The CD19 antigen, expressed on most B-cell acute lymphoblastic leukemias (B-ALL), can be targeted with chimeric antigen receptor–armed T cells (CART-19), but relapses with epitope loss occur in 10% to 20% of pediatric responders. We detected hemizygous deletions spanning the CD19 locus and de novo frameshift and missense mutations in exon 2 of CD19 in some relapse samples. However, we also discovered alternatively spliced CD19 mRNA species, including one lacking exon 2. Pull-down/siRNA experiments identified SRSF3 as a splicing factor involved in exon 2 retention, and its levels were lower in relapsed B-ALL. Using genome editing, we demonstrated that exon 2 skipping bypasses exon 2 mutations in B-ALL cells and allows expression of the N-terminally truncated CD19 variant, which fails to trigger killing by CART-19 but partly rescues defects associated with CD19 loss. Thus, this mechanism of resistance is based on a combination of deleterious mutations and ensuing selection for alternatively spliced RNA isoforms. Significance CART-19 yield 70% response rates in patients with B-ALL, but also produce escape variants. We discovered that the underlying mechanism is the selection for preexisting alternatively spliced CD19 isoforms with the compromised CART-19 epitope. This mechanism suggests a possibility of targeting alternative CD19 ectodomains, which could improve survival of patients with B-cell neoplasms.

Summary Neuroblastoma is a pediatric malignancy that typically arises in early childhood and is derived from the developing sympathetic nervous system. Clinical phenotypes range from localized tumors with excellent outcomes to widely metastatic disease where long-term survival is approximately 40% despite intensive therapy1. A previous genome-wide association study (GWAS) identified common polymorphisms at the LMO1 gene locus that are highly associated with neuroblastoma susceptibility and oncogenic addiction to LMO1 in the tumor cells2. Here we sought to discover the causal DNA variant at this locus and the mechanism by which it leads to neuroblastoma tumorigenesis. We first imputed all possible genotypes across the LMO1 locus and then mapped highly associated single nucleotide polymorphism (SNPs) to areas of chromatin accessibility, evolutionary conservation, and transcription factor binding sites. SNP rs2168101 G>T was the most highly associated variant (combined P=7.47×10-29, Odds Ratio 0.65, 95% CI: 0.60-0.70) and resided in a super-enhancer defined by extensive acetylation of histone H3 lysine 27 within the first intron of LMO1. The ancestral G-allele that is associated with tumor formation resides in a conserved GATA transcription factor binding motif. We show that the newly evolved protective TATA allele is associated with decreased total LMO1 expression (P=0.028) in neuroblastoma primary tumors and ablates GATA3 binding (P<0.0001). We demonstrate allelic imbalance favoring the G-containing strand in tumors heterozygous for this SNP as demonstrated both by RNA sequencing (P<0.0001) and reporter assays (P=0.002). These findings show that a recently evolved polymorphism within a super-enhancer element in the first intron of LMO1 influences neuroblastoma susceptibility through differential GATA transcription factor binding and direct modulation of LMO1 expression in cis, and this leads to an oncogenic dependency in tumor cells.

Summary We developed an RNA sequencing-based pipeline to discover differentially expressed cell surface molecules in neuroblastoma that meet criteria for optimal immunotherapeutic target safety and efficacy. Here we show that GPC2 is a strong candidate immunotherapeutic target in this childhood cancer. We demonstrate high GPC2 expression in neuroblastoma due to MYCN transcriptional activation and/or somatic gain of the GPC2 locus. We confirm GPC2 to be highly expressed on most neuroblastomas, but not detectable at appreciable levels in normal childhood tissues. Additionally, we demonstrate that GPC2 is required for neuroblastoma proliferation. Finally, we develop a GPC2 directed antibody-drug conjugate that is potently cytotoxic to GPC2-expressing neuroblastoma cells. Collectively, these findings validate GPC2 as a non-mutated neuroblastoma oncoprotein and candidate immunotherapeutic target.

Increasing evidence links deregulation of the USP22 deubitiquitylase to cancer development and progression in a select group of tumor types, but its specificity and underlying mechanisms of action are not well-defined. Here we show that USP22 is a critical promoter of lethal tumor phenotypes that acts by modulating nuclear receptor and oncogenic signaling. In multiple xenograft models of human cancer, modeling of tumor-associated USP22 deregulation demonstrated that USP22 controls androgen receptor (AR) accumulation and signaling, and that it enhances expression of critical target genes co-regulated by AR and MYC. USP22 not only reprogrammed AR function, but was sufficient to induce the transition to therapeutic resistance. Notably, in vivo depletion experiments revealed that USP22 is critical to maintain phenotypes associated with end-stage disease. This was a significant finding given clinical evidence that USP22 is highly deregulated in tumors which have achieved therapeutic resistance. Taken together, our findings define USP22 as a critical effector of tumor progression whcih drives lethal phenotypes, rationalizing this enzyme as an appealing therapeutic target to treat advanced disease.