Neoantigen Discovery Using Next-Generation Sequencing Data

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Neoantigen Discovery Using Next-Generation Sequencing Data

Leveraging neoantigens for guided treatment has become the definition of personalized cancer immunotherapy. Given their potential to eradicate nonspecific treatments such as chemotherapy, a host of therapeutic studies are underway to further understand how neoantigens can be used to trigger immunity-induced tumor regression. Seven Bridges has developed a workflow for neoantigen discovery using next-generation sequencing (NGS) data, which analyzes tumor-normal pairs of whole exome sequencing (WES) samples and tumor RNA-seq gene expression data in order to output candidate epitopes for neoantigens.

This neoantigen discovery workflow has been tested throughout the last two rounds (Round 2 and Round X) of the Tumor Neoantigen Selection Alliance (TESLA) Challenge organized by the Parker Institute for Cancer Immunotherapy. The TESLA Challenge has gathered more than 35 leading neoantigen research groups in academia and industry with the goal of validating neoantigen prediction workflows. Within this last round, TESLA provided flow-cytometry-validated peptide-HLA sets for more than 10 patients. The neoantigen discovery workflow developed by Seven Bridges managed to detect the majority of confirmed candidates. Detailed results of the challenge will soon be published by the TESLA organizers.

Neoantigen prediction workflow

Figure 1. Neoantigen prediction workflow
Although we are unable to publish the results, we are able to share the workflow and the various steps and stages involved. Detailed below is what you can expect from the Seven Bridges neoantigen discovery workflow:

  1. First, the workflow will perform read alignment, with the preparation of aligned files, leveraging GATK best practices.
  2. Next, somatic variant calling (using Strelka2 and Mutect2) is performed and variants are merged into one file (by selecting intersect or union).
  3. Once the variants are merged into one file, variant phasing occurs. Variant phasing is the separation of variants belonging to the same chromosome (Figure 2). The purpose of this step is to accurately reconstruct the nucleotide sequence which is translated into the neoantigen candidate.
  4. Now you are ready to move onto the following three steps that occur in parallel with the alignment, somatic variant calling, and variant phasing:
    1. RNA reads are processed with Salmon and RSEM tools, which enable high precision quantification of transcripts. The output from this part is a list of gene isoforms with corresponding RNA expression scores.
    2. GATK4 RNA Variant Calling which outputs tumor RNA variants.
    3. Optitype calculates Human Leukocyte Antigen (HLA) types from tumor FASTQs.
      Two close variants:
      Chr5	20000	C	T	0/1
      Chr5	20011	A	G	0/1
      
      Which two alleles will exist?
      ...C...A… 
      ...C...G…
      ...T...A… 
      ...T...G…
      
      Phasing will tell us! After phasing:
      Chr5	20000	C	T	0|1
      Chr5	20011	A	G	1|0
      
      Two alleles will be called:
      ...C...G… and ...T...A… 
      (...C...A… and ...T...G… will not be considered)
      
      Figure 2. Example of phasing alleles of two variants
  5. Next, phased somatic DNA variants, optionally filtered with RNA expressed regions (to boost-up precision of neoantigen candidates) are passed to protein extraction tools to extract the nucleotide sequences around the positions of the somatic variants and translate them to proteins. During this process the protein extraction tools take into account the full range of complex changes, currently not existent within the published workflows: changes from both germline and somatic variants (SNPs and indels); mutations (SNPs or indels) within the STOP codon (nonstop) and mutations that create STOP codons (nonsense). The extracted protein sequence and identified HLA type are used by the epitope prediction tools (NetCTLpan, IEDB Processing: NetMHCPan, Pickpocket, etc) to compute confidence-ranked epitope candidates.
  6. Finally, a list of the confidence-ranked neoantigen candidates with HLA types, variant information, RNA expression, and variant calling scores are merged and prioritized in the analysis epitopes tool. Its output is shown in Table 1.
Sequence name HLA Type Peptide NetMHC Score Pickpocket Score NetCTLPan Combined Score Transcripts per Million (RNA) Variant Information [GT:AD:AF:…]|transcript_ids]
1_111957245_C_A HLA-A*02:01 MMLSSSPV 0.881 0.633 1.09815 11.5 0/1:261 4:0.018:2|ENST00000341809|ENST00000553168
8_144392368_T_C HLA-A*02:01 WLLEKLEQL 0.828 1.097 1.06133 12.5 0/1:103 12:0.091:6:6:|ENST00000533313|ENST00000332271|ENST00000527701
17_28537638_C_T HLA-A*02:01 VLDEFPHV 0.836 0.374 1.06015 23 0/1:94 4:0.048:|ENST00000579221|ENST00000394821
1_111957245_C_A HLA-A*02:01 MMLSSSPVIQL 0.816 1.273 1.05881 6.7 0/1:261 4:0.018:|ENST00000553168
9_32630149_G_T HLA-B*07:02 RPKQHFML 0.776 0.901 1.01862 9.6 0/1:312 27:0.083:13|ENST00000242310
11_58207203_C_T HLA-A*02:01 YLAIGSYI 0.793 0.705 1.01312 6 0/1:67 6:0.076:|ENST00000302572
Table 1: Example output of scored sorted neoantigen candidates with all necessary information

Non-proprietary components of this portable and reproducible workflow will be publically available on the Seven Bridges Platform. They are described in Common Workflow Language to ensure portability and reproducibility between computation environments and execution times of the workflow per patient varies between 3 and 6 hours ($2.3 to $4.5), depending on the type of tumor and mutation burden.

Combining this workflow with the automation, customization, and scalability of the Seven Bridges Platform enables rapid prioritization of a patient’s neoantigen candidates.

If you are interested in testing the neoantigen discovery workflow, please contact sales@sevenbridges.com.