Influence of <i>TONSL</i> on tumor suppressor function of <i>RAD51</i> and resistance to CDK4/6 inhibitors in ER+ breast cancer.

1097 Background: Homologous Recombination (HR) and Replication Stress (RS) resolution mechanisms help to maintain genomic stability. TONSL, MMS22L, RAD51, BRCA1/2 are the few proteins that cooperatively coordinate HR and RS resolution. Therefore, genes in these pathways are expected to function as tumor suppressors. However, only BRCA 1/2 function as tumor suppressors as their germline mutations increase cancer susceptibility. TONSL and RAD51 are frequently overexpressed in tumors. Here, we investigated the relationship between TONSL-RAD51 axis in breast cancer (BC) outcome and response to CDK4/6 inhibitors (CDK4/6i) and endocrine therapy (ET). Methods: 10,980 ER+ BC samples tested by NGS (592, NextSeq; WES/WTS, NovaSeq, Caris Life Sciences). Dual expression group TONSL/RAD51 -high (H), TONSL -H/ RAD51 -low (L), TONSL -L/ RAD51 -H, and TONSL / RAD51 -L were classified by RNA expression above or below median. HRD score and TONSL /HRD-H/L survival was analyzed using METABRIC (ER+ BC, n = 1904) dataset. TONSL-RAD51 role in sensitivity to CDK4/6i was determined using TONSL -amplified ER+ MCF-7 cells. Real-world median overall survival (mOS) of TONSL/RAD51 groups was derived from insurance claims and calculated from biopsy, start of ET or CDK4/6i to last contact using Kaplan-Meier. Statistical significance was assessed using chi-square and Mann-Whitney U with multiple comparison adjustments (q&lt;0.05). Results: TONSL -H had higher HRD score (33.7 vs 21.1, p&lt;0.01) compared to TONSL -L (n=952 each). TONSL /HRD-H (n=573) had worse mOS (109 m vs 138 m, p&lt;0.05) compared to TONSL /HRD-L (n=630). TONSL -H had higher frequency of LOH (31.6% vs 19.8%), BRCA2 (5.1% vs 2.9%) and BRCA1 (1.5% vs 0.8%) mutation, all q&lt;0.05. TONSL/RAD51 -H had worse mOS compared to TONSL -H/ RAD51 -L, TONSL -L/ RAD51 -H or TONSL / RAD51 -L (Table). TONSL -H/ RAD51 -L had worse survival with ET compared to other groups. With ET+CDK4/6i, TONSL/RAD51 -H and TONSL- H /RAD51 -L had similar but worse mOS compared to TONSL -L/ RAD51 -H and TONSL / RAD51 -L (Table). TONSL knockdown in CDK4/6i resistant MCF-7 cells created TONSL -L/ RAD51 -H phenotype and cells regained the sensitivity to CDK4/6i. Conclusions: These findings suggest that TONSL -H independent of RAD51 expression is associated with poor response to ET or ET+CDK4/6i. Therapeutic targeting of TONSL to create TONSL-L/RAD51-H status may improve response to ET+CDK4/6i. Moreover, as the activity of TONSL is regulated by various protein complexes, destabilizers of TONSL-protein complexes could potentially help in sensitizing ER+ BC to ET or CDK4/6i. TONSL/RAD51 -HOS in months TONSL -H/ RAD51 -LOS in months TONSL-L/RAD51 -HOS in months TONSL/RAD51 -LOS in months p-value Overall 35 m (n=4176) 41.4 m (n=1473) 37.9 m (n=1476) 44.3 m (n=4173) &lt;0.01 ET 63.4 m (n=1361) 58 m (n=461) 65.2 m (n=508) 76.1 m (n=1430) &lt;0.01 ET+CDK4/6i 73.8 m (n=2261) 73.8 m (n=866) 84.6 m (n=799) 80.7 m (n=2291) &lt;0.01