Title: The role of NSD1 in oral squamous homeostasis and HNSCC tumorigenesis
Description: Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide, with over 50,000 new cases in the United States annually. Major risk factors for HNSCC include chronic smoking and/or alcohol consumption, and more recently infection with human papillomaviruses (HPVs). HNSCC comprises a diverse collection of tumors, of which the oral cavity is the most affected site. Most HNSCC patients will die within the first 30 months of disease, an abysmal statistic largely reflecting a lack of effective treatment strategies. This challenge highlights the current unmet need to identify 1) distinct molecular subgroups of HNSCC and 2) prognostic biomarkers that can inform subgroup-specific treatment plans. The Lu Lab has identified a previously unappreciated HNSCC subgroup defined by alteration in NSD1, a histone methyltransferase enzyme mutated in up to 15% of HPV- HNSCC. NSD1 has specific di-methylase activity targeting histone H3 lysine 36 (H3K36). The formation of methylated H3K36 is associated with open chromatin and transcriptional activation.

There are several distinct features of NSD1-mutant HPV- HNSCC tumors, including a) increased patient smoking history and mutational burden; b) a significantly better prognosis; and c) reduced immune cell infiltration in the tumor. These correlative findings may suggest a tumor-suppressive role of NSD1, yet the impact of NSD1-loss in HNSCC pathogenesis and the underlying mechanisms remain unclear. The scope of my thesis work aims to address this gap in knowledge through investigating the consequences of impaired NSD1 in normal epithelial tissue and in the setting of HNSCC tumors. In Chapter 2, we employed both a syngeneic tumor implantation model and a physiologic mouse model of HNSCC carcinogenesis to establish and characterize Nsd1-KO tumors. We found that mice harboring Nsd1-KO tumors are comparatively immune- ‘cold,’ a phenotype that persisted in human HNSCC patient samples. Our in vitro data suggests that depletion of NSD1 epigenetically silences the interferon response, an effect that was rescued through inhibition of epigenetic machinery that limits NSD1-deposited H3K36me2.

These studies provide novel insight into the molecular underpinnings of observed immune-‘coldness’ in NSD1-mutated HNSCC. They also prompted us to probe the impact of NSD1-loss at early stages of tumor development. In Chapter 3, I describe a lingual-derived organoid system to assess unbiased transcriptional changes that occur upon Nsd1-loss during homeostasis, premalignancy, and overt tumor formation. Interestingly, we found that Nsd1-KO organoids harvested from stages preceding HNSCC featured downregulation in gene expression related to epithelial barrier formation and wound healing. Furthermore, organoids derived from Nsd1-KO tumors showed reduced expression of epithelial-to-mesenchymal transition (EMT) signature genes, potentially signifying a role of NSD1 in modulating cell adhesion and mobility. Several additional studies are needed to establish the functional basis of how NSD1 participates in these processes. Taken together, this body of work expands upon our current understanding of NSD1-loss in HNSCC development and presents a conceptual scaffold framing NSD1 as a multi-faceted player in the homeostatic maintenance and neoplastic events of the oral epithelium.