PCSK1 Genotype (p.Ser690Th)

You have two copies of PCSK1. In many European-ancestry studies, about one copy in four carries p.Ser690Thr. That makes it common. It is also linked with obesity in large population studies, though the effect is small.

What it does not do is decide your weight. The effect on any one person is tiny, far too small to read off a scale or predict a life. This is a research-grounded risk marker. It is not a diagnosis.

What p.Ser690Thr Changes

PCSK1 builds an enzyme called proprotein convertase 1/3, or PC1/3. Its job is to cut inactive starter hormones into forms the body can use. It helps convert proinsulin into insulin, and it helps activate the appetite signals your brain uses to know when you are full. The enzyme works in hormone-producing cells in your gut, pancreas, and brain.

The variant swaps one building block of that enzyme, serine, for another, threonine, at position 690. One caveat about identity matters here: p.Ser690Thr almost always travels with a nearby change called p.Gln665Glu. The original cell study tested the pair together. That pair showed about 6 percent lower PC1/3 activity, but the difference was not statistically clear, and the authors concluded that it did not clearly change catalytic activity. So the result should not be read as a direct measure of impaired enzyme function. Its strongest evidence is the population link.

The Obesity Link

The population evidence is replicated but modest. Across eight European groups, adults and children carrying the rs6234/rs6235 pair had higher rates of obesity. A later meta-analysis of up to 331,175 people found that each copy of the risk allele was linked with roughly 7 percent higher odds of obesity.

Sources: Benzinou et al. (2008), Nead et al. (2015), Manco et al. (2024). Takeaway: the effect is small enough that it should not change any day-to-day decision on its own. It is a faint inherited tilt in your baseline, useful as context, not as a reason to expect a particular number on the scale.

Why a Population Signal Isn't a Personal Verdict

There is a tension in the data. A 2023 French case-control study sequenced PCSK1 and tested 65 rare single-copy variants in cells. Null variants and variants that caused complete PC1/3 loss of function went with much higher obesity risk. Variants with partial or neutral activity did not.

That study was about rare variants, not p.Ser690Thr. It still helps explain the scale. Small PCSK1 changes are not the same as losing the gene's function. A common allele that shifts average weight by a hair can still move the average of a million people enough to measure, without predicting the weight of any single one of them. Carrying it does not mean this variant is the reason for your weight. Not carrying it does not clear you of inherited risk from the hundreds of other variants that also matter.

The Sex Difference

In a group of Portuguese children, the variant showed no link to weight when boys and girls were pooled together. Split by sex, boys who carried it had about 1.5 times the odds of being overweight and higher BMI-for-age scores, while girls showed no such rise. Other studies also find that age, sex, and ancestry change the size of the effect, sometimes erasing it. This is more evidence that the variant is a background modifier whose influence depends on context, not a switch that flips the same way in everyone.

Not the Same as PCSK1 Deficiency

PCSK1 deficiency is a rare, severe condition caused by pathogenic PCSK1 variants that badly damage PC1/3. Classic cases usually start in infancy with severe malabsorptive diarrhea, hormone problems across several glands, early-onset obesity, and proinsulin levels many times above normal.

Setmelanotide is approved for obesity due to PCSK1 deficiency confirmed by genetic testing. The current label covers variants read as pathogenic, likely pathogenic, or of uncertain significance when the clinical picture fits. It excludes benign or likely benign variants and general polygenic obesity. Clinical labs classify the common p.Ser690Thr variant as benign or likely benign for PCSK1 deficiency, so carrying it does not mean you have the deficiency and does not qualify you for that treatment.

A Result You Get Once

Your genotype does not change. This is a one-time test, and the result is permanent, so there is nothing to retest and no trend to track on the gene itself. The value comes from what you do with it over years, not from repeating it.

If you want to see how your metabolism is actually behaving, that is where ongoing testing belongs. Proinsulin, insulin, glucose, and HbA1c can be tracked over time, and those numbers reflect hormone processing and blood sugar now. A reasonable rhythm is a baseline metabolic panel, a repeat in 3 to 6 months if you are making changes, then at least yearly.

When a Genetic Result Can Mislead

• Panel coverage: the assay reads only this one spot in the gene. A result that shows you do not carry p.Ser690Thr does not rule out other, rarer PCSK1 changes that a broader sequencing test might catch.
• Ancestry: how common the variant is and how strongly it tracks with obesity differ by population. The signal is clearest in people of European descent and inconsistent elsewhere, so ancestry shapes what your result means.
• The linked neighbor: the functional numbers come from p.Ser690Thr paired with p.Gln665Glu, the way it is usually inherited, not from p.Ser690Thr on its own.
• Clinical meaning: p.Ser690Thr can be discussed as an obesity-risk allele and still be benign or likely benign for PCSK1 deficiency. A weak risk allele is not the same as a disease-causing mutation.
• Broader sequencing: a broader PCSK1 sequencing test may find a variant of uncertain significance. That is a different kind of result from this known common variant and needs to be read with your symptoms and family history.
• Assay grade: a direct-to-consumer report and a clinical-grade genetic test can disagree at the same spot. An unexpected result is worth confirming before you act on it.

What to Do If You Carry It

Carrying the variant is not a diagnosis and not a reason to worry. Treat it as a small clue in your inherited baseline. If you are trying to understand your metabolism, pair it with proinsulin and insulin alongside glucose and HbA1c. Those numbers can change what you do while the gene cannot.

The picture changes if weight has been a lifelong struggle with other red flags, such as severe obesity from very early childhood or hormone problems across several glands. That combination points toward rarer, higher-impact genetic causes, and it is worth involving an endocrinologist and a genetic counselor, who work with sequencing well beyond this single common variant. For most carriers, the action is the same prevention that helps anyone. The variant nudges the odds; it does not decide them.