Abstract 3665: WAC: A candidate tumor suppressor gene in colorectal cancer

Our lab recently performed a DNA transposon forward genetic screen in mice that was designed to identify low-frequency mutations that contribute to colorectal cancer (CRC) initiation and progression. Results from this screen identified the WW domain-containing adaptor with coiled-coil (WAC) gene as a top DNA transposon insertion site. WAC has previously been implicated in several cellular processes including amino acid starvation-induced autophagy, golgi biogenesis, and transcription associated histone modification but has never before been linked to tumorigenesis. Transposon mutagenesis screens performed by others (Takeda et al. Nature Genetics 2015) have also identified Wac as a common insertion site, a result that further implicates WAC as a candidate CRC driver gene. Analyses of transposon insertion patterns within Wac predict loss of gene function and a role as a tumor suppressor. Soft agar colony formation assays reveal that shRNA mediated silencing of Wac cooperates with Apc mutations in mouse colorectal cells to promote cellular transformation. Additional colony formation assays using immortalized human colonic epithelial cells and the adenoma derived AAC1 cell line also shows that silencing WAC is protumorigenic. Using a zebrafish model we demonstrated that overexpression of wild type but not cancer-associated mutant forms of WAC induce expression of the cell cycle inhibitor p21, which suggests that loss of WAC may lead to uncontrolled cellular proliferation. Finally, using publicly available mutation data we determined that WAC is somatically mutated in both breast and lung cancers; a finding that indicates WAC may serve a critical tumor suppressive role in several tissues. Currently we are developing a conditional knockout mouse to further investigate the role of WAC in CRC tumor formation.

Citation Format: Christopher R. Clark, Caitlin Conboy, Makayla Maile, Callie Janik, Julia Hatler, Robert Cormier, David Largaespada, Timothy K. Starr. WAC: A candidate tumor suppressor gene in colorectal cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3665.

Transposon Mutagenesis Screen Identifies Potential Lung Cancer Drivers and CUL3 as a Tumor Suppressor

Non-small cell lung cancers (NSCLC) harbor thousands of passenger events that hide genetic drivers. Even highly recurrent events in NSCLC, such as mutations in PTEN, EGFR, KRAS, and ALK, are detected, at most, in only 30% of patients. Thus, many unidentified low-penetrant events are causing a significant portion of lung cancers. To detect low-penetrance drivers of NSCLC, a forward genetic screen was performed in mice using the Sleeping Beauty (SB) DNA transposon as a random mutagen to generate lung tumors in a Pten-deficient background. SB mutations coupled with Pten deficiency were sufficient to produce lung tumors in 29% of mice. Pten deficiency alone, without SB mutations, resulted in lung tumors in 11% of mice, whereas the rate in control mice was approximately 3%. In addition, thyroid cancer and other carcinomas, as well as the presence of bronchiolar and alveolar epithelialization, in mice deficient for Pten were also identified. Analysis of common transposon insertion sites identified 76 candidate cancer driver genes. These genes are frequently dysregulated in human lung cancers and implicate several signaling pathways. Cullin3 (Cul3), a member of a ubiquitin ligase complex that plays a role in the oxidative stress response pathway, was identified in the screen and evidence demonstrates that Cul3 functions as a tumor suppressor.

IMPLICATIONS

This study identifies many novel candidate genetic drivers of lung cancer and demonstrates that CUL3 acts as a tumor suppressor by regulating oxidative stress.

The LMP1 gene isolated from Russian nasopharyngeal carcinoma has no 30-bp deletion

The Epstein-Barr virus (EBV) is tightly linked to the induction of undifferentiated nasopharyngeal carcinoma (NPC), a tumour endemic in certain areas of southeast Asia. The LMP1 gene encoded by EBV is a classical oncogene due to its ability to transform rodent fibroblasts. LMP1 is absolutely essential for transformation of B cells by the virus and is one of the few EBV genes found to be expressed in NPC. It was originally shown that the LMP1 gene from NPC harbours a deletion of 30 bp in the 3' part of the gene. However, this deletion is also present in the virus spread in healthy people of the areas endemic for NPC and also in other EBV-positive tumours as well as in healthy carriers. We isolated and sequenced the LMP1 gene obtained from tissue of 7 Russian patients with NPC and 1 German patient with an NPC-like tumour of the parotid gland (PG) and compared them with the LMP1 gene isolated from peripheral blood lymphocytes (PBLs) of 6 Russian and 4 German healthy EBV-positive carriers. Neither the Russian NPC cases nor the German NPC-like tumour harboured an LMP1 gene with the 30-bp deletion, while 1 Russian and 2 German carriers contained the LMP1 gene with the 30-bp deletion. In addition, the LMP1 gene isolated from PBLs of the German patient was virtually identical to the gene isolated from the primary tumour. Functional analysis showed no correlation between the presence or absence of the 30-bp deletion and the level of induction of the transcription factors NFkappaB and jun/AP-1 caused by LMP1. These data indicate that the 30-bp deletion is not a factor predisposing for NPC. Comparison of the DNA sequences revealed that the LMP1 genes present in the NPCs most likely represent the "strain" persisting in the general population.