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      The Aromatase Cytochrome P-450 and Its Clinical Impact

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          Cytochrome P-450 aromatase (P450arom), the key enzyme for estrogen biosynthesis, is encoded by a single gene, namely the CYP19 gene, localized on 15q21.2. The human CYP19 gene spans about 123 kb with a coding region of 9 exons (about 30 kb, exon II–exon X). Although there are a number of alternative first exons and nine different transcriptional start sides with individual promoters that permit tissue-specific regulation of expression, the protein expressed in these various tissue sites (placenta, adipose tissue, brain, bone, ovary, etc.) is the same regardless of the promoter used. P450arom catalyzes the conversion of testosterone to estradiol, of androstenedione to estrone, and of 16α-hydroxylated dehydroepiandrosterone to estriol. As not only androgens but also estrogens are of importance, particularly in the male pubertal development, including bone changes which were classically considered mostly androgen dependent, the features of the aromatase deficiency syndrome in affected boys and girls as well as adult males and females are discussed. There is growing awareness that androgens and estrogens have general metabolic roles that reach far beyond reproductive processes. For instance, estrogen has a significant impact on carbohydrate and lipid metabolism, vascular function, and arteriosclerosis. In addition, extragonadal estrogen biosynthesis plays an important but often underestimated physiological and pathophysiological role, for example in breast cancer and endometriosis. Based on that knowledge, progress has been made as far as treatment and follow-up of these disorders are concerned. In addition, there is a focus on the treatment of children suffering from a lack of P450arom activity.

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          Most cited references 16

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          Effect of testosterone and estradiol in a man with aromatase deficiency.

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            Characterization of mice deficient in aromatase (ArKO) because of targeted disruption of the cyp19 gene.

            The formation of estrogens from C19 steroids is catalyzed by aromatase cytochrome P450 (P450arom), the product of the cyp19 gene. The actions of estrogen include dimorphic anatomical, functional, and behavioral effects on the development of both males and females, considerations that prompted us to examine the consequences of deficiency of aromatase activity in mice. Mice lacking a functional aromatase enzyme (ArKO) were generated by targeted disruption of the cyp19 gene. Male and female ArKO mice were born with the expected Mendelian frequency from F1 parents and grew to adulthood. Female ArKO mice at 9 weeks of age displayed underdeveloped external genitalia and uteri. Ovaries contained numerous follicles with abundant granulosa cells and evidence of antrum formation that appeared arrested before ovulation. No corpora lutea were present. Additionally the stroma were hyperplastic with structures that appeared to be atretic follicles. Development of the mammary glands approximated that of a prepubertal female. Examination of male ArKO mice of the same age revealed essentially normal internal anatomy but with enlargement of the male accessory sex glands because of increased content of secreted material. The testes appeared normal. Male ArKO mice are capable of breeding and produce litters of approximately average size. Whereas serum estradiol levels were at the limit of detection, testosterone levels were elevated, as were the levels of follicle-stimulating hormone and luteinizing hormone. The phenotype of these animals differs markedly from that of the previously reported ERKO mice, in which the estrogen receptor alpha is deleted by targeted disruption.
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              Evolution of vertebrate steroid receptors from an ancestral estrogen receptor by ligand exploitation and serial genome expansions.

               J Thornton (2001)
              The evolution of novelty in tightly integrated biological systems, such as hormones and their receptors, seems to challenge the theory of natural selection: it has not been clear how a new function for any one part (such as a ligand) can be selected for unless the other members of the system (e.g., a receptor) are already present. Here I show-based on identification and phylogenetic analysis of steroid receptors in basal vertebrates and reconstruction of the sequences and functional attributes of ancestral proteins-that the first steroid receptor was an estrogen receptor, followed by a progesterone receptor. Genome mapping and phylogenetic analyses indicate that the full complement of mammalian steroid receptors evolved from these ancient receptors by two large-scale genome expansions, one before the advent of jawed vertebrates and one after. Specific regulation of physiological processes by androgens and corticoids are relatively recent innovations that emerged after these duplications. These findings support a model of ligand exploitation in which the terminal ligand in a biosynthetic pathway is the first for which a receptor evolves; selection for this hormone also selects for the synthesis of intermediates despite the absence of receptors, and duplicated receptors then evolve affinity for these substances. In this way, novel hormone-receptor pairs are created, and an integrated system of increasing complexity elaborated. This model suggests that ligands for some "orphan" receptors may be found among intermediates in the synthesis of ligands for phylogenetically related receptors.

                Author and article information

                Horm Res Paediatr
                Hormone Research in Paediatrics
                S. Karger AG
                30 May 2002
                : 57
                : 5-6
                : 145-152
                University Children’s Hospital, Inselspital, Bern, Switzerland
                58374 Horm Res 2002;57:145–152
                © 2002 S. Karger AG, Basel

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                Page count
                Figures: 4, Tables: 1, References: 41, Pages: 8


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