Integrative analysis of the metastatic neuroblastoma transcriptome
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Neuroblastoma (NBL), the most common non-Central Nervous System (CNS) solid tumor of childhood, characteristically displays heterogeneous clinical presentation and biological behavior. Previous work has studied the genetic basis of the disease and revealed a low somatic mutation burden. In order to identify novel therapeutic targets and better understand the biology of high-risk NBLs, I investigated whole transcriptome profiles of two cohorts of metastatic NBLs using RNA sequencing. First, I studied changes in splicing pattern in a cohort of 29 patients. V-Myc Avian Myelocytomatosis Viral Oncogene Neuroblastoma Derived Homolog (MYCN) amplified NBLs showed a distinct splicing pattern affecting multiple cancer hallmarks. Six splicing factors have altered expression patterns in MYCN-amplified tumors and cell lines, and binding motifs for these factors were significantly enriched in differentially-spliced genes. ChIP-seq analysis showed direct binding of MYCN to promoter regions of splicing factors PTBP1 and HNRNPA1, demonstrating that MYCN regulates splicing by directly regulating expression of key splicing factors. Furthermore, high expression of PTBP1 and HNRNPA1 was significantly associated with poor overall survival of stage 4 NBL patients (p≤0.05). Knocking down PTBP1, HNRNPA1 and their downstream target PKM2, a pro-tumor-growth isoform, resulted in repression of NBL cell growth. Second, I used whole transcriptome sequencing in a cohort of 150 patients to assess expressed mutations, fusion genes, and gene expression including long non-coding genes to provide clinically-relevant classification and to offer insights into NBL tumor biology. Twenty-four genes including ALK, ATRX and MYCN were recurrently mutated in NBL transcriptomes. In-frame FOXR1 fusions were detected in 4 samples, including 3 cases or 14% of stage 4S NBLs. Unsupervised gene expression analysis revealed four molecular subgroups. MYCN and tumor microenvironment were the primary discriminating signatures in these molecular subgroups. Fifty-eight percent of MYCN-not-amplified samples showed high MYCN signatures, which were potentially contributed by various genomic events such as MYCN activating mutations and FOXR1 fusions. High MYCN signature was significantly associated with poor overall survival in MYCN-not-amplified tumors (p=0.0017). In addition, the tumor microenvironment including stromal and immune cell infiltration significantly contributed to the NBL transcriptional landscape and tumor progression.