Phenotypically concordant but epigenetically discordant monozygotic dichorionic diamniotic twins with Beckwith-Wiedemann syndrome

Whole-exome sequencing reveals causative genetic variants for several overgrowth syndromes in molecularly negative Beckwith-Wiedemann spectrum

Background Beckwith-Wiedemann syndrome (BWS) is an imprinting disorder caused by (epi)genetic alterations at 11p15. Because approximately 20% of patients test negative via molecular testing of peripheral blood leukocytes, the concept of Beckwith-Wiedemann spectrum (BWSp) was established to encompass a broader cohort with diverse and overlapping phenotypes. The prevalence of other overgrowth syndromes concealed within molecularly negative BWSp remains unexplored. Methods We conducted whole-exome sequencing (WES) on 69 singleton patients exhibiting molecularly negative BWSp. Variants were confirmed by Sanger sequencing or quantitative genomic PCR. We compared BWSp scores and clinical features between groups with classical BWS (cBWS), atypical BWS or isolated lateralised overgrowth (aBWS+ILO) and overgrowth syndromes identified via WES. Results Ten patients, one classified as aBWS and nine as cBWS, showed causative gene variants for Simpson-Golabi-Behmel syndrome (five patients), Sotos syndrome (two), Imagawa-Matsumoto syndrome (one), glycosylphosphatidylinositol biosynthesis defect 11 (one) or 8q duplication/9p deletion (one). BWSp scores did not distinguish between cBWS and other overgrowth syndromes. Birth weight and height in other overgrowth syndromes were significantly larger than in aBWS+ILO and cBWS, with varying intergroup frequencies of clinical features. Conclusion Molecularly negative BWSp encapsulates other syndromes, and considering both WES and clinical features may facilitate accurate diagnosis.

Beckwith-Wiedemann syndrome and twinning: case report and brief review of literature

BACKGROUND

Beckwith-Wiedemann syndrome (BWS, OMIM #130,650) is a pediatric overgrowth disorder involving a predisposition to tumor development. Although the clinical management of affected patients is well established, it is less clear how to handle with the cases of siblings of affected patients, since the prevalence of the condition in twins (1:1000) is ten times higher than in singletones (1:10000).

CASE PRESENTATION

We report the case of a premature twin patient who during her follow-up develops a clinical phenotype compatible with BWS, genetically confirmed in blood. However, the methylation alteration characteristic of the condition was also found in the almost phenotypically normal sibling, making it challening her management.

CONCLUSION

Through our case report we highlight how the diagnosis of BWS can be made without any prenatal suspicion and we propose a review of the literature on how to manage siblings of affected patients in twinning situation.

Aberrant hypomethylation at imprinted differentially methylated regions is involved in biparental placental mesenchymal dysplasia

BACKGROUND

Placental mesenchymal dysplasia (PMD) is a morphological abnormality resembling partial hydatidiform moles. It is often associated with androgenetic/biparental mosaicism (ABM) and complicated by Beckwith-Wiedemann syndrome (BWS), an imprinting disorder. These phenomena suggest an association between PMD and aberrant genomic imprinting, particularly of CDKN1C and IGF2. The existence of another type of PMD containing the biparental genome has been reported. However, the frequency and etiology of biparental PMD are not yet fully understood.

RESULTS

We examined 44 placental specimens from 26 patients with PMD: 19 of these were macroscopically normal and 25 exhibited macroscopic PMD. Genotyping by DNA microarray or short tandem repeat analysis revealed that approximately 35% of the macroscopic PMD specimens could be classified as biparental, while the remainder were ABM. We performed a DNA methylation analysis using bisulfite pyrosequencing of 15 placenta-specific imprinted differentially methylated regions (DMRs) and 36 ubiquitous imprinted DMRs. As expected, most DMRs in the macroscopic PMD specimens with ABM exhibited the paternal epigenotype. Importantly, the biparental macroscopic PMD specimens exhibited frequent aberrant hypomethylation at seven of the placenta-specific DMRs. Allelic expression analysis using single-nucleotide polymorphisms revealed that five imprinted genes associated with these aberrantly hypomethylated DMRs were biallelically expressed. Frequent aberrant hypomethylation was observed at five ubiquitous DMRs, including GRB10 but not ICR2 or ICR1, which regulate the expression of CDKN1C and IGF2, respectively. Whole-exome sequencing performed on four biparental macroscopic PMD specimens did not reveal any pathological genetic abnormalities. Clinical and molecular analyses of babies born from pregnancies with PMD revealed four cases with BWS, each exhibiting different molecular characteristics, and those between BWS and PMD specimens were not always the same.

CONCLUSION

These data clarify the prevalence of biparental PMD and ABM-PMD and strongly implicate hypomethylation of DMRs in the pathogenesis of biparental PMD, particularly placenta-specific DMRs and the ubiquitous GRB10, but not ICR2 or ICR1. Aberrant hypomethylation of DMRs was partial, indicating that it occurs after fertilization. PMD is an imprinting disorder, and it may be a missing link between imprinting disorders and placental disorders incompatible with life, such as complete hydatidiform moles and partial hydatidiform moles.

Characteristics Associated with Tumor Development in Individuals Diagnosed with Beckwith-Wiedemann Spectrum: Novel Tumor-(epi)Genotype-Phenotype Associations in the BWSp Population

Beckwith–Wiedemann Spectrum (BWSp) is the most common epigenetic childhood cancer predisposition disorder. BWSp is caused by (epi)genetic changes affecting the BWS critical region on chromosome 11p15. Clinically, BWSp represents complex molecular and phenotypic heterogeneity resulting in a range of presentations from Classic BWS to milder features. The previously reported tumor risk based on Classic BWS cohorts is 8–10% and routine tumor screening has been recommended. This work investigated the tumor risk and correlation with phenotype within a cohort of patients from Classic BWS to BWSp using a mixed-methods approach to explore phenotype and epigenotype profiles associated with tumor development through statistical analyses with post-hoc retrospective case series review. We demonstrated that tumor risk across BWSp differs from Classic BWS and that certain phenotypic features are associated with specific epigenetic causes; nephromegaly and/or hyperinsulinism appear associated with cancer in some patients. We also demonstrated that prenatal and perinatal factors that are not currently part of the BWSp classification may factor into tumor risk. Additionally, blood testing results are not necessarily synonymous with tissue testing results. Together, it appears that the current understanding from Classic BWS of (epi)genetics and phenotype correlations with tumors is not represented in the BWSp. Further study is needed in this complex population.

Epigenetic mosaicism and cell burden in Beckwith-Wiedemann Syndrome due to loss of methylation at imprinting control region 2

Beckwith–Wiedemann syndrome (BWS) is a rare overgrowth disorder caused by epigenetic alterations on Chromosome 11p15.5. Most molecular changes are sporadic and are thought to occur in a mosaic pattern. Thereby, the distribution of affected cells differs between tissues for each individual, which can complicate genotype–phenotype correlations. In two of the BWS molecular subtypes, tissue mosaicism has been demonstrated; however, mosaicism has not been specifically studied in the most common cause of BWS, loss of methylation (LOM) at KCNQ1OT1:TSS differentially methylated region (DMR) imprinting center 2 (IC2) LOM. The increased prevalence of twinning associated with the IC2 LOM subtype and the discordant phenotypes between the twins previously led to the proposal of diffused epigenetic mosaicism, leading to asymmetric distribution of affected cells during embryonic development. In this study, we evaluated the level of methylation detected in 64 samples collected from 30 individuals with IC2 LOM. We demonstrate that the IC2 LOM defect can occur in mosaic and nonmosaic patterns, and tissues from the same individual can show variable patterns, which suggests that this asymmetric distribution occurs during development. We further suggest that the clinical phenotype in individuals with BWS IC2 LOM is correlated with the epigenetic burden of affected cells in each tissue type. This series is the first report to demonstrate tissue mosaicism within the IC2 LOM epigenotype, and consideration of this mosaicism is necessary to understanding the pathogenesis of BWS.