rs6420424 — BCO1

BCO1 Upstream Regulatory Variant — A Third Independent Dial on Beta-Carotene Conversion

The BCO1 gene encodes beta-carotene 15,15'-monooxygenase¹¹ beta-carotene 15,15'-monooxygenase
The enzyme that cleaves one molecule of dietary beta-carotene into two molecules of retinal, which is subsequently reduced to retinol — the biologically active form of vitamin A, the single enzyme responsible for converting plant-derived provitamin A into a form the body can use. Most of the genetic variation in beta-carotene conversion has been attributed to two coding variants in BCO1 — rs7501331 (Ala379Val)²² rs7501331 (Ala379Val)
Reduces enzyme catalytic efficiency by ~32%; T allele frequency ~24% in Europeans and rs12934922 (Arg267Ser)³³ rs12934922 (Arg267Ser)
Reduces conversion by up to 69% when combined with Ala379Val; T allele frequency ~44% in Europeans — but these are not the full story.

The rs6420424 variant lies approximately 30 kilobases upstream of the BCO1 transcription start site, within the PKD1L2⁴⁴ PKD1L2
Polycystin 1 Like 2 — a gene encoding a membrane protein; rs6420424 is located in its sequence but functions as a regulatory element influencing BCO1 expression in the same chromosomal neighborhood gene region on chromosome 16. It acts not by changing the BCO1 protein sequence, but by modulating how much BCO1 enzyme is made. The A allele is associated with reduced BCO1 catalytic activity, reducing the rate at which beta-carotene is cleaved and converted to retinal.

The Mechanism

Unlike missense variants that alter the enzyme's amino acid sequence, rs6420424 is a regulatory variant⁵⁵ regulatory variant
A non-coding variant that changes the binding of transcription factors, enhancer activity, or chromatin accessibility, altering how much of a nearby protein-coding gene is expressed. Its position ~30 kb upstream of BCO1's first exon places it in a region that likely contains cis-regulatory elements⁶⁶ cis-regulatory elements
DNA sequences that control transcription of nearby genes, including enhancers, silencers, and transcription factor binding sites, acting in the same chromosome copy influencing BCO1 transcription in intestinal enterocytes, where most beta-carotene conversion takes place.

The rs6564851 variant, another upstream regulatory SNP located approximately 22 kb downstream of rs6420424 (at chr16:81,230,991), also reduces BCO1 activity — by ~48%. The two SNPs are in the same genomic neighborhood and were both identified by Lietz et al. 2012⁷⁷ Lietz et al. 2012
Lietz G et al. SNPs upstream from the beta-carotene 15,15'-monoxygenase gene influence provitamin A conversion efficiency. J Nutr, 2012 as independently contributing to conversion efficiency, with rs6420424 showing a positive correlation (r=0.53, P=0.004) between the G allele and the retinyl palmitate:beta-carotene conversion ratio. A allele carriers have a lower ratio — meaning less beta-carotene is converted to retinol after a dietary dose.

The Evidence

The foundational characterization of this variant comes from Lietz and colleagues (2012)⁸⁸ Lietz and colleagues (2012)
Lietz G et al. SNPs upstream from the beta-carotene 15,15'-monoxygenase gene influence provitamin A conversion efficiency in female volunteers. J Nutr, 2012, who measured the TG-rich lipoprotein retinyl palmitate:beta-carotene ratio in female volunteers following a controlled beta-carotene dose. Among three upstream regulatory SNPs tested (rs6420424, rs11645428, rs6564851), rs6420424 showed the largest effect — a 59% reduction in catalytic activity for A allele carriers. The study also documented large inter-ethnic variation in allele frequencies (43-84% for the A allele across populations), with East Asian populations carrying the A allele at particularly high frequency.

Supporting evidence comes from Feigl et al. (2014)⁹⁹ Feigl et al. (2014)
Feigl B et al. The relationship between BCMO1 gene variants and macular pigment optical density in persons with and without AMD. PLoS One, 2014, who examined BCO1 variants in relation to macular pigment optical density (MPOD) — a physiological measure of carotenoid accumulation in the eye. Healthy participants with the AA genotype at rs6420424 had significantly lower MPOD than GG carriers (P<0.01), consistent with reduced carotenoid delivery to the macula from impaired conversion of dietary precursors. The study identified a "high-conversion" genotype pattern (GG at rs6420424 / AA at rs11645428 / TT at rs6564851) with notably higher MPOD, confirming the biological relevance of this variant.

A 2024 population study by Von Holle et al.¹⁰¹⁰ Von Holle et al.
Von Holle A et al. Association between rs6564851 and rs6420424, and lutein/zeaxanthin levels in US postmenopausal women. Front Nutr, 2024 in 519 postmenopausal women found the A allele of rs6420424 inversely associated with circulating lutein/zeaxanthin levels (beta=-0.334, se=0.059, P=2.2×10⁻⁸), a significant genome-wide association replicated independently of rs6564851, which had its own distinct effect (beta=-0.377, P=4.6×10⁻¹⁰). This independent reporting in the same study provides the clearest available evidence that rs6420424 and rs6564851 have separable, non-redundant effects on carotenoid levels — though the formal LD structure between them requires further characterization.

Practical Implications

The clinical relevance of rs6420424 is greatest for people who depend on plant foods as their primary vitamin A source. For a person eating a typical Western mixed diet that includes eggs, dairy, and some meat, reduced BCO1 expression from this regulatory variant is largely compensated by preformed retinol from animal sources, which bypass BCO1 entirely.

The situation changes meaningfully for: - Vegans and vegetarians who derive essentially all vitamin A from beta-carotene in plant foods - People who also carry one or two T alleles at the BCO1 coding variants rs7501331 or rs12934922, creating a cumulative multi-variant poor converter phenotype - People in populations where plant-based provitamin A is the dietary vitamin A standard, particularly in sub-Saharan Africa and Southeast Asia — where this variant's population frequency also varies considerably

Plasma retinol testing is not sensitive for detecting suboptimal status; the liver buffers circulating retinol until stores are substantially depleted. Dietary diversity and preformed vitamin A sources are more practical levers than serum monitoring for mild variants like this one.

Interactions

The rs6420424 variant interacts additively with the other upstream regulatory variant rs6564851, which independently reduces BCO1 activity by 48%. Carriers of risk alleles at both loci are expected to have compounded reductions in BCO1 expression — though the combined effect has not been formally quantified in a single dose-response study.

More importantly, the regulatory variants compound with the BCO1 coding variants rs7501331 (Ala379Val) and rs12934922 (Arg267Ser), which reduce enzyme activity by 32% and contribute to the 69% reduction in compound carriers. An individual who carries risk alleles at both upstream regulatory variants and both coding variants could have substantially impaired beta-carotene conversion — well beyond any single variant's effect. Given the high A allele frequency of rs6420424 in East Asian populations (~86%), combined multi-variant poor converter profiles are likely common in those ancestry groups.