Mitochondrial DNA Mutations in Differentiated Thyroid Cancer with Respect to the Age Factor

Increased numbers of mitochondria in differentiated thyroid cancer and, most strikingly, mutations in human mitochondrial DNA (mtDNA) in older people have led to speculation that mtDNA mutations might contribute to aging or accumulate in postmitotic tissues with age. Mutation analyses of mtDNA in papillary (PTCs) and follicular (FTCs) thyroid carcinomas have been limited to date. The significance and frequency of mtDNA mutations in PTC and FTC are therefore controversial, as is age dependence.

We analyzed eight sample pairs of PTC and six of FTC tissue with the corresponding normal thyroid tissue. DNA was extracted from frozen and formaldehyde-fixed tissue using the QIAmp Tissue Kit. Sequence differences in the mtDNA between tumor and normal tissue were detected using appropriate polymerase chain reaction (PCR) products for heteroduplex analysis in a denaturing high performance liquid chromatography (HPLC) Wave System (Transgenomic). Mutations were confirmed and identified by sequencing the PCR products of conspicuous chromatograms. The samples were obtained from 346 patients with PTC and 105 patients with FTC. We analyzed the whole mitochondrial genome from seven PTC and three FTC tumors along with the corresponding normal thyroid tissue. 3/7 PTC samples showed two heteroplasmic mutations and one polymorphism; all 3 FTCs showed homoplasmic and/or heteroplasmic mutations.

All but one of these tumors are well documented in the mitochondrial database MITOMAP. MtDNA mutations were found in all three patients aged 45 years and older. There was no correlation, however, in this small group to clinical prognostic factors for recurrence and especially for survival in differentiated thyroid carcinomas, such as histology, tumor size, lymph node metastases, distant metastases, and gender, most likely because of the short follow-up. While univariate analysis of the findings in the whole cohort of 346 patients with PTC suggested that age is a significant prognostic factor for survival (P = 0.0237) but not for recurrence (P = 0.65), this was not the case in the 105 patients with FTC.

Although we found accumulation of mutations in two older patients with PTC and one patient with FTC (all three patients older than 45 years had mtDNA mutations), the low frequency of these mutations in the small group of 10 analyzed patients did not correlate with statistically validated clinical prognosticators for recurrence or survival, especially not with age. The low power of our data are therefore not able to support or refute the hypothesis that these mtDNA mutations are related to age-dependent tumor progression in the thyroid or that they “may be involved in thyroid tumorigenesis.”