Predictive and Prognostic Value of the TauProtein in Breast Cancer

Role and Regulation of Tau Protein in Breast Epithelium

Tau protein has the ability to combine with tubulin. It may bind to the outer as well as the inner tubulin surface, at the same binding site as paclitaxel, and consequently it competes with this drug. Tau protein is mainly expressed in the neurons but is also found in normal breast epithelial cells (10). Its role in the cancer process is likely due to the importance of microtubules in mitosis. In a study of 1,942 patients with breast cancer, in 43% of patients, tau protein expression was greater than that of normal breast epithelium (i.e. tau protein-positive) (11).

Tau protein contains an imperfect estrogen-response element upstream of its promoter and is an estrogen-induced protein expressed by cultured neurons and breast cancer cell lines (10, 12, 13). All studies have found that tau protein expression correlates closely with estrogen receptor (ER) expression in breast cancer. Tau protein expression was significantly more frequent among patients with ER-positive cancer, 57-65% of these tumors were tau protein-positive compared with 15-30% of those with ER-negative tumors (11, 14-17). Tau protein positivity was also correlated with positive progesterone receptor (PR) status, lower histological grade and lack of human epidermal growth factor receptor 2 (HER2): 63% of PR-positive tumors were tau-positive compared to 36% of PR-negative tumors; 60% of low-grade tumors were tau-positive compared to 30% of high-grade tumors; and 2.4% of HER2⁺ tumors compared were tau-positive compared to 20% of HER2-negative tumors (11, 15-17). The inverse correlation between tau protein and HER2 expression is interesting as the gene for tau protein is adjacent to HER2 on the 17q12 amplicon, yet rarely appears to be co-expressed in HER2-positive tumors. Tau protein expression did not correlate with tumor size, nodal status or patient age (11, 15-17). The association of Tau protein status with menopausal status is not conclusive (15, 16).

Characterization of Tau Protein and Resistance to Taxanes

Taxanes, paclitaxel or docetaxel, belong to the spindle poison class. While taxanes bind to tubulin, on the microtubule's inner surface, microtubule depolymerization is inhibited (7). Microtubules, usually dynamically unstable, consequently become stable structures. This disables spindle division and causes cell-cycle arrest in phase G₁/G₂ of mitosis (18). The cytotoxic effect of paclitaxel also results from the induction of apoptosis by the regulation of tumor-suppressor genes (p53, B-cell lymphoma-2 (BCL2) and BCL-2–associated X protein (BAX)). The efficiency of taxanes has been demonstrated in many types of cancers and they are widely used in breast cancer chemotherapy.

Kinetic studies showed that tau protein binds to microtubules differently depending on the presence or absence of paclitaxel (19). Tubulin polymerization assays revealed that microtubules pre-assembled in the presence of tau protein bound less paclitaxel than microtubules assembled without tau protein, and this reduced paclitaxel-induced microtubule polymerization (10, 20). Small-interfering RNA experiments showed that down-regulation of tau protein increased the sensitivity of breast cancer cells to paclitaxel and docetaxel, minimally increased that to vinorelbine but not that to epirubicin, in vitro. Conversely, induction of tau protein expression with retinoic acid increased breast cancer cell resistance to paclitaxel, suggesting a protective effect (9). These data support that the loss of tau protein expression may render microtubules hyper-vulnerable to paclitaxel, or that tau protein, at least partially, protects microtubules from paclitaxel binding.

Prognostic Value of Tau Protein Expression

A prognostic factor is a clinical or biological characteristic that provides information on the likely outcome of cancer in an untreated individual. This is helpful for identifying patients with cancer who are at high risk of metastatic relapse and who are, therefore, potential candidates for adjuvant systemic treatments.

The first study to assess the prognostic value of tau protein mRNA expression in 209 patients with node-negative, ER-positive disease who received no adjuvant systemic therapy showed a borderline non-significant association between higher tau protein mRNA expression and better prognosis (14). Another study on 274 patients with high-risk early breast cancer from the HeCOG trial HE10/97 conducted univariate and multivariate Cox regression analysis. The multivariate analysis, including menopausal status; tumor grade; tumor size; number of positive lymph nodes; treatment group; and ER, PR, and tau mRNA expression, revealed that only mRNA expression and the number of involved axillary nodes remained significant independent predictors of overall survival (OS), while tau mRNA expression, number of positive nodes and tumor grade, independently predicted disease-free survival (DFS) (16). Positive tau mRNA expression was associated with a decreased risk of death with an hazard ratio (HR) of 0.46 (95% confidence interval (CI)=0.25-0.85; p=0.01) and a decreased risk of relapse with an HR of 0.53 (95% CI=0.32-0.89; p=0.02). In the ancillary study of the NSABP-B28 trial, among the patients with ER-positive disease (n=1287), patients with tau protein-positive cancer (n=736) had better DFS and OS compared to those with tau protein-negative tumors (11). Among patients with ER-negative disease (n=655), tau protein expression had no prognostic value. In the Yale university cohort including 651 patients, high tau protein expression was associated with improved survival compared to those with low expression (p=0.006) (15). When stratified by ER status, tau protein had prognostic value in patients with ER-negative but not in ER-positive patients. In the ER-negative/high tau protein expressers (n=35), they observed improved survival compared with low expressers (p=0.006). Similarly, patients stratified by HER2 status showed improved survival for those with high tau protein/HER2-positive expression compared with those with low tau protein/HER2-positive expression (p=0.007), although the coexpression of tau and HER2 was a rare event.

High tau protein expression was significantly associated with longer OS, with an HR of 0.42 (p=0.02) in a study including 54 patients with advanced or metastatic breast cancer (17). The estimated median OS time was 57.5 months (95% CI=36.2-78.8 months) for patients with high tau protein expression and 30.4 months (95% CI=26.7-34.1 months) for those with low tau protein expression. Similarly, when the TAX 307S cohort (140 patients with metastatic breast cancer) was stratified by tau protein expression, tau protein had a prognostic value, with improved median time to PFS for high tau protein-expression cases (n=35) compared to low-expression cases (n=73; 33.0 vs. 23.4 months) and a mean time to progression of 31.2 months (p=0.010), suggesting that this marker maintains prognostic value in patients with metastatic disease (15).

Predictive Value of the Tau Protein

Sensitivity to chemotherapy. A predictive marker is a characteristic that provides information on the likely benefit from treatment. It can be used to identify sub-populations of patients who are most likely to benefit from a given therapy. According to in vitro studies, tau protein was a candidate predictive marker for response to chemotherapy.

In 2005, Rouzier et al. conducted the first study involving tau protein in breast cancer (10). They included 122 patients with stage I-III breast cancer treated with neoadjuvant therapy. Indeed, administration of chemotherapy before surgery provides an opportunity to directly measure tumor response and identify molecular predictors. Several large retrospective studies have demonstrated that complete eradication of all invasive cancer from the breast and axillary lymph nodes after preoperative chemotherapy, i.e. pathological complete response (pCR), is associated with excellent cancer-free survival (6, 7). Therefore, molecular predictors of pCR should help identify individuals who are most likely to benefit from a particular therapy. They first used gene-expression profiling (U133A chip) to discover genes associated with extreme chemotherapy sensitivity. The most significantly differentially expressed gene between cases with pCR and those with residual cancer was microtubule-associated tau protein (lower tau mRNA expression in tumors from patients that achieved pCR) (10). They then used tissue arrays from the same patients for validation by immunohistochemistry. Tau-negative status was associated with an odds ratio (OR) for pCR of 3.7 (95% CI=1.6-8.6; p=0.0013). In multivariate analysis (including patient age; tumor size; histological type and grade; and ER, PR, HER2 and tau protein status), nuclear grade, age less than 50 years and tau protein-negative status were independent predictors of pCR (Table I).

In a second study of 82 patients with ER-positive tumors who received neoadjuvant chemotherapy including paclitaxel, the pCR rate was 19% in the lowest tertile of tau protein expression, 3% in the median tertile, and 0% in the highest tertile (14). Multivariate logistic regression analysis including age, tumor size, nodal status, ER expression and tau protein indicated that tau protein remained a significant independent predictor of pCR. High tau expression was associated with a decreased OR for pCR of 0.14, (95% CI=0.03-0.59; p=0.008) (Table I). These results corroborated the previous observation that low tau protein expression in ER-positive breast cancer was associated with paclitaxel sensitivity (10). Thus, tau protein expression was adopted as a candidate novel marker for paclitaxel response in the highlights of the tenth St. Gallen international conference on primary therapy for early breast cancer (21).

Similar results were reported on a small cohort of 35 patients treated with paclitaxel for metastatic breast cancer (22): 60% of patients with tau protein-negative status showed favorable response (complete or partial response according to RECIST criteria) to paclitaxel administration compared to 15% of patients with tau-positive status. The OR for favorable response in patients with tau protein-negative tumors was 8.5 (95% CI=1.7-42.3; p=0.01). In addition, the duration of response was 10.7±5.7 months for the tau protein-negative group and 6.0±2.0 months for the positive group, and time to disease progression was 9.4±6.6 months and 6.0±3.7 months, respectively. Two other studies, including 54 patients with advanced or metastatic breast cancer receiving paclitaxel chemotherapy and 113 patients with breast cancer patients receiving neoadjuvant chemotherapy, showed that tau protein expression was significantly associated with a lower overall response rate and pCR respectively, in univariate and multivariate analysis (Table I) (17, 23).

Conversely, in the ancillary study of the HeCOG trial HE10/97 including 274 patients with high-risk early breast cancer treated with paclitaxel, the predictive role of tau protein for response to chemotherapy was not confirmed (16). The objective of this study was to study the predictive significance and correlations of transcriptional profiling of the genes for ER, PR and tau protein in breast cancer. In multivariate models, they found that neither tau mRNA nor ER mRNA expression status were predictive for benefit from adjuvant treatment with paclitaxel for either DFS or OS. Restricting analysis to ER-positive tumors, tau mRNA expression status remained non-predictive for benefit from adjuvant treatment with paclitaxel. As the majority of this cohort received endocrine therapy after chemotherapy, it is possible that any predictive significance of low tau mRNA expression for taxane benefit was cancelled out by resistance to hormonal therapy, resulting in no detectable differences in outcome. Moreover the authors point-out that in the entire HE10/97 trial, the addition of taxanes reduced the hazard of death only in patients with ER-negative disease (24). Given the correlation of ER and tau gene activity, it is possible that patients with ER-negative disease who benefit from taxanes are mostly tau protein-negative. This effect may have been lost in their cohort as the number of patients evaluated was reduced to less than half due to limitations in tissue availability. Another study of 50 patients treated by neoadjuvant chemotherapy for breast cancer failed to find any predictive value of tau protein for response to taxane (25). However, the population was poor and they used docetaxel rather than paclitaxel, which is more commonly used in other studies. This limits comparability with other studies.

In this context of contradictory results about the predictive value of tau protein, an ancillary study on the NASBP-B28 trial was conducted including 1,942 patients with breast cancer treated with adjuvant chemotherapy (11). Although that study had the greatest number of patient assessed for tau protein in breast cancer, there was no significant interaction between tau protein expression and benefit from paclitaxel in the total population, nor by ER status. This was confirmed on the TAX 307 cohort with 140 patients with metastatic breast cancer (15). No significant interaction between tau protein expression and benefit from docetaxel was observed and response rates to docetaxel therapy as a function of tau protein expression did not differ when partitioned by ER status, adjuvant endocrine therapy, or taxane treatment arm.

Sensitivity to endocrine therapy. Only one study has examined the predictive value of tau protein for sensitivity to endocrine therapy in breast cancer (14). It included 267 ER-positive tumors from patients who received 5 years of adjuvant tamoxifen therapy. Multivariate Cox regression analysis including age, tumor size, nodal status, ER expression, and tau protein expression indicated that tau protein was a significant independent predictor of outcome. Higher tau expression was associated with a decreased risk of distant recurrence in this population of patients with tamoxifen-treated breast cancer, with an HR of 0.60 (95% CI=0.38-0.94), indicating a predictive value for endocrine therapy.