Brake R L, Kees U R, Watt P M
TVW Telethon Institute for Child Health Research, West Perth, Western Australia.
Oncogene. 1998 Oct 8;17(14):1787-95. doi: 10.1038/sj.onc.1202078.
The HOX11 proto-oncogene is normally expressed in embryogenesis where it directs the synthesis of the spleen. In adult tissues, HOX11 expression is silenced by an unknown mechanism. Aberrant expression of HOX11 occurs in T-cell acute lymphoblastic leukaemia (T-ALL), where it is thought to be involved in T-cell immortalization. The deregulated expression of HOX11 is frequently associated with chromosomal translocations which juxtapose a T-cell receptor (TCR) gene upstream of the HOX11 gene. In these cases, it is presumed that the activation of HOX11 expression results from bringing the gene under the control of TCR enhancer elements. However, activation of HOX11 also occurs in the absence of an associated translocation in both T-ALL and erythroleukaemia cells, implying that an alternative activation mechanism may exist. We hypothesized that HOX11 may be repressed in normal T-cells and erythroid cells by the action of negative elements which may be deleted or mutated in leukaemia. We therefore conducted a search for negative elements in the human HOX11 promoter which may function to silence its expression in normal cells of the haematopoietic lineages. Since little sequence of the HOX11 promoter was available, we began our investigation by sequencing over 4.5 kilobases of untranslated DNA from upstream of HOX11. The human sequence that overlaps with the 2.1 kb of murine Hox11 is highly conserved, suggesting that a large region of DNA upstream of HOX11 may have a regulatory function. We then used transfection assays to test the ability of portions of the promoter to drive transcription of a reporter gene. These studies identified four negative elements. Two of them (NRE2 and NRE4) function in all cell lines tested, while the remaining two (NRE1 and NRE3) appear to be cell-type specific. The DNA sequences of three elements are conserved between the human and mouse HOX11/Hox11 promoters. We propose a model in which the combined action of these negative elements contributes to the overall repression of HOX11 expression in normal blood cells.
HOX11原癌基因通常在胚胎发育过程中表达，在此过程中它指导脾脏的合成。在成体组织中，HOX11的表达通过未知机制被沉默。HOX11的异常表达发生在T细胞急性淋巴细胞白血病（T-ALL）中，据认为它参与了T细胞的永生化。HOX11的失调表达经常与染色体易位相关，这些易位使T细胞受体（TCR）基因并列于HOX11基因的上游。在这些情况下，推测HOX11表达的激活是由于该基因受TCR增强子元件的控制。然而，在T-ALL和红白血病细胞中，即使没有相关的易位，HOX11的激活也会发生，这意味着可能存在另一种激活机制。我们假设HOX11在正常T细胞和红系细胞中可能受到负调控元件的抑制，而这些元件在白血病中可能被缺失或突变。因此，我们在人HOX11启动子中寻找可能在造血谱系的正常细胞中使其表达沉默的负调控元件。由于HOX11启动子的序列信息很少，我们从HOX11上游超过4.5千碱基的非翻译DNA测序开始我们的研究。与小鼠Hox11的2.1 kb重叠的人类序列高度保守，这表明HOX11上游的大片段DNA可能具有调控功能。然后我们使用转染实验来测试启动子部分驱动报告基因转录 的能力。这些研究确定了四个负调控元件。其中两个（NRE2和NRE4）在所有测试的细胞系中都起作用，而其余两个（NRE1和NRE3）似乎具有细胞类型特异性。在人HOX11和小鼠Hox11启动子之间，三个元件的DNA序列是保守的。我们提出了一个模型，其中这些负调控元件的共同作用有助于正常血细胞中HOX11表达的整体抑制。
