Yang Y, Guillot P, Boyd Y, Lyon M F, McMahon A P
Department of Molecular and Cellular Biology, The Biolabs, Harvard University, Cambridge, MA 02138, USA.
Development. 1998 Aug;125(16):3123-32. doi: 10.1242/dev.125.16.3123.
Patterning of the vertebrate limb along the anterior-posterior axis is controlled by the zone of polarizing activity (ZPA) located at the posterior limb margin. One of the vertebrate Hh family members, Shh, has been shown to be able to mediate the function of the ZPA. Several naturally occurring mouse mutations with the phenotype of preaxial polydactyly exhibit ectopic Shh expression at the anterior limb margin. In this study, we report the molecular characterization of a spontaneous mouse mutation, Doublefoot (Dbf). Dbf is a dominant mutation which maps to chromosome 1. Heterozygous and homozygous embryos display a severe polydactyly with 6 to 8 digits on each limb. We show here that Shh is expressed normally in Dbf mutants. In contrast, a second Hh family member, Indian hedgehog (Ihh) which maps close to Dbf, is ectopically expressed in the distal limb bud. Ectopic Ihh expression in the distal and anterior limb bud results in the ectopic activation of several genes associated with anterior-posterior and proximal-distal patterning (Fgf4, Hoxd13, Bmp2). In addition, specific components in the Hedgehog pathway are either ectopically activated (Ptc, Ptc-2, Gli1) or repressed (Gli2). We propose that misexpression of Ihh, and not a novel Smoothened ligand as recently suggested (Hayes et al., 1998), is responsible for the Dbf phenotype. We consider that Ihh has a similar activity to Shh when expressed in the early Shh-responsive limb bud. To determine whether Dbf maps to the Ihh locus, which is also on chromosome 1, we performed an interspecific backcross. These results demonstrate that Dbf and Ihh are genetically separated by approximately 1.3 centimorgans, suggesting that Dbf mutation may cause an exceptionally long-range disruption of Ihh regulation. Although this leads to ectopic activation of Ihh, normal expression of Ihh in the cartilaginous elements is retained.
脊椎动物肢体沿前后轴的模式形成由位于肢体后缘的极化活性区（ZPA）控制。脊椎动物Hh家族成员之一，即 Sonic hedgehog（Shh），已被证明能够介导ZPA的功能。一些具有轴前多指表型的天然小鼠突变体在前肢边缘表现出异位的Shh表达。在本研究中，我们报告了一个自发小鼠突变体Doublefoot（Dbf）的分子特征。Dbf是一个显性突变，定位于1号染色体。杂合子和纯合子胚胎均表现出严重的多指畸形，每个肢体有6至8个指（趾）。我们在此表明，Shh在Dbf突变体中正常表达。相反，另一个Hh家族成员，与Dbf紧密连锁的印度刺猬因子（Ihh），在远端肢体芽中异位表达。Ihh在远端和前肢芽中的异位表达导致了几个与前后轴和近远轴模式形成相关的基因（Fgf4、Hoxd13、Bmp2）的异位激活。此外，Hedgehog信号通路中的特定成分要么被异位激活（Ptc、Ptc-2、Gli1），要么被抑制（Gli2）。我们提出，Ihh的错误表达，而非最近所提出的一种新的Smoothened配体（Hayes等人，1998年），是导致Dbf表型的原因。我们认为，Ihh在早期对Shh有反应的肢体芽中表达时，具有与Shh相似的活性。为了确定Dbf是否定位于同样在1号染色体上的Ihh基因座，我们进行了种间回交。这些结果表明，Dbf和Ihh在遗传上相距约1.3厘摩，这表明Dbf突变可能导致Ihh调控出现异常长距离的破坏。尽管这导致了Ihh的异位激活，但Ihh在软骨成分中的正常表达得以保留。
