Sven Bilke

Stability of the Kauffman model

Random Boolean networks, the Kauffman model, are revisited by means of a novel decimation algorithm, which removes variables that cannot be relevant to the asymptotic dynamics of the system. The major part of the removed variables have the same fixed state in all limit cycles. These variables are denoted as the stable core of the network and their number grows approximately linearly with N, the number of variables in the original network. The sensitivity of the attractors to perturbations is investigated. We find that reduced networks lack the well-known insensitivity observed in full Kauffman networks. We conclude that, somewhat counterintuitive, this remarkable property of full Kauffman networks is generated by the dynamics of their stable core. The decimation method is also used to simulate large critical Kauffman networks. For networks up to N=32 we perform full enumeration studies. Strong evidence is provided that the number of limit cycles grows linearly with N. This result is in sharp contrast to the often cited square root of [N] behavior.

Early B-cell factor, E2A, and Pax-5 cooperate to activate the early B cell-specific mb-1 promoter.

ABSTRACT Previous studies have suggested that the early-B-cell-specific mb-1(Igα) promoter is regulated by EBF and Pax-5. Here, we used in vivo footprinting assays to detect occupation of binding sites in endogenous mb-1 promoters at various stages of B-cell differentiation. In addition to EBF and Pax-5 binding sites, we detected occupancy of a consensus binding site for E2A proteins (E box) in pre-B cells. EBF and E box sites are crucial for promoter function in transfected pre-B cells, and EBF and E2A proteins synergistically activated the promoter in transfected HeLa cells. Other data suggest that EBF and E box sites are less important for promoter function at later stages of differentiation, whereas binding sites for Pax-5 (and its Ets ternary complex partners) are required for promoter function in all mb-1-expressing cells. Using DNA microarrays, we found that expression of endogenous mb-1 transcripts correlates most closely with EBF expression and negatively with Id1, an inhibitor of E2A protein function, further linking regulation of the mb-1 gene with EBF and E2A. Together, our studies demonstrate the complexity of factors regulating tissue-specific transcription and support the concept that EBF, E2A, and Pax-5 cooperate to activate target genes in early B-cell development.

RNA analysis of B cell lines arrested at defined stages of differentiation allows for an approximation of gene expression patterns during B cell development

The development of a mature B lymphocyte from a bone marrow stem cell is a highly ordered process involving stages with defined features and gene expression patterns. To obtain a deeper understanding of the molecular genetics of this process, we have performed RNA expression analysis of a set of mouse B lineage cell lines representing defined stages of B cell development using Affymetrix™ microarrays. The cells were grouped based on their previously defined phenotypic features, and a gene expression pattern for each group of cell lines was established. The data indicated that the cell lines representing a defined stage generally presented a high similarity in overall expression profiles. Numerous genes could be identified as expressed with a restricted pattern using dCHIP‐based, quantitative comparisons or presence/absence‐based, probabilistic state analysis. These experiments provide a model for gene expression during B cell development, and the correctly identified expression patterns of a number of control genes suggest that a series of cell lines can be useful tools in the elucidation of the molecular genetics of a complex differentiation process.

Probabilistic estimation of microarray data reliability and underlying gene expression

BackgroundThe availability of high throughput methods for measurement of mRNA concentrations makes the reliability of conclusions drawn from the data and global quality control of samples and hybridization important issues. We address these issues by an information theoretic approach, applied to discretized expression values in replicated gene expression data.ResultsOur approach yields a quantitative measure of two important parameter classes: First, the probability P(σ|S) that a gene is in the biological state σ in a certain variety, given its observed expression S in the samples of that variety. Second, sample specific error probabilities which serve as consistency indicators of the measured samples of each variety. The method and its limitations are tested on gene expression data for developing murine B-cells and a t-test is used as reference. On a set of known genes it performs better than the t-test despite the crude discretization into only two expression levels. The consistency indicators, i.e. the error probabilities, correlate well with variations in the biological material and thus prove efficient.ConclusionsThe proposed method is effective in determining differential gene expression and sample reliability in replicated microarray data. Already at two discrete expression levels in each sample, it gives a good explanation of the data and is comparable to standard techniques.

Somatic mutations in murine models of leukemia and lymphoma: Disease specificity and clinical relevance

Malignant transformation is a multistep process that is dictated by the acquisition of multiple genomic aberrations that provide growth and survival advantage. During the post genomic era, high throughput genomic sequencing has advanced exponentially, leading to identification of countless cancer associated mutations with potential for targeted therapy. Mouse models of cancer serve as excellent tools to examine the functionality of gene mutations and their contribution to the malignant process. However, it remains unclear whether the genetic events that occur during transformation are similar in mice and humans. To address that, we chose several transgenic mouse models of hematopoietic malignancies and identified acquired mutations in these mice by means of targeted re‐sequencing of known cancer‐associated genes as well as whole exome sequencing. We found that mutations that are typically found in acute myeloid leukemia or T cell acute lymphoblastic leukemia patients are also common in mouse models of the respective disease. Moreover, we found that the most frequent mutations found in a mouse model of lymphoma occur in a set of epigenetic modifier genes, implicating this pathway in the generation of lymphoma. These results demonstrate that genetically engineered mouse models (GEMM) mimic the genetic evolution of human cancer and serve as excellent platforms for target discovery and validation.

Abstract 3467: A DAXX-KIFC3 fusion potentiates alternative lengthening of telomeres in osteosarcoma

Proliferating cells must enact a program of telomere lengthening to counteract the chromosome end replication problem. In most types of cancer cells, telomeres are maintained through the action of the ribonucleoprotein telomerase, but some cancer cells, particularly those of mesenchymal origin, utilize an alternative method of telomere repair and lengthening termed the alternative lengthening of telomeres (ALT) pathway. Since telomere maintenance is essential for tumor cell immortality, better understanding of the ALT mechanism could potentially reveal drug targets that could be used to develop novel therapies for tumors that use ALT. It has been previously observed that ALT tumors frequently carry mutations in ATRX, which partners with the protein DAXX in a chromatin remodeling complex, but how these mutations facilitate the ALT pathway is not well understood. Work in our lab identified an ALT-positive osteosarcoma cell line, identified here as OS1, in which DAXX has undergone a fusion event with the non-canonical kinesin KIFC3. We find that knockdown of the DAXX-KIFC3 fusion neither impairs ALT nor cell proliferation, suggesting that the fusion represents a loss of function. Furthermore, inducible restoration of wild-type DAXX, reversibly abrogates ALT function in this cell line. One of the hallmarks of ALT is localization of telomeres and DNA recombination machinery to nuclear PML bodies, resulting in formation of ALT- associated PML Bodies, or APBs. Thus it may be considered that changes in PML body composition represent a key aspect of the ALT mechanism. We observe that in OS1 both DAXX and ATRX fail to localize to PML bodies. This finding is consistent with the fact that the DAXX-KIFC3 fusion results in loss of a C-terminal SUMO interaction motif that normally mediates PML body interaction. Leveraging our inducible system, using biochemical and imaging approaches, we are working to define the role of DAXX in maintaining PML body composition. Citation Format: Sarah F. Clatterbuck Soper, Soyeon A. Showman, Kathryn E. Driest, Joshua J. Waterfall, Robert L. Walker, Marbin A. Pineda, Yuelin J. Zhu, Yonghong Wang, Corbin D. Ester, Sven Bilke, Paul S. Meltzer. A DAXX-KIFC3 fusion potentiates alternative lengthening of telomeres in osteosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3467. doi:10.1158/1538-7445.AM2017-3467

Abstract 4925: Microbiome-TP53 gene interaction in human lung cancer

Lung cancer is the leading cancer diagnosis worldwide and the number one cause of cancer deaths. Exposure to cigarette smoke, the primary risk factor in lung cancer, reduces epithelial barrier function and increases susceptibility to infections. Herein, we hypothesized that somatic mutations together with cigarette smoke create a dysbiotic microbiota that is associated with lung carcinogenesis. Using lung tissue from controls (n=33) or cancer cases (n=143), we conducted 16S rRNA gene sequencing (MiSeq), with RNA-seq data from lung cancer cases in The Cancer Genome Atlas (n=1112) serving as the validation cohort. We demonstrate a lower alpha diversity in normal lung as compared to non-tumor adjacent or tumor tissue, indicating a shift in the overall microbial community in lung cancer patients as compared to those without cancer. Lung cancer cases were classified by the relative abundance of two taxa, Variovorax and Streptococcus, with an increase in Variovorax abundance in tumors as compared to non-tumor adjacent paired lung tissue (FDR corrected P=0.013). The species of Variovorax was identified histologically, and also by two additional 16S rRNA strategies (Resphera Insight analysis and PacBio sequencing). A group of taxa were associated with squamous cell carcinoma (SCC), of which Acidovorax were enriched in smokers (P =0.0013). Further, these taxa, including Acidovorax, exhibited higher abundance among the subset of SCC cases with TP53 mutations, an association not seen in adenocarcinomas (AD). Therefore, we observed a microbiome-gene and a microbiome-exposure interaction in SCC lung cancer tissue. Together, these results open the door for future biomarker research that could be used to improve screening and direct mechanistic studies of lung cancer therapy. Citation Format: Leigh Greathouse, James White, Valery Bliskovsky, Ashley Vargas, Eric Polley, Elise Bowman, Mohammed Khan, Ana Robles, Brid Ryan, Amiran Dzutsev, Giorgio Trinchieri, Marbin Pineda, Sven Bilke, Paul Meltzer, Marina Walther-Antonio, Garth Ehrlich, Joshua Mell, Joshua Earl, Sergey Balashov, Archana Bhat, Alex Valm, Clayton Deming, Sean Conlan, Julia Oh, Julie Segre, Curtis Harris. Microbiome-TP53 gene interaction in human lung cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4925. doi:10.1158/1538-7445.AM2017-4925

Abstract LB-291: Environment or accident? A more stringent bound on environmental contributions to cancerogenesis

Finding the right balance between cancer prevention and early detection is essential for an efficient anti-cancer strategy. Yet, the underlying biological question “how much cancer risk results from chance, how much from environmental factors’’ continues to be controversial. Recently, the “bad luck’’ hypothesis was strengthened based on observing a tight correlation between a tissues cancer risk and its number of stem cell divisions [1]. This interpretation was later challenged by pointing out that certain even drastic changes of environmental risk factors would not affect theat correlation [2]. Based on this argument they derived an estimate of the upper bound of environmental contributions to canceriogenesis. Here we argue that the argument made in [2] did not fully appreciate that typically several mutations are requited before cancer initiation occurs [3]. We show that by incorporating this aspect into the analysis the correlation between the number of stem cell divisions and cancer risk would respond to changes of environmental factors. We re-analyze the data in [1] to derive a slightly more stringent upper bound of the environmental risk contributions for a number of cancers than the ones obtained in [2]. [1] C. Tomasetti, B. Vogelstein, Science 347(6217), 78-81 (2015) [2] S. Wu, S.Powers et al., Nature 529, 43-47 (2015) [3] C. Nordling, British Journal of Cancer 7(1), 68-72 (1953) Citation Format: Sven Bilke, Paul S. Meltzer. Environment or accident? A more stringent bound on environmental contributions to cancerogenesis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-291.

Abstract LB-192: A single catastrophic genomic event is required for the development of osteosarcoma

Over sixty years ago, the groundbreaking works by Nordling 1 , Armitage and Doll 2 found a deep connection between the age dependent incidence rates of cancer with elementary mutational processes in cancer. However, their method to estimate the number of driver mutations from the characteristic increase of adult cancer risk with patient age implicitly assumes tissue homeostasis. It can therefore not be used for developmental cancers emerging from growing tissues. Osteosarcoma (OS), a bone cancer most frequently occurring during adolescence, is one prominent example. Here we combine a model of bone growth and maintenance with somatic mutation theory. Based on age dependent stature 3 ` and morphometric data, we find that our model accurately reproduces epidemiological OS incidence rates 4 `. We conclude that the OS risk can be explained by the cell cycle rate in the osteoblast lineage. Other risk factors, such as endocrine and paracrine signaling have only a weak, if any, impact. The strict association with the cell cycle rate, as opposed to wall clock time, suggests that environmental factors play no dominant role either. In other words, with few exceptions such as genetic syndromes or unusually strong environmental factors such as radiation in cancer treatment, the OS risk is dominated by pure chance. We find that the OS incidence rate is strictly proportional to the age dependent cell cycle rate. This result implies that there is only a single rate limiting factor in the etiology of the disease. This result is a remarkable as it suggests that the disease is not necessarily initiated within a stem cell because the Hayflick limit set by telomere shortening can not suppress single-hit-malignancies. Our results are not only important for a better understanding of the disease and the design of future sequencing efforts. But they also inform development of treatment strategies to reduce the mortality caused by this still deadly disease. 1. Nordling, C. O. C. A New Theory on the Cancer-inducing Mechanism. Br. J. Cancer 7, 68¨C72 (1953). 2. Armitage, P. & Doll, R. The age distribution of cancer and a multi-stage theory of carcinogenesis. Br. J. Cancer VIII, 1¨C12 (1954). 3. CDC Height Data. at 4. Cancer incidence in five continents, vol. I to VIII. CancerBase No. 7 . (Lyon: IARC). at Citation Format: Sven Bilke, Paul Meltzer. A single catastrophic genomic event is required for the development of osteosarcoma. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-192. doi:10.1158/1538-7445.AM2015-LB-192

Abstract 2104: Evidence for E2F/EWS-FLI1 oncoprotein synergism using systems biology

Cell cycle deregulation and enhanced proliferation of cells is one of the hallmarks of oncogenesis. In Ewing Sarcoma, a highly aggressive pediatric cancer, the chimeric transcription factor EWS-FLI1 deregulates cell cycle by targeting several cell cycle regulators including the E2F family of transcription factors. ChIP-seq studies showed a significant overlap of EWS-FLI1 and E2F3/4 binding in Ewing sarcoma cells. We show that EWS-FLI1 is able to directly activate E2F3, followed by the combinatorial binding of EWS-FLI1 and E2F3 on their target genes synergistically activating their transcription. Furthermore we propose a model where EWS-FLI1 directly exchanges repressive E2F4/p130 by E2F3/pRB thereby driving cells into enhanced cell proliferation. However, so far we were unable to experimentally demonstrate a physical interaction of EWS-FLI1 with any E2F or pocket protein. As an alternative approach to the study of functional synergy between EWS-FLI1 and E2F3, we used time resolved RNA and protein expression data as a basis for mathematical modeling of EWS-FLI1 dependent E2F target gene regulation. By Bayesian model selection, we were able to postulate the formation of a complex between EWS-FLI1 and E2F3 as the most likely explanation for the observed ETS/E2F synergy. This study therefore provides an example of how computational systems approaches can complement experimental data in the discovery of disease mechanisms. This study was supported in part by the Austrian Science Fund (FWF), [grant 22328-B09], and by the 7th framework program of the European Commission, [grant 259348] (‘ASSET’). Citation Format: Raphaela Schwentner, Theodore Papamarkou, Maximilian Kauer, Vassilios Stathopoulos, Fan Yang, Sven Bilke, Paul S. Meltzer, Mark Girolami, Heinrich Kovar. Evidence for E2F/EWS-FLI1 oncoprotein synergism using systems biology. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 2104. doi:10.1158/1538-7445.AM2015-2104