Abstract

The Germinal Center (GC) response, a transient cell state that enables B cell affinity maturation in response to an antigen, is a crucial component of the adaptive immune system. GC B cells require dynamic regulation by the histone methyltransferase EZH2, which catalyzes H3K27me3, a histone mark associated with transcriptional repression. However, aberrant EZH2 activity can promote GC transformation to lymphoma. GC B cell-derived lymphomas frequently exhibit gain-of-function alterations in EZH2, including a somatic hotspot mutation (EZH2Y641F/+) or overexpression of wild-type protein (EZH2OE). The distinct role that each event plays in GC B cell oncogenesis is poorly understood. In this thesis, I aimed to determine whether EZH2Y641F/+ and EZH2OE are functionally equivalent or distinct gain-of-function events and to uncover the stage(s) in the B lineage when these events promote GC transformation. Using conditional mouse models that activate EZH2Y641F/+ or EZH2OE in the early B lineage or specifically within GC B cells, I demonstrate that EZH2OE does not impact GC B cell responses or disease onset, while the impact of EZH2Y641F/+ depends on its timing in the B lineage. More specifically, early activation of the mutation reduced GC fitness, while GC-restricted activation drove GC expansion and accelerated death in a lymphoma-relevant transplant model. Transcriptomic profiling and H3K27me3 profiling enabled a deeper investigation into these gain-of-function events. Compared to EZH2WT samples, EZH2OE minimally impacted gene expression or H3K27me3 profiles, while EZH2Y641F/+ drove widespread differential gene expression and H3K27me3 redistribution in both timing contexts. Functional enrichment analysis of regions that gain H3K27me3 in GC-restricted EZH2Y641F/+ cells suggests dysregulated signal transduction pathways as a potential connection between H3K27me3 redistribution and early lymphoma phenotypes. Collectively, my thesis supports a model in which EZH2Y641F/+ mutations are distinct from EZH2 overexpression and promote lymphoma depending on its timing in the B lineage. This work contributes a deeper understanding of the molecular mechanisms and context-dependency behind EZH2 alterations that frequently appear in GC B cell-derived lymphomas, which may inform future efforts to identify allele- or context-specific therapeutic vulnerabilities in patients.

Committee Chair

George Souroullas

Committee Members

Jacqueline Payton; Jeffrey Bednarski II; John Edwards; Laura Schuettpelz

Degree

Doctor of Philosophy (PhD)

Author's Department

Biology & Biomedical Sciences (Molecular Genetics & Genomics)

Author's School

Graduate School of Arts and Sciences

Document Type

Dissertation

Date of Award

8-6-2026

Language

English (en)

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Available for download on Friday, February 05, 2027

Included in

Genetics Commons

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