Genetics Caused It, and the Drug Still Worked

Some obesity has a single identifiable genetic cause, and the commonest is a mutation in the MC4R gene — a receptor sitting at the center of how the brain regulates appetite. Researchers found 32 carriers inside the SURMOUNT-1 trial and looked at what happened to them. They lost 18.3% of their body weight, against 19.9% in everyone else.^[1]

Why this was not the expected answer

The melanocortin 4 receptor is part of the chain that carries signals about the body’s energy stores into decisions about hunger. Leptin, released from fat tissue, feeds into that pathway; MC4R is downstream of it. When the receptor does not work, the brain does not receive the message that there is enough fuel, and hunger persists.

People with these mutations have historically been difficult to treat, which is why a drug exists specifically for disorders of that pathway. The reasonable prior expectation was that a broken appetite receptor would blunt the effect of an appetite-acting drug.

The receptor was broken and the drug worked anyway.

That implies tirzepatide reduces appetite by a route that does not depend on an intact MC4R — which is mechanistically interesting and, for the people concerned, considerably more useful than interesting.

The numbers, and how many they rest on

It is also a research letter of three pages rather than a full trial report, and the carriers began heavier — a BMI of 40 against 38 — which is consistent with a penetrant mutation and is one more way the groups were not identical.

None of that undoes the finding. A 1.6 percentage point gap in a group of 32 is a reasonable basis for saying these patients are not obviously being failed by the drug, which is what people with this diagnosis want to know.

Two kinds of genetics, one conclusion

This register has now covered genetics in obesity from both ends, and they arrive at the same practical place.

So genetics explains a great deal about why someone carries excess weight, and currently very little about which treatment will help them. Those are separate questions and they are routinely merged in marketing for genetic tests, which we take apart in the genetic test that isn’t ready.

What it means practically

• An MC4R diagnosis is not a reason to expect these drugs to fail. On the only direct evidence available, carriers responded like everyone else.
• It is 32 people, so this supports rather than settles.
• It does not speak to other monogenic causes — leptin receptor, POMC and the rest were not examined here.
• Testing is a clinical decision with its own reasons, and this finding is not one of them: knowing you carry the mutation would not change which drug to try.

The broader point about who these trials enrolled, and how far their results travel, is in who was actually in the trials.