Regulatory Mechanisms of Endogenous APOBEC3B Expression in HPV(+) and (–) Oral Premalignancies and Head and Neck Cancer

Abstract

<h3>Introduction</h3> HPV(+) oral and oropharyngeal squamous cell carcinoma (OOPSCC) shows strong association with high-risk HPV16 and 18. The DNA cytosine deaminase APOBEC3B constitutes a major endogenous source of muta- tions in a wide variety of cancers. APOBEC3B causes C-to-T/-G base substitutions in 5'-TCA/T trinucleotide motifs and is overexpressed in head and neck cancer (HNC). High-risk HPV infection and dysregulation of the RB/E2F axis regu- late <i>APOBEC3B in vitro</i>. Here, we investigate changes of APOBEC3B levels in HPV(+) and (–) oral epithelial dysplasia (OED) and OOPSCC, and probe whether APOBEC3B upregulation correlates with cellular proliferation. <h3>Materials and Methods</h3> APOBEC3B and Ki67 expression was assessed by immunohistochemistry in HPV(+) OED (n=10) and OOPSCC (n=28), and HPV(–) low-grade (n=42) and high-grade OED (n=38), and OSCC (n=46). Epithelial hyperplasia (OEH, n=14) and normal adjacent epithelium (n=10) served as controls. APOBEC3B H-scores and per- centage of Ki67(+) cells were calculated using the Aperio ScanScope XT platform. Public NGS datasets available through TCGA were mined to identify molecular mechanisms associated with APOBEC3B upregulation in HNC. <h3>Results</h3> High-risk HPV significantly upregulated APOBEC3B in OED (Kruskal-Wallis test, p<0.05) and OOPSCC (p<0.01) compared to OEH. HPV(+) OEDs and tumors showed strong and diffuse, nuclear APOBEC3B immunore- activity which mirrored p16 staining and coincided with disrupted p53 expression. Progressive APOBEC3B up- regulation was observed in HPV(–) lesions; high-grade premalignancies and OSCCs showed significantly elevated APOBEC3B H-scores compared to low-grade OED (p<0.01) and controls (p<0.01). High mRNA and protein APOBEC3B levels strongly associated with advanced histologic grading in HPV(–) OSCC (p<0.01). A positive linear correlation was evident between APOBEC3B and Ki67 expression (Pearson's r=0.5), while RNAseq analysis suggests cell cycle regulation of APOBEC3B in HPV(–) HNC. <h3>Conclusions</h3> High-risk HPV drives upregulation of APOBEC3B in viral-related OED and HNC. In non-viral lesions, APOBEC3B levels also increase in a mechanism associated in part with cellular proliferation.