Beckwith-Wiedemann syndrome caused by maternally inherited mutation of an OCT-binding motif in the IGF2/H19-imprinting control region, ICR1

Abstract

The imprinted expression of the IGF2 and H19 genes is controlled by the imprinting control region 1 (ICR1) located at chromosome 11p15.5. DNA methylation defects involving ICR1 result in two growth disorders with opposite phenotypes: an overgrowth disorder, the Beckwith-Wiedemann syndrome (maternal ICR1 hypermethylation in 10% of BWS cases) and a growth retardation disorder, the Silver-Russell syndrome (paternal ICR1 loss of methylation in 60% of SRS cases). In familial BWS, hypermethylation of ICR1 has been found in association with microdeletion of repetitive DNA motifs within ICR1 that bind the zinc finger protein CTCF; but more recently, ICR1 point mutations were described in BWS pedigrees. We present a case report of two brothers with BWS and prolonged post-pubertal growth resulting in very large stature. A maternally inherited point mutation was identified in ICR1 in both brothers, which altered binding of OCT transcription factors. The same mutation was present on the paternally inherited allele of their unaffected mother. This is a second report of a point mutation causing ICR1 hypermethylation by altering an OCT-binding motif. The atypical growth phenotype of the brothers may be connected to the unusual underlying cause of their BWS.

Figure 1

(a) Upper left: proband 1 aged 3 year; upper right, proband 2 aged 1 year; lower panels, proband 1 aged 24 year. (b) Examples of pyrograms illustrating allele quantification of bisulfite-induced C/T polymorphisms within the H19 and IGF2P0 differentially methylated regions. Successive rows illustrate a normal control, proband 1, proband 2 and their mother. The left and right columns illustrate the H19-ICR and IGF2P0, respectively. Each table cell contains a representative pyrogram of the first C/T polymorphism quantified by the pyrosequencing reaction. Figures above each pyrogram indicate the proportions of methylated and unmethylated product present in the amplicon. (c) Pedigree of the two probands, and inheritance of microsatellites surrounding H19, illustrating the shared maternal haplotype. (d) Diagram of H19-IGF2-imprinted domain. Top panel: schematic of H19 and ICR1. Differential expression of H19 (maternal allele) and IGF2 (paternal allele) is regulated by ICR1 (the methylated paternal allele is marked with black ‘lollipops'). Differential methylation of the ICR regulates differential access of H19 and IGF2 to shared proximal promoter elements (marked by green hexagons). Second panel: organisation of H19 ICR. The ICR comprises B-type repeats (blue boxes) containing CTCF-binding motifs (blue arrows) interspersed with A-type repeats (orange boxes) and OCT-binding motifs (orange arrows). Third panel: sequence context of mutation chr11:1979624A>C (black arrow); the mutation described by Demars et al is marked by an asterisk. (e) Parental origin of mutation. The left and right pairs of electropherograms illustrate proband 1 and his mother; upper and lower pairs illustrate genomic DNA with and without digestion of unmethylated DNA by BstU1. (f) Electrophoretic mobility shift assay of the wild-type and mutant OCT-binding motif. Arrowheads mark protein–DNA complexes.