Abstract BACKGROUND Retinoblastoma is a rare childhood cancer of the retina that shows the biallelic loss of RB1 in nearly 95% of cases. It was reported that SHH pathway proteins, such as SHH, GLI1, and GLI2, are aberrantly high in human retinoblastoma, and high GLI2 expression is associated with aggressive clinicopathological features. Thus, we assessed whether activation of SHH pathway by overexpression of Gli2 or SMO induces the initiation of retinoblastoma. METHODS We generated a Cre-activated, Doxycycline (Dox)-controlled transgenic model to induce SMO or Gli2 expression in developing retina. In this transgenic mouse, expression of Smo or Gli2A is under the control of TRE, which permits temporal and spatial expression of Smo or Gli2A by administering/withdrawing doxycycline (DOX). RESULTS We fed the triple transgenic mice (Chx10-Cre/R26-LSL-rtTA/tetO-Smo or Chx10-Cre/R26-LSL-rtTA/tetO-Gli2A starting from E0 or P5. Our studies demonstrated that control littermates (Cre-) did not show any abnormalities, while mice with triple transgenes developed tumors in the bilateral eyes with 100% morbidity. The animals showed symptoms of leukocoria at P0-P3 or P25-p30 if the oncogenes were initiated at E0 or P5 respectively. The tumor cells were highly proliferative and expressed retina progenitor cell specific marker of Sox2, showing aberrant expression of pRB, CDK4/6, and CCND1. CONCLUSIONS Our results demonstrate that overexpressing Smo or Gli2 in Chx10+ cells induce rapid tumor formation in mouse retina. In future study, we plan to (i) investigate the impact of Shh pathway activation in RPCs on RB tumorigenesis within the context of Rb1 loss; (ii) define the molecular mechanisms underlying tumor initiation and progression; and (iii) evaluate whether targeting SHH pathway prevents or delays tumor initiation and progression using our Gli2-driven mouse model and patient-derived xenografts.

STEM-06. THE ROLE OF SHH SIGNALING PATHWAY IN RETINOBLASTOMA INITIATION AND PROGRESSION