Biparental expression of IGFBP1 and IGFBP3 renders their involvement in the etiology of Silver-Russell syndrome unlikely.

Maternal uniparental disomy (UPD) of chromosome 7 has recently been reported in about 10% of Silver-Russell (SRS) patients. It can therefore be concluded that at least one gene on chromosome 7 is imprinted and mutations in this gene/these genes might contribute to the phenotype of the disease. Two genes which are involved in growth and localised in 7p12-13 are the insulin-like growth factor binding proteins 1 and 3 (IGFBP1; IGFBP3). Comparison to the mouse genome shows that the syntenic region on mouse chromosome 11 is imprinted, UPD of this region leads to deviations in growth in mice. In the present study we investigated whether the genes for IGFBP1 and IGFBP3 might be involved in the etiology of SRS: after exclusion of SRS specific mutations we could demonstrate biparental expression of both genes in lymphocytes of an SRS patient without UPD7 as well as expression in a patient with maternal UPD7. Our results as well as those from other groups show biparental expression of IGFBP1 in fetal tissues and expression of IGFBP3 in nearly every tissue during puberty and adult life. Thus, no evidence is given for an involvement of the two genes in SRS.     

Duplication of 7p12.1-p13, including GRB10 and IGFBP1, in a mother and daughter with features of Silver-Russell syndrome

Silver-Russell syndrome (SRS) has been associated with maternal uniparental disomy (UPD) of chromosome 7 in approximately 10% of cases, suggesting that at least one imprinted gene on chromosome 7 is involved in the pathogenesis of the disease. We report a proximal 7p interstitial inverted duplication in a mother and daughter both of whom have features of SRS, including marked short stature, low birth weight, facial asymmetry and 5th finger clinodactyly. Fluorescence in situ hybridisation (FISH) with YAC probes enabled delineation of the duplicated region to 7p12.1-p13. This region of proximal chromosome 7 is known to be homologous to an imprinted region in the mouse chromosome 11 and contains the growth-related genes GRB10 (growth factor receptor-bound protein 10), EGFR (epidermal growth factor receptor) and IGFBP1 (insulin-like growth factor binding protein 1), all of which have been suggested as candidate genes for SRS. Molecular analysis showed that the duplication in both mother and daughter spanned a distance of approximately 10 cM and included GRB10 and IGFBP1 but not EGFR. The de novo duplication in the proband's mother was shown to be of paternal origin. In order to test the hypothesis that sub-microscopic duplications of 7p, whether maternal or paternal in origin, are responsible for at least some cases of SRS, we screened a further eight patients referred to our laboratory for SRS. None were found to have duplications of either GRB10 or IGFBP1. The hypothesis that sub-microscopic duplications including GRB10 and IGFBP1 is a cause of SRS remains a possibility and warrants further investigation. Importantly, in contrast to current thinking, our results suggest that imprinted genes may not underlie the SRS phenotype, and we propose an alternative hypothesis to explain the occurrence of maternal UPD 7 seen in some cases of SRS.     

The novel imprinted carboxypeptidase A4 gene ( CPA4) in the 7q32 imprinting domain

By a search for novel human imprinted genes in the vicinity of the imprinted gene MEST, at chromosome 7q32, we identified the carboxypeptidase A4 gene ( CPA4) in a gene cluster of the carboxypeptidase family, 200 kb centromeric to MEST. Because CPA4 was originally identified as a protein induced in a prostate cancer cell line (PC-3) by histone deacetylase inhibitors, and was located at the putative prostate cancer-aggressiveness locus at 7q32, we investigated its imprinting status in fetal tissues and in adult benign hypertrophic prostate (BPH). RT-PCR using four intragenic polymorphisms as markers showed that CPA4 was expressed preferentially from the maternal allele in the fetal heart, lung, liver, intestine, kidney, adrenal gland, and spleen, but not in the fetal brain. It was also preferentially expressed in the BPH. These findings support that CPA4 is imprinted and may become a strong candidate gene for prostate cancer-aggressiveness. As a Silver-Russell syndrome (SRS) locus has been proposed to be located to a region near MEST and to be involved in imprinting, CPA4 would have been a candidate gene for SRS. However, analysis of ten SRS patients revealed no mutations in CPA4.     

Duplication of 7p11.2-p13, including GRB10, in Silver-Russell syndrome

Silver-Russell syndrome (SRS) is characterized by pre- and postnatal growth failure and other dysmorphic features. The syndrome is genetically heterogeneous, but maternal uniparental disomy of chromosome 7 has been demonstrated in approximately 7% of cases. This suggests that at least one gene on chromosome 7 is imprinted and involved in the pathogenesis of SRS. We have identified a de novo duplication of 7p11.2-p13 in a proband with features characteristic of SRS. FISH confirmed the presence of a tandem duplication encompassing the genes for growth factor receptor-binding protein 10 (GRB10) and insulin-like growth factor-binding proteins 1 and 3 (IGFBP1 and -3) but not that for epidermal growth factor-receptor (EGFR). Microsatellite markers showed that the duplication was of maternal origin. These findings provide the first evidence that SRS may result from overexpression of a maternally expressed imprinted gene, rather than from absent expression of a paternally expressed gene. GRB10 lies within the duplicated region and is a strong candidate, since it is a known growth suppressor. Furthermore, the mouse homologue (Grb10/Meg1) is reported to be maternally expressed and maps to the imprinted region of proximal mouse chromosome 11 that demonstrates prenatal growth failure when it is maternally disomic. We have demonstrated that the GRB10 genomic interval replicates asynchronously in human lymphocytes, suggestive of imprinting. An additional 36 SRS probands were investigated for duplication of GRB10, but none were found. However, it remains possible that GRB10 and/or other genes within 7p11.2-p13 are responsible for some cases of SRS.     

Chromosome 7p disruptions in Silver Russell syndrome: delineating an imprinted candidate gene region

Silver-Russell syndrome (SRS) is characterised by pre- and postnatal growth restriction (PNGR) and additional dysmorphic features including body asymmetry and fifth finger clinodactyly. The syndrome is genetically heterogeneous, with a number of chromosomes implicated. However, maternal uniparental disomy for chromosome 7 has been demonstrated in up to 10% of all cases. Three SRS probands have previously been described with a maternally inherited duplication of 7p11.2-p13, defining this as a candidate region. Over-expression of a maternally transcribed, imprinted gene with growth-suppressing activity located within the duplicated region, or breakpoint disruption of genes or regulatory sequences, may account for the phenotype in these cases. Here we describe two additional SRS patients and four probands with PNGR with a range of cytogenetic disruptions of 7p, including duplications, pericentric inversions and a translocation. An incomplete contig consisting of 80 PACs and BACs from the centromere to 7p14 was constructed. Individual clones from this contig were used as FISH probes to map the breakpoints in the six new cases and the three duplication probands previously described. Our data provide further evidence for a candidate SRS region at 7p11.1-p14. A common breakpoint region was identified within 7p11.2 in all nine cases, pinpointing this specific interval. The imprinting status of genes within the 7p11.1-p14 region flanked by the most extreme breakpoints have been analysed using both somatic cell hybrids containing a single full-length maternally or paternally derived chromosome 7 and expressed single nucleotide polymorphisms in paired fetal and maternal samples.     

No Evidence for Mutations of CTCFL/BORIS in Silver-Russell Syndrome Patients with IGF2/H19 Imprinting Control Region 1 Hypomethylation

Background

Silver-Russell syndrome (SRS) is a genetically and clinically heterogeneous disease. Although no protein coding gene defects have been reported in SRS patients, approximately 50% of SRS patients carry epimutations (hypomethylation) at the IGF2/H19 imprinting control region 1 (ICR1). Proper methylation at ICR1 is crucial for the imprinted expression of IGF2, a fetal growth factor. CTCFL, a testis-specific protein, has recently been proposed to play a role in the establishment of DNA methylation at the murine equivalent of ICR1. A screen was undertaken to assess whether CTCFL is mutated in SRS patients with hypomethylation, to explore a link between the observed epimutations and a genetic cause of the disease.

Methodology/Principal Findings

DNA was obtained from 36 SRS patients with hypomethylation at ICR1. All CTCFL coding exons were sequenced and analyzed for duplications/deletions using both multiplex ligation-dependent probe amplification, with a custom CTCFL probe set, and genomic qPCR. Novel SNP alleles were analyzed for potential differential splicing in vitro utilizing a splicing assay. Neither mutations of CTCFL nor duplications/deletions were observed. Five novel SNPs were identified and have been submitted to dbSNP. In silico splice prediction suggested one novel SNP, IVS2-66A>C, activated a cryptic splice site, resulting in aberrant splicing and premature termination. In vitro splicing assays did not confirm predicted aberrant splicing.

Conclusions/Significance

As no mutations were detected at CTCFL in the patients examined, we conclude that genetic alterations of CTCFL are not responsible for the SRS hypomethylation. We suggest that analysis of other genes involved in the establishment of DNA methylation at imprinted genes, such as DNMT3A and DNMT3L, may provide insight into the genetic cause of hypomethylation in SRS patients.     

Differentially methylated regions in maternal and paternal uniparental disomy for chromosome 7

DNA methylation is a hallmark of genomic imprinting and differentially methylated regions (DMRs) are found near and in imprinted genes. Imprinted genes are expressed only from the maternal or paternal allele and their normal balance can be disrupted by uniparental disomy (UPD), the inheritance of both chromosomes of a chromosome pair exclusively from only either the mother or the father. Maternal UPD for chromosome 7 (matUPD7) results in Silver-Russell syndrome (SRS) with typical features and growth retardation, but no gene has been conclusively implicated in SRS. In order to identify novel DMRs and putative imprinted genes on chromosome 7, we analyzed eight matUPD7 patients, a segmental matUPD7q31-qter, a rare patUPD7 case and ten controls on the Infinium HumanMethylation450K BeadChip with 30?017 CpG methylation probes for chromosome 7. Genome-scale analysis showed highly significant clustering of DMRs only on chromosome 7, including the known imprinted loci GRB10, SGCE/PEG10, and PEG/MEST. We found ten novel DMRs on chromosome 7, two DMRs for the predicted imprinted genes HOXA4 and GLI3 and one for the disputed imprinted gene PON1. Quantitative RT-PCR on blood RNA samples comparing matUPD7, patUPD7, and controls showed differential expression for three genes with novel DMRs, HOXA4, GLI3, and SVOPL. Allele specific expression analysis confirmed maternal only expression of SVOPL and imprinting of HOXA4 was supported by monoallelic expression. These results present the first comprehensive map of parent-of-origin specific DMRs on human chromosome 7, suggesting many new imprinted sites.     

Differentially methylated regions in maternal and paternal uniparental disomy for chromosome 7

DNA methylation is a hallmark of genomic imprinting and differentially methylated regions (DMRs) are found near and in imprinted genes. Imprinted genes are expressed only from the maternal or paternal allele and their normal balance can be disrupted by uniparental disomy (UPD), the inheritance of both chromosomes of a chromosome pair exclusively from only either the mother or the father. Maternal UPD for chromosome 7 (matUPD7) results in Silver-Russell syndrome (SRS) with typical features and growth retardation, but no gene has been conclusively implicated in SRS. In order to identify novel DMRs and putative imprinted genes on chromosome 7, we analyzed eight matUPD7 patients, a segmental matUPD7q31-qter, a rare patUPD7 case and ten controls on the Infinium HumanMethylation450K BeadChip with 30 017 CpG methylation probes for chromosome 7. Genome-scale analysis showed highly significant clustering of DMRs only on chromosome 7, including the known imprinted loci GRB10, SGCE/PEG10, and PEG/MEST. We found ten novel DMRs on chromosome 7, two DMRs for the predicted imprinted genes HOXA4 and GLI3 and one for the disputed imprinted gene PON1. Quantitative RT-PCR on blood RNA samples comparing matUPD7, patUPD7, and controls showed differential expression for three genes with novel DMRs, HOXA4, GLI3, and SVOPL. Allele specific expression analysis confirmed maternal only expression of SVOPL and imprinting of HOXA4 was supported by monoallelic expression. These results present the first comprehensive map of parent-of-origin specific DMRs on human chromosome 7, suggesting many new imprinted sites.     

Characterization of genomic variants in CSH1 and GH2, two candidate genes for Silver-Russell syndrome in 17q24-q25

Silver-Russell syndrome (SRS) is a syndrome of severe pre- and postnatal growth retardation and typical dysmorphic features. Rare chromosomal aberrations have been reported in SRS; among these are two balanced translocations involving 17q24-q25. Recently, we described a patient with a paternally inherited heterozygous deletion of the chorionic somatomammotropin hormone 1 (CSH1) gene. The CSH1 gene is member of the growth hormone (GH) gene cluster on 17q, which consists of two growth hormone genes and three CSH genes. Genomic alterations in the GH cluster are well known, causing different phenotypes depending on the size of the deletion and the genes involved. By screening 63 SRS cases with marker D17S254, we have detected 2 further patients with a heterozygous deletion in the GH cluster. Quantitative analysis using restriction assays confirmed these findings. Additionally, in a cohort of 17 patients with isolated intrauterine and postnatal growth retardation, we detected a further patient to be carrier of a CSH1 deletion. Screening of 141 unrelated controls revealed hemizygosity in one person for which data on growth were not available. We additionally analyzed our cohort of SRS patients for mutations in CSH1 and its 3' neighbour GH2. However, analyses failed to reveal any pathogenic mutation. While the central role of GH1 in human growth is well established, the physiological roles of CSH1 and other components of the cluster are unclear. The increased prevalence of hemizygosity of CSH1 in our population in comparison to controls indicates a role for CSH1 haploinsufficiency in the etiology of growth retardation. Investigation of CSH1 deletions in further SRS and growth retarded patients will enable us to establish under which circumstances haploinsufficiency of CSH1 is likely to result in clinical changes.     

Growth retardation versus overgrowth: Silver-Russell syndrome is genetically opposite to Beckwith-Wiedemann syndrome

Human growth is a complex process that requires the appropriate interaction of many players. Central members in the growth pathways are regulated epigenetically and thereby reflect the profound significance of imprinting for correct mammalian ontogenesis. In this review, we show that the growth retardation disorder Silver-Russell syndrome (SRS) is a suitable model to decipher the role of imprinting in growth. As we will show, SRS should not only be regarded as the genetically (and clinically) opposite disease to Beckwith-Wiedemann syndrome, but it also represents the first human disorder with imprinting disturbances that affect two different chromosomes (i.e. chromosomes 7 and 11). Thus, a functional interaction between factors encoded by chromosomes 7 and 11 is likely.     

Transienter neonataler Diabetes und Hypomethylierungssyndrome

Transient neonatal diabetes (TNDM) is manifested before the age of 6 weeks and typically resolves within 18 months. Main clinical features include intrauterine growth retardation, hyperglycemia and dehydration with absent ketoacidosis. Causes of TNDM are heterogeneous but 70% are due to a chromosomal aberration in the region 6q24 which contains the imprinted genes PLAGL1/ZAC and HYMAI. Paternal uniparental disomy 6 (upd(6)pat) or paternal duplications of the imprinted region as well as imprinting defects of the maternal allele all result in an overexpression of the paternally expressed gene PLAGL1. Imprinting defects in 6q24 can occur as isolated events or can affect more than one locus (hypomethylation syndrome). Hypomethylation at multiple loci has so far been observed in patients with TNDM, Silver-Russell syndrome (SRS) and Beckwith-Wiedemann syndrome (BWS).The risk of recurrence depends on the underlying cause of TNDM. Chromosomal aberrations in the parents affecting chromosome 6 increase the risk for UPD or duplication of the imprinted locus in 6q24. Nevertheless, UPD and duplication 6q24 are mostly de novo occurrences.     

Genetik und Epigenetik des Silver-Russell-Syndroms

Silver-Russell syndrome (SRS) is a congenital imprinting disorder mainly characterized by severe intrauterine and postnatal growth retardation, relative macrocephaly, a triangular face and asymmetry of the body. The detection of (epi)genetic aberrations is now possible in about 50% of SRS patients where 7–10% carry a maternal uniparental disomy of chromosome 7 (upd(7)mat) and 40% of the patients show (epi)genetic disturbances in the chromosomal region 11p15. In addition to conventional cytogenetic findings submicroscopic chromosomal imbalances can be detected by molecular karyotyping of the patients. Because there is no unambiguous (epi)genotype-phenotype correlation and clinical diagnosis is complicated due to the variable occurrence of symptoms, genetic testing should be considered in patients showing only some of the typical disease features.     

Imprinted methylation profiles for proximal mouse Chromosomes 11 and 7 as revealed by methylation-sensitive representational difference analysis

Proximal mouse Chromosome (Chr) 11 shares regions of orthology with the candidate gene region for the imprinting growth disorder Silver-Russell syndrome (SRS) on human Chr 7p. It has previously been shown that mice with two maternal or two paternal copies (duplications, Dp) of proximal Chr 11 exhibit reciprocal growth phenotypes. Those with two paternal copies show fetal and placental overgrowth, while those with two maternal copies are growth retarded. The growth retardation observed in the latter is reminiscent of the intrauterine growth restriction (IUGR) observed in SRS patients with maternal uniparental disomy for Chr 7 (mUPD7). We have carried out a methylation-sensitive representational difference analysis (Me-RDA) screen to look for regions of differential methylation (DMRs) associated with imprinted genes. For these experiments, we have used mouse embryos with uniparental duplications of Chrs 11 and 7 proximal to the breakpoint of the reciprocal translocation T(7;11)40Ad. Two previously known imprinted loci associated with paternal allele hypomethylation were recovered on proximal mouse Chr 11, U2af1-rs1 and Meg1/Grb10. These two genes map 15 cM apart, so it seems likely that they are within separate imprinted domains that do not contain additional DMRs. The known imprinted gene Peg3, located on mouse proximal Chr 7, was also detected in our screen. The finding that Peg3 was differentially methylated in embryos with uniparental inheritance of proximal Chr 7 confirms that Peg3 is located proximal to the breakpoint of T40Ad in G-band 7A2. Because GRB10 has previously been reported to be a candidate gene for SRS, we analysed 22 patients for epimutations of the GRB10 differentially methylated region that could lead to the altered expression of this gene. No such mutations were found.     

IGF-1, IGFBP-1, and IGFBP-3 polymorphisms predict circulating IGF levels but not breast cancer risk: findings from the Breast and Prostate Cancer Cohort Consortium (BPC3)

IGF-1 has been shown to promote proliferation of normal epithelial breast cells, and the IGF pathway has also been linked to mammary carcinogenesis in animal models. We comprehensively examined the association between common genetic variation in the IGF1, IGFBP1, and IGFBP3 genes in relation to circulating IGF-I and IGFBP-3 levels and breast cancer risk within the NCI Breast and Prostate Cancer Cohort Consortium (BPC3). This analysis included 6,912 breast cancer cases and 8,891 matched controls (n = 6,410 for circulating IGF-I and 6,275 for circulating IGFBP-3 analyses) comprised primarily of Caucasian women drawn from six large cohorts. Linkage disequilibrium and haplotype patterns were characterized in the regions surrounding IGF1 and the genes coding for two of its binding proteins, IGFBP1 and IGFBP3. In total, thirty haplotype-tagging single nucleotide polymorphisms (htSNP) were selected to provide high coverage of common haplotypes; the haplotype structure was defined across four haplotype blocks for IGF1 and three for IGFBP1 and IGFBP3. Specific IGF1 SNPs individually accounted for up to 5% change in circulating IGF-I levels and individual IGFBP3 SNPs were associated up to 12% change in circulating IGFBP-3 levels, but no associations were observed between these polymorphisms and breast cancer risk. Logistic regression analyses found no associations between breast cancer and any htSNPs or haplotypes in IGF1, IGFBP1, or IGFBP3. No effect modification was observed in analyses stratified by menopausal status, family history of breast cancer, body mass index, or postmenopausal hormone therapy, or for analyses stratified by stage at diagnosis or hormone receptor status. In summary, the impact of genetic variation in IGF1 and IGFBP3 on circulating IGF levels does not appear to substantially influence breast cancer risk substantially among primarily Caucasian postmenopausal women.     

Effect of Single Nucleotide Polymorphisms in IGFBP2 and IGFBP3 Genes on Litter Size Traits in Berkshire Pigs

Litter size is among the most important traits in swine breeding. However, information on the genetics of litter size in pigs is lacking. In this study, we identified single nucleotide polymorphisms (SNPs) in the insulin-like growth factor binding protein 2 and 3 (IGFBP2 and IGFBP3) genes in Berkshire pigs and analyzed their association with litter size traits. The IGFBP2 SNP was located on chromosome 15 intron 2 (455, A?>?T) and the IGFBP3 SNP was on chromosome 18 intron 2 (53, A?>?G). The AT type of IGFBP2 and the GG type of IGFBP3 had the highest values for all litter size traits including total number born (TNB), number of pigs born alive, and breeding value according to TNB. Homozygous GG pigs expressed higher levels of IGFBP3 mRNA in the endometrium than pigs of other genotypes, and a positive correlation was observed between litter size traits and IGFBP3 but not IGFBP2 expression level. These results suggest that SNPs in the IGFBP2 and the IGFBP3 gene are useful biomarkers for increasing the reproductive productivity of Berkshire pigs.     

IGFBP3 Methylation Is a Novel Diagnostic and Predictive Biomarker in Colorectal Cancer

Background and Aim

Aberrant hypermethylation of cancer-related genes has emerged as a promising strategy for the development of diagnostic, prognostic and predictive biomarkers in human cancer, including colorectal cancer (CRC). The aim of this study was to perform a systematic and comprehensive analysis of a panel of CRC-specific genes as potential diagnostic, prognostic and predictive biomarkers in a large, population-based CRC cohort.

Patients and Methods

Methylation status of the SEPT9, TWIST1, IGFBP3, GAS7, ALX4 and miR137 genes was studied by quantitative bisulfite pyrosequencing in a population-based cohort of 425 CRC patients.

Results

Methylation levels of all genes analyzed were significantly higher in tumor tissues compared to normal mucosa (p<0.0001); however, cancer-associated hypermethylation was most frequently observed for miR137 (86.7%) and IGFBP3 (83%) in CRC patients. Methylation analysis using the combination of these two genes demonstrated greatest accuracy for the identification of colonic tumors (sensitivity 95.5%; specificity 90.5%). Low levels of IGFBP3 promoter methylation emerged as an independent risk factor for predicting poor disease free survival in stage II and III CRC patients (HR = 0.49, 95% CI: 0.28–0.85, p = 0.01). Our results also suggest that stage II & III CRC patients with high levels of IGFBP3 methylation do not benefit from adjuvant 5FU-based chemotherapy.

Conclusion

By analyzing a large, population-based CRC cohort, we demonstrate the potential clinical significance of miR137 and IGFBP3 hypermethylation as promising diagnostic biomarkers in CRC. Our data also revealed that IGFBP3 hypermethylation may serve as an independent prognostic and predictive biomarker in stage II and III CRC patients.     

Distinct methylation changes at the IGF2-H19 locus in congenital growth disorders and cancer

Background

Differentially methylated regions (DMRs) are associated with many imprinted genes. In mice methylation at a DMR upstream of the H19 gene known as the Imprint Control region (IC1) is acquired in the male germline and influences the methylation status of DMRs 100 kb away in the adjacent Insulin-like growth factor 2 (Igf2) gene through long-range interactions. In humans, germline-derived or post-zygotically acquired imprinting defects at IC1 are associated with aberrant activation or repression of IGF2, resulting in the congenital growth disorders Beckwith-Wiedemann (BWS) and Silver-Russell (SRS) syndromes, respectively. In Wilms tumour and colorectal cancer, biallelic expression of IGF2 has been observed in association with loss of methylation at a DMR in IGF2. This DMR, known as DMR0, has been shown to be methylated on the silent maternal IGF2 allele presumably with a role in repression. The effect of IGF2 DMR0 methylation changes in the aetiology of BWS or SRS is unknown.

Methodology/Principal Findings

We analysed the methylation status of the DMR0 in BWS, SRS and Wilms tumour patients by conventional bisulphite sequencing and pyrosequencing. We show here that, contrary to previous reports, the IGF2 DMR0 is actually methylated on the active paternal allele in peripheral blood and kidney. This is similar to the IC1 methylation status and is inconsistent with the proposed silencing function of the maternal IGF2 allele. Beckwith-Wiedemann and Silver-Russell patients with IC1 methylation defects have similar methylation defects at the IGF2 DMR0, consistent with IC1 regulating methylation at IGF2 in cis. In Wilms tumour, however, methylation profiles of IC1 and IGF2 DMR0 are indicative of methylation changes occurring on both parental alleles rather than in cis.

Conclusions/Significance

These results support a model in which DMR0 and IC1 have opposite susceptibilities to global hyper and hypomethylation during tumorigenesis independent of the parent of origin imprint. In contrast, during embryogenesis DMR0 is methylated or demethylated according to the germline methylation imprint at the IC1, indicating different mechanisms of imprinting loss in neoplastic and non-neoplastic cells.