Hox Genes in Limb Patterning

描述（由申请人提供）：Hox基因编码含有同源结构域的转录因子，这些转录因子沿着体轴以ap限制性方式表达和发挥作用，与它们在簇中的位置相对应。这种全球Hox表达模式被称为共线性，Hox表达的这种特征在脊椎动物胚胎中一直保留。除了沿主体轴线的共线性外，HoxA和HoxD同源组9至13个基因在早期发育的脊椎动物肢芽中也观察到巢状共线性表达模式。在肢体发育过程中，这些基因的突变引起沿近端远端（PD）轴的特定形态扰动。大量证据支持这样一个模型，即在前肢发育过程中只需要来自HoxA和HoxD复合体的abdb相关Hox基因（Hox9-13）。没有HoxB或HoxC组基因的功能丧失突变体被证明具有前肢缺陷。同样，没有四肢表型与四种HoxA-D复合基因中任何一种的非abdb相关基因（Hox1-8）功能丧失有关。最近的研究还表明，来自HoxA和D复合体的Hox10和Hox13基因共同控制早期肢体芽的生长和Shh表达的启动，这表明控制后期肢体近端远端模式事件的相同Hox基因共同控制早期生长和前后模式事件。研究清楚地表明，肢体共线表达依赖于Hox簇内外的调控元件，但控制Shh/Hoxa10-13启动的因素；Hoxd10-13调控回路未知。我们最近在Hox5 （Hoxa5 -/-; Hoxb5 -/-; Hoxc5 -/-）和Hox9 （Hoxa9 -/-; Hoxb9 -/-; Hoxc9 -/-; Hoxd9 -/-）组基因中产生了同源突变小鼠。Hox5（非abdb相关的Hox基因）功能缺失导致前肢骨骼前侧模式缺陷，与Holt-Oram综合征患者非常相似，初步证据表明Hox5基因参与了前肢发育的Tbx5调控通路。同样出乎意料的是，所有四个Hox9旁系基因的缺失会导致严重的PD生长和影响整个前肢骨骼的AP模式缺陷。初步结果表明，Shh和abdb相关的HoxA/D 10-13基因未在Hox9肢体芽中启动，这表明Hox9是前肢发育中Hox10- 13/Shh环建立的关键节点。从我们的初步研究中出现的观点支持了一个新的模型，其中来自所有四种复合物的Hox基因的表达参与了肢体模式事件，这与目前公认的范式有很大的不同。我们假设Hox5基因通过调节Tbx5通路直接早期前肢AP模式事件，而Hox9基因是早期信号节点，对Shh/Hox10-13调控回路的建立至关重要。公共卫生相关性：我们新的遗传结果揭示了Hox基因在早期肢体模式中的两个意想不到的重要作用：Hox5基因对前肢模式的建立很重要，初步结果表明它们是人类Holt-Oram综合征中中断的通路的调节因子。此外，Hox9基因似乎是建立sh - abdb Hox环的一个节点；这些基因的缺失会导致肢体发育的严重截断。分析这些意想不到的表型和功能对于了解早期肢体发育过程和错误调节如何导致人类疾病至关重要。

DESCRIPTION (provided by applicant): Hox genes encode homeodomain-containing transcription factors that are expressed and function along the body axis in an AP-restricted manner corresponding to their position within the cluster. This pattern of global Hox expression is termed colinearity and this feature of Hox expression has been retained among vertebrate embryos. In addition to the colinearity along the main body axis, a nested, colinear expression pattern is also observed for the HoxA and HoxD paralogous group 9 through 13 genes in the early developing vertebrate limb bud. Mutations in these genes cause specific morphological perturbations along the proximodistal (PD) axis during limb development. The preponderance of evidence supports a model in which only the AbdB-related Hox genes (Hox9-13) from the HoxA and HoxD complex are required in the developing forelimb. No loss-of-function mutants in HoxB or HoxC group genes have ever been demonstrated to have forelimb defects. Similarly, no limb phenotypes have been reported with loss- of-function of non-AbdB-related genes (Hox1-8) from any of the four HoxA-D complex genes. Recent work has additionally shown that Hox10 through Hox13 genes from the HoxA and D complex, together, control early limb bud outgrowth and the initiation of Shh expression, suggesting that the same Hox compliment of genes that control later limb proximodistal patterning events, together, control early outgrowth and anterior-posterior patterning events. It has been clearly shown that colinear expression in the limb relies on regulatory elements both within and outside the Hox cluster, but the factors that control the initiation of the Shh/Hoxa10-13;Hoxd10-13 regulatory loop are unknown. We have recently generated paralogous mutant mice in the Hox5 (Hoxa5 -/-; Hoxb5 -/-; Hoxc5 - /-), and Hox9 (Hoxa9 -/-; Hoxb9 -/-; Hoxc9 -/-; Hoxd9 -/-) group genes. Loss of Hox5 function (non- AbdB-related Hox genes) results in anterior patterning defects of the forelimb skeleton that closely resemble Holt-Oram Syndrome patients, and preliminary evidence suggests Hox5 genes are involved in the Tbx5 regulatory pathway in forelimb development. Also unexpectedly, loss of all four Hox9 paralogous genes results in profound PD outgrowth and AP patterning defects that affect the entire forelimb skeleton. Preliminary results demonstrate that Shh and AbdB-related HoxA/D 10-13 genes are not initiated in Hox9 limb buds, suggesting Hox9 is a node, critical for establishment of the Hox10- 13/Shh loop in forelimb development. The emerging view from our preliminary studies supports a new model wherein expression of Hox genes from all four complexes participates in limb patterning events, a significant departure from the currently accepted paradigm. We hypothesize that Hox5 genes direct early forelimb AP patterning events by regulating the Tbx5 pathway, and that Hox9 genes are an early signaling node, critical for the establishment of the Shh/Hox10-13 regulatory loop. PUBLIC HEALTH RELEVANCE: Our new genetic results uncover two unexpected and important roles for Hox genes in early limb patterning: Hox5 genes are important for the establishment of anterior limb pattern, and preliminary results suggest they are regulators in the pathway disrupted in human Holt-Oram Syndrome. Further, Hox9 genes appear to be a node for the establishing the Shh-Abdb Hox loop; loss of these genes results in severe truncation of limb development. Analyses of these unexpected phenotypes and functions are critical for developing an understanding of early limb developmental processes and how mis-regulation results in human disease.