Low copy number of mitochondrial DNA (mtDNA) predicts worse prognosis in early-stage laryngeal cancer patients

Abstract

Objectives: Alterations in mitochondrial DNA (mtDNA) copy number have been widely reported in various human cancers, and been considered to be an important hallmark of cancers. However, little is known about the value of copy number variations of mtDNA in the prognostic evaluation of laryngeal cancer.

Design and methods: Using real-time quantitative PCR method, we investigated mtDNA copy number in a cohort of laryngeal cancers (n =204) and normal laryngeal tissues (n =40), and explored the association of variable mtDNA copy number with clinical outcomes of laryngeal cancer patients.

Results: Our data showed that the relative mean mtDNA content was higher in the laryngeal cancer patients (11.91 ± 4.35 copies) than the control subjects (4.72 ± 0.70 copies). Moreover, we found that mtDNA content was negatively associated with cigarette smoking (pack-years), tumor invasion, and TNM stage. Notably, variable mtDNA content did not affect overall survival of laryngeal cancer patients. However, when the patients were categorized into early-stage and late-stage tumor groups according to TNM stage, we found that low mtDNA content was strongly associated with poor survival in the former, but not in the latter.

Conclusions: The present study demonstrated that low mtDNA content was strongly correlated with some of clinicopathological characteristics, such as cigarette smoking, tumor invasion and TNM stage. In addition, we found a strong link between low mtDNA content and worse survival of the patients with early-stage tumors. Taken together, low copy number of mtDNA may be a useful poor prognostic factor for early-stage laryngeal cancer patients.

Virtual slides: The virtual slides for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1841771572115955.

Figure 1

Copy number analysis of mtDNA in laryngeal cancer. (A) Copy number of mtDNA corresponding to each individual case of laryngeal cancers and normal laryngeal tissues (circle). Real-time quantitative PCR assay was performed to analyze mtDNA copy number in a cohort of laryngeal cancers and normal laryngeal tissues. Details are as described in Methods. (B) Copy number of mtDNA in 30 pairs of laryngeal cancer tissues and their corresponding normal tissues by real-time quantitative PCR. Horizonal lines represent mean ± S.E. N, normal laryngeal tissues; T, laryngeal cancer tissues.

Figure 2

Association of mtDNA copy number with clinicopathological variables in laryngeal cancer. Copy number of mtDNA was analyzed using real-time quantitative PCR approach. Details are as described in Methods. (A) A frequency distribution of the number of cases by mtDNA copy number, age, and pack-years. (B) The relationship of mtDNA copy number with various clinical features of laryngeal cancer patients. The circle represents mtDNA copy number of each case of laryngeal cancers. Horizonal lines represent mean ± S.E. Sample means were compared using the Mann–Whitney U test. F, female; M, male; Well, well/moderate differentiation; Poor, poor/undifferentiation; N, non-lymph node metastasis; Y, lymph node metastasis.

Figure 3

The effect of variable mtDNA content on poor survival of laryngeal cancer patients. The lower and upper limit (4.02 and 5.42 copies) of the overall 95% confidence interval for all control subjects (A) and the relative mean mtDNA copy number (4.72 copies) of all control subjects (B) were set as the cutoff points, respectively. Kaplan-Meier analysis of survival was then performed according to copy number variations of mtDNA in a large cohort of laryngeal cancers. The results showed that both low and high mtDNA content were not associated with overall survival of the patients. However, when the data were stratified further based on the TNM tumor stage, low mtDNA content was strongly associated with worse survival in the patients who had early-stage tumors, but not in those with late-stage tumors. H, high mtDNA content; M, medium mtDNA content (or reference); L, low mtDNA content.