All-trans-retinoic acid treatment inhibits the growth of retinoic acid receptor beta messenger ribonucleic acid expressing thyroid cancer cell lines but does not reinduce the expression of thyroid-specific genes.

Conventional chemotherapy and radiotherapy are ineffective for the treatment of advanced thyroid tumors like poorly differentiated papillary, anaplastic, and medullary thyroid cancer. In the attempt to evaluate the possibility of using retinoic acid (RA) in the treatment of thyroid cancer refractory to conventional therapy, we studied the effect of all-trans-RA treatment on five human thyroid cancer cell lines. We found that WRO and NPA, derived from follicular and poorly differentiated human thyroid carcinoma, respectively, showed a growth inhibition after 25 and 21 d of RA treatment. Both apoptosis and a decrease in DNA synthesis were observed as mechanisms of growth inhibition. In the NPA cell line, a delay of cell-cycle progression has also been observed. On the contrary, we did not observe any recovery of mRNA expression of thyroid-specific genes and in particular of the sodium iodide symporter gene. The lack of recovery of radioiodide uptake after all-trans-RA treatment confirmed the inability to reexpress sodium iodide symporter mRNA. The main difference between the all-trans-RA responding cells (WRO and NPA) and the nonresponding cells [ARO, FRO (derived from human anaplastic thyroid tumors) and TT (derived from human medullary thyroid tumor)] was the basal and all-trans-RA induced RA receptor (RAR)beta mRNA expression. Interestingly, 14 thyroid tumors (10 papillary and four anaplastic) showed a significant lower expression of RARbeta mRNA when compared with normal thyroid tissues. In agreement with this result, only 30% of papillary thyroid carcinomas analyzed were positive for RARbeta protein expression with a degree of expression that was much lower than that found in normal thyroid tissue. In conclusion we found that all-trans-RA treatment can determine a significant in vitro growth inhibition especially in differentiated thyroid tumor-derived cell lines but it seems unable to reinduce the expression of thyroid-specific genes and in particular to reinduce the ability to take up iodine. The growth inhibition is likely due to apoptosis in an early phase and to a decrease of DNA synthesis later. In some cases, a delay of the cell-cycle progression also may be responsible for the growth inhibition. The finding of a basal and RA-induced RARbeta mRNA expression only in cell lines responding to all-trans-RA suggests that the growth inhibition might be mediated by RARbeta.

Identification of a novel point mutation in the RET gene (Ala883Thr), which is associated with medullary thyroid carcinoma phenotype only in homozygous condition.

The RET protooncogene mutations responsible for multiple endocrine neoplasia type 2 are inherited as autosomic dominant traits. We describe here a novel germline homozygous mutation in exon 15 of the RET gene that determines an amino acid substitution (Ala->Thr) at codon 883. The index case was a 51-yr-old patient with an apparently sporadic form of medullary thyroid cancer (MTC). RET gene mutations screening was performed in exons 10, 11, 13, 14, 15, and 16 by automatic sequence analysis. An unexpected homozygous GCT->ACT point mutation was found at codon 883 in exon 15 and confirmed by restriction analysis (Alu I). The presence of the two chromosomes 10 was confirmed by fluorescence in situ hybridization analysis on lymphocytes. As expected on the basis of the homozygosity of the index case, the parents were consanguineous (second-degree cousins). Eight relatives were further investigated: the mother, two sisters, and the son were positive for heterozygous RET mutation. The mother (82 yr old) showed a nodular goiter but was negative both for basal and pentagastrin stimulated calcitonin. The young son (15 yr old) and the two sisters (63 and 58 yr old) did not show any clinical and/or biochemical sign of MTC. One brother (59 yr old) was negative both for RET mutation and clinical/biochemical examination. The other brother, 56 yr of age, was positive for both homozygous RET mutation and serum calcitonin. When operated, the histological examination of the thyroid showed the presence of MTC and C cell hyperplasia. In conclusion, we identified a new germline RET gene mutation during a routine RET gene screening of an apparently sporadic MTC case. This mutation showed a very low transforming activity as demonstrated by the absence of MTC phenotype in heterozygous subjects. The possibility that the homozygous gene carriers were indeed carrying a germline loss of heterozygosity was excluded by fluorescence in situ hybridization analysis for RET gene performed on lymphocytes derived from one homozygous patient. The analysis of several RET polymorphisms also confirmed the presence of two mutated alleles in MTC affected patients and both mutated and wild-type allele in heterozygous subjects.