The Brain's Hidden Weight Controller
TMEM18 (Transmembrane Protein 18) sits at the second strongest obesity
locus ever discovered by genome-wide association studies, trailing only
FTO. The variant rs6548238 lies 23 kilobases upstream of TMEM18 on
chromosome 2p25.3. What makes this gene remarkable is
where it acts: deep within the
hypothalamus11 hypothalamus
The brain region that controls hunger, thirst, body temperature, and energy balance,
specifically the
paraventricular nucleus (PVN)22 paraventricular nucleus (PVN)
A cluster of neurons in the hypothalamus that integrates hunger and satiety signals and regulates energy expenditure,
the brain's central command for appetite and energy expenditure.
The Mechanism
TMEM18 encodes a protein with four transmembrane domains that localizes
to the nuclear membrane, where it physically interacts with components
of the nuclear pore complex33 nuclear pore complex
The channel through which molecules pass between the cell nucleus and cytoplasm.
In functional studies,
mice lacking TMEM1844 mice lacking TMEM18
Larder et al. TMEM18 has a role in the central control of appetite and body weight regulation. PNAS, 2017
showed increased body weight driven by hyperphagia (overeating),
particularly on a high-fat diet. Conversely, overexpressing TMEM18 in
the PVN reduced food intake, increased energy expenditure, and decreased
both total body mass and fat mass. This demonstrates that TMEM18 acts
as a brake on appetite: less TMEM18 means more eating.
The Evidence
The GIANT consortium GWAS55 GIANT consortium GWAS
Willer et al. Six new loci associated with body mass index highlight a neuronal influence on body weight regulation. Nature Genetics, 2009
first identified this locus in a meta-analysis of over 32,000 individuals,
with replication in 59,000 more, reaching genome-wide significance
(p = 3.2 x 10-26) with a per-allele BMI increase of 0.26 kg/m2
for the C allele.
A comprehensive meta-analysis66 comprehensive meta-analysis
Li et al. The association between polymorphisms near TMEM18 and the risk of obesity: a meta-analysis. BMC Medical Genomics, 2021
confirmed the association with an overall odds ratio of 1.25 (95% CI:
1.08-1.45). The effect was strongest in Europeans (OR 1.32) and
Mexicans (OR 1.39). Notably, the association was more robust in
children (OR 1.28, 95% CI: 1.18-1.39) than in adults, and one study
found TMEM18 variants conferred the
strongest effect on pediatric BMI77 strongest effect on pediatric BMI
Almen et al. TMEM18 variants associated with BMI and waist circumference in children. European Journal of Human Genetics, 2012
out of 25 obesity-associated variants tested in 6,078 children.
The C allele is very common -- around 83-91% frequency depending on ancestry -- meaning most people carry the risk version. The protective T allele is found in only about 9-17% of chromosomes. Because the risk allele is the major allele, the CC genotype (highest risk) is also the most common genotype (~70% of people).
Practical Implications
Because the C risk allele is so prevalent, carrying CC does not mean inevitable obesity. The effect is modest per allele (0.26 kg/m2 BMI), but it compounds across the many obesity-risk loci each person carries.
Interactions
TMEM18 acts independently of FTO (rs9939609) and MC4R (rs17782313) -- the three loci are in linkage equilibrium and their effects on BMI are additive. A person carrying risk alleles at all three loci faces a meaningfully higher cumulative genetic load for obesity than someone with risk alleles at only one.