Nicole Schreiber-Agus

The Ink4a Tumor Suppressor Gene Product, p19Arf, Interacts with MDM2 and Neutralizes MDM2's Inhibition of p53

The INK4a gene encodes two distinct growth inhibitors--the cyclin-dependent kinase inhibitor p16Ink4a, which is a component of the Rb pathway, and the tumor suppressor p19Arf, which has been functionally linked to p53. Here we show that p19Arf potently suppresses oncogenic transformation in primary cells and that this function is abrogated when p53 is neutralized by viral oncoproteins and dominant-negative mutants but not by the p53 antagonist MDM2. This finding, coupled with the observations that p19Arf and MDM2 physically interact and that p19Rrf blocks MDM2-induced p53 degradation and transactivational silencing, suggests that p19Arf functions mechanistically to prevent MDM2s neutralization of p53. Together, our findings ascribe INK4as potent tumor suppressor activity to the cooperative actions of its two protein products and their relation to the two central growth control pathways, Rb and p53.

An amino-terminal domain of Mxi1 mediates anti-myc oncogenic activity and interacts with a homolog of the Yeast Transcriptional Repressor SIN3

Documented interactions among members of the Myc superfamily support a yin-yang model for the regulation of Myc-responsive genes in which transactivation-competent Myc-Max heterodimers are opposed by repressive Mxi1-Max or Mad-Max complexes. Analysis of mouse mxi1 has led to the identification of two mxi1 transcript forms possessing open reading frames that differ in their capacity to encode a short amino-terminal alpha-helical domain. The presence of this segment dramatically augments the suppressive potential of Mxi1 and allows for association with a mammalian protein that is structurally homologous to the yeast transcriptional repressor SIN3. These findings provide a mechanistic basis for the antagonistic actions of Mxi1 on Myc activity that appears to be mediated in part through the recruitment of a putative transcriptional repressor.

myc Family Oncogenes in the Development of Normal and Neoplastic Cells

Publisher Summary This chapter reviews the structural and functional features of myc family genes and their products and discusses these properties in relation to known or suspected roles of myc in normal mammalian development and in malignant disease. The most extensive region of homology among c- myc , N- myc , and L- myc gene products is found near the carboxy-terminal portion of various proteins. This region contains two structural motifs identified in transcription and differentiation factors—namely, the leucine zipper and the helix-loop-helix/basic region motifs. These structures are found in a contiguous arrangement, with the leucine zipper located at the carboxy terminus and the helix-loop-helix just amino terminal to the leucine zipper. myc family genes are important in the regulation of normal cellular growth and differentiation. myc family genes exhibit unique expression patterns during mammalian development and the dramatic changes in their expression coincide with critical developmental transitions in many cell lineages. myc family oncoproteins are localized to the nucleus, and they possess significant homology to known sequence-specific transcription factors and for differentiation factors, thereby suggesting that myc -encoded oncoproteins may serve to regulate specific gene expression during growth and differentiation. Although myc family genes share many structural and functional features, their conservation as distinct sequences in evolution indicates important and unique biological activities.

Role of Mxi1 in ageing organ systems and the regulation of normal and neoplastic growth

Mxi1 belongs to the Mad (Mxi1) family of proteins, which function as potent antagonists of Myc oncoproteins. This antagonism relates partly to their ability to compete with Myc for the protein Max and for consensus DNA binding sites and to recruit transcriptional co-repressors. Mad(Mxi1) proteins have been suggested to be essential in cellular growth control and/or in the induction and maintenance of the differentiated state,. Consistent with these roles, mxi1 may be the tumour-suppressor gene that resides at region 24–26 of the long arm of chromosome 10. This region is a cancer hotspot, and mutations here may be involved in several cancers, including prostate adenocarcinoma. Here we show that mice lacking Mxi1 exhibit progressive, multisystem abnormalities. These mice also show increased susceptibility to tumorigenesis either following carcinogen treatment or when also deficient in Ink4a. This cancer-prone phenotype may correlate with the enhanced ability of several mxi1-deficient cell types, including prostatic epithelium, to proliferate. Our results show that Mxi1 is involved in the homeostasis of differentiated organ systems, acts as a tumour suppressor in vivo, and engages the Myc network in a functionally relevant manner.

Gene-target recognition among members of the Myc superfamily and implications for oncogenesis

Myc and Mad family proteins regulate multiple biological processes through their capacity to influence gene expression directly. Here we show that the basic regions of Myc and Mad proteins are not functionally equivalent in oncogenesis, have separable E-box–binding activities and engage both common and distinct gene targets. Our data support the view that the opposing biological actions of Myc and Mxi1 extend beyond reciprocal regulation of common gene targets. Identification of differentially regulated gene targets provides a framework for understanding the mechanism through which the Myc superfamily governs the growth, proliferation and survival of normal and neoplastic cells.

Repression by the Mad(Mxi1)-Sin3 complex

The functions of Myc in transformation and transactivation are countered by the suppressive actions of the Mad(Mxi1) family. Mad(Mxi1) proteins not only compete with Myc for dimerization to Max and binding to Myc/ Max consensus sites but also recruit powerful repressors of gene expression. A prediction of the yin‐yang relationship between Myc and Mad(Mxi1) families would be that the latter constitutes a new class of tumor suppressors. Here, we review the current literature on the Mad(Mxi1) family, with particular attention paid to the molecular mechanisms by which these proteins antagonize the actions of Myc in normal and neoplastic cells. BioEssays 20:808–818, 1998.

Effects of the MYC oncogene antagonist, MAD, on proliferation, cell cycling and the malignant phenotype of human brain tumour cells

To investigate how overexpression of MAD, an antagonist of MYC oncogenes influences the malignant phenotype of human cancer cells, an adenovirus vector system was used to transfer the human MAD gene (AdMAD) into human astrocytoma cells. Decreased growth potential of AdMAD-infected cells was evidenced by a decrease in [3H]thymidine incorporation, an increase in cell doubling time and alteration of cell-cycle distribution. Diminished malignant potential of AdMAD-infected cells was manifested by their loss of anchorage-independent growth in soft agar and by their inability, in general, to induce tumorigenesis in a xenograft animal model. These studies indicate that adenovirus constructs encoding MAD dramatically inhibit the proliferation and tumorigenicity of human astrocytoma cells and support the use of MAD for gene therapy of human tumours.

Mouse models of prostate cancer

The pathogenetic basis of prostate cancer remains highly elusive; its clarification could be facilitated greatly by laboratory and clinical models of the disease. Although the genetically manipulated mouse has been invaluable for the modeling of other human cancer types, it has fared less well with respect to prostate cancer. Nevertheless, several highly valuable transgenic models exist and are highlighted in this review. Emerging reagents and strategies may allow us to use the mouse more effectively to define the molecular, cellular and physiological events that lead to prostate cancer initiation and progression.

The p.L302P mutation in the lysosomal enzyme gene SMPD1 is a risk factor for Parkinson disease

Objective: To study the possible association of founder mutations in the lysosomal storage disorder genes HEXA, SMPD1, and MCOLN1 (causing Tay-Sachs, Niemann-Pick A, and mucolipidosis type IV diseases, respectively) with Parkinson disease (PD). Methods: Two PD patient cohorts of Ashkenazi Jewish (AJ) ancestry, that included a total of 938 patients, were studied: a cohort of 654 patients from Tel Aviv, and a replication cohort of 284 patients from New York. Eight AJ founder mutations in the HEXA, SMPD1, and MCOLN1 genes were analyzed. The frequencies of these mutations were compared to AJ control groups that included large published groups undergoing prenatal screening and 282 individuals matched for age and sex. Results: Mutation frequencies were similar in the 2 groups of patients with PD. The SMPD1 p.L302P was strongly associated with a highly increased risk for PD (odds ratio 9.4, 95% confidence interval 3.9–22.8, p < 0.0001), as 9/938 patients with PD were carriers of this mutation compared to only 11/10,709 controls. Conclusions: The SMPD1 p.L302P mutation is a novel risk factor for PD. Although it is rare on a population level, the identification of this mutation as a strong risk factor for PD may further elucidate PD pathogenesis and the role of lysosomal pathways in disease development.

Zebra fish myc family and max genes: differential expression and oncogenic activity throughout vertebrate evolution.

To gain insight into the role of Myc family oncoproteins and their associated protein Max in vertebrate growth and development, we sought to identify homologs in the zebra fish (Brachydanio rerio). A combination of a polymerase chain reaction-based cloning strategy and low-stringency hybridization screening allowed for the isolation of zebra fish c-, N-, and L-myc and max genes; subsequent structural characterization showed a high degree of conservation in regions that encode motifs of known functional significance. On the functional level, zebra fish Max, like its mammalian counterpart, served to suppress the transformation activity of mouse c-Myc in rat embryo fibroblasts. In addition, the zebra fish c-myc gene proved capable of cooperating with an activated H-ras to effect the malignant transformation of mammalian cells, albeit with diminished potency compared with mouse c-myc. With respect to their roles in normal developing tissues, the differential temporal and spatial patterns of steady-state mRNA expression observed for each zebra fish myc family member suggest unique functions for L-myc in early embryogenesis, for N-myc in establishment and growth of early organ systems, and for c-myc in increasingly differentiated tissues. Furthermore, significant alterations in the steady-state expression of zebra fish myc family genes concomitant with relatively constant max expression support the emerging model of regulation of Myc function in cellular growth and differentiation.