Monogenic Obesity
Monogenic obesity is severe early-onset obesity caused by a mutation in a single gene, most often within the leptin-melanocortin appetite pathway.
Plain English
Patient-Friendly Explanation
Most obesity comes from many genes and the environment acting together. In a small number of people, however, a single faulty gene is the main cause. These patients usually gain weight very early, often in the first years of life, and feel hungry almost constantly no matter how much they eat. The genes involved include those for leptin, the leptin receptor, POMC, PCSK1 and the melanocortin-4 receptor. Finding a single-gene cause matters, because a few of these conditions have specific treatments.
Medical
Clinical Definition
Monogenic non-syndromic obesity arises from pathogenic variants in genes of the leptin-melanocortin pathway and accounts for roughly 5 to 10 per cent of severe early-onset childhood obesity, depending on the cohort and how strictly the phenotype is defined. The commonest cause is melanocortin-4 receptor deficiency, characterised by hyperphagia, accelerated linear growth and normal or tall final height. Upstream defects include leptin and leptin receptor deficiency, POMC deficiency, which causes obesity with red hair and adrenal insufficiency, and proprotein convertase 1/3 deficiency. Causative variants are also described in SH2B1, KSR2 and SIM1. Unlike polygenic obesity, leptin deficiency responds dramatically to metreleptin replacement, POMC and PCSK1 deficiencies may respond to melanocortin-4 receptor agonists such as setmelanotide, and melanocortin-4 receptor deficiency tends to show a blunted response to bariatric surgery. Diagnosis requires a high index of suspicion, prompted by extreme early-onset obesity, hyperphagia, consanguinity, a positive family history and hypogonadism, and is confirmed by gene panel sequencing.
Key Insight
Why It Matters
The value of the diagnosis is not academic: a confirmed leptin or POMC defect opens a targeted treatment that works, whereas the same patient could otherwise spend years on lifestyle plans that cannot address the underlying cause.
Sources
Reviewed against standard medical references.
Medically reviewed by
Family Physician & Registered Acupuncturist, Accord Medical Clinic. MBBS (NUS), Graduate Diploma in Family Medicine (NUS), Graduate Diploma in Acupuncture (Singapore College of TCM).
Last reviewed: September 2026
This page is general health education, not medical advice. Whether any treatment is appropriate for you is a decision made in consultation after assessment.
