rs1800730 — HFE S65C

HFE S65C — The Third Iron-Regulation Variant

The HFE gene encodes the hereditary hemochromatosis protein¹¹ hereditary hemochromatosis protein
A membrane protein structurally similar to MHC class I molecules that regulates iron absorption by sensing blood iron levels and modulating hepcidin expression, the master regulator of how much iron your gut absorbs each day. Most people are familiar with the two major HFE variants — C282Y (rs1800562) and H63D (rs1799945) — but a third variant, Ser65Cys (S65C), lurks at lower frequency and carries its own clinical implications, particularly when inherited alongside the other two.

S65C is caused by an A-to-T change at chromosome 6, position 26,090,957 (GRCh38), converting serine to cysteine at position 65 of the protein. Unlike C282Y, which catastrophically prevents HFE from reaching the cell surface, S65C substitutes a polar serine with a sulfur-bearing cysteine in the alpha-1 domain²² alpha-1 domain
The membrane-distal domain of HFE involved in interaction with beta-2 microglobulin and transferrin receptors, producing a partial functional impairment rather than complete protein misfolding.

The Mechanism

The HFE protein acts as an iron sensor, forming a complex with beta-2 microglobulin³³ beta-2 microglobulin
A stabilizing partner required for proper folding and cell-surface expression of HFE and other MHC class I-like molecules and interacting with transferrin receptor 1 (TfR1) at the cell surface. When plasma iron is adequate, this complex triggers hepcidin production in the liver. Hepcidin⁴⁴ Hepcidin
The master iron-regulatory hormone; it degrades ferroportin, the only known cellular iron exporter, thereby limiting gut iron absorption and macrophage iron release then suppresses iron uptake from the intestine.

The Ser65Cys substitution partially disrupts the HFE–TfR1 interaction. Unlike C282Y, the S65C protein can still reach the cell surface and bind beta-2 microglobulin — it simply does so less efficiently. The net result is modestly reduced hepcidin signaling and slightly elevated iron absorption. In isolation, this effect is too small to cause clinical disease in most people. But in compound heterozygosity — when S65C is paired with either C282Y or H63D on the opposite chromosome — the combined impairment becomes clinically detectable.

The Evidence

A study of 309 subjects heterozygous for C282Y⁵⁵ study of 309 subjects heterozygous for C282Y
Mura C et al. Frequency of the S65C mutation of HFE and iron overload in 309 subjects heterozygous for C282Y. J Hepatol, 2002 found that among those who also carried S65C on their other chromosome, transferrin saturation was significantly elevated compared with C282Y heterozygotes who carried neither H63D nor S65C. This established S65C as a genuine, if mild, second-hit pathogenic allele.

A retrospective clinical series⁶⁶ retrospective clinical series
Holmström P et al. Mild iron overload in patients carrying the HFE S65C gene mutation: a retrospective study. Gut, 2002 identified patients presenting with suspected iron overload who carried S65C — some in isolation, some compound heterozygous. Patients with S65C alone rarely had severe iron accumulation; compound heterozygotes (particularly C282Y/S65C) were more likely to show elevated ferritin and require clinical attention.

A Mediterranean population study⁷⁷ Mediterranean population study
Aranda N et al. Iron status assessment in Spanish children and adolescents: the AVENA and HELENA studies. Ann Hematol, 2010 showed H63D/S65C compound heterozygotes had increased transferrin saturation relative to wild type, particularly when dietary iron intake or alcohol intake was elevated.

Current ClinVar consensus (VCV000000011)⁸⁸ ClinVar consensus (VCV000000011)
Multiple labs classify S65C as Likely Benign or Uncertain Significance. The older OMIM Pathogenic classification predates large population data. The current weight of evidence supports a low-penetrance risk modifier, not a standalone pathogenic variant reflects conflicting interpretations: most modern clinical laboratories classify S65C as Likely Benign or Uncertain Significance, downgrading from earlier Pathogenic designations that predated large-scale population studies.

Practical Actions

For AT heterozygous individuals: your iron levels may run marginally higher than non-carriers, but clinically significant iron overload from S65C alone is rarely documented. If you also carry H63D or C282Y on your other chromosome, the compound heterozygous state warrants ferritin monitoring — not alarm, but awareness. Baseline serum ferritin and transferrin saturation establish your personal normal.

For TT homozygous individuals (very rare at ~0.1%): both HFE alleles carry the S65C substitution. Published data on this genotype are limited to case reports. Iron monitoring is prudent; clinical iron overload disease from S65C homozygosity alone has not been firmly established but cannot be ruled out.

Interactions

S65C is most clinically meaningful as a compound heterozygote with HFE's major variants. The combination of C282Y (rs1800562)⁹⁹ C282Y (rs1800562)
The primary hemochromatosis variant, present in ~5-8% of Europeans as one copy; homozygosity drives most hereditary hemochromatosis disease on one chromosome and S65C on the other creates a genotype where one allele completely fails (C282Y cannot fold) and the other is partly impaired (S65C has reduced function). This double hit elevates transferrin saturation more than S65C alone.

Similarly, H63D (rs1799945)¹⁰¹⁰ H63D (rs1799945)
The second most common HFE variant; compound heterozygosity with C282Y carries modest iron overload risk paired with S65C produces mildly elevated iron markers in some but not all studies, with modifying factors including alcohol consumption and dietary iron intake. If you carry S65C plus either C282Y or H63D as your full HFE genotype, periodic ferritin monitoring is the appropriate response.