Global measurements of human transcription factor occupancy: Insights into development and genome evolution

Abstract

Transcription factors (TFs) are a class of proteins that interact with the genome, dictating which parts of the genome are utilized by a given cell. Despite the central role TFs play in regulating the genome, technologies available to date have not permitted global measurement of TF occupancy within the cell. Consequently, our understanding of how TFs model cellular development and genome evolution has been limited. To address this, I focused my graduate studies on the development of genomic and proteomic tools that facilitates global measurements of TF occupancy within a cell, including the development of: (i) targeted proteomic assays to quantify TF protein abundances within the nucleus; (ii) a protein-centric method to map TF protein occupancy in functionally distinct chromatin microenvironments; (iii) a genome-wide DNaseI footprinting method that enables construction of global maps of TF occupancy along the genome and core human regulatory networks; and (iv) a method for mapping the in vivo affinity of a TF genome-wide. Using a combination of these techniques, I have identified the sequence and chromatin features that direct TF occupancy within a cell and have characterized the mechanisms underlying TF occupancy dynamics during development and oncogenesis. These global TF occupancy maps have revealed novel insights into how TF occupancy shapes the evolution of both non-coding and coding genomic sequence. Specifically, it appears that TF affinity, not occupancy, plays the dominant role in shaping the evolution of TF binding elements. In addition, coding TF binding elements significantly contribute to protein evolution and codon usage biases in mammals. Together, these results depict the sequence, chromatin, developmental and evolutionary forces that shape TF occupancy within a cell - revealing unforeseen evolutionary constraints on the human genome.