Intraclonal Heterogeneity Caused By Activation-Induced Cytidine Deaminase Is Not a Prognostic Biomarker in Untreated Advanced Stage Follicular Lymphoma: An Analysis of SWOG S0016

Background: Activation induced cytidine deaminase (AICDA or AID) causes somatic hypermutation (SHM) of the immunoglobulin heavy chain variable genes (IGHV). It has been widely hypothesized that aberrant targeting of AICDA creates additional driver mutations and intra-clonal heterogeneity which enables clonal evolution and thus disease progression. This hypothesis predicts that greater AICDA-mediated intraclonal heterogeneity will be associated with shortened progression free survival (PFS). To definitively test this hypothesis that AICDA-mediated heterogeneity is linked to risk of progression, we evaluated available samples from the largest US-based phase 3 trial of untreated follicular lymphoma patients treated with CHOP-based therapies, SWOG trial S0016.

Methods: We have previously shown that ultradeep sequencing of two genes, the expressed IGHV and the 5' UTR of the translocated BCL2 allele, provide unrelated measures of AICDA-mediated genetic heterogeneity (Spence, J Immunol. 2014). Furthermore, the number of AICDA-mediated mutations in the 5' UTR of BCL2, but not in IGHV, correlates with the number of AICDA-mediated mutations at other known AICDA sites throughout the genome (such as PIM1, BCL6, RHOA, etc). Genomic DNA was prepared from all available paraffin-embedded specimens. Ultra-deep, amplicon-based sequencing (~10,000x) with internal controls to establish specimen-specific signal-to-noise parameters allowed sensitive and specific detection of sequence variants at 0.2% (FDR 1%).

Results: For subjects with (152) and without (419) available specimens, the median duration of follow-up (11.3 years), the number of patients who progressed (60%), and other patient characteristics (demographics, histology, stage, FLIPI) were indistinguishable. Sequencing data that fulfilled strict quality metrics for low frequency variants were obtained for >75% of the specimens (114 for BCL2; 120 for IGHV); further analysis was restricted to these subjects.

The extent of intraclonal variation within both the IGHV and the 5' UTR of BCL2 was highly variable. For BCL2, a median of 7 mutations/case were detected (range 0-61); the mutations were highly skewed to the RCYW motif (p<0.001), the preferred target of AICDA. These mutations defined a median of 5 subclones/case (range 1-22). The clonal IGHV was somatically mutated (<0.98 germline identity) in 94% of samples (median 0.89). For IGHV, a median of 113 subclones/case were detected (range 0-1531). Consistent with our prior study, no correlation was seen between the number of mutations detected in the IGHV and BCL2 or the number of subclones defined by IGHV and BCL2.

Neither the number of mutations in nor the number of subclones defined by BCL2 or IGHV were prognostic. Cox regression showed no relationship between PFS and mutation number or subclone count based on either IGHV or BCL2 (hazard ratios are indistinguishable from 1; parameters were treated both as a continuous variables and dichotomized as quartiles). Furthermore, receiver operator curves did not suggest a cut point for any of these measures that would predict disease progression at any time (areas under the curve are indistinguishable from 0.5).

Conclusions: Several measures of AICDA-mediated mutation and resulting subclonal heterogeneity were not found to be prognostic in previously untreated, advanced stage FL treated with CHOP-based regimens. In contrast, AICDA-mediated mutations have been reported to accrue during transformation from low grade FL to large cell lymphoma. This paradox calls for a more refined model to explain the role of AICDA in the progression of FL.

Support: NIH/NCI grants U10CA180888, U10CA180819, U10CA180821, R21CA198072(WRB); and in part by GlaxoSmithKline.