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ATM loss in primary prostate cancer: Analysis of >1000 cases using a validated clinical-grade immunohistochemistry (IHC) assay.

5069 Background: ATM is a protein kinase acting as the main signal transducer of double-strand DNA break repair, in addition to mediating other cellular functions. Germline and somatic pathogenic mutations in ATM occur in a significant fraction of prostate cancers, and targeted therapies for ATM-deficient tumors ( e.g. ATR inhibitors) appear promising. Because DNA sequencing assays frequently cannot distinguish mono-allelic from bi-allelic ATM alterations, a clinical-grade protein IHC assay for ATM loss is needed to select men for these trials and better characterize ATM-deficient tumors. Methods: We validated an automated dichotomously-scored IHC assay to detect ATM protein loss in primary prostate cancer using prostate cancer cell lines with and without bi-allelic ATM inactivation and 49 high-grade (primary Gleason pattern 5) prostate tumors with known ATM genomic status. We then examined the frequency of ATM loss among 23 tumors with pathogenic germline ATM mutations, as well as > 1000 additional primary prostate carcinomas using tissue microarrays (TMA). Results: ATM loss by IHC was found in 14% (7/49) of primary Gleason pattern 5 (5+4 = 9 and 5+5 = 10) tumors. Of these, all cases with adequate tumor content and DNA yield had underlying pathogenic ATM mutations. Of the remaining 42 cases without ATM protein loss, none had ATM alterations. Among men with pathogenic germline ATM mutations, 74% (17/23) had ATM loss by IHC. Of these, 76% (13/17) had homogeneous loss of ATM protein in all tumor cells within a dominant tumor nodule, suggesting that ATM loss was an early clonal event. On TMA analysis, 90% (944/1044) of tumors were evaluable for ATM status by IHC. Among these, ATM loss was seen in 3.3% (31/944), and was significantly more common in tumors with Gleason scores 9-10 (20/198; 10.1%) than in those of all other Gleason grades (11/746; 1.5%) ( P< 0.0001). Conclusions: Validated ATM IHC is a sensitive assay for detecting underlying genomic ATM alterations. ATM protein loss appears to be an early event occurring in the majority of tumors with underlying germline pathogenic ATM mutations, and is significantly enriched in high-grade prostate cancers (especially Gleason grades 9-10).