5325

PLAGL1

LOT1|ZAC|ZAC1;pleiomorphic adenoma gene-like 1;PLAGL1;pleiomorphic adenoma gene-like 1

LOT-1|PLAG-like 1|lost on transformation 1|pleiomorphic adenoma-like protein 1|tumor suppressor ZAC|zinc finger protein PLAGL1

6q24-q25

protein-coding

Imprinted genes are expressed from one allele according to their parent of origin, and many are essential to mammalian embryogenesis. Here we show that the epsilon-sarcoglycan gene (Sgce) and Zac1 (Lot1) are both paternally expressed imprinted genes. They were identified in a subtractive screen for imprinted genes using a cDNA library made from novel parthenogenetic and wild-type fibroblast lines. Sgce is a component of the dystrophin-sarcoglycan complex, Zac1 is a nuclear protein inducing growth arrest and/or apoptosis, and Zac1 is a potential tumor suppressor gene. Sgce and Zac1 are expressed predominantly from their paternal alleles in all adult mouse tissues, except that Zac1 is biallelic in the liver and Sgce is weakly expressed from the maternal allele in the brain. Sgce and Zac1 are broadly expressed in embryos, with Zac1 being highly expressed in the liver primordium, the umbilical region, and the neural tube. Sgce, however, is strongly expressed in the allantoic region on day 9.5 but becomes more widely expressed throughout the embryo by day 11.5. Sgce is located at the proximal end of mouse chromosome 6 and is a candidate gene for embryonic lethality associated with uniparental maternal inheritance of this region. Zac1 maps to the proximal region of chromosome 10, identifying a new imprinted locus in the mouse, homologous with human chromosome 6q24-q25. In humans, unipaternal disomy for this region is associated with fetal growth retardation and transient neonatal diabetes mellitus. In addition, loss of expression of ZAC has been described for a number of breast and ovarian carcinomas, suggesting that ZAC is a potential tumor suppressor gene.

ZAC encodes a zinc finger protein with antiproliferative activity, is maternally imprinted and is a candidate for the tumor suppressor gene on 6q24. ZAC expression is frequently lost in breast and ovary tumor-derived cell lines and down-regulated in breast primary tumors. In this report, we describe ZACDelta2, an alternatively spliced variant of ZAC lacking the sequence encoding the two N-terminal zinc fingers. Messenger RNAs encoding ZAC or ZACDelta2 were equally abundant and both proteins were nuclear. ZACDelta2 displayed an improved transactivation activity and an enhanced affinity for a ZAC binding site, suggesting that the two N-terminal zinc fingers negatively regulated ZAC binding to its target DNA sequences. Both proteins were equally efficient in preventing colony formation, indicating similar overall antiproliferative activities. However, these activities resulted from a differential regulation of apoptosis vs cell cycle progression since ZACDelta2 was more efficient at induction of cell cycle arrest than ZAC, whereas it was the reverse for apoptosis induction. Hence, these data further underline that ZAC gene is critically controlled, both at the transcriptional level through imprinting and at the functional level through alternative splicing.

LOT1 is a zinc-finger nuclear transcription factor, which possesses anti-proliferative effects and is frequently silenced in ovarian and breast cancer cells. The LOT1 gene is localized at chromosome 6q24-25, a chromosomal region maternally imprinted and linked to growth retardation in several organs and progression of disease states such as transient neonatal diabetes mellitus. Toward understanding the molecular mechanism underlying the loss of LOT1 expression in cancer, we have characterized the genomic structure and analyzed its epigenetic regulation. Genome mapping of LOT1 in comparison with the other splice variants, namely ZAC1 and PLAGL1, revealed that its mRNA ( approximately 4.7 kb; GenBank accession number U76261) is potentially spliced using six exons spanning at least 70 kb of the human genome. 5-RACE (rapid amplification of cDNA ends) data indicate the presence of at least two transcription start sites. We found that in vitro methylation of the LOT1 promoter causes a significant loss in its ability to drive luciferase transcription. To determine the nature of in vivo methylation of LOT1, we used bisulfite-sequencing strategies on genomic DNA. We show that in the ovarian and breast cancer cell lines and/or tumors the 5-CpG island of LOT1 is a differentially methylated region. In these cell lines the ratio of methylated to unmethylated CpG dinucleotides in this region ranged from 31 to 99% and the ovarian tumors have relatively higher cytosine methylation than normal tissues. Furthermore, we show that trichostatin A, a specific inhibitor of histone deacetylase, relieves transcriptional silencing of LOT1 mRNA in malignantly transformed cells. It appears that, unlike DNA methylation, histone deacetylation does not target the promoter, and rather it is indirect and may be elicited by a mechanism upstream of the LOT1 regulatory pathway. Taken together, the data suggest that expression of LOT1 is under the control of two epigenetic modifications and that, in the absence of loss of heterozygosity, the biallelic (two-hit) or maximal silencing of LOT1 requires both processes.

ZAC is a zinc finger transcription factor that induces apoptosis and cell cycle arrest in various cell lines. The corresponding gene is maternally imprinted and localized on chromosome 6q24-q25, a region harboring an unidentified tumor suppressor gene for a variety of solid neoplasms. ZAC expression is lost or down-regulated in some breast, ovary, and pituitary tumors and in an in vitro model of ovary epithelial cell transformation. In the present study, we examined ZAC expression in normal skin and found a high expression level in basal keratinocytes and a lower, more heterogeneous, expression in the first suprabasal differentiating layers of epidermis. In vitro, ZAC was up-regulated following induction of keratinocyte differentiation. Conversely, ZAC expression triggered keratinocyte differentiation as indicated by induction of involucrin expression. Interestingly, we found a dramatic loss of ZAC expression in basal cell carcinoma, a neoplasm characterized by a relatively undifferentiated morphology. In contrast, ZAC expression was maintained in squamous cell carcinomas that retain the squamous differentiated phenotype. Altogether, these data suggest a role for ZAC at an early stage of keratinocyte differentiation and further support its role in carcinogenesis.

Capillary hemangioblastomas (CHBs) are vascular, usually benign, tumors of the CNS, occurring either as a component of familial von Hippel-Lindau (VHL) disease or as a sporadic entity. Both familial and sporadic forms of VHL-associated tumors involve inactivation of the VHL gene; for CHB, 20% to 50% of sporadic cases are affected. However, other molecular alterations involved in the pathogenesis of sporadic CHBs, which represent up to 70% of CHBs, remain largely unknown. We previously identified a minimal deleted area at 6q23-24 in CHB, and the present study focused on the ZAC1 gene (6q24-25). ZAC1 is a maternally imprinted tumor suppressor gene with antiproliferative properties. We investigated loss of heterozygosity (LOH), promoter methylation, and expression status of ZAC1 in mainly sporadic cases of CHB. Our LOH analysis with 6 microsatellite markers spanning the ZAC1 gene region revealed a high frequency (6 of 10, 60%) of LOH. The promoter methylation analysis detected predominance of the methylated ZAC1 sequence in the majority (9 of 10, 90%) of the tumors. Immunohistochemistry exhibited a strongly reduced expression of ZAC1 in stromal cells of all CHBs studied. Collectively, our current results indicate that the absence of the unmethylated ZAC1 sequence was highly concurrent with ZAC1 region LOH or 6q loss and with lack of ZAC1 expression, suggesting preferential loss of the nonimprinted, expressed ZAC1 allele in CHB. This novel finding highlights the importance of ZAC1 in development of CHB, particularly in non-VHL-associated cases.

ZAC/PLAGL1 is a novel imprinted tumor suppressor gene encoding an important inducer of cell cycle arrest and apoptosis, and found to be lost during tumorigenesis. We analyzed the significance of ZAC in the development of a rare, usually benign tumor of the adrenal gland: pheochromocytoma (PCC). Twenty-four PCCs were analyzed for the loss of the active nonimprinted allele of ZAC, and nine of the twenty-four PCCs were also assayed for expression of the protein. In thirteen of the cases, a paired nonmalignant tissue was available for analysis. Methylation-specific polymerase chain reaction revealed frequent (15 of 23, 65%) loss of unmethylated DNA in the imprinting control region of ZAC. Immunohistochemistry identified reduced ZAC expression in 56% (5 of 9) of the subset cases. Four of the five PCC cases where reduced expression of ZAC was observed were also positive for the loss of the active ZAC allele. Additionally, the loss of ZAC expression was also found to be frequent in a series of capillary hemangioblastomas and gliomas (6 of 6, 100%, and 17 of 27, 63%, respectively) examined for comparison. In conclusion, our study suggests the involvement of the imprinted ZAC gene in the pathogenesis of PCC.