Somatic mutations, clinicopathologic characteristics, and survival in patients with untreated breast cancer with bone-only and non-bone sites of first metastasis

Background: Bone is the most common site of metastasis of breast cancer. Biological mechanisms of metastasis to bone may be different from mechanisms of metastasis to non-bone sites, and identification of distinct signaling pathways and somatic mutations may provide insights on biology and rational targets for treatment 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 sites of first metastasis. Methods: Primary tumor samples were collected before treatment from 389 patients with untreated primary breast cancer and distant metastasis at diagnosis. In each sample, 46 or 50 cancer-related genes were analyzed for mutations by AmpliSeq Ion Torrent next-generation sequencing. Fisher's exact test was used to identify somatic mutations associated with bone-only first metastasis. Logistic regression models were used to identify differences in detected somatic mutations, clinicopathologic characteristics, and survival 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, 72 (18.5%) had bone-only first metastasis, 223 (57.3%) had other-only first metastasis, and 94 (24.2%) had first metastasis in both bone and non-bone sites. The most commonly mutated genes were TPS3 (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 lower grade (grade 1 or 2; Nottingham grading system) (all three comparisons, p<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<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 tend to 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.