The role of HIWI in stem cell maintenance and sarcomagenesis.

Abstract

e20500 Background: PIWI proteins comprise a highly conserved protein family that promote genomic integrity during early development via small RNA-mediated gene silencing and DNA methylation-mediated inhibition of transposon migration. Surprisingly, the human ortholog of Piwi - HIWI, is highly expressed at the mRNA level in soft tissue sarcomas (and other cancers) and is associated with poor clinical outcomes. However, the reason that high Hiwi levels would be associated with cancer promotion rather than genomic stability remains unknown. Methods: We have employed immunohistochemistry, PCR and western blotting techniques. Results: Our studies reveal that Hiwi expression inversely correlates with tumor grade/differentiation (i.e., expressed universally in poorly differentiated, high grade sarcomas). Exogenous over-expression of HIWI in mesenchymal stem cells (MSCs) prevents their in vitro maturation and transforms MSCs into undifferentiated sarcomas in a xenograft model. Functionally, over-expression of Hiwi in MSCs lowers transposon expression levels, increases global DNA methylation levels, and results in promoter hypermethylation and gene dowregulation of many tumor suppressor genes; all of which is reversible via treatment with DNA demethylating agents. Finally, inducible downregulation of endogenous Hiwi in a Hiwi-expressing undifferentiated sarcoma cell line inhibits its growth. Conclusions: These studies identify a novel oncogenic role for Hiwi and support a model in which HIWI mediates tumorigenesis via global silencing of multiple tumor suppressors through DNA methylation of their promoter regions. These studies also provide a rationale for the use of epigenetic agents in Hiwi-expressing cancers.