Advancing blood biopsy through the canine comparative model

Pet dogs are a powerful spontaneous model for human cancers. Dogs develop the same cancers that humans do, with striking similarities at the clinical, histopathologic, and genomic levels. These cancers have an accelerated clinical course compared with their human counterparts, and canine patients undergo many of the same interventions and treatments as human patients. The dog model thus offers the opportunity to implement rapid clinical trials informative for human medicine. We are developing liquid biopsy in dogs to further refine its clinical use by addressing questions that are difficult to assess in human patients. We have four ongoing pilot studies examining several different aspects of blood biopsy in canine patients. (1) We characterized the yield and tumor fraction of cell-free DNA (cfDNA) in nine canine cancer histologies using prospectively collected and banked plasma samples. (2) We tested the correlation between large-scale somatic copy number aberrations (SCNAs) in 24 osteosarcoma tumor samples and matched plasma samples and are now preparing to perform whole-exome sequencing (WES) of these samples to assess the concordance of SNVs and INDELs. (3) We investigated the use of cfDNA as a biomarker for response to therapy in four dogs with multicentric lymphoma treated with L-asparaginase via longitudinal blood biopsy. (4) We are optimizing phlebotomy techniques and characterizing short-term cfDNA dynamics by testing the effect of blood draw site and time of day on yield and tumor fraction. Samples from three sites (jugular, saphenous, and cephalic veins) were collected in a cohort of ten patients, and collection of samples at three time points throughout the day in a separate cohort is ongoing. (1) The majority (93%) of plasma samples yielded or were projected to yield sufficient DNA (>2ng) for ultra-low-pass whole-genome sequencing, and 37% had a tumor fraction ≥10%, allowing for WES given sufficient input DNA. (2) We found a strong correlation (Spearman coefficient of 0.74) between SCNAs in 13 tumor/plasma pairs with tumor fraction ≥10%. We are moving forward with WES of these cfDNA samples to examine simple somatic mutations. (3) Longitudinal cfDNA yield and tumor fraction correlated with response to therapy in three of four lymphoma patients. (4) Sample collection and sequencing are under way for the studies on the effect of blood draw site and time of day. Our studies demonstrated the feasibility of blood biopsy in dogs, the high concordance between cfDNA and tumor samples, and suggested that blood biopsy is a promising method for monitoring response to therapy. Emerging results will help to optimize phlebotomy protocols and characterize cfDNA dynamics over short time scales, as well as assessing in more detail the concordance between cfDNA and matched tumor samples. Further development of blood biopsy in dogs has the potential to improve and accelerate clinical application of this technology in human medicine and to facilitate further high-impact comparative translational studies in the dog model. Citation Format: Kate Megquier, Kan Xiong, Heather L. Gardner, Justin Rhoades, Viktor Adalsteinsson, Cheryl A. London, Elinor K. Karlsson. Advancing blood biopsy through the canine comparative model [abstract]. In: Proceedings of the AACR Special Conference on Advances in Liquid Biopsies; Jan 13-16, 2020; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(11_Suppl):Abstract nr A58.