Altered expression of retinoblastoma (RB) protein in acute myelogenous leukemia does not result from methylation of the Rb promotor

MCM8- and MCM9 Deficiencies Cause Lifelong Increased Hematopoietic DNA Damage Driving p53-Dependent Myeloid Tumors

Hematopoiesis is particularly sensitive to DNA damage. Myeloid tumor incidence increases in patients with DNA repair defects and after chemotherapy. It is not known why hematopoietic cells are highly vulnerable to DNA damage. Addressing this question is complicated by the paucity of mouse models of hematopoietic malignancies due to defective DNA repair. We show that DNA repair-deficient Mcm8- and Mcm9-knockout mice develop myeloid tumors, phenocopying prevalent myelodysplastic syndromes. We demonstrate that these tumors are preceded by a lifelong DNA damage burden in bone marrow and that they acquire proliferative capacity by suppressing signaling of the tumor suppressor and cell cycle controller RB, as often seen in patients. Finally, we found that absence of MCM9 and the tumor suppressor Tp53 switches tumorigenesis to lymphoid tumors without precedent myeloid malignancy. Our results demonstrate that MCM8/9 deficiency drives myeloid tumor development and establishes a DNA damage burdened mouse model for hematopoietic malignancies.

Aberrant expression of tumor suppressor genes and their association with chimeric oncogenes in pediatric acute lymphoblastic leukemia

Aberrant expression of tumor suppressor genes WT 1, RB 1, p53, homozygous deletion of p16 gene and their relationship with expression of oncogenes BCR-ABL, TEL-AML 1, MLL-AF 4, E2A-PBX 1, SIL-TAL 1 were determined in bone marrow samples of children with de novo B-lineage (n=170) and T-lineage (n=25) acute lymphoblastic leukemia (ALL). In contrast to expression of chimeric oncogenes alterations in p16, WT 1, RB 1 and p53 expression were T/B-lineage-unrestricted. Significant association between expression of MLL-AF 4 and WT 1, E2A-PBX 1 and p53; SIL-TAL 1 and homozygous deletion of p16 has been demonstrated.

Delineation of the minimal commonly deleted segment and identification of candidate tumor-suppressor genes in del(9q) acute myeloid leukemia

Deletion of the long arm of chromosome 9, del(9q), is a recurring chromosomal aberration in acute myeloid leukemia (AML) that is frequently associated with t(8;21). The critical gene products affected by del(9q) are unknown but likely cooperate with the AML1/ETO fusion gene created by t(8;21) in leukemogenesis. In 43 AML samples with del(9q), we used high-density microsatellite markers to define the commonly deleted region (CDR) to less than 2.4 Mb. We found no homozygous loss at any locus tested. The CDR contains 7 known genes, FRMD3, UBQLN1, GKAP42, KIF27, HNRPK, SLC28A3, and NTRK2, and 4 novel genes, RASEF, C9orf103, C9orf64, and C9orf76. In addition, TLE1 and TLE4 are adjacent to the CDR. We performed a comprehensive mutational analysis of the coding regions of all these genes. No sequence variations absent in normal controls were seen in more than a single del(9q) AML sample. Expression of 7 of the 10 genes examined was significantly down-regulated in del(19q)AML as compared with the CD34-purified progenitors from normal individuals, a pattern distinct from that seen in AML samples with a normal karyotype. The results of our studies are consistent with a model of tumor suppression mediated by haploinsufficiency of critical genes in del(9q) AML.

Epigenetic Inactivation of Tumor Suppressor Genes in Hematologic Malignancies

A number of genetic alterations are involved in the development of hematologic malignancies. These alterations include the activation of oncogenes by chromosomal translocation or gene amplification and the inactivation of tumor suppressor genes by gene deletion or mutations. Recently, epigenetic change has been proven to be another important means of inactivating tumor suppressor genes in tumor cells, and hypermethylation of promoter DNA is one of the most important mechanisms. In hematologic malignancies, many kinds of tumor suppressor genes and candidate suppressor genes are epigenetically inactivated. Inactivation of tumor suppressor genes usually occurs in a disease-specific manner and plays important roles in the development and progression of the disease. Some of these alterations have clinical effects on treatment results or the prognoses of the patients.

Caspase-independent cell death in AML: caspase inhibition in vitro with pan-caspase inhibitors or in vivo by XIAP or Survivin does not affect cell survival or prognosis

Survivin and XIAP, members of the protein family known as the inhibitors of apoptosis, interfere with the activation of caspases, called the "cell death executioners." We examined Survivin (n = 116) and XIAP (n = 172) expression in primary acute myeloid leukemia (AML) blasts and assessed the impact of their expression on prognosis. They were detected in all samples analyzed. However, no correlation was observed with cytogenetics, remission attainment, or overall survival of patients with AML. To investigate the importance of caspases in chemotherapy-induced apoptosis in AML, we treated OCI-AML3 cells with Ara-C, doxorubicin, vincristine, and paclitaxel, which induced caspase cleavage and apoptosis. Blocking of caspase activation by pan-caspase inhibitor abolished poly(adenosine diphosphate [ADP]-ribose) polymerase cleavage and DNA fragmentation but did not prevent chemotherapy-induced cell death and did not inhibit, or only partially inhibited, mitochondrial release of cytochrome c, Smac, apoptosis-inducing factor (AIF), or loss of mitochondrial membrane potential. Caspase inhibition also did not protect AML blasts from chemotherapy-induced cell death in vitro. These results suggest that expression levels of Survivin or XIAP have no prognostic impact in AML patients. Although anticancer drugs induced caspase cleavage and apoptosis, cell killing was caspase independent. This may partially explain the lack of prognostic impact of XIAP and Survivin and may suggest caspase-independent mechanisms of cell death in AML.

Tumor suppressor genes in normal and malignant hematopoiesis

Over the last decade, a growing number of tumor suppressor genes have been discovered to play a role in tumorigenesis. Mutations of p53 have been found in hematological malignant diseases, but the frequency of these alterations is much lower than in solid tumors. These mutations occur especially as hematopoietic abnormalities become more malignant such as going from the chronic phase to the blast crisis of chronic myeloid leukemia. A broad spectrum of tumor suppressor gene alterations do occur in hematological malignancies, especially structural alterations of p15(INK4A), p15(INK4B) and p14(ARF) in acute lymphoblastic leukemia as well as methylation of these genes in several myeloproliferative disorders. Tumor suppressor genes are altered via different mechanisms, including deletions and point mutations, which may result in an inactive or dominant negative protein. Methylation of the promoter of the tumor suppressor gene can blunt its expression. Chimeric proteins formed by chromosomal translocations (i.e. AML1-ETO, PML-RARalpha, PLZF-RARalpha) can produce a dominant negative transcription factor that can decrease expression of tumor suppressor genes. This review provides an overview of the current knowledge about the involvement of tumor suppressor genes in hematopoietic malignancies including those involved in cell cycle control, apoptosis and transcriptional control.