Transposable elements and the regulatory logic of hematopoietic differentiation

Abstract

The temporal regulation of gene expression by transcription factors, chromatin modifiers and cis-regulatory elements is central to establish cellular identity and function. Understanding this regulatory logic is critical for deriving select cell types in vitro for translational applications. The human hematopoietic system has long been a model system and an important source for adoptive cell therapies, yet, our understanding of the regulatory mechanisms that elicit commitment toward distinct hematopoietic lineages is continuously evolving.

In this thesis, I describe several studies on transposable elements (TEs) as natural and engineered sources of regulatory innovation that contribute to, and aid in the investigation of, dynamic cellular processes. Toward this end, I built comprehensive genome-wide enhancer-gene maps spanning the human hematopoietic system and identified that TEs in the human genome contribute to the transcriptional networks regulating lymphoid cells. De-repression of TEs in hematopoietic stem cells, enacted via modulation of TE chromatin silencing machinery, facilitates the development of natural killer (NK) cells during lymphoid differentiation. Specifically, knockout of the H3K9 methyltransferase EHMT1 or transcriptional co-repressor TRIM28 induced NK-fated progenitors that ultimately generated NK cells with diverse effector properties. We further leveraged TEs by repurposing the packaging function of the MLV gag polyprotein to create a non-destructive reporter of the transcriptional states of living cells, enabling the measurement of dynamic transcriptional processes. Through engineering and scientific inquiry, I established the utility of TEs as synthetic biology tools, furthering our understanding of hematopoietic lineage decisions and highlighting that modulation of TEs can be enabling for hematopoietic cell engineering.