Thoc1 encodes an RNA binding protein (Thoc1) which interacts with the tumor suppressor retinoblastoma (Rb1). Thoc1 levels have been found to be elevated in multiple cancers relative to normal tissue. Additionally cancer cells but not most normal cells undergo apoptosis following Thoc1 deletion, suggesting Thoc1 may be a good therapeutic target. In normal tissue, Thoc1 is needed for embryonic inner cell mass viability and Thoc1 hypomorphic mice are small and sterile. Based on these data we hypothesize highly proliferative cells, possibly those with an extended replicative potential, such as stem cells, require Thoc1. To test this, we examined the effect of Thoc1 deletion in the intestine since it contains well-defined stem cells Thoc1 was deleted in the adult mouse using the Tamoxifen/CreER system with Rosa26 driving CreER expression. Mice were treated with 2 mg/day Tamoxifen for 1, 2, 3, 4, 5, or 6 days. Thoc1 deletion completely disrupts small intestine (SI) structure. Specifically, Thoc1 deletion impairs SI crypt cell function and viability, which leads to a complete loss of SI epithelial cell turnover. The SI crypts contain rapidly proliferating intestinal stem (ISC) and progenitor cells that continuously feed and replace the differentiated SI villi epithelium every 3-5 days. Within the proliferative crypts, multiple ISC populations exist. These include the active, homeostatic Lgr5+ ISCs and the reserve, more quiescent mTert+ and Bmi1+ ISCs. Lgr5+ ISCs are the most actively proliferating ISC population, whereas mTert+ and Bmi1+ ISCs are believed to become proliferative upon tissue insult. QRT-PCR data of ISC (Lgr5, mTert, Bmi1) marker transcripts from crypt purified epithelial cells suggests Thoc1 loss initially induces ISC populations to proliferate and expand (transcripts increase), likely a direct response to the initial Thoc1 deletion. However, once active it appears proliferating ISCs become susceptible to Thoc1 deletion, as ISC marker transcripts ultimately approach zero on day 6 in test mice. In addition, ISC fate mapping studies show recombined ISCs in test mice are less likely to give rise to long lived stable progeny than recombined ISCs in control mice. Despite a similar structure to the SI, Thoc1 deletion does not appear to affect the colon. The colon too contains ISCs that give rise to all cells in the tissue; however, colon ISCs are more quiescent versus the SI ISC, which suggests actively proliferating stem/progenitor cell types may specifically require Thoc1. The results of this study suggest actively proliferating stem cell types require Thoc1, most likely to support the coordinated gene expression necessary for rapid proliferation and differentiation. This finding is significant as it sheds light on Thoc1 function and may help explain why Thoc1 levels are increased in various cancers, as proliferating stem cells and cancer cells share similar traits.

Citation Format: Laura B. Pitzonka, Sumana Ullas, David Goodrich. The RNP biogenesis factor Thoc1 is required for proliferative intestinal stem cell viability. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5193. doi:10.1158/1538-7445.AM2013-5193