Circulating mitochondrial DNA (mtDNA) variants to predict metastatic progression of rectal cancer.

Abstract

e16132 Background: High rates of systemic failure after curative-intent therapy in locally advanced rectal cancer (LARC) call for new diagnostic tools to improve risk-based treatment stratification. We hypothesized that blood mtDNA polymorphisms, generated by the high mtDNA mutation rate when the mitochondrial biogenesis is mobilized during clonal immune cell activation, correlate with systemic immune control of LARC dissemination and thus can be used as diagnostic marker. Methods: 41 LARC patients given neoadjuvant (chemo)radiotherapy had DNA extracted from whole blood (WB) sampled at the time of diagnosis, for high-throughput full-length mtDNA sequencing followed by annotation of mtDNA variants using the curated reference genome (mitomap.org). The composition of peripheral blood mononuclear cell (PBMC) subpopulations was analyzed by high-dimensional single-cell mass cytometry. Metastatic events were recorded over 24-60 months of follow-up after completion of the multimodal treatment. Results: The total number of WB-mtDNA variants was strikingly different across the study cases but without correlation to patient’s age, body mass index, or metformin use, which may affect mitochondrial function, or to the disease extension at presentation or the local tumor response to the neoadjuvant therapy. All patients had microsatellite-stable tumor. The higher the number of variants in MT-RNR2, encoding one of the two ribosomal subunits of mitochondria, the better distant metastasis-free survival (DMFS). For each patient, two MT-RNR2 sites were homoplasmic for either the normal or a deletion (3105AC > A, 3106CN > C) variant, categorizing the patients into two groups that comprised the same cases for both mtDNA sites. Nine of a total of 10 DMFS events occurred during the 18 first months of follow-up in one of the patient groups ( n = 23). The single event in the other group ( n = 18) appeared in a patient who had declined primary tumor surgery. In the former group, a low number of variants in the ribosomal subunit MT-RNR1 identified all patients who had metastatic progression. These patients had reduced PBMC frequency of CD8+ (cytolytic) T cells that were positive for CXCR4 ( p = 0.001), a chemokine receptor that promotes their homing to tumor sites. Conclusions: The WB-ribosomal mtDNA polymorphisms reflected systemic immune cells endowed with cytolytic and tumor-homing properties. The actual mtDNA variants were single-base alterations and might thus be analyzed by PCR and developed as a feasible assay for metastatic risk assessment in rectal cancer. Clinical trial information: 01816607 .