There are several hundred known genetic syndromes that affect neural development and result in intellectual disability (ID), epilepsy, or other neurological or psychiatric symptoms.
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These include recognized syndromes that often manifest with symptoms of autism spectrum disorders (ASD) or schizophrenia (SZ), such as Fragile X syndrome, Rett syndrome, tuberous sclerosis, velocardio-facial syndrome, and many others. For ASD, it has been known for many years that these syndromes account for a significant but still small fraction (5–10%) of all cases (Miles, 2011). What has not been clear is whether such cases, associated with single mutations, represent a typical mode by which such conditions arise or are, alternatively, exceptional and quite distinct from the general etiology of idiopathic ASD, epilepsy, SZ, or ID (Wray and Visscher, 2010). Other common disorders including dyslexia, specific language impairment, obessive-compulsive disorder, and so on, will not be considered here in detail, though the general principles probably apply.
In general, the genetic architecture of com- mon NDDs has been considered to be “complex”or multifactorial (Plomin et al., 2009; Sullivan et al., 2003). This is usually taken to mean that many causal factors, both genetic and non-genetic, are involved in each affected individual.
Under this view, the large group of currently idiopathic cases have a very different genetic architecture from the small number of known monogenic cases. An alternative view is that the vast majority of cases of these conditions are caused by independent mutations in any one of a very large number of genes. According to this model, these diagnostic categories of idiopathic
cases represent artificial groupings reflecting our current ignorance, rather than natural kinds.
Here, I consider the theoretical underpin- nings and empirical evidence relating to the genetic architecture of NDDs. These have been greatly influenced by technological advance- ments which have allowed various types of genetic variation to be assayed. Studies over the past several years have revealed an extreme level of genetic heterogeneity and complexity for common NDDs, with the discovery of high-risk mutations in a large number of single loci and additional complexities in the causal architecture in individuals.
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