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Hormone Linked to Male Bias in Autism

Autism

AMH may have slown the cognitive development of boys so that 6-year-old boys with very high levels of AMH have brains that are as mature as an average 4.5-year-old girl.

By Ian McLennan

Anti-Müllerian hormone affects the rate of development of boys, leading to a male bias in autism spectrum disorders.

Men and women have different propensities to develop medical conditions, particularly brain disorders. Autism spectrum disorders and motor neuron disease are more common in males, while anoxeria nervosa and depression have a female bias. Schizophrenia tends to be more severe in young males. Our research is asking whether protein hormones released from the testes of boys creates the male gender and contributes to the male biases in some diseases.

The male and female forms are different throughout the entire body, but the nature of the differences varies between tissues. The reproductive organs have distinct female and male forms that have little in common: women have ovaries and a uterus, whereas men have testes and no uterus.

The male and female forms overlap elsewhere in the body, with sex differences only existing when groups of men and women are compared. Men, for instance, are taller than women on average, but a tall woman is still feminine. Such differences do not define a person’s sex. I refer to this type of subtle bias as a gender difference to distinguish them from the larger differences that define a person’s biological sex.

The Creation of Sex and Gender

Embryos are initially asexual, having the precursors of both the male and the female forms. The gross features of females are the default setting, and develop automatically without the need for a female signal. Males, in contrast, require signals from their testes to acquire their maleness, with testosterone being particularly important.

When an individual with male sex chromosomes (XY) lacks the ability to respond to testosterone, she will have a normal female appearance even though she has testes rather than an ovary. Her first inkling that she is different to other women may not occur until she wonders why she is the last girl in her class to begin to menstruate. This is because a hormone from her testes has triggered the degeneration of the uterus during embryonic development.

Testosterone creates the overt sex differences that catch the eye and define a person’s sex. But does it also create all of the gender biases? Initially it was presumed to, for testosterone and maleness seemed to be inseparable.

One of the many problems with this notion is that testosterone is only transiently present during male development. In humans, male foetuses have low levels of testosterone that overlap with females from around the 20th week of gestation. Newborn male babies have high levels of testosterone, but boys from 6-months of age until puberty have less testosterone than their mothers.

If not testosterone, then what creates gender biases? Women are wonderfully diverse in their femininity, as are men in their masculinity. This suggests that gender is created by a number of factors. Social forces are involved, but some gender biases in the brain are evident at birth, suggesting that other biological factors exist.

Some of the genes of the sex chromosomes are important here. Inactivation of such genes influences a person’s gender but not their sex. Rarely studied hormones are also important.

The testes have two hormone-producing cells. The first to form are the Sertoli cells. These release protein hormones, most notably anti-Müllerian hormone (AMH).

The Müllerian duct is the precursor of the uterus. Male embryos initially have this duct, but it degenerates as soon as their testes release AMH. This is what gives anti-Müllerian hormone its name: it opposes the Müllerian duct.

AMH is an almost perfect mirror-image of testosterone, in the most paradoxical of ways. Testosterone is only transiently present during male development. Its influence would therefore be expected to be restricted. However, when testosterone is absent, an XY individual is overtly female.

AMH is present at high levels in the blood of males from the eighth week of gestation until puberty, with girls having no AMH until they enter puberty. Human males that lack AMH have an unremarkable appearance, except that their testes do not descend into the scrotum. On closer examination, their testes are being held within the abdomen, as they have a rudimentary uterus attached to them.

Developing males thus have a hormone that is not shared with their sisters, but at first sight does nothing after the first trimester of pregnancy. This begged an explanation, but this issue had never been explored. It was an exoteric observation buried in the depth of the library. Testosterone was so pre-eminent that few biologists were actively seeking novel male influences, and those who did were exploring the sex chromosomes.

Shaping the Male Brain

A few years ago my research group was working on motor neuron disease. We were seeking novel regulators of the motor neurons that control movement, and used a simple strategy. Cells respond to biological regulators through proteins known as receptors. We therefore isolated motor neurons from the spinal cord of mice and asked whether they produced any receptors that were not known to be present in the brain.

Surprisingly, the data suggested that the receptor for AMH was present in motor neurons. Even more surprisingly the AMH receptor was present in all neurons. In contrast, the receptors for testosterone are concentrated in discrete regions of the brain.

AMH receptors are present throughout the brain, and AMH is present throughout the entire period of brain development, but its absence has no obvious effect. For a while, deducing its function seemed like a huge challenge until we realised that the absence of an overt effect was a stunning clue.

The male or female sexual forms are very distinct, and when an aspect of sex is abnormal it will be noticed. Gender has multiple determinants and varies from person to person, with overlap between the sexes. If one aspect of the biology of gender were missing, then who would notice?

If AMH is an important determinant of gender, then male mice that were AMH-deficient should be sexually male (which they are) but have some feminine aspects to their gender. One gender difference is the number of brain cells, with many parts of the brain being subtly larger in males. In AMH-deficient mice, some of these gender differences in the number of neurons are absent, indicating that AMH is a factor shaping the male brain.

In the wild, female and male mice will defend their territories, with male mice generally having larger territories than their sisters. In the laboratory, this behavioural trait is expressed as a male bias in the propensity to explore novel objects or novel territory. The exploratory behaviour of AMH-deficient male mice is like that of a female. AMH-deficient male mice are thus sexually male but have female-like non-sexual behaviour. Our attention was immediately drawn to human gender.

Men and women are different, but which of the myriad of differences between the sexes are a reflection of real biology? Which are important, and which are trivial? For us, the sex biases in disease were the most interesting targets, with the taxing issue being how to explore the function of a hormone in humans when manipulation of its levels is not possible.

The function of a hormone can be examined by observing what events occur in the body when the level of the hormone changes. Insulin, for example, rises after eating, with glucose levels decreasing after the rise in insulin. Insulin controls glucose levels in blood.

AMH is an atypical hormone that does not vary during the day. Even the level of the hormone in each boy is largely stable over many years. This suggests that AMH regulates processes that occur over a very long period of time.

Although each boy has a stable level of AMH, the level of AMH is highly variable between boys. This created a way forward for us. We postulated that traits regulated by AMH should be more evident in boys with high levels of AMH than those with lower levels. If so, some of the functions of AMH should be detectable by correlating the characteristics of boys with their level of AMH.

Boys on average develop more slowly than girls, both physically and cognitively. Our experiments suggest that AMH creates this sex difference by slowing male development. Boys with high levels of AMH are short relative to the height of the parents, whereas boys with low levels of AMH tend to be tall relative to their parents.

We estimate that the physical development of boys with an average level of AMH has been slowed by about 9 months at the age of 6. This is a large effect, but consistent with the fact that boys take over 2 years longer on average than girls to reach their adult height.

The difference in the speed of development is most evident in the higher brain centres, with boys on average taking 2–4 years longer to reach peak brain volume than girls, depending on which part of the brain is examined. Our exploration of this issue is at any early stage, but the initial data suggest that AMH may generate this difference.

Boys aged 5–6 year who have high levels of AMH tend to draw immature stick-like representations of themselves. As with height, we estimate that AMH may slow the cognitive development of boys so that 6-year-old boys with very high levels of AMH have brains that are as mature as an average 4.5-year-old girl. We wonder how these boys cope with school.

Severity of Developmental Disorders

Rapid development confers many advantages, including diminished risk of dying during development. Equally, slow development is also advantageous, but for different reasons. When complex organisms develop rapidly, the rate of microscopic defects in their cellular anatomy increases, making them less robust. Minor genetic or environmental defects can easily escalate to severe problems. Fast development may thus be a risk factor for congenital disorders.

The brain is uniquely difficult to develop. Consequently, the speed at which vertebrates develop has slowed as they have evolved a larger brain. The brain of men, on average, is 11% larger than women although there is much overlap between the sexes.

Men may therefore need to develop slowly, yet some boys develop quickly and this may put them at risk of congenital disorders. If AMH slows male development as we suspect, then boys with low levels of AMH should suffer from congenital disorders to a greater extent than boys with naturally high levels of AMH. We therefore sought a developmental disorder with a male bias.

The autistic spectrum describes a family of developmental disorders that are characterised by repetitive behaviours and difficulties in socialisation and communication. Autism has a significant male bias and precocious physical development of the brain.

Some girls develop autism. This suggests to us that the underlying cause of autism is common to female and males, with the severity of autism being affected by whether an individual is developing fast or slow relative to their intrinsic need.

Consistent with this, we have recently found that the severity of autistic symptoms is inversely correlated to a boy’s level of AMH. AMH is not a cause of autism: the range of AMH levels in autistic boys is not different to non-autistic boys. Rather, the correlation between AMH and the severity of autism appears to arise because boys with low AMH develop more rapidly, with rapid development amplifying any mild disorder of development.

The Future

The proof of ideas in biology requires many layers involving different types of experiments. The putative role of AMH in shaping the male gender has only a single layer. It is a beginning of a story, not the middle and nowhere near the end. The testes of boys do not make sperm and only trace levels of testosterone, but they do produce protein hormones, of which AMH is but one.

What we can be reasonably certain of is that a boy’s testes are not sitting inert, waiting for adulthood. In some yet to be defined way, they are shaping his development, and contributing to the nature of the future man.

Ian McLennan is Professor of Anatomy at the University of Otago.