rs6235 — PCSK1 PCSK1 S690T (Ser690Thr)

PCSK1 S690T — The Prohormone Scissor's Hidden Flaw

Every time your pancreatic beta cells sense rising blood glucose, they do not release insulin directly — they release proinsulin¹¹ proinsulin
An inactive precursor of insulin that must be cleaved at two sites to release active insulin and C-peptide; elevated circulating proinsulin is a clinical marker of impaired beta-cell processing, a folded precursor that needs enzymatic cutting to become active. The enzyme responsible for most of that cleavage is prohormone convertase 1/3 (PC1/3), encoded by the PCSK1 gene on chromosome 5. PC1/3 also cleaves pro-opiomelanocortin (POMC) into the satiety peptide α-MSH in hypothalamic neurons, and proglucagon into GLP-1 in gut enteroendocrine cells. A single enzyme sits at the convergence of insulin release, appetite regulation, and incretin signaling.

rs6235 causes a serine-to-threonine substitution at position 690 of PC1/3 (S690T). It almost always travels alongside rs6234 (Q665E) in the same haplotype — in European populations the two variants are in near-complete linkage disequilibrium. Together they define the Q665E-S690T haplotype, which is the functional basis for the common PCSK1 obesity association²² common PCSK1 obesity association
Benzinou M et al. Common nonsynonymous variants in PCSK1 confer risk of obesity. Nat Genet, 2008 first demonstrated in 13,659 Europeans across eight cohorts.

The Mechanism

The S690T substitution lies within the C-terminal propeptide region of PC1/3. This domain acts as an intramolecular chaperone that guides proper folding of the mature enzyme after synthesis — it is cleaved off as part of the activation process, but its sequence influences how efficiently the enzyme assembles. The Q665E-S690T haplotype together alters this C-terminal region, reducing overall catalytic efficiency without abolishing enzyme function. Functional studies by Mbikay et al. 2011³³ Mbikay et al. 2011
Effects of rs6234/rs6235 and rs6232/rs6234/rs6235 PCSK1 single-nucleotide polymorphism clusters on proprotein convertase 1/3 biosynthesis and activity. Mol Genet Metab, 2011 showed that the double-variant isoform undergoes increased C-terminal proteolytic processing relative to the wild-type form, consistent with impaired folding — and proposed that this creates a subtle chronic deficit in PC1/3 enzymatic activity in endocrine and neuroendocrine cells.

The downstream consequences propagate through three pathways: slower proinsulin-to-insulin conversion (raising the circulating proinsulin fraction), reduced POMC cleavage into α-MSH (partially blunting melanocortin-4 receptor-mediated satiety signaling), and potentially impaired proglucagon processing to GLP-1 (modifying incretin-amplified insulin release after meals). None of these effects is catastrophic alone, but their convergence on energy balance creates a consistent metabolic tilt.

The Evidence

The founding population-genetic study⁴⁴ population-genetic study
Benzinou M et al. Common nonsynonymous variants in PCSK1 confer risk of obesity. Nat Genet, 2008 established the rs6234-rs6235 haplotype as a robust obesity locus (p = 2.31 × 10⁻¹²) and confirmed functional PC1/3 impairment in cell-based assays. The largest meta-analysis to date, Nead et al. 2015⁵⁵ Nead et al. 2015
Contribution of common non-synonymous variants in PCSK1 to body mass index variation and risk of obesity: a systematic review and meta-analysis with evidence from up to 331,175 individuals. Hum Mol Genet, 2015, quantified the effect: OR = 1.07 (95% CI 1.04–1.10) per rs6235 G allele for obesity, and a BMI increase of 0.02 per allele (p = 5.57 × 10⁻⁴). These are modest per-allele effects consistent with the polygenic architecture of common obesity, but they replicate at genome-wide significance across diverse ancestries.

Critically, the metabolic consequence was directly measured by Heni et al. 2010⁶⁶ Heni et al. 2010
Association of obesity risk SNPs in PCSK1 with insulin sensitivity and proinsulin conversion. BMC Med Genet, 2010: in 1,498 German subjects, each rs6235 G allele was associated with 8% higher proinsulin area under the curve (AUC) and a significantly elevated proinsulin-to-insulin ratio (p ≤ 0.009). This is in vivo biochemical evidence that S690T reduces prohormone processing efficiency, not merely a statistical association with BMI.

The evidence level is rated strong: there is consistent replication across multiple large cohorts and meta-analyses (total n > 300,000), clear biological mechanism, and direct in vivo biomarker confirmation. Effect sizes are modest (OR 1.07 per allele), but the molecular chain from variant to protein change to proinsulin accumulation to obesity risk is more complete than most common GWAS loci.

Practical Actions

The primary actionable lever for G-allele carriers is glycemic load management. Each postprandial glucose peak triggers a pulse of proinsulin secretion from beta cells — in carriers with reduced PC1/3 efficiency, those pulses are converted to active insulin less completely, leaving more proinsulin in circulation. Reducing the amplitude of postprandial glucose excursions reduces the secretory burden on an already less-efficient processing system. Fasting proinsulin and the proinsulin:C-peptide ratio are direct biomarkers for PC1/3 processing efficiency — they detect impairment before HbA1c rises and are available through specialist endocrinology labs.

Protein intake supports satiety through pathways that bypass PC1/3: peptide YY (PYY), cholecystokinin (CCK), and peripheral GLP-1 receptor activation do not require the PC1/3-dependent α-MSH pathway, and can partially compensate for reduced melanocortin-mediated satiety.

Interactions

rs6235 is essentially always co-inherited with rs6234 (Q665E) in European populations — the two define the Q665E-S690T haplotype. rs10515237, an intronic PCSK1 variant in strong LD with this haplotype (r² ≈ 0.84), is the tag SNP for this locus on the HapMap/OmniExpress arrays. rs6232 (N221D, encoded by a separate PCSK1 variant with MAF ~6%) independently reduces PC1/3 catalytic activity at the active site and shows an additive effect with the rs6234/rs6235 haplotype — carriers of both face compound PC1/3 impairment. The MC4R variant rs17782313, downstream of PC1/3 in the POMC→α-MSH→MC4R satiety axis, would compound the satiety impairment created by reduced POMC processing.