Estudo das alterações do ADN mitocondrial e do GRIM-19 em oncocitomas renais: comparação com os tumores de células de Hürthle da tireóide

Abstract

Renal oncocytomas are composed by cells whose cytoplasm is packed with an abnormally high mumber of biochemically and morphologically deficient mitochondria (oxyphilic, oncocytic or Hürthle cells). Tumours with an oncocytic fenotype, resulting from mitochondria accumulation, may occur in many organs, predominantly in tissues with low proliferation index. In the thyroid, where they are called Hürthle cell tumours, our group has identified alterations in the mitochondrial DNA (mtDNA) and in a nuclear gene that encodes for a mitochondrial protein (GRIM-19), with significant differences when compared to non-oncocytic tumours. Regarding renal oncocytomas there are no publications describing the occurrence of such alterations. The mitochondria accumulation in the cytoplasm of cells may be the result of a primary alteration in mtDNA that encodes for mitochondrial enzymes, or it can be a consequence of mutations in nuclear DNA (nDNA) that encodes for mitochondrial proteins. In an attempt to contribute to a better understanding of the tumourigenesis of oxyphilic cell tumours in general and of renal oncocytomas in particular, we studied 14 renal oncocytomas and the respective non neoplastic parenchyma regarding some mtDNA and nDNA alterations. The cases were reviewed and DNA extraction was performed in microdissected tissues obtained from the paraffin-embedded material. We searched for mtDNA alterations such as the common deletion, mutations in the D-loop non codifying region and in the subunits 6 and 8 of complex V (ATPase). GRIM-19 alterations were evaluated by immunohistochemistry. The mtDNA common deletion was detected in 78.6% of renal oncocytomas and in 50% of the cases in the respective non neoplastic parenchyma. Instability in the Dloop region was detected in 42.9% of the tumours. We identified somatic mutations in ATPase 6 and/or 8 in 14,3% of the cases. GRIM-19 expression was slightly less intense in the oncocytomas than in the adjacent proximal renal tubules. Our results do not differ substantially from those obtained in Hürthle cell tumours of the thyroid, namely in the kind of alteration and its relative frequency. The homogenous cellular fenotype of renal oncocytomas and the obvious oncocytic transformation observed in the non neoplastic renal parenchyma in most of the studied cases (12/14) indicates that the carcinogenic event must have occurred in a Estudo das alterações do ADN mitocondrial e do GRIM-19 em oncocitomas renais: Comparação com os tumores de células de Hürthle da tireóide 4 cell with a previous anomaly of mtDNA and/or nDNA that encodes for mitochondrial enzymes. This hypothesis is favoured by the presence of the common deletion in a higher percentage of the non-neoplastic parenchyma of oncocytomas (50%) than in nonneoplastic thyroid parenchyma in the cases of Hürthle cell tumours (25-33%). It is possible that the common deletion may be an indirect biomarker of the alterations occurring in mitochondrial biogenesis in these conditions. Mutations in ATPase 6 seem to occur predominantly in oxiphylic cell tumours, at least in kidney and thyroid. Our results do not favour the existence of a determinant role for the instability of the non codifying D-loop region in the oncocytic transformation, despite finding such alterations in 42.9% of the tumours. It is probable that mutations in the D-loop region are mainly the result of increased reactive oxygen species production by tumoural cells in general. The main role of GRIM-19 in renal tumourigenesis does not seem to be related to the mitochondrial respiratory chain, or at least, does not seem to be related with oncocytic features, since the expression of GRIM-19 is significantly more affected in renal cell carcinomas than in oncocytomas.