| Bone is a common site for the spread of breast cancer. The identification of a highly discriminatory plasma protein profile could provide a tool for the early detection of bone metastasis. The purpose of this study was to identify a protein signature indicative of breast cancer metastasis to bone. In all, plasma was obtained from 76 breast cancer patients treated with second-line hormone therapy: 38 patients with clinical evidence of bone metastasis and high levels of the bone resorption marker N-Telopeptide (NTx), and 38 patients with no clinical evidence of bone metastasis and low levels of NTx. Samples were analyzed in duplicate on weak cation-exchange (CM10) arrays using surface-enhanced laser desorption ionization time-of-flight mass spectrometry (SELDI-TOF MS) in three mass ranges: low 1.5--10 kDa, mid 7--30 kDa, and high 25--150 kDa. All spectra were baseline corrected, normalized to the total ion current, log2 transformed, and analyzed by analysis of variance (ANOVA), principle component analysis (PCA), partial least squares (PLS), random forest (RF), classification and regression trees (CART), and See5. The multivariate statistical analysis and data mining procedures ultimately resulted in the detection of thirteen specific protein peaks (MW ranging from 4.1-39.1kDa) that optimally separated the two patient groups with a high degree of significance (p<0.05). In all, 11 peaks were identified as increased and 2 peaks decreased in the plasma of breast cancer patients with bone metastasis. The plasma protein profile identified was independently validated in a blinded fashion using a second set of anonymous plasma samples from 20 breast cancer patients with and 20 breast cancer patients without bone metastasis; 11 of the original 13 protein peaks identified were confirmed. Importantly, one discriminatory protein peak found in the plasma of bone metastasis patients was identified as parathyroid hormone-related protein (PTHrP), a tumor derived factor expressed in the majority of breast cancer bone metastasis. SELDI-TOF MS analysis identified and confirmed a plasma protein profile that distinguishes breast cancer patients with and without bone metastasis. Further elucidation of the identity of these proteins has the potential to improve the early diagnosis of breast cancer bone metastasis. |
