Mitochondrial-Associated Cell Death Mechanisms Are Reset to an Embryonic-Like State in Aged Donor-Derived iPS Cells Harboring Chromosomal Aberrations

Somatic cells reprogrammed into induced pluripotent stem cells (iPSCs) acquire features of human embryonic stem cells (hESCs) and thus represent a promising source for cellular therapy of debilitating diseases, such as age-related disorders. However, reprogrammed cell lines have been found to harbor various genomic alterations. In addition, we recently discovered that the mitochondrial DNA of human fibroblasts also undergoes random mutational events upon reprogramming. Aged somatic cells might possess high susceptibility to nuclear and mitochondrial genome instability. Hence, concerns over the oncogenic potential of reprogrammed cells due to the lack of genomic integrity may hinder the applicability of iPSC-based therapies for age-associated conditions. Here, we investigated whether aged reprogrammed cells harboring chromosomal abnormalities show resistance to apoptotic cell death or mitochondrial-associated oxidative stress, both hallmarks of cancer transformation. Four iPSC lines were generated from dermal fibroblasts derived from an 84-year-old woman, representing the oldest human donor so far reprogrammed to pluripotency. Despite the presence of karyotype aberrations, all aged-iPSCs were able to differentiate into neurons, re-establish telomerase activity, and reconfigure mitochondrial ultra-structure and functionality to a hESC-like state. Importantly, aged-iPSCs exhibited high sensitivity to drug-induced apoptosis and low levels of oxidative stress and DNA damage, in a similar fashion as iPSCs derived from young donors and hESCs. Thus, the occurrence of chromosomal abnormalities within aged reprogrammed cells might not be sufficient to over-ride the cellular surveillance machinery and induce malignant transformation through the alteration of mitochondrial-associated cell death. Taken together, we unveiled that cellular reprogramming is capable of reversing aging-related features in somatic cells from a very old subject, despite the presence of genomic alterations. Nevertheless, we believe it will be essential to develop reprogramming protocols capable of safeguarding the integrity of the genome of aged somatic cells, before employing iPSC-based therapy for age-associated disorders.将体细胞（somatic cells）重编程为诱导多能干细胞（induced pluripotent stem cells, iPSCs）后，这类细胞会获得人类胚胎干细胞（human embryonic stem cells, hESCs）的特征，因此成为治疗衰弱性疾病（如年龄相关性疾病）的极具潜力的细胞治疗来源。然而，已有研究证实重编程细胞系存在多种基因组改变。此外，我们近期发现，人类成纤维细胞的线粒体DNA（mitochondrial DNA）在重编程过程中同样会发生随机突变事件。衰老体细胞可能对细胞核与线粒体基因组的不稳定性具有较高易感性。因此，因重编程细胞缺乏基因组完整性而产生的致瘤潜能相关顾虑，可能会阻碍基于iPSCs的疗法在年龄相关性疾病中的应用。本研究旨在探究携带染色体异常的衰老重编程细胞是否会表现出对细胞凋亡（apoptotic cell death）或线粒体相关氧化应激（mitochondrial-associated oxidative stress）的抵抗性——二者均为癌症转化的标志性特征。我们从一名84岁女性的皮肤成纤维细胞中构建了4株iPSC系，该供体是目前已报道的年龄最大的可被重编程至多能状态的人类受试者。尽管存在核型异常（karyotype aberrations），所有衰老来源iPSCs均能够分化为神经元，重建端粒酶活性（telomerase activity），并将线粒体超微结构与功能重塑至类似hESCs的状态。重要的是，衰老来源iPSCs表现出对药物诱导凋亡的高敏感性，以及低水平的氧化应激与DNA损伤，其特征与年轻供体来源的iPSCs及hESCs高度相似。因此，衰老重编程细胞中出现的染色体异常或许不足以突破细胞监视机制，也无法通过改变线粒体相关细胞死亡通路诱发恶性转化（malignant transformation）。综上，本研究揭示：即便存在基因组改变，体细胞重编程仍能够逆转极高龄供体体细胞的衰老相关特征。尽管如此，我们认为在将基于iPSCs的疗法应用于年龄相关性疾病之前，开发能够保障衰老体细胞基因组完整性的重编程方案仍是十分必要的。