Abstract S1-10: Clinical implications of somatic mutations in post-menopausal early-stage estrogen receptor (ER)-positive HER2-negative breast cancer (BC): Results from the BIG 1-98 study

Abstract

Abstract Background Next generation sequencing (NGS) has revealed that ER+/HER2- BCs have diverse somatic copy number and mutation profiles but thus far the clinical relevance of such findings is unknown. We characterized the molecular alterations in post-menopausal primary BC patients treated in the BIG 1-98 adjuvant letrozole and tamoxifen study and evaluated their associations with prognosis. Materials and Methods NGS was used to genotype DNA from archival primary tumor blocks for 286 cancer-related genes. From 2706 available eligible samples (confirmed ER+, excluding HER2-positivity or neoadjuvant treatment, adequate DNA quality/quantity), a case-cohort design selected 764 samples: all distant relapses and a stratified sampling of non-relapses after 8yr median follow-up. Mutation prevalence and associations with clinicopathological factors (CP) as well as distant recurrence-free interval (DRFS) were analyzed using weighted tests and Cox regression models, with sampling weights to represent the ER+/HER2-negative trial population. Multivariable analyses were adjusted for tumor size, nodal status, grade and age. Results NGS data was available from 538/764 samples (70%), there were 140 (26%) distant relapses. Median sequencing depth was 483x. There was a mean of 11 mutations (1-46) per sample, with 25% having 13 or more mutations and no tumors without a mutation. Overall 28 genes were altered at a frequency of >10%. The most commonly mutated genes were PIK3CA (49.3%), NCOR1 (27.2%), MAP3K1 (23.8%) TP53 (16.6%), CCND1 (17.8%) and GATA3 (17.1%). Alterations that were significantly associated with both Luminal B (Ki67 >14%) and grade 3 included TP53 mutations (p<0.001), FGFR1 (p=0.001) and MYC amplifications (p=0.004). Gene alterations that were significantly associated with shorter DRFS included TP53 (HR:2.16), ARID1A (HR:2.43), CHEK2 (HR:2.54), BRCA2 (HR:1.93), PTEN (HR:2.03), CCND1 (HR:1.82) and FGFR1 (HR:1.78). PIK3CA was significantly associated with lower risk of distant relapse (HR:0.64; 0.43-0.97). Increasing number of total mutations was significantly associated with shorter DRFS (HR:1.04; 95% CI 1.01-1.07; p=0.006). In the multivariable model adjusted for CP factors, ARID1A, BRCA2, CCND1, CHEK2 and PTEN remained independent for shorter DRFS. Greater than 90% of PIK3CA mutations co-existed with another alteration, most common being NCOR1 (29%), MAP3K1 (24%), CDH1 (16%), GATA3 (16%), TP53 (14%) and CCND1 (13%). Patients with a PIK3CA mutation had greater benefit with letrozole over tamoxifen monotherapy (HR 0.32; 0.13-0.8) than those without (HR:0.70; 0.33-1.48) (Pint=0.06). This effect was strongest in the subgroup of PIK3CA mutant patients who were CCND1 and TP53 wild-type (HR:0.24, 0.12-0.48; Pint=0.02) with only 1% relapsing at 5 years. Conclusion: For the first time, we report the prognostic relevance of oncogenic mutations in ER+/HER2- postmenopausal early-stage BCs from a clinical trial. Tumors with PIK3CA mutations derived greater benefit from letrozole over tamoxifen monotherapy, especially if wild-type for CCND1 and TP53. These findings could significantly improve prognostic risk classification and guide future clinical trials of targeted therapies in ER+/HER2- BCs. Citation Format: Loi S, Asher R, Lee CK, Luen S, Savas P, Kammler R, Dell'Orto P, Blasi OM, Demanse D, JeBailley L, Dolan S, Hackl W, Thuerlimann B, Viale G, Regan M, Colleoni MA. Clinical implications of somatic mutations in post-menopausal early-stage estrogen receptor (ER)-positive HER2-negative breast cancer (BC): Results from the BIG 1-98 study [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr S1-10.