Missense mutation serine106—-proline causes 17 alpha-hydroxylase deficiency

17alpha-hydroxylase/17,20-lyase deficiency as a model to study enzymatic activity regulation: role of phosphorylation

Cytochrome P450 17alpha-hydroxylase (CYP17) is a single gene-encoded protein with two activities: 17alpha-hydroxylase and 17,20-lyase. The two catalytic activities are differentially regulated in health and disease. We took advantage of naturally occurring human mutations to understand the molecular bases of this differential regulation. We identified eight novel mutations in the CYP17 gene, different in nature and spread throughout the gene. As posttranslational modifications appear to be important for activity control, we investigated the phosphorylation state of wild-type and mutant CYP17 proteins. Although phospholabeled protein was seen when the wild-type and most mutant proteins were expressed, no phosphorylation was detected for the F417C mutant. F417C is the only 17,20-lyase deficiency case confirmed at the molecular level and represents the first phosphorylation CYP17-deficient mutant. In search of the physiological agents involved in this process, the effect of cAMP was tested on activity and phosphorylation state of our mutant CYP17 proteins. cAMP stimulates activity and phosphorylation in all cases, except in the F417C and R35L mutants. The lack of response to the physiological second messenger might explain the different phenotypes. The F417C mutant protein, which is already shown to be associated with the lack of electron transfer, provides for the first time a link between the electron transfer system and the phosphorylation state of the CYP17 enzyme in the control of 17,20-lyase activity.

The molecular basis of premature adrenarche: an hypothesis

Adrenarche is characterized by a prepubertal rise in adrenal secretion of dehydroepiandrosterone (DHEA) and DHEA sulfate (DHEAS) that is independent of the gonads or gonadotropins. Adrenopause is the corresponding diminution in DHEA and DHEAS concentrations in later life. The mechanisms by which adrenarche and adrenopause are induced and regulated are unknown. Early work focused on identifying hypothetical adrenal androgen regulatory hormones that would induce DHEA in much the same way that adrenocorticotropin induces cortisol, but no such factors have been found. Current studies of adrenarche focus on intra-adrenal events, particularly those concerning 3beta-hydroxysteroid dehydrogenase (3beta-HSD) and 17alpha-hydroxylase/17,20-lyase (P450c17). Molecular data implicate a decrease in 3beta-HSD specifically in the adrenal zona reticularis. However, a decrease in 3beta-HSD is insufficient to explain why the reticularis catalyzes 17,20-lyase activity and hence makes DHEA, rather than catalyzing only 17alpha-hydroxylase activity, as does the zona fasciculata. P450c17 appears to catalyze 17,20-lyase activity only if P450c17 has undergone serine phosphorylation and has access to cytochrome b5 as an allosteric cofactor. Although these two factors have not yet been investigated in adrenarche, it appears that both a zone-specific diminution in 3beta-HSD and a zone-specific induction of 17,20-lyase activity are required to account for the physiological data. Exaggerated premature adrenarche appears to be an early sign of polycystic ovary syndrome (PCOS). Mechanistic considerations of PCOS suggest a key role for serine phosphorylation of P450c17 in both adrenarche and some forms of heritable PCOS.

A(-6)G variant of angiotensinogen gene and aldosterone levels in hypertensives

Recently, a novel mutation in the promoter region of the angiotensinogen gene that involves the presence of an adenine instead of a guanine 6 bp upstream from the transcription initiation site (A(-6)G) has been shown to induce an increase in gene transcription. The aim of this study was to determine the prevalence of the A(-6)G mutation in essential hypertensive patients and to correlate it with aldosterone and renin activity levels. We studied 191 hypertensives. We measured levels of aldosterone (plasma and urinary) and plasma renin activity. We determined the variants A and G using a mutagenically separated polymerase chain reaction technique. In 191 hypertensives, the A variant was detected in 266 of 382 (69.6%) and the G variant in 116 of 382 alleles (30.4%). Plasma aldosterone was significantly higher in patients homozygous for AA than in those homozygous for GG (369+/-208 versus 246+/-142 pmol/L). Urinary aldosterone was significantly higher in homozygous AA than in AG or GG patients (62.4+/-39.4 versus 50.8+/-25.2 and 37.4+/-22.3 nmol/d, respectively). When the patients were grouped according to the presence or absence of the A allele, the aldosterone levels and the plasma aldosterone/plasma renin activity ratio were significantly higher in patients with the A allele. The presence of the A variant was associated with higher levels of aldosterone. These results suggest that the presence of the A variant could determine the appearance of arterial hypertension through higher transcription activity of the angiotensinogen gene and concomitant aldosterone production.

Congenital adrenal hyperplasia: molecular genetics and alternative approaches to treatment

Several autosomal recessive disorders affecting the adrenal cortex and its development and leading to defective cortisol biosynthesis are known under the collective term "congenital adrenal hyperplasia" (CAH). Over the last two decades, the genes causing most of these disorders have been identified and molecular genetics may supplement their clinical and biochemical diagnosis. In addition, new treatments have emerged; although gene therapy has yet to be applied in humans, studies are ongoing in gene transfer in adrenocortical cell lines and animal models. In this review, after a brief introduction on the developmental biology and biochemistry of the adrenal cortex and its enzymes, we will list the new developments in the genetics and treatment of diseases causing CAH, starting with the most recent findings. This order happens to follow adrenal steroidogenesis from the mitochondrial entry of cholesterol to cortisol synthesis; it is unlike other presentations of CAH syndromes that start with the most frequently seen syndromes, because the latter were also the first to be investigated at the genetic level and have been extensively reviewed elsewhere. We will start with the latest syndrome to be molecularly investigated, congenital lipoid adrenal hyperplasia (CLAH), which is caused by mutations in the gene coding for the steroidogenic acute regulatory (StAR) protein. We will then present new developments in the genetics of 3-beta-hydroxysteroid dehydrogenase (3 beta HSD), 17 hydroxylase and 17,20-lyase (P450c17), 11 hydroxylase (P450c11 beta), and 21 hydroxylase (P450c21) deficiencies. Alternative treatment approaches and gene therapy experiments are reviewed collectively in the last section, because they are still in their infantile stages.

Decrease in the expression of the 3beta-hydroxysteroid dehydrogenase gene in human adrenal tissue during prepuberty and early puberty: implications for the mechanism of adrenarche

Adrenarche is the increase of adrenal androgen secretion, mainly dehydroepiandrosterone and dehydroepiandrosterone sulfate, that occurs during prepuberty in higher primates. This event takes place at about 6-8 y of age in humans. It had been postulated that adrenarche might reflect an increase in the 17,20 lyase:17OH-ase activity ratio of microsomal cytochrome P450c17. However, studies to demonstrate this mechanism have been unsuccessful. Because it has been described that virilizing adrenocortical carcinomas have high dehydroepiandrosterone sulfate secretion and low 3beta-hydroxysteroid dehydrogenase (3betaHSD) activity, in this study we evaluated the possible existence of maturative changes of the level of 3betaHSD type II mRNA in 11 normal prepubertal and early pubertal human adrenals. Adrenal glands from subjects aged 0.1 to 13 y were obtained from organ donors, patients undergoing resection of the kidney for renal neoplasms or necropsies with less than 6 h of postmortem time. The expression of 3betaHSD type II gene was studied by dot blot in all samples and by relative reverse transcriptase (RT)-PCR in nine samples. The size of the transcripts was evaluated by Northern blot. Hybridization was performed using labeled human 3betaHSD cDNA probes. The uniformity of loading was tested using labeled human beta actine cDNA. The relative intensities of hybridization signals were quantified by scanning densitometry. The expected bands after relative RT-PCR were eluted, and radioactivity was measured in a scintillation counter. For the analysis of the results, subjects were divided into two groups as a function of age: group 1, less than 8 y (n = 6; range 0.1-2.48 y) and group 2, equal or older than 8 y (n = 5; range 8-13 y). 3BetaHSD type II mRNA expression, analyzed by dot blot and relative RT-PCR, was significantly higher (p < 0.05) in group 1 (median and range 4.99, 0.50-8.00 and 16.3, 13.5-40.0 arbitrary units, respectively) than in group 2 (0.15, 0.12-0.75 and 5.66, 3.18-13.0, respectively). In conclusion, we have shown a decrease of the expression 3betaHSD type II gene as a function of age in prepubertal and early pubertal normal human adrenal tissue. This maturative change might be involved in the mechanism of human adrenarche.

Role of cytochrome P450c17 in polycystic ovary syndrome

The hyperandrogenism of polycystic ovary syndrome (PCOS) appears to be due to dysregulation of steroidogenesis within the ovaries and adrenal glands. P450c17 is the key enzyme that regulates androgen synthesis. It is the only enzyme known to have the capacity to convert C21-precursors to the androgen pre-hormones, the 17-ketosteroids. It is a single enzyme with two activities, 17-hydroxylase and 17,20-lyase. Thus, its regulation is a significant factor in the expression of hyperandrogenism. Androgen secretion is LH-dependent in the ovary and ACTH-dependent in the adrenal glands. The androgenic response to each of these tropic hormones seems to be modulated by intra-ovarian or intra-adrenal autocrine and paracrine mechanisms. This modulation serves to regulate steroid hormone secretion in tissue-specific ways. Insulin, IGFs and inhibin are among the many growth factors capable of augmenting the response to LH and ACTH. The insulin/IGF system stimulates P450c17 mRNA expression and activities in the ovaries and adrenal glands. An integrating link between insulin resistance and hyperandrogenemia may be serine phosphorylation, which inhibits activity of the insulin receptor and promotes the 17,20-lyase activity of P450c17. However, it must be kept in mind that there is some evidence for the existence of P450c17-independent pathways of androgen biosynthesis.

The molecular basis of isolated 17,20 lyase deficiency

Human P450c17 catalyzes the 17alpha-hydroxylation of pregnenolone to 17OH pregnenolone and of progesterone to 17alpha-OH progesterone; the same P450c17 enzyme also catalyzes 17,20 lyase activity on the same active site, converting 17OH-pregnenolone to DHEA. Rodent and porcine P450c17 also catalyze 17,20 lyase activity with delta4 substrates, converting 17OH-progesterone to delta4 androstenedione, but human P450c17 catalyzes this reaction very inefficiently, so that virtually all human C19 sex steroids are made via 17OH pregnenolone and DHEA. P450c17 is encoded by a single gene and a single species of mRNA. Many mutations of this gene have been described, but until recently all of these either entirely eliminated both 17alpha-hydroxylase and 17,20 lyase activity, or affected each activity equivalently. We have identified and characterized the first patients with P450c17 mutations that selectively ablate 17,20 lyase activity while retaining 17alpha-hydroxylase activity. Through a combination of enyzmologic experiments in transfected mammalian cells and in genetically manipulated yeast, plus a computer model of human P450c17, we have proven that the responsible mutations, R347H and R358Q lie in the redox-partner binding site of P450c17. This site, through which P450c17 interacts with P450 oxidoreductase to receive the electrons needed for catalysis, can be allosterically influenced by cytochrome b5. These two mutations have contributed substantially to our understanding of the mechanisms by which 17alpha-hydroxylase and 17,20 lyase activities are regulated independently, and thus have contributed to the study of regulated 17,20 lyase activity in adrenarche, aging, and the polycystic ovary syndrome.

T235 variant of the angiotensinogen gene and blood pressure in the Chilean population

BACKGROUND

The angiotensinogen gene has recently been linked to essential hypertension. A variant within this gene, encoding threonine rather than methionine at amino acid position 235, was associated with essential hypertension. However, results of new studies have not confirmed this association, suggesting that ethnic differences may explain the different results.

OBJECTIVE

To evaluate whether the T235 variant is associated with a higher incidence of essential hypertension among Hispanics (a group that has scarcely been evaluated) and to determine whether T235 is associated with variations in the plasma renin activity or the serum aldosterone level.

PATIENTS AND METHOD

We studied 64 patients with essential hypertension and 62 normotensives, matched for age and sex. We obtained samples for determinations of plasma renin activity, serum aldosterone level and genome DNA from all subjects. The genomic DNA was amplified using the polymerase chain reaction technique and digested by the restriction enzyme streptococcus faecalis (Sfa NI) which cuts M235 only, not T235.

RESULTS

The patients with essential hypertension had a higher prevalence of the risk variant T235 (alleles 77/128 = 60.2%) than did the normotensive controls (alleles 65/124 = 52.4%), but the difference was not statistically significant (chi2=1.53, P=0.22). The plasma renin activity levels in hypertensives were not statistically different for homozygous T235, heterozygous and homozygous M235 (1.0 +/- 0.96, 2.0 +/- 2.25 and 1.55 +/- 1.49 ng/ml per h, respectively, P=0.5 1). However, when we considered those hypertensives with low plasma renin activity levels (< 1 ng/ml per h), we found a high prevalence (72.7%) of subjects homozygous for the T235 variant. We found no association between the T235 variant and the serum aldosterone levels in hypertensive and normotensive subjects.

CONCLUSIONS

We demonstrated that there is a high prevalence of T235 variant in our Hispanic population. The slight difference between prevalences of T235 variant among hypertensive and normotensive subjects that we found was not statistically significant and did not permit us to establish an association between T235 variant and essential hypertension. We believe that only studying a larger cohort of subjects could show whether there is a quantitative effect of the T allele on plasma renin activity levels.

Early steps in androgen biosynthesis: from cholesterol to DHEA

Sex steroids, both androgens and oestrogens, are made from dehydroepiandrosterone (DHEA). The biosynthesis of DHEA from cholesterol entails four steps. First, cholesterol enters the mitochondria with the assistance of a recently described factor called the steroidogenic acute regulatory protein (StAR). Mutations in the StAR gene cause congenital lipoid adrenal hyperplasia. Next, cholesterol is converted to pregnenolone by the cholesterol side chain cleavage enzyme, P450scc. Mutations in the gene for P450scc and for its electron transfer partners, ferredoxin reductase and ferredoxin, have not been described and are probably incompatible with term gestation. Third, pregnenolone undergoes 17 alpha-hydroxylation by microsomal P450c17. Finally, 17-OH pregnenolone is converted to DHEA by the 17,20 lyase activity of P450c17. Isolated 17,20 lyase deficiency is rare, but the identification of its genetic basis and the study of P450c17 enzymology have recently clarified the mechanisms by which DHEA synthesis may be regulated in adrenarche, and have suggested that the lesion underlying polycystic ovary syndrome might involve a serine kinase.

Cytochrome b5 augments the 17,20-lyase activity of human P450c17 without direct electron transfer

In the biosynthesis of steroid hormones, P450c17 is the single enzyme that catalyzes both the 17alpha-hydroxylation of 21-carbon steroids and the 17,20-lyase activity that cleaves the C17-C20 bond to produce C19 sex steroids. Cytochrome b5 augments the 17,20-lyase activity of cytochrome P450c17 in vitro, but this has not been demonstrated in membranes, and the mechanism of this action is unknown. We expressed human P450c17, human P450-oxidoreductase (OR), and/or human cytochrome b5 in Saccharomyces cerevisiae and analyzed the 17alpha-hydroxylase and 17,20-lyase activities of the resulting yeast microsomes. Yeast expressing only P450c17 have 17alpha-hydroxylase and trace 17,20-lyase activities toward both Delta4 and Delta5 steroids. Coexpression of human OR with P450c17 increases the Vmax of both the 17alpha-hydroxylase and 17,20-lyase reactions 5-fold; coexpression of human b5 with P450c17 also increases the Vmax of the 17,20-lyase reactions but not of the 17alpha-hydroxylase reactions. Simultaneous expression of human b5 with P450c17 and OR, or addition of purified human b5 to microsomes from yeast coexpressing human P450c17 and OR, further increases the Vmax of the 17,20-lyase reaction without altering 17alpha-hydroxylase activity. Genetically engineered yeast and mixing experiments demonstrate that OR is both necessary and sufficient for microsomal 17,20-lyase activity. Addition of purified human holo-b5, apo-b5, or cytochrome c to microsomes containing both human P450c17 and OR demonstrate that the stimulatory action of b5 does not require electron transfer from b5 to P450c17. These data suggest that human b5 acts principally as an allosteric effector that interacts primarily with the P450c17.OR complex to stimulate 17, 20-lyase activity.

Complete structure of the human gene for the vitamin D 1alpha-hydroxylase, P450c1alpha

The rate-limiting, hormonally regulated step in the biosynthesis of the biologically active form of vitamin D, 1,25(OH)2D, is its 1alpha-hydroxylation in the kidney by the mitochondrial P450 enzyme, P450c1alpha. We have recently cloned the human P450c1alpha cDNA and shown that this enzyme is the factor disrupted in vitamin D-dependent rickets, type 1 (VDDR-1). To facilitate the analysis of further patients with VDDR-1 and to permit studies of the regulation of the gene for P450c1alpha, we have used PCR-based tactics to clone the gene. Southern blotting studies indicate that there is only one copy of this gene in the human genome. The complete sequence of all exons and introns show that the gene consists of 9 exons spanning only 5 kb; the entire protein-coding region can be PCR-amplified as a single 4-kb fragment. The transcriptional start site, located by primer extension and S1 nuclease protection, lies 62-bp upstream from the ATG transitional start codon. Analysis of rodent/human somatic cell hybrid DNAs show that this gene lies on chromosome 12. Although the gene is substantially smaller than the human genes for other mitochondrial enzymes, its intron/exon organization is very similar, especially to that of P450scc. This indicates that although the mitochondrial P450 enzymes retain only 30%-40% amino acid sequence identity, they all belong to a single evolutionary lineage.

The genetic and functional basis of isolated 17,20-lyase deficiency

Human male sexual differentiation requires production of fetal testicular testosterone, whose biosynthesis requires steroid 17,20-lyase activity. Patients with putative isolated 17,20-lyase deficiency have been reported. The existence of true isolated 17,20-lyase deficiency, however, has been questioned because 17 alpha-hydroxylase and 17,20-lyase activities are catalyzed by a single enzyme, microsomal cytochrome P450c17, and because the index case of apparent isolated 17,20-lyase deficiency had combined deficiencies of both activities. We studied two patients with clinical and hormonal findings suggestive of isolated 17,20-lyase deficiency. We found two patients homozygous for substitution mutations in CYP17, the gene encoding P450c17. When expressed in COS-1 cells, the mutants retained 17 alpha-hydroxylase activity but had minimal 17,20-lyase activity. Substrate competition experiments suggested that the mutations did not alter the enzyme's substrate-binding capacity, but co-transfection of cells with P450 oxidoreductase, the electron donor used by P450c17, indicated that the mutants had a diminished ability to interact with redox partners. Computer-graphic modelling of P450c17 suggests that both mutations lie in or near the redox-partner binding site, on the opposite side of the haem from the substrate-binding pocket. These mutations alter electrostatic charge distribution in the redox-partner binding site, so that electron transfer for the 17,20-lyase reaction is selectively lost or diverted to uncoupling reactions. These are the first proven cases of isolated 17,20-lyase deficiency, and they demonstrate a novel mechanism for loss of enzymatic activity.

Steroid 17alpha-hydroxylase deficiency: first Australian case report

17alpha-hydroxylase deficiency is a rare form of congenital adrenal hyperplasia (CAH) that affects both glucocorticoid and sex hormone biosynthesis. We report a case of an unambiguous female with testes and hypertension. She was found to have deficient 17alpha-hydroxylase activity. The diagnosis was not made easily, the condition being unexpected due to its rarity. The discriminating feature of this form of sex-reversal is the presence of hypertension due to the elevated serum deoxycorticosterone levels. A failure to detect this will inappropriately focus attention on other, more common causes of sex reversal such as androgen insensitivity and gonadal dysgenesis, and expose the patient to the long-term sequelae of uncontrolled arterial hypertension.

New developments in congenital lipoid adrenal hyperplasia and steroidogenic acute regulatory protein

To date, studies of patients with lipoid CAH have shown the indispensable role of StAR in the production of steroids by adrenal gland and gonads. Lipoid CAH is the first and so far only inborn disorder of steroid hormone synthesis and metabolism that is not caused by a defective steroidogenic enzyme but rather by a defect in cholesterol transport.

The regulation of 17,20 lyase activity

P450c17 is a single microsomal enzyme that catalyzes two distinct steroid biosynthetic activities: 17 alpha-hydroxylase and 17,20 lyase. Human beings have only one gene that encodes only one form of P450c17. Three clinical observations indicated that these were independently regulated activities. First, several cases of isolated 17,20 lyase deficiency were reported, in which 17 alpha-hydroxylase activity was spared. Second, most adrenal steroidogenesis in children stops after 17 alpha-hydroxylation, thus permitting the synthesis of cortisol, whereas most gonadal steroidogenesis proceeds to C19 sex steroids as a result of both activities. Third, the 17,20 lyase activity of the human adrenal is developmentally activated during adrenarche. To catalyze these two activities, P450c17 must receive reducing equivalents from electron donors (redox partners). Previous observations showed that the molar ratio of P450 oxidoreductase to P450c17 was 3-fold higher in the testis than in the adrenal, and that increasing the molar ratio of the redox partner to P450c17 would increase the ratio of 17,20 lyase activity to 17 alpha-hydroxylase. We have recently shown that P450c17 must be phosphorylated on serine and threonine residues by a cAMP-dependent protein kinase to acquire 17,20 lyase activity. We have also recently found two cases of isolated 17,20 lyase deficiency that have mutations of residues in the proposed redox partner binding site. Together, these studies suggest a unified view of the regulation of 17,20 lyase activity. The ratio of 17,20 lyase to 17 alpha-hydroxylase activity of P450c17 is regulated by the availability of reducing equivalents flowing to the enzyme. This can be increased by increasing the molar concentration of electron-donating redox partners, such as P450 oxidoreductase or possibly cytochrome b5, as appears to be the case in the gonads. Alternatively, the affinity of P450c17 for redox partners may be selectively increased by Ser/Thr phosphorylation, or selectively decreased by certain mutations in the redox partner binding site, in either case altering an electrostatic interaction between P450c17 and the redox partner. This model is consistent with all present observations about the biochemistry, genetics, enzymology, and clinical phenomenology of P450c17.

Identification, characterization, cloning, and expression of apolipoprotein C-IV, a novel sialoglycoprotein of rabbit plasma lipoproteins

We have identified and characterized a novel proline- and arginine-rich protein component of lipoproteins, present in up to five sialylated isoforms, in rabbit blood plasma. The pI of the desialylated protein is 5.7. Based upon its N-terminal sequence, a complete cDNA sequence of 555 nucleotides was cloned from rabbit liver. The synthesized protein is predicted to contain 124 amino acids, including a typical signal peptide of 27 residues. The mature protein of 97 amino acids, designated apolipoprotein C-IV, is associated with the lipoproteins of blood plasma, primarily very low density and high density lipoproteins. It contains two potential amphipathic helices characteristic of plasma apolipoproteins and forms discoidal micelles with phosphatidylcholine. Northern analysis shows a single 0.6-kilobase apolipoprotein C-IV mRNA, detected only in the liver, and Southern analysis suggests a single copy gene. Sialylated apolipoprotein C-IV is secreted from transfected mammalian cells. Nucleotide sequence comparisons demonstrate a strong homology to portions of the upstream regions of the mouse and human apolipoprotein C2 genes, within each of which a distinct gene has recently been identified. The nucleotide sequences and the predicted amino acid sequences, as well as corresponding cDNA sequences in the rat and monkey, indicate that the apolipoprotein C4 gene has been highly conserved during mammalian evolution.

17 alpha-Hydroxylase/17,20-lyase defects

Congenital adrenal hyperplasia due to 17 alpha-hydroxylase/17/20-lyase deficiency is caused by genetic defects in the gene encoding P450c17 (CYP17). To date, 18 different mutations in 27 individuals have been identified and all of them are located in the coding region of CYP17. Several mutations have been reconstructed in human P450c17 cDNA and expressed in COS cells to characterize the kinetic properties of 17 alpha-hydroxylase and 17,20-lyase activities. The molecular bases of cases clinically reported as 17 alpha-hydroxylase deficiency have turned out to result from complete or partial combined deficiencies of 17 alpha-hydroxylase/17,20-lyase. The elucidation of the molecular bases generally explains the patient's clinical profiles including the sexual phenotype of the external genitalia. In one case initially reported as isolated 17,20-lyase deficiency, the molecular basis was found to be partial combined deficiency of both activities, somewhat discordant with the patient's clinical profile. However, the patient was subsequently found to have 17 alpha-hydroxylase deficiency, suggesting involvements of age-dependent unknown factors affecting P450c17 activity.

Molecular cloning and expression of guinea pig cytochrome P450c17 cDNA (steroid 17 alpha-hydroxylase/17,20 lyase): tissue distribution, regulation, and substrate specificity of the expressed enzyme

In mammalian and fish species, P450c17 mediates both 17 alpha-hydroxylase and 17,20-lyase activities in the synthesis of steroid hormones. Previous results have shown that among the adrenal steroid hydroxylase enzymes involved in adrenal C19 steroid and glucocorticoid synthesis, regulation of cytochrome P450c17 is of primary importance because it is localized at the key branch between glucocorticoid and C19 steroid synthesis. A cDNA library from guinea pig adrenal was constructed, and the complete 17 alpha-hydroxylase cytochrome P450 cDNA was isolated. The guinea pig P450c17 cDNA includes the full-length coding region (1,524 nucleotide), the complete 3' untranslated region (169 nucleotide), and 39 bases of the 5' untranslated region. Our clone shares most of the features of the other P450c17 cDNAs; however, in addition, we identified a novel conserved region of 18 amino acids located in exon I between residues 80 and 97. This region presents the highest percentage of identity among the other P450c17 enzymes and is positioned one helixturn upstream of the important Ser106 on the corresponding human form. On Northern blot, the cDNA hybridizes with a major 1.8-kb mRNA and with two other related P450c17 mRNA of about 3 and 4 kb. P450c17 mRNA is equally distributed in male and female gonads and adrenals. Characterization of the enzymatic activity shows that 17 alpha-hydroxylase and 17,20-lyase are carried by a single protein, but in homogenates 17,20-lyase activity is barely detectable. Moreover, we demonstrate in vitro and in vivo that the guinea pig enzyme preferentially has very high levels of 17 alpha-hydroxylase and 17,20-lyase activities only toward delta 4 steroids. Second-messenger cyclic adenosine monophosphate and adrenocorticotropin specifically increased the abundance of P450c17 mRNA levels in guinea pig adrenal cells.

Genetic diseases of steroid metabolism

All major classes of biologically active steroid hormones (progestins, mineralocorticoids, glucocorticoids, and sex steroids) are synthesized from cholesterol through 11 different bioconversions. With the exception of 5 alpha-reductase, all the enzymes mediating these reactions fall into two classes, cytochromes P450 and short-chain dehydrogenases. Cytochromes P450 are heme-containing membrane-bound proteins with molecular weights of approximately 50,000 that utilize molecular oxygen and electrons from NADPH-dependent accessory proteins to hydroxylate substrates. Short-chain dehydrogenases have molecular weights of 30,000-40,000, have tyrosine and lysine residues at the active site, and remove a hydride from the substrate, transferring the electrons of the hydride to NAD+ or NADP+. In most cases, this reaction is reversible so that the dehydrogenase can also function as a reductase under appropriate conditions. Inherited disorders in enzymes required for steroid biosynthesis have varying effects. Defects that prevent cortisol from being synthesized are referred to collectively as congenital adrenal hyperplasia. Because the enzymes required for cortisol biosynthesis in the adrenal cortex are in many cases required for the synthesis of mineralocorticoids and/or sex steroids, these classes of steroids may also not be synthesized normally. Thus, cholesterol desmolase and 3 beta-hydroxysteroid dehydrogenase deficiencies affect synthesis of all classes of steroids in both the adrenals and gonads. Steroid 21-hydroxylase deficiency, the most common cause (> 90% of cases) of congenital adrenal hyperplasia, can affect both mineralocorticoid and glucocorticoid synthesis, but androgen secretion is usually abnormally high due to shunting of accumulated precursors into this pathway. Excessive secretion of androgens and mineralocorticoids occurs in 11 beta-hydroxylase deficiency (the second most frequent form of congenital adrenal hyperplasia). Mineralocorticoid excess is also seen in 17 alpha-hydroxylase deficiency, but in this disorder sex steroid synthesis is defective. All defects that affect estrogen synthesis (deficiencies of cholesterol desmolase, 3 beta-hydroxysteroid dehydrogenase, 17 alpha-hydroxylase, aromatase, and 17 beta-hydroxysteroid dehydrogenase) are very rare, suggesting that the inability to synthesize placental estrogens may adversely affect fetal survival. A number of enzymes are expressed at sites of steroid action and regulate the amount of active steroid available to steroid receptors. Steroid 5 alpha-reductase converts testosterone to the more active dihydrotestosterone. Deficiency of this activity leads to incomplete development of male genitalia; 17 beta-hydroxysteroid dehydrogenase deficiency has similar phenotypic effects.(ABSTRACT TRUNCATED AT 400 WORDS)

The human peripheral benzodiazepine receptor gene: cloning and characterization of alternative splicing in normal tissues and in a patient with congenital lipoid adrenal hyperplasia

The mitochondrial benzodiazepine receptor (mBzR) appears to be a key factor in the flow of cholesterol into mitochondria to permit the initiation of steroid hormone synthesis. The mBzR consists of three components; the 18-kDa component on the outer mitochondrial membrane appears to contain the benzodiazepine binding site, and is hence often termed the peripheral benzodiazepine receptor (PBR). Using a cloned human PBR cDNA as probe, we have cloned the human PBR gene. The 13-kb gene is divided into four exons, with exon 1 encoding only a short 5' untranslated segment. The 5' flanking DNA lacks TATA and CAAT boxes but contains a cluster of SP-1 binding sites, typical of "house-keeping" genes. The encoded PBR mRNA is alternately spliced into two forms: "authentic" PBR mRNA retains all four exons, while a short form termed PBR-S lacks exon 2. While PBR-S contains a 102-codon open reading frame with a typical initiator sequence, the reading frame differs from that of PBR, so that the encoded protein is unrelated to PBR. RT-PCR and RNase protection experiments confirm that both PBR and PBR-S are expressed in all tissues examined and that expression PBR-S is about 10 times the level of PBR. Expression of PBR cDNA in pCMV5 vectors transfected into COS-1 cells resulted in increased binding of [3H]PK11195, but expression of PBR-S did not. It has been speculated that patients with congenital lipoid adrenal hyperplasia, who cannot make any steroids, might have a genetic lesion in mBzR. RT-PCR analysis of testicular RNA from such a patient, sequencing of the cDNA, and blotting analysis of genomic DNA all indicate that the gene and mRNA for the PBR component of mBzR are normal in this disease.

Combined 17 alpha-hydroxylase/17,20-lyase deficiency caused by heterozygous stop codons in the cytochrome P450 17 alpha-hydroxylase gene

OBJECTIVE

To determine the genetic defect underlying congenital adrenal hyperplasia due to 17 alpha-hydroxylase deficiency in a genetic female.

DESIGN

Blood samples were used as a source of genomic DNA. A library of size selected genomic DNA sequences was prepared. In addition, portions of the 17 alpha-hydroxylase gene were amplified by the polymerase chain reaction and the gene products sequenced.

PATIENTS

Samples were obtained from a patient with sexual infantilism, lack of secondary sexual characteristics and hypertension. Streak gonads were found on laparoscopy.

RESULTS

Two point mutations were found, one in exon 3 and one in exon 4 which generate premature stop codons at codons 194 and 239 in place of glutamate and arginine respectively. The mutation in exon 3 has not previously been reported in patients with 17 alpha-hydroxylase deficiency.

CONCLUSION

The protein product of these defective genes could be expected to be severely truncated with no catalytic activity. This is in keeping with the complete lack of cortisol and sex steroid output in this patient. The polymerase chain reaction provides faster access to gene sequence information than previous procedures based on library screening prior to sequencing.

Hybrid enzymes for structure-function analysis of cytochrome P-450 2B11

Previous work has shown that P-450 2B11 is responsible for the unique ability of dogs to metabolize and eliminate certain highly-chlorinated biphenyls such as 2,2',4,4',5,5'-hexachlorobiphenyl (245-HCB), whereas the related P-450 2B forms in rat and rabbit are unable to metabolize the compound to any significant degree. To determine the structural basis for this functional diversity, hybrid enzymes were generated. Success with this approach required a careful choice of second enzyme and common substrate with which to assess the functional integrity of the hybrid proteins. The choices of P-450 2B5 from rabbit as the second enzyme and androstenedione as the substrate were based in part on the finding that P-450 2B11 and P-450 2B5 hydroxylate androstenedione with similar overall activities but distinct profiles. Enzymatic studies with eight hybrid enzymes provided evidence for two regions of P-450 2B11 and 2B5, between residues 95-239 and 240-370, that appear to be involved in defining substrate specificity for androstenedione, and three regions of P-450 2B11, between residues 95-239, 240-370, and 371-494, that contain amino acids necessary for metabolism of 245-HCB. This deliberate approach to the creation of hybrid cytochromes P-450 has generated a series of enzymes that will be central to further structure-function studies of the cytochromes P-450 2B.

Isolation and expression of rainbow trout (Oncorhynchus mykiss) ovarian cDNA encoding 3β-hydroxysteroid dehydrogenase/Δ(5-4)-isomerase

A distinct shift in steroidogenesis from testosterone to 17α-hydroxyprogesterone occurs in the salmonid ovarian thecal cell layers immediately prior to oocyte maturation, and is a prerequisite for the production of 17α,20β-dihydroxy-4-pregnen-3-one (maturation-inducing hormone of salmonid fishes) by granulosa cells during oocyte maturation. 17α-Hydroxylase/17,20-lyase cytochrome P-450 (P-45017α) and 3β-hydroxysteroid dehydrogenase/Δ(5-4)-isomerase (3β-HSD) are the two major steroidogenic enzymes involved in the production of 17α-hydroxyprogesterone and testosterone. Using mammalian cDNA probes, we isolated and characterized full-length cDNAs encoding these two enzymes from a rainbow trout (Oncorhynchus mykiss) ovarian thecal cell cDNA library. The cloning of 2.4-kilobase cDNA encoding P-45017α and transient expression of this clone in nonsteroidogenic monkey kidney tumor COS-1 cells have recently been reported (Sakai et al. 1992). We have isolated a 1.4-kilobase cDNA which is hybridized to the mammalian 3β-HSD cDNAs. Expression of this cDNA in COS-1 cells led to the production of an enzyme which is capable of converting dehydroepiandrosterone to androstenedione. In this study, enzymatic activities and expression of rainbow trout ovarian P-45017α and 3β-HSD are discussed in relation of the steroidogenic shift occurring in the ovarian follicle layers.

Diagnosis and treatment of 17-hydroxylase deficiency

Leading symptoms of 17-hydroxylase/17,20-lyase deficiency in childhood are hypertension and hypokalemia. We found this enzyme defect in 3 phenotypically female siblings aged 12, 15 and 16 years. Two of the sibs have a 46,XY chromosome pattern, the third is genetically female. Pubertal development did not occur. Both of the 46,XY sibs have male internal and female external genitalia. The 46,XX sister has normal female internal genitalia. At the time of diagnosis, two of the three siblings had hypertension (RR between 190/135 and 160/110 mmHg). Two of the three siblings had low serum potassium and metabolic alkalosis. All three patients had excessively high plasma levels of 11-deoxycorticosterone (DOC) and corticosterone. Aldosterone was also elevated whereas plasma renin activity was suppressed. Plasma cortisol and its 17-hydroxylated precursors were low, as were plasma testosterone, dihydroepiandrosterone sulphate and estradiol, while the gonadotropins LH and FSH were elevated in all three patients. We studied the steroid profiles of these three patients during long term glucocorticoid treatment with dexamethasone, which is now followed for 13 years. Blood pressure and serum potassium became normal. Plasma aldosterone, corticosterone and DOC were clearly lower but not fully normalized. The two genetically male sisters obtained estrogens for induction of female secondary sex characteristics. The third 46,XX sister has normal menstruations during substitution with cyclic estrogen/gestagen therapy. All three patients lack pubic and axillary hair, and reached normal adult heights both for phenotypic sex and for target height. The psychosocial orientation is female in all of them. Apart from rare reports of development of malignant hypertension, prognosis is better than in other enzyme deficiencies causing congenital adrenal hyperplasia since no Addisonian crises occur due to DOC and corticosterone overproduction resulting in apparently normal endogenous glucocorticoid activity.

The P450 superfamily: update on new sequences, gene mapping, accession numbers, early trivial names of enzymes, and nomenclature

We provide here a list of 221 P450 genes and 12 putative pseudogenes that have been characterized as of December 14, 1992. These genes have been described in 31 eukaryotes (including 11 mammalian and 3 plant species) and 11 prokaryotes. Of 36 gene families so far described, 12 families exist in all mammals examined to date. These 12 families comprise 22 mammalian subfamilies, of which 17 and 15 have been mapped in the human and mouse genome, respectively. To date, each subfamily appears to represent a cluster of tightly linked genes. This revision supersedes the previous updates [Nebert et al., DNA 6, 1-11, 1987; Nebert et al., DNA 8, 1-13, 1989; Nebert et al., DNA Cell Biol. 10, 1-14 (1991)] in which a nomenclature system, based on divergent evolution of the superfamily, has been described. For the gene and cDNA, we recommend that the italicized root symbol "CYP" for human ("Cyp" for mouse), representing "cytochrome P450," be followed by an Arabic number denoting the family, a letter designating the subfamily (when two or more exist), and an Arabic numeral representing the individual gene within the subfamily. A hyphen should precede the final number in mouse genes. "P" ("p" in mouse) after the gene number denotes a pseudogene. If a gene is the sole member of a family, the subfamily letter and gene number need not be included. We suggest that the human nomenclature system be used for all species other than mouse. The mRNA and enzyme in all species (including mouse) should include all capital letters, without italics or hyphens. This nomenclature system is identical to that proposed in our 1991 update. Also included in this update is a listing of available data base accession numbers for P450 DNA and protein sequences. We also discuss the likelihood that this ancient gene superfamily has existed for more than 3.5 billion years, and that the rate of P450 gene evolution appears to be quite nonlinear. Finally, we describe P450 genes that have been detected by expressed sequence tags (ESTs), as well as the relationship between the P450 and the nitric oxide synthase gene superfamilies, as a likely example of convergent evolution.

Molecular basis of 17α-hydroxylase/17,20-lyase deficiency

17α-Hydroxylase deficiency is characterized by a defect in either or both of 17α-hydroxylase and 17,20-lyase activities, based on the fact that a single polypeptide P450c17 can catalyze both reactions. The clinical manifestations of 17α-hydroxylase/17,20-lyase deficiency seem to be more heterogeneous than expected, varying from the classical type to less symptomatic forms as also observed in 21-hydroxylase deficiency. We have sequenced all eight exons of the CYP17 (P450c17) gene in DNA from several patients, reconstructed the mutations in a human P450c17 cDNA and expressed the mutant P450c17 in COS 1 cells to characterize the kinetic properties of 17α-hydroxylase and 17,20-lyase activities. The molecular bases of cases clinically reported as 17α-hydroxylase deficiency have turned out to be complete or partial combined deficiencies of 17α-hydroxylase/17,20-lyase. The elucidation of the molecular basis generally explains the patient's clinical profiles including the sexual phenotype of the external genitalia. In one case clinically reported as isolated 17,20-lyase deficiency, the molecular basis was found to be partial combined deficiency of both activities, somewhat discordant with the patient's clinical profile. Based on the results obtained so far we can predict that those 17α-hydroxylase deficient individuals having a homozygous stop codon in the CYP17 gene positioned at the amino terminal side of the P450c17 heme-binding cysteine (442) will all have the same phenotype. However those individuals having homozygous missense mutations or those who are compound heterozygotes having a missense mutation in at least one CYP17 allele will display their own unique phenotype which clinically will be subtly different from all others.

Molecular basis of apparent isolated 17,20-lyase deficiency: compound heterozygous mutations in the C-terminal region (Arg(496)----Cys, Gln(461)----Stop) actually cause combined 17 alpha-hydroxylase/17,20-lyase deficiency

The molecular defect in a reported case of isolated 17,20-lyase deficiency in a 46XY individual has been elucidated. The patient was found to be a compound heterozygote, carrying two different mutant alleles in the CYP17 gene. One allele contains a point mutation of arginine (CGC) to cysteine (TGC) at amino acid 496 in exon 8. The second allele contains a stop codon (TAG) in place of glutamine (CAG) at position 461 in exon 8 which is located 19 amino acids to the carboxy-terminal side of the P-450(17) alpha heme binding cysteine. COS-1 cells transfected with cDNAs containing one or the other of these mutations showed dramatically reduced 17 alpha-hydroxylase and 17,20-lyase activities relative to cells transfected with the wild type P-450(17) alpha cDNA. While the in vitro data in COS 1 cells can explain the patient's physical phenotype, with female external genitalia, it was somewhat discordant with the clinical expression of isolated 17,20-lyase deficiency with relative preservation of 17 alpha-hydroxylase activity in vivo. In addition to the expression studies of these two examples of mutants in the C-terminal region of cytochrome P-450(17) alpha, a third mutant cDNA construct containing a 4-base duplication at codon 480 previously found in patients with combined 17 alpha-hydroxylase/17,20-lyase deficiency was also expressed in COS-1 cells. This expressed protein was completely inactive with respect to both activities, supporting the biochemical findings in serum and in vitro biochemical data obtained using a testis from the patient. The results from these patients clearly indicate the importance of the C-terminal region of human P-450(17) alpha in its enzymatic activities.

Rainbow trout cytochrome P-450c17 (17 alpha-hydroxylase/17,20-lyase). cDNA cloning, enzymatic properties and temporal pattern of ovarian P-450c17 mRNA expression during oogenesis

A cDNA clone encoding cytochrome P-450c17 (17 alpha-hydroxylase/17,20-lyase) was isolated from a rainbow trout ovarian follicle cDNA library. The cDNA contained an open reading frame of 1,542 nucleotides encoding a protein of 514 amino acid residues. The amino acid sequence of trout P-450c17 shows a much greater homology with chicken P-450c17 than with that of human, bovine and rat. The trout P-450c17 expressed in non-steroidogenic mammalian COS-1 cells showed both 17 alpha-hydroxylase and 17,20-lyase activities. The cDNA only hybridized to a single species of mRNA (2.4 kb) isolated from rainbow trout ovaries; the 2.4 kb transcripts were abundant in trout ovaries during the later stages of oogenesis.