p18INK4C and BRCA1 inhibit follicular cell proliferation and dedifferentiation in thyroid cancer

Only 3% of thyroid cancers are medullary thyroid carcinomas (MTCs), the rest are follicular epithelial cell derived non-MTCs (NMTCs). A dysfunctional INK4-CDK4-RB pathway is detected in most of NMTCs. DNA repair defects and genome instability are associated with NMTC dedifferentiation and aggressiveness. Whether inactivation of the INK4-CDK4-RB pathway induces NMTCs and how differentiation of NMTC cells is controlled remain elusive. In this study, we generated p18Ink4c and Brca1 singly and doubly deficient mice as well as p16Ink4a and Brca1 singly and doubly deficient mice. By using these mice and human thyroid carcinoma cell lines, we discovered that loss of p18Ink4c, not p16Ink4a, in mice stimulated follicular cell proliferation and induced NMTCs. Depletion of Brca1 alone or both p16Ink4a and Brca1 did not induce thyroid tumor. Depletion of Brca1 in p18Ink4c null mice results in poorly differentiated and aggressive NMTCs with epithelial-mesenchymal transition (EMT) features and enhanced DNA damage. Knockdown of BRCA1 in thyroid carcinoma cells activated EMT and promoted tumorigenesis whereas overexpression of BRCA1 inhibited EMT. BRCA1 and EMT marker expression were inversely related in human thyroid cancers. Our finding, for the first time, demonstrates that inactivation of INK4-CDK4-RB pathway induces NMTCs and that Brca1 deficiency promotes dedifferentiation of NMTC cells. These results suggest that BRCA1 and p18INK4C collaboratively suppress thyroid tumorigenesis and progression and CDK4 inhibitors will be effective for treatment of INK4-inactivated or cyclin D-overexpressed thyroid carcinomas.