Cancers may arise due to the """"""""normal"""""""" expression of signaling proteins that sensitize specific cell types to specific oncogenic mutations. However, the means by which cell type-specific signaling pathways predispose to particular cancers is poorly understood. Retinoblastoma is a suitable model with which to address this issue, as the retinoblastoma cell of origin is exceptionally predisposed to initiate tumorigenesis in the event that the RB1 gene is inactive. Accordingly, this project aims to identify and characterize specific signals that sensitize the retinoblastoma cell of origin to the loss of Rb function. Preliminary Studies identified cone photoreceptor precursors (CPs) as a candidate retinoblastoma cell of origin and showed that human CPs are unique among retinal cells in that they prominently express the MDM2 oncoprotein and a thyroid hormone receptor isoform, TR?2, that is also prominent in the precursors to Rb- deficient mouse pituitary tumors. They also revealed that MDM2 and TR?2 are essential for retinoblastoma cell proliferation and survival, that MDM2 is needed to suppress the oncogenic stress response, and that TR?2 is needed to sustain the expression of yet another protein that is essential to the development of Rb-deficient tumors, the F-box protein Skp2. To determine whether these CP features underlie retinoblastoma tumorigenesis, Aim 1 will define the roles of the cone circuitry in the response of primary human CPs to Rb depletion.
Aim 2 will extend these in vitro studies by examining whether ectopic MDM2 expression in Rb- deficient mouse CPs - either from a cone-targeted transgene or from an intact human MDM2 locus - deregulates CP proliferation and elicits CP-derived retinoblastomas in vivo.
Aim 3 seeks to define the role of TR?2 in retinoblastoma cell proliferation and survival, and will specifically evaluate whether TR?2 is needed to promote Skp2 expression and acts by antagonizing the related TR?1 isoform.
Aim 4 will evaluate whether TR?2 contributes to the development of Rb-deficient mouse pituitary tumors in vivo, similar to its effects on human retinoblastoma cells in vitro, and will define the cellular effects of TR?2 loss in the abnormally proliferating Rb-deficient cells that give rise to these tumors. Together, these studies will evaluate whether """"""""normal"""""""" CP circuitry sensitizes cone precursors to the oncogenic effects of RB1 inactivation. The studies are likely to have an impact extending beyond retinoblastoma, by defining cell type-specific features that commonly collaborate with Rb loss to enable the development of Rb-deficient cancers.

Public Health Relevance

These studies will identify signaling features that sensitize normal cells to the effects of specific oncogenic mutations. To the extent that this has not been widely achieved in other cancers, the research will help to establish an important and novel paradigm for investigating human tumorigenesis. The cell type-specific features that are identified through these studies could provide novel targets for cancer therapy.

National Institute of Health (NIH)
National Cancer Institute (NCI)
Research Project (R01)
Project #
Application #
Study Section
Molecular Oncogenesis Study Section (MONC)
Program Officer
Mietz, Judy
Project Start
Project End
Budget Start
Budget End
Support Year
Fiscal Year
Total Cost
Indirect Cost
Children's Hospital of Los Angeles
Los Angeles
United States
Zip Code
Singh, Hardeep P; Wang, Sijia; Stachelek, Kevin et al. (2018) Developmental stage-specific proliferation and retinoblastoma genesis in RB-deficient human but not mouse cone precursors. Proc Natl Acad Sci U S A 115:E9391-E9400
Xu, Xiaoliang L; Li, Zhengke; Liu, Aihong et al. (2017) SKP2 Activation by Thyroid Hormone Receptor ?2 Bypasses Rb-Dependent Proliferation in Rb-Deficient Cells. Cancer Res 77:6838-6850
Qi, D-L; Cobrinik, D (2017) MDM2 but not MDM4 promotes retinoblastoma cell proliferation through p53-independent regulation of MYCN translation. Oncogene 36:1760-1769
Dimaras, Helen; Corson, Timothy W; Cobrinik, David et al. (2015) Retinoblastoma. Nat Rev Dis Primers 1:15021
Xu, Xiaoliang L; Singh, Hardeep P; Wang, Lu et al. (2014) Rb suppresses human cone-precursor-derived retinoblastoma tumours. Nature 514:385-8