Pervasive enhanced transcription in inflammatory breast cancer tumors and PBMCs impacts RNA splicing and intronic RNAs in plasma.

Inflammatory breast cancer (IBC), an aggressive and lethal breast cancer subtype, lacks unequivocal genomic differences or robust biomarkers that differentiate it from non-IBC. Here, TGIRT-seq revealed myriad differences in tumors, PBMC, and plasma RNAs that distinguished IBC patients from non-IBC patients and healthy donors across different tested breast cancer subtypes. By mapping reads to genome and transcriptome reference sequences and quantitating intron-to-exon read depth ratios (IDRs), we developed methods for parallel analysis of transcriptional and posttranscriptional gene regulation. This analysis identified numerous protein-coding genes in IBC patient tumors and PBMCs with high IDRs, suggesting rate-limiting RNA splicing that decreases mRNA production. Mirroring gene expression differences in tumors and PBMCs, overrepresented protein-coding gene RNAs in IBC patient plasma were largely intron RNA fragments, while those in non-IBC patient and healthy donor plasma were largely mRNA fragments. Our findings provide insights into IBC and should enable monitoring disease progression by liquid biopsy.