Date of Award

Winter 12-15-2017

Author's School

Graduate School of Arts and Sciences

Author's Department

Biology & Biomedical Sciences (Computational & Systems Biology)

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

The interaction between transcription factors (TFs) and DNA plays an important role in gene expression regulation. In the past, experiments on protein–DNA interactions could only identify a handful of sequences that a TF binds with high affinities. In recent years, several high-throughput experimental techniques, such as high-throughput SELEX (HT-SELEX), protein-binding microarrays (PBMs) and ChIP-seq, have been developed to estimate the relative binding affinities of large numbers of DNA sequences both in vitro and in vivo. The large volume of data generated by these techniques proved to be a challenge and prompted the development of novel motif discovery algorithms. These algorithms are based on a range of TF binding models, including the widely used probabilistic model that represents binding motifs as position frequency matrices (PFMs). However, the probabilistic model has limitations and the PFMs extracted from some of the high-throughput experiments are known to be suboptimal. In this dissertation, we attempt to address these important questions and develop a generalized biophysical model and an expectation maximization (EM) algorithm for estimating position weight matrices (PWMs) and other parameters using HT-SELEX data. First, we discuss the inherent limitations of the popular probabilistic model and compare it with a biophysical model that assumes the nucleotides in a binding site contribute independently to its binding energy instead of binding probability. We use simulations to demonstrate that the biophysical model almost always provides better fits to the data and conclude that it should take the place of the probabilistic model in charactering TF binding specificity. Then we describe a generalized biophysical model, which removes the assumption of known binding locations and is particularly suitable for modeling protein–DNA interactions in HT-SELEX experiments, and BEESEM, an EM algorithm capable of estimating the binding model and binding locations simultaneously. BEESEM can also calculate the confidence intervals of the estimated parameters in the binding model, a rare but useful feature among motif discovery algorithms. By comparing BEESEM with 5 other algorithms on HT-SELEX, PBM and ChIP-seq data, we demonstrate that BEESEM provides significantly better fits to in vitro data and is similar to the other methods (with one exception) on in vivo data under the criterion of the area under the receiver operating characteristic curve (AUROC). We also discuss the limitations of the AUROC criterion, which is purely rank-based and thus misses quantitative binding information. Finally, we investigate whether adding DNA shape features can significantly improve the accuracy of binding models. We evaluate the ability of the gradient boosting classifiers generated by DNAshapedTFBS, an algorithm that takes account of DNA shape features, to differentiate ChIP-seq peaks from random background sequences, and compare them with various matrix-based binding models. The results indicate that, compared with optimized PWMs, adding DNA shape features does not produce significantly better binding models and may increase the risk of overfitting on training datasets.

Language

English (en)

Chair and Committee

Gary D. Stormo

Committee Members

Jeremy D. Buhler, Barak A. Cohen, S. Joshua Swamidass, Ting Wang,

Comments

Permanent URL: https://doi.org/10.7936/K7FX78W6

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