Re-expression of hSNF5/INI1/BAF47 in pediatric tumor cells leads to G1arrest associated with induction of p16ink4a and activation of RB

Abstract

Truncating mutations and homozygous deletions in the hSNF5/INI1/BAF47 subunit of human SWI/SNF complexes occur in most malignant rhabdoid tumors and some other malignancies. How loss of hSNF5 contributes to tumorigenesis remains unknown. Because the SWI/ SNF subunit BRG1 is required for RB-mediated cell cycle arrest, we hypothesized that hSNF5 deficiency disrupts RB signaling. Here we demonstrate that unlike BRG1, hSNF5 deficient cells retain functional RB since ectopic expression of either p16ink4a or a constitutively active form of RB (PSM -RB) led to cell cycle arrest. To determine how hSNF5 loss might contribute to tumorigenesis, we used a retrovirus to introduce hSNF5 into multiple deficient cell lines. In all cases, re-expression inhibited colony formation and induced cell cycle arrest characterized by a flattened morphology. Flow cytometry revealed that these cells accumulated in G 0 /G 1 . Importantly, arrested cells exhibited strong induction of p16ink4a, hypophosphorylated RB, and down-regulation of cyclin A, suggesting that hSNF5 signals upstream of RB to induce growth arrest. Co-expression of SV40 T/t abolished hSNF5-induced G 1 arrest and activation of RB. Likewise, HPV-16 E7 was sufficient to partially overcome cell cycle arrest. These results suggest that hSNF5 loss is not equivalent to BRG1/ BRM loss in human tumor cell lines. Furthermore, hSNF5-induced cell cycle arrest of deficient cells is mediated in part through activation of p16ink4a expression. These findings provide insight into mechanisms of hSNF5-mediated tumor suppression.

SMARCB1/INI1 Genetic Inactivation Is Responsible for Tumorigenic Properties of Epithelioid Sarcoma Cell Line VAESBJ

“…The p16/CCND1/CDK4/RB/E2F pathway inactivation has been initially reported as the prominent downstream effect of SMARCB1 inactivation in MRTs (20,30,36). However, more recently, engineered mouse models showed that SMARCB1 haploinsufficiency cooperates with both RB and TP53 haploinsufficiency to increase penetrance and reduce latency of tumor onset (9,22,37) and in vitro studies showed that SMARCB1 inactivation globally affects the epigenetic status of the cancer genome (38,39).…”

“…This result indicates that VAESBJ represents a unique model to investigate the role of SMARCB1 inactivation in the context of epithelioid sarcoma. Because in MRTs cells, SMARCB1 tumor suppressor activity has been shown to impinge on cell proliferation (29,30), apoptosis (31), senescence (20,27), and cell motility (21), we investigated these phenotypes in SMARCB1-deleted VAESBJ epithelioid sarcoma cell line. Here, we provide evidence that SMARCB1 restoration in this cell line affects cell proliferation, enhances the sensitivity to genotoxic stress, and reduces cell migration.…”

The requirement for SNF5/INI1 in adipocyte differentiation highlights new features of malignant rhabdoid tumors

“…Indeed, several groups have documented that hSNF5/INI1 re-expression in MRT cell lines leads to the accumulation of cells in the G 0 /G 1 phase of the cell-cycle (Betz et al, 2002;Versteege et al, 2002;Zhang et al, 2002). This hSNF5/INI1-induced cell-cycle arrest requires a functional RB/E2F pathway and is reversible (Medjkane et al, 2004).…”

Srg3, a mouse homolog of BAF155, is a novel p53 target and acts as a tumor suppressor by modulating p21WAF1/CIP1 expression

“…Re-expression of BRG1 in the BRG1-deficient cancer cell lines restores RB function, repressing E2F targets, such as cyclin E (Hendricks et al, 2004). Although SNF5 is dispensable for RB-induced cell-cycle arrest, re-expression of SNF5 into malignant rhabdoid tumor cell lines leads to G1 cell-cycle arrest, possibly through p16 INK4a induction and Cyclin D1 repression (Betz et al, 2002;Zhang et al, 2002). Furthermore, several reports suggest that ablation of p53 dramatically accelerates tumor formation caused by Snf5 deficiency (Isakoff et al, 2005;Klochendler-Yeivin et al, 2006).…”