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Underexpressed Protein May Play Role in Down Syndrome

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.

Next, the researchers examined adult and fetal human brain tissue from healthy and Down syndrome samples obtained from a national tissue bank. “In both adult and fetal Down syndrome brain samples, it didn’t matter which area of the brain we were looking at, the MeCP2 proteins were down. These are just observations with no manipulation on our part, and the MeCP2 is almost non-existent in the Down syndrome brain,” senior author Dr. Terry Elton said. “We marked the protein with a fluorescent molecule, and by comparison, we could visualize and appreciate how much MeCP2 was being made by neurons in the control samples.”

MeCP2 is a transcription factor, meaning that it turns genes on and off. If its levels are too low in the brain, this suggests that genes influenced by its presence should be malfunctioning too. Based on previous research by another group, Dr. Elton and colleagues focused on two genes affected by the MeCP2 protein for their next set of experiments. Looking again at the human brain tissue samples, they found that the genes were indeed affected by the lowered protein level in Down syndrome brains--one gene that MeCP2 normally silences was in abundance, and the gene that should have been activated was underexpressed. Because the two genes examined have known roles in neural development, Dr. Elton said the results suggested even more strongly that the lowered protein’s effects on the genes likely contribute to cognitive problems associated with Down syndrome.

Finally, the researchers tested an experimental drug called an antagomir on mice that serve as models for Down syndrome research. Antagomirs are relatively new agents that render microRNAs inactive. The scientists injected an antagomir into the brains of these mice to silence the miR-155 with the intent to increase levels of the MeCP2 protein. Seven days after the injection, the level of the protein in the treated mouse brains resembled levels in normal mouse brains. “We showed that we can fix the protein abnormality in mice that model Down syndrome. But we can’t undo the pathology that has already occurred,” Elton said. “It’s a starting point, but it appears that we have new therapeutic targets to consider.”

This work was published in the January 8, 2010 issue of the Journal of Biological Chemistry. [Press release] [JBC abstract]