Role of FIR in limb morphogenesis

发育中的肢体是一个很好的研究模型的形态发生，其中许多调控骨骼模式和生长的组件已被阐明，提供了一个强大的生物学框架解释基因功能的突变表型的基础上。 FIR（FBP interacting repressor）是一种在进化上高度保守的myc转录负调控因子，存在于脊椎动物和果蝇中。果蝇同源基因（半品脱）控制着发育中的胚胎翅盘中的非增殖区。人类同源物与结直肠癌有关; FIR的选择性剪接显性干扰形式与这些癌症有关。阐明FIR在哺乳动物组织中的正常功能受到FIR无效突变胚胎的非常早期胚胎致死（植入前）的阻碍，限制了评估该基因在正常组织形态发生中的作用的能力。 莱文斯博士（CCR，NCI）已经产生了小鼠FIR的条件（floxed）等位基因，我们正在与他合作，使用几种Cre驱动程序来选择性地从早期肢芽中去除FIR。我们想测试FIR是否在调节肢体生长中起作用，如果是，了解它如何调节生长。在初步结果中，我们已经发现，令人惊讶的是，FIR功能的损失，在早期肢芽中胚层的结果在相当大的细胞凋亡，导致截断的肢体骨骼。在果蝇中，半品脱是Wg（wingless，Wnt）途径的靶标。有趣的是，Wnt信号传导效应物β-连环蛋白从肢芽中胚层的丢失也导致小鼠胚胎中的肢截短表型，这与肢芽中早期FGF信号传导中心（AER，顶端外胚层嵴）形成异常有关，肢芽中早期FGF信号传导中心是肢生长所必需的。我们正在分析Wnt通路的组成部分，以确定它们是否在FIR突变肢体中发生改变，沿着其他对早期肢体细胞存活至关重要的因素和信号中心的分析。这些研究将揭示FIR在调节生长中的不同作用，并将对开发可能靶向这种Myc调节剂功能的癌症疗法产生影响。

The developing limb is a well studied model for morphogenesis in which many of the regulatory components governing skeletal pattern and growth have been elucidated, providing a strong biological framework for interpreting gene function based on mutant phenotypes. FIR (FBP interacting repressor) is a highly evolutionarily conserved negative regulator of myc transcription present in both vertebrates and Drosophila. The Drosophila homolog (half pint) controls the zone of non-proliferation in the developing embryonic wing disc. The human homolog has been implicated in colorectal cancer; alternatively spliced dominant-interfering forms of FIR are associated with these cancers. Elucidation of the normal function of FIR in mammalian tissues has been hampered by the very early embryonic lethal (prior to implantation) of FIR null mutant embryos, limiting the ability to assess the role of this gene in normal tissue morphogenesis. In collaboration with Dr. Levens (CCR, NCI), who has generated a conditional (floxed) allele of mouse FIR, we are using several Cre drivers to selectively remove FIR from the early limb bud. We want to test whether FIR plays a role in regulating limb outgrowth, and if it does, learn how it acts to regulate growth. In preliminary results, we have found that, surprisingly, loss of FIR function in early limb bud mesoderm results in considerable apoptosis leading to truncations of the limb skeleton. In Drosophila, half pint is a target of Wg (wingless, Wnt) pathway. Interestingly, loss of the Wnt signaling effector, beta-catenin, from limb bud mesoderm also causes limb truncation phenotypes in mouse embryos, related to abnormalities in formation of an early Fgf signaling center in the limb bud (AER, apical ectodermal ridge) that is necessary for limb outgrowth. We are analyzing components of the Wnt pathway to determine whether they are altered in the FIR mutant limbs, along with other analyses of factors and signaling centers critical for cell survival in the early limb. These studies will shed new light on the varied roles of FIR in regulating growth, and will have implications for developing cancer therapies that may target function of this Myc-regulator.