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Epigenetics of autism
Autism spectrum disorder (ASD) includes autism, Asperger disorder, childhood disintegrative disorder and pervasive developmental disorder-not otherwise specified. While the exact cause of ASD has remained somewhat of a mystery, it appears to be genetic in origin. Most data supports a polygenic, epistatic model, meaning that the disorder is caused by two or more genes and that those genes are interacting in a complex manner. Several genes, between two to fifteen in number, have been identified and could potentially contribute to disease susceptibility.〔 However, an exact determination of the cause of ASD has yet to be discovered and there probably is not one single genetic cause of any particular set of disorders, leading many researchers to believe that epigenetic mechanisms, such as genomic imprinting or epimutations, may play a major role.
Epigenetic mechanisms can contribute to disease phenotypes. Epigenetic modifications include DNA cytosine methylation and post-translational modifications to histones. These mechanisms contribute to regulating gene expression without changing the sequence of the DNA and may be influenced by exposure to environmental factors and may be heritable from parents.〔 Rett syndrome and Fragile X syndrome (FXS) are single gene disorders related to ASD with overlapping symptoms that include deficient neurological development, impaired language and communication, difficulties in social interactions, and stereotyped hand gestures. It is not uncommon for a patient to be diagnosed with both ASD and Rett syndrome and/or FXS. Epigenetic regulatory mechanisms play the central role in pathogenesis of these two diseases.〔 Rett syndrome is caused by a mutation in the gene that encodes methyl-CpG-binding protein (MECP2), one of the key epigenetic regulators of gene expression. MeCP2 binds methylated cytosine residues in DNA and interacts with complexes that remodel chromatin into repressive structures. On the other hand, FXS is caused by mutations that are both genetic and epigenetic. Expansion of the CGG repeat in the 5’-untranslated region of the FMR1 genes leads to susceptibility of epigenetic silencing, leading to loss of gene expression.〔
Genomic imprinting may also contribute to ASD. Genomic imprinting is another example of epigenetic regulation of gene expression. In this instance, the epigenetic modification(s) causes the offspring to express the maternal copy of a gene or the paternal copy of a gene, but not both. The imprinted gene is silenced through epigenetic mechanisms. Candidate genes and susceptibility alleles for autism are identified using a combination of techniques, including genome-wide and targeted analyses of allele sharing in sib-pairs, using association studies and transmission disequilibrium testing (TDT) of functional and/or positional candidate genes and examination of novel and recurrent cytogenetic aberrations. Results from numerous studies have identified several genomic regions known to be subject to imprinting, candidate genes, and gene-environment interactions. Particularly, chromosomes 15q and 7q appear to be epigenetic hotspots in contributing to ASD. Also, genes on the X chromosome may play an important role, as in Rett Syndrome.〔
==Chromosome 15==
In humans, chromosome 15q11-13 is the location of a number of mutations that have been associated with Autism spectrum disorders (ASD).
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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