Why autism affects boys more than girls?

It really bewilders me to witness more boys affected with autism than girls. Whether autism is down to a cruel hereditary, environmental, lifestyle factors or ‘vaccine reaction’ is open to debate and less is known, but my intention here is the gender discrepancy and what has attributed for it. This mystery has also befuddled autism researchers.

Autism is a lifelong neurological disorder that typically manifests by three years of age. Individuals with autism are severely impaired in reciprocal social interaction, cognitive development and verbal and non-verbal communication. Other difficulties may include gross motor problems, unusual fear or anxiety, hyper-orality and the inability to modulate sensory input.

From the data I found, boys are three to four times more likely to be diagnosed with autistic spectrum disorders than girls. While experts do not yet have a solid answer for the obvious gender discrepancy, some believe that girls with mild autism may be better able to mask their symptoms and thus go undiagnosed.

Some link this with genetic factors that predispose boys more. The cause of autistic spectrum disorders is not yet clear. Genetics play an important role, and researchers are examining a number of chromosome sites that could be implicated. It’s likely that autism occurs when a small number of genes interact in a specific way, possibly linked to some external event or factor. A new finding reported by the journal Molecular Psychiatry in 2009 found a genetic clue that may help explain the disparity.

The newly discovered autism-risk gene, identified by authors as CACNA1G, is more common in boys than in girls (why that’s so is still not clear), and the authors suggest it plays a role in boys’ increased risk of the developmental disorder. CACNA1G, which sits on chromosome 17, amid other genes that have been previously linked to autism, is responsible for regulating the flow of calcium into and out of cells. Nerve cells in the brain rely on calcium to become activated, and research suggests that imbalances in the mineral can result in the overstimulation of neural connections and create developmental problems, such as autism and even epilepsy, which is also a common feature of autism.

The contemporary theories about autism suggest that the disorder is related to overexcitability at nerve endings and it will be interesting to see that the gene they identified appears to modulate excitability of neurons. The previous understanding rather emphasises on the fact that mutations on chromosome 16 have been tied to autism. The theory behind might force us delve into the argument: neurological disorder or natural diversity?

The glitch is in a DNA region that contains “morpheus” genes, or genes which historically have changed very rapidly as humans evolved. In other words, the same method that helped evolve human intelligence may contribute to autism.

However, it will be apparent that a single gene (CACNA1G) might not most likely be responsible for a disorder as complex as autism — a spectrum of developmental disorders involving impairment in language, social behavior and certain physical behaviors — with symptoms that range widely in number and severity but it would serve as a determinant for the gender disparity, and many more genes around the area they investigate must be accountable for the different severities the autism spectrum show.

Currently there is no clear relationship between structural abnormalities in the brain and autism. However, some previous literature has suggested a possible link between autism and abnormal development of the amygdala (Baron-Cohen, Social intelligence in the normal and autistic brain: an fMRI study). These reviews primarily site evidence from post-mortem qualitative observations of autistic brain tissue and structural magnetic resonance imaging (MRI) studies of live subjects with autism vs controls. According to Coleman, The Neurology of Autism, “Researchers have found that the area of the brain called the amygdala was on average 13% larger in young children with autism compared with children without autism.”

The amygdala, an almond-sized and -shaped brain structure, has long been linked with a person’s mental and emotional state. But thanks to scientific advances, researchers have recently grasped how important this 1-inch-long structure really is. Associated with a range of mental conditions from normalcy to depression to even autism, the amygdala has become the focal point of numerous research projects.

This mentions nothing about the possible link-up with gender. Related research shows boys and men that suffer from autism have fewer neurons in the amygdala area of the brain than healthy men. Many scholars believe autism is a combination of genetic vulnerability that is triggered by some kind of social or toxic influence.  Boys are more likely to be affected than girls, though research suggests that when girls have the condition they may be more severely affected. Girls with autistic symptoms may be suffering from Rett Syndrome. This syndrome affects mainly females, because male fetuses with the disorder rarely survive to term. Development is typical until 6-18 months, when language and motor milestones are lost.

Researchers are unclear why boys are diagnosed with autism more than girls, but some think that the problem lies with the X chromosome (x-linked), of which boys only have one. According to the NewScientist in its Sex chromosome key to Autism article, David Skuse, from the Institute of Child Health in London, explains this affiliation in a lucid manner as:

A small group of genes on the X chromosome regulate the brain’s “threat-detector” and might explain the high prevalence of autism among males, researchers have discovered. Some people lacking these genes have problems recognising fear in another person’s face, a common trait in autism. They also have abnormal amygdalas – a brain area known as the “fear centre”. The results provide a possible genetic mechanism for the sex bias of autism. Other recent research has identified a gene in the same region of the X chromosome that correlates with the severity of autism. However, confirmation of this explanation of autism’s sex bias is still far off – researchers have not yet determined which specific gene or genes are responsible and have not looked at the function of these genes in autistic people.

The X chromosome carries many genes that are vital for a wide range of physiological functions. Women have two X chromosomes while men have an X and a Y. Because almost all women have two copies of X chromosome genes, their cells turn off, or inactivate, one copy. However, not all X-linked genes undergo this inactivation, meaning that women could have higher levels of some gene products in their cells. Skuse suggests it is these “dosage-sensitive” genes that are responsible for the sex differences in autism.

Skuse and his colleagues studied women with only one X chromosome, a condition known as Turner’s syndrome. These women are susceptible to problems with X-linked genes just like men and are much more likely to develop autism than unaffected people.  In fact, both autistic people and women with Turner’s syndrome share a common trait – they avoid eye contact and have trouble reading fear in another person’s facial expression. These problems can also be observed in the brain, says Skuse, as both groups show abnormalities in the function of the amygdala and its cortical connections.

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