Understanding Genetic Factors that Modulate Hedgehog-Driven Skin Cancer

The Hedgehog (Hh) signaling pathway is essential for instructing tissues to properly develop during vertebrate embryogenesis. Perturbations that cause constitutive activation of Hh signaling, primarily through loss-of-function mutations in PTCH1, drive the formation of basal cell carcinoma (BCC). While mutations in the Hh pathway play a necessary role for this disease, BCCs also possess high tumor mutational burdens, suggesting that additional mutations may collaborate with Hh signaling to promote oncogenesis. My thesis therefore examines whether other genetic factors can collaborate with Hh signaling to promote the formation of macroscopic BCC-like tumors in mice. Using BCC mouse models, I found that loss of Ptch1 is not sufficient to promote macroscopic tumor formation. Because NOTCH1, TRP53, and MYCN are frequently mutated in human BCCs, I investigated if these genetic factors can collaborate with Hh signaling to drive tumor progression. Indeed, ablating either Notch1 or Trp53 in our BCC mouse model system caused rare “successful” macroscopic tumors to form. Upon ablating Notch signaling, I found that microscopic BCC-like tumors persist and resist tumor regression. Losing Trp53 likely promotes tumor mutability and helps enable genomic amplification of Gli1 or Gli2. Finally, I found that overexpression of MYCN promotes proliferation and tumor progression. Collectively, these data suggest acquired secondary mutations that upregulate Gli1, Gli2 and/or Mycn levels enable tumors to expand indefinitely. Losing Notch1 and p53 also facilitates tumor progression. I also determined whether immune privilege extends to BCC and addressed if inhibiting CD200 suppresses tumor initiation. CD200 is thought to be a potential immunosuppressive molecule in the hair follicle and is expressed in BCC. I found that suppression of CD200 did not affect nascent tumor size during tumor initiation. Furthermore, suppression of CD200 did not affect immune cell numbers in tumor-containing skin. Finally, gene amplification is a common genetic alteration in cancer, and some amplified genes may cause cells to divide or become resistant to therapy. However, there are limited in situ hybridization approaches to detect and visualize copy number changes. I therefore optimized the new DNAscope® duplex assay to examine GLI1, GLI2, and MYCN copy number in human BCC samples. In summary, my findings expand our understanding of key aspects of BCC biology and define genetic factors that drive BCC progression. Furthermore, the tools and assays I have optimized can be leveraged in futures studies in skin cancer biology.PHDCell and Developmental BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/174527/1/ketrieu_1.pd