No abstract

The recent demonstration of genomic imprinting of DLK1 and MEG3 on human chromosome 14q32 indicates that these genes might contribute to the discordant phenotypes associated with uniparental disomy (UPD) of chromosome 14. Regulation of imprinted expression of DLK1 and MEG3 involves a differentially methylated region (DMR) that encompasses the MEG3 promoter. We exploited the normal differential methylation of the DLK1/MEG3 region to develop a rapid diagnostic PCR assay based upon an individual's epigenetic profile. We used methylation-specific multiplex PCR in a retrospective analysis to amplify divergent lengths of the methylated and unmethylated MEG3 DMR in a single reaction and accurately identified normal, maternal UPD14, and paternal UPD14 in bisulfite converted DNA samples. This approach, which is based solely on differential epigenetic profiles, may be generally applicable for rapidly and economically screening for other imprinting defects associated with uniparental disomy, determining loss of heterozygosity of imprinted tumor suppressor genes, and identifying gene-specific hypermethylation events associated with neoplastic progression.

The gene MEG3 is located in the imprinted human chromosomal region on 14q32. Imprinting of a structurally homologous region IGF2/H19 on 11p15 is mediated through cytosine methylation-controlled binding of the protein CTCF to target sites upstream of H19. We identified five new CTCF binding sites around the promoter of MEG3. Using an electrophoretic mobility shift assay, we showed that these sites bind CTCF in vitro. Using one of these sites, chromatin immunoprecipitation (ChIP) analysis confirmed CTCF binding in-vivo, and differential allele-specific methylation was demonstrated in seven individuals with either maternal or paternal uniparental disomy 14 (UPD14). The site was unmethylated on the maternally inherited chromosomes 14 and methylated on the paternally inherited chromosomes 14, suggesting parent-specific methylation of sequences upstream of MEG3. We speculate that this CTCF-binding region may provide a mechanism for the transcriptional regulation of MEG3 and DLK1.

No abstract

Uniparental disomy of chromosome 14 (UPD 14) results in one of two distinct abnormal phenotypes, depending upon the parent of origin. This discordance may result from the reciprocal over-expression and/or under-expression of one or more imprinted genes. We report a case of segmental paternal isodisomy for chromosome 14 with features similar to those reported in other paternal disomy 14 cases. Microsatellite marker analysis revealed an apparent somatic recombination event in 14q12 leading to proximal biparental inheritance, but segmental paternal uniparental isodisomy distal to this site. Analysis of monochromosomal somatic cell hybrids containing either the paternally inherited or the maternally inherited chromosome 14 revealed no deletion of the maternally inherited chromosome 14 and demonstrated the presence of paternal sequences from D14S121 to the telomere on both chromosomes 14. Thus, the patient has paternal isodisomy for 14q12-14qter. Because the patient shows most of the features associated with paternal disomy 14, this supports the presence of the imprinted domain(s) distal to 14q12 and suggests that the proximal region of chromosome 14 does not contain imprinted genes that contribute significantly to the paternal UPD 14 phenotype.