Dosage-sensitive Sex Reversal Research Articles

Sex determining gene on the X chromosome short arm: Dosage sensitive sex reversal

The present review article summarizes current knowledge concerning the sex determining gene on Xp21, termed DSS (dosage sensitive sex reversal). The presence of DSS has been based on the finding that, in the presence of SRY, partial active Xp duplications encompassing the middle part of Xp result in sex reversal, whereas those of the distal or proximal part of Xp permit male sex development. Because Klinefelter patients develop as males, it is believed that DSS is normally subject to X-inactivation, and that two active copies of DSS override the function of SRY, resulting in gonadal dysgenesis because of meiotic pairing failure. It may be possible that DSS encodes a target sequence for repressing function of SRY or that DSS is involved in an X chromosome-counting mechanism. Molecular approaches have localized DSS to a 160 kb region and isolated candidate genes such as DAX-1 and MAGE-Xp, but there has been no formal evidence equating the candidate gene with DSS. In addition to its clinical importance, the exploration of DSS must provide a useful clue to phylogenetic studies of sex chromosomes and dosage compensation.

Early step in mammalian sex determination

Since the cloning of the Y-located testis-determining factor, sex determining region Y ( SRY), several other genes have been implicated in the process of mammalian sex determination. Mutations of an SRY-related gene, SRY-related high-mobility group box 9 ( SOX9), result in autosomal sex reversal and campomelic dysplasia. The genes Steroidogenic factor 1 ( SF1) and Wilm's tumour 1 ( WT1) are required for early gonadal development as well as for the formation of adrenals and kidneys respectively. The gene responsible for adrenal hypoplasia congenita, DAX1, is a candidate for the X-linked dosage sensitive sex reversal gene ( DSS).

Mouse Dax1 expression is consistent with a role in sex determination as well as in adrenal and hypothalamus function

Duplications of a chromosome Xp21 locus DSS (Dosage Sensitive Sex reversal) are associated with male to female sex reversal. An unusual member of the nuclear hormone receptor superfamily, DAX1, maps to the DSS critical region and is responsible for X-linked adrenal hypoplasia congenita. Here we describe the isolation of the mouse Dax1 gene and its pattern of expression during development. Expression was detected in the first stages of gonadal and adrenal differentiation and in the developing hypothalamus. Moreover, Dax1 expression is down-regulated coincident with overt differentiation in the testis, but persists in the developing ovary. Comparison of the predicted protein products of the human and mouse genes show that specific domains are evolving rapidly. Our results suggest a basis for adrenal insufficiency and hypogonadotropic hypogonadism in males affected by adrenal hypoplasia congenita and are consistent with a role for DAX1 in gonadal sex determination.

Xp-duplications with and without sex reversal

Duplications in Xp including the DSS (dosage sensitive sex reversal) region cause male to female sex reversal. We investigated two patients from families with Xp duplications. The first case was one of two sisters with karyotype 46,XY,der(22),t(X;22)(p11.3;p11)mat and unambiguous female genitalia. The living sister was developmentally retarded, and showed multiple dysmorphic features and an acrocallosal syndrome. The second case was a boy with a maternally inherited direct duplication of Xp21.3-pter with the breakpoint close to the DSS locus. He had multiple abnormalities and micropenis, but otherwise unambiguous male genitalia. We performed quantitative Southern blot analysis with probes from Xp22.13 to p21.2 to define the duplicated region. Clinical, cytogenetic, and molecular data from both patients were compared with those of previously reported related cases. A comparison of the extragenital symptoms revealed no differences between patients with or without sex reversal. In both cases, the symptoms were non-specific. Among 22 patients with a duplication in Xp, nine had unambiguous female genitalia and a well-documented duplication of the DSS region. Two patients with duplication of DSS showed ambiguous external genitalia. From these data, we conclude that induction of testicular tissue may start in these patients, but that the type of genitalia depends on the degree of subsequent degeneration by a gene in DSS.

Xp duplications and sex reversal.

Male to female sex reversal has been observed in individuals with duplications of the short arm of the X chromosome. The study of Xp duplicated patients demonstrated that sex reversal results from the presence of two active copies of the DSS (dosage sensitive sex reversal) locus. A double dosage of DSS disrupts testis formation whereas its absence is compatible with a male phenotype, suggesting a role for DSS in ovarian development and as a link between ovary and testis formation. DSS was localized to a 160 kb region of Xp21, overlapping the adrenal hypoplasia congenita locus. The search for expressed sequences in the DSS critical region led to the identification of two types of genes: the DAM family and DAX-1, an atypical member of the nuclear receptor superfamily. Although no function is currently known for DAM genes, functional deficiency for DAX-1 has been shown to be responsible for adrenal hypoplasia congenita and hypogonadotropic hypogonadism. The search for the DSS gene(s) is still open and both the DAM genes and DAX-1 represent DSS candidate genes.

New bedfellows in the mammalian sex-determination affair

In eutherian mammals, sex determination is defined as the initiation of testis development. This is because all subsequent sexual differentiation is dependent upon and secondary to this initial event, which is controlled by the presence or absence of the SRY gene on the Y chromosome 1. SRYis thought to exert its dominant effect by diverting the development of the indifferent gonad from the female (default) pathway to the male pathway of development. Several human loci not located on the Y chromosome have been known for many years to be associated with disturbances in gonadal development. Recently, genes have been isolated at two of these loci, both of which appear to exert their effect on sex determination through an altered dosage in XY individuals who carry a normal SRYgene on their Y chromosome. One of these genes maps to a locus on chromosome 17, SRA1, which is associated with the skeletal malformation syndrome campomelic dysplasia (CD), that is sometimes accompanied by a gradation of genital defects or primary gonadal dysgenesis 2. By cloning the breakpoints of several translocations associated with CD and sex reversal, soxg, a gene continuing an SRY-like HMG (high mobility group) box domain, has been identified at this locus 3. This gene was analysed by assaying for single-strand conformation polymorphisms and by direct sequencing for mutations in CD patients3,4. Mutations within the gene involving premature stop codons, amino acid substitutions, one splice acceptor and one donor alteration have been identified, and all are expected to result in loss of function of the SOX9 gene product. All the mutant individuals except one were heterozygous, carrying one normal allele of SOXg. These findings suggest that both CD and sex reversal result from a haploinsufficiency of the gene product rather than from a dominant negative effect. The second gene that has recently been isolated is DAX1, an X-linked gene responsible for adrenal hypoplasia congenita (AHC) 5,6. This gene maps to Xp21, a critical interval of the X chromosome that has been known for some time to cause male to female sex reversal when it is duplicated 7. Mutational analysis of DAXI in patients with X-linked AHC has demonstrated that functional disturbance of this gene in the absence of any other detectable change in Xp21 also results in hypogonadotropic hypogonadism (HHG), and lends credence to the idea that DAX1 might be the gene responsible for the dosage-sensitive sex reversal associated with the DSS locus on Xp21.

A Dosage Sensitive Locus at Chromosome Xp21 Is Involved in Male to Female Sex Reversal

Male to female sex reversal has been observed in individuals with duplications of the short arm of the X chromosome. Here we demonstrate that sex reversal results from the presence of two active copies of an Xp locus rather than from its rearrangement and that alterations at this locus constitute one of the causes of sex reversal in individuals with a normal 46,XY karyotype. We have named this locus DSS (Dosage Sensitive Sex reversal) and localized it to a 160 kilobase region of chromosome Xp21, adjacent to the adrenal hypoplasia congenita locus. The identification of male individuals deleted for DSS suggests that this locus is not required for testis differentiation. We propose that DSS has a role in ovarian development and/or functions as a link between ovary and testis formation.

Mutations in the DAX-1 gene give rise to both X-linked adrenal hypoplasia congenita and hypogonadotropic hypogonadism.

Adrenal hypoplasia congenita (AHC) is an X-linked disorder characterized by primary adrenal insufficiency. Hypogonadotropic hypogonadism (HHG) is frequently associated with this disorder but is thought not to be caused by the low adrenal androgen levels due to adrenal hypoplasia. It is uncertain whether there are two distinct yet physically linked genes responsible for AHC and HHG or a single gene responsible for both diseases. AHC can occur as a part of a contiguous deletion syndrome together with Duchenne muscular dystrophy (DMD) and/or glycerol kinase deficiency (GKD). From the analysis of deletions, the following gene order has been deduced: Xpter-AHC-GKD-DMD-cen. An AHC critical region of 200-500 kilobases has been defined by physical mapping and partially overlaps with a 160-kilobase dosage-sensitive sex (DSS) reversal critical region. The DAX-1 (DSS-AHC critical region on the X, gene 1) gene was isolated and found to encode a new member of the nuclear hormone receptor family. Here we report that DAX-1 is deleted in 14 patients and point mutations were found in the coding region in DNA from 12 unrelated individuals. All AHC patients over 14 years old and with only point mutations in DAX-1 were also diagnosed with HHG, confirming that the DAX-1 gene is responsible for both X-linked AHC and HHG. But in four sporadic cases and a single familial case, no point mutations were found, suggesting genetic heterogeneity or differential expression of DAX-1.

A dosage sensitive locus at chromosome Xp21 is involved in male to female sex reversal

Male to female sex reversal has been observed in individuals with duplications of the short arm of the X chromosome. Here we demonstrate that sex reversal results from the presence of two active copies of an Xp locus rather than from its rearrangement and that alterations at this locus constitute one of the causes of sex reversal in individuals with a normal 46,XY karyotype. We have named this locus DSS (Dosage Sensitive Sex reversal) and localized it to a 160 kilobase region of chromosome Xp21, adjacent to the adrenal hypoplasia congenita locus. The identification of male individuals deleted for DSS suggests that this locus is not required for testis differentiation. We propose that DSS has a role in ovarian development and/or functions as a link between ovary and testis formation.