The roles of LZAP in vertebrate embryogenesis and head and neck carcinogenesis

Abstract

To improve our understanding of LZAP’s role in normal and cancer cell biology, the work in this thesis explored LZAP activities in development and carcinogenesis. This dissertation focused on understanding the role of LZAP in vertebrate development because no LZAP-/- pups were observed after crossing LZAP+/- mice. In zebrafish, we discovered that LZAP loss results in early embryonic lethality at least partially by inhibiting normal cell cycle progression while potentiating apoptosis. LZAP morphants do not initiate epiboly, the earliest developmental cell movement. Together these findings suggest that LZAP may be a critical regulator of embryonic stem cell function(s) required for their survival. Examination of mice after targeting LZAP led to the unexpected finding that p53 levels are decreased in heterozygous mice embryonic stem cells and tissues. In vitro experiments confirmed that LZAP depletion decreases both wild-type and mutant p53 protein levels by decreasing both p53 protein half-life and p53 mRNA translation. Mechanistically, loss of LZAP is associated with increased expression of nucleolin, a known p53 inhibitor, and depletion of nucleolin partially abrogates the effect of LZAP loss. Consistent with downregulation of p53, LZAP loss is associated with resistance to radiation in cells expressing wild-type p53. Remarkably, decreased mutant p53 levels are observed following LZAP depletion, which sensitize cancer cells to radiation. These data suggest that LZAP may be an ideal target for treating p53 mutant tumors, because temporary inhibition of LZAP activity toward p53 could simultaneously sensitize the tumor to DNA damaging agents (chemotherapy or radiation) while protect normal surrounding tissue. To determine if loss of LZAP is biologically significant, primary HNSCCs were examined revealing that LZAP and p53 levels significantly correlate and that cancers with downregulated LZAP have reduced pressure to inactivate p53 via mutation. This work will impact several areas of cancer research in both the relatively new LZAP field and the persistently important p53 field. In summary, these data suggest that LZAP is critical for early embryogenesis in zebrafish and mice, and that loss of LZAP is a new mechanism of p53 loss and a driver of HNSCC tumorigenesis.