Imprinting is a classic mammalian epigenetic phenomenon that results in expression from a single parental allele. Imprinting defects can lead to inappropriate expression from the normally silenced allele, but it remains unclear whether every cell in a mutant organism follows the population average, which would have profound implications for human imprinting disorders. Here, we apply a new fluorescence in situ hybridization method that measures allelespecific expression in single cells to address this question in mutants exhibiting aberrant H19/Igf2 (insulin-like growth factor 2) imprinting. We show that mutant primary embryonic mouse fibroblasts are comprised of two subpopulations: one expressing both H19 alleles and another expressing only the maternal copy. Only in the latter cell population is Igf2 expression detected. Furthermore, the two subpopulations are stable in that cells do not interconvert between the two expression patterns. Combined small input methylation analysis and transcriptional imaging revealed that these two mutant subpopulations exhibit distinct methylation patterns at their imprinting control regions. Consistently, pharmacological inhibition of DNA methylation reduced the proportion of monoallelic cells. Importantly, we observed that the same two subpopulations are also present in vivo within murine cardiac tissue. Our results establish that imprinting disorders can display striking single-cell heterogeneity in their molecular phenotypes and suggest that such heterogeneity may underlie epigenetic mosaicism in human imprinting disorders.

Beckwith-Wiedemann syndrome (BWS) is the most common epigenetic overgrowth disorder and presents with patients affected by a variety of clinical features. Although genotype-phenotype correlations have been demonstrated in BWS and although BWS has been reported to occur equally among racial and ethnic backgrounds, no study to date has evaluated the frequency of findings in different backgrounds. In this study, we evaluated the incidence of clinical features and molecular diagnoses among patients with BWS in Caucasian, Mixed, and non-Caucasian groups. These results suggest that clinical features and molecular diagnoses differ between race/ethnicity groups and raise the possibility of race and ethnicity effects on genotype-phenotype correlations in BWS. K E Y W O R D SBeckwith-Wiedemann syndrome, diverse populations, imprinting, methylation, overgrowth, race

The cover image is based on the Original Article Beckwith‐Wiedemann syndrome in diverse populations by Kelly A. Duffy et al., DOI: https://doi.org/10.1002/ajmg.a.61053.

Beckwith-Wiedemann Syndrome (BWS) is a cancer predisposition syndrome that affects at least 1 in 10,500 children. Up to 25% of children with BWS develop tumors, primarily Wilms tumor and hepatoblastoma. BWS is due to genetic or epigenetic changes that affect imprinted loci on chromosome 11 and these same changes are also found in other types of cancer. There are no cell-based models of BWS and most mouse models do not recapitulate the tumor phenotype. To understand more about the mechanisms leading to tumor formation in BWS, we developed the first human cell-based model of BWS. Human induced pluripotent stem cell (iPSC) models are commonly used to study disease mechanisms in tissue types that are not normally accessible for study. In the case of BWS, we plan to use such models to study how the genetic and epigenetic changes in BWS lead to tumor formation in hepatic and renal cells. Using skin fibroblasts from four BWS patients, we derived the first iPSC models of BWS. Prior to iPSC derivation, we characterized different tissues available from these four patients, and demonstrated genetic mosaicism in different tissue types, including blood, skin, and pancreas. During the derivation process, we demonstrated that both normal and BWS iPSC lines could be derived from the same patient fibroblast sample and the number of clones of each type from each sample approximated the initial level of mosaicism in the original sample. For each patient, these lines are isogenic except for the BWS critical region. The BWS and isogenic normal iPSCs were characterized for pluripotency markers and demonstrated to have normal karyotypes. Following this analysis, BWS and isogenic normal lines were characterized extensively for DNA methylation at specific imprinted loci in both early passage and extended culture. Methylation analysis was performed by both pyrosequencing and COBRA assays. Methylation was maintained at some imprinted loci but not at others in extended culture. Importantly, relatively stable methylation levels were observed at the BWS critical imprinted regions (H19/IGF2 and KvDMR), regardless of methods of reprogramming, indicating a relatively stable state of DNA methylation in this region. Additionally, normal methylation was seen at the SNRPN locus. In contrast, another imprinted locus, IG/MEG3, displayed abnormal hypermethylation in iPSCs. These data indicate that reprogramming and extended culture of iPSCs can affect stability of DNA methylation at certain imprinted loci. Therefore caution should be used in interpreting studies using iPSCs as these aberrant methylation states at imprinted loci can affect the downstream functionality of iPSC models. BWS iPSCs and isogenic controls with normal methylation will be used for further study of the mechanism of tumor formation in BWS. Note: This abstract was not presented at the meeting. Citation Format: Stella K. Hur, Joanne L. Thorvaldsen, Suhee Chang, Carolyn Lye, Alice Yu, Monserrat C. Anguera, Marisa S. Bartolomei, Jennifer M. Kalish. Epigenetic challenges in derivation of the first cell-based model of Beckwith-Wiedemann Syndrome [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1936. doi:10.1158/1538-7445.AM2017-1936