Novel Chromosomal Rearrangements and Sequence Mutations in High-Risk Ph-Like Acute Lymphoblastic Leukemia

Abstract 67

Acute lymphoblastic leukemia (ALL) is the most common pediatric malignancy, and relapsed B-lineage ALL remains a leading cause of cancer death in young people. Recent genomic analyses by our group and others identified a unique subtype of BCR-ABL-negative, high-risk B-ALL, with deletion or mutation of IKZF1 and a gene expression profile similar to BCR-ABL1-positive ALL (Ph-like ALL). Up to 50% of Ph-like patients harbor rearrangements of the cytokine receptor gene, CRLF2, with concomitant JAK mutations detected in ∼30%. However, the nature of genetic alterations activating kinase signaling in the remaining cases is unknown.

To identify novel genetic alterations in Ph-like ALL, we performed transcriptome sequencing (RNA-seq) on 11 cases of Ph-like B-ALL (10 from the P9906 Children's Oncology Group trial and 1 from the St Jude Total XV study), and whole genome sequencing (WGS) on two of these. Using multiple complementary analysis pipelines including deFuse, Mosaik, CREST and CONSERTING, we identified novel rearrangements, structural variations and sequence mutations dysregulating cytokine receptor and kinase signaling in 10 cases. Putative rearrangements and sequence mutations were validated using RT-PCR, genomic PCR and Sanger sequencing. The spectrum of alterations included 3 cases with known IGH@CRLF2 rearrangement, 2 cases with the NUP214-ABL1 rearrangement, 1 case each with the in-frame fusions EBF1-PDGFRB, BCR-JAK2 or STRN3-JAK2, and 1 case with a cryptic IGH@-EPOR rearrangement. Detailed analysis of RNA-seq data revealed a 7.5 kb insertion of EPOR downstream of the enhancer domain in the IGH@ locus, which was not detected by fluorescence in situ hybridization. WGS identified an in-frame activating insertion in the transmembrane domain of IL7R (L242>FPGVC) in 1 index case, and recurrence screening identified similar IL7R sequence mutations in 8 cases from the P9906 cohort (N=188). This patient also harbored a focal homozygous deletion removing the first two exons of SH2B3 that was not evident by SNP array analysis. SH2B3 encodes LNK, a negative regulator of JAK2 signaling. Notably, all patients harbor genetic lesions affecting B-lymphoid development (e.g IKZF1), suggesting these events cooperate to drive B-lineage ALL.

To determine the frequency of each fusion, candidate RT-PCR was performed on 231 cases from the COG AALL0232 trial of high-risk B-ALL, 40 (17%) of which were identified as Ph-like using Predictor Analysis of Microarrays (PAM). The EBF1-PDGFRB fusion was detected in 3 additional patients, each containing an intact PDGFRB kinase domain. No additional cases of NUP214-ABL1, BCR-JAK2, or STRN3-JAK2 were identified. Phosphoflow analysis on 3 primary ALL samples demonstrated increased CKRL phosphorylation in the NUP214-ABL1 case and tyrosine phosphorylation in the cases with BCR-JAK2 and STRN3-JAK2 fusions. Importantly, this activation was reduced with the tyrosine kinase inhibitors (TKI) imatinib, dasatinib and the T315I inhibitor XL228 in cells harboring the ABL1 fusion, and the JAK2 inhibitor, XL019, in the JAK2-rearranged samples. Furthermore, the novel EBF1-PDGFRB fusion transformed Ba/F3 cells to growth factor independence, induced constitutive activation of pSTAT5, pAkt, pERK1/2, and responded with low IC₅₀ values to imatinib, dasatinib and the specific PDGFRB/FGFR inhibitor, dovitinib.

Using complementary genomic approaches we show that rearrangements, sequence mutations and DNA copy number alterations dysregulating cytokine receptor and kinase signaling are a hallmark of Ph-like ALL. These data support the screening of patients at diagnosis to identify those with Ph-like ALL, characterize the genomic lesions driving this phenotype, and to determine those that may benefit from TKI treatment.