Shirley Horn-Saban

Genome-wide midrange transcription profiles reveal expression level relationships in human tissue specification

MOTIVATION Genes are often characterized dichotomously as either housekeeping or single-tissue specific. We conjectured that crucial functional information resides in genes with midrange profiles of expression. RESULTS To obtain such novel information genome-wide, we have determined the mRNA expression levels for one of the largest hitherto analyzed set of 62 839 probesets in 12 representative normal human tissues. Indeed, when using a newly defined graded tissue specificity index tau, valued between 0 for housekeeping genes and 1 for tissue-specific genes, genes with midrange profiles having 0.15< tau<0.85 were found to constitute >50% of all expression patterns. We developed a binary classification, indicating for every gene the I(B) tissues in which it is overly expressed, and the 12-I(B) tissues in which it shows low expression. The 85 dominant midrange patterns with I(B)=2-11 were found to be bimodally distributed, and to contribute most significantly to the definition of tissue specification dendrograms. Our analyses provide a novel route to infer expression profiles for presumed ancestral nodes in the tissue dendrogram. Such definition has uncovered an unsuspected correlation, whereby de novo enhancement and diminution of gene expression go hand in hand. These findings highlight the importance of gene suppression events, with implications to the course of tissue specification in ontogeny and phylogeny. AVAILABILITY All data and analyses are publically available at the GeneNote website, http://genecards.weizmann.ac.il/genenote/ and, GEO accession GSE803. CONTACT doron.lancet@weizmann.ac.il SUPPLEMENTARY INFORMATION Four tables available at the above site.

The H3K27 demethylase Utx regulates somatic and germ cell epigenetic reprogramming

Induced pluripotent stem cells (iPSCs) can be derived from somatic cells by ectopic expression of different transcription factors, classically Oct4 (also known as Pou5f1), Sox2, Klf4 and Myc (abbreviated as OSKM). This process is accompanied by genome-wide epigenetic changes, but how these chromatin modifications are biochemically determined requires further investigation. Here we show in mice and humans that the histone H3 methylated Lys 27 (H3K27) demethylase Utx (also known as Kdm6a) regulates the efficient induction, rather than maintenance, of pluripotency. Murine embryonic stem cells lacking Utx can execute lineage commitment and contribute to adult chimaeric animals; however, somatic cells lacking Utx fail to robustly reprogram back to the ground state of pluripotency. Utx directly partners with OSK reprogramming factors and uses its histone demethylase catalytic activity to facilitate iPSC formation. Genomic analysis indicates that Utx depletion results in aberrant dynamics of H3K27me3 repressive chromatin demethylation in somatic cells undergoing reprogramming. The latter directly hampers the derepression of potent pluripotency promoting gene modules (including Sall1, Sall4 and Utf1), which can cooperatively substitute for exogenous OSK supplementation in iPSC formation. Remarkably, Utx safeguards the timely execution of H3K27me3 demethylation observed in embryonic day 10.5–11 primordial germ cells (PGCs), and Utx-deficient PGCs show cell-autonomous aberrant epigenetic reprogramming dynamics during their embryonic maturation in vivo. Subsequently, this disrupts PGC development by embryonic day 12.5, and leads to diminished germline transmission in mouse chimaeras generated from Utx-knockout pluripotent cells. Thus, we identify Utx as a novel mediator with distinct functions during the re-establishment of pluripotency and germ cell development. Furthermore, our findings highlight the principle that molecular regulators mediating loss of repressive chromatin during in vivo germ cell reprogramming can be co-opted during in vitro reprogramming towards ground state pluripotency.

Epigenetic polymorphism and the stochastic formation of differentially methylated regions in normal and cancerous tissues

DNA methylation has been comprehensively profiled in normal and cancer cells, but the dynamics that form, maintain and reprogram differentially methylated regions remain enigmatic. Here, we show that methylation patterns within populations of cells from individual somatic tissues are heterogeneous and polymorphic. Using in vitro evolution of immortalized fibroblasts for over 300 generations, we track the dynamics of polymorphic methylation at regions developing significant differential methylation on average. The data indicate that changes in population-averaged methylation occur through a stochastic process that generates a stream of local and uncorrelated methylation aberrations. Despite the stochastic nature of the process, nearly deterministic epigenetic remodeling emerges on average at loci that lose or gain resistance to methylation accumulation. Changes in the susceptibility to methylation accumulation are correlated with changes in histone modification and CTCF occupancy. Characterizing epigenomic polymorphism within cell populations is therefore critical to understanding methylation dynamics in normal and cancer cells.

A Critical Role for Ceramide Synthase 2 in Liver Homeostasis II. INSIGHTS INTO MOLECULAR CHANGES LEADING TO HEPATOPATHY

We have generated a mouse that cannot synthesize very long acyl chain (C22–C24) ceramides (Pewzner-Jung, Y., Park, H., Laviad, E. L., Silva, L. C., Lahiri, S., Stiban, J., Erez-Roman, R., Brugger, B., Sachsenheimer, T., Wieland, F. T., Prieto, M., Merrill, A. H., and Futerman, A. H. (2010) J. Biol. Chem. 285, 10902–10910) due to ablation of ceramide synthase 2 (CerS2). As a result, significant changes were observed in the sphingolipid profile of livers from these mice, including elevated C16-ceramide and sphinganine levels. We now examine the functional consequences of these changes. CerS2 null mice develop severe nonzonal hepatopathy from about 30 days of age, the age at which CerS2 expression peaks in wild type mice, and display increased rates of hepatocyte apoptosis and proliferation. In older mice there is extensive and pronounced hepatocellular anisocytosis with widespread formation of nodules of regenerative hepatocellular hyperplasia. Progressive hepatomegaly and noninvasive hepatocellular carcinoma are also seen from ∼10 months of age. Even though CerS2 is found at equally high mRNA levels in kidney and liver, there are no changes in renal function and no pathological changes in the kidney. High throughput analysis of RNA expression in liver revealed up-regulation of genes associated with cell cycle regulation, protein transport, cell-cell interactions and apoptosis, and down-regulation of genes associated with intermediary metabolism, such as lipid and steroid metabolism, adipocyte signaling, and amino acid metabolism. In addition, levels of the cell cycle regulator, the cyclin dependent-kinase inhibitor p21WAF1/CIP1, were highly elevated, which occurs by at least two mechanisms, one of which may involve p53. We propose a functional rationale for the synthesis of sphingolipids with very long acyl chains in liver homeostasis and in cell physiology.

Signaling to transcription networks in the neuronal retrograde injury response.

Robustness in nerve injury responses results from control of axon-to-soma signaling networks by multiple regulatory components. Calling In the Repair Crew The ability of a damaged neuron to regenerate depends on the initiation of a repair program in the cell body, so that the injured neuron switches from a “growth-as-normal” mode to an “injury-response” mode. Initiation of such a repair program depends in turn on the receipt by the cell body of injury signals from the lesion. Michaelevski et al. combined phosphoproteomic analyses of injured and uninjured rat sciatic nerve with microarray analyses of transcripts in the dorsal root ganglia to identify retrograde signaling networks implicated in activating the transcriptional response to axonal injury. Pharmacological manipulation of various protein kinases that appeared in many of these networks and were predicted to play a key role in affecting signaling network size and connectivity affected neurite outgrowth of cultured sensory neurons. Paradoxically, the combined manipulation of pairs of these kinases was sometimes less effective at affecting neurite outgrowth than manipulation of either alone—an observation that has substantial implications for development of appropriate therapies for treating nerve injury. Retrograde signaling from axon to soma activates intrinsic regeneration mechanisms in lesioned peripheral sensory neurons; however, the links between axonal injury signaling and the cell body response are not well understood. Here, we used phosphoproteomics and microarrays to implicate ~900 phosphoproteins in retrograde injury signaling in rat sciatic nerve axons in vivo and ~4500 transcripts in the in vivo response to injury in the dorsal root ganglia. Computational analyses of these data sets identified ~400 redundant axonal signaling networks connected to 39 transcription factors implicated in the sensory neuron response to axonal injury. Experimental perturbation of individual overrepresented signaling hub proteins, including Abl, AKT, p38, and protein kinase C, affected neurite outgrowth in sensory neurons. Paradoxically, however, combined perturbation of Abl together with other hub proteins had a reduced effect relative to perturbation of individual proteins. Our data indicate that nerve injury responses are controlled by multiple regulatory components, and suggest that network redundancies provide robustness to the injury response.

Expression Profiling and Biochemical Analysis Suggest Stress Response as a Potential Mechanism Inhibiting Proliferation of Polyamine-depleted Cells

Background: Depletion of cellular polyamines results in cessation of cellular proliferation. Results: Genomic and biochemical analysis demonstrates stress establishment in the polyamine-depleted cells. Conclusion: Establishment of stress response accounts at least in part for growth arrest establishment in polyamine-depleted cells. Significance: Our results suggest a novel view on the mechanistic basis accounting for growth arrest of polyamine-depleted cells. Polyamines are small organic polycations that are absolutely required for cell growth and proliferation; yet the basis for this requirement is mostly unknown. Here, we combined a genome-wide expression profiling with biochemical analysis to reveal the molecular basis for inhibited proliferation of polyamine-depleted cells. Transcriptional responses accompanying growth arrest establishment in polyamine-depleted cells or growth resumption following polyamine replenishment were monitored and compared. Changes in the expression of genes related to various fundamental cellular processes were established. Analysis of mirror-symmetric expression patterns around the G1-arrest point identified a set of genes representing a stress-response signature. Indeed, complementary biochemical analysis demonstrated activation of the PKR-like endoplasmic reticulum kinase arm of the unfolded protein response and of the stress-induced p38 MAPK. These changes were accompanied by induction of key growth-inhibitory factors such as p21 and Gadd45a and reduced expression of various cyclins, most profoundly cyclin D1, setting the basis for the halted proliferation. However, although the induced stress response could arrest growth, polyamine depletion also inhibited proliferation of PKR-like endoplasmic reticulum kinase and p38α-deficient cells and of cells harboring a nonphosphorylatable mutant eIF2α (S51A), suggesting that additional yet unidentified mechanisms might inhibit proliferation of polyamine-depleted cells. Despite lengthy persistence of the stress and activation of apoptotic signaling, polyamine-depleted cells remained viable, apparently due to induced expression of protective genes and development of autophagy.

VAV1 and BAFF, via NFκB pathway, are genetic risk factors for myasthenia gravis.

To identify novel genetic loci that predispose to early‐onset myasthenia gravis (EOMG) applying a two‐stage association study, exploration, and replication strategy.

Using Synthetic Mouse Spike-In Transcripts to Evaluate RNA-Seq Analysis Tools.

One of the key applications of next-generation sequencing (NGS) technologies is RNA-Seq for transcriptome genome-wide analysis. Although multiple studies have evaluated and benchmarked RNA-Seq tools dedicated to gene level analysis, few studies have assessed their effectiveness on the transcript-isoform level. Alternative splicing is a naturally occurring phenomenon in eukaryotes, significantly increasing the biodiversity of proteins that can be encoded by the genome. The aim of this study was to assess and compare the ability of the bioinformatics approaches and tools to assemble, quantify and detect differentially expressed transcripts using RNA-Seq data, in a controlled experiment. To this end, in vitro synthesized mouse spike-in control transcripts were added to the total RNA of differentiating mouse embryonic bodies, and their expression patterns were measured. This novel approach was used to assess the accuracy of the tools, as established by comparing the observed results versus the results expected of the mouse controlled spiked-in transcripts. We found that detection of differential expression at the gene level is adequate, yet on the transcript-isoform level, all tools tested lacked accuracy and precision.

Olfactory Reception: From Signal Modulation to Human Genome Mapping

Olfactory science has evolved into a comprehensive molecular definition brought about by the realization that broad odorant spectra may still be compatible with receptor genes encoding stereospecific proteins [1–3]. Evidence for the existence of specific molecular reception pathways arose with the discovery of G-protein-coupled second messenger transduction cascades in olfactory ciliary membranes [4–6]. Subsequently, a family of hundreds of G-protein-coupled receptors, first proposed based on the available biochemical data [7–9], became a reality with the cloning of olfactory receptors (ORs) in several vertebrate species [10–12]. It is proposed that when this molecular understanding is combined with human psychophysics and genome analysis, a better picture of human olfactory function will emerge.

Major differences in microRNA quantification are platform and sequence dependent

Background Small RNAs (sRNAs) are known to play an important regulatory role in a wide range of organisms and biological processes through expression regulation of a diverse array of genes. Several classes of sRNA including plant microRNAs (miRNAs), small interfering RNAs (siRNAs) and Piwi-interacting RNAs (piRNAs) carry an O-methyl modification at their 3 termini, and this is suspected to complicate their accurate quantification. miRNA precursors have been detected in cell lines and other tissues, where their amount does not necessarily correlate with the amount of their mature miRNA due to different regulation processes controlling their biogenesis. Methods currently used to identify and quantify sRNAs face unique challenges due to their short length, as well as the high sequence similarities between them. The correct identification, discrimination and profiling of the different types of sRNAs, including their available isoforms, are crucial for understanding of the regulatory networks they are involved in.