rs7834555 — BCO1

BCO1 Region Variant — A Third Independent Influence on Beta-Carotene Status

The BCO1 gene (also known as BCMO1) encodes beta-carotene 15,15'-monooxygenase¹¹ beta-carotene 15,15'-monooxygenase
The enzyme that cleaves dietary beta-carotene into two molecules of retinal, which is then reduced to retinol — the form of vitamin A used by the body, the central enzyme in the conversion of plant-based provitamin A into biologically active vitamin A. Most people are familiar with BCO1 through its two well-studied coding variants — rs7501331 (Ala379Val) and rs12934922 (Arg267Ser) — which directly reduce enzyme activity by up to 69% in compound carriers. The rs7834555 variant represents a third, independently acting influence on circulating beta-carotene and retinol levels, identified through genome-wide association analysis of circulating carotenoid concentrations.

The Mechanism

Unlike the coding BCO1 variants that alter the enzyme's amino acid sequence, rs7834555 is an intergenic variant²² intergenic variant
Located between protein-coding genes; does not directly change any protein but can influence gene regulation through effects on enhancers, transcription factor binding sites, or chromatin accessibility located on chromosome 8 at position 81,785,390 (GRCh38). Its mechanistic connection to carotenoid metabolism is not yet characterized at the molecular level. The variant likely acts as a GWAS tag SNP³³ GWAS tag SNP
A marker in linkage disequilibrium with a functional variant nearby, which has not yet been pinpointed; the tag SNP's association reflects the true causal variant's effect, meaning it serves as a detectable signal for a nearby regulatory element that modulates expression or activity of a gene in the broader carotenoid absorption and conversion pathway.

The intestinal absorption and conversion of beta-carotene is a multi-step process involving several proteins beyond BCO1 itself: membrane transporters such as SR-B1 (SCARB1)⁴⁴ SR-B1 (SCARB1)
Scavenger receptor class B type 1 — a lipid transport protein that facilitates uptake of carotenoids and other fat-soluble compounds into enterocytes and CD36⁵⁵ CD36
A fatty acid translocase that also facilitates carotenoid uptake at the intestinal brush border, retinaldehyde reductases, and CRBP chaperone proteins. Genetic variation that influences any of these steps will independently affect circulating carotenoid and retinol levels.

The Evidence

The association of rs7834555 with circulating beta-carotene and retinol levels comes from genome-wide association study data. The broader framework for understanding how genetic variation near BCO1 modulates carotenoid metabolism was established by Ferrucci et al. 2009⁶⁶ Ferrucci et al. 2009
Ferrucci L et al. Common variation in the β-carotene 15,15′-monooxygenase 1 gene affects circulating levels of carotenoids. Am J Hum Genet, 2009, who performed the first GWAS of circulating carotenoids, identifying the BCMO1/BCO1 locus on chromosome 16 as the strongest genetic determinant of plasma beta-carotene levels.

Subsequent work by Lietz et al. 2012⁷⁷ Lietz et al. 2012
Lietz G et al. Single nucleotide polymorphisms upstream from the β-carotene 15,15′-monoxygenase gene influence provitamin A conversion efficiency in female volunteers. J Nutr, 2012 demonstrated that upstream regulatory SNPs near BCO1 (rs6420424, rs11645428, rs6564851) reduced BCMO1 catalytic activity by 48-59% independently of the coding variants, confirming that non-coding variation is a major contributor to the wide interindividual variability in beta-carotene conversion.

Hendrickson et al. 2012⁸⁸ Hendrickson et al. 2012
Hendrickson SJ et al. β-Carotene 15,15′-monooxygenase 1 SNPs in relation to plasma carotenoid and retinol concentrations in women of European descent. Am J Clin Nutr, 2012 showed that a genetic score using multiple BCO1-region SNPs predicted plasma beta-carotene concentrations with a 48% difference across extreme quintiles in 2,344 European women — underscoring that the full genetic picture of carotenoid metabolism requires considering multiple independent signals.

The evidence for rs7834555 specifically as an independent signal should be considered emerging until replicated in peer-reviewed publications with reported effect sizes and p-values.

Practical Implications

If the A allele of rs7834555 is confirmed as reducing beta-carotene conversion efficiency or retinol status, the practical implications follow the same logic as the better-characterized BCO1 coding variants. People who rely heavily on plant-based provitamin A sources — vegans, vegetarians, and those with limited access to animal-source foods — are most affected. Since this variant acts independently of the coding variants at rs7501331 and rs12934922, individuals who carry all three risk alleles would face cumulative impairment of their beta-carotene-to-retinol conversion pathway.

The most direct way to compensate is to include preformed vitamin A⁹⁹ preformed vitamin A
Retinol from animal sources (liver, egg yolks, dairy, fatty fish) or retinyl palmitate/acetate supplements — bypasses BCO1 entirely and does not require conversion in the diet, which bypasses the BCO1 enzyme entirely and provides retinol directly to the body. For those eating a mixed diet with regular animal products, even substantially reduced BCO1 activity is unlikely to produce clinical vitamin A deficiency.

Interactions

The rs7834555 signal is proposed to be independent of the two major BCO1 coding variants: rs7501331 (Ala379Val, on chromosome 16) and rs12934922 (Arg267Ser, also chromosome 16). If confirmed, individuals carrying the A allele at rs7834555 alongside T alleles at rs7501331 and/or rs12934922 would face additive impairment of their beta-carotene conversion capacity — the combined effect exceeding any single variant alone.

The upstream regulatory BCO1 SNPs (rs6564851, rs6420424, rs11645428) on chromosome 16 are separate signals that also independently influence conversion and may interact further. The full genetic architecture of interindividual variation in beta-carotene conversion thus spans both structural (amino acid) and regulatory (expression-level) effects, with rs7834555 proposed as an additional independent determinant from a distinct genomic locus.