THE NATURAL PROTEIN-KINASE C-ALPHA MUTANT IS PRESENT IN HUMAN THYROIDNEOPLASMS

An altered protein expression of Ca2+-dependent protein kinase C (PKC) isoforms and a point mutation in the PKC alpha cDNA (position 908 of the nucleotide sequence, position 294 of the amino acid sequence, subs titution of an aspartic acid by a glycine) have been previously descri bed in a subpopulation of human pituitary tumors. In this work, we scr eened 16 thyroid tissue samples (four follicular adenomas, five colloi d adenomas, three papillary carcinomas, one follicular carcinoma and t hree normal tissues adjacent to the tumors) for the presence of the PK C alpha point mutation and for PKC alpha, beta 1, beta 2, epsilon and delta protein expression. Screening for the presence of the PKC alpha mutant was performed by a subcloning technic. The polymerase chain rea ction products were generated using reverse-transcribed cDNAs, subclon ed and sequenced (10 clones were routinely sequenced). The PKC alpha p oint mutation at position 908 of the cDNA sequence was found in four o ut of the nine adenomas and in the follicular carcinoma. It was neithe r detected in the papillary carcinomas nor in the adjacent normal tiss ues (one was the adjacent normal tissue of the follicular carcinoma; i n this sample, genomic DNA and cDNA were used to look for the presence of the mutant), demonstrating the somatic nature of this mutant. West ern blot analysis of PKC isoforms showed that the expression of all is oforms was higher in the thyroid neoplasms as compared with their adja cent normal tissue (n=3). It was also higher in the samples containing the PKC mutant (two follicular adenomas, two colloid adenomas and the follicular carcinoma) as compared with the tumors where it was not de tected (three papillary carcinomas and five adenomas). Samples could b e ordered according to their increasing PKC expression as follows: nor mal adjacent tissue < follicular adenomas without PKC alpha mutant les s than or equal to papillary carcinoma < follicular adenomas with PKC mutant < follicular carcinoma with PKC mutant. In conclusion, the disc overy of the PKC alpha mutant in thyroid neoplasms demonstrates that t his mutant is not particular to human pituitary tumors where it was or iginally detected. It is a somatic mutation and its presence is concom itant with high levels of all of the PKC isoforms analysed. The presen ce of the PKC mutant in thyroid neoplasms raises the question of its i mportance in thyroid tumorigenesis.