Cancer is a disease that results from alteration of the cellular genome. The sources of these changes are multifarious, and in many cases unknown. This thesis focuses on the polynucleotide cytosine deaminase, APOBEC3B, as a newly discovered source of mutation in multiple human cancers. As a deaminase, APOBEC3B converts cytosines to uracils in single-stranded DNA. These uracil lesions are mutagenic as failure to properly repair them can result in a wide variety of mutation types.
The initial discovery of this mutational phenomenon was described mechanistically using a variety of biochemical, genetic, and cellular assays in breast cancer cell lines. Follow-up work using publicly available next generation sequencing and clinical data indicated that this effect is operating in a large proportion of breast cancers. Expanded bioinformatic analysis that assessed APOBEC3B's potential impact was expanded to include 18 other human cancer types in addition to breast. This work shows that APOBEC3B is likely a significant contributor to the genetic heterogeneity in breast, head & neck, bladder, cervical, and lung (adeno- and squamous cell) carcinomas as evidenced by differential levels of expression in cancer tissues, increased mutation load, mutation clusters (kataegis), and an APOBEC3B mutation signature in tumors expressing high levels of this enzyme. Taken together, this thesis is a body of work describing a previously unappreciated source of genetic heterogeneity in several human cancers.