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

Tumor-specific cooperation of retinoblastoma protein family and Snf5 inactivation

Malignant rhabdoid tumors (MRT) are rare aggressive cancers that occur in young children. Seventy-five percent of sporadic MRTs harbor inactivating SNF5 mutations, and mice heterozygous for an Snf5-null allele develop MRTs with partial penetrance. The diagnosis of choroid plexus carcinomas (CPC) in addition to MRTs in families with a single mutant SNF5 allele prompted us to assess the role of SNF5 loss in CPC in genetically engineered mice. With high frequency, TgT(121) mice develop CPCs that are initiated by inactivation of retinoblastoma protein (pRb) and related proteins p107 and p130. However, CPC penetrance and latency were not significantly affected by Snf5 heterozygosity, consistent with recent evidence that CPCs in SNF5 families were, in many cases, misdiagnosed MRTs. Surprisingly, although the CPC phenotype was unaffected, TgT(121);Snf5(+/-) mice developed MRTs with increased penetrance and decreased latency compared with TgT(121);Snf5(+/+) littermates. MRTs expressed the T(121) protein with a concomitant increase in mitotic activity. The predominant appearance of TgT(121);Snf5(+/-) MRTs in the spinal cord led to the discovery that these tumors likely arose from a subset of spinal cord neural progenitor cells expressing T(121) rather than from transdifferentiation of CPC. Significantly, the target cell type(s) for MRT is unknown. Hence, this study not only shows that pRb(f) and SNF5 inactivation cooperate to induce MRTs but also provides new insight into the MRT target population.

Mutation of the INI1 gene in composite rhabdoid tumor of the endometrium

Composite rhabdoid tumors are typically adult tumors that contain a component of rhabdoid cells, which are characteristic of the aggressive childhood malignant rhabdoid tumor. Pediatric rhabdoid tumors are characterized by the inactivation of the hSNF5/INI1/SMARCB1 gene, with subsequent loss of expression of the protein. In contrast, only a single composite rhabdoid tumor has demonstrated involvement of the INI1 gene. In our study, INI1 protein expression was studied in 2 uterine carcinosarcomas with rhabdoid components (composite rhabdoid tumors). The rhabdoid component of 1 tumor showed lack of immunoreactivity for the INI1 protein and strong positivity for cyclin D1, whereas the adenocarcinomatous component of the tumor and both components of the second tumor were immunoreactive for the INI1 protein and negative for cyclin D1. Loss of one INI1 allele and a mutation in exon 7 of the remaining allele were detected in the first tumor, consistent with the immunohistochemistry results. Our results demonstrate that deletions and mutations of the INI1 gene can occur also in rare composite rhabdoid tumors of adulthood. Further studies are necessary, however, to determine the prognostic significance of this finding.