I'll load the search tools and check the literature before writing the report.

Obesity, Hyperphagia, and Developmental Delay (OBHD; MONDO:0013483)

How reliable this is. I ran two web searches. Most PubMed, PMC and OMIM pages were not reachable, and one fetch of the POU3F2 paper failed. Items marked [verified] come from those search results. Items marked [recalled] come from my background knowledge and have not been checked. Items marked [not found] are things I could not locate. Check every PMID with just fetch-reference before use. I give no verbatim abstract quotes, because I could not read any abstract in full. The one quoted phrase in section 3 is search-snippet text, not a confirmed abstract quote, so re-copy it from the abstract before using it as a snippet. Where I could not confirm a CURIE, I say so.

1. Disease information

• Overview. OBHD is an autosomal dominant neurodevelopmental disorder. It presents as global developmental delay and hyperphagia that leads to obesity. Some patients have seizures. Reported minor features are hypotonia, autistic-like behavior, and, in a few cases, streak ovaries. [verified] (MalaCards, MedGen, PhenoDis aggregates).
• Identifiers [verified]:
• OMIM 613886
• MONDO:0013483
• MedGen C3151303
• MeSH supplementary record C563938
• Synonym: OBHD
• Orphanet and ICD codes: not checked.
• Data origin. The OMIM entry is built from a handful of case-level reports, not from large cohorts.

Sources: MedGen C3151303, MalaCards OBHD, PhenoDis, LOVD disease 03483.

2. Etiology

• Causal gene. NTRK2 (TrkB, 9q21.33). It is the only gene linked to the OMIM entry [verified].
• Index case. Yeo et al. (Nat Neurosci 2004, PMID:15494731 per search; I recalled 15494728, so confirm which is right) described an 8-year-old boy. He had a de novo heterozygous missense variant, p.Tyr722Cys (Y722C), severe obesity and a complex developmental syndrome. The variant impaired receptor autophosphorylation and MAPK signaling [verified].
• Cohort data. Gray et al. (Int J Obes 2007) tested four rare NTRK2 variants found among 198 obese children. Only Y722C impaired signaling, and it also reduced BDNF-induced neurite outgrowth [verified]. A Manchester-affiliated paper, "Functional characterization of human NTRK2 mutations identified in patients with severe early-onset obesity", appeared in the search results. I did not get its PMID, so I do not know if it is the same study.
• Pathway parallel. Gray et al. (Diabetes 2006, PMID:17130481) described a girl with a de novo inversion disrupting BDNF. She had hyperphagia, severe early-onset obesity, hyperactivity and cognitive impairment [verified]. This implicates the ligand as well as the receptor.
• Risk factors. No environmental or gene-environment data were found. No protective factors are known.
• Related genes (not NTRK2). These are separate entities. Check whether they should be handled as their own entries before attaching them here:
• POU3F2: a 2023 AJHG report of a monoallelic truncating variant in a family with syndromic obesity, plus ten more individuals with autism spectrum disorder (ASD), neurodevelopmental disorder (NDD) and adolescent-onset obesity [verified, via search summary].
• GNB1 and GNAS: conference abstracts only. Treat them as weak and uncurated [verified as abstracts].

3. Phenotypes (suggested HPO terms)

HPO IDs below are from memory. Look each one up before binding it. Frequencies are unknown, because the evidence is a few cases.

• Onset: childhood.
• Quality-of-life data: none found.

The search summary phrase usable as a lead is "global developmental delay and hyperphagia resulting in obesity". It came from an aggregator, so re-copy it from the abstract of a cached source.

4. Genetic and molecular information

• Gene: NTRK2, HGNC lowercase form hgnc:<id>. I did not confirm the ID, so look it up with runoak.
• Variant: p.Tyr722Cys (Y722C). It is missense, de novo, heterozygous and germline. It causes partial loss of function: impaired autophosphorylation and MAPK signaling [verified].
• Allele frequency, ClinVar classification, ClinGen validity: not checked. Run just check-gene-validity and just list-gene-validity for ClinGen status.
• Modifier genes: none known.
• Epigenetic and chromosomal data: none found.

5. Environmental information

No environmental, lifestyle or infectious contributors were found. Leave environmental: empty. If you want a waiver, use the review_notes "Left deliberately uncited." convention, with a recorded search.

6. Mechanism and pathophysiology

Causal chain: 1. A de novo heterozygous NTRK2 missense variant (Y722C) is the initiating lesion. This is demonstrated [verified]. 2. The variant impairs TrkB kinase autophosphorylation and downstream MAPK signaling. This is demonstrated in vitro [verified]. 3. Weaker BDNF-TrkB signaling reduces neurite outgrowth and neuronal function. Neurite outgrowth reduction was shown in vitro [verified]. The broader neuronal-function claim is inferred. 4. The reduced signaling plausibly affects hypothalamic feeding circuits. This is inferred here. The BDNF-haploinsufficiency case and mouse literature [recalled, not checked] support the idea. Leaving it as "inferred" avoids overclaiming. 5. Disrupted feeding control leads to hyperphagia, then obesity. 6. In parallel, impaired TrkB-dependent neurodevelopment contributes to developmental delay, autism-like behavior, hypotonia and seizures. This is inferred.

• Suggested GO terms (unchecked): neurotrophin TRK receptor signaling pathway, MAPK cascade, positive regulation of neuron projection development, regulation of feeding behavior.
• Cell types (unchecked): hypothalamic neurons; neuron (CL:0000540, recalled).
• Immune, metabolomic and single-cell data: none found.

7. Anatomical structures

Central nervous system, especially the hypothalamus, plus cerebral cortex and hippocampus (BDNF-TrkB targets, recalled). Obesity is the downstream peripheral consequence. UBERON IDs need to be looked up.

8. Temporal development

Childhood onset. Course and critical periods are not characterized in the sources I found.

9. Inheritance and population

• Inheritance: autosomal dominant, with the index variant de novo [verified].
• Prevalence: not found. This is extremely rare, with a handful of reported cases.
• Penetrance, expressivity and founder effects: not found.

10. Diagnostics

Genetic testing is the diagnostic route. Sequencing NTRK2 in a child with early-onset obesity, hyperphagia and developmental delay is reasonable. In practice this means a severe early-onset obesity gene panel or exome. No dedicated criteria or biomarkers were found. Recommended differentials, from my background knowledge: - MC4R and other leptin-melanocortin pathway defects - Prader-Willi syndrome - SIM1 variants - 16p11.2 deletions - Chung-Jansen syndrome (PHIP)

I have not verified any of these.

11. Prognosis

Not found.

12. Treatment

No NTRK2-specific therapy was found. Supportive care is the usual approach: dietary and behavioral management, environmental food restriction, developmental therapies and seizure management. Setmelanotide has been approved for some monogenic obesities, but I did not find it linked to NTRK2. Do not curate it for this disease. Suggested NCIT terms, from the CLAUDE.md common list: Supportive Care (NCIT:C15747), Dietary Intervention (NCIT:C15447), Rehabilitation (NCIT:C15315). I did not search ClinicalTrials.gov.

13. Prevention

Genetic counseling (NCIT:C15240) applies. Because the index variant was de novo, recurrence risk is low unless there is germline mosaicism.

14. Other species

Not researched. Mouse is the relevant species. NCBITaxon:10090 and NCBI Gene IDs were not looked up.

15. Model organisms

I could not find the citation for the TrkB hypomorph mouse studies (the Xu/Xie 2008 reference). [not found]. From memory, TrkB hypomorphic mice and BDNF-deficient mice develop hyperphagic obesity. Treat that as a lead, not a citation, until a PMID is found and the abstract checked. Run just fetch-reference on any candidate PMID before it goes in the entry.

Suggested next steps

1. Fetch and cache PMID 15494731 (confirm the correct Yeo number), 17130481, and the Gray 2007 and Manchester NTRK2 papers.
2. Look up every CURIE, including the HGNC ID for NTRK2.
3. Search for a recent NTRK2 review and any newer NTRK2 cases. I did not find any.
4. Decide how to handle POU3F2, GNB1 and GNAS: separate entries or OUT_OF_SCOPE. Do not fold them into this entry.

Sources

• MedGen C3151303
• MalaCards: OBHD
• PhenoDis: OBHD
• LOVD disease 03483
• MeSH C563938
• Functional characterization of human NTRK2 mutations (Manchester)
• Gray et al., Int J Obes (Nature)
• PMC2413291, appeared in results, not read
• POU3F2 paper, PMC10257002, appeared in results but fetch failed
• ESPE 2023 abstract on GNB1

Reference Validation

Checked with linkml-reference-validator 0.3.0.

2 of 4 references resolved; the rest could not be looked up either way.

Term Validation

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

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:0002591 (1 mention) - the report calls it "Hyperphagia"; HP calls it Polyphagia, and lists "Hyperphagia" among its other names
• HP:0000752 (1 mention) - the report calls it "Hyperactivity (BDNF-pathway cases)"; HP calls it Hyperactivity

Every term resolved, and every label the report gave matched.

1. Disease Information

Overview. OBHD is a syndromic monogenic obesity disorder defined by the triad of early-onset severe obesity, hyperphagia (insatiable appetite), and global developmental delay, with characteristic impairment of short-term memory and language. The MONDO record (EBI OLS) defines it as "A syndromic disease caused by a variation in the NTRK2 gene, characterized by early-onset severe obesity, hyperphagia, and global developmental delay with specific impairment of short-term memory and language delay."

Key identifiers.

Synonyms / alternative names. OBHD; Obesity-Hyperphagia-Developmental Delay syndrome; Early-onset obesity–hyperphagia–severe developmental delay syndrome; NTRK2-related obesity.

Data source type. Information is derived from aggregated disease-level resources (OMIM, Orphanet, MONDO, HPO, ClinVar, gnomAD) combined with individual-patient case reports and a 44-individual genotype–phenotype cohort. The index case was a single detailed patient report (PMID: 15494731); the allelic-series delineation came from an aggregated cohort (PMID: 39540377).

2. Etiology

Primary cause — genetic. The disease is caused by heterozygous loss-of-function variants in NTRK2. The index patient carried a de novo missense variant p.Tyr722Cys (Y722C) that, as the authors state, "markedly impaired receptor autophosphorylation and signaling to MAP kinase" (PMID: 15494731). The larger cohort established that truncating variants, or variants that disturb the 3D protein structure leading to loss of function, produce "a remarkably milder phenotype of developmental delay, obesity, and hyperphagia" (PMID: 39540377).

Genetic risk factors. The causal variants are the risk factors; there are no recognized common susceptibility loci for this specific Mendelian entity. On the ligand side of the same axis, BDNF haploinsufficiency (e.g., WAGR 11p14 deletions) causes an overlapping obesity phenotype, confirming the pathway: WAGR patients with BDNF deletion "had significantly higher BMI z scores throughout childhood than did patients with intact BDNF" (PMID: 18753648).

Environmental risk factors. As a highly penetrant Mendelian disorder, environmental contribution to causation is minimal. However, general obesogenic exposures (hypercaloric diet, sedentary behavior) plausibly modify the severity of weight gain. Rodent data show early-life hypercaloric diet alters BDNF/TrkB signaling and feeding behavior, illustrating gene–environment convergence on the same molecular axis (PMID: 32926590).

Protective factors. No specific genetic protective alleles are established for this disorder. Environmentally, dietary restriction and structured feeding management reduce weight gain but do not correct the underlying hyperphagia driver.

Gene–environment interactions. The shared node is BDNF/TrkB tone. Reduced BDNF/TrkB signaling (genetic) and obesogenic diet (environmental) both push the same circuit toward hyperphagia, suggesting additive/synergistic weight gain in affected individuals exposed to obesogenic environments.

3. Phenotypes

The official HPO disease annotation for OMIM:613886 (JAX HPO) and the index/cohort reports define the phenotype spectrum below. Frequencies "1/1" reflect the single annotated index proband; the cohort confirms the core triad broadly.

The index report emphasized that "the associated impairment in memory, learning and nociception seen in the proband reflects the crucial role of TrkB in the human nervous system" (PMID: 15494731). The loss-of-function cohort confirmed "developmental delay, obesity, and hyperphagia" as the core triad, with cognitive impairment milder than in the gain-of-function group (PMID: 39540377).

Phenotype characteristics. Onset is infantile/early childhood (HP:0003593 infantile onset). Severity ranges from moderate to severe; obesity and hyperphagia are typically severe and progressive if unmanaged, whereas developmental delay is static/non-progressive. Quality-of-life impact is substantial: insatiable hyperphagia dominates daily functioning (analogous to other MC4R-pathway hyperphagic diseases, where patients describe "unrelenting hunger"; PMID: 42821228), and developmental delay/ID impairs independence, learning, and communication lifelong.

4. Genetic / Molecular Information

Causal gene. NTRK2 (neurotrophic receptor tyrosine kinase 2), encoding TrkB. HGNC:8032; chromosome 9q21.33 (GRCh38 chr9:~84.67–85.10 Mb); Ensembl ENSG00000148053; UniProt Q16620.

Pathogenic variants. - Index variant: c.2165A>G, p.Tyr722Cys (Y722C), de novo heterozygous missense in the tyrosine kinase domain. As reported: "An 8-year-old male with a complex developmental syndrome and severe obesity was heterozygous for a de novo missense mutation resulting in a Y722C substitution"; the variant functionally impairs autophosphorylation and MAPK signaling (loss of function) (PMID: 15494731). - LoF group (this disease): "The second group had truncating variants or variants that presumably disturb the 3D structure of the protein leading to loss of function" → obesity/hyperphagia/developmental delay (PMID: 39540377). - GoF group (distinct disease): variants in the transmembrane cholesterol-binding motif, recurrent c.1301A>G p.(Tyr434Cys) → upregulation of TrkB activity → severe DD/ID, refractory epilepsy, blindness (PMID: 39540377).

Variant classification & burden. ClinVar (retrieved 2026-10-17): NTRK2[gene] returns 978 total variant records, of which 130 are Pathogenic/Likely pathogenic, and 49 records annotated to an obesity phenotype. Classifications follow ACMG/AMP criteria. As the cohort noted, "Heterozygous pathogenic variants in NTRK2 (HGNC: 8032) have been associated with global developmental delay" (PMID: 39540377).

Allele frequency. Pathogenic variants are ultra-rare/absent in gnomAD, consistent with de novo dominant disease. NTRK2 is extremely LoF-constrained: gnomAD v2 pLI = 1.00, LOEUF (oe_lof_upper) = 0.219, observed/expected LoF = 0.138 (13 observed vs 94.5 expected), LoF-Z = 7.11; also missense-constrained (missense-Z = 5.63, oe_mis = 0.60).

Somatic vs germline. Germline (de novo or, rarely, inherited). Not a somatic/cancer entity in this context.

Functional consequences. Loss of function / haploinsufficiency for this disease (impaired kinase activity, autophosphorylation, and downstream MAPK/PI3K signaling). The allelic series demonstrates that gain of function produces a mechanistically and clinically opposite neurological disorder.

Modifier genes / epigenetics / chromosomal abnormalities. No established Mendelian modifier genes specific to NTRK2 OBHD. Epigenetic regulation of BDNF/NTRK2 (e.g., diet-induced HDAC5 changes in rodents; PMID: 32926590) may modulate pathway tone but is not a primary disease mechanism. Large 9q21 deletions encompassing NTRK2 would be detected by chromosomal microarray (DECIPHER/ClinVar), though point variants predominate in reported cases.

5. Environmental Information

• Environmental factors: No toxicological, radiation, or occupational cause. This is a monogenic disorder.
• Lifestyle factors: Diet and physical activity modify the degree of obesity but not its genetic cause. Hyperphagia makes dietary control exceptionally difficult.
• Infectious agents: Not applicable.

6. Mechanism / Pathophysiology

Ordered causal chain (initiating lesion → clinical manifestation)

1. A heterozygous loss-of-function variant in NTRK2 (de novo) results in a TrkB receptor with impaired kinase activity (e.g., Y722C abolishes autophosphorylation) — demonstrated in vitro (PMID: 15494731).
2. Impaired TrkB leads to failure of BDNF→TrkB signal transduction (reduced MAPK/ERK and PI3K-AKT and PLCγ signaling) in TrkB-expressing neurons — demonstrated for MAPK; PI3K/PLCγ inferred by homology to canonical TrkB signaling.
3. In the hypothalamic PVH and DMH, loss of TrkB signaling removes an anorexigenic "satiety brake" that normally operates downstream of MC4R — demonstrated by mouse circuit genetics (PMID: 32265438, PMID: 30718415).
4. Absent satiety signaling results in hyperphagia (insatiable appetite), which precedes weight gain — demonstrated in Nkx2.1-Ntrk2 mice (PMID: 26629410).
5. Sustained hyperphagia, with no compensatory increase in energy expenditure, leads to positive energy balance and severe early-onset obesity, with secondary hyperinsulinemia, hyperglycemia, and hyperleptinemia — demonstrated in mice (PMID: 26629410).

Branch (neurodevelopment): In parallel, loss of TrkB signaling in broader CNS neurons impairs neuronal survival, differentiation, and synaptic plasticity, leading to global developmental delay, intellectual disability, impaired memory/learning, behavioral abnormalities, hypotonia, and blunted nociception — inferred from TrkB's established neurobiology and index-patient phenotype (PMID: 15494731).

Mechanistic detail

• Molecular pathways. Canonical TrkB signaling: MAPK/ERK (demonstrated defective in Y722C), PI3K-AKT-mTOR, and PLCγ. These transduce BDNF's anorexigenic and neurotrophic effects. KEGG: Neurotrophin signaling pathway (hsa04722); MAPK (hsa04010); PI3K-AKT (hsa04151).
• Cellular processes. Synaptic plasticity, neuronal survival/differentiation (neurotrophic support), and hypothalamic satiety neurotransmission. GO: neurotrophin TRK receptor signaling pathway (GO:0048011), positive regulation of synaptic transmission (GO:0050806), regulation of eating behavior (GO:0042755), energy homeostasis (GO:0097009).
• Protein dysfunction. Loss of receptor tyrosine kinase autophosphorylation and downstream phospho-signaling (loss of function). Gain-of-function variants (distinct disease) increase constitutive activity.
• Metabolic changes. Secondary: hyperinsulinemia, hyperglycemia, hyperleptinemia, increased adiposity; reduced locomotor activity and core body temperature contribute in mouse models (PMID: 26629410).
• Immune system. Not a primary feature.
• Upstream vs downstream. Upstream: leptin → POMC/α-MSH → MC4R. Node: BDNF → TrkB acts downstream of MC4R in PVH/DMH. Downstream of TrkB defect: failed satiety → hyperphagia → obesity. This downstream position is therapeutically important (see §12).
• Cell types & GO/CL terms. TrkB-expressing hypothalamic neurons in PVH and DMH; broadly, central neurons. CL: neuron (CL:0000540), glutamatergic neuron (CL:0000679). GO biological processes above.

Supporting circuit evidence (model organisms): - "selective Ntrk2 deletion within paraventricular hypothalamus (PVH) leads to severe hyperphagic obesity" (PMID: 32265438). - TrkB neurons in the DMH are necessary and sufficient to suppress homeostatic feeding (PMID: 30718415). - Hypothalamic TrkB ablation causes hyperphagia preceding weight gain, with hyperglycemia/hyperinsulinemia/hyperleptinemia (PMID: 26629410). - Human ligand evidence: BDNF-haploinsufficient (WAGR) children "had significantly higher BMI z scores throughout childhood than did patients with intact BDNF" (PMID: 18753648).

Shared-circuit context: SIM1 (differential, not causal for MONDO:0013483)

The phenotypically overlapping 6q16 deletion / SIM1 haploinsufficiency Prader–Willi-like syndrome converges on the same PVH circuit from a different angle: SIM1 builds the PVH and sustains MC4R and oxytocin expression. In Sim1⁺/⁻ mice, "Oxt mRNA and peptide were decreased by 80% in Sim1(+/-) mice" and central oxytocin rescues feeding (PMID: 18451093); conditional/germline Sim1 loss shows "a remarkable decrease in hypothalamic oxytocin (Oxt) and PVN melanocortin 4 receptor (Mc4r) mRNA" (PMID: 20220015), and inducible inactivation "caused increased food and water intake and decreased expression of PVN neuropeptides, especially oxytocin and vasopressin, with no change in energy expenditure" (PMID: 24773343). This explains the clinical mimicry and underscores why NTRK2 must be distinguished molecularly from SIM1/6q16 disease.

7. Anatomical Structures Affected

• Organ level (primary): Brain, specifically the hypothalamus — paraventricular nucleus (UBERON:0001930) and dorsomedial nucleus (UBERON:0002708). Broader CNS involvement underlies developmental delay.
• Body systems: Nervous (central regulation of appetite and neurodevelopment) and endocrine/metabolic (secondary — adipose tissue, pancreatic islets via hyperinsulinemia).
• Secondary organ involvement: Adipose tissue expansion (UBERON:0001013); metabolic sequelae in liver/pancreas (insulin resistance).
• Tissue/cell level: Nervous tissue; TrkB-expressing hypothalamic neurons (CL:0000540 neuron). Peripheral adipose BDNF/TrkB axis also modulates feeding, especially in females (PMID: 28721258).
• Subcellular level (GO Cellular Component): plasma membrane receptor (GO:0005886), receptor complex (GO:0043235), postsynaptic membrane (GO:0045211), cytoplasmic kinase signaling.
• Localization / lateralization: Bilateral, symmetric CNS involvement (not lateralized).

UBERON suggestions: UBERON:0000955 (brain), UBERON:0001898 (hypothalamus), UBERON:0001930 (paraventricular nucleus), UBERON:0002708 (dorsomedial hypothalamic nucleus).

8. Temporal Development

• Onset: Infantile/early childhood (HP:0003593 infantile onset). Obesity and hyperphagia emerge in early life; developmental delay is apparent in infancy (hypotonia, delayed milestones).
• Onset pattern: Insidious/chronic.
• Progression: Obesity is progressive if hyperphagia is unmanaged; developmental delay/ID is static (non-progressive). Seizures, when present, are variable.
• Disease course: Chronic, lifelong. No spontaneous remission of the genetic defect; weight can be modulated by intensive management.
• Critical periods: Early childhood is the key window for feeding-behavior intervention and developmental/early-intervention therapies; hyperphagia precedes measurable weight gain (mouse data; PMID: 26629410), supporting early vigilance.

9. Inheritance and Population

• Inheritance: Autosomal dominant (HP:0000006), typically de novo heterozygous (PMID: 15494731; PMID: 39540377).
• Penetrance: High for the dominant LoF variants reported; precise penetrance estimates are limited by rarity. (Contrast: SIM1 haploinsufficiency shows incomplete penetrance — "only 8 of 13 patients with SIM1 deletion exhibited obesity, in agreement with incomplete penetrance of SIM1 haploinsufficiency"; PMID: 25351778.)
• Expressivity: Variable (severity of obesity, DD, seizures differs across individuals).
• Genetic anticipation / mosaicism / founder effects: Not applicable / not reported (no repeat expansion; ultra-rare de novo variants preclude founder effects).
• Consanguinity / carrier frequency: Not relevant for a dominant de novo disorder; carrier frequency effectively zero in gnomAD (pLI = 1.00).
• Epidemiology: Ultra-rare. Exact prevalence/incidence undefined; only a small number of probands worldwide (one index case plus the LoF subset of a 44-individual NTRK2 cohort). Orphanet classifies it among rare syndromic obesities.
• Population demographics: No ethnic/geographic predilection; de novo origin implies sporadic global occurrence. Sex ratio not established (mouse models suggest females may show a more robust metabolic phenotype — PMID: 26629410, PMID: 28721258 — but this is not confirmed in humans).

10. Diagnostics

Clinical/laboratory tests. No specific diagnostic biomarker. Supportive findings: elevated fasting insulin, glucose, and leptin (secondary to obesity). Standard obesity work-up (lipids, HbA1c, thyroid, cortisol) to exclude other causes.

Genetic testing (definitive). - Recommended approach: Molecular confirmation of a pathogenic NTRK2 variant. In a child with severe early-onset obesity + hyperphagia + developmental delay, phenotype-driven genetic testing is indicated (PMID: 42761221). - NGS obesity gene panels (leptin–melanocortin pathway panels including NTRK2, BDNF, MC4R, POMC, LEPR, SIM1, etc.). The Greek cohort used a 101-gene monogenic-obesity panel (PMID: 42761221). - Whole-exome (WES)/whole-genome (WGS) sequencing: high utility for detecting de novo point variants and resolving syndromic presentations. - Chromosomal microarray (CMA): to detect 9q21 deletions involving NTRK2 and to exclude 6q16 (SIM1) and 15q11-q13 (Prader–Willi) in the differential. - Variant interpretation: ACMG/AMP criteria; many obesity-panel variants are VUS and must be interpreted cautiously (PMID: 42761221).

Imaging. Brain MRI may show delayed myelination (rare); not diagnostic.

Clinical criteria & differential diagnosis. Diagnosis rests on the clinical triad plus molecular confirmation. Differential diagnosis includes: - Prader–Willi syndrome (15q11-q13) and Prader–Willi-like / SIM1 6q16 deletion (clinically very similar; distinguished by CMA/molecular testing). - MC4R, POMC, LEPR, BDNF monogenic obesity (leptin–melanocortin pathway). - ARNT2-related syndrome (SIM1 heterodimer partner; PMID: 34214899). - MRAP2 deficiency. - Acquired/hypothalamic obesity and ROHHAD.

Screening. Cascade testing of at-risk relatives if an inherited (non-de novo) variant is found; prenatal/preimplantation testing possible for known familial variants.

11. Outcome / Prognosis

• Survival/mortality: No direct disease-specific mortality data; prognosis is driven by obesity-related morbidity (type 2 diabetes, cardiovascular disease, metabolic dysfunction-associated steatotic liver disease) and complications of developmental disability. Life expectancy may be reduced by severe obesity complications if uncontrolled.
• Morbidity/function: Substantial lifelong disability from intellectual disability/developmental delay and from refractory hyperphagic obesity. Quality of life is heavily impacted by insatiable hunger (analogous MC4R-pathway diseases; PMID: 42821228).
• Complications: Insulin resistance/type 2 diabetes, dyslipidemia, hepatic steatosis, orthopedic and sleep complications of obesity; seizures in a minority; behavioral/psychiatric challenges.
• Prognostic factors: Severity of hyperphagia, success of weight management, degree of cognitive impairment, and presence of epilepsy. Developmental delay is static but limits adaptive outcomes.
• Recovery potential: The genetic defect is not reversible; weight and metabolic parameters are partially modifiable with intensive management.

12. Treatment

General principle — multidisciplinary, supportive. Management combines structured dietary/behavioral control of hyperphagia, physical activity, developmental/early-intervention therapies (speech, occupational, physical therapy), anti-seizure medication where indicated, and management of metabolic complications.

Pharmacotherapy / mechanism-based therapy — an important caveat. The MC4R agonist setmelanotide (NCIT: Setmelanotide) acts at MC4R. Because the NTRK2/TrkB lesion lies downstream of MC4R, setmelanotide's efficacy in NTRK2-OBHD is mechanistically uncertain and unproven. Setmelanotide is effective where downstream MC4R signaling is intact (e.g., POMC/LEPR deficiency, acquired hypothalamic obesity reconceptualized as "α-MSH deficiency"; PMID: 42114845, PMID: 42878773, PMID: 42496807). In TrkB loss of function, the deficit is at/after the TrkB node, so melanocortin agonism may not restore the missing satiety signal. This should be regarded as experimental for this indication.

Advanced/experimental therapeutics. No approved gene therapy, cell therapy, or RNA therapy. A TrkB agonist strategy is mechanistically rational for loss-of-function variants but unproven clinically; it would be contraindicated for the gain-of-function allelic group. No NCT trials specific to NTRK2-OBHD identified.

Surgical/interventional. Bariatric/metabolic surgery is considered in hypothalamic/refractory obesity but with heterogeneous response; evidence is limited and individualized (PMID: 42496807).

Supportive & rehabilitative. Nutritional counseling and tightly structured food environment; developmental and behavioral therapies; management of comorbidities (diabetes, dyslipidemia, sleep apnea).

Personalized medicine. Genotype-guided: distinguishing LoF vs GoF NTRK2 variants is essential because they define opposite disorders with opposite therapeutic logic (TrkB agonism vs. seizure/antagonist management).

NCIT suggestions: Setmelanotide (melanocortin receptor agonist), Behavioral/Dietary Weight Management, Rehabilitation Therapy, Anticonvulsant/Antiepileptic Therapy.

13. Prevention

• Primary prevention: Not possible for a de novo genetic disorder. Genetic counseling for families with an identified variant.
• Secondary prevention: Early molecular diagnosis enables early feeding-behavior management and metabolic monitoring to limit obesity severity and complications; early developmental intervention optimizes neurodevelopmental outcomes.
• Tertiary prevention: Aggressive management of obesity complications (diabetes, cardiovascular, hepatic, sleep), seizure control, and behavioral support.
• Genetic counseling: Recurrence risk is low for de novo variants (but germline mosaicism cannot be excluded); prenatal/PGT available for known familial variants.
• Screening: Consider NTRK2 within monogenic-obesity panels for children with severe early-onset obesity, hyperphagia, and neurodevelopmental delay (PMID: 42761221).

14. Other Species / Natural Disease

• Taxonomy / orthologs: Ntrk2 is highly conserved. Mouse Ntrk2 (Mus musculus, NCBI Taxon 10090) and rat ortholog are the principal experimental orthologs; BDNF/TrkB function in energy balance is evolutionarily conserved across mammals (PMID: 38672441).
• Natural disease in other species: No well-characterized spontaneous NTRK2-obesity syndrome reported in companion animals (OMIA search did not yield an established natural ortholog disease).
• Comparative biology: Mouse models faithfully reproduce the hyperphagia-driven obesity and confirm the hypothalamic BDNF/TrkB circuit, supporting strong cross-species conservation of the mechanism.
• Transmission/zoonosis: Not applicable (non-infectious genetic disease).

15. Model Organisms

Mouse (Mus musculus) is the primary model.

Phenotype recapitulation. Mouse TrkB/Ntrk2 models reproduce the hyperphagia and obesity core and localize the effect to hypothalamic PVH/DMH — excellent recapitulation of the metabolic phenotype. Limitations: rodent models less fully capture the human cognitive/developmental delay, memory, language, and nociception phenotypes; sex-dependence in mice is not clearly mirrored in humans. Resources: MGI (Ntrk2), IMPC/KOMP for knockout alleles; patient-derived iPSCs for human neuronal modeling.

Mechanistic Model (synthesis)

   LEPTIN  ──►  POMC neurons  ──►  α-MSH  ──►  MC4R (PVH)        [upstream, intact]
                                 │
                                 ▼
              BDNF ──► TrkB (NTRK2) in PVH/DMH   ◄── LESION (LoF variant)
                                 │                      (Y722C: no autophosphorylation;
                                 │                       truncating → haploinsufficiency)
              (MAPK/ERK, PI3K-AKT, PLCγ signaling)
                                 │
        ┌────────────────────────┴──────────────────────────┐
        ▼ (metabolic branch)                                 ▼ (neurodevelopmental branch)
  Loss of anorexigenic "satiety brake"                 Loss of neurotrophic support
        │                                      (survival, differentiation, plasticity)
        ▼                                                     ▼
    HYPERPHAGIA (precedes weight gain)         Global developmental delay / ID,
        │                               impaired memory & language,
    No ↑ energy expenditure                     hypotonia, blunted nociception,
        ▼                               behavioral abnormalities, ± seizures
      SEVERE EARLY-ONSET OBESITY
      (2° hyperinsulinemia, hyperglycemia, hyperleptinemia)

Key point for therapy: the TrkB defect is downstream of MC4R, so MC4R agonists (setmelanotide) may not rescue satiety — unlike in POMC/LEPR deficiency or acquired hypothalamic obesity, where MC4R signaling is intact.

SIM1 vs NTRK2 — same circuit, distinct genes

Evidence Base

Limitations and Knowledge Gaps

1. Rarity limits epidemiology. Prevalence, incidence, sex ratio, and penetrance for NTRK2-OBHD are not quantitatively established; the HPO frequencies are anchored largely to the index proband.
2. Mid-course gene correction. Early iterations pursued SIM1/6q16; the authoritative MONDO definition anchors MONDO:0013483 to NTRK2. SIM1 material is retained as shared-circuit/differential context, not as the cause of this entity. Readers should not conflate the two.
3. Human neurodevelopmental mechanism is partly inferred. The feeding mechanism is strongly demonstrated in mice; the cognitive/memory/nociception phenotypes are inferred from TrkB's general neurobiology and single-patient observations rather than mechanistic human studies.
4. Therapeutics unproven. No controlled trials in NTRK2-OBHD. The downstream-of-MC4R position makes setmelanotide's benefit uncertain; TrkB-agonist therapy is conceptual.
5. Variant interpretation. Many obesity-panel variants are VUS; functional validation (as done for Y722C) is needed to classify novel NTRK2 variants and to assign them to the LoF vs GoF axis.
6. Signaling branches partly inferred. MAPK defect is demonstrated; PI3K-AKT and PLCγ contributions are inferred from canonical TrkB biology.

Proposed Follow-up Experiments / Actions

1. Functional variant classification pipeline. Systematically assay novel NTRK2 variants (autophosphorylation, MAPK/ERK, PI3K-AKT, PLCγ readouts) to assign LoF vs GoF — critical because the two define opposite disorders and opposite therapeutic logic.
2. Patient-derived iPSC hypothalamic organoids/neurons. Model TrkB LoF in human PVH/DMH-like neurons to test satiety-signaling defects and candidate TrkB-agonist rescue in a human system.
3. Natural-history registry. Aggregate NTRK2-OBHD cases (via GeneMatcher/Orphanet) to define penetrance, sex ratio, progression, seizure frequency, and long-term outcomes.
4. Therapeutic test of downstream vs upstream logic. Preclinically compare MC4R agonism (setmelanotide) vs small-molecule TrkB agonists in hypothalamic Ntrk2-LoF models to determine whether rescuing below the lesion is required.
5. Deep phenotyping of cognition/nociception. Standardized neurocognitive and sensory testing across the LoF cohort to characterize the neurodevelopmental branch quantitatively.
6. Biomarker discovery. Explore circulating BDNF, leptin, and insulin trajectories as potential monitoring biomarkers in affected children.

Evidence source types are indicated throughout: human clinical (index case, cohorts, WAGR), model organism (mouse Ntrk2/Sim1 genetics), in vitro (luciferase/autophosphorylation assays), and computational (gnomAD constraint, ClinVar burden). Ontology suggestions (HPO, GO, CL, UBERON, NCIT, MONDO) are embedded in the relevant sections.

Artifacts

• OpenScientist final report
• OpenScientist final report

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:0013483 (9 mentions) - the report calls it "if available", "MONDO"; MONDO calls it obesity, hyperphagia, and developmental delay
• HP:0001328 (1 mention) - the report calls it "Impaired learning"; HP calls it Specific learning disability
• UBERON:0001013 (1 mention) - the report calls it "Secondary organ involvement: Adipose tissue expansion"; UBERON calls it adipose tissue**

Unresolved terms

These identifiers do not exist in an ontology that resolved other terms from the same prefix, so they were most likely invented:

• HP:0009074 (1 mention), reported as "Impaired short-term memory" - HP does not contain this term

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:0002591 (1 mention) - the report calls it "Hyperphagia / polyphagia"; HP calls it Polyphagia, and lists "Hyperphagia" among its other names
• HP:0001250 (1 mention) - the report calls it "Seizure (general)"; HP calls it Seizure
• GO:0042755 (1 mention) - the report calls it "regulation of eating behavior"; GO calls it eating behavior
• UBERON:0001930 (2 mentions) - the report calls it "paraventricular nucleus"; UBERON calls it paraventricular nucleus of hypothalamus, and lists "paraventricular nucleus" among its other names
• UBERON:0002708 (2 mentions) - the report calls it "dorsomedial hypothalamic nucleus"; UBERON calls it posterior periventricular nucleus, and lists "posterior periventricular hypothalamic nucleus" among its other names
• GO:0043235 (1 mention) - the report calls it "receptor complex"; GO calls it signaling receptor complex, and lists "receptor complex" among its other names
• HP:0000006 (1 mention) - the report calls it "Autosomal dominant", "Inheritance: Autosomal dominant"; HP calls it Autosomal dominant inheritance, and lists "Autosomal dominant" among its other names

Terms named inconsistently

The report gives these identifiers more than one name of its own:

• MONDO:0013483 - called "if available", "MONDO"
• HP:0000006 - called "Autosomal dominant", "Inheritance: Autosomal dominant"

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: OMIM, Orphanet, UMLS, ORPHA.

Reference Validation

Checked with linkml-reference-validator 0.3.0.

All extracted references resolved successfully.