Molecular Biology of the Chromosomal Instability Phenotype of Follicular Cell Thyroid Tumors

Abstract

The overall aim of the present thesis was to progress in the understanding of the pathogenic and diagnostic meaning of some numerical and structural chromosomal abnormalities that are frequently detected in follicular cell derived tumors of the thyroid. The flow cytometry study of 143 thyroid lesions revealed that the aneuploidy in follicular tumors is strongly associated with a particular morphologic pattern, the so-called fetal type of growth pattern, leading to the individualization of fetal adenomas (FTA) from the DNA content standpoint. Moreover, we found that a high percentage of FTA, as well as follicular thyroid carcinomas (FTC) with a fetal adenoma growth pattern, were within the triploid range. The results obtained by CGH (and confirmed by FISH) revealed that aneuploidy in FTA and FTC is usually represented by whole chromosome gains. Not all chromosomes are affected in the same way. Despite being frequently triploid, FTA display a highly repetitive pattern characterized by four copies of some chromosomes, three copies of several other chromosomes, and two copies of others. The follicular tumors displaying a distinct DNA content had a usual follicular growth pattern and showed a CGH profile with very few alterations and a trend toward the loss of some chromosomes. The CGH results suggest that follicular tumorigenesis may follow at least two pathways: one characterized by prominent aneuploidy and numerous gains, in which the tumors display a fetal adenomalike growth pattern, and another pathway accompanied by less obvious aneuploidy or even quasi diploidy and dominant chromosome losses, in which the tumors display a “common follicular architecture.” To investigate the putative molecular alteration underlying aneuploidization in follicular tumors, the RAS mutational status was investigated in 85 tumors. The results showed that RAS mutations are not a major cause of aneuploidy in thyroid tumors, but there is a strong association between the H-RAS 81C polymorphism and the presence of aneuploidy. Since the H-RAS 81C polymorphism does not alter the RAS protein, the association has to be explained in another way: We advanced that such association could reflect the increased H-RAS mRNA expression found in these tumors, as well as by the preferential expression of the H-RAS p21 isoform, which is the isoform that is able to activate the downstream effectors of RAS. Besides FTA and FTC, there is a third follicular patterned thyroid lesion, the follicular variant of papillary thyroid carcinoma (FVPTC), which is characterized by consistent diploidy and a disputable pathogenesis. To investigate if this FVPTC, from a molecular