Abstract LB-222: Long-term subclonal evolution of CLL from immune selective pressure after allogeneic stem cell transplant and donor lymphocyte infusion

Next-generation sequencing (NGS) has revealed that the malignant subclones comprising a patient's cancer can possess tremendous genetic heterogeneity at different sites of disease and over time. In leukemia, chemotherapy can hasten subclonal evolution allowing for rare leukemic subclones with aggressive driver mutations to gain a competitive advantage and to predominate at relapse, often portending an inferior treatment response. The impact of immunotherapy on subclonal evolution is less well studied. To determine the effects of allogeneic stem cell transplant (alloSCT) and donor lymphocyte infusion (DLI) on subclonal evolution, we performed whole exome sequencing (WES) on longitudinal peripheral blood and bone marrow from 4 patients with CLL. Specifically, timepoints analyzed included pre-transplant, post-transplant relapse, and post-DLI relapse over a period of up to 8.5 years. B-CLL cells (CD19+CD5+) and normal T cells (CD3+) were sort-purified by fluorescence-activated cell sorting prior to genomic DNA extraction. Libraries for WES were constructed and sequenced to an average depth of 300x on an Illumina HiSeq 2000 using 76 bp paired-end reads. Somatic single nucleotide variants (sSNVs) and indels were called using MuTect and Pindel, respectively, and copy number changes were assessed using an in-house algorithm. In general, these patients had more nonsynonymous mutations per pre-alloSCT sample than reported in other CLL NGS studies (average 30.3; range 8-45), likely related to the significant amount of pre-transplant therapies. Heterogeneous patterns of linear and branched subclonal evolution were seen after alloSCT and DLI in both responders and non-responders. Mutations in several candidate CLL driver genes were seen in this cohort, including SF3B1, SAMHD1, BCOR, EGR2, TP53, and DDX3X. Interestingly, sSNVs in multiple recurrently mutated CLL or cancer census genes (e.g. MAP2K1) rose to levels of detection only after alloSCT or DLI, suggesting they may play a role in immune evasion. In addition, several subclonal genetic variants, including missense mutations in FAM126B and LTBP3, were no longer detected after alloSCT or DLI and may thus represent potential neoantigens. In one treatment-refractory patient, a somatic nonsynonymous clonal CHEK2 mutation was found in 8 longitudinal samples and may represent a novel unique driver mutation. Finally, in one patient who experienced a durable complete remission after DLI, concurrent CLL WES and T-cell receptor beta chain CDR3 NGS was performed, which demonstrated a rapidly evolving T-cell repertoire at the time of complete remission after DLI. For CLL, alloSCT and DLI offer a potentially curative treatment strategy and a better understanding of the genes that confer susceptibility or resistance to these immunotherapies may help unlock the mechanisms that underlie these durable responses. Citation Format: Haven R. Garber, Hannah Beird, Yu Cao, Jianhua Zhang, Rachel Sargent, Pei Lin, Sahil Seth, Xingzhi Song, Huandong Sun, Xizeng Mao, Lisa St John, Karen Clise-Dwyer, Gheath Alatrash, P. Andrew Futreal, Jeffrey J. Molldrem. Long-term subclonal evolution of CLL from immune selective pressure after allogeneic stem cell transplant and donor lymphocyte infusion. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-222. doi:10.1158/1538-7445.AM2015-LB-222