rs7594951 — SRD5A2 SRD5A2 intron 4 regulatory variant

SRD5A2 rs7594951 — A Regulatory Variant Tuning DHT Production

The SRD5A2 gene encodes steroid 5-alpha-reductase type 2, the enzyme that converts testosterone into dihydrotestosterone (DHT) — the androgen driving prostate growth, scalp hair loss, and male external genital development in utero. Variants in SRD5A2 that alter enzyme activity or expression can shift how much DHT an individual produces throughout life, with downstream effects on androgen-sensitive tissues. The rs7594951 variant lies deep within intron 4 of SRD5A2 on chromosome 2 (GRCh38 position 31,566,723). The SRD5A2 gene runs on the minus strand; in the coding-strand frame this is a G>A change, while in genome files (WGS and consumer chips including 23andMe) it appears as the plus-strand C>T variant described here.

The Mechanism

As an intronic variant, rs7594951 does not alter the SRD5A2 protein sequence directly. Instead, it may affect pre-mRNA splicing, intronic enhancer elements, or mRNA stability — mechanisms known to modify the final amount of functional enzyme produced from a gene. The precise molecular mechanism has not been characterized in published literature, making this a variant of [emerging | an association identified in a single moderately-sized study that requires replication in larger cohorts] evidence status rather than an established functional variant.

SRD5A2 is the dominant 5-alpha-reductase isoform in the prostate, seminal vesicles, epididymis, and fetal external genitalia. Its activity level determines how much DHT is produced locally in androgen-sensitive tissues — and therefore how sensitive those tissues are to androgenic stimulation.

The Evidence

The primary evidence linking rs7594951 to DHT metabolism comes from a 2010 study by Setlur et al. examining 426 Austrian men (205 controls, 221 prostate cancer cases) for variants in DHT-metabolizing genes¹¹ Setlur et al. examining 426 Austrian men (205 controls, 221 prostate cancer cases) for variants in DHT-metabolizing genes
Setlur SR et al. Cancer Epidemiol Biomarkers Prev. 2010;19(1):229-39. Men in the control group who carried the AA genotype in the paper's coding-strand notation — corresponding to the TT genotype in plus-strand notation — tended toward higher serum DHT levels (P = 0.03). This suggests the minor T allele, or a variant in strong linkage disequilibrium with it, is associated with modestly increased 5-alpha-reductase activity or efficiency, resulting in more testosterone being converted to DHT.

rs7594951 was also among the SRD5A2 tag SNPs examined in a 2014 population-based study by Carmichael et al. investigating sex hormone gene variants in hypospadias — a congenital defect of the male urethra caused by insufficient androgenization during fetal development²² Carmichael et al. investigating sex hormone gene variants in hypospadias — a congenital defect of the male urethra caused by insufficient androgenization during fetal development
Carmichael SL et al. Andrology. 2014;2(1):130-7. The study genotyped 332 tagSNPs across 20 sex hormone metabolism genes in 633 cases and 855 controls born in California. Ten SRD5A2 SNPs showed p < 0.01 in single-SNP analyses; three SRD5A2 haplotype blocks showed odds ratios of 1.4–1.7 for hypospadias. While the paper reports haplotype blocks rather than individual SNP effects for rs7594951, the direction is consistent with DHT availability during genital development influencing hypospadias risk. The effect direction here is opposite to the DHT-level finding: haplotypes conferring lower SRD5A2 function associate with higher hypospadias risk (less DHT during fetal development impairs urethral fusion), while the TT genotype at rs7594951 may associate with higher DHT.

The overall evidence base for rs7594951 as an independent causal variant is limited to these two studies, neither of which reports rs7594951 in isolation with a definitive effect size. The variant should be understood as a possible marker of 5-alpha-reductase activity variation rather than a clinically established risk allele.

Practical Implications

If the minor T allele does increase local DHT production — consistent with the Setlur 2010 findings — CT and TT carriers may experience somewhat stronger androgenic effects in DHT-sensitive tissues over their lifetime. This could manifest as more pronounced androgenetic alopecia in genetically predisposed individuals, a modestly greater tendency toward benign prostatic hyperplasia with age in men, or potentially higher baseline DHT levels on serum hormone testing. The effect magnitude is likely modest given the intronic location and the minor allele frequency of ~10.7%.

For individuals prescribed finasteride (for hair loss or BPH) or dutasteride, intrinsic 5-alpha-reductase activity level at baseline may influence the degree of DHT suppression achieved at standard doses — higher baseline activity could theoretically require different titration. This is speculative at the individual SNP level, as pharmacogenomic data for rs7594951 specifically are not available.

Regarding hypospadias, the developmental window during which DHT levels matter (weeks 8–16 of gestation) is not modifiable postnatally. The hypospadias association is of academic and family planning interest but does not inform actionable intervention in adults.

Interactions

rs7594951 sits in the same gene as the better-characterized V89L variant (rs523349) and the A49T variant (rs9282858). The V89L variant reduces SRD5A2 enzyme activity by approximately 30%, and A49T also alters enzyme kinetics. Haplotypes combining rs7594951 with these coding variants may produce combined effects on DHT output that differ from either variant alone. Literature on the specific haplotype structure involving all three variants is limited, but the SRD5A2 locus should be interpreted as a unit across these SNPs where possible.

The CYP17A1 variant rs743572 influences androgen precursor production upstream of SRD5A2, and variants in the androgen receptor (AR) gene modulate DHT sensitivity in target tissues downstream. Individuals with multiple variants affecting DHT production or response may experience amplified or attenuated androgen-sensitive phenotypes.