A novel L23-related gene 40 kb downstream of the imprinted H19 gene is biallelically expressed in mid-fetal and adult human tissues

Structural and functional analysis of a 0.5-Mb chicken region orthologous to the imprinted mammalian Ascl2/Mash2-Igf2-H19 region

Previous studies revealed that Igf2 and Mpr/Igf2r are imprinted in eutherian mammals and marsupials but not in monotremes or birds. Igf2 lies in a large imprinted cluster in eutherians, and its imprinting is regulated by long-range mechanisms. As a step to understand how the imprinted cluster evolved, we have determined a 490-kb chicken sequence containing the orthologs of mammalian Ascl2/Mash2, Ins2 and Igf2. We found that most of the genes in this region are conserved between chickens and mammals, maintaining the same transcriptional polarities and exon-intron structures. However, H19, an imprinted noncoding transcript, was absent from the chicken sequence. Chicken ASCL2/CASH4 and INS, the orthologs of the imprinted mammalian genes, showed biallelic expression, further supporting the notion that imprinting evolved after the divergence of mammals and birds. The H19 imprinting center and many of the local regulatory elements identified in mammals were not found in chickens. Also, a large segment of tandem repeats and retroelements identified between the two imprinted subdomains in mice was not found in chickens. Our findings show that the imprinted genes were clustered before the emergence of imprinting and that the elements associated with imprinting probably evolved after the divergence of mammals and birds.

Loss of wild-type Trp53 protein in mouse fibroblasts leads to increased radioresistance with consequent decrease in repair of potentially lethal damage

It has been reported that the loss of function of Trp53 protein is associated with a reduction in the expression of radiation-induced potentially lethal damage (PLD). These studies, however, were carried out using either transformed or transfected cell lines, and other factors may have existed that could interfere with PLD repair. In this study, we used isogenic fibroblasts derived from Trp53 knockout mice to study radiation sensitivity, PLD repair, and repair of DNA double-strand breaks (DSBs). Experiments were carried out using wild-type (Trp53(+/+)), heterozygous (Trp53(+/-)) and homozygous mutant (Trp53(-/-)) cells. This is an ideal system because the only difference in the three cell strains is the status of the Trp53 protein. DSB repair was measured by pulsed-field-gel electrophoresis (PFGE), while radiosensitivity and PLD repair were studied using the clonogenic survival assay. Cells were irradiated in plateau phase and then trypsinized and plated either immediately or 24 h later to allow for PLD repair. The results of Western blot analyses showed that Trp53(-/-) cells expressed a putative mutant form of Trp53 that was unable to transcriptionally activate Cdkn1a (p21) protein in response to irradiation. The Trp53(-/-) cells were significantly more radioresistant than the Trp53(+/+) cells, and this was associated with a moderate reduction in PLD repair. DNA repair experiments showed no difference in DSB rejoining capability between the two cell lines. In conclusion, our results show that loss of wild-type Trp53 leads to increased radioresistance with consequent reduction in PLD repair but with no effect on DNA DSB repair.

Biallelic expression of HRAS and MUCDHL in human and mouse

At least eight genes clustered in 1 Mb of DNA on human chromosome (Chr) 11p15.5 are subject to parental imprinting, with monoallelic expression in one or more tissues. Orthologues of these genes show conserved linkage and imprinting on distal Chr 7 of mice. The extended imprinted region has a bipartite structure, with at least two differentially methylated DNA elements (DMRs) controlling the imprinting of two sub-domains. We previously described three biallelically expressed genes ( MRPL23, 2G7 and TNNT3) in 100 kb of DNA immediately downstream of the imprinted H19 gene, suggesting that H19 marks one border of the imprinted region. Here we extend this analysis to two additional downstream genes, HRAS and MUCDHL (mu-protocadherin). We find that these genes are biallelically expressed in multiple fetal and adult tissues, both in humans and in mice. The mouse orthologue of a third gene, DUSP8, located between H19 and MUCDHL, is also expressed biallelically. The DMR immediately upstream of H19 frequently shows a net gain of methylation in Wilms tumors, either via Chr 11p15.5 loss of heterozygosity (LOH) or loss of imprinting (LOI), but changes in methylation in CpG-rich sequences upstream and within the MUCDHL gene are rare in these tumors and do not correlate with LOH or LOI. These findings are further evidence for a border of the imprinted region immediately downstream of H19, and the data allow the construction of an imprinting map that includes more than 20 genes, distributed over 3 Mb of DNA on Chr 11p15.5.

Short interspersed transposable elements (SINEs) are excluded from imprinted regions in the human genome

To test whether regions undergoing genomic imprinting have unique genomic characteristics, imprinted and nonimprinted human loci were compared for nucleotide and retroelement composition. Maternally and paternally expressed subgroups of imprinted genes were found to differ in terms of guanine and cytosine, CpG, and retroelement content, indicating a segregation into distinct genomic compartments. Imprinted regions have been normally permissive to L1 long interspersed transposable element retroposition during mammalian evolution but universally and significantly lack short interspersed transposable elements (SINEs). The primate-specific Alu SINEs, as well as the more ancient mammalian-wide interspersed repeat SINEs, are found at significantly low densities in imprinted regions. The latter paleogenomic signature indicates that the sequence characteristics of currently imprinted regions existed before the mammalian radiation. Transitions from imprinted to nonimprinted genomic regions in cis are characterized by a sharp inflection in SINE content, demonstrating that this genomic characteristic can help predict the presence and extent of regions undergoing imprinting. During primate evolution, SINE accumulation in imprinted regions occurred at a decreased rate compared with control loci. The constraint on SINE accumulation in imprinted regions may be mediated by an active selection process. This selection could be because of SINEs attracting and spreading methylation, as has been found at other loci. Methylation-induced silencing could lead to deleterious consequences at imprinted loci, where inactivation of one allele is already established, and expression is often essential for embryonic growth and survival.

The human mitochondrial ribosomal protein genes: mapping of 54 genes to the chromosomes and implications for human disorders

Mitochondria possess their own translational machinery, which is composed of components distinct from their cytoplasmic counterparts. To investigate the possible involvement of mitochondrial ribosomal defects in human disease, we mapped nuclear genes that encode mitochondrial ribosomal proteins (MRPs). We generated sequence-tagged sites (STSs) of individual MRP genes that were able to be detected by PCR. They were placed on an STS content map of the human genome by typing of radiation hybrid panels. We located 54 MRP genes on the STS-content map and assigned these genes to cytogenetic bands of the human chromosomes. Although mitochondria are thought to have originated from bacteria, in which the genes encoding ribosomal proteins are clustered into operons, the mapped MRP genes are widely dispersed throughout the genome, suggesting that transfer of each MRP gene to the nuclear genome occurred individually. We compared the assigned positions with candidate regions for mendelian disorders and found certain genes that might be involved in particular diseases. This map provides a basis for studying possible roles of MRP defects in mitochondrial disorders.

Structural compensation for the deficit of rRNA with proteins in the mammalian mitochondrial ribosome. Systematic analysis of protein components of the large ribosomal subunit from mammalian mitochondria

The mammalian mitochondrial ribosome (mitoribosome) is a highly protein-rich particle in which almost half of the rRNA contained in the bacterial ribosome is replaced with proteins. It is known that mitochondrial translation factors can function on both mitochondrial and Escherichia coli ribosomes, indicating that protein components in the mitoribosome compensate the reduced rRNA chain to make a bacteria-type ribosome. To elucidate the molecular basis of this compensation, we analyzed bovine mitoribosomal large subunit proteins; 31 proteins were identified including 15 newly identified proteins with their cDNA sequences from human and mouse. The results showed that the proteins with binding sites on rRNA shortened or lost in the mitoribosome were enlarged when compared with the E. coli counterparts; this suggests the structural compensation of the rRNA deficit by the enlarged proteins in the mitoribosome.

Mammalian mitochondrial ribosomal proteins (4). Amino acid sequencing, characterization, and identification of corresponding gene sequences

Mitochondrial ribosomal proteins (MRPs) are required for the translation of all 13 mitochondrial encoded genes in humans. It has been speculated that mutations and polymorphisms in the human MRPs may be a primary cause of some oxidative phosphorylation disorders or modulate the severity and tissue specificity of pathogenic mitochondrial DNA mutations. Although the sequences of most of the yeast MRPs are known, only very few mammalian and nearly no human MRPs have been completely characterized. MRPs differ greatly in sequence, and sometimes biochemical properties, between different species, not allowing easy recognition by sequence homology. Therefore, the Mammalian Mitochondrial Ribosomal Consortium is using a direct approach of purifying individual mammalian (bovine) MRPs, determining their N-terminal and/or internal peptide sequences using different protein sequencing techniques, and using the resulting sequence information for screening expressed sequence tags and genomic data bases to determine human, mouse, and rat homologues of the bovine proteins. Two proteins of the large and three proteins of the small ribosomal subunit have been analyzed in this manner. Three of them represent "new," i.e. formerly unknown mammalian mitochondrial ribosomal protein classes. Only one of these three different MRPs shows significant sequence similarities to known ribosomal proteins. In one case, the corresponding human genomic DNA sequences were found in the data bases, and the exon/intron structure was determined.

Mammalian mitochondrial ribosomal proteins (2). Amino acid sequencing, characterization, and identification of corresponding gene sequences

Four different classes of mammalian mitochondrial ribosomal proteins were identified and characterized. Mature proteins were purified from bovine liver and subjected to N-terminal or matrix-assisted laser-desorption mass spectroscopic amino acid sequencing after tryptic in-gel digestion and high pressure liquid chromatography separation of the resulting peptides. Peptide sequences obtained were used to virtually screen expressed sequence tag data bases from human, mouse, and rat. Consensus cDNAs were assembled in silico from various expressed sequence tag sequences identified. Deduced mammalian protein sequences were characterized and compared with ribosomal protein sequences of Escherichia coli and yeast mitochondria. Significant sequence similarities to ribosomal proteins of other sources were detected for three out of four different mammalian protein classes determined. However, the sequence conservation between mitochondrial ribosomal proteins of mammalian and yeast origin is much less than the sequence conservation between cytoplasmic ribosomal proteins of the same species. In particular, this is shown for the mammalian counterparts of the E. coli EcoL2 ribosomal protein (MRP-L14), that do not conserve the specific and functional highly important His(229) residue of E. coli and the corresponding yeast mitochondrial Rml2p.

Genomic imprinting and chromatin insulation in Beckwith-Wiedemann syndrome

Genes are recognized as undergoing genomic imprinting when they are capable of being expressed only from the paternal or only from the maternal chromosome. The process can occur coordinately within large physical domains in mammalian chromosomes. One interesting facet of the study of genomic imprinting is that it offers insight into the regulation of large chromosomal regions. Understanding this regulation involves elucidating the cis-acting regulators of gene expression and defining the elements that maintain chromatin insulation, both required for understanding more practically applicable areas of biological research, such as efficient transgene production. This review is focused on the regulation of the imprinted domain of human chromosome 11p15.5, responsible for Beckwith-Wiedemann syndrome (BWS). Recent findings indicate that the maintenance of imprinting within this domain is critically dependent on the stable maintenance of chromatin insulation.

Syntenic organization of the mouse distal chromosome 7 imprinting cluster and the Beckwith-Wiedemann syndrome region in chromosome 11p15.5

In human and mouse, most imprinted genes are arranged in chromosomal clusters. Their linked organization suggests co-ordinated mechanisms controlling imprinting and gene expression. The identification of local and regional elements responsible for the epigenetic control of imprinted gene expression will be important in understanding the molecular basis of diseases associated with imprinting such as Beckwith-Wiedemann syndrome. We have established a complete contig of clones along the murine imprinting cluster on distal chromosome 7 syntenic with the human imprinting region at 11p15.5 associated with Beckwith-Wiedemann syndrome. The cluster comprises approximately 1 Mb of DNA, contains at least eight imprinted genes and is demarcated by the two maternally expressed genes Tssc3 (Ipl) and H19 which are directly flanked by the non-imprinted genes Nap1l4 (Nap2) and Rpl23l (L23mrp), respectively. We also localized Kcnq1 (Kvlqt1) and Cd81 (Tapa-1) between Cdkn1c (p57(Kip2)) and Mash2. The mouse Kcnq1 gene is maternally expressed in most fetal but biallelically transcribed in most neonatal tissues, suggesting relaxation of imprinting during development. Our findings indicate conserved control mechanisms between mouse and human, but also reveal some structural and functional differences. Our study opens the way for a systematic analysis of the cluster by genetic manipulation in the mouse which will lead to animal models of Beckwith-Wiedemann syndrome and childhood tumours.

Multiple mechanisms regulate imprinting of the mouse distal chromosome 7 gene cluster

Genomic imprinting is an epigenetic process that results in the preferential silencing of one of the two parental copies of a gene. Although the precise mechanisms by which genomic imprinting occurs are unknown, the tendency of imprinted genes to exist in chromosomal clusters suggests long-range regulation through shared regulatory elements. We characterize a 800-kb region on the distal end of mouse chromosome 7 that contains a cluster of four maternally expressed genes, H19, Mash2, Kvlqt1, and p57(Kip2), as well as two paternally expressed genes, Igf2 and Ins2, and assess the expression and imprinting of Mash2, Kvlqt1, and p57(Kip2) during development in embryonic and extraembryonic tissues. Unlike Igf2 and Ins2, which depend on H19 for their imprinting, Mash2, p57(Kip2), and Kvlqt1 are unaffected by a deletion of the H19 gene region, suggesting that these more telomeric genes are not regulated by the mechanism that controls H19, Igf2, and Ins2. Mutations in human p57(Kip2) have been implicated in Beckwith-Wiedemann syndrome, a disease that has also been associated with loss of imprinting of IGF2. We find, however, that a deletion of the gene has no effect on imprinting within the cluster. Surprisingly, the three maternally expressed genes are regulated very differently by DNA methylation; p57(Kip2) is activated, Kvlqt1 is silenced, and Mash2 is unaffected in mice lacking DNA methyltransferase. We conclude that H19 is not a global regulator of imprinting on distal chromosome 7 and that the telomeric genes are imprinted by a separate mechanism(s).

Divergently transcribed overlapping genes expressed in liver and kidney and located in the 11p15.5 imprinted domain

Human chromosomal band 11p15.5 has been shown to contain genes involved in the development of several pediatric and adult tumors and in Beckwith-Wiedemann syndrome (BWS). Overlapping P1 artificial chromosome clones from this region have been used as templates for genomic sequencing in an effort to identify candidate genes for these disorders. PowerBLAST identified several matches with expressed sequence tags (ESTs) from fetal brain and liver cDNA libraries. Northern blot analysis indicated that two of the genes identified by these ESTs encode transcripts of 1-1.5 kb with predominant expression in fetal and adult liver and kidney. With RT-PCR and RACE, full-length transcripts were isolated for these two genes, with the largest open reading frames encoding putative proteins of 253 and 424 amino acids. Database comparison of the predicted amino acid sequence of the larger transcript indicated homology to integral membrane organic cation transporters; hence, we designate this gene ORCTL2 (organic cation transporter-like 2). An expressed sequence polymorphism provided evidence that the ORCTL2 gene exhibits "leaky" imprinting in both human fetal kidney and human fetal liver. The mouse orthologue (Orctl2) was identified, and a similar polymorphism was used to demonstrate maternal-specific expression of this gene in fetal liver from interspecific F1 mice. The predicted protein of the smaller gene showed no significant similarity in the database. Northern and RACE analyses suggest that this gene may have multiple transcription start sites. Determination of the genomic structure in humans indicated that the 5'-end of this transcript overlaps in divergent orientation with the first two exons of ORCTL2, suggesting a possible role for antisense regulation of one gene by the other. We, therefore, provisionally name this second transcript ORCTL2S (ORCTL2-antisense). The expression patterns of these genes and the imprinted expression of ORCTL2 are suggestive of a possible role in the development of Wilms tumor (WT) and hepatoblastoma. Although SSCP analysis of 62 WT samples and 10 BWS patients did not result in the identification of any mutations in ORCTL2 or ORCTL2S, it will be important to examine their expression pattern in tumors and BWS patients, since epigenetic alteration at these loci may play a role in the etiology of these diseases.

Epigenetic reprogramming of the human H19 gene in mouse embryonic cells does not erase the primary parental imprint

BACKGROUND

Genomic imprinting in mammals is thought to result from epigenetic modifications to chromosomes during gametogenesis, which leads to differential allelic expression during development. There is a requirement for an appropriate experimental system to enable the analysis of the mechanisms of genomic imprinting during embryogenesis.

RESULTS

To develop a novel in vitro system for studying the molecular basis of genomic imprinting, we constructed mouse cell lines containing either a paternal or maternal human chromosome 11, by microcell-mediated chromosome transfer. Allele-specific expression and DNA methylation studies revealed that the imprinting status of the human H19 gene was maintained in mouse A9 mono-chromosomal hybrids. Each parental human chromosome was introduced independently into mouse near-diploid immortal fibroblasts (m5S) and two embryonal carcinoma (EC) cell lines (OTF9-63 and P19). The paternal allele of human H19 remained in a repressed state in m5S cells, but was de-repressed in both EC cells. The paternal H19 allele was demethylated extensively in OTF9-63 cells, whereas the only alteration in P19 hybrids was de novo methylation on both alleles in the 3' region. Following in vitro differentiation, the expressed paternal H19 allele was selectively repressed in differentiated derivatives of EC hybrids.

CONCLUSION

These results indicated that human imprint marks could function effectively in mouse cells, and that the imprinting process was epigenetically reprogrammed in embryonal carcinoma cells, without erasure of the primary imprint that marked the parental origin. Therefore, these mono-chromosomal hybrids could provide a valuable in vitro system to study the mechanisms involved in the regulation of imprinted gene expression.

The mouse H19 locus mediates a transition between imprinted and non-imprinted DNA replication patterns

Genes subject to genomic imprinting generally occur in clusters of hundreds of kilobases. These domains exhibit several gamete of origin-dependent manifestations, including a pattern of asynchronous replication when studied by fluorescence in situ hybridization (FISH). We find a transition from asynchronous replication at the imprinted mouse H19 gene to synchronous replication at the downstream Rpl23 gene, the human homologue of which appears to be non-imprinted. Two-colour FISH demonstrates that this transition is due solely to a difference in replication timing between the upstream and downstream chromatin on the later-replicating (maternal) chromosome. This difference is lost in mice deleted for the H19 gene body and 9.9 kb of upstream DNA when this deletion is maternally inherited, with synchronous replication patterns extending over 110 kb upstream from the deleted area. No effect is seen when the deletion is paternally inherited. The presence of a boundary element in this region has been suggested by observations of position-independent expression of H19 -containing transgenes and the blocking of accessibility of downstream enhancers to the upstream Igf2 and Ins2 genes on the maternal chromosome. The FISH studies presented here demonstrate the insulation of replication patterns within the imprinted domain from downstream, non-imprinted chromatin, mediated by an element at the H19 locus which is subject to genomic imprinting.

The IPL gene on chromosome 11p15.5 is imprinted in humans and mice and is similar to TDAG51, implicated in Fas expression and apoptosis

We searched for novel imprinted genes in a region of human chromosome 11p15.5, which contains several known imprinted genes. Here we describe the cloning and characterization of the IPL ( I mprinted in P lacenta and L iver) gene, which shows tissue-specific expression and functional imprinting, with the maternal allele active and the paternal allele relatively inactive, in many human and mouse tissues. Human IPL is highly expressed in placenta and shows low but detectable expression in fetal and adult liver and lung. Mouse Ipl maps to the region of chromosome 7 which is syntenic with human 11p15.5 and this gene is expressed in placenta and at higher levels in extraembryonic membranes (yolk sac), fetal liver and adult kidney. Mouse and human IPL show sequence similarity to TDAG51 , a gene which was shown to be essential for Fas expression and susceptibility to apoptosis in a T lymphocyte cell line. Like several other imprinted genes, mouse and human IPL genes are small and contain small introns. These data expand the repertoire of known imprinted genes and will be helpful in testing the mechanism of genomic imprinting and the role of imprinted genes in growth regulation.

Structure and expression of the mouse L23mrp gene downstream of the imprinted H19 gene: biallelic expression and lack of interaction with the H19 enhancers

The human L23 (mitochondrial)-related protein gene, located 40 kb downstream of the imprinted H19 gene, is biallelically expressed. We have cloned and characterized its mouse homolog, L23mrp, which maps to the conserved syntenic region on mouse chromosome 7. The promoter of L23mrp is a CpG island that is transcribed ubiquitously, but at different levels, in different fetal tissues. Allele-specific expression analysis revealed that both parental alleles are equally active. Since the enhancers located between H19 and L23mrp had been shown to be involved in the imprinted expression of Ins-2, Igf-2, and H19, we asked whether they also influence L23mrp. Analysis of mice with a targeted deletion of the enhancers demonstrated that they were not disrupted in the expression of L23mrp. These findings indicate that L23mrp is functionally insulated from the Ins-2/Igf-2/H19 domain in terms of both imprinting and enhancer action.

A 1-Mb physical map and PAC contig of the imprinted domain in 11p15.5 that contains TAPA1 and the BWSCR1/WT2 region

We have constructed a 1-Mb contig in human chromosomal band 11p15.5, a region implicated in the etiology of several embryonal tumors, including Wilms tumor, and in Beckwith-Wiedemann syndrome. Cosmid, P1, PAC, and BAC clones were characterized by NotI/SalI digestion and hybridized to a variety of probes to generate a detailed physical map that extends from D11S517 to L23MRP. Included in the map are the CARS, NAP2, p57/KIP2, KVLQT1, ASCL2, TH, INS, IGF2, H19, and L23MRP genes as well as end probes isolated from PACs. The TAPA1 gene, whose protein product can transmit an antiproliferative signal, was also localized in the contig. However, Northern blot analysis demonstrated that its expression did not correlate with tumorigenicity in G401 Wilms tumor hybrids, suggesting that TAPA1 is not responsible for the tumor suppression associated with 11p15.5. Genomic clones were used as probes in FISH analysis to map the breakpoints from three Beckwith-Wiedemann syndrome patients and a rhabdoid tumor. Interestingly, each of the breakpoints disrupts the KVLQT1 gene, which is spread over a 400-kb region of the contig. Since 11p15.5 contains several genes with imprinted expression and one or more tumor suppressor genes, our contig and map provide a framework for characterizing this intriguing genetic environment.

Genomic organization and chromosomal localization of a member of the MAP kinase phosphatase gene family to human chromosome 11p15.5 and a pseudogene to 10q11.2

Mitogen-activated protein kinase phosphatases (MKPs) play a central role in a variety of signaling pathways. We recently described a novel murine MKP, M3/6, which is uniquely specific for c-Jun N-terminal kinase/stress-activated protein kinase and p38 kinase. Here we report the localization of the human orthologue of this gene, HB5, to within 150 kb of H19 on human chromosome 11p15.5. The gene consists of six exons. Two of the introns in HB5 are not found in other genes of this family, suggesting an evolutionary split between MKPs displaying specificity toward different MAP kinases. An intronless pseudogene is present on chromosome 10q11.2. Although 11p15.5 is an imprinted region, HB5 is almost entirely unmethylated on both alleles in lymphocytes. Chromosome 11p15 has been implicated in the development of a number of tumor types, including lung, a tissue known to express this gene. Loss of heterozygosity was found in one of eight informative lung tumors studied.

DNA methylation in genomic imprinting

As a reversible epigenetic modification which can affect gene expression, DNA methylation has been an attractive candidate for the biochemical mechanism of genomic imprinting. Many correlations in mice and humans link allele-specific DNA methylation to the allele-restricted RNA expression which is the hallmark of imprinted genes. Moreover, abnormal DNA methylation accompanies the pathological functional imprinting of certain human genes on chromosome 11p15.5 in Wilms' tumors and in the Beckwith-Weidemann syndrome and on chromosome 15q11-13 in the Prader-Willi and Angelman syndromes. A role for DNA methylation in maintaining the transcriptional silence of imprinted alleles at some loci has been supported by pharmacological manipulation with 5-aza-2'-deoxycytidine and by experiments with methyltransferase deletion mice. Gametic differences in DNA methylation could also account for the initiation of imprints, but this remains unproven. Comprehensive physical models for the role of DNA methylation in imprinting must account not only for local allele-restricted gene expression but also for the existence of large chromosomal domains containing multiple coordinately imprinted genes.

Matrix-attachment regions in the mouse chromosome 7F imprinted domain

We have mapped the matrix-attachment regions (MARs) in 200 kilobases of the mouse Chromosome (Chr) 7F imprinted domain. MARs are genetic elements known to have effects in cis on methylation at nonimprinted loci. The imprinting of the Igf2 and Ins2 genes is dependent on the transcription of the downstream H19 gene. The transcription of H19 is dependent in turn on its methylation status. The cis-acting regulators of methylation at this site are not known. As MARs are potential regulators not only of methylation but also other elements of genomic imprinting, we mapped the MARs within the 200 kilobases around H19. This report describes the mapping of four MARs from this region.

Genomic imprinting: nature and clinical relevance

Molecular genetic techniques allow investigators to trace chromosomes and genes from parent to child and, in a single individual, from tissue to tissue. These techniques have uncovered a new type of gene control in which the allele from one parent is expressed and the allele from the other parent is not. This differential expression is called genomic imprinting. It may lead to phenotypic differences when inheritance is from the mother versus the father. Genomic imprinting has been observed in a number of disorders having to do with growth, behavior, and abnormal cell growth. It is important to be aware that such a phenomenon exists and to consider it when making diagnoses and determining therapy.

Genomic imprinting and Wilms' tumor

The selective loss of maternal and reduplication of paternal chromosome 11p15.5 alleles in Wilms' tumors (WTs) points to the existence of a paternally imprinted tumor suppressor gene(s) and/or a maternally imprinted dose-dependent growth-promoting gene(s) in this chromosomal region. Two reciprocally imprinted chromosome 11p15.5 genes, H19, a candidate tumor suppressor gene, and IGF2, a candidate dominant oncogene, have been well-characterized in terms of their imprinting and expression status in WTs. Here we review and extend data indicating that a majority of WTs show a bipaternal epigenotype at these loci, with H19 inactive and IGF2 biallelically active. This can arise either through loss of heterozygosity (LOH) or by a non-LOH pathway involving localized biallelic hypermethylation of H19 DNA. Conversion to this bipaternal endpoint has recently been found to affect not only these two genes, but also at least one other imprinted 11p15.5 gene, KIP2. Since 11p15.5 LOH and biallelic H19 hypermethylation can occur both early and late in tumor progression and since early loss is not associated with bilaterality or multifocality of WTs, these types of lesions appear to be permissive rather than rate-limiting in Wilms' tumorigenesis.

Imprinted genes and regulation of gene expression by epigenetic inheritance

Six new imprinted genes have recently been identified by association with established imprinted regions, in systematic screens or by serendipity. This brings the total to seventeen imprinted genes, which display a wide variety of functions. Some imprinted genes have been shown to be both physically and mechanistically linked within domains that are under the control of an imprinting centre. Others may apparently undergo imprinting independently. Methylation is clearly required for maintenance of mono-allelic expression while chromatin structure and non-coding RNAs may also play a role.