# Longitudinal analysis of retinal cell state transitions in RB1-deficient retinal organoids reveals the nascent cone precursors are the earliest cell-origin of human retinoblastoma

## TL;DR

This study identifies the earliest cell origin of retinoblastoma in human retinal organoids and shows how RB1 deficiency leads to abnormal cell proliferation and tumor formation.

## Contribution

The study demonstrates for the first time that nascent cone precursors are the earliest cell origin of human retinoblastoma.

## Key findings

- RB1 loss induces overproliferation of ATOH7+ neurogenic retinal progenitor cells, disrupting retinal development.
- ATOH7+/RXRγ+ nascent cone precursors survive and drive retinoblastoma tumorigenesis.
- A potential therapeutic target for retinoblastoma was identified and validated using multi-omics data and experiments.

## Abstract

All cancers arise from the malignant transformation of normal cells, yet their cells-of-origin remain challenging to identify due to the inability to directly observe dynamic changes in human tumors. Retinoblastoma (Rb), a malignant intraocular cancer, serves as a well-established model for investigating the molecular and cellular mechanisms underlying tumorigenesis. While the maturing cone precursors (CPs) have been proposed as the cellular origin of human Rb, it is unclear whether other retinal cell types are similarly sensitive to RB1 inactivation. In this study, we developed RB1-deficient human retinal organoids (ROs) models using RB1−/− or RB1+/- human induced pluripotent stem cells (hiPSCs). RB1−/− hiPSCs generated tumor cells that recapitulated key features of human Rb and formed serial orthotopic xenografts. Importantly, RB1 loss induced overproliferation of ATOH7+ neurogenic retinal progenitor cells (nRPCs), which disrupted retinal development by generating ectopic dividing early-born retinal cells (retinal ganglion cells and CPs). Single-cell RNA sequencing analysis confirmed that ATOH7+/RXRγ+ nascent CPs survived and ultimately drove Rb tumorigenesis. In contrast, monoallelic RB1 inactivation resulting in low pRB expression did not induce proliferation of nascent CPs, but only triggered overproliferation of nRPCs, leading to a retinocytoma-like phenotype. Finally, a potential therapeutic target for Rb was identified from multi-omics data and validated through knockdown experiment and a small-molecule inhibitor. Our findings demonstrate, for the first time, that nRPCs are the most sensitive cells to RB1 loss inducing abnormal proliferation of nascent retinal cells, while ATOH7+ nascent CPs represent the earliest cellular origin of human Rb. These insights may facilitate the development of targeted therapies for Rb.