A Leu----His substitution at residue 93 in human corticosteroid binding globulin results in reduced affinity for cortisol

Cortisol, Stress, and Disease-Bidirectional Associations; Role for Corticosteroid-Binding Globulin?

Selye described stress as a unified neurohormonal mechanism maintaining homeostasis. Acute stress system activation is adaptive through neurocognitive, catecholaminergic, and immunomodulation mechanisms, followed by a reset via cortisol. Stress system components, the sympathoadrenomedullary system, hypothalamic-pituitary-adrenal axis, and limbic structures are implicated in many chronic diseases by establishing an altered homeostatic state, allostasis. Consequent "primary stress system disorders" were popularly accepted, with phenotypes based on conditions such as Cushing syndrome, pheochromocytoma, and adrenal insufficiency. Cardiometabolic and major depressive disorders are candidates for hypercortisolemic etiology, contrasting the "hypocortisolemic symptom triad" of stress sensitivity, chronic fatigue, and pain. However, acceptance of chronic stress etiology requires cause-and-effect associations, and practical utility such as therapeutics altering stress system function. Inherent predispositions to stress system perturbations may be relevant. Glucocorticoid receptor (GR) variants have been associated with metabolic/neuropsychological states. The SERPINA6 gene encoding corticosteroid-binding globulin (CBG), was the sole genetic factor in a single-nucleotide variation-genome-wide association study linkage study of morning plasma cortisol, a risk factor for cardiovascular disease, with alterations in tissue-specific GR-related gene expression. Studies showed genetically predicted high cortisol concentrations are associated with hypertension and anxiety, and low CBG concentrations/binding affinity, with the hypocortisolemic triad. Acquired CBG deficiency in septic shock results in 3-fold higher mortality when hydrocortisone administration produces equivocal results, consistent with CBG's role in spatiotemporal cortisol delivery. We propose some stress system disorders result from constitutional stress system variants rather than stressors themselves. Altered CBG:cortisol buffering may influence interstitial cortisol ultradian surges leading to pathological tissue effects, an example of stress system variants contributing to stress-related disorders.

CBG Montevideo: A Clinically Novel SERPINA6 Mutation Leading to Haploinsufficiency of Corticosteroid-binding Globulin

Corticosteroid-binding globulin (CBG) is the main transport protein for cortisol, binding up to 90% in a 1:1 ratio. CBG provides transport of cortisol within the circulation and targeted cortisol tissue delivery.

Here, we describe the clinically novel “CBG Montevideo” a SERPINA6 pathogenic variant that results in a 50% reduction in plasma CBG levels. This was associated with low serum total cortisol and clinical features of hypoglycemia, exercise intolerance, chronic fatigue, and hypotension in the proband, a 7-year-old boy, and his affected mother.

Previous reports of 9 human CBG genetic variants affecting either CBG concentrations or reduced CBG-cortisol binding properties have outlined symptoms consistent with attenuated features of hypocortisolism, fatigue, and hypotension. Here, however, the presence of hypoglycemia, despite normal circulating free cortisol, suggests a specific role for CBG in effecting glucocorticoid function, perhaps involving cortisol-mediated hepatic glucose homeostasis and cortisol-brain communication.

Avian corticosteroid-binding globulin: biological function and regulatory mechanisms in physiological stress responses

Corticosteroid-binding globulin (CBG) is a high-affinity plasma protein that binds glucocorticoids (GCs) and regulates their biological activities. The structural and functional properties of CBG are crucial to understanding the biological actions of GCs in mediating stress responses and the underlying mechanisms. In response to stress, avian CBGs modulate the free and bound fractions of plasma corticosterone (CORT, the main GC), enabling them to mediate the physiological and behavioral responses that are fundamental for balancing the trade-off of energetic investment in reproduction, immunity, growth, metabolism and survival, including adaptations to extreme high-elevation or high-latitude environments. Unlike other vertebrates, avian CBGs substitute for sex hormone-binding globulin (SHBG) in transporting androgens and regulating their bioavailability, since birds lack an Shbg gene. The three-dimensional structures of avian and mammalian CBGs are highly conserved, but the steroid-binding site topographies and their modes of binding steroids differ. Given that CBG serves as the primary transporter of both GCs and reproductive hormones in birds, we aim to review the biological properties of avian CBGs in the context of steroid hormone transportation, stress responses and adaptation to harsh environments, and to provide insight into evolutionary adaptations in CBG functions occurred to accommodate physiological and endocrine changes in birds compared with mammals.

Neutrophil elastase-cleaved corticosteroid-binding globulin is absent in human plasma

Corticosteroid-binding globulin (CBG) transports glucocorticoids in blood and is a serine protease inhibitor family member. Human CBG has a reactive center loop (RCL) which, when cleaved by neutrophil elastase (NE), disrupts its steroid-binding activity. Measurements of CBG levels are typically based on steroid-binding capacity or immunoassays. Discrepancies in ELISAs using monoclonal antibodies that discriminate between intact vs RCL-cleaved CBG have been interpreted as evidence that CBG with a cleaved RCL and low affinity for cortisol exists in the circulation. We examined the biochemical properties of plasma CBG in samples with discordant ELISA measurements and sought to identify RCL-cleaved CBG in human blood samples. Plasma CBG-binding capacity and ELISA values were consistent in arterial and venous blood draining skeletal muscle, liver and brain, as well as from a tissue (adipose) expected to contain activated neutrophils in obese individuals. Moreover, RCL-cleaved CBG was undetectable in plasma from critically ill patients, irrespective of whether their ELISA measurements were concordant or discordant. We found no evidence of RCL-cleaved CBG in plasma using a heat-dependent polymerization assay, and CBG that resists immunoprecipitation with a monoclonal antibody designed to specifically recognize an intact RCL, bound steroids with a high affinity. In addition, mass spectrometry confirmed the absence of NE-cleaved CBG in plasma in which ELISA values were highly discordant. Human CBG with a NE-cleaved RCL and low affinity for steroids is absent in blood samples, and CBG ELISA discrepancies likely reflect structural differences that alter epitopes recognized by specific monoclonal antibodies.

Corticosteroid-binding globulin: acute and chronic inflammation

Corticosteroid-binding globulin (CBG) is the principal transport protein for cortisol binding 80% in a 1:1 ratio. Since its discovery in 1958, CBG's primary function has been considered to be cortisol transport within the circulation. More recent data indicate a cortisol tissue delivery function, particularly at inflammatory sites. CBG's structure as a non-inhibitory serine protease inhibitor allows allosteric structural change after reactive central loop (RCL) cleavage by neutrophil elastase (NE) and RCL insertion into CBG's protein core. Transition from the high to low affinity CBG form reduces cortisol-binding. Areas covered: In acute systemic inflammation, high affinity CBG (haCBG) is depleted proportionate to sepsis severity, with lowest levels seen in non-survivors. Conversely, in chronic inflammation, CBG cleavage is paradoxically reduced in proportion to disease severity, implying impaired targeted delivery of cortisol. CBG's structure allows thermosensitive release of bound cortisol, by reversible partial insertion of the RCL and loosening of CBG:cortisol binding. Recent studies indicate a significant frequency of function-altering single nucleotide polymorphisms of the SERPINA6 gene which may be important in population risk of inflammatory disease. Expert commentary: Further exploration of CBG in inflammatory disease may offer new avenues for treatment based on the model of optimal cortisol tissue delivery.

Plasma steroid-binding proteins: primary gatekeepers of steroid hormone action

Biologically active steroids are transported in the blood by albumin, sex hormone-binding globulin (SHBG), and corticosteroid-binding globulin (CBG). These plasma proteins also regulate the non-protein-bound or 'free' fractions of circulating steroid hormones that are considered to be biologically active; as such, they can be viewed as the 'primary gatekeepers of steroid action'. Albumin binds steroids with limited specificity and low affinity, but its high concentration in blood buffers major fluctuations in steroid concentrations and their free fractions. By contrast, SHBG and CBG play much more dynamic roles in controlling steroid access to target tissues and cells. They bind steroids with high (~nM) affinity and specificity, with SHBG binding androgens and estrogens and CBG binding glucocorticoids and progesterone. Both are glycoproteins that are structurally unrelated, and they function in different ways that extend beyond their transportation or buffering functions in the blood. Plasma SHBG and CBG production by the liver varies during development and different physiological or pathophysiological conditions, and abnormalities in the plasma levels of SHBG and CBG or their abilities to bind steroids are associated with a variety of pathologies. Understanding how the unique structures of SHBG and CBG determine their specialized functions, how changes in their plasma levels are controlled, and how they function outside the blood circulation provides insight into how they control the freedom of steroids to act in health and disease.

Corticosteroid-binding globulin: modulating mechanisms of bioavailability of cortisol and its clinical implications

Corticosteroid-binding globulin (CBG) is the principal transport protein of glucocorticoids. Approximately 80-90% of serum cortisol binds to CBG with high affinity and only about 5% of cortisol remain unbound and is considered biologically active. CBG seems to modulate and influence the bioavailability of cortisol to local tissues. In this review, we will discuss physicochemical properties of CBG and structure of CBG in the mechanisms of binding and release of cortisol. This review describes several factors affecting CBG functions, such as genetic factors or temperature. Furthermore, clinical implications of CBG abnormalities and the measurement of CBG and its use for assessment of free cortisol levels are described in this review.

Two different corticosteroid-binding globulin variants that lack cortisol-binding activity in a greek woman

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.

Single nucleotide polymorphisms in the human corticosteroid-binding globulin promoter alter transcriptional activity

Corticosteroid-binding globulin (CBG) is a high-affinity plasma protein that transports glucocorticoids and progesterone. Others and we have reported non-synonymous single nucleotide polymorphisms (SNPs) that influence CBG production or steroid-binding activity. However, no promoter polymorphisms affecting the transcription of human CBG gene (Cbg) have been reported. In the present study we investigated function implications of six promoter SNPs, including -26 C/G, -54 C/T, -144 G/C, -161 A/G, -205 C/A, and -443/-444 AG/-, five of which are located within the first 205 base pairs of 5'-flanking region and close to the highly conserved footprinted elements, TATA-box, or CCAAT-box. Luciferase reporter assays demonstrated that basal activity of the promoter carrying -54 T or -161 G was significantly enhanced. The first three polymorphisms, -26 C/G, -54 C/T, and -144 G/C located close to the putative hepatic nuclear factor (HNF) 1 binding elements, altered the transactivation effect of HNF1β. We also found a negative promoter response to dexamethasone-activated glucocorticoid receptor (GR) α, although none of the SNPs affected its transrepression function. Our results suggest that human Cbg -26 C/G, -54 C/T, -144 G/C, and -161 A/G promoter polymorphisms alter transcriptional activity, and further studies are awaited to explore their association with physiological and pathological conditions.

Clinical manifestations of highly prevalent corticosteroid-binding globulin mutations in a village in southern Italy

CONTEXT

Corticosteroid-binding globulin (CBG) is the binding protein for cortisol. Rare kindreds with CBG mutations reducing CBG levels or altering binding affinity have been described, along with clinical manifestations encompassing fatigue, chronic pain, and hypotension. The largest kindred, exhibiting two mutations (null and Lyon) were Australian immigrants from Italy.

OBJECTIVE

Our objective was to determine the prevalence of the null/Lyon mutations in the village where the original null/Lyon family emigrated and compare subjects with and without CBG mutations, without previous knowledge of their mutation status.

DESIGN, SETTING, AND PARTICIPANTS

We conducted a survey field study that included 495 adult residents.

MAIN OUTCOMES

We assessed clinical history, CBG mutation analysis, plasma CBG, salivary cortisol, body mass index, waist circumference, blood pressure, and the Krupp fatigue scale.

RESULTS

Eighteen of 495 participants (3.6%, seven males and 11 females) had one of two function-altering CBG mutations. All were heterozygous for the null (n = 6) or Lyon mutations (n = 12). Of 12 Lyon participants (four males and eight females), eight (two males and six females) had chronic widespread pain and five osteoarthritis with associated pain (one male and four females). Of six null participants (three males and three females), three (one male and two females) had chronic pain and four osteoarthritis with associated pain (two males and two females).

CONCLUSIONS

A high combined prevalence (3.6%) of these two CBG mutations was detected. The presence of either mutation conferred a propensity to chronic pain. In other communities, individuals with the same genetic background complain more of fatigue than pain, suggesting an environmental effect on the phenotype. These findings, combined with animal CBG gene knockout and human CBG single-nucleotide polymorphism haplotype studies, suggest that CBG influences the endocrine and neurobehavioral response to stress, including the development of pain/fatigue syndromes.

Molecular and structural basis of steroid hormone binding and release from corticosteroid-binding globulin

Corticosteroid-binding globulin (CBG), a non-inhibitory member of the serine proteinase inhibitor (serpin) super-family, is the high-affinity transport protein for glucocorticoids in vertebrate blood. Plasma CBG is a glycoprotein with 30% of its mass represented by N-linked oligosaccharide chains. Its well-characterized steroid-binding properties represent a "bench-mark data set" used extensively for in silico studies of protein-ligand interactions and drug design. Recent crystal structure analyses of intact rat CBG and cleaved human CBG have revealed the precise topography of the steroid-binding site, and shown that cortisol-bound CBG displays a typical stressed (S) serpin conformation with the reactive center loop (RCL) fully exposed from the central beta-sheet A, while proteolytic cleavage of the RCL results in CBG adopting a relaxed (R) conformation with the cleaved RCL fully inserted within the protein core. These crystal structures have set the stage for mechanistic studies of CBG function which have so far shown that helix D plays a key role in coupling RCL movement and steroid-binding site integrity, and provided evidence for an allosteric mechanism that modulates steroid binding and release from CBG. These studies have also revealed how the irreversible release of steroids occurs after proteolysis and re-orientation of the RCL within the R conformation. This recent insight into the structure and function of CBG reveals how naturally occurring genetic CBG mutations affect steroid binding, and helps understand how proteolysis of CBG enhances the targeted delivery of biologically active steroids to their sites of action.

Corticosteroid-binding globulin: the clinical significance of altered levels and heritable mutations

Corticosteroid-binding globulin (CBG) is the specific high-affinity plasma transport glycoprotein for cortisol. Stress-induced falls in CBG levels may heighten hypothalamic-pituitary-adrenal axis responses and CBG:tissue interactions may allow targeted cortisol delivery. Three genetic variants of CBG have been identified that reduce cortisol binding affinity and/or CBG levels. These include the Leuven and Lyon mutations which reduce CBG:cortisol binding affinity 3- and 4-fold, respectively, and the null mutation resulting in a 50% (heterozygote) or 100% (homozygote) reduction in CBG levels. The three reported null homozygotes demonstrate that complete CBG deficiency is not lethal, although it may be associated with hypotension and fatigue. The phenotype of a CBG null murine model included fatigue and immune defects. One community-based study revealed that severe CBG mutations are rare in idiopathic fatigue disorders. The mechanisms by which CBG mutations may cause fatigue are unknown. There are preliminary data of altered CBG levels in hypertension and in the metabolic syndrome; however, the nature of these associations is uncertain. Further studies may clarify the functions of CBG, and clinical observations may validate and/or extend the phenotypic features of various CBG mutations.

Residues in the human corticosteroid-binding globulin reactive center loop that influence steroid binding before and after elastase cleavage

Corticosteroid-binding globulin (CBG) is a non-inhibitory serine proteinase inhibitor (serpin) that transports cortisol and progesterone in blood. Crystal structures of rat CBG and a thrombin-cleaved human CBG:anti-trypsin (Pittsburgh) chimera show how structural transitions after proteolytic cleavage of the CBG reactive center loop (RCL) could disrupt steroid binding. This ligand release mechanism is assumed to involve insertion of the cleaved RCL into the beta-sheet A of the serpin structure. We have, therefore, examined how amino acid substitutions in the human CBG RCL influence steroid binding before and after its cleavage by neutrophil elastase. Elastase-cleaved wild-type CBG or variants with substitutions at P15 and/or P16 (E334G/G335N or E334A) lost steroid binding completely, whereas deletion of Glu-334 resulted in no loss of steroid binding after RCL cleavage, presumably because this prevents its insertion into beta-sheet A. Similarly, the steroid binding properties of CBG variants with substitutions at P15 (G335P), P14 (V336R), or P12 (T338P) in the RCL hinge were largely unaffected after elastase cleavage, most likely because the re-orientation and/or insertion of the cleaved RCL was blocked. Substitutions at P10 (G340P, G340S) or P8 (T342P, T342N) resulted in a partial loss of steroid binding after proteolysis which we attribute to incomplete insertion of the cleaved RCL. Remarkably, several substitutions (E334A, V336R, G340S, and T342P) increased the steroid binding affinities of human CBG even before elastase cleavage, consistent with the concept that CBG normally toggles between a high affinity ligand binding state where the RCL is fully exposed and a lower affinity state in which the RCL is partly inserted into beta-sheet A.

The S-to-R transition of corticosteroid-binding globulin and the mechanism of hormone release

Corticosteroids are transported in the blood by a serpin, corticosteroid-binding globulin (CBG), and their normally equilibrated release can be further triggered by the cleavage of the reactive loop of CBG. We report here the crystal structures of cleaved human CBG (cCBG) at 1.8-A resolution and its complex with cortisol at 2.3-A resolution. As expected, on cleavage, CBG undergoes the irreversible S-to-R serpin transition, with the cleaved reactive loops being fully incorporated into the central beta-sheet. A connecting loop of helix D, which is in a helix-like conformation in native CBG, unwinds and grossly perturbs the hormone binding site following beta-sheet expansion in the cCBG structure but shifts away from the binding site by more than 8 A following the binding of cortisol. Unexpectedly, on cortisol binding, the hormone binding site of cCBG adopts a configuration almost identical with that of the native conformer. We conclude that CBG has adapted an allosteric mechanism of the serpins to allow equilibrated release of the hormones by a flip-flop movement of the intact reactive loop into and out of the beta-sheet. The change in the hormone binding affinity results from a change in the flexibility or plasticity of the connecting loop, which modulates the configuration of the binding site.

Corticosteroid-binding globulin gene polymorphisms: clinical implications and links to idiopathic chronic fatigue disorders

Corticosteroid-binding globulin (CBG) binds cortisol with high affinity, facilitating transport of cortisol in blood, although tissue-specific CBG-cortisol interactions have long been postulated. There are three heritable, human CBG gene mutations that can reduce CBG-cortisol binding affinity and/or reduce circulating CBG levels. In some families, fatigue and low blood pressure have been associated with affinity altering or CBG level reducing mutations. The limited numbers of reports raise the possibility of ascertainment bias as many cases presented with features suggesting cortisol deficiency. The recent description of a genetically CBG-deficient mouse listed fatigue, manifest as reduced activity levels, as part of the phenotype, which also included immune aberrations. Severe CBG mutations may produce fatigue, but one study suggests that these are a rare cause of idiopathic fatigue. A mechanism for the effect of CBG mutations on fatigue is not readily apparent because free cortisol levels are normal, although we speculate that CBG may have an effect on cortisol-brain transport.

Corticosteroid-binding globulin, a structural basis for steroid transport and proteinase-triggered release

Corticosteroid-binding globulin (CBG) is a serine proteinase inhibitor (serpin) family member that transports glucocorticoids in blood and regulates their access to target cells. The 1.9A crystal structure of rat CBG shows that its steroid-binding site resembles the thyroxin-binding site in the related serpin, thyroxin-binding globulin, and mutagenesis studies have confirmed the contributions of key residues that constitute the steroid-binding pocket. Unlike thyroxin-bound thyroxin-binding globulin, the cortisol-bound CBG displays an "active" serpin conformation with the proteinase-sensitive, reactive center loop (RCL) fully expelled from the regulatory beta-sheet A. Moreover, the CBG structure allows us to predict that complete insertion of the proteolytically cleaved RCL into the serpin fold occurs in concert with a displacement and unwinding of helix D that would disrupt the steroid-binding site. This allosteric coupling between RCL positioning and occupancy of the CBG steroid-binding site, which resembles the ligand (glycosamino-glycan)-dependent activation of the thrombin inhibitory serpins heparin cofactor II and anti-thrombin RCLs, ensures both optimal recognition of CBG by target proteinases and efficient release of steroid to sites of action.

Hyporesponsiveness to glucocorticoids in mice genetically deficient for the corticosteroid binding globulin

Corticosteroid binding globulin (CBG) is the carrier for glucocorticoids in plasma. The protein is believed to keep the steroids inactive and to regulate the amount of free hormone acting on target tissues (free hormone hypothesis). Here, we generated a mouse model genetically deficient for CBG to test the contribution of the carrier to glucocorticoid action and adrenocortical stress response. The absence of CBG resulted in a lack of corticosterone binding activity in serum and in an approximately 10-fold increase in free corticosterone levels in CBG-null mice, consistent with its role in regulation of circulating free hormone levels. Surprisingly, cbg(-/-) animals did not exhibit features seen in organisms with enhanced glucocorticoid signaling. Rather, the mice exhibited increased activity of the pituitary axis of hormonal control, normal levels of gluconeogenetic enzymes, and fatigue, as well as an aggravated response to septic shock, indicating an inability to appropriately respond to the excess free corticosterone in the absence of CBG. Thus, our data suggest an active role for CBG in bioavailability, local delivery, and/or cellular signal transduction of glucocorticoids that extends beyond a function as a mere cargo transporter.

Functional implication of an Arg307Gly substitution in corticosteroid-binding globulin, a candidate gene for a quantitative trait locus associated with cortisol variability and obesity in pig

We previously reported that corticosteroid-binding globulin gene (Cbg) may be the causal gene of a quantitative trait locus associated with cortisol levels, fat deposition, and muscle content in a pig intercross. Sequence analysis of parental animals allowed us to identify four amino-acid substitutions. Here we have examined if any of these single amino acid substitutions could be responsible for the difference in CBG binding and affinity for cortisol between the parental breeds, using in vitro assays of Cbg variants after transfection of mammalian cells. Additionally, the Cbg coding region was analyzed in samples from a synthetic pig line to study association between polymorphism and CBG biochemical properties, carcass composition, and meat quality. Both in vitro transfection assays and the association studies suggest a role of the Arg307Gly mutation in increasing CBG capacity (by >70%) and decreasing CBG affinity for cortisol (by 30%). The Ile265Val substitution may also have an effect on decreasing CBG affinity for cortisol by 25%. The mutations Ser15Ile and Thr257Met do not seem to have an effect on CBG parameters. The Arg307Gly substitution was the only mutation associated with a parameter of meat quality and no mutation was linked to carcass composition.

Association between chronic fatigue syndrome and the corticosteroid-binding globulin gene ALA SER224 polymorphism

Chronic fatigue syndrome (CFS) is characterized by idiopathic fatigue of greater than 6 months' duration with postexertional exacerbation and many other symptoms. A trend toward relative hypocortisolism is described in CFS. Twin and family studies indicate a substantial genetic etiologic component to CFS. Recently, severe corticosteroid-binding globulin (CBG) gene mutations have been associated with CFS in isolated kindreds. Human leukocyte elastase, an enzyme important in CBG catabolism at inflammatory sites, is reported to be elevated in CFS. We hypothesized that CBG gene polymorphisms may act as a genetic risk factor for CFS. A total of 248 patients with CFS defined by Centers for Disease Control criteria, and 248 controls were recruited. Sequencing and restriction enzyme testing of the CBG gene coding region allowed detection of severe CBG gene mutations and a common exon 3 polymorphism (c.825G-->T, Ala-Ser224). Plasma CBG levels were measured in 125 CFS patients and 198 controls by radioimmunoassay. Total and free (calculated and measured) cortisol levels were ascertained in single samples between 8-10 a.m. The age of onset (mid 30s) and gender ratio (2.2:1, female:male) of the patients were similar to those reported in U.S. epidemiologic studies. A trend toward a preponderance of serine224 homozygosity among the CFS patients was noted, compared with controls (chi2 = 5.31, P = 0.07). Immunoreactive-CBG (IR-CBG) levels were higher in Serine/Alanine (Ser/Ala) than Ala/Ala subjects and higher again in Ser/Ser subjects, this effect was strongest in controls; Ser/Ser: 46.1+/-1.8 (n = 31, P = 0.03) vs. Ser/Ala: 42.4+/-1.0 (n = 56, P = 0.05) vs. Ala/Ala: 40.8+/-1.7 microg/mL (n = 21). Despite higher CBG levels, there was a nonsignificant trend toward lower total and free plasma cortisol in serine allele positive patients, total cortisol: Ser/Ser: 13.3+/-1.4 (n = 34) vs. Ser/Ala: 14.0+/-0.7 (n = 66) vs. Ala/Ala: 15.4+/-1.0 (n = 23). Homozygosity for the serine allele of the CBG gene may predispose to CFS, perhaps due to an effect on hypothalamic-pituitary-adrenal axis function related to altered CBG-cortisol transport function or immune-cortisol interactions.

Hereditary corticosteroid-binding globulin deficiency due to a missense mutation (Asp367Asn, CBG Lyon) in a Brazilian kindred

OBJECTIVE

Abnormal corticosteroid-binding globulin (CBG) is an extremely rare condition and only three mutations have been described in four families. The molecular basis of an abnormal CBG in a Brazilian family was studied and correlations between genotype and serum cortisol, cortisol binding capacity (CBC) and CBG levels were determined.

SUBJECTS

All 10 family members, comprising three generations, and nine healthy volunteers were studied.

MEASUREMENTS

Genomic DNA was extracted from white blood cells from all family members. The human cbg exons 2-5 were amplified by PCR, submitted to automatic sequencing. Cortisol and CBG levels in serum were measured by radioimmunoassay (RIA). CBC in serum was determined using tritiated cortisol and other cortisol binding parameters were calculated through Scatchard analysis.

RESULTS

A missense mutation in exon 5 of cbg (1254G --> A; Asp367Asn), recently described as CBG Lyon, was found in all family members. The proband and one sister were homozygous whereas all other family members, including parents, were heterozygous for this mutation. Cortisol levels in the only two homozygotes were lower than in heterozygotes and both were significantly lower as compared to controls (69 and 182 nmol/l vs. 267 +/- 129 nmol/l vs. 459 +/- 195 nmol/l, respectively, P < 0.05). CBC was decreased in the two homozygotes as compared to heterozygotes and in both groups as compared to controls (< 90 and 114 nmol/l vs. 305.0 +/- 81.4 nmol/l vs. 594.8 +/- 59.5 nmol/l, respectively, P < 0.05). CBG levels were lower in homozygotes as compared to heterozygotes and in both as compared to controls (325 and 375 nmol/l vs. 496.75 +/- 50.75 nmol/l vs. 647.25 +/- 87.50 nmol/l, respectively, P < 0.05).

CONCLUSIONS

An abnormal CBG resulting from a missense mutation and known as CBG Lyon was found in this Brazilian kindred. This abnormal CBG has decreased affinity for cortisol and results in low or low normal serum cortisol levels in homozygous and heterozygous subjects. Although relative hypotension and fatigue have recently been associated with CBG deficiency in a family with two CBG mutations (null and Lyon), the two homozygous subjects in this kindred were both normotensive and only the proband presented with fatigue.

Crystal structure of protein C inhibitor provides insights into hormone binding and heparin activation

Protein C inhibitor (PCI) is a member of the serpin family that has many biological functions. In blood it acts as a procoagulant, and, in the seminal vesicles, it is required for spermatogenesis. The activity of PCI is affected by heparin binding in a manner unique among the heparin binding serpins, and, in addition, PCI binds hydrophobic hormones with apparent specificity for retinoids. Here we present the 2.4 A crystallographic structure of reactive center loop (RCL) cleaved PCI. A striking feature of the structure is a two-turn N-terminal shortening of helix A, which creates a large hydrophobic pocket that docking studies indicate to be the retinoid binding site. On the basis of surface electrostatic properties, a novel mechanism for heparin activation is proposed.

Familial corticosteroid-binding globulin deficiency due to a novel null mutation: association with fatigue and relative hypotension

Corticosteroid-binding globulin is a 383-amino acid glycoprotein that serves a hormone transport role and may have functions related to the stress response and inflammation. We describe a 39-member Italian-Australian family with a novel complete loss of function (null) mutation of the corticosteroid-binding globulin gene. A second, previously described, mutation (Lyon) segregated independently in the same kindred. The novel exon 2 mutation led to a premature termination codon corresponding to residue -12 of the procorticosteroid-binding globulin molecule (c.121G-->A). Among 32 family members there were 3 null homozygotes, 19 null heterozygotes, 2 compound heterozygotes, 3 Lyon heterozygotes, and 5 individuals without corticosteroid-binding globulin mutations. Plasma immunoreactive corticosteroid-binding globulin was undetectable in null homozygotes, and mean corticosteroid-binding globulin levels were reduced by approximately 50% at 18.7 +/- 1.3 microg/ml (reference range, 30-52 microg/ml) in null heterozygotes. Morning total plasma cortisol levels were less than 1.8 microg/dl in homozygotes and were positively correlated to the plasma corticosteroid-binding globulin level in heterozygotes. Homozygotes and heterozygote null mutation subjects had a high prevalence of hypotension and fatigue. Among 19 adults with the null mutation, the systolic blood pressure z-score was 12.1 +/- 3.5; 11 of 19 subjects (54%) had a systolic blood pressure below the third percentile. The mean diastolic blood pressure z-score was 18.1 +/- 3.4; 8 of 19 subjects (42%) had a diastolic blood pressure z-score below 10. Idiopathic chronic fatigue was present in 12 of 14 adult null heterozygote subjects (86%) and in 2 of 3 null homozygotes. Five cases met the Centers for Disease Control criteria for chronic fatigue syndrome. Fatigue questionnaires revealed scores of 25.1 +/- 2.5 in 18 adults with the mutation vs. 4.2 +/- 1.5 in 23 healthy controls (P < 0.0001). Compound heterozygosity for both mutations resulted in plasma cortisol levels comparable to those in null homozygotes. Abnormal corticosteroid-binding globulin concentrations or binding affinity may lead to the misdiagnosis of isolated ACTH deficiency. The mechanism of the association between fatigue and relative hypotension is not established by these studies. As idiopathic fatigue disorders are associated with relatively low plasma cortisol, abnormalities of corticosteroid-binding globulin may be pathogenic.

Potential functions of plasma steroid-binding proteins

The plasma steroid-binding proteins, sex hormone-binding globulin (SHBG) and corticosteroid-binding globulin (CBG), transport steroid hormones in the blood and regulate their access to target tissues. Recent biochemical and molecular analyses of these proteins and their genes, and studies of their biosynthesis and localization in the liver and other tissues during development, have led to the realization that CBG and SHBG function in much more sophisticated ways. In particular, the presence of plasma membrane binding sites for both CBG and SHBG on steroid target cells, and evidence for interactions between CBG and specific proteinases at sites of inflammation or tissue remodeling, suggest that these proteins control steroid hormone bioavailability and/or action in a highly selective or targeted fashion. This new information should not only serve to extend our understanding of the basis of steroid-hormone dependent diseases, but may influence the design of steroid hormone agonists and antagonist of therapeutic potential.

What do dysfunctional serpins tell us about molecular mobility and disease?

Proteinase inhibitors of the serpin family have a unique ability to regulate their activity by changing the conformation of their reactive-centre loop. Although this may explain their evolutionary success, the dependence of function on structural mobility makes the serpins vulnerable to the effects of mutations. Here, we describe how studies of dysfunctional variants, together with crystal structures of serpins in different forms, provide insights into the molecular functions and remarkable folding properties of this family. In particular, comparisons of variants affecting different serpins allow us to define the domains which control this folding and show how spontaneous but inappropriate changes in conformation cause diverse diseases.

Structure and chromosomal location of the gene encoding mouse corticosteroid-binding globulin: strain differences in coding sequence and steroid-binding activity

Corticosteroid-binding globulin (CBG) is a member of the serine proteinase inhibitor superfamily and is responsible for the plasma transport of glucocorticoids. The mouse Cbg gene structure has been deduced from two non-overlapping DNA fragments of a lambda EMBL-3 genomic library, as well as PCR amplification of the approx. 2 kb of genomic DNA that lies between them. Mouse Cbg comprises five exons that span a region of approx. 10.5 kb, and has been localized in tight linkage with the Aat (alpha 1-antitrypsin) and Spi (serine proteinase inhibitor) gene complex on chromosome 12, in a region syntenic with this genetic locus on human chromosome 14. Intron-specific oligodeoxyribonucleotide primers were also used to PCR-amplify Cbg coding regions from several mouse strains. No differences were found in the Cbg coding sequences of BALB/c and C57BL/6J-cpk/cpk mice, while two mutations were found within RIIIS/J Cbg that result in Lys201-->Glu and Ala357-->Thr substitutions in the mature mouse CBG polypeptide. To assess what impact these substitutions might have on the steroid-binding activity of RIIIS/J CBG, these mutations were introduced separately or together into a BALB/c mouse Cbg cDNA. Expression of these mutants in the MDCK cell line indicated that the Lys201-->Glu substitution accounts for the abnormal steroid-binding affinity of CBG in RIIIS/J mice.

Decreased cortisol-binding affinity of transcortin Leuven is associated with an amino acid substitution at residue-93

Genomic DNA was isolated from two related individuals who are homozygous for transcortin Leuven, a corticosteroid-binding globulin variant with decreased cortisol-binding affinity. This material was amplified using intron-specific oligonucleotide primers in a polymerase chain reaction to obtain the four exons that encode transcortin. Sequence analysis of these exons showed several mutations within the coding sequence of both individuals, but only one of these will result in an amino acid substitution. This mutation is located within exon 2 and alters the codon (CTC) normally associated with Leu-93 in the transcortin polypeptide to a codon (CAC) for histidine in the variant genes.