The mouse ortholog of the human SMARCB1 gene encodes two splice forms

A Carboxy-terminal Smarcb1 Point Mutation Induces Hydrocephalus Formation and Affects AP-1 and Neuronal Signalling Pathways in Mice

The BAF (BRG1/BRM-associated factor) chromatin remodelling complex is essential for the regulation of DNA accessibility and gene expression during neuronal differentiation. Mutations of its core subunit SMARCB1 result in a broad spectrum of pathologies, including aggressive rhabdoid tumours or neurodevelopmental disorders. Other mouse models have addressed the influence of a homo- or heterozygous loss of Smarcb1, yet the impact of specific non-truncating mutations remains poorly understood. Here, we have established a new mouse model for the carboxy-terminal Smarcb1 c.1148del point mutation, which leads to the synthesis of elongated SMARCB1 proteins. We have investigated its impact on brain development in mice using magnetic resonance imaging, histology, and single-cell RNA sequencing. During adolescence, Smarcb11148del/1148del mice demonstrated rather slow weight gain and frequently developed hydrocephalus including enlarged lateral ventricles. In embryonic and neonatal stages, mutant brains did not differ anatomically and histologically from wild-type controls. Single-cell RNA sequencing of brains from newborn mutant mice revealed that a complete brain including all cell types of a physiologic mouse brain is formed despite the SMARCB1 mutation. However, neuronal signalling appeared disturbed in newborn mice, since genes of the AP-1 transcription factor family and neurite outgrowth-related transcripts were downregulated. These findings support the important role of SMARCB1 in neurodevelopment and extend the knowledge of different Smarcb1 mutations and their associated phenotypes.

Recent insights into the SWI/SNF complex and the molecular mechanism of hSNF5 deficiency in rhabdoid tumors

Genetic information encoded by DNA is packaged in the nucleus using the chromatin structure. The accessibility of transcriptional elements in DNA is controlled by the dynamic structural changes of chromatin for the appropriate regulation of gene transcription. Chromatin structure is regulated by two general mechanisms, one is histone modification and the other is chromatin remodeling in an ATP-dependent manner. Switch/sucrose nonfermentable (SWI/SNF) complexes utilize the energy from ATP hydrolysis to mobilize nucleosomes and remodel the chromatin structure, contributing to conformational changes in chromatin. Recently, the inactivation of encoding genes for subunits of the SWI/SNF complexes has been documented in a series of human cancers, accounting for up to almost 20% of all human cancers. For example, human SNF5 (hSNF5), the gene that encodes a subunit of the SWI/SNF complexes, is the sole mutation target that drives malignant rhabdoid tumors (MRT). Despite remarkably simple genomes, the MRT has highly malignant characteristics. As a key to understanding MRT tumorigenesis, it is necessary to fully examine the mechanism of chromatin remodeling by the SWI/SNF complexes. Herein, we review the current understanding of chromatin remodeling by focusing on SWI/SNF complexes. In addition, we describe the molecular mechanisms and influences of hSNF5 deficiency in rhabdoid tumors and the prospects for developing new therapeutic targets to overcome the epigenetic drive of cancer that is caused by abnormal chromatin remodeling.

Pathogenic noncoding variants in the neurofibromatosis and schwannomatosis predisposition genes

Neurofibromatosis type 1 (NF1), type 2 (NF2), and schwannomatosis are a group of autosomal dominant disorders that predispose to the development of nerve sheath tumors. Pathogenic variants (PVs) that cause NF1 and NF2 are located in the NF1 and NF2 loci, respectively. To date, most variants associated with schwannomatosis have been identified in the SMARCB1 and LZTR1 genes, and a missense variant in the DGCR8 gene was recently reported to predispose to schwannomas. In spite of the high detection rate for PVs in NF1 and NF2 (over 90% of non-mosaic germline variants can be identified by routine genetic screening) underlying PVs for a proportion of clinical cases remain undetected. A higher proportion of non-NF2 schwannomatosis cases have no detected PV, with PVs currently only identified in around 70%-86% of familial cases and 30%-40% of non-NF2 sporadic schwannomatosis cases. A number of variants of uncertain significance have been observed for each disorder, many of them located in noncoding, regulatory, or intergenic regions. Here we summarize noncoding variants in this group of genes and discuss their established or potential role in the pathogenesis of NF1, NF2, and schwannomatosis.

Loss of SMARCB1/INI1 Immunoexpression in Chordoid Meningiomas

Background

Chordoid meningiomas have an aggressive clinical course characterized by frequent recurrences. Recent whole-genome sequencing studies demonstrated Chr22 loss in chordoid meningiomas not accounted for by NF2 mutations. SMARCB1/INI1 is a candidate gene on Chr22, which has not been analyzed extensively in meningiomas. AKT1 mutation has been recently identified to be a driver of meningiomagenesis.

Materials and Methods

Cases of chordoid meningioma were retrieved along with meningiomas of other subtypes for comparison. INI1 immunohistochemistry was performed. SMARCB1 and AKT1 were analyzed by sequencing.

Results

Sixteen chordoid meningiomas were identified (1.1% of all meningiomas). Six cases (37.5%) showed loss of INI1 immunoexpression. All other meningioma subtypes (n = 16) retained INI1 immunoexpression. AKT1 E17K mutation was identified in one case (16.7%). Notably, SMARCB1 mutations were not identified in any of the chordoid meningiomas analyzed, including those showing INI1 loss immunohistochemically.

Conclusion

This is the first study to demonstrate loss of SMARCB1/INI1 immunoexpression in chordoid meningiomas, adding to the tumors with INI1 loss. However, in absence of INI1 mutation, mechanisms for INI1 loss require further evaluation. Identification of AKT1 mutation opens up new avenues for targeted therapy in patients with such aggressive tumors.

Double somatic SMARCB1 and NF2 mutations in sporadic spinal schwannoma

In sporadic schwannomas, inactivation of both copies of the NF2 tumor suppressor gene on 22q is common. Constitutional mutations of SMARCB1 are responsible of schwannomatosis, an inherited tumor predisposition syndrome, characterized by the development of multiple schwannomas. We analysed the frequency of copy number changes on chromosome 22 and the mutation of NF2 and SMARCB1 in 26 sporadic schwannomas. We found two spinal schwannomas with an identical somatic missense mutation in SMARCB1 exon 9: p.(Arg377His). Both SMARCB1 mutated schwannomas had LOH of 22q and one of them harbored an inactivating mutation of NF2. The p.(Arg377His) change was not found in a series of 28 vestibular schwannomas. Our data indicate that mutations affecting SMARCB1 play a role in the development or progression of a small subset of spinal schwannomas and that biallelic inactivation of SMARCB1 may cooperate with deficiency of NF2 function in schwannoma tumorigenesis according to the "four-hit/three events" mechanism of tumorigenesis that we demonstrated in schwannomatosis-associated schwannomas.

Comparative molecular genetic analysis of simian and human HIV-1 integrase interactor INI1/SMARCB1/SNF5

Human integrase interactor 1 (INI1/SMARCB1/SNF5) is a chromatin-remodeling molecule that binds to HIV-1 integrase and enhances proviral DNA integration. INI1 is also known as a tumor suppressor gene and has been found to be mutated in several aggressive tumors such as rhabdoid and lymphoid tumors. To study the function of simian INI1, we screened and cloned simian INI1 cDNA from B lymphoma cells of rhesus monkeys using RT-PCR. Sequence analysis showed 23 single nucleotide differences compared to the human ortholog, which, however, did not result in amino acid changes, and the amino acid sequence is therefore 100% conserved between human and simian INI1. Two alternatively spliced isoforms, INI1a and INI1b, were also found in simian INI1. These two isoforms did not show any functional difference in HIV-1 proviral DNA integration and nuclear localization, suggesting that the specificity of simian INI1 would not be a factor preventing HIV-1 infection of a simian host. Nevertheless, INI1b is expressed only in established cancer cell lines such as Jurkat and COS-7 cells, and not in primary cells, suggesting that INIlb could be an indicator of cell transformation.

Premature termination of SMARCB1 translation may be followed by reinitiation in schwannomatosis-associated schwannomas, but results in absence of SMARCB1 expression in rhabdoid tumors

In schwannomatosis, germline SMARCB1 mutations predispose to the development of multiple schwannomas, but not vestibular schwannomas. Many of these are missense or splice-site mutations or in-frame deletions, which are presumed to result in the synthesis of altered SMARCB1 proteins. However, also nonsense and frameshift mutations, which are characteristic for rhabdoid tumors and are predicted to result in the absence of SMARCB1 protein via nonsense-mediated mRNA decay, have been reported in schwannomatosis patients. We investigated the consequences of four of the latter mutations, i.e. c.30delC, c.34C>T, c.38delA, and c.46A>T, all in SMARCB1-exon 1. We could demonstrate for the c.30delC and c.34C>T mutations that the respective mRNAs were still present in the schwannomas of the patients. We hypothesized that these were prevented from degradation by translation reinitiation at the AUG codon encoding methionine at position 27 of the SMARCB1 protein. To test this, we expressed the mutations in MON cells, rhabdoid cells without endogenous SMARCB1 protein, and found that all four resulted in synthesis of the N-terminally truncated protein. Mutation of the reinitiation methionine codon into a valine codon prevented synthesis of the truncated protein, thereby confirming its identity. Immunohistochemistry with a SMARCB1 antibody revealed a mosaic staining pattern in schwannomas of the patients with the c.30delC and c.34C>T mutations. Our findings support the concept that, in contrast to the complete absence of SMARCB1 expression in rhabdoid tumors, altered SMARCB1 proteins with modified activity and reduced (mosaic) expression are formed in the schwannomas of schwannomatosis patients with a germline SMARCB1 mutation.

Update from the 2011 International Schwannomatosis Workshop: From genetics to diagnostic criteria

Schwannomatosis is the third major form of neurofibromatosis and is characterized by the development of multiple schwannomas in the absence of bilateral vestibular schwannomas. The 2011 Schwannomatosis Update was organized by the Children's Tumor Foundation (www.ctf.org) and held in Los Angeles, CA, from June 5-8, 2011. This article summarizes the highlights presented at the Conference and represents the "state-of-the-field" in 2011. Genetic studies indicate that constitutional mutations in the SMARCB1 tumor suppressor gene occur in 40-50% of familial cases and in 8-10% of sporadic cases of schwannomatosis. Tumorigenesis is thought to occur through a four-hit, three-step model, beginning with a germline mutation in SMARCB1 (hit 1), followed by loss of a portion of chromosome 22 that contains the second SMARCB1 allele and one NF2 allele (hits 2 and 3), followed by mutation of the remaining wild-type NF2 allele (hit 4). Insights from research on HIV and pediatric rhabdoid tumors have shed light on potential molecular pathways that are dysregulated in schwannomatosis-related schwannomas. Mouse models of schwannomatosis have been developed and promise to further expand our understanding of tumorigenesis and the tumor microenvironment. Clinical reports have described the occurrence of intracranial meningiomas in schwannomatosis patients and in families with germline SMARCB1 mutations. The authors propose updated diagnostic criteria to incorporate new clinical and genetic findings since 2005. In the next 5 years, the authors expect that advances in basic research in the pathogenesis of schwannomatosis will lead toward clinical investigations of potential drug therapies.

Establishment and characterization of MRT cell lines from genetically engineered mouse models and the influence of genetic background on their development

Malignant rhabdoid tumors (MRTs) are rare, aggressive cancers occuring in young children primarily through inactivation of the SNF5(INI1, SMARCB1) tumor suppressor gene. We and others have demonstrated that mice heterozygous for a Snf5 null allele develop MRTs with partial penetrance. We have also shown that Snf5(+/-) mice that lack expression of the pRb family, due to TgT121 transgene expression, develop MRTs with increased penetrance and decreased latency. Here, we report that altering the genetic background has substantial effects upon MRT development in Snf5(+/--) and TgT121 ;Snf5(+/-) mice, with a mixed F1 background resulting in increased latency and the appearance of brain tumors. We also report the establishment of the first mouse MRT cell lines that recapitulate many features of their human counterparts. Our studies provide further insight into the genetic influences on MRT development as well as provide valuable new cell culture and genetically engineered mouse models for the study of CNS-MRT etiology.

Alterations in the SMARCB1 (INI1) tumor suppressor gene in familial schwannomatosis

Schwannomatosis is a third major form of neurofibromatosis that has recently been linked to mutations in the SMARCB1 (hSnf5/INI1) tumor suppressor gene. We analyzed the coding region of SMARCB1 by direct sequencing and multiplex ligation-dependent probe amplification (MLPA) in genomic DNA from 19 schwannomatosis kindreds. Microsatellite markers in the SMARCB1 region were developed to determine loss of heterozygosity (LOH) in associated tumors. We detected four alterations in conserved splice acceptor or donor sequences of exons 3, 4 and 6. Two alterations that likely affect splicing were seen in introns 4 and 5. An additional four alterations of unclear pathogenicity were found to segregate on the affected allele in eight families including two non-conservative missense alterations in three families. No constitutional deletions or duplications were detected by MLPA. Nine of 13 tumors examined showed partial LOH of the SMARCB1 region consistent with 'second hits.' Alterations were detected in tumors both with and without somatic NF2 gene changes. These findings support the hypothesis that SMARCB1 is a tumor suppressor for schwannomas in the context of familial disease. Further work is needed to determine its role in other multiple and single tumor syndromes.

Cloning of a group of telomere-associated proteins

AIM: To elucidate the biological function and molecular mechanism of telomerase in immortal and/or tumor cells.

METHODS: Based on the principle of the yeast two-hybrid technology, the recombinant of human telomerase catalytic subunit bait fusion gene was constructed by DNA recombination technique, which was employed to screen cDNA libraries. The positive clones were confirmed by β-galactosidase colony-lift filter assay and furthermore the obtained plasmids cDNA sequences were compared with the isogenous sequences in GenBank.

RESULTS: The gene phenotypes of AH109 and Y187 yeast strains were stable and there was no leak expression of His. The recombinant of human telomerase catalytic subunit bait fusion gene, which was named as pGBKT7-hTERT, was constructed, and its bait fusion protein had no toxic effects on AH109 yeast cells and no activation effect on the autonomous reporter gene LacZ. Twenty-eight Ade+, Leu+, Trp+, and His+ clones were screened from the cDNA libraries and 12 Ade+, His+ and LacZ+ clones were obtained by the print analysis of β-galactosidase colony-lift filter assay. Of the 12 positive clones, 5 library plasmids possessed the effect of autonomously activating the reporter gene, so they were removed. There were 7 true positive clones among the library plasmids, 2 of which were found repeated after sequencing. The six obtained plasmids cDNA sequences were compared with the isogenous sequences in GenBank by Blast software via Internet. Finally, 6 recorded cDNA sequences were obtained, including T-STAR, PAWR, I-1, SMARCB1, LOXL3 and HKR3.

CONCLUSION: The 6 obtained telomere-associated proteins may help us understand the structure of telomerase holoenzyme and the biological function and molecular mechanism of telomerase in tumor and/or immortal cells, providing a theoretical basis for the tumor genesis, aging and prevention.

Brm transactivates the telomerase reverse transcriptase (TERT) gene and modulates the splicing patterns of its transcripts in concert with p54(nrb)

We report that a DBHS (Drosophila behaviour, human splicing) family protein, p54(nrb), binds both BRG1 (Brahma-related gene 1) and Brm (Brahma), catalytic subunits of the SWI/SNF (switch/sucrose non-fermentable) chromatin remodelling complex, and also another core subunit of this complex, BAF60a. The N-terminal region of p54(nrb) is sufficient to pull-down other core subunits of the SWI/SNF complex, suggesting that p54(nrb) binds SWI/SNF-like complexes. PSF (polypyrimidine tract-binding protein-associated splicing factor), another DBHS family protein known to directly bind p54(nrb), was also found to associate with the SWI/SNF-like complex. When sh (short hairpin) RNAs targeting Brm were retrovirally expressed in a BRG1-deficient human cell line (NCI-H1299), the resulting clones showed down-regulation of the TERT (telomerase reverse transcriptase) gene and an enhancement of ratios of exon-7-and-8-excluded TERT mRNA that encodes a beta-site-deleted inactive protein. All of these clones display growth arrest within 2 months of the Brm-knockdown. In NCI-H1299 cells, Brm, p54(nrb), PSF and RNA polymerase II phosphorylated on CTD (C-terminal domain) Ser(2) specifically co-localize at a region incorporating an alternative splicing acceptor site of TERT exon 7. These findings suggest that, at the TERT gene locus in human tumour cells containing a functional SWI/SNF complex, Brm, and possibly BRG1, in concert with p54(nrb), would initiate efficient transcription and could be involved in the subsequent splicing of TERT transcripts by accelerating exon-inclusion, which partly contributes to the maintenance of active telomerase.

Functional interaction of the retinoblastoma and Ini1/Snf5 tumor suppressors in cell growth and pituitary tumorigenesis

The Ini1 subunit of the SWI/SNF chromatin remodeling complex suppresses formation of malignant rhabdoid tumors in humans and mice. Transduction of Ini1 into Ini1-deficient tumor-derived cell lines has indicated that Ini1 arrests cell growth, controls chromosomal ploidy, and suppresses tumorigenesis by regulating components of the retinoblastoma (Rb) signaling pathway. Furthermore, conditional inactivation of Ini1 in mouse fibroblasts alters the expression of various Rb-E2F-regulated genes, indicating that endogenous Ini1 levels may control Rb signaling in cells. We have reported previously that loss of one allele of Ini1 in mouse fibroblasts results only in a 15% to 20% reduction in total Ini1 mRNA levels due to transcriptional compensation by the remaining Ini1 allele. Here, we examine the effects of Ini1 haploinsufficiency on cell growth and immortalization in mouse embryonic fibroblasts. In addition, we examine pituitary tumorigenesis in Rb-Ini1 compound heterozygous mice. Our results reveal that heterozygosity for Ini1 up-regulates cell growth and immortalization and that exogenous Ini1 down-regulates the growth of primary cells in a Rb-dependent manner. Furthermore, loss of Ini1 is redundant with loss of Rb function in the formation of pituitary tumors in Rb heterozygous mice and leads to the formation of large, atypical Rb(+/-) tumor cells lacking adrenocorticotropic hormone expression. These results confirm in vivo the relationship between Rb and Ini1 in tumor suppression and indicate that Ini1 plays a role in maintaining the morphologic and functional differentiation of corticotrophic cells.

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.

Germline mutation of INI1/SMARCB1 in familial schwannomatosis

Patients with schwannomatosis develop multiple schwannomas but no vestibular schwannomas diagnostic of neurofibromatosis type 2. We report an inactivating germline mutation in exon 1 of the tumor-suppressor gene INI1 in a father and daughter who both had schwannomatosis. Inactivation of the wild-type INI1 allele, by a second mutation in exon 5 or by clear loss, was found in two of four investigated schwannomas from these patients. All four schwannomas displayed complete loss of nuclear INI1 protein expression in part of the cells. Although the exact oncogenetic mechanism in these schwannomas remains to be elucidated, our findings suggest that INI1 is the predisposing gene in familial schwannomatosis.

LIM domains-containing protein 1 (LIMD1), a tumor suppressor encoded at chromosome 3p21.3, binds pRB and represses E2F-driven transcription

LIM domains-containing protein 1 (LIMD1) is encoded at chromosome 3p21.3, a region commonly deleted in many solid malignancies. However, the function of LIMD1 is unknown. Here we show that LIMD1 specifically interacts with retinoblastoma protein (pRB), inhibits E2F-mediated transcription, and suppresses the expression of the majority of genes with E2F1-responsive elements. LIMD1 blocks tumor growth in vitro and in vivo and is down-regulated in the majority of human lung cancer samples tested. Our data indicate that LIMD1 is a tumor-suppressor gene, the protein product of which functionally interacts with pRB and the loss of which promotes lung carcinogenesis.

Meningiomas: Updating Basic Science, Management, and Outcome

BACKGROUND

Meningiomas are biologically complex and clinically and surgically challenging. These features, combined with the rewarding potential for cure, make them of great interest to neurologists, neurosurgeons, and neuroscientists alike.

REVIEW SUMMARY

Initially, we review the clinical context of meningiomas, particularly recent changes in histopathological classification, diagnosis, and neuroimaging. Secondly, the underlying basic science as it has evolved over the last decades is summarized. The status of areas recently of intense interest, such as steroid hormone receptors and oncogenic viruses is described. Additionally, emerging areas of great promise, such as cytogenetics and molecular biology are presented. Lastly, we describe recent advances in management. In particular, skull-base surgery, image-guided surgery, and advances in radiotherapy are emphasized. The possible impact of basic research on management and outcome is also outlined.

CONCLUSIONS

Although usually benign and amenable to cure, meningiomas still present significant diagnostic and treatment challenges. Advances in basic science, surgery, and adjuvant therapy are widening the potential for safe, effective, evidence-based management leading to even better outcomes

Malignant rhabdoid meningioma arising in the setting of preexisting ganglioglioma: a diagnosis supported by fluorescence in situ hybridization. Case report

A highly malignant brain neoplasm with rhabdoid morphological features emerged in the bed of a subtotally resected ganglioglioma in a 54-year-old retired nuclear submarine officer. A combined application of neuroimaging, immunohistochemical studies, electron microscopy, and fluorescence in situ hybridization (FISH) was used to establish the morphological identity of the tumor. The rhabdoid appearance of the tumor cells indicated either an especially malignant variant of rhabdoid meningioma or an atypical teratoid/rhabdoid tumor with an unusually late onset. Whereas immunohistochemical studies and electron microscopy could only be used to narrow down the differential diagnosis, FISH revealed loss of one copy of NF2 with preservation of the INI1 region on 22q, thus establishing the identity of the tumor.

Human medulloblastomas lack point mutations and homozygous deletions of the hSNF5/INI1 tumour suppressor gene

Medulloblastomas (MBs) are malignant primitive neuroectodermal tumours (PNETs) of the cerebellum occurring predominantly in childhood. The association of monosomy of chromosome 22 with MB is controversial. Atypical teratoid/rhabdoid tumours (AT/RTs) of the brain share clinical and histological features with MBs and supratentorial PNETs (sPNETs). In particular, AT/RTs can be misdiagnosed as MBs and sPNETs because AT/RTs frequently contain areas of primitive neuroepithelial cells similar to PNETs. Recently, mutations of the tumour suppressor gene hSNF5/INI1, located on 22q11.23, have been described in AT/RTs, MBs and sPNETs, with conflicting data on the prevalence of hSNF5/INI1 mutations in the latter entities. Therefore, we screened MBs for point mutations and homozygous deletions of the hSNF5/INI1 tumour suppressor gene. In 90 MBs, no mutations of the hSNF5/INI1 gene were identified. Thus, our study virtually rules out hSNF5/INI1 as a tumour suppressor gene involved in the pathogenesis of medulloblastoma.

Disruption of Ini1 leads to peri-implantation lethality and tumorigenesis in mice

SNF5/INI1 is a component of the ATP-dependent chromatin remodeling enzyme family SWI/SNF. Germ line mutations of INI1 have been identified in children with brain and renal rhabdoid tumors, indicating that INI1 is a tumor suppressor. Here we report that disruption of Ini1 expression in mice results in early embryonic lethality. Ini1-null embryos die between 3.5 and 5.5 days postcoitum, and Ini1-null blastocysts fail to hatch, form the trophectoderm, or expand the inner cell mass when cultured in vitro. Furthermore, we report that approximately 15% of Ini1-heterozygous mice present with tumors, mostly undifferentiated or poorly differentiated sarcomas. Tumor formation is associated with a loss of heterozygocity at the Ini1 locus, characterizing Ini1 as a tumor suppressor in mice. Thus, Ini1 is essential for embryo viability and for repression of oncogenesis in the adult organism.