Role of hsnf5/BAF47 Loss in Human Cancer Development

人类肿瘤抑制基因的发现使人们对人类癌症的发生机制有了新的认识。第一批肿瘤抑制基因的分离源于对儿童恶性肿瘤的研究，包括Rb和WT1基因。在横纹肌样瘤中，22号染色体上频繁的LOH导致了一个新的肿瘤抑制基因的发现，命名为INI1/hSNF5/BAF47。该基因编码酵母SNF5基因的人类同源基因，该基因是SWI/SNF染色质重塑复合体的成员。SWI/SNF复合体作为一种全球转录激活剂，以能量依赖的方式改变核小体在DNA上的位置。在过去的几年中，染色质重塑改变在肿瘤进展中的作用得到了越来越多的认识。最近的报道强烈支持INI1/hSNF5/BAF47作为典型的肿瘤抑制基因的观点。其中包括证明横纹肌样肿瘤、脉络丛肿瘤和横纹肌肉瘤中发生突变和缺失，杂合性缺失导致剩余的野生型等位基因的移除，携带生殖系突变的家族高频率地发生这些肿瘤，以及在小鼠中的胚系失活导致横纹肌样肿瘤的发展。我们发现，SNF5在横纹肌样肿瘤细胞系中的重新表达会导致生长抑制，同时伴随着p16INK4A蛋白水平的急剧上升。基于这些初步研究以及SWI/SNF复合体的已知功能和其他相关科学文献，我们假设hSWI/SNF复合体INI1/SNF5成分的改变通过阻断p16INK4A的诱导并扰乱正常的细胞周期控制而参与人类肿瘤的发生。在这个应用中，我们将通过生化、生物学和动物模型分析来确定该基因活性丧失影响p16INK4A蛋白水平的机制来检验这一假说。在特定的目标1中，我们将确定INI1/SNF5调控的细胞周期调控通路以及这些活动的相关结构域。在特定的目标2中，我们将确定INI1/SNF5缺失对SWI/SNF功能的生化影响，以及该蛋白是否与p16INK4A启动子直接相互作用。在第三个具体目标中，我们将通过将Tag转基因小鼠与SNF5/-小鼠杂交来建立脉络丛癌的小鼠模型。研究INI1/SNF5基因在基因表达调控中的作用将拓宽我们对抑癌基因功能的理解，为染色质重塑复合体在正常和肿瘤发生中的作用以及对这些毁灭性的儿科癌症的治疗和检测的影响提供重要线索。

The identification of human tumor suppressor genes has led to new insights into the mechanisms of human cancer development. Isolation of the first tumor suppressor genes resulted from studies of pediatric malignancies including the RB and WT1 genes. In the case of rhabdoid tumors, frequent LOH on chromosome 22 has led to the discovery of a novel tumor suppressor gene designated INI1/hSNF5/BAF47. This gene codes for the human homolog of the yeast SNF5 gene, a member of the SWI/SNF chromatin remodeling complex. The SWI/SNF complex acts as a global transcriptional activator that alters nucleosome positioning on DNA in an energy-dependent manner. The role of altered chromatin remodeling during neoplastic progression has gained increasing recognition over the last several years. Recent reports strongly support the notion that INI1/hSNF5/BAF47 acts as a prototypical tumor suppressor gene. These include demonstrations that mutations and deletions occur in rhabdoid tumors, choroid plexus tumors and rhabdomyosarcomas, that LOH drives the removal of the remaining wild-type allele, that families carrying germline mutations develop these tumors at a high frequency and that germline inactivation in mice leads to the development of rhabdoid-like tumors. We have found that re- expression of SNF5 in rhabdoid tumor cell lines causes growth inhibition accompanied by a dramatic rise in p16INK4A protein levels. Based on these preliminary studies as well as the known functions of the SWI/SNF complex and other relevant scientific literature, we hypothesize that alterations in the INI1/SNF5 component of the hSWI/SNF complex contribute to human tumor development by blocking the induction of p16INK4A and disrupting normal cell cycle control. In this application, we will test this hypothesis by determining the mechanism by which loss of activity of this gene affects p16INK4A protein levels using biochemical, biological and animal model assays. In Specific Aim number 1, we will determine the cell cycle control pathways regulated by INI1/SNF5 and the relevant domains for these activities. In Specific Aim number 2, we will ascertain the biochemical effects of INI1/SNF5 loss on SWI/SNF function and whether the protein directly interacts with the p16INK4A promoter. In Specific Aim number 3, we will develop a mouse model for choroid plexus carcinomas by crossing TAg transgenic mice to SNF5+/- mice. The characterization of the INI1/SNF5 gene role in regulation of gene expression will broaden our understanding of tumor suppressor gene functions, provide important clues about the role of chromatin remodeling complexes in normal and neoplastic development and impact upon treatment and detection of these devastating pediatric cancers.