Gene expression and ChIP-seq analyses of mesenchymal stem cells, osteoblasts and the U2OS osteosarcoma cell line
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ABSTRACT:
ORGANISM(S): Homo sapiens
SUBMITTER: hari easwaran
PROVIDER: E-GEOD-35576 | biostudies-arrayexpress |
REPOSITORIES: biostudies-arrayexpress
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Project description:Many DNA-hypermethylated cancer genes are occupied by the polycomb (PcG) repressor complex in embryonic stem cells (ESCs). Their prevalence in the full spectrum of cancers, the exact context of chromatin involved, and their status in adult cell renewal systems are unknown. Using a genome-wide analysis, we demonstrate that approximately 75% of hypermethylated genes are marked by PcG in the context of bivalent chromatin in both ESC and adult stem/progenitor cells. A large number of these genes are key developmental regulators and a subset, which we call the "DNA hypermethylation module", comprise a portion of the PcG target genes that are downregulated in cancer. Genes with bivalent chromatin have a low, poised gene transcription state that has been shown to maintain stemness and self-renewal in normal stem cells. However, when DNA-hypermethylated in tumors, we find these genes are further repressed. We also show that the methylation status of these genes can cluster important subtypes of colon and breast cancers. By evaluating the subsets of genes that are methylated in different cancers with consideration of their chromatin status in ESCs, we provide evidence that DNA-hypermethylation preferentially targets the subset of PcG genes that are developmental regulators, and this may contribute to the stem-like state of cancer. Additionally, the capacity for global methylation profiling to cluster tumors by phenotype may have important implications for further refining tumor behavior patterns that may ultimately aid therapeutic interventions. The goal was to analyze the chromatin in MSCs, osteoblasts and U2OS cells of genes that are methylated in the osteosarcoma cell line U2OS. Cell culture: MSCs derived from adult bone marrow were cultured and differentiated to osteoblasts as described by Jaiswal et al., 1997 (J Cell Biochem 64, 295-312). U2OS cells were cultured in McCoy's 5A medium supplemented with 10% FBS. ChIP-seq: Cells were crosslinked in 3.7% formaldehyde, lysed and sonicated to obtain chromatin fragments in the range of 200 to 600 base pairs. Sonicated chromatin was incubated with primary antibody, H3K4Me3 (Millipore, 07-473) or H3K27Me3 (kind gift from Thomas Jenuwein/ Nicholas Shukeir), overnight at 4°C, followed by capturing primary antibodies by adding ProteinA/G magnetic beads (DynaBeads) and incubating at room temperature for 3-4 hrs. Captured chromatin was washed in low- and high-salt buffers, as well as TE. Chromatin fragments were subjected to simultaneous elution, de-crosslinking and Proteinase-K treatment at 65°C in elution buffer followed by phenol-chloroform-isoamyl alcohol extraction and ethanol precipitation. Sequencing and alignment: Purified DNA was used to prepare the sequencing library and sequenced on Applied Biosystems SOLiD (V3). Sequencing reads were aligned to hg18 (NCBI 36) using Bioscope 1.2.1. The software guarantees finding all alignments between the first 25 base pairs of the read (seed) and the reference sequence with up to 2 mismatches. Each match is extended to the full length of the read, scoring 1 point for matching and -2 points for mismatching bases. The read is trimmed to the length with the highest score. If there is only one alignment or if an alignment scores significantly higher than the others for the same read, it is considered unique and reported. Peak identification: Aligned reads were analyzed using Model-based Analysis of ChIP-Seq (MACS) to detect regions enriched for the histone marks (called peaks) with default settings. Input was used as background for peak identification.
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Project description:Transcriptional profiling of MSC, Osteoblasts and U2OS cells. The aim was to quantitate relative gene expression in MSC, osteoblasts and U2OS. MSC and osteoblasts were used as normal cells in this study because osteosarcoma most likely originates from MSC or osteoblasts. Cell culture and treatment: U2OS cells were cultured in McCoys 5A media (Invitrogen) containing 10% FBS. MSC and osteoblasts were cultured as described by Jaiswal et al. (J Cell Biochem 64, 295-312; 1997). Cells were treated with 5 uM 5-Azadeoxycytidine (5-Aza-CdR) or 300 nM Trischostatin-A (TSA) for 96 hours or 18 hours, respectively. Microarray analysis: Total RNA was extracted using the Qiagen kit according to the manufacturer's instructions, including a DNase step. RNA was quantified using the NanoDrop ND-100, followed by quality assessment with the 2100 Bioanalyzer (Agilent Technologies). RNA concentrations for individual samples were greater than 200ng/ul, with 28s/18s ratios greater than 2.2 and RNA integrity numbers of 10 (highest). Sample amplification and labeling procedures were carried out using the Low RNA Input Fluorescent Linear Amplification Kit (Agilent Technologies) according to the manufacturer's instructions. The labeled cRNA was purified using the RNeasy mini kit (Qiagen) and quantified. RNA spike-in controls (Agilent Technologies) were added to the RNA samples before amplification. 0.75 microgram of samples labeled with Cy3 or Cy5 were mixed with control targets (Agilent Technologies), assembled on Oligo Microarray, hybridized, and processed according to the Agilent microarray protocol. Scanning was performed with the Agilent Scanner G2505B under the default settings recommended by Agilent Technologies. Data analysis: All arrays were subject to the quality checks recommended by the manufacturer. Images were visually inspected for artifacts, and distributions of signal and background intensity of both red and green channels were examined to identify anomalous arrays. No irregularities were observed, and all arrays were retained and used. All calculations were performed using the R statistical computing platform and packages from the Bioconductor bioinformatics software project. To determine the expression levels of genes in MSC, Osteoblasts and U2OS, the data were LOWESS-normalized, the probe intensities of the mock channel (no drug treatment) were extracted and set to the same scale by quantile normalization. Mean of replicate samples were calculated for each probe.
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