Novel human corticosteroid-binding globulin variant with low cortisol-binding affinity.
Corticosteroid-binding globulin (CBG) is the plasma transport protein that regulates the access of glucocorticoid hormones to target cells. Genetic deficiencies of CBG are rare, and only a single human CBG variant (Trancortin Leuven) has been related so far to decreased cortisol-binding affinity. We report here on a 43-yr-old woman, referred for chronic asthenia and hypotension, with repeatedly low morning serum cortisol levels (22-61 nmol/L; normal range, 204-546 nmol/L), normal plasma ACTH levels (38-49 pg/mL; normal, <50 pg/mL), and normal urinary cortisol (10-76 nmol/24 h; normal range, 10-105 nmol/24 h). An increased percent-free (dialysable fraction) serum cortisol (8.7-9.7%, normal range, 2.9-3.9%) suggested abnormal CBG binding activity. Indeed, she had a low serum CBG concentration (24 mg/L vs. 44+/-6 mg/L in normal women), and the affinity of her CBG for cortisol was decreased (association constant, Ka = 0.12 L/nmol vs. 0.82+/-0.29 L/nmol). In her immediate family members, the serum CBG concentration and cortisol-binding activity were normal in her husband, but the four living children had slightly lower serum CBG concentrations than the reference ranges for their pre- and postpubertal status. Measurements of cortisol distribution in undiluted serum indicated that an increase in the percentage of nonprotein-bound cortisol offsets the low cortisol levels to give approximately normal concentrations of free cortisol in serum. Direct sequencing of PCR-amplified exons encoding CBG revealed that the proband was homozygous for a polymorphism (GAC-AAC) in the codon for residue 367, which results in a Asp367-->Asn substitution. Her children were heterozygous for this polymorphism. When this nucleotide change was introduced into a normal human CBG complementary DNA, for expression in Chinese hamster ovary cells, Scatchard analysis demonstrated that the Asn367 substitution reduced the affinity of human CBG for cortisol by approximately 4-fold (Ka = 0.15 L/nmol), as compared to normal recombinant CBG (Ka = 0.66 L/nmol). These results suggest that Asp367 is an important determinant of CBG steroid-binding activity and that normal negative regulation of the hypothalamic-pituitary-adrenal axis is maintained by relatively normal serum-free cortisol concentrations, despite a marked reduction in the steroid-binding affinity of this novel human CBG variant, which we have designated as CBG-Lyon.
Emptoz-Bonneton
A
A
Hospices Civils de Lyon, Laboratoire de la Clinique Endocrinologique, Hôpital de l'Antiquaille, Lyon, France.
Cousin
P
P
Seguchi
K
K
Avvakumov
G V
GV
Bully
C
C
Hammond
G L
GL
Pugeat
M
M
D000303:Adrenal Cortex Diseases / Q000235:genetics / Q000523:psychology; D000328:Adult; D019943:Amino Acid Substitution / Q000235:genetics; D015153:Blotting, Western; D017931:DNA Primers; D018076:DNA, Complementary / Q000096:biosynthesis / Q000235:genetics; D005260:Female; D006728:Hormones / Q000097:blood; D006801:Humans; D006854:Hydrocortisone / Q000097:blood / Q000378:metabolism*; D016296:Mutagenesis; D011110:Polymorphism, Genetic / Q000235:genetics; D012150:Polymorphism, Restriction Fragment Length; D011485:Protein Binding; D014156:Transcortin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
D017931:DNA Primers; D018076:DNA, Complementary; D006728:Hormones; D014156:Transcortin; D006854:Hydrocortisone