Neurofibromatosis Type 1 Gene Regulates Myelopoiesis

描述（由申请人提供）：NF1肿瘤抑制基因突变导致1型神经纤维瘤病（NF-1）。NF1编码p21 ras （ras）的GTPase激活蛋白（GAP），称为神经纤维蛋白。神经纤维蛋白将p21 ras从活性GTP转化为非活性GDP结合构象。患有NFl的个体有获得良性和恶性肿瘤的倾向。此外，患有NFl的儿童易患幼年粒细胞白血病（JMML）。来自JMML骨髓细胞的髓系祖细胞（CFU-GM）的一个特征是它们在低剂量生长因子粒细胞巨噬细胞集落刺激因子（GM-CSF）的反应中倾向于过度增殖。纯合子破坏在子宫内是致命的；然而，我们发现Nf1缺陷的小鼠胎儿造血细胞在多种生长因子（包括GM-CSF和c-kit受体酪氨酸激酶配体干细胞因子（SCF））的作用下，表现出异常的CFU-GM生长模式和Ras效应物的过度激活。C-kit由小鼠显性白色斑点位点W编码。由于W和Nfl基因座似乎沿着一条共同的发育途径发挥作用，因此产生了两个基因座都发生突变的小鼠。我们发现，Nfl单倍体缺失部分修复了W41小鼠肥大细胞和毛色缺陷。这些数据提供了遗传学证据，证明Nfl的单倍体缺陷在体外和体内调节两个谱系中受Nfl个体影响的细胞命运。这些结果支持了肿瘤抑制基因杂合失活可能具有重要生物学效应的新概念。虽然神经纤维蛋白缺失会增加特定细胞系中p21 ras的活性，但在nf1缺陷细胞中，控制增殖和存活的不同p21 ras效应通路的改变是不完整的，这对理解疾病发病机制至关重要。大多数先前的研究认为，神经纤维蛋白的缺失导致经典p21 ras-Raf-Mek-ERK通路的激活增加。然而，我们有初步的数据来支持另一种生化模型，即nfl1缺陷细胞的生长优势是通过p21 ras到小Rho GTPase Rac2的信号增加来介导的，Rac2是一种仅在造血细胞中表达的Rac亚型。我们建议研究p21 ras和Rac亚型的激活如何协同改变Nfl +/-肥大细胞和Nf 1 -/-干细胞和髓系祖细胞的生物学特性，利用这些位点的基因突变小鼠。

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.