USP44 overexpression drives a MYC-like gene expression program in neuroblastoma through epigenetic reprogramming

Neuroblastoma is an embryonic cancer that disproportionately contributes to death in young children. Sequencing data have uncovered few recurrently mutated genes in this cancer though epigenetic pathways have been implicated in disease pathogenesis. We performed a computational screen of deubiquitinating enzymes to identify potential new targets and identified the histone H2B deubiquitinating enzyme USP44 as significantly over-expressed in high-risk tumors. High levels of USP44 significantly correlate with metastatic disease, unfavorable histology, advanced patient age, and MYCN-amplification. The subset of patients with tumors expressing high levels of USP44 have a significantly worse survival, including those with tumors lacking MYCN-amplification. Depleting USP44 in neuroblastoma cell lines leads to diminished proliferation, migration, invasion, as well as impaired neuritic development in response to retinoic acid. Integrated analysis of RNA-seq and ChIP-seq demonstrates a distinct set of genes that is regulated by USP44, including those in Hallmark pathways critical to tumorigenesis – findings that were completely reversed by the re-introduction of USP44. We conclude that USP44 is a novel epigenetic regulator that restrains differentiation and promotes aggressive features. Our data further suggests that USP44 may be a novel target in this disease. Overall design: mRNA-seq was performed in immortalized mouse embryonic fibroblasts (MEF) that were USP44 null or were rescued with re-introduction of USP44 to determine the contribution of USP44 on gene regulation.