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.
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.
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.
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.
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
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
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
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)
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
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.
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
SRC1 (NCOA1) Deficiency
- Clinical hallmarks: Severe hyperphagia; cognitive impairment variable; reduced physical activity; AR
- Lab: NCOA1 sequencing (rare — often identified via research panels)
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.
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.
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.
Sources