Oncocytic cell
tumor of the thyroid is composed of large polygonal cells with eosinophilic cytoplasm that is rich in mitochondria. These
tumors frequently have the mutations in
mitochondrial DNA encoding the mitochondrial electron transport system complex I. However, the mechanism for accumulation of abnormal mitochondria is unknown. A noncanonical mitophagy system has recently been identified, and mitochondria-eating
protein (MIEAP) plays a key role in this system. We therefore hypothesized that accumulation of abnormal mitochondria could be attributed to defective MIEAP expression in these
tumors. We first show that MIEAP was expressed in all the conventional thyroid
follicular adenomas (FAs)/adenomatous
goiters (AGs) but not in oncocytic FAs/AGs; its expression was defective not only in oncocytic
thyroid cancers but also in the majority of conventional
thyroid cancers. Expression of MIEAP was not correlated with methylation status of the 5'-UTR of the gene. Our functional analysis showed that exogenously induced MIEAP, but not PARK2, reduced the amounts of abnormal mitochondria, as indicated by decreased
reactive oxygen species levels, mitochondrial DNA / nuclear
DNA ratios, and cytoplasmic acidification. Therefore, together with previous studies showing that impaired mitochondrial function triggers compensatory mitochondrial biogenesis that causes an increase in the amounts of mitochondria, we conclude that, in oncocytic cell
tumors of the thyroid, increased abnormal mitochondria cannot be efficiently eliminated because of a loss of MIEAP expression, ie impaired MIEAP-mediated noncanonical mitophagy.