There is quite a lot that is still unknown about the etiology of autism – just how much of it precisely is due to genetics and how much of it is influenced by other factors, or even the timeline of prenatal changes that are necessary to develop the postnatal neurodevelopment disorders commonly seen. There can be such a vast difference between a person who is barely on the spectrum and another who may develop severe impairments in social interactions and communication, and pinpointing the precise mechanism behind the development of Autism Spectrum Disorder (ASD) has been a challenge.
One thing that is very certain is there is a clear risk disparity between boys and girls: of the 1 in 68 children in the United States diagnosed with autism, the prevalence for boys is 4:1. In terms of percentages, that means about 80% of them would be boys. Among those with Asperger Syndrome, a mild form of ASD, the prevalence for boys is even higher at 11:1.1
There are several hypotheses for this disparity between the sexes: some ASD risk genes may have sexually dimorphic expression; perhaps they are located on the X-chromosome. Women have two X-chromosomes in their cells, with one of them being rendered inactive (two X chromosomes would otherwise mean double the dosage of every gene on that chromosome, so the body inactivates one X-chromosome at random; this inactive X-chromosome is called a Barr body). So it’s possible that many X’s with the ASD risk gene would be effectively ‘turned off’ in women, but in men this doesn’t happen because they only have a single X-chromosome.
It’s also hypothesized that perhaps the risk genes play a role in protein pathways that are themselves sexually dimorphic, so even if they are located on an autosome (every other chromosome that is not the X or Y chromosome), those genes may be involved in processes in the body that do not occur identically in male and female bodies. An example would be a protein pathway that includes sex hormones.
It is known that your brain before and after birth is highly influenced by sex hormones – the estrogens and the androgens. Estrogens increase the formation of your synapses, helps regulate the ability of the synapses to change and adapt to new information (termed ‘synaptic plasticity’), and estrogen in the form of estradiol is known to have a neuroprotective effect for brain injuries in both males and females.
Among those with ASD, there are some who have a high level of androgens – the male sex hormones. It is thought that high testosterone levels may indeed be a risk factor for development of ASD.
In the brain, testosterone has two pathways: it is either converted into DHT (dihydroxotestosterone) or it is converted into estradiol. It’s weird to think about it this way because so often people speak of testosterone and estrogens so separately, but in fact testosterone can be converted into an estrogen by an enzyme called estrogen synthase.
All of this together led a research team to take a closer look at SHANK genes, a family whose genetic mutations and disruptions have been linked to Autism and their possible relationship with sex hormones. The SHANK gene family is located on the autosomes so they are not linked to X- and Y-chromosome inheritance, and many ASD people have been found to have some defect in the SHANK genes, especially Shank3.
Their in vitro and in vivo studies found that both androgens and estrogens are able to affect SHANK expression.
There are three SHANK proteins – SHANK1, SHANK2, and SHANK3. In literature it is important to note that proteins are written in normal font; the genes that code for those proteins are written in italics; for example, Shank1 is the gene for the protein SHANK1.
SHANK proteins are important because they are scaffolding proteins, so their proper functioning is inherently crucial to important pathways. Some protein pathways in the body are pretty complicated, involving multiple members simultaneously that are held together in a protein complex. Though they have other possible functions, the most basic function of a scaffold protein is to tether and organize multiple members in this complex; you can think of them almost like the glue that holds it all together. Except that it’s a smart glue that can do more than just binding; some scaffold proteins can organize, localize, insulate signaling proteins from competing proteins, and coordinate feedback loops. SHANK proteins are even more special, because they are ‘master’ scaffolding proteins that tether and organize other scaffolding proteins at your synapses. These proteins are key to your synapses developing and functioning normally.2
In their in vitro study, the research team exposed a cell line to three different levels the androgen DHT and the estrogen 17β-estradiol and found that testosterone had a much greater effect on the expression of the SHANK genes. At the highest exposure tested, DHT increases the total expression of all three genes by 35%, while 17β-estradiol only increased it as much as 15%.
Another proof that androgens regulate SHANK genes was that when they used the anti-adrogen flutamide, which blocks the ability of the androgen receptor to interact with the DHT, the gene expressions plummeted. Therefore the increase of the SHANK gene expression was directly correlated to the increase in the androgen DHT and the interaction with the androgen receptor. They were able to show the same pattern with the 17β-estradiol.
When they looked at wild-type mice, meaning mice with no particular genetic manipulation, they found that females had significantly higher mRNA levels – the copy of your DNA that is used as a template to make a particular protein – of Shank1 and Shank3 late in prenatal development, while Shank1 alone was higher after birth.
Counter to the seemingly logical conclusion that higher mRNA levels mean higher protein levels, that is not actually how it works in the body. There are lots of processes involved in the translation of the genetic copy into the final protein that affect how much of the protein is actually made.
So when the researchers measured the actual protein levels, they found that in fact all of the SHANK proteins were significantly higher in males both before birth and after birth, but the difference was even more pronounced during the prenatal stages.
The central dogma of molecular biology is that the flow of genetic information is DNA –> RNA –> Proteins. A quick BIO 101 lesson: all of the information in your DNA is not readily legible to the parts of your cells responsible for making your proteins, so the body has to do a little extra work in the process of gene expression in steps called ‘transcription’ and ‘translation’. A copy of your DNA is transcribed into messenger RNA (mRNA); that mRNA is then translated into your sequence of amino acids that make up the final protein.
So what do their findings really mean? It shows that while both the estrogen and the androgen play a role at the transcriptional level because they were both able to affect the level of mRNA, only the androgen has a major effect on the translational level when the mRNA is made into the protein form.
This could be a contributing factor for why boys are affected by ASD at a higher rate than girls; boys have higher prenatal and postnatal testosterone levels and therefore mutations in the SHANK gene family could be expressed at higher levels in males than in females.