Cytogenetics of thyroid nodules in Hashimoto thyroiditis

[Inflammatory diseases of the thyroid gland]

This review article deals with the classification, clinical features and morphology of thyroiditis. These inflammatory diseases account for approximately 20 % of all thyroid diseases. The vast majority of cases of thyroiditis are of immunogenic origin while non-immunogenic thyroiditis (caused by pathogens or iatrogenic) is a rarity.

Cytogenetic and molecular events in adenoma and well-differentiated thyroid follicular-cell neoplasia

In spite of its simple organization, the thyroid gland can give rise to a wide spectrum of neoplasms, ranging from innocuous to highly malignant lesions. Approximately 94% of the malignancies is represented by well-differentiated thyroid carcinoma originating from follicular cells. These neoplasms are divided into two main categories, papillary thyroid carcinoma and follicular thyroid carcinoma. Despite their origin from the same type of cells, the two neoplasias show different biological behavior and a different set of genetic features, including specific cytogenetic patterns. Thyroid adenoma is the benign counterpart of follicular carcinoma. No benign counterpart of papillary carcinoma has yet been identified. The chromosomes of thyroid nodules have been investigated since 1965, and different cytogenetic subgroups have been recognized, some of which show structural chromosomal rearrangements. These structural changes lead to the formation of fusion genes RET-PTC, TRK(-T), and BRAF-AKAP9, which originate as a result of intrachromosomal or interchromosomal rearrangements and are found in papillary thyroid carcinoma. Fusion genes involving PPARγ are caused mainly by translocations and are characteristic of follicular neoplastic tissue. Radiation exposure and the particular architectural arrangement of chromatin regions in which the affected genes lie during interphase are thought to favor the formation of fusion genes in papillary thyroid carcinoma and possibly also in follicular thyroid carcinoma.

Karyotypic characterization of 64 nonmalignant thyroid goiters

Cytogenetic analyses were performed on 64 nonmalignant thyroid goiters (11 common and 53 multinodular goiters) after short-term culture. The majority of goiters (67%) were characterized by a normal karyotype, but in 5 common (45%) and 16 nodular (30%) goiters, small clones with various numerical and/or structural aberrations were found, in addition to many normal cells. Trisomy or tetrasomy 7 was the most frequent numerical aberration, seen in five cases. Deletion of 18p11 was found in four cases, and in three of them as the sole change. Selection and clonal evolution of aneuploid cells present in nonmalignant goiters could underlie progression into adenoma formation.

Detection and molecular characterisation of thyroid cancer precursor lesions in a specific subset of Hashimoto's thyroiditis

Hashimoto's thyroiditis (HT) represents the most common cause of hypothyroidism and nonendemic goiter, but its clinical and pathological heterogeneity opens the question if this disease should be more properly considered as a spectrum of different thyroid conditions rather than as a single nosological entity. In this study, we analysed 133 cases of HT for the expression of galectin-3, a lectin molecule involved in malignant transformation, apoptosis and cell cycle control. An unexpected expression of galectin-3 was demonstrated in a subset of HT together with the presence of HBME-1, c-met and cyclin-D1 that are also involved in malignant transformation and deregulated cell growth. Furthermore, a loss of allelic heterozygosity in a specific cancer-related chromosomal region was demonstrated in some HT harbouring galectin-3-positive follicular cells, by using laser capture microdissection. On the basis of the morphological and molecular findings we identified four subsets of HT: (a) HT with classic features of chronic autoimmune thyroiditis; (b) HT associated to hyperplastic/adenomatous lesions; (c) HT harbouring thyroid cancer precursors; (d) HT associated to unequivocal thyroid microcarcinomas. Our findings provide a well-substantiated morphological and molecular demonstration that HT may include a spectrum of different thyroid conditions ranging from chronic autoimmune thyroiditis to thyroiditis triggered by specific immune-response to cancer-related antigens.

Aneuploidy in oncocytic lesions of the thyroid gland: diffuse accumulation of mitochondria within the cell is associated with trisomy 7 and progressive numerical chromosomal alterations

Oncocytic cells are characterized by a greatly increased number of mitochondria that distend the cell cytoplasm and result in a distinctive granular appearance of the cell on conventional histology sections. Oncocytes are frequently found in metabolically active human tissues including the thyroid gland, and, as a general rule, when their proportion in a thyroid tumor is greater than 75% the tumor is referred to as oncocytic (Hürthle cell) adenoma or carcinoma. Such tumors represent a subset of thyroid lesions, and recently, both interphase fluorescence in situ hybridization (FISH) and comparative genomic hybridization (CGH) studies reported that they may show aneuploidy, with widespread numerical chromosomal alterations. In contrast, very few cases have been studied by conventional cytogenetic analysis. Whether the cells with chromosomal changes are the same as those with mitochondrial accumulation or whether lesions only partially composed of oncocytic cells also have cytogenetic alterations is unclear. To investigate the relationship between acquisition of the oncocytic phenotype and numerical chromosomal changes, we analyzed a random selection of thyroid lesions with (18 cases) and without (11 cases) morphological evidence of oncocytic differentiation. Lesions with oncocytes included hyperplastic nodules, adenomas, Hürthle cell tumors, and papillary carcinomas with lymphocytic stroma (Whartin-like tumors of the thyroid). Karyotypic changes were analyzed by cytogenetic analysis, FISH, or CGH, and the results were compared with in situ analysis of mitochondrial accumulation by immunofluorescence. A striking correlation between the presence of oncocytes and the presence of aneuploid katyotypes was seen in the oncocytic follicular thyroid nodules, but not in the oncocytic papillary tumors. Structural chromosome changes or normal karyotypes were observed in the lesions lacking oncocytic features. Extending the FICTION technique to the evaluation of a cytoplasmic antigen (mitochondrial membrane antigen), we pursued the simultaneous visualization of both mitochondrial increase and numerical chromosomal alterations, and showed that oncocytes of follicular lesions are prone to become aneuploid. Our data support the contention that follicular tumors composed of oncocytes should be regarded as a distinct subset.

Prevalence of RET/PTC rearrangements in Hashimoto's thyroiditis and papillary thyroid carcinomas

The relationship between Hashimoto's thyroiditis (HT) and follicular cell-derived thyroid cancer remains unclear. Recently, 2 studies reported a 95% prevalence of RET/PTC rearrangements in histologically benign tissue affected by HT, suggesting that multiple occult tumors exist in HT patients with high frequency. We tested the prevalence of RET/PTC rearrangements in 26 HT, in 6 papillary carcinomas arising in the background of HT, and in 27 papillary carcinomas not associated with HT. We detected no RET/PTC rearrangements in HT or papillary carcinomas arising in the background of HT, in contrast to a 33% prevalence among papillary carcinomas not associated with HT. However, the expression of wild-type RET was found in more than half of papillary carcinomas. These results suggest that, if the association between HT and thyroid cancer exists, its molecular basis is different from RET/PTC rearrangement.