Comparison of Gold Standard Genescan with NGS-Based TCR-Beta Clonality Analysis Using Oncomine TCR Beta-Short Read Assay

Introduction

All mature T-cell-based lymphoid malignancies harbor identical (clonal) rearrangements of their T cell receptor (TCR) genes (van Dongen et al. Clinica Chimica Acta. 1991, 198, 1-92). The clonality assessment of the rearranged TCR gene is particularly important for the identification, characterization and monitoring of T-cell neoplasms since histology and immunophenotyping alone is not enough to make conclusive diagnosis in all cases. Diagnostic clonality testing is currently based on the parallel analysis of the rearranged TCR-gamma and TCR-beta chain genes. This is done using a multiplex PCR developed within the European BIOMED-2/EuroClonality consortium (van Dongen et al. Leukemia. 2003, 17, 2257-2317) followed by capillary electrophoresis of the resultant PCR products. Although, this method is considered the "gold standard", which is established worldwide in many molecular diagnostic laboratories, the resolution of this approach is limited, especially in cases with low percentage of clonally rearranged TCRs and a high mixture of non-clonally rearranged T-cells. Next generation sequencing (NGS) is a powerful tool to provide resolution at single gene level. This also holds true for NGS assays, which are able to detect all unique TCR rearrangements in a given sample with very high resolution and sensitivity. This approach can be employed for precise assessment of the immune repertoire, minimal residue disease and T-cell clonality. Detailed insights into the clonotypes provides a great potential for early diagnosis of T-cell neoplasms, and the identification of individual clones is essential for monitoring of the disease.

Methods

We investigated 19 formalin-fixed paraffin-embedded (FFPE) tissue samples from celiac disease patients (n = 14) and routine diagnostic cases suspicious of lymphoma (n = 5). After DNA extraction, we performed multiplex PCRs using the BIOMED-2 TCR-beta primer sets (van Dongen et al. Leukemia. 2003, 17, 2257-2317) followed by capillary electrophoresis. For TCR-beta NGS, the Oncomine™ TCR Beta-SR DNA Assay (Thermo Fisher Scientific) was used according to the manufacturer's instructions and sequencing was performed on the IonTorrent S5. Identification of individual clonotypes and bioinformatics analysis of the data was done with the help of the IonReporter software (Thermofisher Scientific). It is worth noting that the 14 celiac disease samples were previously analysed by a different TCR-beta NGS approach (Ritter et al. Gut. 2018, 67, 644-653) and utilized for the comparison of the NGS-data.

Results

17 out of 19 cases analysed by TCR-beta multiplex PCR (BioMed-2) followed by capillary electrophoresis and by the Oncomine™ TCR Beta-SR assay displayed a very similar length distribution of the PCR products. This holds true for samples with a clonal appearance and for samples with an oligo-/polyclonal pattern. In two discrepant cases, the Primerset B of the BioMed-2 approach showed a dominant amplification product, which was not as clear by TCR-beta NGS in which both cases displayed an oligoclonal TCR-beta gene rearrangement pattern with a few dominant T-cell populations. Both samples were from celiac disease patients, which mainly present a pronounced oligoclonal TCR-beta gene rearrangement pattern. Strikingly, the NGS data from the Oncomine™ TCR Beta-SR DNA Assay delivered highly comparable results when compared to the sequencing data of Ritter et al. 2018 despite having completely different primer sets and a different NGS platform.

Conclusion

Our comparison of the conventional multiplex PCR (BioMed-2) and TCR-beta NGS (Oncomine™ TCR Beta-SR DNA Assay) demonstrated a very high concordance (17/19 cases) of the molecular data. The two discordant cases can be explained by an over-interpretation of dominant species BioMed-2 Primerset B amplifications, which often show up in cases with low T-cell content or oligoclonal T-cell counts. These T-cell clonotypes are detectable by TCR-beta NGS only at a low percentage because of combination of all type of rearrangements in one assay. The robustness and reliability of NGS-based TCR-beta clonality testing was demonstrated by comparison of two completely different assays, leading to very similar results for all celiac disease patients. We are thus very confident that NGS-based clonality testing will be the "gold standard" of the future.