Two Different Corticosteroid-Binding Globulin Variants that Lack Cortisol-Binding Activity in a Greek Woman

Abstract

Corticosteroid-binding globulin (CBG), encoded by SERPINA6, is the principal plasma binding protein for cortisol. Most nonsynonymous single-nucleotide polymorphisms that alter the production or function of CBG occur rarely, and their clinical significance remains obscure. Serum and DNA were obtained from a Greek woman with low morning cortisol levels and from family members. SERPINA6 exons were sequenced, and serum CBG was measured by ELISA and cortisol-binding capacity assay. Recombinant CBG variants were produced for detailed functional studies. A novel heterozygous c.1282G>C transversion in exon 5 of SERPINA6, resulting in a p.Trp393Ser (W371S) substitution, was identified in the proband, who was also heterozygous for single-nucleotide polymorphisms encoding the CBG Lyon (D367N) and CBG A224S variants. The proband had no measurable plasma cortisol-binding activity despite a CBG level of 273 nm by ELISA. She inherited CBG W371S from her mother whose plasma cortisol-binding capacity was approximately 50% lower than the CBG measurements by ELISA (314 nm). The proband's father and four children were heterozygous for CBG D367N; their CBG levels by ELISA were normal, but corresponding cortisol-binding capacity measurements were 50% lower. Pedigree analysis revealed that W371S segregates with A224 and that D367N and W371S segregate separately. Recombinant CBG D367N and CBG W371S had no measureable cortisol-binding activity. A new CBG Athens (W371S) variant that lacks cortisol-binding activity has been identified in a carrier of the cortisol-binding deficient CBG Lyon (D367N) variant. Analyses of CBG levels in this pedigree illustrate how immunoassays fail to accurately reflect cortisol-binding activity.