Targeted parallel DNA sequencing detects circulating tumor‐associated variants of the mitochondrial and nuclear genomes in patients with neuroblastoma

BACKGROUND: The utility for liquid biopsy of tumor‐associated circulating single‐nucleotide variants, as opposed to mutations, of the mitochondrial (mt) and nuclear genomes in neuroblastoma (NB) is unknown. PROCEDURE: Variants of the mt and nuclear genomes from tumor, blood cells, and consecutive plasma samples of five patients with metastatic NB that relapsed or progressed were analyzed. Targeted parallel sequencing results of the mt genome, and of the coding region of 139 nuclear genes and 22 miRNAs implicated in NB, were correlated with clinical imaging and laboratory data. RESULTS: All tumors harbored multiple somatic mt and nuclear single nucleotide variants with low allelic frequency, most of them not detected in the circulation. In one patient a tumor‐associated mt somatic variant was detected in the plasma before and during progressive disease. In a second patient a circulating nuclear tumor‐associated DNA variant heralded clinical relapse. In all patients somatic mt and nuclear variants not evident in the tumor biopsy at time of diagnosis were found circulating at varying timepoints. This suggests either tumor heterogeneity, evolution of tumor variants or a confounding contribution of normal tissues to somatic variants in patient plasma. The number and allelic frequency of the circulating variants did not reflect the clinical course of the tumors. Mutational signatures of mt and nuclear somatic variants differed. They varied between patients and were detected in the circulation without mirroring the patients' course. CONCLUSIONS: In this limited cohort of NB patients clinically informative tumor‐associated mt and nuclear circulating variants were detected by targeted parallel sequencing in a minority of patients.