Backtracking Leukemia Clonal Evolution from Post-Transplantation Relapse to Initial Diagnosis to Identify Founder Mutations and Mechanisms of Immune Evasion

INTRODUCTION: Although allogeneic Hematopoietic Stem Cell Transplantation (allo-HSCT) can accomplish apparent leukemia eradication in most patients, residual tumor cells can persist over time and eventually outgrow, resulting in clinical relapse. The genetic landscape of relapsing leukemia is often markedly different from its counterpart at diagnosis, due to clonal evolution (Ding et al, Nature, 2012) and selection of treatment-resistant variants (Vago et al, N Engl J Med, 2009). A potential solution to treat, or even prevent, relapse is to identify and specifically target mutations occurring very early in the leukemic transformation process, and thus putative hallmarks of cancer progenitors. Next-Generation Sequencing (NGS) provides the opportunity to track disease subclones during the clinical history of patients, pinpoint founder alterations and identify the mechanisms by which leukemia evades elimination.

METHODS: In the present study, we combined immunogenetic analyses and NGS to detail the complex clinical history of a patient with therapy-related myelodysplasia (t-MDS), diagnosed years after sequential intensive chemotherapy for B cell Acute Lymphoblastic Leukemia (B-ALL). Leukemic cells collected at serial time-points during the patient disease history (initial diagnosis of B-ALL, first presentation of t-MDS, relapse after first allo-HSCT, relapse after second allo-HSCT) were characterized by means of genomic HLA typing, HLA allele-specific quantitative PCR and whole exome sequencing, with a minimum depth of coverage of 70x. Patient fibroblasts and peripheral blood mononuclear cells (PBMCs) collected before the occurrence of t-MDS, as well as PBMCs from the two allogeneic HSC donors, served as controls and reference exomes. Targeted resequencing of mutations of interest was performed using the Fluidigm Access Array system. Gene segments encompassing newly-identified mutations in TP53, NHEJ1 and BTNL8and their respective wild-type counterparts were cloned into plasmids, and used to design and validate specific droplet digital PCR assays, using the Bio-Rad QX100 instrument.

RESULTS: A 54-year-old patient with high-risk t-MDS received two subsequent allo-HSCTs from partially-incompatible family donors, and after each transplant experienced disease recurrences. Genomic HLA typing and HLA allele-specific qPCR demonstrated that both relapses were due to mutant variants of the original leukemia that had evaded immune pressure through genomic loss of the HLA haplotype targeted by the respective donor’s T cells. Immunogenetic studies ruled out any linear relationship between the two relapses, suggesting that both might have derived from a common HLA-heterozygous progenitor. Accordingly, whole exome sequencing demonstrated direct clonal evolution from the initial presentation of t-MDS to first relapse, whereas the large majority of mutations present at second relapse were unique to the sample. Only five non-synonymous coding mutations were present in all three t-MDS samples, comprising disrupting mutations in TP53, in NHEJ1 (a key factor in DNA double strand-break repair), and in the T cell costimulatory receptor BTNL8. Of notice secondary mutations, detected only in one or two of the three disease presentations, were predicted to result in partially overlapping functional effects, and could be modeled in a conjoint DNA damage repair network, centered around the putative founder mutation in TP53. By ultra-sensitive droplet digital PCR, the same TP53 mutation was backtracked throughout the whole nine years of patient clinical history, including the phases of apparent complete remission, up to a preleukemic progenitor present in the patient bone marrow at the time of B-ALL initial diagnosis.

CONCLUSIONS: Our results demonstrate that therapies, and the immune pressure of allo-HSCT in particular, can have a dramatic effect in shaping leukemia clonal evolution. Importantly, by identifying leukemia founder mutations, we might further our insights into leukemogenesis, and identify novel targets for post-transplantation diagnostics and leukemia-eradicating therapies.