Normal tissue clinical radiosensitivity in patients with nasopharyngeal carcinoma : potential prediction using genetic and cellular approaches

Abstract

Nasopharyngeal carcinoma (NPC) is one of the common cancers in Hong Kong. Radiation therapy (RT) is the primary treatment of NPC. Radiation-induced toxicities are the major obstacles affecting treatment effectiveness and quality of life in post-radiotherapy patients. The aims of this study were to investigate the potentials of genetic variants and direct cellular response in predicting radiosensitivity in Chinese NPC patients. Acute skin reactions, acute mucositis, and chronic neck fibrosis were the three phenotypes of radiation-induced toxicities evaluated in the first part of this study. Associations between each phenotype and six candidate genes (ATM, SOD2, TGFβ1, TP53, XRCC1 and XRCC3 ) were investigated. Besides tagging single nucleotide polymorphisms (SNPs), previously reported functional SNPs were also included to investigate their role as universal biomarkers to predict risk of each phenotype. One hundred and twenty eight Chinese NPC patients were recruited and they were classified into controls or cases according to toxicity grading criteria established by Radiation Therapy Oncology Group (RTOG). Genotyping was performed by restriction fragment length polymorphism or unlabelled probe melting curve analysis. Single-marker and haplotype analyses were performed by PLINK. Multiple testing was corrected by 10,000 permutations. None of the investigated SNPs and constructed haplotypes showed association with the risk of any phenotype after permutation (Pemp>0.05). A meta-analysis was conducted to overcome the sample size limitation. A comprehensive literature search was performed to identify all existing literature. XRCC1 and XRCC3 were selected to investigate their potential roles in radiation-induced mucositis in head and neck cancer patients. Three functional SNPs and 332 head and neck cancer patients were included in this meta-analysis. Meta-analysis was performed with RevMan version 5.2. However, none of the SNPs showed any association with the risk of radiation-induced mucositis in dominant model, recessive model and allelic model (P>0.05). Quantification of radiation-induced apoptosis as direct cellular response was the third part of this study. Thirty NPC patients were recruited. Peripheral blood mononuclear cells (PBMCs) were isolated from patients blood samples. Patients were classified as normal or sensitive according to the highest grade of acute skin reactions and mucositis observed during RT. Flow cytometry was used to measure the percentage of radiation-induced apoptosis in PBMCs at two time points (2hr and 18 hr) after irradiation (0 Gy, 2 Gy, and 8 Gy). Two-way ANOVA was used to compare difference of percent of radiation-induced apoptosis between radiation dose and time points. Percentage of radiation-induced apoptosis between normal and sensitive groups was not significantly different at all time points at all doses (P>0.05). To conclude, our findings suggested that individual genetic variant and measurement of direct cellular response alone are insufficient to predict radiosensitivity in Chinese NPC patients. Meta-analysis is a useful tool to investigate the effect of reported biomarkers in different ethnic origins with a higher statistical power. Further studies are needed to explore the cumulative effect of genetic variants, in addition to individual direct cellular response, to understand the underlying complex mechanism of radiation-induced toxicities.