Deletion hybrid genes, due to unequal crossing over between CYP11B1 (11beta-hydroxylase) and CYP11B2(aldosterone synthase) cause steroid 11beta-hydroxylase deficiency and congenital adrenal hyperplasia.

Chromosomal rearrangements are natural experiments that can provide unique insights into in vivo regulation of genes and physiological systems. We have studied a patient with congenital adrenal hyperplasia and steroid 11beta-hydroxylase deficiency who was homozygous for a deletion of the CYP11B1 and CYP11B2 genes normally required for cortisol and aldosterone synthesis, respectively. The genes were deleted by unequal recombination between the tandemly arranged CYP11B genes during a previous meiosis, leaving a single hybrid gene consisting of the promoter and exons 1-6 of CYP11B2 and exons 7-9 of CYP11B1. The hybrid gene also carried an I339T mutation formed by intracodon recombination at the chromosomal breakpoint. The mutant complementary DNA corresponding to this gene was expressed in COS-1 cells and was found to have relatively unimpaired 11beta-hydroxylase and aldosterone synthase activities. Apparently the 11beta-hydroxylase deficiency and the adrenal hyperplasia are due to the lack of expression of this gene in the adrenal zona fasciculata/reticularis resulting from replacement of the CYP11B1 promoter and regulatory sequences by those of CYP11B2.

Sequence similarities between a novel putative G protein-coupled receptor and Na+/Ca2+ exchangers define a cation binding domain.

cDNA clones encoding a novel putative G protein-coupled receptor have been characterized. The receptor is widely expressed in normal solid tissues. Consisting of 1967 amino acid residues, this receptor is one of the largest known and is therefore referred to as a very large G protein-coupled receptor, or VLGR1. It is most closely related to the secretin family of G protein-coupled receptors based on similarity of the sequences of its transmembrane segments. As demonstrated by cell surface labeling with a biotin derivative, the recombinant protein is expressed on the surface of transfected mammalian cells. Whereas several other recently described receptors in this family also have large extracellular domains, the large extracellular domain of VLGR1 has a unique structure. It has nine imperfectly repeated units that are rich in acidic residues and are spaced at intervals of approximately 120 amino acid residues. These repeats resemble the regulatory domains of Na+/Ca2+ exchangers as well as a component of an extracellular aggregation factor of marine sponges. Bacterial fusion proteins containing two or four repeats specifically bind 45Ca in overlay experiments; binding is competed poorly by Mg2+ but competed well by neomycin, Al3+, and Gd3+. These results define a consensus cation binding motif employed in several widely divergent types of proteins. The ligand for VLGR1, its function, and the signaling pathway(s) it employs remain to be defined.

The L10F mutation of angiotensinogen is rare in pre-eclampsia.

BACKGROUND: A mutation in the gene for angiotensinogen, changing the leucine residue at position 10 to a phenylalanine (L10F), has been reported in a patient with proteinuric pre-eclampsia. In vitro enzymatic studies suggest this mutation would increase production of the vasoactive peptide, angiotensin II in vivo, and therefore explain the etiology of the maternal hypertension. OBJECTIVE: To determine whether mutation of codon 10 of angiotensinogen is common in pre-eclampsia, and therefore likely to be involved in disease susceptibility. DESIGN: We collected a cohort of 32 women with 'true' pre-eclampsia. All were normotensive prior to the 20th week of pregnancy, developed blood pressures consistently above 140/90 mmHg and had proteinuria of greater than 300 mg/day during the third trimester. All had blood pressures that returned to normal within 1 month of delivery; 31 women were primigravida. Genomic DNA was isolated from their peripheral blood lymphocytes for genetic analyses. METHODS: A polymerase chain reaction-restriction enzyme-based assay was devised to screen for mutation of codon 10 of the angiotensinogen gene. In addition, we determined the frequency of a threonine residue at position 235 in the angiotensinogen gene, given previous controversial findings of association of this polymorphism with disease. CONCLUSIONS: We detected no mutation of codon 10 in angiotensinogen in any of the 32 women studied, indicating that this mutation is not commonly associated with proteinuric pre-eclampsia. Furthermore, there was no increased frequency of threonine 235 in the affected individuals studied compared with respective normotensive Caucasian-American and African-American populations.

Prenatal diagnosis and treatment of 11beta-hydroxylase deficiency congenital adrenal hyperplasia resulting in normal female genitalia.

Congenital adrenal hyperplasia (CAH) consists of autosomal recessive disorders of cortisol biosynthesis, which in the majority of cases result from 21-hydroxylase deficiency. Another enzymatic defect causing CAH is 11beta-hydroxylase deficiency. In both forms, the resulting excessive androgen secretion causes genital virilization of the female fetus. For over 10 yr female fetuses affected with 21-hydroxylase deficiency have been safely and successfully prenatally treated with dexamethasone. We report here the first successful prenatal treatment with dexamethasone of an affected female with 11beta-hydroxylase deficiency CAH. The family had two girls affected with 1beta-hydroxylase deficiency born with severe ambiguous genitalia who were both homozygous for the T318M mutation in the CYP11B1 gene, which codes for the 11beta-hydroxylase enzyme. In the third pregnancy in this family, the female fetus was treated in utero by administering dexamethasone to the mother, starting at 5 weeks gestation. The treatment was successful, as the newborn was not virilized and had normal female external genitalia. A second family with two affected sons was also studied in preparation for a future pregnancy. We report a novel 1-bp deletion in codon 394 (R394delta1) in the CYP11B1 gene in this family.

Recombinant CYP11B genes encode enzymes that can catalyze conversion of 11-deoxycortisol to cortisol, 18-hydroxycortisol, and 18-oxocortisol.

CYP11B1 (11beta-hydroxylase) and CYP11B2 (aldosterone synthase) are 93% identical mitochondrial enzymes that both catalyze 11beta-hydroxylation of steroid hormones. CYP11B2 has the additional 18-hydroxylase and 18-oxidase activities required for conversion of 11-deoxycorticosterone to aldosterone. These two additional C18 conversions can be catalyzed by CYP11B1 if serine-288 and valine-320 are replaced by the corresponding CYP11B2 residues, glycine and alanine. Here we show that such a hybrid enzyme also catalyzes conversion of 11-deoxycortisol to cortisol, 18-hydroxycortisol, and 18-oxocortisol. These latter two steroids are present at elevated levels in individuals with glucocorticoid suppressible hyperaldosteronism (GSH) and some forms of primary aldosteronism. Their production by the recombinant CYP11B enzyme is enhanced by substitution of further amino acids encoded in exons 4, 5, and 6 of CYP11B2. A converted CYP11B1 gene, containing these exons from CYP11B2, would be regulated like CYP11B1, yet encode an enzyme with the activities of CYP11B2, thus causing GSH or essential hypertension. In a sample of 103 low renin hypertensive patients, 218 patients with primary aldosteronism, and 90 normotensive individuals, we found a high level of conversion of CYP11B genes and four cases of GSH caused by unequal crossing over but no gene conversions of the type expected to cause GSH.

Isolated aldosterone synthase deficiency caused by simultaneous E198D and V386A mutations in the CYP11B2 gene.

Isolated deficiencies in aldosterone biosynthesis are caused by mutations in the CYP11B2 (aldosterone synthase) gene. Patients with this deficiency have impaired aldosterone synthesis, exhibit increased plasma renin activity, secrete increased amounts of the steroid precursors DOC, corticosterone, and 18OHDOC, and are subject to salt wasting and poor growth. Two forms are generally distinguished. The first, corticosterone methyloxidase type I (CMO I or type 1 deficiency), is characterized by no detectable aldosterone secretion, a low or normal secretion of the steroid 18OHB, and are always found to have mutations that completely inactivate the encoded CYP11B2 enzyme. The second form (CMO II or type 2 deficiency) may have low to normal levels of aldosterone, but at the expense of greatly increased secretion of its immediate precursor 18OHB. These patients usually have a CYP11B2 enzyme with some residual enzymatic activity, especially 11beta-hydroxylase activity. We have studied two twins with an isolated aldosterone synthase activity who have a clinical profile typical of the type 1 deficiency. Their CYP11B2 genes are homozygous for three sequence changes, R173K, E198D, and V386A. In transfection assays these substitutions individually have modest effects on the encoded enzyme, but when found together they result in an enzyme with a decreased 11beta-hydroxylase activity, a large decrease of 18-hydroxylase activity, and no detectable 18-oxidase activity. This residual activity is more typical of that observed in patients classified as having CMO II deficiency, rather than CMO I deficiency, where no activity is detectable. This disparity between the CYP11B2 enzyme with residual activity and a clinical phenotypic typical of the type 1 deficiency, suggests that phenotype genotype relationships are not yet fully understood.

The amino acid substitutions Ser288Gly and Val320Ala convert the cortisol producing enzyme, CYP11B1, into an aldosterone producing enzyme.

Transfection studies with cDNAs encoding hybrids between the highly similar cytochrome P450 enzymes, CYP11B1 (steroid 11 beta-hydroxylase) and CYP11B2 (aldosterone synthase) have identified which amino acids determine the different activities of the enzymes.

Aldosterone secretion: a molecular perspective.

The major mineralocorticoid hormone aldosterone is secreted from the zona glomerulosa of the adrenal cortex. Aldosterone is synthesized from cholesterol via a series of hydroxylations and oxidations. The enzymes involved in these reactions are mostly members of the cytochrome P450 superfamily. The final steps of this pathway, the conversion of 11-deoxycorticosterone (DOC) to aldosterone, require conversion via the intermediates 18-hydroxy-DOC or corticosterone and 18-hydroxycorticosterone. There are significant differences between species in the number of genes that encode the P450(11beta)-related enzymes (CYP11B) involved in these steps and the zonal distribution of their expression. One enzyme is capable of 11-hydroxylation, 18-hydroxylation, and 18-oxidation of DOC to aldosterone. The genetic basis of four diseases-congenital adrenal hyperplasia due to 11beta-hydroxylase deficiency, glucocorticoid-remediable aldosteronism, aldosterone synthase deficiency type I and type II-is explicable by mutations in these cytochrome P450(11beta)-related genes.

Angiotensin II receptors: protein and gene structures, expression and potential pathological involvements.

Two distinct types of cell-surface angiotensin II receptors (AT1 and AT2) have been defined pharmacologically and cDNAs encoding each type have been identified by expression cloning. These pharmacological studies showed the AT1 receptors to mediate all the known functions of angiotensin II in regulating salt and fluid homeostasis. Further complexity in the angiotensin II receptor system was revealed when homology cloning showed the existence of two AT1 subtypes in rodents and in situ hybridization and reverse transcription-polymerase chain reaction analyses showed their level of expression to be regulated differently in different tissues: AT1A is the principal receptor in the vessels, brain, kidney, lung, liver, adrenal gland and fetal pituitary, while AT1B predominates in the adult pituitary and is only expressed in specific regions of the adrenal gland (zona glomerulosa) and kidney (glomeruli). Expression of AT1A appears to be induced by angiotensin II in vascular smooth-muscle cells but is inhibited in the adrenal gland. Preliminary analysis of the AT1 promoters is also suggestive of a high degree of complexity in their regulation. Investigation of a potential role for altered AT1 receptor function has commenced at a genetic level in several diseases of the cardiovascular system. No mutations affecting the coding sequence have been identified in Conn adenoma and no linkage has been demonstrated with human hypertension by sib-pair analysis. None the less, certain polymorphisms that do not alter the protein structure have been found to be associated with hypertension and to occur at an increased frequency in conjunction with specific polymorphisms in the ACE gene in individuals at increased risk for myocardial infarction. Further characterization of the regions of the AT1 gene that regulate its expression are therefore needed. The physiological importance of the AT2 gene product still remains a matter of debate.

Human type-1 angiotensin II (AT1) receptor gene structure and function.

1. The type-1 angiotensin II (AngII) receptors, designated AT1, mediate most of the biological actions of the peptide hormone AngII. They are the most recent drug target for the treatment of hypertension and cardiac failure and basic research is now focusing on the mechanisms that regulate their expression. 2. In humans there is a single AT1 gene. It encodes a 47 kb pre-mRNA containing five exons, with the previously described AT1 open reading frame (ORF) on exon 5. Alternative splicing results in the production of mature mRNA that are translated at different efficiencies and encode two receptor isoforms. The inclusion of exon 2 markedly inhibits translation of the down-stream ORF, both in vitro and in vivo. Nonetheless, this exon is present in up to one-half of AT1 mRNA in all tissues studied. 3. Transcripts containing exon 3 spliced to exon 5 encode a receptor with an amino-terminal extension of 32 amino acids and represent up to one-third of total AT1 mRNA in each tissue examined. In vitro, these latter transcripts are translated to produce a longer receptor and, in transfected cells, they encode a functional AT1 receptor with ligand-binding and signalling properties similar to those of the short isoform. 4. Exon 4 is of minor significance as it is rarely spliced into AT1 mRNA. 5. These data indicate that, in addition to characterizing factors that modulate AT1 promoter activity and RNA stability, it is important to analyse the splicing patterns of this gene when studying the regulation of its expression.

Polar residues in the transmembrane domains of the type 1 angiotensin II receptor are required for binding and coupling. Reconstitution of the binding site by co-expression of two deficient mutants.

Type 1 angiotensin receptors (AT1) are G-protein coupled receptors, mediating the physiological actions of the vasoactive peptide angiotensin II. In this study, the roles of 7 amino acids of the rat AT1A receptor in ligand binding and signaling were investigated by performing functional assays of individual receptor mutants expressed in COS and Chinese hamster ovary cells. Substitutions of polar residues in the third transmembrane domain with Ala indicate that Ser105, Ser107, and Ser109 are not essential for maintenance of the angiotensin II binding site. Replacement of Asn111 or Ser115 does not alter the binding affinity for peptidic analogs, but modifies the ability of the receptor to interact with AT1 (DuP753)- or AT2 (CGP42112A)-specific ligands. These 2 residues are probably involved in determining the binding specificity for these analogs. The absence of G-protein coupling to the Ser115 mutant suggests that this residue, in addition to previously identified residues, Asp74 and Tyr292, participates in the receptor activation mechanism. Finally, Lys102 (third helix) and Lys199 (fifth helix) mutants do not bind angiotensin II or different analogs. Co-expression of these two deficient receptors permitted the restoration of a normal binding site. This effect was not due to homologous recombination of the cDNAs but to protein trans-complementation.

Human type-1 angiotensin II (AT1) receptor gene structure and function.

1. The type-1 angiotensin II (AngII) receptors, designated AT(1), mediate most of the biological actions of the peptide hormone AngII. They are the most recent drug target for the treatment of hypertension and cardiac failure and basic research is now focusing on the mechanisms that regulate their expression. 2. In humans there is a single AT(1) gene. It encodes a 47 kb pre-mRNA containing five exons, with the previously described AT(1) open reading frame (ORF) on exon 5. Alternative splicing results in the production of mature mRNA that are translated at different efficiencies and encode two receptor isoforms. The inclusion of exon 2 markedly inhibits translation of the downstream ORF, both in vitro and in vivo. Nonetheless, this exon is present in up to one-half of AT(1) mRNA in all tissues studied. 3. Transcripts containing exon 3 spliced to exon 5 encode a receptor with an amino-terminal extension of 32 amino acids and represent up to one-third of total AT(1) mRNA in each tissue examined. In vitro, these latter transcripts are translated to produce a longer receptor and, in transfected cells, they encode a functional AT(1) receptor with ligand-binding and signalling properties similar to those of the short isoform. 4. Exon 4 is of minor significance as it is rarely spliced into AT(1) mRNA. 5. These data indicate that, in addition to characterizing factors that modulate AT(1) promoter activity and RNA stability, it is important to analyse the splicing patterns of this gene when studying the regulation of its expression.

Glucocorticoid-suppressible hyperaldosteronism and adrenal tumors occurring in a single French pedigree.

Glucocorticoid-suppressible hyperaldosteronism is a dominantly inherited form of hypertension believed to be caused by the presence of a hybrid CYP11B1/CYP11B2 gene which has arisen from an unequal crossing over between the two CYP11B genes in a previous meiosis. We have studied a French pedigree with seven affected individuals in which two affected individuals also have adrenal tumors and two others have micronodular adrenal hyperplasia. One of the adrenal tumors and the surrounding adrenal tissue has been removed, giving a rare opportunity to study the regulation and action of the hybrid gene causing the disease. The hybrid CYP11B gene was demonstrated to be expressed at higher levels than either CYP11B1 or CYP11B2 in the cortex of the adrenal by RT-PCR and Northern blot analysis. In situ hybridization showed that both CYP11B1 and the hybrid gene were expressed in all three zones of the cortex. In cell culture experiments hybrid gene expression was stimulated by ACTH leading to increased production of aldosterone and the hybrid steroids characteristic of glucocorticoid-suppressible hyperaldosteronism. The genetic basis of the adrenal pathologies in this family is not known but may be related to the duplication causing the hyperaldosteronism.

Alternatively spliced human type 1 angiotensin II receptor mRNAs are translated at different efficiencies and encode two receptor isoforms.

The peptide hormone angiotensin II (AngII) plays a principal role in regulating blood pressure and fluid homeostasis. Most of its known effects are mediated by a guanine nucleotide-regulatory protein (G protein)-coupled receptor pharmacologically defined as the type-1 AngII receptor or AT1. Characterization of cDNA and genomic clones shows that the human AT1 gene contains five exons and encodes two receptor isoforms as a result of alternative splicing. Exon 5 contains the previously characterized open reading frame for AT1, and exons 1 to 3 are alternatively spliced upstream of it to generate several mRNA species, while transcripts containing exon 4 are of minor abundance. In an in vitro translation system, the presence of exon 1 was found to be extremely inhibitory to translation, probably because it can form a stable secondary structure at the RNA level. The alternatively spliced second exon also had a strong inhibitory effect on translation, presumably because it contains a minicistron commencing with an ATG in an optimal context for translation initiation. Exon 2 was similarly inhibitory to protein production in transfected cells, but exon 1 was found to enhance protein synthesis in this system. Transcripts containing exon 3 and 5, which comprise up to one-third of AT1 mRNAs in all tissues examined, encode a receptor with an amino-terminal extension of 32-35 amino acids. These transcripts were translated into a larger receptor isoform in vitro and produced a functional receptor with normal ligand binding and signaling properties in transfected cells.

Genetic recombination as a cause of inherited disorders of aldosterone and cortisol biosynthesis and a contributor to genetic variation in blood pressure.

CYP11B1 (11 beta-hydroxylase) and CYP11B2 (aldosterone synthase) are steroidogenic enzymes which mediate the final step (11 beta-hydroxylation) in cortisol synthesis and the final three steps (11 beta-hydroxylation, 18-hydroxylation, and 18-oxidation) in aldosterone synthesis, respectively. The enzymes share 93% identity in amino acid sequence and are encoded by two structurally similar genes which are located in tandem on chromosome 8q22, approximately 40 kb apart. Expression of the aldosterone synthase gene (CYP11B2) is limited to the zona glomerulosa of the adrenal cortex, thereby limiting the synthesis of aldosterone to that zone, where it is principally regulated by plasma levels of angiotensin II and potassium. The 11 beta-hydroxylase gene (CYP11B1) is expressed in the zona fasciculata, the zone which also expresses a 17-hydroxylase activity, where it mediates cortisol synthesis under the control of ACTH. Genetic recombination involving a mispairing of the two CYP11B genes can lead to duplications and deletions of the genes, creation of hybrid genes of several forms, or transfer of coding and regulatory sequences from one gene to the other. Since the two genes have related but different activities, are normally expressed in different zones, and respond to different physiological signals, such recombination has the potential to generate a variety of inherited disorders of steroid production. In this paper we review the range of mutations which can occur and the resulting disorders of steroid biosynthesis, and suggest some novel mutations which might be sought in variants of these endocrinological syndromes.

Disorders of steroid 11 beta-hydroxylase isozymes.

The most active corticosteroids are 11 beta-hydroxylated. Humans have two isozymes with 11 beta-hydroxylase activity that are respectively required for cortisol and aldosterone synthesis. CYP11B1 (11 beta-hydroxylase) is expressed at high levels and is regulated by ACTH, whereas CYP11B2 (aldosterone synthase) is normally expressed at low levels and is regulated by angiotensin II. In addition to 11 beta-hydroxylase activity, the latter enzyme has 18-hydroxylase and 18-oxidase activities and thus can synthesize aldosterone from deoxycorticosterone. Insights into the normal functioning of these enzymes are gained from studies of disorders involving them. Mutations in the CYP11B1 gene cause steroid 11 beta-hydroxylase deficiency, a form of congenital adrenal hyperplasia characterized by signs of androgen excess and by hypertension. Mutations in CYP11B2 result in aldosterone synthase (corticosterone methyloxidase) deficiency, an isolated defect in aldosterone biosynthesis that can cause hyponatremia, hyperkalemia, and hypovolemic shock in infancy and failure to thrive in childhood. These are both recessive disorders. Unequal crossing over between the CYP11B genes can generate a duplicated chimeric gene with the transcriptional regulatory region of CYP11B1 but sufficient coding sequences from CYP11B2 so that the encoded enzyme has aldosterone synthase (i.e. 18-oxidase) activity. This results in aldosterone biosynthesis being regulated by ACTH, a condition termed glucocorticoid-suppressible hyperaldosteronism. This form of genetic hypertension is inherited in an autosomal dominant manner.

Mutations in the CYP11B1 gene causing congenital adrenal hyperplasia and hypertension cluster in exons 6, 7, and 8.

Steroid 11 beta-hydroxylase deficiency (failure to convert 11-deoxycortisol to cortisol) is the second most common cause of congenital adrenal hyperplasia and results in a hypertensive form of the disease. The 11 beta-hydroxylase enzyme is encoded by the CYP11B1 gene on chromosome 8q22. Two mutations in CYP11B1 have previously been reported in patients with 11 beta-hydroxylase deficiency--Arg-448-->His and a 2-bp insertion in codon 394. We now report eight previously uncharacterized mutations causing this disorder. Seven are point mutations (three nonsense and four missense) and one is a single base pair deletion causing a frameshift. We have used an in vitro transfection assay to show that all five known missense mutations causing 11 beta-hydroxylase deficiency abolish enzymatic activity. In principle, deletions of CYP11B1 could be generated by unequal crossing-over between CYP11B1 and the adjacent CYP11B2 gene, but no such deletions were found among the deficiency alleles in this study. Seven of the 10 known mutations are clustered in exons 6-8, a nonrandom distribution within the gene. This may reflect the location of functionally important amino acid residues within the enzyme or an increased tendency to develop mutations within this region of the gene.

Transcripts originating in intron 1 of the HSD11 (11 beta-hydroxysteroid dehydrogenase) gene encode a truncated polypeptide that is enzymatically inactive.

There is evidence for the presence in the kidney of more than one isoform of 11 beta-hydroxysteroid dehydrogenase (11HSD), an enzyme that interconverts cortisol and cortisone (in rodents, corticosterone and 11-dehydrocorticosterone). A specific isoform might arise from transcripts in the kidney that are known to originate in intron 1; translation from these transcripts is predicted to initiate at the codon that in the full-length rat enzyme encodes Met27. Alignment of the full-length rat and human 11HSD sequences with other members of the short chain dehydrogenase family suggests that initiation of translation at Met27 might yield a functional enzyme, since the amino-termini of most of these enzymes occur at equivalent positions. We confirmed that short transcripts are found in the kidney and are detectable at lower levels in the liver and lung. In vitro transcription and translation of short cDNA demonstrated that the AUG encoding Met27 is indeed a functional initiation codon. However, Chinese hamster ovary cells transfected with short cDNA in the pCMV4 vector expressed apparently low levels of the corresponding truncated polypeptide and had no 11HSD activity. Thus, the functional significance of transcripts originating in intron 1 is unclear.