Beckwith-Wiedemann syndrome (BWS) is a congenital stem cell disorder characterized by a defective developmental program that results in enlarged organs and the spontaneous incidence of childhood tumors. This disease is associated with aberrant expression of several growth regulatory genes, including silencing of β2-spectrin (β2SP, encoded by SPTBN1), a scaffolding protein for the SMAD3/4 tumor suppressors that signal downstream of TGF-β. While deregulation of TGF-β-mediated SMAD3/4 signaling is a validated contributor in certain cancers, it is unclear whether this pathway is responsible for driving BWS. Jian Chen and colleagues at the MD Anderson Cancer Center generated mice that are heterozygous for loss of both Sptbn1 and Smad3 (Sptbn1+/- Smad3+/-) and found that this model phenocopies BWS, including increased tumor incidence. Loss of both of these genes often occurs simultaneously in several human cancers, and whole transcriptome sequencing of both Sptbn1+/- Smad3+/- mouse embryonic fibroblasts (MEFs) and human BWS cells revealed downregulation of TGF-β signaling and upregulation of several cytokines and growth factors implicated in oncogenesis. Stimulation with TGF-β, shRNA-mediated suppression of β2SP, or pharmacologic inhibition of the TGF-β receptor in both human cell lines and BWS patient-derived cells revealed that β2SP and SMAD3 interact in a TGF-β-dependent manner and translocate to the nucleus where they bind to chromatin. Levels of CTCF, which binds 5’-CCCTC-3’ motifs in chromatin and prohibits transcription, were substantially reduced upon disruption of the TGF-β/β2SP/SMAD3 pathway. In hepatocytes, loss of CTCF expression derepressed TERT and MYC expression, both of which are known to contribute to unrestrained cell growth. Additionally, dysfunctional β2SP/SMAD3/CTCF signaling in MEFs and human BWS cell lines resulted in upregulation of many stem cell-associated genes, likely accounting for the increased tumorigenic potential of these cells. Overall, this study provides increased mechanistic insight into the genetic aberrations driving BWS and spontaneous tumor development. The accompanying image compares immunohistochemical staining of liver tissue from WT mice (top left) and mice with heterozygous for deletion of Sptbn1 (top right), Smad3 (bottom left), or both (Sptbn1-/+ Smad3-/+, bottom right), and reveals that mice heterozygous for one or both of these genes exhibit decreased expression of CTCF (brown).
Beckwith-Wiedemann syndrome (BWS) is a human stem cell disorder, and individuals with this disease have a substantially increased risk (~800-fold) of developing tumors. Epigenetic silencing of β2-spectrin (β2SP, encoded by
Jian Chen, Zhi-Xing Yao, Jiun-Sheng Chen, Young Jin Gi, Nina M. Muñoz, Suchin Kundra, H. Franklin Herlong, Yun Seong Jeong, Alexei Goltsov, Kazufumi Ohshiro, Nipun A. Mistry, Jianping Zhang, Xiaoping Su, Sanaa Choufani, Abhisek Mitra, Shulin Li, Bibhuti Mishra, Jon White, Asif Rashid, Alan Yaoqi Wang, Milind Javle, Marta Davila, Peter Michaely, Rosanna Weksberg, Wayne L. Hofstetter, Milton J. Finegold, Jerry W. Shay, Keigo Machida, Hidekazu Tsukamoto, Lopa Mishra