Alexander S. Brodsky

Genome-wide analysis of estrogen receptor binding sites

The estrogen receptor is the master transcriptional regulator of breast cancer phenotype and the archetype of a molecular therapeutic target. We mapped all estrogen receptor and RNA polymerase II binding sites on a genome-wide scale, identifying the authentic cis binding sites and target genes, in breast cancer cells. Combining this unique resource with gene expression data demonstrates distinct temporal mechanisms of estrogen-mediated gene regulation, particularly in the case of estrogen-suppressed genes. Furthermore, this resource has allowed the identification of cis-regulatory sites in previously unexplored regions of the genome and the cooperating transcription factors underlying estrogen signaling in breast cancer.

Chromosome-Wide Mapping of Estrogen Receptor Binding Reveals Long-Range Regulation Requiring the Forkhead Protein FoxA1

Estrogen plays an essential physiologic role in reproduction and a pathologic one in breast cancer. The completion of the human genome has allowed the identification of the expressed regions of protein-coding genes; however, little is known concerning the organization of their cis-regulatory elements. We have mapped the association of the estrogen receptor (ER) with the complete nonrepetitive sequence of human chromosomes 21 and 22 by combining chromatin immunoprecipitation (ChIP) with tiled microarrays. ER binds selectively to a limited number of sites, the majority of which are distant from the transcription start sites of regulated genes. The unbiased sequence interrogation of the genuine chromatin binding sites suggests that direct ER binding requires the presence of Forkhead factor binding in close proximity. Furthermore, knockdown of FoxA1 expression blocks the association of ER with chromatin and estrogen-induced gene expression demonstrating the necessity of FoxA1 in mediating an estrogen response in breast cancer cells.

Chromatin remodeling in the aging genome of Drosophila

Chromatin structure affects the accessibility of DNA to transcription, repair, and replication. Changes in chromatin structure occur during development, but less is known about changes during aging. We examined the state of chromatin structure and its effect on gene expression during aging in Drosophila at the whole genome and cellular level using whole‐genome tiling microarrays of activation and repressive chromatin marks, whole‐genome transcriptional microarrays and single‐cell immunohistochemistry. We found dramatic reorganization of chromosomal regions with age. Mapping of H3K9me3 and HP1 signals to fly chromosomes reveals in young flies the expected high enrichment in the pericentric regions, the 4th chromosome, and islands of facultative heterochromatin dispersed throughout the genome. With age, there is a striking reduction in this enrichment resulting in a nearly equivalent level of H3K9me3 and HP1 in the pericentric regions, the 4th chromosome, facultative heterochromatin, and euchromatin. These extensive changes in repressive chromatin marks are associated with alterations in age‐related gene expression. Large‐scale changes in repressive marks with age are further substantiated by single‐cell immunohistochemistry that shows changes in nuclear distribution of H3K9me3 and HP1 marks with age. Such epigenetic changes are expected to directly or indirectly impinge upon important cellular functions such as gene expression, DNA repair, and DNA replication. The combination of genome‐wide approaches such as whole‐genome chromatin immunoprecipitation and transcriptional studies in conjunction with single‐cell immunohistochemistry as shown here provide a first step toward defining how changes in chromatin may contribute to the process of aging in metazoans.

Genome-wide identification of functionally distinct subsets of cellular mRNAs associated with two nucleocytoplasmic-shuttling mammalian splicing factors.

BackgroundPre-mRNA splicing is an essential step in gene expression that occurs co-transcriptionally in the cell nucleus, involving a large number of RNA binding protein splicing factors, in addition to core spliceosome components. Several of these proteins are required for the recognition of intronic sequence elements, transiently associating with the primary transcript during splicing. Some protein splicing factors, such as the U2 small nuclear RNP auxiliary factor (U2AF), are known to be exported to the cytoplasm, despite being implicated solely in nuclear functions. This observation raises the question of whether U2AF associates with mature mRNA-ribonucleoprotein particles in transit to the cytoplasm, participating in additional cellular functions.ResultsHere we report the identification of RNAs immunoprecipitated by a monoclonal antibody specific for the U2AF 65 kDa subunit (U2AF65) and demonstrate its association with spliced mRNAs. For comparison, we analyzed mRNAs associated with the polypyrimidine tract binding protein (PTB), a splicing factor that also binds to intronic pyrimidine-rich sequences but additionally participates in mRNA localization, stability, and translation. Our results show that 10% of cellular mRNAs expressed in HeLa cells associate differentially with U2AF65 and PTB. Among U2AF65-associated mRNAs there is a predominance of transcription factors and cell cycle regulators, whereas PTB-associated transcripts are enriched in mRNA species that encode proteins implicated in intracellular transport, vesicle trafficking, and apoptosis.ConclusionOur results show that U2AF65 associates with specific subsets of spliced mRNAs, strongly suggesting that it is involved in novel cellular functions in addition to splicing.

Long-lived Indy induces reduced mitochondrial reactive oxygen species production and oxidative damage

Decreased Indy activity extends lifespan in D. melanogaster without significant reduction in fecundity, metabolic rate, or locomotion. To understand the underlying mechanisms leading to lifespan extension in this mutant strain, we compared the genome-wide gene expression changes in the head and thorax of adult Indy mutant with control flies over the course of their lifespan. A signature enrichment analysis of metabolic and signaling pathways revealed that expression levels of genes in the oxidative phosphorylation pathway are significantly lower in Indy starting at day 20. We confirmed experimentally that complexes I and III of the electron transport chain have lower enzyme activity in Indy long-lived flies by Day 20 and predicted that reactive oxygen species (ROS) production in mitochondria could be reduced. Consistently, we found that both ROS production and protein damage are reduced in Indy with respect to control. However, we did not detect significant differences in total ATP, a phenotype that could be explained by our finding of a higher mitochondrial density in Indy mutants. Thus, one potential mechanism by which Indy mutants extend life span could be through an alteration in mitochondrial physiology leading to an increased efficiency in the ATP/ROS ratio.

Identification of Ovarian Cancer Metastatic miRNAs

Serous epithelial ovarian cancer (EOC) patients often succumb to aggressive metastatic disease, yet little is known about the behavior and genetics of ovarian cancer metastasis. Here, we aim to understand how omental metastases differ from primary tumors and how these differences may influence chemotherapy. We analyzed the miRNA expression profiles of primary EOC tumors and their respective omental metastases from 9 patients using miRNA Taqman qPCR arrays. We find 17 miRNAs with differential expression in omental lesions compared to primary tumors. miR-21, miR-150, and miR-146a have low expression in most primary tumors with significantly increased expression in omental lesions, with concomitant decreased expression of predicted mRNA targets based on mRNA expression. We find that miR-150 and miR-146a mediate spheroid size. Both miR-146a and miR-150 increase the number of residual surviving cells by 2–4 fold when challenged with lethal cisplatin concentrations. These observations suggest that at least two of the miRNAs, miR-146a and miR-150, up-regulated in omental lesions, stimulate survival and increase drug tolerance. Our observations suggest that cancer cells in omental tumors express key miRNAs differently than primary tumors, and that at least some of these microRNAs may be critical regulators of the emergence of drug resistant disease.

MicroRNA profiling in mucosal biopsies of eosinophilic esophagitis patients pre and post treatment with steroids and relationship with mRNA targets.

Background The characterization of miRNAs and their target mRNAs involved in regulation of the immune process is an area of intense research and relatively little is known governing these processes in allergic inflammation. Here we present novel findings defining the miRNA and mRNA transcriptome in eosinophilic esophagitis (EoE), an increasing recognized allergic disorder. Methods Esophageal epithelial miRNA and mRNA from five paired biopsies pre- and post-treatment with glucocorticosteroids were profiled using Taqman and Affymetrix arrays. Validation was performed on additional paired biopsies, untreated EoE specimens and normal controls. Differentially regulated miRNAs and mRNAs were generated, within which miRNA-mRNA target pairs with high predicted confidence were identified. Results Compared to the post-glucocorticoid treated esophageal mucosa, of all the 377 miRNA sequences examined, 32 miRNAs were significantly upregulated and four downregulated in the pre-treated biopsies. MiR-214 was the most upregulated (150 fold) and miR-146b-5b, 146a, 145, 142-3p and 21 were upregulated by at least 10 fold. Out of 12 miRNAs chosen for validation by qRT-PCR, five (miR-214, 146b-5p, 146a, 142-3p and 21) were confirmed and 11 shared the same trend. When the expression of the 12 miRNAs in the EoE mucosa was compared to unrelated normal mucosa, six (miR-214, 146b-5p, 146a, 21, 203, and 489) showed similar significant changes as in the paired samples and 10 of them shared the same trend. In the same five pairs of samples used to profile miRNA, 311 mRNAs were down-regulated and 35 were up-regulated in pre-treated EoE mucosa. Among them, 164 mRNAs were identified as potential targets of differentially regulated miRNAs. Further analysis revealed that immune-related genes, targeted and non-targeted by miRNAs, were among the most important genes involved in the pathogenesis of EoE. Conclusions Our findings add to the accumulating body of data defining a regulatory role for miRNA in immune and allergic processes.

Exon expression profiling reveals stimulus-mediated exon use in neural cells

Background:Neuronal cells respond to changes in intracellular calcium ([Ca2+]i) by affecting both the abundance and architecture of specific mRNAs. Although calcium-induced transcription and transcript variation have both been recognized as important sources of gene regulation, the interplay between these two phenomena has not been evaluated on a genome-wide scale.Results:Here, we show that exon-centric microarrays can be used to resolve the [Ca2+]i-modulated gene expression response into transcript-level and exon-level regulation. Global assessments of affected transcripts reveal modulation within distinct functional gene categories. We find that transcripts containing calcium-modulated exons exhibit enrichment for calcium ion binding, calmodulin binding, plasma membrane associated, and metabolic proteins. Additionally, we uncover instances of regulated exon use in potassium channels, neuroendocrine secretory proteins and metabolic enzymes, and demonstrate that regulated changes in exon expression give rise to distinct transcript variants.Conclusion:Our findings connect extracellular stimuli to specific exon behavior, and suggest that changes in transcript and exon abundance are reflective of a coordinated gene expression response to elevated [Ca2+]i. The technology we describe here lends itself readily to the resolution of stimulus-induced gene expression at both the transcript and exon levels.

Identifying proteins that affect mRNA localization in living cells.

Messenger RNA transport has emerged as a significant mechanism for regulating gene expression. Many of the protein factors affecting RNA transport remain unknown. The emergence of green fluorescent protein (GFP) fluorescence microscopy allows imaging in living cells and an increased understanding of in vivo molecular transport. GFP imaging is now applied to RNA transport by engineering RNA hairpins into the RNA of interest and observing fluorescence from GFP fused to an RNA-binding protein that recognizes the hairpins. In yeast, different genetic backgrounds can be tested to identify various proteins that affect RNA transport and localization. The technology also allows the swapping of different regions of the RNA to determine the cis requirements for transport. GFP RNA imaging opens many possibilities to examine RNA transport in real time in a variety of different organisms.

Expression profiling of primary and metastatic ovarian tumors reveals differences indicative of aggressive disease.

The behavior and genetics of serous epithelial ovarian cancer (EOC) metastasis, the form of the disease lethal to patients, is poorly understood. The unique properties of metastases are critical to understand to improve treatments of the disease that remains in patients after debulking surgery. We sought to identify the genetic and phenotypic landscape of metastatic progression of EOC to understand how metastases compare to primary tumors. DNA copy number and mRNA expression differences between matched primary human tumors and omental metastases, collected at the same time during debulking surgery before chemotherapy, were measured using microarrays. qPCR and immunohistochemistry validated findings. Pathway analysis of mRNA expression revealed metastatic cancer cells are more proliferative and less apoptotic than primary tumors, perhaps explaining the aggressive nature of these lesions. Most cases had copy number aberrations (CNAs) that differed between primary and metastatic tumors, but we did not detect CNAs that are recurrent across cases. A six gene expression signature distinguishes primary from metastatic tumors and predicts overall survival in independent datasets. The genetic differences between primary and metastatic tumors, yet common expression changes, suggest that the major clone in metastases is not the same as in primary tumors, but the cancer cells adapt to the omentum similarly. Together, these data highlight how ovarian tumors develop into a distinct, more aggressive metastatic state that should be considered for therapy development.