rs28934880 — HSD3B2 Ala10Glu

HSD3B2 Ala10Glu — A Founder Variant That Eliminates Steroidogenesis

3β-hydroxysteroid dehydrogenase type 2¹¹ 3β-hydroxysteroid dehydrogenase type 2
encoded by HSD3B2; the rate-limiting enzyme for converting Δ5-precursors (pregnenolone, 17-OH-pregnenolone, DHEA) into the active Δ4-steroids (progesterone, 17-OH-progesterone, androstenedione) is indispensable for the biosynthesis of all major steroid hormones — mineralocorticoids (aldosterone), glucocorticoids (cortisol), and sex steroids (testosterone, estrogens). The enzyme is expressed in both the adrenal cortex and the gonads. Without it, the body cannot complete the conversion from cholesterol-derived precursors to functional steroids in either tissue.

The rs28934880 variant introduces a C-to-A transversion in exon 1 of HSD3B2, changing codon 10 from GCA (alanine) to GAA (glutamic acid). This single amino acid substitution — p.Ala10Glu — was first characterized in two French-Canadian families presenting with severe salt-wasting congenital adrenal hyperplasia (CAH), and evidence of a shared ancestral haplotype spanning 3.3 cM around the locus points to a founder event in that population. The variant is exceptionally rare globally (gnomAD genomes: 1 observation in 149,092 alleles).

The Mechanism

Alanine at position 10 is highly conserved across the entire 3βHSD gene family in vertebrates²² Alanine at position 10 is highly conserved across the entire 3βHSD gene family in vertebrates
Conservation at this site implies structural or catalytic importance across species. The residue sits in the enzyme's NAD-binding domain — the cofactor-binding pocket that the enzyme requires to carry out oxidation and isomerization of Δ5-steroids. Replacing the small, non-polar alanine with a bulkier, negatively charged glutamic acid at this site is expected to disrupt the geometry of the NAD-binding pocket and destabilize the protein's tertiary structure.

Functional studies in transfected Ad293 cells confirmed this prediction: the Ala10Glu mutant enzyme exhibited no detectable 3β-HSD activity³³ the Ala10Glu mutant enzyme exhibited no detectable 3β-HSD activity
Intact-cell transfection assays quantify the enzyme's ability to convert substrate in a cellular environment; zero detectable activity means essentially no functional enzyme is produced or retained. This is a complete loss-of-function variant. When both copies of HSD3B2 carry this (or another severe) mutation, the adrenal cortex and gonads cannot produce glucocorticoids, mineralocorticoids, or sex steroids from their Δ5-steroid precursors. The consequent accumulation of DHEA and 17-OH-pregnenolone alongside deficiency of cortisol and aldosterone defines the biochemical signature of the disorder.

Because HSD3B1 (the type 1 isoenzyme, expressed in placenta and peripheral tissues) is encoded by a different gene and is not affected, partial peripheral conversion of androgens can still occur via HSD3B1 — explaining why some affected individuals show partial masculinization during puberty despite adrenal/gonadal deficiency.

The Evidence

The index case characterization comes from Alos et al. 2000 (J Clin Endocrinol Metab 85:1968–74)⁴⁴ Alos et al. 2000 (J Clin Endocrinol Metab 85:1968–74)
A novel A10E homozygous mutation in HSD3B2 gene causing severe salt-wasting 3beta-hydroxysteroid dehydrogenase deficiency in 46,XX and 46,XY French-Canadians: evaluation of gonadal function after puberty. Two unrelated homozygous patients — one 46,XX, one 46,XY — both presented in infancy with salt-wasting CAH. The 46,XY patient achieved partial masculinization during puberty through peripheral HSD3B1 activity, but semen analysis at age 18.5 showed azoospermia; the 46,XX patient reached menarche and had progesterone secretion, demonstrating residual gonadal function via peripheral conversion.

A comprehensive 2019 review by Al Alawi, Nordenström and Falhammar (Endocrine 64:622–634)⁵⁵ Al Alawi, Nordenström and Falhammar (Endocrine 64:622–634)
Clinical perspectives in congenital adrenal hyperplasia due to 3β-hydroxysteroid dehydrogenase type 2 deficiency synthesized data from all published HSD3B2 cases. A critical point for carriers: "the hormonal profile cannot distinguish heterozygous carriers from normal people" — molecular genetic testing is required to identify carrier status. The review documents that over 40 HSD3B2 mutations have been characterized, with null mutations (frameshift, nonsense) invariably causing salt-wasting, while missense mutations with residual activity (~10%) are associated with non-salt-wasting phenotypes.

Mermejo et al. 2005 (J Clin Endocrinol Metab 90:1287–93)⁶⁶ Mermejo et al. 2005 (J Clin Endocrinol Metab 90:1287–93) established biochemical thresholds: ACTH-stimulated 17-OH-pregnenolone ≥201 nmol/L distinguishes genotype-confirmed HSD3B2-deficient patients from hormonal-only evaluations, helping reduce false-positive diagnoses. Importantly, their data showed heterozygous parents of affected children had intermediate but not clearly elevated hormone levels — again supporting that carriers do not have detectable clinical disease.

Male fertility in treated patients is not uniformly lost. Donadille et al. 2018 (Endocrine Connections 7:R255–R265)⁷⁷ Donadille et al. 2018 (Endocrine Connections 7:R255–R265) reported a 24-year-old male homozygote with a different HSD3B2 deletion who maintained sperm concentrations of 57.6 million/mL under replacement therapy — the first report of adequate spermatogenesis in a clinically confirmed case, suggesting that early diagnosis and steroid replacement may preserve fertility in some males.

Practical Actions

For heterozygous carriers (AC genotype), no clinical management is required — adrenal and gonadal steroid production is entirely normal with one functional gene copy. The clinical relevance is reproductive: each pregnancy with another carrier carries a 25% probability of an affected child. Informing a reproductive endocrinologist or genetic counselor before conception allows informed decisions.

For homozygous individuals (AA genotype), this is a medical condition requiring specialist management. Long-term glucocorticoid replacement (hydrocortisone 10–15 mg/m²/day, divided three times daily) suppresses excess Δ5-androgens and replaces cortisol. Salt-wasting forms additionally require mineralocorticoid replacement (fludrocortisone 0.1 mg/day with sodium supplementation in infancy). Androgen management is typically more challenging than in 21-hydroxylase deficiency. Fertility preservation counseling should be initiated early in adulthood.

Interactions

Compound heterozygosity — one Ala10Glu allele plus a second distinct HSD3B2 pathogenic variant — produces the same phenotypic spectrum as homozygosity for either single variant, since both copies of HSD3B2 are nonfunctional. The severity depends on the residual activity of the second allele: a missense mutation with ~10% activity combined with the Ala10Glu null allele may produce a milder non-salt-wasting phenotype. Full genotype characterization of both alleles is essential for prognosis.