Abstract 2683: Ductal carcinoma in situ is a multiclonal disease

Introduction. Ductal carcinoma in situ (DCIS) of the breast has a limited propensity to progress to invasive breast cancer. However, due to a lack of markers that predict the risk of progression to invasive cancer, many DCIS patients may receive unnecessary surgery. Here, we explore whether the multiclonal intratumoral heterogeneity commonly observed in invasive breast cancers is also present in the precursor DCIS, and whether it differs between pure DCIS (without concurrent invasion) and mixed DCIS (with concurrent invasion). Methods. A total of 19 tumors (10 pure, 9 mixed) were analyzed. Based on whole exome sequencing, tumor-specific mutation panels were identified and contained a median of 60 point mutations per tumor (range: 35-80). From each tumor, 3-4 histologic sections were obtained, with a mean distance between slides of 1.26cm (range: 0.34-6.0cm). Individual DCIS ducts were microdissected from the sections using selective ultraviolet radiation fractionation (SURF); median number of ducts per tumor was 23 (range: 10-50). For each duct, the tumor specific mutation panel was assayed using targeted deep sequencing (median depth: 9,000x). Clonal deconvolution within each tumor was performed using the software package CloneFinder. Individual glands were classified as multiclonal if they contained 2 or more tumor subclones. In addition, the following evolutionary measures were computed for each tumor: (i) clone diversity as the mean pairwise Hamming distance between all ducts; (ii) clonal mixing as the number of clones that were present in more than one section. Comparisons between pure and mixed cases were performed using a two-sided Wilcoxon rank sum test. Results. All DCIS lesions were multiclonal. The number of subclones per tumor ranged from 2 to 9 overall, with a median of 5 and 6 subclones in pure and mixed DCIS, respectively (p-value for difference=.1). The median fraction of multiclonal glands per tumor was higher in mixed vs. pure DCIS (10% vs. 0%; p=.03). Median subclone diversity was higher in mixed vs. pure DCIS (1.48 vs. 1.04; p=.03), and the fraction of mixing subclones trended higher in mixed DCIS (median: 63%, range: 38-100%) compared to pure DCIS (median: 37%, range: 0-80%; p=.1). Subclones were spatially extensive: 18/19 tumors had at least one subclone that spanned across the analyzed sections, covering a mean distance of 2.5cm. Conclusions. All analyzed DCIS cases were multiclonal and individual subclones were found to span large regions of the tumor. Despite small sample sizes we found differences between pure and mixed DCIS. In particular, mixed DCIS harbored a higher fraction of multiclonal glands and a higher subclone diversity between individual ducts. Our findings characterize the evolutionary dynamics of breast cancer initiation and may provide evolutionary markers that distinguish indolent (pure) and progressive (mixed) disease. As such, this line of work has the potential to risk-stratify DCIS lesions. Citation Format: Marc D. Ryser, Inmaculada C. Sorribes, Matthew Greenwald, Allison Hall, Diego Mallo, Lorraine M. King, Timothy Hardman, Lunden Simpson, Carlo C. Maley, Jeffrey R. Marks, Darryl Shibata, E. Shelley Hwang. Ductal carcinoma in situ is a multiclonal disease [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 2683.