Understanding the role of long non-coding RNAs in neuroblastoma development and progression

Abstract

Neuroblastoma (NB), a common cancer of childhood, contributes to 15% of all pediatric cancer deaths. The improper neuronal differentiation of neural crest cells to mature neurons in the sympathetic nervous system leads to NB tumor formation. NB is an extremely heterogeneous disease and high-risk NB is very difficult to treat, with the incidence of relapse in 50% of cases despite of intensive chemotherapeutic treatment. Long non-coding RNAs (lncRNAs) are a class of biological molecules that are transcribed but not translated to any functional protein. The mechanism of functions for these lncRNAs are diverse and context-specific. De-regulation of lncRNAs has been proposed to play a critical role in cancer development and progression. The goal of the current thesis was to identify novel neuroblastoma-specific lncRNAs for better stratification of the disease and characterizing their functional role in greater detail. In the first study, we characterized differentially expressed lncRNAs between low-risk and high-risk NB tumors using transcriptome profiling. Among the differentially expressed lncRNAs, we chose a lncRNA, neuroblastoma associated transcript 1 (NBAT1), that maps to NB hotspot locus, 6p22.3, which has been shown to harbor several NB-specific risk-associated SNPs. We showed that NBAT1 is a tumor suppressor lncRNA and it carries out this tumor suppressor function through regulating cellular proliferation and differentiation. Consistent with its tumor suppressor properties, its higher expression in NB patients predicts a good prognosis. Mechanistically, NBAT1 controls NB cell growth through epigenetically silencing cell proliferating genes, as well as NB cell differentiation by repressing the neuron-restrictive silencer factor NRSF, also known as REST. In the second study, we sought to investigate the functional connection between NBAT1 and its sense partner CASC15 lncRNA in NB development and progression. Like NBAT1, CASC15 harbors NB-specific tumor suppressor properties and its higher expression in NB patients correlates with good clinical outcomes. We show that CASC15/NBAT1 (6p22lncRNAs) promote cell differentiation by the specific regulatory interactions with SOX9 and USP36 located on 17q, which is frequently gained in NB. We could show mechanistically that 6p22lncRNAs dictate SOX9 expression by controlling CHD7 stability via modulating cellular localization of USP36, which is a deubiquitinase. In the third and final study, we found that NBAT1 is a p53 responsive lncRNA and regulates p53 subcellular localization. We observed that a decrease in NBAT1 expression in NB cells results in resistance to genotoxic drugs, which in part occurs due to cytoplasmic p53 accumulation and concomitant loss of p53 dependent gene expression. Higher expression of the p53 exporter CRM1 in NBAT1 depleted cells contributes to p53 cytoplasmic localization, while CRM1 inhibition in these cells restores p53 localization. We observed that combined inhibition of CRM1 and MDM2 sensitized aggressive NB cells with cytoplasmic p53, suggesting that this drug combination could be a potential therapeutic strategy for high-risk NB patients. In summary, these findings highlight the regulatory role of lncRNAs in NB disease development.