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Large-scale CRISPR screens show RASA2 ablation boosts the efficacy of T cell therapies

Written by Miyako Rogers, Science Writer

In a new study published in Nature, researchers used CRISPR screens to identify the gene RASA2 as a target to improve the efficacy of T-cell therapies. Knock-out studies both in vitro and in vivo showed that RASA2 ablation significantly improves T-cell functions, including enhanced cancer-killing capacity, prolonged survival, and more.

Genome-wide CRISPR screens converge on RASA2

Adoptive T-cell therapies, including those in early-stage clinical trials, such as chimeric antigen receptor T-cells (CAR T-cells) or T cell receptor (TCR) transgenic cells, have been a promising course of treatment for cancer. However, in many cases, initial results may look successful, but shortly afterwards the treatment appears to stop working, and the cancer rapidly progresses. This may be due to the tumour microenvironment, which contains many immunosuppressive signals which could limit the efficacy of T-cell therapies. One solution to overcome this problem is targeted gene editing; however, researchers needed to first identify which gene to target.

Figure 1 ¦ Schematic showing workflow for identification of RASA2 by CRISPR screens

To systematically explore which genes to target, researchers performed multiple genome-wide CRISPR knock-out screens under different immunosuppressive conditions. These immunosuppressive conditions were designed to mimic the tumour microenvironment (Figure 1). The CRISPR knock-out screens were performed, and T-cell function was analysed. This was repeated until the screens converged on the gene RASA2. RASA2 codes for RAS GTP-ase activating protein (RAS-GAP), which is a signalling checkpoint in human T cells (Figure 2). RASA2 has been shown to be downregulated upon TCR stimulation, and prolonged exposure to antigens can cause a gradual increase in Ras-GAP levels.

RASA2 ablation enhances T-cell function

The researchers then performed several experiments which showed that blocking RASA2 makes T-cells do their job a lot better. Knock-out (KO) experiments showed that following RASA2 ablation, T-cells exhibited enhanced MAPK signalling (Figure 2) and CAR T-cell cytolytic activity when exposed to target antigens. They also profiled the transcriptome of RASA2 KO T-cells and showed that after repeated tumour antigen stimulation, RASA2 KO T-cells had increased activation, cytokine production, metabolic activity and cancer-killing capacity than control T-cells.

Figure 2 ¦ Schematic of RAS signalling in T-cells

Researchers also used animal models to show that RASA2 ablation improved the performance of engineered T-cells in multiple preclinical models of adoptive T-cell therapies. Mice were xenografted with liquid or solid tumours, and researchers showed that RASA2 KO significantly slowed tumour growth, leading to prolonged survival.

Future implications

Altogether, this research suggests that RASA2 is a promising target for improving the performance of T-cell therapies. Moreover, this method of unbiased genetic screening to systematically explore potential targets could be used in other studies to identify therapeutic targets and/or elucidate the genetic basis for disease.