It is common knowledge among medical professionals that Down syndrome is characterized by an extra copy of chromosome (chr) 21. Thus, the conventional thinking has been that the constellation of Down syndrome phenotypes is caused by over-expression of trisomy chr 21 genes. However, a research team led by Terry Elton, Ph.D., professor of pharmacology at Ohio State University, Columbus, has reported that the opposite – gene under-expression – is partly to blame. Elton and his team found that two micro RNAs (miRNAs), miR-155 and miR-802, encoded in trisomy chr 21 genes inhibit the expression of the transcription factor methyl-CpG-binding protein 2 (MeCP2), resulting in the upregulation ofCREB1/Creb1 and downregulation of MEF2C/Mef2c, two genes that control neuronal plasticity and development (Kuhn et al. 2010).
MiRNAs are a family of small, approximately 21-nucleotide-long, non protein-coding RNAs. When they bind to messenger RNA (mRNA), they inhibit translation, making them key post-transcriptional regulators of gene expression. They are believed to modulate the expression of about 30% of all mammalian protein-coding genes and virtually all cellular processes. Altered miRNA expression has been associated with several human diseases. So far, some 424 human mature miRNAs have been investigated.
Elton and his team found that five miRNAs – miR-99a, let-7c, miR-125b-2, miR-155, and miR-802 – are encoded on chr 21 genes and that all five are over-expressed in the brains of individuals with Down syndrome. By sifting through relevant databases, Elton and his team found that these five miRNAs downregulate the expression of some 1,695 protein-encoding genes, a smaller number of which are expressed only in the brain and heart. With the help of computer-assisted analyses, the team decided to focus on a likely candidate – methyl-CpG-binding protein 2 (MeCP2). All the evidence indicated that downregulation of this gene might result in Down syndrome phenotypes. Additionally, an MeCP2 mutation is known to lead to Rett syndrome, a cognitive disorder. "So we thought that it was more than a coincidence that this protein plays a role in normal brain development, and that if the protein doesn't function right, there's going to be cognitive impairment," Elton says. "We still don't know if this is the most important protein related to Down syndrome, but we were able to go on." (Innovations report 2010)
Although all five chr 21-encoded miRNAs could putatively down-regulate MeCP2, only two of them, miR-155 and miR-80, are encoded on trisomy 21 of the Ts65Dn mouse, JAX® Mice strain B6EiC3Sn a/A-Ts(1716)65Dn , the most widely used mouse model of Down syndrome. The team planned to use this mouse later in their study, so they proceeded to determine the effects of these two miRNAs on MeCP2 expression. By manipulating the levels of the two miRNAs in human brain cell lines, Elton's team found an inverse relationship between their levels and those of the MeCP2 protein: the higher the levels of either miRNA, the lower the levels of the protein and vice versa. Next, Elton's team examined fetal and adult brain tissues from a national tissue bank and found that the MeCP2 protein is virtually absent from the tissues of individuals with Down syndrome.
Being a transcription factor, the MeCP2 protein either activates or silences other genes. So, Elton's team examined the samples from the national tissue bank to see how MeCP2's absence affects the activity of two of its target genes, MEF2C/Mef2c and CREB1/Creb1, both of which play an important role in neural development. They found that MEF2C/Mef2c, which MeCP2 normally silences, is very active, and that CREB1/Creb1, which it usually activates, is silenced in individuals with Down syndrome, strongly suggesting that MeCP2's absence contributes to cognitive problems associated with Down syndrome.
To determine if MeCP2 protein levels could be restored, Elton's team injected an antagomir, a relatively new type of drug that inactivates miRNAs, into the brains of Ts65Dn mice. Seven days post injection, MeCP2 levels in the treated mice were normal. "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 says. "It's a starting point, but it appears that we have new therapeutic targets to consider." (Innovations report 2010)
The results of Elton's team shed new light on key biochemical pathways that underlie Down syndrome. Future research on the role of other chr 21 miRNAs in the pathogenesis of Down syndrome will undoubtedly uncover other key pathways and perhaps help scientists develop therapies that can attenuate, perhaps even reverse, some of Down syndrome's effects.
Innovations report. 2010. http://www.innovations-report.com/html/reports/medicine_health/theory_syndrome_lead_therapies_151394.html. Retrieved March 24, 2010.
Kuhn DE, Nuovo GJ, Terry AV Jr, Martin MM, Malana GE, Sansom SE, Pleister AP, Beck WD, Head E, Feldman DS, Elton TS. 2010. Chromosome 21-derived microRNAs provide an etiological basis for aberrant protein expression in human Down syndrome brains. J Biol Chem