Oral Presentation 33rd Lorne Cancer Conference 2021

SMAD4 as a potential gatekeeper for genomic instability and mTOR-mediated tumourigenesis in oesophageal adenocarcinoma (#10)

Julia Milne 1 2 , Jovana Gotovac 1 2 , Kenji Fujihara 1 2 , Kaylene Simpson 1 2 3 , Cuong Duong 1 2 , Wayne Phillips 1 2 4 , Nicholas Clemons 1 2
  1. Division of Cancer Research, Peter MacCallum Cancer Centre, Melbourne, VICTORIA, Australia
  2. Sir Peter MacCallum Department of Oncology, University of Melbourne, Parkville, VICTORIA, Australia
  3. Victorian Centre for Functional Genomics (VCFG), Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia
  4. Department of Surgery (St. Vincent's Hospital), University of Melbourne, Parkville, Victoria, Australia

Oesophageal cancer is the 8th most common cancer worldwide and has the 6th highest mortality rate of all cancers. The 5-year survival rate following oesophageal adenocarcinoma (OAC) diagnosis is dismal at <15%, indicating a dire need for improved therapeutic strategies and early detection. OAC develops stepwise following exposure to chronic gastric reflux: From pre-malignant Barrett’s metaplasia, through stages of low- and high-grade dysplasia until developing into invasive cancer. Mutation or loss of common tumour suppressor genes TP53 and SMAD4 act as markers for cancer progression, occurring in high-grade dysplastic tissue and invasive OAC, respectively. Our novel in vivo tumourigenesis model demonstrates progression of Barrett’s metaplasia to OAC, in which SMAD4-deficient Barrett’s metaplasia cells form tumours in immunodeficient mice after a period of latency and in a dose-dependent manner. This delayed tumour growth onset suggests further drivers are required for oncogenesis, and these SMAD4-deficient cells and tumours display a greater degree of genomic instability than wildtype-SMAD4 controls. A genome-wide CRISPR-Cas9 knockout screen unveiled 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 OAC. Additionally, a concurrent in vivo CRISPR-Cas9 tumourigenesis screen produced tumours 4-fold faster than the previous model and identified regulators of mTOR signalling as co-operative drivers of tumourigenesis in OAC. Wildtype-SMAD4 cells failed to generate tumours despite undergoing the same genetic perturbations, indicating a potential gatekeeping effect of SMAD4 in mTOR-mediated OAC tumourigenesis. In sum, loss of SMAD4 acts as a double-edged sword, increasing genomic instability and thereby rendering OAC cells sensitive to cell cycle checkpoint inhibition, whilst simultaneously co-operating with modulated mTOR signalling to promote tumourigenesis in OAC xenograft models.

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