DAX-1, an Unusual Orphan Receptor at the Crossroads of Steroidogenic Function and Sexual Differentiation

Enzo Lalli and Paolo Sassone-Corsi

Institut de Génétique et de Biologie Moléculaire et Cellulaire, Centre National de la Recherche Scientifique, Institut National de la Santé et de la Recherche Médicale, Université Louis Pasteur, 67404 Illkirch, Strasbourg, France

Address all correspondence and requests for reprints to: Enzo Lalli, Institut de Génétique et de Biologie Moléculaire et Cellulaire, Centre National de la Recherche Scientifique, Institut National de la Santé et de la Recherche Médicale, Université Louis Pasteur, B.P. 163, 67404 Illkirch, Strasbourg, France. E-mail: ninino{at}igbmc.u-strasbg.fr; or to Paolo Sassone-Corsi at paolosc{at}igbmc.u-strasbg.fr.


    ABSTRACT
 TOP
 ABSTRACT
 INTRODUCTION
 DAX-1 AND THE ADRENAL...
 DAX-1: PROTEIN STRUCTURE AND...
 DAX-1 AS A TRANSCRIPTIONAL...
 THE PATHOGENESIS OF AHC...
 DAX-1 AS A CANDIDATE...
 REGULATION OF DAX-1 EXPRESSION
 FUTURE DIRECTIONS
 REFERENCES
 
The unusual orphan member of the nuclear hormone receptor superfamily DAX-1 (NR0B1) owes its name to its double role in human pathology. On one side, duplications in Xp21, containing the DAX-1 gene, cause phenotypic sex reversal in XY individuals. On the other side, DAX-1 gene mutations are responsible for adrenal hypoplasia congenita, invariably associated with hypogonadotropic hypogonadism. DAX-1 functions as a global negative regulator of steroid hormone production by repressing the expression of multiple genes involved in the steroidogenic pathway. Here we review the mechanism of DAX-1 function in adrenal and gonadal differentiation, with special emphasis on recent results showing the critical role of DAX-1 protein misfolding in the pathogenesis of adrenal hypoplasia congenita.


    INTRODUCTION
 TOP
 ABSTRACT
 INTRODUCTION
 DAX-1 AND THE ADRENAL...
 DAX-1: PROTEIN STRUCTURE AND...
 DAX-1 AS A TRANSCRIPTIONAL...
 THE PATHOGENESIS OF AHC...
 DAX-1 AS A CANDIDATE...
 REGULATION OF DAX-1 EXPRESSION
 FUTURE DIRECTIONS
 REFERENCES
 
IN 1994 A REGION of approximately 160 kb, the duplication of which causes a reversal of the phenotypical sex from male to female in individuals with a normal SRY (male sex determinant) gene, was identified on chromosome Xp21. The clinical syndrome characterized by duplication of this chromosomal region was termed DSS (dosage-sensitive sex reversal) (1). The analysis of genes present in this region of the X chromosome led to the cloning of the DAX-1 gene (2). This gene derives its name by its double role in human pathology, i.e. the DSS syndrome and adrenal hypoplasia congenita (AHC). In fact, mutations in DAX-1 are the cause of the type of AHC linked to the X chromosome (2, 3).


    DAX-1 AND THE ADRENAL GLAND: STEROIDOGENESIS AND GROWTH CONTROL
 TOP
 ABSTRACT
 INTRODUCTION
 DAX-1 AND THE ADRENAL...
 DAX-1: PROTEIN STRUCTURE AND...
 DAX-1 AS A TRANSCRIPTIONAL...
 THE PATHOGENESIS OF AHC...
 DAX-1 AS A CANDIDATE...
 REGULATION OF DAX-1 EXPRESSION
 FUTURE DIRECTIONS
 REFERENCES
 
During primate development, the fetal zone occupies most of the adrenal cortex. This specialized cellular compartment produces steroid hormones, which play an important role in the regulation of feto-placental homeostasis and in fetal lung maturation (4). The fetal zone regresses during the early postnatal period. Subsequent to this phenomenon, proliferation and differentiation of cells laying under the connective capsula (definitive or permanent zone) occur to form the glomerulosa, fasciculata, and reticularis layers of the adult adrenal cortex. The phenomenon of development and regression of a portion of the adrenal cortex has correlates in other mammalian species: in the mouse a transitory cell layer, the X zone, has been described surrounding the medulla, which develops after birth and regresses after sexual maturity. The functional kinship of primate fetal adrenal with mouse X zone, however, is not clear (5).

AHC is a hereditary disease of the adrenal cortex, which in the majority of cases becomes clinically evident as a syndrome of adrenal insufficiency in the first days of life (http://www.ncbi.nlm.nih.gov/htbin-post/Omim/dispmim?300200). Two forms of the disease have been identified based on pathological and inheritance criteria: 1) an X-linked form, which is characterized by the absence of the permanent zone and by the presence of large vacuolated cells, similar to fetal adrenal cells; and 2) a rare autosomal recessive or sporadic form with miniature-type cells.

AHC is a lethal disease if left untreated, because of dehydration and electrolyte imbalance due to mineralocorticoid deficit. Although the disease can present a wide clinical spectrum (6), a constant feature of the X-linked syndrome is the association with hypogonadotropic hypogonadism (HHG). This is diagnosed in AHC patients in whom appropriate hormonal replacement therapy allows survival beyond pubertal age and is caused by combined hypothalamic failure in GnRH release and pituitary defect in gonadotropin production (7). In rare instances HHG may be the presenting or the only symptom in some patients (8, 9). In AHC-associated HHG the onset of gonadotropin production at puberty is selectively impaired, whereas the physiological postnatal transient activation of the hypothalamic-pituitary-gonadal axis is normal in these patients (10, 11).


    DAX-1: PROTEIN STRUCTURE AND EXPRESSION
 TOP
 ABSTRACT
 INTRODUCTION
 DAX-1 AND THE ADRENAL...
 DAX-1: PROTEIN STRUCTURE AND...
 DAX-1 AS A TRANSCRIPTIONAL...
 THE PATHOGENESIS OF AHC...
 DAX-1 AS A CANDIDATE...
 REGULATION OF DAX-1 EXPRESSION
 FUTURE DIRECTIONS
 REFERENCES
 
Mutations in the DAX-1 (NR0B1) gene are responsible for X-linked AHC/HHG (2, 3). A nuclear receptor ligand-binding domain motif is present in the DAX-1 C terminus, even if no ligands are known to bind to the protein, thereby justifying its classification among the orphan receptors. Homology models of the DAX-1 C-terminal domain based on the structures of apo-retinoic X receptor-{alpha} and holo-retinoic acid receptor-{gamma} and thyroid hormone receptor-{alpha} reveal the presence of all features constituting the nuclear receptor ligand-binding domain fold (12, 13). In particular, DAX-1 contains a conserved {Phi}{Phi}XE{Phi}{Phi} motif in the C-terminal H12 helix, which in other nuclear receptors is essential for ligand-dependent transcriptional activation (activation function 2). Conversely, the DAX-1 N terminus is occupied by three repeats of a unique cysteine-rich motif about 70 amino acids long. The number of repeats varies during evolution, only one repeat being present in Dax-1 from nonmammalian species (14, 15, 16, 17).

DAX-1 has an expression pattern restricted to tissues directly involved in steroid hormone production and reproductive function, i.e. adrenal cortex, testicular Leydig and Sertoli cells, ovarian theca and granulosa cells, pituitary gonadotropes, ventromedial hypothalamic nucleus, and possibly some other brain areas (arcuate nuclei, amygdala, hippocampus, cerebral cortex) (18, 19, 20, 21). This pattern overlaps with the expression domains of another orphan nuclear receptor, steroidogenic factor 1 (Ad4BP/SF-1). SF-1 is an important transcriptional activator of a large number of genes involved in steroid hormone production and plays an essential role in the organogenesis of adrenal glands and gonads (22).


    DAX-1 AS A TRANSCRIPTIONAL REGULATOR
 TOP
 ABSTRACT
 INTRODUCTION
 DAX-1 AND THE ADRENAL...
 DAX-1: PROTEIN STRUCTURE AND...
 DAX-1 AS A TRANSCRIPTIONAL...
 THE PATHOGENESIS OF AHC...
 DAX-1 AS A CANDIDATE...
 REGULATION OF DAX-1 EXPRESSION
 FUTURE DIRECTIONS
 REFERENCES
 
DAX-1 works as a negative regulator of SF-1-induced transactivation through a powerful transcriptional repression domain present in its C terminus and overlapping with its nuclear receptor ligand-binding domain motif (12, 23). DAX-1 can repress SF-1 transactivation both by binding to gene promoters regulated by SF-1 [e.g. steroidogenic acute regulatory protein (StAR) and Dax-1 promoters (24)] and by direct interaction with SF-1 via one of the LXXLL motifs present in the DAX-1 N terminus (25). The presence of functional SF-1 binding sites in the Dax-1 promoter (26) and the reduced expression of Dax-1 in Sf-1 null mice (27, 28) lend support to the existence of a negative feedback loop to control SF-1 activity in steroidogenic and reproductive tissues. However, the presence of cells expressing Dax-1 but not SF-1 in different organs (22) suggests that Dax-1 functions extend beyond the regulation of SF-1-dependent genes. In fact, ligand-dependent transactivation by other nuclear receptors is also negatively regulated by DAX-1 (2, 29, 30). Moreover, a role for DAX-1 in posttranscriptional regulations is suggested by its nucleocytoplasmic shuttling, its RNA binding activity, and its association with actively translating polyribosomes as part of a messenger ribonucleoprotein complex in steroidogenic cells (31) (Fig. 1Go).



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Fig. 1. Multifunctional Role of DAX-1

DAX-1 represses ligand-dependent transcriptional activation by retinoic acid receptor-retinoid X receptor (RAR-RXR) heterodimers through interference with their DNA binding. DAX-1 also interacts with SF-1, estrogen receptors (ER) {alpha} and ß, and androgen receptor (AR) via LXXLL motifs in its N-terminal domain. Direct binding of DAX-1 to stem-loop DNA sites in the StAR and the Dax-1 promoters is responsible for repression of these genes. Corepressors are recruited via the DAX-1 C-terminal domain, which is invariably deleted or mutated in AHC. A posttranscriptional role for DAX-1 is suggested by its RNA-binding activity and association with polyribosomes through binding to messenger ribonucleoprotein complexes (mRNPs).

 
Table 1Go lists all gene promoters shown to be down-regulated by DAX-1 by direct or indirect mechanisms (24, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42). The picture emerges of DAX-1 as a global negative regulator of genes involved in steroid hormone production and metabolism in steroidogenic tissues. Modulation of DAX-1 expression by diverse bioactive substances and physiological conditions is likely to represent an important mechanism to control steroid hormone production (43, 44, 45). Interestingly, the hormonal steroidogenic pathway appears to be specifically inhibited by DAX-1 in steroid-producing cells, because the production of other cholesterol-derived steroid compounds with cardiovascular effects (cardiotonic steroids) in adrenocortical Y-1 cells is not repressed by DAX-1 overexpression (46).


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Table 1. Gene Promoters Down-Regulated Directly or Indirectly by DAX-1

 
DAX-1 mutations causing AHC include frameshifts and nonsense and missense mutations (see Ref. 47 for a review). All these mutations alter the protein C terminus, which bears the transcriptional silencing activity, and impair transcriptional repression by DAX-1 (12, 23). Transcriptional repressors play a fundamental role in developmental processes in Drosophila and in other organisms (48). Within the nuclear receptor family, the chicken ovalbumin upstream promoter transcription factors I and II orphans also act as transcriptional repressors and are essential for nervous and cardiovascular system development, respectively (49, 50). Repression of the expression of differentiation genes (StAR, steroid hydroxylases) in the adrenal cortex by DAX-1 may be a prerequisite for proliferation of the definitive adrenocortical zone in the critical postnatal period in humans. We speculate that, in the presence of a DAX-1 null allele, abnormally early expression of genes involved in steroid hormone production is activated in adrenal-definitive zone cells. This process would impair their proliferation and further zonation. Adrenal hypoplasia would then ensue after the physiological regression of the fetal zone (Fig. 2Go).



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Fig. 2. A Model of DAX-1 Function during Human Adrenal Development

DAX-1 represses steroidogenic gene expression in definitive zone cells, allowing for their proliferation and differentiation into glomerulosa, fasciculate, and reticularis zones. In the absence of functional DAX-1, abnormally early expression of steroidogenic genes is activated in the definitive zone and its proliferation is down-regulated. Under these conditions, adrenal hypoplasia follows the physiological regression of the fetal zone.

 
Dax-1 null mice do not manifest adrenal hypoplasia and their serum LH/FSH levels are normal (51). The only anomaly in the adrenal cortex of these animals is failure in regression of their X zone after puberty. However, males are sterile (due to progressive impairment of spermatogenesis), whereas females have normal fertility. The lack of an evident adrenal phenotype in these animals is probably only apparently surprising, given the different structure and ontogenesis of the primate vs. rodent adrenal cortex (4). The human DAX-1 gene may be part of a regulatory circuit involved in the establishment of these morphogenetic differences in the adrenal gland, or other genes may compensate for its function in the mouse. Conversely, DAX-1 function in the testis seems to be conserved between human and mouse species (see below).


    THE PATHOGENESIS OF AHC CAUSED BY DAX-1 MISSENSE MUTANTS: ALTERED SUBCELLULAR LOCALIZATION BY PROTEIN MISFOLDING
 TOP
 ABSTRACT
 INTRODUCTION
 DAX-1 AND THE ADRENAL...
 DAX-1: PROTEIN STRUCTURE AND...
 DAX-1 AS A TRANSCRIPTIONAL...
 THE PATHOGENESIS OF AHC...
 DAX-1 AS A CANDIDATE...
 REGULATION OF DAX-1 EXPRESSION
 FUTURE DIRECTIONS
 REFERENCES
 
Many examples exist of clinical syndromes due to mutations affecting ligand binding in the case of ligand-activated nuclear hormone receptors (52, 53). On the other hand, one argument suggesting that DAX-1 may lack a physiological ligand is that none of the missense mutations identified in AHC patients localizes to positions predicted to contact a putative ligand, but they map to a protein subdomain involved in protein fold stabilization (13).

Based on the analysis of several different DAX-1 missense mutants found in AHC patients, we recently showed that their impairment in transcriptional repression is dependent upon an altered nuclear localization of the mutant protein (54). This is surprising, because the DAX-1 nuclear localization signal (NLS) lies in the protein N-terminal domain and is not affected by the AHC mutations. An inverse relationship exists between the extent of nuclear localization and the transcriptional repression activity of individual DAX-1 AHC mutants. Significantly, the I439S DAX-1 mutation, found in a patient with late-onset adrenal insufficiency and incomplete HHG (55), impairs nuclear localization of the protein only partially and causes an incomplete loss of its transcriptional repression activity. In addition, AHC mutations also impair nuclear localization of GAL4 DNA-binding domain-DAX-1 C terminus fusion proteins, endowed with a heterologous NLS in the GAL4 DNA-binding domain. The integrity of the DAX-1 H12 helix is critical for DAX-1 nuclear localization, as shown by two AHC mutations affecting H12 (M462stop and L466R), which impair nuclear localization (54).

Transcriptional repression by DAX-1 involves interaction with intermediary factors (corepressors) (12) that silence the activity of the basal transcriptional machinery and/or induce chromatin modifications. It has been proposed that interaction with the nuclear receptor corepressor (56) and Alien (57) corepressors is relevant to mediate DAX-1 transcriptional silencing, because this interaction was found to be impaired by DAX-1 AHC mutations, using the mammalian and the yeast two-hybrid systems. However, localization of DAX-1 mutants in the cytoplasm explains the lack of interaction with the nuclear corepressors. In addition, we have directly tested interaction in vitro between DAX-1 and nuclear receptor corepressor and found it to be very weak and relatively insensitive to the introduction of AHC mutations (54). Furthermore, mutagenesis of conserved surface residues in DAX-1, which in other nuclear receptors directly contact corepressors, did not affect its transcriptional silencing properties (58). These data suggest that cofactors distinct from known nuclear receptor corepressors are involved in the mechanism of transcriptional silencing by DAX-1.

Folded proteins have a different sensitivity to protease digestion than unfolded or misfolded polypeptides (59). DAX-1 AHC mutants are significantly more sensitive to limited proteolysis than the wild-type protein (58). This finding indicates that AHC mutations produce protein misfolding, consistent with their localization at the level of residues stabilizing the structure of the DAX-1 C terminus. The case of DAX-1 AHC missense mutations represents an example of protein misfolding as a cause of human disease, a frequent pathogenetic mechanism (59). We suggest that nuclear translocation of the misfolded DAX-1 proteins may be impaired consequent to the exposure of new molecular surfaces mediating interactions with cytoplasmic anchoring sites (Fig. 3Go). Interestingly, misfolding of DAX-1 AHC mutants may also explain their reduced RNA-binding activity (31). Consequently, in addition to transcriptional repression impairment, another important consequence of DAX-1 AHC mutations may be to affect protein nucleo-cytoplasmic shuttling and its association with polyribosomes.



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Fig. 3. Impaired Nuclear Localization of DAX-1 Missense Mutants Found in AHC Patients

A, An NLS present in the DAX-1 N-terminal domain mediates nuclear import of the protein. B, Missense mutations in the DAX-1 C-terminal structural subdomain induce protein misfolding. Misfolded DAX-1 proteins interact with cytoplasmic anchoring sites, preventing nuclear import.

 

    DAX-1 AS A CANDIDATE MODULATOR OF THE STEROIDOGENIC PHENOTYPE OF ADRENOCORTICAL TUMORS
 TOP
 ABSTRACT
 INTRODUCTION
 DAX-1 AND THE ADRENAL...
 DAX-1: PROTEIN STRUCTURE AND...
 DAX-1 AS A TRANSCRIPTIONAL...
 THE PATHOGENESIS OF AHC...
 DAX-1 AS A CANDIDATE...
 REGULATION OF DAX-1 EXPRESSION
 FUTURE DIRECTIONS
 REFERENCES
 
Most adrenocortical tumors are clinically silent and are detected fortuitously during autopsy, surgery, or ultrasound/computed tomography scan investigations (incidentalomas). In a minority of cases, adrenocortical tumors produce symptoms related to mineralocorticoid (Conn syndrome), glucocorticoid (Cushing syndrome), or androgen hyperproduction (60). Excessive steroid hormone production results from disordered expression of specific steroidogenic enzymes, which may be the consequence of the abnormal expression of their transcriptional regulators. Two studies have examined the expression of DAX-1 in adrenocortical tumors. Compared with normal adrenal levels, Reincke et al. (61) found DAX-1 expression elevated in nonfunctioning adenomas, intermediate in cortisol-producing adenomas, and low or absent in aldosterone-producing adenomas and carcinomas. Another study described low DAX-1 expression in cortisol-producing adenomas but elevated expression in two cases of the rare deoxycorticosterone-producing adenomas, where CYP17 expression is very low, and which are clinically evident as a syndrome of mineralocorticoid excess (62). These studies indicate that an inverse correlation exists between DAX-1 expression and steroid hormone production in adrenocortical tumors and that DAX-1 may influence the pattern of steroids produced by the tumor.


    REGULATION OF DAX-1 EXPRESSION
 TOP
 ABSTRACT
 INTRODUCTION
 DAX-1 AND THE ADRENAL...
 DAX-1: PROTEIN STRUCTURE AND...
 DAX-1 AS A TRANSCRIPTIONAL...
 THE PATHOGENESIS OF AHC...
 DAX-1 AS A CANDIDATE...
 REGULATION OF DAX-1 EXPRESSION
 FUTURE DIRECTIONS
 REFERENCES
 
SF-1 plays an important role in regulating Dax-1 expression by binding to at least three functional binding sites in the Dax-1 promoter (26, 27, 28). However, Dax-1 expression is reduced, but not abrogated, in Sf-1 null mice and keeps its specific tissue-restricted pattern (19). This indicates that other factors are involved in Dax-1 gene regulation. One of these is Dax-1 itself, which is able to auto-regulate its own expression by binding to a stem-loop DNA site in its promoter (24). The Wilms’ tumor gene (wt1) is expressed in the embryonic gonad and its (-KTS) protein isoforms are able to activate Dax-1 promoter and transcript expression in transfected cells (63). Wnt-4, a member of the Wnt family of secreted growth factors, is also able to activate Dax-1 expression in transfected TM4 Sertoli cells (64). Consistent with this finding, recent data have been published showing that Dax-1 expression is activated by ß-catenin, which lies downstream in the Wnt signaling pathway, and is reduced in female embryonic gonads from Wnt4 null mice (65).

DAX-1 AND SEXUAL DIFFERENTIATION
As mentioned before, the DAX-1 gene lies inside the critical region in Xp21, the duplication of which causes the DSS syndrome (1). Based upon this finding, a role for DAX-1 has been hypothesized in the sexual differentiation process.

In mammals, an undifferentiated gonad (genital ridge) forms from the intermediate mesoderm. During this bipotential stage, two ductal structures are present: the Wolffian duct, which will differentiate into the epidydimis and vas deferens in the male; and the Müllerian duct, which is the progenitor of the oviducts, uterus, and upper vagina in the female. The critical event in sex determination is the formation of Sertoli cells in the male gonad. These cells produce the Müllerian inhibiting substance (MIS), which triggers the regression of Müllerian ducts in males. In the absence of MIS production, the default female differentiation program is activated. This critical stage is regulated by the Y chromosome testis determining factor Sry. Once gonadal sex has been determined, gonad differentiation ensues into a testis or an ovary (66, 67).

A sexually dimorphic expression of Dax-1 in mouse gonadal development was described by Swain et al. (18), with its transcript first appearing in the genital ridge at 11.5 d post coitus (dpc) and then being down-regulated in the male gonad, but remaining expressed in the developing ovary. Moreover, overexpression of a genomic DNA fragment containing the Dax-1 gene in mouse strains harboring a weak Sry allele (Mus domesticus poschiavinus, Sry transgenic XX animals) induces gonadal female differentiation of some, but not all, animals expressing Dax-1 (68). These data, taken together with the human DSS syndrome phenotype, led to the hypothesis that DAX-1 represents an antitestis gene, antagonizing Sry function and required for ovarian differentiation (69). However, at variance with these results, other studies showed that the Dax-1 transcript is still expressed at equivalent levels in mouse and rat testis and ovary at 12.5–15.5 dpc and is down-regulated in the ovary at later stages (19, 38). A recent immunohistochemical study showed that the Dax-1 protein is expressed in the mouse both in testis Sertoli and Leydig cells and throughout the ovarian primordium at 12.5–14.5 dpc (21). Moreover, DAX-1 transcripts are expressed both in the male and the female gonadal ridges during the critical period of sex determination in human embryos (70). In addition, DAX-1 homologs are also expressed both in the male and in the female gonad during embryogenesis in another mammalian species [pig (71)] and in chicken (14), alligator (15), frog (16), and fish (17).

These recent results contradict the simplistic model of differential Dax-1 expression in testis vs. ovary being required for female gonad development and indicate that this factor has a specific function in distinct cell populations both in the male and in the female gonads. Dax-1 activity in gonadal development is essential in males but dispensable in females. In fact, Dax-1 is required for normal testis morphology and function in mice (51, 72) and may even be essential for testis determination in the poschiavinus mouse (73), whereas Dax-1 null females have normal ovaries and are fertile (51). Moreover, a primary testicular defect is present in AHC patients, because in these subjects gonadotropin treatment is not able to normalize spermatogenesis, and testicular biopsy shows disorganization of seminiferous tubular structure and Leydig cell hyperplasia (74, 75). On the other hand, ovarian differentiation occurred in a female patient homozygous for a DAX-1 mutation (9). However, in the pathological context of duplications affecting Xp21 (1) and 1p31-p35 (64), DAX-1 overexpression due to gene dosage effects or to up-regulation of the WNT-4 pathway triggers female or ambiguous gonadal differentiation in XY individuals. We suggest a model according to which two molecular mechanisms could be responsible for this phenotype (Fig. 4Go):



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Fig. 4. A Model of the Pathogenesis of DSS Syndrome

Double dosage of DAX-1 inhibits MIS production by fetal Sertoli cells and testosterone secretion by fetal Leydig cells. This leads to formation of female or ambiguous genitalia in XY individuals. Other genes probably influence the sexual phenotype in these patients.

 
1) Repression of MIS production by fetal Sertoli cells. Dax-1 antagonizes the synergy between SF-1 and WT1 in activating the MIS gene promoter, consistent with its role as a general negative regulator of SF-1-induced transactivation (38). DAX-1 overexpression would then lead to repression of MIS expression during the period critical for gonadal specification.

2) Repression of testosterone production by fetal Leydig cells. Considering its negative effect upon steroidogenesis, DAX-1 overexpression would inappropriately repress testosterone biosynthesis in fetal Leydig cells, which is essential for sexual secondary character masculinization (76).

In addition, other genetic factors may contribute to the DSS phenotype. A family of genes encoding proteins similar to the MAGE family of tumor-associated antigens lie inside the critical DSS interval in Xp21 (77, 78). These genes are expressed in fetal and adult testis and their duplication may also play an important role in defining the sex reversal phenotype.


    FUTURE DIRECTIONS
 TOP
 ABSTRACT
 INTRODUCTION
 DAX-1 AND THE ADRENAL...
 DAX-1: PROTEIN STRUCTURE AND...
 DAX-1 AS A TRANSCRIPTIONAL...
 THE PATHOGENESIS OF AHC...
 DAX-1 AS A CANDIDATE...
 REGULATION OF DAX-1 EXPRESSION
 FUTURE DIRECTIONS
 REFERENCES
 
An important target of future studies will be the identification of cofactors involved in mediating transcriptional silencing of target genes by DAX-1, as well as of the factors directing its tissue-specific expression. In addition, the identification of the functional role of DAX-1 in posttranscriptional regulations (mRNA export, translational control) in steroidogenic cells will require the characterization of the RNA and protein components of the DAX-1-containing ribonucleoprotein complex. In this context, structural studies would be essential to understand the mechanisms of protein misfolding in DAX-1 AHC missense mutants.


    ACKNOWLEDGMENTS
 
E.L. thanks B. Bardoni for putting him on the DAX-1 track.


    FOOTNOTES
 
This work was supported by by Centre National de la Recherche Scientifique, Institut National de la Santé et de la Recherche Médicale, Centre Hospitalier Universitaire Régional, Fondation de la Recherche Médicale and Association pour la Recherche sur le Cancer, Centre Hospitalier Universitaire Régional, Human Frontiers Science Program, and Organon (Akzo/Nobel).

Abbreviations: AHC, Adrenal hypoplasia congenita; dpc, days post coitus; DSS, dosage-sensitive sex reversal; HHG, hypogonadotropic hypogonadism; MIS, Müllerian inhibiting substance; NLS, nuclear localization signal; SF-1, steroidogenic factor 1; StAR, steroidogenic acute regulatory protein.

Received for publication April 25, 2003. Accepted for publication May 14, 2003.


    REFERENCES
 TOP
 ABSTRACT
 INTRODUCTION
 DAX-1 AND THE ADRENAL...
 DAX-1: PROTEIN STRUCTURE AND...
 DAX-1 AS A TRANSCRIPTIONAL...
 THE PATHOGENESIS OF AHC...
 DAX-1 AS A CANDIDATE...
 REGULATION OF DAX-1 EXPRESSION
 FUTURE DIRECTIONS
 REFERENCES
 

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