Hex - a Homeobox Gene Essential for Liver Development

我们和其他人的研究都表明，携带同源盒基因Hhex零突变的小鼠在妊娠中期死亡，并伴有多种发育缺陷，包括肝脏完全缺失（REF）。早期的致死率和肝芽形成的缺失使得分析Hhex在肝脏发育和功能后期的作用变得不可能。我们已经证明Hhex在整个肝脏发育过程中表达，在肝母细胞、成熟肝细胞以及肝内和肝外胆管上皮（BDE）中表达，这表明Hhex在肝脏形态发生的多个阶段发挥作用（REF）。为了确定Hhex在肝芽形成后肝脏发育中的作用，我们对小鼠进行了Hhex的“条件敲除”。使用两株Hhex条件敲除小鼠，我们已经证明Hhex在肝芽形成后具有重要功能。首先，我们发现Foxa3-Cre；Hhex？2,3/-小鼠，在E8.5 - 9.0的早期肝母细胞中缺失Hhex，在E18.5死亡，肝脏明显发育不全，主要由胆管组成，肝细胞缺乏。其次，我们发现AFP-Cre；Hhex？在E10.5及以后肝脏中缺失Hhex的2,3/-小鼠出现导管板畸形和多囊肝。基于这两种小鼠品系的显著特征，我们推测：1)Hhex是肝母细胞正常分离为肝细胞和胆管上皮（BDE）所必需的；2)由于Hhex是成熟肝脏中胆管板重塑和/或BDE的正常发育和功能所必需的，因此在缺乏Hhex的情况下胆管增大和胆道囊肿发生；3) Hhex在肝脏发育和胆囊胚形成过程中调控多种富肝基因的表达。这些基因包括至关重要的HNF家族成员HNF6、HNF1a和HNF4a以及其他因素。为了检验这些假设，我们提出以下3个具体目标。具体目标目的：观察肝母细胞中Hhex表达水平的改变是否会干扰其向肝细胞和BDE的正常分化。具体目标2。确定Foxa3-Cre中胆管增大和胆囊炎形成的机制；Hhex吗?2,3/-和AFP-Cre；Hhex吗?2、3 / -老鼠。具体目标3。阐明Hhex在肝脏和胆道发育过程中调控的转录网络。这些研究阐明的发育机制将为鉴定肝脏疾病细胞治疗中产生分化细胞所需的遗传因素提供至关重要的知识。此外，本研究将对包括胆道闭锁和多囊性肝病在内的胆道疾病的发病机制提供重要的见解。

Both we and others have shown that mice with a null mutation of the homeobox gene Hhex die mid-gestation with multiple developmental defects, including complete absence of the liver (REF). The early lethality and absence of liver bud formation makes analysis of the role of Hhex in later stages of liver development and function impossible. We have shown that Hhex is expressed throughout liver development and in the adult liver in hepatoblasts, mature hepatocytes and both intra- and extra-hepatic bile duct epithelia (BDE), indicating that Hhex plays a role at multiple stages of liver morphogenesis (REF). To determine the role that Hhex plays in liver development after the formation of a liver bud, we generated mice a "conditional knockout" of Hhex. Using two strains of Hhex conditional knockout mice, we have demonstrated that Hhex has important functions after the formation of the liver bud. First, we found that Foxa3-Cre;Hhex?2,3/- mice, in which Hhex is deleted in early hepatoblasts at E8.5 - 9.0, die by E18.5 and have a liver that is significantly hypoplastic and composed predominately of bile ducts with a paucity of hepatocytes. Second, we found that AFP-Cre;Hhex?2,3/- mice, in which Hhex is deleted in the liver at E10.5 and later, have ductal plate malformations and polycystic livers. Based on the striking features of these two mouse strains, we hypothesize that 1) Hhex is required for the normal segregation of hepatoblasts into hepatocytes and bile duct epithelia (BDE); 2) enlarged bile ducts and biliary cysts develop in the absence of Hhex because Hhex is required for ductal plate remodeling and/or the normal development and function of BDE in the mature liver; and 3) Hhex regulates the expression of a number of liver-enriched genes during liver development and biliary cystogenesis. Those genes include the critically important HNF family members HNF6, HNF1a and HNF4a as well as other factors. In order to test these hypotheses, we propose the following 3 specific aims. Specific Aim 1. To determine if alterations in Hhex expression levels in hepatoblasts perturbs their normal differentiation into hepatocytes and BDE. Specific Aim 2. Determine the mechanism(s) involved in the formation of enlarged bile ducts and biliary cysts in Foxa3-Cre; Hhex?2,3/- and AFP-Cre; Hhex?2,3/- mice. Specific Aim 3. Elucidate the transcriptional network regulated by Hhex during liver and biliary tract development. The developmental mechanisms elucidated by these studies will provide knowledge that is critical to the identification of genetic factors required to generate differentiated cells for cell therapy of liver diseases. Additionally, this research will shed important insight into the pathogenesis of diseases of the biliary tract, including biliary atresia and polycystic liver disease.