Tissue-specific imprinting of the ZAC/PLAGL1 tumour suppressor gene results from variable utilization of monoallelic and biallelic promoters

The tumour suppressor gene ZAC/PLAGL1 is widely expressed in many human tissues during fetal development and throughout life. It encodes a DNA-binding protein which shares with p53 the ability to regulate apoptosis and cell cycle arrest concurrently. Owing to its anti-proliferative properties, down-regulation or loss of ZAC is believed to deregulate cell growth, and loss of expression has been observed in a number of different cancers. In addition, overexpression of ZAC during fetal development is believed to underlie the rare disorder transient neonatal diabetes mellitus (TNDM). Imprinted expression of ZAC has been demonstrated in many human and mouse tissues, although biallelic transcription has been noted in human peripheral blood leucocytes (PBL). We report here the identification of a second ZAC promoter, which is responsible for the observed biallelic expression. The promoter lies within a previously uncharacterized CpG island similar to 55 kb upstream of the imprinted CpG island. In PBL, the imprinted CpG island (P1) is differentially methylated and produces monoallelic transcripts, as in other tissues. However, biallelic transcripts predominate and are derived from the alternative CpG island (P2), which is unmethylated. Biallelic P2 expression was also found in adult pancreas, and ZAC expression from this promoter was identified at a low level in all adult human tissues tested. These findings show that regulation of ZAC expression is more complex than previously realized. The existence of the apparently independently-regulated P2 promoter has important implications for the study of ZAC dysregulation in cancer and TNDM.