翻译重编程破坏有丝分裂以驱动SMAD4缺陷型食管腺癌

Esophageal adenocarcinoma (EAC) develops from a precursor lesion named Barrett's esophagus; a progression that can be mapped by increasing genomic instability. We have previously shown that SMAD4 loss induces tumorigenesis and increased copy number alterations (CNA) in Barrett's esophagus cells with an existing TP53 mutation. The observed CNA increase was especially pronounced in the resultant SMAD4-deficient tumor cells, suggesting that accumulation of mitotic errors occurs in these cells throughout their oncogenic progression. However, such an effect cannot be explained by our current understanding of canonical SMAD4 signalling. Thus, in this study, we aimed to delineate the role of SMAD4 in maintaining chromosomal stability in EAC. Our multi-omics (RNA-sequencing, proteomics and reverse-phase protein array) approach detected both deregulated translation and mitosis signatures in SMAD4-deficient cells. Using live cell imaging, we demonstrated that TP53 -mutant SMAD4-deficient cells exhibit significantly more mitotic errors than TP53 -mutant only cells, particularly chromosome segregation defects, which frequently result in chromosomal instability (CIN). Consistent with this, analysis of TCGA data demonstrated that CIN was more prevalent in tumours with both SMAD4 and TP53 mutation than either mutation alone. Further biochemical analyses revealed an increase in cap-dependent translation in these cells, and a decrease in internal ribosome entry site (IRES)-mediated translation. IRES-mediated translation is essential for synthesis of many important mitotic proteins, including a mitosis-specific isoform of a cell cycle kinase. Polysome sequencing showed decreased translation (but not transcription) of this kinase in SMAD4-deficient cells. Ectopic expression of mitosis-specific isoform of this kinase rescued the mitotic errors that arose as a consequence of SMAD4 loss. In sum, we present a model where translational reprogramming disrupts mitosis, inducing CIN and potentially driving EAC tumorigenesis in response to loss of SMAD4. This represents a novel mechanism of tumorigenesis in EAC and a previously unknown role of SMAD4 in maintaining chromosomal integrity.