Conlon R A, Rossant J
Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Ontario, Canada.
Development. 1992 Oct;116(2):357-68. doi: 10.1242/dev.116.2.357.
Exogenous retinoic acid (RA) has teratogenic effects on vertebrate embryos and alters Hox-C gene expression in vivo and in vitro. We wish to examine whether RA has a role in the normal regulation of Hox-C genes, and whether altered Hox-C gene expression in response to RA leads to abnormal morphology. The expression of 3' Hox-2 genes (Hox-2.9, Hox-2.8, Hox-2.6 and Hox-2.1) and a 5' gene (Hox-2.5) were examined by whole-mount in situ hybridization on embryos 4 hours after maternal administration of teratogenic doses of RA on embryonic day 7 to 9. The expression of the 3' Hox-2 genes was found to be ectopically induced in anterior regions in a stage-specific manner. The Hox-2.9 and Hox-2.8 genes were induced anteriorly in the neurectoderm in response to RA on day 7 but not at later stages. Expression of Hox-2.6 and Hox-2.1 was ectopically induced anteriorly in neurectoderm in response to RA on day 8. Hox-2.1 remained responsive on day 9, whereas Hox-2.6 was no longer responsive at this stage. The expression of the 5' gene Hox-2.5 was not detectably altered at any of these stages by RA treatments. We also examined the response of other genes whose expression is spatially regulated in early embryos. The expression of En-2 and Wnt-7b was not detectably altered by RA, whereas RAR beta expression was induced anteriorly by RA on day 7 and 8. Krox-20 expression was reduced in a stage- and region-specific manner by RA. The ectopic anterior expression of Hox-2.8 and Hox-2.9 induced by RA on day 7 was persistent to day 8, as was the altered expression of Krox-20. The altered pattern of expression of these genes in response to RA treatment on day 7 may be indicative of a transformation of anterior hindbrain to posterior hindbrain, specifically, a transformation of rhombomeres 1 to 3 towards rhombomere 4 identity with an anterior expansion of rhombomere 5. The ectopic expression of the 3' Hox-2 genes in response to RA is consistent with a role for these genes in mediating the teratogenic effects of RA; the rapid response of the Hox-C genes to RA is consistent with a role for endogenous RA in refining 3' Hox-C gene expression boundaries early in development.
外源性视黄酸（RA）对脊椎动物胚胎具有致畸作用，并在体内和体外改变Hox-C基因的表达。我们希望研究RA是否在Hox-C基因的正常调控中发挥作用，以及RA诱导的Hox-C基因表达改变是否会导致形态异常。在胚胎第7至9天给母体注射致畸剂量的RA后4小时，通过全胚胎原位杂交检测3'Hox-2基因（Hox-2.9、Hox-2.8、Hox-2.6和Hox-2.1）和一个5'基因（Hox-2.5）的表达。发现3'Hox-2基因的表达以阶段特异性方式在前部区域异位诱导。Hox-2.9和Hox-2.8基因在第7天因RA而在前神经外胚层中前部诱导表达，但在后期则没有。Hox-2.6和Hox-2.1在第8天因RA而在前神经外胚层中前部异位诱导表达。Hox-2.1在第9天仍有反应，而Hox-2.6在此阶段不再有反应。5'基因Hox-2.5的表达在这些阶段的任何一个都未被RA处理检测到改变。我们还研究了其他在早期胚胎中表达受空间调控的基因的反应。En-2和Wnt-7b的表达未被RA检测到改变，而RARβ表达在第7天和第8天因RA而在前部诱导。Krox-20的表达因RA以阶段和区域特异性方式降低。第7天RA诱导的Hox-2.8和Hox-2.9的异位前部表达持续到第8天，Krox-20的表达改变也是如此。这些基因在第7天对RA处理的表达模式改变可能表明前脑后部向前脑后部的转变，具体而言，是菱脑节1至3向菱脑节4身份的转变以及菱脑节5的前部扩展。3'Hox-2基因对RA的异位表达与这些基因在介导RA致畸作用中的作用一致；Hox-C基因对RA的快速反应与内源性RA在发育早期细化3'Hox-C基因表达边界中的作用一致。
