We report a novel PPARG germline mutation in a patient affected by colorectal cancer that replaces serine 289 with cysteine in the mature protein (S289C). The mutant has impaired transactivation potential and acts as dominant negative to the wild type receptor. In addition, it no longer restrains cell proliferation both in vitro and in vivo. Interestingly, the S289C mutant poorly activates target genes and interferes with the inflammatory pathway in tumor tissues and proximal normal mucosa. Consistently, only mutation carriers exhibit colonic lesions that can evolve to dysplastic polyps. The proband presented also dyslipidemia, hypertension and overweight, not associated to type 2 diabetes; of note, family members tested positive for the mutation and display only a dyslipidemic profile at variable penetrance with other biochemical parameters in the normal range. Finally, superimposing the mutation to the crystal structure of the ligand binding domain, the new Cys289 becomes so closely positioned to Cys285 to form an S-S bridge. This would reduce the depth of the ligand binding pocket and impede agonist positioning, explaining the biological effects and subcellular distribution of the mutant protein. This is the first PPARG germline mutation associated with dyslipidemia and colonic polyp formation that can progress to full-blown adenocarcinoma.
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