Novel role of SAMHD1 as a tumor suppressor in cutaneous T-cell lymphomas

描述(由申请人提供)：表观遗传机制通过调节基因表达，在癌症的发生和发展中发挥重要作用。皮肤T细胞淋巴瘤(CTCL)的定义是活化的和恶性的CD4+T细胞在皮肤中的渗透。CTCL中肿瘤CD4+T细胞增殖的分子机制尚不完全清楚，但涉及多种肿瘤抑制基因的表观遗传沉默。不育α基序(SAM)和含HD结构域的蛋白1(SAMHD1)是第一个被发现的哺乳动物三磷酸水解酶，它能降解细胞内的脱氧核苷三磷酸(DNTPs)。通过耗尽逆转录病毒复制所需的细胞内dNTP，SAMHD1阻止了静止的CD4+T细胞和髓样细胞中的逆转录病毒感染。这些细胞表达高水平的SAMHD1蛋白，细胞内dNTP水平比激活的CD4+T细胞低6-100倍，而一些白血病和淋巴瘤CD4+T细胞株缺乏SAMHD1蛋白，dNTP水平增加。本项目的目的是更好地了解SAMHD1在CTCL中的表观遗传沉默的机制和致病后果，这可能有助于开发治疗CTCL的新的治疗方法。一个关键的问题是，CTCL患者的CD4+T细胞是否下调或缺乏SAMHD1的表达，以维持细胞内高水平的dNTPs，并支持有效的DNA复制和不受控制的细胞生长。我们的初步结果表明，人SAMHD1可能在CTCL中发挥肿瘤抑制作用。有趣的是，我们观察到9名CTCL患者外周血单个核细胞SAMHD1mRNA水平比14名健康献血者低3倍(p=0.0013)，这表明CTCL患者SAMHD1mRNA表达下调。我们还发现，SAMHD1基因的表观遗传修饰对于抑制SAMHD1的表达至关重要，这可能反过来调节恶性CD4+T细胞的DNA复制和细胞生长。因此，我们假设SAMHD1作为一种肿瘤抑制因子发挥作用，在恶性CD4+T细胞中SAMHD1表达的表观遗传沉默导致细胞内dNTPs增加，并导致CTCL中T细胞的不受控制的生长。为了验证这一假设，我们提出了两个特定的目标：目的1.确定50例CTCL患者CD4+T细胞中SAMHD1的表达与dNTP水平之间的相关性；以及目的2.研究SAMHD1在恶性CD4+T细胞中的表观遗传沉默机制，并用CTCL小鼠模型和SAMHD1基因敲除小鼠模型验证结果。实现我们的目标将为调节恶性CD4+T细胞中dNTP代谢的新机制提供关键知识，这将有助于开发更有效的抗CTCL和其他T细胞淋巴瘤的药物。SAMHD1蛋白表达缺失机制的发现将为进一步诱导SAMHD1基因在CTCL中的表达作为一种新的治疗手段提供支持。SAMHD1的S作为一种新的肿瘤抑制因子，可能在其他类型的癌症中也很重要，并具有超越CTCL的更广泛的影响。

DESCRIPTION (provided by applicant): Epigenetic mechanisms play an important role in cancer initiation and progression through modulation of gene expression. Cutaneous T cell lymphoma (CTCL) is defined by infiltration of activated and malignant CD4+ T- cells in the skin. The molecular mechanisms underlying the proliferation of neoplastic CD4+ T-cells in CTCL are not fully understood, but involve epigenetic silencing of multiple tumor suppressor genes. Sterile alpha motif (SAM) and HD domain-containing protein 1 (SAMHD1) is the first identified mammalian triphosphohydrolase that hydrolyzes cellular deoxynucleoside triphosphates (dNTPs). By depleting intracellular dNTPs required for retroviral replication, SAMHD1 blocks retroviral infection in resting CD4+ T-cells and myeloid cells. These cells express high levels of SAMHD1 protein and have 6- to 100-fold lower levels of intracellular dNTPs compared to activated CD4+ T-cells, while several leukemia and lymphoma CD4+ T-cell lines lack SAMHD1 protein expression and have increased dNTP levels. The goal of this project is to better understand the mechanisms and pathogenic consequences of epigenetic silencing of SAMHD1 in CTCL, which may aid in developing novel therapeutic approaches to treat CTCL. A key question is whether CD4+ T-cells from CTCL patients down- regulate or lack SAMHD1 expression to maintain high levels of intracellular dNTPs and support efficient DNA replication and uncontrolled cell growth. Our preliminary results suggest that human SAMHD1 may function as a tumor suppressor in CTCL. Interestingly, we observed 3-fold lower SAMHD1 mRNA levels in peripheral blood mononuclear cells from 9 CTCL patients compared to 14 healthy donors (p=0.0013), indicating that SAMHD1 expression is down-regulated in CTCL patients. We also found that epigenetic modification of the SAMHD1 gene is critical for silencing of SAMHD1 expression, which may in turn regulate DNA replication and cell growth of malignant CD4+ T-cells. Thus, we hypothesize that SAMHD1 functions as a tumor suppressor, and that epigenetic silencing of SAMHD1 expression in malignant CD4+ T-cells leads to increased intracellular dNTPs and contributes to uncontrolled T-cell growth in CTCL. To test this hypothesis, we propose two specific aims: Aim 1. To determine the correlation between SAMHD1 expression and dNTP levels in CD4+ T-cells from 50 patients with CTCL; and Aim 2. To investigate the mechanisms of epigenetic silencing of SAMHD1 in malignant CD4+ T-cells and to validate the results using a CTCL mouse model and a SAMHD1 knockout mouse model. Achieving our goals will provide critical knowledge into a novel mechanism that regulates dNTP metabolism in malignant CD4+ T-cells, which will facilitate the development of more effective drugs against CTCL and other T-cell lymphomas. Discovery of the mechanisms leading to loss of SAMHD1 protein expression could support the development of strategies to re-induce SAMHD1 gene expression as a new therapeutic modality in CTCL. SAMHD1's function as a novel tumor suppressor may also be important in other types of cancer and have a broader impact beyond CTCL.