The MAD (600021) and MXI1 genes encode basic helix-loop-helix leucine zipper transcription factors that bind MAX (154950) in vitro, forming a sequence-specific DNA-binding complex similar to the MYC-MAX heterodimer. MXI1 and MAD may antagonize MYC function and are candidate tumor suppressor genes.
NCBI Summary:
Expression of the c-myc gene, which produces an oncogenic transcription factor, is tightly regulated in normal cells but is frequently deregulated in human cancers. The protein encoded by this gene is a trancriptional repressor thought to negatively regulate MYC function, and is therefore a potential tumor suppressor. The protein inhibits the transcriptional activity of MYC by competing for MAX, another basic helix-loop-helix protein that binds to MYC and is required for its function. Defects in this gene are frequently found in patients with prostate tumors. Two transcript variants encoding different isoforms have been identified for this gene.
General function
Tumor suppressor, DNA binding, Transcription factor
Comment
Cellular localization
Nuclear
Comment
Ovarian function
Luteinization, Oocyte maturation
Comment
Changing mad2 levels affects chromosome segregation and spindle assembly checkpoint control in female mouse meiosis I. Niault T et al. The spindle assembly checkpoint (SAC) ensures correct separation of sister chromatids in somatic cells and provokes a cell cycle arrest in metaphase if one chromatid is not correctly attached to the bipolar spindle. Prolonged metaphase arrest due to overexpression of Mad2 has been shown to be deleterious to the ensuing anaphase, leading to the generation of aneuploidies and tumorigenesis. Additionally, some SAC components are essential for correct timing of prometaphase. In meiosis, we and others have shown previously that the Mad2-dependent SAC is functional during the first meiotic division in mouse oocytes. Expression of a dominant-negative form of Mad2 interferes with the SAC in metaphase I, and a knock-down approach using RNA interference accelerates anaphase onset in meiosis I. To prove unambigiously the importance of SAC control for mammalian female meiosis I we analyzed oocyte maturation in Mad2 heterozygote mice, and in oocytes overexpressing a GFP-tagged version of Mad2. In this study we show for the first time that loss of one Mad2 allele, as well as overexpression of Mad2 lead to chromosome missegregation events in meiosis I, and therefore the generation of aneuploid metaphase II oocytes. Furthermore, SAC control is impaired in mad2+/- oocytes, also leading to the generation of aneuploidies in meiosis I.
Expression regulated by
LH
Comment
Ovarian localization
Oocyte, Granulosa, Luteal cells
Comment
Chaffin CL, et al 2003 reported dynamics of Myc/Max/Mad Expression during Luteinization of Primate Granulosa Cells in Vitro: Association with Periovulatory Proliferation.
Granulosa cell luteinization involves the attenuation of gonadotropin-induced proliferation. Although recent evidence indicates that primate granulosa cells stop dividing within 12 h of an ovulatory stimulus, early events in cell cycle arrest remain unknown. In the current study an in vitro model of primate granulosa cell luteinization is established that allows assessment of early events in terminal differentiation. A luteinizing dose of human chorionic gonadotropin (hCG) results in a secondary rise in proliferation before cell cycle arrest that is paralleled by a transient increase in the expression of c-Myc. In contrast, the c-Myc antagonists Mad1, Mad4, and Mxi1 are transiently repressed by hCG. Max, the common dimerization partner for Myc and Mad, is similarly repressed by hCG, suggesting that changes in the expression of this gene may further regulate the activity of Myc and Mad. To determine whether other cell cycle regulatory families are involved in luteinization, the expression of p53 and the wild-type p53-inducible phosphatase (wip1) was examined. Similar to Mad and Max, p53 and wip1 are transiently repressed by hCG, suggesting that the p53 and Mad pathways have either parallel or cooperative roles in luteinization. Thus, luteinization of primate granulosa cells is preceded by a burst of proliferation that is regulated by changes in the relative levels of c-Myc, Max, and Mad as well as p53.