rs28942111 — PCSK9 S127R

Gain-of-function PCSK9 missense variant (Ser127Arg) causing autosomal dominant familial hypercholesterolemia through enhanced LDL receptor degradation; rare but highly penetrant, carriers have severely elevated LDL-C and premature coronary artery disease

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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.

Genotype Interpretations

What each possible genotype means for this variant.

TT Normal PCSK9 S127 Normal

No PCSK9 S127R variant detected — standard cholesterol regulation

You carry two copies of the reference T allele at rs28942111, meaning your PCSK9 protein has the normal serine at position 127. This is the genotype found in essentially the entire global population — the S127R variant is so rare it was undetected in a study of 78,702 individuals across multiple ancestries.

Your PCSK9 function follows standard regulation: activity rises and falls with hepatic cholesterol levels, leaving LDL receptors available to clear LDL-C from circulation normally. Any elevated LDL you experience would be due to other factors — lifestyle, diet, other genetic variants (LDLR, APOB, APOE), or secondary causes — not this variant.

AT S127R Carrier High Risk

Gain-of-function PCSK9 S127R variant — familial hypercholesterolemia, substantially elevated LDL-C from birth

S127R is one of the rare gain-of-function PCSK9 mutations that increases the protein's ability to degrade LDL receptors beyond the normal set-point. Unlike the common PCSK9 gain-of-function variant D374Y (which enhances LDLR binding affinity), S127R operates through an intracellular mechanism: despite being poorly secreted, S127R PCSK9 reduces surface LDLR expression 30% beyond what wild-type PCSK9 achieves, by degrading receptors inside the ER or early endosomes before they even reach the cell surface (Homer et al. 2008).

iPSC-derived hepatocytes from an S127R carrier showed 71% reduced LDL uptake vs controls (Si-Tayeb et al. 2016). Critically, S127R also breaks the normal LDL-mediated feedback: wild-type PCSK9's LDLR degradation is inhibited when LDL is abundant; S127R ignores this signal, continuing to degrade LDLRs regardless of ambient cholesterol.

For cardiovascular risk: familial hypercholesterolemia from PCSK9 mutations (HCHOLA3) carries the same cardiovascular burden as classical FH from LDLR mutations — estimated 10- to 20-fold higher CHD risk compared to age-matched population if untreated. Statin therapy reduces this dramatically, and PCSK9 inhibitors can achieve near-normalization of LDL in combination with statins.

AA Homozygous S127R High Risk

Homozygous PCSK9 S127R — extremely rare, very severe LDL elevation requiring specialist management

Homozygous PCSK9 gain-of-function is analogous in severity to homozygous familial hypercholesterolemia caused by LDLR mutations. With both copies of PCSK9 carrying enhanced LDLR-degradation activity, hepatic LDL receptor density is reduced to near-zero, LDL-C cannot be cleared, and cholesterol accumulates in xanthomas, coronary arteries, and aortic valves from early childhood.

Statin therapy alone is often insufficient in homozygous FH because statins rely on upregulating LDL receptors — but if PCSK9 is maximally degrading them, the net effect is limited. PCSK9 inhibitor drugs should in principle be highly effective here (blocking the very protein that is constitutively active), but clinical data specifically for homozygous PCSK9 S127R are unavailable given the extreme rarity. Lomitapide and ezetimibe, which act independently of LDLR, are important adjuncts. LDL apheresis (mechanical removal of LDL from blood) may be required if pharmacological therapy cannot reach LDL targets.

Management must occur at a specialised FH centre with experience in homozygous hypercholesterolemia.