Deposition of replication-independent histone variant H3.3 into chromatin is essential for many biological processes, including development and reproduction. Unlike replication-dependent H3.1/2 isoforms, H3.3 is expressed throughout the cell cycle and becomes enriched in postmitotic cells with age. However, lifelong dynamics of H3 variant replacement and the impact of this process on chromatin organization remain largely undefined. Using quantitative middle-down proteomics we demonstrate that H3.3 accumulates to near saturation levels in the chromatin of various mouse somatic tissues by late adulthood. Accumulation of H3.3 is associated with profound changes in global levels of both individual and combinatorial H3 methyl modifications. A subset of these modifications exhibit distinct relative abundances on H3 variants and remain stably enriched on H3.3 throughout the lifespan, suggesting a causal relationship between H3 variant replacement and age-dependent changes in H3 methylation. Furthermore, the H3.3 level is drastically reduced in human hepatocarcinoma cells as compared to nontumoral hepatocytes, suggesting the potential utility of the H3.3 relative abundance as a biomarker of abnormal cell proliferation activity. Overall, our study provides the first quantitative characterization of dynamic changes in H3 proteoforms throughout lifespan in mammals and suggests a role for H3 variant replacement in modulating H3 methylation landscape with age.

中文翻译：

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随着年龄的增长，组蛋白变体H3.3的积累与组蛋白甲基化格局的深刻变化有关

将不依赖复制的组蛋白变体H3.3沉积到染色质中对于许多生物学过程（包括发育和繁殖）都是必不可少的。与复制依赖性H3.1 / 2同工型不同，H3.3在整个细胞周期中表达，并随着年龄的增长而富集在有丝分裂后细胞中。但是，H3变异替代品的终生动力学以及该过程对染色质组织的影响仍然不确定。使用定量的中下蛋白质组学，我们证明到成年后期，H3.3在各种小鼠体细胞组织的染色质中积累至接近饱和水平。H3.3的积累与单个和组合的H3甲基修饰的整体水平的深刻变化有关。这些修饰的子集在H3变体上表现出不同的相对丰度，并且在整个生命周期中都稳定地富集在H3.3上，表明H3变体替换与H3甲基化的年龄依赖性变化之间存在因果关系。此外，与非肿瘤肝细胞相比，人肝癌细胞中的H3.3水平大大降低，表明H3.3相对丰度作为异常细胞增殖活性的生物标志物的潜在用途。总的来说，我们的研究提供了哺乳动物整个生命中H3蛋白形式动态变化的第一个定量表征，并提出了H3变体替换在随着年龄的增长而调节H3甲基化景观中的作用。提示H3变体替换与H3甲基化的年龄依赖性变化之间存在因果关系。此外，与非肿瘤肝细胞相比，人肝癌细胞中的H3.3水平大大降低，表明H3.3相对丰度作为异常细胞增殖活性的生物标志物的潜在用途。总体而言，我们的研究提供了哺乳动物整个生命中H3蛋白形式动态变化的第一个定量表征，并提出了H3变体替代在随年龄调节H3甲基化景观中的作用。提示H3变体替换与H3甲基化的年龄依赖性变化之间存在因果关系。此外，与非肿瘤肝细胞相比，人肝癌细胞中的H3.3水平大大降低，表明H3.3相对丰度作为异常细胞增殖活性的生物标志物的潜在用途。总体而言，我们的研究提供了哺乳动物整个生命中H3蛋白形式动态变化的第一个定量表征，并提出了H3变体替代在随年龄调节H3甲基化景观中的作用。