Defective Oxidative Phosphorylation in Thyroid Oncocytic Carcinoma Is Associated with Pathogenic Mitochondrial DNA Mutations Affecting Complexes I and III

Abstract

Oncocytic tumors are characterized by cells with an aberrant accumulation of mitochondria. To assess mitochondrial function in neoplastic oncocytic cells, we studied the thyroid oncocytic cell line XTC.UC1 and compared it with other thyroid non-oncocytic cell lines. Only XTC.UC1 cells were unable to survive in galactose, a condition forcing cells to rely solely on mitochondria for energy production. The rate of respiration and mitochondrial ATP synthesis driven by complex I substrates was severely reduced in XTC.UC1 cells. Furthermore, the enzymatic activity of complexes I and III was dramatically decreased in these cells compared with controls, in conjunction with a strongly enhanced production of reactive oxygen species. Osteosarcoma-derived transmitochondrial cell hybrids (cybrids) carrying XTC.UC1 mitochondrial DNA (mtDNA) were generated to discriminate whether the energetic failure depended on mitochondrial or nuclear DNA mutations. In galactose medium, XTC.UC1 cybrid clones showed reduced viability and ATP content, similarly to the parental XTC.UC1, clearly pointing to the existence of mtDNA alterations. Sequencing of XTC.UC1 mtDNA identified a frameshift mutation in ND1 and a nonconservative substitution in cytochrome b, two mutations with a clear pathogenic potential. In conclusion, this is the first demonstration that mitochondrial dysfunction of XTC.UC1 is due to a combined complex I/III defect associated with mtDNA mutations, as proven by the transfer of the defective energetic phenotype with the mitochondrial genome into the cybrids. (Cancer Res 2006; 66(12): 6087-96)

Disruptive mitochondrial DNA mutations in complex I subunits are markers of oncocytic phenotype in thyroid tumors

“…Two samples (Table 1) harbored the same mutation (3571insC) in the ND1 gene that we recently reported in the XTC.UC1 oncocytic cell line, in which we demonstrated complete absence of the protein and defective activity of complex I (10).…”

“…Recently, we have attempted to clarify this point by clearly demonstrating the association between mtDNA mutations and defective oxidative phosphorylation in a cell line model of thyroid oncocytic tumor (10). The term ''oncocytic'' is used to designate lesions composed of cells with aberrant accumulation of mitochondria, resulting in a distinctive granular eosinophilic appearance on conventional histology.…”

Lack of complex I is associated with oncocytic thyroid tumours

“…Renal oncocytoma, another type of oncocytic tumour with a benign course, was recently found to be associated with complex I deficiency (Gasparre et al, 2008;Mayr et al, 2008). The oncocytic tumour cell line XTC.UC1, which is derived from a metastasis of a Hürthle cell carcinoma, shows both an insertion mutation in the gene encoding ND1 subunit of complex I and a reduction in complex I activity (Bonora et al, 2006). Ishikawa et al (2008) reported complex I deficiency caused by mutations in the ND6 gene of complex I in non-oncocytic mouse lung tumours.…”

“…An association between mitochondrial DNA (mtDNA) mutations and oncocytic thyroid tumours was reported in several studies (Tallini et al, 1994;Maximo et al, 2002;Bonora et al, 2006). A study on 45 oncocytic thyroid tumours identified potentially pathogenic mutations in subunits of complex I (NADH-ubiquinone oxidoreductase) in 53% of the investigated samples (Gasparre et al, 2007).…”

Metabolic reprogramming of the tumor

“…Warburg averred that an irreversible injury to respiration had to occur in order to trigger the progressive striving of cells that had succeeded to engage in lactic fermentation to fulfill their energy needs, leading ultimately to the formation of a tumor (Warburg, 1956a). Mutations in mitochondrial DNA (Bonora et al, 2006), and defective mitochondrial lipid content and structure (Kiebish et al, 2008), were found to squelch respiration in some tumors, while malfunction of some Krebs cycle enzymes also contributes to apoptosis resistance and aerobic glycolysis (King et al, 2006). This suggests that mitochondrial defects may have a role in certain cancers.…”