Real-Time Molecular Classification of Diffuse Large B-Cell Lymphoma (DLBCL) By Gene Expression Profiling (GEP): Successful Delivery of a Routine Service for Randomization of Patients Onto the Multicenter Remodl-B Trial (ISRCTN 51837425)

Germinal Center (GCB) and activated (ABC) B-cell subtypes of DLBCL can be identified by Gene expression profiling (GEP). These subgroups are biologically distinct, harboring mutations in different pathways. Patients classified as ABC more often have mutations of the NF-kB pathway and an inferior response to standard R-CHOP therapy. The REMoDL-B trial utilised GEP to stratify patients for the addition of bortezomib to R-CHOP, based on the hypothesis that this agent may selectively improve the outcome of the ABC subtype.

GEP was performed on RNA extracted from diagnostic formalin-fixed paraffin-embedded (FFPE) biopsies using Illumina WG-DASL^TM during the 1st cycle of R-CHOP. Patients were classified as GCB, ABC or Unclassified before cycle 2 using the cross-platform DAC classifier (Care et al, PLOS ONE 2013) and randomised to continue R-CHOP+/-bortezomib. Work is underway to compare data generated on Affymetrix arrays and targeted RNA-seq (Illumina TRex), as well as validation by targeted mutational analysis of 18 genes associated with DLBCL (TNFAIP3, CARD11, CD79A, CD79B, MYD88, TRAF3, TNFRSF11A, PRDM1, TP53, FAS, B2M, CD58, EZH2, MLL2, MEF2B, EP300, CREBBP, KDM2B) using Fluidigm multiplex PCR and Illumina MiSeq on DNA also from the FFPE blocks.

The trial closed to recruitment in May 2015 and 1147 samples have been analysed. One hundred and fifty three (13%) biopsies were unsuitable for GEP (for insufficient tumor tissue, inappropriate block sent). The remaining samples were classified as ABC (n=261, 23%), GCB (n=471, 44%) and Unclassified (n=214, 19%), with only 11 samples (1%) failing to yield a GEP result. GEP was successful in a range of sample types, including needle and endoscopic biopsies, bone marrow trephines and formal biopsies, with results obtained from as little as 40ng of total RNA, all from FFPE samples. Mutational data were available in 199 samples, with 73% of these having a mutation detectable in 1 or more genes (range 0-5) at a AAF (alternative allele frequency) cutoff at 10%. MYD88 was most commonly mutated (in 30% of ABC and 7% of GCB). EZH2 mutations were restricted to the GCB category (26%) and MYD88, CD79a/b and PRDM1 were more commonly associated with the ABC group. MYD88/PRDM1 were the most frequently associated events, with MYD88/CD79a/b and MYD88/NF-kB being mutually exclusive. Where MYD88 was seen in GCB cases, coexisting mutations imply an origin from transformed follicular lymphoma. B2M mutations were commonly identified across all subtypes (n=26), but specifically enriched in Type III (unclassified) cases (25%), which supports the hypothesis that mutational immune escape may be a feature of DLBCL, in common with other tumor types. Cross platform validation is highly concordant using Affymetrix arrays from a pilot series (27/27 gave the same classifier output, with correlating confidences also seen between platforms). RNA-seq analysis is ongoing, however initial analysis shows 86% concordance with the DASL output. Comprehensive cross platform comparison data will be available for presentation at the meeting.

This study demonstrates the feasibility of GEP classification of DLBCL at diagnosis in a large international trial. The molecular classification can also be replicated using different technologies. Mutational analysis confirmed the association between DLBCL subtype and specific mutational hotspots.