Mutation R96W in cytochrome P450c17 gene causes combined 17 alpha-hydroxylase/17-20-lyase deficiency in two French Canadian patients

High incidence of hypertension-mediated organ damage in a series of Chinese patients with 17α-hydroxylase deficiency

OBJECTIVE

To analyze the prevalence of hypertension-mediated organ damage (HMOD) and its relationship with enzyme activity of mutant CYP17A1 and other risk factors in patients with 17α-hydroxylase/17,20-lyase deficiency (17-OHD).

METHODS

A total of 68 patients with 17-OHD were recruited in the Peking Union Medical College Hospital from 2003 to 2021. The incidence of hypertension and HMOD was respectively analyzed. CYP17A1 sequencing was performed and the enzyme activity of mutant CYP17A1 was determined by analyzing the characteristics of mutation itself and the functional data reported previously. A logistic regression model was employed to analyze the factors related to HMOD and the specific damaged organs in 17-OHD patients.

RESULT(S)

Sixty-five patients (95.6%) exhibited hypertension, 32 of whom were diagnosed with HMOD. c.985_987delTACinsAA (p.Y329KfsX418) (53.8%) and c.1459_1467del (p. del D487_F489) (11.4%) were the top two mutations, and no correlation was found between enzyme activity of mutant CYP17A1 and HMOD. The risk of HMOD increased by 32% for each additional year of hypertension duration, 10.2-fold for each one-grade increase in hypertension level, 2.3-fold for each grade of exacerbation of hypokalemia.

CONCLUSION

Patients with 17-OHD experience a high incidence of HMOD. There was no correlation between the HMOD occurrence and enzyme activity of mutant CYP17A1. Longer duration of hypertension, more severe hypertension, and hypokalemia were independent risk factors for the occurrence of HMOD in patients with 17-OHD.

An Asian case of combined 17α-hydroxylase/17,20-lyase deficiency due to homozygous p.R96Q mutation: A case report and review of the literature

BACKGROUND

Combined 17α-hydroxylase/17,20-lyase deficiency (17-OHD) is a very rare form of congenital adrenal hyperplasia (CAH) caused by mutations in the CYP17A1 gene. Almost 100 different mutations of the CYP17A1 gene have been reported, including p.R96Q mutation, but no case of p.R96Q mutation has been described in Asian populations.

CASE PRESENTATION

We describe a 22-year-old female patient of 46,XY karyotype, who presented with pseudohermaphrodism, primary amenorrhea, underdeveloped secondary sexual characteristics, delayed epiphyseal healing, hypertension, and hypokalemia. The diagnosis of 17-OHD was reached by measurement of steroid hormones and abdominal CT scan and confirmed by genetic sequencing, which revealed a homozygous p.R96Q missense mutation in the CYP17A1 gene. The patient received treatment with dexamethasone and estradiol, and 4 months of follow-up showed that both blood pressure and potassium were well controlled.

CONCLUSIONS

This is the first Asian case of CAH caused by a homozygous p.R96Q missense mutation in the CYP17A1 gene. Herein, we highlight the role of inguinal hernia in the early diagnosis of female 17-OHD and the necessity of removing the ectopic testis.

Clinical and Genetic Characteristics of 17 α-Hydroxylase/17, 20-Lyase Deficiency: c.985_987delTACinsAA Mutation of CYP17A1 Prevalent in the Chinese Han Population

OBJECTIVE

17 α-hydroxylase/17, 20-lyase deficiency (17-OHD) is a rare recessive hereditary disease that can be attributed to cytochrome P450 17 α-hydroxylase deficiency caused by CYP17A1 gene mutations.

METHODS

A large cohort of 10 Chinese Han patients with 17-OHD from 2012 to 2020 were enrolled. The clinical and biochemical features were investigated, and genetic mutations of CYP17A1 were analyzed by polymerase chain reaction-Sanger sequencing. Karyotype identification and the SRY gene test were also carried out. In silico analysis was used to predict the effects of genetic mutations on the protein function.

RESULTS

All patients were female. Common complaints were hypertension, hypokalemia, and primary amenorrhea. The karyotype was 46, XY, and the SRY gene was detected in 7 patients; the karyotype was XX in the remaining 3 patients. A total of 7 mutations including Y329N, Y329X, Y329Lfs∗, R96W, A82D, S380N, and A487_P489del have been identified in the CYP17A1 gene. The Y329Lfs∗ mutation was found in 9/10 (90%) of patients with a high allele frequency of 70%. In silico prediction showed that a novel variant of c.1139G>A (S380N) occurs at a conserved residue and can cause disease.

CONCLUSION

We presented a detailed description of the clinical and genetic characteristics in Chinese patients with 17-OHD and concluded that Y329Lfs∗ mutation of CYP17A1 is prevalent in the Chinese Han population. Therefore, hotspot screening by polymerase chain reaction-Sanger sequencing for exon 6 of CYP17A1 could contribute to the rapid diagnosis of 17-OHD in China. Genetic counseling based on the genetic diagnosis for at-risk relatives is advised.

Mutations in PIK3C2A cause syndromic short stature, skeletal abnormalities, and cataracts associated with ciliary dysfunction

PIK3C2A is a class II member of the phosphoinositide 3-kinase (PI3K) family that catalyzes the phosphorylation of phosphatidylinositol (PI) into PI(3)P and the phosphorylation of PI(4)P into PI(3,4)P2. At the cellular level, PIK3C2A is critical for the formation of cilia and for receptor mediated endocytosis, among other biological functions. We identified homozygous loss-of-function mutations in PIK3C2A in children from three independent consanguineous families with short stature, coarse facial features, cataracts with secondary glaucoma, multiple skeletal abnormalities, neurological manifestations, among other findings. Cellular studies of patient-derived fibroblasts found that they lacked PIK3C2A protein, had impaired cilia formation and function, and demonstrated reduced proliferative capacity. Collectively, the genetic and molecular data implicate mutations in PIK3C2A in a new Mendelian disorder of PI metabolism, thereby shedding light on the critical role of a class II PI3K in growth, vision, skeletal formation and neurological development. In particular, the considerable phenotypic overlap, yet distinct features, between this syndrome and Lowe’s syndrome, which is caused by mutations in the PI-5-phosphatase OCRL, highlight the key role of PI metabolizing enzymes in specific developmental processes and demonstrate the unique non-redundant functions of each enzyme. This discovery expands what is known about disorders of PI metabolism and helps unravel the role of PIK3C2A and class II PI3Ks in health and disease.

Identifying the genetic basis of rare disorders can provide insight into gene function, susceptibility to disease, guide the development of new therapeutics, improve opportunities for genetic counseling, and help clinicians evaluate and potentially treat complicated clinical presentations. However, it is estimated that the genetic basis of approximately one-half of all rare genetic disorders remains unknown. We describe one such rare disorder based on genetic and clinical evaluations of individuals from 3 unrelated consanguineous families with a similar constellation of features including short stature, coarse facial features, cataracts with secondary glaucoma, multiple skeletal abnormalities, neurological manifestations including stroke, among other findings. We discovered that these features were due to deficiency of the PIK3C2A enzyme. PIK3C2A is a class II member of the phosphoinositide 3-kinase (PI3K) family that catalyzes the phosphorylation of the lipids phosphatidylinositol (PI) into PI(3)P and the phosphorylation of PI(4)P into PI(3,4)P2 that are essential for a variety of cellular processes including cilia formation and vesicle trafficking. This syndrome is the first monogenic disorder caused by mutations in a class II PI3K family member and thus sheds new light on their role in human development.

Congenital adrenal hyperplasia causing hypertension: an illustrative review

Congenital adrenal hyperplasia (CAH) is often considered a pediatric endocrinology condition, but we present two cases of young adults who presented with hypertension. An 18-year-old woman was found to have hypertension and hypokalemia when she presented for gonadectomy for 46, XY gonadal dysgenesis. She was subsequently found to have low cortisol, elevated progesterone, and elevated aldosterone. Genetic testing confirmed 17-alpha hydroxylase deficiency (17OHD). Her case was unique in that 17OHD usually presents with hypoaldosteronism. We also present the case of a 15-year-old man (46, XX) with resistant hypertension due to 11-beta hydroxylase deficiency (11OHD) who underwent bilateral adrenalectomy for control of hypertension. The relevant literature is reviewed including the pathophysiology, clinical presentation, and treatment of the hypertensive variants of congenital adrenal hyperplasia. We also review the unique areas of hyperaldosteronism in 17OHD and the use of bilateral adrenalectomy for control of hypertension in CAH.

Delayed Diagnosis of a 17-Hydroxylase/17,20-Lyase Deficient Patient Presenting as a 46,XY Female: A Low Normal Potassium Level Can Be an Alerting Diagnostic Sign

17-hydroxylase/17,20-lyase deficiency (17-OHD), a rare autosomal recessive defect in adrenal and gonadal steroidogenesis, causes absence of secondary sexual characteristics and frequently associated with hypertension and hypokalemia. Here, we report a 46,XY case who had normal potassium levels and no hypertension. Our patient was a 2.5-year-old female admitted with female external genitalia and inguinal swelling. Pathology of biopsy revealed that this gonad was a testis. Karyotype was 46,XY. She had no hypertension and no hypokalemia. Serum luteinizing hormone and follicle-stimulating hormone levels were high; testosterone, dehydroepiandrosterone sulfate, and androstenedione were low. Human chorionic gonadotrophin stimulation resulted in partial testosterone response. She was initially diagnosed as partial gonadal dysgenesis or testosterone synthesis defect. In her follow-up after noticing low normal potassium levels at age 9 years, progesterone level was measured and detected to be high. Adrenocorticotropic hormone-stimulated steroid measurements were consistent with 17-OHD. Genetic analyses revealed p. R96Q (c.287G>A) homozygous mutation on exon 1 of CYP17A1 gene. In conclusion, evaluation of 46,XY disorder of sex development patients must include serum potassium levels, and near low levels of potassium levels should also suggest 17-OHD despite absence of hypertension or remarkable hypokalemia. Testosterone synthesis defects must be excluded before establishing the diagnosis of partial gonadal dysgenesis.

Clinical and molecular manifestation of fifteen 17OHD patients: a novel mutation and a founder effect

17-hydroxylase deficiency (17OHD) has long been regarded as a rare form of congenital adrenal hyperplasia, inherited in an autosomal recessive pattern. Fifteen patients with 17OHD were described from clinical manifestations and hormone profile. Then, CYP17A1 gene was amplified and sequenced in a new patient. Heterozygous mutations c. 431_433del, p.K144del/c. 985_987delinsAA, p.Y329 fs were identified. Functional study indicated the novel mutation K144del completely abolished enzyme activity. In the three-dimensional model, the K144del mutation completely destroyed the alpha helix in the steroid binding domain. Sixteen SNPs within CYP17A1 gene were selected and genotyped in 7 unrelated families to determine whether Y329 fs had founder effect in China. Haplotyping study showed that all c. 985_987delinsAA mutation shared the same haplotype. However, from GWAS data of 2760 controls, this special haplotype was found only in one chromosome. In conclusion, we identified a novel (K144del) and a widely reported (Y329 fs) heterozygous mutations of CYP17A1 gene from a 17OHD patient. Haplotyping analysis showed the common mutation Y329 fs in China came from the same ancestor, which explains the reason that 17OHD was the second cause of CAH in China.

New, recurrent, and prevalent mutations: Clinical and molecular characterization of 26 Chinese patients with 17alpha-hydroxylase/17,20-lyase deficiency

BACKGROUND

Combined 17alpha-hydroxylase/17,20-lyase deficiency (17OHD), caused by mutations in the CYP17A1 gene, is a rare autosomal recessive form of congenital adrenal hyperplasia and characterized by hyporeninemic hypokalemic hypertension, primary amenorrhea and absence of secondary sexual characteristics.

SUBJECTS AND METHODS

Twenty six 17OHD subjects from 23 Chinese families were recruited. The CYP17A1 gene was sequenced and 17alpha-hydroxylase/17,20-lyase enzymatic activities were assessed in vitro.

RESULTS

Eight CYP17A1 mutations were identified in 23 patients. Of eight mutations, c.985_987delinsAA/p.Y329Kfs and c.1460_1469del/p.D487_F489del mutations accounted for 60.8% (28/46) and 21.7% (10/46) of the mutant alleles, respectively. The enzymatic activities for both mutations were completely abolished. We also identified three novel mutations c.971_972insG/p.K325Afx, c.1464_1466delT/p.F489Sfx and c.1386G>T/p.R462S. The enzymatic activities for c.971_972insG/p.K325Afx and c.1464_1466delT/p.F489Sfx mutations were almost completely abolished, whereas the mutation c.1386G>T/p.R462S only resulted in partial reduction of 17alpha-hydroxylase (34.6%) and 17,20 lyase activities (27.0%), which is correlated with the partial 17OHD phenotype in this patient.

CONCLUSION

The c.985_987delinsAA/p.Y329Kfs and c.1460_1469del/p.D487_F489del mutations are prevalent in Chinese 17OHD patients. The genetic defects are well correlated with the phenotypes in both complete and partial forms of 17OHD.

A family-based association study identified CYP17 as a candidate gene for obesity susceptibility in Caucasians

The cytochrome P450c17α gene (CYP17) encodes a key biosynthesis enzyme of estrogen, which is critical in regulating adipogenesis and adipocyte development in humans. We therefore hypothesized that CYP17 is a candidate gene for predicting obesity. In order to test this hypothesis, we performed a family-based association test to investigate the relationship between the CYP17 gene and obesity phenotypes in a large sample comprising 1873 subjects from 405 Caucasian nuclear families of European origin recruited by the Osteoporosis Research Center of Creighton University, USA. Both single SNPs and haplotypes were tested for associations with obesity-related phenotypes, including body mass index (BMI) and fat mass. We identified three SNPs to be significantly associated with BMI, including rs3740397, rs6163, and rs619824. We further characterized the linkage disequilibrium structure for CYP17 and found that the whole CYP17 gene was located in a single-linkage disequilibrium block. This block was observed to be significantly associated with BMI. A major haplotype in this block was significantly associated with both BMI and fat mass. In conclusion, we suggest that the CYP17 gene has an effect on obesity in the Caucasian population. Further independent studies will be needed to confirm our findings.

Combined 17α-hydroxylase/17,20-lyase deficiency due to p.R96W mutation in the CYP17 gene in a Brazilian patient

Congenital adrenal hyperplasia (CAH) resulting from 17α-hydroxylase/17,20-lyase deficiency is a rare autosomal recessive disease and the second most common form of CAH in Brazil. We describe the case of a Brazilian patient with CYP17 deficiency (17α-hydroxylase/17,20-lyase deficiency) caused by a homozygous p.R96W mutation on exon 1 of the CYP17 gene, an unusual genotype in Brazilian patients with this form of CAH. The patient, raised as a normal female, sought medical care for lack of pubertal signs and primary amenorrhea at the age of 16 years. At evaluation, the presence of a 46,XY karyotype, hypertension and hypokalemia were observed. We emphasize the recognition of CYP17 deficiency in the differential diagnosis of cases of hypergonadotrophic hypogonadism and hypertension in young patients who need specific treatment for both situations.

17alpha-hydroxylase deficiency : biochemical and molecular findings in two sisters and their family

OBJECTIVE

To search for molecular changes in two Argentinian sisters with a clinical and biochemical diagnosis of 17alpha-hydroxylase deficiency.

SUBJECTS

Both patients had 46 XX karyotype, with sexual infantilism, primary amenorrhea, and hypertension. Other member of the first degree family did not have this deficiency. HORMONAL RESULTS: The patients showed high levels of gonadotrophins and progesterone along with very low cortisol and androgen levels. Basal levels of corticosterone were very high, but aldosterone was normal. Both steroids had a high response after adrenocorticotropic hormone (ACTH) stimulation, with no changes in 17-hydroxyl progesterone and cortisol levels. Progesterone, corticosterone, and aldosterone decreased with the dexamethasone test, without modifications in 17-hydroxyl progesterone and cortisol levels. A corticosterone/aldosterone ratio was calculated from the results of the stimulation test; the ratios were similar in both patients. On administration of the ACTH test, both parents and one sister (S2) showed a marked response in corticosterone levels, their corticosterone/aldosterone ratios were also similar to each other and similar to the patients. MOLECULAR RESULTS: Molecular studies in the cytochrome P450 17 (CYP17) gene showed that exon 8 had a 4 bp duplication at codon 480 (CATC) in the two patients and their mother and in exon 1, a C to T transition at codon 96 was identified, changing CGG into TGG in the two patients, S2, and their father.

CONCLUSIONS

Both patients were shown to be compound heterozygous, carrying different alleles in exon 1 and exon 8, inherited from their father and mother, respectively. The molecular results obtained on S2 confirmed the heterozygosity suggested by the stimulated hormonal test and corticosterone/aldosterone ratio.

Functional characterization of mutant CYP17 genes isolated from a 17 alpha-hydroxylase/17,20-lyase-deficient patient

CYP17 has a dual enzymatic activity that is necessary for steroid hormone biosynthesis. It catalyzes the 17 alpha-hydroxylation of progesterone or pregnenolone and also removes an acetyl moiety of hydroxy-progesterone or hydroxypregnenolone by its 17,20-lyase activity to produce androstenedione or dehydroepiandrosterone (DHEA), respectively. We previously isolated a compound heterozygous mutant of CYP17 from a Korean female patient: 1-base deletion and 1-base transversion mutation at 1 allele and 3-base deletion mutation at the other allele. Here we tested the functional activities of these 2 mutant CYP17 alleles using a transfection analysis in COS-1 cells with radiolabeled substrates and thin layer chromatography. Both mutant CYP17 genes lost not only 17 alpha-hydroxylation activity, but also 17,20-lyase activity in this assay system. This nonfunctional nature of 2 mutant CYP17 genes explains the clinical manifestation of a patient who had 17 alpha-hydroxylase deficiency.

P450c17 deficiency in Brazilian patients: biochemical diagnosis through progesterone levels confirmed by CYP17 genotyping

P450c17 deficiency is an autosomal recessive disorder and a rare cause of congenital adrenal hyperplasia characterized by hypertension, hypokalemia, and impaired production of sex hormones. We performed a clinical, hormonal, and molecular study of 11 patients from 6 Brazilian families with the combined 17alpha-hydroxylase/17,20-lyase deficiency phenotype. All patients had elevated basal serum levels of progesterone (1.8-38 ng/ml; 0.57-12 pmol/liter) and suppressed plasma renin activity. CYP17 genotyping identified 5 missense mutations. The compound heterozygous mutation R362C/W406R was found in 1 family, whereas the homozygous mutations R96W, Y329D, and P428L were seen in the other 5 families. The R96W mutation has been described as the cause of p450c17 deficiency in Caucasian patients. The other mutations were not found in 50 normal subjects screened by allele-specific oligonucleotide hybridization (Y329D, R362C, and W406R) or digestion with HphI (P428L) and were recently found in other Brazilian patients. Therefore, we elucidated the genotype of 11 individuals with p450c17 deficiency and concluded that basal progesterone measurement is a useful marker of p450c17 deficiency and that its use should reduce the misdiagnosis of this deficiency in patients presenting with male pseudohermaphroditism, primary or secondary amenorrhea, and mineralocorticoid excess syndrome.

Genetic analysis of the cytochrome P-450c17alpha (CYP17) and aldosterone synthase (CYP11B2) in Japanese patients with 17alpha-hydroxylase deficiency

OBJECTIVE

To determine the clinical and molecular genetic characterization of two Japanese patients with 17alpha-hydroxylase deficiency, we analysed the 17alpha-hydroxylase/17,20-lyase gene (CYP17). Next, to clarify the mechanism of hypoaldosteronism in 17alpha-hydroxylase deficiency, we analysed the expression of aldosterone synthase (CYP11B2) messenger RNA and sequenced CYP11B2 in these patients.

PATIENTS

Patient 1 (46 XY), phenotypically female, sought medical attention for hypertension, amenorrhea and infantile genitalia. Patient 2 (46 XX), phenotypically female, presented for hypertension and amenorrhea. Hormonal data in both patients showed decreased levels of sex steroids, cortisol, aldosterone and plasma renin activity and extreme elevation of deoxycortisol.

DESIGN

Direct sequencing of CYP17 and CYP11B2 was performed using genomic DNA from the patients. An expression studies of mutated forms of CYP17 was performed using COS-1 cells. The expression of CYP11B2 messenger RNA in mononuclear leucocytes (MNLs) of these patients and normal subjects was measured using the competitive polymerase chain reaction

METHOD

The effect of renin secretion stimulation on the levels of CYP11B2 messenger RNA in MNLs of normal subjects was also studied.

RESULTS

We detected two novel genetic defects in 17alpha-hydroxylase. Sequence analysis revealed one base pair deletion (T) at codon 243 in exon 4 in patient 1. CYP17 in patient 2 contained a point mutation (C to T) at position 415 in exon 8. Transfected cells of mutant from patient 1 had no 17alpha-hydroxylase or 17,20-lyase activity. The R415C mutant protein showed very weak activity of 17alpha-hydroxylase or 17,20-lyase activity. In the renin secretion stimulating test, the increase in CYP11B2 messenger RNA levels in MNLs was parallel with that of plasma aldosterone concentration. The expression of CYP11B2 mRNA in NMLs of these patients was lower compared to controls. No mutations in CYP11B2, including the 5' flanking region, were found.

CONCLUSIONS

These results indicate that the novel mutations of the CYP17 gene found in these patients inactivate cytochrome P450c17 function, and that hypoaldosteronism in these patients may be partly explained by a decreased activity of aldosterone synthase, which is regulated at the transcriptional level.

O espectro das síndromes de hipertensão esteróide na infância e adolescência

Hipertensão arterial não é privilégio de adultos. Além de causas renais e vasculares, doenças adrenocorticais ou correlatas devem ser consideradas na investigação da criança e adolescente hipertensos. O receptor mineralocortidóide (MC) pode ser ativado tanto por MC típicos como pelo cortisol, e mesmo funcionar de maneira autônoma, decorrente de distúrbio nos canais de sódio. Assim, hiperatividade MC (hipertensão, hipocalemia e supressão de renina) pode resultar do excesso de: (1) aldosterona, (2) deoxicorticosterona (DOC) e (3) cortisol. O primeiro grupo, denominado hiperaldosteronismo primário (HAP), inclui o adenoma, o carcinoma e a hiperplasia produtora de aldosterona, além de causa familiares: HA supressível por dexametasona (ou tipo I) e o tipo II. O segundo grupo engloba os tumores produtores tanto de DOC, como de andrógenos ou estrógenos, e a produção de DOC secundária ao excesso de ACTH (síndrome de Cushing, hiperplasia adrenal congênita por deficiência de 11beta- e 17alfa-hidroxilases e síndrome de resistência periférica ao cortisol). Na síndrome do excesso aparente de MC, cortisol age como um MC graças à deficiência congênita ou à inibição (pelo alcaçuz) da enzima 11beta-hidroxisteróide desidrogenase, responsável pela oxidação do cortisol em cortisona. Sódio e fluidos podem ser absorvidos nos túbulos renais de forma inapropriada, tanto na síndrome de Liddle (mutações ativadoras do gene do canal epitelial de sódio) como na de Arnold-Healy-Gordon (onde a hiperreabsorção de cloretos e sódio no túbulo renal impede a excreção de H+ e K+, produzindo hipertensão com acidose e hipercalemia). Todo este espectro de doenças adrenais hipertensivas, apesar de pouco prevalentes, deve ser lembrado com possível causa da hipertensão que pode ocorrer na infância e adolescência.

Compound heterozygous mutations (PHE53/54DEL and HIS373LEU) of the P450c17 gene result in a 17alpha-hydroxylase/17,20-lyase deficient male pseudohermaphrodite with unambiguous external genitalia

The autosomal recessive disease 17alpha-hydroxylase/17,20-lyase deficiency is characterized by mutation of the P450c17 enzyme, which catalyzes 17alpha-hydroxylation and 17,20-lysis in the steroidogenic pathways. Although 17 mutations of this enzyme have been reported, only a few of them resulted in a completely unambiguous phenotype of female external genitalia in 46, XY individuals. We report here a Japanese patient with a 46,XY karyotype, who showed such a unambiguous female external genitalia. Nucleotide sequencing of the P450c17 gene revealed the patient to be a compound heterozygote carrying two different mutations (PHE53/54DEL in exon 1 and HIS373LEU in exon 6). As these mutations have been previously detected in unrelated Japanese patients, it is confirmed that these mutations accumulate regionally. Since these mutations could be screened by a multiple genotyping method, the method is applicable when 17alpha-hydroxylase/17, 20-lyase deficiency is suspected in Japanese patients.

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.