Contrary to conventional wisdom that the symptoms of Down syndrome are likely caused by an overabundance of certain proteins due to the additional copy of chromosome 21, scientists at Ohio State University and collaborators have found evidence that at least some of the symptoms may actually be associated with underexpression of a certain protein or proteins due to the presence of five microRNA genes on chromosome 21. MicroRNAs bind to messenger RNA and cause the inhibition of protein synthesis for that messenger RNA. Computer analysis revealed over 1,600 proteins that were potential targets of the five microRNAs on chromosome 21, all of which could cause problems in Down syndrome because they would be underexpressed. Based on other evidence, the researchers selected one of the protein genes (for methyl-CpG-binding protein 2, known as MeCP2) for further study. Among the reasons for selecting this gene was that it is known to be mutated in Rett syndrome, an inherited cognitive disorder. The researchers used just two of the five microRNAs on chromosome 21 for the experiments in this study, miR-155 and miR-802, to match the only microRNAs available in the genetically engineered mouse model of Down syndrome. First, the researchers made copies of the relevant microRNAs. In human brain cell lines, they manipulated levels of those two molecules to show the inverse relationship with MeCP2. If the microRNAs were overexpressed, the level of the MeCP2 protein went down. When the microRNAs were underexpressed, the protein levels went up.
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