rs489693 — MC4R MC4R AIWG variant

MC4R rs489693 — The Antipsychotic Weight Gain Locus

The melanocortin-4 receptor (MC4R) sits at the centre of the hypothalamus's appetite control system. When the brain releases satiety signals — leptin-driven alpha-melanocyte stimulating hormone¹¹ alpha-melanocyte stimulating hormone
α-MSH, derived from the POMC precursor peptide in hypothalamic neurons — MC4R receives them, suppressing hunger and raising energy expenditure. Rare coding mutations in MC4R cause severe monogenic obesity; common variants near the gene produce subtler, population-wide effects on body weight. rs489693 belongs to this second category: an intergenic variant located approximately 155 kilobases downstream of MC4R on chromosome 18, within a regulatory region that influences MC4R locus activity.

What makes rs489693 distinctive within the MC4R neighbourhood is its clinical specificity. Unlike rs17782313 (the strongest common MC4R-region obesity signal), rs489693 was discovered through a pharmacogenomics GWAS²² pharmacogenomics GWAS
genome-wide association study restricted to patients being treated with antipsychotic medications rather than a general population obesity scan. The A allele acts recessively — heterozygous AC carriers show modest effects, while AA homozygotes (about 10% of Europeans) experience markedly amplified weight gain when prescribed second-generation antipsychotics.

The Mechanism

rs489693 has no protein-coding consequence — it lies in intergenic DNA without a documented transcript. Its functional effect is presumed to be regulatory: the variant likely resides in an enhancer or repressor element³³ enhancer or repressor element
non-coding DNA that controls when and how much a nearby gene is expressed that modulates MC4R expression in hypothalamic or limbic tissue. Second-generation antipsychotics (SGAs) such as olanzapine and clozapine are known to antagonise histamine H1 and serotonin 5-HT2C receptors, triggering increased appetite and reducing energy expenditure. Reduced MC4R-mediated satiety tone appears to compound this drug effect: when the melanocortin brake on appetite is already weakened by genetic variation at this locus, the appetite-stimulating actions of SGAs go less opposed, producing larger weight increases.

The Malhotra et al. 2012 GWAS⁴⁴ Malhotra et al. 2012 GWAS
139 pediatric patients in discovery, three independent replication cohorts, total n=344 found that AA homozygotes showed not only greater weight gain but also consistent elevations in triglycerides, leptin, and insulin — a full metabolic syndrome profile. This suggests the variant influences an MC4R-mediated pathway that governs both appetite and peripheral metabolic signalling.

The Evidence

The pharmacogenomics signal at rs489693 is among the most robustly replicated in antipsychotic-induced weight gain (AIWG) research. The pivotal study by Malhotra et al.⁵⁵ Malhotra et al.
GWAS with three independent replication cohorts across US and European psychiatric centres achieved genome-wide significance (p=5.59×10^-12) in meta-analysis across cohorts — one of the few pharmacogenomics variants to reach this threshold for any drug-induced side effect.

A subsequent naturalistic study by Czerwensky et al. 2013⁶⁶ Czerwensky et al. 2013
341 Caucasian inpatients on SGAs including olanzapine, clozapine, risperidone, paliperidone, quetiapine, and amisulpride found that AA homozygotes gained 2.2 times as much weight as CC carriers after four weeks of treatment (+2.2 kg vs +1.0 kg, p=0.039). In the subgroup without weight-gain-inducing co-medications, the difference was 3.1-fold; in first-episode patients, 2.7-fold — both p<0.05. This confirms the signal is not confounded by prior drug exposure and is strongest when the genetic variant can be evaluated in isolation.

A 2026 meta-analysis⁷⁷ 2026 meta-analysis
75 studies examined in systematic review, all adult patients with severe mental illness prescribed antipsychotics confirmed rs489693 as one of six pharmacogenomics loci reaching significance across the AIWG literature (Hedge's g=0.127). Notably, five of the six replicated genes regulate hypothalamic appetite and satiety pathways — MC4R, HTR2C, LEPR, ADRA2A, and CNR1 — pointing to a convergent mechanism through central appetite regulation rather than peripheral metabolic effects.

A real-world polygenic risk score⁸⁸ real-world polygenic risk score
incorporating rs489693 plus five other AIWG variants in first-episode psychosis patients predicted weight gain with adjusted r²=0.59 in females, though the PRS showed no significant effect in males — a sex-specific finding that requires replication but highlights the importance of considering sex as a biological variable in AIWG prediction.

Practical Actions

For AA homozygotes, the most clinically useful application is pre-treatment risk stratification. Before starting a high-weight-gain-liability SGA (olanzapine, clozapine, quetiapine, or risperidone), knowing you carry the AA genotype provides a genetic rationale for discussing lower-weight-gain alternatives with your prescriber — aripiprazole, lurasidone, and ziprasidone have substantially lower AIWG profiles in the general population, and genetic risk strengthens the case for preferring them when clinically equivalent.

When SGA treatment is medically necessary and cannot be changed, proactive metabolic monitoring becomes essential. AA carriers should have baseline and regular follow-up measures of weight, waist circumference, fasting glucose, triglycerides, and HbA1c — the full metabolic syndrome panel. Early detection of metabolic drift enables earlier intervention (lifestyle, metformin adjunct, or medication review) before frank metabolic syndrome develops.

Timing matters: the Czerwensky 2013 data shows 2.2 kg excess gain in just four weeks, before many patients would be flagged by routine clinical review. More frequent early weight monitoring (weekly for the first month) in AA carriers could prompt earlier clinical response.

Interactions

rs17782313 (MC4R near-gene variant): Both variants tag the MC4R locus, but they are not in strong linkage disequilibrium and appear to capture partially independent signals. rs17782313 is primarily an obesity signal in the general population; rs489693 is primarily an antipsychotic-weight-gain signal. Carriers of risk alleles at both loci may have compound MC4R-pathway vulnerability — both general appetite dysregulation and amplified drug-induced weight gain. The combined effect has not been quantified in a single study.

rs8087522 (MC4R upstream regulatory variant): A second MC4R upstream variant ~155 kb from the TSS with an exploratory association with clozapine-induced weight gain in European-ancestry patients (PMID 22310352). The rs489693 and rs8087522 signals may partially overlap or independently contribute to MC4R regulatory tone. Combined effects have not been formally assessed.

HTR2C rs3813929: The serotonin 2C receptor variant is the strongest single-gene AIWG signal (Hedge's g=0.76 in the Warner-Levy 2026 meta-analysis). Antipsychotics cause weight gain partly through 5-HT2C antagonism; MC4R pathway impairment at rs489693 operates through a different mechanism (reduced satiety tone). Carriers of risk alleles at both loci face additive AIWG risk through independent pathways, making a combined pharmacogenomics assessment more predictive than either alone.