Medullary carcinomas of the thyroid: a monoclonal origin.

We studied the clonality of medullary thyroid carcinomas (MTC) from 16 female patients by determining X chromosome inactivation by polymerase chain reaction (PCR) amplification of a CAG repeat in exon 1 of the human androgen-receptor gene. One patient with sporadic medullary thyroid carcinoma (MTC) was homozygous for this microsatellite and was not considered for the assessment of clonality. Sixteen tumor samples from the informative 15 patients were studied: 11 were from sporadic cases and 5 were from familial cases (3 cases of multiple endocrine neoplasia type 2A [MEN 2A]; 1 case of familial medullary thyroid carcinoma [FMTC]). Fourteen tumor samples (10/11 sporadic, 3/4 MEN 2A and 1/1 FMTC) were clearly monoclonal with allelic cleavage ratios between 2.5 and 49.1. Sixty-four percent of these cases (9/14) had the preferential amplification of the shorter allele while 36 percent (5/14) had the preferential amplification of the longer allele. Two frozen tumor samples (1 sporadic and 1 MEN 2A) were polyclonal. However, the corresponding tumor embedded in paraffin from the sporadic case was monoclonal. The other polyclonal tumor was found in the right thyroid lobe of a patient with MEN 2A who had a monoclonal tumor in the left lobe. Our results clearly demonstrate that MTC have a monoclonal origin in the majority of the cases.