Discovery and use of immunogenic peptides for the treatment and prevention of cancers
Unique amino acid sequences at the junctions of fusion or truncated proteins translated from chimeric RNAs form neoantigen peptide sites capable of inducing anti-tumor immune responses. Our objective is to find recurrent fusion transcripts that have the potential to generate candidate neoantigens presented by major histocompatibility complex class I (MHC I) during breast cancer progression. We comprehensively characterized the landscape of fusion transcripts in 225 samples of breast tumors representing 3 subtypes. For each patient we tested 4 sites including Tumor (T), Adjacent Normal (Adj-NL), Distant Normal (Dist-NL-2 sites). Using breast tissue from unaffected individuals (NL) we uncovered 20 novel fusion transcript variants detected from RNAseq data analyzed through two fusion callers. NSF-LRRC37A3 the fusion transcript with the largest number of junction crossing reads per sample and the highest recurrence was selected for further study. The fusion was detected in ~20% of the 75 tumor samples (TNBC=4/25, HER2+=7/25 and HR+=7/25), 5 samples in the TCGA breast cancer dataset and absent in NL (n=4). Some fraction of the patients that presented with NSF-LRRC37A3 in the tumor also contained the fusion in Adj-NL and/or Dist-NL. Interestingly, some patients who were fusion negative for the tumor scored fusion positive for the matched Adj-NL and Distant-NL. The 5'- and 3'-boundaries were found located on the coding strands of Exon 13 of NSF and Exon 5 of LRRC37A3. The two major ORFs were predicted including NSF-Exon 1-13-KFPRKLYFLH (NSF with a C-terminal truncation) and MISNQN-LRRC37A3 Exon 5-15 (LRRC37A3 with an N-terminal truncation). The 2 ORFs were analyzed through MHCnuggets, a deep neural network method that predicts peptide–MHC binding to MHC class I/II. A total of 18 different 8-11 mer neoantigen peptides discovered from the fusion ORFs were predicted to bind to a total of 30 unique MHC class I alleles with a binding affinity of IC50<500nM.