rs104894138 — CYP17A1 Arg96Trp (R96W)

Pathogenic missense variant abolishing 17α-hydroxylase/17,20-lyase activity, causing combined cortisol and sex steroid deficiency with mineralocorticoid excess

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The Enzyme That Makes Cortisol and Sex Steroids — Broken at the Switch

CYP17A1 encodes cytochrome P450 17α-hydroxylase/17,20-lyase, an enzyme that sits at a critical junction in human steroid biosynthesis. It performs two consecutive chemical reactions: first converting pregnenolone to 17α-hydroxypregnenolone (the 17-hydroxylase step), then cleaving that intermediate to yield DHEA and other 17-ketosteroids11 17-ketosteroids
17-ketosteroids are precursors to testosterone, estradiol, and other sex steroids; without them, the gonads and adrenals cannot make sex hormones (the 17,20-lyase step). Both activities reside in the same enzyme active site, and the Arg96 residue sits in the substrate-binding region responsible for positioning steroid molecules for catalysis.

The Arg96Trp variant — a single nucleotide change on chromosome 10 — replaces arginine with tryptophan at position 96, disrupting the substrate-binding pocket and almost completely abolishing both enzymatic activities22 almost completely abolishing both enzymatic activities
Laflamme et al. expressed the R96W mutant in COS-1 cells and found near-total loss of both 17α-hydroxylase and 17,20-lyase function compared to wild-type. This is a rare autosomal recessive variant: homozygotes develop the full clinical syndrome of 17α-hydroxylase/17,20-lyase deficiency (17-OHD), while heterozygous carriers are generally unaffected but carry the variant for transmission to offspring.

The Mechanism

When CYP17A1 is non-functional, steroid precursors are shunted entirely into the mineralocorticoid pathway. The adrenal glands overproduce 11-deoxycorticosterone (DOC) and corticosterone33 11-deoxycorticosterone (DOC) and corticosterone
DOC and corticosterone are potent mineralocorticoids that act on the kidney to retain sodium and water, raising blood pressure and suppressing renin; they are normally minor products but become the dominant adrenal steroids when CYP17A1 fails. The result is volume-dependent hypertension, hypokalemia (low potassium), and suppressed renin — a distinctive biochemical fingerprint. Simultaneously, cortisol cannot be made, so ACTH rises chronically (ACTH is the pituitary hormone that drives adrenal function), further amplifying DOC and corticosterone overproduction.

Sex steroid synthesis collapses entirely: no DHEA, no androstenedione, no testosterone, no estradiol from either gonads or adrenals. In 46,XY individuals, the testes cannot produce testosterone during fetal development, producing female external genitalia (complete sex reversal) and undescended testes. In 46,XX individuals, the ovaries cannot produce estrogen, causing primary amenorrhea and absent pubertal development, though the uterus is present and fertility may be possible with treatment.

The Evidence

The R96W variant was first characterized in 1996 by Laflamme et al.44 1996 by Laflamme et al.
JCEM 1996; 81(1):264-268 — two homozygous French-Canadian 46,XY siblings with combined 17-OHD; site-directed mutagenesis in COS-1 cells showed near-complete abolition of both enzymatic activities. Subsequent case reports have confirmed the same clinical syndrome across populations. A 2010 Brazilian case55 2010 Brazilian case
Costenaro et al., Arq Bras Endocrinol Metabol 2010; 54(8):744-748 — a 16-year-old 46,XY phenotypic female with primary amenorrhea, hypertension, and hypokalemia documented the classic presentation: raised as female, never menstruated, found to have a 46,XY karyotype with no sex steroid production. ClinVar (VCV000001785) classifies R96W as pathogenic/likely pathogenic based on 11 out of 12 submissions from major genetics laboratories, supported by functional evidence of ~10–30% residual enzyme activity.

Codon 96 appears structurally critical: the R96Q variant at the same position66 R96Q variant at the same position
Brooke et al., JCEM 2006; 91(6):2428-2431 — R96Q identified independently, causes complete P450c17 inactivity; demonstrates that any substitution at Arg96 disrupts the substrate-binding region causes an identical clinical syndrome, reinforcing that Arg96 is essential for positioning substrates in the active site.

ClinVar review status: "criteria provided, multiple submitters, no conflicts" — the highest level of confidence for a rare variant short of expert panel review.

Practical Actions

For homozygous individuals (full 17-OHD), treatment requires lifelong glucocorticoid replacement to suppress ACTH, normalize mineralocorticoid excess, and control hypertension. Sex steroid replacement is also needed: estrogen (with progesterone if a uterus is present) for 46,XX individuals and for 46,XY individuals raised as female; testosterone replacement if the individual identifies as male. Blood pressure should be monitored at every visit, as it may normalize with glucocorticoid therapy alone but sometimes requires additional antihypertensives.

Fertility is possible for 46,XX women with 17-OHD. A 2025 case series77 2025 case series
Chai et al., J Assist Reprod Genet 2025; 42(7) — four Chinese women with 17-OHD completed eight IVF frozen embryo transfer cycles; glucocorticoid pretreatment enabled adequate endometrial preparation; three live births resulted demonstrated successful pregnancies through IVF with glucocorticoid-supplemented frozen embryo transfer protocols. Pregnancy in 17-OHD requires specialist obstetric care with careful blood pressure management.

For heterozygous carriers, the condition is autosomal recessive: carriers have one functional gene copy and do not develop the clinical syndrome. The reproductive significance is that two carrier parents have a 25% chance of an affected child per pregnancy. Reproductive genetic counseling and preimplantation genetic testing (PGT-M) are available options.

Interactions

17-OHD caused by R96W belongs to the same clinical syndrome as 17-OHD caused by other CYP17A1 mutations. The POR gene (encoding P450 oxidoreductase, the electron donor for CYP17A1) and CYB5A gene (encoding cytochrome b5, an allosteric activator of the 17,20-lyase step) can cause overlapping phenotypes when mutated. These represent different genetic causes of the same enzymatic pathway dysfunction and are not compound interactions with rs104894138.

The drug abiraterone acetate pharmacologically replicates CYP17A1 deficiency by inhibiting the same enzyme — its clinical use in prostate cancer produces the same mineralocorticoid excess profile seen in genetic 17-OHD, requiring concurrent prednisone to suppress ACTH-driven DOC overproduction.

Genotype Interpretations

What each possible genotype means for this variant.

GG No R96W Variant Normal

No Arg96Trp mutation — normal CYP17A1 function at this position

You carry two copies of the normal CYP17A1 allele at this position and do not have the Arg96Trp mutation. Your CYP17A1 enzyme is not impaired by this variant. This result is expected in over 99.99% of the general population, as the R96W allele is extremely rare globally (frequency approximately 0.004% or 4 in 100,000 alleles). Normal CYP17A1 function means your adrenal glands and gonads can produce cortisol, sex steroids, and androgens through the standard enzymatic pathway.

AG Carrier Carrier

Carries one copy of Arg96Trp — unaffected carrier of a recessive disorder

The R96W variant almost completely abolishes CYP17A1 enzymatic activity when present on both chromosomes. However, one functional copy of CYP17A1 produces enough enzyme to maintain normal cortisol and sex steroid biosynthesis — the single functional allele is fully sufficient, and carriers do not show elevated DOC or corticosterone, do not have hypertension from mineralocorticoid excess, and do not have sex steroid deficiency.

The primary clinical significance of carrier status is reproductive: if your biological partner is also an Arg96Trp carrier (or carries any other pathogenic CYP17A1 variant), there is a 25% chance per pregnancy that a child will inherit two non-functional CYP17A1 alleles and develop 17-OHD. Given the rarity of this allele, the population-level risk is very low, but targeted genetic counseling is appropriate for family planning.

AA Homozygous Affected Homozygous

Carries two copies of Arg96Trp — 17α-hydroxylase/17,20-lyase deficiency

CYP17A1 Arg96Trp (R96W) was functionally characterized in 1996: site-directed mutagenesis in cell culture showed the mutant protein retains approximately 10–30% of wild-type activity, with near-complete loss of both the 17-hydroxylase and 17,20-lyase reactions. In homozygous individuals, steroid synthesis is redirected entirely into the mineralocorticoid pathway: 11-deoxycorticosterone (DOC) and corticosterone accumulate, causing sodium retention, volume expansion, hypertension, and hypokalemia. The renin-angiotensin system is suppressed by the volume load. ACTH rises chronically (because cortisol feedback is absent), amplifying DOC and corticosterone production further.

Sex steroid production collapses: no DHEA, no testosterone, no estradiol from either adrenals or gonads. In 46,XY individuals, absent fetal testosterone causes female external genitalia and undescended testes (46,XY disorder of sex development). In 46,XX individuals, absent ovarian estrogen causes primary amenorrhea and no breast development at puberty, though internal reproductive organs are present; the progesterone precursors that accumulate can sometimes sustain a luteal-like endometrium, making fertility possible with treatment.

Key laboratory findings include: elevated progesterone, elevated DOC and corticosterone, suppressed renin, low cortisol (or low serum response to ACTH stimulation), low DHEA-S, low testosterone and estradiol, elevated LH and FSH. Hypertension and hypokalemia complete the clinical picture.

Treatment centers on glucocorticoid replacement (dexamethasone or prednisone) to suppress ACTH, which reduces mineralocorticoid overproduction and typically normalizes blood pressure over weeks. Sex steroid replacement is added: estrogen (plus progesterone if a uterus is present) for individuals raised female, or testosterone for those raised male. IVF with glucocorticoid-supported embryo transfer has produced successful pregnancies in 46,XX women.