The role of SMARCAL1 in glioma telomere maintenance.

项目摘要 神经胶质瘤是成人中最常见的原发性恶性脑肿瘤，导致超过14，000例死亡 每年在美国。胶质母细胞瘤（GBM，IV级胶质瘤）是最常见和致命的胶质瘤 亚型，并且与侵袭性治疗的中位总生存期仅约15个月相关。端粒 维持机制是癌症的一个标志，并且是使细胞能够复制永生所必需的。 恶性细胞，包括神经胶质瘤。为了维持端粒，大多数神经胶质瘤使用端粒酶， 它使用RNA模板和逆转录来延伸染色体末端的端粒重复序列。 相反，胶质瘤的一个子集采用端粒酶非依赖性机制，称为替代性Lengths 端粒（ALT）ALT利用同源重组来维持端粒长度，其特征在于： 端粒区域复制应激增加以及端粒染色质结构重塑 允许同源重组的表观遗传状态。约10%的GBM病例和几乎所有 进行性II/III级IDH突变型星形细胞瘤具有参与ALT诱导的遗传改变。在 除了ATRX突变外，我们最近还发现SMARCAL 1中的复发性功能丧失突变， (SWI/SNF相关的、基质相关的、肌动蛋白依赖的染色质调节子，亚家族A样1）基因作为新的 与IDH-野生型- TERT启动子野生型GBM中ALT表型相关的遗传突变。 SMARCAL 1编码退火解旋酶，其定位于DNA损伤和复制应激位点， 解决了停滞的复制叉结构，以促进难以复制的DNA中的叉进展 序列，如端粒。最近的研究表明SMARCAL 1定位于ALT-1中的端粒DNA。 具有天然ATRX失活突变且SMARCAL 1解旋酶活性抑制的阳性癌细胞 复制压力和端粒DNA损伤的标志物。基于这些观察，我们假设， SMARCAL 1活性对于解决端粒区域的复制应激和SMARCAL 1缺失是必不可少的。 功能导致未解决的复制叉崩溃和端粒DNA损伤的增加， 允许同源重组和ALT。在目标1中，我们将使用基因工程细胞系 和具有天然SMARCAL 1突变的癌细胞系，以鉴定所需的特定ALT途径组分 在SMARCAL 1功能丧失突变的背景下，用于端粒合成和细胞永生化。在 目的2，我们将使用患者来源的胶质瘤细胞系和异种移植物来确定SMARCAL 1在多大程度上被激活。 解旋酶活性使用ALT解决ATRX突变GBM细胞系中端粒处的复制应激。总的来说， 这些研究有望大大提高我们对SMARCAL 1在GBM中作用的认识 端粒的维持，旨在建立SMARCAL 1治疗靶向的概念证明， 携带ATRX突变的ALT阳性胶质瘤。

PROJECT SUMMARY Gliomas are the most common primary malignant brain tumors in adults and account for over 14,000 deaths annually in the United States. Glioblastoma (GBM, grade IV glioma) is the most common and deadly glioma subtype and is associated with a median overall survival of only ~15 months with aggressive therapy. Telomere maintenance mechanisms are a hallmark of cancer and are required to enable replicative immortality of malignant cells, including gliomas. To maintain telomeres, the majority of gliomas use the enzyme telomerase, which uses an RNA template and reverse transcription to extend telomeric repeats at the ends of chromosomes. In contrast, a subset of gliomas employs telomerase-independent mechanisms, termed Alternative Lengthening of Telomeres (ALT). ALT uses homologous recombination to maintain telomere length and is characterized by increased replicative stress in telomere regions and remodeling of telomeric chromatin architecture to an epigenetic state that is permissive to homologous recombination. ~10% of GBM cases and virtually all progressive grade II/III IDH-mutant astrocytomas harbor genetic alterations involved in the induction of ALT. In addition to ATRX mutations, we recently identified recurrent loss-of-function mutations in the SMARCAL1 (SWI/SNF-related, matrix-associated, actin-dependent regulator of chromatin, subfamily A-like1) gene as novel genetic mutations associated with the ALT phenotype in IDH-wildtype – TERT promoter wild-type GBM. SMARCAL1 encodes an annealing helicase that localizes to sites of DNA damage and replication stress and resolves stalled replication fork structures to facilitate fork progression within difficult-to-replicate DNA sequences, such as telomeres. Recent studies indicate that SMARCAL1 localizes to telomeric DNA in ALT- positive cancer cells with native ATRX-inactivating mutations and that SMARCAL1 helicase activity suppresses markers of replication stress and DNA damage at telomeres. Based on these observations, we hypothesize that SMARCAL1 activity is essential for resolving replication stress at telomeric regions and SMARCAL1 loss-of- function leads to an increase in unresolved replication fork collapse and DNA damage at telomeres in a manner that is permissive to homologous recombination and ALT. In Aim 1 we will use genetically engineered cell lines and cancer cell lines with native SMARCAL1 mutations to identify specific ALT pathway components required for telomere synthesis and cellular immortalization in the context of SMARCAL1 loss-of-function mutations. In Aim 2, we will use patient-derived glioma cell lines and xenografts to determine the extent to which SMARCAL1 helicase activity resolves replication stress at telomeres in ATRX-mutant GBM cell lines using ALT. Collectively, the proposed studies are expected to significantly advance our understanding of the role of SMARCAL1 in GBM telomere maintenance and are designed to establish proof of concept for therapeutic targeting of SMARCAL1 in ALT-positive gliomas harboring ATRX mutations.