The precise molecular abnormalities that cause primary cortisol resistance have not been completely described. In a subject with primary cortisol resistance we have observed glucocorticoid receptors (hGR) with a decreased affinity for dexamethasone. We hypothesize that a mutation of the hGR glucocorticoid-binding domain is the cause of cortisol resistance. Total RNA isolated from the index subject's mononuclear leukocytes was used to produce first strand hGR cDNAs, and the entire hGR cDNA was amplified in segments and sequenced. At nucleotide 2,317 we identified a homozygous A for G point mutation that predicts an isoleucine (ATT) for valine (GT1) substitution at amino acid 729. When the wild-type hGR and hGR-Ile 729 were expressed in COS-1 cells and assayed for 13H1-Dexamathasone binding, the dissociation constants were 0.799±0.068 and 1.54±0.06 nM (mean±SEM) (P < 0.01), respectively. When the wild-type hGR and hGR-Ile 729 were expressed in CV-1 cells that were cotransfected with the mouse mammary tumor virus long terminal repeat fused to the chloramphenicol acetyl transferase (CAT) gene, the hGR-Ile 729 conferred a fourfold decrease in apparent potency on dexamethasone stimulation of CAT activity. The isoleucine for valine substitution at amino acid 729 impairs the function of the hGR and is the likely cause of primary cortisol resistance in this subject. (J. Clin. Invest. 1993.91:1918-1925

Primary cortisol resistance (PCR) is a rare cause of hypercortisolism and usually does not produce clinical manifestations. This report describes primary cortisol resistance in a boy with isosexual precocity. A 6 7/12-yr-old boy had Tanner stage 3 pubic hair, accelerated linear growth, and advanced bone age (10 yr), but normal (for age) tests. There were no features of glucocorticoid excess. Serum androstenedione and dehydroepiandrosterone concentrations were 4.7 +/- 0.3 nmol/L (mean +/- SEM of four measurements; normal less than 1.2) and 13.5 nmol/L (single measurement; normal, 1.0-2.2), respectively. The serum testosterone concentration was 0.9 nmol/L (normal, less than 0.7), and FSH and LH were normal. Serum cortisol concentrations were 1590 +/- 110 nmol/L (normal, 190-630) and 580 +/- 60 nmol/L (normal, 50-410) at 0800 and 2000 h, respectively. Serum cortisol responded normally to insulin-induced hypoglycemia. Glucocorticoids and adrenal androgens were resistant to suppression by dexamethasone. The Kd of [3H]dexamethasone binding to the glucocorticoid receptors of mononuclear leukocytes was increased (6.4 +/- 0.8 nM; mean +/- SEM of four determinations; normal, 1.4-3.4; P less than 0.001), but the binding capacity was normal. This patient with isosexual precocity has PCR, as indicated by functionally abnormal glucocorticoid receptors and hypercortisolism without other clinical or biochemical manifestations of Cushing's syndrome. Excessive adrenal stimulation by ACTH caused increased secretion of both cortisol and adrenal androgens, and the latter caused the clinical manifestations. PCR should be considered in other male children with isosexual precocity or female children with heterosexual precocity.