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，其以对应于其在簇内的位置的AP限制性方式沿体轴表达和起沿着作用。这种全局Hox表达的模式被称为共线性，并且Hox表达的这种特征在脊椎动物胚胎中被保留。除了共线性沿着主体轴，嵌套，共线性表达模式也观察到HoxA和HoxD旁系同源组9通过13个基因在早期发育的脊椎动物肢芽。这些基因的突变会导致肢体发育过程中沿近远轴（PD）的特定形态学扰动沿着。绝大多数证据支持这样一种模型，即在前肢发育中只需要来自HoxA和HoxD复合体的AbdB相关Hox基因（Hox 9 -13）。HoxB或HoxC组基因的功能缺失突变体从未被证明具有前肢缺陷。类似地，没有报道来自四个HoxA-D复合体基因中的任何一个的非AbdB相关基因（Hox 1 -8）的功能丧失的肢体表型。最近的工作还表明，Hox 10到Hox 13基因从HoxA和D复合体，一起控制早期肢芽生长和启动Shh表达，这表明相同的Hox互补基因，控制后来的肢体proximodistal图案事件，一起控制早期生长和前后图案事件。已经清楚地表明，在肢体共线表达依赖于调控元件内和外的Hox集群，但控制启动的Shh/Hoxa 10 -13; Hoxd 10 -13调控环的因素是未知的。我们最近在Hox 5（Hoxa 5-/-; Hoxb 5-/-; Hoxc 5- /-）和Hox 9（Hoxa 9-/-; Hoxb 9-/-; Hoxc 9-/-; Hoxd 9-/-）组基因中产生了旁系同源突变小鼠。Hox 5功能（非AbdB相关Hox基因）的丧失导致前肢骨骼的前部图案化缺陷，其非常类似于Holt-Oram综合征患者，并且初步证据表明Hox 5基因参与前肢发育中的Tbx 5调节途径。同样出乎意料的是，所有四个Hox 9旁系同源基因的丢失导致影响整个前肢骨骼的严重PD生长和AP图案缺陷。初步结果表明，Shh和AbdB相关的HoxA/D 10-13基因在Hox 9肢芽中不启动，表明Hox 9是一个节点，对前肢发育中Hox 10 - 13/Shh环的建立至关重要。从我们的初步研究中出现的观点支持一个新的模型，其中表达的Hox基因从所有四个复杂的参与肢体图案的事件，一个显着偏离目前公认的范式。我们假设Hox 5基因通过调节Tbx 5通路来指导早期前肢AP模式化事件，并且Hox 9基因是早期信号传导节点，对于Shh/Hox 10 -13调节环的建立至关重要。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：Hox 5基因对建立前肢模式很重要，初步结果表明它们是人类Holt-Oram综合征中被破坏的通路的调节因子。此外，Hox 9基因似乎是建立Shh-Abdb Hox环的节点;这些基因的缺失导致肢体发育严重截断。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以及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.