rs137852689 — STAR R217T (Arg217Thr)

STAR R217T — When Cholesterol Cannot Enter the Mitochondria

The steroidogenic acute regulatory protein (StAR) performs one of the most consequential jobs in human physiology: it shuttles cholesterol across the outer mitochondrial membrane into the matrix, where the enzyme P450scc converts it into pregnenolone — the universal precursor for every steroid hormone in the body. Without functional StAR, the adrenal cortex and gonads cannot make cortisol, aldosterone, estrogens, testosterone, or progesterone. The R217T variant is a pathogenic mutation that abolishes StAR function, causing lipoid congenital adrenal hyperplasia¹¹ lipoid congenital adrenal hyperplasia
lipoid CAH — named for the massive cholesterol ester deposits that accumulate in steroidogenic cells when substrate cannot be processed, the most severe form of CAH and a life-threatening condition from the first days of life.

The Mechanism

The R217T mutation (c.650G>C at the nucleotide level) sits at the final nucleotide of exon 5 of the STAR gene. This position is the splice donor site — the precise boundary signal that tells the cell's splicing machinery where exon 5 ends. A G>C transversion here is not merely an amino acid change; it destroys the consensus splice donor sequence, causing complete skipping of exon 5 during RNA processing²² destroys the consensus splice donor sequence, causing complete skipping of exon 5 during RNA processing
Without exon 5, the reading frame of exon 6 is shifted, a premature stop codon appears at amino acid 174, and the resulting truncated protein has no steroidogenesis-enhancing activity. The protein change notation p.Arg217Thr describes the amino acid substitution that would result if splicing were unaffected, but in practice the splice disruption is the dominant consequence.

Studies of StAR missense mutants causing lipoid CAH³³ Studies of StAR missense mutants causing lipoid CAH
Bose et al. Biochemistry 1998 show that these proteins maintain overall globular structure but develop gross errors in tertiary folding, likely from loss of salt bridges that stabilize the normal three-dimensional architecture. Mutant proteins form abnormal intermolecular beta-sheets rather than the correctly folded alpha-helical structure of wild-type StAR. The practical consequence: StAR's cholesterol-transferring activity localizes to its C-terminal region⁴⁴ StAR's cholesterol-transferring activity localizes to its C-terminal region
The 30-kDa mature StAR must contact the outer mitochondrial membrane and transfer cholesterol without itself crossing into the matrix. When this domain is misfolded or truncated, cholesterol cannot enter the mitochondria, P450scc sits idle, and the cell accumulates massive cholesterol ester deposits — the lipoid appearance that names the disease.

The Evidence

Lipoid CAH caused by STAR mutations was one of the first conditions to illustrate the concept of a "human gene knockout" — the disease phenotype in compound heterozygous or homozygous patients mirrors precisely what happens when StAR is ablated in mice. More than 34 distinct STAR mutations have been identified, and R217T was first described in a Japanese patient⁵⁵ R217T was first described in a Japanese patient
Katsumata et al. JCEM 1999 with compound heterozygous mutations (R217T on one allele, A218V on the other). Both mutations individually abolish StAR function.

The clinical consequences are consistent across case series: most patients present with adrenal insufficiency between 1 day and 2 months of age⁶⁶ most patients present with adrenal insufficiency between 1 day and 2 months of age
Fujieda et al. J Steroid Biochem Mol Biol 2003, often in life-threatening adrenal crisis with severe electrolyte imbalance (hyponatremia, hyperkalemia) and cardiovascular collapse. The 46,XY genotype presents with female external genitalia⁷⁷ 46,XY genotype presents with female external genitalia
Because fetal testicular testosterone production requires StAR, 46,XY fetuses with lipoid CAH cannot masculinize the external genitalia, resulting in a phenotypically female presentation at birth — a DSD that is frequently the first diagnostic clue alongside the adrenal crisis. Affected 46,XX individuals are phenotypically female at birth (external genitalia are independent of StAR in early female development) but cannot produce ovarian estrogen at puberty without replacement therapy.

In contrast, some 46,XX patients develop spontaneous puberty and cyclical uterine bleeding. This is because the ovary — unlike the adrenal gland and testis — is quiescent during fetal life and early childhood, so the two-hit model of StAR inactivation-then-lipid-accumulation unfolds later. Some follicular steroidogenesis can proceed via a StAR-independent pathway before lipid loading destroys the tissue.

Practical Actions

Because lipoid CAH presents neonatally or in early infancy, the primary interventions are initiated by pediatric endocrinologists at diagnosis. For heterozygous carriers identified incidentally — including parents of affected children — the relevant actions center on genetic counseling, reproductive planning, and ensuring their own children receive immediate evaluation at birth.

Established management for affected individuals (homozygous or compound heterozygous) includes lifelong glucocorticoid replacement (hydrocortisone 10–15 mg/m²/day divided three times daily), mineralocorticoid replacement (fludrocortisone 0.05–0.2 mg/day with sodium chloride supplementation in infancy), stress dosing protocols for illness or surgery, and sex hormone replacement starting at the expected age of puberty. Stress dosing — tripling hydrocortisone during illness or injury — is the critical skill for preventing life-threatening adrenal crisis in affected individuals.

Interactions

The biology of lipoid CAH involves the full steroidogenesis cascade. StAR works upstream of every adrenal and gonadal steroid hormone, so its loss affects: cortisol (adrenal zona fasciculata), aldosterone (zona glomerulosa), DHEA/DHEAS (zona reticularis), testosterone (Leydig cells), and estrogen/progesterone (ovary). Other STAR loss-of-function alleles — including the nonsense mutations Q258X and R193X and the missense mutation R182L — account for the majority of lipoid CAH alleles worldwide, with Q258X representing approximately 70% of affected alleles in Japanese and Korean patients. Compound heterozygosity for R217T plus any other pathogenic STAR allele produces the same clinical syndrome as R217T homozygosity.

For heterozygous R217T carriers, no gene-gene interaction with other steroidogenesis pathway variants is documented to produce clinical disease. Clinical disease requires two non-functional STAR alleles (compound heterozygous or homozygous).