monogenic-obesity-diagnosis

Monogenic & Syndromic Obesity — Diagnostic Pathway

Structured diagnosis for children and adolescents with suspected genetically-driven obesity. Uses the leptin-melanocortin pathway framework and key clinical features to reach a specific genetic diagnosis and matched pharmacotherapy.

Sources: Kalinderi et al., Children 2024; Fitch et al., Obesity Pillars 2024; UpToDate Feb 2026.

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Step 1 — Entry Criteria: Is This Case Atypical Enough to Pursue?

Proceed with this pathway if ≥1 of the following:

• Obesity onset before age 5 (especially before age 2)
• Severe, insatiable hyperphagia — child cannot feel full, food-seeking behaviour
• BMI z-score ≥ +3 early in life
• Positive family history of severe early-onset obesity (autosomal recessive or dominant pattern)
• Prior red flag screen (etiology screener) flagged a monogenic or syndromic cause
• Failed standard behavioural intervention with no plausible exogenous explanation

If you haven't run the etiology screener yet, consider doing so first.

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Step 2 — Primary Branch: Is Developmental Delay or Intellectual Disability Present?

This is the single most important fork in the diagnostic algorithm.

➤ YES — Developmental delay, intellectual disability, or learning difficulties present

→ Suspect Syndromic obesity. Continue to Step 3A.

➤ NO — Normal development and cognition

→ Suspect Monogenic (non-syndromic) obesity. Continue to Step 3B.

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Step 3A — Syndromic Obesity: Identify the Syndrome

Collect: dysmorphic features, vision/hearing, organ anomalies, hypotonia history, polydactyly, behaviour pattern.

Syndrome	Key Distinguishing Features	Inheritance
Prader-Willi (PWS)	Neonatal hypotonia, poor feeding → hyperphagia age 2–5; short stature; hypogonadism; almond-shaped eyes; skin picking; no polydactyly	Chromosome 15q11-q13 (imprinting)
Bardet-Biedl (BBS)	Rod-cone dystrophy (night blindness, visual field loss); polydactyly (pre- or post-axial); renal anomalies; hypogonadism; mild-moderate cognitive impairment	AR — BBS1–BBS21 genes
Albright's Hereditary Osteodystrophy (AHO/GNAS)	Short stature; round facies; brachydactyly; ectopic ossification; hormone resistance (PTH, TSH)	AD — GNAS (maternally inherited → pseudohypoparathyroidism type 1A)
Smith-Magenis (SMS)	Severe behavioural problems; inverted sleep-wake cycle; self-injurious behaviour; hugging behaviour	AD (usually de novo) — RAI1 deletion 17p11.2
SIM1 haploinsufficiency	PWS-like phenotype but NO neonatal hypotonia; severe hyperphagia; developmental delay variable	AD
SH2B1 deletion (16p11.2)	Hyperphagia; severe insulin resistance; developmental and language delay	AD — 16p11.2 chromosomal deletion

If PWS is suspected → confirm with methylation testing (not standard sequencing).

If BBS is suspected → retinal assessment + renal ultrasound before genetics.

→ Once syndrome identified, go to Step 5 for genomic test and Step 6 for pharmacotherapy.

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Step 3B — Monogenic Obesity: Work Through the Leptin-Melanocortin Pathway

All monogenic non-syndromic obesity disrupts the hypothalamic satiety axis. Ask about: onset age, severity of hyperphagia, immune phenotype, adrenal/gonadal function, GI symptoms.

Key Pathway: LEP → LEPR → POMC/PCSK1 → MC4R

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Leptin Deficiency (LEP mutation)

• Clinical hallmarks: Profound hyperphagia from birth; normal birth weight then rapid weight gain; recurrent infections (impaired immune function); hypogonadotropic hypogonadism; low/undetectable serum leptin; AR
• Distinguishing feature: Very low leptin level despite severe obesity (leptin is normally elevated in obesity)
• Lab: Serum leptin (undetectable or extremely low)
• → Pharmacotherapy: Metreleptin (recombinant leptin) — 0.03–0.06 mg/kg/day SC — highly effective

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Leptin Receptor Deficiency (LEPR mutation)

• Clinical hallmarks: Similar to leptin deficiency — severe early-onset hyperphagia, rapid weight gain; normal or elevated serum leptin (receptor non-functional); hypogonadotropic hypogonadism; growth hormone deficiency; AR
• Distinguishing feature: High leptin but same phenotype as leptin deficiency
• Lab: Serum leptin (elevated), LEPR sequencing
• → Pharmacotherapy: Setmelanotide — up to 3 mg SC daily — targets downstream MC4R axis

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POMC Deficiency

• Clinical hallmarks: Early-onset hyperphagia + severe obesity; adrenal insufficiency (ACTH deficiency — POMC is precursor); red/fair hair in European populations (MSH derived from POMC); pale skin; AR
• Distinguishing feature: Adrenal insufficiency + red hair + hyperphagia triad
• Lab: Low cortisol/ACTH, POMC sequencing
• → Pharmacotherapy: Setmelanotide — up to 3 mg SC daily (targets MC4R directly below POMC)

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PCSK1 Deficiency (PC1/3)

• Clinical hallmarks: Severe early-onset obesity; malabsorptive diarrhoea in infancy (impaired proinsulin processing → GI dysmotility); hypoglycaemia; hypogonadotropic hypogonadism; central diabetes insipidus possible; AR
• Distinguishing feature: Neonatal diarrhoea + hypoglycaemia + obesity
• Lab: Proinsulin:insulin ratio elevated, PCSK1 sequencing
• → Pharmacotherapy: Setmelanotide — up to 3 mg SC daily

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MC4R Mutation

• Clinical hallmarks: Most common monogenic cause (2–5% of severe childhood obesity); hyperphagia; tall stature (linear growth acceleration); increased lean mass and bone density (distinguishes from other causes); hyperinsulinaemia; AD (heterozygous = incomplete penetrance)
• Distinguishing feature: Tall for age + increased muscle mass + hyperphagia — NOT short or dysmorphic
• Lab: MC4R sequencing
• → Pharmacotherapy: Setmelanotide shows benefit (especially homozygous/compound het). Also consider GLP-1 agonists (semaglutide/liraglutide) for general obesity component.

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Alström Syndrome (ALMS1)

• Clinical hallmarks: Resembles BBS but NO polydactyly, NO intellectual disability; cone-rod dystrophy (nystagmus, photophobia early); cardiomyopathy (dilated, often presenting in infancy); sensorineural hearing loss; type 2 diabetes early; AR
• Distinguishing feature: BBS-like but cognitively normal + cardiomyopathy
• Lab: ALMS1 sequencing, echocardiogram, audiology

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SRC1 (NCOA1) Deficiency

• Clinical hallmarks: Severe hyperphagia; cognitive impairment variable; reduced physical activity; AR
• Lab: NCOA1 sequencing (rare — often identified via research panels)

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Step 4 — Confirm with Genetic Testing

Once a likely diagnosis is identified, order the appropriate NHS genomic test.

Invoke the nhs-genomic-test-finder skill and search for the relevant R-code. Key tests to look up:

Suspected Diagnosis	Search Term for NHS Finder
Leptin/LEPR/POMC/MC4R/PCSK1 deficiency	"monogenic obesity" or "R191"
Bardet-Biedl syndrome	"Bardet-Biedl"
Prader-Willi syndrome	"Prader-Willi"
Alström syndrome	"Alström" or "ALMS1"
Syndromic obesity panel (broad)	"obesity panel" or "R190"

The NHS Genomic Test Finder will surface: Test ID (R-code), target genes, test method, commissioning category, and referral pathway.

Commissioning reminder: Monogenic obesity panels are typically Specialised or Highly Specialised — referral to a paediatric genetics or obesity medicine centre is usually required before testing.

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Step 5 — Targeted Pharmacotherapy Summary

Diagnosis	Drug	Dose	Route	Notes
LEP deficiency	Metreleptin	0.03–0.06 mg/kg/day	SC daily	Highly effective; restores satiety and immune function; requires specialist prescribing
LEPR deficiency	Setmelanotide	Start 0.5 mg → titrate to max 3 mg/day	SC daily	EU/US approved; LEPR indication
POMC deficiency	Setmelanotide	Start 0.5 mg → titrate to max 3 mg/day	SC daily	Licensed in EU/US for POMC deficiency
PCSK1 deficiency	Setmelanotide	Start 0.5 mg → titrate to max 3 mg/day	SC daily	Licensed in EU/US
BBS	Setmelanotide	Start 0.5 mg → titrate to max 3 mg/day	SC daily	Licensed for BBS specifically
MC4R mutation	Setmelanotide	As above	SC daily	Consider + GLP-1 agonist for insulin resistance
General severe obesity (≥12 years)	Semaglutide	Up to 2.4 mg/week	SC weekly	STEP TEENS data; NICE approved in UK ≥12
General severe obesity (≥12 years)	Liraglutide	Up to 3 mg/day	SC daily	SCALE TEENS data

⚠️ Setmelanotide (Imcivree) and Metreleptin (Myalepta) are specialist medications — prescribe only through a centre experienced in monogenic obesity. Check current NHS England commissioning for access criteria.

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Step 6 — Clinical Guardrails

Don't miss LEP deficiency because leptin wasn't checked. Serum leptin is cheap, fast, and diagnostic — order it in any child with severe early-onset obesity + recurrent infections.

Elevated leptin ≠ no monogenic cause. LEPR deficiency has high leptin; interpret in clinical context.

MC4R is heterozygous and common. Finding one pathogenic MC4R variant doesn't mean you've found the whole picture — check for comorbid contributors. Incomplete penetrance is real.

PWS methylation testing, not sequencing. Standard sequencing misses PWS (imprinting defect). Request methylation-specific testing.

BBS needs eyes and kidneys assessed before genetics. Retinal dystrophy and renal anomalies are the dangerous comorbidities — don't let the genetics delay urgent organ assessment.

Adrenal insufficiency in POMC deficiency is life-threatening. Check cortisol before starting setmelanotide — ACTH deficiency requires hydrocortisone replacement first.

Setmelanotide in BBS: Works even without POMC/LEPR deficiency — MC4R pathway activation is the mechanism. Approved for BBS as a class, not gene-by-gene.

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Sources

• Kalinderi K et al. "Monogenic and Syndromic Forms of Childhood Obesity: An Update." Children 2024;11:153. https://doi.org/10.3390/children11020153
• Fitch AK et al. "Differentiating Monogenic and Syndromic Causes of Obesity from Common Polygenic Obesity." Obesity Pillars 2024. https://doi.org/10.1016/j.obpill.2024.100120
• UpToDate: "Genetic contribution and pathophysiology of obesity." Updated Feb 2026.
• NHS England National Genomic Test Directory for Rare and Inherited Disease, v9.0, April 2026.