Sex Differences in Human Development, Physiology and Diseases

Sex differences are prevalent in human development, physiology and diseases. Although differential actions of the sex hormones and their receptors could play important roles in these biological processes, the sex chromosomes could contribute to various genetic aspects of sex differences. In particular, the human Y chromosome is unique to male and could exert positive influence(s) in a male-specific manner(s) in normal development/physiology and diseases. The human Y chromosome contains only 17 unique protein-coding genes in the male-specific (MSY) region. Most have highly conserved homologues on the X chromosome and could serve similar functions in dosage dependent manners. There are a few exceptions, i.e. SRY, testis-specific protein Y (TSPY), Heat shock factor Y (HSFY) and the RNA-binding motif Y (RBMY), which have diverged significantly from the respective X counterparts and serve male-specific functions. SRY is a single-copy gene important for male sex determination while TSPY, HSFY and RBMY are repetitive genes important for spermatogenesis. Ectopic expression of these MSY-unique genes, such as SRY, TSPY and RBMY, in non-gonadal tissues could exert unexpected effects on the somatic tissues/organs, thereby influencing the respective biological processes in a male-specific manner(s).

Figure 1. Genes on the male-specific region of the human Y chromosome. Unique male-specific genes are boxed.

Important clues on such postulation have derived from our studies on identifying the target genes for SRY and SOX9 during mouse sex determination, in which we showed that about 50% of SOX9 targets being bound by SRY at the same positions in their respective promoters (Figure 2). Since SRY is only needed for testis determination, while SOX9 serves more diverse differentiation functions in other organs, such as the brain, neural crest, prostate and pancreas. If SRY is ectopically activated in these organs, it could competitively bind to the promoters of SOX9 and/or other SOX targets, thereby potentially disrupting their gene regulatory programs in the affected organs. Accordingly, SRY could serve as a male-specific genetic modifier in development, physiology and diseases.

Figure 2. SRY and SOX9 share close to 50% of target genes in fetal mouse gonads

SRY Regulates the X-located Monoamine Oxidase A Gene

Among the genes bound by SRY transcription complexes, numerous neural genes were identified and confirmed to be targets for this Y-encoded transcription factor, such as the X-located monoamine oxidase A (MAOA) gene. MAOA catalyzes the oxidative deamination of monoamine neurotransmitters such as serotonin and plays a critically important role in brain development and functions. Abnormal MAOA activity has been implicated in several neuropsychiatric disorders, such as depression, autism, attention deficit hyperactivity disorder and schizophrenia, which show various degrees of sexual dimorphisms. However, the molecular basis for such sex differences in the disease processes is unclear. Through a collaboration with Dr. Jean Shih, University of Southern California, we demonstrated that SRY can transcriptionally up regulate and further exacerbate the MAOA expression via interaction with the transcription factor Sp1, which collectively stimulate the catalytic activities of the enzyme. Hence, we showed that a Y- located transcription factor SRY is capable of regulating an X-located gene, suggesting a novel molecular mechanism for sexual dimorphism in neural development, brain functions and initiation/progression of neural/psychiatric disorders associated with MAOA dysfunctions.

Figure 3. SRY collaborates with Sp1 and upregulates the MAOA gene expression

SRY Competitively Disrupts the SOX10 Regulation of RET, a Significant Gene for the Hirschsprung’s Disease

Hirschsprung’s disease (HSCR, or megacolon disease) is a complex congenital disorder, arising from the abnormalities in enteric (gastrointestinal) nervous system (ENS) development. There is a significant sex difference with male to female ratio as high as 5 to 1. Loss of function mutations in genes and/or haploinsufficiency of their gene products involved in ENS development play significant roles in the pathogenesis of HSCR. We showed that over half of the HSCR susceptibility genes, such as RET, GDNF, NRTN, EDNRB, L1CAM, IHH, FOXF2, and other genes involved in ENS development, are targets of SRY, suggesting that SRY could be a major genetic modifier in the disease processes, thereby increasing the male biases in HSCR. Among the SRY targets, the tyrosine kinase receptor RET represents the most important HSCR susceptibility gene, whose mutations or haploinsufficiency account for over one third of the sporadic and half of the familial forms of HSCR. RET is regulated by a distal and a proximal enhancer, in which PAX3 and NKX2-1 are the respective resident transcription factors. SOX10 interacts with these transcription factors and co-activates RET transactivation in a transcriptional complex (Figure 4, left). We showed that SRY competitively displaces SOX10 in its interactions with PAX3 and NKX2-1 and represses the transcriptional activities on the RET gene (Figure 4, right). We surmise that SRY repression of RET results in haploinsufficiency of RET protein, thereby promoting HSCR development in a male-biased manner(s).

Figure 4. SRY competitively displaces/disrupts SOX10 (a major transcription factor) in a transcription complex, regulating the RET (a major HSCR disease gene) expression.

Aberrant Activation of the Human SRY Gene in Early Embryonic Development Results in Postnatal Growth Retardation and Lethality in Mice

Our studies on MAOA and RET genes suggest that SRY could indeed be a genetic modifier, capable of exerting male-specific effects on various disease processes. To experimentally evaluate such a possibility, we have established a transgene activation system, and activated the human SRY gene at single-cell embryonic stage in transgenic mice (Figure 5, left). Pups with human SRY activated (SRY-ON) are born in same size as their non-transgenic (control) littermates, but they retard in growth and all die of multi-organ failure(s) before two weeks of age (Figure 5, right). Pathological and molecular characterization show that they lack innate suckling activities, and develop fatty liver disease, arrested lung development, impaired neurogenesis in the brain, and frequent myocardial fibrosis and infarction. These findings suggest that aberrant and high-level activation of SRY disrupt normal development and physiology in multiple organs in prenatal/postnatal life and could potentially contribute to the sex differences in various human diseases with significant sex biases. 

Figure 5. Aberrant activation of the human SRY gene in single-cell mouse embryos (left) retards postnatal growth and results in death before two-weeks of age (right).

These observations suggest that the Cre-LoxP transgene activation system (Figure 5, left) could be used to address the effects of SRY and other Y chromosome genes in embryonic development and postnatal life in specific organs/cell types in transgenic mice, using tissue-specific Cre transgenic mouse lines widely available in the scientific community.  Such investigations should shed critical insights on the roles of the Y chromosome genes in sexual dimorphisms in human development, physiology and diseases.