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

描述（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）的定义是激活和恶性CD4+ T细胞浸润在皮肤。肿瘤CD4+ t细胞在CTCL中增殖的分子机制尚不完全清楚，但涉及多个肿瘤抑制基因的表观遗传沉默。无菌α基序（SAM）和含HD结构域蛋白1 （SAMHD1）是第一个被发现的哺乳动物三磷酸水解酶，可水解细胞脱氧核苷三磷酸（dNTPs）。通过消耗逆转录病毒复制所需的细胞内dNTPs， SAMHD1阻断静止的CD4+ t细胞和髓细胞中的逆转录病毒感染。这些细胞表达高水平的SAMHD1蛋白，与活化的CD4+ t细胞相比，细胞内dNTPs水平低6- 100倍，而一些白血病和淋巴瘤CD4+ t细胞系缺乏SAMHD1蛋白表达，dNTP水平升高。该项目的目标是更好地了解SAMHD1在CTCL中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的新治疗方法。一个关键问题是CTCL患者的CD4+ t细胞是否下调或缺乏SAMHD1表达以维持高水平的细胞内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，人类SAMHD1可能在CTCL中起肿瘤抑制作用。有趣的是，我们观察到9名CTCL患者外周血单个核细胞中SAMHD1 mRNA水平比14名健康供者低3倍（p=0.0013），表明SAMHD1表达在CTCL患者中下调。我们还发现SAMHD1基因的表观遗传修饰对于SAMHD1表达的沉默至关重要，这可能反过来调节DNA复制和恶性CD4+ t细胞的细胞生长。因此，我们假设SAMHD1作为肿瘤抑制因子，并且SAMHD1在恶性CD4+ t细胞中表达的表观遗传沉默导致细胞内dNTPs增加，并有助于CTCL中t细胞生长不受控制。为了验证这一假设，我们提出了两个具体目标：探讨50例CTCL患者CD4+ t细胞中SAMHD1表达与dNTP水平的相关性；和Aim 2。研究SAMHD1在恶性CD4+ t细胞中表观遗传沉默的机制，并通过CTCL小鼠模型和SAMHD1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治疗CTCL和其他t细胞淋巴瘤的药物。发现导致SAMHD1蛋白表达缺失的机制可以支持重新诱导SAMHD1基因表达的策略的发展，作为CTCL的一种新的治疗方式。SAMHD1作为一种新型肿瘤抑制因子的功能可能在其他类型的癌症中也很重要，并且具有比CTCL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.