CDK4 phosphorylation status and rational use for combining CDK4/6 and BRAF/MEK inhibition in advanced thyroid carcinomas
Despite overall good prognosis associated to thyroid cancer, poorly differentiated carcinomas (PDTC) and anaplastic carcinomas (ATC) represent major clinical challenges. We have shown that the presence of active T172-phosphorylated CDK4 predicts sensitivity to CDK4/6 inhibitory drugs (CDK4/6i) including palbociclib. Here, CDK4 phosphorylation was detected in all well-differentiated thyoid carcinomas (n=29), 19/20 PDTC, 16/23 ATC, and 18/21 thyroid cancer cell lines including 11 ATC-derived ones. The cell lines lacking CDK4 phosphorylation were insensitive to CDK4/6i. RNA-sequencing and immunohistochemistry revealed that tumors and cell lines without phosphorylated CDK4 presented very high p16CDKN2A levels that were associated with proliferative activity. One of the main mechanisms of resistance to CDK4/6i is RB1 defects or inactivation. RB1 mutations were present in the 3 insensitive cell lines but were not found in 5 of the 7 tumors without phosphorylated CDK4. p16/KI67 immunohistochemistry and a previously developed 11-gene signature identified the likely insensitive tumors and cell lines lacking CDK4 phosphorylation. In cell lines, palbociclib synergized with dabrafenib/trametinib, completely and irreversibly arresting proliferation. The combined drugs prevented resistance mechanisms induced by palbociclib, most notably Cyclin E1-CDK2 activation and a paradoxical stabilization of phosphorylated CDK4 complexes. Our study supports the evaluation of CDK4/6i for ATC/PDTC treatment, including in combination with MEK/BRAF inhibitors. Overall design: 16 thyroid cancer cell lines in basal conditions were profilled by RNA-seq.