Naturally Occurring Mutations of Human Corticosteroid-binding Globulin

Context: Corticosteroid-binding globulin (CBG) is encoded by SERPINA6. It is the major plasma binding protein of glucocorticoids and regulates plasma cortisol levels and bioavailability in humans. Several proteases target CBG and disrupt its steroid-binding properties. To date, most genetic deficiencies that alter plasma CBG levels or function have been identified in patients presenting with a variety of clinical conditions.

Objective: The objective of the study was to test 32 previously uncharacterized nonsynonymous, single-nucleotide polymorphisms in SERPINA6 for their ability to alter CBG production and/or function.

Design: Human CBG mutants were produced in Chinese hamster ovary cells for ELISA, cortisol-binding activity measurements, and Western blotting as well as assays of their protease sensitivities.

Results: Eight naturally occurring CBG mutants with abnormal production and/or function were identified. Cortisol-binding affinity was markedly reduced for CBG H14Q and CBG H89Y, moderately decreased for CBG I279F, and undetectable for CBG R260L. By contrast, CBG H14R exhibited a decreased cortisol-binding capacity. Comparison of CBG levels in cell extracts and media by Western blotting revealed that CBG I48N and CBG P246Q have secretion defects. Two mutants (CBG I179V and CBG I279F) displayed reduced rates of cortisol-binding activity loss after exposure to three different proteases (neutrophil elastase, chymotrypsin, and LasB produced by Pseudomonas aeruginosa).

Conclusion: Our data provide insight into how specific residues affect CBG secretion or function and illustrate the need to consider the various naturally occurring human CBG mutations in clinical evaluations of diseases associated with abnormalities in cortisol levels or activity.