rs80358216 — HSD3B2 Trp171X

HSD3B2 Trp171X — The Steroidogenesis Bottleneck That Breaks

At a single enzymatic step buried deep in the steroid hormone assembly line, 3β-hydroxysteroid dehydrogenase type II — encoded by HSD3B2 — performs one of the most consequential conversions in human biochemistry: it transforms inactive Δ5-steroids into the active Δ4-steroids that become cortisol, aldosterone, testosterone, and estrogen. 3β-HSD type II¹¹ 3β-HSD type II
The type II isoenzyme is the adrenal and gonadal form; HSD3B1 is a distinct gene expressed in placenta and peripheral tissues that cannot compensate for HSD3B2 loss in the adrenal or gonad. The Trp171X variant (c.512G>A, p.Trp171Ter) introduces a premature stop codon at codon 171, producing a severely truncated protein that lacks the final 202 amino acids — including the entire substrate-binding domain. No residual enzymatic activity remains.

The Mechanism

HSD3B2 catalyzes the oxidative conversion of Δ5-ene-3β-hydroxy steroids (pregnenolone, DHEA, 17α-hydroxypregnenolone, androstenediol) into their Δ4-ketone counterparts (progesterone, androstenedione, 17α-hydroxyprogesterone, testosterone). This single step is required at the entrance to every branch of steroid hormone synthesis in the adrenal cortex and gonads. When this step is blocked²² When this step is blocked
Without functional 3β-HSD type II, the adrenal glands can still produce cholesterol and cleave it to pregnenolone via StAR and CYP11A1, but the downstream cascade to cortisol, aldosterone, and sex steroids halts completely at the Δ5→Δ4 branch point.

The Trp171X nonsense mutation was first described by Simard et al. 1993³³ Simard et al. 1993
Molecular Endocrinology 7(5):716-28 in a compound heterozygous patient with severe salt-losing disease. Functional studies confirmed no detectable 3β-HSD enzymatic activity from the truncated allele. The GRCh38 plus-strand change (G→A at chr1:119,422,013) converts the TGG tryptophan codon to TAG (stop), verified against ClinVar VCV000012184 with "criteria provided, multiple submitters, no conflicts" review status — the highest non-expert-panel ClinVar tier.

The Evidence

3β-HSD2 deficiency (OMIM 201810) is among the rarest forms of congenital adrenal hyperplasia. The comprehensive mutation registry reviewed by Simard, Moisan & Morel 2002⁴⁴ Simard, Moisan & Morel 2002
Seminars in Reproductive Medicine catalogued 34 pathogenic HSD3B2 variants across 56 patients from 44 families — making it one of the smallest CAH gene mutation databases despite its clinical severity.

Two clinically distinct presentations emerge from the genotype-phenotype data:

• Salt-wasting form: Complete enzyme loss (nonsense, frameshift, large missense changes) produces the most severe phenotype. Neonatal salt-wasting crises occur within the first 2–4 weeks of life as aldosterone and cortisol fail. In 46,XY patients, incomplete masculinization occurs because testosterone synthesis in the fetal gonad is also abrogated. In 46,XX patients, mild virilization may paradoxically occur due to adrenal DHEA accumulation converted peripherally by HSD3B1.

• Non-salt-wasting form: Partial-activity missense mutations leave enough residual enzyme to prevent mineralocorticoid crisis but still cause incomplete androgen synthesis. These patients often present later — at puberty, or even in adulthood.

The diagnostic challenge is illustrated by Jeandron & Sahakitrungruang 2012⁵⁵ Jeandron & Sahakitrungruang 2012
Hormone Research in Paediatrics: a 46,XX newborn with a homozygous HSD3B2 nonsense mutation (Q334X) had elevated 17-hydroxyprogesterone on newborn screen and was initially diagnosed as 21-hydroxylase deficiency. The elevation occurs because peripheral HSD3B1 converts the accumulating 17α-hydroxypregnenolone to 17α-hydroxyprogesterone — mimicking 21-OHD biochemistry. Correct diagnosis requires HSD3B2 gene sequencing.

A late-diagnosed case reported by Fanis et al. 2020⁶⁶ Fanis et al. 2020
J Pediatr Endocrinol Metab illustrates the diagnostic fingerprint: GC-MS urinary steroid metabolome showing the characteristic accumulation of Δ5-steroid metabolites — the most reliable non-genetic diagnostic tool for 3β-HSD deficiency when gene sequencing is not immediately available.

Practical Actions

Treatment of classic salt-wasting 3β-HSD2 deficiency is analogous to other forms of CAH requiring both glucocorticoid and mineralocorticoid replacement. Hydrocortisone suppresses the chronically elevated ACTH, reducing substrate accumulation. Fludrocortisone replaces the absent aldosterone to normalize sodium and potassium balance. Androgen suppression in 3β-HSD2 deficiency is notably more difficult than in 21-hydroxylase deficiency because adrenal DHEA continues to accumulate and is converted peripherally.

For carriers — heterozygous individuals with one functional and one Trp171X allele — enzyme activity is approximately halved but clinically silent. The primary clinical relevance is reproductive: two carrier parents face a 25% probability of an affected child with each pregnancy. Newborn screening identifies affected neonates before salt-wasting crisis in most high-income countries, but rapid confirmatory testing is essential.

Interactions

HSD3B2 Trp171X interacts critically with compound heterozygous variants in the same gene. A single copy of Trp171X paired with any other HSD3B2 loss-of-function variant (frameshift, nonsense, or complete-loss missense) produces compound heterozygous disease clinically indistinguishable from homozygous disease. Simard et al. 1993 described exactly this pattern: W171X + E142K compound heterozygosity producing severe salt-wasting phenotype.

HSD3B2 Trp171X does not interact with variants in other steroidogenic genes (CYP21A2, CYP17A1, CYP11B1) in any documented compound heterozygous sense, as 3β-HSD2 deficiency is a discrete enzymatic block upstream of all other steroid synthesis steps.