Mutant thyroid hormone receptors (TRs) isolated from distinct cancer types display distinct target gene specificities: a unique regulatory repertoire associated with renal clear cell carcinomas

Thyroid hormone receptors (TRs) are hormone-regulated transcription factors that regulate a diverse array of biological activities, including metabolism, homeostasis, and development. TRs also serve as tumor suppressors, and aberrant TR function (via mutation, deletion, or altered expression) is associated with a spectrum of both neoplastic and endocrine diseases. A particularly high frequency of TR mutations has been reported in renal clear cell carcinoma (RCCC) and in hepatocellular carcinoma (HCC). We have shown that HCC-TR mutants regulate only a fraction of the genes targeted by wild-type TRs, but have gained the ability to regulate other, unique, targets. We have suggested that this altered gene recognition may contribute to the neoplastic phenotype. Here, to determine the generality of this phenomenon, we examined a distinct set of TR mutants associated with RCCCs. We report that two different TR mutants, isolated from independent RCCC tumors, possess greatly expanded target gene specificities that extensively overlap one another, but only minimally overlap that of the WT-TRs, or those of two HCC-TR mutants. Many of the genes targeted by either or both RCCC-TR mutants have been previously implicated in RCCC, and include a series of metallothioneins, solute carriers, and genes involved in glycolysis and energy metabolism. We propose that TR mutations from RCCC and HCC are likely to play tissue-specific roles in carcinogenesis, and that the divergent target gene recognition patterns of TR mutants isolated from the two different types of tumors arises from different selective pressures during development of RCCC versus HCC. Gene expression was analyzed in HepG2 transformants expressing ectopic wildtype THRA, wildtype THRB, HCC-TR mutants αΙ and βN, and RCCC-TR mutants 6α and 15β using Affymetrix Human Gene 1.0 ST arrays in the presence or absence of T3. Each HepG2 transformant was assayed in triplicate.