Function and membrane topology of wild-type and mutated cytochrome P-450c21

Genetic and clinical characteristics including occurrence of testicular adrenal rest tumors in Slovak and Slovenian patients with congenital adrenal hyperplasia due to 21-hydroxylase deficiency

OBJECTIVE

To analyze the mutational spectrum, clinical characteristics, genotype-phenotype correlations, testicular adrenal rests tumor prevalence, and role of neonatal screening in congenital adrenal hyperplasia (CAH) patients from Slovakia and Slovenia.

DESIGN AND METHODS

Data were obtained from 104 patients with CAH registered in Slovak and Slovenian databases. Low-resolution genotyping was performed to detect the most common point mutations. To detect deletions, conversions, point mutations, or other sequence changes in the CYP21A2 gene, high-resolution genotyping was performed. Genotypes were classified according to residual 21-hydroxylase activity (null, A, B, C).

RESULTS

64% of the individuals had the salt-wasting form (SW-CAH), 15% the simple virilizing form (SV-CAH), and 21% the non-classic (NC-CAH). CYP21A2 gene deletion/conversion and c.293-13A/C>G pathogenic variant accounted together for 55.5% of the affected alleles. In SV-CAH p.Ile172Asn was the most common pathogenic variant (28.13%), while in NC-CAH p.Val282Leu (33.33%), CYP21A2 gene deletion/conversion (21.43%), c.293-13A/C>G (14.29%), Pro30Leu (11.90%). The frequency of alleles with multiple pathogenic variants was higher in Slovenian patients (15.83% of all alleles). Severe genotypes (0 and A) correlated well with the expected phenotype (SW in 94.74% and 97.3%), while less severe genotypes (B and C) correlated weaklier (SV in 50% and NC in 70.8%). The median age of SW-CAH patients at the time of diagnosis was 6 days in Slovakia vs. 28.5 days in Slovenia (p=0.01). Most of the Slovak patients in the cohort were detected by NBS. (24 out of 29). TARTs were identified in 7 out of 24 male patients, of whom all (100%) had SW-CAH and all had poor hormonal control. The median age at the diagnosis of TARTs was 13 years.

CONCLUSION

The study confirmed the importance of neonatal screening, especially in the speed of diagnosis of severe forms of CAH. The prediction of the 21-OH deficiency phenotype was reasonably good in the case of severe pathogenic variants, but less reliable in the case of milder pathogenic variants, which is consistent compared to data from other populations. Screening for TARTs should be realized in all male patients with CAH, since there is possible remission when identified early.

Salt-Losing 21-Hydroxylase Deficiency Caused by Double Homozygosity for Two "Mild" Mutations

CONTEXT

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency presents with different severities that correlate with the genotype. The salt-losing phenotype requires 2 alleles with "severe" mutations.

CASE DESCRIPTION

We present a case of salt-losing 21-hydroxylase deficiency that was found to be homozygous for 2 "mild" pathogenic variants: V281L and S301Y. Both in silico and heterologous expression functional analysis demonstrated that co-occurrence of these 2 mutations in cis severely impairs the function of the 21-hydroxylase enzyme.

CONCLUSIONS

This case has important implications for genetic counseling. Regarding this combination of 2 "mild" variants as having mild phenotypic effects could lead to inappropriate counseling of heterozygote carriers.

Genotype/phenotype correlations in 538 congenital adrenal hyperplasia patients from Germany and Austria: discordances in milder genotypes and in screened versus prescreening patients

Congenital adrenal hyperplasia (CAH) due to CYP21A2 gene mutations is associated with a variety of clinical phenotypes (salt wasting, SW; simple virilizing, SV; nonclassical, NC) depending on residual 21-hydroxylase activity. Phenotypes and genotypes correlate well in 80-90% of cases. We set out to test the predictive value of CAH phenotype assignment based on genotype classification in a large multicenter cohort. A retrospective evaluation of genetic data from 538 CAH patients (195 screened) collected from 28 tertiary centers as part of a German quality control program was performed. Genotypes were classified according to residual 21-hydroxylase activity (null, A, B, C) and assigned clinical phenotypes correlated with predicted phenotypes, including analysis of Prader stages. Ultimately, concordance of genotypes with clinical phenotypes was compared in patients diagnosed before or after the introduction of nationwide CAH-newborn screening. Severe genotypes (null and A) correlated well with the expected phenotype (SW in 97 and 91%, respectively), whereas less severe genotypes (B and C) correlated poorly (SV in 45% and NC in 57%, respectively). This was underlined by a high degree of virilization in girls with C genotypes (Prader stage >1 in 28%). SW was diagnosed in 90% of screening-positive babies with classical CAH compared with 74% of prescreening patients. In our CAH series, assigned phenotypes were more severe than expected in milder genotypes and in screened vs prescreening patients. Diagnostic discrimination between phenotypes based on genotypes may prove overcome due to the overlap in their clinical presentations.

Prenatal Diagnosis of Congenital Adrenal Hyperplasia

Congenital adrenal hyperplasia (CAH) owing to 21-hydroxylase deficiency is a monogenic disorder of adrenal steroidogenesis. To prevent genital ambiguity, in girls, prenatal dexamethasone treatment is administered early in the first trimester. Prenatal genetic diagnosis of CAH and fetal sex determination identify affected female fetuses at risk for genital virilization. Advancements in prenatal diagnosis are owing to improved understanding of the genetic basis of CAH and improved technology. Cloning of the CYP21A2 gene ushered in molecular genetic analysis as the current standard of care. Noninvasive prenatal diagnosis allows for targeted treatment and avoids unnecessary treatment of males and unaffected females.

Genotype-phenotype correlation in 1,507 families with congenital adrenal hyperplasia owing to 21-hydroxylase deficiency

Over the last two decades, we have extensively studied the genetics of congenital adrenal hyperplasia caused by 21-hydroxylase deficiency (CAH) and have performed 8,290 DNA analyses of the CYP21A2 gene on members of 4,857 families at risk for CAH--the largest cohort of CAH patients reported to date. Of the families studied, 1,507 had at least one member affected with one of three known forms of CAH, namely salt wasting, simple virilizing, or nonclassical CAH. Here, we report the genotype and phenotype of each affected patient, as well as the ethnic group and country of origin for each patient. We showed that 21 of 45 genotypes yielded a phenotypic correlation in our patient cohort. In particular, contrary to what is generally reported in the literature, we found that certain mutations, for example, the P30L, I2G, and I172N mutations, yielded different CAH phenotypes. In salt wasting and nonclassical CAH, a phenotype can be attributed to a genotype; however, in simple virilizing CAH, we observe wide phenotypic variability, particularly with the exon 4 I172N mutation. Finally, there was a high frequency of homozygous I2G and V281L mutations in Middle Eastern and Ashkenazi Jewish populations, respectively. By identifying the predominant phenotype for a given genotype, these findings should assist physicians in prenatal diagnosis and genetic counseling of parents who are at risk for having a child with CAH.

Duplication of 111 bases in exon 1 of the CYP21 gene is combined with deletion of CYP21P-C4B genes in steroid 21-hydroxylase deficiency

Congenital adrenal hyperplasia (CAH) is a common autosomal recessive disorder mainly caused by defects in the steroid 21-hydroxylase (CYP21) gene. A 9.3-kb fragment generated by NdeI and AseI digestion by Southern blot analysis indicated that a consequence of deletion of the C4-CYP21 repeat module was the production of a distinct chimeric CYP21P/CYP21 molecule. In the present study, we report a novel CYP21 genotype in two CAH families in which the gene appeared as 9.4- and 3.3-kb fragments by TaqI digestion, rather than as a chimeric gene. From the analysis of PCR amplification patterns and DNA sequencing, we found that there was a duplication of 111 bases from codons 21 to 57 inserted at codon 58 in exon 1 of the CYP21 gene. In addition, codon 21 in the repeated sequence changed from TGG to AGG. Furthermore, this novel CYP21 gene present in both CAH families showed no mutations at IVS2-12A/C>G, 707-714delGAGACTAC, and P30L. Interestingly, the 5' end region of these two CYP21 genes showed the sequence of the CYP21P gene at nucleotides (nt) -103, -110, -123, and thereafter. Our data suggest that these two CYP21 genes are caused by deletion of the CYP21P, XA, RP2, and C4B genes. Possibly, the additional 111-base duplicated coding sequence may be generated by multiple intergenic recombinations, while there seems to be no relationship with deletion of the CYP21P-C4B regions.

CYP21 mutations and congenital adrenal hyperplasia

Congenital adrenal hyperplasia (CAH) is a common autosomal recessive disorder caused mainly by defects in the steroid 21-hydroxylase (CYP21) gene. More than 90% of CAH cases are caused by mutations of the CYP21 gene on chromosome 6p21.3. The wide range of CAH phenotypes is associated with multiple mutations known to affect 21-hydroxylase enzyme activity. To date, 56 different CYP21 mutations have been reported, mostly point mutations, but small deletions or insertions have been described too, as well as complete gene deletions. Fifteen mutations, constituting 90-95% of alleles, are derived from intergenic recombination of DNA sequences between the CYP21 gene and the highly homologous CYP21P pseudogene, while the remaining are spontaneous mutations. A reliable and accurate detection of CYP21 mutations is not only important for clinical diagnosis, but also for carrier detection as there is a high variability in the basal level of 17-hydroxyprogesterone between normal and heterozygous individuals. Several strategies based on polymerase chain reaction (PCR)-driven amplification with allele-specific oligonucleotides to the CYP21 gene have been developed. It has been demonstrated that one reaction for PCR amplification of the CYP21 gene and the chimeric CYP21P/CYP21 gene using mixed primers in combination with nested PCR and single-strand conformation polymorphism is considered highly efficient and accurate for molecular diagnosis of CAH due to 21-hydroxylase deficiency.

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency

More than 90% of cases of congenital adrenal hyperplasia (CAH, the inherited inability to synthesize cortisol) are caused by 21-hydroxylase deficiency. Females with severe, classic 21-hydroxylase deficiency are exposed to excess androgens prenatally and are born with virilized external genitalia. Most patients cannot synthesize sufficient aldosterone to maintain sodium balance and may develop potentially fatal "salt wasting" crises if not treated. The disease is caused by mutations in the CYP21 gene encoding the steroid 21-hydroxylase enzyme. More than 90% of these mutations result from intergenic recombinations between CYP21 and the closely linked CYP21P pseudogene. Approximately 20% are gene deletions due to unequal crossing over during meiosis, whereas the remainder are gene conversions--transfers to CYP21 of deleterious mutations normally present in CYP21P. The degree to which each mutation compromises enzymatic activity is strongly correlated with the clinical severity of the disease in patients carrying it. Prenatal diagnosis by direct mutation detection permits prenatal treatment of affected females to minimize genital virilization. Neonatal screening by hormonal methods identifies affected children before salt wasting crises develop, reducing mortality from this condition. Glucocorticoid and mineralocorticoid replacement are the mainstays of treatment, but more rational dosing and additional therapies are being developed.

Effects of missense mutations and deletions on membrane anchoring and enzyme function of human steroid 21-hydroxylase (P450c21)

We studied membrane binding and enzyme function of six variant forms of human steroid 21-hydroxylase (P450c21), a mutant (P30Q) from a patient with congenital adrenal hyperplasia, four artificial deletions in the amino terminal region (delS1 and del S2; the first and second hydrophobic segment, delS3; the region in between, delS4; the combination of these), and one naturally ocurring polymorphism in a region implicated to be critical for membrane integration (delL10). Enzyme function was assayed after transient expression in COS-1 cells, and membrane binding was studied by coupled in vitro transcription-translation in the presence of microsomal membranes. P450c21(delS1) retained some enzyme activity but showed severely reduced membrane binding. P450c21(P30Q), P450c21 (delS2), P450c21(delS3), and P450c21(delS4) had abolished enzyme function. P450c21(P30Q) and P450c21 (delS2) did not affect membrane binding, P450c21 (delS3) had slightly reduced binding with a qualitative difference suggested by the absence of a glycosylated form of the protein, and P450c21(delS4) had abolished membrane integration. No significant differences could be identified for the delL10 variant. These data support that P450c21 spans the membrane through its first hydrophobic domain only, and that the protein lacking this segment retains sufficiently normal structure to enable catalysis. They also confirm that P30Q is responsible for the severe phenotype of the patient in which it was found, and indicate that the common delL10 polymorphism does not have a major effect on enzyme function.

Cell-type specificity of human CYP11A1 TATA box

Expression of the CYP11A1 (SCC) genes, which encode the enzyme important for the first step of steroid biosynthesis, occurs in the adrenal gland and gonads, and is stimulated by cAMP. Transfection of serial deletions of the SCC promoter, which drives reporter gene expression, showed that a minimal promoter containing only the TATA box could direct cAMP-dependent transcription. Transcription factor SF1, which binds to a site next to the TATA box, can stimulate basal transcription but not cAMP response, either in adrenal cell lines or in COS-1 co-transfected with the SF1 expression plasmid. These data lead to the conclusion that the minimal promoter containing only the TATA box can drive cell type-specific, cAMP-dependent transcription. Additional experiments replacing the TATA sequence of SCC with other TATA sequences suggested that the TATA sequence itself is important for this cAMP-dependent transcription.

Characterization of the consequence of a novel Glu-380 to Asp mutation by expression of functional P450c21 in Escherichia coli

P450c21 catalyzes an important step in steroid synthesis. Its deficiency leads to symptoms of steroid imbalance. To obtain enough P450c21 for structure and function studies, we developed a method to express P450c21 in Escherichia coli. The 5'-region of the human P450c21 cDNA was modified to ensure efficient translation and the C terminus of the protein was extended with four His residues for easy purification. Mutant proteins with substitutions at residues 172 and 281 exhibited decreased enzymatic activities similar to those found in mammalian cells. One new mutation changing Glu-380 to Asp (D380) caused 3-fold reduction in enzymatic activity. The amount of apoprotein production detected by immunoblotting and the affinity of the mutant protein towards substrate as measured by Km were normal. The defect lies in the decreased ability of the apoprotein to bind heme, which was measured by CO difference and substrate-binding spectra. The D380 mutant protein had 3-fold reduction in peak heights in both spectra. This reduced heme binding resulted in 3-fold lower enzymatic activity.

Molecular modelling of steroidogenic cytochromes P450 from families CYP11, CYP17, CYP19 and CYP21 based on the CYP102 crystal structure

The results of homology modelling of mammalian steroidogenic cytochromes P450 (CYP) from families CYP11, CYP17, CYP19 and CYP21 are reported, based on a novel protein sequence alignment with CYP102, a bacterial P450 of known crystal structure. The molecular models generated from the CYP102 crystal structure template are consistent with experimental information from site-directed mutagenesis studies, steroidal substrate specificity and active site inhibitor studies. Interactive docking studies with both substrates and inhibitors of these enzymes indicate key residue interactions with the putative active site regions of each isoform investigated, which point to potential determinants of substrate specificity within these related enzymes.

Autoantibody binding to steroid 21-hydroxylase--effect of five mutations

Steroid 21-hydroxylase (21-OH) is a key haem containing steroidogenic enzyme and a major adrenal specific autoantigen. Cys 428 in 21-OH is thought to have an important role in haem binding and we now describe the effects of mutations at Cys 428 (to Ser, Arg and Phe) on 21-OH autoantibody binding. Expression of wild type and mutated 21-OH was carried out using an in vitro transcription/translation (TnT) system and reactivity of 21-OH autoantibodies with mutated 21-OH analysed by western blotting (in the case of unlabelled proteins) or immunoprecipitation assay (IPA) (in the case of 35S-labelled proteins). All 3 substitutions at Cys 428 had similar effects on 21-OH autoantibody binding and each one caused a reduction in autoantibody binding to about 50% of wild type in the case of IPA and to about 70% of wild type in the case of western blotting analysis. In addition to mutations at Cys 428, we studied 2 naturally occurring mutations at Pro 30 to Leu and Ile 172 to Asn which are associated with diminished 21-OH enzyme activity. The Pro 30 mutation had no effect, but the Ile 172 mutation caused a reduction in 21-OH autoantibody binding in the IPA to about 80% of wild type. Overall, our studies emphasise the close relationship between the 21-OH aminoacid sequences important for 21-OH enzyme activity and 21-OH autoantibody binding.