Characterization and chromosomal localization of the human proto-oncogene BMI-1

Independent prognostic value of the basal-like phenotype of breast cancer and associations with EGFR and candidate stem cell marker BMI-1

AIMS

To study the relationship between basal-like breast cancers, epidermal growth factor receptor (EGFR) and candidate stem cell markers (BMI-1, EZH2, Oct-4) in a population-based setting.

METHODS AND RESULTS

Immunohistochemistry was evaluated in a series of 190 breast cancers. Basal-like phenotype (BLP) 1-5 was found in 4.3-14.3% of cases. EGFR was expressed in 9% of cases and associated with cytokeratin (CK) 5 and P-cadherin positivity, but not with survival; 28% of CK5+ cases were EGFR+. On multivariate analysis, basal-like differentiation and lymph node status were independent prognostic factors of comparable strength. BMI-1 positivity (42.6%) was associated with absence of basal-like features, oestrogen receptor positivity and low Ki67, but not related to survival. BMI was not associated with EZH2 expression, and these markers tended to show opposite associations with other variables, suggesting different roles in breast cancer. Oct-4 expression was not detected in this series.

CONCLUSIONS

Basal-like features and lymph node status were strong and independent prognostic factors in this population-based series of breast cancer. Neither EGFR nor BMI-1 had significant prognostic impact, whereas EZH2 expression was associated with decreased survival. BMI-1 was inversely related to basal-like factors, and a stem cell phenotype of the basal-like subgroup could not be verified by this marker.

Expression of Bmi-1 protein in tumor tissues is associated with favorable prognosis in breast cancer patients

PURPOSE

Abnormal expression of the cell cycle regulatory protein Bmi-1 has been studied in breast cancer, but the clinical relevance has not been fully elucidated. We studied the expression of Bmi-1 protein in breast cancer patients to define its clinical significance in breast cancer.

EXPERIMENTAL DESIGN

Tissue microarrays were performed to evaluate the expression of Bmi-1 by immunohistochemistry in tumor tissues from 960 patients with stage I-III breast cancer. We assessed the relationship between the expression of Bmi-1 and pathologic prognostic indices as well as clinical long-term follow up outcome.

RESULTS

Bmi-1 was expressed in 53.2% of breast cancer patients by immunohistochemistry, and the expression of Bmi-1 was significantly correlated with favorable prognostic indices at diagnosis. In univariate analysis, patients with Bmi-1 expression showed favorable relapse-free survival (88.6+/-2.7% vs. 72.3+/-4.3%, P=0.041) and favorable overall survival (93.5+/-2.2% vs. 82.6+/-3.5%, P<0.001) than patients without Bmi-1 expression. According to multivariate analyses, Bmi-1 expression was identified as independent prognostic factor for overall survival with a statistical significance (hazard ratio of Bmi-1 (-) patients compared to Bmi-1 (+) patients, 1.744; 95% CI, 1.013-3.003; P=0.045). This correlation of Bmi-1 expression with favorable overall survival was maintained in patients underwent uniform chemotherapy, regardless of undergoing adjuvant chemotherapy. In a subset analysis according to ER status, Bmi-1 expression associated with favorable overall survival only in ER-positive patients.

CONCLUSIONS

Bmi-1 expression assessed with Immunohistochemistry may be associated with favorable overall survival in breast cancer patients, especially in patients with ER-positive breast cancer.

Expression of Bmi-1 in epidermis enhances cell survival by altering cell cycle regulatory protein expression and inhibiting apoptosis

The polycomb group (PcG) genes are epigenetic suppressors of gene expression that play an important role in development. In this study, we examine the role of Bmi-1 (B-cell-specific Moloney murine leukemia virus integration site 1) as a regulator of human epidermal keratinocyte survival. We identify Bmi-1 mRNA and protein expression in epidermis and in cultured human keratinocytes. Bmi-1 is located in the nucleus in cultured keratinocytes, and in epidermis it is expressed in the basal and suprabasal layers. Adenovirus-delivered Bmi-1 promotes keratinocyte survival and protects keratinocytes from stress agent-mediated cell death. This is associated with increased levels of cyclin D1 and selected cyclin-dependent kinases, and reduced caspase activity and poly(ADP-ribose) polymerase (PARP) cleavage. Bmi-1 may be involved in the maintenance of disease state, as Bmi-1 levels are elevated in transformed keratinocytes, skin tumors, and psoriasis. The presence of Bmi-1 in suprabasal non-proliferative cells of the epidermis and within a high percentage of cells within skin tumors suggests a non-stem cell pro-survival role for Bmi-1 in this tissue. Based on the suprabasal distribution of Bmi-1 in epidermis, we propose that Bmi-1 may promote maintenance of suprabasal keratinocyte survival to prevent premature death during differentiation. Such a function would help assure proper formation of the stratified epidermis.

BMI-1 is highly expressed in M0-subtype acute myeloid leukemia

Recent studies have suggested that one of the polycomb group genes, BMI-1, has an important role in the maintenance of normal and leukemic stem cells by repressing the INK4a/ARF locus. Here, we quantitatively examined BMI-1 expression level in samples from patients with acute myeloid leukemia (AML) and other hematologic malignancies. Moderate to high BMI-1 expression was detected in AML patients, and the BMI-1 expression levels in AML samples were significantly higher than in normal bone marrow controls (P = .0011). Specimens of French-American-British classification subtype M0 showed higher relative expression of the BMI-1 transcript (median, 390.2 3 10(-3)) than the other subtypes (median, 139.0 3 10(-3)) (P < .0001). Leukemia other than AML showed low to moderate expression. INK4a-ARF transcript expression tended to be inverse proportion to that of BMI-1. In an M0 patient with a high BMI-1 transcript level, the INK4a-ARF transcript level fell promptly and maintained a low value after the patient achieved complete remission. These results indicated that a subgroup of M0 patients has a high expression level of polycomb group gene BMI-1, which may contribute to leukemogenesis.

Direct evidence for cooperating genetic events in the leukemic transformation of normal human hematopoietic cells

Although genetic abnormalities associated with hematological malignancies are readily identified, the natural history of human leukemia cannot be observed because initiating and subsequent transforming events occur before clinical presentation. Furthermore, it has not been possible to study leukemogenesis in vitro as normal human cells do not spontaneously transform. Thus, the nature and sequence of genetic changes required to convert human hematopoietic cells into leukemia cells have never been directly examined. We have developed a system where the first step in the leukemogenic process is an engineered disruption of differentiation and self-renewal due to expression of the TLS-ERG oncogene, followed in some cases by overexpression of hTERT. In two of 13 experiments, transduced cells underwent step-wise transformation and immortalization through spontaneous acquisition of additional changes. The acquired karyotypic abnormalities and alterations including upregulation of Bmi-1 and telomerase all occur in acute myeloid leukemia (AML), establishing the relevance of this system. One resultant cell line studied in depth exhibits cellular properties characteristic of AML, notably a hierarchical organization initiated by leukemic stem cells that differentiate abnormally. These findings provide direct evidence for multiple cooperating events in human leukemogenesis, and provide a foundation for studying the genetic changes that occur during leukemic initiation and progression.

Mammary stem cells, self-renewal pathways, and carcinogenesis

The mammary gland epithelial components are thought to arise from stem cells that undergo both self-renewal and differentiation. Self-renewal has been shown to be regulated by the Hedgehog, Notch, and Wnt pathways and the transcription factor B lymphoma Mo-MLV insertion region 1 (Bmi-1). We review data about the existence of stem cells in the mammary gland and the pathways regulating the self-renewal of these cells. We present evidence that deregulation of the self-renewal in stem cells/progenitors might be a key event in mammary carcinogenesis. If 'tumor stem cells' are inherently resistant to current therapies, targeting stem cell self-renewal pathways might provide a novel approach for breast cancer treatment.

HOXA genes are included in genetic and biologic networks defining human acute T-cell leukemia (T-ALL)

Using a combination of molecular cytogenetic and large-scale expression analysis in human T-cell acute lymphoblastic leukemias (T-ALLs), we identified and characterized a new recurrent chromosomal translocation, targeting the major homeobox gene cluster HOXA and the TCRB locus. Real-time quantitative polymerase chain reaction (RQ-PCR) analysis showed that the expression of the whole HOXA gene cluster was dramatically dysregulated in the HOXA-rearranged cases, and also in MLL and CALM-AF10-related T-ALL cases, strongly suggesting that HOXA genes are oncogenic in these leukemias. Inclusion of HOXA-translocated cases in a general molecular portrait of 92 T-ALLs based on large-scale expression analysis shows that this rearrangement defines a new homogeneous subgroup, which shares common biologic networks with the TLX1- and TLX3-related cases. Because T-ALLs derive from T-cell progenitors, expression profiles of the distinct T-ALL subgroups were analyzed with respect to those of normal human thymic subpopulations. Inappropriate use or perturbation of specific molecular networks involved in thymic differentiation was detected. Moreover, we found a significant association between T-ALL oncogenic subgroups and ectopic expression of a limited set of genes, including several developmental genes, namely HOXA, TLX1, TLX3, NKX3-1, SIX6, and TFAP2C. These data strongly support the view that the abnormal expression of developmental genes, including the prototypical homeobox genes HOXA, is critical in T-ALL oncogenesis.

Maintenance of gene expression patterns

In development, cells pass on established gene expression patterns to daughter cells over multiple rounds of cell division. The cellular memory of the gene expression state is termed maintenance, and the proteins required for this process are termed maintenance proteins. The best characterized are proteins of the Polycomb and trithorax Groups that are required for silencing and maintenance of activation of target loci, respectively. These proteins act through DNA elements termed maintenance elements. Here, we re-examine the genetics and molecular biology of maintenance proteins. We discuss molecular models for the maintenance of activation and silencing, and the establishment of epigenetic marks, and suggest that maintenance proteins may play a role in propagating the mark through DNA synthesis.

DNA microarray analysis of natural killer cell-type lymphoproliferative disease of granular lymphocytes with purified CD3-CD56+ fractions

Natural killer (NK) cell-type lymphoproliferative disease of granular lymphocytes (LDGL) is characterized by the outgrowth of CD3(-)CD16/56(+) NK cells, and can be further subdivided into two distinct categories: aggressive NK cell leukemia (ANKL) and chronic NK lymphocytosis (CNKL). To gain insights into the pathophysiology of NK cell-type LDGL, we here purified CD3(-)CD56(+) fractions from healthy individuals (n=9) and those with CNKL (n=9) or ANKL (n=1), and compared the expression profiles of >12 000 genes. A total of 15 'LDGL-associated genes' were identified, and a correspondence analysis on such genes could clearly indicate that LDGL samples share a 'molecular signature' distinct from that of normal NK cells. With a newly invented class prediction algorithm, 'weighted distance method', all 19 samples received a clinically matched diagnosis, and, furthermore, a detailed cross-validation trial for the prediction of normal or CNKL status could achieve a high accuracy (77.8%). By applying another statistical approach, we could extract other sets of genes, expression of which was specific to either normal or LDGL NK cells. Together with sophisticated statistical methods, gene expression profiling of a background-matched NK cell fraction thus provides us a wealth of information for the LDGL condition.

The Bmi-1 oncoprotein is overexpressed in human colorectal cancer and correlates with the reduced p16INK4a/p14ARF proteins

To clarify the roles of Bmi-1 in colorectal carcinoma, we examined the expression of Bmi-1 in 41 samples out of 46 colorectal carcinomas by reverse transcription-PCR, whereas all 46 were analyzed by immunostaining. In addition, we analyzed the expression patterns of Bmi-1 in association with p16INK4a and p14ARF (in mouse p19ARF) in a series of colorectal carcinomas. The level of Bmi-1 mRNA in the carcinoma tissues was significantly higher than those of the adjacent non-neoplastic colonic mucosal tissues. Immunohistochemistry for Bmi-1 showed moderate or strong expression levels in 65% (30/46) of colorectal carcinomas. Colorectal carcinomas with moderate or strong Bmi-1 expression were more likely to have low levels of the INK4 locus proteins (p16INK4a/p14ARF) (P<0.07). These results suggested that modulation of Bmi-1 protein might be involved in human colorectal carcinogenesis by repressing the INK4a/ARF proteins.

A panel of monoclonal antibodies against human polycomb group proteins

Polycomb-group (PcG) proteins are chromatin-associated proteins that heritably repress gene activity in many organisms, including man. Two distinct human PcG complexes have been identified. The HPC/HPH PcG complex I contains the HPC, HPH, RING1, and BMI1 proteins, the EED/EZH2 PcG complex II contains the EED, EZH2, and YY1 proteins. Previously we found that the relative expression levels of proteins of the human PcG complexes I and II are severely deregulated in human tumors. These findings signify an important role for antibodies against human PcG proteins as diagnostic tools. To be able to produce standardized anti-human PcG antibodies, we developed a panel of five mouse monoclonal antibodies (MAbs) against the human PcG proteins HPC2, BMI1, RING1A, EED, and EZH2. All MAbs can be used for Western blot analysis and immunofluorescence labeling of tissue culture cells. With the exception of the MAb against HPC2, all MAbs can also be used in immunoprecipitation experiments and immunohistochemistry of human tissues. The novel MAbs are therefore valuable tools for the cell biological, biochemical, and pathological analysis of human PcG proteins.

Molecular cloning, mapping and characterization of a novel mouse RING finger gene, Mrf1

With a combined approach of database search, heterologous polymerase chain reaction (PCR), reverse transcription-PCR, rapid amplification of complementary DNA ends and genomic library screening, we have successfully cloned a mouse RING finger gene, mouse RING finger 1 (Mrf1). The Mrf1 gene has two exons of 63 and 2665 bp, respectively, and one intron of over 13 kb. An open reading frame was identified exclusively in exon 2, which encodes a putative protein of the RING-B box-coiled coil or the tripartite motif type of 403 amino acids. Mrf1 is moderately expressed in the spleen, brain and heart as a single 3.0 kb product and very highly expressed in the testis as two transcripts of 3.0 and 1.5 kb, respectively. The Mrf1 gene was mapped to mouse chromosome 3, between markers D3Mit70 and D3Mit277. Western blotting analysis indicated that an expected protein of approximately 44 kD was detected in the brain extracts of mouse, rat and human. The possible functions of Mrf1 are discussed in the contexts of protein-protein interactions, oncogenesis and ubiquitination.

Differential expression of human Polycomb group proteins in various tissues and cell types

Polycomb group proteins are involved in the maintenance of cellular identity. As multimeric complexes they repress cell type-specific sets of target genes. One model predicts that the composition of Polycomb group complexes determines the specificity for their target genes. To study this hypothesis, we analyzed the expression of Polycomb group genes in various human tissues using Northern blotting and immunohistochemistry. We found that Polycomb group expression varies greatly among tissues and even among specific cell types within a particular tissue. Variations in mRNA expression ranged from expression of all analyzed Polycomb group genes in the heart and testis to no detectable Polycomb group expression at all in bone marrow. Furthermore, each Polycomb group gene was expressed in a different number of tissues. RING1 was expressed in practically all tissues, while HPH1 was expressed in only a few tissues. Also within one tissue the level of Polycomb group expression varied greatly. Cell type-specific Polycomb group expression patterns were observed in thyroid, pancreas, and kidney. Finally, in various developmental stages of fetal kidney, different Polycomb group expression patterns were observed. We conclude that Polycomb group expression can vary depending on the tissue, cell type, and development stage. Polycomb group complexes can only be composed of the Polycomb group proteins that are expressed. This implies that with cell type-specific Polycomb group expression patterns, cell type-specific Polycomb group complexes exist. The fact that there are cell type-specific Polycomb group targets and cell type-specific Polycomb group complexes fits well with the hypothesis that the composition of Polycomb group complexes may determine their target specificity. J. Cell. Biochem. Suppl. 36: 129-143, 2001.

The bmi-1 oncoprotein is differentially expressed in non-small cell lung cancer and correlates with INK4A-ARF locus expression

Genes of the polycomb group function by silencing homeotic selector genes that regulate embryogenesis. In mice, downregulation of one of the polycomb genes, bmi-1, leads to neurological alterations and severe proliferative defects in lymphoid cells, whilst bmi-1 overexpression, together with upregulation of myc-1, induces lymphoma. An oncogenic function has been further supported in primary fibroblast studies where bmi-1 overexpression induces immortalization due to repression of p16/p19ARF, and where together with H-ras, it readily transforms MEFs. It was the aim of this study to assess the expression of bmi-1 in resectable non-small cell lung cancer (NSCLC) in association with p16 and p14ARF (=human p19ARF). Tumours (48 resectable NSCLC (32 squamous, 9 adeno-, 2 large cell, 4 undifferentiated carcinomas and 1 carcinoid); stage I, 29, II, 7, III, 12; T1, 18, T2, 30; differentiation: G1 12, G2 19, G3 17) were studied by immunohistochemistry for protein expression and by comparative multiplex PCR for gene amplification analysis. In tumour-free, normal lung tissue from patients, weak - moderate bmi-1 staining was seen in some epithelial cells, lymphocytes, glandular cells and in fibroblasts, whereas blood, endothelial, chondrocytes, muscle cells and adipocytes did not exhibit any bmi-1 expression. In tumours, malignant cells were negative/weakly, moderately and strongly positive in 20, 22 and 6 cases, respectively. As assessed by multiplex PCR, bmi-1 gene amplification was not the reason for high-level bmi-1 expression. Tumours with moderate or strong bmi-1 expression were more likely to have low levels of p16 and p14ARF (P = 0.02). Similarly, tumours negative for both, p16 and p14ARF, exhibit moderate-strong bmi-1 staining. 58% of resectable NSCLC exhibit moderate-high levels of bmi-1 protein. The inverse correlation of bmi-1 and the INK4 locus proteins expression (p16/p14ARF) supports a possible role for bmi-1 misregulation in lung carcinogenesis.

Clinicopathological features of myeloid/natural killer (NK) cell precursor acute leukemia

Four patients (three males and one female) were diagnosed as myeloid/natural killer (NK) cell precursor acute leukemia in our department. Two patients showed the extramedullary involvement at initial presentation. Leukemic cells expressed CD7, CD33, CD45 and CD56 in all patients. Additionally, CD13, CD34, HLA-DR, cytoplasmic CD3 and myeloperoxidase were expressed in some patients. Trisomy 10 was found in two patients, which has not been reported in this disease. Therefore, myeloid/NK cell precursor acute leukemia might be rather heterogeneous especially in chromosomal abnormality though it seemed to constitute the distinct clinical entity among acute myeloid leukemia of M0 subtype.

Lack of the Polycomb-group gene rae28 causes maturation arrest at the early B-cell developmental stage

The rae28 gene (rae28) is a murine homologue of the Drosophila polyhomeotic gene, which is a member of the Polycomb-group genes. In this study, we examined the role of rae28 in lymphocyte development. Because homozygous rae28-deficient (rae28-/-) mice died in the perinatal period, we examined lymphocyte development by generating chimeric mice reconstituted with green fluorescence protein-labeled mutant fetal liver cells as well as in in vitro culture systems. We further examined RAE28 expression by reverse transcriptase polymerase chain reaction assay in human leukemic cells with B-lineage acute lymphoblastic leukemia (ALL). Severe B-cell maturation arrest was observed in rae28-/- between pro- and pre-B lymphocyte stages. B-cell development was also delayed in heterozygous neonates. Furthermore, interleukin-7-dependent colony-forming ability was impaired not only in homozygous lymphocytes but also in heterozygotes. Its human homologue, RAE28, is located on chromosome 12p13, which frequently is associated with chromosomal abnormalities and loss of heterozygosity in patients with hematologic malignancies. To determine whether a link exists between RAE28 and leukemia, we examined RAE28 expression in leukemic cells from pediatric patients with B-lineage ALL. RAE28 expression was not detected in four B-cell precursor ALL cases of a total of 43 examined, although RAE28 is normally expressed constitutively during the process of B-cell maturation as assessed in isolated cell populations. rae28 plays an important role in the early B-cell developmental stage in a gene dosage-dependent manner. Furthermore, the human RAE28 locus may provide a candidate gene causing the molecular pathogenesis of childhood B-cell precursor ALL.

Trisomy 10 as the sole abnormality in biphenotypic leukemia

We observed a case of acute biphenotypic leukemia with trisomy 10 as the sole abnormality. The patient was an adult male diagnosed with ALL(L2), Cell marker studies showed positivity for CD3, CD7, CD13 and CD33, so the phenotypic diagnosis was determined to be biphenotypic leukemia. No case of biphenotypic leukemia with trisomy 10 has been previously reported, until now.

Autosomal dominant thrombocytopenia: incomplete megakaryocyte differentiation and linkage to human chromosome 10

We studied a large kindred with nonsyndromic autosomal dominant thrombocytopenia to define the phenotype and used genomic linkage analysis to determine the locus of the abnormal gene. Affected family members are characterized by lifelong moderate thrombocytopenia (mean = 42.7 × 109/L) with moderate propensity toward easy bruising and minor bleeding. Megakaryocytes are present in bone marrow with reduced frequency, and there are no apparent abnormalities of myeloid or erythroid cells. This type of inherited thrombocytopenia has no evident association with hematopoietic malignancy or progression to aplastic anemia. In the past, members of this family have failed therapeutic trials of immunosuppression and splenectomy. In our investigation, we found that affected individuals had normal platelet size compared with unaffected family members and modestly increased thrombopoietin levels. Hematopoietic colony assays from bone marrow and peripheral blood demonstrated that megakaryocyte precursors (CFU-Mk) were dramatically increased in both number and size in affected individuals. Bone marrow cells grown in liquid culture with thrombopoietin failed to develop polyploid cells greater than 8N. Also, electron microscopy demonstrated that megakaryocytes from an affected individual had markedly delayed nuclear and cytoplasmic differentiation. Genome-wide linkage analysis established a single locus for the disease gene on the short arm of chromosome 10 with a maximum 2-point lod score of 5.68 (at θ = 0). By recruiting additional family members, the genomic region was narrowed to 17 centimorgans. We conclude that a gene in this locus plays an important role in megakaryocyte endomitosis and terminal maturation.

Autosomal dominant thrombocytopenia: incomplete megakaryocyte differentiation and linkage to human chromosome 10

We studied a large kindred with nonsyndromic autosomal dominant thrombocytopenia to define the phenotype and used genomic linkage analysis to determine the locus of the abnormal gene. Affected family members are characterized by lifelong moderate thrombocytopenia (mean = 42.7 × 109/L) with moderate propensity toward easy bruising and minor bleeding. Megakaryocytes are present in bone marrow with reduced frequency, and there are no apparent abnormalities of myeloid or erythroid cells. This type of inherited thrombocytopenia has no evident association with hematopoietic malignancy or progression to aplastic anemia. In the past, members of this family have failed therapeutic trials of immunosuppression and splenectomy. In our investigation, we found that affected individuals had normal platelet size compared with unaffected family members and modestly increased thrombopoietin levels. Hematopoietic colony assays from bone marrow and peripheral blood demonstrated that megakaryocyte precursors (CFU-Mk) were dramatically increased in both number and size in affected individuals. Bone marrow cells grown in liquid culture with thrombopoietin failed to develop polyploid cells greater than 8N. Also, electron microscopy demonstrated that megakaryocytes from an affected individual had markedly delayed nuclear and cytoplasmic differentiation. Genome-wide linkage analysis established a single locus for the disease gene on the short arm of chromosome 10 with a maximum 2-point lod score of 5.68 (at θ = 0). By recruiting additional family members, the genomic region was narrowed to 17 centimorgans. We conclude that a gene in this locus plays an important role in megakaryocyte endomitosis and terminal maturation.

Nucleotide sequence of the feline Bmi-1 coding region

The bmi-1 gene was discovered as a clonal integration site of the Moloney murine leukaemia virus in B-cell lymphomas. The Bmi-1 protein contains the RING finger motif and is homologous to two Drosophila proteins known to be part of a multimeric protein complex involved in repressing gene transcription. A similar role for the highly conserved Bmi-1 protein in mammalian cells has been suggested. The coding regions for the mouse and human genes are known and are 92% homologous. This study involved PCR amplification, cloning, and sequencing of the 980bp feline bmi-1 coding region which was shown to be 92% and 97% homologous to the mouse and human genes respectively. From the open reading frame the feline protein is 326 amino acids in length and is 99% homologous to the human protein and 97% homologous to the mouse protein. This data is consistent with the closer relationship between the feline and human genomes and provides another experimental system in which to analyse Bmi-1 function.

Chromatin-association of the Polycomb group protein BMI1 is cell cycle-regulated and correlates with its phosphorylation status

The human proto-oncogene Bmi1 is a member of the mammalian Polycomb Group (Pc-G) genes. The subnuclear distribution of the BMI1 protein was studied in several primary human and tumor-derived cell lines using immunohistochemical and biochemical methods. In primary and tumor cells, nuclear BMI1 shows a fine-grain distribution over chromatin, usually dense in interphase nuclei and significantly weaker along mitotic chromosomes. In addition, BMI1 preferentially associates with several distinct heterochromatic domains in tumor cell lines. In both primary and tumor cell lines a marked cell cycle-regulation of Pc-G-chromatin interaction is observed: nuclear BMI1-staining dissipates in late S phase and is re-established early in G(1)-phase. Chromatin-association of BMI1 inversely correlates with its phosphorylation status in a cell cycle-dependent fashion: at G(1)/S, hypophosphorylated BMI1 is specifically retained in the chromatin-associated nuclear protein fraction, whereas during G(2)/M, phosphorylated BMI1 is not chromatin-bound. Our findings indicate a strict cell cycle-controlled regulation of Pc-G complex-chromatin association and provide molecular tools for improving our understanding of Pc-G complex regulation and function in mammalian cells.

Trisomy 10 in a child with acute nonlymphocytic leukemia followed by relapse with a different clone

We report a 2-year-old Japanese boy with acute nonlymphocytic leukemia (ANLL) having trisomy 10 as the sole chromosomal abnormality. The majority of the marrow blasts had lobulated nuclei and Auer rods in their cytoplasm. The blasts were positive for peroxidase, CD13, CD15, and CD33, but negative for esterase, CD3, CD7, CD34, and HLA-DR, indicating a diagnosis of ANLL, atypical M2 in French-American-British (FAB) classification. He was treated with combination chemotherapy, including anthracyclines, etoposide, and cytosine arabinoside. Four months after achieving the first remission, the disease relapsed during the consolidation therapy, and he died 9 months later. Trisomy 10 was not detected at relapse, and blasts showed phenotypes different from those at initial diagnosis. The present case suggests that the prognosis of acute leukemia with trisomy 10 in the pediatric age group may not be good, and that further studies are required to clarify the association of trisomy 10 with leukemogenesis and disease progression.

Cloning of the rat proto-oncogene bmi-1

The bmi-1 gene was identified as a common proviral integration site in Moloney murine leukemia virus. In the present studies, we cloned and sequenced the rat bmi-1 gene by reverse transcriptase-polymerase chain reaction (RT-PCR) using degenerate PCR primers of homologous sequences between mouse and human. We found 93% identity to the mouse bmi-1 cDNA and 90% identity to the human bmi-1. The open reading frame encodes a protein of 324 amino acids. In the deduced amino acid sequence we observed 95% and 94% homology to the mouse and human, respectively. The structural motifs, a novel zinc finger motif and a putative helix-turn-helix motif, were conserved in the predicted rat BMI-1 protein. We also confirmed ubiquitous expression of bmi-1 in normal tissues except brain. These results suggest functional conservation of the bmi-1 gene in the rat.

Characterization of interactions between the mammalian polycomb-group proteins Enx1/EZH2 and EED suggests the existence of different mammalian polycomb-group protein complexes

In Drosophila melanogaster, the Polycomb-group (PcG) and trithorax-group (trxG) genes have been identified as repressors and activators, respectively, of gene expression. Both groups of genes are required for the stable transmission of gene expression patterns to progeny cells throughout development. Several lines of evidence suggest a functional interaction between the PcG and trxG proteins. For example, genetic evidence indicates that the enhancer of zeste [E(z)] gene can be considered both a PcG and a trxG gene. To better understand the molecular interactions in which the E(z) protein is involved, we performed a two-hybrid screen with Enx1/EZH2, a mammalian homolog of E(z), as the target. We report the identification of the human EED protein, which interacts with Enx1/EZH2. EED is the human homolog of eed, a murine PcG gene which has extensive homology with the Drosophila PcG gene extra sex combs (esc). Enx1/EZH2 and EED coimmunoprecipitate, indicating that they also interact in vivo. However, Enx1/EZH2 and EED do not coimmunoprecipitate with other human PcG proteins, such as HPC2 and BMI1. Furthermore, unlike HPC2 and BMI1, which colocalize in nuclear domains of U-2 OS osteosarcoma cells, Enx1/EZH2 and EED do not colocalize with HPC2 or BMI1. Our findings indicate that Enx1/EZH2 and EED are members of a class of PcG proteins that is distinct from previously described human PcG proteins.

Trisomy 10 in leukemia

Previous studies had raised questions about whether the relatively rare finding of trisomy 10 in leukemia is nonrandomly associated with a specific immunophenotype or ethnic origin. To shed light on the above questions and to obtain additional clinical and pathologic information on this unique class of leukemic patients, we conducted a retrospective study of leukemia cases at our laboratory from July 1, 1990, to July 31, 1996. The results not only support the rarity of trisomy 10, but they also reject the hypotheses that all trisomy 10 cases are CD7-positive, or found in Orientals.

MPc2, a new murine homolog of the Drosophila polycomb protein is a member of the mouse polycomb transcriptional repressor complex

The evolutionarily conserved polycomb and trithorax-group genes are required to maintain stable expression patterns of homeotic genes and other target genes throughout development. Here, we report the cloning and characterization of a novel mouse polycomb homolog, MPc2, in addition to the previously described M33 polycomb gene. Co-immunoprecipitations and subnuclear co-localization studies show that MPc2 interacts with the mouse polycomb-group oncoprotein Bmi1 and is a new member of the mouse polycomb multiprotein complex. Gal4DB-MPc2 or -M33 fusion proteins mediate a five- to tenfold repression of stably integrated reporter constructs carrying GAL4 binding sites, demonstrating that these proteins are transcriptional repressors. The MPc2 gene is localized on chromosome 11, in close proximity to the classical mouse mutations tail short (Ts) and rabo torcido (Rbt). Ts and Rbt hemizygous mice display anemia and transformations of the axial skeleton reminiscent of phenotypes observed in mice with mutated polycomb or trithorax-group genes, suggesting that MPc2 is a candidate gene for Ts and Rbt.

RING1 is associated with the polycomb group protein complex and acts as a transcriptional repressor

The Polycomb (Pc) protein is a component of a multimeric, chromatin-associated Polycomb group (PcG) protein complex, which is involved in stable repression of gene activity. The identities of components of the PcG protein complex are largely unknown. In a two-hybrid screen with a vertebrate Pc homolog as a target, we identify the human RING1 protein as interacting with Pc. RING1 is a protein that contains the RING finger motif, a specific zinc-binding domain, which is found in many regulatory proteins. So far, the function of the RING1 protein has remained enigmatic. Here, we show that RING1 coimmunoprecipitates with a human Pc homolog, the vertebrate PcG protein BMI1, and HPH1, a human homolog of the PcG protein Polyhomeotic (Ph). Also, RING1 colocalizes with these vertebrate PcG proteins in nuclear domains of SW480 human colorectal adenocarcinoma and Saos-2 human osteosarcoma cells. Finally, we show that RING1, like Pc, is able to repress gene activity when targeted to a reporter gene. Our findings indicate that RING1 is associated with the human PcG protein complex and that RING1, like PcG proteins, can act as a transcriptional repressor.

Identification and characterization of interactions between the vertebrate polycomb-group protein BMI1 and human homologs of polyhomeotic

In Drosophila melanogaster, the Polycomb-group (PcG) genes have been identified as repressors of gene expression. They are part of a cellular memory system that is responsible for the stable transmission of gene activity to progeny cells. PcG proteins form a large multimeric, chromatin-associated protein complex, but the identity of its components is largely unknown. Here, we identify two human proteins, HPH1 and HPH2, that are associated with the vertebrate PcG protein BMI1. HPH1 and HPH2 coimmunoprecipitate and cofractionate with each other and with BMI1. They also colocalize with BMI1 in interphase nuclei of U-2 OS human osteosarcoma and SW480 human colorectal adenocarcinoma cells. HPH1 and HPH2 have little sequence homology with each other, except in two highly conserved domains, designated homology domains I and II. They share these homology domains I and II with the Drosophila PcG protein Polyhomeotic (Ph), and we, therefore, have named the novel proteins HPH1 and HPH2. HPH1, HPH2, and BMI1 show distinct, although overlapping expression patterns in different tissues and cell lines. Two-hybrid analysis shows that homology domain II of HPH1 interacts with both homology domains I and II of HPH2. In contrast, homology domain I of HPH1 interacts only with homology domain II of HPH2, but not with homology domain I of HPH2. Furthermore, BMI1 does not interact with the individual homology domains. Instead, both intact homology domains I and II need to be present for interactions with BMI1. These data demonstrate the involvement of homology domains I and II in protein-protein interactions and indicate that HPH1 and HPH2 are able to heterodimerize.

Identification of Bmi1-interacting proteins as constituents of a multimeric mammalian polycomb complex

The Bmi1 gene has been identified as a mouse Polycomb group (Pc-G) gene implicated in the regulation of Hox gene expression. Here we describe the characterization of a Bmi binding protein Mph1, which shares similarity to Drosophila polyhomeotic. Coimmunoprecipitation experiments indicate that Bmi1 and Mph1, as well as the Mel18 and M33 proteins described previously, are constituents of a multimeric protein complex in mouse embryos and human cells. A central domain of Bmi1 interacts with the carboxyl terminus of Mph1, whereas a conserved alpha-helical domain in the Mph1 protein is required for its homodimerization. Transgenic mice overexpressing various mutant Bmi1 proteins demonstrate that the central domain of Bmil is required for the induction of anterior transformations of the axial skeleton. Bmi1, M33, and Mph1 show an overlapping speckled distribution in interphase nuclei. These data provide molecular evidence for the existence of a mammalian Polycomb complex.

Transformation by the Bmi-1 oncoprotein correlates with its subnuclear localization but not its transcriptional suppression activity

The bmi-1 oncogene cooperates with c-myc in transgenic mice, resulting in accelerated lymphoma development. Altering the expression of Bmi-1 affects normal embryogenesis. The protein product of bmi-1 is homologous to certain Drosophila Polycomb group proteins that regulate homeotic gene expression through alteration of chromatin structure. Chimeric LexA-Bmi-1 protein has previously been shown to repress transcription. How Bmi-1 functions in embryogenesis and whether this relates to the ability of Bmi-1 to mediate cellular transformation is unknown. We demonstrate here that Bmi-1 is able to transform rodent fibroblasts in vitro, providing a system that has allowed us to correlate its molecular properties with its ability to transform cells. We map functional domains of Bmi-1 involved in transcriptional suppression by using the GAL4 chimeric transcriptional regulator system. Deletion analysis shows that the centrally located helix-turn-helix-turn-helix-turn (HTHTHT) motif is necessary for transcriptional suppression whereas the N-terminal RING finger domain is not required. We demonstrate that nuclear localization requires KRMK (residues 230 to 233) and that the absence of nuclear entry ablates transformation. In addition, we find that the subnuclear localization of wild-type Bmi-1 to the rim of the nucleus requires the RING finger domain and correlates with its ability to transform. Our studies with Bmi-1 deletion mutants suggest that the ability of Bmi-1 to mediate cellular transformation correlates with its unique subnuclear localization but not its transcriptional suppression activity.

The t(10;11)(p13;q14) in the U937 cell line results in the fusion of the AF10 gene and CALM, encoding a new member of the AP-3 clathrin assembly protein family

The translocation t(10;11)(p13;q14) is a recurring chromosomal abnormality that has been observed in patients with acute lymphoblastic leukemia as well as acute myeloid leukemia. We have recently reported that the monocytic cell line U937 has a t(10;11)(p13;q14) translocation. Using a combination of positional cloning and candidate gene approach, we cloned the breakpoint and were able to show that AF10 is fused to a novel gene that we named CALM (Clathrin Assembly Lymphoid Myeloid leukemia gene) located at 11q14. AF10, a putative transcription factor, had recently been cloned as one of the fusion partners of MLL. CALM has a very high homology in its N-terminal third to the murine ap-3 gene which is one of the clathrin assembly proteins. The N-terminal region of ap-3 has been shown to bind to clathrin and to have a high-affinity binding site for phosphoinositols. The identification of the CALM/AF10 fusion gene in the widely used U937 cell line will contribute to our understanding of the malignant phenotype of this line.

Polycomb and bmi-1 homologs are expressed in overlapping patterns in Xenopus embryos and are able to interact with each other

The Polycomb group genes in Drosophila are involved in the stable and inheritable repression of gene expression. The Polycomb group proteins probably operate as multimeric complexes that bind to chromatin. To investigate molecular mechanisms of stable repression of gene activity in vertebrates we have begun to study Xenopus homologs of Polycomb group genes. We identified the Xenopus homologs of the Drosophila Polycomb gene and the bmi-1 gene. bmi-1 is a proto-oncogene which has sequence homology with the Polycomb group gene Posterior Sex Combs. We show that the XPolycomb and Xbmi-1 genes are expressed in overlapping patterns in the central nervous system of Xenopus embryos. However, XPolycomb is also expressed in the somites, whereas Xbmi-1 is not. We further demonstrate that the XPolycomb and Xbmi-1 proteins are able to interact with each other via conserved sequence motifs. These data suggest that also vertebrate Polycomb group proteins form multimeric complexes.

Characterization of a t(10;11)(p13-14;q14-21) in the monoblastic cell line U937

Previous analysis of the monoblastic cell line U937 has shown that several sublines contain a rearranged chromosome arm 11q. In order to determine the true nature of the rearrangement, fluorescence in situ hybridization (FISH) was carried out with various combinations of single copy anonymous markers, clones containing genes, a chromosome 10 paint, and an 11 centromere specific sequence. The rearrangement was deduced to be a reciprocal translocation between chromosomes 10 and 11 described as t(10;11)(p13-14;q14-21). The breakpoint on chromosome 11 is telomeric to the INT2 gene and the pHS11 probe at 11q13, and centromeric to the marker D11S36 localized to 11q14.3-q22.1 and the MLL gene at 11q23. Similar translocations have been reported in various acute leukemias, principally of the monocytic lineage, and also in T-cell precursor acute lymphocytic leukemias. Further characterization of the genetic rearrangements in U937 may lead to the isolation of genes important in leukemogenesis and provide an in vitro system for their study.