Loss of heterozygosity of the long arm of chromosome 7 in follicular and anaplastic thyroid cancer, but not in papillary thyroid cancer

Papillary thyroid cancer (PTC), but neither the follicular nor the anaplastic histotype [follicular thyroid cancer (FTC), anaplastic thyroid cancer (ATC)], overexpresses simultaneously the protooncogene HGF (hepatocyte growth factor) and its receptor HGF-R (or c-met). Because 1) HGF and c-met map to chromosome 7q21 and 7q31, respectively, 2) FTC loses genetic material at multiple loci with a frequency much higher than PTC, and 3) loss of heterozygosity (LOH) on 7q has been previously found in various tumors, we tested the hypothesis that both FTC and ATC, but not PTC, could harbor LOH in segments of 7q encompassing the loci for HGF and c-met. We screened 6 normal thyroids, 10 colloid nodules, 10 follicular hyperplasias, 10 oncocytic adenomas, 10 follicular adenomas (FA), 10 FTC, 6 ATC, 12 PTC using two microsatellite markers for HGF, and two for c-met. LOH for all 4 markers was found in 100% of FTC, 100% of ATC, and (for only 1 or 2 markers) in 10-29% of FA. This is the first demonstration of an LOH that separates both FTC and ATC from PTC, in the best possible manner: 100% vs. 0%. Clearly, each of the two segments we have probed contains at least one tumor suppressor gene, whose inactivation is crucial for the establishment of the FTC (and ATC) phenotype. This loss of genetic material explains why FTC and ATC, but not PTC, fail to express both HGF and c-met. Our findings may also have immediate diagnostic application, in the context of assisting pathologists in the often difficult task of distinguishing FA from FTC.