INTRODUCTION

Despite a steadily increasing interest in sexual dimorphism of the human brain, our knowledge about the underlying mechanisms is still scant. It is an area that warrants further attention, considering that the vast majority of psychiatric disorders are characterized by a skewed sex distribution with regard to prevalence, age at onset, and symptom presentation [Young and Pfaff, 2014]. Many of these disorders affect the neuronal networks that process cognitive and emotional functions, and it is possible that inherent sex differences in these networks constitute vulnerability factors for disease development. Understanding the underpinnings of cerebral sex dimorphism may, therefore, provide us with important tools for identifying possible sex‐specific protective factors that could contribute to therapeutic improvement. Through brain imaging methods, sex differences have been shown in regard to cortical thickness [Good et al., 2001; Luders et al., 2006; Savic and Arver, 2014], and regional gray and white matter volumes [Goldstein et al., 2001; Good et al., 2001; Lentini et al., 2013; Nopoulos et al., 2000; Savic and Arver, 2011], and in subcortical volumes, namely larger volumes of the caudate and hippocampus in women, and larger putamen and amygdala volumes in men, [Filipek et al., 1994; Giedd et al., 1997; Goldstein et al., 2001; Lentini et al., 2013; Raz et al., 1995]. They have also been detected in neuronal connections, usually measured with diffusion tensor imaging (DTI), [Kanaan et al., 2012; Rametti et al., 2011; Takeuchi et al., 2013] and resting state PET and functional MRI [Erickson et al., 2005; Filippi et al., 2013; Kilpatrick et al., 2006; Kong et al., 2010; Savic and Lindstrom, 2008]. These differences cannot be attributed to differences in brain size [Luders et al., 2009] and are believed to be influenced by an intricate interplay between sex chromosome genes, sex hormones, and the external environment.

In the quest to distinguish between these etiological factors, studies of individuals with certain disorders of sex development (DSD), in which the development of chromosomal, gonadal, and/or anatomical sex is atypical [Hughes et al., 2012; McCarthy et al., 2012], can provide insights. Based on direct comparisons of cortical thickness (Cth) and gray matter volume (GMV) among 46,XX women, 46,XY men, and men with 47,XXY (Klinefelter syndrome), we recently put forward the hypothesis that programming of the motor cortex and the basal ganglia is influenced by processes linked to X‐chromosome inactivation escapee genes, which do not have Y‐chromosome homologs, whereas sex differences in the sensory cortices (the parietal and occipital corticesin particular) are inversely correlated to testosterone levels, [Savic and Arver, 2014]. These observations, together with corresponding data from women with the 45,X karyotype [Marzelli et al., 2011] who, like 46,XY men, have a thinner motor cortex, smaller nucleus caudate volumes, and larger amygdala volumes than 46,XX women, raise the interesting question as to whether Y‐chromosome genes have any masculinizing effect on the brain in addition to the effects mediated by testosterone and the genes involved in testis formation. To address this issue we chose to investigate individuals with complete androgen insensitivity syndrome (CAIS).

Women with CAIS have a 46,XY karyotype; they are born with testes that secrete male‐typical or elevated amounts of testosterone prenatally and postnatally because they lack functional androgen receptors due to mutations of the androgen receptor (AR) gene [Cheikhelard et al., 2009; Hughes et al., 2012; Quigley et al., 1995]. Individuals with CAIS are thus born with female external genitalia, develop a female phenotype, are reared as girls, and undergo a feminizing puberty as a result of the aromatization of their testosterone to estradiol [Cheikhelard et al., 2009]. Due to secretion of antimüllerian hormone (AMH) from the sertoli cells of the testes, female internal genitalia do not develop and no uterus is formed, and therefore CAIS is often presented as primary amenorrhea. Studies using quantitative measures of psychosexual development indicate that individuals with CAIS have a female gender identity, show female‐typical gender role behavior, and are most often androphilic (sexually attracted to men) [Koolschijn et al., 2014; Masica et al., 1971; Walhovd et al., 2010]; CAIS occurs in approximately 1–5 per 100,000 births [Mendoza and Motos, 2013].

Comparisons among women with CAIS, 46,XY males, and 46,XX female controls provide a unique opportunity to study the separate effects of testosterone and sex chromosome genes on the sexual differentiation of the brain.

We investigated this, using MR measurements of cortical thickness (Cth) and subcortical structural volumes. We also carried out DTI of fractional anisotropy (FA) indexing structural (axonal) connections in the brain, and resting state fMRI indexing cerebral functional connections. Sex differences have been reported in all these metrices, but the underpinnings of these differences may vary with the specific metric, and also between different neuronal networks. To better understand these complex interactions, multimethodological investigations of the same study groups would therefore be required, which are scarce in the literature, especially for populations with rare conditions. Based on previous studies suggesting pruning effects of testosterone [Fernandez et al., 2003; Rasgon et al., 2005], and its stimulating effect on white matter tracts [Rametti et al., 2012; van Hemmen et al., in press], we hypothesized that Cth of the parietal lobe (and probably also the occipital lobe) would be greater in both 46,XX women and women with CAIS, whereas the FA values in the long white matter tracts (the corticospinal tract, superior and inferior longitudinal fascicle, the fronto‐occipital fascicle, and perhaps also the corpus callosum [CC]) would be lower compared to 46,XY men. Given our previous comparative studies between XXY men and controls suggesting that both testosterone and X‐chromosome gene dosage could influence the amygdala [Savic and Arver, 2014], it was an open question as to whether the limbic and paralimbic networks would be affected by testosterone. This also applied to the caudate and putamen volumes for which we did not have a primary hypothesis.