Transactivation of Igf2 in a mouse model of Beckwith-Wiedemann syndrome

Acrocentric chromosome polymorphisms: beware of cryptic translocations

Cryptic translocations may escape diagnosis, especially when they implicate chromosomal regions that are known to be polymorphic in the human karyotype. We describe a case of postnatal diagnosis of Beckwith-Wiedemann syndrome (BWS) due to an unbalanced translocation that had not been diagnosed in the fetal karyotype. This first cytogenetic analysis revealed that one chromosome 14 presented as a common acrocentric short arm polymorphism. Further analyses after birth, using C-banding, NOR staining and fluorescence in situ hybridization (FISH) with telomeric probes, revealed that it was the result of an unbalanced de novo t(11;14)(p15;p13) translocation leading to partial 11p trisomy and to BWS. Prenatal cytogenetic management of such apparently inoffensive chromosome markers is discussed.

EGR1 target genes in prostate carcinoma cells identified by microarray analysis

The EGR1 transactivator is overexpressed in prostate cancer, and its expression pattern suggests that EGR1 could potentially regulate a number of steps involved in initiation and progression of prostate cancer, such as mitogenesis, invasiveness, angiogenesis, and metastasis. To identify potential EGR1 target genes in an unbiased manner, we have utilized adenovirus-mediated expression of EGR1 in a prostate cancer cell line to identify specific genes that are induced by EGR1. Using oligonucleotide arrays, a number of EGR1-regulated genes were identified and their regulation was confirmed by quantitative reverse transcription-polymerase chain reaction analysis. One of the largest gene classes identified in this screen includes several neuroendocrine-associated genes (neuron-specific enolase, neurogranin), suggesting that EGR1 overexpression may contribute to the neuroendocrine differentiation that often accompanies prostate cancer progression. This screen also identified several growth factors such as insulin-like growth factor-II, platelet-derived growth factor-A, and transforming growth factor-beta1, which have previously been implicated in enhancing tumor progression. The insulin-like growth factor-II gene lies within the 11p15.5 chromosomal locus, which contains a number of other imprinted genes, and EGR1 expression was found to induce at least two other genes in this locus (IPL, p57(KIP2)). Based on our results, coupling adenoviral overexpression with microarray and quantitative reverse transcription-polymerase chain reaction analyses could be a versatile strategy for identifying target genes of transactivators.

Epigenotype-phenotype correlations in Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is a model imprinting disorder resulting from mutations or epigenetic events involving imprinted genes at chromosome 11p15.5. Thus, germline mutations in CDKN1C, uniparental disomy (UPD), and loss of imprinting of IGF2 and other imprinted genes have been implicated. Many familial BWS cases have germline CDKN1C mutations. However, most BWS cases are sporadic and UPD or putative imprinting errors predominate in this group. We have identified previously a subgroup of sporadic cases with loss of imprinting (LOI) of IGF2 and epigenetic silencing of H19 proposed to be caused by a defect in a distal 11p15.5 imprinting control element (designated BWSIC1). However, many sporadic BWS patients show biallelic IGF2 expression in the presence of normal H19 methylation and expression patterns. This and other evidence suggested the existence of a further imprinting control element (BWSIC2) at 11p15. 5. Recently, we showed that a subgroup of BWS patients have loss of methylation (LOM) at a differentially methylated region (KvDMR1) within the KCNQ1 gene centromeric to the IGF2 and H19 genes. We have now analysed a large series of sporadic cases to define the frequency and phenotypic correlates of epigenetic abnormalities in BWS. LOM at KvDMR1 was detected by Southern analysis or a novel PCR based method in 35 of 69 (51%) sporadic BWS without UPD. LOM at KvDMR1 was often, but not invariably associated with LOI of IGF2. KvDMR1 LOM was not detected in BWS patients with putative BWSIC1 defects and cases with KvDMR1 LOM (that is, putative BWSIC2 defects) invariably had a normal H19 methylation pattern. The incidence of exomphalos in putative BWSIC2 defect patients was not significantly different from that in patients with germline CDKN1C mutations (20/29 and 13/15 respectively), but was significantly greater than that in patients with putative BWSIC1 defects (0/5, p=0.007) and UPD (0/22, p<0.0001). These findings are consistent with the hypothesis that LOM of KvDMR1 (BWSIC2 defect) results in epigenetic silencing of CDKN1C and variable LOI of IGF2. BWS patients with embryonal tumours have UPD or a BWSIC1 defect but not LOM of KvDMR1. This study has further shown how (1) variations in phenotypic expression of BWS may be linked to specific molecular subgroups and (2) molecular analysis of BWS can provide insights into mechanisms of imprinting regulation.

Overexpression of insulin-like growth factor-II in mouse embryonic stem cells promotes myogenic differentiation

Embryonic stem (ES) cells derived from androgenetic or parthenogenetic mouse embryos are important tools for studying the roles of imprinted genes in early development. Androgenetic ES cells have been shown to preferentially differentiate into the myogenic lineage both in vitro and after formation of teratocarcinomas in vivo. To clarify if the maternally imprinted Igf2 gene which is expected to be overexpressed in androgenetic ES cells is sufficient to induce myogenic differentiation, R1 ES cells were transfected with human IGF-II expression vectors. Stable ES cell clones exhibiting human IGF-II mRNA and protein expression were studied vs ES cell clones without IGF-II overexpression in a standard in vitro differentiation system involving culture in "hanging drops" and observation of differentiation of the recovered embryoid bodies (EBs). EBs derived from IGF-II overexpressing ES cells showed stimulated myogenic differentiation evident by the appearance of myoblasts already 3 days after plating and by higher levels of skeletal muscle-specific transcripts (myf5, myoD, myogenin) at earlier stages. Our study demonstrates for the first time that overexpression of IGF-II enhances and accelerates myogenic differentiation of ES cells, which has implications for ES cell-derived tissue engineering.

Analysis of CDKN1C in Beckwith Wiedemann syndrome

In this study we have examined 32 patients with Beckwith Wiedemann Syndrome (BWS) for mutations affecting the CDKN1C gene, including seven cases of familial BWS. Mutations were not detected in the coding region of the CDKN1C gene in any individual with BWS. However in two patients, two G/A base substitutions at adjacent positions in the 5'UTR were detected. These substitutions were also found in normal controls. Expression of CDKN1C in somatic tissues was examined in 18 of the 32 cases using semi-quantitative RT-PCR. CDKN1C expression was significantly reduced in the peripheral blood of three cases compared with controls. These results suggest that, although coding region mutations in the CDKN1C gene are rare in BWS, mutations disrupting CDKN1C expression may be found. Three of five informative patients exhibited biallelic CDKN1C expression in lymphocytes, cord blood, and kidney tissue, respectively. Biallelic expression was not associated with overall CDKN1C levels significantly different to those in controls. Patients who expressed CDKN1C biallelically, or who were low CDKN1C expressors, maintained monoallelic methylation in the Differentially Methylated Region 2 (DMR2) of the IGF2 locus. One patient expressing CDKN1C biallelically, maintained imprinted gene expression at the IGF2 locus. These results suggest that biallelic CDKN1C expression does not significantly perturb the overall levels of CDKN1C expression in somatic tissue. They also confirm other studies showing that the mechanisms associated with regulating CDKN1C expression and imprinting are separate from those regulating IGF2 imprinting.

H19 sense and antisense transgenes modify insulin-like growth factor-II mRNA levels

The oppositely-imprinted genes insulin-like growth factor-II (IGF2) and H19, a putative tumor suppressor, often show coordinate, reciprocal regulation and are believed to play a role in carcinogenesis. To explore the possible interactions between these genes, we stably transfected diHepG2 cells with a plasmid containing either the sense or the antisense H19 cDNA sequences and verified their expression by Northern analysis and by RNase protection analysis. Levels of H19, IGF2 and gamma-actin mRNA were quantified by competitive RT-PCR analysis. Although H19 sense transgene overexpression (n = 24 clones) did not decrease the low, basal levels of IGF2 mRNA compared to control cells, levels of IGF2 mRNA were positively correlated with the levels of H19 antisense mRNA (P < 0.0001, n = 40 clones). Furthermore, the increase in IGF2 mRNA level was accompanied by an elevation of IGF-II peptide in conditioned media. To see if H19 mRNA had a specific effect on transcription, we also performed transient transfections with reporter gene constructs containing IGF2 promoter 3 in the presence of sense or antisense H19 cDNA sequences under control of a cytomegalovirus promoter. We show a lower reporter gene activity from reporter gene constructs in the presence of sense H19 cDNA than from those with antisense or neomycin. Our results suggest that H19 participates in the repression of IGF2, at least in part through effects on IGF2 transcription, an effect which may contribute to its action as a tumor suppressor.

Subtle familial unbalanced translocation t(8;11)(p23.2;p15.5) in two fetuses with Beckwith-Wiedemann features

We describe a subtle translocation t(8;11)(p23.2;p15.5) ascertained after two induced abortions in the same sibship because of the discovery of fetal hydrops on ultrasound examination. Initial cytogenetic studies performed on cultured amniotic fluid cells were considered as normal in both fetuses. High resolution banding analysis and FISH studies performed on the parents' chromosomes revealed a paternal translocation t(8;11)(p23.2;p15.5). Retrospective FISH analysis of both fetuses showed that they carried the same chromosomal imbalance including a distal monosomy 8pter and a distal trisomy 11pter. The phenotypes of the fetuses were re-examined and found to be compatible with Beckwith-Wiedemann syndromes (BWS). FISH analysis using an IGF2 probe demonstrated the presence of three copies of the IGF2 gene. This study highlights the value of searching for subtle chromosome rearrangements in families with recurrent unexplained multiple malformation syndromes discovered prenatally. Also, it contributes to a better delineation of the prenatal phenotype of BWS. Finally, it sheds new light on the aetiology of non-immune hydrops fetalis.

Increased IGF-II protein affects p57kip2 expression in vivo and in vitro: implications for Beckwith-Wiedemann syndrome

In both human and mouse, the Igf2 gene, localized on chromosomes 11 and 7, respectively, is expressed from the paternally inherited chromosome in the majority of tissues. Insulin-like growth factor-II (IGF-II) plays an important role in embryonic growth, and aberrant IGF2 expression has been documented in several human pathologies, such as Beckwith-Wiedemann syndrome (BWS), and a wide variety of tumors. Human and mouse genetic data strongly implicate another gene, CDKN1C (p57(kip2)), located in the same imprinted gene cluster on human chromosome II, in BWS. p57(KIP2) is a cyclin-dependent kinase inhibitor and is required for normal mouse embryonic development. Mutations in CDKN1C (p57(kip2)) have been identified in a small proportion of patients with BWS, and removal of the gene from mice by targeted mutagenesis produces a phenotype with elements in common with this overgrowth syndrome. Patients with BWS with biallelic expression of IGF2 or with a CDKN1C (p57(kip2)) mutation, as well as overlapping phenotypes observed in two types of mutant mice, the p57(kip2) knockout and IGF-II-overexpressing mice, strongly suggest that the genes may act in a common pathway of growth control in situations where Igf2 expression is abnormal. Herein, we show that p57(kip2) expression is reduced on IGF-II treatment of primary embryo fibroblasts in a dose-dependent manner. In addition, p57(kip2) expression is down-regulated in mice with high serum levels of IGF-II. These data suggest that the effects of increased IGF-II in BWS may, in part, be mediated through a decrease in p57(kip2) gene expression.

Alterations of H19 imprinting and IGF2 replication timing are infrequent in Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is an overgrowth disorder resulting from dysregulation of multiple imprinted genes through a variety of distinct mechanisms. A frequent alteration in BWS involves changes in the imprinting status of the coordinately regulated IGF2 and H19 genes on 11p15. Patients have been categorized according to alterations in the imprinted expression, allele-specific methylation, and regional replication timing of these genes. In this work, IGF2/H19 expression, H19 DNA methylation, and IGF2 regional replication timing were studied in nine karyotypically normal BWS fibroblasts and two BWS patients with maternally inherited 11p15 chromosomal rearrangements. Informative patients (9/9) maintained normal monoallelic H19 expression/methylation, despite biallelic IGF2 expression in 6/9. Replication timing studies revealed no changes in the pattern of asynchronous replication timing for both a patient with biallelic IGF2 expression and a patient carrying an 11p15 inversion. In contrast, a patient with a chromosome 11;22 translocation and normal H19 expression/methylation exhibited partial loss of asynchrony and a shift toward earlier replication times. These results indicate that in BWS, (1) H19 imprinting alterations are less frequent than previously estimated, (2) IGF2 imprinting and H19 imprinting are not necessarily coordinated, and (3) alterations in regional replication timing are generally not correlated with either chromosomal rearrangements or the imprinting status of IGF2 and H19.

Disruption of a novel imprinted zinc-finger gene, ZNF215, in Beckwith-Wiedemann syndrome

The genetics of Beckwith-Wiedemann syndrome (BWS) is complex and is thought to involve multiple genes. It is known that three regions on chromosome 11p15 (BWSCR1, BWSCR2, and BWSCR3) may play a role in the development of BWS. BWSCR2 is defined by two BWS breakpoints. Here we describe the cloning and sequence analysis of 73 kb containing BWSCR2. Within this region, we detected a novel zinc-finger gene, ZNF215. We show that two of its five alternatively spliced transcripts are disrupted by both BWSCR2 breakpoints. Parts of the 3' end of these splice forms are transcribed from the antisense strand of a second zinc-finger gene, ZNF214. We show that ZNF215 is imprinted in a tissue-specific manner.

Hypoglycemia in Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is an overgrowth syndrome associated with macrosomia, macroglossia, abdominal wall defects, hypoglycemia in the neonatal period and embryonal cancers of infancy and early childhood. The frequency of hypoglycemia in this population is between 30% and 50%. The majority of infants with hypoglycemia will be asymptomatic and have resolution of the hypoglycemia within the first 3 days of life. Less than 5% will have hypoglycemia beyond the neonatal period requiring either continuous feeding or a partial pancreatectomy. The cause of hypoglycemia is unclear, but direct and indirect evidence supports a hyperinsulinemia as the major factor. Recent identification of the majority of genes associated with BWS in the 11p15 region and the genotype of persistent hyperinsulinemia hypoglycemia of childhood also in the 11p15 region may provide a molecular basis for hypoglycemia in BWS, particularly for the occasional patients with hypoglycemia requiring a partial pancreatectomy. Detailed genotype phenotype evaluations are needed and should provide an insight as to why patients with BWS have hypoglycemia.

A case of Perlman syndrome: fetal gigantism, renal dysplasia, and severe neurological deficits

We report on a neonate presenting with polyhydramnios; macrosomia; macrocephaly; visceromegaly including bilateral nephromegaly, hepatomegaly, cardiomegaly; thymus hyperplasia; cryptorchidism; generalized muscle hypotonia; and a distinctive facial appearance. The clinical course was marked by severe neurodevelopmental deficits combined with progressive respiratory decompensation leading to death at the age 6 months. Magnetic resonance imaging (MRI) disclosed a generalized cerebral atrophy with a marked deficit of the white matter. Renal ultrasound and MRI showed markedly enlarged kidneys with multiple small cystic lesions, a pattern indistinguishable from polycystic kidney disease. The postmortem kidney biopsy revealed dysplastic changes, microcysts, and a focal nephrogenic rest, characteristic features of the Perlman syndrome. In children with fetal gigantism, renal abnormalities, and neurological deficits, Perlman syndrome should be considered and may be confirmed by kidney biopsy.

Relaxation of insulin-like growth factor 2 imprinting and discordant methylation at KvDMR1 in two first cousins affected by Beckwith-Wiedemann and Klippel-Trenaunay-Weber syndromes

Beckwith-Wiedeman syndrome (BWS) and Klippel-Trenaunay-Weber syndrome (KTWS) are different human disorders characterized, among other features, by tissue overgrowth. Deregulation of one or more imprinted genes located at chromosome 11p15.5, of which insulin-like growth factor 2 (IGF2) is the most likely candidate, is believed to cause BWS, whereas the etiology of KTWS is completely obscure. We report a case of BWS and a case of KTWS in a single family. The probands, sons of two sisters, showed relaxation of the maternal IGF2 imprinting, although they inherited different 11p15.5 alleles from their mothers and did not show any chromosome rearrangement. The patient with BWS also displayed hypomethylation at KvDMR1, a maternally methylated CpG island within an intron of the KvLQT1 gene. The unaffected brother of the BWS proband shared the same maternal and paternal 11p15.5 haplotype with his brother, but the KvDMR1 locus was normally methylated. Methylation of the H19 gene was normal in both the BWS and KTWS probands. Linkage between the insulin-like growth factor 2 receptor (IGF2R) gene and the tissue overgrowth was also excluded. These results raise the possibility that a defective modifier or regulatory gene unlinked to 11p15.5 caused a spectrum of epigenetic alterations in the germ line or early development of both cousins, ranging from the relaxation of IGF2 imprinting in the KTWS proband to disruption of both the imprinted expression of IGF2 and the imprinted methylation of KvDMR1 in the BWS proband. Analysis of these data also indicates that loss of IGF2 imprinting is not necessarily linked to alteration of methylation at the KvDMR1 or H19 loci and supports the notion that IGF2 overexpression is involved in the etiology of the tissue hypertrophy observed in different overgrowth disorders, including KTWS.

Large-scale analysis of differential gene expression in the hindlimb muscles and diaphragm of mdx mouse

The mdx mouse is an animal model for Duchenne muscular dystrophy (DMD), which is caused by the absence of dystrophin. Mdx limb muscles substantially compensate for the lack of dystrophin while the diaphragm is affected like DMD skeletal muscles. To understand better the complex cascade of molecular events leading to muscle degeneration and compensatory processes in mdx muscles, we analyzed alterations of gene expression in mdx hindlimb and diaphragm muscles as compared to their normal counterparts. The strategy was based on suppression subtractive hybridization followed by reverse Northern quantitative hybridization. Four subtracted/normalized libraries, containing cDNA clones up- or downregulated in mdx hindlimb muscles or diaphragm, were constructed and a total of 1536 cDNA clones were analyzed. Ninety-three cDNAs were found to be differentially expressed in mdx hindlimb muscles and/or diaphragm. They corresponded to 54 known genes and 39 novel cDNAs. The potential role of the known genes is discussed in the context of the mdx phenotype.

Insulin-like growth factor-I in diabetes mellitus: its physiology, metabolic effects, and potential clinical utility

Type 1 diabetes mellitus (DM) is a disease of insulin deficiency, resulting from the autoimmune-mediated destruction of pancreatic beta cells. However, as a likely consequence of intraportal insulin deficiency, patients with type 1 DM also exhibit abnormalities of the growth hormone (GH)/IGF/IGF-binding protein (IGFBP) axis, including GH hypersecretion, reduced circulating levels of insulin-like growth factor-I (IGF-I) and IGFBP-3, and elevated levels of IGFBP-1. These abnormalities not only exacerbate hyperglycemia in patients with type 1 DM, but may contribute to the pathogenesis of diabetes-specific complications, including diabetic neuropathy, nephropathy, and retinopathy. Therefore, therapeutic modalities aimed at restoring the GH-IGF-IGFBP axis are being considered. Herein, we review the efficacy of one such therapy, specifically IGF-I replacement therapy. To date, short-term beneficial metabolic effects of recombinant human IGF (rhIGF)-I therapy have been demonstrated in numerous diabetic conditions, including type 1 DM, type 2 DM, and type A insulin resistance. However, the long- term safety and metabolic efficacy of rhIGF-I therapy remains to be established. Moreover, the potential impact of rhIGF-I on the natural history of diabetic complications has yet to be explored.

Oppositely imprinted genes p57(Kip2) and igf2 interact in a mouse model for Beckwith-Wiedemann syndrome

Beckwith-Wiedemann syndrome (BWS) is a clinically variable disorder characterized by somatic overgrowth, macroglossia, abdominal wall defects, visceromegaly, and an increased susceptibility to childhood tumors. The disease has been linked to a large cluster of imprinted genes at human chromosome 11p15.5. A subset of BWS patients has been identified with loss-of-function mutations in p57(KIP2), a maternally expressed gene encoding a G(1) cyclin-dependent kinase inhibitor. Some patients display loss of imprinting of IGF2, a fetal-specific growth factor that is paternally expressed. To understand how the same disease can result from misregulation of two linked, but unrelated, genes, we generated a mouse model for BWS that both harbors a null mutation in p57(Kip2) and displays loss of Igf2 imprinting. These mice display many of the characteristics of BWS, including placentomegaly and dysplasia, kidney dysplasia, macroglossia, cleft palate, omphalocele, and polydactyly. Some, but not all, of the phenotypes are shown to be Igf2 dependent. In two affected tissues, the two imprinted genes appear to act in an antagonistic manner, a finding that may help explain how BWS can arise from mutations in either gene.

A human p57(KIP2) transgene is not activated by passage through the maternal mouse germline

Genomic imprinting results in expression of some autosomal genes from one parental allele only. Human chromosome 11p15, and the syntenic region on mouse distal chromosome 7, contain several imprinted genes, including p57 (KIP2) ( CDKN1C ) and IGF2. These two genes, which are separated by >700 kb, are both implicated in the pathogenesis of Beckwith-Wiedemann syndrome. We have shown previously that an Igf2/H19 transgene is expressed appropriately and can imprint at ectopic chromosomal locations. To investigate the p57 (KIP2) region, we similarly tested the imprinting and function of a 38 kb human genomic fragment containing the p57 (KIP2) gene in transgenic mice. This transgene showed appropriate tissue-specific expression and transgene copy number-dependent expression at ectopic sites. However, the levels of expression are reminiscent of that found for the paternal allele in humans (10%). There was no change in expression levels when the transgene was inherited from the maternal germline. These results suggest that the cis -elements required for enhanced expression of the maternally inherited p57 (KIP2) allele lie at a distance from the gene. This finding has important implications for the role of this gene in the human disease, in particular with respect to the translocation breakpoints identified in some patients.

Disruption of imprinted genes at chromosome region 11p15.5 in paediatric rhabdomyosarcoma

Rhabdomyosarcomas are characterized by loss of heterozygosity (LOH) at chromosome region 11p15.5, a region known to contain several imprinted genes including insulin-like growth factor 2 (IGF2), H19, and p57(KIP2). We analyzed 48 primary tumour samples and found distinct genetic changes at 11p15.5 in alveolar and embryonal histological subtypes. LOH was a feature of embryonal tumours, but at a lower frequency than previous studies. Loss of imprinting (LOI) of the IGF2 gene was detected in 6 of 13 informative cases, all harbouring PAX3-FKHR or PAX7-FKHR fusion genes characteristic of alveolar histology. In contrast, H19 imprinting was maintained in 14 of 15 informative cases and the case with H19 LOI had maintenance of the IGF2 imprint indicating separate mechanisms controlling imprinting of IGF2 and H19. The adult promoter of IGF2, P1, was used in 5 of 14 tumours and its expression was unrelated to IGF2 imprinting status implying a further mechanism of altered IGF2 regulation. The putative tumour suppressor gene p57(KIP2) was expressed in 15 of 29 tumours and expression was unrelated to allele status. Moreover, in tumours with p57(KIP2) expression, there was no evidence for inactivating mutations, suggesting that p57(KIP2) is not a tumour suppressor in rhabdomyosarcoma.

The undermethylated state of a CpG island region in igf2 transgenes is dependent on the H19 enhancers

CpG islands are GC-rich regions located in the promoter regions of housekeeping genes and many tissue-specific genes. While most CpG islands are normally unmethylated, island methylation can occur and is associated with silencing of the corresponding gene. Experiments with transgenic mice and DNA transfection in pluripotential embryonic cells have led to the conclusion that the information required for protecting the islands from methylation is contained within the CpG islands themselves and have identified Sp1 binding sites as an important element in establishing and/or maintaining the methylation-free state of CpG islands. To examine the generality of these observations, we analyzed the methylation of one of the mouse Igf2 CpG islands and its flanks in transgenic mice. We observed that the undermethylated state of this region is dependent on the presence of a separate cis-regulatory element, the H19 enhancers. These tissue-specific enhancers had a ubiquitous, non-tissue-specific effect on island region methylation. Structural alterations outside of the island and these enhancers also affected this region's methylation. These findings indicate that the methylation of some CpG island-containing regions is more sensitive than previously believed to the activity of distant cis-regulatory elements and to structural alterations in nonisland sequences in cis.

Cloning of breakpoints in and downstream the IGF2 gene that are associated with overexpression of IGF2 transcripts in colorectal tumours

The human IGF2 gene belongs to a group of imprinted genes clustered on the short arm of chromosome 11, band p15.5. It contains 9 exons and spans over 30 kb. IGF2 mRNA overexpression has been reported in human tumours and in some inherited growth disorders. It was recently demonstrated that IGF2 mRNA overexpression contributes to tumour progression and that loss of parental imprinting as well as altered transcription factors are contributing to this overexpression. We have reported structural alterations in the 3' region of the IGF2 gene in two colorectal tumours that overexpressed the IGF2 transcript by 200- and 800-fold. We cloned by the vectorette-PCR strategy, genomic DNA fragments containing the breakpoints from these tumours. The sequencing of these fragments positioned the breakpoint 2 kb downstream the IGF2 gene in one tumour, and in exon 9 in the second. Both breakpoints occurred in regions containing repetitive elements: a TGGA repeat we have identified downstream the gene, and the (CA)n repetition in exon 9. We hypothesize that a negative regulatory element, located downstream the IGF2 gene, has been deleted following these structural alterations and leads to IGF2 gene overexpression.

Glypican-3-deficient mice exhibit developmental overgrowth and some of the abnormalities typical of Simpson-Golabi-Behmel syndrome

Glypicans are a family of heparan sulfate proteoglycans that are linked to the cell surface through a glycosyl-phosphatidylinositol anchor. One member of this family, glypican-3 (Gpc3), is mutated in patients with the Simpson-Golabi-Behmel syndrome (SGBS). These patients display pre- and postnatal overgrowth, and a varying range of dysmorphisms. The clinical features of SGBS are very similar to the more extensively studied Beckwith-Wiedemann syndrome (BWS). Since BWS has been associated with biallelic expression of insulin-like growth factor II (IGF-II), it has been proposed that GPC3 is a negative regulator of IGF-II. However, there is still no biochemical evidence indicating that GPC3 plays such a role.Here, we report that GPC3-deficient mice exhibit several of the clinical features observed in SGBS patients, including developmental overgrowth, perinatal death, cystic and dyplastic kidneys, and abnormal lung development. A proportion of the mutant mice also display mandibular hypoplasia and an imperforate vagina. In the particular case of the kidney, we demonstrate that there is an early and persistent developmental abnormality of the ureteric bud/collecting system due to increased proliferation of cells in this tissue element. The degree of developmental overgrowth of the GPC3-deficient mice is similar to that of mice deficient in IGF receptor type 2 (IGF2R), a well characterized negative regulator of IGF-II. Unlike the IGF2R-deficient mice, however, the levels of IGF-II in GPC3 knockouts are similar to those of the normal littermates.

Analysis of germline CDKN1C (p57KIP2) mutations in familial and sporadic Beckwith-Wiedemann syndrome (BWS) provides a novel genotype-phenotype correlation

Beckwith-Wiedemann syndrome (BWS) is a human imprinting disorder with a variable phenotype. The major features are anterior abdominal wall defects including exomphalos (omphalocele), pre- and postnatal overgrowth, and macroglossia. Additional less frequent complications include specific developmental defects and a predisposition to embryonal tumours. BWS is genetically heterogeneous and epigenetic changes in the IGF2/H19 genes resulting in overexpression of IGF2 have been implicated in many cases. Recently germline mutations in the cyclin dependent kinase inhibitor gene CDKN1C (p57KIP2) have been reported in a variable minority of BWS patients. We have investigated a large series of familial and sporadic BWS patients for evidence of CDKN1C mutations by direct gene sequencing. A total of 70 patients with classical BWS were investigated; 54 were sporadic with no evidence of UPD and 16 were familial from seven kindreds. Novel germline CDKN1C mutations were identified in five probands, 3/7 (43%) familial cases and 2/54 (4%) sporadic cases. There was no association between germline CDKN1C mutations and IGF2 or H19 epigenotype abnormalities. The clinical phenotype of 13 BWS patients with germline CDKN1C mutations was compared to that of BWS patients with other defined types of molecular pathology. This showed a significantly higher frequency of exomphalos in the CDKN1C mutation cases (11/13) than in patients with an imprinting centre defect (associated with biallelic IGF2 expression and H19 silencing) (0/5, p<0.005) or patients with uniparental disomy (0/9, p<0.005). However, there was no association between germline CDKN1C mutations and risk of embryonal tumours. No CDKN1C mutations were identified in six non-BWS patients with overgrowth and Wilms tumour. These findings (1) show that germline CDKN1C mutations are a frequent cause of familial but not sporadic BWS, (2) suggest that CDKN1C mutations probably cause BWS independently of changes in IGF2/H19 imprinting, (3) provide evidence that aspects of the BWS phenotype may be correlated with the involvement of specific imprinted genes, and (4) link genotype-phenotype relationships in BWS and the results of murine experimental models of BWS.

Overexpression of insulin-like growth factor-II in transgenic mice is associated with pancreatic islet cell hyperplasia

We have used an insulin-like growth factor (IGF)-II transgenic mouse model in which mouse IGF-II is widely overexpressed, resulting in increased fetal size and selective organ overgrowth, to investigate the effects on the development of the endocrine pancreas. Fetuses examined on day 19.5-20 of gestation had significantly elevated circulating levels of IGF-II, compared with control mice. The pancreatic islets in transgenic animals were of irregular shape and had a mean area five times greater than in controls, whereas the mean number of islets per tissue section was not altered. The size of individual endocrine cells was not altered. Although the islets in animals expressing the IGF-II transgene were considerably larger, immunohistochemistry for insulin and glucagon showed that the relative proportion of beta-cells was significantly less, and that of alpha-cells was higher. Normal islet morphology was disrupted, with alpha-cells appearing in small groups within the islets, as well as on the periphery, whereas beta-cells were often seen at the edge of the islets. Twice as many islet cells (21.9% vs. 11.4%) were involved in cell replication, detected by the presence of immunoreactive proliferating cell nuclear antigen, in pancreata from transgenic mice vs. controls, whereas the number of cells undergoing apoptosis was significantly reduced. Abundant IGF-II messenger RNAwas found within the islets of transgenic animals by in situ hybridization, and the relative area of islets demonstrating immunoreactive IGF-II was significantly greater. Immunoreactive IGF-I was much less abundant and was further reduced in islets of transgenic animals. The area of islets immunopositive for IGF binding protein-2 was unaltered. Despite the presence of islet hyperplasia, circulating insulin levels and serum glucose levels were not significantly different between transgenic and control mice. These results show that an overexpression of IGF-II in fetal life has a profound effect on islet morphology and causes islet hyperplasia while reducing the attrition of islet cells by apoptosis.

Loss of imprinting of a paternally expressed transcript, with antisense orientation to KVLQT1, occurs frequently in Beckwith-Wiedemann syndrome and is independent of insulin-like growth factor II imprinting

Genomic imprinting plays a fundamental role in cancer and some hereditary diseases, including Beckwith-Wiedemann syndrome (BWS), a disorder of prenatal overgrowth and predisposition to embryonal malignancies such as Wilms tumor. We have previously shown that the KVLQT1 gene on chromosomal band 11p15 is imprinted, with expression of the maternal allele, and that the maternal allele is disrupted in rare BWS patients with balanced germ-line chromosomal rearrangements. We now show that an antisense orientation transcript within KVLQT1, termed LIT1 (long QT intronic transcript 1) is expressed normally from the paternal allele, from which KVLQT1 transcription is silent, and that in the majority of patients with BWS, LIT1 is abnormally expressed from both the paternal and maternal alleles. Eight of sixteen informative BWS patients (50%) showed biallelic expression, i.e., loss of imprinting (LOI) of LIT1. Similarly, 21 of 36 (58%) BWS patients showed loss of maternal allele-specific methylation of a CpG island upstream of LIT1. Surprisingly, LOI of LIT1 was not linked to LOI of insulin-like growth factor II (IGF2), which was found in 2 of 10 (20%) BWS patients, even though LOI of IGF2 occurs frequently in Wilms and other tumors, and in some patients with BWS. Thus, LOI of LIT1 is the most common genetic alteration in BWS. We propose that 11p15 harbors two imprinted gene domains-a more centromeric domain including KVLQT1 and p57(KIP2), alterations in which are more common in BWS, and a more telomeric domain including IGF2, alterations in which are more common in cancer.

Genomic imprinting and cancer

Although we inherit two copies of all genes, except those that reside on the sex chromosomes, there is a subset of these genes in which only the paternal or maternal copy is functional. This phenomenon of monoallelic, parent-of-origin expression of genes is termed genomic imprinting. Imprinted genes are normally involved in embryonic growth and behavioral development, but occasionally they also function inappropriately as oncogenes and tumor suppressor genes. The evidence that imprinted genes play a role in carcinogenesis will be discussed in this review. Additional information about imprinted genes can be found on the Genomic Imprinting Website at: (http://www.geneimprint.com).

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.

Genomic imprinting: implications for human disease

Genomic imprinting refers to an epigenetic marking of genes that results in monoallelic expression. This parent-of-origin dependent phenomenon is a notable exception to the laws of Mendelian genetics. Imprinted genes are intricately involved in fetal and behavioral development. Consequently, abnormal expression of these genes results in numerous human genetic disorders including carcinogenesis. This paper reviews genomic imprinting and its role in human disease. Additional information about imprinted genes can be found on the Genomic Imprinting Website at http://www.geneimprint.com.

Proinsulin-like growth factor-II overexpression does not alter monoallelic H19 gene expression in transfected human embryonic kidney fibroblasts

Insulin-like growth factor-II (IGF-II) is a potent mitogen for cells in culture. The H19 gene is a developmentally regulated gene with putative tumor suppressor activity, and loss of H19 expression may be involved in tumorigenesis. The H19 gene is closely linked to the human IGF-II gene (IGF2) on chromosome 11p15.5 and these genes are reciprocally imprinted in most fetal tissues. H19 is expressed only from the maternal and IGF2 from the paternal chromosome. We have asked whether overexpression of proIGF-II alters H19 imprinting status and/or expression. Human embryonal kidney fibroblasts (293 cells) were stably transfected with a PCMV5 vector containing the full length human IGF-II cDNA or a control cDNA. Transfectant clones expressed large quantities of IGF-II mRNA and secrete 1-5 ug/ml and 150-230 ng/ml proIGF-II within 24 hours of serum-free culture (transfectant 293-9 and -11 respectively) (1). Cells were genotyped at the exon 5, RsaI restriction fragment length polymorphism (RFLP) and found to be informative (+/-). H19 expression was monoallelic (+) indicating preservation of H19 imprinting in all cell lines. Using quantitative RT-PCR with internal competitors for H19 and for IGF-II cDNA, overexpression of IGF2 in 293-11 and 293-9 cells was confirmed. In contrast, no significant difference with respect to H19 expression was detected between the overexpressing cells and control lines. In conclusion, (1) human embryonal fibroblasts express the H19 gene. (2) H19 imprinting is preserved in these cells. (3) proIGF-II overexpression does not alter H19 expression.

Mechanisms of in utero overgrowth

Determination of the mechanisms that lead to in utero overgrowth has proved elusive. Recently, however, our knowledge has significantly expanded as a result of the generation of experimental mouse models, engineered to disrupt the expression of one or more genes (knockout mice), and by detailed molecular and genetic analyses of infants and children with overgrowth syndromes. Studies of knockout mice have largely defined the essential roles of the insulin-like growth factors (IGF-I and IGF-II), insulin and their receptors in embryonic and fetal growth, and have provided compelling evidence that increased IGF-II gene expression and/or abundance can stimulate excessive fetal somatic growth. The IGF-II gene is usually expressed only by the paternally derived allele; however, when this imprinting is erased and IGF-II expression is biallelic, fetal overgrowth ensues. Such increased IGF-II expression would appear to explain the overgrowth in Beckwith-Wiedemann syndrome. Using the information gathered from knockout mice as a guide to human studies, detailed genetic investigations are likely to unravel the mechanisms behind other human overgrowth syndromes.

A family of insulin-like growth factor II mRNA-binding proteins represses translation in late development

Insulin-like growth factor II (IGF-II) is a major fetal growth factor. The IGF-II gene generates multiple mRNAs with different 5' untranslated regions (5' UTRs) that are translated in a differential manner during development. We have identified a human family of three IGF-II mRNA-binding proteins (IMPs) that exhibit multiple attachments to the 5' UTR from the translationally regulated IGF-II leader 3 mRNA but are unable to bind to the 5' UTR from the constitutively translated IGF-II leader 4 mRNA. IMPs contain the unique combination of two RNA recognition motifs and four hnRNP K homology domains and are homologous to the Xenopus Vera and chicken zipcode-binding proteins. IMP localizes to subcytoplasmic domains in a growth-dependent and cell-specific manner and causes a dose-dependent translational repression of IGF-II leader 3 -luciferase mRNA. Mouse IMPs are produced in a burst at embryonic day 12.5 followed by a decline towards birth, and, similar to IGF-II, IMPs are especially expressed in developing epithelia, muscle, and placenta in both mouse and human embryos. The results imply that cytoplasmic 5' UTR-binding proteins control IGF-II biosynthesis during late mammalian development.

Glypican 3 and glypican 4 are juxtaposed in Xq26.1

Recently, we have shown that mutations in the X-linked glypican 3 (GPC3) gene cause the Simpson-Golabi-Behmel overgrowth syndrome (SGBS; ). The next centromeric gene detected is another glypican, glypican 4 (GPC4), with its 5' end 120763bp downstream of the 3' terminus of GPC3. One recovered GPC4 cDNA with an open reading frame of 1668nt encodes a putative protein containing three heparan sulfate glycosylation signals and the 14 signature cysteines of the glypican family. This protein is 94.3% identical to mouse GPC4 and 26% identical to human GPC3. In contrast to GPC3, which produces a single transcript of 2.3kb and is stringently restricted in expression to predominantly mesoderm-derived tissues, Northern analyses show that GPC4 produces two transcripts, 3.4 and 4.6kb, which are very widely expressed (though at a much higher level in fetal lung and kidney). Interestingly, of 20 SGBS patients who showed deletions in GPC3, one was also deleted for part of GPC4. Thus, GPC4 is not required for human viability, even in the absence of GPC3. This patient shows a complex phenotype, including the unusual feature of hydrocephalus; but because an uncle with SGBS is less affected, it remains unclear whether the GPC4 deletion itself contributes to the phenotype.

Deletion of the H19 differentially methylated domain results in loss of imprinted expression of H19 and Igf2

Differentially methylated sequences associated with imprinted genes are proposed to control genomic imprinting. A 2-kb region located 5' to the imprinted mouse H19 gene is hypermethylated on the inactive paternal allele throughout development. To determine whether this differentially methylated domain (DMD) is required for imprinted expression at the endogenous locus, we have generated mice harboring a 1.6-kb targeted deletion of the DMD and assayed for allelic expression of H19 and the linked, oppositely imprinted Igf2 gene. H19 is activated and Igf2 expression is reduced when the DMD deletion is paternally inherited; conversely, upon maternal transmission of the mutation, H19 expression is reduced and Igf2 is activated. Consistent with the DMD's hypothesized role of setting up the methylation imprint, the mutation also perturbs allele-specific methylation of the remaining H19 sequences. In conclusion, these experiments show that the H19 hypermethylated 5' flanking sequences are required to silence paternally derived H19. Additionally, these experiments demonstrate a novel role for the DMD on the maternal chromosome where it is required for the maximal expression of H19 and the silencing of Igf2. Thus, the H19 differentially methylated sequences are required for both H19 and Igf2 imprinting.

Do we understand the evolution of genomic imprinting?

The conflict theory is the only hypothesis to have attracted any critical attention for the evolution of genomic imprinting. Although the earliest data appeared supportive, recent systematic analyses have not confirmed the model's predictions. The status of theory remains undecided, however, as post-hoc explanations can be provided as to why these predictions are not borne out.

Cip/Kip cyclin-dependent kinase inhibitors: brakes of the cell cycle engine during development

Precise control of cell-cycle progression is believed to be critical for normal development, while oncogenesis may be a direct result of its disturbance. Cell-cycle progression is regulated predominantly by a series of serine/threonine kinases, the cyclin-dependent kinases (CDKs). The activities of the CDKs are controlled by a variety of mechanisms, and a group of molecules that inhibit CDK activity, CDK inhibitors (CKIs), has recently become the focus of interest, particularly in the fields of development and tumorigenesis. To date, seven CKIs have been identified in mammals and categorized into two families, the Cip/Kip and Ink4 families. The Cip/Kip family is well conserved phylogenetically, suggesting that it is biologically important. Despite the structural and biochemical similarities among the Cip/Kip members, the phenotypes of knockout mice of each Cip/Kip member are surprisingly different, which suggests that the Cip/Kip CKIs have a variety of physiological functions. In this review, the biological roles of Cip/Kip CKIs in development and tumor suppression are discussed.

Seven cases of Wiedmann-Beckwith syndrome, including the first reported case of mosaic paternal isodisomy along the whole chromosome 11

Genomic imprinting of chromosome arm 11p is involved in the Wiedemann-Beckwith syndrome (WBS). About 20% of patients with sporadic WBS have paternal uniparental disomy (UPD) of 11p. Mitotic recombination at the 11p region has been suggested to be responsible for the somatic mosaicism in these patients. Our current study concerning sporadic WBS patients demonstrated six patients with mosaic isodisomy restricted to part of 11p and one patient with mosaic paternal uniparental disomy for the whole chromosome 11. Apparently the clinical findings for this patient did not differ from data reported for other WBS patients. This case makes it unlikely that the proximal short arm and the long arm of chromosome 11 contain imprinted genes with a phenotype recognizable prenatally or in infancy, and gives some support to the hypothesis that non-mosaic UPD-11 is prenatally lethal.

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.

Insulin-like growth factor II (IGF-II)

Insulin-like growth factor II (IGF-II) plays a key role in mammalian growth, influencing foetal cell division and differentiation and possibly metabolic regulation. The mature 67 amino acid peptide shares sequence homology with both insulin and IGF-I. The liver is the main endocrine source of IGFs, but autocrine/paracrine activity is found in most tissues. The type 1 receptor mediates most of the biological effects of IGF-I and IGF-II; the type 2 receptor is involved with IGF-II degradation. Binding proteins may both localise IGFs to the receptors and regulate their activities. The IGF2 gene is maternally imprinted in mouse and human. Relaxation of IGF2 imprinting occurs in the Beckwith-Wiedemann syndrome of somatic overgrowth, sporadic Wilms' tumour and a number of other cancers. In the general adult population, the IGF2-INS gene cluster may also influence body weight, in which case IGF-II function could become a target for therapeutic intervention in obesity.

The INS 5' variable number of tandem repeats is associated with IGF2 expression in humans

The minisatellite DNA polymorphism consisting of a variable number of tandem repeats (VNTR) at the human INS (insulin gene) 5'-flanking region has demonstrated allelic effects on insulin gene transcription in vitro and has been associated with the level of insulin gene expression in vivo. We now show that this VNTR also has effects on the nearby insulin-like growth factor II gene (IGF2) in human placenta in vivo and in the HepG2 hepatoma cell line in vitro. We show that higher steady-state IGF2 mRNA levels are associated with shorter alleles (class I) than the longer class III alleles in term placentae. In vitro, reporter gene activity was greater from reporter gene constructs with IGF2 promoter 3 in the presence of class I alleles than from those with class III. Taken together with the documented transcriptional effects on the insulin gene, we propose that the VNTR may act as a long range control element affecting the expression of both INS and IGF2. The localization of a type 1 diabetes susceptibility locus (IDDM2) to the VNTR itself suggests that either or both of these genes may be involved in the biologic effects of IDDM2.

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).

Animal models for dysmorphology

Areas where studies of animal models have been vital to the understanding of human malformation syndromes include the dissection of complex genetic mechanisms such as imprinting, the creation of animal models of known human conditions, and the provision of further clues involving gene families and pathways.

PD-loop: a complex of duplex DNA with an oligonucleotide

A stable complex between duplex DNA and an oligonucleotide is assembled with the aid of a DNA synthetic mimic, peptide nucleic acid (PNA). Homopyrimidine PNAs are known to invade into short homopurine tracts in duplex DNA forming P-loops. We have found that P-loops, formed at two closely located purine tracts in the same DNA strand separated by a mixed purine-pyrimidine sequence, merge and open the double helix between them. The opposite DNA strand, which is not bound with PNA, exposes and becomes accessible for complexing with an oligonucleotide via Watson-Crick pairing. As a result, the PD-loop emerges, which consists of locally open duplex DNA, PNA "openers," and an oligonucleotide. The PD-loop stability and sequence specificity are demonstrated by affinity capture of duplex DNAs by using biotinylated oligonucleotides and streptavidin-covered magnetic beads. The type of complex formed by PNAs, an oligonucleotide and duplex DNA we describe, opens ways for development of various in vitro and in situ hybridization techniques with duplex DNA and may find applications in DNA nanotechnology and genomics.

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.

Imprinting mechanisms in mammals

Imprinting is a genetic mechanism that determines expression or repression of genes according to their parental origin. Some imprinted genes occur in clusters in the genome. Recent work using transgenic mice shows that multiple cis-acting sequences are needed for correct imprinting. Mutation analysis in a normal chromosomal context reveals the importance of imprinting centres for regional establishment or maintenance of imprinting in a cluster. Elements that contribute to the function of imprinting centres and regional propagation of the imprints are CpG-rich differentially methylated regions (that during development retain germline imposed methylation or demethylation), direct repeat clusters, and unusual RNAs (antisense, non-translated etc.). The interaction of these cis elements with transacting factors such as methylase and chromatin factors establishes a hierarchical control system with local and regional effects.