Abstract 3536: Genetic validation of an ATM immunohistochemistry assay for genomic and clinical-pathologic characterization of primary prostate cancers with ATM loss

Background: ATM, a protein kinase involved in double-strand DNA break repair, is mutated in a significant fraction of prostate cancers, and targeted therapies such as ATR inhibitors appear promising for ATM-deficient tumors. However, few ATM-deficient primary prostate cancers have been clinically and genomically characterized. Design: We validated an automated and dichotomously-scored clinical-grade immunohistochemistry (IHC) assay to detect ATM protein loss using prostate cancer cell lines and 52 very high-grade prostate tumors (Gleason score 5+4=9 or 5+5=10) with known ATM genomic status. We examined the frequency of ATM protein loss among 29 tumors with known pathogenic germline ATM mutations or variants of unknown significance (VUS). Finally, we screened more than 800 additional primary prostate carcinomas using tissue microarrays (TMA) for ATM protein loss. Targeted somatic genomic sequencing using HRD Plus (Myriad Genetics) or UW-OncoPlex was conducted in a subset of immunostained tumors in each cohort. Results: ATM protein loss by IHC was found in 13% (7/52) of very high-grade (primary Gleason pattern 5) tumors with somatic genomic sequencing data. Of 7 cases with ATM protein loss, 4 had a deleterious ATM mutation and 3 had apparent single copy ATM genomic loss. Of the remaining 45 cases without ATM protein loss, none had ATM genomic alterations. In the cohort of tumors with pathogenic germline ATM mutations, 74% (17/23) had ATM protein loss, but only 45% (9/20) with evaluable sequencing had a second pathogenic alteration in ATM detected (all of these had protein loss). Of those with germline mutations and ATM protein loss, the majority (76% or 13/17) had an apparent clonal alteration with homogeneous protein loss in all tumor cells sampled. In contrast, among 6 cases with germline ATM VUS, only one had protein loss. On TMA analysis, ATM loss was present in 3% (25/824) of evaluable tumors overall, and was significantly more common in tumors from Grade Group 5 (12/130; 9.2%) compared to all other Grade Groups (13/694; 1.9%) (P Conclusions: Validated ATM IHC is a sensitive assay for detecting underlying ATM alterations and may be useful to evaluate VUS in ATM. However, not all tumors with ATM protein loss have detectable genomic alterations. ATM protein loss is likely an early clonal event in tumors with germline mutations and is significantly enriched in high-grade prostate cancers. Citation Format: Harsimar Kaur, Jessica Hicks, Colin Pritchard, Angelo De Marzo, William Isaacs, Jerry Lanchbury, Kirsten Timms, Emmanuel Antonarakis, Tamara L. Lotan. Genetic validation of an ATM immunohistochemistry assay for genomic and clinical-pathologic characterization of primary prostate cancers with ATM loss [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 3536.