Gene expression analysis in Merkel cell polyomavirus‐positive non‐small cell lung cancer from Japanese patients

Abstract

We read with great interest the recent article by Lasithiotaki et al. entitled “The presence of Merkel cell polyomavirus is associated with deregulated expression of BRAF and Bcl-2 genes in non-small cell lung cancer.”1 This very interesting report gives a new insight into the currently important issue of the infectious causes of cancer. The analysis presented in this study was based on 110 patients with non-small cell lung cancer (NSCLC) in Crete, Greece. The authors found that 9.1% (10/110) of NSCLC specimens were positive for the presence of Merkel cell polyomavirus (MCPyV), identified by polymerase chain reaction (PCR). The article also provided interesting data on the expression patterns of seven cellular genes in the MCPyV-positive NSCLC specimens by comparing 6 MCPyV-positive and 10 MCPyV-negative samples. Around the same time, we reported on our analysis of 112 Japanese NSCLC patients with an MCPyV detection rate of 17.9% (20/112).2 The viral prevalence rate was higher in these Japanese NSCLC patients than in the Greek patients reported by Lasithiotaki et al. In our study, MCPyV was detected more frequently in never smokers than in smokers, although Lasithiotaki et al. showed a positive trend for smoking status. Recent studies have indicated that the increase in the incidence of lung cancer in never smokers is higher in East Asia, including Japan, than in the United States and European countries.3-5 Thus, the incidence of lung cancer in never smokers seems to vary between geographic locations or ethnic groups; the higher prevalence in Asia may account for the discordance between our study and that by Lasithiotaki et al. Japanese ethnicity compared with Caucasian ethnicity and never-smoker status are independent favorable prognostic factors for overall survival in NSCLC patients.6 In addition, Asians and Caucasians may have different genetic susceptibilities to lung cancer, as evidenced by gene polymorphisms and genome-wide association studies. For example, East Asian patients with NSCLC have a much higher prevalence of mutation of the “epidermal growth factor receptor” gene and a lower prevalence of mutation of the K-ras gene. These clinical and genetic differences between East Asians and Caucasians prompted us to investigate the gene-expression patterns in MCPyV-positive NSCLC samples from Japanese patients and to compare the results with those reported by Lasithiotaki et al. Our study included 27 surgically resected frozen tumors from Japanese NSCLC patients, including 12 squamous cell carcinomas (SCCs) and 15 adenocarcinomas. Nine samples (four SCCs and five adenocarcinomas) were positive for MCPyV and 18 samples (8 SCCs and 10 adenocarcinomas) were negative. Total RNA was extracted with the High Pure RNA Tissue Kit (Roche Diagnostics, Tokyo, Japan). The extracted RNA was treated with DNase to avoid amplification of DNA. One microgram of RNA was reverse transcribed using the SuperScript III First-Strand Synthesis System (Life Technologies, Tokyo, Japan). The transcript levels of the K-ras, BRAF, RKIP, p53, RB1, Bcl-2, and Bax genes were measured using the StepOne Plus real-time PCR system with SYBER Green PCR Master Mix (Life Technologies). Primer sequences and PCR conditions were the same as those used by Lasithiotaki et al. All reactions were run in triplicate. mRNA of the housekeeping gene GAPDH was used as an internal control to normalize mRNA expression levels. Relative gene expression was calculated by the 2–ΔΔCt method, as described.1, 7 The gene expression levels were first evaluated by the F-test to determine whether they followed a normal distribution pattern. Depending on the results, the Aspin–Welch's t test or Student's t test was used to compare the gene expression levels between MCPyV-positive and MCPyV-negative samples. A p-value less than 0.05 was considered significant. This study was approved by the Ethics Committee of Kochi Medical School, Kochi University. As shown in Table 1, the transcript levels of the K-ras, BRAF, RKIP, p53, RB1, Bcl-2, and Bax genes did not differ statistically between MCPyV-positive and MCPyV-negative NSCLC samples. However, we found a trend toward higher expression levels of the Bcl-2 gene in MCPyV-positive samples than in negative samples (p = 0.073). This trend was noticeable in MCPyV-positive adenocarcinoma (p = 0.012). Considering the antiapoptotic effect of the Bcl-2 gene, it is conceivable that this proto-oncogene might be associated, at least partially, with the pathogenesis of MCPyV-infected NSCLC in a subset of Japanese patients. Our results did not match exactly the findings of Lasithiotaki et al., who found downregulation of the Bcl-2 gene and upregulation of the BRAF gene in MCPyV-positive patients. These two reports clearly illustrate the need for further worldwide large-scale studies to elaborate on possible ethnic differences in the genetic pathogenecity of MCPyV-associated NSCLC. Yours sincerely, Yumiko Hashida, Masayuki Imajoh, and Masanori Daibata