Intellectual Disability, Autosomal Recessive 65 (MRT65): Comprehensive Research Report

1. Disease Information

Overview. Intellectual Developmental Disorder, Autosomal Recessive 65 (MRT65) is a rare Mendelian neurodevelopmental disorder caused by biallelic (homozygous or compound heterozygous) loss-of-function variants in KDM5B (lysine demethylase 5B), located on chromosome 1q32.1. It is characterized by moderate-to-severe intellectual disability, global developmental delay, characteristic facial dysmorphism, and camptodactyly, with onset in infancy (OMIM #618109; MalaCards).

Key identifiers: - OMIM: #618109 (phenotype); 605393 (KDM5B gene locus) - MONDO: MONDO:0020850 - Gene: KDM5B, HGNC (HGNC:29332), NCBI Gene, chromosome 1q32.1 - Related Orphanet entries: KDM5B is listed as causal for "Autosomal recessive non-syndromic intellectual disability" (ORPHA:88616) and as a candidate gene for autosomal dominant non-syndromic intellectual disability (ORPHA:178469) (Orphanet gene page) - Synonyms:* MRT65; Intellectual disability, autosomal recessive 65; KDM5B-related recessive intellectual disability syndrome; KDM5B-associated developmental delay with facial dysmorphism and camptodactyly

Note on nomenclature confusion (important for curation): MRT65/KDM5B-biallelic disease is distinct from "Alwadei syndrome" (also loosely called MRT61 in some lay sources), which is caused by biallelic RUSC2 variants on chromosome 9p13.3 (OMIM #617773) and is a separate, phenotypically overlapping but genetically unrelated autosomal recessive ID syndrome (Wikipedia summary of Alwadei syndrome, citing Alwadei et al. 2016, Dev Med Child Neurol). Some tertiary aggregator pages conflate the two; this should not be repeated in a curated entry.

Source of information: Nearly all clinical description derives from aggregated case-series/cohort publications (not large-scale EHR data) — a small number of published families/probands (originally 3, now up to ~10–11 published biallelic cases across multiple reports), supplemented by structured databases (OMIM, ClinVar, Orphanet, gnomAD) and model-organism data.

2. Etiology

Disease Causal Factors

MRT65 is a monogenic, purely genetic disorder — no environmental, infectious, or acquired mechanism is implicated. Disease results from biallelic loss-of-function (LoF) variants in KDM5B: nonsense, frameshift, and canonical splice-site variants that are predicted to trigger nonsense-mediated decay or truncate the protein before/within critical catalytic domains, as well as at least one compound-heterozygous case pairing a damaging missense variant (p.Ala635Thr, in the catalytic JmjC domain) with a frameshift null allele (Martin syndrome case report, PMC8467522).

Genetic Risk Factors

• Causal variants (original cohort, Faundes et al. 2018, Am J Hum Genet 102(1):175-187, PMID:29276005):
• Patient 12: homozygous c.4109T>G, p.(Leu1370Ter)
• Patient 13: compound heterozygous c.2475-2A>G (splice acceptor) / c.895C>T, p.(Arg299Ter)
• Patient 14: compound heterozygous c.3906delC, p.(Asn1302LysfsTer45) / c.622dupT, p.(Tyr208LeufsTer5)
• Additional reported variant (ClinVar, RCV001764151): c.3424-2A>G, a splice-acceptor variant
• Compound heterozygous missense + frameshift in siblings with agenesis of the corpus callosum: c.1903G>A p.(Ala635Thr) [CADD 28.8, JmjC domain, conserved across KDM5 paralogues] with c.3463del p.(Ser1155AlafsTer4) (PMC8467522)
• Statistical evidence of pathogenicity: Fisher's exact test showed a 96.89-fold enrichment of homozygous protein-truncating variants (PTVs) in affected cohorts versus gnomAD background (95% CI 3.95–2,378.87; p=0.03); no homozygous KDM5B knockouts were observed among 3,222 adults from a high-consanguinity reference cohort, consistent with strong purifying selection against biallelic LoF (Faundes et al. 2018).
• Cumulative literature: A 2024 genotype/phenotype study (Chong et al., Genes 15(8):1033, PMC11353349) catalogs 2 newly reported plus ~8 previously published biallelic cases (total ~10), all with LoF-type variants (nonsense/frameshift), contrasted against 19 individuals (18 novel) with monoallelic (dominant) KDM5B variants including 8 missense, 3 nonsense, 2 splice-site, 4 frameshift, and 1 whole-gene deletion (PMC11353349).
• Modifier genes: None established.
• Consanguinity: Multiple reported homozygous cases arise in the setting of parental consanguinity or regions of homozygosity, consistent with an autosomal recessive founder-type mechanism in some families (not formally reported as a specific founder allele/population to date).

Environmental Risk Factors

None identified; this is a Mendelian disorder with no reported environmental/lifestyle contribution to causation.

Protective Factors

None specifically described for the recessive form. Population data show that heterozygous (single-allele) LoF carriers are largely unaffected or mildly affected — heterozygous PTVs are common enough in the population (and in unaffected relatives/controls) to be associated only with subtle cognitive effects (see below), implying the wild-type allele in trans provides substantial functional buffering in carriers.

Gene-Environment Interactions

Not reported; no GxE data available for this ultra-rare monogenic disorder.

3. Phenotypes

Below are the principal reported phenotypes, aggregated primarily from Faundes et al. 2018 (original 3 patients), the sibling agenesis-of-corpus-callosum report (PMC8467522), and the 2024 genotype/phenotype cohort (PMC11353349).

Notably absent in the biallelic (recessive) cohort, in contrast to the monoallelic/dominant KDM5B phenotype: autism spectrum disorder and seizures were not observed in the 2024 systematic genotype/phenotype study's recessive cases, whereas ASD (53–64%) and sleep disorders (47%) are common in dominant KDM5B disease (PMC11353349).

Progression/severity: Developmental milestones are markedly delayed but generally achieved (walking and speech onset between 2–4 years in the original cohort); the disorder is considered a static/stable neurodevelopmental disability rather than a progressive neurodegenerative process, though hippocampal/memory-related deficits demonstrated in mouse models raise the possibility of an ongoing (not purely developmental) component to the cognitive phenotype (see Mechanism section).

Quality of life impact: Not formally studied with standardized instruments (EQ-5D, SF-36) in this ultra-rare population; qualitatively, affected individuals require lifelong support for intellectual disability, communication impairment, and (in a subset) structural brain and congenital anomalies requiring surgical/medical management.

4. Genetic/Molecular Information

Causal gene: KDM5B (also known as JARID1B, PLU1, RBBP2H1A, CT31), OMIM *605393, chromosome 1q32.1.

Gene product/domain architecture: KDM5B contains 7 annotated domains: JmjN domain, catalytic JmjC domain, ARID (DNA-binding) domain, C5HC2 zinc finger, and three PHD (plant homeodomain) fingers (PHD1–3) (Sci Rep 2019, molecular architecture paper; Functions and Interactions of Mammalian KDM5 Demethylases, PMC9309374).

Molecular function: KDM5B is an Fe(II)/2-oxoglutarate-dependent JmjC-domain histone demethylase that removes methyl groups from trimethylated, dimethylated, and monomethylated lysine 4 of histone H3 (H3K4me3/me2/me1) — a promoter-proximal mark generally associated with transcriptional activation. Catalysis proceeds via decarboxylation of 2-oxoglutarate to succinate + CO₂ coupled to hydroxylation of the methylated lysine, yielding an unstable hemiaminal that spontaneously decomposes to release formaldehyde and demethylated lysine. KDM5B additionally regulates RNA polymerase II initiation/elongation rates and alternative splicing.

Variant classification (ACMG/AMP) in the recessive disease: Reported biallelic variants are predominantly classified pathogenic/likely pathogenic loss-of-function alleles: - Nonsense: p.(Leu1370Ter), p.(Arg299Ter) - Frameshift: p.(Asn1302LysfsTer45), p.(Tyr208LeufsTer5), p.(Ser1155AlafsTer4) - Canonical splice-site: c.2475-2A>G, c.3424-2A>G (ClinVar RCV000678688, RCV001764151) - One missense (p.Ala635Thr, JmjC domain, CADD 28.8) reported in trans with a frameshift null, in a family with corpus callosum agenesis; classified as likely pathogenic on the basis of conservation, domain location, and segregation (PMC8467522)

Population allele frequency (gnomAD): The pathogenic missense variant (p.Ala635Thr) was observed in a single gnomAD heterozygote; the frameshift null was absent from gnomAD, consistent with strong selection against biallelic loss-of-function. No homozygous KDM5B PTV carriers were found among 3,222 highly consanguineous individuals in reference data. KDM5B shows strong constraint against loss-of-function in gnomAD generally (specific pLI/LOEUF values were not independently verifiable via the sources retrieved in this session and should be confirmed directly against the gnomAD browser before curation).

Somatic vs. germline: All reported disease-causing variants are germline; KDM5B is separately studied as a somatically amplified oncogene in some cancers (breast, prostate), but that is a distinct biological context from the germline recessive ID disorder.

Functional consequences: Loss-of-function variants are predicted/demonstrated to cause haploinsufficiency-on-a-null-background — i.e., the recessive disease requires near-complete loss of KDM5B demethylase activity from both alleles, since heterozygous LoF carriers are generally unaffected or only mildly affected (see Section 2). This is mechanistically consistent with the semi-dominant/dosage-sensitive biology of KDM5 family demethylases.

Modifier genes: None established.

Epigenetic information: KDM5B is itself a chromatin-modifying enzyme; its loss is predicted to cause genome-wide dysregulation of the H3K4me3 landscape, particularly at promoters of activity-dependent and developmentally regulated genes (directly demonstrated in mouse hippocampal models — see Mechanism section). No human DNA methylation/EWAS "episignature" for MRT65 was identified in the sources reviewed in this session.

Chromosomal abnormalities: No microdeletion/microduplication syndrome mechanism is described for MRT65; disease arises from intragenic sequence variants, not copy-number rearrangement (in contrast to some dominant KDM5B cases, one of which involved a whole-gene deletion).

Suggested ontology bindings: - Gene: hgnc:29332 (KDM5B) - GO Molecular Function: GO:0032454 (histone H3-K9 demethylase activity — not correct; correct term is GO:0032453, "histone H3K4 demethylase activity, trimethyl-H3K4-specific") — verify exact GO ID against OAK/AmiGO before binding - GO Biological Process: GO:0016575 (histone deacetylation — not applicable); more precisely, "negative regulation of gene expression, epigenetic" GO:0045814, or "histone H3-K4 demethylation" (verify exact CURIE)

5. Environmental Information

No environmental factors, lifestyle exposures, or infectious agents are implicated in causation of MRT65. This is a purely monogenic Mendelian disorder.

6. Mechanism / Pathophysiology

Causal chain (ordered, from molecular lesion to clinical phenotype)

1. Biallelic KDM5B loss-of-function or damaging missense variant (nonsense, frameshift, canonical splice-site, or a JmjC-domain missense in trans with a null allele) leads to near-complete loss of functional KDM5B protein or catalytic activity in the affected individual (demonstrated statistically by enrichment of homozygous PTVs over population background; PMID:29276005).
2. Loss of KDM5B H3K4 demethylase activity leads to failure to remove trimethyl/dimethyl/monomethyl marks from histone H3 lysine 4 at target gene promoters, resulting in genome-wide dysregulation of the H3K4me3 chromatin landscape — this step is demonstrated directly in a demethylase-dead mouse model (Kdm5bΔARID/ΔARID), which shows abnormal baseline H3K4me3 accumulation in hippocampal neurons (PMID:38575342/PMC11079963). This mouse-model step is inferred to translate to human neurons; it has not been directly demonstrated in human tissue.
3. Dysregulated H3K4me3 leads to abnormal baseline transcription of activity-dependent immediate-early genes (Egr1, Npas4, cFos are downregulated at baseline) and, upon a learning/activity stimulus, hyperactivated and pathologically prolonged transcription of the same genes (persisting ~3 hours vs. transient in controls) — a bidirectional dysregulation of neuronal activity-dependent gene programs (model-organism evidence, PMC11079963).
4. Dysregulated activity-dependent transcription leads to impaired postsynaptic long-term potentiation (LTP) stability in hippocampal CA1 neurons following theta-burst stimulation, with intact presynaptic release and short-term plasticity — i.e., a selective postsynaptic synaptic-plasticity defect (model-organism evidence).
5. Impaired synaptic plasticity leads to deficits in long-term (but not short-term) spatial memory consolidation, demonstrated behaviorally in Morris water maze and object-location memory tasks in mice, while short-term memory and cognitive flexibility are preserved (model-organism evidence).
6. In humans, this cascade is inferred (by analogy/extrapolation from the mouse mechanistic data, not directly demonstrated) to manifest as global developmental delay and moderate-to-severe intellectual disability, with a possible ongoing (not purely developmental) memory-consolidation component — raising the hypothesis that some cognitive deficits could remain modifiable in postnatal/adult life, a claim explicitly framed as therapeutically speculative by the authors (PMC11079963).
7. In parallel, independent of the neuronal/synaptic branch, KDM5B's broader role in regulating cell differentiation, developmental gene expression programs, and (in mouse knockouts) cranial neural crest/skeletal and craniofacial development leads to the syndromic congenital anomalies seen in humans — camptodactyly, characteristic facial dysmorphism (broad forehead/metopic prominence, high nasal bridge, bulbous nasal tip, thin lips), and in a subset, structural brain malformation (corpus callosum agenesis/hypoplasia) and other congenital anomalies (cardiac septal defects, genitourinary anomalies, hernias). This branch is supported by Kdm5b-null mouse phenotypes: disorganized cranial nerves, eye developmental defects, exencephaly, and skeletal anomalies, with most homozygous-null pups dying within the first postnatal day (perinatal lethality), and surviving pups showing developmental defects into adulthood (PMID:29276005; MGI:1922855).
8. A distinct, dosage-dependent branch: heterozygous (monoallelic) KDM5B protein-truncating variants, insufficient to cause the full recessive syndrome, are nonetheless associated with measurably reduced educational attainment, slower reaction time, and worse verbal-numerical reasoning at the population level, and are enriched among individuals with ADHD in two independent large case-control sequencing studies — indicating a dose-dependent (semi-dominant) relationship between KDM5B function and cognitive/behavioral phenotype severity, from mild population-level cognitive effects (heterozygous LoF) through syndromic dominant disease (heterozygous disruptive or missense variants with more severe/penetrant effects, including ASD and renal/dermatologic anomalies) to the fully penetrant recessive syndrome described here (biallelic LoF) (PMC10260403; PMC11353349; medRxiv 2025.01.14.25320294; Nat Commun 2024, 10.1038/s41467-024-50247-7).

Molecular pathways

Histone H3K4 methylation/demethylation cycle (writers: SET1/MLL family KMTs; erasers: KDM5 family, including KDM5A/B/C/D); RNA Pol II transcriptional initiation/elongation regulation; alternative splicing regulation.

Cellular processes

Neuronal activity-dependent transcription (immediate-early gene induction), synaptic plasticity (LTP), neural differentiation, cranial neural crest-derived craniofacial/skeletal development.

Protein dysfunction

Loss of catalytic JmjC-domain demethylase activity (nonsense/frameshift → truncated/degraded protein via NMD; missense in JmjC domain → predicted catalytic impairment).

Cell types and biological processes implicated (suggested ontology terms)

• Cell types: hippocampal pyramidal neuron (CL:0002614 or more specific CA1 pyramidal neuron term), cranial neural crest cell (CL:0000333)
• GO Biological Process candidates: "histone H3-K4 demethylation," "regulation of synaptic plasticity" (GO:0048167), "positive regulation of transcription by RNA polymerase II" (GO:0045944), "long-term memory" (GO:0007616), "neural crest cell development" (GO:0014032) — all CURIEs should be verified against OAK before binding.

Single-cell / omics data

No human single-cell, spatial transcriptomic, or multi-omic dataset specific to MRT65 patient tissue was identified in this session's searches. The relevant "omics" evidence base is exclusively mouse hippocampal transcriptomic (RNA-seq of immediate-early and late-response genes) and ChIP-based H3K4me3 profiling from the 2024 J Neurosci study (PMC11079963), plus a related 2024/2025 preprint on autism-like phenotypes and increased NMDAR2D expression in KDM5B-deficient mice (Sci Adv, PMC13189110) describing altered glutamatergic signaling gene expression.

7. Anatomical Structures Affected

Organ level: - Primary: central nervous system (intellectual disability, developmental delay; structural anomaly — corpus callosum agenesis/hypoplasia, absent interthalamic adhesion, hypoplastic anterior commissure in severe cases; optic nerve hypoplasia) - Secondary: musculoskeletal system (camptodactyly, joint hypermobility), craniofacial skeleton (dolichocephaly, metopic prominence, facial dysmorphism), cardiovascular system (atrial septal defect in one patient), genitourinary system (cryptorchidism, hypospadias in one patient), abdominal wall (inguinal/umbilical hernia) - Body systems: nervous, skeletal, craniofacial, cardiovascular (occasional), genitourinary (occasional)

Tissue/cell level: Neurons (particularly hippocampal), cranial neural crest-derived mesenchyme/craniofacial tissue, developing corpus callosum axonal tracts (with Probst bundle formation in complete agenesis — anomalous longitudinally-oriented axon bundles that fail to cross the midline).

Subcellular level: Nucleus (chromatin/nucleosome — GO Cellular Component: "nucleus" GO:0005634; more specifically chromatin, GO:0000785), consistent with KDM5B's role as a chromatin-modifying nuclear enzyme.

Localization/UBERON suggestions: brain (UBERON:0000955), corpus callosum (UBERON:0002336), hippocampus (UBERON:0002421), hand digit / phalanx (for camptodactyly — UBERON:0002389 or more specific digit terms), face (UBERON:0001456).

Lateralization: Not reported as asymmetric; anomalies (corpus callosum agenesis, camptodactyly) are typically bilateral/midline.

8. Temporal Development

• Onset: Congenital/infantile — feeding difficulties and dysmorphic features present from birth or early infancy; developmental delay recognized in infancy.
• Onset pattern: Insidious/developmental rather than acute.
• Progression: The intellectual disability and developmental delay are generally considered a static (non-progressive) neurodevelopmental disability once diagnosed, though affected individuals continue to acquire skills at a delayed rate (walking and first words achieved between ages 2–4 years in the original cohort). No degenerative course has been reported.
• Disease course pattern: Stable/lifelong; no remission.
• Critical periods: Mouse mechanistic data suggest KDM5B has functions in both prenatal neurodevelopment and ongoing adult synaptic plasticity/memory consolidation (demonstrated via adult hippocampal knockdown reproducing memory deficits) — raising the hypothesis of a postnatal "window of continued relevance" for gene function, distinct from a purely prenatal critical period, though this remains an extrapolation from animal data with unproven human therapeutic implications (PMC11079963).

9. Inheritance and Population

Epidemiology: MRT65 is an ultra-rare disorder. As of the original description, only 3 unrelated patients (all boys) had been reported (Faundes et al. 2018); by the 2024 genotype/phenotype study, roughly 10 biallelic cases total were catalogued in the literature (PMC11353349). No formal population prevalence or incidence estimate exists; this would be classified prevalence_class: NOT_YET_DOCUMENTED or an ultra-rare qualitative tier per dismech convention, sourced to measure_type: CASES_IN_LITERATURE.

Inheritance pattern: Autosomal recessive (biallelic KDM5B variants — homozygous or compound heterozygous).

Penetrance: Appears fully penetrant for the biallelic LoF genotype based on reported cases, though ascertainment bias in an ultra-rare, case-report-driven literature cannot be excluded.

Expressivity: Variable — e.g., presence/absence of craniofacial dysmorphism, corpus callosum anomalies, cardiac/genitourinary anomalies differ among the reported patients despite shared biallelic LoF mechanism (Patient 13 notably lacked craniofacial dysmorphism).

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

Germline mosaicism: Not specifically reported for KDM5B.

Founder effects: Not established; however, several published families are consanguineous, consistent with the general expectation for an ultra-rare autosomal recessive disorder.

Consanguinity role: Multiple reported homozygous cases arose from consanguineous unions; the population-genetics analysis in Faundes et al. specifically leveraged highly consanguineous reference cohorts (3,222 individuals) to show absence of homozygous PTV carriers as pathogenicity evidence.

Carrier frequency: Not established for MRT65 specifically; heterozygous KDM5B PTVs are present in the general population at a frequency sufficient to be studied for population-level cognitive/ADHD associations (i.e., not vanishingly rare), consistent with a modest carrier frequency, though a precise number was not identified in this session's sources — verify via gnomAD before curation.

Population demographics: All three original patients were male (boys), though this is very likely an ascertainment artifact from a small case series rather than a true sex-linked susceptibility, since the gene is autosomal. No specific ethnic/geographic enrichment has been reported; cases described in the literature originate from genetically heterogeneous ascertainment (Deciphering Developmental Disorders (DDD)-type cohorts and clinical case reports internationally).

Sex ratio: Not formally established (small sample skewed male in original description; not confirmed as a true biological sex bias).

10. Diagnostics

Genetic testing (primary diagnostic modality): - Whole exome sequencing (WES) or whole genome sequencing (WGS) — the diagnostic method by which essentially all reported cases were identified (via trio-based DDD-type cohorts or diagnostic clinical WES), given the ultra-rare and clinically non-specific nature of the phenotype. - Gene panel testing for intellectual disability/developmental delay panels that include KDM5B (e.g., Genomics England PanelApp Intellectual Disability panel, green/confidence level 10 — well-established gene-disease association) (PanelApp). - Single-gene KDM5B sequencing reasonable only when phenotype (camptodactyly + facial gestalt + developmental delay) is strongly suggestive, given the rarity. - Chromosomal microarray — not the primary diagnostic tool for this sequence-variant-driven disorder (as opposed to dominant cases with a reported whole-gene deletion), but useful to exclude CNV-based differentials. - RNA/splicing studies: Used in at least one report to functionally validate splice-site variants ("Novel KDM5B splice variants identified in patients with developmental disorders: Functional consequences," ScienceDirect/ResearchGate) — demonstrating aberrant transcript products from canonical splice-site variants.

Clinical/imaging tests: - Brain MRI — recommended given the reported association with corpus callosum agenesis/hypoplasia, absent interthalamic adhesion, hypoplastic anterior commissure, and optic nerve hypoplasia in a subset of patients; prenatal fetal MRI has also identified partial corpus callosum agenesis (splenium absence) at 22 weeks gestation in one case. - Cardiac echocardiogram — indicated given a reported atrial septal defect. - Skeletal/hand examination — camptodactyly assessment. - Developmental/cognitive assessment — standardized developmental and IQ testing to characterize the moderate-to-severe intellectual disability.

Differential diagnosis: Other genetic causes of syndromic intellectual disability with camptodactyly and/or corpus callosum anomalies, including but not limited to other chromatin-modifier-related neurodevelopmental disorders (KMT2D/Kabuki syndrome spectrum, other KDM5-family disorders — KDM5A, KDM5C [X-linked, Claes-Jensen syndrome], KDM5D), and RUSC2-related Alwadei syndrome (biallelic, overlapping ID + facial dysmorphism + occasional corpus callosum hypoplasia phenotype, OMIM #617773) — this is a genuine differential to flag given surface-level phenotypic overlap and nomenclature confusion in secondary sources.

Screening: No population or newborn screening program exists for this ultra-rare disorder; diagnosis is reactive, prompted by clinical developmental delay/dysmorphism evaluation.

11. Outcome/Prognosis

• Survival/mortality: No mortality directly attributable to MRT65 itself has been reported in surviving liveborn patients (oldest reported patient in the 2024 cohort was 28 years old); one reported pregnancy was electively terminated at 24 weeks for severe prenatal brain malformation (partial corpus callosum agenesis), reflecting reproductive-decision outcomes rather than a mortality statistic of the postnatal disease.
• Morbidity/function: Lifelong moderate-to-severe intellectual disability with need for ongoing developmental, educational, and adaptive support; camptodactyly may require orthopedic/hand-therapy management; structural brain anomalies (when present) correlate with more severe neurological phenotype.
• Complications: Congenital anomalies in a subset (cardiac septal defect, cryptorchidism/hypospadias, hernias) may require surgical correction.
• Recovery potential: No cure; supportive management only. The mouse memory-consolidation data (PMC11079963) raise a speculative, not yet clinically validated, hypothesis that some adult cognitive deficits could theoretically remain responsive to intervention beyond the developmental period — this should be flagged in any curated entry as a HUMAN_MODEL_MISMATCH-type caveat rather than an established human therapeutic avenue.
• Prognostic factors: Presence/severity of structural brain anomaly (corpus callosum agenesis) appears to correlate with greater phenotypic severity in the small reported series, though this is not statistically established given sample size.

12. Treatment

No disease-specific or targeted pharmacotherapy exists. Management is entirely supportive and symptom-directed:

• Supportive care / multidisciplinary developmental management: NCIT:C15747 (Supportive Care)
• Early intervention / developmental therapies: physical therapy (NCIT:C15302), occupational therapy (NCIT:C121351), speech therapy (NCIT:C159273) — addressing motor delay, camptodactyly-related hand function, and speech/communication delay
• Special education / behavioral support for intellectual disability and behavioral problems
• Orthopedic surgical procedure (NCIT:C16186) — potential management of camptodactyly if functionally limiting
• Cardiac surgical correction — for structural cardiac anomalies (atrial septal defect) if hemodynamically significant, NCIT:C15329 (Surgical Procedure)
• Genetic counseling (NCIT:C15240) — essential given autosomal recessive inheritance and 25% recurrence risk for future pregnancies of carrier parents; prenatal diagnosis is feasible once the familial variants are known (as demonstrated by the prenatal case detected via fetal MRI/genetic testing).

Experimental/clinical trials: No KDM5B/MRT65-specific clinical trials were identified in ClinicalTrials.gov searches performed in this session; none should be assumed to exist without direct verification.

Genotype-guided considerations: None currently established; the disorder is too rare for stratified precision-medicine approaches to have been developed.

13. Prevention

• Primary prevention: Not applicable (no modifiable risk factor); genetic counseling for known carrier couples (e.g., after an affected child) regarding 25% recurrence risk in future pregnancies.
• Secondary prevention: Prenatal diagnosis via chorionic villus sampling/amniocentesis for known familial variants once identified in a proband, as illustrated by the prenatal detection case (partial corpus callosum agenesis on fetal MRI, followed by molecular confirmation) (PMC8467522). Preimplantation genetic diagnosis (PGD) is a theoretical option for known carrier couples, though not specifically documented as used in the literature reviewed.
• Tertiary prevention: Early developmental intervention services to optimize functional outcomes once diagnosed.
• Population/genetic screening: No population carrier-screening panel currently includes KDM5B as a routine component (it is not among common expanded-carrier-screening panel genes at this time), consistent with its very recent (2018) disease-gene establishment and ultra-rare status.

14. Other Species / Natural Disease

No naturally occurring veterinary/companion-animal disease caused by biallelic KDM5B loss-of-function has been reported in the sources reviewed (no OMIA entry identified). KDM5B orthologs are broadly conserved (mouse Kdm5b, MGI:1922855; Drosophila kdm5/lid; and other KDM5-family members across vertebrates), but disease association is currently a human/laboratory-model finding rather than a recognized natural veterinary disease.

15. Model Organisms

Mouse models (primary model system used): - Constitutive Kdm5b knockout mice (MGI:1922855): Homozygous knockout causes postnatal lethality in the majority of pups, most dying within the first 24 hours of life; surviving pups develop normally into adulthood but show increased incidence of exencephaly, eye developmental defects, disorganized cranial nerves, and skeletal anomalies — recapitulating several structural/congenital features of the human syndrome, but with far greater lethality than seen in humans, indicating the mouse null is not a fully faithful model of viable human biallelic loss (relevant for a FAILS_TO_RECAPITULATE/PARTIALLY_RECAPITULATES model-link annotation, fidelity: LOW–MODERATE, with a SPECIES_MISMATCH or BOUNDARY_OMISSION divergence given the survival discrepancy) (PMID:29276005; MGI:1922855). - Kdm5bΔARID/ΔARID mice (demethylase-catalytically-dead, but not full protein-null): viable, adult-survivable model used to study postnatal/adult hippocampal function — hyperactivity, long-term (but not short-term) spatial memory deficits, impaired hippocampal LTP, and dysregulated activity-dependent immediate-early gene transcription (PMID:38575342/PMC11079963). This model partially recapitulates the human cognitive phenotype at the behavioral/circuit level (RECAPITULATES for memory/behavioral readouts) but does not model the congenital/dysmorphic features of the human syndrome, since it preserves partial protein structure and is not a full LoF-null. - Adult hippocampal shRNA knockdown (CA1-targeted, wild-type mice): Demonstrates that postnatal/adult-only knockdown is sufficient to cause spontaneous seizures, hyperactivity, and hippocampus-dependent long-term memory/LTP deficits — showing the phenotype is not exclusively attributable to developmental loss, supporting a model where ongoing adult gene function is separately required (PMC11079963). - Mouse model of autism-like phenotypes: A 2024–2025 study (Science Advances, PMC13189110) reports KDM5B-deficient mice show autism-like behavioral phenotypes and increased NMDAR2D (Grin2d) expression, suggesting altered glutamatergic signaling as an additional mechanistic branch — this maps more directly to the dominant/monoallelic human KDM5B-ASD phenotype than to the biallelic MRT65 phenotype (which notably lacks ASD in the 2024 clinical cohort), and should be flagged as a model-to-phenotype scale/relevance mismatch if used to support the recessive MRT65 entry specifically.

Invertebrate models: Drosophila KDM5 (fly ortholog, "lid") knockdown/mutant studies demonstrate roles in synaptic structure and function at the neuromuscular junction, larval growth, and chromatin insulator activity in the brain — supporting deep evolutionary conservation of KDM5's neurodevelopmental role, but these are invertebrate systems with substantial biological distance from the human CNS phenotype (fidelity: LOW, useful mechanistically but not for direct phenotype recapitulation claims).

Model limitations (general): No existing animal model — including the closest available (Kdm5bΔARID/ΔARID) — reproduces the full human MRT65 syndromic phenotype (camptodactyly, characteristic facial dysmorphism, corpus callosum agenesis) together with viable adult survival; the constitutive null best matches structural/congenital features but is embryonic/perinatal lethal in the majority of animals, while the catalytically-dead ARID model best matches the cognitive/memory phenotype but lacks the structural and craniofacial anomalies. This split strongly suggests the human syndrome as a whole has never been fully modeled in one animal system to date, an appropriate HUMAN_MODEL_MISMATCH discussion point for curation.

Summary of Key Primary Citations

Curation caveats to flag explicitly: (1) The disorder is defined by an extremely small published case count (~10 biallelic patients across all literature to date) — treat prevalence as CASES_IN_LITERATURE/undocumented, not a population rate. (2) Distinguish this recessive entry sharply from dominant/monoallelic KDM5B disease and from RUSC2-related Alwadei syndrome, both of which are conflated in some secondary/tertiary web sources. (3) Mouse mechanistic data (hippocampal memory consolidation, autism-like NMDAR2D findings) come from models that do not fully recapitulate the human biallelic-null phenotype and should be evidence-graded MODEL_ORGANISM with explicit HUMAN_MODEL_MISMATCH/divergence annotations rather than treated as directly confirmed human mechanism. (4) Several ontology CURIEs suggested above (GO terms in particular) are provisional and must be verified against OAK/live ontology lookup before binding, per this repository's term-validation policy.

Reference Validation

Checked with linkml-reference-validator 0.2.1.

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:

• HP:0001263 (1 mention) - the report calls it "~100% (all reported cases)"; HP calls it Global developmental delay
• HP:0000750 (1 mention) - the report calls it "Most patients (e.g., "5 words" at age 4 in one case)"; HP calls it Delayed speech and language development
• HP:0001510 (1 mention) - the report calls it "Mild, in a subset"; HP calls it Growth delay
• HP:0000268 (1 mention) - the report calls it "Present in a subset"; HP calls it Dolichocephaly
• HP:0012385 (1 mention) - the report calls it "Present in majority (hallmark feature)"; HP calls it Camptodactyly
• HP:0001382 (1 mention) - the report calls it "~50% in 2024 cohort"; HP calls it Joint hypermobility
• HP:0000708 (1 mention) - the report calls it "Present in all biallelic cases assessed"; HP calls it Atypical behavior
• HP:0001631 (1 mention) - the report calls it "One patient"; HP calls it Atrial septal defect
• HP:0002558 (1 mention) - the report calls it "One patient"; HP calls it Supernumerary nipple
• HP:0000609 (1 mention) - the report calls it "One patient (sibling case)"; HP calls it Optic nerve hypoplasia
• HP:0000767 (1 mention) - the report calls it "One patient"; HP calls it Pectus excavatum
• NCIT:C15302 (1 mention) - the report calls it "Early intervention / developmental therapies: physical therapy"; NCIT calls it Physical Therapy**

Obsolete terms

These terms are real but deprecated. Citing one is not a fabrication; it does mean the report is naming something the ontology has retired:

• GO:0016575 (obsolete histone deacetylation) (1 mention)

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:

• HP:0000321 (1 mention) - the report calls it "Some patients"; HP calls it Square face, and lists "Square facies" among its other names
• GO:0016575 (1 mention) - the report calls it "histone deacetylation — not applicable"; GO calls it obsolete histone deacetylation

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: ORPHA, MGI.