Female Development — All by Default?
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《新英格兰医药杂志》
It has been an indisputable fact that the constitutive sex in mammalian fetal development is female. Furthermore, a functioning ovary is not required for the female phenotype, whereas a testis is mandatory for male development. More than 50 years after Jost performed experiments in rabbit embryos in which castration was followed by testis engraftment, his observations remain a beacon of clarity illuminating the mechanism of fetal sexual differentiation — the physical phenotype consistent with male or female sex.1
Another indisputable fact of mammalian fetal sexual development is the morphogenesis of a bipotential gonad in the genital ridge, dual internal genital ducts, and a common anlagen for the external genitalia. So what factors determine sexual dimorphism? Clearly, the production of an XY zygote at fertilization is the simplest explanation for the development of a bipotential gonad into a testis. The production of müllerian inhibiting substance by Sertoli cells and androgens by Leydig cells in a critical-concentration–dependent and time-dependent manner induces male sexual differentiation by means of a hormone-dependent process. In contrast, a panoply of genes are involved in gonadal development and, hence, sex determination.2 There is compelling evidence that a gene on the short arm of the Y chromosome close to the pseudo-autosomal region — called SRY, for the sex-determining region of the Y chromosome — is the key player in testis development. The translocation of SRY to the X chromosome during paternal meiosis explains testis development in 90 percent of XX males. Mutations in SRY are present in 10 to 15 percent of XY females who have complete gonadal dysgenesis, and the insertion of the corresponding mouse gene sry into XX mouse embryos results in male offspring. A number of other genes are involved in testis determination, such as SOX9, which is induced by SRY; steroidogenic factor 1 (SF1); Wilms' tumor 1 (WT1); and DAX1. Perturbations in the structure or function of these genes or their products cause sex-disorder syndromes in humans. More testis-determining genes have been identified through studies in transgenic mice. However, their relevance to disorders such as XY gonadal dysgenesis in humans remains unknown.
Are there no key genes for human ovarian determination and differentiation of the female reproductive tract? Some syndromes associated with gene duplication cause XY sex reversal. For instance, the duplication of a 160-kb region on the short arm of the X chromosome (Xp21.3) leads to the development of an XY female. Since this dose-sensitive sex locus contains the DAX1 gene, one might ask whether DAX1 is an ovarian-determining gene. Apparently not, since female mice with a mutant dax1 gene have normal ovaries. Studies of dax1 in concert with sry in transgenic mice suggest that DAX1 normally complements the role of SRY in testis determination but can act in an antitestis manner when it is overexpressed. A factor whose role in female sexual development seems more critical is highlighted by the duplication of chromosome 1p31-p35. Within this region lies the WNT4 gene, which encodes one member of a large family of locally acting growth factors that are involved in intracellular signaling.3 The overexpression of WNT4 up-regulates DAX1 and may cause sex reversal by means of the same mechanism to which Xp21 duplication is ascribed.
When the corresponding mouse gene, wnt4, is inactivated, male mice are normal, but females become "masculinized."4 They have ovaries that morphologically resemble testes and contain steroidogenic enzymes that are normally expressed only in the testis in early gestation. Their wolffian ducts are stabilized, and müllerian ducts are absent. Wnt4 is also essential for kidney development, and mice with a mutant wnt4 gene die of kidney failure after birth. These studies indicate that wnt4 has a role in the development of the müllerian ducts and that it inhibits steroid-specific activity in the developing ovary by suppressing Leydig cells. The ovaries of these mice are also depleted of oocytes, suggesting that wnt4 may be involved in the development of female germ cells.
Prompted by these observations in mice with mutant wnt4, Biason-Lauber et al. studied a woman with the Mayer–Rokitansky–Küster–Hauser syndrome in whom a missense mutation was identified in the WNT4 gene; they report their results in this issue of the Journal (pages 792–798). The syndrome is characterized by the absence of a uterus and vagina in an otherwise normal girl or woman. With an estimated frequency of 1 in 5000 girls or women, the Mayer–Rokitansky–Küster–Hauser syndrome is second only to gonadal dysgenesis in the causation of primary amenorrhea. About one third of patients with the syndrome also have renal abnormalities. This young woman with a WNT4 mutation presented with primary amenorrhea and no uterus, vagina, or right kidney, according to findings on magnetic resonance imaging. Endocrine studies showed increased androgen production but no signs of virilization. No direct visualization of the internal genitalia was performed, so it is not possible to comment on the presence of wolffian ducts or on the ovarian morphology. The mutant WNT4 protein containing a Glu226Gly amino acid change was transfected into suitable ovarian and adrenal cell lines and, in contrast to the normal protein, failed to suppress steroidogenic enzyme activity. Thus, this single case illustrates several of the features observed in female mice with mutant wnt4.
Although the specific gene mutation described in this case is indeed a likely cause of the Mayer–Rokitansky–Küster–Hauser syndrome, a relatively common congenital-malformation disorder, this observation actually makes a greater contribution to the general understanding of the genetic control of human fetal sexual development (see Figure). It is unlikely that a high prevalence of WNT4 mutations will be found among patients with the syndrome. However, researchers should now study a large series of carefully characterized patients whose phenotypes — in relation to the development of müllerian and wolffian ducts, renal anomalies, ovarian morphology, and androgen secretion — are known. Astute observations in sex-reversed persons, along with appropriate extrapolation from the results of experiments in transgenic mice, have been the mainstay of research into human fetal sexual development. WNT4 should now be added to a list of genes such as SRY, SOX9, WT1, DAX1, and SF1 whose study in isolated cases of sex reversal has contributed a number of pieces to the jigsaw puzzle of human sexual development. Indeed, perhaps it is now possible to dispense with the unfortunate truism that the female represents the default pathway in mammalian fetal sexual development.
Figure. Possible Roles of Key Genes in Human Fetal Sexual Development.
The development of the genital ridge and bipotential gonad is under similar control in the two sexes. An ovary develops in the absence of SRY and SOX9 action, possibly because of the antitestis effects of DAX1 and WNT4. Steroidogenesis is delayed in the ovary by the action of WNT4, which is also needed for the development of the müllerian ducts. A testis develops as a result of SRY and SOX9 action, complemented by DAX1. The regression of the müllerian ducts is mediated by MIS and its receptor (R), whereas the androgenic stabilization of the wolffian ducts and the differentiation of the external genitalia are mediated by the androgen receptor (AR). The descent of the testes is partially mediated by the insulin-like 3 ligand (Insl3) and its receptor (Lgr8).
Source Information
From the Department of Paediatrics, University of Cambridge, Cambridge, United Kingdom.
References
Jost A. Problems of fetal endocrinology: the gonadal and hypophyseal hormones. Recent Prog Horm Res 1953;8:379-418.
MacLaughlin DT, Donahoe PK. Sex determination and differentiation. N Engl J Med 2004;350:367-378.
Dale TC. Signal transduction by the Wnt family of ligands. Biochem J 1998;329:209-223.
Vainio S, Heikkila M, Kispert A, Chin N, McMahon AP. Female development in mammals is regulated by Wnt-4 signalling. Nature 1999;397:405-409.(Ieuan A. Hughes, M.D.)
Another indisputable fact of mammalian fetal sexual development is the morphogenesis of a bipotential gonad in the genital ridge, dual internal genital ducts, and a common anlagen for the external genitalia. So what factors determine sexual dimorphism? Clearly, the production of an XY zygote at fertilization is the simplest explanation for the development of a bipotential gonad into a testis. The production of müllerian inhibiting substance by Sertoli cells and androgens by Leydig cells in a critical-concentration–dependent and time-dependent manner induces male sexual differentiation by means of a hormone-dependent process. In contrast, a panoply of genes are involved in gonadal development and, hence, sex determination.2 There is compelling evidence that a gene on the short arm of the Y chromosome close to the pseudo-autosomal region — called SRY, for the sex-determining region of the Y chromosome — is the key player in testis development. The translocation of SRY to the X chromosome during paternal meiosis explains testis development in 90 percent of XX males. Mutations in SRY are present in 10 to 15 percent of XY females who have complete gonadal dysgenesis, and the insertion of the corresponding mouse gene sry into XX mouse embryos results in male offspring. A number of other genes are involved in testis determination, such as SOX9, which is induced by SRY; steroidogenic factor 1 (SF1); Wilms' tumor 1 (WT1); and DAX1. Perturbations in the structure or function of these genes or their products cause sex-disorder syndromes in humans. More testis-determining genes have been identified through studies in transgenic mice. However, their relevance to disorders such as XY gonadal dysgenesis in humans remains unknown.
Are there no key genes for human ovarian determination and differentiation of the female reproductive tract? Some syndromes associated with gene duplication cause XY sex reversal. For instance, the duplication of a 160-kb region on the short arm of the X chromosome (Xp21.3) leads to the development of an XY female. Since this dose-sensitive sex locus contains the DAX1 gene, one might ask whether DAX1 is an ovarian-determining gene. Apparently not, since female mice with a mutant dax1 gene have normal ovaries. Studies of dax1 in concert with sry in transgenic mice suggest that DAX1 normally complements the role of SRY in testis determination but can act in an antitestis manner when it is overexpressed. A factor whose role in female sexual development seems more critical is highlighted by the duplication of chromosome 1p31-p35. Within this region lies the WNT4 gene, which encodes one member of a large family of locally acting growth factors that are involved in intracellular signaling.3 The overexpression of WNT4 up-regulates DAX1 and may cause sex reversal by means of the same mechanism to which Xp21 duplication is ascribed.
When the corresponding mouse gene, wnt4, is inactivated, male mice are normal, but females become "masculinized."4 They have ovaries that morphologically resemble testes and contain steroidogenic enzymes that are normally expressed only in the testis in early gestation. Their wolffian ducts are stabilized, and müllerian ducts are absent. Wnt4 is also essential for kidney development, and mice with a mutant wnt4 gene die of kidney failure after birth. These studies indicate that wnt4 has a role in the development of the müllerian ducts and that it inhibits steroid-specific activity in the developing ovary by suppressing Leydig cells. The ovaries of these mice are also depleted of oocytes, suggesting that wnt4 may be involved in the development of female germ cells.
Prompted by these observations in mice with mutant wnt4, Biason-Lauber et al. studied a woman with the Mayer–Rokitansky–Küster–Hauser syndrome in whom a missense mutation was identified in the WNT4 gene; they report their results in this issue of the Journal (pages 792–798). The syndrome is characterized by the absence of a uterus and vagina in an otherwise normal girl or woman. With an estimated frequency of 1 in 5000 girls or women, the Mayer–Rokitansky–Küster–Hauser syndrome is second only to gonadal dysgenesis in the causation of primary amenorrhea. About one third of patients with the syndrome also have renal abnormalities. This young woman with a WNT4 mutation presented with primary amenorrhea and no uterus, vagina, or right kidney, according to findings on magnetic resonance imaging. Endocrine studies showed increased androgen production but no signs of virilization. No direct visualization of the internal genitalia was performed, so it is not possible to comment on the presence of wolffian ducts or on the ovarian morphology. The mutant WNT4 protein containing a Glu226Gly amino acid change was transfected into suitable ovarian and adrenal cell lines and, in contrast to the normal protein, failed to suppress steroidogenic enzyme activity. Thus, this single case illustrates several of the features observed in female mice with mutant wnt4.
Although the specific gene mutation described in this case is indeed a likely cause of the Mayer–Rokitansky–Küster–Hauser syndrome, a relatively common congenital-malformation disorder, this observation actually makes a greater contribution to the general understanding of the genetic control of human fetal sexual development (see Figure). It is unlikely that a high prevalence of WNT4 mutations will be found among patients with the syndrome. However, researchers should now study a large series of carefully characterized patients whose phenotypes — in relation to the development of müllerian and wolffian ducts, renal anomalies, ovarian morphology, and androgen secretion — are known. Astute observations in sex-reversed persons, along with appropriate extrapolation from the results of experiments in transgenic mice, have been the mainstay of research into human fetal sexual development. WNT4 should now be added to a list of genes such as SRY, SOX9, WT1, DAX1, and SF1 whose study in isolated cases of sex reversal has contributed a number of pieces to the jigsaw puzzle of human sexual development. Indeed, perhaps it is now possible to dispense with the unfortunate truism that the female represents the default pathway in mammalian fetal sexual development.
Figure. Possible Roles of Key Genes in Human Fetal Sexual Development.
The development of the genital ridge and bipotential gonad is under similar control in the two sexes. An ovary develops in the absence of SRY and SOX9 action, possibly because of the antitestis effects of DAX1 and WNT4. Steroidogenesis is delayed in the ovary by the action of WNT4, which is also needed for the development of the müllerian ducts. A testis develops as a result of SRY and SOX9 action, complemented by DAX1. The regression of the müllerian ducts is mediated by MIS and its receptor (R), whereas the androgenic stabilization of the wolffian ducts and the differentiation of the external genitalia are mediated by the androgen receptor (AR). The descent of the testes is partially mediated by the insulin-like 3 ligand (Insl3) and its receptor (Lgr8).
Source Information
From the Department of Paediatrics, University of Cambridge, Cambridge, United Kingdom.
References
Jost A. Problems of fetal endocrinology: the gonadal and hypophyseal hormones. Recent Prog Horm Res 1953;8:379-418.
MacLaughlin DT, Donahoe PK. Sex determination and differentiation. N Engl J Med 2004;350:367-378.
Dale TC. Signal transduction by the Wnt family of ligands. Biochem J 1998;329:209-223.
Vainio S, Heikkila M, Kispert A, Chin N, McMahon AP. Female development in mammals is regulated by Wnt-4 signalling. Nature 1999;397:405-409.(Ieuan A. Hughes, M.D.)