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Research Report: Intellectual Developmental Disorder, Autosomal Dominant 48 (MRD48; RAC1-related neurodevelopmental disorder)

Target: Intellectual Disability, Autosomal Dominant 48. MONDO:0030913 as given in the template; OMIM #617751. Report date: 2026-10-01

How this report was built

I read the nine primary references already cached in this worktree's references_cache/ (PMIDs 28886345, 30042656, 34725860, 35139179, 37059841, 37328543, 39307291, 39838818, 42120539). Five have full text and four have abstracts only. Every quoted snippet below was copied from those cached files.

• Live web and database searches were not run. I did not query OMIM, Orphanet, ClinVar, gnomAD or MGI, so no figures from those sources appear here.
• Unchecked CURIEs. The ontology identifiers marked [verify] were recalled from memory and not looked up. Under the repo's term contract (CLAUDE.md), each must be checked with runoak or the term caches before it is bound.

1. Disease information

Overview. MRD48 is a rare, highly variable neurodevelopmental disorder (NDD). It is caused by heterozygous, almost always de novo, missense variants in RAC1, a Rho-family small GTPase. The core features are: - developmental delay or intellectual disability (DD/ID) with speech delay; - structural brain anomalies; - hypotonia, feeding difficulties and behavioural problems; - a recognisable but subtle facial gestalt.

Its most striking feature is that head size depends on the variant. Occipitofrontal circumference ranges from about −5 SD to +4.5 SD, and this follows whether the variant is dominant-negative or activating.

• "De novo missense variants in RAC1 are associated with a rare neurodevelopmental disorder (MRD48) characterized by DD/ID and brain abnormalities coupled with a wide range of additional features." (PMID:37059841, Priolo et al., Eur J Hum Genet 2023)
• "Collectively, we observed an extraordinary spread of ∼10 SD of head circumferences orchestrated by distinct mutations in the same gene." (PMID:28886345, Reijnders et al., Am J Hum Genet 2017; the founding report)

Identifiers

Synonyms: Intellectual developmental disorder, autosomal dominant 48; MRD48; RAC1-related neurodevelopmental disorder (RAC1-NDD, PMID:35139179); RAC1-related intellectual developmental disorder (PMID:39838818).

Basis of knowledge. Everything comes from individual-patient case series: about 7 patients in 2017, 8 in 2022, 2 in 2023, and further single cases from 2023 to 2025. The 2026 switch II paper reports 15 new individuals but gives clinical detail for only 6. No registry or EHR-level data exist.

Lump/split note for curation. The 2026 paper says "variants affecting N- and C-terminal parts of RAC1 switch II cause phenotypically and mechanistically distinct disorders" (PMID:42120539). Recommendation: keep one Disease entry anchored to OMIM 617751 / MONDO, and model functional-class strata as has_subtypes: - dominant-negative / microcephalic; - activating switch II N-terminal (Q61–R68) / normocephalic; - macrocephalic (V51).

There are no separate MONDO terms for these strata.

2. Etiology

• Cause: a monoallelic germline missense variant in RAC1 (7p22.1), almost always de novo.
• Variant spectrum: missense only. "RAC1 is highly conserved across species and is under strict mutational constraint" (PMID:28886345).
• Variant classes: loss-of-function (truncating) variants and deletions have not been reported as a cause. Complete Rac1 loss is embryonic lethal in mice, so the disease appears to require dominant-negative or activating missense alleles rather than haploinsufficiency. That last point is an inference, not a demonstrated result.
• Risk factors: none are known apart from the de novo event (advanced parental age is not documented). No environmental, protective or gene–environment factors have been reported.
• Double diagnosis. One patient's sensorineural deafness was explained by a separate pathogenic GRHL2 nonsense variant (DFNA28) inherited from the father (PMID:37059841). This matters when attributing hearing loss to RAC1.

3. Phenotypes

Frequencies come from the 10-patient review in PMID:37059841. The denominator is the number of patients assessed for each feature:

"all subjects shared ID with speech delay (10/10 and 7/7, respectively), a wide array of brain abnormalities (8/8) … behavioral abnormalities (5/6), hypotonia and neonatal/childhood feeding difficulties (5/7 and 5/8, respectively). CHDs were also common (4/8), while epilepsy was reported in 3/8 subjects. We also observed recurrent ectodermal anomalies, mainly represented by eczematous rashes, with or without ichthyosiform manifestations (4/10), and various skeletal anomalies (3/10), such as small hands and feet (3/10)."

Facial gestalt. From systematic review of photographs: "high anterior hairline (9/9), arched eyebrows (8/9) with tendency to lateral sparse (7/9) … mild widely spaced eyes (8/9), wave-shaped palpebral fissures (7/9), overhanging columella (6/9) … short philtrum (7/9), deep nasolabial folds (8/9), thin upper lip (7/9), abnormally spaced teeth … (6/6) … and a long pointed chin (7/9)." The same paper notes that "widely spaced eyes, bulbous nasal tip and prominent nasolabial folds tend to smoothen in adolescents and young adults" (PMID:37059841).

Some HPO terms to check for these features: HP:0000294 Low anterior hairline (do not use; the feature here is a high hairline), HP:0002668 Arched eyebrow, HP:0000316 Hypertelorism, HP:0000322 Short philtrum, HP:0000219 Thin upper lip vermilion, HP:0000687 Widely spaced teeth. [verify all]

Severe or atypical presentations. - p.Tyr40His (Seyama 2023, PMID:37328543): a VACTERL-like presentation with fatal tracheal aplasia. "the patient died of respiratory failure caused by tracheal aplasia type III." Other features were TAPVR, oesophageal atresia, scoliosis and polydactyly. - Mild cases: Upadia et al. 2025 (PMID:39838818) widen the mild end of the spectrum: "We present one case with typical phenotype and two cases with a mild phenotype."

Quality of life. No studies using standard instruments (EQ-5D, PedsQL) were found. The burden is inferred from ID, behavioural disorder and congenital heart disease.

4. Genetic and molecular information

• Gene: RAC1 (Rac family small GTPase 1), 7p22.1, OMIM 602048, hgnc:9801 [verify]. Reference transcripts are NM_006908 (RAC1) and NM_018890 (RAC1B, which carries an extra exon 4) (PMID:28886345).
• Inheritance: autosomal dominant (HP:0000006 [verify]). All reported cases are de novo (HP:0025352 de novo [verify]).

Reported pathogenic variants and functional class

• Classification and ClinVar. The two Priolo variants are ACMG Pathogenic (ClinVar SCV002605544 and SCV002605545). They are absent from gnomAD v2.1.1 (PMID:37059841).
• ClinGen. I did not check whether a gene–disease validity curation exists. Run just list-gene-validity / clingen-list before filling gene_disease_validity, and do not assign a tier yourself.
• Mechanistic summary: "Structural and functional studies have documented either a dominant negative or constitutively active behavior for a subset of mutations" (PMID:37059841). In the 2026 switch II study: "N-terminal variants are activating, while C-terminal variants are dominant-negative" (PMID:42120539).
• Hotspots shared with paralogs: "recurring variants at specific residues (E62 and Y64 in G3/Switch II) were reported in all proteins" (RAC1–3, CDC42; PMID:37059841).
• Not reported: modifier genes, epigenetic signatures (no published episignature) and chromosomal abnormalities (CNVs) as a cause.

Suggested functional_impact_category values: - DOMINANT_NEGATIVE for the C18Y, N39S, E31G and C-terminal switch II variants; - GAIN_OF_FUNCTION (or HYPERMORPHIC) for Y64D, E62K, A159T and the other Q61–R68 variants.

5. Environmental information

Not applicable. No environmental, lifestyle or infectious contributors have been reported.

6. Mechanism and pathophysiology

Ordered causal chain

1. A de novo heterozygous missense variant in RAC1 alters the GTP/GDP switch. It does so either by disrupting nucleotide binding or effector binding (C18, N39, Y40, E31, P69–Q74, C157), or by locking RAC1 in the GTP-bound state (Q61–R68, A159).
2. Branch A (dominant-negative alleles): the mutant protein fails to activate effectors such as PAK1. For C18Y it occludes the GTP pocket ("this mutation will strongly inhibit Rac1 activation by occluding Rac1's GTP binding pocket", PMID:30042656). This leads to reduced RAC1 signalling: less actin polymerisation and lamellipodia formation.
3. 2a → reduced neural progenitor proliferation → microcephaly and cerebellar anomalies. This was demonstrated in zebrafish overexpression (PMID:28886345). Its relevance to human cortex is inferred.
4. 2b → impaired axon elongation and dendritic arborisation (E31G, in utero electroporation in mouse, PMID:39307291) → abnormal connectivity.
5. 2c → in hippocampal neurons, loss of synaptic AMPA receptor function and failed LTP induction ("results in a selective reduction in synaptic AMPA receptor function… prevents the induction of long-term potentiation (LTP)", PMID:30042656) → this is argued to contribute to ID (the link to ID is inferred).
6. Branch B (activating alleles): a higher fraction of GTP-bound RAC1 → over-activation of PAK1/2/3 and of the WAVE regulatory complex (WRC)/Arp2/3 (PMID:35139179) → excess actin branching:
7. shorter axons with more filopodia, axon fasciculation defects, and dendrite over-branching with loss of self-avoidance (Drosophila Y64D);
8. these defects are rescued by Cyfip knockdown ("RNAi knock down of the WAVE regulatory complex component Cyfip significantly rescues these morphological defects").
9. → abnormal neuronal morphology and cortical malformation (polymicrogyria) → ID, seizures, and normocephaly or mild macrocephaly.
10. Extra-neural branch (inferred from mouse models): RAC1 dysfunction in neural crest cells and the second heart field → outflow tract and septal defects and craniofacial dysmorphism. "the perturbed development of neural crest cell (NCC)-derived tissues documented in NCC-targeted Rac1 conditional KO mice may explain both craniofacial and cardiac features" (PMID:37059841). This step is inferred and has not been shown with patient alleles.
11. Macrocephaly (V51): the mechanism is unresolved, described as "context dependent" (PMID:28886345). Mention of mTOR involvement in RAC1 biology is background only and has not been shown for these alleles.

Pathways and processes, with ontology suggestions [verify all]

• Molecular function: GTPase activity (GO:0003924); GTP binding (GO:0005525).
• Biological processes:
• small GTPase-mediated signal transduction (GO:0007264);
• actin cytoskeleton organization (GO:0030036);
• lamellipodium assembly (GO:0030032);
• Arp2/3 complex-mediated actin nucleation (GO:0034314);
• axonogenesis (GO:0007409);
• dendrite morphogenesis (GO:0048813);
• neural precursor cell proliferation (GO:0061351);
• long-term synaptic potentiation (GO:0060291);
• neural crest cell migration (GO:0001755).
• Effectors: PAK1 (hgnc:8590 [verify]), WAVE2/WASF2, CYFIP1/2, Arp2/3.
• Cell types: neural progenitor cell (CL:0011020), neuron (CL:0000540), hippocampal pyramidal neuron, cerebellar neurons, cranial or cardiac neural crest cell (CL:0000333 neural crest cell). [verify]
• Subcellular location: actin cytoskeleton / lamellipodium (GO:0030027), plasma membrane, postsynaptic density (GO:0014069), dendritic spine (GO:0043197). [verify]
• Omics: no transcriptomic, proteomic, metabolomic, single-cell or CRISPR-screen data for MRD48 were found.
• Biological scale tags for the pathograph:
• RAC1 GTPase switch defect: MOLECULAR;
• PAK/WRC over-activation or under-activation: CELLULAR;
• progenitor proliferation and neuronal morphology: CELLULAR;
• cortical malformation and microcephaly: TISSUE.

7. Anatomical structures affected

• Primary: brain, including cerebral cortex (polymicrogyria/pachygyria), corpus callosum, cerebellar vermis, brainstem/pons and lateral ventricles. UBERON candidates [verify]: UBERON:0000955 brain, UBERON:0000956 cerebral cortex, UBERON:0002336 corpus callosum, UBERON:0004720 cerebellar vermis.
• Secondary: heart (septa, valves, outflow tract; UBERON:0000948), craniofacial skeleton, skin, kidney (hydronephrosis), inner ear and eye. Rare cases involve the trachea and oesophagus (p.Y40H).
• Laterality: malformations are generally bilateral or midline. One case had left fronto-insular pachygyria.

8. Temporal development

• Onset: congenital, with brain and heart malformations present at birth. DD becomes apparent in infancy. Microcephaly can be congenital or acquired (E31G, PMID:39307291).
• Course: a static neurodevelopmental disorder. No degenerative course is documented. Behavioural and psychiatric problems may emerge in adolescence; one patient's competences worsened from age 12 (PMID:37059841). The facial gestalt changes with age.
• Mortality: one neonatal death, from tracheal aplasia (p.Y40H).
• Critical period: prenatal neurogenesis and neural crest development (inferred).

9. Inheritance and population

• Inheritance pattern: autosomal dominant, essentially always de novo.
• Penetrance: apparently complete for reported alleles, though the evidence is limited. No familial transmission has been reported.
• Expressivity: highly variable, with a strong allele-to-phenotype correlation for head size.
• Not reported: anticipation, founder effects, a role for consanguinity, and parental germline mosaicism. For counselling, a low recurrence risk (about 1%, the generic figure for de novo disorders) applies.
• Prevalence: unknown. Roughly 30 individuals have been described in the literature (7 + 8 + 2 + ~3 single cases + 15 in 2026, allowing for overlap).
• Suggested KB record: measure_type: CASES_IN_LITERATURE with prevalence_class: ULTRA_RARE or NOT_YET_DOCUMENTED.
• Source the count from the specific papers; do not invent a rate.
• Sex ratio and ancestry: both sexes are affected; no ancestry bias has been noted.

10. Diagnostics

• Method: diagnosis is molecular, by trio exome or genome sequencing. Most cases came through DDD trio WES or clinical WES after a normal chromosomal microarray (CMA) (PMID:28886345, 37059841). RAC1 is on most ID/NDD panels.
• Variant interpretation:
• de novo status plus a constrained missense position support pathogenicity;
• functional assays used in research are RAC1-GTP pulldown (PAK-CRIB), fibroblast spreading/roundness index, and phospho-PAK and WAVE2 staining (PMID:35139179, 37059841).
• Imaging: brain MRI (polymicrogyria, corpus callosum and cerebellar anomalies), echocardiography and renal ultrasound.
• Other workup: EEG if seizures occur; audiology; ophthalmology.
• Biomarkers: none (no metabolic or laboratory markers).
• Differential diagnosis:
• RAC3-related NEDBAF (OMIM 618577): more severe ID, more frequent corpus callosum and brainstem/cerebellar dysplasia, upslanted fissures and upturned nares (PMID:37059841);
• CDC42-related Takenouchi–Kosaki syndrome;
• TRIO-related NDD;
• HACE1 disorder;
• RASopathies (Noonan, CFC, Costello);
• other polymicrogyria or microcephaly genes;
• VACTERL for the Y40H-type presentation.
• Screening: there is no newborn or carrier screening. Prenatal ultrasound may detect anomalies (as in the Y40H case).

11. Outcome and prognosis

• Natural history: no formal studies. Most individuals survive into adolescence or adulthood; patients aged 14 and 27 are described in PMID:37059841.
• Mortality: the main reported fatality was neonatal, from tracheal aplasia (PMID:37328543).
• Morbidity: driven by ID (mild to severe), behavioural disorder or ASD, epilepsy, and congenital heart disease.
• Prognostic factors (tentative):
• dominant-negative / microcephalic variants and severe brain malformations tend to go with more severe ID;
• RAC1 overall is milder than RAC3: "subjects with pathogenic RAC1 variants show a milder developmental involvement (5/10 with mild to moderate DD/ID) compared to individuals with RAC3 mutations" (PMID:37059841).
• Not available: survival rates, life-expectancy data, and QoL instrument studies.

12. Treatment

There is no disease-specific or approved therapy, and no clinical trials. Supportive, multidisciplinary management:

Experimental and preclinical leads only: - For activating alleles, the WAVE regulatory complex/Arp2/3 pathway has been proposed as a target: "reveal the WAVE regulatory complex/Arp2/3 pathway as a possible therapeutic target for activating RAC1 variants" (PMID:35139179). This rests on Drosophila Cyfip knockdown only. - RAC1 inhibitors (e.g., NSC23766, EHT1864) are research tools and have not been tested in patients. - Because the functional classes are opposite, any targeted therapy would need to be allele-class-specific.

13. Prevention

• Primary prevention: none, because the disease arises de novo.
• Reproductive options: genetic counselling, with low recurrence risk but parental mosaicism to be considered. Prenatal or preimplantation testing can be offered once a familial variant is known.
• Tertiary prevention:
• echocardiography and renal ultrasound at diagnosis;
• hearing and vision screening;
• developmental and behavioural surveillance;
• seizure monitoring.

14. Other species and natural disease

• Animals: no naturally occurring RAC1 disease is known (OMIA not checked).
• Conservation: RAC1 is highly conserved. Orthologs: mouse Rac1, zebrafish rac1a/rac1b, Drosophila Rac1. The disease is not zoonotic.

15. Model organisms

Gap: no patient-allele knock-in mouse or iPSC/organoid model was identified. A HUMAN_MODEL_MISMATCH discussion would fit here: the microcephaly and neuron-morphology mechanisms rest on zebrafish and Drosophila overexpression rather than human cortical tissue.

Key references (all cached locally)

Curation cautions

1. Evidence classification for mixed papers. Reijnders, Banka and Althebaiti each report patients, cell assays and animal models. Split their evidence items by evidence_source.
2. Frequency denominators. Use the per-feature denominators from PMID:37059841 (for example 5/7 for hypotonia), not a blanket n=10.
3. Deafness attribution. Do not attribute sensorineural deafness solely to RAC1. One of the two cases is explained by GRHL2.
4. Abstract-only references. PMID:39838818, 39307291, 34725860 and 42120539 are abstract-only in the cache, so snippets must come from those abstracts.
5. Unchecked CURIEs. Every CURIE marked [verify] must be looked up. This includes MONDO:0030913's label, the HGNC ID for RAC1, all HP/GO/CL/UBERON IDs, and the NCIT treatment terms.

Reference Validation

Checked with linkml-reference-validator 0.3.0rc3.

All extracted references resolved successfully.

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

• MONDO:0030913 (3 mentions) - the report calls it "MONDO"; MONDO calls it intellectual disability, autosomal dominant 48
• HP:0001263 (1 mention) - the report calls it "Infancy"; HP calls it Global developmental delay
• GO:0030027 (1 mention) - the report calls it "Subcellular location: actin cytoskeleton / lamellipodium"; GO calls it lamellipodium**

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

• GO:0003924 (1 mention) - the report calls it "Molecular function: GTPase activity"; GO calls it GTPase activity**, and lists "tubulin GTPase activity" among its other names
• CL:0011020 (1 mention) - the report calls it "Cell types: neural progenitor cell"; CL calls it neural progenitor cell**
• NCIT:C15240 (1 mention) - the report calls it "Genetic counselling"; NCIT calls it Genetic Counseling