While CRISPR-Cas9 gene knockout screens have become a cornerstone of cancer research, identifying genes that act as tumor drivers when activated or whose inhibition might be therapeutically beneficial requires different approaches. CRISPR activation (CRISPRa) and CRISPR interference (CRISPRi) are powerful CRISPR-based technologies that allow researchers in the United States and worldwide to precisely upregulate or downregulate gene expression without permanently altering the DNA sequence.
Cancer is not solely driven by genes that are completely inactivated. Overexpression of certain oncogenes or subtle downregulation of tumor suppressor genes can also play critical roles. CRISPRa and CRISPRi enable researchers to explore this broader landscape of gene function:
CRISPR Activation (CRISPRa): Turning Genes ON: CRISPRa utilizes a catalytically inactive form of Cas9 (dCas9) fused to transcriptional activators, such as VP64, p300, or SAM (Synergistic Activation Mediator) complexes.
Identify Oncogenes upon Overexpression: By systematically overexpressing genes across the genome in cancer cells, CRISPRa screens can pinpoint genes that, when upregulated, promote cell growth, survival, invasion, or metastasis. These genes represent potential therapeutic targets whose activity could be blocked.
Uncover Hidden Tumor Suppressor Mechanisms: In some cases, increasing the expression of a partially functional or silenced tumor suppressor gene might restore its activity and inhibit cancer progression. CRISPRa screens can help identify such genes.
Study Regulatory Networks: By activating specific transcription factors, researchers can use CRISPRa to dissect the downstream targets and regulatory networks that contribute to cancer phenotypes.
CRISPR Interference (CRISPRi): Turning Genes DOWN (Without Complete Knockout): CRISPRi also employs dCas9, but this time fused to transcriptional repressors, such as the Krüppel-associated box (KRAB) domain.
Identifying Essential Genes and Oncogenes upon Knockdown: Similar to knockout screens, CRISPRi screens can identify genes essential for cancer cell survival or proliferation.
Uncovering Vulnerabilities through Partial Inhibition: Partial downregulation of certain oncogenes might be sufficient to inhibit cancer growth without the severe off-target effects associated with complete ablation. CRISPRi screens can help identify such "rheostatic" targets.
Studying Non-Coding RNAs: CRISPRi can be used to precisely silence the transcription of non-coding RNAs, such as microRNAs and long non-coding RNAs, whose dysregulation is increasingly recognized as a driver of cancer.
CRISPRa and CRISPRi screens follow a similar workflow to knockout screens:
gRNA Library Design: Libraries of gRNAs are designed to target the promoter regions (for activation or repression) or coding regions (for repression) of thousands of genes in the genome.
Lentiviral Delivery: The gRNA libraries, along with the dCas9-activator or dCas9-repressor machinery, are packaged into lentiviruses and used to transduce cancer cell lines.
Selection and Perturbation: Cells with stable integration of the CRISPRa/i components and gRNAs are selected. The expression of target genes is then either upregulated (CRISPRa) or downregulated (CRISPRi).
Applying Selective Pressure: The perturbed cell populations are subjected to various selective pressures relevant to cancer, such as drug treatment, nutrient deprivation, or conditions promoting metastasis.
gRNA Enrichment/Depletion Analysis: After a period of selection, the gRNAs enriched or depleted in the surviving/dying cell populations are identified through next-generation sequencing. Enriched gRNAs in CRISPRa screens point to genes whose overexpression confers a selective advantage, while enriched gRNAs in CRISPRi screens (under negative selection pressure) often target genes whose downregulation is detrimental to the cells.
CRISPRa and CRISPRi screens are significantly expanding our understanding of the functional genome in cancer:
Identifying Novel Therapeutic Targets: These screens are uncovering new genes and regulatory pathways that were not identified through knockout screens alone, providing a broader landscape of potential drug targets.
Understanding Non-Oncogene Dependencies: CRISPRi screens are particularly valuable for identifying "non-oncogene addiction" – situations where cancer cells rely on genes that are not themselves mutated oncogenes but are essential for the survival of cancer cells in a specific context.
Developing Combination Therapies: By identifying genes whose activation or inhibition synergizes with existing cancer therapies, CRISPRa/i screens can guide the development of more effective combination treatment strategies.
Personalized Medicine Approaches: In the future, CRISPRa/i screens could potentially be applied to patient-derived tumor cells to identify specific vulnerabilities or dependencies that can be targeted with personalized therapies.
As CRISPRa and CRISPRi technologies continue to evolve with improved specificity, efficiency, and delivery methods, they will undoubtedly play an increasingly crucial role in dissecting the complex genetic circuitry of cancer and accelerating the development of the next generation of cancer therapeutics in the United States and beyond.
