rs104894085 — STAR Q258X (c.772C>T)

STAR Q258X — The Most Common Allele That Silences All Steroid Synthesis

Every steroid hormone your body makes — cortisol, aldosterone, testosterone, estrogen, progesterone — begins at the same bottleneck: getting cholesterol across the outer mitochondrial membrane into the matrix, where the first enzyme of the steroidogenic cascade converts it to pregnenolone. The steroidogenic acute regulatory protein (StAR)¹¹ steroidogenic acute regulatory protein (StAR)
Encoded by the STAR gene on chromosome 8p11.23; a 30 kDa protein induced within minutes by ACTH and LH to enable acute steroidogenesis in adrenal and gonadal cells performs this critical transport. Without functional StAR, the transport step fails completely, cholesterol piles up as massive lipid deposits inside steroidogenic cells, and no steroid hormone — cortisol, aldosterone, or sex steroids — can be made.

The Q258X variant (c.772C>T on the coding strand; G>A on the genomic plus strand at rs104894085) introduces a premature stop codon at position 258 of the 285-amino-acid StAR protein, eliminating the entire C-terminal region where cholesterol-transferring activity resides. This is not a partial-function variant: the truncated protein is completely inactive. Q258X is the most prevalent allele causing lipoid congenital adrenal hyperplasia (lipoid CAH), accounting for approximately 70% of affected alleles globally²² 70% of affected alleles globally
Kim 2014: this figure rises to 92.3% in the Korean population due to a pronounced founder effect.

The Mechanism

The cholesterol-transferring activity of StAR is entirely dependent on its C-terminal domain — the region from roughly amino acid 230 onward forms the functional core of the START (StAR-related lipid transfer) domain³³ START (StAR-related lipid transfer) domain
A conserved ~210-amino-acid module that binds one cholesterol molecule and facilitates its transfer across the outer mitochondrial membrane. Q258X truncates the protein 28 amino acids from the end, removing the final portion of this domain. The resulting protein cannot transfer cholesterol into the mitochondrial matrix, so the cytochrome P450 side-chain cleavage enzyme (CYP11A1/P450scc) sitting in the inner membrane has no substrate to process.

Two consequences follow. First, steroid synthesis stops. Second, cholesterol continues to enter the cell under ACTH and LH stimulation but cannot be metabolized — it accumulates as massive cholesterol ester deposits. This lipid engorgement progressively destroys the steroidogenic cells of the adrenal cortex and gonads, producing the characteristic "lipoid" (fatty) appearance of the adrenal glands at autopsy or imaging and explaining why 46,XX individuals eventually lose ovarian function even though their initial estrogen production may be partially preserved during fetal life.

The Evidence

Lipoid CAH caused by STAR mutations was characterized by several groups working in parallel in the mid-1990s, and Q258X emerged as the dominant allele in East Asian cohorts. Nakae et al. 1997⁴⁴ Nakae et al. 1997
Human Molecular Genetics; analysis of 19 Japanese lipoid CAH patients (PMID 9097960) found Q258X in 16 of 19 patients as either homozygous or compound heterozygous. A homozygous Q258X case in a Japanese child was published the same year by Katsumata et al.⁵⁵ Katsumata et al.
Endocrine Journal, 1997 (PMID 9279522), who also showed that heterozygous carrier parents had completely normal hormonal responses to ACTH stimulation — making biochemical carrier detection impossible and confirming that genetic sequencing is required to identify carriers before they have an affected child.

The Korean data is even more striking. Kim et al. 2011⁶⁶ Kim et al. 2011
European Journal of Endocrinology (PMID 21846663) found Q258X in 46 of 50 alleles (92%) from 25 unrelated Korean lipoid CAH patients, confirming an extreme founder effect. The clinical presentation is invariant: all patients had normal female external genitalia regardless of karyotype (both 46,XY and 46,XX), absent production of cortisol and aldosterone presenting as neonatal or early-infantile adrenal crisis, and lifelong dependence on glucocorticoid, mineralocorticoid, and sex hormone replacement. Fujieda et al. 2003⁷⁷ Fujieda et al. 2003
JSBMB review of 34+ STAR mutations (PMID 12943739) summarizes that most affected patients present with adrenal insufficiency between 1 day and 2 months of age.

An important exception to complete gonadal failure in 46,XX individuals: some Q258X homozygotes undergo spontaneous feminization at puberty. Because the fetal ovary is largely quiescent — unlike the fetal adrenal gland and testis — it escapes the early lipid engorgement that destroys adrenal cells. A proportion of follicular steroidogenesis can proceed before lipid accumulation catches up. Kaku et al. 2008⁸⁸ Kaku et al. 2008
Endocrine Journal (PMID 18724044) documented a 46,XX adult with homozygous Q258X who menstruated spontaneously until ovarian torsion at age 22, at which point histology revealed massive lipoid deposition — the second hit completing ovarian destruction.

Practical Actions

Because lipoid CAH presents in the neonatal period or early infancy, diagnosis and initial management are always established by pediatric endocrinologists before the affected individual can read their genome report. For most users, the relevant information falls into one of two profiles:

Homozygous Q258X (AA genotype): you are almost certainly already under specialist care for this condition. The management principles below reflect established clinical practice — lifelong glucocorticoid (hydrocortisone) and mineralocorticoid (fludrocortisone) replacement, stress-dosing protocols, sex hormone replacement at puberty, and emergency kit preparedness. Missed doses or failure to stress-dose during illness are the principal causes of preventable adrenal crisis deaths.

Heterozygous carrier (AG genotype): you have one non-functional STAR allele and one functional copy, which is sufficient for completely normal steroid synthesis. The clinical relevance is reproductive: if your partner also carries any pathogenic STAR allele, each pregnancy has a 25% chance of producing an affected child. Partner testing and genetic counseling are the relevant actions.

Interactions

STAR Q258X is one of three most clinically important STAR null alleles: Q258X (rs104894085, ~70% of alleles globally), R217T (rs137852689, common in Japanese patients), and A218V (rs137852690, identified in Japanese and Iranian cohorts). All three alleles individually abolish StAR function. Compound heterozygosity for any two null alleles — including Q258X paired with R217T or A218V — produces the same complete lipoid CAH phenotype as Q258X homozygosity. Clinical management is identical regardless of which two alleles are compound heterozygous.

The STAR pathway interacts with the entire steroidogenic cascade: CYP11A1, HSD3B, CYP17A1, CYP11B1, and CYP11B2 all depend on the cholesterol import step that StAR enables. No single-gene interaction modifies the severity of Q258X homozygosity — complete StAR loss is the rate-limiting defect that renders all downstream enzymes irrelevant because they have no substrate.