rs3829251 — NADSYN1

Intronic NADSYN1 variant at the DHCR7/NADSYN1 vitamin D locus; A allele was the top GWAS hit for lower circulating 25-hydroxyvitamin D in Ahn et al. 2010 (P = 3.4×10⁻⁹), reducing 7-dehydrocholesterol availability for skin vitamin D3 synthesis

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NADSYN1 rs3829251 — The Original Vitamin D GWAS Signal at the DHCR7 Locus

When Ahn and colleagues scanned the genomes of 6,722 individuals for variants associated with circulating vitamin D levels, the strongest signal they found on chromosome 11 was rs3829251 — a variant sitting in an intron of NADSYN1, a gene encoding [NAD synthetase 1 | NAD synthetase 1 catalyzes the final step of the NAD biosynthesis salvage pathway, converting nicotinic acid adenine dinucleotide to NAD⁺ using glutamine as a nitrogen donor. The gene sits adjacent to DHCR7 on chromosome 11q13.4 and the two genes are often discussed together because their regulatory regions overlap in the GWAS signal]. The p-value reached 3.4×10⁻⁹ — genome-wide significant — and the variant explained a meaningful fraction of variance in [25-hydroxyvitamin D | 25(OH)D, or calcidiol, is the main circulating form of vitamin D measured in blood tests. It reflects your overall vitamin D storage from both sun exposure and dietary/supplemental intake] levels. Despite being named for NADSYN1, the functional biology at this locus is attributed to the neighboring gene: DHCR7.

The DHCR7 enzyme (7-dehydrocholesterol reductase) governs a metabolic competition in your skin. Its substrate, [7-dehydrocholesterol (7-DHC) | A cholesterol precursor concentrated in the outer layers of the epidermis; UVB radiation (290–315 nm) breaks its B-ring to form previtamin D3, which then thermally isomerizes into vitamin D3 (cholecalciferol)], can either be converted to vitamin D3 by UVB light or to cholesterol by DHCR7. Every molecule that DHCR7 captures for cholesterol synthesis is one less molecule available for vitamin D production. Variants in this regulatory region that increase DHCR7 activity or expression tilt the balance toward cholesterol, reducing the skin's vitamin D yield from a given amount of sunlight.

rs3829251 itself does not change any amino acid sequence in NADSYN1 or DHCR7. It is an intronic tag variant that marks a haplotype associated with altered DHCR7 expression. The Ahn et al. 2010 paper noted that rs3829251 was in high [linkage disequilibrium | LD: a measure of how strongly two alleles at nearby chromosomal positions are inherited together across generations. When LD is high, knowing one variant predicts the other with high accuracy] with rs1790349, a DHCR7 intronic variant. This locus overlaps with the signal for rs12785878, the top hit in the concurrent Wang et al. 2010 Lancet GWAS, though whether rs3829251 and rs12785878 represent the same or independent signals within the locus has not been definitively resolved.

The Mechanism

DHCR7 catalyzes the [final step in the Kandutsch-Russell cholesterol synthesis pathway | One of two cellular routes to cholesterol synthesis; DHCR7 uses NADPH to reduce the C7-8 double bond in 7-DHC on the endoplasmic reticulum membrane, producing cholesterol irreversibly], consuming a molecule of 7-DHC that can no longer become vitamin D3. Regulatory variants at this locus that elevate DHCR7 transcription or enzymatic efficiency therefore create a constitutive drain on the skin's UV-responsive vitamin D synthesis capacity, even under identical sun exposure conditions.

A normal [cholesterol-mediated feedback loop | Rising intracellular cholesterol accelerates DHCR7 proteasomal degradation, allowing 7-DHC to accumulate and favoring vitamin D synthesis — a homeostatic mechanism that rs3829251 risk allele carriers may have blunted] would normally slow DHCR7 activity as cholesterol rises. Variants that constitutively upregulate DHCR7 may partially uncouple this feedback, producing both lower vitamin D and a modest shift toward cholesterol synthesis. Notably, this variant affects only the skin-synthesis pathway; it has no effect on intestinal absorption of dietary or supplemental vitamin D.

The Evidence

The Ahn et al. 2010 GWAS11 Ahn et al. 2010 GWAS
Ahn J et al. Genome-wide association study of circulating vitamin D levels. Hum Mol Genet, 2010 identified rs3829251 as the lead variant at the DHCR7/NADSYN1 locus with P = 8.8×10⁻⁷ in the discovery cohort and P = 3.4×10⁻⁹ in the meta-analysis with validation samples (total 6,722 individuals). The A allele at this SNP was the risk allele, with a frequency of approximately 0.19 in Europeans.

Replication in diverse populations confirmed the locus. A study in 3,210 Chinese Hans22 study in 3,210 Chinese Hans
Lu L et al. Associations between common variants in GC and DHCR7/NADSYN1 and vitamin D concentration in Chinese Hans. Hum Genet, 2012 found rs3829251 significantly associated with lower plasma 25(OH)D levels (β = −0.036 to −0.076 per risk allele, P ≤ 5.7×10⁻⁵), demonstrating that the signal generalizes beyond European ancestry populations. A pediatric study in 506 northeastern Han Chinese children33 pediatric study in 506 northeastern Han Chinese children
Zhang Y et al. The GC, CYP2R1 and DHCR7 genes are associated with vitamin D levels in northeastern Han Chinese children. Swiss Med Wkly, 2012 confirmed significant associations for both rs3829251 and rs12785878 under additive and recessive models.

Beyond vitamin D levels, rs3829251 has been associated with height. The Tromsø Study44 Tromsø Study
Jorde R et al. Associations between polymorphisms related to calcium metabolism and human height: the Tromsø Study. Ann Hum Genet, 2012, in 9,471 subjects, found that homozygotes for the two alleles at rs3829251 differed by 1.5–2.0 cm in height (P < 0.01), the largest height effect among all calcium-metabolism SNPs studied, seen consistently in both sexes and all age groups. This association likely reflects vitamin D's role in bone mineralization during growth.

An exploratory case-control study55 case-control study
Anic GM et al. An exploratory analysis of common genetic variants in the vitamin D pathway including genome-wide associated variants in relation to glioma risk and outcome. Cancer Causes Control, 2012 of 622 glioma cases and 628 controls found rs3829251 variant alleles associated with increased risk of astrocytic tumors — an exploratory finding consistent with vitamin D's documented neuroprotective roles, but requiring confirmation in larger studies.

Practical Implications

The per-allele effect of rs3829251 on vitamin D levels is modest — approximately 2–4 nmol/L (about 1 ng/mL) lower 25(OH)D per A allele. This is not alarming in isolation, but it compounds with the environmental and behavioral factors that dominate overall vitamin D status: high latitude, winter season, indoor lifestyle, darker skin pigmentation, and obesity. For AA homozygotes, who have two copies of the lower-vitamin-D allele, the cumulative genetic reduction in vitamin D synthesis capacity is clinically meaningful, particularly during low-UV months.

This variant affects only the skin synthesis pathway. It does not impair intestinal absorption of vitamin D from diet or supplements, making supplementation with cholecalciferol (D3) an effective and direct countermeasure regardless of genotype.

The A allele is approximately 0.19 in Europeans but around 0.36 in East Asian populations (Korean, Japanese) and intermediate (~0.27) in African populations, a frequency pattern that mirrors the established latitude gradient at the DHCR7/NADSYN1 locus and is consistent with positive selection for higher vitamin D synthesis in populations that migrated to low-UV northern latitudes.

Interactions

rs3829251 sits at the same DHCR7/NADSYN1 locus as rs12785878 and rs7940244. These variants are in varying degrees of LD with each other and may capture overlapping signals. If a genome report includes both rs3829251 and rs12785878 (or rs7940244), they should not be treated as fully independent effects — a combined risk score should be used cautiously.

The four major vitamin D pathway loci interact to determine overall circulating 25(OH)D: CYP2R1 (rs10741657) performs the liver 25-hydroxylation step; GC (rs2282679) encodes the vitamin D binding protein that transports 25(OH)D; CYP24A1 (rs6013897) encodes the enzyme that degrades active 1,25(OH)₂D. Wang et al. 2010 found that individuals in the highest quartile of a combined genetic risk score across these loci had 2.47-fold higher odds of vitamin D insufficiency than those in the lowest quartile. Carrying rs3829251 risk alleles alongside risk alleles at these other loci warrants more aggressive monitoring and supplementation.

Nutrient Interactions

vitamin D impaired_conversion

Genotype Interpretations

What each possible genotype means for this variant.

GG Optimal Vitamin D Synthesis Normal

Favorable genotype — normal vitamin D synthesis capacity at this locus

The GG genotype at rs3829251 places you on the favorable end of the DHCR7/NADSYN1 vitamin D haplotype. The A allele at this locus — identified as the risk allele in the Ahn et al. 2010 GWAS — reduces circulating 25(OH)D by approximately 2–4 nmol/L per copy. As a GG carrier, you have zero copies of this lower-vitamin-D allele.

Your actual vitamin D status still depends predominantly on sun exposure, latitude, season, skin pigmentation, dietary intake, and body composition. A serum 25(OH)D test remains the most reliable way to assess your individual status, regardless of genotype.

AG Mildly Reduced Synthesis Intermediate

One copy of the lower-vitamin-D allele — modest reduction in skin vitamin D synthesis

The AG genotype represents one copy of the DHCR7/NADSYN1 risk allele, with an intermediate effect between the GG and AA genotypes. This is an additive-effect variant, meaning one copy has roughly half the biological impact of two copies. For most AG carriers, adequate sun exposure and standard dietary vitamin D intake will maintain sufficient levels.

The effect becomes more meaningful when combined with other risk factors: living above 40°N latitude, spending most of the day indoors, having darker skin pigmentation, or carrying additional risk alleles at the other major vitamin D pathway genes (CYP2R1, GC, CYP24A1). If any of these apply, more consistent monitoring and seasonal supplementation are advisable.

AA Reduced Vitamin D Synthesis Reduced

Two copies of the lower-vitamin-D allele — genetically reduced skin vitamin D synthesis

The AA genotype carries two copies of the risk allele at rs3829251 — the maximum genetic effect at this locus on vitamin D synthesis. The A allele was the top GWAS hit at the NADSYN1/DHCR7 locus in the Ahn et al. 2010 study (P = 3.4×10⁻⁹), and homozygous AA carriers are expected to have approximately 6–8 nmol/L lower 25(OH)D than GG homozygotes under equivalent sun exposure conditions, based on the additive per-allele effect.

This effect is sufficient to shift many individuals from vitamin D sufficiency into insufficiency during winter months, particularly at high latitudes. The Tromsø Study also found that homozygotes for the two rs3829251 alleles differed by 1.5–2.0 cm in height, the largest height effect among calcium-metabolism SNPs studied, reflecting the long-term skeletal consequences of genetically lower vitamin D during bone growth.

Critically, this variant affects only the skin-synthesis pathway. It does not impair absorption of dietary or supplemental vitamin D (cholecalciferol or ergocalciferol), making supplementation a fully effective strategy to compensate for reduced skin synthesis capacity.