Laboratory of Stem Cell Biology, Department of Anatomy; University of Cambridge
Beckwith-Wiedemann syndrome (BWS) is a fetal overgrowth and tumour predisposing condition leading to a characteristic, but variable, constellation of disorders. Cytogenetic analysis originally implicated two regions on chromosome 11p, BWSCR1 and BWSCR2. The former contains a cluster of imprinted genes bounded by NAP2 centromerically and L23/MRP telomerically. Family studies indicated that the gene responsible for BWS was likely to be imprinted and consequently we have examined the imprinted genes of this cluster for lesions in BWS patients. The most common lesion in BWS is paternal uniparental disomy (patUPD), but as the chromosome region containing this cluster is included in the smallest region of disomy, it is not possible to separate the contribution of epigenetic alterations at each of the candidate loci. However, imprinting of IGF2 is also lost in the majority of patients without patUPD11p; in some cases this is associated with loss of expression of H19, in others H19 is unaffected (1). Only the former combination is consistent with the enhancer competition model of Tilghman and co-workers, suggesting that alterations in the mechanism linking IGF2 and H19 imprinting may cause IGF2 LOI ("imprinting center" type mutations). We consequently sequenced the H19 gene in 15 patients without 11p15 patUPD and found no identifiable mutations, suggesting that IGF2 LOI was not dependent on mutations in the H19 structural gene or promoter.
We have previously noted hypermethylation of the H19 promoter in a small subset (5-10%) of patients without patUPD (2), and have now examined the methylation of the region between -210 bp and + 200 bp of the H19 transcription start site. H19 methylation was examined in 9 patients and 2 normal controls, including patients showing IGF2 LOI and no expression of H19, biallelic IGF2/monoallelic H19, conserved imprinting at both loci, and as positive controls, those patients showing hypermethylation of the promoter region and paternal 11p15.5 duplication. 14 CpGs were investigated in the promoter region of H19 using bisulphite modification of genomic DNA and sequencing. The expected hypermethylation of H19 null patients was not found, separating methylation of this region from extinction of expression from the maternal allele. H19 methylation was unaffected in patients with biallelic IGF2 expression and monoallelic H19, suggesting that dissociation of methylation from expression was not a potential mechanism for uncoupling imprinting of IGF2 and H19.
Around 1% of BWS cases show chromosomal breakpoints which lie within the KvLQT1 gene. We have identified a conserved CpG-island (KvDMR1) in an intron of KvLQT1 which exhibits differential methylation in all somatic tissues tested (3). A KvDMR1-associated antisense RNA is transcribed exclusively from the paternal allele of the maternally expressed KvLQT1 gene. We have examined 12 cases of BWS with normal H19 methylation; 5 showed hypomethylation of KvDMR1, while in four cases of BWS with H19 hypermethylation, methylation at KvDMR1 was normal. Of the 5 patients showing loss of KvLQT1 methylation, both informative at IGF2 showed LOI. We suggest that inactivation of H19 and hypomethylation at KvDMR1 represent distinct epigenetic anomalies associated with biallelic expression of IGF2. We propose that KvDMR1 and/or its associated antisense RNA represents an additional imprinting control element or center in the human 11p15.5 imprinted domain.
Joyce et al., Hum. Mol. Genet. 6:1543-1548, 1997.
Catchpoole et al., J. Med Genet. 34: 353-359, 1997.
Smilinich et al., Proc. Natl. Acad. Sci. USA 96: 8064-8069, 1999.