Functional analysis of oxidative phosphorylation system genes in thyroid tumors

Abstract

Hürthle cell thyroid tumors (HCT) constitute an unusual form of neoplasm composed of cells with voluminous, granular, eosinophilic cytoplasm due to the huge amount of abnormal mitochondria. The molecular basis for the occurrence of HCT is still unknown. Mitochondria have been proposed to play an important role in HCT formation. The increased number of mitochondria and the mitochondrial structural abnormalities observed in HCT mimic those detected in the cells of patients with several mitochondrial diseases and myopathies. Mitochondria play an essential role in cellular energy production and it has been proposed thatmitochondrial proliferation in HCT might be a compensatory mechanism for a decline in oxidative phosphorylation. A high prevalence of non-silent point mutations of Complex I genes has been reported in thyroid carcinomas in comparison with follicular adenomas. These mtDNA somatic mutations of Complex I genes appear to be involved in thyroid tumorigenesis in general and in the occurrence of Hürthle cell tumors in particular. Recently, a novel mitochondrial-related gene, GRIM-19, has been identified. GRIM-19 is a cell death regulatory gene that promotes apoptosis; is a negative regulator of cell growth; and is also involved in mitochondrial metabolism. GRIM-19 is the human homologue of the bovine subunit of the mitochondrial Complex I. GRIM-19 has been found to fulfil two roles within the cell: as a member of the interferon-b and retinoic acid-induced pathway of cell death, and as part of the mitochondrial Complex I assembly. These two seemingly disparate functions may be associated via the involvement of mitochondria in apoptotic cell death. The dual role of GRIM-19 in apoptosis and mitochondrial biogenesis makes it a good candidate for being involved in Hürthle cell tumorigenesis. The GRIM-19 mutations are the first nuclear gene mutations specific to Hürthle cell tumors to be reported to date. The association between sporadic and familial Hürthle cell carcinomas and GRIM-19 mutations and the upregulation of ICAM1 in the cases in which GRIM-19 mutations were demonstrated, suggest that these mutations are inactivating mutations. In the present study, we intend to verify if GRIM-19 mutations are exclusive of Hürthle cell tumors and if they are restricted to carcinomas. We also intend to demonstrate, by in vitro assays, the functional alterations induced by GRIM-19 "silencing", namely in mitochondrial respiratory chain activity, expression of GRIM-19-regulated genes, apoptosis and cell proliferation. (AU)