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Institute of Molecular and Translational Medicine, Faculty of Medicine and Dentistry, Palacky University and University Hospital in Olomouc, Olomouc, Czech Republic
Address for correspondence: Vladimira Koudelakova, PhD, Institute of Molecular and Translational Medicine, Faculty of Medicine and Dentistry, Palacky University and University Hospital in Olomouc, Hnevotinska 5, 775 15 Olomouc, Czech Republic
Institute of Molecular and Translational Medicine, Faculty of Medicine and Dentistry, Palacky University and University Hospital in Olomouc, Olomouc, Czech Republic
Institute of Molecular and Translational Medicine, Faculty of Medicine and Dentistry, Palacky University and University Hospital in Olomouc, Olomouc, Czech Republic
Institute of Molecular and Translational Medicine, Faculty of Medicine and Dentistry, Palacky University and University Hospital in Olomouc, Olomouc, Czech Republic
Triple-negative breast cancer (TNBC) accounts for 15% to 20% of breast cancer cases and is characterized by the absence of estrogen, progesterone, and human epidermal growth factor 2 receptors. Though TNBC is a highly heterogenic and aggressive disease, TNBC patients have better response to neoadjuvant therapy compared to other breast cancer subtypes. Nevertheless, patients with residual disease have a very poor prognosis, with higher probability of relapse and lower overall survival in the first years after diagnosis. TNBC has 6 subtypes with distinct molecular signatures with different prognoses and probably different responses to therapy. The precise stratification of TNBC is therefore crucial for the development of potent standardized and targeted therapies. In spite of intensive research into finding new molecular biomarkers and designing personalized therapeutic approaches, BRCA mutational status is the only clinically validated biomarker for personalized therapy in TNBC. Recent studies have reported several promising biomarkers that are currently being validated through clinical trials. The objective of this review was to summarize the clinically relevant genetic markers for TNBC that could serve as diagnostic, prognostic, or predictive or could improve personalized therapeutic strategies.
Breast cancer is the leading cause of cancer death in women worldwide, and triple-negative breast cancer (TNBC) accounts for approximately 15% to 20% of all new cases. All TNBC subtypes share a common gene expression pattern: the absence of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2; also known as ERBB2) expression.
Despite these shared features, TNBC is a highly heterogeneous disease that can be divided into many distinct subgroups according to clinical, histopathologic, and molecular profiles.
TNBC patients are typically young (< 40 years), are African American, and have shorter progression-free survival and overall survival (OS) relative to non-TNBC breast cancer patients.
Descriptive analysis of estrogen receptor (ER)-negative, progesterone receptor (PR)-negative, and HER2-negative invasive breast cancer, the so-called triple-negative phenotype: a population-based study from the California Cancer Registry.
Descriptive analysis of estrogen receptor (ER)-negative, progesterone receptor (PR)-negative, and HER2-negative invasive breast cancer, the so-called triple-negative phenotype: a population-based study from the California Cancer Registry.
Additional features of TNBC include a peak in recurrence between 1 and 3 years after diagnosis (hazard ratio = 2.6; P < .0001), as well as a majority of deaths occurring within 5 years of therapy (hazard ratio = 3.2; P < .0001) compared to non-TNBC phenotypes.
TNBC patients usually experience better pathologic complete response rates (pCR) after neoadjuvant chemotherapy (pCR rates in 30%-40%). Moreover, TNBC patients who experience pCR have excellent long-term clinical outcome. However, TNBC patients with residual disease after neoadjuvant chemotherapy have very poor prognosis.
The recurrence of TNBC is associated with a high risk of metastasis to the lungs or central nervous system, a lower risk of bone metastasis, and a dismal median survival of approximately 1 year.
Clinicopathologic features, patterns of recurrence, and survival among women with triple-negative breast cancer in the National Comprehensive Cancer Network.
The intricacy of this disease is further illustrated by the high prevalence of rare histopathologic subtypes such as metaplastic (90%), medullary (95%), and apocrine (40%-60%) carcinomas.
When both the poor prognosis facing TNBC patients and the lack of a recognized predictor of therapy response are considered, the need to identify specific markers that can be targeted by tailored therapies or used to predict response to chemotherapy is indisputable.
This review focuses on genetic alterations in TNBC that could serve as predictive markers of prognosis, which will help in selecting a suitable chemotherapy approach and/or inspire further research.
Intrinsic Subgroups of Breast Cancer
Breast cancer comprises a heterogeneous group of diseases that can be, according to gene expression profiles, classified into luminal A, luminal B, basal-like, normal-like and HER2-enriched subgroups.
At the morphologic level, TNBC and basal-like breast cancer (BLBC) are similar in terms of larger tumor size, higher grade, presence of geographic necrosis, enhanced invasive potential, and stromal lymphocytic infiltration.
However, the gene expression profiles of only 71% of TNBC samples are clustered as basal-like. Moreover, only 77% of the basal-like tumors bear TNBC signatures.
This observation was confirmed through the molecular characterization of 412 TNBC and 473 basal-like (based on PAM50 subtype prediction) breast cancer samples.
Using this approach, 21.4% of TNBC samples were not assigned as BLBC, and 31.5% of BLBC samples did not display a TNBC profile. Out of 412 TNBC samples, 78.6% were identified as BLBC, 7% as normal-like, 7.8% as HER2 enriched, 4.4% as luminal B, and 2.2% as luminal A.
TNBC Subtypes
Once the molecular heterogeneity of TNBC was recognized, subsequent research focused on classifying TNBC subtypes on the basis of disease prognosis or the expected response to systemic therapy. Groundbreaking work identified 6 different TNBC gene expression profile subtypes from 587 TNBC cases identified in 21 gene expression data sets using a top-down approach of hierarchical clustering.
The subtypes were named according to their expression patterns: basal-like 1 and 2 (BL1/2), immunomodulatory (IM), mesenchymal (M), mesenchymal stem–like (MSL), and luminal androgen receptor (LAR) (Table 1). Following this classification, approximately 30 TNBC cell lines have been identified as models of the distinct subtypes and are used to investigate which pharmacologic strategies are most effective against each subtype.
Table 1Triple-Negative Breast Cancer Subtype Characterization
Both BL1 and BL2 subtypes are sensitive to DNA-damaging agents (such as cisplatin) and show elevated expression of cell-cycle and DNA damage-response genes. While BL1 is characterized by heightened expression of both cell division and DNA damage-response genes, as well as elevated Ki-67 expression, BL2 displays up-regulated growth factor signaling, glycolysis, and gluconeogenesis along with increased expression of myoepithelial markers.
Both M and MSL subtypes are characterized by decreased distant metastasis-free survival and positive response to phosphoinositide 3-kinase (PI3K)/mechanistic target of rapamycin (mTOR) inhibitors and dasatinib. The gene expression profiles of the M and MSL subtypes overlap with that of chemoresistant metaplastic breast cancer and display the up-regulation of genes involved in epithelial–mesenchymal transition, cell motility, extracellular matrix remodeling, and cellular differentiation. Unlike the M subtype, which displays overexpression of proliferation genes, the MSL subtype is enriched in mesenchymal stem-cell–associated genes and shows up-regulation of genes involved in angiogenesis and growth factor pathways. The MSL subtype overlaps with the previously described claudin-low subtype, as both demonstrate reduced claudin 3, 4, and 7 expression.
The IM subtype is characterized by increased expression of immune signaling genes (immune cell and cytokine signaling, antigen processing and presentation, core immune signaling pathways). The IM expression profile overlaps with the molecular signature of medullary breast cancer, and both classifications share a good prognosis.
The robust presence of TILs has been found in approximately 20% of TNBC and was found to be an independent prognostic marker in TNBC. The BIG 02-98, ECOG 2197, and ECOG 1199 trials demonstrated very similar results, with 15% to 20% reduction in any recurrence and mortality for every 10% increase in stromal TILs.
Prognostic value of tumor-infiltrating lymphocytes in triple-negative breast cancers from two phase III randomized adjuvant breast cancer trials: ECOG 2197 and ECOG 1199.
Neoadjuvant bevacizumab and anthracycline–taxane–based chemotherapy in 678 triple-negative primary breast cancers; results from the geparquinto study (GBG 44).
Tumor infiltrating lymphocytes are prognostic in triple negative breast cancer and predictive for trastuzumab benefit in early breast cancer: results from the FinHER trial.
The presence of TILs is associated with better response to both adjuvant and neoadjuvant therapy, and could serve as marker of better outcome when detected in residual tumor after neoadjuvant therapy.
Prognostic value of tumor-infiltrating lymphocytes in triple-negative breast cancers from two phase III randomized adjuvant breast cancer trials: ECOG 2197 and ECOG 1199.
Prognostic value of tumor-infiltrating lymphocytes on residual disease after primary chemotherapy for triple-negative breast cancer: a retrospective multicenter study.
Tumor-infiltrating lymphocytes and response to neoadjuvant chemotherapy with or without carboplatin in human epidermal growth factor receptor 2–positive and triple-negative primary breast cancers.
The final subtype, LAR, is enriched in genes involved in hormone signaling, steroid synthesis, and androgen/estrogen metabolism, including overexpression of androgen receptor (AR) and its downstream targets and coactivators. Patients with the LAR subtype show shorter relapse-free survival. This subtype overlaps with the previously described molecular apocrine group.
Moreover, LAR-subtype cell lines are sensitive to PI3K inhibitors as a result of a mutation in the phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit α (PIK3CA) kinase domain.
A study clarifying the clinical relevance of the 7-subtype classification of TNBC revealed differences in pCR after neoadjuvant chemotherapy (P = .044) and found the TNBC subtype to be an independent predictor of pCR status (P = .022) by a likelihood ratio test. BL1 showed the highest pCR rate (52%), while BL2 and LAR subtypes showed the lowest rates (0% and 10%, respectively).
The molecular and genetic profiles of TNBC, known for its enormous complexity and diversity, continue to challenge researchers all around the world. As mentioned above, TNBC tumors are characterized by the lack of ER, PR, and HER2 expression. The lack of therapeutic targets complicates efforts to characterize TNBC with certain molecular markers in a bid to improve disease outcome. To date, two large studies have focused on the genetic basis of TNBC.
Prognostic interaction between expression of p53 and estrogen receptor in patients with node-negative breast cancer: results from IBCSG trials VIII and IX.
p53 deficiency linked to B cell translocation gene 2 (BTG2) loss enhances metastatic potential by promoting tumor growth in primary and metastatic sites in patient-derived xenograft (PDX) models of triple-negative breast cancer.
Higher response to neoadjuvant anthracycline and taxane therapy, response to platinum-based therapy, potential predictor for response to PARP inhibitors
Both carboplatin and bevacizumab improve pathological complete remission rate in neoadjuvant treatment of triple negative breast cancer: a meta-analysis.
Randomized phase III trial of paclitaxel once per week compared with nanoparticle albumin-bound nab-paclitaxel once per week or ixabepilone with bevacizumab as first-line chemotherapy for locally recurrent or metastatic breast cancer: CALGB 40502/NCCTG N063H (Alliance).
PIK3CA mutations in androgen receptor–positive triple negative breast cancer confer sensitivity to the combination of PI3K and androgen receptor inhibitors.
Neoadjuvant bevacizumab and anthracycline–taxane–based chemotherapy in 678 triple-negative primary breast cancers; results from the geparquinto study (GBG 44).
Both carboplatin and bevacizumab improve pathological complete remission rate in neoadjuvant treatment of triple negative breast cancer: a meta-analysis.
Impact of the addition of carboplatin and/or bevacizumab to neoadjuvant once-per-week paclitaxel followed by dose-dense doxorubicin and cyclophosphamide on pathologic complete response rates in stage II to III triple-negative breast cancer: CALGB 40603.
The exome-sequencing, RNA-sequencing, high-resolution single nucleotide polymorphism arrays and targeted deep resequencing were performed on 104 primary TNBC samples grouped into various subsets to reveal the patterns of somatic mutation.
The most frequent copy number aberrations were identified for the PARK2 (Parkinson disease 2) (6%), RB1 (retinoblastoma gene 1) (5%), PTEN (phosphatase and tensin homolog) (3%), and EGFR (epidermal growth factor receptor) (5%) genes. TP53 mutations were found to be the most common somatic aberration, observed in 53.8% of cases, while the TNBC samples also showed frequent mutations in the PIK3CA (10.2%), USH2A (usher syndrome 2A) (9.2%), MYO3A (myosin IIIA) (9.2%), PTEN, and RB1 genes (7.7%). However, only a minority of mutations (36%) were transcribed into mRNA.
The Cancer Genome Atlas Group analyzed samples from 463 patients using genomic DNA copy number arrays, DNA methylation, exome sequencing, mRNA arrays, microRNA sequencing, and reverse-phase protein arrays.
In a group containing 93 basal-like tumors (76 TNBCs), the most commonly mutated genes were found to be TP53 (80%), PIK3CA (9%), MLL3 (lysine methyltransferase 2C) (5%), AFF2 (AF4/FMR2 family member 2) (4%), RB1 (4%), and PTEN (1%). Copy number alterations were observed in several chromosomal regions or genes, namely amplification or gain of MYC (MYC protooncogene) (40%), (E3 ubiquitin-protein ligase Mdm2) (14%), CCNE (cyclin E1) (9%), as well as the 1q and 10p regions, along with loss of PTEN, RB1, INPP4B (inositol polyphosphate-4-phosphatase type II B) (30%), and the 8p and 5q regions. Heightened CDKN2A (cyclin-dependent kinase inhibitor 2A) expression, decreased RB1 expression, and high genomic instability were also found to be typical features of the BLBC profile.
The discovery of the fusion gene EML4-ALK (echinoderm microtubule-associated protein-like 4-anaplastic lymphoma kinase) in non–small-cell lung cancer fueled interest in finding such a structural rearrangement in breast carcinomas, particularly in TNBC.
An enrichment in most known MAGI3-AKT3 (membrane-associated guanylate kinase–AKT serine/threonine kinase 3) translocation as well as rearrangements involving the NOTCH1/2 (Notch 1/2) and MAST (microtubule-associated serine–threonine kinase) genes, were identified in TNBC by whole exome sequencing.
As was mentioned above, TNBC disease showed higher sensitivity to neoadjuvant chemotherapy, but patients with residual disease have very poor prognosis. Identification of targetable alteration in residual tumor is therefore necessary.
The genomic profile of tumor has been shown to be frequently altered during chemotherapy. Several studies comparing pretreatment and posttreatment biopsy samples found significant changes mainly in cell-cycle regulators and PI3K/mTOR pathway.
Molecular profiling of the residual disease of triple-negative breast cancers after neoadjuvant chemotherapy identifies actionable therapeutic targets.
These genomic changes could be the reason for resistance to conventional chemotherapies and identification of new druggable targets in posttreatment biopsy samples could significantly improve TNBC outcome. Molecular analysis of posttreatment biopsy samples is therefore necessary in TNBC patients who do not experience pCR after neoadjuvant chemotherapy.
TP53 Gene
TP53 is one of the most important genes involved in maintaining homeostasis and genomic integrity throughout cell-cycle arrest, DNA repair, and apoptosis. Alterations of TP53 associated with aberrant p53 expression have been described in numerous types of human cancers as well as in all breast cancer subtypes.
Expression of mutant p53 was found to be associated with high proliferation rate, early disease recurrence, and early death in node-negative breast cancer.
In breast cancer, the DNA-binding domain is the most frequently mutated area of the TP53 gene, and missense substitutions were identified as the culprit behind unfavorable breast cancer outcomes.
While missense mutations have been found to be predominantly associated with the luminal subtype, nonsense and frameshift changes are prevalent in basal-like tumors.
Prognostic interaction between expression of p53 and estrogen receptor in patients with node-negative breast cancer: results from IBCSG trials VIII and IX.
Moreover, ER-negative patients with p53 expression (TNBC and HER2-positive subtypes) were reported to have a better prognosis, while p53 expression in ER-positive patients was related to a worse prognosis.
Prognostic interaction between expression of p53 and estrogen receptor in patients with node-negative breast cancer: results from IBCSG trials VIII and IX.
In the context of TNBC, these mutations result in increased genetic instability and cytogenetic changes, as well as a higher probability of loss of heterozygosity.
p53 deficiency linked to B cell translocation gene 2 (BTG2) loss enhances metastatic potential by promoting tumor growth in primary and metastatic sites in patient-derived xenograft (PDX) models of triple-negative breast cancer.
However, another study did not confirm that mutations of TP53 and/or p53 expression are prognostic factors; nevertheless, discrepancies between TP53 mutation and p53 expression could be a potential predictor of poor outcome in TNBC.
Taken together, TP53 is mutated in a majority of TNBC cases and is therefore an attractive candidate for antitumor therapies.
BRCA1/2
The BRCA1 and BRCA2 gene products are vital to the activation and transcriptional regulation of DNA damage, control of the cell cycle, and cellular proliferation and differentiation.
More specifically, BRCA1/2 proteins play an essential role in DNA double-strand break repair by homologous recombination (HRR) and the maintenance of DNA stability.
Over 80% of hereditary BRCA1-mutated breast cancers are classified as TNBC and/or BLBC, and approximately 15% TNBC patients are carriers of a BRCA germ-line mutation (gBRCA).
Inherited mutations in 17 breast cancer susceptibility genes among a large triple-negative breast cancer cohort unselected for family history of breast cancer.
The remaining sporadic TNBC cases frequently share certain characteristics with BRCA1/2 mutation carriers in HRR defects, sometimes collectively termed BRCAness.
This BRCAness status can involve the epigenetic inactivation of BRCA1 by promoter methylation, which has been associated with poor prognosis in terms of relapse-free survival and OS after anthracycline- or taxane-based therapy.
Breast cancers with BRCA1 mutations as well as BRCAness often express basal markers that correspond with the BL1 subtype and therefore respond to neoadjuvant anthracycline and taxane therapy.
Interesting results were recently published in the POSH study determining the effect of gBRCA on breast cancer outcome after systemic therapy. OS at 10 years was 78% in gBRCA carriers compared to 69% in BRCA-negative cases, suggesting that BRCA mutation provided some survival advantage to their carriers.
Better survival of gBRCA TNBC and probably also BRCAness might be caused by better sensitivity of gBRCA carriers to chemotherapy as a result of defects in HRR or higher immune activation.
Veliparib with temozolomide or carboplatin/paclitaxel versus placebo with carboplatin/paclitaxel in patients with BRCA1/2 locally recurrent/metastatic breast cancer: randomized phase II study.
Moreover, patients with deficient BRCA1/2 function should be more susceptible to DNA-damaging agents like platinum derivatives and poly(ADP ribose) polymerase (PARP) inhibitors.
The Treating to New Targets (TNT) trial shown the double objective response rate to carboplatin compared to docetaxel in metastatic TNBC tumors carrying gBRCA mutations.
The TNT trial: a randomized phase III trial carboplatin (C) compared with docetaxel (D) for patients with metastatic or recurrent locally advanced triple negative or BRCA1/2 breast cancer (CRUK/07/012).
Cancer Res.2015; 75 (Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium; December 9-13, 2014; San Antonio, TX. Philadelphia, PA: AACR; 2015) (abstract S3-01)
Moreover, not only BRCA1/2 mutation carriers but also patients with advanced TNBC with defects in the BRCA1/2 pathway (determined by higher values of loss of heterozygosity score and large-scale state transitions score) showed a positive response to platinum therapy in the TBCR009 study.
Biomarkers of genomic instability that predict a positive response to platinum-based therapy should therefore be validated for a subset of TNBC tumors.
In the neoadjuvant setting, the role of gBRCA mutation in response to platinum-based therapy is unclear. Several studies demonstrated higher responses in gBRCA carriers; nevertheless, the GeparSixto study showed higher responses in patients with wild-type BRCA.
PARP inhibitors present another promising therapeutic tool. PARP inhibition increases the occurrence of irreparable toxic DNA double-strand breaks resulting in cell death in BRCA-mutated patients. PARP inhibitors therefore effectively kill the tumor cells through the principle of synthetic lethality. The PARP inhibitors olaparib and rucaparib have been approved for the treatment of advanced previously treated ovarian cancer with gBRCA. More recently, olaparib was also approved by the US Food and Drug Administration for treatment of metastatic HER2-negative breast cancer with BRCA mutation previously treated with chemotherapy.
The poly(ADP-ribose) polymerase inhibitor niraparib (MK4827) in BRCA mutation carriers and patients with sporadic cancer: a phase 1 dose-escalation trial.
Promising results with PARP inhibitor talazoparib were shown in the EMBRACA study. Talazoparib therapy resulted in significantly prolonged progression-free survival in advanced HER2-negative breast cancer patients with gBRCA compared to treatment of physician's choice.
EMBRACA: a phase 3 trial comparing talazoparib, an oral PARP inhibitor, to physician’s choice of therapy in patients with advanced breast cancer and a germline BRCA mutation.
Cancer Res.2018; 78 (Proceedings of the 2017 San Antonio Breast Cancer Symposium; December 5-9, 2017; San Antonio, TX. Philadelphia, PA: AACR; 2018) (abstract GS6-07)
Dysregulation of the PI3K/AKT/mTOR pathway causes changes in cell survival, differentiation, and/or proliferation that are frequently observed during carcinogenesis.
PTEN is an important negative regulator of the PI3K pathway. Loss of PTEN expression has been shown to be significantly associated with ER negativity as well as basal-like phenotype.
The phosphatase INPP4B, another negative regulator of the PI3K pathway, has been shown to be frequently lost in ER-negative primary breast carcinomas. INPP4B loss is associated with high clinical grade, increased tumor size, loss of hormone receptors, and aggressive basal-like breast cancers.
In addition, oncogenic mutations of PIK3CA, which encodes for a catalytic subunit of PI3K (p110α), occur in about 10% of TNBC cases and can further activate the PI3K pathway. Among the TNBC subtypes, LAR shows the highest prevalence of PIK3CA mutations, and in this way the simultaneous therapeutic targeting of AR and PIK3CA could prove beneficial to patients.
PIK3CA mutations in androgen receptor–positive triple negative breast cancer confer sensitivity to the combination of PI3K and androgen receptor inhibitors.
In addition to the known TNBC cancer-related genes involved in PI3K pathway regulation, the novel MAGI3-AKT3 translocation has been described. This rearrangement occurs in about 7% of TNBC cases and leads to constitutive AKT3 activation and hyperactivation of the PI3K pathway.
In TNBC, PI3K/AKT/mTOR pathway alterations occur frequently and are promising therapeutic targets. Preclinical data have demonstrated that TNBC tumors are more sensitive to combination therapy.
PIK3CA mutations in androgen receptor–positive triple negative breast cancer confer sensitivity to the combination of PI3K and androgen receptor inhibitors.
Clinical trials are currently evaluating the potency of mTOR, PI3K, AKT, and mTOR/PI3K inhibitors for treating TNBC alone or in combination with other therapies (eg, cisplatin, PARP, and AR inhibitors).
Tyrosine kinase receptors from the EGFR, FGFR (fibroblast growth factor receptor), and VEGFR (vascular endothelial growth factor receptor) families have been reported to be potential clinical targets for treating TNBC.
The tyrosine kinase receptor EGFR (HER1) mediates cell proliferation, angiogenesis, and metastasis as well as the inhibition of apoptosis by transducing an extracellular signal through a kinase cascade to ultimately initiate the transcription of specific genes. While EGFR overexpression has been described in approximately 60% of TNBC cases, EGFR amplification or high copy number has been reported in only 5% to 30% of cases. Moreover, EGFR mutations were found to be rare, occurring in about 11% of samples.
Frequently increased epidermal growth factor receptor (EGFR) copy numbers and decreased BRCA1 mRNA expression in Japanese triple-negative breast cancers.
Nevertheless, a recent study reported EGFR copy number to correlate with EGFR overexpression and to be associated with poor clinical outcome in TNBC. EGFR overexpression is also influenced by factors other than genomic changes, and EGFR copy number status seems to predict the response of TNBC patients to anti-EGFR therapy.
However, the results from these clinical trials, which tested EGFR tyrosine kinase inhibitors alone or in combination with chemotherapy, have so far been disappointing.
Similarly, clinical trials investigating anti-EGFR monoclonal antibodies in monotherapy or combination therapy have not yet provided any promising results.
Randomized phase II study of the anti–epidermal growth factor receptor monoclonal antibody cetuximab with cisplatin versus cisplatin alone in patients with metastatic triple-negative breast cancer.
Multicentric neoadjuvant phase II study of panitumumab combined with an anthracycline/taxane-based chemotherapy in operable triple-negative breast cancer: identification of biologically defined signatures predicting treatment impact.
Despite predominantly unsuccessful studies, a small proportion of TNBC patients have disease that responds positively to anti-EGFR therapy. Therefore, identification of patients with EGFR activations who may profit from anti-EGFR therapy is crucial.
FGF receptors mediate proliferation, survival, migration, and differentiation. As such, they could be a promising target for treating a subset of TNBC patients. Amplifications of FGFR1 or FGFR2, with respective frequencies of 9% and 4% in TNBC, may act as driver mutations, whereas mutations in FGFR genes are less common in TNBC.
Two studies have shown that FGFR2 amplification leads to constitutive activation of the receptor in TNBC cell lines, and that this subset of cells is sensitive to the FGFR ATP-competitive inhibitor PD173074.
These results concerning FGFRs, along with the fact that alterations in FGFR genes occur in more than 10% of TNBC cases, make this family of tyrosine kinase receptors an attractive therapeutic target. Ongoing clinical trials will, we hope, clarify the effectiveness of FGFR inhibitors in breast cancer patients.
The VEGFR family has also been explored as a potential therapeutic target because it plays an essential role in angiogenesis, which affects cancer development, invasion, and metastasis.
Even though VEGFR amplifications or mutations are rare in TNBC, a number of clinical trials have confirmed that the addition of bevacizumab to chemotherapy significantly elevates pCR rates in TNBC patients.
Neoadjuvant bevacizumab and anthracycline–taxane–based chemotherapy in 678 triple-negative primary breast cancers; results from the geparquinto study (GBG 44).
Impact of the addition of carboplatin and/or bevacizumab to neoadjuvant once-per-week paclitaxel followed by dose-dense doxorubicin and cyclophosphamide on pathologic complete response rates in stage II to III triple-negative breast cancer: CALGB 40603.
Higher pCR rates occur in TNBC patients treated with bevacizumab; interestingly, the best responses to bevacizumab were associated with high VEGFR1 levels.
The effect of the multitarget tyrosine kinase receptor inhibitor sunitinib on TNBC has been evaluated in several studies, but this inhibitor was not found to be any more effective than other previously reported therapeutic approaches.
Phase I/II trial of neoadjuvant sunitinib administered with weekly paclitaxel/carboplatin in patients with locally advanced triple-negative breast cancer.
In breast cancer, AR positivity is more common in older women and is associated with lower stage, nuclear grade, and risk of lymph node involvement as well as smaller tumor size at diagnosis, decreased risk of recurrence, and better OS and disease-free survival.
Elevated serum estradiol and testosterone concentrations are associated with a high risk for breast cancer. Study of Osteoporotic Fractures Research Group.
The prognostic significance of AR positivity is controversial; AR positivity has been associated with both favorable and poor prognoses in previous studies.
Elevated serum estradiol and testosterone concentrations are associated with a high risk for breast cancer. Study of Osteoporotic Fractures Research Group.
Small interfering RNA knockdown and pharmacologic inhibition of AR both substantially decreased cell viability and tumor growth. Moreover, all analyzed LAR cell lines showed an activating mutation in the kinase domain of PIK3CA (H1047R) and were therefore sensitive to PI3K inhibitors.
PIK3CA mutations in androgen receptor–positive triple negative breast cancer confer sensitivity to the combination of PI3K and androgen receptor inhibitors.
Studies using in vitro experiments and xenograft models have shown that the treatment of both LAR and non-LAR TNBC subtypes with the AR inhibitors enzalutamide and bicalutamide reduces proliferation, anchorage-independent growth, migration, and invasion, and increases apoptosis.
Therefore, a positive response to AR antagonists is probably not limited to the LAR TNBC subtype. However, the TBCRC011 study showed a relatively weak response, with a 6-month clinical benefit rate of 19% for bicalutamide in AR-positive patients compared to 18% in the intention-to-treat population.
In a MDV3100-11 study, enzalutamide showed higher clinical activity, with a 6-month clinical benefit rate of 28%, for AR-positive patients compared to 20% in the intention-to-treat population.
Future studies should focus on elucidating the mechanisms of AR therapy resistance and how to select patients who will show the optimal response. Other therapeutic approaches, such as CYP17 (cytochrome P450 family 17 subfamily a member 1) inhibitors or the combination of AR inhibitors with CDK4/CDK6 (cyclin-dependent kinase) inhibitors, PI3K inhibitors or neoadjuvant chemotherapy, are still being investigated.
The therapeutic value of screening for AR positivity is that this is an easily detectable marker than can identify subgroups of TNBC patients who will derive minimal clinical benefit from standard chemotherapy. AR-dependent TNBC patients could benefit from targeted therapy based on AR antagonists alone or in combination with other chemical agents.
BCL2 Gene
B-cell lymphoma 2 (BCL2) is a mitochondrial protein known for its antiapoptotic and oncogenic effects. BCL2 exerts inhibitory effects on cell growth and proliferation and DNA damage, resulting in increased genetic instability.
Many studies have proven BCL2 expression to be a promising prognostic and predictive marker, especially in hormone receptor–positive, node-negative breast cancer.
BCL2 expression is directly up-regulated by estrogens and therefore commonly shows elevated levels in ER-positive breast cancers.
The role of BCL2 in the context of TNBC has not yet been well established. BCL2 positivity was found to be a positive prognostic factor in TNBC, as the ER−BCL2+ group demonstrated a better prognosis than the ER+BCL2− group.
Moreover, BCL2 positivity was shown to be a predictor of response to neoadjuvant and adjuvant anthracycline-based chemotherapy. The absence of BCL2 expression in prechemotherapy TNBC samples was associated with a higher probability of pCR to neoadjuvant doxorubicin-based chemotherapy, and the lack of BCL2 expression was also found to be an independent predictor of pCR.
The mechanism of this response is not entirely clear, but it may be influenced by expression changes in genes associated with BCL2 levels—for example, altered expression of HER3 (human epidermal growth factor receptor 3), MDM4 (Mdm2-like P53-binding protein), and p27 proteins.
Bcl2 is an independent prognostic marker of triple negative breast cancer (TNBC) and predicts response to anthracycline combination (ATC) chemotherapy (CT) in adjuvant and neoadjuvant settings.
The addition of BCL2 to the screening panel in clinical practice would be simple and could provide important prognostic and predictive information about the TNBC patient.
Cyclin-Dependent Kinases
CDKs and cyclins play key roles in cell-cycle regulation and are altered in almost all cancer types. Altered expression of cyclin D, cyclin E, CDK4/6, CDK2, and others was observed in TNBC, and CDK inhibition therapy therefore seems to be promising strategy in TNBC.
Molecular profiling of the residual disease of triple-negative breast cancers after neoadjuvant chemotherapy identifies actionable therapeutic targets.
More than 10 CDK inhibitors are evaluating in ongoing clinical trials; the most promising are ribociclib, palbociclib, abemaciclib, and dinaciclib. The CDK4/6 inhibitors ribociclib and palbociclib have been already approved for treatment of advanced breast cancer patients with hormone receptor positivity and HER2 negativity.
In TNBC, the LAR subgroup was found to be sensitive to CDK4/6 inhibition (palbociclib/ribociclib). Moreover, CDK4/6 inhibitors were synergistic with PI3K inhibitors in TNBC cell lines with PIK3CA mutation.
Recently, inhibition of CDK4/6 was found to block breast tumor metastasis in TNBC xerograph model. Palbociclib inhibition did not affect growth of primary tumor but nevertheless significantly inhibited the spread of TNBC to distant organs through destabilization of the SNAIL1 protein.
Currently, palbociclib and ribociclib in combination with bicalutamide (AR antagonist) are being tested for the treatment of advanced AR-positive TNBC.
A phase 1 study with dose expansion of the CDK inhibitor dinaciclib (SCH 727965) in combination with epirubicin in patients with metastatic triple negative breast cancer.
TNBC encompasses a complex group of heterogeneous diseases characterized by various genetic alterations and a lack of validated biomarkers. Current research is focused on identifying genes that may serve as therapeutic targets, prognostic markers, or predictors of therapeutic response and are common in all or particular TNBC subtypes. High-throughput analysis tools such as sequencing and microarray technology have the potential to elucidate the nature of TNBC; however, results of these technologies rarely have therapeutic impact. Well-defined and extensive data sets are required for clinical validation of founded biomarkers. To date, several promising markers have been described, but they still lack validation with the stringent criteria of clinical studies. The heterogeneity of TNBC is also evident in its treatment. The different subtypes differ in both proliferative activity and response to conventional chemotherapy; as such, classic therapeutic approaches should consider which subtype is being targeted until personalized options become available.
Disclosure
The authors have stated that they have no conflict of interest.
Acknowledgment
Supported by grant NPS I LO1304 from the Czech Ministry of Education,Youth and Sports.
Descriptive analysis of estrogen receptor (ER)-negative, progesterone receptor (PR)-negative, and HER2-negative invasive breast cancer, the so-called triple-negative phenotype: a population-based study from the California Cancer Registry.
Clinicopathologic features, patterns of recurrence, and survival among women with triple-negative breast cancer in the National Comprehensive Cancer Network.
Prognostic value of tumor-infiltrating lymphocytes in triple-negative breast cancers from two phase III randomized adjuvant breast cancer trials: ECOG 2197 and ECOG 1199.
Neoadjuvant bevacizumab and anthracycline–taxane–based chemotherapy in 678 triple-negative primary breast cancers; results from the geparquinto study (GBG 44).
Tumor infiltrating lymphocytes are prognostic in triple negative breast cancer and predictive for trastuzumab benefit in early breast cancer: results from the FinHER trial.
Prognostic value of tumor-infiltrating lymphocytes on residual disease after primary chemotherapy for triple-negative breast cancer: a retrospective multicenter study.
Tumor-infiltrating lymphocytes and response to neoadjuvant chemotherapy with or without carboplatin in human epidermal growth factor receptor 2–positive and triple-negative primary breast cancers.
Prognostic interaction between expression of p53 and estrogen receptor in patients with node-negative breast cancer: results from IBCSG trials VIII and IX.
p53 deficiency linked to B cell translocation gene 2 (BTG2) loss enhances metastatic potential by promoting tumor growth in primary and metastatic sites in patient-derived xenograft (PDX) models of triple-negative breast cancer.
Both carboplatin and bevacizumab improve pathological complete remission rate in neoadjuvant treatment of triple negative breast cancer: a meta-analysis.
Randomized phase III trial of paclitaxel once per week compared with nanoparticle albumin-bound nab-paclitaxel once per week or ixabepilone with bevacizumab as first-line chemotherapy for locally recurrent or metastatic breast cancer: CALGB 40502/NCCTG N063H (Alliance).
PIK3CA mutations in androgen receptor–positive triple negative breast cancer confer sensitivity to the combination of PI3K and androgen receptor inhibitors.
Impact of the addition of carboplatin and/or bevacizumab to neoadjuvant once-per-week paclitaxel followed by dose-dense doxorubicin and cyclophosphamide on pathologic complete response rates in stage II to III triple-negative breast cancer: CALGB 40603.
Molecular profiling of the residual disease of triple-negative breast cancers after neoadjuvant chemotherapy identifies actionable therapeutic targets.
Inherited mutations in 17 breast cancer susceptibility genes among a large triple-negative breast cancer cohort unselected for family history of breast cancer.
Veliparib with temozolomide or carboplatin/paclitaxel versus placebo with carboplatin/paclitaxel in patients with BRCA1/2 locally recurrent/metastatic breast cancer: randomized phase II study.
The TNT trial: a randomized phase III trial carboplatin (C) compared with docetaxel (D) for patients with metastatic or recurrent locally advanced triple negative or BRCA1/2 breast cancer (CRUK/07/012).
Cancer Res.2015; 75 (Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium; December 9-13, 2014; San Antonio, TX. Philadelphia, PA: AACR; 2015) (abstract S3-01)
The poly(ADP-ribose) polymerase inhibitor niraparib (MK4827) in BRCA mutation carriers and patients with sporadic cancer: a phase 1 dose-escalation trial.
EMBRACA: a phase 3 trial comparing talazoparib, an oral PARP inhibitor, to physician’s choice of therapy in patients with advanced breast cancer and a germline BRCA mutation.
Cancer Res.2018; 78 (Proceedings of the 2017 San Antonio Breast Cancer Symposium; December 5-9, 2017; San Antonio, TX. Philadelphia, PA: AACR; 2018) (abstract GS6-07)
Frequently increased epidermal growth factor receptor (EGFR) copy numbers and decreased BRCA1 mRNA expression in Japanese triple-negative breast cancers.
Randomized phase II study of the anti–epidermal growth factor receptor monoclonal antibody cetuximab with cisplatin versus cisplatin alone in patients with metastatic triple-negative breast cancer.
Multicentric neoadjuvant phase II study of panitumumab combined with an anthracycline/taxane-based chemotherapy in operable triple-negative breast cancer: identification of biologically defined signatures predicting treatment impact.
Phase I/II trial of neoadjuvant sunitinib administered with weekly paclitaxel/carboplatin in patients with locally advanced triple-negative breast cancer.
Elevated serum estradiol and testosterone concentrations are associated with a high risk for breast cancer. Study of Osteoporotic Fractures Research Group.
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