella-specific episignature exists. Some individual chromatin disorders have blood DNA-methylation signatures, but those results should be recorded under the specific gene-defined disorder rather than generalized to AD-NSID.

5. Environmental information

Environmental exposures, lifestyle factors, and infectious agents are not primary etiologies once a causal AD variant is established. Their clinical relevance is principally differential diagnosis, prevention of additional brain injury, and optimization of general health. There is no AD-NSID-specific evidence that smoking, alcohol use by the affected person, diet, or exercise changes penetrance. Prenatal alcohol or teratogen exposure may create a blended phenotype. Routine immunization is appropriate but does not prevent the genetic disorder.

6. Mechanism and pathophysiology

Integrated causal chain

Upstream genetic lesion: a coding, splice, CNV, structural, or fetal-brain regulatory variant alters dosage or function of an AD-ID gene.
Molecular dysregulation: disturbed chromatin remodeling/transcription, enhancer activity, RNA/protein regulation, Rho-GTPase signaling, microtubule/actin dynamics, vesicle transport, glutamatergic signaling, or postsynaptic scaffolding.
Cellular effects: altered neural progenitor programs, neuronal migration, neurite extension, dendritic arborization, spine morphogenesis, synapse formation and plasticity.
Circuit effects: inefficient cortical/hippocampal connectivity and, in some genes, excitation–inhibition imbalance.
Clinical manifestation: impaired learning, memory, language, executive function, and adaptive behavior.

The cytoskeleton review describes this hierarchy from molecular defect to “cell compartment and functions, circuits, cognition, and behavior.” It identifies cytoskeletal disruption affecting neuronal migration, neuritogenesis, and synaptic plasticity, and notes convergence on Rho-GTPase signaling. Published June 7, 2021; DOI: https://doi.org/10.3390/ijms22116167. (liaci2021neuronalcytoskeletonin pages 1-2)

Chromatin modifiers can also act upstream of synaptic pathology. Animal models and human postmortem evidence show abnormal dendritic-spine/synapse morphogenesis and plasticity; ARID1B, KANSL1, and WDR5 are highlighted examples. This evidence is mechanistically informative but is not specific to every MONDO:0015802 gene. Published January 19, 2023; DOI: https://doi.org/10.3389/fnmol.2022.1048713. (ford2023dendriticspineand pages 1-2)

Suggested GO biological-process terms: chromatin organization (GO:0006325); regulation of transcription by RNA polymerase II (GO:0006357); nervous system development (GO:0007399); neuron migration (GO:0001764); neuron projection development (GO:0031175); axonogenesis (GO:0007409); dendrite morphogenesis (GO:0048813); synapse organization (GO:0050808); regulation of synaptic plasticity (GO:0048167); actin cytoskeleton organization (GO:0030036); learning or memory (GO:0007611).

Suggested cell types: neuron (CL:0000540), pyramidal neuron (CL:0000598), excitatory neuron and inhibitory interneuron where gene-specific evidence exists, neural stem/progenitor cell for prenatal chromatin/enhancer mechanisms. No immune-cell or inflammatory mechanism is consistently implicated in the umbrella disorder.

No reproducible umbrella-level metabolomic, lipidomic, proteomic, or circulating biomarker signature exists. WGS plus regulatory epigenomics and functional CRISPR assays represent the most relevant recent multi-omic advance. (vas2023regulatorydenovo pages 1-2)

7. Anatomical structures affected

The central nervous system is primary, especially cerebral cortex and hippocampal networks involved in cognition and memory. Cerebellar involvement may occur in individual genotypes. Peripheral organs are not obligatorily involved; consistent congenital or multisystem abnormalities should prompt a gene-specific syndromic diagnosis.

Suggested annotations are brain (UBERON:0000955), cerebral cortex (UBERON:0000956), hippocampus (UBERON:0002421), nervous tissue, neuron (CL:0000540), pyramidal neuron (CL:0000598), nucleus (GO:0005634), chromatin, cytoskeleton, dendrite (GO:0030425), dendritic spine (GO:0043197), and synapse (GO:0045202). Effects are bilateral and network-level; there is no characteristic lateralization. (liaci2021neuronalcytoskeletonin pages 1-2, ford2023dendriticspineand pages 1-2)

8. Temporal development

The biological lesion is present from conception or arises early postzygotically. Vulnerability is greatest during prenatal neurogenesis, neuronal migration, circuit assembly, and early-childhood synaptic maturation. Clinical onset is insidious and developmental rather than acute. The course is usually stable/nondegenerative but lifelong; developmental gains occur at a slower rate and may plateau. There are no validated early/intermediate/end-stage categories and no expected remission. Early childhood is the principal intervention window because communication systems, adaptive skills, and educational supports can be introduced during maximal developmental plasticity.

9. Inheritance and population

Inheritance is autosomal dominant. An affected heterozygous parent generally has a 50% transmission probability per pregnancy, although expression may vary. Many moderate/severe cases are de novo, so family history is often negative. In the 2024 trio-WES cohort, 95.4% had no family history of epilepsy or DD/ID, emphasizing that a negative pedigree does not exclude dominant disease. (li2024triowholeexomesequencing pages 2-3)

Penetrance is high for many severe de novo loss-of-function disorders but gene- and allele-specific. Expressivity can vary widely. Anticipation is not a general feature; repeat-expansion disorders belong in the differential rather than the core AD-NSID category. Consanguinity is not a risk factor for AD-NSID, though it raises the probability of an alternative recessive diagnosis. Founder effects and carrier frequencies are variant-specific; there is no meaningful aggregate carrier frequency because affected heterozygotes are not asymptomatic “carriers.”

For a confirmed de novo variant absent from parental blood, counseling commonly starts with an empirical recurrence risk near 1%, reflecting possible gonadal mosaicism. If a parent is mosaic, recurrence can be substantially higher and depends on germ-cell involvement; Cook et al. explain a theoretical range from approximately 1–2% to as high as 50%. Deep sequencing of multiple parental tissues or paternal sperm can refine risk in selected families. (cook2021somaticmosaicismdetected pages 1-2)

There is no reliable incidence or prevalence specifically for MONDO:0015802, no established ethnic concentration, geographic endemicity, or robust sex ratio. Autosomal inheritance predicts broadly similar biological risk in males and females; ascertainment may differ because males are more often referred for neurodevelopmental assessment.

10. Diagnostics

Clinical assessment

Assessment should include prenatal/perinatal and three-generation history; growth and dysmorphology examination; formal developmental, cognitive, language, and adaptive testing; hearing and vision evaluation; neurologic examination; and autism/behavioral screening. EEG is indicated for seizures or suspicious episodes, MRI for abnormal neurologic examination, regression, focal signs, abnormal head growth, or a genotype-specific indication. Routine biopsy is not useful. Metabolic testing should be phenotype-directed, particularly for regression, episodic decompensation, organomegaly, movement disorder, or unusual biochemical findings.

Current genetic-testing algorithm

Trio ES or GS early/first tier, with CNV calling and ACMG/AMP interpretation.
If ES is used, ensure adequate CNV detection; add CMA where platform/payer practice requires it or CNV sensitivity is insufficient.
Use Fragile X testing, repeat-expansion assays, methylation testing, mitochondrial testing, or single-gene testing when clinically indicated because standard ES may miss these mechanisms.
For nondiagnostic ES, perform periodic reanalysis, then GS to detect structural, deep-intronic, regulatory, repeat, low-level mosaic, or poorly covered variants.
RNA sequencing or methylation studies may resolve splice variants or gene-specific episignatures, but are not universal diagnostic tests.

Recent implementation data are strong:

Li et al., November 2024: trio-WES diagnosed 86/173 (49.7%) unexplained DD/ID cases. SNVs accounted for 54/173 (31.2%) and CNVs for 36/173 (20.8%); 22 SNVs were novel. Yield was 57.8% in syndromic versus 47.2% in non-syndromic DD/ID. Exact abstract conclusion: “De novo variants in autosomal dominant genes are significant contributors to DD/ID.” DOI: https://doi.org/10.1038/s41598-024-79431-x. (li2024triowholeexomesequencing pages 1-2)
Wayhelova et al., February 2024: trio-based ES yielded 44/90 (48.9%) diagnoses in severe pediatric NDD/MCA; 25/47 (53.2%) causative variants were novel, and ES detected intragenic CNVs through a 6-Mb duplication. DOI: https://doi.org/10.1186/s13023-024-03056-6. (wayhelova2024exomesequencingimproves pages 1-2)
Kim et al., August 2, 2024: after chromosome analysis, CMA, and clinical exome were nondiagnostic, trio-WGS solved an additional 10/52 (19.2%); 8/10 were AD de novo diagnoses. Reasons included small structural variants, uncovered/new genes, low variant allele fraction, coverage, and interpretation. DOI: https://doi.org/10.3390/diagnostics14151680. (kim2024diagnosticyieldof pages 1-2)
Wigby et al., February 2024: across 71 GS studies and more than 13,000 patients, unweighted mean yield was 45% for first-line GS, 33% after prior testing, and 33% in exome-negative cohorts; reported management changes ranged 24–100%. DOI: https://doi.org/10.1038/s41525-024-00396-x. (wigby2024evidencereviewand pages 1-2)
Slaba et al., November 2024: singleton WES diagnosed 25/58 (43%) undiagnosed pediatric patients; 76% of diagnosed cases had at least one management change. DOI: https://doi.org/10.1038/s41598-024-79872-4. (slaba2024diagnosticefficacyand pages 1-2)

Differential diagnosis includes syndromic AD-ID, X-linked or recessive ID, chromosomal disorders, Fragile X, imprinting disorders, developmental epileptic encephalopathy, autism without ID, cerebral palsy/perinatal injury, fetal alcohol spectrum disorder, congenital infection, metabolic disease, hypothyroidism, hearing/vision impairment, and neurodegenerative disorders. Clinical phenotype alone often cannot reliably distinguish them.

Population newborn screening is not available. After finding a familial variant, cascade testing, prenatal diagnosis, and preimplantation genetic testing are technically possible. Predictive testing of asymptomatic minors requires careful consideration because onset is developmental and penetrance may be allele-specific.

11. Outcome and prognosis

No umbrella-specific 5-year survival, life expectancy, or disease-specific mortality rate is available. In genuinely non-syndromic ID without epilepsy, severe motor impairment, swallowing dysfunction, or organ disease, life expectancy may approach that of the general population, but this cannot be assumed for every genotype. Morbidity is chiefly lifelong cognitive, communication, adaptive, educational, vocational, and social disability.

Major prognostic factors are severity of early developmental impairment, language acquisition, epilepsy, autism/behavioral comorbidity, motor and feeding ability, access to communication and educational services, and the specific molecular diagnosis. There is no validated molecular prognostic biomarker across AD-NSID. Recovery to fully typical cognition is uncommon, but meaningful functional gains are expected with individualized support. Regression, new neurologic signs, or unexpectedly rapid deterioration warrants reassessment for seizures, medication effects, psychiatric illness, sleep disorder, or an alternative/blended diagnosis.

12. Treatment and current applications

There is no approved disease-modifying pharmacotherapy, gene therapy, RNA therapy, cell therapy, immunotherapy, or surgery for the MONDO:0015802 umbrella. The 2021 mechanistic review states that “no pharmacological therapies are currently available” for ID broadly. Molecular diagnosis nevertheless changes surveillance, avoids unnecessary testing, informs recurrence counseling, and occasionally identifies a genotype-specific comorbidity or treatment. (liaci2021neuronalcytoskeletonin pages 1-2, slaba2024diagnosticefficacyand pages 1-2)

Standard multidisciplinary management

Individualized early developmental intervention, special education, and behavioral supports.
Speech-language therapy and augmentative/alternative communication.
Occupational and physical therapy for adaptive, sensory, fine-motor, gross-motor, or hypotonia-related needs.
Standard evidence-based treatment for epilepsy, ADHD, anxiety, sleep disturbance, constipation, feeding problems, and other comorbidities.
Hearing, vision, dental, nutritional, sleep, and safeguarding assessment.
Transition planning, supported decision-making, vocational support, respite, and caregiver mental-health support.

Suggested MAXO annotations include genetic counseling, developmental assessment, neuropsychological assessment, speech-language therapy, occupational therapy, physical therapy, educational intervention, augmentative communication, EEG, brain MRI, genomic sequencing, and seizure management. Precise MAXO IDs should be verified against the current ontology release before database ingestion.

No clinical trial retrieved was a disease-modifying trial specifically for AD-NSID. Relevant ID-wide studies largely concern behavioral, communication, lifestyle, or comorbidity interventions—for example written-language intervention (NCT05851937) and metformin for antipsychotic-associated weight gain (NCT05744479)—and should not be represented as treatments for the underlying genetic disease.

Research directions include restoring gene dosage, allele-specific silencing for gain-of-function alleles, ASOs, CRISPR-based correction, and pathway modulation of chromatin, Rho-GTPase/cytoskeletal, or synaptic defects. These remain gene- and variant-specific, largely preclinical concepts; the same intervention could be harmful if applied across opposite loss- and gain-of-function mechanisms.

13. Prevention

Primary prevention by lifestyle change or vaccination is not applicable. Prevention is principally reproductive and complication-focused:

Preconception and postdiagnostic genetic counseling.
Parental testing with consideration of low-level mosaicism.
Prenatal testing by CVS/amniocentesis or PGT-M when a familial pathogenic variant is known.
Early developmental surveillance of at-risk children.
Prevention of secondary harm through seizure control, communication support, safe feeding, sleep care, sensory screening, vaccination, and avoidance of additional neurotoxic exposures.

Noninvasive prenatal screening does not comprehensively detect heterogeneous single-gene AD-NSID. Prenatal diagnosis should not be based on a VUS.

14. Other species and natural disease

There is no recognized naturally occurring veterinary disease that is directly equivalent to the heterogeneous human AD-NSID umbrella. Orthologous genes are widely conserved in mammals, zebrafish, Drosophila, and C. elegans, and variants may cause learning, behavior, synaptic, or developmental phenotypes. These are comparative models rather than zoonotic disease. There is no infectious transmission, cross-species transmission, or zoonotic potential. Gene-, species-, NCBI Taxon-, NCBI Gene-, and VBO-level entries should be curated separately for each causal gene/model.

15. Model organisms

Available systems include constitutive or conditional knockout/heterozygous mice, variant knock-in mice, zebrafish, Drosophila, cultured primary neurons, patient-derived iPSCs, induced neurons, and cerebral organoids. Chromatin-modifier models involving ARID1B, KANSL1, WDR5, and related genes reproduce aspects of dendritic arborization, spine morphology, synaptic signaling, learning, or behavior. Cytoskeletal models test neuronal migration, neuritogenesis, Rho-GTPase signaling, and network plasticity. Regulatory-variant studies combine human WGS with luciferase assays and CRISPR epigenome editing. (liaci2021neuronalcytoskeletonin pages 1-2, vas2023regulatorydenovo pages 1-2, ford2023dendriticspineand pages 1-2)

Model limitations are substantial: human adaptive behavior cannot be directly modeled; heterozygous null animals may not mimic a human dominant-negative or gain-of-function allele; developmental timing and cortical architecture differ across species; and behavioral assays have limited construct validity. The preferred model is therefore variant-specific and should demonstrate directionally correct molecular dysfunction before therapeutic testing.

Evidence-quality conclusions

The strongest current evidence concerns the high contribution of de novo AD variants, the diagnostic effectiveness of trio ES/GS, and convergence on prenatal gene regulation, cytoskeletal development, and synaptic function. The weakest areas are disease-specific epidemiology, phenotype frequencies, adult natural history, QoL, penetrance, modifier genes, and treatment outcomes. A knowledge base should consequently retain MONDO:0015802 as an umbrella, attach variants and mechanisms to the relevant gene-specific disease whenever possible, and label statistics derived from mixed DD/ID cohorts as indirect rather than AD-NSID-specific evidence.

References

(OpenTargets Search: autosomal dominant intellectual disability): Open Targets Query (autosomal dominant intellectual disability, 32 results). Buniello, A. et al. (2025). Open Targets Platform: facilitating therapeutic hypotheses building in drug discovery. Nucleic Acids Research.
(maia2021intellectualdisabilitygenomics pages 1-2): Nuno Maia, Maria João Nabais Sá, Manuel Melo-Pires, Arjan P. M. de Brouwer, and Paula Jorge. Intellectual disability genomics: current state, pitfalls and future challenges. BMC Genomics, Dec 2021. URL: https://doi.org/10.1186/s12864-021-08227-4, doi:10.1186/s12864-021-08227-4. This article has 125 citations and is from a peer-reviewed journal.
(li2024triowholeexomesequencing pages 1-2): Chengyan Li, You Wang, Cizheng Zeng, Binglong Huang, Yinhui Chen, Chupeng Xue, Ling Liu, Shiwen Rong, and Yongwen Lin. Trio-whole exome sequencing reveals the importance of de novo variants in children with intellectual disability and developmental delay. Scientific Reports, Nov 2024. URL: https://doi.org/10.1038/s41598-024-79431-x, doi:10.1038/s41598-024-79431-x. This article has 22 citations and is from a peer-reviewed journal.
(ko2023genomewidesequencingmodalities pages 2-4): Mary Hsin-Ju Ko and Hui-Ju Chen. Genome-wide sequencing modalities for children with unexplained global developmental delay and intellectual disabilities—a narrative review. Mar 2023. URL: https://doi.org/10.3390/children10030501, doi:10.3390/children10030501. This article has 14 citations.
(ko2023genomewidesequencingmodalities pages 1-2): Mary Hsin-Ju Ko and Hui-Ju Chen. Genome-wide sequencing modalities for children with unexplained global developmental delay and intellectual disabilities—a narrative review. Mar 2023. URL: https://doi.org/10.3390/children10030501, doi:10.3390/children10030501. This article has 14 citations.
(liaci2021neuronalcytoskeletonin pages 1-2): Carla Liaci, Mattia Camera, Giovanni Caslini, Simona Rando, Salvatore Contino, Valentino Romano, and G. Merlo. Neuronal cytoskeleton in intellectual disability: from systems biology and modeling to therapeutic opportunities. International Journal of Molecular Sciences, Jun 2021. URL: https://doi.org/10.3390/ijms22116167, doi:10.3390/ijms22116167. This article has 36 citations.
(wayhelova2024exomesequencingimproves pages 1-2): Marketa Wayhelova, Vladimira Vallova, Petr Broz, Aneta Mikulasova, Jan Smetana, Hana Dynkova Filkova, Dominika Machackova, Kristina Handzusova, Renata Gaillyova, and Petr Kuglik. Exome sequencing improves the molecular diagnostics of paediatric unexplained neurodevelopmental disorders. Orphanet Journal of Rare Diseases, Feb 2024. URL: https://doi.org/10.1186/s13023-024-03056-6, doi:10.1186/s13023-024-03056-6. This article has 18 citations and is from a peer-reviewed journal.
(kim2024diagnosticyieldof pages 1-2): Jaewon Kim, Jaewoong Lee, Myungshin Kim, and Dae-Hyun Jang. Diagnostic yield of trio whole-genome sequencing in children with undiagnosed developmental delay or congenital anomaly: a prospective cohort study. Diagnostics, 14:1680, Aug 2024. URL: https://doi.org/10.3390/diagnostics14151680, doi:10.3390/diagnostics14151680. This article has 6 citations.
(vas2023regulatorydenovo pages 1-2): Matias G De Vas, Fanny Boulet, Shweta S Joshi, Myles G Garstang, Tahir N Khan, Goutham Atla, David Parry, David Moore, Inês Cebola, Shuchen Zhang, Wei Cui, Anne K Lampe, Wayne W Lam, Jorge Ferrer, Madapura M Pradeepa, and Santosh S Atanur. Regulatory de novo mutations underlying intellectual disability. Life Science Alliance, 6:e202201843, Feb 2023. URL: https://doi.org/10.26508/lsa.202201843, doi:10.26508/lsa.202201843. This article has 7 citations and is from a peer-reviewed journal.
(ford2023dendriticspineand pages 1-2): Thomas James L. Ford, Byeong Tak Jeon, Hyunkyoung Lee, and Woo-Yang Kim. Dendritic spine and synapse pathology in chromatin modifier-associated autism spectrum disorders and intellectual disability. Frontiers in Molecular Neuroscience, Jan 2023. URL: https://doi.org/10.3389/fnmol.2022.1048713, doi:10.3389/fnmol.2022.1048713. This article has 30 citations.
(jarvela2021exomesequencingreveals pages 1-2): Irma Järvelä, Tuomo Määttä, Anushree Acharya, Juha Leppälä, Shalini N. Jhangiani, Maria Arvio, Auli Siren, Minna Kankuri-Tammilehto, Hannaleena Kokkonen, Maarit Palomäki, Teppo Varilo, Mary Fang, Trevor D. Hadley, Angad Jolly, Tarja Linnankivi, Ritva Paetau, Anni Saarela, Reetta Kälviäinen, Jan Olme, Liz M. Nouel-Saied, Diana M. Cornejo-Sanchez, Lorida Llaci, James R. Lupski, Jennifer E. Posey, Suzanne M. Leal, and Isabelle Schrauwen. Exome sequencing reveals predominantly de novo variants in disorders with intellectual disability (id) in the founder population of finland. Human Genetics, 140:1011-1029, Mar 2021. URL: https://doi.org/10.1007/s00439-021-02268-1, doi:10.1007/s00439-021-02268-1. This article has 55 citations and is from a peer-reviewed journal.
(cook2021somaticmosaicismdetected pages 1-2): Courtney B. Cook, Linlea Armstrong, Cornelius F. Boerkoel, Lorne A. Clarke, Christèle du Souich, Michelle K. Demos, William T. Gibson, Harinder Gill, Elena Lopez, Millan S. Patel, Kathryn Selby, Ziad Abu-Sharar, Alison M. Elliott, and Jan M. Friedman. Somatic mosaicism detected by genome-wide sequencing in 500 parent–child trios with suspected genetic disease: clinical and genetic counseling implications. Molecular Case Studies, 7:a006125, Oct 2021. URL: https://doi.org/10.1101/mcs.a006125, doi:10.1101/mcs.a006125. This article has 25 citations.
(wigby2024evidencereviewand pages 1-2): Kristen M. Wigby, Deanna Brockman, Gregory Costain, Caitlin Hale, Stacie L. Taylor, John Belmont, David Bick, David Dimmock, Susan Fernbach, John Greally, Vaidehi Jobanputra, Shashikant Kulkarni, Elizabeth Spiteri, and Ryan J. Taft. Evidence review and considerations for use of first line genome sequencing to diagnose rare genetic disorders. npj Genomic Medicine, Feb 2024. URL: https://doi.org/10.1038/s41525-024-00396-x, doi:10.1038/s41525-024-00396-x. This article has 26 citations and is from a peer-reviewed journal.
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(li2024triowholeexomesequencing pages 2-3): Chengyan Li, You Wang, Cizheng Zeng, Binglong Huang, Yinhui Chen, Chupeng Xue, Ling Liu, Shiwen Rong, and Yongwen Lin. Trio-whole exome sequencing reveals the importance of de novo variants in children with intellectual disability and developmental delay. Scientific Reports, Nov 2024. URL: https://doi.org/10.1038/s41598-024-79431-x, doi:10.1038/s41598-024-79431-x. This article has 22 citations and is from a peer-reviewed journal.

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Autosomal Dominant Non-Syndromic Intellectual Disability: Disease Characteristics Report

Executive summary and scope

1. Disease information

2. Etiology, risk, protection, and gene–environment interaction

Genetic causes

Regulatory and chromosomal causes

Environmental, infectious, and lifestyle factors

3. Phenotypes and quality-of-life effects

4. Genetic and molecular information

Representative gene architecture

Pathogenic variants and classification

5. Environmental information

6. Mechanism and pathophysiology

Integrated causal chain

7. Anatomical structures affected

8. Temporal development

9. Inheritance and population

10. Diagnostics

Clinical assessment

Current genetic-testing algorithm

11. Outcome and prognosis

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