CRISPR-Cas13d functional transcriptomics reveals widespread isoform-selective cancer dependencies on lncRNAs.

Long noncoding RNAs (lncRNAs) are a significant yet largely uncharted component of the cancer transcriptome, with their isoform-specific functions remaining poorly understood. In this study, we used RNA-targeting CRISPR-Cas13d to uncover and characterize hundreds of tumor-essential lncRNA (te-lncRNA) isoforms with clinical relevance. Focusing on multiple myeloma (MM), we targeted the lncRNA transcriptome expressed in tumor cells from patients with MM and revealed both MM-specific and pan-cancer dependencies across diverse cancer cell lines, which we further validated in animal models. Additionally, we mapped the subcellular localization of these te-lncRNAs, identifying >30 cytosolic isoforms that proved essential when targeted by cytosol-localized Cas13d. Notably, a specific isoform of small nucleolar RNA host gene 6, enriched in the endoplasmic reticulum, interacts with heat shock proteins to maintain cellular proteostasis. We also integrated functional and clinical data into the publicly accessible LongDEP Portal, providing a valuable resource for the research community. Our study offers a comprehensive characterization of te-lncRNAs, underscoring their oncogenic roles and therapeutic potential.