Questioning How to Define the "Ultra-High-Risk" Subgroup of Neuroblastoma Patients

Neuroblastic tumours exhibit heterogeneity, which results in different therapeutic outcomes. Neuroblastoma is categorized into three major risk groups (low, intermediate, high risk). Recent identification of new genes raised the possibility of new biomarkers to identify sub-risk groups. In this retrospective cross-sectional study, we aimed to assess new biomarkers defining the ultra-high-risk subgroup within the high-risk group that differ in clinical situation with very bad prognosis. Twenty-five low- and 29 high-risk groups of patients were analysed for their expression of ALK, ATRX, HIF1a, HIF2a (EPAS), H2AFX, and ETV5 genes at the RNA level. Immunohistochemistry was performed to confirm the protein expression level of ALK. The risk group of patients was determined according to the International Neuroblastoma Risk Group Stratification System. Spearman correlation analysis and Mann-Whitney-U nonparametric test were used to assess the importance of expression levels among the groups. P < 0.05 was considered as significant. Sensitivity of the results was checked by ROC curve analysis. All analysed genes were found to be highly expressed in the high-risk group compared to the low-risk group, except for ETV5. When the ultra-high-risk and highrisk groups were compared, ALK was found to be highly expressed in the ultra-high-risk group. Our results show that ALK may be a candidate gene whose mRNA expression levels can distinguish the ultrahigh- risk subgroup of patients in the high-risk group of patients with non-familial neuroblastoma.