Molecular genetic studies of human thyroid tumors

Abstract

Tumors of the thyroid gland are common among the general population, but only a small proportion is malignant. The vast majority of thyroid tumors are benign, so called follicular adenomas. Sometimes subtypes such as atypical adenomas and adenomas with oxyphil, i.e. Hurthle cell differentiation, are detected. In Sweden 400 malignant thyroid tumors are diagnosed each year, representing 1% of all cancers reported. Thyroid carcinomas are divided into four different variants: papillary thyroid carcinoma (PTC), follicular thyroid carcinoma (FTC), medullary thyroid carcinoma (MTC) and anaplastic thyroid carcinoma (ATC), the latter being undifferentiated. In absence of clear signs of metastases or invasive growth, it is preoperatively impossible to separate benign from malignant follicular tumors. Therefore, virtually all patients with a follicular tumor are operated on. FTCs are generally sporadic tumors but can be found in patients with Cowden disease (CD), an inherited tumor syndrome characterized by multiple hamartomas and an increased risk of developing breast and thyroid cancer. PTEN is the disease gene for CD and is thereby also a candidate gene for the oncogenesis of sporadic FTC.

To study the role of PTEN in sporadic FTC development, 95 benign and malignant thyroid tumors of different types were investigated by loss of heterozygosity (LOH) and by direct sequencing of PTEN (paper 1). Structural mutations of PTEN were found in I I cases only. As PTEN, despite the rare structural alterations, may be involved in thyroid tumorigenesis via other mechanisms, expression of PTEN was studied in 87 thyroid tumors by reverse transcriptase polymerase chain reaction (RT- PCR) and restriction enzyme cleavage (paper ll). Fourteen of the tumors showed reduced PTEN expression, but only few of these were differentiated tumors. In contrast, the ATCs had a significantly reduced expression. PTEN seems to be involved in the development of some thyroid tumors, and may contribute to the progression towards the undifferentiated ATC.

To detect chromosomal regions potentially harboring tumor suppressor genes or oncogenes, comparative genomic hybridization (CGH) was used to screen 21 follicular tumors (paper 111). Chromosomal aberrations were detected equally often in the benign variants as in the malignant variants, indicating that there is no stepwise progression from adenomas to carcinomas. FTCs with Hurthle cell differentiation all had deletions of chromosome 9q13-21.3. This genomic region may thus harbor a tumor suppressor gene important for these tumors. MTC is sporadic in 75% of the cases. In the remaining ones, MTC is part of multiple endocrine neoplasia type 2 (MEN 2) and its variants. The RET proto-oncogene is the disease gene in the familial cases, but mutations of RET have also been detected in a proportion of sporadic MTCs. RET encodes for a receptor tyrosine kinase and several ligands are known to activate RET. By using mRNA in situ hybridization, the expression of RET and its ligand complexes were analyzed in 15 MTCs with or without the RET M918T mutation (paper IV).

All MTCs showed expression of RET, and the majority also expressed the ligand complexes. The mRNA expression was verified by RT-PCR. A strong expression of the genes was seen in tumor cells adjacent to the stroma. in 7 MTCs, indicating that not only mutations, but also the expression level of RET and its ligand complexes are of importance in the development and progression of MTC. CGH was performed on 24 MTCs with or without the RET M918T mutation(paper V). MTC seems to be a genetically relatively stable tumor, with fewer alterations found in the tumors without the RET mutation. The tumors from patients who died from their MTC all showed a gain of chromosome 11cq12.

In summary these studies show that although thyroid tumors are genetically relatively stable, specific genes and chromosomal regions are involved in their development and presumably also in their progression. This work emphasizes that the identification of such genetic alterations are crucial for the understanding of the variable clinical features of these tumors, and the need for a more individualized treatment.