Neurofibromatosis Type 1 Gene Regulates Myelopoiesis

描述(由申请人提供)：NF1肿瘤抑制基因中的突变会导致1型神经纤维瘤病(NF-1)。NF1编码p21ras(RAS)的GTP酶激活蛋白(GAP)，称为神经纤维素。神经纤维素将p21ras从其活性GTP转变为其非活性GDP结合构象。NFL患者有患良性肿瘤和恶性肿瘤的倾向。此外，NFL儿童易患幼年性粒单核细胞白血病(JMML)。JMML骨髓细胞的髓系祖细胞(CFU-GM)的一个特点是在低剂量的生长因子粒细胞巨噬细胞集落刺激因子(GM-CSF)刺激下有过度增殖的倾向。NFL的纯合破坏在子宫内是致命的；然而，我们发现Nf1缺陷的小鼠胎儿血球细胞显示出异常的CFU-GM生长模式和RAS效应器的过度激活，以响应多种生长因子，包括GM-CSF和干细胞因子(SCF)，c-kit受体酪氨酸激酶的配体。C-Kit由小鼠显性白斑基因W)编码。由于W和NFL基因座似乎沿着共同的发育途径发挥作用，因此产生了这两个基因座都有突变的小鼠。我们发现，NFL的单倍性不足部分挽救了W41小鼠的肥大细胞和毛色缺陷。这些数据提供了遗传证据，表明NFL单倍体不足在体外和体内调节了两个NFL个体受影响的谱系的细胞命运。研究结果支持肿瘤抑制基因杂合性失活可能具有重要生物学效应的新兴概念。虽然神经纤维素的丢失增加了特定细胞系中p21ras的活性，但识别控制NF1缺陷细胞增殖和存活的不同p21ras效应通路的变化是不完整的，对于理解疾病的发病机制至关重要。大多数先前的研究认为，神经纤维蛋白的丢失会导致经典的p21ras-Raf-MEK-ERK通路的激活增加。然而，我们有初步的数据支持另一种生化模型，在该模型中，Nfl1缺陷细胞的生长优势是通过从p21ras到小Rho GTP酶rac2的信号增加而介导的，rac2是一种仅在造血细胞中表达的Rac亚型。我们建议进行研究，以检验p21ras和RAC亚型的激活如何协同改变NFL+/-肥大细胞以及NF1-/-干细胞和髓系祖细胞的生物学，利用这些基因突变的小鼠。

DESCRIPTION (provided by applicant): Mutatons in the NF1 tumor suppressor gene cause neurofibromatosis type 1 (NF-1). NF1 encodes a GTPase activating protein (GAP) for p21 ras (Ras) called neurofibromin. Neurofibromin converts p21 ras from its active GTP to its inactive GDP bound conformation. Individuals with NFl have a propensity to acquire benign and malignant tumors. Additionally, children with NFl are predisposed to juvenile myelomonocytic leukemia (JMML). A hallmark of myeloid progenitors (CFU-GM) from JMML bone marrow cells is their propensity to hyperproliferate in response to low doses of the growth factor granulocyte macrophage colony stimulating factor (GM-CSF). Homozygous disruption of Nfl is lethal in utero; however we found that murine Nf1 -deficient fetal hematopoletic cells show an abnormal pattern of CFU-GM growth and hyperactivation of Ras effectors in response to multiple growth factors, including GM-CSF and stem cell factor (SCF), the ligand for the c-kit receptor tyrosine kinase. C-kit is encoded by the murine dominant white spotting locus, W). Since the W and Nfl loci appeared to function along a common developmental pathway, mice with mutations at both loci were generated. We found that haploinsufficiency of Nfl partially rescued the mast cell and coat color defects in W41 mice. These data offered genetic evidence that haploinsufficiency at Nfl modulates cell fates in vitro and in vivo in two lineages that are affected in individuals with NFl. The results support the emerging concept that heterozygous inactivation of tumor suppressor genes may have important biological effects.While loss of neurofibromin increases p21 ras activity in specific cell lineages, identification of alterations in distinct p21 ras effector pathways that control proliferation and survival in NF1-deficient cells is incomplete and critical for understanding disease pathogenesis. Most previous studies argue that loss of neurofibromin results in increased activation of the classical p21 ras-Raf-Mek-ERK pathway. However, we have preliminary data to support an alternative biochemical model where the growth advantage of Nfl1-deficient cells is mediated through increased signals from p21 ras to the small Rho GTPase, Rac2, a Rac isoform expressed only in hematopoietic cells. We propose studies to examine how activation of p21 ras and Rac isoforms cooperate to alter the biology of Nfl +/- mast cells and Nf 1 -/- stem and myeloid progenitor cells utilizing mice with genetic mutations in these loci.