Expression and LOI of Igf2 in mouse tumors

Igf2在小鼠胎儿期的大多数组织中都有表达，而在成年小鼠中，除了大脑外，Igf2是被抑制的。Igf2是父本等位基因表达，而其物质等位基因被抑制的印迹基因。这表明Igf2的印迹是通过Igf2基因内特定区域的甲基化以及位于Igf2下游的H19基因的甲基化来维持的。Igf2基因包含6个外显子，lgf2 mRNA包含1-3外显子和4-6外显子中的一个。另一方面，由于大多数小鼠肿瘤表达Igf2，并且由于肿瘤偶尔会在Igf2转基因小鼠中发生，因此Igf2被认为在肿瘤发生中起作用。本研究的目的是研究Igf2等位基因特异性表达、Igf2基因甲基化和转录起始点在小鼠肿瘤细胞中的作用。以下几点在1998- 1999年间变得清晰起来。(1)通过分析Igf2外显子6的多态性CA重复，发现小鼠HCC细胞系中经常检测到印迹丢失（LOI）。另一方面，来自正常肝脏的肝上皮细胞（LE）中几乎没有发现LOI。(2) P1、P2、P3启动子在HCC细胞中均有活性，而在LE细胞中只有P2和P3启动子有活性。(3) Igf2在正常肝细胞原代培养中表达，但在这种情况下其印迹被维持。(4)通过Igf2上游非编码区甲基化特异性PCR-DNA测序，鉴定了父本等位基因特异性CpG甲基化。

Igf2 is expressed in most tissues during a fetal period in mice, while it is suppressed except for the brain in adult. Igf2 is an imprinted gene of which paternal allele is expressed, while its material allele is suppressed. It is suggested that the imprinting of Igf2 is maintained by methylation of the specific regions within the Igf2 gene as well as methylation of the H19 gene which is located downstream of Igf2. The Igf2 gene contains 6 exons, and lgf2 mRNA includes one of exons 1-3 with exons 4-6 in common. On the other hand, since most mouse tumors express Igf2 and since tumors occasionally develop in Igf2 transgenic mice, Igf2 is thought to have a role in tumor development. A purpose of this study was to investigate the allele specific Igf2 expression, methylation of Igf2 gene and transcription start points in mouse tumor cells.Following points became clear in 1998- 1999.(1) By analyzing the polymorphic CA repeat within Igf2 exon 6, loss of imprinting (LOI) was frequently detected in mouse HCC cell lines. On the other hand, LOI was hardly found in liver epithehal (LE) cells derived from the normal liver.(2) All the P1, P2, P3 promoters were active in the HCC cells, while only P2 and P3 promoters were active in LE cells.(3) Igf2 was expressed in the primary cultures of normal hepatocytes, but its imprinting is maintained in this case.(4) Paternal allele specific methylation of CpG was identified by methylation specific PCR-DNA sequencing in the upstream non-coding region in Igf2.