rs104894143 — CYP17A1 W406R (Trp406Arg)

CYP17A1 W406R — The Steroid Synthesis Null Allele

CYP17A1 encodes 17α-hydroxylase/17,20-lyase¹¹ 17α-hydroxylase/17,20-lyase
a dual-function cytochrome P450 enzyme that sits at a critical branch point in adrenal and gonadal steroid synthesis. Without this enzyme, the steroid pathway cannot produce cortisol, sex hormones (androgens and estrogens), or DHEA. Instead, precursors accumulate upstream — particularly mineralocorticoids²² mineralocorticoids
aldosterone-pathway steroids that regulate blood pressure and electrolytes, including deoxycorticosterone (DOC), which causes the mineralocorticoid excess syndrome characteristic of the disease.

The W406R variant (c.1216T>C in coding-strand notation; A>G on the GRCh38 plus strand at chr10:102,831,535) substitutes a tryptophan for arginine at position 406. This single amino acid change completely abolishes both catalytic activities of the enzyme. rs104894143 is the most prevalent pathogenic CYP17A1 allele identified in Brazilian and Portuguese-ancestry populations, accounting for approximately 50% of mutant alleles in the largest case series to date.

The Mechanism

CYP17A1 catalyzes two sequential reactions. The first (17α-hydroxylation³³ 17α-hydroxylation
converts pregnenolone → 17-hydroxypregnenolone and progesterone → 17-hydroxyprogesterone) is required for cortisol synthesis in the adrenal cortex. The second (17,20-lyase activity) converts 17-hydroxylated substrates to DHEA and androstenedione — the precursors to all androgens and estrogens in both the gonads and adrenal gland.

Tryptophan 406 lies in the enzyme's active site in a region critical for haem coordination and substrate binding. Replacing it with the bulkier, charged arginine disrupts the active site geometry. In vitro expression studies⁴⁴ In vitro expression studies
using COS-7 cells and yeast microsomes confirm that W406R produces a completely inactive enzyme — no detectable 17α-hydroxylase or 17,20-lyase activity above background, a finding consistent across multiple independent laboratories.

Carriers of one W406R allele (AG genotype) retain one functional copy of CYP17A1. A 2010 study of 14 genotype-proven heterozygous carriers⁵⁵ 2010 study of 14 genotype-proven heterozygous carriers
Qiao et al., Clinical Endocrinology found measurable but subclinical reduction in 17α-hydroxylase reserve: carriers showed elevated corticosterone-to-cortisol and progesterone-to-17-hydroxyprogesterone ratios after ACTH stimulation compared with matched controls, indicating half-normal enzymatic capacity. None had clinical symptoms.

The Evidence

The clearest description of the W406R allele's phenotypic consequences comes from Costa-Santos et al. 2004⁶⁶ Costa-Santos et al. 2004
24 patients from 19 Brazilian families, Emory University collaboration, which found that W406R accounts for half of all pathogenic CYP17A1 alleles in their cohort and appears to represent a founder mutation in Portuguese-descent populations.

Among homozygous and compound-heterozygous affected individuals studied by Carvalho et al. 2016⁷⁷ Carvalho et al. 2016
n=16 female patients with CYP17A1 mutations, Fertility and Sterility, the clinical picture is consistent: 71% had primary amenorrhea, 88% had hypertension at diagnosis, 62% had ovarian macrocysts, and three patients required surgery for ovarian torsion or rupture. Pubic hair was absent or sparse in all patients. Despite this severe phenotype, the same study concluded that fertility is achievable through assisted reproductive techniques with appropriate hormonal management.

A 2016 case report⁸⁸ 2016 case report
Bianchi et al., J Clin Endocrinol Metab documented the first successful live birth in a W406R carrier (compound heterozygote W406R/P428L) using a progestin-primed ovarian stimulation protocol, glucocorticoid pre-treatment to suppress the elevated endogenous progesterone below 1 ng/mL, and frozen-thawed embryo transfer. The child was born healthy after delivery at 30 weeks.

Epidemiologically, 17α-hydroxylase deficiency is rare — approximately 1 in 50,000 newborns globally — but considerably more common in populations with Portuguese founder ancestry. The G allele at rs104894143 is absent from gnomAD exomes (>1.4 million alleles) and detectable at only 7 per million alleles in gnomAD genomes, consistent with a severe recessive disease allele under strong purifying selection.

Practical Implications

For heterozygous carriers (AG genotype): Carriers are clinically asymptomatic but carry a 25% risk of having an affected child with another carrier. Endocrinology evaluation should include ACTH-stimulated steroid profiling (corticosterone, cortisol, progesterone, 17-OHP) to characterize individual enzymatic reserve. Genetic counseling and partner testing are appropriate before family planning. Carriers considering IVF have the option of preimplantation genetic testing (PGT) to select unaffected embryos.

For homozygotes (GG genotype): Full 17α-hydroxylase/17,20-lyase deficiency requires lifelong glucocorticoid replacement (to suppress ACTH and mineralocorticoid precursor overproduction) and sex hormone replacement (estrogen/progesterone in 46,XX; testosterone in 46,XY). Blood pressure and electrolytes require monitoring due to mineralocorticoid excess. With modern hormone replacement, fertility is possible for 46,XX women using IVF with glucocorticoid pre-treatment to normalize elevated progesterone before embryo transfer.

Interactions

rs743572 (CYP17A1 promoter -34 T>C): This variant in the same gene affects CYP17A1 expression levels through a regulatory mechanism rather than enzyme structure. Individuals who are AG or GG at rs104894143 and also carry the expression-altering promoter variant may have a modified phenotype, though this compound scenario has not been systematically studied — the rarity of the coding mutation makes compound observations uncommon.

Compound heterozygosity within CYP17A1: The literature documents numerous patients who carry W406R on one chromosome and a different CYP17A1 pathogenic variant on the other (e.g. R362C, P428L, Y329D). These compound heterozygotes phenotypically resemble homozygotes and require the same clinical management. Genetic testing should sequence the full CYP17A1 gene to exclude compound heterozygosity in any carrier of W406R.