肿瘤异质性

肿瘤异质性（Tumor heterogeneity）是一个描述不同肿瘤细胞之间，表现不同型态与特性的生物现象，例如基因表达、代谢、运动、增殖与转移潜能的差异^[1]。这种现象存在于肿瘤之间以及同一肿瘤内。肿瘤内异质性的最低水平是DNA复制错误的结果：每当一个细胞（正常或癌细胞）进行细胞分裂时，都会产生一些突变^[2]，从而形成多样化的癌细胞子类群^[3]。肿瘤异质性的存在导致临床治疗受到阻碍。然而，针对肿瘤异质性的研究将可以更好地理解癌症的起源和进展，并提供更有效的治疗策略^[4]。

这项特性已经在多种癌症中被证实，包含白血病^[5]、乳癌^[6]、前列腺癌^[7][8][9]、结肠癌^[10][11][12]、脑瘤^[13]、食道癌^[14]、头颈癌^[15]、膀胱癌^[16]、妇科癌症^[17]、脂肪肉瘤^[18]与多发性骨髓瘤^[19]。

肿瘤异质性模型

目前，有两种模型用于解释肿瘤异质性的起源，分别为克隆演化模型以及癌症干细胞模型。这两个模型并不互斥，且被认为在不同肿瘤类型中产生不同程度的影响^[20]。

分支演化相较于线性演化被认为与肿瘤异质性更相关。

复制演化模型

复制演化模型于1976年由彼得·诺威尔提出。在此模型中，肿瘤最初源自于一个突变细胞，并在其增殖的过程中累积额外的突变，产生新的亚群，并且每个亚群都具有进一步分裂和突变的能力。这种异质性可能在肿瘤环境中产生相对于其他亚群更具有演化优势的亚群，而优势亚群可能随著时间的推移而成为肿瘤中主要的细胞群。该模型提出后，它使人们能够理解肿瘤生长、治疗失败以及在肿瘤形成的自然过程中发生的肿瘤侵袭。最初肿瘤细胞的演化可能透过两种模式发生：

线性演化

此模式认为突变连续且有序的突变在致癌基因、肿瘤抑制基因和DNA修复酶中累积，导致肿瘤细胞的复制与扩张。此模型无法反映恶性肿瘤的演化终点，因为肿瘤中突变的累积是随机的^[21]。

分支演化

此模式比线性演化更与肿瘤异质性相关，透过细胞分裂将突变扩展到多个癌细胞亚群^[22]。由于每一代的基因组不稳定性增加，基因组中突变的累积是随机发生的。长期的突变累积可能对肿瘤进展的特定阶段提供优势。肿瘤微环境也可能有助于肿瘤生长，因为它能够改变肿瘤细胞所面临的天择压力^[23]。

复制演化模型与癌症干细胞模型中，癌细胞形成肿瘤的能力。

癌症干细胞模型

癌症干细胞模型认为，肿瘤之中存在一小群具有致瘤性（tumorigenicity），这类细胞被称为癌症干细胞（cancer stem cells, CSCs），具有自我更新和分化为非致瘤子代的能力。CSC 模型认为，肿瘤细胞之间观察到的异质性是它们起源的干细胞差异的结果。干细胞变异通常是由表观遗传变化引起的，但也可能是由 CSC 群体的复制演化所引起的，其中有利的基因突变将可以在 CSC 及其后代中累积^[24]。

癌症干细胞模型的证据已经在多种癌症类型中被证实，包含白血病^[25][26]、胶质母细胞瘤^[27]、乳腺癌^[28]和前列腺癌^[29]。然而，CSC的存在仍存在争议。其中一个原因是CSC的生物标记难以在多种肿瘤中重现。此外，确定致瘤潜力的方法是透过异种移植模型进行。这些方法存在固有的局限性，例如需要抑制移植动物的免疫反应，以及从原发性肿瘤部位到异种移植部位的环境条件存在显著差异（例如缺乏所需的生长因子或讯号）^[30]。

肿瘤异质性的类型与原因

在肿瘤细胞之间观察到多种类型的异质性，这些异质性源自于遗传和非遗传变异^[31]。

遗传异质性

遗传异质性是肿瘤基因组的共同特征，并且可能由多种因素导致。有些癌症是在外源性因素引入突变时引发的，例如紫外线辐射（皮肤癌）和烟草（肺癌）。一个更常见的来源是基因组不稳定，当细胞中的关键调控路径（例如DNA修复机制受损）被破坏时，经常会导致基因组不稳定，这可能造成复制错误增加，以及有丝分裂机制的缺陷，导致整个染色体大规模增加或缺失^[32]。此外，一些癌症治疗（例如替莫唑胺或其他化疗药物）可能会进一步增加遗传变异^[33][34]。

突变的肿瘤异质性是指不同基因和样本中突变频率的变化，可以透过MutSig进行探究^[35]。此外，来自相同或不同癌症类型的肿瘤样本之间的突变过程的病因可能有很大差异，并且可以表现在不同的背景依赖性突变谱中，这能够透过COSMIC Mutational Signatures^[36]或MutaGene^[37]进行分析。

其他类型的异质性

肿瘤细胞的基因表达谱之间也存在异质性，这经常是由潜在的表观遗传变化引起的^[38]。在同一个体肿瘤样本的不同区域中可以检测到表达特征的变化。研究发现，影响H3K36甲基转移酶SETD2和组蛋白H3K4去甲基化酶KDM5C的趋同突变出现在空间上分离的肿瘤切片。相似地，MTOR（一种编码细胞调节激酶的基因）表现出组成型活性（constitutive activity），从而增加S6磷酸化，这种磷酸化可以作为卵巢透明细胞癌的生物标记^[39]。

机械化学异质性是生物体内真核细胞的一个特征。它可以影响表观遗传基因的调控。异质性的动态机械化学过程通过黏附的方式调节细胞表面分子群体的相互关系^[40]。肿瘤的发展和扩散伴随著群体细胞内的机械化学相互作用过程的变化，并且在癌症患者当中是分层的^[41]。机械化学异质性的生物现象可能用于与胃黏膜炎症患者的不同胃癌诊断^[42]，以及使用机械异质化的肿瘤细胞微粒进行装载时，增加基于疫苗的树突细胞的抗转移活性增强^[43]。

肿瘤微环境异质性

由于肿瘤微环境的异质性，肿瘤细胞之间的异质性会进一步增加。肿瘤内的区域差异（例如氧气的可用性）对肿瘤细胞施加不同的天择压力，导致肿瘤的不同空间区域产生更广泛的优势亚群。微环境对克隆优势的影响也是许多患者中原发性肿瘤和转移性肿瘤之间异质性以及相同肿瘤类型患者之间观察到的肿瘤间异质性的可能原因^[44]。

影响与挑战

治疗抗性

异质性肿瘤接受化疗后，很少能够杀死所有肿瘤细胞。多数的癌细胞亚群不具备抗药性而死亡，这使得抗药性肿瘤细胞亚群能够透过分支演化机制复制并生长出新的肿瘤。由此产生的再增殖肿瘤是异质的并且对所使用的药物治疗具有抗性，重新生长的肿瘤也可能更具侵袭性^[1]。

化疗会导致肿瘤细胞遭遇瓶颈效应，多数的细胞亚群会死亡，而抗药性亚群存活并继续增生。

生物标记的发现

由于肿瘤内部和肿瘤间的遗传差异，可用于预测治疗效果或预后的生物标记可能无法广泛应用。然而，有研究提出异质性的高低本身就可以用作生物标记^[45]，因为异质性越高的肿瘤可能更可能包含抗药性的细胞亚群，而其他可解释异质性的生物标记仍在研究中。

研究模型的缺乏与局限

目前的模型系统通常缺乏人类癌症中所见的异质性^[46]。为了准确地研究肿瘤异质性，必须开发更准确的临床前模型。其中一种模型是患者来源的肿瘤异种移植物（patient-derived xenograft，PDX），在保留肿瘤异质性方面显示出极佳的实用性，同时能够详细研究复制适应性的驱动因素^[47]。然而，即使这个模型也无法重现癌症的全部复杂性。

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