Cell-Free DNA Variant Sequencing Using CTC-Depleted Blood for Comprehensive Liquid Biopsy Testing in Metastatic Breast Cancer

Keup and colleagues provide liquid biopsy preliminary results by sequencing variants in circulating tumor cells (CTCs) and cell-free deoxyribonucleic acid (cfDNA) "all from one tube" format, in order to use the same blood sample under the same isolation conditions of both analytes to reach an unbiased comparability and consistency. We appreciated the attempt of the authors to improve technical procedures in liquid biopsy research area, but we wanted to raise several issues related to cfDNA detection, reporting our research experience. This is a feasibility study as the authors analyzed only one sample from a small case series at an advanced line of treatment. In the clinical practice to monitor the disease and predict the treatment response, the analysis should be done at multiple time points. We have previously demonstrated that the quantity and the integrity of the cfDNA are not useful to determine the evolution of early breast cancer (bc), maybe due to the fact that cfDNA is not strictly related to cancer but also to an inflammatory status. Given that a high content of cfDNA could reflect inflammatory processes, we decided to investigate the role of stimulator of interferon gene (STING), an important regulator of cancer cell growth and senescence, in bc tissue in relation to cfDNA. STING biomarker analyzed by immunohistochemistry on tumor tissue could reflect a circulating inflammatory status and needs to be further investigated, not only on CTCs but also on cfDNA. One of the major issues of cfDNA is to decide what to analyze on it, in terms of type of cells and genetic alterations. Considering that multiple tests could be done to study gene copy number alterations, mutations, and variant fusions, the proper molecular test should be chosen, on the basis of the clinical need, starting from the treatment choice to disease monitoring.