Structural and functional analysis of a novel mutation of CYP21B in a heterozygote carrier of 21-hydroxylase deficiency

EMQN best practice guidelines for molecular genetic testing and reporting of 21-hydroxylase deficiency

Molecular genetic testing for congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency (21-OHD) is offered worldwide and is of importance for differential diagnosis, carrier detection and adequate genetic counseling, particularly for family planning. In 2008 the European Molecular Genetics Quality Network (EMQN) for the first time offered a European-wide external quality assessment scheme for CAH (due to 21-OH deficiency). The interest was great and over the last years at about 60 laboratories from Europe, USA and Australia regularly participated in that scheme. These best practice guidelines were drafted on the basis of the extensive knowledge and experience got from those annually organized CAH-schemes. In order to obtain the widest possible consultation with practicing laboratories the draft was therefore circulated twice by EMQN to all laboratories participating in the EQA-scheme for CAH genotyping and was updated by that input. The present guidelines address quality requirements for diagnostic molecular genetic laboratories, as well as criteria for CYP21A2 genotyping (including carrier-testing and prenatal diagnosis). A key aspect of that article is the use of appropriate methodologies (e.g., sequencing methods, MLPA (multiplex ligation dependent probe amplification), mutation specific assays) and respective limitations and analytical accuracy. Moreover, these guidelines focus on classification of variants, and the interpretation and standardization of the reporting of CYP21A2 genotyping results. In addition, the article provides a comprehensive list of common as well as so far unreported CYP21A2-variants.

CYP21A2 mutation update: Comprehensive analysis of databases and published genetic variants

Congenital adrenal hyperplasia (CAH) is a group of autosomal recessive disorders of adrenal steroidogenesis. Disorders in steroid 21-hydroxylation account for over 95% of patients with CAH. Clinically, the 21-hydroxylase deficiency has been classified in a broad spectrum of clinical forms, ranging from severe or classical, to mild late onset or non-classical. Known allelic variants in the disease causing CYP21A2 gene are spread among different sources. Until recently, most variants reported have been identified in the clinical setting, which presumably bias described variants to pathogenic ones, as those found in the CYPAlleles database. Nevertheless, a large number of variants are being described in massive genome projects, many of which are found in dbSNP, but lack functional implications and/or their phenotypic effect. In this work, we gathered a total of 1,340 GVs in the CYP21A2 gene, from which 899 variants were unique and 230 have an effect on human health, and compiled all this information in an integrated database. We also connected CYP21A2 sequence information to phenotypic effects for all available mutations, including double mutants in cis. Data compiled in the present work could help physicians in the genetic counseling of families affected with 21-hydroxylase deficiency.

Structure-based activity prediction of CYP21A2 stability variants: A survey of available gene variations

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency accounts for 90-95% of CAH cases. In this work we performed an extensive survey of mutations and SNPs modifying the coding sequence of the CYP21A2 gene. Using bioinformatic tools and two plausible CYP21A2 structures as templates, we initially classified all known mutants (n = 343) according to their putative functional impacts, which were either reported in the literature or inferred from structural models. We then performed a detailed analysis on the subset of mutations believed to exclusively impact protein stability. For those mutants, the predicted stability was calculated and correlated with the variant's expected activity. A high concordance was obtained when comparing our predictions with available in vitro residual activities and/or the patient's phenotype. The predicted stability and derived activity of all reported mutations and SNPs lacking functional assays (n = 108) were assessed. As expected, most of the SNPs (52/76) showed no biological implications. Moreover, this approach was applied to evaluate the putative synergy that could emerge when two mutations occurred in cis. In addition, we propose a putative pathogenic effect of five novel mutations, p.L107Q, p.L122R, p.R132H, p.P335L and p.H466fs, found in 21-hydroxylase deficient patients of our cohort.

17-Hydroxyprogesterone in children, adolescents and adults

17-Hydroxyprogesterone (17-OHP) is an intermediate steroid in the adrenal biosynthetic pathway from cholesterol to cortisol and is the substrate for steroid 21-hydroxylase. An inherited deficiency of 21-hydroxylase leads to greatly increased serum concentrations of 17-OHP, while the absence of cortisol synthesis causes an increase in adrenocorticotrophic hormone. The classical congenital adrenal hyperplasia (CAH) presents usually with virilisation of a girl at birth. Affected boys and girls can have renal salt loss within a few days if aldosterone production is also compromised. Diagnosis can be delayed in boys. A non-classical form of congenital adrenal hyperplasia (NC-CAH) presents later in life usually with androgen excess. Moderately raised or normal 17-OHP concentrations can be seen basally but, if normal and clinical suspicion is high, an ACTH stimulation test will show 17-OHP concentrations (typically >30 nmol/L) above the normal response. NC-CAH is more likely to be detected clinically in females and may be asymptomatic particularly in males until families are investigated. The prevalence of NC-CAH in women with androgen excess can be up to 9% according to ethnic background and genotype. Mutations in the 21-hydroxylase genes in NC-CAH can be found that have less deleterious effects on enzyme activity. Other less-common defects in enzymes of cortisol synthesis can be associated with moderately elevated 17-OHP. Precocious puberty, acne, hirsutism and subfertility are the commonest features of hyperandrogenism. 17-OHP is a diagnostic marker for CAH but opinions differ on the role of 17OHP or androstenedione in monitoring treatment with renin in the salt losing form. This review considers the utility of 17-OHP measurements in children, adolescents and adults.

Structure-phenotype correlations of human CYP21A2 mutations in congenital adrenal hyperplasia

Mutations in the cytochrome p450 (CYP)21A2 gene, which encodes the enzyme steroid 21-hydroxylase, cause the majority of cases in congenital adrenal hyperplasia, an autosomal recessive disorder. To date, more than 100 CYP21A2 mutations have been reported. These mutations can be associated either with severe salt-wasting or simple virilizing phenotypes or with milder nonclassical phenotypes. Not all CYP21A2 mutations have, however, been characterized biochemically, and the clinical consequences of these mutations remain unknown. Using the crystal structure of its bovine homolog as a template, we have constructed a humanized model of CYP21A2 to provide comprehensive structural explanations for the clinical manifestations caused by each of the known disease-causing missense mutations in CYP21A2. Mutations that affect membrane anchoring, disrupt heme and/or substrate binding, or impair stability of CYP21A2 cause complete loss of function and salt-wasting disease. In contrast, mutations altering the transmembrane region or conserved hydrophobic patches cause up to a 98% reduction in enzyme activity and simple virilizing disease. Mild nonclassical disease can result from interference in oxidoreductase interactions, salt-bridge and hydrogen-bonding networks, and nonconserved hydrophobic clusters. A simple in silico evaluation of previously uncharacterized gene mutations could, thus, potentially help predict the often diverse phenotypes of a monogenic disorder.

Analysis of HIV protease binding pockets based on 3D shape and electrostatic potential descriptors

Using a shape analysis technique, a Dataset of 79 (19 wild-type and 60 mutated) HIV-1 protease crystal structures was analyzed for the shape changes in their binding pockets. The structures are reported with different bound inhibitors and consist of a variety of mutations. Several 3D-shape descriptors based on the volumetric shape function are calculated, in addition to the electrostatic potential (EP)-based descriptors. A cluster analysis is performed using the calculated descriptors of the binding pocket of the proteins to investigate the effect of mutations or bound inhibitors on the shape of the binding pocket of proteins. We observed that mutations and/or bound ligands influence the 3D shape of the binding pocket of these HIV proteases. This study shows that the shape changes in the binding pocket caused by mutations or bound ligands can be quantitatively captured using 3D shape- and EP-based descriptors.

Premature pubarche in Mediterranean girls: high prevalence of heterozygous CYP21 mutation carriers

AIM

This study investigated the prevalence and consequences of heterozygous CYP21A2 mutations in premature pubarche (PP) girls.

MAIN FINDING

We investigated 36 French Mediterranean girls with isolated PP. We performed synacthen testing with 17OHP and 21-deoxycortisol evaluation, along with molecular analysis of the CYP21A2 gene in girls with abnormal elevation of one of these two adrenal steroids. Three girls (8.3%) had nonclassical adrenal hyperplasia, secondary to compound heterozygosity that associated at least one severe mutation for the three girls. A heterozygous mutation of the CYP21A2 gene was confirmed by molecular biology in eight girls (22%); a deletion of the CYP21A2 gene was found in one of them. Biological hyperandrogenism was found in the prepubertal CYP21A2 mutation carriers, whereas the four heterozygous girls who were followed long enough to have reached pubertal age presented biological and clinical hyperandrogenism.

CONCLUSIONS

We underline the high prevalence of heterozygous CYP21A2 mutations in girls with PP and demonstrate the usefulness of systematic screening by synacthen testing, both to improve their future clinical management and to prevent the transmission of classical adrenal hyperplasia to future offspring. Because of the severe metabolic and cardiovascular consequences of hyperandrogenism, long-term follow-up of these heterozygous patients is mandatory.

Association of the GAA1013→GAG polymorphism of the insulin-like growth factor-1 receptor (IGF1R) gene with premature pubarche

OBJECTIVE

A single-nucleotide polymorphism (SNP), the G variant in codon 1013 (GAA1013-->GAG) of the insulin-like growth factor-1 (IGF-1) receptor (IGFIR) gene, has been associated with higher IGF-1 concentrations in Caucasian subjects. Because elevated serum levels of IGF-1 have been described in children with premature pubarche (PP) and in adolescent girls with hyperandrogenism, we tested the a priori hypothesis that the frequency of the A-->G variant would be overrepresented among children with PP.

DESIGN

Case-control association study.

SETTING

University-based pediatric endocrinology practice.

PATIENT(S)

Sixty-nine children (63 girls and 6 boys) with PP, 52 adolescent girls with hyperandrogenism, and 92 healthy subjects.

INTERVENTION(S)

Blood was obtained for genotype analysis, glucose measurement, and hormone (A, insulin, 17-hydroxyprogesterone, and T) determinations.

MAIN OUTCOME MEASURE(S)

Frequency of the SNP in the IGF1R gene and correlation of this SNP with hormone concentrations.

RESULT(S)

Distribution of the G allele was statistically significantly different between the children with PP and the healthy control subjects, independent of insulin sensitivity.

CONCLUSION(S)

This common SNP in the IGF1R gene may be associated with PP caused by premature adrenarche in children. Because PP has been associated with higher IGF-1 concentrations, these data suggest a potential molecular basis for prior clinical observations of elevated IGF-1 concentrations in children with PP.

Molecular model of human CYP21 based on mammalian CYP2C5: structural features correlate with clinical severity of mutations causing congenital adrenal hyperplasia

Enhanced understanding of structure-function relationships of human 21-hydroxylase, CYP21, is required to better understand the molecular causes of congenital adrenal hyperplasia. To this end, a structural model of human CYP21 was calculated based on the crystal structure of rabbit CYP2C5. All but two known allelic variants of missense type, a total of 60 disease-causing mutations and six normal variants, were analyzed using this model. A structural explanation for the corresponding phenotype was found for all but two mutants for which available clinical data are also discrepant with in vitro enzyme activity. Calculations of protein stability of modeled mutants were found to correlate inversely with the corresponding clinical severity. Putative structurally important residues were identified to be involved in heme and substrate binding, redox partner interaction, and enzyme catalysis using docking calculations and analysis of structurally determined homologous cytochrome P450s (CYPs). Functional and structural consequences of seven novel mutations, V139E, C147R, R233G, T295N, L308F, R366C, and M473I, detected in Scandinavian patients with suspected congenital adrenal hyperplasia of different severity, were predicted using molecular modeling. Structural features deduced from the models are in good correlation with clinical severity of CYP21 mutants, which shows the applicability of a modeling approach in assessment of new CYP21 mutations.

Hyperandrogenism in carriers of CYP21 mutations: the role of genotype

OBJECTIVE

It has been hypothesized that carriers of CYP21 mutations are at increased risk of developing clinically evident hyperandrogenism. In the present study, we assessed the risk for symptoms of androgen excess in carriers of CYP21 gene mutations and the effect of different mutations on phenotype.

DESIGN

All subjects underwent clinical evaluation, ACTH stimulation testing, and molecular analysis of the CYP21 gene.

PATIENTS

One hundred and eighty-seven subjects were included in the study. Five groups were defined according to genotype: A, 38 patients with classical-congenital adrenal hyperplasia (CAH); B, 16 patients with non classical CAH; C, 66 family member carriers; D, 24 children and adolescents with symptoms suggestive of hyperandrogenism who were found to be heterozygous and E, 43 subjects with androgen excess symptoms who had normal genotype.

MEASUREMENTS

Cortisol and 17-hydroxyprogesterone (17OHP) were measured basally and 60 min after ACTH stimulation. Analysis of seven common mutations of the CYP21 gene, which collectively cover 95% of the mutations in the Israeli population, was performed. The hormonal results were correlated with the genotype.

RESULTS

The symptomatic carriers had a significantly higher rate of the mild mutation V281L compared with the family members (58% vs. 22%) and significantly higher levels of ACTH-stimulated 17OHP (mean 37.2 and 21 nmol/l, respectively; P = 0.025). Higher values of peak 17OHP levels were found in the carriers of the V281L mutation compared with carriers of other mutations (P = 0.025). Stimulated 17OHP levels were significantly higher in carriers compared with normal genotype subjects (P < 0.0001).

CONCLUSIONS

Our findings are the first to show that there is increased risk of androgen excess in carriers of CYP21 mutations and that the risk is related to genotype. Carriers for the mild, V281L mutation, are at higher risk of symptoms of androgen excess than carriers of the severe mutations. It appears that the variable phenotypes can be at least partially attributed to the degree of impairment of enzyme activity in different mutations. The severe consequences of hyperandrogenism in some of the carrier subjects indicate the need for long-term follow-up in these patients.