References 6.a: Causes – Genetic factors – Human studies

Early androgen exposure and human gender development

During early development, testosterone plays an important role in sexual differentiation of the mammalian brain and has enduring influences on behavior. Testosterone exerts these influences at times when the testes are active, as evidenced by higher concentrations of testosterone in developing male than in developing female animals. This article critically reviews the available evidence regarding influences of testosterone on human gender-related development. In humans, testosterone is elevated in males from about weeks 8 to 24 of gestation and then again during early postnatal development. Individuals exposed to atypical concentrations of testosterone or other androgenic hormones prenatally, for example, because of genetic conditions or because their mothers were prescribed hormones during pregnancy, have been consistently found to show increased male-typical juvenile play behavior, alterations in sexual orientation and gender identity (the sense of self as male or female), and increased tendencies to engage in physically aggressive behavior. Studies of other behavioral outcomes following dramatic androgen abnormality prenatally are either too small in their numbers or too inconsistent in their results, to provide similarly conclusive evidence. Studies relating normal variability in testosterone prenatally to subsequent gender-related behavior have produced largely inconsistent results or have yet to be independently replicated. For studies of prenatal exposures in typically developing individuals, testosterone has been measured in single samples of maternal blood or amniotic fluid. These techniques may not be sufficiently powerful to consistently detect influences of testosterone on behavior, particularly in the relatively small samples that have generally been studied. The postnatal surge in testosterone in male infants, sometimes called mini-puberty, may provide a more accessible opportunity for measuring early androgen exposure during typical development. This approach has recently begun to be used, with some promising results relating testosterone during the first few months of postnatal life to later gender-typical play behavior. In replicating and extending these findings, it may be important to assess testosterone when it is maximal (months 1 to 2 postnatal) and to take advantage of the increased reliability afforded by repeated sampling.

Author/-s:       Melissa Hines; Mihaela Constantinescu; Debra Spencer

Publication:     Biology of sex differences, 2015

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Androgen receptor function links human sexual dimorphism to DNA methylation

Sex differences are well known to be determinants of development, health and disease. Epigenetic mechanisms are also known to differ between men and women through X-inactivation in females. We hypothesized that epigenetic sex differences may also result from sex hormone functions, in particular from long-lasting androgen programming. We aimed at investigating whether inactivation of the androgen receptor, the key regulator of normal male sex development, is associated with differences of the patterns of DNA methylation marks in genital tissues. To this end, we performed large scale array-based analysis of gene methylation profiles on genomic DNA from labioscrotal skin fibroblasts of 8 males and 26 individuals with androgen insensitivity syndrome (AIS) due to inactivating androgen receptor gene mutations. By this approach we identified differential methylation of 167 CpG loci representing 162 unique human genes. These were significantly enriched for androgen target genes and low CpG content promoter genes. Additional 75 genes showed a significant increase of heterogeneity of methylation in AIS compared to a high homogeneity in normal male controls. Our data show that normal and aberrant androgen receptor function is associated with distinct patterns of DNA-methylation marks in genital tissues. These findings support the concept that transcription factor binding to the DNA has an impact on the shape of the DNA methylome. These data which derived from a rare human model suggest that androgen programming of methylation marks contributes to sexual dimorphism in the human which might have considerable impact on the manifestation of sex-associated phenotypes and diseases.

Author/-s:       O. Ammerpohl; S. Bens; M. Appari; R. Werner; B. Korn; S. L. Drop; F. Verheijen; Y. van der Zwan; T. Bunch; I. Hughes; M. Cools; F. G. Riepe; O. Hiort; R. Siebert; P. M. Holterhus

Publication:     Plos One, 2013

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Intersex and Transsex: Atypical Gender Development and Social Construction

[…] In summary, the behaviors of intersexed and transgendered persons provide a wide range of evidence against many aspects of social science and social construction theory. Intersexed and transgendered persons, as well as typical persons, are each born with a certain background based upon evolutionary heritage, family genetics, uterine environment, and health factors that they will evidence in a socially permissive culture and limit in a restrictive one. The strongest gestational influences are from genetic and endocrinal organizing forces. Organizing factors are those genetic and hormonal influences established prenatally that influence postnatal behaviors set in motion by social or other environmental activation processes (such as puberty) or events (such as serious threats). Organizing factors influence or bias subsequent responses of the individual to environmental/social forces; they predispose the person to manifest behaviors and attitudes (biases) that have come to be recognized as appropriate. Sex-related activation effects occur postnatally; most noticeably at or after puberty. The lives of intersex and transgendered persons provide strong evidence for a realistic theory of sexual development: biased-interaction theory.

Author/-s:       Milton Diamond

Publication:     Women’s Studies Review, 2012

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Prenatal endocrine influences on sexual orientation and on sexually differentiated childhood behaviour

Both sexual orientation and sex-typical childhood behaviors, such as toy, playmate and activity preferences, show substantial sex differences, as well as substantial variability within each sex. In other species, behaviors that show sex differences are typically influenced by exposure to gonadal steroids, particularly testosterone and its metabolites, during early development (prenatally or neonatally). This article reviews the evidence regarding prenatal influences of gonadal steroids on human sexual orientation, as well as sex-typed childhood behaviors that predict subsequent sexual orientation. The evidence supports a role for prenatal testosterone exposure in the development of sex-typed interests in childhood, as well as in sexual orientation in later life, at least for some individuals. It appears, however, that other factors, in addition to hormones, play an important role in determining sexual orientation. These factors have not been well-characterized, but possibilities include direct genetic effects, and effects of maternal factors during pregnancy. Although a role for hormones during early development has been established, it also appears that there may be multiple pathways to a given sexual orientation outcome and some of these pathways may not involve hormones.

Author/-s:       Melissa Hines

Publication:     Frontiers in neuroendocrinology, 2011

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Reframing sexual differentiation of the brain

In the twentieth century, the dominant model of sexual differentiation stated that genetic sex (XX versus XY) causes differentiation of the gonads, which then secrete gonadal hormones that act directly on tissues to induce sex differences in function. This serial model of sexual differentiation was simple, unifying and seductive. Recent evidence, however, indicates that the linear model is incorrect and that sex differences arise in response to diverse sex-specific signals originating from inherent differences in the genome and involve cellular mechanisms that are specific to individual tissues or brain regions. Moreover, sex-specific effects of the environment reciprocally affect biology, sometimes profoundly, and must therefore be integrated into a realistic model of sexual differentiation. A more appropriate model is a parallel-interactive model that encompasses the roles of multiple molecular signals and pathways that differentiate males and females, including synergistic and compensatory interactions among pathways and an important role for the environment.

Author/-s:       Margaret M. McCarthy; Arthur P. Arnold

Publication:     Nature neuroscience, 2011

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Sexual differentiation of the brain related to gender identity: beyond hormones

The sexual differentiation of the brain starts in the second semester of pregnancy, which is, after the development of the genitals which differentiate in the second month of pregnancy. Because these two processes have different timetables, it could be that these are initiated through different pathways. Male gonads synthesize testosterone, which can be converted into estrogen by aromatase in the brain. In humans, the exact mechanism of male and female brain development has still to be elucidated. Based on clinical evidence from genetic men (XY) suffering from a mutation in the androgen receptor gene (complete androgen-insensitivity syndrome) and who show a female phenotype of the external genitals as well as the brain, it can be proposed that direct action of testosterone is probably causing the brain to differentiate in the male direction. However, when the process of genital development and of brain sexual development does not match the same sex, females with a male brain and vice versa can arise. These transsexual people have problems with their gender identity and have the conviction of being born in the wrong body. Twin and family studies show that there are genetic factors influencing the chances of a gender identity problem. Genetic factors could play a large role in the sexual differentiation of the brain, as can be shown from studies where differential genetic expression is found before development of the gonads. These genes could also function in other tissues than gonads and influence the sexual differentiation of the brain. The DMRT gene family which encodes transcription factors or the amount of sex hormone binding globulin (SHBG) is possibly influencing the development of sex differences, just as sex-biased differential splicing. Epigenetic mechanisms such as X-inactivation and genomic imprinting are also good candidates for causing differences in the sexual differentiation of the brain. These observations indicate that probably many processes operate together in the sexual differentiation of the brain and that diverse mutations can lead to gender identity problems.

Author/-s:       L. A. Worrell

Publication:     Master Thesis, Faculty of Medicine, Universiteit Utrecht, 2010

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The Epigenetics of Sex Differences in the Brain

Epigenetic changes in the nervous system are emerging as a critical component of enduring effects induced by early life experience, hormonal exposure, trauma and injury, or learning and memory. Sex differences in the brain are largely determined by steroid hormone exposure during a perinatal sensitive period that alters subsequent hormonal and nonhormonal responses throughout the lifespan. Steroid receptors are members of a nuclear receptor transcription factor superfamily and recruit multiple proteins that possess enzymatic activity relevant to epigenetic changes such as acetylation and methylation. Thus steroid hormones are uniquely poised to exert epigenetic effects on the developing nervous system to dictate adult sex differences in brain and behavior. Sex differences in the methylation pattern in the promoter of estrogen and progesterone receptor genes are evident in newborns and persist in adults but with a different pattern. Changes in response to injury and in methyl-binding proteins and steroid receptor coregulatory proteins are also reported. Many steroid-induced epigenetic changes are opportunistic and restricted to a single lifespan, but new evidence suggests endocrine-disrupting compounds can exert multigenerational effects. Similarly, maternal diet also induces transgenerational effects, but the impact is sex specific. The study of epigenetics of sex differences is in its earliest stages, with needed advances in understanding of the hormonal regulation of enzymes controlling acetylation and methylation, coregulatory proteins, transient versus stable DNA methylation patterns, and sex differences across the epigenome to fully understand sex differences in brain and behavior.

Author/-s:       Margaret M. McCarthy; Anthony P. Auger; Tracy L. Bale; Geert J. De Vries; Gregory A. Dunn; Nancy G. Forger; Elaine K. Murray; Bridget M. Nugent; Jaclyn M. Schwarz; Melinda E. Wilson

Publication:     The Journal of Neuroscience, 2009

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Mecp2 organizes juvenile social behavior in a sex-specific manner

Methyl-CpG-binding protein 2 (MeCP2) binds methylated DNA and recruits co-repressor proteins to modify chromatin and alter gene transcription. Mutations of the MECP2 gene can cause Rett syndrome (RTT), while subtle reductions of MeCP2 expression may be associated with male dominated social and neurodevelopmental disorders. We report that transiently decreased amygdala Mecp2 expression during a sensitive period of brain sexual differentiation disrupts the organization of sex differences in juvenile social play behavior. Interestingly, neonatal treatment with Mecp2 siRNA within the developing amygdala reduced juvenile social play behavior in males but not females. Reduced Mecp2 expression did not change juvenile sociability or anxiety-like behavior, suggesting this disruption is associated with subtle behavioral modification. This suggests Mecp2 may have an overlooked role in the organization of sexually dimorphic behaviors and that male juvenile behavior is particularly sensitive to Mecp2 disruption during this period of development.

Author/-s:       Joseph R. Kurian; Meaghan E. Bychowski; Robin M. Forbes-Lorman; Catherine J. Auger; Anthony P. Auger

Publication:     The Journal of Neuroscience, 2008

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The control of sexual differentiation of the reproductive system and brain

This review summarizes current knowledge of the genetic and hormonal control of sexual differentiation of the reproductive system, brain and brain function. While the chromosomal regulation of sexual differentiation has been understood for over 60 years, the genes involved and their actions on the reproductive system and brain are still under investigation. In 1990, the predicted testicular determining factor was shown to be the SRY gene. However, this discovery has not been followed up by elucidation of the actions of SRY, which may either stimulate a cascade of downstream genes, or inhibit a suppressor gene. The number of other genes known to be involved in sexual differentiation is increasing and the way in which they may interact is discussed. The hormonal control of sexual differentiation is well-established in rodents, in which prenatal androgens masculinize the reproductive tract and perinatal oestradiol (derived from testosterone) masculinizes the brain. In humans, genetic mutations have revealed that it is probably prenatal testosterone that masculinizes both the reproductive system and the brain. Sexual differentiation of brain structures and the way in which steroids induce this differentiation, is an active research area. The multiplicity of steroid actions, which may be specific to individual cell types, demonstrates how a single hormonal regulator, e.g. oestradiol, can exert different and even opposite actions at different sites. This complexity is enhanced by the involvement of neurotransmitters as mediators of steroid hormone actions. In view of current environmental concerns, a brief summary of the effects of endocrine disruptors on sexual differentiation is presented.

Author/-s:       C. A. Wilson; D. C. Davies

Publication:     Reproduction, 2007

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Transsexualism. Epidemiology, phenomenology, regret after surgery, aetiology, and public attitudes

Transsexualism denotes a condition in which the gender identity-the personal sense of being a man or a woman-contradicts the bodily sex characteristics. This thesis is based on three independent surveys about transsexualism.

FIRST, all 233 subjects applying for sex reassignment in Sweden during 1972-1992 were retrospectively examined through medical records. The incidence of applying for sex reassignment was 0.17/100,000 individuals over 15 years of age and per year. The male-to-female (M-F)/female-to-male (F-M) ratio was 1.4/1. With the exception of an incidence peak related to the legislation regulating sex reassignment in the early 1970s, the incidence has remained fairly stable since the first estimates in Sweden in the late 1960s. The M-F (n=134) and F-M (n=99) groups were phenomenologically compared. M-F transsexuals were older, and more often had a history of marriage and children than their F-M counterparts. M-F transsexuals also had more heterosexual experience. F-M transsexuals, on the other hand, more frequently reported cross-gender behaviour in childhood than did M-F transsexuals. It is concluded that transsexualism is manifested differently in males and females. The regret frequency (defined as applying for reversal to the original sex) was 3.8%. Prognostic factors for regret were, 'a poor support from the family', and 'belonging to the secondary group of transsexuals' (denotes people who develop transsexualism only after a significant period of transvestism or homosexuality).

SECOND, 28 M-F transsexuals and 30 male controls were investigated. To test the hypothesis that genes coding for proteins involved in the sexual differentiation of the brain influence the susceptibility of transsexualism, we analysed (1) a tetra nucleotide polymorphism of the aromatase gene, (2) a CAG repeat sequence in the first exon of the gene coding for the androgen receptor, and (3) a CA repeat polymorphism of the estrogen receptor beta gene. Results support the notion that the gender identity is related to the sex steroid-driven sexual differentiation of the brain, and that certain genetic variants of three of the genes critically involved in this process, may enhance the susceptibility for transsexualism.

THIRD, a questionnaire comprising questions about attitudes towards transsexualism and transsexuals was mailed to a random national sample (n=998) of Swedish residents, 18–75 years of age. The response rate was 67%. The results showed that a majority supports the possibility for transsexuals to undergo sex reassignment. However, 63% thought that the individual should bear the expenses for it. In addition, a majority supported the transsexuals' right to get married in their new sex, and their right to work with children. Transsexuals' right to adopt and raise children was supported by 43% whereas 41% opposed this. The results indicated that those who believed that transsexualism is caused by psychological factors had a more restrictive view on transsexualism than people who held a biological view.

Author/-s:       Mikael Landén

Publication:     Doctoral Thesis, University of Gothenburg, 1999

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Gene- and environment-dependent neuroendocrine etiogenesis of homosexuality and transsexualism

Sexual brain organization is dependent on sex hormone and neurotransmitter levels occurring during critical developmental periods. The higher the androgen levels during brain organization, caused by genetic and/or environmental factors, the higher is the biological predisposition to bi- and homosexuality or even transsexualism in females and the lower it is in males. Adrenal androgen excess, leading to heterotypical sexual orientation and/or gender role behavior in genetic females, can be caused by 21-hydroxylase deficiency, especially when associated with prenatal stress. The cortisol (F) precursor 21-deoxycortisol (21-DOF) was found to be significantly increased after ACTH stimulation in homosexual as compared to heterosexual females. 21-DOF was increased significantly before and even highly significantly after ACTH stimulation in female-to-male transsexuals. In view of these data, heterozygous and homozygous forms, respectively, of 21-hydroxylase deficiency represent a genetic predisposition to androgen-dependent development of homosexuality and transsexualism in females. Testicular androgen deficiency in prenatal life, giving rise to heterotypical sexual orientation and/or gender role behavior in genetic males, may be induced by prenatal stress and/or maternal or fetal genetic alterations. Most recently, in mothers of homosexual men--following ACTH stimulation--a significantly increased prevalence of high 21-DOF plasma values and 21-DOF/F ratios was found, which surpassed the mean + 1 SD level of heterosexual control women. In homosexual men as well--following ACTH stimulation--most of the 21-DOF plasma values and 21-DOF/F ratios also surpassed the mean + 1 SD level of heterosexual men. In only one out of 9 homosexual males, neither in his blood nor in that of his mother increased 21-DOF values and 21-DOF/F ratios were found after ACTH stimulation. In this homosexual man, however, the plasma dehydroepiandrosterone sulfate (DHEA-S) values and the DHEA-S/1000 x A (A = androstenedione) ratio were increased before and after ACTH stimulation. Furthermore, highly significantly increased basal plasma levels of dehydroepiandrosterone sulfate were found in male-to-female transsexuals as compared to normal males, suggesting partial 3 beta-ol hydroxysteroid dehydrogenase deficiency to be a predisposing factor for the development of male-to-female transsexualism.

Author/-s:       G. Dörner; I. Poppe; F. Stahl; J. Kölzsch; R. Uebelhack

Publication:     Experimental and Clinical Endocrinology, 1991

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