Chromatin remodeling protein HELLS is upregulated by inactivation of the RB-E2F pathway and is nonessential for osteosarcoma tumorigenesis

Abstract

Osteosarcoma is the most common primary bone malignancy in children and adolescents. Among the various molecular mechanisms implicated in osteosarcomagenesis, the RB-E2F pathway is of particular importance as virtually all cases of osteosarcoma display alterations in the RB-E2F pathway. In this study, we examined the transcription factor E2F family members that are associated with increased malignancy in Rb1-null osteosarcoma tumors. Using genetically engineered mouse models of osteosarcoma, we found that loss of activator E2Fs, E2F1 and E2F3, significantly delays tumor progression and increases the overall survival of the p53/Rb1-deficient osteosarcoma mouse model. We also studied the role of helicase, lymphoid specific (HELLS), a chromatin remodeling protein identified as a critical downstream effector of the RB-E2F signaling pathway in various cancers. In this study, we confirmed that the RB-E2F pathway directly regulates HELLS gene expression. We also found that HELLS mRNA is upregulated and its protein overexpressed in osteosarcoma. Using loss-of-function assays to study the role of HELLS in human osteosarcoma, we observed that HELLS has no effect on tumor proliferation and migration. Further, we pioneered the study of Hells in developmental tumor models by generating Hells conditional knockout osteosarcoma mouse models to examine the role of HELLS in osteosarcoma tumor development. We found that loss of Hells in osteosarcoma has no effect in tumor initiation and overall survival of mice. This suggests that while HELLS may serve as a biomarker for tumorigenesis and for RB-E2F pathway status, it is unlikely to serve as a relevant target for therapeutics in osteosarcoma.

Retinoblastoma Tumor Suppressor Protein Roles in Epigenetic Regulation

“…DNA methylation is catalyzed by enzymes in the DNA methyltransferase family (DNMTs), including DNMT1, which is associated with the maintenance of DNA methylation [ 82 , 83 ], and DNMT3A and DNMT3B, which are responsible for de novo DNA methylation [ 84 ]. pRB not only transcriptionally represses the expression of DNMT1 and DNMT3A / B [ 85 , 86 , 87 ], but also physically interacts with DNMT1 and chromatin remodelers that can recruit DNMT1 and DNMT3A/B to affect DNA methylation [ 88 , 89 , 90 , 91 ].…”

“…Unpublished studies from our lab using genetic engineered mouse models of retinoblastoma also indicate that UHRF1 overexpression is required for the epigenetic changes that drive retinoblastoma tumor progression and in the absence of UHRF1, retinoblastoma tumors fail to form. HELLS is also transcriptionally regulated by the pRB/E2F pathway and overexpressed in multiple types of cancer, including retinoblastoma [ 90 , 96 , 97 ]. HELLS remodels chromatin to render DNA accessible to DNMT3 to support de novo DNA methylation and stable gene silencing during cellular differentiation.…”

Chromatin remodeling protein HELLS is critical for retinoblastoma tumor initiation and progression

“…Previous studies have shown that HELLS is regulated through the RB/E2F pathway and that loss of RB1 results in upregulation and overexpression of HELLS, a process that may contribute to epigenetic changes that drive tumor progression 7,8,10 . We verified that HELLS overexpression occurs soon after tumor initiation in Chx10-Cre Rb1 lox/lox p107 −/− (Rb1/p107 DKO; Fig.…”

“…Other groups and we have identified HELLS as a transcriptional downstream target gene of E2F1 that is overexpressed in cancer and contributes to tumor progression 9,10,30 . The evidence of direct regulation of HELLS by the RB family was limited to in vitro studies in gliomas and osteosarcoma 7,9 . In this study, we confirmed that the RB/E2F signaling pathway directly regulates transcriptional activation of HELLS in the developing retinae.…”

“…HELLS (helicase, lymphoid specific; also known as LSH, ICF4, PASG, and SMARCA6), a gene transcriptionally controlled by the RB/E2F pathway [7][8][9] , encodes a chromatin remodeling protein thought to be responsible for the epigenetic changes seen in retinoblastoma and required for tumor survival 10 . HELLS is a SWI/SNFrelated matrix-associated regulator of chromatin that contributes to global genome methylation 11 .…”