Control of sex determination in animals

46,XX Differences of Sex Development outside congenital adrenal hyperplasia: pathogenesis, clinical aspects, puberty, sex hormone replacement therapy and fertility outcomes

The term 'differences of sex development' (DSD) refers to a group of congenital conditions that are associated with atypical development of chromosomal, gonadal, and/or anatomical sex. DSD in individuals with a 46,XX karyotype can occur due to fetal or postnatal exposure to elevated amount of androgens or maldevelopment of internal genitalia. Clinical phenotype could be quite variable and for this reason these conditions could be diagnosed at birth, in newborns with atypical genitalia, but also even later in life, due to progressive virilization during adolescence, or pubertal delay. Understand the physiological development and the molecular bases of gonadal and adrenal structures is crucial to determine the diagnosis and best management and treatment for these patients. The most common cause of DSD in 46,XX newborns is congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency, determining primary adrenal insufficiency and androgen excess. In this review we will focus on the other rare causes of 46,XX DSD, outside CAH, summarizing the most relevant data on genetic, clinical aspects, puberty and fertility outcomes of these rare diseases.

Actions and Roles of FSH in Germinative Cells

Follicle stimulating hormone (FSH) is produced by the pituitary gland in a coordinated hypothalamic-pituitary-gonadal (HPG) axis event, plays important roles in reproduction and germ cell development during different phases of reproductive development (fetal, neonatal, puberty, and adult life), and is consequently essential for fertility. FSH is a heterodimeric glycoprotein hormone of two dissociable subunits, α and β. The FSH β-subunit (FSHβ) function starts upon coupling to its specific receptor: follicle-stimulating hormone receptor (FSHR). FSHRs are localized mainly on the surface of target cells on the testis and ovary (granulosa and Sertoli cells) and have recently been found in testicular stem cells and extra-gonadal tissue. Several reproduction disorders are associated with absent or low FSH secretion, with mutation of the FSH β-subunit or the FSH receptor, and/or its signaling pathways. However, the influence of FSH on germ cells is still poorly understood; some studies have suggested that this hormone also plays a determinant role in the self-renewal of germinative cells and acts to increase undifferentiated spermatogonia proliferation. In addition, in vitro, together with other factors, it assists the process of differentiation of primordial germ cells (PGCLCs) into gametes (oocyte-like and SSCLCs). In this review, we describe relevant research on the influence of FSH on spermatogenesis and folliculogenesis, mainly in the germ cell of humans and other species. The possible roles of FSH in germ cell generation in vitro are also presented.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

SRY and the standoff in sex determination

SRY was identified as the mammalian sex-determining gene more than 15 yr ago and has been extensively studied since. Although many of the pathways regulating sexual differentiation have been elucidated, direct downstream targets of SRY are still unclear, making a top down approach difficult. However, recent work has demonstrated that the fate of the gonad is actively contested by both male-promoting and female-promoting signals. Sox9 and Fgf9 push gonads towards testis differentiation. These two genes are opposed by Wnt4, and possibly RSPO1, which push gonads toward ovary differentiation. In this review, we will discuss the history of the field, current findings, and exciting new directions in vertebrate sex determination.

The C-terminal nuclear localization signal of the sex-determining region Y (SRY) high mobility group domain mediates nuclear import through importin beta 1

The sex-determining factor SRY is a DNA-binding protein that diverts primordial gonads from the ovarian pathway toward male differentiation to form testes. It gains access to the nucleus through two distinct nuclear localization signals (NLSs) that flank the high mobility group (HMG) DNA-binding domain, but the mechanisms through which these NLSs operate have not been studied. In this study, we reconstitute the nuclear import of SRY in vitro, demonstrating a lack of requirement for exogenous factors for nuclear accumulation and a significant reduction in nuclear transport in the presence of antibodies to importin beta but not importin alpha. Using a range of quantitative binding assays including enzyme-linked immunosorbent assay, fluorescence polarization, and native gel mobility electrophoresis, we assess the binding of importins to SRY, demonstrating a high affinity recognition (in the low nm range) by Imp beta independent of Imp alpha. In assessing the contribution of each NLS, we found that the N-terminal NLS was recognized poorly by importins, whereas the C-terminal NLS was bound by importin beta with similar affinity to SRY. We also found that RanGTP, but not RanGDP, could dissociate the SRY-importin beta complex in solution using FP. We describe a novel double-fluorescent label DNA binding assay to demonstrate mutual exclusivity between importin beta recognition and DNA binding on the part of SRY, which may represent an alternative release mechanism upon nuclear entry. This study represents the first characterization of the nuclear import pathway for a HMG domain-containing protein. Importantly, it demonstrates for the first time that recognition of SRY by Imp beta is of comparable affinity to that with which Imp alpha/beta recognizes conventional NLS-containing substrates.

Sex from W to Z: evolution of vertebrate sex chromosomes and sex determining genes

Sex determination in major vertebrate groups appears to be very variable, including systems of male heterogamety, female heterogamety and a variety of genetic and environmental sex determining systems. Yet comparative studies of sex chromosomes and sex determining genes now suggest that these differences are more apparent than real. The sex chromosomes of even widely divergent groups now appear to have changed very little over the last 300+ million years, and even independently derived sex chromosomes seem to have followed the same set of evolutionary rules. The sex determining pathway seems to be extremely conserved, although the control of the genes in this pathway is vested in different elements. We present a scenario for the independent evolution of XY male heterogamety in mammals and ZW female heterogamety in birds and some reptiles. We suggest that sex determining genes can be made redundant, and replaced by control at another step of a conserved sex determining pathway, and how choice of a gene as a sex switch has led to the evolution of new sex chromosome systems. J. Exp. Zool. 290:449-462, 2001.

Deletions of 9p and the quest for a conserved mechanism of sex determination

Distal chromosome 9p contains a locus that, when deleted, is a cause of 46,XY gonadal dysgenesis in the absence of extragenital anomalies. This locus might account for the frequently observed cases of 46,XY pure gonadal dysgenesis who do not harbor mutations in SRY, the sex master regulator gene found in mammalian species. The genomic organization of 9p positional candidate genes is currently being studied and mutational screens are ongoing. Among other positional candidates, including two additional doublesex-related genes, the evidence to support a role for the gene DMRT1 in vertebrate male sexual development is accumulating. Although formal proof of the requirement of DMRT1 in gonadal sex fate choice has not been obtained so far, the particular interest in this gene and perhaps other doublesex-related genes identified in vertebrates lies in that they may provide an entry point to a conserved mechanism of sex determination across animal phyla. We discuss recent results and emerging views on the genetics of sex determination, while stressing that the majority of cases of 46,XY gonadal dysgenesis remain unexplained. The latter is likely to be efficiently addressed by positional cloning efforts, particularly by considering the wealth of sequence data provided by the Human Genome Project.

Incomplete masculinisation of XX subjects carrying the SRY gene on an inactive X chromosome

46,XX subjects carrying the testis determining SRY gene usually have a completely male phenotype. In this study, five very rare cases of SRY carrying subjects (two XX males and three XX true hermaphrodites) with various degrees of incomplete masculinisation were analysed in order to elucidate the cause of sexual ambiguity despite the presence of the SRY gene. PCR amplification of 20 Y chromosome specific sequences showed the Yp fragment to be much longer in XX males than in true hermaphrodites. FISH analysis combined with RBG banding of metaphase chromosomes of four patients showed that in all three true hermaphrodites and in one XX male the Yp fragment was translocated onto a late replicating inactive X chromosome in over 90% of their blood lymphocytes. However, in a control classical XX male with no ambiguous features, the Yp fragment (significantly shorter than in the XX male with sexual ambiguity and only slightly longer than in XX hermaphrodites) was translocated onto the active X chromosome in over 90% of cells.

These studies strongly indicate that inactivation on the X chromosome spreading into a translocated Yp fragment could be the major mechanism causing a sexually ambiguous phenotype in XX (SRY+) subjects.

Sox9 expression during gonadal development implies a conserved role for the gene in testis differentiation in mammals and birds

Heterozygous mutations in SOX9 lead to a human dwarfism syndrome, Campomelic dysplasia. Consistent with a role in sex determination, we find that Sox9 expression closely follows differentiation of Sertoli cells in the mouse testis, in experimental sex reversal when fetal ovaries are grafted to adult kidneys and in the chick where there is no evidence for a Sry gene. Our results imply that Sox9 plays an essential role in sex determination, possibly immediately downstream of Sry in mammals, and that it functions as a critical Sertoli cell differentiation factor, perhaps in all vertebrates.

SRY-negative true hermaphrodites and an XX male in two generations of the same family

Two 46,XX true hermaphrodites and one XX male without genital ambiguities are reported. They coexist in two generations of the same pedigree, with paternal transmission and in the absence of SRY (sex-determining region, Y chromosome). These familial cases provide evidence to support the hypothesis that these disorders are alternative manifestations of the same genetic defect, probably an autosomal dominant mutation (with incomplete penetrance) or an X-linked mutation (limited by the presence of the Y chromosome).

Intercalation, DNA kinking, and the control of transcription

Biological processes involved in the control and regulation of transcription are dependent on protein-induced distortions in DNA structure that enhance the recruitment of proteins to their specific DNA targets. This function is often accomplished by accessory factors that bind sequence specifically and locally bend or kink the DNA. The recent determination of the three-dimensional structures of several protein-DNA complexes, involving proteins that perform such architectural tasks, brings to light a common theme of side chain intercalation as a mechanism capable of driving the deformation of the DNA helix. The protein scaffolds orienting the intercalating side chain (or side chains) are structurally diverse, presently comprising four distinct topologies that can accomplish the same task. The intercalating side chain (or side chains), however, is exclusively hydrophobic. Intercalation can either kink or bend the DNA, unstacking one or more adjacent base pairs and locally unwinding the DNA over as much as a full turn of helix. Despite these distortions, the return to B-DNA helical parameters generally occurs within the adjacent half-turns of DNA.

A Bkm-associated human y-chromosomal DNA is conserved and transcribed in the testis of mouse

Y chromosome associated genes and repetitive sequences are continually viewed from the point of view of their possible involvement in sex determination and in the evolution of such a mechanism, thus sustaining an interest in the identification of novel sequences to gain newer insights. Here we have used the highly conserved class of Bkm repeats to isolate its associated sequences from the Y chromosome under the assumption that these sequences could be involved in sex determination and might also reflect the evolutionary status of the Y chromosome. Towards this end we have screened a genomic library enriched with human Y chromosome DNA with Bkm. One of the positive clones, C65, has a pericentromeric location on the Y chromosome and is present in a number of human sex-reversed XX males. The 10.5kb insert of clone C65 has been further subcloned (pFI, pFII, pFIII, pFIV). The subclone pFIII is present in both sexes in human and mouse, whereas pFIV is primate specific and present in both sexes. pFII contains sequences homologous to Bkm. pFI is conserved in mouse and man, but is Y specific only in primates. Although present in both sexes in mouse, pFI is transcribed specifically in the male testis suggesting that it may be involved in the process of sex determination or testis differentiation and spermatogenesis.

Homology model building of the HMG-1 box structural domain

Nucleoproteins belonging to the HMG-1/2 family possess homologous domains approximately 75 amino acids in length. These domains, termed HMG-1 boxes, are highly structured, compact, and mediate the interaction between HMG-1 box-containing proteins and DNA in a variety of biological contexts. Homology model building experiments on HMG-1 box sequences 'threaded' through the 1H-NMR structure of an HMG-1 box from rat indicate that the domain does not have rigid sequence requirements for its formation. Energy calculations indicate that the structure of all HMG-1 box domains is stabilized primarily through hydrophobic interactions. We have found structural relationships in the absence of statistically significant sequence similarity, identifying several candidate proteins which could possibly assume the same three-dimensional conformation as the rat HMG-1 box motif. The threading technique provides a method by which significant structural similarities in a diverse protein family can be efficiently detected, and the 'structural alignment' derived by this method provides a rational basis through which phylogenetic relationships and the precise sites of interaction between HMG-1 box proteins and DNA can be deduced.