Abstract PS19-19: Whole exome sequencing and BRCAness estimation in TNBCs and their correlation with response to platinum
Authors: Korlimarla Aruna , Radhakrishnan Sabarinathan , VP Snijesh , Prabhu Jyothi S , Demonte Naveen Luke , Rajarajan Savitha , Patil Yatish , Diwakar Ravi , Apachu Sandhya , Srinath BS , TS Sridhar , Sadanandam Anguraj 
Published: February 2021

Background: Triple-Negative Breast Cancer (TNBC) comprises approximately 30% of all breast cancers in Indian women. The hallmark of TNBC is genomic instability with very high rates of TP53 mutation. BRCAness has been defined by Ashworth et al (2004 &2014), as the phenocopying of BRCA1&2 mutation by alternate genetic or Epigenetic mechanisms. The consequence of this is DNA damage repair (DDR) deficiency. Though PARP inhibitors have been of utility in treating this subset of TNBC, trials have in general not found an unequivocal support for the use of platinum. We have developed an assay for BRCAness, the BRCA1 deficiency score (BDS) (Korlimarla et al 2016). In this study, we have applied BDS assay in tandem with whole exome sequencing to a small retrospective series of TNBC patients more than half of whom were treated with Platinum. Methods: 40 TNBC primary specimens along with complete clinical data from a retrospective series at the SSCHRC were obtained under all IERB approvals and informed patient consent for BDS assay and mutational analysis. BDS assay is a multianalyte assay involving measurement of BRCA1 (Transcript and protein) as well as additional epigenetic regulators of BRCA1, mir182 and ID4. Whole Exome Sequenced (WES) using the Agilent Sureselect V6 kit on Illumina HiSeq platform. Variant calling analysis was performed using GATK Mutect2 with Hg38 reference genome following the best practice workflow. Variants annotated as protein affecting based on their functional impact prediction and mapped to known cancer related genes (from COSMIC), were selected. Results: Clinical details: Mean age of patient in the series was 49. 37/40 qualified for BDS assay. 21/37 (57%) patients were treated with Carboplatin in combination with Docetaxel in an adjuvant setting. Remaining patients were treated with Cyclophosphamide Adriamycin with or without Docetaxel. 15/37 (45%) were BRCA1 deficient and10/15 were treated with Platinum and 8 (80%) were responders.

39 passed QC and considered for analysis. Spectrum of variants were missense mutations (86.5%), followed by stop gained (5.91%) and frame shift (3.76%). Results were compared to TCGA TNBC set (n=123). The most frequently mutated gene was TP53 (62%) as reported in TCGA. We also report higher frequency of deleterious mutations on DNA damage repair (DDR) genes like ATM and BRCA2 (15%). Response to Platinum therapy in this subset, correlated with Mutations in DNA damage repair genes (p ≤ 0.003). 4/6 samples mutated were also In the BDS deficient group. BRCAness score for predicting Platinum response also correlated significantly (p ≤ 0.05). Other genes which showed significant alterations were KMT2A and KMT2D (15% and 13%) which encode the histone methyl transferase and are responsible in altering the chromatin structure and RECQL4 (10%) which is a helicase involved in DDR Conclusion: Both BRCAness and mutation profile identified a DDR deficient group within TNBC patients. In addition to TP53 which is the most frequently mutated gene, our small sample set has shown higher frequencies of mutations in DDR genes like ATM and BRCA2. This group of patients showed favourable response to platinum therapy. Since very little is known about the molecular heterogeneity of TNBCs in Indian patients, our analysis aids in identification of actionable mutations in TNBCs and may be of use in selection of patients for platinum therapy.

CHARACTERSn%Platinum Treatment - ( n )Treatment response%BDS3721Deficient154513BRCA1 deficient response861Proficient19518BRCA1 Proficient response450Undet34

Citation Format: Aruna Korlimarla, Sabarinathan Radhakrishnan, Snijesh VP, Jyothi S Prabhu, Naveen Luke Demonte, Savitha Rajarajan, Yatish Patil, Ravi Diwakar, Sandhya Apachu, BS Srinath, Sridhar TS, Anguraj Sadanandam. Whole exome sequencing and BRCAness estimation in TNBCs and their correlation with response to platinum [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS19-19.