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发生和进展中起重要作用。皮肤T细胞淋巴瘤（CTCL）定义为皮肤中活化和恶性CD 4 + T细胞的浸润。CTCL中肿瘤性CD 4 + T细胞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结构域的蛋白1（SAMHD 1）是第一个被鉴定的哺乳动物三磷酸水解酶，其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dNTP，SAMHD 1阻断了静息CD 4 + T细胞和骨髓细胞中的逆转录病毒感染。这些细胞表达高水平的SAMHD 1蛋白，并且与活化的CD 4 + T细胞相比具有低6至100倍的细胞内dNTP水平，而几种白血病和淋巴瘤CD 4 + T细胞系缺乏SAMHD 1蛋白表达并且具有增加的dNTP水平。该项目的目标是更好地了解CTCL中SAMHD 1表观遗传沉默的机制和致病后果，这可能有助于开发新的治疗方法来治疗CTCL。关键问题是来自CTCL患者的CD 4 + T细胞是否下调或缺乏SAMHD 1表达以维持高水平的细胞内dNTP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SAMHD 1可能作为一个肿瘤抑制CTCL。有趣的是，我们观察到与14名健康供体相比，9名CTCL患者的外周血单核细胞中SAMHD 1 mRNA水平低3倍（p=0.0013），表明CTCL患者中SAMHD 1表达下调。我们还发现，SAMHD 1基因的表观遗传修饰对于SAMHD 1表达的沉默至关重要，这反过来可能调节恶性CD 4 + T细胞的DNA复制和细胞生长。因此，我们假设SAMHD 1作为肿瘤抑制因子发挥作用，并且恶性CD 4 + T细胞中SAMHD 1表达的表观遗传沉默导致细胞内dNTP增加，并有助于CTCL中不受控制的T细胞生长。为了验证这一假设，我们提出了两个具体目标：目标1。确定来自50名CTCL患者的CD 4 + T细胞中SAMHD 1表达和dNTP水平之间的相关性;目的2.研究SAMHD 1在恶性CD 4 + T细胞中表观遗传沉默的机制，并使用CTCL小鼠模型和SAMHD 1敲除小鼠模型验证结果。实现我们的目标将为调节恶性CD 4 + T细胞中dNTP代谢的新机制提供关键知识，这将有助于开发针对CTCL和其他T细胞淋巴瘤的更有效药物。发现导致SAMHD 1蛋白表达丧失的机制可以支持重新诱导SAMHD 1基因表达作为CTCL的新治疗方式的策略的发展。SAMHD 1作为一种新型肿瘤抑制因子的功能在其他类型的癌症中也可能很重要，并且具有超越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.