Fusion of NUP214 to ABL1 on amplified episomes in T-cell acute lymphoblastic leukemia.

In T-cell acute lymphoblastic leukemia (T-ALL), transcription factors are known to be deregulated by chromosomal translocations, but mutations in protein tyrosine kinases have only rarely been identified. Here we describe the extrachromosomal (episomal) amplification of ABL1 in 5 of 90 (5.6%) individuals with T-ALL, an aberration that is not detectable by conventional cytogenetics. Molecular analyses delineated the amplicon as a 500-kb region from chromosome band 9q34, containing the oncogenes ABL1 and NUP214 (refs. 5,6). We identified a previously undescribed mechanism for activation of tyrosine kinases in cancer: the formation of episomes resulting in a fusion between NUP214 and ABL1. We detected the NUP214-ABL1 transcript in five individuals with the ABL1 amplification, in 5 of 85 (5.8%) additional individuals with T-ALL and in 3 of 22 T-ALL cell lines. The constitutively phosphorylated tyrosine kinase NUP214-ABL1 is sensitive to the tyrosine kinase inhibitor imatinib. The recurrent cryptic NUP214-ABL1 rearrangement is associated with increased HOX expression and deletion of CDKN2A, consistent with a multistep pathogenesis of T-ALL. NUP214-ABL1 expression defines a new subgroup of individuals with T-ALL who could benefit from treatment with imatinib.

Haploinsufficiency of CBFA2 causes familial thrombocytopenia with propensity to develop acute myelogenous leukaemia.

Familial platelet disorder with predisposition to acute myelogenous leukaemia (FPD/AML, MIM 601399) is an autosomal dominant disorder characterized by qualitative and quantitative platelet defects, and propensity to develop acute myelogenous leukaemia (AML). Informative recombination events in 6 FPD/AML pedigrees with evidence of linkage to markers on chromosome 21q identified an 880-kb interval containing the disease gene. Mutational analysis of regional candidate genes showed nonsense mutations or intragenic deletion of one allele of the haematopoietic transcription factor CBFA2 (formerly AML1) that co-segregated with the disease in four FPD/AML pedigrees. We identified heterozygous CBFA2 missense mutations that co-segregated with the disease in the remaining two FPD/AML pedigrees at phylogenetically conserved amino acids R166 and R201, respectively. Analysis of bone marrow or peripheral blood cells from affected FPD/AML individuals showed a decrement in megakaryocyte colony formation, demonstrating that CBFA2 dosage affects megakaryopoiesis. Our findings support a model for FPD/AML in which haploinsufficiency of CBFA2 causes an autosomal dominant congenital platelet defect and predisposes to the acquisition of additional mutations that cause leukaemia.

TG101209, a small molecule JAK2-selective kinase inhibitor potently inhibits myeloproliferative disorder-associated JAK2V617F and MPLW515L/K mutations.

JAK2V617F and MPLW515L/K represent recently identified mutations in myeloproliferative disorders (MPD) that cause dysregulated JAK-STAT signaling, which is implicated in MPD pathogenesis. We developed TG101209, an orally bioavailable small molecule that potently inhibits JAK2 (IC(50)=6 nM), FLT3 (IC(50)=25 nM) and RET (IC(50)=17 nM) kinases, with significantly less activity against other tyrosine kinases including JAK3 (IC(50)=169 nM). TG101209 inhibited growth of Ba/F3 cells expressing JAK2V617F or MPLW515L mutations with an IC(50) of approximately 200 nM. In a human JAK2V617F-expressing acute myeloid leukemia cell line, TG101209-induced cell cycle arrest and apoptosis, and inhibited phosphorylation of JAK2V617F, STAT5 and STAT3. Therapeutic efficacy of TG101209 was demonstrated in a nude mouse model. Furthermore, TG101209 suppressed growth of hematopoietic colonies from primary progenitor cells harboring JAK2V617F or MPL515 mutations.

Fatal myeloproliferation, induced in mice by TEL/PDGFbetaR expression, depends on PDGFbetaR tyrosines 579/581.

The t(5;12)(q33;p13) translocation associated with chronic myelomonocytic leukemia (CMML) generates a TEL/PDGFbetaR fusion gene. Here, we used a murine bone marrow transplant (BMT) assay to test the transforming properties of TEL/PDGFbetaR in vivo. TEL/PDGFbetaR, introduced into whole bone marrow by retroviral transduction, caused a rapidly fatal myeloproliferative disease that closely recapitulated human CMML. TEL/PDGFbetaR transplanted mice developed leukocytosis with Gr-1(+) granulocytes, splenomegaly, evidence of extramedullary hematopoiesis, and bone marrow fibrosis, but no lymphoproliferative disease. We assayed mutant forms of the TEL/PDGFbetaR fusion protein - including 8 tyrosine to phenylalanine substitutions at phosphorylated PDGFbetaR sites to which various SH2 domain-containing signaling intermediates bind - for ability to transform hematopoietic cells. All of the phenylalanine (F-) mutants tested conferred IL-3-independence to a cultured murine hematopoietic cell line, but, in the BMT assay, different F-mutants displayed distinct transforming properties. In transplanted animals, tyrosines 579/581 proved critical for the development of myeloproliferative phenotype. F-mutants with these residues mutated showed no sign of myeloproliferation but instead developed T-cell lymphomas. In summary, TEL/PDGFbetaR is necessary and sufficient to induce a myeloproliferative disease in a murine BMT model, and PDGFbetaR residues Y579/581 are required for this phenotype.

Socs-1 inhibits TEL-JAK2-mediated transformation of hematopoietic cells through inhibition of JAK2 kinase activity and induction of proteasome-mediated degradation.

TEL-JAK2 fusion proteins, which are a result of t(9;12)(p24;p13) translocations associated with human leukemia, activate Stat5 in vitro and in vivo and cause a myelo- and lymphoproliferative disease in a murine bone marrow transplant model. We report that Socs-1, a member of the SOCS family of endogenous inhibitors of JAKs and STATs, inhibits transformation of Ba/F3 cells by TEL-JAK2 but has no effect on Ba/F3 cells transformed by BCR-ABL, TEL-ABL, or TEL-platelet-derived growth factor receptor beta. TEL-JAK2, in addition to activating Stat5, associates with Shc and Grb2 and induces activation of Erk2, and expression of Socs-1 inhibits engagement of each of these signaling molecules. TEL-JAK2 kinase activity is inhibited by Socs-1, as assessed by in vitro kinase assays. In addition, Socs-1 induces proteasomal degradation of TEL-JAK2. Mutational analysis indicates that the SOCS box of Socs-1 is required for proteasomal degradation and for abrogation of growth of TEL-JAK2-transformed cells. Furthermore, murine bone marrow transplant assays demonstrate that expression of Socs-1 prolongs latency of TEL-JAK2-mediated disease in vivo. Collectively, these data indicate that Socs-1 inhibits TEL-JAK2 in vitro and in vivo through inhibition of kinase activity and induction of TEL-JAK2 protein degradation.

Oligomerization of the ABL tyrosine kinase by the Ets protein TEL in human leukemia.

TEL is a member of the Ets family of transcription factors which are frequently rearranged in human leukemia. The mechanism of TEL-mediated transformation, however, is unknown. We report the cloning and characterization of a chromosomal translocation associated with acute myeloid leukemia which fuses TEL to the ABL tyrosine kinase. The TEL-ABL fusion confers growth factor-independent growth to the marine hematopoietic cell line Ba/F3 and transforms Rat-1 fibroblasts and primary murine bone marrow cells. TEL-ABL is constitutively tyrosine phosphorylated and localizes to the cytoskeleton. A TEL-ABL mutant containing an ABL kinase-inactivating mutation is not constitutively phosphorylated and is nontransforming but retains cytoskeletal localization. However, constitutive phosphorylation, cytoskeletal localization, and transformation are all dependent upon a highly conserved region of TEL termed the helix-loop-helix (HLH) domain. TEL-ABL formed HLH-dependent homo-oligomers in vitro, a process critical for tyrosine kinase activation. These experiments suggest that oligomerization of TEL-ABL mediated by the TEL HLH domain is required for tyrosine kinase activation, cytoskeletal localization, and transformation. These data also suggest that oligomerization of Ets proteins through the highly conserved HLH domain may represent a previously unrecognized phenomenon.

The t(12;21) translocation converts AML-1B from an activator to a repressor of transcription.

The t(12;21) translocation is present in up to 30% of childhood B-cell acute lymphoblastic and fuses a potential dimerization motif from the ets-related factor TEL to the N terminus of AML1. The t(12;21) translocation encodes a 93-kDa fusion protein that localizes to a high-salt- and detergent-resistant nuclear compartment. This protein binds the enhancer core motif, TGTGGT, and interacts with the AML-1-binding protein, core-binding factor beta. Although TEL/AML-1B retains the C-terminal domain of AML-1B that is required for transactivation of the T-cell receptor beta enhancer, it fails to activate transcription but rather inhibits the basal activity of this enhancer. TEL/AML-1B efficiently interferes with AML-1B dependent transactivation of the T-cell receptor beta enhancer, and coexpression of wild-type TEL does not reverse this inhibition. The N-terminal TEL helix-loop-helix domain is essential for TEL/AML-1B-mediated repression. Thus, the t(12;21) fusion protein dominantly interferes with AML-1B-dependent transcription, suggesting that the inhibition of expression of AML-1 genes is critical for B-cell leukemogenesis.

A model system involving anti-concanavalin A for antibody targeting of diphtheria toxin fragment A1.

Results obtained in a model system strongly suggest that antibodies to cell surface determinants may be used to direct the toxic potential of the A chain of diphtheria toxin (DTA). The A chain (M.W. 21,000) was covalently attached to antibody against concanavalin A (anti-Con A) by means of a disulfide-containing cross-bridge. This DTA-SS-(anti-Con A) conjugate was toxic for 3T3 cells containing Con A on their surface but was not toxic in the same concentration range for: (a) cells lacking Con A; (b) Con A-treated cells washed with buffer containing alpha-methyl-D-mannoside; (c) cells containing wheat germ agglutinin on their surface; or (d) Con A-treated mutant Chinese hamster ovary cells containing altered, toxin-insensitive elongation factor 2. Conjugates containing DTA disulfide linked to anti-wheat germ agglutinin antibody or to nonspecific rabbit immunoglobulin G were not toxic for cells coated with Con A. The results suggest a new approach to the construction of antibody-directed, tumor-specific chemotherapeutic agents. Conjugates containing DTA disulfide linked to antibody against specific cell surface antigens may also be generally useful as specific selective agents for the isolation of mutant cell lines.

Analysis of meningiomas by methylation- and transcription-based clonality assays.

The clonal derivation of tumors can be determined by X chromosome inactivation analysis based on differential expression of genes or differential methylation of cytosine residues in CpG islands near polymorphic loci. In this report, we compared a transcription-based RNA analysis with a methylation-based DNA assay to determine clonality of meningiomas. Both clonality assays use PCR-based analysis at the hunan androgen-receptor gene (HUMARA) on the X chromosome. Among 23 meningiomas from female patients, 19 were informative heterozygotes at this locus (83%). The patterns of X chromosome inactivation in four patients were extremely skewed towards one allele in blood (unequal Lyonization), which precluded clonality determination in the tumor samples. Concordant clonality results with methylation- and transcription-based clonality assays were demonstrated in 9 of 13 informative tumors expressing the androgen receptor. Seven meningiomas were monoclonal, but surprisingly, two pathologically documented cases of meningiomas were polyclonal. There was disparity in 4 of 13 tumor specimens that were polyclonal by the methylation-based assay but monoclonal by the transcription assay. Clonality examination of these tumors by the methylation-based phosphoglycerate kinase assay provided identical results to the methylation-based analysis at the HUMARA locus. In addition, loss of heterozygosity (LOH) studies of chromosome 22, which is frequently deleted in meningiomas, showed that four of four informative samples of the six polyclonal tumors had partial LOH in tumor tissues. However, complete LOH was observed in primary cultured cells, which were also monoclonal by the methylation assay. Taken together, these data suggest that the disparity of the two assays in these four cases may be due to differences in the level of expression of the androgen receptor gene in tumors. Therefore, we conclude that: (a) clonal derivation of meningiomas determined by both transcription- and methylation-based clonality assays are in full agreement in many (9 of 13) but not all cases (4 of 13); and (b) most meningiomas (9 of 15) are monoclonal in origin, whereas some meningioma samples (6 of 15) are polyclonal or may contain heterogeneous components.

Divergent roles of STAT1 and STAT5 in malignancy as revealed by gene disruptions in mice.

Stat proteins are latent transcription factors activated by tyrosine phosphorylation downstream of cytokine and growth factor receptors and have been implicated in a variety of cell growth regulatory pathways. Constitutive phosphorylation has also been observed in various transformed cell line and in primary malignant tissue, suggesting that Stat protein activation may contribute to the transformed phenotype. One method to distinguish between a causative role in malignancy as opposed to bystander phosphorylation from the increased tyrosine phosphorylation that accompanies transformation is to investigate cell growth and malignancy in the absence of particular Stat proteins using targeted gene disruptions in transgenic mice. Such studies show that Stat1 primarily mediates growth inhibitory signals and contributes to the host rejection of tumors, and that its activation in transformed cells is not necessary for malignancy. Activation of Stat5 can be both necessary and sufficient for malignant transformation, and single Stat5-target genes have been identified that are critical for heightened proliferation. Nonetheless, some malignancies that are characterized by constitutively phosphorylated Stat5 are not altered by the loss of Stat5 protein. Its role in these cases may be redundant with other transforming events that are in themselves sufficient to cause disease, rendering tyrosine phosphorylation of Stat5 unnecessary in these transformed cells. Oncogene (2000).

p210BCR/ABL, p190BCR/ABL, and TEL/ABL activate similar signal transduction pathways in hematopoietic cell lines.

The Philadelphia chromosome translocation generates a chimeric oncogene, BCR/ABL which causes chronic myelogenous leukemia. Two different fusion proteins can be produced, p190BCR/ABL and p210BCR/ABL, depending on the location of the breakpoint in BCR. Although the ABL tyrosine kinase activity of the resulting oncoprotein is essential for transformation, the exact functional contribution of BCR to transformation is unclear. A novel oncogene containing ABL is formed by the (9;12) translocation which fuses part of the ets-family member TEL to c-ABL in patients with acute leukemia. In an effort to compare the biological effects of various ABL oncogenes, we transformed two different factor-dependent murine hematopoietic cell lines with cDNA's encoding p210BCR/ABL, p190BCR/ABL, or TEL/ABL. Transfection of each of the three activated ABL oncogenes resulted in rapid emergence of growth factor-independence, and 2-4 sublines from each cell line with each oncogene were further studied. Each oncogene induced an increase in the tyrosine phosphorylation of cellular proteins and autophosphorylation of the oncoprotein itself. Overall, the pattern of increased tyrosine phosphorylation was similar in the cell lines, suggesting that many of the major substrates were identical. We specifically examined a series of proteins known to be p210BCR/ABL substrates, including rasGAP, Shc, SH-PTP2, SH-PTP1, CRK-L, CBL, paxillin, and STATs, and found that each were also tyrosine phosphorylated in response to p190BCR/ABL and TEL/ABL. These results suggest that the function of BCR can be largely replaced by the unrelated protein TEL with regards to transformation of murine hematopoietic cell lines to factor-independence, and support the hypothesis that a major contribution of both fusion partners is to activate the ABL tyrosine kinase.

A hybrid toxin containing fragment A from diphtheria toxin linked to the B protomer of cholera toxin.

We have constructed and characterized a hybrid toxin containing the A chain of diphtheria toxin linked via a disulfide bridge to the B protomer of cholera toxin. Cholera toxin B protomer, previously derivatized with 4-5 cystaminyl groups per pentameric protomer, was reacted with reduced diphtheria toxin chain A to give the desired hybrid, containing an average of 2 molecules of diphtheria toxin chain A per cholera toxin B protomer. A concentration of 0.3 nM hybrid inhibited protein synthesis by 50% in 24 h in several cultured cell lines; thus the hybrid was about 10-fold more toxic than of a (diphtheria toxin chain A)-SS-(concanavalin A) conjugate described previously. Evidence was obtained that toxicity of the hybrid was dependent on the functional contributions of both the diphtheria toxin chain A and cholera toxin B protomer moieties.

Prognostic factors for treatment outcome in autotransplantation of intermediate-grade and high-grade non-Hodgkin's lymphoma with cyclophosphamide, carmustine, and etoposide.

PURPOSE: We examined a consecutive series of 78 patients with non-Hodgkin's lymphoma treated on prospective protocols with high-dose cyclophosphamide, carmustine (BCNU), and etoposide (CBV) plus autotransplantation to determine prognostic factors for time to treatment failure. PATIENTS AND METHODS: Patients with relapsed, refractory, or poor-risk intermediate- and high-grade non-Hodgkin's lymphoma were treated with CBV with autologous marrow or peripheral-blood progenitor cell support. Patient characteristics before transplantation were examined in univariate analyses by the log-rank test and simultaneously in a Cox proportional hazards regression analysis. A best-predictive model was determined from those variables significant (P < .10) in the univariate test. RESULTS: In univariate analysis, intermediate-grade and immunoblastic lymphoma, responsiveness to pretransplant salvage chemotherapy, and transplantation after primary therapy (first complete response [CR] or partial response [PR]) were associated with prolonged time to treatment failure. In proportional hazards multiple regression analysis, intermediate-grade and immunoblastic histology, responsive disease, and autotransplantation in first CR or PR were positive prognostic factors, and these characteristics are the basis of the best-predictive model for prolonged time to failure. Actuarial 3-year failure-free survival of patients with stable or responding disease at autotransplant was 54%. CONCLUSION: CBV is an effective conditioning regimen in intermediate-grade and immunoblastic non-Hodgkin's lymphoma. Patients with these histologies transplanted while responding to primary therapy, or those with stable disease or disease responding to salvage therapy at the time of autotransplant, are most likely to benefit. Patients with lymphoblastic lymphoma or diffuse undifferentiated lymphoma did poorly with CBV and should be offered alternative therapy.

Molecular genetics of human leukemia.

Human leukemias are clonal hemopathies generally characterized by acquired somatic mutations, including translocations, deletions, and insertions. Ten years ago, the first leukemia-related chromosomal translocation, the Philadelphia chromosome, was cloned and fully characterized at the molecular level. Since then, a plethora of chromosomal translocations and mutations has been associated with leukemogenesis. Analysis and comparison of this bewildering array of genetic changes have helped identify shared paradigms and themes in the mechanisms of transformation of hematopoietic cells. This information will guide the development of improved therapies that take into account the cytogenetic and molecular characteristics of human leukemias. This review summarizes the current knowledge of the nature of the genetic changes associated with leukemogenesis and discusses their consequences at the molecular and cellular levels.

TEL is one of the targets for deletion on 12p in many cases of childhood B-lineage acute lymphoblastic leukemia.

Abnormalities of the short arm of chromosome 12 including loss of heterozygosity (LOH) and TEL/AML-1 fusion resulting from a t(12;21)(p13;q22) translocation are frequently observed in childhood acute lymphoblastic leukemia (ALL). We investigated 21 DNA samples of childhood ALL which had LOH at 12p13. Rearrangement of TEL was observed in eight cases and another case showed a homozygous deletion of TEL. Two informative samples with TEL rearrangement had a deletion localized to the 5' region of this gene. The deletion in these two cases includes the helix-loop-helix (HLH) domain. This is consistent with the hypothesis that the normal tel can heterodimerize with the TEL/AML-1 gene product and inhibit the transforming capacity of the chimeric protein. Presumably, loss of the HLH of the normal remaining TEL allele abrogates this tumor suppressor-like function. The case with homozygous deletion of TEL is also consistent with this gene having qualities of a tumor suppressor. One unusual case had T-ALL rather than B-lineage ALL and the leukemic cells had rearrangement of TEL, but they did not have an alteration of the remaining TEL allele suggesting that the etiology of this disease may be different. This analysis further emphasizes the importance of loss of the normal TEL allele in childhood precursor B-lineage ALL.

CBFA2, frequently rearranged in leukemia, is not responsible for a familial leukemia syndrome.

We have identified a family with an autosomal dominant platelet disorder with a predisposition for developing myeloid malignancies and have previously demonstrated linkage of this trait to chromosome 21q22.1-22.2. The nearest flanking markers, D21S1265 and D21S167, define the familial platelet disorder (FPD) critical region at a genetic distance of approximately 15.2 centimorgans and physical distance of approximately 6 megabases. This locus is of particular interest as it has previously been implicated in the pathogenesis of acute myelogenous leukemia (AML) and acute lymphoblastic leukemia (ALL) through the (8;21), (3;21) and (12;21) chromosomal translocations. In each of these cases, the CBFA2 gene is rearranged. As well, there is a potential association of this locus with the hematologic abnormalities seen in Down syndrome (trisomy 21). To identify the mutant gene in this pedigree, a positional cloning strategy has been undertaken. Several candidate genes map to this locus including: CBFA2, IFNAR1, IFNAR2, CRFB4, GART, SON, KCNE1, SCL5A3 and ATP50. CBFA2, as well as IFNAR1 and CRFB4, were the focus of initial mutational analysis efforts. In this report, we exclude CBFA2 as a candidate by Northern and Southern blotting, RNase protection, single-strand conformational polymorphism (SSCP), direct sequencing and gel-shift analysis. Exons of the IFNAR1 and CRFB4 genes were also analyzed by SSCP and demonstrated no evidence of mutation. SSCP analysis identified a new polymorphism in the second exon of the CRFB4 gene and confirmed a previously described polymorphism in the fourth exon of IFNAR1. Efforts are currently underway to delimit further the FPD critical region and to analyze the other known candidate genes, as well as novel candidate genes, which map to this locus.

Clinical and molecular features of FIP1L1-PDFGRA (+) chronic eosinophilic leukemias.

Detection of the FIP1L1-PDGFRA fusion gene or the corresponding cryptic 4q12 deletion supports the diagnosis of chronic eosinophilic leukemia (CEL) in patients with chronic hypereosinophilia. We retrospectively characterized 17 patients fulfilling WHO criteria for idiopathic hypereosinophilic syndrome (IHES) or CEL, using nested RT-PCR and interphase fluorescence in situ hybridization (FISH). Eight had FIP1L1-PDGFRA (+) CEL, three had FIP1L1-PDGFRA (-) CEL and six had IHES. FIP1L1-PDGFRA (+) CEL responded poorly to steroids, hydroxyurea or interferon-alpha, and had a high probability of eosinophilic endomyocarditis (n=4) and disease-related death (n=4). In FIP1L1-PDGFRA (+) CEL, palpable splenomegaly was present in 5/8 cases, serum vitamin B(12) was always markedly increased, and marrow biopsies revealed a distinctively myeloproliferative aspect. Imatinib induced rapid complete hematological responses in 4/4 treated FIP1L1-PDGFRA (+) cases, including one female, and complete molecular remission in 2/3 evaluable cases. In the female patient, 1 log reduction of FIP1L1-PDGFRA copy number was reached as by real-time quantitative PCR (RQ-PCR). Thus, correlating IHES/CEL genotype with phenotype, FIP1L1-PDGFRA (+) CEL emerges as a homogeneous clinicobiological entity, where imatinib can induce molecular remission. While RT-PCR and interphase FISH are equally valid diagnostic tools, the role of marrow biopsy in diagnosis and of RQ-PCR in disease and therapy monitoring needs further evaluation.

TEL/PDGFbetaR fusion protein activates STAT1 and STAT5: a common mechanism for transformation by tyrosine kinase fusion proteins.

OBJECTIVE: TEL/PDGFbetaR is a tyrosine kinase fusion protein associated with the pathogenesis of chronic myelomonocytic leukemia. The following experiments were undertaken to understand the mechanisms whereby TEL/PDGFbetaR transforms cells. MATERIALS AND METHODS: Activation of JAK and STAT proteins was studied in an interleukin 3 (IL-3)-dependent cell line, Ba/F3, transformed to IL-3 independence by TEL/PDGFbetaR. RESULTS: TEL/PDGFbetaR activates STAT1 and STAT5 in transformed Ba/F3 cells through a JAK-independent pathway. Activation of STAT proteins requires the kinase activity of TEL/PDGFbetaR. JAK1, JAK2, JAK3, and TYK2 are not phosphorylated by TEL/PDGFbetaR. However, TEL/PDGFbetaR can phosphorylate STAT5 in transiently transfected COS cells, suggesting that TEL/PDGFbetaR may itself be the kinase involved in tyrosine phosphorylation of STAT proteins. In contrast, native PDGFbetaR stimulated by PDGF ligand does not activate STAT proteins to a significant degree in this hematopoietic context. STAT1 and STAT5 also are activated by TEL/ABL and TEL/JAK2 fusion proteins associated with human leukemia. CONCLUSIONS: STAT activation may be a common mechanism of transformation by leukemogenic tyrosine kinase fusion proteins.

Human pituitary adenomas show no loss of heterozygosity at the retinoblastoma gene locus.

The retinoblastoma tumor suppressor gene (RB1) is inactivated in hereditary and sporadic forms of retinoblastoma as well as in a number of other sporadic tumors. The majority of human pituitary tumors have been shown to be monoclonal neoplasms, suggesting that 1 or more somatic mutations are involved in the clonal expansion of a single progenitor cell. Recently, a high percentage of transgenic mice containing a disrupted RB1 allele have been shown to develop pituitary tumors. To investigate whether RB1 inactivation contributes to the development of human pituitary adenomas, we searched for loss of heterozygosity (LOH) within the RB1 gene locus in a variety of human pituitary adenomas. We screened 34 adenomas for LOH using a polymerase chain reaction (PCR)-based microsatellite polymorphic marker at the RB1 gene locus. In addition, a variable number of tandem repeat markers from within the RB1 gene was also used to search for LOH in 14 tumors. We found no LOH or microsatellite instability at the RB1 locus in any of the informative cases (30 of 34). Additionally, we showed that 4 representative adenomas from female patients are monoclonal in origin using a PCR-based clonality analysis assay. We conclude that the RB1 gene shows no LOH in a variety of human pituitary adenomas and that PCR-based microsatellite markers can serve as a useful tool for LOH analysis in human pituitary tumors.

Molecular biology of acute myeloid leukemia.

Clonal chromosome translocations, deletions, and inversions have been repeatedly observed for decades in approximately two thirds of all cases of acute myeloid leukemia (AML). With the dramatic advances in molecular biology that have occurred during the past two decades, these structural cytogenetic abnormalities have now provided invaluable clues as to the location of genes known or suspected of inducing leukemia. In most instances, leukemogenesis in AML results from gene fusion, when segments from two different genes are fused together to give rise to a chimeric structure consisting of the 5' end of one gene and the 3' end of another. Exceptions to this, however, do exist. In cases of AML that lack cytogenetic abnormalities, investigators are now also beginning to elucidate the genes involved in malignant transformation. Together, these observations support the notion that AML is heterogeneous at the molecular level, and suggest that clinicians will need to continue to take cytogenetic and molecular characteristics into consideration to optimize patient therapy.