Hox Genes in Limb Patterning

描述(申请人提供)：HOX基因编码含有同源结构域的转录因子，这些转录因子在体轴上以AP受限的方式表达并发挥作用，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，这种HOX表达的特征在脊椎动物胚胎中得到了保留。除了沿着主体轴的共线性外，在脊椎动物早期发育的肢芽中，还观察到HoxA和HoxD并列的第9至13组基因的嵌套、共线表达模式。在肢体发育过程中，这些基因的突变会导致近端远端(PD)轴上特定的形态扰动。大多数证据支持这样一种模型，即发育中的前肢只需要来自HoxA和HoxD复合体的与AbdB相关的Hox基因(Hox9-13)。没有HoxB或HoxC组基因的功能丧失突变被证明有前肢缺陷。类似地，还没有报告四个HoxA-D复合体基因中的任何一个的非AbdB相关基因(Hox1-8)功能丧失的肢体表型。最近的研究还表明，来自HoxA和D复合体的Hox10到Hox13基因共同控制早期肢体芽的生长和Shh表达的启动，这表明同样的HOX互补基因共同控制后期肢体近端模式事件，共同控制早期生长和前后模式事件。已经清楚地表明，肢体中的共线表达依赖于Hox簇内外的调控元件，但控制Shh/Hoxa10-13；Hoxd10-13调控环的启动的因素尚不清楚。我们最近在Hox5(Hoxa5-/-；HOXB5-/-；Hoxc5-/-)和Hox9(Hoxa9-/-；HOXB9-/-；Hoxc9-/-)组基因中产生了平行突变小鼠。Hox5功能缺失(非AbdB相关的HOX基因)导致前肢骨骼的前花纹缺陷，与Holt-Oram综合征患者非常相似，初步证据表明Hox5基因参与了Tbx5调控途径在前肢发育中的作用。同样出乎意料的是，所有四个Hox9平行基因的丢失会导致严重的帕金森病生长和AP图案缺陷，从而影响整个前肢骨骼。初步结果表明，与Shh和AbdB相关的HoxA/D10-13基因不是在Hox9肢芽中启动的，这表明Hox9是一个节点，在前肢发育中Hox10-13/Shh环的建立至关重要。我们初步研究的新观点支持一种新的模型，在该模型中，来自所有四个复合体的HOX基因的表达参与了肢体模式事件，这与目前接受的范例有很大不同。我们假设Hox5基因通过调节Tbx5通路来指导早期前肢AP模式形成事件，而Hox9基因是一个早期信号节点，对Shh/Hox10-13调控环的建立至关重要。公共卫生相关性：我们的新的遗传学结果揭示了HOX基因在早期肢体模式中的两个意想不到的重要作用：Hox5基因对于建立前肢模式是重要的，初步结果表明它们是人类Holt-Oram综合征中断的通路的调节者。此外，Hox9基因似乎是建立Shh-Abdb Hox环的节点；这些基因的丢失会导致肢体发育的严重截断。对这些意想不到的表型和功能的分析对于了解早期肢体发育过程以及不当调控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.