LAUSR

Introduction Ductal carcinoma in situ (DCIS) is a pre-invasive breast lesion, frequently detected by breast cancer screening, with thousands of new cases each year. While DCIS is not life threatening, it does increase a women’s risk of developing invasive breast cancer; this in turn can lead to breast-cancer specific mortality. Therefore, almost all DCIS lesions are treated to prevent progression to invasive disease. However, many DCIS lesion will never develop into invasive breast cancer if left untreated, indicating that many women are overtreated. In this study, we performed RNA sequencing (RNAseq) on a large set of primary DCIS lesions, comparing lesions with a subsequent invasive breast cancer recurrence (IBCR) to those without, to identify factors distinguishing harmless from potentially hazardous DCIS. Material and methods We made use of a DCIS case-control series, nested in a nation-wide population-based cohort of Dutch women diagnosed with primary DCIS and treated with breast conserving surgery alone (standard clinical practice) between 1989–2005. Mean follow up time was 12.0 years. RNA was extracted from FFPE laser-microdissected tissue fragments of 183 primary DCIS lesions: 100 associated with subsequent IBCR and 83 which remained free of invasive recurrence. RNAseq was performed to identify differentially expressed genes between DCIS with and without IBCR. Subsequently, gene set enrichment analysis (GSEA) was performed. Finally, findings were correlated with patients’ clinical and histopathological characteristics. Results and discussions DCIS lesions were categorised into PAM50 intrinsic subtypes: Luminal A 25%, Luminal B 38%, Her2 22%, Basal 10%, Normal 5%. Within each PAM50 subtype, 1 to 64 genes were statistically significant (p<0.05) differentially expressed between DCIS lesions with and without IBCR. GSEA revealed these genes were enriched in immune-related pathways. In Luminal A DCIS the pathways interferon alpha response and interferon gamma response and were associated with subsequent IBCR. Correlation of these findings with clinical and histopathological characteristics is ongoing. Conclusion To our knowledge, we performed the first large-scale transcriptome analysis of primary DCIS lesions with and without subsequent IBCR and long-term follow-up. Our data suggest that immune-related pathways are involved in DCIS progression. The results of our study add to the current understanding of DCIS and to risk stratification of DCIS in the near future.