NATUREIVol 447 24 May 2007 INSIGHT REVIEW Figure 2 Beckwith-Wiedemann a Normal Enlarged kidneys, overgrowth syndrome as an example of a monogenic Wilms tumou disease that reveals mechanisms of normal epigenetic regulation. Depicted are a pair of normal chromosomes(a) maternal chromosomes are n:(b-f); al illustrative lesie (Me). Imprinted genes are depicted, not b UPD scale or in their entirety, with green representing active and red representing silent alleles, respectively. Patients wit uniparental disomy(UPD, b)have complete genetic replacement of the maternal allele region with a second mal copy(dashed enclosure). Loss of imprinting(LOf)of IG F2(c)causes a C LOI IGF2 switch in epigenotype of the IGF2/H19 subdomain(dashed enclosure). LOI of LITI(d) of the p57/K,LQTI/LITI subdomain. Some patients show LOI of the entire domain in the absence UPD(e). Other patients show localized chromatin disruption(small d LOI LITI yellow circle, f)sil imprinting is organized hierarchically ning two smaller subdomains In addit patients show microdeletions in either of the two domains(black crosses), revealing the location ofimprinting control centres. The domain organization e loi domain similarly reveals the contiguous gene ndrome nature of the disease. patients with involvement(genetic or epigenetic of the IGF2/H19 domain have enlarg kidneys and wilms' tumours Patients with involvement of the P57/K,LQT1/ I domain show somatic overgrowth, an enlarged tongue and omphalocele(in f p57 silencing which abdominal organs protrude from the navel). And children with involvement of both domains show both phenotypes. d It (see refs 16, 17 for reviews). In addition, many C/T(cancer/testis)genes should also be noted that as many genes are silenced as are activated in that are expressed normally in the healthy testis are activated in other tumours by both drug-induced hypomethylation and by knockdown of cells by hypomethylation in cancer, including the melanoma-associated DNA methyltransferases", thus both hypomethylation and hypermethyl antigen(MAGE)gene family, which has antigenic and immunothera- ation can lead to gene activation and ancer eutic value in melanoma and glioblastoma.and the oncogenic micro RNAlet-7a-3(ref. 18). Activation of the human papilloma virus HPV16 Loss of imprinting in cancer by hypomethylation is a major mechanism affecting tumour latency in The earliest clue that genomic imprinting might be involved in can- cervical cancer. 7. Recently, oestrogen-and tamoxifen-induced activa- cer came from two rare types of tumour: hydatidiform moles, which tion of PAX2 and endometrial proliferation, leading to cell proliferation, are malignant trophoblastic tumours caused by a pregnancy arising was found to be cancer-specific because of PAX2 promoter hypomethyl- from two complete sets of the paternal genome, and ovarian teratomas, which are benign tumours with many tissue types that arise from two By contrast, tumour suppressor gene silencing has been linked to pro- complete sets of the maternal genome. Molecular evidence for a role of moter hypermethylation, first described for RB, the gene associated with genomic imprinting in cancer emerged from studies showing a universal tinoblastoma and many other tumour suppressor genes, including loss of the maternal allele in wilms tumours and embryonal rhabdo- P16, VHL (von Hippel-Lindau), MLH1, APC (adenomatosis polyposis myosarcoma, with loss of heterozygosity(LOH)of 11p15, implying coli)and E-cadherin(see ref 21 for a review). Recent high-throughput that normally only the maternal allele of an as yet unidentified tumour approaches have been used to identify other candidate genes. An exci- suppressor gene might be expressed". So it was surprising that the first ting demonstration of domain-wide silencinginvolves an entire chromo- molecular evidence for a role of genomic imprinting in cancer was loss somal band, suggesting a disturbance ofhigher-order chromatin( Fig. 1). of imprinting(LOD), causing abnormal activation of the normally silent However, studies focused on loss of DNA methylation in cancer may copy of IGF2, an important autocrine growth factor, leading to patho have overlooked hypomethylation of tissue-specific methylation marks logical biallelic expression of IGF2 in Wilms'tumours, the most common at CpG islands. Indeed, whole-genome analysis suggests that CpG island childhood solid tumour.. LOI refers either to aberrant activation of the 435 @2007 Nature Publishing Grouprenal-cell cancer, and S100 calcium-binding protein A4 in colon cancer (see refs 16, 17 for reviews). In addition, many C/T (cancer/testis) genes that are expressed normally in the healthy testis are activated in other cells by hypomethylation in cancer, including the melanoma-associated antigen (MAGE) gene family, which has antigenic and immunothera￾peutic value in melanoma and glioblastoma16,17 and the oncogenic micro RNA let-7a-3 (ref. 18). Activation of the human papilloma virus HPV16 by hypomethylation is a major mechanism affecting tumour latency in cervical cancer16,17. Recently, oestrogen- and tamoxifen-induced activa￾tion of PAX2 and endometrial proliferation, leading to cell proliferation, was found to be cancer-specific because of PAX2 promoter hypomethyl￾ation in the tumours19. By contrast, tumour suppressor gene silencing has been linked to pro￾moter hypermethylation, first described for RB, the gene associated with retinoblastoma20, and many other tumour suppressor genes, including p16, VHL (von Hippel–Lindau), MLH1, APC (adenomatosis polyposis coli) and E-cadherin (see ref. 21 for a review). Recent high-throughput approaches have been used to identify other candidate genes22,23. An exci￾ting demonstration of domain-wide silencing involves an entire chromo￾somal band24, suggesting a disturbance of higher-order chromatin (Fig. 1). However, studies focused on loss of DNA methylation in cancer may have overlooked hypomethylation of tissue-specific methylation marks at CpG islands25. Indeed, whole-genome analysis suggests that CpG island hypermethylation may be less widespread than had been suspected26. It should also be noted that as many genes are silenced as are activated in tumours by both drug-induced hypomethylation and by knockdown of DNA methyltransferases27, thus both hypomethylation and hypermethyl￾ation can lead to gene activation and gene silencing in cancer. Loss of imprinting in cancer The earliest clue that genomic imprinting might be involved in can￾cer came from two rare types of tumour: hydatidiform moles, which are malignant trophoblastic tumours caused by a pregnancy arising from two complete sets of the paternal genome, and ovarian teratomas, which are benign tumours with many tissue types that arise from two complete sets of the maternal genome. Molecular evidence for a role of genomic imprinting in cancer emerged from studies showing a universal loss of the maternal allele in Wilms’ tumours and embryonal rhabdo￾myosarcoma, with loss of heterozygosity (LOH) of 11p15, implying that normally only the maternal allele of an as yet unidentified tumour suppressor gene might be expressed28. So it was surprising that the first molecular evidence for a role of genomic imprinting in cancer was loss of imprinting (LOI), causing abnormal activation of the normally silent copy of IGF2, an important autocrine growth factor, leading to patho￾logical biallelic expression of IGF2 in Wilms’ tumours, the most common childhood solid tumour29,30. LOI refers either to aberrant activation of the a Normal b UPD p57K1P2 LIT1 IGF2 H19 c LOI IGF2 d LOI LIT1 e LOI domain f p57 silencing Enlarged kidneys, Wilm’s tumour Omphalocele, overgrowth, macroglosia Me Me Me Me Me Me Me Me Me Me Me Me Figure 2 | Beckwith–Wiedemann syndrome as an example of a monogenic disease that reveals mechanisms of normal epigenetic regulation. Depicted are a pair of normal chromosomes (a) and several illustrative lesions (b–f); maternal chromosomes are pink and paternal blue, and DMRs are indicated (Me). Imprinted genes are depicted, not to scale or in their entirety, with green representing active and red representing silent alleles, respectively. Patients with uniparental disomy (UPD, b) have complete genetic replacement of the maternal allele region with a second paternal copy (dashed enclosure). Loss of imprinting (LOI) of IGF2 (c) causes a switch in epigenotype of the IGF2/H19 subdomain (dashed enclosure). LOI of LIT1 (d) causes a switch in epigenotype of the p57/KVLQT1/LIT1 subdomain. Some patients show LOI of the entire imprinted gene domain in the absence of UPD (e). Other patients show localized chromatin disruption (small yellow circle, f) silencing p57KIP2. Thus, imprinting is organized hierarchically into a large domain containing two smaller subdomains. In addition, some patients show microdeletions in either of the two domains (black crosses), revealing the location of imprinting control centres. The domain organization similarly reveals the contiguous gene syndrome nature of the disease. Patients with involvement (genetic or epigenetic) of the IGF2/H19 domain have enlarged kidneys and Wilms’ tumours. Patients with involvement of the p57/KVLQT1/ LIT1 domain show somatic overgrowth, an enlarged tongue and omphalocele (in which abdominal organs protrude from the navel). And children with involvement of both domains show both phenotypes. 435 NATURE|Vol 447|24 May 2007 INSIGHT REVIEW ￾