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Retinoblastoma Study Reveals Pathogenesis Gene Downstream of Mutated RB1

NEW YORK – Investigators at the University of Miami Miller School of Medicine, the University of Iowa, and the University of Texas Southwestern Medical Center turned to a multiomics approach to untangle the downstream consequences of RB1 tumor suppressor inactivation in retinoblastoma, a common and dangerous eye cancer that can occur in children.

"Despite considerable improvements in treatment and patient survival over the past century, severe visual impairment and loss of the eye are still common, due in part to treatment resistance and toxicity associated with currently used chemotherapeutic agents," senior and corresponding author William Harbour, a University of Miami Miller School of Medicine researcher, and his colleagues wrote in Science Advances on Friday. "A better understanding of molecular dependencies in Rb could lead to more specific and effective targeted therapies."

Along with exome sequencing on 103 primary retinoblastoma tumor samples, the researchers performed whole-genome sequencing on four retinoblastoma tumors. They also profiled gene expression with the help of RNA sequencing on 20 samples and five single-cell RNA-seq experiments.

Their results suggested that the characteristic RB1 inactivation in retinoblastoma frees the estrogen-related receptor gamma (ESRRG) from its usual regulation by RB1, apparently boosting ESRRG expression, particularly under the oxygen-depleted, hypoxic conditions found in Rb tumor cells.

"We found that the estrogen-related receptor (ESR) gamma (ESRRG) is an essential mediator of hypoxic adaptation and cell survival in Rb that is constitutively activated by RB1 loss and is subsequently affected by recurrent genomic aberrations in Rb," the authors reported. "These findings suggest a selective pressure to increase ESRRG activity during Rb progression and offer a potential target for therapy."

Along with RB1 mutations, which turned up in 94 percent of the samples profiled, the team tracked down recurrent mutations in genes such as BCOR, FCGBP, NSD1, BRCA2, and PI4KA, as well as recurrent gains or losses affecting chromosomes 1, 6, 16, and other chromosomes.

"Strikingly," the investigators noted, "most of the proteins encoded by recurrently mutated genes participate in an estrogen receptor/ESRR (ESR/ESRR) protein interaction network involved in development, neurogenesis, metabolism, hypoxia, and cell cycle regulation."

The team focused its further analyses on ESRRG, based on results from its transcriptome and single-cell RNA-seq experiments, as well as results from chromosomal analyses that pointed to recurrent gains or rearrangements involving the chromosome 1 region containing the ESRRG gene.

From there, the investigators relied on approaches such as ESRRG-targeted chromatin immunoprecipitation (ChIP) sequencing, ChIP-quantitative PCR, interaction network analyses, co-immunoprecipitation experiments, and reporter assays to better understand the gene's regulation and potential role in Rb. Among other clues, their results highlighted interactions between ESRRG and RB1, which appeared to inhibit ESRRG activity.

In the absence of RB1, the team saw a significant uptick in ESRRG expression and enhanced hypoxia response in human cell line experiments, while cell death and hypoxia susceptibility occurred after short hairpin RNA-based ESRRG knockdown or ESRRG targeting with an agonist called GSK5182.

"Together, these findings suggest that RB1 dampens the hypoxia-induced surge in ESRRG activity and that loss of RB1 abolishes this homeostatic mechanism, resulting in nascent Rb cells becoming increasingly dependent on ESRRG in the hypoxic tumor microenvironment," adding that "[f]urther studies are warranted to investigate the potential role for inhibiting ESRRG in the management of Rb."