Multiple negative elements contribute to repression of the HOX11 proto-oncogene

Abstract

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 identi®ed 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 speci®c. 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.

Expression of HOX11 in childhood T-lineage acute lymphoblastic leukaemia can occur in the absence of cytogenetic aberration at 10q24: a study from the Children's Cancer Group (CCG)

“…Our recent studies demonstrated that demethylation of the promoter accompanies reactivation of the gene. 30 In addition, we identified several negative elements upstream of the HOX11 gene, 31 but have so far failed to find evidence for their mutation in primary patient specimens. The high frequency of HOX11 deregulation in T-ALL suggests its involvement is a key pathway for leukaemogenesis.…”

Extensive molecular mapping of TCRα/δ- and TCRβ-involved chromosomal translocations reveals distinct mechanisms of oncogene activation in T-ALL

“…4 Such translocations are generally believed to result from illegitimate V(D)J recombination events and to lead to ectopic activation of oncogenes because of their the potent positive regulatory elements of the TCR locus or loss of the negative regulatory element. 5,6 Specific mechanistic differences in V(D)Jmediated translocation mechanisms have been shown to guide break location and clustering in T-ALL. 7 Two main types of oncogenic translocations involving the TCR␤ and TCR␣/␦ have been described.…”

Molecular characterization of a complex chromosomal translocation breakpoint t(10;14) including the HOX11 oncogene locus

“…Our analysis now reveals that this 263 bp sequence stems from a region proximal to the breakpoint at 3.4 kb described here for patient P4. Deletion of the intervening sequence in the HOX11 locus detected in patient DW removes segments of the promoter which we have recently shown to contain several negative elements 23 suggesting a possible selection mechanism occurring in the case described by Kagan and colleagues. 18 Hence this region 3.4 kb upstream of HOX11 may define a novel recombination 'hot-spot'.…”

“…AF067443). 23 This sequence was also identical to normal genomic DNA from a clone designated A1, which we constructed using a 4 kb DNA (HindIII-EcoRI) fragment from a region approximately 5 kb upstream of the HOX11 gene (see Methods section, Figure 1). By using a further primer PS293, sequencing towards the TCR␦ region was performed.…”