In this review, the advances in the study of breast cancer molecular classifications and the molecular signatures of the luminal subtypes A and B of breast cancer were summarized. Effective clinical outcomes depend mainly on successful preclinical diagnosis and therapeutic decisions. Over the last few years, the ever-expanding investigations focusing on breast cancer diagnosis and the clinical trials have provided accumulating information on the molecular characteristics of breast cancer. Specifically, among the estrogen receptor (ER)-positive types of breast cancer, the luminal subtype A breast cancer has been shown to exhibit good clinical outcomes with endocrine therapy, whereas the luminal subtype B breast cancer represents the more complicated type, diagnostically as well as therapeutically. Furthermore, even in luminal subtype A breast cancer, the resistance to treatment has become the major limitation for endocrine-based therapy. Accumulating molecular data and further clinical trials may enable more accurate diagnostic and therapeutic decisions. The molecular signatures have emerged as a powerful tool for future diagnosis and therapeutic decisions, although currently available data are limited.
Breast cancer is one of the major causes of cancer-related morbidity and mortality among women worldwide (
The current molecular classifications of breast cancer molecular subtypes are generally based on the gene expression profiles according to i) luminal cell-related markers, such as cytokeratins (CKs); ii) hormone receptors, such as estrogen receptor (ER), progesterone receptor (PR) and androgen receptor (AR); iii) growth factor receptors, such as human epidermal growth factor receptor (HER); iv) anti-apoptosis markers, such as Bcl-2 and p53; v) cell proliferation indicators, such as Ki-67 and survivin; vi) cell invasion-related factors, such as matrix metalloproteinases (MMPs) and integrins; vii) signal transduction pathway members, such as the PI3K/AKT pathway members phosphatidylinositol-3-kinase (PI3K) and AKT; viii) cell cycle control members, such as cyclins and cyclin-dependent kinases (CDKs); ix) epithelial-to-mesenchymal transition-indicating factors and regulating factors, such as cadherins and zinc-finger transcription factors Snail, Slug, Zeb1 and Twist; x) metastatic control factors; and xi) blood vessel-forming control factors (
The luminal type of breast cancer tends to be morphologically well differentiated and exhibits a relatively good prognosis, whereas the ER− tumors are poorly differentiated and exhibit a poor prognosis. The designation of the luminal type of breast cancer was derived from the finding that this type of breast cancer exhibits mRNA and protein expression of CKs 8/18 (
Among the different molecular subtypes of breast cancer, ER+ breast cancer comprises ~75% of all breast cancers (
The luminal A subtype of breast cancer is characterized by the luminal-type conventional molecular signatures (ER, PR, Bcl-2 and CK8/18) and the luminal A subtype-specific signatures of ER+ and/or PR+, HER2− and Ki-67 ≤14%, which distinguishes luminal A from luminal B subtype. The recognized luminal A subtype breast cancer molecular signatures include GATA binding protein 3 (GATA-3), X-box binding protein 1 (XBP-1), forkhead box A1 (FOXA1) and ADH1B (
Studies of the crosstalk between estrogen receptor α (ERα), FOXA1 and GATA-3 revealed that, in addition to the ER and PR status, FOXA1 and GATA-3 are also correlated with the luminal A subtype (
The 70-gene signature (MammaPrint®) assay provides a powerful prognostic gene expression signature profile for the prediction of distant recurrence and survival of primary breast cancer, including luminal subtype A breast cancer (
Estrogen is a steroid hormone that is crucial for growth, development and reproduction (
Since luminal subtype A and B breast cancer cells are ER+ and/or PR+, patients with these two types of breast cancer are always subjected to endocrine therapy with tamoxifen, to inhibit the functions of ER (
However, in ER+ patients, the clinical therapeutic practices with endocrine therapy to antagonize ER signaling were proven to be 30% effective (
Based on the molecular signatures of luminal A subtype breast cancer, the luminal A subtype signature genes, such as GATA-3 and FOXA1, also appear to be promising therapeutic targets. Among these, the expression of FOXA1 was positively correlated with ER-positivity, particularly luminal A type ER-positivity, and negatively correlated with tumor size, tumor grade, nodal status, the expression of Ki-67 and HER2 and basal-like subtype of breast cancer (
An alternative way for the therapeutic considerations in luminal A subtype of breast cancer is targeting other members that are coexpressed with ER in the superfamily of steroid receptors, including estrogen-related receptors, PRs (
The major molecular distinctions between luminal type A and B tumors are that luminal type A tumors exhibit a higher expression of ER-related genes and luminal type B tumors exhibit a higher expression of proliferation-related genes, such as CCNB1, MKI67 and myeloblastosis oncogene-like 2 (MYBL2) (
In addition to sharing similar signatures with luminal subtype A, such as ER and Bcl-2, luminal subtype B tumors also share similar signatures with the basal-like subtype tumors, including the proliferation markers Ki-67, survivin and CCNB1, as well as similar signatures with the HER2 subtype, such as the overexpression of HER2 (
Although luminal B cancers are ER+, they do not appear to exhibit a corresponding expression of estrogen-regulated genes and may therefore depend on alternative pathways for growth. Candidate pathways that may be targeted in luminal B cancer cells include those involving growth factor receptors, such as HER2 and EGFR, as well as the PI3K/AKT/mTOR pathway. The standards for distinguishing luminal A from luminal B cancers are the Ki-67 index (cut-off value, 14%), clinicopathological factors, such as age at diagnosis, intensity score of ER and PR, histological grade, Bcl-2 (cut-off value, 33%) and disease-free survival (DFS).
Similar to luminal A, luminal B breast cancer is currently treated as an ER+, hormone-sensitive disease (
A major characteristic of luminal type B breast cancer cells is the expression of the HER2 gene (
Another characteristic of luminal B tumors is the high expression levels of Ki-67 combined with HER2 expression, exhibiting high scores in the Oncotype DX gene expression profile. Thus, for patients with HER2+ and ER+ tumors, combination treatment with endocrine and anti-HER2 therapy may achieve therapeutic benefits (
In luminal B-type tumors, the high expression levels of Ki-67 combined with HER2 expression exhibit high scores in the Oncotype DX gene expression profile; thus, for patients with HER2+ and ER+ tumors, the combination of endocrine and anti-HER2 therapy may achieve therapeutic benefits (
Higher PI3K signature scores have been observed in ER+ tumors and the cell lines of the more aggressive luminal B compared to those of the less aggressive luminal A molecular subtype, suggesting that targeting PI3K in these tumors may reverse the loss of ER expression and signaling and restore hormonal sensitivity (
Charafe-Jauffret
The present molecular classifications assigned ER+ breast cancer into two categories: the luminal subtype A, characterized by ER+/PR+/HER2−/low Ki-67; and the luminal subtype B, characterized by ER+/PR+/HER2+ or ER+/PR+/high Ki-67. Based on these molecular signatures, effective diagnostic and therapeutic decisions are enabled, with subsequent improved clinical outcomes. However, these simplified signatures are unable to accurately represent the complex intrinsic processes of tumor cell growth. The details of the molecular processes within the tumor cell have not been elucidated, nor have the mechanisms of treatment resistance, the clear panorama of the molecular networks and the molecular cascade upon treatment. In addition, the treatment interventions in the tumor suppressor genes have not been adequately investigated and the effect of the microenvironment on treatment efficacy has not been determined. The applications of several molecular signature sets, including the Oncotype DX and MammaPrint assays mentioned earlier, have been demonstrated to be powerful tools enabling more accurate predictions and effective therapeutic decisions. The prediction values of these assays have also been confirmed when combined with other approaches. However, due to the heterogeneous properties of the breast cancer subtypes and the complex molecular processes underlying breast cancer development, these molecular signature sets are not considered sufficient for practical clinical application. In addition, accumulating data have demonstrated the individual differences based on genomic sequencing results of clinical samples. Although the individual genomic differences may not reflect the decisive molecular mechanisms, they are a reminder of the heterogeneity and complexity of the decision-making process. The combination of the present molecular signature sets with accumulating data obtained by the new-generation sequencing technology and the high throughput gene expression quantification technology may enable obtaining more reliable molecular signature sets for future diagnosis and prediction of human breast cancer.
This study was funded by the ‘Financial support for selected researchers back from abroad (2011)’ of Liaoning Province.
The estrogen receptor (ER) pathway includes the canonical and the non-canonical ER pathways. In the canonical pathway, the cytoplasmic estrogens bind directly to the nuclear membrane estrogen receptor and activate signaling. In the non-canonical pathway, the extracellular estrogens bind to the plasma membrane receptors and activate the phosphatidylinositol-3-kinase (PI3K) or Ras signaling pathway. Alternatively, the estrogens first penetrate through the cell membrane and then bind to the estrogen receptor monomer, which is then dimerized and transported into the nucleus to activate signaling. Treatment with tamoxifen blocks the binding of estrogen and, thus, inhibits the estrogen-activated signaling. Treatment with fulvestrant inhibits the cytoplasmic dimerization of estrogen. Treatment with aromatase inhibitors, such as letrozole, anastrozole and exemestane blocks estrogen production. Another characteristic correlated to the ER pathway is the progesterone receptor (PR) signaling, which is also involved in the regulation of luminal type breast cancer development. CoR, coregulators; RE, response element; TFs, transcription factors; mER, membrane ER; HER, human epidermal growth factor receptor; MAPK, mitogen-activated protein kinase; ERE, estrogen response element; PRE, progesterone response element.
Crosstalk of signaling pathways in breast cancer and the potential clinical therapeutic targets. The receptors of extracellular small molecules shown here include: epidermal growth factor (EGF), transforming growth factor (TGF)-β, vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), insulin-like growth factor (IGF)-1, insulin and estrogen, which activate their corresponding receptors and further transduce the signals mainly through the phosphatidylinositol-3-kinase (PI3K)/AKT pathway or the Ras/MEK pathway. A number of PI3K/AKT and Ras/MEK signaling pathway inhibitors have been developed, some of which may be used in combination (details also summarized in
Molecular subtype signatures of breast cancer.
Classification | Signature genes | Signaling pathways | Clinical grade | Therapeutic options | 5-year survival rate | p53-mutation | Refs. |
---|---|---|---|---|---|---|---|
Luminal A | Marker genes: ER+ and/or PR+, HER2−, CK8/18+; GATA-3, XBP-1, FOXA1 and ADH1B gene overexpression | Estradiol response | I | Tamoxifen; anastrozole (Arimidex) | 95% | 13% | ( |
Luminal B | Marker genes: ER+ and/or PR+, HER2+, CK8/18+, FGFR1, HER1, Ki-67 and/or cyclin E1, CCNB1 and MYBL2 gene overexpression | IGF-1 |
I (III also observed) | Bevacizumab combined with paclitaxel, tamoxifen combined with small-molecule inhibitors or antibodies against IGF-1R/IR, FGF, FGFR, PI3K and EGFR/HER2 | 50% | 40% | ( |
ErbB2/HER2+ | Marker genes: ER− and/or PR−, HER2+ and GRB7 overexpression | IGF-1 |
More likely III | Trastuzumab (Herceptin), lapatinib |
30% | 71% | ( |
Basal-like | Marker genes: ER− and/or PR−, HER2−, CK5/6+, CK14+, CK17+, EGFR+ HER1 and/or c-Kit, FOXC1, p63, P-cadherin, vimentin and laminin overexpression | IGF-1 |
More likely III | Chemotherapy; antiangiogenic agents; platinum salts; PARP inhibitors | 30% | 83% | ( |
Normal | Marker genes: ER− and/or PR−, breast-like HER2−, CK5/6−, CK14−, CK17−, EGFR and ADH1B overexpression | 50% | 33% | ( |
ER, estrogen receptor; PR, progesterone receptor; HER2, human epidermal growth factor receptor 2; GATA-3, GATA binding protein 3; XBP-1, X-box binding protein 1; FOXA1, forkhead box A1; FGFR1, fibroblast growth factor receptor 1; MYBL2, myeloblastosis oncogene-like 2; CK, cytokeratin; ADH, alcohol dehydrogenase; GRB, growth factor receptor-bound protein; IGF, insulin-like growth factor; PI3K, phosphatidylinositol-3-kinase; mTOR, mammalian target of rapamycin; PARP, poly ADP ribose polymerase.
Inhibitors or antibodies for signaling pathway elements in breast cancer.
No. | Target | Target agents | Refs. |
---|---|---|---|
1 | HER2 | Trastuzumab (Herceptin); pertuzumab (Omnitarg); CP-724,714; TAK165; CI-1033; 2C4; AG1478; ARRY-380 | ( |
EGFR/HER2 | Lapatinib (Tykerb); AEE-788; BMS-599626; ARRY-334543; BIBW2992; HKI-272; MP-412; CI-1033 (canertinib); neratinib (HKI-272); CUDC-101; AZD8931; BMS-599626; ARRY-334543; dacomitinib (PF-00299804); TAK-285 | ( | |
EGFR | Gefitinib (ZD-1839, Iressa); CPI-358,774; cetuximab; OSI-774; PD153035; erlotinib | ( | |
VEGFRs | Regorafenib (BAY 73-4506) |
( | |
EGFR/VEGFRs | AEE-788; EXEL-7647 (XL647) | ( | |
2 | FGF | 1A6; FP-1039; palifermin (Kepivance) | ( |
3 | FGFRs | SU5402; PD173074; TKI-258; BIBF 1120; BMS-582,664 (brivanib); E7080; TSU-68; IMC-A1; PRO-001; R3Mab; AZD-4547; ENMD-2076; AZD4547 | ( |
4 | IGF-1R | BMS-754807; NVP-AEW541 | ( |
IGF-1R/IR | Cixutumumab; MK-0646; dalotuzumab; CP-758171; BMS-554417; BMS-536924; GSK1904529A; OSI-906 (linsitinib); AG-1024 (Tyrphostin); GSK1838705A | ( | |
5 | ER | ICI 182,780; tamoxifen; Casodex; fulvestrant; letrozole; anastrozole; exemestane | ( |
6 | PI3K | Wortmannin; BKM120; LY294002; XL-147; GDC-0941; PX-866; ZSTK474; SF1126; IC486068; fused heteroaryl and imidazopyridine-based inhibitors; WAY-266176; WAY-266175; PIK75 | ( |
PI3K/mTOR | BEZ235; XL-765; GSK2126458; PKI-402; GDC-0980; PF-05212384 (PKI-587) | ( | |
7 | AKT | MK-2206; PX-316; perifosine (KYX-0401); UCN-01; GSK690693; AT7867; PHT-427; triciribine | ( |
8 | PDK1 | OSU-03012; PHT-427; BX-795; BX-912; BX-320; | ( |
9 | mTORC1 | Everolimus; rapamycin; RAD-001; temsirolimus (CCI-779); ridaforolimus (AP23573; MK-8669); PF-04691502; | ( |
mTORC1/2 | OXA-01; OSI-027; AZD8055; WYE-125132 (WYE-132); GSK2126458; GDC-0980; WAY-600; WYE-687; WYE-354; AZD2104; IKK-128; XL388 | ( | |
10 | Raf | Regorafenib (BAY 73-4506) |
( |
11 | MEK1/2(MAPK) | UO126; PD98059; CI-1040 (PD184352); UCN-01; PD318088 | ( |
Regorafenib (BAY 73-4506) is an oral multikinase inhibitor of of VEGFR1/2/3, PDGFRβ, KIT, RET and Raf-1 (
SU11248, an oral multitargeted TKI with antiangiogenic and antitumor activity, inhibits VEGF, PDGF, KIT and FLT3 receptor TKs (
Apatinib (YN968D1) is a small-molecule TKI that inhibits VEGFR2 (Flk-1/KDR), RET (rearranged during transfection), c-Kit (stem cell factor receptor) and c-Src tyrosine kinases (
HER, human epidermal growth factor receptor; ER, estrogen receptor; EGFR, epidermal growth factor receptor; VEGFR, vascular endothelial growth factor receptor; FGF, fibroblast growth factor receptor; IGF, insulin-like growth factor; PI3K, phosphatidylinositol-3-kinase; mTOR, mammalian target of rapamycin; PDK, phosphoinositide-dependent kinase; MAPK, mitogen-activated protein kinase; PDGFR, platelet-derived growth factor receptor; FLT, FMS-like tyrosine kinase; TKI, tyrosine kinase inhibitor.