Genetics of Intellectual and Developmental Disabilities #AtoZChallenge

Hi everyone. I was at my and my husband’s house in Lobith yesterday and, though I fully intended on writing my letter G post while there, I didn’t get to it. Rather than give up on the challenge altogether, I’m going to make up for it today by writing my post now. For my letter G post, my topic is “genetics”. It’s not a topic I know all that much about. I mean, I know the basics of heridity through dominant and recessive, autosomal and X-linked genes. I also know a little about trisomies such as trisomy 21. However, I really don’t think I know much beyond high school biology. For this reason, I am going to provide a very basic introduction to the genetics of intellectual disability based on the info I could find online.

First, of course, not all intellectual disabilities are caused by genetic factors, or solely by genetic factors. Environmental factors such as birthweight and gestational age (ie. whether the child was born prematurely), exposure to substances such as medications or drugs during the fetal period, etc., can contribute to or even cause an intellectual disability too.

That being said, even if the cause of someone’s intellectual disability is (most likely) genetic, it is not always known. There are thought to be approximately 2,500 genes that contribute to intellectual disability, but about half of these haven’t yet been identified. Due to genome and exome sequencing, however, the diagnosis of intellectual disability-related genetic mutations is making advancements.

There are some genetic intellectual disability syndromes that run in families, such as Fragile X Syndrome. However, the majority of individuals with a genetic mutation causing their intellectual disability, did not inherit it from their parents. This means that a future child born to the same family, isn’t at increased risk of being intellectually disabled.

Why, then, would you want to know whether there’s a known genetic cause? Well, a recent article I found on the Dutch Center for Consultation and Expertise website, explains it very well: knowing what syndrome a person has, makes the person’s perspective clearer and may provide ideas for future medical or behavioral intervention. For instance, a doctor cited in the article talked about a girl with a particular genetic mutation causing her intellectual disability which he knew also causes leukemia. The doctor mentioned this to the patient’s primary care physician, who remembered this two years later when the girl complained of significant fatigue. This allowed her to be treated early for what turned out to be leukemia indeed. Another example is the fact that people with Phelan McDermid Syndrome usually experience bipolar-like mood dysregulation in adolescence, which, if not treated, leads to loss of skills. Since these people often have severe intellectual disability, their behaviors could easily be misinterpreted if their syndrome isn’t identified.

Of course, there remains a significant portion of the intellectually disabled population for which no genetic syndrome can be identified. For those with milder intellectual disability and no clear physical features, genetic testing may not even be routinely done. Same for those with other developmental disabilities. In my own case, the possibility of genetic testing was mentioned in my application for one-on-one support, but was immediately dismissed because it’d be “too much for me to handle”. Not that it was ever discussed with me. For all I knew, there was no need for it in my case as my conditions are all attributed to premature birth, with the exception maybe of autism, and people who are just autistic don’t get genetic testing done either.