A Comparison of Minimal Residual Disease Detection Among ASO-PCR, Dd-PCR and Deep-Sequencing in Patients with Multiple Myeloma Who Underwent Autologous Stem Cell Transplantation

Background: Most patients with multiple myeloma (MM) are considered to be incurable, and relapse, due to minimal residual disease (MRD), is the main cause of death among these patients. Even though allele-specific oligonucleotide real-time quantitative PCR (ASO-qPCR) of immunoglobulin heavy chain gene rearrangement has been used to assess the MRD in MM due to its excellent sensitivity and specificity, ASO-qPCR has a major limitation in its relative quantification method because it needs a reference standard curve, which is usually made with dilutions of diagnostic myeloma DNA or from plasmids containing the target IgH rearrangement gene. Recently, droplet digital PCR (ddPCR) was developed to perform reliable absolute quantification of target genes. Based on the principle of single target gene detection, the sensitivity can be increased when the larger amount of DNA is analyzed. Here we assessed the prognostic value of MRD assessment in autografts from MM patients in the autologous stem cell transplantation (ASCT) setting using ASO-qPCR, ddPCR and next-generation sequencing (NGS) approaches.

Methods: Twenty-three Japanese patients with newly diagnosed MM who received various induction regimens prior to ASCT without any post-ASCT therapy were retrospectively analyzed. Median age 57 (range 39-67); males 11, females 12; ISS 1 (n=7), 2 (n=12), 3 (n=3), not assessed (n=1). 11 patients were analyzed by G-banding and FISH (t(4;14), del17p, t(14;16)) and 4 patients showed high-risk chromosomal abnormalities (t(4;14) (n=2), t(14;16) (n=1), -13 by G-banding (n=1)). All patients had achieved a very good partial response (VGPR) or better after ASCT. Analyzed samples included: (1) BM slides from 20 MM patients at diagnosis, (2) fresh/frozen BM cells from 3 MM patients at diagnosis, and (3) obtained autografts. IGH-based ASO-qPCR was performed as described previously (Methods Mol Biol 2009). ddPCR was performed by the QX200 Droplet Digital PCR system (Bio-RAD Inc.) with a total 6000 ng of genomic DNA combined with the same ASO-primers and TaqMan-probes used in the ASO-qPCR. Droplets were generated by the QX200 droplet generator. End-point PCR (40 cycles) was performed on a C1000 Touch Thermal cycler (Bio-RAD Inc). The PCR product was loaded in the QX200 droplet reader and analyzed by QuantaSoft 1.7.4 (Bio-Rad Inc). NGS-based MRD assessment was performed using the immunosequencing platform (Adaptive Biotechnologies, South San Francisco, CA) (Martinez-Lopez et al Blood 2014).

Results: Nineteen patients could be analyzed by ASO-qPCR and ddPCR, while all 23 patients could be analyzed by NGS. We compared MRD results in autografts between ddPCR and NGS. We observed a high correlation between ddPCR and NGS results of MRD (r=0.82, P<0.0001). Although 19 samples were MRD negative by ASO-qPCR (MRD^ASO(-)), 12 samples were MRD positive by NGS (0.6-46 x 10^-6, median 3.5 x 10^-6) (MRD^NGS(+)) and 7 samples were MRD positive by ddPCR (1.2-102 x 10^-6, median 7.5 x 10^-6) (MRD^ddPCR(+)), demonstrating the higher sensitivity of NGS (10^-6 or higher) and ddPCR compared to ASO-qPCR (10^-4-10^-5). Two high-risk chromosomal abnormality cases (t(4;14) (n=1), -13 by G-banding (n=1)) could achieve MRD^NGS(-) in autograft. We evaluated the association of clinical outcome with MRD in autografts using ddPCR and NGS. To investigate the value of sensitive MRD detection by ddPCR and NGS, we compared PFS in 7 MRD^ddPCR(+) cases (Group 1) with 12 MRD^ddPCR(-) cases (Group 2) and 12 MRD^NGS(+) cases (Group 3) with 11 MRD^NGS(-) (Group 4). Group 2 and Group 4 showed significantly better PFS than Group 1 (P = 0.028) and Group 3 (P=0.033), respectively (Figure 1A and 1B). We also compared 5 MRD^ddPCR(-) & MRD^NGS(+) cases (Group 5) with 11 MRD^NGS(-) cases (Group 4). Group 4 tended to show better PFS than Group 5 (P=0.063, Figure 1C).

Conclusions: In this study, we showed the prognostic value of ddPCR and NGS-based MRD assessment in autografts of patients with MM. Although the ddPCR had improved sensitivity in detecting MRD in autografts and demonstrated higher prognostic value compared with ASO-qPCR, the NGS platform showed the highest sensitivity and prognostic value among these methods.