Regulation of the Sox3 Gene in an X0/X0 Mammal without Sry, the Amami Spiny Rat, Tokudaia osimensis

Identification of a New Enhancer That Promotes Sox9 Expression by a Comparative Analysis of Mouse and Sry-Deficient Amami Spiny Rat

INTRODUCTION

Testis differentiation is initiated by the SRY gene on the Y chromosome in mammalian species. However, the Amami spiny rat, Tokudaia osimensis, lacks both the Y chromosome and the Sry gene and acquired a unique Sox9 regulatory mechanism via a male-specific duplication upstream of Sox9, without Sry. In general mammalian species, the SRY protein binds to a testis-specific enhancer to promote SOX9 gene expression. Several enhancers located upstream of Sox9/SOX9 have been reported in mice and humans. In particular, the binding of SRY to the highly conserved enhancer Enh13 is thought to be a common mechanism underlying testis differentiation and sex determination in mammals.

METHODS

Sequences of T. osimensis homologues of three Sox9 enhancers that were previously reported in mice, Enh8, Enh14, and Enh13, were determined. We performed in vitro assays to confirm enhancer activity involved in Sox9 regulation in T. osimensis.

RESULTS

T. osimensis Enh13 showed enhancer activity when co-transfected with NR5A1 and SOX9. Mouse Enh13 was activated by NR5A1 and SRY; however, T. osimensis Enh13 did not respond to SRY, even though the binding sites of SRY and NR5A1 were conserved. To identify the key sequence that is present in mouse but absent from T. osimensis, we performed reporter gene assays using vectors in which partial sequences of T. osimensis Enh13 were replaced with mouse sequences. For T. osimensis Enh13 in which the second half (approximately 430 bp) was replaced with the corresponding mouse sequence, activity in response to NR5A1 and SRY was recovered. Further, reporter assays revealed that multiple regions in the second half of the mouse Enh13 sequence are required for the response to NR5A1 and SRY. The latter 49 bp was particularly important and contained four binding sites for three transcription factors, POU2F1, HOXA3, and GATA1.

CONCLUSION

We showed that there are unknown sequences responsible for the interaction between NR5A1 and SRY and mEnh13 based on comparative analyses of Sry-dependent and Sry-independent species. Our comparative analyses revealed new molecular mechanisms underlying mammalian sex determination.

45,X male - rare case of unbalanced translocation of Y chromosome to chromosome 2 presenting with developmental delay, learning difficulty and obesity

Summary

A male phenotype accompanied by a 45,X karyotype is rare. It may occur due to Y chromosomal translocation or insertion to X/autosome. Clinical presentation may vary depending on the presence of the Y chromosomal locus and the degree of loss of autosome material. 45,X males can present with short stature and Turner syndrome phenotype due to haploinsufficiency of genes which are normally expressed in both X and Y chromosomes. The presence of the sex-determining region Y (SRY) gene leads to the differentiation of bipotential gonads to testis. Most individuals go through puberty normally, but some may need pubertal induction for delayed puberty. Rarely some can have a pubertal arrest. The risk of gonadoblastoma is minimal in these individuals due to functioning testicular tissue. The azoospermia factor (AZF) region is found on the long arm of the Yq chromosome and is needed for spermatogenesis. In a 45,X male with unbalanced translocation of Y chromosome, spermatogenesis can be affected due to the lack of AZF leading to Sertoli cell-only syndrome. This will have an implication on fertility in adult life. We present a 14-year-old boy with developmental delay, learning difficulties and subtle dysmorphic features who was diagnosed with 45,X,der(2)t(Y:2)(?:p25). Fluorescence in situ hybridisation analysis revealed translocation of SRY (Yp11.3) to the terminal part of the short arm of chromosome 2 resulting in the deletion of most of the Y chromosome (Yp11.2-q12) and part of chromosome 2(2p25.3). This is the first case where SRY translocation to chromosome 2 presents with the above clinical presentation.

Learning points

45,X karyotype is rare in male. It may occur due to SRY translocation or an insertion to X/autosome. SRY gene translocation to chromosome 2 has been not reported in the literature. Clinical presentation can be varied due to degree of loss of chromosomal material. Due to loss of AZF region found on the long arm of the Yq, spermatogenesis can be affected. Loss of 2p25 leads to learning difficulty and obesity.

Unusual Mammalian Sex Determination Systems: A Cabinet of Curiosities

Therian mammals have among the oldest and most conserved sex-determining systems known to date. Any deviation from the standard XX/XY mammalian sex chromosome constitution usually leads to sterility or poor fertility, due to the high differentiation and specialization of the X and Y chromosomes. Nevertheless, a handful of rodents harbor so-called unusual sex-determining systems. While in some species, fertile XY females are found, some others have completely lost their Y chromosome. These atypical species have fascinated researchers for over 60 years, and constitute unique natural models for the study of fundamental processes involved in sex determination in mammals and vertebrates. In this article, we review current knowledge of these species, discuss their similarities and differences, and attempt to expose how the study of their exceptional sex-determining systems can further our understanding of general processes involved in sex chromosome and sex determination evolution.