Abstract 1977: Size selection of cell-free DNA increases the proportion of tumor specific variants in cancer patients
Authors: Holden Kimberly A. , Fitzgerald Kerry D. , McLennan Graham , Faulkner Nathan , Guan Xiaojun , Eisenberg Marcia , Jensen Taylor J. 
Published: September 2020

Cell-free DNA (cfDNA) derived from tumor cells is present in the plasma of patients with cancer; however, the proportion of this circulating tumor DNA (ctDNA) is typically less than 2%. To overcome this limitation, methods have been developed to better distinguish the biological signal derived from ctDNA from the technical and statistical noise present in most current methodologies. While successful in minimizing the technical noise, these methods often require increased sequencing depth and other advanced analytical techniques. Thus, alternative or complimentary approaches would be beneficial for improving noninvasive cancer diagnostics (i.e. liquid biopsies). One biological feature that may be able to be leveraged to improve this signal-to-noise challenge is the fragment length of cfDNA. The median length of cfDNA in circulation from healthy tissue is typically about 167bp, while ctDNA has been demonstrated to be, on average, shorter. We sought to evaluate whether size selecting cfDNA could enrich for shorter ctDNA fragments and thereby enhance signal for the detection of tumor-specific variants. To test this hypothesis, adapter-ligated libraries were size selected using the Coastal Genomics NIMBUS Select, an automated platform for gel-based electrophoresis and size selection, targeting cfDNA fragment sizes up to 142bp (+/- 15bp). The size selected libraries from each patient were first assayed with low-coverage (~0.3X) genome-wide sequencing and analyzed for insert size to ensure proper enrichment of shorter cfDNA fragments. Libraries prior to size selection yielded, on average, 25.9% of reads with cfDNA fragment sizes shorter than 150bp. After size selection, the proportion of cfDNA shorter than 150bp was significantly increased to 96.4% (p&lt;0.001; Wilcoxon Rank Sum). Copy number alterations (CNAs) were identified in the cfDNA data and characterized using analytical methods originally developed for noninvasive prenatal testing and subsequently optimized for ctDNA. The amplitude of a detectable autosomal CNA represents the relative magnitude of the CNA. When evaluating cfDNA from healthy patients, the amplitudes of CNAs before and after size selection were on average within 6%, consistent with a lack of signal enrichment in the absence of disease. Conversely, detectable CNAs in cancer patients were on average 44% greater in amplitude in size selected samples than in the same samples prior to size selection, consistent with an enrichment of signal. These data demonstrate a proof-of-concept for using size selection to enhance signal for the detection of tumor-specific variants in cancer patients.

Citation Format: Kimberly A. Holden, Kerry D. Fitzgerald, Graham McLennan, Nathan Faulkner, Xiaojun Guan, Marcia Eisenberg, Taylor J. Jensen. Size selection of cell-free DNA increases the proportion of tumor specific variants in cancer patients [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1977.