Novel alternative PBX3 isoforms in leukemia cells with distinct interaction specificities

Abstract

PBX3 is a member of the PBX family of TALE homeobox genes. The prototypic member, PBX1, was first identified in chromosomal translocations in B-lineage leukemia and is required for normal hematopoiesis. PBX2 and PBX3 were later identified as members of this highly conserved family by their strong homology to PBX1. While the expression pattern of PBX1 is restricted, PBX2 and PBX3 are ubiquitously expressed. Little is known about the functional role of PBX3. Our studies identified two PBX3 transcripts alternative to the canonical forms, PBX3A and PBX3B, resulting from a novel splice in PBX3. These new isoforms, named PBX3C and PBX3D, have been detected in all tissues and cell lines tested. Intriguingly, expression of PBX3D is favored in normal cells, whereas PBX3C expression is favored in leukemia cells. Functional studies showed that PBX3C and PBX3D proteins were unable to interact with the PBX-interacting factor PREP1 and weakly interacted with MEIS proteins. We propose that PBX3C and PBX3D may affect PBX3-mediated transcriptional regulation by acting in opposition to the known PBX proteins through alternative PBX3 complex formation. The identification and characterization of these novel PBX3 isoforms provide a foundation for a better understanding of the biological role of PBX3.

Up-regulation of a HOXA-PBX3 homeobox-gene signature following down-regulation of miR-181 is associated with adverse prognosis in patients with cytogenetically abnormal AML

“…As a cofactor of HOX proteins, PBX3 is a member of the PBX family of 3-amino-acid loop extension homeobox proteins, 17 which can form stable heterocomplexes with HOX and MEIS proteins to regulate the transcription of downstream targets. 18 However, although aberrant overexpression of PBX3 in CA-AML, such as MLL-associated leukemia, has been observed frequently, 20,21,[23][24][25] its biologic function remains unexplored.…”

“…15,16 Hox proteins can form heterodimers or heterotrimers with members of the 3-amino-acid loop extension family of cofactors, including PBX and MEIS proteins to regulate the transcription of downstream targets directly. [15][16][17][18] The aberrant overexpression of these homeobox genes (particularly HOXA5, HOXA7, HOXA9, HOXA10, MEIS1, and PBX3) has been frequently reported previously in various subtypes of intermediate or poor prognosis CA-AML, including those bearing MLL (mixed lineage leukemia) rearrangements [19][20][21][22][23][24][25][26][27] or trisomy 8. 28 However, although the expression profile of HOXA7, HOXA9, or PBX3 has been implied to be associated with poor survival of AML patients in several individual studies, 20,[29][30][31] their prognostic impact has not been tested in multivariable models or been validated by further large-scale independent studies.…”

PBX proteins: much more than Hox cofactors

“…Some of these genes give rise to several isoforms by alternative splicing. Indeed, Pbx proteins arise from differential splicing of Pbx transcripts to yield large (Pbx1a, Pbx2, Pbx3a and Pbx4) and short (Pbx1b, Pbx3b, Pbx3c and Pbx3d) forms of the respective proteins (Monica et al, 1991, Wagner et al, 2001, Milech et al, 2001. Based on biochemical studies, it is possible to discriminate clearly between different Pbx proteins and isoforms.…”

“…Furthermore, expression of some Pbx isoforms seems to be favoured in certain pathologic contexts. Indeed, expression of Pbx3d is observed in normal cells, whereas Pbx3c expression is mostly found in leukaemia cells (Milech et al, 2001).…”

miR-495 is a tumor-suppressor microRNA down-regulated in MLL -rearranged leukemia

“…HOX proteins can form heterodimers or heterotrimers with members of the three-amino-acid loop extension family of cofactors, including PBX and MEIS proteins, to regulate the transcription of multiple downstream targets directly (22)(23)(24). Similar to HOXA9 and MEIS1, overexpression of PBX3 has also been frequently observed in various subtypes of AML with unfavorable prognosis, particularly in MLL-rearranged leukemia (13,18,(25)(26)(27)(28).…”