Loss of the tumor-suppressor gene SMAD4 promotes progression of high-grade Barrett's esophagus toward esophageal adenocarcinoma. This study provides an Q7 in vivo model of dysplastic Barrett's esophagus progression toward invasive esophageal adenocarcinoma upon SMAD4 inactivation.BACKGROUND & AIMS: Esophageal adenocarcinoma (EAC) develops from its precursor Barrett's esophagus through intermediate stages of low-and high-grade dysplasia. However, knowledge of genetic drivers and molecular mechanisms implicated in disease progression is limited. Herein, we investigated the effect of SMAD4 loss on transforming growth factor b (TGF-b) signaling functionality and in vivo tumorigenicity in high-grade dysplastic Barrett's cells. METHODS:An in vivo xenograft model was used to test tumorigenicity of SMAD4 knockdown or knockout in CP-B highgrade dysplastic Barrett's cells. RT 2 Q8polymerase chain reaction arrays were used to analyze TGF-b signaling functionality, and low-coverage whole-genome sequencing was performed to detect copy number alterations upon SMAD4 loss. RESULTS:We found that SMAD4 knockout significantly alters the TGF-b pathway target gene expression profile. SMAD4 knockout positively regulates potential oncogenes such as CRYAB, ACTA2, and CDC6, whereas the CDKN2A/B tumorsuppressor locus was regulated negatively. We verified that SMAD4 in combination with CDC6-CDKN2A/B or CRYAB genetic alterations in patient tumors have significant predictive value for poor prognosis. Importantly, we investigated the effect of SMAD4 inactivation in Barrett's tumorigenesis. We found that genetic knockdown or knockout of SMAD4 was sufficient to promote tumorigenesis in dysplastic Barrett's esophagus cells in vivo. Progression to invasive EAC was accompanied by distinctive and consistent copy number alterations in SMAD4 knockdown or knockout xenografts.CONCLUSIONS: Altogether, up-regulation of oncogenes, downregulation of tumor-suppressor genes, and chromosomal instability within the tumors after SMAD4 loss implicates SMAD4 as a protector of genome integrity in EAC development and progression. Foremost, SMAD4 loss promotes tumorigenesis from dysplastic Barrett's toward EAC.

Efficacious therapeutic approaches are urgently needed to improve outcomes in patients with oesophageal adenocarcinoma (OAC). However, oncogenic drivers amenable to targeted therapy are limited and their functional characterisation is essential. Among few targeted therapies available, anti-human epidermal growth factor receptor 2 (HER2) therapy showed only modest benefit for patients with OAC. Herein, we investigated the potential oncogenic role of growth factor receptor bound protein 7 (GRB7), which is reported to be co-amplified with HER2 (ERBB2) in OAC. GRB7 was highly expressed in 15% of OAC tumours, not all of which could be explained by co-amplification with HER2, and was associated with a trend for poorer overall survival. Knockdown of GRB7 decreased proliferation and clonogenic survival, and induced apoptosis. Reverse phase protein array (RPPA) analyses revealed a role for PI3K, mammalian target of rapamycin (mTOR), MAPK, and receptor tyrosine kinase signalling in the oncogenic action of GRB7. Furthermore, the GRB7 and HER2 high-expressing OAC cell line Eso26 showed reduced cell proliferation upon GRB7 knockdown but was insensitive to HER2 inhibition by trastuzumab. Consistent with this, GRB7 knockdown in vivo with an inducible shRNA significantly inhibited tumour growth in cell line xenografts. HER2 expression did not predict sensitivity to trastuzumab, with Eso26 xenografts remaining refractory to trastuzumab treatment. Taken together, our study provides strong evidence for an oncogenic role for GRB7 in OAC and suggests that targeting GRB7 may be a potential therapeutic strategy for this cancer.

Extensive genomic analysis of patient samples has identified genes whose mutation or loss map malignant progression from Barrett’s metaplasia, through low- (CDKN2A) and high-grade dysplasia (TP53), to invasive adenocarcinoma (SMAD4). Interestingly, loss of SMAD4 has been found to occur exclusively in the invasive disease stage, but the reason for this is unknown. This work aimed to characterise the role of SMAD4 in esophageal adenocarcinoma (EAC) tumorigenesis and identify novel therapeutic targets for SMAD4-deficient EAC. Methods We developed a novel in vivo tumorigenesis model that demonstrates progression of dysplastic Barrett’s esophagus (BE) to invasive EAC upon knockout of SMAD4. We conducted parallel genome-wide CRISPR-Cas9 knockout screens, both in vitro and in vivo, on a background of either wildtype-SMAD4 or SMAD4-knockout dysplastic BE cells to identify co-operative drivers of tumorigenesis in vivo, as well as synthetic lethal interactions to identify potential therapeutic targets in SMAD4-deficient EAC. Functional validation of hits was performed using cell-based assays and drugs targeting candidate molecular targets. Results We identified a synthetic lethal relationship between SMAD4-deficiency and cell cycle checkpoint inhibition, suggesting a role for SMAD4 in maintaining genomic stability and a potential novel therapeutic avenue for SMAD4-deficient EAC. A concurrent in vivo CRISPR-Cas9 tumorigenesis screen produced tumors 4-fold faster than loss of SMAD4 alone and identified regulators of mTOR signalling and SMAD4 as co-operative drivers of tumorigenesis in EAC. Interestingly, these tumorigenic cells exhibited an inherent dependency on specific translation mechanisms downstream of mTOR. Meanwhile, wildtype-SMAD4 BE cells failed to thrive in vivo with mTOR modifications alone, indicating a true co-operative effect at play with SMAD4 loss. Conclusion This study uncovered a potential gatekeeping role of SMAD4 in maintaining genomic stability and inhibiting mTOR-mediated EAC tumorigenesis. In sum, loss of SMAD4 was found to increase genomic instability, thereby rendering EAC cells sensitive to cell cycle checkpoint impediment, whilst simultaneously co-operating with modulated mTOR signalling to promote tumorigenesis in EAC xenograft models.