Chromosome 21-derived MicroRNAs Provide an Etiological Basis for Aberrant Protein Expression in Human Down Syndrome Brains*

Down syndrome (DS), or Trisomy 21, is the most common genetic cause of cognitive impairment and congenital heart defects in the human population. Bioinformatic annotation has established that human chromosome 21 (Hsa21) harbors five microRNA (miRNAs) genes: miR-99a, let-7c, miR-125b-2, miR-155, and miR-802. Our laboratory recently demonstrated that Hsa21-derived miRNAs are overexpressed in DS brain and heart specimens. The aim of this study was to identify important Hsa21-derived miRNA/mRNA target pairs that may play a role, in part, in mediating the DS phenotype. We demonstrate by luciferase/target mRNA 3′-untranslated region reporter assays, and gain- and loss-of-function experiments that miR-155 and -802 can regulate the expression of the predicted mRNA target, the methyl-CpG-binding protein (MeCP2). We also demonstrate that MeCP2 is underexpressed in DS brain specimens isolated from either humans or mice. We further demonstrate that, as a consequence of attenuated MeCP2 expression, transcriptionally activated and silenced MeCP2 target genes, CREB1/Creb1 and MEF2C/Mef2c, are also aberrantly expressed in these DS brain specimens. Finally, in vivo silencing of endogenous miR-155 or -802, by antagomir intra-ventricular injection, resulted in the normalization of MeCP2 and MeCP2 target gene expression. Taken together, these results suggest that improper repression of MeCP2, secondary to trisomic overexpression of Hsa21-derived miRNAs, may contribute, in part, to the abnormalities in the neurochemistry observed in the brains of DS individuals. Finally these results suggest that selective inactivation of Hsa21-derived miRNAs may provide a novel therapeutic tool in the treatment of DS.

The presence of three copies of all, or part, of human chromosome 21 (Hsa21) 2 results in the constellation of physio-logic traits known as Down syndrome (DS) or Trisomy 21 (1). With an incidence of approximately one in 750 live births, DS is the most frequently survivable congenital chromosomal abnormality (2,3). The phenotypes of DS are complex and variable; they include cognitive impairment, congenital heart defects, craniofacial abnormalities, gastrointestinal anomalies, leukemia, and Alzheimer disease (1)(2)(3). Experimental studies utilizing tissues derived from individuals with DS have confirmed that expression of trisomic genes is increased by ϳ50% (i.e. consistent with gene dosage) (4 -6). Recent bioinformatic annotation has established that Hsa21 harbors more than 500 genes (7,8), including five miRNA genes (miR-99a, let-7c, miR-125b-2, miR-155, and miR-802).
miRNAs are a family of small, ϳ21-nucleotide long, nonprotein-coding RNAs that have emerged as key post-transcriptional regulators of gene expression (9 -11). miRNAs are processed from precursor molecules (pri-miRNAs), which are either transcribed from independent miRNA genes or are portions of introns of protein-coding RNA polymerase II transcripts. Following their processing, miRNAs are assembled into ribonucleoprotein complexes called microribonucleoproteins (miRNPs) or miRNA-induced silencing complexes. The miRNA acts as an adaptor for miRNA-induced silencing complex to specifically recognize and regulate particular mRNAs. Mature miRNAs recognize their target mRNAs by basepairing interactions between nucleotides 2 and 8 of the miRNA (the seed region) and complementary nucleotides in the 3Ј-untranslated region (3Ј-UTR) of mRNAs. miRISCs subsequently inhibit gene expression by targeting mRNAs for translational repression or destabilization (12)(13)(14). In mammals, miRNAs are predicted to control the activity of ϳ30% of all protein-coding genes, and functional studies indicate that miRNAs participate in the regulation of almost every cellular process investigated. Importantly, alterations in miRNA expression have also been observed in a number of human pathologies (9 -14).
Bioinformatic annotation has established that Hsa21 harbors five miRNA genes (miR-99a, let-7c, miR-125b-2, miR-155, and miR-802). We have previously demonstrated, by miRNA expression profiling experiments, that of the 424 human mature miRNAs investigated, only 10 miRNAs were overexpressed in human brain DS specimens when compared with age-and sexmatched controls (15). Importantly, RT-PCR, and miRNA in situ hybridization experiments validated that all five Hsa21derived miRNAs were overexpressed in these brain specimens (15). In this study, we test the hypothesis that Trisomy 21 gene dosage overexpression of Hsa21-derived miRNAs result in the decreased expression of specific target proteins in both individuals with DS, and in a mouse model of DS. We demonstrate that the Hsa21-derived miRNA predicted the mRNA target, the transcription factor methyl-CpG-binding protein 2 (MeCP2) (16,17), is underexpressed in DS brain specimens. Furthermore, we demonstrate that two MeCP2 target genes that play key roles in neuronal plasticity and development (18 -21), CREB1/Creb1 and MEF2C/Mef2c, are also aberrantly regulated in these same samples. We conclude from these data that the improper attenuation of MeCP2 expression ultimately results in the dysregulation of important "regulatory circuits" that contribute, in part, to the cognitive defects that occur in DS individuals.

EXPERIMENTAL PROCEDURES
Human Brain Specimens-Human brain cerebellum (CBLM), hippocampus (HIPP), and pre-frontal cortex (Pre-FCTX) samples, age-and sex-matched, were obtained from the Brain and Tissue Bank for Developmental Disorders, University of Maryland at Baltimore, in contract with the National Institutes of Health, NICHD. Additional human brain samples used in this project were provided by the Institute for Brain Aging and Dementia and the University of California Alzheimer Disease Research Center (UCI-ADRC). The fetal samples ranged from 18 to 22 weeks of gestation. Children, adolescent, and adult brain samples were obtained from patients ranging 1-8, 9 -19, and 20 -50 years of age, respectively. For postmortem specimen information see supplemental Table S1.
Cell Culture-The human neuroblastoma cell line, SK-N-SH, was purchased from American Type Culture Collection (ATCC, Manassas, VA) and cultured in Dulbecco's modified Eagle's medium (Invitrogen) supplemented with 10% fetal bovine serum (HyClone Laboratories, Logan, UT), 80 units/ml of penicillin, and 80 g/ml of streptomycin. All cultured cells were maintained in a humidified atmosphere of 5% CO 2 .
Hsa21-derived miRNA Bioinformatic Analyses-To predict putative Hsa21-derived miRNA target mRNAs, multiple computational algorithms were utilized at the default settings (miRBase (22,23), TargetScan (24 -26), PicTar (27,28), and PITA (29)). These computational analyses demonstrated that Hsa21-derived miRNAs could theoretically interact with thousands of distinct mRNA targets and, unfortunately, many of the identified targets did not overlap between analyses. Given that the combinations of computational analyses perform worse than the prediction of a single algorithm (58), we chose to focus on TargetScan-predicted miRNA targets because this algorithm has a precision rate of around 50% with a sensitivity of ϳ12% (58). To reduce the number of TargetScan putative targets, the list of candidate mRNAs was subsequently prioritized with respect to their potential clinical relevance to DS and the number of multiple Hsa21-derived miRNA recognition sites harbored in possible mRNA targets (supplemental Tables S2 and S3).
Real Time PCR-Total RNA was isolated from frozen human control and DS brain, or transfected cell, samples using TriZOL (Invitrogen). The RNA was subsequently treated with RNasefree DNase I, and mature human let-7c, miR-99a, miR-125b, miR-155, and miR-802 were quantified utilizing TaqMan microRNA assay kits specific for each Hsa21-derived miRNA (Applied Biosystems, Foster City, CA) as previously described (15, 30 -33). Briefly, 100 ng of total RNA was heated for 5 min at 80°C with 2.5 M 18S rRNA antisense primer followed by 5 min at 60°C then cooling to room temperature. The resulting solution was then added to a reverse transcriptase mixture and transcription was performed in 20 l according to the manufacturer's recommendations. Quantitative real time PCR (20 l total reaction) was performed using 5 l of a 1:50 dilution of cDNA. Gene expression was calculated relative to 18S rRNA and C t values were normalized to "1" for normal control samples to simplify data presentation. Alternatively, total RNA samples isolated from human brains were utilized to measure MeCP2, CREB1, and MEF2C steady state mRNA levels using TaqMan Gene Expression Assays (MeCP2, Hs00172845_m1; CREB1, Hs00231713; and MEF2C, Hs00231149_m1).
Luciferase Reporter Constructs-A 3602-bp fragment (Fig.  1A) encompassing a portion of the human MeCP2 3Ј-UTR (accession number NM_004992; the entire length of this region is almost 9 kb) was PCR amplified utilizing sense (5Ј-GAC-CGACAGCTTTCCAGTACC-3Ј) and antisense (5Ј-CCTC-AGAAGAAGCAATGACAGCA-3Ј) primers using standard procedures and a proofreading polymerase (Platinum Pfu, Invitrogen). Human genomic DNA was used as template. The PCR product was subcloned into the pCR TM .1 vector (Invitrogen). Following the manufacturer's protocol, the PCR product was treated for 10 min with Taq polymerase. Plasmid DNA was subsequently isolated from recombinant colonies and sequenced to ensure authenticity. The MeCP2 3Ј-UTR inserts were removed from the pCR 2.1 plasmid by EcoRI digestion. The fragments were subsequently gel purified, filled in, and blunt-end ligated into a filled-in XhoI site that is located downstream of the Renilla luciferase (r-luc) reporter gene (psiCHECK-2 TM , Promega). The authenticity and orientation of the inserts relative to the Renilla luciferase gene were confirmed by dideoxy sequencing. The resulting recombinant plasmid was designated psiCHECK/MeCP2. The mutant reporter constructs, psiCHECK/155mut1 and psiCHECK/155mut2, and psiCHECK/155mut1&2 were generated by utilizing the psiCHECK/MeCP2 plasmid as template and mutating the first (located at 4693-4699 bp) and/or second (located at 6321-6327 bp) miR-155 recognition site (Fig. 1C) harbored in the MeCP2 3Ј-UTR using the QuikChange sitedirected mutagenesis kit (Stratagene). Briefly, a forward miR-155 number 1 mutagenic primer (5Ј-GGCCTGAGATG-CCTGGTATAATAAACAGGCAAGGGGAATCTG-3Ј) and a complementary reverse miR-155 number 1 mutagenic primer (5Ј-CAGATTCCCCTTGCCTGTTTATTATACC-AGGCATCTCAGGCC-3Ј), or a forward miR-155 mutagenic  number 2 primer (5Ј-TGTTCTTCCAAAGCAGAATAATA-AATAATCACCAGGGCCAAA-3Ј) and a complementary  reverse miR-155 number 2 mutagenic primer (5Ј-TTTGGCC-CTGGTGATTATTTATTATTCTGCTTTGGAAGAACA-3Ј), were synthesized and utilized in a PCR experiment as described by the manufacturer. The amplification reactions were treated with the DpnI restriction enzyme to eliminate the parental template and the remaining DNA was used for transformation. Mutation of the AGCAUUA miR-155 seed binding site was confirmed by dideoxy chain termination sequencing. Finally, transformed bacterial cultures were grown and each reporter construct was purified using the PureLink TM Hipure Plasmid Maxiprep kit (Invitrogen). Additionally, miR-802 mutant binding site reporter constructs, psiCHECK/802mut1 and psiCHECK/802mut2, and psiCHECK/802mut1&2, were also generated by utilizing the psiCHECK/MeCP2 plasmid as template and mutating the first (located at 3439 -3445 bp) and/or second (located at 6889 -6895 bp) miR-802 recognition site ( Fig. 1E) harbored in the MeCP2 3Ј-UTR using the QuikChange site-directed mutagenesis kit (Stratagene).
Transfection and Luciferase Assay-Hsa21-derived miRNA mimics (partially double-stranded RNAs that mimic the Dicer cleavage product and are subsequently processed into their respective mature miRNAs), scrambled sequence negative control mimics, Hsa21-derived miRNA inhibitors (antisense single-stranded chemically enhanced oligonucleotides, ASO), and negative control miRNA inhibitors were obtained from Dharmacon (Lafayette, CO). Transfection of CHO and SK-N-SH cells with small RNAs was optimized utilizing Lipofectamine 2000 (Invitrogen) and a fluorescein-labeled double-stranded RNA oligomer designated BLOCKiT TM (Invitrogen). Once conditions were optimized, CHO cells (approaching 100% transfection efficiency) were transfected with the luciferase reporter constructs described above and the appropriate miRNA precursor as indicated. After 24 h, cells were washed and lysed with Passive Lysis Buffer (Promega), and firefly and Renilla luciferase activities were determined using the Dual Luciferase Reporter Assay System (Promega) and a luminometer. Renilla luciferase expression in the psiCHECK vector was generated via an SV40 promoter. Additionally, the psiCHECK-2 vector possesses a secondary firefly reporter expression cassette under the control of the herpes simplex virus-thymidine kinase promoter. This firefly reporter cassette has been specifically designed to be an intraplasmid transfection normalization reporter; thus when using the psiCHECK-2 vector, the Renilla luciferase signal is normalized to the firefly luciferase signal. Alternatively, SK-N-SH cells were transiently transfected with miRNA reagents utilizing Lipofectamine 2000 (Invitrogen) following the manufacturer's protocol. Twentyfour hours after transfection total RNA was isolated and Hsa21derived miRNAs and MeCP2, CREB1, and MEF2C mRNA levels were quantitated by RT-PCR as described above. Protein lysates were also isolated for Western blot experiments.
Western Blot Analyses-Frozen human control and DS brain specimens were solubilized with RIPA buffer using freshly added protease and phosphatase inhibitors. Equal quantities (10 g/well) of cell lysate were separated by 10% SDS-PAGE. Following transfer to nitrocellulose membrane and blocking, the blot was incubated with an anti-MeCP2 antibody (Upstate Biotechnology, number 07-013), anti-CREB1 antibody (Upstate Biotechnology, number 06-863), anti-MEF2C antibody (Santa Cruz, number SC-132660), or anti-GAPDH antibody (Santa Cruz, number SC-20357). The immunoblots were incubated with a secondary antibody conjugated with horseradish peroxidase, visualized with enhanced chemiluminescence (ECL), and the autoradiographs were quantitated by densitometric analysis. The blots were subsequently stripped and reprobed with a GAPDH-specific antibody (Cell Signaling) to normalize the level of MeCP2, CREB1, or MEF2C proteins to total protein. Western blots probing MeCP2 levels in fetal brain samples were routinely exposed to film an additional 5-10 min so that the MeCP2 bands could be more easily detected and quantitated. All of the antibodies utilized detected the appropriate protein based on molecular mass of the protein visualized (e.g. CREB1, 43 kDa; GAPDH, 37 kDa; MeCP2, 75 kDa; MEF2C, 44 kDa).
Immunohistochemistry-Immunohistochemical testing was performed using the Ventana Benchmark System (Ventana Medical Systems, Tucson, AZ). All human control and DS brain specimens were fixed in formalin and embedded in wax. After the blocks were cooled, 5-m sections were cut. Sections were placed on 3% aminopropylethoxysilane-coated glass slides before being dewaxed in a 60°C oven for 20 min. They were then transferred to xylene and rehydrated in descending ratios of alcohol (99 -95%; one change of each) to distilled water before being either stained histologically (with hematoxylin and eosin) or immunohistochemically. Briefly, sections were rehydrated by washing in Tris-buffered saline. Nonspecific binding was blocked by exposure to goat serum (Sigma) in Trisbuffered saline (v/v) in a humidity chamber. The primary antibody, MeCP2, CREB1, or MEF2C (1:100 dilution) was added overnight at 4°C. After three Tris-buffered saline washes, secondary amplification (Envision; Dako) of alkaline phosphataselabeled polymer conjugates to affinity-purified goat anti-mouse immunoglobulins was performed at room temperature. Immunoreactivity was visualized with fast red chromogen and dilute hematoxylin. The sections were then placed under a glass coverslip using mounting medium (i.e. neuron-specific enolase). The cell type (i.e. neurons) that exhibited positive immunostaining was determined by morphologic and cytologic criteria as well as immunostaining. In negative control experiments, preimmune serum replaced the specific antibodies. All specimens were sequentially viewed in their entirety under a ϫ100 objective and number of positively stained cells (deep pink color) per field was scored.
Chromatin Immunoprecipitation (ChIP)-Control or DS brain tissue (75-125 mg) was minced into very fine pieces on a glass plate using a sterile razor blade. The pieces were scraped into a 1.8-ml microcentrifuge tube containing 1.2 ml of 1% formaldehyde in phosphate-buffered saline, and rocked for 10 min at room temperature. The tissue was pelleted by pulse centrifugation and re-suspended in 1 ml of phosphate-buffered saline containing 0.125 M glycine and protease inhibitors (all remaining solutions contained protease inhibitors). Samples were incubated at room temperature for 5 min, pelleted by pulse centrifugation, and washed twice with 1 ml of phosphatebuffered saline containing protease inhibitors. Following the final wash the samples were re-suspended in 500 l of homogenization buffer containing 10 mM Tris-HCl (pH 7.5), 10 mM NaCl, 3 mM MgCl 2 , and 0.5% Nonidet P-40. The samples were homogenized using a glass-ground hand-held homogenizer (Wheaton Industries, Millville, NJ). Fifty microliters of the resulting solution was removed and used in real time PCR for input quantity normalization. Samples were subsequently incubated on ice for 10 min and pelleted by centrifugation at 3000 ϫ g for 5 min. The pellets were washed twice with ice-cold homogenization buffer, and re-suspended in 500 l of micrococcal nuclease buffer containing 10 mM Tris-HCl (pH 7.5), 4 mM MgCl 2 , and 1 mM CaCl 2 . The samples were then sonicated in a Branson Sonifier 450 at a power setting of 6 at 50% duty cycle for 6 s. Micrococcal nuclease (Sigma) was added to a concentration of 5 units/ml, and the samples incubated at 37°C for 7 min. Fifty microliters was removed and used to check for nuclease digestion efficiency on a 3% agarose gel after DNA extraction. The nuclease reaction was terminated by the addition of 1 l of 1 M EDTA, 45 l of 10% SDS, and 45 l of 100 mM NaCl. Samples were again sonicated as described above. The resulting solution was centrifuged at 13,000 ϫ g for 10 min at 4°C and the supernatant collected. The supernatant was separated into two aliquots of 200 l each and used as input material for a ChIP Assay Kit from Upstate Cell Signaling Solutions/ Millipore. Briefly, each 200-l aliquot was added to 1.8 ml of the provided dilution buffer containing 0.01% SDS, 1.1% Triton X-100, 1.2 mM EDTA, 16.7 mM Tris (pH 8.1), and 162 mM NaCl. These aliquots were pre-cleared by incubating with 75 l of the provided Protein A-agarose/salmon sperm DNA (50% slurry) for 60 min at 4°C on a rotating platform. The Protein A-agarose was pelleted by centrifugation at 1000 ϫ g for 1 min at 4°C, and the supernatant collected. One 2-ml sample was incubated on a rotating platform overnight at 4°C with 5 g of rabbit polyclonal anti-MeCP2 antibody. The other 2-ml sample was used as a negative control by omission of the capturing antibody or by utilizing preimmune serum. Antibody-MeCP2 complexes were collected following an incubation with 60 l of Protein A-agarose/salmon sperm DNA (50% slurry) for 2 h at 4°C on a rotating platform. The agarose was pelleted by centrifugation at 1000 ϫ g for 1 min at 4°C, and the supernatant removed. The agarose was then successively washed at 4°C for 5 min on a rotating platform in 1 ml of the provided low salt buffer, high salt buffer, LiCl buffer, and finally washed twice with TE buffer (pH 8.0) as described in the manufacturer's protocol. Antibody-MeCP2 complexes were eluted from the Protein A-agarose by adding 250 l of elution buffer (0.1 M NaHCO 3 , 1% SDS) and incubating at room temperature for 15 min on a rotating platform. The agarose was pelleted as before, the supernatant collected, and the elution step repeated; aliquots were combined. MeCP2/DNA cross-linking was reversed by adding 20 l of 5 M NaCl and incubating at 65°C for 4 h. Samples were digested with proteinase K by the addition of 10 l of 0.5 M EDTA, 20 l of 1 M Tris-HCl (pH 6.5), and 1 l of 20 mg/ml of proteinase K solution (Ambion/Applied Biosystems) and incubated at 45°C for 1 h. DNA was recovered from this solution using a ChIP DNA Clean & Concentrator kit from Zymol Research (Orange, CA). The resulting DNA pellets were re-suspended in nucleasefree distilled water and used in real time PCR to quantify captured CREB1 and MEF2C promoter products. The CREB1 and MEF2C PCR primers were based on those utilized by Chahrour et al. (39).
Antagomirs-Based on the optimization strategies of Krutzfeldt et al. (34,35), chemically modified (all ribonucleotide basepairs were 2Ј-O-methyl modified, six phosphorothioate backbone modifications were also included with two phosphorothioates located at the 5Ј-end and four at the 3Ј-end and a cholesterol moiety at the 3Ј-end) single-stranded RNA analogs complementary to mouse miR-155 (5Ј-CCCCUAU-CACAAUUAGCAUUAA-3Ј, designated antagomir-155), and miR-802 (5Ј-AAGGAUGAAUCUUUGUUACUGA-3Ј, designated antagomir-802), were synthesized and reverse phasehigh pressure liquid chromatography purified for in vivo use (Dharmacon, Lafayette, CO). A scrambled control antagomir (5Ј-GACUCCACUCUUCUAGAAUAAC-3Ј) was also synthesized with the same chemical modifications as described above, however, a biotin moiety was included at the 5Ј-end of the oligonucleotide so that proper location after in vivo injection and antagomir stability could be verified.
Transgenic Mice-Male B6EiC3Sn a/A-Ts(1716)65Dn and control littermates, 5-6 months old, were obtained from The Jackson Laboratories (Bar Harbor, ME), housed in pairs in a temperature-controlled room (25°C), and maintained on a 12-h light/dark cycle with free access to food (Teklad Rodent Diet 8604 pellets, Harlan, Madison, WI) and water. All procedures employed during this study were reviewed and approved by the Medical College of Georgia Institutional Animal Care and Use Committee and are consistent with AAALAC guidelines. Measures were taken to minimize pain or discomfort in accordance with the National Institutes of Health Guide for the Care and Use of Laboratory Animals (NIH Publication number 80-23, revised 1996). Significant efforts were also made to minimize the total number of animals used while maintaining statistically valid group numbers.
Stereotaxic Intracerebroventricular (ICV) Injection-The method of Vanderwolf (36) was used as a technical guide for animal care and stereotaxic surgery. An intraperitoneal injection mixture (1 ml/kg) of ketamine (100 M), xylazine (10 mg/ml) was used to deeply anesthetize each mouse before shaving its head and positioning it in a Dual Ultra Precise Small Animal Stereotaxic Instrument (model 962, David Kopf Instruments, Tujunga, CA). On the dorsal surface of the head, a midline incision was made to separate the subcutaneous fascia to expose the skull and visualize bregma and lamda. Stereotaxic coordinates (37) for injection into the lateral ventricle were: 1.0-mm lateral from bregma (x plane), Ϫ0.34-mm rostral from bregma (y plane), and Ϫ2.5-mm ventral from bregma (z plane). After dialing in the coordinates, an 18-guage needle (catalog number 305196, BD Biosciences) was used to make a bur hole for the syringe needle to pass through the skull. A gas-tight 10-l Hamilton syringe was used with a microsyringe holder (model 1772-F1, David Kopf Instruments, Tujunga, CA) to inject 10 l of miR-155 antagomir at a rate of 5 l/min. The injection needle was left in place for 5 min after the injection, then withdrawn. The bur hole was covered by bone wax and the scalp sutured. Following injections, subjects were returned to their home cages to recover and allowed food and water ad libitum. The mice were sacrificed on day 7 after antagomir injection by isofurane inhalation and decapitation. The brains were removed from the skull and placed in a mouse brain blocker and sectioned in 1.0-mm blocks. From the subsequent blocks, the prefrontal cortex, striatum, hippocampus, and cortex (remaining regions combined) were dissected out. The brain regions were placed in individual 0.5-ml flat top microcentrifuge tubes, flash frozen in liquid nitrogen, and stored at Ϫ80°C until further use.
Statistical Analysis-All data are reported as mean Ϯ S.E. When comparisons were made between two different groups, statistical significance was determined using Student's t test. When multiple comparisons were made, statistical significance was determined using two-way analysis of variance. All statistical analysis was performed using the software package Prism 4.0b (GraphPad Software, San Diego, CA).

Hsa21-derived miRNAs Are Overexpressed in Human DS
Brain Specimens-To extend our previous observations that demonstrated that Hsa21-derived miRNAs were overexpressed in human DS fetal heart and hippocampus specimens (15), mature RT-PCR assays specific for these miRNAs were performed utilizing total RNA isolated from human prefrontal cortex specimens from brains of fetuses, infants/children, adolescents, and adults with DS (supplemental Table S1, Postmortem Specimen Information). These experiments demonstrated that all five Hsa21-derived miRNAs were overexpressed by at least 50% in prefrontal cortex samples at all ages examined when compared with age-and sex-matched control brain specimens (supplemental Fig. S1, A-E).
MeCP2 Is a Target of Hsa21-derived miRNAs-miRNA-mediated regulation of gene expression results when a miRNA interacts with a specific recognition element within the 3Ј-UTR of a target mRNA and suppresses its translation or initiates its degradation (12)(13)(14). To predict putative Hsa21-derived miRNA target mRNAs, multiple computational algorithms were utilized (22)(23)(24)(25)(26)(27)(28)(29). These analyses demonstrated that the expression of several thousand proteins may be regulated by Hsa21-derived miRNAs (data not shown). These potential mRNA targets were subsequently prioritized based on the assumption that the degree of miRNA-mediated gene repression is proportional to the number of Hsa21-derived miRNA recognition sites harbored in a given mRNA target (i.e. combinatorial miRNA inhibition; see Refs. 28 and 38) (supplemental Table S2). Based on TargetScan analyses (24 -26), no putative mRNA targets harbored binding sites for all five Hsa21-derived miRNAs. However, 33 mRNA targets were identified that harbored four of the five Hsa21-derived miRNA binding sites (supplemental Table S3). This list of candidate targets was further prioritized with respect to the potential clinical relevance of a given target gene in playing a role in DS. Based on these criteria, we chose to investigate the MeCP2 mRNA as a potentially important Hsa21-derived miRNA target because its 3Ј-UTR harbors two miR-155 and -802, and single miR-125b and let-7c, putative recognition sites (Fig. 1A). Additionally, MeCP2 is a provocative clinical miRNA target because it is highly expressed in neurons and has been shown to play a role in neurogenesis (16,17), a process that is abnormal in DS individuals (1)(2)(3). Although MeCP2 mRNA is a putative target for several of the Hsa21-derived miRNAs, this study focused on the functionality of miR-155 and -802 because the DS mice utilized later in our studies were only trisomic for these two miRNAs (Fig. 6A).
Although multiple miR-155 and -802 recognition sites were predicted by the TargetScan algorithm in the MeCP2 3Ј-UTR, it is important to note that the TargetScan "Total Context" score for these sites is very low (e.g. miR-155 site 1/0.04, mir-155 site 2/Ϫ0.06, mir-802 site 1/0.03, and mir-802 site 2/0.00). Therefore, the functional importance of these sites in repressing MeCP2 expression is questionable. To begin to determine whether or not miR-155 and/or -802 could regulate the expression of MeCP2 we chose to utilize a luciferase reporter assay. The rationale for utilizing this assay is that the binding of a given miRNA to its specific mRNA target site will repress reporter protein production thereby reducing activity and expression that can ultimately be measured and compared with a control. Therefore, the region (i.e. Fig. 1A, 3602 bp) of the MeCP2 3Ј-UTR that encompassed the four putative miR-155 and -802 binding sites was subcloned downstream from the Renilla luciferase open reading frame harbored in the psiCHECK plasmid and the resulting construct was designated psiCHECK/MeCP2. The potency of miR-155 and -802 was then tested by co-transfecting psiCHECK/MeCP2 into CHO cells with increasing concentrations of each specific miRNA mimic (partially double-stranded RNAs that mimic the Dicer cleavage product and are subsequently processed into their respective mature miRNAs), and luciferase activities were determined. Dose-response experiments demonstrated that relative luciferase activity was significantly decreased with as little as 1 nM miR-155 or -802 and a maximal decrease was obtained with a 25 nM concentration of these mimics (Fig. 1B). In contrast, increasing concentrations of scrambled control mimic had no effect on luciferase activity (Fig. 1B).
To validate that miR-155 and/or -802 interacted with specific target sequences localized within the MeCP2 3Ј-UTR, additional luciferase reporter constructs were generated in which the 7-bp "seed" sequences, which are complementary to the 5Ј-end of miR-155 (Fig. 1C) or miR-802 (Fig. 1E), were mutated. The resulting constructs were subsequently cotransfected with miR-155 or -802 into CHO cells and luciferase activity was measured. Importantly, miR-155 could no longer decrease the luciferase activity of only the psiCHECK/155mut1,2-transfected cells (Fig. 1D), suggesting that miR-155 can interfere with luciferase expression via direct interaction with both miR-155 recognition sites (located at positions 4679 -4700 and 6305-6328 bp) harbored within the MeCP2 3Ј-UTR. Additionally, the data suggests that the effect of multiple miR-155 recognition sites is additive because luciferase activity is lowest when both miR-155 sites are present. In contrast, miR-802 and the mutant construct co-transfection experiments demonstrated that miR-802 can interfere with luciferase expression via direct interaction with only the second miR-802 site (i.e. 6875-6896 bp) in this in vitro surrogate assay (Fig. 1F).
individually manipulated by transfection of miRNA mimics (gain-of-function experiment) or miRNA inhibitors (ASO, loss-of-function experiment) and changes in mature miR-155, -802, MeCP2 mRNA, and protein levels were determined. With transfection of either the miR-155 or -802 mimic, endogenous miR-155 and -802 levels increased by 200 -310% (Fig. 1G) and MeCP2 mRNA (Fig. 1G) and MeCP2 protein (Fig. 1H) levels were significantly decreased (30 -50% at the mRNA level and 40 -50% at the protein level) when compared with non-transfected or scrambled miRNAtransfected cells. In contrast, subsequent to transfection with either miR-155 or -802 ASO inhibitors, endogenous miR-155 and -802 levels decreased (Fig. 1G) and MeCP2 mRNA (Fig. 1G) and MeCP2 protein (Fig. 1H) levels were significantly increased (45-120% at the mRNA level and 50 -70% at the protein level). Collectively, these results strongly support the hypothesis that MeCP2 mRNA is a target of both miR-155 and -802 and suggest that these miRNAs markedly decrease MeCP2 expression by targeting MeCP2 mRNAs for degradation.
MeCP2 Is Underexpressed in Human DS Brain Specimens-To demonstrate the potential significance of MeCP2 as a target of Hsa21-derived miRNAs in vivo, we investigated whether or not MeCP2 was underexpressed in brain samples isolated from individuals with DS. RT-PCR experiments demonstrated that, independent of the age or brain region investigated, MeCP2 mRNA levels were decreased by 60 -70% in brain specimens from DS individuals relative to age-and sex-matched controls (Fig. 2, A and B). Consistent with these results, Western blot analyses of the same human brain specimens showed that  Table S1. A, MeCP2 mRNA levels were quantitated utilizing a MeCP2 gene-specific RT-PCR assay. Gene expression was calculated relative to 18S rRNA as described above. The error bars represent the average Ϯ S.E. of triplicate samples repeated in at least three independent experiments (*, p Ͻ 0.01 DS versus control). B, in a complimentary set of experiments, protein and total RNA was isolated from fetal, child, adolescent, and adult prefrontal cortex specimens from control and DS (age-and sex-matched, n ϭ 3-5) patients. Subsequently, MeCP2 mRNA expression levels were determined by RT-PCR as described above. C and D, representative autoradiographs of Western blot experiments of MeCP2 protein expression in identical control and DS fetal brain specimens as indicated. Representative photomicrographs of MeCP2 expression in (E) control (ϫ1000), (F) DS fetal hippocampus (HIPP) (ϫ400, so more negative cells can be visualized), (G) control (ϫ1000), and (H) DS adult prefrontal cortex (Pre-FCTX) (ϫ400) age-and sex-matched fixed human brain specimens (n ϭ 3-5). The fixed tissues were stained with the nuclear dye hematoxylin (light blue signal), then stained again for MeCP2 immunoreactivity. Immunoreactivity was visualized with fast red chromogen (positive staining deep pink color). All immunohistochemistry experiments were repeated a minimum of five times. All MeCP2 staining was lost if the primary or secondary antibodies were omitted. Preimmune serum did not give a MeCP2 positive signal.
MeCP2 protein levels were also attenuated 50 -70% in the DS samples relative to controls (Fig. 2, C and D).
To further validate our Western blot results, and determine which cell type(s) expressed MeCP2 protein, immunohistochemistry experiments were performed utilizing a MeCP2-specific antibody and formalin-fixed age-and sex-matched control and DS samples generated from the same brain specimens used in the RT-PCR and Western studies. Representative photomicrographs of the human control brain samples (Fig. 2, E and G) demonstrated many positive MeCP2-stained neurons (fetal, 9 -12%; adult, 11-20%). The positive signal was evident in both the cytoplasm (large arrow) as well as in the nucleus of neurons (small arrow). In contrast, there were far less positive MeCP2 staining cells (fetal, Ͻ1%; adult, 1-5%) in brain specimens from individuals with DS (Fig. 2, F and H). Quantitative analysis of age-and sex-matched DS and control brain specimens demonstrated that MeCP2 expression was decreased at least 4-fold in DS samples.

Mature miR-155 and -802 Manipulation or MeCP2 siRNA Knockdown Modulate MeCP2 Target Gene Expression-Ini-
tially identified on the basis of this ability of the protein to bind methylated DNA, MeCP2 was thought to only transcriptionally repress target genes (16,17). Recently, however, Chahrour et al. (39) demonstrated that MeCP2 can activate and repress the transcription of a large number of genes that play a role in neurobiology. Based on these observations, we now hypothesize that as a consequence of Trisomy 21-mediated attenuation of MeCP2 protein expression, genes that were activated by MeCP2 would be underexpressed and genes that were repressed by MeCP2 would be overexpressed in individuals with DS (Fig. 3A). To begin to test this hypothesis we have chosen to focus on one transcriptionally activated (cAMP response element-binding protein; CREB1) and one transcriptionally silenced (myocyte enhancer factor 2C; MEF2C) MeCP2 target gene that was identified by Chahrour et al. (39). These specific MeCP2 target genes were chosen because they encode transcription factors that have been shown to play a role in neurodevelopment and neuronal plasticity (18 -21), processes that are abnormal in DS individuals (1)(2)(3). To begin to test this hypothesis, the endogenous levels of MeCP2 were manipulated in a human neuronal cell line by transfection with miR-155 or -802 mimics (Fig. 3B), miR-155 or -802 ASO inhibitors (Fig.  3D), or MeCP2 siRNAs (Fig. 3F) and the expression levels of CREB1 and MEF2C were subsequently measured. As expected, SK-N-SH cells transfected with miR-155 or -802 mimics, or increasing concentrations of MeCP2 siRNAs resulted in decreased MeCP2 mRNA (Fig. 3, B, 40 -50% reduction, and F, 70% reduction with 10 nM MeCP2 siRNA) and protein expression (Fig. 3, C, 40 -50% reduction, and G, 60% reduction). Additionally, CREB1 mRNA (Fig. 3, B and F) and protein levels (Fig.  3, C and G) were also attenuated in the transfected cells (25-40% at the mRNA level and 40 -60% at the protein level). In contrast, MEF2C mRNA (Fig. 3, B and F) and protein levels (Fig.  3, C and G) were significantly increased in the transfected cells (50 -350% at the mRNA level and 50 -110% at the protein level). Finally, transfection studies utilizing miR-155 or -802 ASO inhibitors resulted in reciprocal observations (Fig. 3, D  and E). Together these results strongly suggest that the CREB1 and MEF2C genes are transcriptional targets of MeCP2 and that by reducing or increasing MeCP2 protein levels, MeCP2 target gene expression is modulated.
MeCP2 Target Genes Are Aberrantly Expressed in Human DS Brain Specimens-Because MeCP2 was underexpressed in human brain samples (Fig. 2, A-H), we investigated whether or not a dysregulation of CREB1 and MEF2C also occurred in brain specimens isolated from individuals with DS. RT-PCR assays demonstrated that, independent of age or brain region investigated, CREB1 mRNA was underexpressed (Fig. 4, A and  B, 45-60%) and MEF2C mRNA was overexpressed (Fig. 5, A  and B, 160 -210%) in DS samples when compared with controls. In addition, Western blot analyses demonstrated that CREB1 protein levels were also decreased 40 -60% in DS fetal hippocampus and adult prefrontal cortex samples (Fig. 4C). In contrast, MEF2C protein levels were increased 60 -70% in identical DS brain specimens (Fig. 5C). To further substantiate the Western blot results, and determine which cell type(s) expressed CREB1 and MEF2C, immunohistochemistry experiments were performed. Representative photomicrographs of the human adult control brain samples demonstrated positive staining for CREB1 (fuschia color) in neurons (fetal, 25-33%; adult, 26 -45%) (Fig. 4, D and F), whereas DS brain samples showed a decreased number of CREB1 positive cells (fetal, 1%; adult, 1-9%) (Fig. 4, E and G). Additional representative photomicrographs of the human control brain samples demonstrated that very few MEF2C positive staining neurons were observed (fetal, 1%; adult, 1%) (Fig. 5, D and F). In contrast, a significant increase in MEF2C staining neurons were detected in DS specimens (fetal, 5-20%; adult, 16 -25%) (Fig. 5, E and G).
To demonstrate that the decreased MeCP2 expression levels observed in DS brain specimens actually result in attenuated levels of MeCP2 protein bound to the promoter regions of MeCP2 target genes in vivo, ChIP experiments were performed. When compared with age-and sex-matched control brain specimens, decreased MeCP2 interactions were observed with the CREB1 (Fig. 4H) and MEF2C (Fig. 5H) promoters in DS samples. Taken together, these data support the hypothesis that Hsa21-derived miRNA overexpression leads to the attenuation of MeCP2 expression which, in turn, results in the aberrant regulation of MeCP2 target genes.
The Ts65Dn Mouse Model of DS Aberrantly Expresses miR-155, -802, MeCP2, and MeCP2 Target Genes-To further support the argument that a causal connection exists between Hsa21-derived miRNA overexpression and the dysregulation of MeCP2 and MeCP2 target genes in vivo, a series of experiments was designed utilizing the Ts65Dn mouse model. Ts65Dn mice are trisomic for 104 orthologs of Hsa21 genes and are the most widely used mouse model of DS because they present craniofacial, cognitive, and heart defects (40 -44) similar to those observed in DS (1-3). Bioinformatic analyses suggested that Ts65Dn mice are trisomic for only two of the five Hsa21-derived miRNAs (miR-155 and miR-802). Thus, experiments were conducted to: 1) determine whether or not the Ts65Dn mouse model also overexpressed miR-155 and -802 similar to DS individuals and, 2) if so, whether silencing of miR-155 or -802 in vivo with antagomirs could normalize MeCP2, and MeCP2 target gene expression levels. RT-PCR experiments demonstrated that of the five Hsa21-derived miRNAs only the mouse orthologs of miR-155 and -802 were overexpressed by 40 -60% in the Ts65Dn mice thereby validating computational data (Fig. 6A). As miR-155 and -802 were overexpressed in the Ts65Dn and human DS brain specimens, we hypothesized that MeCP2 and CREB1 would be underexpressed and MEF2C would be overexpressed in these mice, similar to what was observed in human brain samples. To test this hypothesis RT-PCR and Western blot experiments were performed on hippocampus samples isolated from Ts65Dn and euploid control mice. Importantly, these experiments demonstrated that MeCP2 and CREB1 mRNA (Fig. 6B) and MeCP2 and CREB1 protein (Fig. 6, E and F) levels were underexpressed in the Ts65Dn samples compared with the euploid controls (30 -40% at the mRNA level and 40 -90% at the protein level). In contrast, in these same samples, MEF2C mRNA (Fig. 6B) and protein ( Fig. 6, E and F) levels were overexpressed (220 -235% at the mRNA level and 200 -300% at the protein level). Taken together, these results further support the premise that MeCP2 mRNA is a target for miR-155 and -802 and underexpression of MeCP2 may be involved, in part, in mediating DS.
Silencing of miR-155 or -802 in Vivo with Antagomirs Normalizes miR-155, miR-802, MeCP2, and MeCP2 Target Gene Expression-Previous studies demonstrated that chemically modified, cholesterol-conjugated, single-stranded RNA analogs complementary to miRNAs, designated "antagomirs," can silence endogenous miRNAs in vivo (34,35,45). To determine whether or not the silencing of miR-155 or -802 expression in the brains of Ts65Dn mice resulted in augmented MeCP2 expression levels, Ts65Dn and euploid control littermates were ICV injected with antagomir-155 or miR-802. Due to the biological stability of antagomirs (34,35), Ts65Dn brains were harvested 7 days after injection to maximize their effect. ICV injection of antagomir-155 resulted in the attenuation (30 -40%) of endogenous expression of mature miR-155 in the hippocampus of the Ts65Dn animals and no changes were observed in miR-802 expression levels (Fig. 6C). Importantly, the decrease in miR-155 expression resulted in augmented MeCP2 (65-170%) FIGURE 4. CREB1, an activated MeCP2 target gene, is underexpressed in human DS brain specimens. CREB1 mRNA levels were quantitated utilizing gene-specific RT-PCR assays and total RNA isolated from (A) human control and DS fetal brain specimens and (B) control and DS fetal and adult prefrontal cortex specimens. The error bars represent the average Ϯ S.E. of triplicate samples repeated in at least three independent experiments (*, p Ͻ 0.01 DS versus control). CBLM, cerebellum. C, representative autoradiographs of Western blot experiments of CREB1 expression in control and DS fetal hippocampus (HIPP) and control and DS adult prefrontal cortex specimens. D-G, representative photomicrographs (ϫ1000) of total CREB1 expression in human brain specimens by immunohistochemistry. Fixed (D) control and (E) DS fetal hippocampus, and (F) control and (G) DS adult prefrontal cortex brain samples age-and sex-matched. All immunohistochemistry experiments were repeated a minimum of five times. All CREB1 staining was lost if the primary or secondary antibodies were omitted. Preimmune serum did not give a CREB1 positive signal. H, human control and DS prefrontal cortex brain specimens were subjected to ChIP as described under "Experimental Procedures." Formaldehyde-fixed, sheared control and DS chromatin samples were immunoprecipitated with control or MeCP2 antibodies. Additionally, control and DS chromatin samples were utilized as "input" controls to ensure that equal amounts of starting material was immunoprecipitated. The CREB1 promoter region was PCR amplified and the resulting product was quantified. The gene expression levels were calculated relative to 18S rRNA and the data are expressed as fold-decrease under input control, which was assigned a value of 1. The error bars represent the average Ϯ S.E. of three independent experiments (*, p Ͻ 0.01 DS versus control). and CREB1 (150 -200%) mRNA (Fig. 6D) and MeCP2 (70 -90%) and CREB1 (40 -70%) protein levels (Fig. 6E), and attenuated MEF2C mRNA (150 -165%) and protein levels (180 -250%) (Fig. 6, D and E) in the hippocampus of the Ts65Dn animals. Similar results were obtained in Ts65Dn animals ICV injected with antagomir-802 (Fig. 6, C, D, and F). Importantly, no changes were observed in the miR-155, miR-802, MeCP2, CREB1, or MEF2C expression levels in Ts65Dn animals ICV injected with a control scrambled antagomir (Fig. 6, C-E). In summary, these cumulative results clearly suggest that MeCP2 mRNA is a direct target of miR-155 and -802 in vivo, and that silencing of endogenous miRNAs may have therapeutic value.

DISCUSSION
The major findings in the present study are that Hsa21-derived miRNAs (miR-155, and -802), and proteins MeCP2, CREB1, and MEF2C are all aberrantly expressed in a cascadedependent manner in brain specimens isolated from DS individuals. Similar results were obtained utilizing Ts65Dn mouse brain samples that were trisomic for only miR-155 and -802. Preimmune serum did not give a MEF2C positive signal. H, human control and DS prefrontal cortex brain specimens were subjected to ChIP as described under "Experimental Procedures." Formaldehyde-fixed, sheared control and DS chromatin samples were immunoprecipitated with control or MeCP2 antibodies. Additionally, control and DS chromatin samples were utilized as input controls to ensure that equal amounts of starting material was immunoprecipitated. The MEF2C promoter region was PCR amplified and the resulting product was quantified. The gene expression levels were calculated relative to 18S rRNA and the data are expressed as fold-decrease under input control, which was assigned a value of 1. The error bars represent the average Ϯ S.E. of three independent experiments (*, p Ͻ 0.01 DS versus control).
Although bioinformatic analyses demonstrated that Hsa21derived miRNAs could theoretically interact with thousands of distinct mRNA targets, we chose to initially focus on MeCP2 as a potentially important DS target mRNA because mutations in this gene have already been shown to cause the postnatal neurodevelopmental disorder Rett syndrome (16,17). Specifically, MeCP2 is a transcription factor that binds to methylated CpG dinucleotides and induces the recruitment of protein complexes that are involved in histone modifications and chromatin remodeling (16,17). Therefore, MeCP2 was thought to play an important role in the transcriptional silencing of specific target genes. However, Chahrour et al. (39) recently demonstrated that MeCP2 could activate and repress the transcription of a large number of genes. MeCP2 is expressed in most tissues and cell types with the highest expression levels detected in the brain, where it is present primarily in neurons (46,47). MeCP2 is spatially and developmentally regulated, and is characterized by heterogeneous expression in subpopulations of neurons in the brain. The timing of MeCP2 expression correlates with the maturation of the central nervous system (47,48), and recent reports suggest that MeCP2 may be involved in synaptic plasticity (49). Finally, transgenic mouse models have demonstrated that either underexpression or overexpression of MeCP2 are detrimental to cognitive development indicating that levels of MeCP2 in the central nervous system are tightly regulated and crucial for neuronal function (16,17).
Consistent with our observation that MeCP2 is underexpressed in DS brain specimens, other investigators performing mRNA expression survey experiments utilizing RNA isolated from DS fetal fibroblasts and hearts demonstrated that, although a large number of Hsa21 genes were consistently overexpressed, many non-Hsa21 genes were underexpressed, including MeCP2 (50,51). Additionally, Nagarajan et al. (52) demonstrated that the MeCP2 protein was underexpressed in several DS frontal cortex samples. Interestingly, these investigators not only demonstrated decreased MeCP2 expression in DS samples, but also showed that MeCP2 protein expression was reduced in brain specimens isolated from individuals with Rett syndrome, Angelman syndrome, Prader-Willi syndrome, autism, and attention deficit hyperactivity disorder (52). Finally, Samaco et al. (53) demonstrated that precise control of MeCP2 is critical for normal behavior and they predicted that human neurodevelopmental disorders would result from a subtle reduction in MeCP2 expression. Therefore, in the DS setting, the documented underexpression of MeCP2 may play a major role in mediating the observed neurodevelopmental disorders. Additionally, decreased MeCP2 expression may represent a common thread in a number of neurodevelopmental disorders.
Traditionally, MeCP2 was thought to play an important role in transcriptional silencing of specific target genes (16,17). Recently, however, MeCP2 was shown to activate and repress the transcription of a large number of genes (39). Although MeCP2 is known to regulate many downstream target genes (39), we have chosen to focus on CREB1 and MEF2C, as they have been shown to play a critical role in various aspects of neural development (18 -21). We believe that these key proteins are aberrantly expressed in DS brains as a result of the dysregulation of MeCP2 expression mediated by Hsa21-derived miRNAs. This may represent, in part, the next crucial sequence of steps in this neural gene network. Specifically, CREB factors are critical to a variety of functions in the nervous system (including functions that are especially relevant to conditions such as DS), including neurogenesis and neuronal survival, development, and differentiation, axonal outgrowth, synaptic plasticity, and memory formation (18,19). Our CREB1 expression and ChIP experimental data supports the study published by Chahrour et al. (39) where they also demonstrated that MeCP2 regulated CREB1 gene expression. Interestingly, these investigators also established that MeCP2 and CREB1 were direct binding partners and act synergistically on the promoter region of the somatostatin gene (i.e. a MeCP2-activated target gene) (39). Finally, Klein et al. (54) demonstrated that MeCP2 expression was repressed by a CREB-induced miR-132mediated mechanism. Taken together these observations support the hypothesis that there is a critical relationship between MeCP2 and CREB1, and that any aberrant fluctuation of these proteins may result in neuronal disorders.
Emerging evidence also suggests that MEF2C plays a role in programming early neuronal differentiation and proper distribution within the layers of the neocortex (21) and facilitates hippocampal-dependent learning and memory (20). In contrast to the positive impact of the CREB pathway on synaptic inputs, MEF2C keeps the synapse number and function under control (20,21). Therefore, again in the DS setting, the appropriate integration of these essential transcription factor signals may be abnormal.
For miRNAs whose up-regulation in a disease state plays a causal role in the disease, specific reduction of the miRNA in vivo would be therapeutically desirable. Inhibition of miRNA activity can be achieved through the use of chemically modified single-stranded reverse complement oligonucleotides or ASOs. In general, an effective ASO is resistant to nonspecific cellular ribonucleases, resistant to miRNA-directed cleavage by RISC, and binds miRNAs in RISCs with high affinity, effectively out-competing binding to target mRNAs (55). ASO inhibitors containing exclusively 2Ј-O-methyl (2Ј-O-Me) ribose sugars are resistant to cleavage by both RISC and other cellular ribonucleases and 2Ј-O-methyl-modified RNA-RNA hybrids are more thermodynamically stable than either RNA-RNA or DNA-RNA duplexes (56). Nuclease-resistant phosphorothioate backbone linkages, in combination with 2Ј-O-Me ribose modifications, have also been employed in ASOs (34,35). Finally, a 3Ј-terminal cholesterol group conjugation appears to aid delivery of ASOs into cells; however, it may have properties that further enhance ASO activity, such as improved intracellular escape from liposomes, relocalization of the targeted miRNAs, or enhancement of ASO stability (55). Utilizing these ASO strategies, previous studies have demonstrated that chemically modified, cholesterol-conjugated, single-stranded RNA analogs complementary to miRNAs, designated antagomirs, can silence endogenous miRNAs in vivo (34,35,45). The mech-anism(s) by which ASOs affect miRNA expression can theoretically occur at multiple levels (57): 1) by binding to the mature miRNA within the RISC and acting as a competitive inhibitor; 2) by binding to the pre-miRNA and preventing its processing or entry into the RISC; 3) by interfering with the processing or export of the pre-or pri-miRNA from the nucleus. Regardless of the mechanism, the net result is a reduction in the concentration of a specific miRNA-programmed RISC.
In conclusion, our data show that the Hsa21-derived miRNAs are overexpressed due to Trisomy 21, and results in the underexpression of the target protein, MeCP2. The attenuation of this protein, and subsequent aberrant expression of the CREB1 and MEF2C transcription factors, may lead to abnormal brain development through anomalous neuronal gene expression during the critical period of synaptic maturation (i.e. alterations in neurogenesis, neuronal differentiation, myelination, and synaptogenesis), which are thought to result in the cognitive impairment of DS patients (2,3). Although we have demonstrated that miR-155 and -802 can directly regulate the expression of MeCP2, Hsa21-derived miRNAs may regulate thousands of mRNA targets. However, therapeutically this is not a disadvantage because inhibition or knock-down of these overexpressed miRNAs should normalize the expression levels of all miRNA/mRNA targets back to non-trisomic 21 levels.