DNMT3B7, an aberrant DNMT3B isoform, suppresses growth, induces differentiation, and alters DNA methylation in human neuroblastoma

In adult cancers, epigenetic changes and aberrant splicing of the DNMT3B is commonly observed, and the pattern of gene methylation and expression has been shown to be modified by DNMT3B7, a truncated protein of DNMT3B. Much less is known about the mechanism of epigenetic changes in the pediatric cancer neuroblastoma. To investigate if aberrant DNMT3B transcripts alter DNA methylation, gene expression and tumor phenotype in neuroblastoma, we measured DNMT3B isoform expression in primary tumors and cell lines. Higher levels of DNMT3B7 were detected in differentiated ganglioneuroblastomas compared to undifferentiated neuroblastomas, suggesting that expression of DNMT3B7 may induce a less clinically aggressive tumor phenotype. To test this hypothesis, we investigated the effects of forced DNMT3B7 in neuroblastoma cells. We found that DNMT3B7 expression significantly inhibited neuroblastoma cell proliferation in vitro, and in neuroblastoma xenografts, DNMT3B7 decreased angiogenesis and tumor growth. DNMT3B7-positive cells had higher levels of total genomic methylation, and RNA-sequencing revealed a dramatic decrease in expression of FOS and JUN family members, AP1 complex components. Consistent with the established antagonistic relationship between AP1 expression and retinoic acid receptor activity, decreased proliferation and increased differentiation was seen in the DNMT3B7-expressing neuroblastoma cells following treatment with all trans retinoic acid (ATRA) compared to controls. Our results demonstrate that high levels of DNMT3B7 modify the epigenome in neuroblastoma cells, induce changes in gene expression, inhibit tumor growth, and increase sensitivity to ATRA. Further knowledge regarding mechanisms by which DNMT3B7 regulates gene methylation may ultimately lead to the development of therapeutic strategies that reverse the epigenetic aberrations that drive neuroblastoma pathogenesis. DNMT3B7, a truncated DNMT3B isoform, was stably transfected into an N-type neuroblastoma cell line (LA1-55n) using a Tet-off inducible system. DNMT3B7 expressing cells were compared to vector control cells after 21 days of induction.