Loss of p73 gene expression in lymphoid leukemia cell lines is associated with hypermethylation

A study of deregulated MMR pathways and anticancer potential of curcuma derivatives using computational approach

Plant derived products have steadily gained momentum in treatment of cancer over the past decades. Curcuma and its derivatives, in particular, have diverse medicinal properties including anticancer potential with proven safety as supported by numerous in vivo and in vitro studies. A defective Mis-Match Repair (MMR) is implicated in solid tumors but its role in haematologic malignancies is not keenly studied and the current literature suggests that it is limited. Nonetheless, there are multiple pathways interjecting the mismatch repair proteins in haematologic cancers that may have a direct or indirect implication in progression of the disease. Here, through computational analysis, we target proteins that are involved in rewiring of multiple signaling cascades via altered expression in cancer using various curcuma derivatives (Curcuma longa L. and Curcuma caesia Roxb.) which in turn, profoundly controls MMR protein function. These biomolecules were screened to identify their efficacy on selected targets (in blood-related cancers); aberrations of which adversely impacted mismatch repair machinery. The study revealed that of the 536 compounds screened, six of them may have the potential to regulate the expression of identified targets and thus revive the MMR function preventing genomic instability. These results reveal that there may be potential plant derived biomolecules that may have anticancer properties against the tumors driven by deregulated MMR-pathways.

Dalhat M, Abdullah O, Kaleem M, Hosawi S, A Al-Abbasi F, Wu W, Choudhry H, Alhosin M. Targeting Post-Translational Modifications of the p73 Protein: A Promising Therapeutic Strategy for Tumors

The tumor suppressor p73 is a member of the p53 family and is expressed as different isoforms with opposing properties. The TAp73 isoforms act as tumor suppressors and have pro-apoptotic effects, whereas the ΔNp73 isoforms lack the N-terminus transactivation domain and behave as oncogenes. The TAp73 protein has a high degree of similarity with both p53 function and structure, and it induces the regulation of various genes involved in the cell cycle and apoptosis. Unlike those of the p53 gene, the mutations in the p73 gene are very rare in tumors. Cancer cells have developed several mechanisms to inhibit the activity and/or expression of p73, from the hypermethylation of its promoter to the modulation of the ratio between its pro- and anti-apoptotic isoforms. The p73 protein is also decorated by a panel of post-translational modifications, including phosphorylation, acetylation, ubiquitin proteasomal pathway modifications, and small ubiquitin-related modifier (SUMO)ylation, that regulate its transcriptional activity, subcellular localization, and stability. These modifications orchestrate the multiple anti-proliferative and pro-apoptotic functions of TAp73, thereby offering multiple promising candidates for targeted anti-cancer therapies. In this review, we summarize the current knowledge of the different pathways implicated in the regulation of TAp73 at the post-translational level. This review also highlights the growing importance of targeting the post-translational modifications of TAp73 as a promising antitumor strategy, regardless of p53 status.

p53 Activity Dominates That of p73 upon Mdm4 Loss in Development and Tumorigenesis

Mdm4 negatively regulates the p53 tumor suppressor. Mdm4 loss in mice leads to an embryonic lethal phenotype that is p53-dependent. Biochemical studies indicate that Mdm4 also binds p73, a member of the p53 family, with higher affinity than p53. In this study, the significance of the Mdm4 and p73 interaction in vivo during embryogenesis and tumorigenesis was examined. The data revealed that p73 loss did not rescue either the early Mdm4-deficient embryonic lethality or the runted phenotype of Mdm4(Δ2/Δ2) p53(+/-) embryos. Furthermore, studies in the developing central nervous system wherein both genes have prominent roles indicated that loss of p73 also did not rescue the Mdm4-null brain phenotype as did p53 loss. This p53 dependency occurred despite evidence for p73-specific transcriptional activity. In tumor studies, the combination of Mdm4 overexpression and p73 loss did not alter survival of mice or the tumor spectrum as compared with Mdm4 overexpression alone. In summary, these data demonstrate that the Mdm4-p73 axis cannot override the dominant role of p53 in development and tumorigenesis.

IMPLICATIONS

Genetic characterization of the Mdm4 and p73 interaction during development and tumorigenesis suggests new insight into the role of p53 family members, which may influence treatment options for patients.

Aberrant methylation and decreased expression of the RIZ1 gene are frequent in adult acute lymphoblastic leukemia of T-cell phenotype

Retinoblastoma protein-interacting zinc finger, RIZ1, is a tumor suppressor gene that is inactivated in various solid tumors. However, the role of the RIZ1 gene has not been well examined in adult acute lymphoblastic leukemia (ALL). We analyzed the expression and promoter methylation status of the RIZ1 gene in patients with newly diagnosed ALL by quantitative real-time reverse transcription polymerase chain reaction (PCR) and methylation-specific PCR, respectively. RIZ1 expression in 67 cases of ALL (mean 1.043) was decreased compared with that in normal bone marrow (mean 1.471) (p = 0.030). Methylation was detected in 11 of 71 patients (15.5%) but not in healthy controls. Methylation was associated with decreased RIZ1 expression in many ALL cases examined, but this was not statistically significant. In T-ALL, RIZ1 methylation was more frequent (63.6%) than in B-ALL (6.7%) (p < 0.0001) and the decrease of RIZ1 expression was more significant than in B-ALL (p = 0.045). 5-Aza-2'-deoxycytidine treatment of MOLT-4 cells with RIZ1 methylation induced demethylation of RIZ1 and restoration of expression. Forced RIZ1 expression in T-ALL cell lines suppressed cell growth accompanied by G2/M arrest and apoptosis. No mutations were found by PCR-single strand conformation polymorphism analysis in hotspots of the gene. These results suggest that RIZ1 is inactivated in adult ALL, and this inactivation is associated with methylation in T-ALL.

Notch and the p53 clan of transcription factors

Notch 1 to 4 and the p53 clan, comprising p53, p63 and p73 plus numerous isoforms thereof, are gene transcription regulators that are critically involved in various aspects of cell differentiation, stem cell maintenance and tumour suppression. It is thus perhaps no surprise that extensive crosstalk between the Notch and p53 pathways is implemented during these processes. Typically, Notch together with p53 and even more so with transactivation competent p63 or p73, drives differentiation, whereas Notch combined with transactivation impaired p63 or p73 helps maintain undifferentiated stem cell compartments. With regard to cancer, it seems that Notch acts as a tumour suppressor in cellular contexts where Notch signalling supports p53 activation and both together can bring on its way an anti-proliferative programme of differentiation, senescence or apoptosis. In contrast, Notch often acts as an oncoprotein in contexts where it suppresses p53 activation and activity and where differentiation is unwanted. It is no accident that the latter pathways-the inhibition by Notch of p53 and differentiation-are operative in somatic stem cells as well as in tumour cells.

The p73 tumor suppressor is targeted by Pirh2 RING finger E3 ubiquitin ligase for the proteasome-dependent degradation

The p73 gene, a homologue of the p53 tumor suppressor, is expressed as TA and ΔN isoforms. TAp73 has similar activity as p53 and functions as a tumor suppressor whereas ΔNp73 has both pro- and anti-survival functions. While p73 is rarely mutated in spontaneous tumors, the expression status of p73 is linked to the sensitivity of tumor cells to chemotherapy and prognosis for many types of human cancer. Thus, uncovering its regulators in tumors is of great interest. Here, we found that Pirh2, a RING finger E3 ubiquitin ligase, promotes the proteasome-dependent degradation of p73. Specifically, we showed that knockdown of Pirh2 up-regulates, whereas ectopic expression of Pirh2 down-regulates, expression of endogenous and exogenous p73. In addition, Pirh2 physically associates with and promotes TAp73 polyubiquitination both in vivo and in vitro. Moreover, we found that p73 can be degraded by both 20 S and 26 S proteasomes. Finally, we showed that Pirh2 knockdown leads to growth suppression in a TAp73-dependent manner. Taken together, our findings indicate that Pirh2 promotes the proteasomal turnover of TAp73, and thus targeting Pirh2 to restore TAp73-mediated growth suppression in p53-deficient tumors may be developed as a novel anti-cancer strategy.

The association between non-Hodgkin lymphoma and methylation of p73

To investigate the effects of methylation of the p73 gene on the pathogenesis of non-Hodgkin lymphoma (NHL), the methylation status of the p73 gene promoter and the expression of p73 mRNA were examined in NHLs by methylation-specific polymerase chain reaction (MSP) and reverse transcription-polymerase chain reaction, respectively; p73 protein was detected by Western blotting analysis. Furthermore, the expression of p73 mRNA in NHL cells treated with 5-Aza-2'-deoxycytidine was analyzed. MSP results revealed that the promoter of p73 was methylated in 87.5% of NHLs but was not methylated in reactive hyperplasia lymph node samples. The expression of p73 mRNA was not detected in 83.33% of NHLs but was detected in all of the reactive hyperplasia lymph node samples. The p73 protein was not detected in 91.67% of NHLs but was detected in all of the reactive hyperplasia lymph node samples. The expression of p73 mRNA was detected in NHL cells treated with 5-Aza-2'-deoxycytidine. The inactivation of p73, predominantly by methylation, may be involved in the pathogenesis of NHLs.

Epigenomics in cancer management

The identification of all epigenetic modifications implicated in gene expression is the next step for a better understanding of human biology in both normal and pathological states. This field is referred to as epigenomics, and it is defined as epigenetic changes (ie, DNA methylation, histone modifications and regulation by noncoding RNAs such as microRNAs) on a genomic scale rather than a single gene. Epigenetics modulate the structure of the chromatin, thereby affecting the transcription of genes in the genome. Different studies have already identified changes in epigenetic modifications in a few genes in specific pathways in cancers. Based on these epigenetic changes, drugs against different types of tumors were developed, which mainly target epimutations in the genome. Examples include DNA methylation inhibitors, histone modification inhibitors, and small molecules that target chromatin-remodeling proteins. However, these drugs are not specific, and side effects are a major problem; therefore, new DNA sequencing technologies combined with epigenomic tools have the potential to identify novel biomarkers and better molecular targets to treat cancers. The purpose of this review is to discuss current and emerging epigenomic tools and to address how these new technologies may impact the future of cancer management.

Absence of methylation-dependent transcriptional silencing in TP73 irrespective of the methylation status of the CDKN2A CpG island in plasma cell neoplasia

Few studies exist regarding the methylation status of the TP73 CpG island in plasma cell dyscrasias. We have tested whether CpG methylation of both CDKN2A and TP73 occurs in 45 individuals with multiple myeloma (24 male and 21 female, mean age 66.4 years) and in 4 patients (2 male and 2 female, mean age 61.7 years) with Waldenström's macroglobulinemia. No patient was found to be methylated for the promoter of TP73 while CDKN2A promoter was found to be methylated in 12/45 MM patients (26.6%) at diagnosis and in 1/4 WM patients. To verify the absence of detectable methylation observed using MSP, we performed bisulphite sequence analysis on a subset of the cases and confirmed the absence of methylation. Interesting trends were identified where patients with methylated CDKN2A had an increased risk of death (HR = 1.9, p = 0.32), advanced stage disease (DS > or = II) (OR = 1.9, p = 0.3) and anemia (OR = 1.4, p = 0.6). TP73 CpG methylation is an infrequent event in patients with MM and WM. Further evaluation in a larger sample of patients is needed in order to enhance our statistical power and to test our hypothesis that CDKN2A methylation status can become a useful prognostic biomarker.

Regulating p73 isoforms in human tumours

Although mutations in the TP73 gene are extremely rare in human tumours, altered expression is common. In some tumours, most notably leukaemias and lymphomas, expression of TP73 is reduced, suggesting a tumour suppressor role. In contrast, TP73 is over-expressed in many other tumour types, implying that it has oncogenic functions in human tumourigenesis. These conflicting scenarios can be reconciled by the observations that the TP73 gene produces p53-like isoforms (TAp73) and anti-p53 isoforms (DeltaTAp73). Thus, loss of TAp73 or over-expression of DeltaTAp73 should each promote oncogenic transformation, and the balance of expression of the opposing isoforms is the crucial factor. The mechanisms that regulate expression of TP73 isoforms are therefore of great interest. Recent data provide evidence for interacting roles of ZEB1, p300, and a polymorphic 73 bp deletion in intron 1 of the human TP73 gene in this process. Importantly, alterations to the proposed regulatory pathway for controlling TP73 isoform expression in colorectal cancer are associated with adverse clinico-pathological characteristics. Because p73 is also associated with tumour chemosensitivity, these new findings should provide prognostic information and have the potential to guide future therapeutic decisions.

The role of p73 in hematological malignancies

The P73 gene is a homologue of the P53 tumor suppressor. Owing to its structural similarity with p53, p73 was originally considered to have tumor suppressor function. However, the discovery of N-terminal truncated isoforms with oncogenic properties showed a 'two in one' structure of its product, p73 protein. The full-length variants are strong inducers of apoptosis, whereas the truncated isoforms inhibit proapoptotic activity of p53 and the full-length p73. Thus, p73 is involved in the regulation of cell cycle, cell death and development. Moreover, it plays a role in carcinogenesis and controls tumor sensitivity to treatment. p73 is commonly expressed in tumor cells in hematological malignancies. Overexpression of p73 protein and aberrant expression of its particular isoforms, with very low frequency of P73 hypermethylation or mutations, were found in malignant myeloproliferations, including acute myeloblastic leukemia. In contrast, hypermethylation and subsequent inactivation of the P73 gene are the most common findings in malignant lymphoproliferative disorders, especially acute lymphoblastic leukemia (ALL) and non-Hodgkin's lymphomas. Assessment of P73 methylation may provide important prognostic information, as was confirmed in patients with ALL. This review summarizes some aspects of p73 biology with particular reference to its possible pathogenetic role and prognostic significance in hematological malignancies.

Differential expression of p73 isoforms in relation to drug resistance in childhood T-lineage acute lymphoblastic leukaemia

The T-lineage phenotype of childhood acute lymphoblastic leukaemia (ALL) is associated with an increased relapse-risk and in vitro resistance to drugs as compared to a precursor B phenotype. Antiapoptotic isoforms of p73 that lack part of the transactivation (TA) domain (DeltaTA-p73, i.e. p73Deltaex2, p73Deltaex3, p73Deltaex2/3 and DeltaN-p73) may cause resistance to anticancer agents through inhibition of p53 and/or proapoptotic p73 family members (TA-p73). We demonstrate in our study that the expression of total p73 mRNA was higher in childhood T-ALL compared to controls (P=0.004). In T-ALL, the relative contribution of antiapoptotic DeltaTA-p73 (88%) was larger than of proapoptotic TA-p73 (12%). Leukaemic cells of T-ALL patients expressing higher levels of antiapoptotic p73 were more resistant to the DNA-damaging drug daunorubicin compared to cells of patients with low or negative expression or these isoforms (P(trend)=0.045). Interestingly, p73Deltaex2 was the most abundantly expressed antiapoptotic isoform in daunorubicin-resistant patient cells (44% of total p73). No association was found between high expression of proapoptotic TA-p73 or antiapoptotic DeltaTA-p73 and relapse-risk. Our results suggest that childhood T-ALL is associated with a high expression of DeltaTA-p73. These isoforms may play a role in cellular resistance to DNA-damaging drugs in children at initial diagnosis of T-ALL.

Lack of genetic and epigenetic changes in meningiomas without NF2 loss

Approximately 60% of sporadic meningiomas are caused by inactivation of the NF2 tumour suppressor gene. The causative gene for the remaining meningiomas is unknown. Previous studies have shown that these tumours have no recurrent karyotypic abnormalities. They differ from their NF2-related counterparts in that they are more often of the meningothelial subtype and are located preferentially in the anterior skull base. To gain more insight into the aetiology of these tumours, we studied genetic and epigenetic alterations in 25 meningiomas without NF2 involvement. We first established a genome-wide allelotype using 3 microsatellite markers per chromosome arm. Loss of heterozygosity (LOH) was detected at a low frequency and no indication for the location of putative tumour suppressor genes could be established. We next screened the subtelomeric regions by using 2-3 polymorphic markers close to each telomere. Again no evidence for LOH of a particular chromosome arm was obtained, and no LOH was found in the genomic regions containing the NF2-related ERM family members ezrin and radixin, DAL-1, protein 4.1R, and TSLC1. Mutations in the X-chromosome based family member, moesin, were analysed by SSCP and were not detected. Microsatellite instability was studied using 6 commonly used markers but none of these was altered in any meningioma. Methylation was detected in 5 of 16 genes (NF2, p14(ARF), CDH1, BRCA1, RB1) previously shown to be silenced in a variety of tumour types. However, methylation percentages for these genes were generally higher in a group of NF2-related meningiomas, with the exception of the BRCA1 gene. The NF2 gene was methylated in only 1 of 21 tumours. In conclusion, meningiomas with an intact NF2 gene have a normal karyotype and no obvious genetic or epigenetic aberrations, suggesting that the gene(s) involved in the pathogenesis of these tumours are altered by smaller events than can be detected with the techniques used in our study.

Alteration of p73 in pediatric de novo acute lymphoblastic leukemia

The frequency of p73 mutation is low in hematologic malignancies as well as solid tumors. Aberrant DNA methylation of multiple promoter associated CpG islands is a frequent phenomenon in acute lymphoblastic leukemia (ALL). In the present study, we scanned for mutations in the exons 4, 5, 6, and 7 of p73 gene. Using PCR-based methylation analysis, we have explored the prevalence of methylation of this pathway in a cohort of children with ALL (N=120). One pediatric patient showed mutation in exon 4, two showed mutation in exon 5, and none of the patients showed mutation in exons 6 and 7. Methylation of p73 gene is absent in the patients studied. Expression level of p73 m RNA was also examined in 40 ALL samples using reverse transcriptase/polymerase chain reaction. All the patients showed p73 mRNA expression. However, p73 overexpression was observed in 58% of pediatric patients as demonstrated by immunocytochemistry and Western blot analysis. Further, mutation of p73 has been correlated with p73 m RNA and p73 protein status. The results show the presence of overexpressed protein in the samples with mutated p73 gene. Thus, it is presumed that mutation of p73 might lead to production of defective p73 protein and this might have a role in the process of leukemogenesis of ALL. Methylation of p73 does not play a role in pediatric ALL patients of our population. This report is the first demonstrating the presence of p73 gene mutations in exons 4 and 5 with overexpression of p73 protein and absence of p73 methylation in pediatric ALL patients of eastern Indian population.

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.

p63 and p73: Roles in Development and Tumor Formation

The tumor suppressor p53 is critically important in the cellular damage response and is the founding member of a family of proteins. All three genes regulate cell cycle and apoptosis after DNA damage. However, despite a remarkable structural and partly functional similarity among p53, p63, and p73, mouse knockout studies revealed an unexpected functional diversity among them. p63 and p73 knockouts exhibit severe developmental abnormalities but no increased cancer susceptibility, whereas this picture is reversed for p53 knockouts. Neither p63 nor p73 is the target of inactivating mutations in human cancers. Genomic organization is more complex in p63 and p73, largely the result of an alternative internal promoter generating NH2-terminally deleted dominant-negative proteins that engage in inhibitory circuits within the family. Deregulated dominant-negative p73 isoforms might play an active oncogenic role in some human cancers. Moreover, COOH-terminal extensions specific for p63 and p73 enable further unique protein-protein interactions with regulatory pathways involved in development, differentiation, proliferation, and damage response. Thus, p53 family proteins take on functions within a wide biological spectrum stretching from development (p63 and p73), DNA damage response via apoptosis and cell cycle arrest (p53, TAp63, and TAp73), chemosensitivity of tumors (p53 and TAp73), and immortalization and oncogenesis (ΔNp73).

DNA-methylation analysis identifies the E-cadherin gene as a potential marker of disease progression in patients with monoclonal gammopathies

BACKGROUND

Silencing of tumor suppressor genes (TSG) by aberrant methylation (referred to as methylation) contributes to the pathogenesis of various human malignancies. However, little is known about the methylation of known and putative TSGs in monoclonal gammopathies. Thus, the authors investigated the methylation frequencies of 10 genes in patients with monoclonal gammopathies.

METHODS

The methylation patterns of the genes p16(INK4a) (p16), tissue inhibitor of metalloproteinase 3 (TIMP3), p15(INK4b) (p15), E-cadherin (ECAD), death-associated protein kinase (DAPK), p73, RAS-association domain family 1A (RASSF1A), p14, O(6)-methylguanine DNA methyltransferase (MGMT), and retinoid acid receptor beta2 (RARbeta) were determined in patients with monoclonal gammopathy of undetermined significance (MGUS; n = 29), smoldering multiple myeloma (SMM; n = 5), multiple myeloma (MM; n = 113), or plasma cell leukemia (PCL; n = 7) by methylation-specific polymerase chain reaction analysis.

RESULTS

Methylation frequencies for p16, TIMP3, p15, ECAD, DAPK, p73, RASSF1A, p14, MGMT, and RARbeta were as follows: 28%, 35%, 10%, 0%, 17%, 21%, 14%, 14%, 7%, and 0%, respectively, in patients with MGUS and 36%, 29%, 27%, 27%, 22%, 15%, 15%, 9%, 4%, and 0%, respectively, in patients with MM. Methylation of at least 1 of these genes was detected in 79% of patients with MGUS and in 80% of patients with MM. Although methylation of ECAD was not detected in patients with MGUS, it was observed frequently in patients with MM and with even greater frequency in patients with PCL. It is noteworthy that an association was found between ECAD methylation and poor prognostic markers in patients with MM.

CONCLUSIONS

Methylation of certain genes can be detected frequently in patients with monoclonal gammopathies. The current data suggest that methylation of ECAD is a marker of disease progression in patients with MM and PCL.

Hypermethylation of the 5' CpG island of the FHIT gene is associated with hyperdiploid and translocation-negative subtypes of pediatric leukemia

The human FHIT (fragile histidine triad) gene is a putative tumor suppressor gene located at chromosome region 3p14.2. Previous studies have shown that loss of heterozygosity, homozygous deletions, and abnormal expression of the FHIT gene are involved in several types of human malignancies. A CpG island is present in the 5' promoter region of the FHIT gene, and methylation in this region correlates with loss of FHIT expression. To test whether aberrant methylation of the FHIT gene may play a role in pediatric leukemia, we assessed the FHIT methylation status of 10 leukemia cell lines and 190 incident population-based cases of childhood acute lymphocytic and myeloid leukemias using methylation-specific PCR. Conventional and fluorescence in situ hybridization cytogenetic data were also collected to examine aneuploidy, t(12, 21), and other chromosomal rearrangements. Four of 10 leukemia cell lines (40%) and 52 of 190 (27.4%) bone marrows from childhood leukemia patients demonstrated hypermethylation of the promoter region of FHIT. Gene expression analyses and 5-aza-2'-deoxycytidine treatment showed that promoter hypermethylation correlated with FHIT inactivation. Among primary leukemias, hypermethylation of FHIT was strongly correlated with acute lymphoblastic leukemia (ALL) histology (P = 0.008), high hyperdiploid (P < 0.0001), and translocation-negative (P < 0.0001) categories. Hyperdiploid B-cell ALLs were 23-fold more likely to be FHIT methylated compared with B-cell ALL harboring TEL-AML translocations. FHIT methylation was associated with high WBC counts at diagnosis, a known prognostic indicator. These results suggest that hypermethylation of the promoter region CpG island of the FHIT gene is a common event and may play an important role in the etiology and pathophysiology of specific cytogenetic subtypes of childhood ALL.

Exclusion of c-Abl from the nucleus restrains the p73 tumor suppression function

The p73alpha protein is a functional homolog of the p53 tumor suppressor. Although the TP53 gene is frequently mutated in human cancers, the TP73 gene is rarely inactivated. We have found that p73alpha is highly expressed in a significant fraction of anaplastic thyroid cancer, whereas it is not detectable in normal thyroid epithelial cells or in papillary and follicular thyroid cancer cells. Interestingly, the tumor suppression function of p73alpha is actively restrained in anaplastic thyroid cancer cells. We have also found that c-Abl tyrosine kinase, an activator of p73, is excluded from the nucleus of p73alpha-positive thyroid cancer cells; whereas c-Abl undergoes nuclear-cytoplasmic shuttling in normal thyroid and p73-negative thyroid cancer cells. We constructed an AblNuk-FK506-binding protein (FKBP) fusion protein to enforce the nuclear accumulation of an inducible Abl kinase. Activation of this nuclear AblNuk-FKBP by dimerization with AP20187 in anaplastic thyroid cancer cells increased the levels of p73alpha and p21Cip1 and caused p73-dependent apoptosis. These results suggest subcellular segregation of c-Abl from p73 to be a strategy for disrupting the tumor suppression function of p73alpha.

Preclinical evaluation of antineoplastic activity of inhibitors of DNA methylation (5-aza-2'-deoxycytidine) and histone deacetylation (trichostatin A, depsipeptide) in combination against myeloid leukemic cells

During the development of leukemia, genes that suppress growth and induce differentiation can be silenced by aberrant DNA methylation and by changes in chromatin structure that involve histone deacetylation. It has been reported that a positive interaction between DNA methylation and histone deacetylation takes place to inhibit transcription. Based on this observation, our working hypothesis was that a combination of inhibitors of these processes should produce an enhancement of their antineoplastic activity on leukemic cells. The cytosine nucleoside analog, 5-aza-2'-deoxycytidine (5AZA), is a potent inhibitor of DNA methylation, which can activate tumor suppressor genes in leukemic cells that have been silenced by aberrant methylation. In clinical trials, 5AZA was demonstrated to be an active antileukemic agent. Histone deacetylase inhibitors (HDI) can also activate gene expression in leukemic cell lines by producing changes in chromatin configuration, and show antineoplastic activity in preclinical studies. In this report, we investigated the in vitro antineoplastic activity of 5AZA, alone and in combination with the HDI, trichostatin A (TSA) and depsipeptide (FR901228, depsi), on the human myeloid leukemic cell lines, HL-60 and KG1a. The results showed that the combination of 5AZA with TSA or depsi produced a greater inhibition of growth and DNA synthesis and a greater loss of clonogenicity than either agent alone. These results suggest that 5AZA used in combination with HDI may be an interesting chemotherapeutic regimen to investigate in patients with acute myeloid leukemia that is resistant to conventional chemotherapy.

Alterations of DNA methylation in hematologic malignancies

The DNA methylation profile of cancer cells is frequently characterized by global hypomethylation and simultaneous hypermethylation of selected CpG island gene promoters. In recent years, the epigenetic phenomenon of DNA promoter methylation has gained increasing recognition as an important mechanism for transcriptional inactivation of cancer related genes. Studies on both liquid and solid tumors have revealed myriad aberrant methylation events, some of which may provide important clues to the pathogenesis of these tumors. The identification of these methylation alterations and elucidation of the mechanistic events surrounding them are of prime importance, as the methylation status of cancer cells can now be manipulated in vivo with demethylating chemotherapeutics.

Frequent inactivation of the p73 gene by abnormal methylation or LOH in non-Hodgkin's lymphomas

p73 is a candidate tumor suppressor and imprinted gene that shares significant homology with the p53 gene. It is located on 1p36, a region frequently deleted in neuroblastoma and other tumors. To investigate the pattern of inactivation of this gene in human lymphomas, we studied 59 tumors to identify abnormal methylation in exon 1 and loss of heterozygosity (LOH) at this locus. p73 was methylated in 13/50 (26%) B cell lymphomas. There was no evidence of p73 methylation in the 9 T cell lymphomas analyzed. Burkitt's lymphomas showed the highest proportion of methylated cases (36%), although this alteration also affected other aggressive lymphomas such as diffuse large cell and some marginal zone lymphomas. LOH at the p73 locus was detected in 4/34 (11%) B and 1/9 (11%) T cell lymphomas. The p73 expression analysis showed absence or low level of p73 product in methylated lymphomas, whereas p73 was always detected in unmethylated tumors. We found monoallelic expression in normal peripheral blood samples, consistent with imprinting. None of the tumors showed LOH and methylation of the remaining allele simultaneously, suggesting that alteration of the expressed allele could lead to the total inactivation of the gene. Our results show that deletion or methylation of the p73 gene could be important mechanisms in suppressing p73 expression in B cell non-Hodgkin's lymphomas.

DNA methylation changes in leukaemia

Leukaemogenesis is a multi-step process whereby a clonal population arises that has undergone successive alterations to the genotype and the phenotype of the cells that make up the clone. Leukaemia has traditionally been viewed as a genetic disease, however epigenetic defects also play an important role. Expression of the DNA methyltransferase enzymes is elevated in leukaemia, and aberrant methylation is common with both a decrease in the total genomic 5-methylcytosine, and a concomitant hypermethylation of CpG island-associated tumour suppressor genes. This review will discuss the multitude of DNA methylation changes in haematopoietic malignancies and the implications they have for diagnosis and treatment.

DNA methylation and breast carcinogenesis

Knowledge about breast carcinogenesis has accumulated during the last decades but has barely been translated into strategies for early detection or prevention of this common disease. Changes in DNA methylation have been recognized as one of the most common molecular alterations in human neoplasia and hypermethylation of gene-promoter regions is being revealed as one of the most frequent mechanisms of loss of gene function. The heritability of methylation states and the secondary nature of the decision to attract or exclude methylation support the idea that DNA methylation is adapted for a specific cellular memory. According to Hanahan and Weinberg, there are six novel capabilities a cell has to acquire to become a cancer cell: limitless replicative potential, self-sufficiency in growth signals, insensitivity to growth-inhibitory signals, evasion of programmed cell death, sustained angiogenesis and tissue invasion and metastasis. This review highlights how DNA-methylation contributes to these features and offers suggestions about how these changes could be prevented, reverted or used as a 'tag' for early detection of breast cancer or, preferably, for detection of premalignant changes.

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.

Transcriptional inactivation of TP73 expression in oligodendroglial tumors

The TP73 gene, located on chromosome 1p36.3, encodes a product that shares significant structural homology with the tumor suppressor TP53. The aim of this study was to investigate whether TP73 is involved in the development of oligodendroglial tumors, which frequently carry deletions involving 1p36.3. Semi-quantitative reverse transcription-polymerase chain reaction was used to determine TP73 transcript levels. Ten of 24 (42%) tumors showed negligible to more than 5-fold reduction in TP73 expression when compared to normal brain level. To identify potential mechanisms that may modulate TP73 transcription in oligodendroglial tumors, we performed mutation analysis on the TP73 gene. No somatic mutations were however detected in the gene sequence. We then evaluated the possible involvement of epigenetic change in TP73 expression. Bisulfite genomic sequencing detected aberrant hypermethylation at the 5' region upstream and including the first exon of the TP73 gene in 17 of 44 (39%) oligodendroglial tumors, whereas normal brain tissues showed no methylation in the same region examined. Moreover, 6 of 10 (60%) tumors with negligible or decreased levels of TP73 transcripts were methylation-positive. In conclusion, our results showed that inactivation of TP73 occurs at the transcriptional level and is associated with promotor hypermethylation. Loss of or reduced TP73 transcript expression may contribute to the tumorigenesis of oligodendroglial tumors.

On the shoulders of giants: p63, p73 and the rise of p53

The discoveries of the p53 homologs, p63 and p73, have both fueled new insights and exposed enigmas in our understanding of the iconic p53 tumor suppressor. Although the pivotal role of p53 in cancer pathways remains unchallenged, because p63 and p73 are now implicated in stem cell identity, neurogenesis, natural immunity and homeostatic control. Despite their seemingly separate tasks, there are hints that the p53 family members both collaborate and interfere with one another. The question remains, therefore, as to whether these genes evolved to function independently or whether their familial ties still bind them in pathways of cell proliferation, death and tumorigenesis.

Specific patterns of gene methylation in natural killer cell lymphomas : p73 is consistently involved

Aberrant methylation of promoter CpG regions is a putative mechanism whereby tumor suppressor genes are inactivated. We used a candidate gene approach to investigate the patterns and significance of this epigenetic change in natural killer (NK) cell malignancies. Thirty-three patients were studied for promoter methylation in five putative tumor suppressor genes by methylation-specific polymerase chain reaction (MSP), which has a sensitivity of 10(-3). The p73 gene was methylated in 94% of cases, a frequency that is the highest known for any human malignancy. In the NK cell lymphoma line NK92, p73 was also completely methylated, and the p73 transcript was correspondingly not detectable by quantitative polymerase chain reaction. Treatment of the cell line with 5-azacytidine, a demethylation reagent, led to demethylation of the p73 promoter and reinduction of p73 gene expression. These results suggested that promoter CpG methylation might be an important mechanism in suppressing p73 gene expression in NK cells. Other methylated genes included hMLH1 (63%), p16 (63%), p15 (48%), and RAR beta (47%). Methylation of two or more genes occurred in 88% of cases. With promoter methylation as a molecular marker, MSP identified two cases of occult marrow metastasis. Interestingly, the primary tumor and metastasis showed different methylation patterns, implying that separate clonal evolutions might have occurred at these sites. Furthermore, MSP also identified tumor infiltration in random oropharyngeal biopsies in a case where histological examination could not show evidence of tumor involvement. We conclude that NK cell malignancies show a specific pattern of promoter methylation, with p73 being consistently involved. These results suggest that p73 may be an important target in the neoplastic transformation of NK cells, and the demonstration of its methylation may serve as a potential molecular tool for NK cell lymphoma detection.

p53, p63 and p73--solos, alliances and feuds among family members

p53 controls crucial stress responses that play a major role in preventing malignant transformation. Hence, inactivation of p53 is the single most common genetic defect in human cancer. With the recent discovery of two close structural homologs, p63 en p73, we are getting a broader view of a fascinating gene family that links developmental biology with tumor biology. While unique roles are apparent for each of these genes, intimate biochemical cross-talk among family members suggests a functional network that might influence many different aspects of individual gene action. The most interesting part of this family network derives from the fact that the p63 and p73 genes are based on the "two-genes-in-one" idea, encoding both agonist and antagonist in the same open reading frame. In this review, we attempt to present an overview of the current status of this fast moving field.