Mitochondrial DNA and human thyroid diseases

Cells of the thyroid tissue, either diseased or normal, can accumulate altered mitochondrial genomes in primary lesions and in surrounding parenchyma. Depending on the experimental approaches and the extent of the mutational process, it has been possible to demonstrate the occurrence of homoplasmic or heteroplasmic point mutations, presence of a common deletion and random large-scale mtDNA aberrations in various pathological states. Point somatic mutations documented in 5-60% of thyroid tumors do not concentrate in obvious hotspots but tend to cluster in certain regions of the mitochondrial genome and their distribution may differ between carcinomas and controls. Large-scale deletions in mtDNA are quite prevalent in healthy and diseased thyroid; however, the proportion of aberrant mtDNA molecules accounts for a very small part of total mtDNA and does not seem to correlate with pathological characteristics of thyroid tumors. Common deletion is most abundant in Hurthle cell tumors, yet it also occurs in other thyroid diseases as well as in normal tissue. The principal difference between the common deletion and other deletion-type mtDNA molecules is that the former does not depend on the relative mtDNA content in the tissue whereas in a subset of thyroid tumors, such as radiation-associated papillary carcinomas and follicular adenomas, there is a strong correlation between mtDNA levels and prevalence of large-scale deletions. Relative mtDNA levels by themselves are elevated in most thyroid tumors compared to normal tissue. Distinct differential distribution and prevalence of mutational mtDNA burden in normal tissue and thyroid lesions are suggestive of the implication of altered mtDNA in thyroid diseases, especially in cancer.