rs3877899 — SELENOP Ala234Thr

SELENOP Ala234Thr — When Your Selenium Carrier Protein Carries Less

Selenoprotein P (SELENOP, formerly SEPP1) is the primary selenium transport protein in human plasma. Unlike most proteins, which are built from the standard 20 amino acids, SELENOP incorporates selenocysteine¹¹ selenocysteine
the 21st amino acid, structurally similar to cysteine but with selenium replacing sulfur — it gives selenoproteins their exceptional antioxidant power at 10 positions. SELENOP accounts for roughly 50–60% of all plasma selenium and is the main vehicle by which the liver ships selenium to peripheral tissues — brain, testes, kidney, and thyroid among them. rs3877899 changes a single amino acid at position 234 of the protein, swapping alanine for threonine, and this substitution measurably alters how much selenium your body successfully delivers.

The Mechanism

SELENOP circulates as a mixture of isoforms. The full-length 60-kDa form contains all 10 selenocysteine residues and binds the apoER2 receptor (LRP8) on target cells, enabling receptor-mediated uptake of selenium into tissues. Shorter truncated isoforms containing six or fewer selenocysteine residues cannot bind apoER2 and are therefore poor selenium donors to tissues. The Ala234Thr substitution falls within a region of SELENOP that influences this isoform balance. In colorectal cancer patients, the GG coding genotype (CC plus-strand) combined with a 3'UTR variant in rs7579 was associated with decreased expression of the full-length 60-kDa isoform²² colorectal cancer patients, the GG coding genotype (CC plus-strand) combined with a 3'UTR variant in rs7579 was associated with decreased expression of the full-length 60-kDa isoform
Short SP et al. Free Radic Biol Med. 2018, suggesting the variant's effects on isoform ratios are context-dependent.

A randomized dietary intervention in 170 Danish adults aged 50–74³³ randomized dietary intervention in 170 Danish adults aged 50–74
Kopp TI et al. Genes Nutr. 2018 provided the clearest functional evidence: participants who ate 1,000 g of selenium-rich fish and mussels weekly for 26 weeks showed a genotype-dependent rise in plasma SELENOP. CC homozygotes achieved plasma selenoprotein P levels averaging 4.68 ng/mL higher than T-allele carriers at week 26 (95% CI −8.49 to −0.871, p=0.018). The T allele blunts the protein-level response to dietary selenium — meaning T carriers absorb selenium but synthesize or retain SELENOP less efficiently.

The Evidence

Selenium transport efficiency: The Kopp et al. RCT (n=170) is the most direct human evidence that the T allele compromises SELENOP upregulation in response to selenium intake. The CC genotype more effectively converts dietary selenium into circulating SELENOP, the protein responsible for distributing it.

Pregnancy and GPx response: A UK cohort of 230 pregnant women by Mao et al. 2016⁴⁴ Mao et al. 2016
Mao J et al. Am J Clin Nutr. 2016 found that T-allele carriers (coding A allele) maintained blood selenium better during gestation (p=0.005, explaining 8% of variance) and showed greater glutathione peroxidase-3 (GPx3) activity increases in response to selenium supplementation (p=0.01). This suggests the T allele may partially reroute selenium toward GPx synthesis rather than SELENOP.

Prostate cancer: A Chicago cohort study of prostate cancer patients by Ekoue et al. 2018⁵⁵ Ekoue et al. 2018
Ekoue DN et al. Prostate. 2018 found that men homozygous for the TT genotype (coding AA, Thr/Thr) had nearly 6-fold higher odds of treatment failure within 2 years of surgery (OR=5.75, 95% CI 1.09–30.5, p=0.021) compared to CC (Ala/Ala) homozygotes. This was independent of serum selenium levels, suggesting the functional isoform produced by the Ala234Thr variant — rather than circulating selenium per se — influences cancer progression.

Retinopathy of prematurity: In a cohort of 173 premature infants (gestational age ≤32 weeks), Strauss et al. 2023⁶⁶ Strauss et al. 2023
Strauss E et al. Int J Mol Sci. 2023 found the rs3877899 A allele (coding A = plus-strand T) significantly associated with advanced retinopathy of prematurity requiring treatment (OR=1.8, p=0.045). TT homozygote infants showed OR=7.0 for treatment-requiring ROP and OR=13.6 for treatment failure (p=0.005). The proposed mechanism is impaired antioxidant protection during critical retinal vascularization due to reduced selenium bioavailability.

Metabolic effects: A Brazil nut supplementation RCT in 130 healthy volunteers by Donadio et al. 2018⁷⁷ Donadio et al. 2018
Donadio JLS et al. Eur J Nutr. 2018 found that CT+TT carriers had significantly lower fasting glucose compared to CC carriers, suggesting the Thr234 protein variant may interact with selenoprotein P's emerging role in glucose metabolism.

Practical Implications

The T allele does not simply lower selenium — it changes how selenium is packaged and delivered. T-allele carriers may benefit from ensuring consistent selenium intake through food sources with high selenomethionine bioavailability, since selenomethionine (from Brazil nuts, fish, eggs) is incorporated non-specifically into all proteins and provides a buffer that selenocysteine-dependent SELENOP cannot. Selenium supplementation above 200 mcg/day carries toxicity risk regardless of genotype; targeting the 55–200 mcg/day range through diet is appropriate for T carriers.

Interactions

The companion variant rs7579 in SELENOP (a 3'UTR variant at c.*14G>A) interacts with rs3877899 to influence isoform expression in colorectal cancer patients. The combined GG(rs3877899) + GA(rs7579) genotype was associated with the greatest reduction in full-length 60-kDa SELENOP. rs3877899 also functionally interacts with the selenium pathway partner rs1050450 (GPX1 Pro198Leu) — both variants modulate how effectively dietary selenium is converted into active selenoproteins, and their combined effect on cancer risk is an active area of research. The SELENOP-GPX4 axis (rs713041) is a third interaction point, as GPX4 competes with SELENOP for selenium incorporation and affects ferroptosis susceptibility.