Histone deacetylase inhibitors induce remission in transgenic models of therapy-resistant acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is associated with chromosomal translocations, invariably involving the retinoic acid receptor alpha (RAR alpha) gene fused to one of several distinct loci, including the PML or PLZF genes, involved in t(15;17) or t(11;17), respectively. Patients with t(15;17) APL respond well to retinoic acid (RA) and other treatments, whereas those with t(11;17) APL do not. The PML-RAR alpha and PLZF-RAR alpha fusion oncoproteins function as aberrant transcriptional repressors, in part by recruiting nuclear receptor-transcriptional corepressors and histone deacetylases (HDACs). Transgenic mice harboring the RAR alpha fusion genes develop forms of leukemia that faithfully recapitulate both the clinical features and the response to RA observed in humans with the corresponding translocations. Here, we investigated the effects of HDAC inhibitors (HDACIs) in vitro and in these animal models. In cells from PLZF-RAR alpha/RAR alpha-PLZF transgenic mice and cells harboring t(15;17), HDACIs induced apoptosis and dramatic growth inhibition, effects that could be potentiated by RA. HDACIs also increased RA-induced differentiation. HDACIs, but not RA, induced accumulation of acetylated histones. Using microarray analysis, we identified genes induced by RA, HDACIs, or both together. In combination with RA, all HDACIs tested overcame the transcriptional repression exerted by the RAR alpha fusion oncoproteins. In vivo, HDACIs induced accumulation of acetylated histones in target organs. Strikingly, this combination of agents induced leukemia remission and prolonged survival, without apparent toxic side effects.

The theory of APL

Acute promyelocytic leukemia (APL) is associated with reciprocal and balanced chromosomal translocations always involving the Retinoic Acid Receptor alpha (RARalpha) gene on chromosome 17 and variable partner genes (X genes) on distinct chromosomes. RARalpha fuses to the PML gene in the vast majority of APL cases, and in a few cases to the PLZF, NPM, NuMA and STAT5b genes. As a consequence, X-RARalpha and RARalpha-X fusion genes are generated encoding aberrant fusion proteins that can interfere with X and/or RARalpha function. Here we will review the relevant conclusions and the open questions that stem from a decade of in vivo analysis of APL pathogenesis in the mouse in transgenic and knock-out models.

In vivo analysis of the molecular genetics of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is a distinct and paradigmatic subtype of myeloid leukemia associated with reciprocal chromosomal translocations always involving the Retinoic Acid Receptor(alpha) (RARalpha) gene on chromosome 17 and variable partner genes (X genes) on different chromosomes. As a consequence of these translocations X-RARalpha and RARalpha-X fusion genes are generated. RARalpha fuses to the PML gene in the vast majority of APL cases, and in a few cases to the PLZF, NPM, NuMA and STAT5b genes respectively. In the last few years, the functions of these aberrant fusion proteins and of the normal gene products involved in these translocations have been extensively characterized in vivo in transgenic and KO animal models. Here we will review the important conclusions, the novel questions and paradoxes that stem from this analysis.

Histone deacetylases and transcriptional therapy with their inhibitors

Acute promyelocytic leukemia (APL) is characterized by the expansion of malignant myeloid cells blocked at the promyelocytic stage of hemopoietic development and is invariably associated with reciprocal chromosomal translocations involving the retinoic acid receptor alpha (RARalpha) gene on chromosome 17. RARalpha variably fuses to PML, PLZF, NPM, NuMA, and Stat5B genes (X genes/proteins). These translocations are balanced and reciprocal, thus leading to the generation of X-RARalpha and RARalpha-X fusion genes of which the products coexist in the APL blast. The invariable involvement in these translocations of RARalpha, a prototypical transcription factor, makes APL a compelling example of aberrant transcriptional mechanisms in the etiopathogenesis of cancer. This paper focuses on the recent progress in defining the molecular mechanisms underlying APL pathogenesis and addresses how this new understanding has allowed the proposal and development of novel therapeutic strategies with compounds such as histone deacetylase inhibitors and inorganic arsenicals such as As2O3 which are currently being tested in murine leukemia models as well as in human APL patients. In particular, the crucial role played by the aberrant transcriptional activities of X-RARalpha and RARalpha-X fusion proteins in APL pathogenesis is discussed by reviewing the relevant therapeutic implications resulting from this analysis.

The role of promyelocytic leukemia zinc finger and promyelocytic leukemia in leukemogenesis and development

Acute promyelocytic leukemia (APL) was originally distinguished by an extremely poor clinical outcome. In the past few years, however, important progress has been made in defining the molecular basis of APL pathogenesis and in optimizing its treatment to an extent that this leukemia is now considered curable. Two features are unique to this leukemia: its remission after retinoic acid (RA) treatment through induction of blast differentiation, and the presence in the leukemic blast of fusion proteins in which the retinoic acid receptor alpha (RARalpha) fuses to distinct partners. Here we review how a detailed analysis of the functions of two of these RARalpha partners, the promyelocytic leukemia (PML) and promyelocytic leukemia zinc finger (PLZF) proteins, has allowed a greater understanding of the molecular mechanisms implicated in APL pathogenesis.

Mzf1 controls cell proliferation and tumorigenesis

MZF1 is a transcription factor belonging to the Krüppel family of zinc finger proteins, expressed in totipotent hemopoietic cells as well as in myeloid progenitors. Here we have inactivated Mzfi1 by gene targeting. Mzf1(-/-) mice develop lethal neoplasias characterized by the infiltration and complete disruption of the liver architecture by a monomorphic population of cells of myeloid origin reminiscent of human chloromas. Mzf1 inactivation results in a striking increase of the autonomous cell proliferation and of the ability of Mzf1(-/-) hemopoietic progenitors to sustain long-term hemopoiesis. These findings demonstrate that Mzf1 can act as a tumor/growth suppressor in the hemopoietic compartment.

Role of the promyelocytic leukemia protein PML in the interferon sensitivity of lymphocytic choriomeningitis virus

Lymphocytic choriomeningitis virus (LCMV) induces type I interferon (alpha and beta interferon [IFN-alpha and IFN-beta]) upon infection and yet is sensitive to the addition of type II interferon (gamma interferon [IFN-gamma]) to the culture media. This sensitivity is biologically important because it correlates inversely with the ability of certain LCMV strains to persist in mice (D. Moskophidis, M. Battegay, M. A. Bruendler, E. Laine, I. Gresser, and R. M. Zinkernagel, J. Virol. 68:1951-1955, 1994). The cellular oncoprotein PML is induced by both IFN-alpha/beta and IFN-gamma, and PML binds the LCMV Z protein and becomes redistributed within cells from nucleus to cytoplasm upon LCMV infection. In the present study, increased PML expression results in diminished LCMV replication, implicating PML in the IFN sensitivity of LCMV. Virus production in PML -/- murine embryonic fibroblasts (MEF) exceeds virus production in PML +/+ MEF, and this difference is exacerbated by IFN treatment that upregulates PML expression. IFN-gamma also diminishes LCMV production in PML -/- cells; therefore, viral IFN sensitivity is not entirely due to PML. Both viral mRNA production and viral protein production decrease as PML expression increases. Here we propose that PML reduces LCMV transcription through its interaction with the Z protein.

Altered myelopoiesis and the development of acute myeloid leukemia in transgenic mice overexpressing cyclin A1

A mammalian A-type cyclin, cyclin A1, is highly expressed in testes of both human and mouse and targeted mutagenesis in the mouse has revealed the unique requirement for cyclin A1 in the progression of male germ cells through the meiotic cell cycle. While very low levels of cyclin A1 have been reported in the human hematopoietic system and brain, the sites of elevated levels of expression of human cyclin A1 were several leukemia cell lines and blood samples from patients with hematopoietic malignances, notably acute myeloid leukemia. To evaluate whether cyclin A1 is directly involved with the development of myeloid leukemia, mouse cyclin A1 protein was overexpressed in the myeloid lineage of transgenic mice under the direction of the human cathepsin G (hCG) promoter. The resulting transgenic mice exhibited an increased proportion of immature myeloid cells in the peripheral blood, bone marrow, and spleen. The abnormal myelopoiesis developed within the first few months after birth and progressed to overt acute myeloid leukemia at a low frequency ( approximately 15%) over the course of 7-14 months. Both the abnormalities in myelopoiesis and the leukemic state could be transplanted to irradiated SCID (severe combined immunodeficient) mice. The observations suggest that cyclin A1 overexpression results in abnormal myelopoiesis and is necessary, but not sufficient in the cooperative events inducing the transformed phenotype. The data further support an important role of cyclin A1 in hematopoiesis and the etiology of myeloid leukemia.

Evidence for BLM and Topoisomerase IIIalpha interaction in genomic stability

The genomic instability of persons with Bloom's syndrome (BS) features particularly an increased number of sister-chromatid exchanges (SCEs). The primary cause of the genomic instability is mutation at BLM, which encodes a DNA helicase of the RecQ family. BLM interacts with Topoisomerase IIIalpha (Topo IIIalpha), and both BLM and Topo IIIalpha localize to the nuclear organelles referred to as the promyelocytic leukemia protein (PML) nuclear bodies. In this study we show, by analysis of cells that express various deletion constructs of green fluorescent protein (GFP)-tagged BLM, that the first 133 amino acids of BLM are necessary and sufficient for interaction between Topo IIIalpha and BLM. The Topo IIIalpha-interaction domain of BLM is not required for BLM's localization to the PML nuclear bodies; in contrast, Topo IIIalpha is recruited to the PML nuclear bodies via its interaction with BLM. Expression of a full-length BLM (amino acids 1-1417) in BS cells can correct their high SCEs to normal levels, whereas expression of a BLM fragment that lacks the Topo IIIalpha interaction domain (amino acids 133-1417) results in intermediate SCE levels. The deficiency of amino acids 133-1417 in the reduction of SCEs was not explained by a defect in DNA helicase activity, because immunoprecipitated 133-1417 protein had 4-fold higher activity than GFP-BLM. The data implicate the BLM-Topo IIIalpha complex in the regulation of recombination in somatic cells.

Role of PML and PML-RARalpha in Mad-mediated transcriptional repression

Fusion of the promyelocytic leukemia (PML) protein to the retinoic acid receptor-alpha (RARalpha) generates the transforming protein of acute promyelocytic leukemias. PML appears to be involved in multiple functions, including apoptosis and transcriptional activation by RAR, whereas PML-RARalpha blocks these functions of PML. However, the mechanisms of leukemogenesis by PML-RARalpha remain elusive. Here we show that PML interacts with multiple corepressors (c-Ski, N-CoR, and mSin3A) and histone deacetylase 1, and that this interaction is required for transcriptional repression mediated by the tumor suppressor Mad. PML-RARalpha has the two corepressor-interacting sites and inhibits Mad-mediated repression, suggesting that aberrant binding of PML-RARalpha to the corepressor complexes may lead to abrogation of the corepressor function. These mechanisms may contribute to events leading to leukemogenesis.

Transcription therapy for cancer

In the post genome era it will soon be possible to associate a specific tumor type with a specific gene expression profile and to define each molecular lesion characteristic of any given cancer. It is intuitive that a successful therapeutic strategy for cancer should aim at blocking the aberrant biochemical activity triggered by the oncogene or the lack of tumor suppressor gene activity that ultimately leads to full-blown neoplastic transformation. However, an attractive alternative approach entails the blockade of the transcriptional consequences of such oncogenic activities irrespective of their original biochemical nature, thus antagonizing the key transcriptional events underlying cancer pathogenesis in any specific neoplastic cellular population. This approach is now rendered possible by major advances along several lines of investigation: (i) the possibility of analysing gene expression through high throughput methods; (ii) a more detailed knowledge of the regulatory regions and of the transcription factors that control gene expression also facilitated in the future by a comprehensive whole genome comparative analysis of these regulatory sequences; (iii) the ability of modulating gene expression at the single gene level through various approaches both pharmacological and biochemical; (iv) the opportunity of directly antagonizing the aberrant activities of oncogenic transcription factors through a detailed knowledge of their abnormal transcriptional function; (v) the possibility of validating, in vivo, in animal models the relevance for neoplastic transformation of specific transcriptional events as well as of testing the efficacy of 'transcription therapy' in faithful animal models of human cancer. Here, we will review the facts, the existing applications and the hypothesis underlying such therapeutic modality for cancer therapy.

Oncogenes and tumor suppressors in the molecular pathogenesis of acute promyelocytic leukemia

Acute promyelocytic leukemia (APL) is associated with reciprocal chromosomal translocations always involving the retinoic acid receptor alpha (RARalpha) gene on chromosome 17 and variable partner genes (X genes) on distinct chromosomes. RARalpha fuses to the PML gene in the vast majority of APL cases, and in a few cases to the PLZF, NPM, NuMA and Stat5b genes, respectively, leading to the generation of RARalpha-X: and X:-RARalpha fusion genes. Both fusion proteins can exert oncogenic functions through their ability to interfere with the activities of X and RARalpha proteins. Here, it will be discussed in detail how an extensive biochemical analysis as well as a systematic in vivo genetic approach in the mouse has allowed the definition of the multiple oncogenic activities of PML-RARalpha, and how it has become apparent that this oncoprotein is able to impair RARalpha at the transcription level and the tumor suppressive function of the PML protein.

Pten and p27KIP1 cooperate in prostate cancer tumor suppression in the mouse

The genetic bases underlying prostate tumorigenesis are poorly understood. Inactivation of the tumor-suppressor gene PTEN and lack of p27(KIP1) expression have been detected in most advanced prostate cancers. But mice deficient for Cdkn1b (encoding p27(Kip1)) do not develop prostate cancer. PTEN activity leads to the induction of p27(KIP1) expression, which in turn can negatively regulate the transition through the cell cycle. Thus, the inactivation of p27(KIP1) may be epistatic to PTEN in the control of the cell cycle. Here we show that the concomitant inactivation of one Pten allele and one or both Cdkn1b alleles accelerates spontaneous neoplastic transformation and incidence of tumors of various histological origins. Cell proliferation, but not cell survival, is increased in Pten(+/-)/Cdkn1b(-/-) mice. Moreover, Pten(+/-)/Cdkn1b(-/-) mice develop prostate carcinoma at complete penetrance within three months from birth. These cancers recapitulate the natural history and pathological features of human prostate cancer. Our findings reveal the crucial relevance of the combined tumor-suppressive activity of Pten and p27(Kip1) through the control of cell-cycle progression.

Regulation of Pax3 transcriptional activity by SUMO-1-modified PML

Pax3 is an evolutionarily conserved transcription factor that plays a major role in a variety of developmental processes. Mutations in Pax3 lead to severe malformations as seen in human Waardenburg syndrome and in the Splotch mutant mice. The transcriptional activity of Pax3 was recently shown to be repressed by Daxx whereas the oncogenic fusion protein Pax3-FKHR is unresponsive to this repressive action. Here we demonstrate that Daxx-mediated repression of Pax3 can be inhibited by the nuclear body (NB)-associated protein PML. Interestingly, this suppression of Daxx properties correlates with its recruitment to the NBs. Factors such as arsenicals and interferons that enhance NB formation, trigger both the targeting of Daxx to these nuclear structures and the relief of the repressive activity of Daxx. Conversely, lack of structurally intact NBs profoundly impairs Pax3 transcriptional activity, likely by increasing the pool of available nucleoplasmic Daxx. Moreover, a PML mutant that can not be modified by the ubiquitin-related SUMO-1 modifier is no more able to interact with Daxx. Consistently, such a mutant fails both to inhibit the Daxx repressing effect on Pax3 and to induce its accumulation into the NBs. Taken together, these results argue that SUMO-1 modified PML can derepress Pax3 transcriptional activity through sequestration of the Daxx repressor into the NBs and suggest a role for these nuclear structures in the transcriptional control by Pax proteins. Oncogene (2001) 20, 1 - 9.

Analysis of the molecular genetics of acute promyelocytic leukemia in mouse models

Acute promyelocytic leukemia (APL) is characterized by reciprocal chromosomal translocations that always Involve the retinoic acid receptor-alpha (RARalpha) gene on chromosome 17. RARalpha variably fuses to the PML, PLZF, NPM, NuMA, and STAT 5b genes (X genes), leading to the generation of X-RARalpha and RARalpha-X fusion genes. The aberrant X-RARalpha proteins retain the dimerization domains of their parental proteins and therefore can act as dominant negative oncogenic products on both RARalpha/RXR and X pathways. Studies in transgenic mice harboring X-RARalpha and RARalpha-X fusion genes and In mice lacking X genes have helped unravel the molecular mechanisms underlying APL leukemogenesis, which lead to the development of novel therapeutic strategies. Moreover, transgenic mouse models of APL were useful to test in vivo the efficacy of these novel therapeutic approaches as well as of drug combinations such as retinoic acid and As2O3 that were previously known to be effective as single agents in human APL.

Modeling Acute Promyelocytic Leukemia in the Mouse: New Insights in the Pathogenesis of Human Leukemias

Acute promyelocytic leukemia (APL) is characterized by the expansion of malignant myeloid cells blocked at the promyelocytic stage of differentiation and is associated with reciprocal chromosomal translocations always involving the retinoic acid receptor alpha (RARalpha) gene on chromosome 17. As a consequence of the translocation, RARalpha variably fuses to the PML, PLZF, NPM, NuMA, and Stat5b genes (X genes), respectively, leading to the generation of RARalpha-X and X-RARalpha fusion genes. The aberrant chimeric proteins encoded by these genes, as well as the inactivation of the X and RARalpha functions, may exert a crucial role in leukemogenesis. To define the molecular genetics of APL and the contribution of each molecular event in APL pathogenesis, we have generated transgenic mice harboring X-RARalpha and/or RARalpha-X genes as well as mice where the various X genes have been inactivated by homologous recombination. Here we show that while the X-RARalpha fusion gene is crucial for leukemogenesis, the presence of RARalpha-X and the inactivation of X function are critical in modulating the onset as well as the phenotype of the leukemia.

Regulation of p53 activity in nuclear bodies by a specific PML isoform

Covalent modification of the promyelocytic leukaemia protein (PML) by SUMO-1 is a prerequisite for the assembly of nuclear bodies (NBs), subnuclear structures disrupted in various human diseases and linked to transcriptional and growth control. Here we demonstrate that p53 is recruited into NBs by a specific PML isoform (PML3) or by coexpression of SUMO-1 and hUbc9. NB targeting depends on the direct association of p53, through its core domain, with a C-terminal region of PML3. The relocalization of p53 into NBs enhances p53 transactivation in a promoter-specific manner and affects cell survival. Our results indicate the existence of a cross-talk between PML- and p53-dependent growth suppression pathways, implying an important role for NBs and their resident proteins as modulators of p53 functions.

Two critical hits for promyelocytic leukemia

Acute promyelocytic leukemia (APL) is associated with chromosomal translocations that always involve the RARalpha gene, which variably fuses to one of several distinct loci, including PML or PLZF (X genes). Due to the reciprocity of the translocation, X-RARalpha and RARalpha-X fusion proteins coexist in APL blasts. PLZF-RARalpha transgenic mice (TM) develop leukemia that lacks the differentiation block at the promyelocytic stage that characterizes APL. We generated TM expressing RARalpha-PLZF and PLZF-RARalpha in their promyelocytes. RARalpha-PLZF TM do not develop leukemia. However, PLZF-RARalpha/RARalpha-PLZF double TM develop leukemia with classic APL features. We demonstrate that RARalpha-PLZF can interfere with PLZF transcriptional repression and that this is critical for APL pathogenesis, since leukemias in PLZF(-/-)/PLZF-RARalpha mutants and in PLZF-RARalpha/RARalpha-PLZF TM are indistinguishable. Thus, both products of a cancer-associated translocation are crucial in determining the distinctive features of the disease.

The function of PML in p53-dependent apoptosis

The PML gene of acute promyelocytic leukaemia (APL) encodes a growth- and tumour-suppresor protein that is essential for several apoptotic signals. The mechanisms by which PML exerts its pro-apoptotic function are still unknown. Here we show that PML acts as a transcriptional co-activator with p53. PML physically interacts with p53 both in vitro and in vivo and co-localizes with p53 in the PML nuclear body (PML-NB). The co-activatory role of PML depends on its ability to localize in the PML-NB. p53-dependent, DNA-damage-induced apoptosis, transcriptional activation by p53, the DNA-binding ability of p53, and the induction of p53 target genes such as Bax and p21 upon gamma-irradiation are all impaired in PML-/- primary cells. These results define a new PML-dependent, p53-regulatory pathway for apoptosis and shed new light on the function of PML in tumour suppression.

BCL-6 regulates chemokine gene transcription in macrophages

The transcriptional repressor protein BCL-6, implicated in the pathogenesis of B cell lymphoma, regulates lymphocyte differentiation and inflammation. We investigated the mechanism for the T helper cell subset 2 (TH2)-type inflammation that occurs in BCL-6-/- mice. Using chimeric mice we found that the TH2-type inflammation is dependent upon nonlymphoid cells. We identified three chemokines, MCP-1, MCP-3 and MRP-1, which are negatively regulated by BCL-6 in macrophages. Promoter analysis revealed that BCL-6 is a potent repressor of MCP-1 transcription. Our results provide a mechanism for the regulation of TH2-type inflammation by BCL-6 and link TH2 differentiation to innate immunity.

The puzzling multiple lives of PML and its role in the genesis of cancer

PML, the gene associated with acute promyelocytic leukemia (APL); PML, the target of numerous viral agents; PML, the growth suppressor; PML, the mediator of multiple apoptotic pathways; PML, the tumor suppressor; PML, the protein which epitomizes a novel nuclear structure, the nuclear body; PML, the transcription co-factor. Despite the recent flurry of reports attributing multiple biological roles to the PML protein, PML still lacks a definitive biochemical function. This is probably the reason why PML is so attractive to many investigators. Here, we will summarize the facts and speculations on this puzzling protein.

Retinoic acid (RA) and As2O3 treatment in transgenic models of acute promyelocytic leukemia (APL) unravel the distinct nature of the leukemogenic process induced by the PML-RARalpha and PLZF-RARalpha oncoproteins

Acute promyelocytic leukemia (APL) is associated with chromosomal translocations always involving the RARalpha gene, which variably fuses to one of several distinct loci, including PML or PLZF (X genes) in t(15;17) or t(11;17), respectively. APL in patients harboring t(15;17) responds well to retinoic acid (RA) treatment and chemotherapy, whereas t(11;17) APL responds poorly to both treatments, thus defining a distinct syndrome. Here, we show that RA, As(2)O(3), and RA + As(2)O(3) prolonged survival in either leukemic PML-RARalpha transgenic mice or nude mice transplanted with PML-RARalpha leukemic cells. RA + As(2)O(3) prolonged survival compared with treatment with either drug alone. In contrast, neither in PLZF-RARalpha transgenic mice nor in nude mice transplanted with PLZF-RARalpha cells did any of the three regimens induce complete disease remission. Unexpectedly, therapeutic doses of RA and RA + As(2)O(3) can induce, both in vivo and in vitro, the degradation of either PML-RARalpha or PLZF-RARalpha proteins, suggesting that the maintenance of the leukemic phenotype depends on the continuous presence of the former, but not the latter. Our findings lead to three major conclusions with relevant therapeutic implications: (i) the X-RARalpha oncoprotein directly determines response to treatment and plays a distinct role in the maintenance of the malignant phenotype; (ii) As(2)O(3) and/or As(2)O(3) + RA combination may be beneficial for the treatment of t(15;17) APL but not for t(11;17) APL; and (iii) therapeutic strategies aimed solely at degrading the X-RARalpha oncoprotein may not be effective in t(11;17) APL.

PML regulates p53 acetylation and premature senescence induced by oncogenic Ras

The tumour suppressor p53 induces cellular senescence in response to oncogenic signals. p53 activity is modulated by protein stability and post-translational modification, including phosphorylation and acetylation. The mechanism of p53 activation by oncogenes remains largely unknown. Here we report that the tumour suppressor PML regulates the p53 response to oncogenic signals. We found that oncogenic Ras upregulates PML expression, and overexpression of PML induces senescence in a p53-dependent manner. p53 is acetylated at lysine 382 upon Ras expression, an event that is essential for its biological function. Ras induces re-localization of p53 and the CBP acetyltransferase within the PML nuclear bodies and induces the formation of a trimeric p53-PML-CBP complex. Lastly, Ras-induced p53 acetylation, p53-CBP complex stabilization and senescence are lost in PML-/- fibroblasts. Our data establish a link between PML and p53 and indicate that integrity of the PML bodies is required for p53 acetylation and senescence upon oncogene expression.

Plzf regulates limb and axial skeletal patterning

The promyelocytic leukaemia zinc finger (Plzf) protein (encoded by the gene Zfp145) belongs to the POZ/zinc-finger family of transcription factors. Here we generate Zfp145-/- mice and show that Plzf is essential for patterning of the limb and axial skeleton. Plzf inactivation results in patterning defects affecting all skeletal structures of the limb, including homeotic transformations of anterior skeletal elements into posterior structures. We demonstrate that Plzf acts as a growth-inhibitory and pro-apoptotic factor in the limb bud. The expression of members of the abdominal b (Abdb) Hox gene complex, as well as genes encoding bone morphogenetic proteins (Bmps), is altered in the developing limb of Zfp145-/- mice. Plzf regulates the expression of these genes in the absence of aberrant polarizing activity and independently of known patterning genes. Zfp145-/- mice also exhibit anterior-directed homeotic transformation throughout the axial skeleton with associated alterations in Hox gene expression. Plzf is therefore a mediator of anterior-to-posterior (AP) patterning in both the axial and appendicular skeleton and acts as a regulator of Hox gene expression.