Genetic analysis of p73 localized at chromosome 1p36.3 in primary neuroblastomas

p73, like its p53 homolog, shows preference for inverted repeats forming cruciforms

p73 is a member of the p53 protein family and has essential functions in several signaling pathways involved in development, differentiation, DNA damage responses and cancer. As a transcription factor, p73 achieves these functions by binding to consensus DNA sequences and p73 shares at least partial target DNA binding sequence specificity with p53. Transcriptional activation by p73 has been demonstrated for more than fifty p53 targets in yeast and/or human cancer cell lines. It has also been shown previously that p53 binding to DNA is strongly dependent on DNA topology and the presence of inverted repeats that can form DNA cruciforms, but whether p73 transcriptional activity has similar dependence has not been investigated. Therefore, we evaluated p73 binding to a set of p53-response elements with identical theoretical binding affinity in their linear state, but different probabilities to form extra helical structures. We show by a yeast-based assay that transactivation in vivo correlated more with the relative propensity of a response element to form cruciforms than to its expected in vitro DNA binding affinity. Structural features of p73 target sites are therefore likely to be an important determinant of its transactivation function.

Meta-analysis of the correlation between the rs17401966 polymorphism in kinesin family member 1B and susceptibility to hepatitis B virus related hepatocellular carcinoma

Background/Aims

The association between the kinesin family member 1B (KIF1B) gene polymorphism and the risk of hepatitis B virus-related hepatocellular carcinoma (HCC) has been investigated in many peer-reviewed studies. However, scholars have failed to replicate these results in validation tests. The purpose of the present study was to explore whether the KIF1B rs17401966 polymorphism was associated with susceptibility to HCC.

Methods

The results of case-controlled studies on the correlation between the KIF1B rs17401966 polymorphism and HCC susceptibility were collected using Google Scholar and the EMBASE, PubMed and CNKI databases. Based on inclusion and exclusion criteria, 5 papers with a total of 12 cohorts were included in this study.

Results

The 12 cohorts were integrated, and the results showed that the rs17401966 polymorphism reduced the risk for HCC under the allele, heterozygous, homozygous, and dominant models but not under the additive or recessive models. Moreover, the merged results showed strong heterogeneity, and the cumulative meta-analysis results were unreliable. A genetic differentiation analysis of the 12 cohorts found different degrees of genetic differentiation between the 5 cohorts in Zhang et al.’s study and the cohorts in the other studies. We further divided the 12 study cohorts into 2 subgroups based on fixation index value; however, the results of that analysis were inconsistent.

Conclusions

The results of this meta-analysis were not able to verify the association between the KIF1B rs1740199 polymorphism and HCC risk. Therefore, a well-designed, large-scale, multicenter validation study is needed to confirm the relationship.

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.

Tracing the protectors path from the germ line to the genome

One of the basic principles that nature uses in evolution is to recycle successful concepts and create new functions by modifying existing units. This conservatism in evolution has resulted in an astonishingly high sequence identity of genes, even between evolutionarily distant species such as the nematode Caenorhabditis elegans and Homo sapiens. The recycling of successful concepts in conjunction with gene duplication events has also led to the existence of highly homologous proteins within the genome of many species. Often, these homologous proteins show similar, yet distinct functions that, in combination with their individual tissue distribution, define their specific physiological role. One prominent example is the p53 protein family, which consists of p53, p63, and p73. Recent advances in understanding the specific biological functions of these members have shed some light onto the evolution of this crucial protein family, from a germ line-specific quality-control factor to a somatic tumor suppressor. Furthermore, structures of the oligomerization domains of the mammalian paralogs, p53 and p73, and invertebrate orthologs, CEP-1 and DMP53, have delineated evolutionary changes and revealed that the oligomerization domain of p53 lacks additional stabilizing structural elements present in all other p53 family members. This suggests that p53 is the most recent evolutionary member of this protein family and predicts a mechanism for p53 activation.

p53 family: Therapeutic targets in neuroblastoma

Survival rates for metastatic neuroblastoma remain poor, despite significant increase in the intensity of therapy. Although it represents approximately 7% of pediatric cancer, neuroblastoma accounts for approximately 15% of childhood cancer deaths. Thus, novel approaches to enhance neuroblastoma chemotherapy sensitivity and prevent or bypass chemoresistance are required. Disruption of the p53 pathway is a common mechanism leading to defects in apoptosis in cancer cells. Increasing evidence suggests that the p53 pathway may be inactivated in neuroblastoma. Inactivation of the p53 pathway occurs most commonly at the time of relapse, and probably contributes to chemoresistance. The p53 family proteins, p73 and p63, can also induce apoptosis, and early studies suggest that p73 may be important in neuroblastoma pathogenesis and response to treatment. This article focuses on current therapies and novel drugs targeting p53 and p73 signaling pathways in neuroblastoma. Understanding the balance between the p53 family proteins in neuroblastoma and how their expression and activity are regulated will hopefully lead to the discovery of agents that target these pathways to induce neuroblastoma cell death, alone or in combination with chemotherapies.

p73 expression and its clinical significance in colorectal cancer

OBJECTIVE

To explore p73 mRNA and p73 protein expression and their clinical significance in colorectal cancer.

METHOD

p73 mRNA and p73 protein expression were detected using hybridization in situ and immunohistochemical method in cancerous tissue from 60 patients of colorectal cancer and in paracancerous tissue from 23 patients among the corresponding 60 patients of colorectal cancer. Quantitative analysis was performed using Smartscape image analysis system.

RESULTS

Compared with paracancerous tissue, p73 mRNA and p73 protein expression in cancerous tissue was increased with a statistically significant difference (P < 0.01) and was associated with differentiation level and lymphatic metastasis.

CONCLUSION

High expression of p73 mRNA and p73 protein in colorectal cancer may be involved in the progression of colorectal cancer and p73 mRNA and p73 protein may serve as a potential index to predict differentiation level and prognosis of colorectal cancer.

Lysosomal-associated protein multispanning transmembrane 5 gene (LAPTM5) is associated with spontaneous regression of neuroblastomas

Background

Neuroblastoma (NB) is the most frequently occurring solid tumor in children, and shows heterogeneous clinical behavior. Favorable tumors, which are usually detected by mass screening based on increased levels of catecholamines in urine, regress spontaneously via programmed cell death (PCD) or mature through differentiation into benign ganglioneuroma (GN). In contrast, advanced-type NB tumors often grow aggressively, despite intensive chemotherapy. Understanding the molecular mechanisms of PCD during spontaneous regression in favorable NB tumors, as well as identifying genes with a pro-death role, is a matter of urgency for developing novel approaches to the treatment of advanced-type NB tumors.

Principal Findings

We found that the expression of lysosomal associated protein multispanning transmembrane 5 (LAPTM5) was usually down-regulated due to DNA methylation in an NB cell-specific manner, but up-regulated in degenerating NB cells within locally regressing areas of favorable tumors detected by mass-screening. Experiments in vitro showed that not only a restoration of its expression but also the accumulation of LAPTM5 protein, was required to induce non-apoptotic cell death with autophagic vacuoles and lysosomal destabilization with lysosomal-membrane permeabilization (LMP) in a caspase-independent manner. While autophagy is a membrane-trafficking pathway to degrade the proteins in lysosomes, the LAPTM5-mediated lysosomal destabilization with LMP leads to an interruption of autophagic flux, resulting in the accumulation of immature autophagic vacuoles, p62/SQSTM1, and ubiqitinated proteins as substrates of autophagic degradation. In addition, ubiquitin-positive inclusion bodies appeared in degenerating NB cells.

Conclusions

We propose a novel molecular mechanism for PCD with the accumulation of autophagic vacuoles due to LAPTM5-mediated lysosomal destabilization. LAPTM5-induced cell death is lysosomal cell death with impaired autophagy, not cell death by autophagy, so-called autophagic cell death. Thus LAPTM5-mediated PCD is closely associated with the spontaneous regression of NBs and opens new avenues for exploring innovative clinical interventions for this tumor.

KIF1Bbeta functions as a haploinsufficient tumor suppressor gene mapped to chromosome 1p36.2 by inducing apoptotic cell death

Deletion of the distal region of chromosome 1 frequently occurs in a variety of human cancers, including aggressive neuroblastoma. Previously, we have identified a 500-kb homozygously deleted region at chromosome 1p36.2 harboring at least six genes in a neuroblastoma-derived cell line NB1/C201. Among them, only KIF1Bbeta, a member of the kinesin superfamily proteins, induced apoptotic cell death. These results prompted us to address whether KIF1Bbeta could be a tumor suppressor gene mapped to chromosome 1p36 in neuroblastoma. Hemizygous deletion of KIF1Bbeta in primary neuroblastomas was significantly correlated with advanced stages (p = 0.0013) and MYCN amplification (p < 0.001), whereas the mutation rate of the KIF1Bbeta gene was infrequent. Although KIF1Bbeta allelic loss was significantly associated with a decrease in KIF1Bbeta mRNA levels, its promoter region was not hypermethylated. Additionally, expression of KIF1Bbeta was markedly down-regulated in advanced stages of tumors (p < 0.001). Enforced expression of KIF1Bbeta resulted in an induction of apoptotic cell death in association with an increase in the number of cells entered into the G2/M phase of the cell cycle, whereas its knockdown by either short interfering RNA or by a genetic suppressor element led to an accelerated cell proliferation or enhanced tumor formation in nude mice, respectively. Furthermore, we demonstrated that the rod region unique to KIF1Bbeta is critical for the induction of apoptotic cell death in a p53-independent manner. Thus, KIF1Bbeta may act as a haploinsufficient tumor suppressor, and its allelic loss may be involved in the pathogenesis of neuroblastoma and other cancers.

Wild-type p53 and p73 negatively regulate expression of proliferation related genes

When normal cells come under stress, the wild-type (WT) p53 level increases resulting in the regulation of gene expression responsible for growth arrest or apoptosis. Here we show that elevated levels of WT p53 or its homologue, p73, inhibit expression of a number of cell cycle regulatory and growth promoting genes. Our analysis also identified a group of genes whose expression is differentially regulated by WT p53 and p73. We have infected p53-null H1299 human lung carcinoma cells with recombinant adenoviruses expressing WT p53, p73 or beta-galactosidase, and have undertaken microarray hybridization analyses to identify genes whose expression profile is altered by p53 or p73. Quantitative real-time PCR verified the repression of E2F-5, centromere protein A and E, minichromosome maintenance proteins (MCM)-2, -3, -5, -6 and -7 and human CDC25B after p53 expression. 5-Fluorouracil treatment of colon carcinoma HCT116 cells expressing WT p53 results in a reduction of the cyclin B2 protein level suggesting that DNA damage may indeed cause repression of these genes. Transient transcriptional assays verified that WT p53 repressed promoters of a number of these genes. Interestingly, a gain-of-function p53 mutant instead upregulated a number of these promoters in transient transfection. Using promoter deletion mutants of MCM-7 we have found that WT p53-mediated repression needs a minimal promoter that contains a single E2F site and surrounding sequences. However, a single E2F site cannot be significantly repressed by WT p53. Many of the genes identified are also repressed by p21. Thus, our work shows that WT p53 and p73 repress a number of growth-related genes and that in many instances this repression may be through the induction of p21.

Microarray-based CGH of sporadic and syndrome-related pheochromocytomas using a 0.1-0.2 Mb bacterial artificial chromosome array spanning chromosome arm 1p

Pheochromocytomas (PCC) are relatively rare neuroendocrine tumors, mainly of the adrenal medulla. They arise sporadically or occur secondary to inherited cancer syndromes, such as multiple endocrine neoplasia type II (MEN2), von Hippel-Lindau disease (VHL), or neurofibromatosis type I (NF1). Loss of 1p is the most frequently encountered genetic alteration, especially in MEN2-related and sporadic PCC. Previous studies have revealed three regions of common somatic loss on chromosome arm 1p, using chromosome-based comparative genomic hybridization (CGH) and LOH analysis. To investigate these chromosomal aberrations with a higher resolution and sensitivity, we performed microarray-based CGH with 13 sporadic and 11 syndrome-related (10 MEN2A-related and 1 NF1-related) tumors. The array consisted of 642 overlapping bacterial artificial chromosome (BAC) clones mapped to 1p11.2-p36.33. Chromosomal deletions on 1p were detected in 18 of 24 cases (75%). Among 9 tumors with partial 1p loss, the deleted region was restricted to 1cen-1p32.3 in six cases (25%), indicating a region of genetic instability. The consensus regions of deletion in this study involved 1cen-1p21.1, 1p21.3-1p31.3, and 1p34.3-1p36.33. In conclusion, these data strongly suggest that chromosome arm 1p is the site for multiple tumor suppressor genes, although the potential candidate genes CDKN2C and PTPRF/LAR are not included in these regions.

Microsatellite analysis at 1p36.3 in malignant melanoma of the skin: fine mapping in search of a possible tumour suppressor gene region

Deletions in 1p36 in malignant melanoma have been found in high percentages in nodular melanomas and melanoma metastases. Despite many efforts, no candidate tumour suppressor gene associated with malignant melanoma has so far been found in this region. To further determine a possible tumour suppressor gene locus, we carried out a deletion mapping of chromosome 1p36 at nine microsatellite loci in 74 malignant melanomas. Loss of heterozygosity (LOH) in this region was found in 77% of nodular melanomas (NMs), 86% of metastatic melanomas, but only 20% of superficial spreading melanomas (SSMs). Regarding the allelic losses, the nodular and metastatic melanoma samples could be divided into three groups: one showing LOH at the more telomeric loci D1S243 and D1S468 (1p36.33), one displaying allelic loss at the more centromeric loci D1S214 and D1S253 (1p36.32-31) and one with LOH over all informative loci between D1S243 and D1S160. We did not find any significant correlation between a deletion in any of the investigated loci and the survival data of the patients. However, our results confine the deleted region in malignant melanoma to a very small area around 1p36.32, thus facilitating the search for the tumour suppressor gene with importance in malignant melanoma.

A C-terminal inhibitory domain controls the activity of p63 by an intramolecular mechanism

The human genome is far smaller than originally estimated, and one explanation is that alternative splicing creates greater proteomic complexity than a simple count of open reading frames would suggest. The p53 homologue p63, for example, is a tetrameric transcription factor implicated in epithelial development and expressed as at least six isoforms with widely differing transactivation potential. In particular, p63alpha isoforms contain a 27-kDa C-terminal region that drastically reduces their activity and is of clear biological importance, since patients with deletions in this C terminus have phenotypes very similar to patients with mutations in the DNA-binding domain. We have identified a novel domain within this C terminus that is necessary and sufficient for transcriptional inhibition and which acts by binding to a region in the N-terminal transactivation domain of p63 homologous to the MDM2 binding site in p53. Based on this mechanism, we provide a model that explains the transactivation potential of homo- and heterotetramers composed of different p63 isoforms and their effect on p53.