Abstract P1-16-04: Somatic mutations, clinicopathologic characteristics, and survival in patients with untreated breast cancer with bone-only and non-bone sites of first metastasis
Authors: Kono M , Fujii T , Matsuda N , Harano K , Chen H , Wathoo C , Aron JY , Tripathy D , Meric-Bernstam F , Ueno NT 
Published: February 2018

Background: Bone is the most common site of metastasis of breast cancer, and bone metastasis is associated with a high rate of skeletal-related events, all of which contribute to decreased quality of life and poor outcomes. Biological mechanisms of metastasis to bone may be unique, and identification of distinct signaling pathways and somatic mutations may provide biological insight into or rational targets for treatment of and prevention of bone metastasis. The aims of this study were to compare and contrast somatic mutations, clinicopathologic characteristics, and survival in breast cancer patients with bone only versus non-bone as first metastatic site.

Methods: Tumor samples were collected from 389 patients who had metastasis and untreated primary breast cancer. In each sample, 46 or 50 cancer-related genes were selectively amplified and analyzed for mutations by AmpliSeq Ion Torrent next-generation sequencing. We used Fisher's exact test to identify somatic mutations associated with bone-only first metastasis and logistic regression models to identify differences in clinicopathologic characteristics, survival, and somatic mutations between patients with bone-only first metastasis and patients with first metastasis in non-bone sites only (“other-only first metastasis”).

Results: Among the 389 patients, the first metastasis was located in bone only in 72 patients (18.5%), non-bone sites only in 223 patients (57.3%), and both in 94 patients (24.2%). Of the cancer-related genes analyzed, the most commonly mutated were TP53 (N=103), PIK3CA (N=79), AKT (N=13), and PTEN (N=2). Compared to patients with other-only first metastasis, patients with bone-only first metastasis had higher rates of hormone-receptor-positive disease, non-triple-negative subtype, and low nuclear grade (grade 1 or 2) (all 3 comparisons, p&lt;0.001); had a lower ratio of cases of invasive ductal carcinoma to cases of invasive lobular carcinoma (p=0.002); and tended to have a higher 5-year overall survival (OS) rate (78.2% [95% confidence interval (CI), 68.6%-89.0%] vs 55.0% [95% CI, 48.1%-62.9%]; p=0.051). However, in the subgroup of patients with TP53 mutation and in the subgroup of patients with PIK3CA mutation, OS did not differ between patients with bone-only and other-only first metastasis (p=0.49 and p=0.68; respectively). In univariate analysis, the rate of TP53 mutation tended to be lower in patients with bone-only first metastasis than in those with other-only first metastasis (15.3% vs 29.1%; p=0.051). In multivariate analysis, TP53 mutation was not significantly associated with site of first metastasis (p=0.54) but was significantly associated with hormone-receptor-negative disease (p&lt;0.001).

Conclusions: We did not find associations between somatic mutations and bone-only first metastasis in patients with untreated breast cancer. Patients with bone-only first metastasis have longer OS than patients with other-only first metastasis. More comprehensive molecular analysis may be needed to further understand the factors associated with bone-only metastatic disease in breast cancer.

Citation Format: Kono M, Fujii T, Matsuda N, Harano K, Chen H, Wathoo C, Aron JY, Tripathy D, Meric-Bernstam F, Ueno NT. Somatic mutations, clinicopathologic characteristics, and survival in patients with untreated breast cancer with bone-only and non-bone sites of first metastasis [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-16-04.