Mitochondrial DNA sequence variants in epithelial ovarian tumor subtypes and stages

Abstract

Background: A majority of primary ovarian neoplasms arise from cell surface epithelium of the ovaries. Although old age and a positive family history are associated risk factors, the etiology of the epithelial ovarian tumors is not completely understood. Additionally, knowledge of factors involved in the histogenesis of the various subtypes of this tumor as well as those factors that promote progression to advanced stages of ovarian malignancy are largely unknown. Current evidence suggests that mitochondrial alterations involved in cellular signaling pathways may be associated with tumorigenesis.

Methods: In this study, we determined the presence of polymorphisms and other sequence variants of mitochondrial DNA (mtDNA) in 102 epithelial ovarian tumors including 10 matched normal tissues that paired with some of the tumors. High-resolution restriction endonucleases and PCR-based sequencing were used to assess the mtDNA variants spanning 3.3 kb fragment that comprised the D-Loop and 12S rRNA-tRNAphe, tRNAval, tRNAser, tRNAasp, tRNAlys, ATPase 6, ATPase 8, cytochrome oxidase I and II genes.

Results: Three hundred and fifty-two (352) mtDNA sequence variants were identified, of which 238 of 352 (68%) have not been previously reported. There were relatively high frequencies of three mutations in the 12S rRNA gene at np 772, 773, and 780 in stage IIIC endometrioid tumors, two of which are novel (773delT and 780delC), and occurred with a frequency of 100% (7/7). Furthermore, two mutations were observed in serous tumors only at np 1657 in stage IV (10/10), and at np 8221delA in benign cystadenomas (3/3) and borderline tumors (4/4). A high frequency, 81% (13/16) of TC insertion at np 310 was found only in early stages of serous subtype (benign cystadenomas, 3/3; borderline tumors, 4/4; stage I tumors, 2/5 and matched normal tissues 4/4).

Conclusion: Our findings indicate that certain mtDNA mutations can reliably distinguish the different histologic subtypes of epithelial ovarian tumors. In addition, these data raise the possibility that certain mtDNA mutations may be useful biomarkers for predicting tumor aggressiveness and may play a potential role in tumorigenesis.

Figure 1

Distributions of the most frequent mtDNA mutations (984delC-12S rRNA; 1648delC, T1653A, 1659delT-tRNA^val; 8877delC-ATPase 6; T16519C, A73G, A263G-D-loop region) in three epithelial ovarian tumor subtypes compared to 100 human genome sequences [24]. Mutation A263G appears to be less frequent in the epithelial ovarian tumor subtypes compared to the general population and the mtDNA-tRNA^val 1653delT variant may be considered as a causative event for the three epithelial ovarian tumor subtypes.

Figure 2

A. MtDNA sequence electropherograms showing variations in consecutive C-stretch at np 303–310 of the D-loop region obtained from the three subtypes of the epithelial ovarian neoplasms (a) C₆TC₆ and (b) C₇TC₅ sequences from stage I of endometrioid tumor; (c) C₇TC₆ and (d) C₈TC₆ sequences from stage I of mucinous tumor; (e) C₉TC₆ sequence obtained from stage III of mucinous tumor; (f) C₁₀TC₆ sequence from stage III of serous tumor. Interestingly, the C-stretch instability at np 303–315 was observed in 97% of our study samples. B. Sequence electropherograms showing the mtDNA instability at np (a) 309insCT in stage II of endometrioid tumor and (b) 310insTC observed only in early stages of serous subtype in this study. Arrow shows the unique 309insCT and 310insTC patterns respectively.