Papillary thyroid cancer: high inter-(simple sequence repeat) genomic instability in a typically indolent cancer

The object of this study is to measure genomic instability in papillary thyroid cancer and correlate these measurements with known clinical prognosticators such as patient age, tumor size, histologic subtype, and three commonly used thyroid risk assessment indices. A secondary objective of this study was to use the measurements of genomic instability to estimate the number of mutational events present in the papillary thyroid cancer genome.

Inter-simple sequence repeat polymerase chain reaction (ISSR-PCR) is a rapid and reproducible technique for quantitation of genomic instability, or the degree of genome alteration, in solid tumors. This includes quantitation of amplifications, deletions, translocations, and insertions. Twenty-eight papillary carcinomas were evaluated by ISSR-PCR.

Evaluation of 28 papillary carcinomas by ISSR-PCR demonstrated a wide range of genomic instability. Of the panel of clinicopathologic factors examined, only patient age was significantly associated with genomic instability. The mean genomic instability index value was greatest in the youngest age group, which was significantly different from the median value measured in the oldest age group (3.7, 2.5, respectively, p =.05). The mean value in the intermediate age group fell between the younger and older groups (3.1). By use of ISSR-PCR, we have calculated 15,000 individual genomic events as having occurred in each papillary tumor cell.

Despite its generally indolent biologic behavior, papillary thyroid cancer exhibits a high degree of genomic instability comparable to that seen in colorectal cancer. These results suggest that elevated genomic instability, as measured by ISSR-PCR, may not be sufficient to enable thyroid tumor progression to less indolent disease and that this process is severely constrained by some additional essential factor such as the differentiated state of the tissue or the need to bring into play an additional form of genomic destabilization.