Infectious origins of childhood leukemia

描述（由申请人提供）：多条证据支持感染在引发儿童白血病的继发性病变病因学中的关键作用。这导致了一个假设，即延迟暴露于常见感染易患儿童前B ALL。早期暴露于传染性病原体，例如通过日托或疫苗接种，导致轻度和典型的亚临床免疫反应，并与儿童ALL风险显著降低相关。相比之下，延迟感染往往更严重，引起更强烈的免疫反应，并易患儿童前B ALL。该提案的中心目标是通过实验测试“延迟感染”假说，并描述感染背景下人类前B细胞遗传脆弱性的机制。前B细胞经历Rag 1/2依赖性免疫球蛋白V（D）J基因重组，代表儿童ALL的起源细胞。Rag 1/2介导的重组酶活性导致DNA双链断裂，并与获得染色体易位的低风险相关。然而，当Rag 1/2酶与B细胞特异性突变酶AID同时表达时，这种风险显著增加。因此，Rag 1/2和AID的表达在B细胞发育的早期和晚期分别是相互排斥的和暂时分离的：Rag 1和Rag 2的表达仅限于免疫球蛋白V（D）J基因在原和前B细胞中的重组。相反，AID表达被认为局限于遇到抗原的成熟B细胞中的体细胞超突变和类别转换重组。在这项建议的初步工作，我们发现，IL 7 R？/ Stat 5/Akt信号传导对于将正常前B细胞保持在抗原无应答状态是至关重要的。在撤回IL 7或条件性缺失Stat 5后，前B细胞变得对抗原（例如LPS）完全应答，并在LPS刺激后以与成熟B细胞相似的水平表达AID。虽然骨髓中高水平的IL 7使前B细胞处于抗原无反应状态，但我们发现了正常前B细胞分化过程中的脆弱性窗口：前B细胞受体信号传导下调IL 7 R？表达并诱导Rag 1/2介导的免疫球蛋白轻链基因在小的静息（组分D）前B细胞中重组。在遇到抗原时，除了Rag 1/2之外，组分D前B细胞还表达高水平的AID，这显著增加了它们的染色体易位倾向。基于这些发现，我们假设组分D前B细胞在感染背景下对遗传病变高度敏感。我们提出了以下三个目的来测试预测，即组分D前B细胞池的大小以及对感染的免疫应答的频率和强度将决定前白血病（例如TEL-AML 1）前B细胞克隆获得关键继发性病变的可能性。

DESCRIPTION (provided by applicant): Multiple lines of evidence support a critical role of infections in the etiology of secondary lesions that initiate childhood leukemia. This led to the hypothesis that delayed exposure to common infections predisposes to childhood pre-B ALL. Early exposure to infectious pathogens, e.g. through daycare attendance or vaccinations, leads to mild and typically subclinical immune responses and is associated with a significantly diminished risk to develop childhood ALL. By contrast, delayed infections are often more serious, cause more vigorous immune responses and predispose to childhood pre-B ALL. The central goal of this proposal is to experimentally test the 'delayed infections' hypothesis and to delineate mechanisms of genetic vulnerability of human pre-B cells in the context of infection. Pre-B cells undergo Rag1/2-dependent immunoglobulin V(D)J gene recombination and represent the cell of origin of childhood ALL. Rag1/2-mediated recombinase activity causes DNA double strand breaks and is associated with a low risk to acquire chromosomal translocations. This risk, however, dramatically increases when Rag1/2 enzymes are expressed concomitantly with the B cell-specific mutator enzyme AID. Hence, expression of Rag1/2 and AID is mutually exclusive and temporally separated in early and late B cell development, respectively: Rag1 and Rag2 expression is limited to immunoglobulin V(D)J gene recombination in pro- and pre-B cells. Conversely, AID expression is thought to be restricted to somatic hypermutation and class-switch recombination in mature B cells that have encountered antigen. In preliminary work for this proposal, we found that IL7R?/Stat5/Akt signaling is critical to keep normal pre-B cells in an antigen- unresponsive state. Upon withdrawal of IL7 or conditional deletion of Stat5, pre-B cells become fully responsive to antigen (e.g. LPS) and express AID at similar levels as mature B cells upon LPS stimulation. While high levels of IL7 in the bone marrow secure pre-B cells in an antigen-unresponsive state, we discovered a window of vulnerability during normal pre-B cell differentiation: Pre-B cell receptor signaling downregulates IL7R? expression and induces Rag1/2-mediated immunoglobulin light chain gene recombination in small resting (Fraction D) pre-B cells. Upon antigen encounter, Fraction D pre-B cells express high levels of AID in addition to Rag1/2, which dramatically increases their propensity to chromosomal translocations. Based on these findings, we hypothesize that Fraction D pre-B cells are highly susceptible to genetic lesions in the context of infection. We propose the following three Aims to test the prediction that the size of the Fraction D pre-B cell pool and the frequency and intensity of immune responses to infections will determine the likelihood of a pre-leukemic (e.g. TEL-AML1) pre-B cell clone to acquire critical secondary lesions.