Point mutations of ras oncogenes are an early event in thyroid tumorigenesis.

Identifying the nature of the genetic mutations in thyroid neoplasms and their prevalence in the various tumor phenotypes is critical to understanding their pathogenesis. Mutational activation of ras oncogenes in human tumors occurs predominantly through point mutations in two functional regions of the molecules, codons 12, 13 (GTP-binding domain) or codon 61 (GTPase domain). We examined the prevalence of point mutations in codons 12, 13, and 61 of the oncogenes K-ras, N-ras, and H-ras in benign and malignant human thyroid tumors by hybridization of PCR-amplified tumor DNA with synthetic oligodeoxynucleotide probes. None of the eight normal thyroid tissues harbored point mutations. Four of nineteen nodules from multinodular goiters (21%), 6/24 microfollicular adenomas (25%), 3/14 papillary carcinomas (21%), and 0/3 follicular carcinomas contained ras point mutations. The predominant mutation was a valine for glycine substitution in codon 12 of H-ras. None of the multinodular goiter tumors known to be polyclonal (and thus due to hyperplasia) had point mutations, whereas one of the two monoclonal adenomas arising in nodular glands contained in H-ras codon 12 valine substitution, which was confirmed by sequencing the tumor DNA. These data show that ras activation is about equally prevalent in benign and malignant thyroid neoplasms, and thus may be an early event in the tumorigenic process.