Medscape, NY, USA
Endocrine PracticeEvidence Supporting the Biologic Nature of Gender Identity
Aruna Saraswat, MD, Jamie D. Weinand, BA, BS, Joshua D. Safer, MD |
DisclosuresEndocr Pract. 2015;21(2):199-204.
Abstract and IntroductionAbstractObjective To review current literature that supports a biologic basis of gender identity.
Methods A traditional literature review.
Results Evidence that there is a biologic basis for gender identity primarily involves (1) data on gender identity in patients with disorders of sex development (DSDs, also known as differences of sex development) along with (2) neuroanatomical differences associated with gender identity.
Conclusions Although the mechanisms remain to be determined, there is strong support in the literature for a biologic basis of gender identity.
__________
IntroductionGender identity is a fundamental human attribute that has a profound
impact on personal well-being. Transgender individuals are those whose
lived and identified gender identity differs from their natal sex.
Various etiologies for transgender identity have been proposed, but
misconceptions that gender identity can be altered persist. However,
clinical experience with treatment of transgender persons has clearly
demonstrated that the best outcomes for these individuals are achieved
with their requested hormone therapy and surgical sexual transition as
opposed to psychiatric intervention alone.
[1] In this review, we will discuss the data in support of a fixed, biologic basis for gender identity.
MethodsThis traditional literature review was conducted using a search of PubMed and Google Scholar for the following key terms: gender identity, gender dysphoria, transsexual, transgender, transmen, and transwomen.
ResultsDisorders (or Differences) of Sex Development (DSDs)
A seminal study by Meyer-Bahlburg et al involving outcomes of XY
individuals raised as females due to severe nonhormonal, anatomic
abnormalities of sex development provided the most convincing evidence
that gender identity is fixed.
[2]
These congenital abnormalities include penile agenesis, cloacal
exstrophy, and penile ablation. For many years, female gender assignment
along with surgical feminization was the dominant approach for these
patients. In this study, 78% of all female-assigned 46 XY patients were
living as females. While the majority of these patients did not initiate
a gender change to male, none of the 15 male-raised 46 XY patients
initiated a gender change to female. Thus, the risk of questioning
gender identity was higher in those 46 XY subjects raised as females
than in those raised as males. The same group examined the degree of
satisfaction with surgical intervention reported by patients with 46 XY
genotypes and found that those subjects raised as boys were considerably
more comfortable with their gender identity.
[3]Another seminal study relevant to this topic was by Reiner and Gearhart.
[4]
In their review of 16 XY genotype subjects with cloacal exstrophy who
underwent female gender reassignment surgery, 4 of the 14 individuals
raised as girls announced they were male, and 4 later chose to live as
boys when they became aware of their genotype. The 2 individuals who
were raised as males identified as males throughout life. The sexual
behavior and attitudes of all 16 subjects ultimately reflected strong
masculine characteristics regardless of gender assignment. Thus,
children who were born genetically and hormonally male identified as
males despite being raised as females and undergoing feminizing
genitoplasty at birth. Although the cohort sizes in these studies were
small, the data provide the strongest evidence for the biologic
underpinnings of gender identity.
In congenital adrenal hyperplasia (CAH), the adrenal glands produce
excessive amounts of androgens, causing genital virilization with a
spectrum of different phenotypes in 46 XX neonates. Dessens et al
[5]
reported that the prevalence of male gender identity in 46 XX
female-raised subjects with CAH was higher than the prevalence of
female-to-male (FTM) transgender individuals in the general population
of chromosomal females. In this study, the large majority (95%) of 250
female-raised patients later maintained a female gender identity.
However, 13 (5.2%) had serious problems with their gender identity.
Deficiencies of 5 alpha-reductase-2 and 17-beta-hydroxy-steroid
dehyrogenase-3 are similar conditions in which the synthesis and
conversion of testosterone to dihydrotestosterone is inhibited,
preventing the development of external male genitalia and resulting in
potential genital ambiguity. As with CAH, affected individuals are often
raised as females. In a study of affected subjects, gender role changes
were reported in 56 to 63% of cases with 5 alpha-reductase-2 deficiency
and 39 to 64% of cases with 17-beta-hydroxysteroid dehydrogenase-3
deficiency who were raised as girls.
[6] These data support the concept that gender identity might be attributed to hormone milieu during intrauterine development.
Data from DSDs highlight the potential influence of abnormal hormone exposure on the development of transgender identity in some individuals. However, it is important to note that most transgender individuals develop a gender identity that cannot be explained by atypical sexual differentiation. It is possible for individuals with normal sexual differentiation to develop transgender identity later in life.
Neuroanatomical Differences. Many of the current hypotheses for
the biologic origin of transgender identity are based on atypical sexual
differentiation of the brain. The perception of one's own gender is
linked to sexual differentiation of the brain, which differs from the
body phenotype in transgender individuals.
[7]
Swaab et al have proposed that this discrepancy could be due to the
fact that sexual differentiation of the brain takes place only after
sexual differentiation of the gonads in early fetal life.
[8] Along these lines, the degree of genital masculinization may not reflect that of the brain.
The notion of transgender-specific cerebral phenotypes is further
supported by postmortem brain studies investigating the underlying
neuroanatomical correlates of gender identity.
[9,10,12]
The vast majority of these studies have compared particular regions of
interest only in male-to-female (MTF) transgender individuals.
[13–15] These studies support the hypothesis that atypical cerebral networks in transgender individuals have a neuroanatomical basis.
Gray Matter Studies. Studies of cerebral gray matter in transgender individuals have provided
the strongest neuroanatomical case for transgender gender identity.
Postmortem brain studies suggest that some subcortical structures are
feminized in MTF individuals. One of the earliest and most influential
studies in this area investigated the bed nucleus of the stria
terminalis (BSTc), which was reported to be a sexually dimorphic nucleus
in humans with a larger volume in males than in females. In 1995, Zhou
et al reported that the size and number of neurons in the BSTc of 6 MTF
estrogen-treated transgender individuals was typical for the size and
neuron numbers generally found in control females.
[9] The authors further reported that these findings could not be explained by differences in adult sex hormone levels.
A similar study by Kruijver et al provided further data supporting the role of the BSTc in transgender identity.
[10]
They examined tissue from the same 6 MTF estrogen-treated transgender
persons studied by Zhou et al and found that the number of neurons in
the BSTc was more similar to genetic XX female controls. BSTc neuron
number was also in the male range in the 1 FTM androgen-treated
transgender individual studied.
Most transgender individuals experience feelings of gender dysphoria
that begin in childhood. However, in a study of BSTc volume in
postmortem brains of 50 control subjects, Chung et al reported that
sexual dimorphism in the BSTc did not develop until adulthood.
[11]
Yet, the same group remarked that changes in fetal hormone levels could
have delayed effects on BSTc volume and neurons in adulthood, thereby
suggesting a role for BSTc as a marker for gender identity. Still,
delayed development of sexual dimorphism in the BSTc would not explain
childhood development of gender dysphoria or gender identity
discrepancy.
In 2008, Garcia-Falgueras and Swaab were the first to report a sex
reversal in the uncinate nucleus. They examined the third interstitial
nucleus of the anterior hypothalamus (INAH 3), which is a sexually
dimorphic component of the uncinate nucleus, in relation to the brains
of transgender individuals.
[12] They reported that the mean INAH3 volume and neuron number in 11 MTF transgender subjects were in the female ranges.
The above studies are limited by the fact that they involved postmortem examinations of a small number of brains from MTF individuals, some of whom had either received hormone treatment or surgery. Therefore, the study findings may represent confounding effects from exogenous hormones in a small group of transgender individuals. Despite their small sample size, these studies provide valuable evidence that gender identity is linked to neuroanatomy.
Studies by Luders et al provided further evidence that transgender identity is associated with distinct cerebral patterns.
[13,14]
In 2009, the group analyzed magnetic resonance imaging (MRI) data of 24
MTF transgender individuals who had not yet begun hormone treatment.
These subjects were shown to have a pattern that was more similar to
control males. However, they also observed a significantly larger, more
"feminized" volume of regional gray matter in the right putamen in these
subjects. In 2012, the same group observed thicker cortices in 24 MTF
transgender individuals who had not yet received exogenous hormones
compared with 24 age-matched control males in a number of regions across
the lateral and medial cortical surfaces. The data supported a
dichotomy between MTF transgender individuals and gender congruent males
with regard to brain structure.
Differences in brain volume and cerebral activation patterns have been
proposed as potential explanations for transgender identity. In 2011,
Savic et al examined brains of 24 living MTF transgender individuals and
found significant volume reductions of the putamen in MTF transgender
individuals and significant increases in gray matter volumes compared
with male and female controls.
[15]
Although these findings differ from the findings of smaller,
"feminized," putamens in MTF transgender individuals, they still
indicate that certain brain areas in the transgender group have
characteristic structural features compared with controls.
The same group investigated 12 living MTF transgender individuals who
smelled 2 steroidal compounds: the progesterone derivative
4,16-androstadien3-one (AND) and the estrogen-like compound estra-1,3,5,
[10]
16-tetraen-3-ol (EST). These compounds have been reported to activate
the hypothalamic networks in a sex-differentiated way. MTF transgender
individuals who had not received hormone treatment were found to respond
similarly to female controls, with AND activating the anterior
hypothalamus.
[16]
Another study by Gizewski et al showed a similar cerebral activation in
MTF transgender individuals relative to female controls while they
viewed erotic stimuli.
[17]
While the above studies only involved MTF transgender individuals, they
nonetheless provided evidence of neuroanatomical pathway alteration as
an explanation for transgender identity.
The following 2 studies were unique from the aforementioned ones because
they included both MTF and FTM transgender individuals who had not
received hormone treatment. Zubiaurre et al reported that FTM
transgender individuals showed evidence of subcortical gray matter
masculinization in the right putamen, while MTF transgender individuals
had feminized cortical thickness.
[18] In 2013, Simon et al reported differences in gray matter in 17 living transgender subjects compared with controls.
[19]
Differences were seen in transgender patients in the cerebellum,
angular gyrus, and parietal lobe compared with controls, independent of
their biologic gender.
White Matter Studies. Although an early study by Emory et al
[20]
found no difference in the whole corpus callosum or splenium region
between MTF and FTM transgender individuals, the following MRI studies
of white matter brain characteristics of transgender individuals
suggested a strong neuroanatomical explanation for transgender identity.
Yokota et al reported that the pattern of corpus callosum shape in both
FTM and MTF transgender individuals was closer to subjects with shared
gender identities than to subjects who shared the same natal sex.
[21]
Among FTM transgender individuals who had not received hormone
treatment, certain white matter fasciculi involved in higher cognitive
functions were closer to the pattern of control males than to control
females.
[22]
Among MTF transgender individuals who had not received treatment,
diffusion tensor imaging revealed an intermediate white matter pattern
that was between those of male and female controls.
[23]__________
Genetic Factors and ExposuresAlthough limited in size and scope, the role of genetic factors in transgender identity is supported by small studies of gene abnormalities associated with steroid hormones, twin case studies, neuroproteins, and prenatal exposures.
Steroid Hormone Genetics. Select genes have been associated with transgender identity. Although
these studies have been small, they are most convincing findings to date
linking atypical genes with transgender identity in both MTF and FTM
transgender individuals. The
CYP17 gene encodes the 17-alpha
hydroxylase enzyme and is associated with elevated serum levels of
estradiol, progesterone, and testosterone. In a case-control study of
151 transgender individuals, Bentz et al reported a significant
association between the
CYP17 gene and FTM transgender individuals but not in MTF transgender individuals.
[24]
Another study by the same group examined a polymorphism in the gene
coding for 5-alpha reductase and found no association in a sample of
both MTF and FTM transgender individuals.
[25]Various groups have investigated steroid hormone receptor gene variants
to determine if they confer risk of developing transgender identity.
Steroid hormones exert profound influences on fetal sexual development
and act via specific receptors. It is therefore plausible that abnormal
sex hormone receptor function may predispose to transgender identity.
However, the existing studies on this topic have been contradictory and
require replication. Henningson et al found an association between MTF
transgender individuals and a dinucleotide CA polymorphism in the
estrogen receptor beta gene (ERb).
[26]
However, 2 subsequent studies by separate groups reported different
results. Hare et al performed a larger study of MTF transgender
individuals and found no relationship with the ERb, but they did find a
significant association with an androgen receptor repeat.
[27]
In a similar study of 242 MTF and FTM transgender individuals, Ujike et
al examined sex steroid receptor genes and found no association with
transgender identity.
[28]There have been several small case reports of atypical sex chromosomes
in transgender individuals. The most common association reported was
with disomy-Y (47, XXY); however, no statistically significant
association between particular genes has been described.
[29]
Two recent studies of MTF and FTM transgender individuals reported that
aneuplodies are slightly more common in transgender individuals than in
the general population, but neither was controlled. In the first,
karyotype abnormalities were found in 2.5% of the 368 transgender
individuals studied.
[30] A second study of 302 transgender individuals also showed a low overall incidence (1.5%) of chromosomal abnormalities.
[31]Twin Studies. Twin literature supports the potential contribution of genetic factors
to the development of transgender identity. In 2 separate retrospective
studies of twin pairs, Bailey et al and Coolidge et al demonstrated a
strong heritable component among twins with transgender identity.
[32,33]
Hylens et al performed a similar study of 23 monozygotic twin pairs and
showed that 9 were concordant for transgender identity compared to no
concordance among dizygotic twin pairs.
[34] Two small studies
[35,36]
also demonstrated a higher concordance for transgender identity among
monozygotic twins versus dizygotic twins. Nevertheless, the overall
prevalence of monozygotic twins discordant for transgender identity
still outnumbers those who are concordant.
Neuroproteins. Brain-derived neurotrophic factor (BDNF) is a member of the growth
factor family involved in synaptic plasticity and neuronal development.
Altered BDNF signaling is thought to be a contributor to psychiatric
conditions. Fontanari et al
[37]
reported that serum BDNF levels were 15% lower in an uncontrolled study
of 45 MTF transgender individuals. However, all study subjects were
treated with hormones, and no female subjects were included.
Neurokinin B (NKB) is a potent regulator of gonadotropin-releasing
hormone secretion, which is essential for reproductive function. A
postmortem brain study of 4 MTF transgender individuals by Taziaux et al
[38]
showed a mean infundibular NKB volume similar to control females. The
observed feminization may have been explained either by medical estrogen
therapy or lack of androgens due to orchiectomy.
Prenatal Exposures. Dessens et al
[39]
reported that 3 prenatally anticonvulsant-exposed subjects were
transgender individuals. For many years, researchers have been assessing
the impact of prenatal exposure to the estrogenic antimiscarriage drug
DES (diethylstilbestrol) on the development of gender dysphoria in
affected offspring. While the vast majority of DES-exposed children have
not developed transgender identity, a 5-year online study of
DES-exposed sons by Kerlin et al reported at least 150 cases of
moderate-to-severe gender dysphoria among 500 sons with confirmed or
suspected prenatal DES exposure.
[40]Although no studies to date demonstrate mechanism, multiple studies have
reported associations with gender identity that support it being a
biologic phenomenon.
Table 1 organizes areas studied by study type and lists the associations that have been made.
----------
Table 1. Evidence for a Biologic Basis of Gender Identity
Studies showing rigid gender identity in patients with disorders (or differences) of sexual development (DSD) |
• Congenital, nonhormonal conditions |
Penile ablation/agenesis, cloacal exstrophy |
Studies showing that gender
identity may be associated with prenatal hormone exposure in some
(perhaps otherwise predisposed) individuals |
• Congenital adrenal hyperplasia |
|
• Hormone deficiencies |
5 alpha-reductase-2, |
|
17-beta-hydroxy-steroid dehydrogenase-3 |
Studies with gender identity associated with neuroanatomical differences |
• Gray matter studies |
BSTc |
|
Uncinate nucleus |
|
Putamen volumes |
|
Cortical thickness |
|
Hypothalamic response to odorous steroids |
• White matter studies |
Corpus callosum |
|
Microstructure differences |
Studies with gender identity associated with genetic factors and exposures |
• Steroid hormone genetics |
Genes: CYP17, SRD5A2, ERb, androgen receptor |
• Sex chromosome aneuploidy |
Disomy-Y |
• Twin case studies |
|
• Neuroproteins |
BDNF, NKB |
• Prenatal exposures |
Anticonvulsants, DES |
Abbreviations: BDNF = brain-derived neurotrophic factor; BSTc =
bed nucleus of stria terminalis; CYP17 = cytochrome P-450 17 alpha gene;
DES = diethylstilbestrol; ERb = estrogen receptor beta gene; NKB =
neurokinin B; SRD5A2 = steroid-5-alpha reductase, alpha polypeptide 2
gene.
----------
ConclusionCurrent data suggest a biologic etiology for transgender identity. Studies of DSD patients and neuroanatomical studies provide the strongest evidence for the organic basis of transgender identity. Because the sample sizes of most studies on this subject were small, the conclusions must be interpreted with caution. Further research is required to assign specific biologic mechanisms for gender identity.
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