Abstract A064: Identifying cooperating genetic events contributing to disease progression in mutant p53-driven breast cancer
Authors: Morrissey Rhiannon L , McDaniel Joy M , Chachad Dhruv , Su Xiaoping , Chau Patty , El-Naggar Adel , Lozano Guillermina 
Published: February 2024

Abstract
Ductal carcinoma in situ (DCIS) is an early breast neoplasm that may, but does not always, progress to invasion. It is difficult to predict if DCIS will become invasive. Treatment includes surgical removal and radiation. To avoid overtreatment, a strategy is needed to stratify tumors likely to spread from those that will remain indolent. The Lozano lab created a somatic mouse model with a conditional p53R245W mutation in the mammary epithelium, resulting in breast tumorigenesis. Pathological analysis identified both invasive carcinomas (IC) and DCIS. DCIS and ICs were analyzed for mutations and copy-number changes cooperating with mutant p53 to drive DCIS progression. We hypothesize that additional genetic events are required for DCIS progression to IC. 54 ICs have been exome-sequenced. Laser capture microdissection (LCM) was performed on serial sections of mammary gland tissue contralateral to a breast tumor, resulting in excision of 12 DCIS lesions. Genome Access Toolkit (GATK) and MuTect2 variant calling analysis revealed mutations and copy-number events present in the 12 DCIS lesions. GISTIC2.0 identified recurrent amplification and deletion peaks among these samples, and genes within those peaks. These mutations and copy-number events were compared to those in the 54 ICs to identify potential cooperating events driving DCIS progression. Exonic mutations in DCIS deemed deleterious by the Ensembl algorithm were compared to those in the ICs. Fourteen mutations were shared between the DCIS lesions and ICs. One recurrent amplification peak, on chromosome 6 (chr 6), was shared between the DCIS and ICs. This region is syntenic between mouse chr 6 and human chr 7. Shared events will be assessed in functional studies for their role in tumor initiation and progression. 2 tumor-derived cell lines from this model were subjected to knockdown of genes in the amplified locus, followed by a transwell invasion assay. Preliminary data indicated markedly decreased invasion after knockdown of target genes, as compared to the nontargeting control. This suggests that genes amplified on the chr 6 locus may play a role in invasion in mutant p53-driven tumor cells. Recurrent mutations and copy-number changes private to the tumor dataset will also be considered, as these genomic events are likely involved in gain of invasive potential. Sixteen genes had a mutational recurrence rate &gt;10% in the tumor dataset. Six amplification peaks were recurrent between the tumors. The highest recurrence in amplifications was in two peaks on chr 11 and chr 15. Thirteen recurrent deletion peaks were identified. Further analysis into these amplified and deleted loci is ongoing. Private tumor events will be functionally assessed for their role in tumor progression. Our conditional somatic p53R245W mouse model provides a robust system for identifying cooperating events driving DCIS progression. Genetic events, both shared between DCIS and IC, as well as private to IC, have been identified as potential lesions mediating DCIS progression and require functional assessment.
Citation Format: Rhiannon L Morrissey, Joy M McDaniel, Dhruv Chachad, Xiaoping Su, Patty Chau, Adel El-Naggar, Guillermina Lozano. Identifying cooperating genetic events contributing to disease progression in mutant p53-driven breast cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Breast Cancer Research; 2023 Oct 19-22; San Diego, California. Philadelphia (PA): AACR; Cancer Res 2024;84(3 Suppl_1):Abstract nr A064.