The sins of the fathers and mothers: genomic imprinting in mammalian development.

That some imprinting is dependent upon a signal capable of acting over a large distance follows from studies of a subset of PWS and AS patients (Figure 3Figure 3). These PWS patients have small microdeletions of the promoter and first exon of the SNRPN gene, which result in the loss of expression of that gene, as well as at least three paternally expressed genes that lie as far as 1 Mb away on the chromosome (Buiting et al. 1995xInherited microdeletions in the Angelman and Prader-Willi syndromes define an imprinting centre on human chromosome 15. Buiting, K, Saitoh, S, Gross, S, Dittrich, B, Schwartz, S, Nicholls, R.D, and Horsthemke, B. Nat. Genet. 1995; 9: 395–400Crossref | PubMed | Scopus (436)See all ReferencesBuiting et al. 1995). Furthermore, a paternal chromosome that harbors such a mutation displays the methylation pattern of a wild-type maternal chromosome. Even more surprising is the finding that deletions just upstream of the SNRPN promoter lead to AS by presumably eliminating the expression of UBE3A, and possibly other maternally expressed genes that lie hundreds of kilobases telomeric to SNRPN.Figure 3Imprinting in the Prader-Willi and Angelman Syndrome Region(A) The structure of the SNRPN gene and the positions of the upstream “BD” exons are depicted, with the transcriptional start sites indicated by the horizontal arrows. Below the diagram is the extent of the minimal region held in common among PWS and AS microdeletion families. Adapted from Dittrich et al. 1996xImprint switching on human chromosome 15 may involve alternative transcripts of the SNRPN gene. Dittrich, B, Buiting, K, Korn, B, Rickard, S, Buxton, J, Saitoh, S, Nicholls, R.D, Poustka, A, Winterpacht, A, Zabel, B, and Horsthemke, B. Nat. Genet. 1996; 14: 163–170Crossref | PubMed | Scopus (180)See all ReferencesDittrich et al. 1996.(B) The PWS and AS genes are indicated by rectangles, with the transcribed alleles filled in with purple (paternal expression) or red (maternal expression). The SNRPN promoter deleted in PWS patients is indicated by the blue horizontal arrow. The maternal germline epigenetic mark is indicated by the red vertical lines, whereas the positions of DNA methylation that may be the consequence of germline methylation are indicated by gray vertical lines.(C) The functional consequence of the PWS deletion on the paternal chromosome and the AS deletion on the maternal chromosome. The question mark indicates that the paternal expression of UBE3A has not been verified in PWS.View Large Image | View Hi-Res Image | Download PowerPoint SlideHow do these cis-acting sites act at such distances? The answer is far from clear, but in each case, the primary gametic signal, presumably the methylation status of the SNRPN promoter, is able to propagate changes in DNA methylation and/or chromatin structure throughout the region (5xInherited microdeletions in the Angelman and Prader-Willi syndromes define an imprinting centre on human chromosome 15. Buiting, K, Saitoh, S, Gross, S, Dittrich, B, Schwartz, S, Nicholls, R.D, and Horsthemke, B. Nat. Genet. 1995; 9: 395–400Crossref | PubMed | Scopus (436)See all References, 12xImprint switching on human chromosome 15 may involve alternative transcripts of the SNRPN gene. Dittrich, B, Buiting, K, Korn, B, Rickard, S, Buxton, J, Saitoh, S, Nicholls, R.D, Poustka, A, Winterpacht, A, Zabel, B, and Horsthemke, B. Nat. Genet. 1996; 14: 163–170Crossref | PubMed | Scopus (180)See all References). In the soma, transcription of the SNRPN could be required to propagate an unmethylated and open chromatin conformation throughout the paternal chromosome, akin to the model in Figure 2FFigure 2F. Maternal methylation at the SNRPN could lead to the propagation of a different, more closed and methylated chromatin state. There is precedence for the ability of a CpG island, like the SNRPN promoter, to inhibit the methylation of non-island sequences at a distance, although the distances in the PWS region are very much greater than any that have been tested experimentally (4xSp1 elements protect a CpG island from de novo methylation. Brandeis, M, Frank, D, Keshet, I, Siegfried, Z, Mendelsohn, M, Nemes, A, Temper, V, Razin, A, and Cedar, H. Nature. 1994; 371: 435–438Crossref | PubMed | Scopus (539)See all References, 35xSp1 sites in the mouse aprt gene promoter are required to prevent methylation of the CpG island. Macleod, D, Charlton, J, Mullins, J, and Bird, A.P. Genes Dev. 1994; 8: 2282–2292Crossref | PubMedSee all References).The foregoing attributes a key function to the SNRPN promoter in the soma, but an analysis of sporadic microdeletion and nondeletion PWS patients implied that the promoter may also function in the germ line to erase the maternal imprint (5xInherited microdeletions in the Angelman and Prader-Willi syndromes define an imprinting centre on human chromosome 15. Buiting, K, Saitoh, S, Gross, S, Dittrich, B, Schwartz, S, Nicholls, R.D, and Horsthemke, B. Nat. Genet. 1995; 9: 395–400Crossref | PubMed | Scopus (436)See all References, 6xSporadic imprinting defects in Prader-Willi syndrome and Angelman syndrome (implications for imprint-switch models, genetic counseling, and prenatal diagnosis) . Buiting, K, Dittrich, B, Gross, S, Lich, C, Farber, C, Buchholz, T, Smith, E, Reis, A, Burger, J, Nothen, M.M et al. Am. J. Hum. Genet. 1998; 63: 170–180Abstract | Full Text | Full Text PDF | PubMed | Scopus (108)See all References). That is, the imprinting defect in sporadic PWS patients, as reflected by the methylated status of the paternal chromosome, is always inherited from the grandmother. The simplest explanation for this observation is that the region deleted in the microdeletion PWS patients is required for switching the “epigenotype” of the grandmaternal chromosome to a paternal epigenotype in the father, but models for how this might occur are hampered by our lack of knowledge about imprint erasure.The function of the maternal imprint control region (IC), as well as how its deletion leads to AS syndrome, is even more difficult to understand. As with PWS, the mutant maternal AS chromosome has the wrong methylation pattern, being largely unmethylated throughout the PWS region. Dittrich et al. 1996xImprint switching on human chromosome 15 may involve alternative transcripts of the SNRPN gene. Dittrich, B, Buiting, K, Korn, B, Rickard, S, Buxton, J, Saitoh, S, Nicholls, R.D, Poustka, A, Winterpacht, A, Zabel, B, and Horsthemke, B. Nat. Genet. 1996; 14: 163–170Crossref | PubMed | Scopus (180)See all ReferencesDittrich et al. 1996 identified a series of transcripts initiating upstream of the SNRPN promoter, whose structure would be disrupted in AS microdeletion patients (Figure 3Figure 3). On this basis, they proposed that the upstream “BD exon” transcripts act as cis-acting RNA “imprintors” in the maternal germ line to switch the paternal chromosome to a maternal epigenotype. This proposal seems less likely in light of the observations that the lack of methylation on the maternal AS chromosome does not appear to arise from a switch failure, but occurs equally frequently on the mother’s grandparental chromosomes in both deletion and nondeletion sporadic cases (Buiting et al. 1998xSporadic imprinting defects in Prader-Willi syndrome and Angelman syndrome (implications for imprint-switch models, genetic counseling, and prenatal diagnosis) . Buiting, K, Dittrich, B, Gross, S, Lich, C, Farber, C, Buchholz, T, Smith, E, Reis, A, Burger, J, Nothen, M.M et al. Am. J. Hum. Genet. 1998; 63: 170–180Abstract | Full Text | Full Text PDF | PubMed | Scopus (108)See all ReferencesBuiting et al. 1998). This is consistent with the idea that the grandmaternal methylation imprint is first erased, and then the two parental chromosomes have an equal likelihood of becoming methylated. Those chromosomes that fail to become methylated lead to AS, but the function of the IC in this process is unclear.While these PWS and AS mutations have been instrumental in localizing key cis-acting regulatory sequences for imprinting in the region, the recent generation of a mouse with a deletion analogous to that found in PWS patients will greatly accelerate studies to understand long-range gene silencing in this domain (Yang et al. 1998xA mouse model for Prader-Willi syndrome imprinting-centre mutations. Yang, T, Adamson, T.E, Resnick, J.L, Leff, S, Wevrick, R, Francke, U, Jenkins, N.A, Copeland, N.G, and Brannan, C.I. Nat. Genet. 1998; 19: 25–31Crossref | PubMed | Scopus (217)See all ReferencesYang et al. 1998).The variety of mechanisms that have been invoked to explain genomic imprinting (Figure 2Figure 2) may reflect the fact that imprinting has arisen very recently in evolution. Mammals have exploited whatever cis-acting sequences and biochemical machinery that are available in order to achieve allele-specific gene silencing, although with the one exception of Mash2 (Caspary et al. 1998xMultiple mechanisms regulate imprinting of the mouse distal chromosome 7 gene cluster. Caspary, T, Cleary, M.A, Baker, C.C, Guan, X.-J, and Tilghman, S.M. Mol. Cell. Biol. 1998; 18: 3466–3474PubMedSee all ReferencesCaspary et al. 1998), every mechanism appears to involve differential DNA methylation. On the other hand, the apparent variety may reflect something more fundamental about epigenetic gene regulation. As the mechanisms that regulate X chromosome dosage compensation are being revealed in invertebrates and vertebrates, it appears that each organism has commandeered a different preexisting cellular machinery to achieve its goal (Lieb et al. 1998xMIX-1 (an essential component of the C. elegans mitotic machinery executes X chromosome dosage compensation) . Lieb, J.D, Albrecht, M.R, Chuang, P.T, and Meyer, B.J. Cell. 1998; 92: 265–277Abstract | Full Text | Full Text PDF | PubMed | Scopus (131)See all ReferencesLieb et al. 1998).