Genetic ablation of Cyclin D1 abrogates genesis of rhabdoid tumors resulting from Ini1 loss

Abstract

Rhabdoid tumors are aggressive pediatric malignancies for which, currently, there are no effective or standard treatment strategies. Rhabdoid tumors arise because of the loss of the tumor suppressor gene INI1. We have previously demonstrated that INI1 represses Cyclin D1 transcription in rhabdoid cells by directly recruiting histone deacetylase 1 complex to its promoter, leading to G 0-G1 arrest. Expression of Cyclin D1 overcomes cell cycle arrest mediated by INI1 and Cyclin D1 overexpression in human rhabdoid tumors is a common phenomenon. However, it is not clear whether Cyclin D1 is a critical downstream target of INI1 in vivo and whether the derepression of this gene is essential for rhabdoid tumorigenesis. To determine the requirement of Cyclin D1 for genesis of rhabdoid tumors in vivo, we developed Ini1 heterozygous mice by targeted disruption. We found that the tumors developed in these Ini1؉͞؊ mice are rhabdoid, defective for Ini1 protein, and like the human tumors, express Cyclin D1. We crossed Ini1؉͞؊ mice to Cyclin D1؊͞؊ mice and found that Ini1؉͞؊ mice with Cyclin D1 deficiency did not develop any spontaneous tumors, in contrast to the parental Ini1؉͞؊ mice. These results strongly support the hypothesis that Cyclin D1 is a key mediator in the genesis of rhabdoid tumors. Our results provide an in vivo proof of concept that drugs that target Cyclin D1 expression or activity could be potentially effective as novel therapeutic agents for rhabdoid tumors.INI1͞hSNF5 ͉ cell cycle ͉ atypical teratoid ͉ tumorigenesis R habdoid tumors, including malignant rhabdoid tumors of the kidneys (MRT) and atypical teratoid and rhabdoid tumors (AT͞RT) of the brain and soft tissues, are highly aggressive pediatric malignancies (1-3). Prognosis for infants with these tumors is poor and Ϸ80% of the children die within 1 yr of diagnosis despite intensive treatment. Chemotherapy is effective only transiently in some AT͞RT patients and the use of radiotherapy is limited in children under 3 yr of age because of subsequent neurological damage and because surgical resection is challenging (3). In the past several years, there has been significant progress in the cytogenetic and molecular genetic studies, which indicated that the majority of MRT and AT͞RT tumors harbor recurrent biallelic alterations in the INI1͞hSNF5 gene, located in chromosome 22q11.2 (4-6). Several studies support the model that INI1 is a tumor suppressor. Children who harbor germ-line constitutive mutation in one allele of INI1 develop MRT or AT͞RT when the second allele of INI1 is also mutated (5), and families who are carriers of a mutation in this gene are predisposed to rhabdoid syndrome (7). Furthermore, mice heterozygous for Ini1͞hSNF5 mutations develop rhabdoid tumors with a high frequency because of loss of heterozygosity at the Ini1 locus (8-10).INI1͞hSNF5 is a component of the chromatin-remodeling mammalian SWI͞SNF complex (11). Reintroduction of this gene into rhabdoid cell lines causes G 0 -G 1 arrest and affects the transcription of several cell c...

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

“…Similar to MRTs cell lines (7,10), flavopiridol treatment in VAESBJ induced both CCND1 protein downmodulation and cell-cycle arrest. Unexpectedly, we observed a synergistic effect on cell-cycle block with combined Flavopiridol treatment and SMARCB1 ectopic reexpression, indicating that flavopiridol affects cell proliferation and viability through pathways at least in part independent of SMARCB1 tumor suppressor.…”

“…As a result, in MRT cell lines, it has been shown that SMARCB1 loss is associated with responsiveness to Cdk/cyclin inhibitors, such as 4-HPR (7) and flavopiridol (8). In addition, the in vivo spontaneous tumorigenesis in SMARCB1 þ/À knockout mice is increased by TP53-null genetic background (9) and is prevented by CCND1 ablation (10), further supporting the interaction with crucial molecules controlling cell-cycle progression.…”

Targeting cyclin D1, a downstream effector of INI1/hSNF5, in rhabdoid tumors

“…Cyclin D1 is derepressed in human and mouse INI1À/À RTs (Zhang et al, 2002;Tsikitis et al, 2005). In addition, genetic ablation of cyclin D1 is sufficient to abrogate the genesis of RTs in vivo mouse models (Tsikitis et al, 2005). These studies suggest that RTs are critically dependent on the presence of cyclin D1 for genesis and/or survival.…”

“…Effect of cyclin D1 siRNA on proliferation of rhabdoid cells Mouse knockout studies indicated that deletion of cyclin D1 abrogates the genesis of RTs (Tsikitis et al, 2005). To test if cyclin D1 itself is required for the survival of RT cell lines, we transfected a rhabdoid cell line with small interfering RNA oligonucleotides against cyclin D1.…”

“…We found that INI1/hSNF5 directly represses cyclin D1 transcription and that cyclin D1 expression from a heterologous promoter is sufficient to overcome INI1/hSNF5-mediated cell cycle arrest (Zhang et al, 2002). Cyclin D1 is derepressed in human and mouse INI1À/À RTs (Zhang et al, 2002;Tsikitis et al, 2005). In addition, genetic ablation of cyclin D1 is sufficient to abrogate the genesis of RTs in vivo mouse models (Tsikitis et al, 2005).…”