Dynamics of somatic mosaicism in the murine hematopoietic system

现在已经在许多表型正常的组织中发现了携带被称为癌症驱动因子的体细胞突变的细胞克隆，并且随着年龄的增长而积累。造血已经成为这种体细胞嵌合的范例;在健康老年人中经常发现具有白血病驱动突变的大造血干细胞（HSC）克隆。这些观察结果提出了一个问题，即哪些细胞机制对抗白血病发生，以及是否有早期预警信号表明这些保护机制正在崩溃。我们和其他人最近的观察提供了一个线索，即小鼠中的HSC显示出显着的活动模式：当静止数月时，HSC很少但可靠地变得活跃以产生多能祖细胞（MPP）。MPP在体内表现出广泛的自我更新，即使在HSC耗尽时也能长期维持血细胞的产生。因此，我们假设，罕见的HSC输出功能，以防止过度积累的突变更迅速地分裂的祖细胞。为了测试这一点，我们将利用小鼠模型：（i）允许选择性耗尽HSC，（ii）通过Tet 2功能丧失遗传干扰表观遗传调控，减少HSC输出并增加自我更新，以及（iii）在整个基因组中整体增加突变率。我们将量化单个HSC和祖细胞以及散装细胞中体细胞突变的积累，分别产生关于单个细胞中突变计数和分选群体中体细胞变体频率分布的补充信息。利用群体遗传学的数学方法，我们将从这些全面的数据中推断出正常衰老过程中造血的克隆进化和对扰动的反应。此外，我们将重建由Tet 2功能丧失和突变率增加引发的恶性肿瘤的进化路径。该项目将对小鼠的体细胞嵌合现象和白血病发生的起源产生前所未有的洞察力。鉴于人类骨髓增生异常疾病经常引起急性白血病，我们希望我们的研究结果将为理解和早期诊断这种向白血病的转变提供新的方法。

Cell clones harboring somatic mutations known as cancer drivers have now been found in many phenotypically normal tissues and accumulate with aging. Hematopoiesis has emerged as a paradigm of such somatic mosaicism; large hematopoietic stem cell (HSC) clones with leukemic driver mutations are frequently found in healthy elderly. These observations raise the question which cellular mechanisms counteract leukemogenesis, and whether there are early warning signs that these protective mechanisms are breaking down. A clue is provided by the recent observation, by us and others, that HSC in mice show a remarkable pattern of activity: While being quiescent for several months, HSCs rarely but reliably become active to spawn a multipotent progenitor (MPP). MPPs exhibit extensive self-renewal in vivo and maintain production of blood cells long-term even when HSCs are depleted. Hence, we hypothesize that the rare HSC output functions to prevent excessive accumulation of mutations in more rapidly dividing progenitors. To test this, we will make use of mouse models that: (i) allow the selective depletion of HSCs, (ii) genetically perturb epigenetic regulation by Tet2 loss-of-function, decreasing HSC output and increasing self-renewal, and (iii) globally increase mutation rate across the genome. We will quantify the accumulation of somatic mutations in single HSCs and progenitors as well as in bulk cells, yielding complementary information on mutation count in single cells and on the frequency distribution of somatic variants in sorted populations, respectively. Using mathematical approaches from population genetics, we will infer from these comprehensive data the clonal evolution of hematopoiesis during normal aging and in response to the perturbations. Moreover, we will reconstruct evolutionary paths to malignancy triggered by Tet2 loss-of-function and increased mutation rate. This project will yield unprecedented insight into somatic mosaicism in mice and origins of leukemogenesis. Given that myelodysplastic disorders in humans frequently give rise to acute leukemia, we expect that our findings will inform novel approaches at understanding, and early diagnosing, this transition to leukemia.