rs28942111 — PCSK9 S127R

PCSK9 S127R — A Gain-of-Function Mutation That Breaks the LDL Receptor Thermostat

PCSK9 (proprotein convertase subtilisin/kexin type 9) is the liver's master dial for LDL cholesterol. When LDL-C is high, PCSK9 activity should stay low, leaving LDL receptors (LDLRs) on hepatocyte surfaces to clear cholesterol from the blood. The S127R gain-of-function mutation throws this dial permanently toward maximum activity: PCSK9 becomes more potent at targeting LDLR for degradation, LDL receptor density on the liver falls, and LDL cholesterol accumulates in circulation from birth — causing autosomal dominant hypercholesterolemia type 3¹¹ autosomal dominant hypercholesterolemia type 3
HCHOLA3, the familial hypercholesterolemia subtype caused by PCSK9 mutations, distinct from HCHOLA1 (LDLR mutations) and HCHOLA2 (APOB mutations).

This is one of the rarest variants in this encyclopedia — essentially undetectable in large population databases — but among the most clinically important to identify. Carriers face a lifetime of severely elevated LDL, a high probability of coronary artery disease before age 50, and a very strong response to PCSK9-targeted therapy. Knowing you carry S127R changes medical management significantly.

The Mechanism

PCSK9 is synthesised as a proenzyme (zymogen) in the liver, autocatalytically cleaves itself to reach its mature form, and is then secreted into plasma. Its canonical action is straightforward: PCSK9 binds the EGF-A domain of LDLR on the hepatocyte surface, escorts the receptor into endosomes, and redirects it toward lysosomal degradation instead of recycling back to the surface. The result is fewer LDLRs available to clear circulating LDL.

The S127R mutation (c.381T>A, serine→arginine at position 127) sits in PCSK9's prodomain. Structurally, the wild-type serine at position 127 forms a stabilising hydrogen bond with aspartate-129; arginine substitution destroys this bond and alters the prodomain fold. This produces a paradox: S127R PCSK9 is actually non-secreted — retained inside the cell — yet still reduces surface LDLR more potently than secreted wild-type PCSK9²² non-secreted — retained inside the cell — yet still reduces surface LDLR more potently than secreted wild-type PCSK9
Homer et al. 2008: LDLR expression was reduced 30% beyond wild-type PCSK9 levels when S127R was present, LDL cellular binding fell by 45%. This finding revealed that PCSK9 can degrade LDLRs intracellularly — inside the ER or early endosomes — without needing to be secreted and re-endocytosed.

The functional consequence is that S127R PCSK9 is more potent than wild-type protein in reducing LDL uptake³³ more potent than wild-type protein in reducing LDL uptake
Pandit et al. 2008: "S127R and S127K proteins were more potent in decreasing LDL uptake than was wild-type PCSK9", with an additive effect when combined with the D374Y gain-of-function variant (a different hypercholesterolemia mutation in the same gene). The enhanced activity is attributed to stabilisation of the prodomain fold in a conformation that prevents normal PCSK9 turnover, extending the protein's intracellular half-life.

At a molecular level, LDL particle binding was nearly abolished by S127R⁴⁴ LDL particle binding was nearly abolished by S127R
Sarkar et al. 2022: "LDL binding was nearly abolished by a prodomain S127R GOF mutation, one of the first PCSK9 mutations identified in FH patients", meaning the normal feedback loop — where LDL in the medium dampens PCSK9's LDLR degradation — is broken. S127R PCSK9 keeps degrading LDLRs regardless of ambient LDL concentration.

The Evidence

S127R was identified in the landmark 2003 Nature Genetics paper by Abifadel et al.⁵⁵ landmark 2003 Nature Genetics paper by Abifadel et al.
Mutations in PCSK9 cause autosomal dominant hypercholesterolemia that discovered PCSK9 as a third gene for familial hypercholesterolemia. The variant was later identified in additional families from New Zealand and South Africa by Homer et al. 2008⁶⁶ Homer et al. 2008
Identification and characterization of two non-secreted PCSK9 mutants associated with familial hypercholesterolemia in cohorts from New Zealand and South Africa. Atherosclerosis 2008.

In an iPSC disease-modelling study, Si-Tayeb et al. 2016⁷⁷ Si-Tayeb et al. 2016
Urine-sample-derived human iPSCs as a model to study PCSK9-mediated autosomal dominant hypercholesterolemia. Dis Model Mech 2016 generated hepatocyte-like cells from a patient carrying S127R and demonstrated 71% reduction in LDL uptake (P<0.001) compared to control cells — the most direct human-cell evidence linking this genotype to impaired cholesterol clearance. Importantly, S127R cells secreted 38.5% less PCSK9 into the medium, confirming the non-secreted nature of the variant.

Clinically, the same study found that pravastatin treatment restored LDL uptake by 2.19-fold in S127R-derived cells (vs 1.38-fold in controls), and a retrospective look at five patients carrying S127R showed a mean LDL cholesterol reduction of 60.4% on pravastatin — a robust response.

ClinVar lists S127R as Pathogenic for autosomal dominant hypercholesterolemia (HCHOLA3) and as likely pathogenic for familial hypercholesterolemia, reviewed by multiple expert submitters. OMIM 607786.0001 documents the founding S127R families and phenotype.

Practical Implications for Carriers

Carriers of one S127R allele (AT genotype) should expect LDL-C in the range typical of untreated familial hypercholesterolemia — commonly 190–350 mg/dL without treatment, with coronary artery disease frequently presenting before age 50. The mutation follows an autosomal dominant pattern, meaning a single copy is sufficient to cause the full phenotype.

The excellent statin response observed in S127R carriers (60.4% LDL reduction with pravastatin) is mechanistically expected: statins reduce hepatic cholesterol synthesis, triggering SREBP-2–driven LDLR upregulation through the remaining functional receptor pool. Because S127R carriers are heterozygous and still have one wild-type PCSK9 allele, this compensatory upregulation can be substantial.

PCSK9 inhibitor drugs (evolocumab, alirocumab, inclisiran) are highly relevant for S127R carriers: by neutralising circulating PCSK9 protein, they protect the LDLR from degradation regardless of intracellular S127R activity. Adding a PCSK9 inhibitor to statin therapy can reduce LDL by an additional 50–60%. Any carrier with residual elevated LDL on maximum-tolerated statin therapy should be considered for PCSK9 inhibitor eligibility.

Screening of first-degree relatives (parents, siblings, children) is strongly indicated — each first-degree relative has a 50% chance of carrying the mutation.

Interactions

S127R is a gain-of-function mutation in PCSK9 and interacts with other LDL metabolism variants. Carriers who also have:

• Loss-of-function variants in LDLR (the most common cause of FH): the combined phenotype is more severe, as both the receptor itself and the PCSK9-mediated protection pathway are impaired simultaneously.
• APOE ε4 alleles (rs429358, rs7412): APOE4 increases hepatic cholesterol flux and further elevates LDL-C, compounding the S127R phenotype.
• PCSK9 R46L loss-of-function (rs11591147): the protective R46L allele is in trans at the same gene locus; however, S127R and R46L are distinct variants on separate chromosomes, so compound heterozygosity at the PCSK9 locus is theoretically possible and would be expected to partially attenuate the S127R phenotype.

PCSK9 D374Y is a distinct gain-of-function variant at a different residue; functional studies show S127R and D374Y have additive effects, and carriers of both would be expected to have a very severe phenotype.