20.3.1 Risk Factors for Breast Cancer
Female gender and increasing age are the major risk factors for human breast cancer. Also, factors that increase the cumulative exposure and/or sensitivity of the breast epithelium to estrogen have been established as risk factors, including early menarche, late menopause, and obesity in postmenopausal women. Factors that reduce the cumulative exposure to estrogens, such as early first pregnancy, multiple full-term pregnancies, oophorectomy in premenopausal women, and physical activity, are associated with a reduced risk of breast cancer. Consistent with these observations, increased serum levels of estrogens are associated with postmenopausal breast cancer (Table 20–1). Use of hormone replacement therapies in menopausal women, particularly those that combine an estrogen and a progestin, also increase the risk of breast cancer (Banks et al, 2008).
TABLE 20–1Hormonally related epidemiological risk factors for breast cancer. ||Download (.pdf) TABLE 20–1 Hormonally related epidemiological risk factors for breast cancer.
| ||Risk Group || |
|Factor ||Low ||High ||Relative Risk* |
|Sex ||Male ||Female ||183 |
|Oophorectomy ||Age <35 y ||No ||2.5 |
|Age at menarche ||≥14 y ||≤11 y ||1.5 |
|Age at first birth ||<20 y ||≥30 y ||1.9 |
|Parity ||≥5 y ||nulliparity ||1.4 |
|Age at natural menopause ||<45 y ||≥55 y ||2.0 |
|Obesity (BMI)† ||<22.9 ||>30.7 ||1.6 |
|Oral contraceptive use ||Never ||Ever ||1.0 |
| ||Never ||≥4 y before first pregnancy ||1.7 |
|Estrogen+ progestin replacement ||Never ||Ever >5 y ||1.3 |
|Dense mammogram‡ ||None ||≥75% density ||5.3 |
Increased ER expression in normal breast epithelium is a risk factor for breast cancer (Khan et al, 1998), as is increased mammographic breast density (Martin et al, 2009) and increased circulating IGF-I levels (Key et al, 2010). These latter 2 factors are correlated and may be associated functionally (Becker and Kaaks, 2009). Estrogens have been shown to increase, and tamoxifen to decrease, mammographic density, and the intimate association and crosstalk of the IGF-I signaling pathway with the ER signaling pathway in target cells may also play a role in the risk of breast cancer (Key et al, 2010).
Women with a family history of breast cancer are at increased risk of breast cancer and 2 genes, BRCA1 and BRCA2, when carrying a germline mutation, are associated with an inherited predisposition to breast cancer (see Chap. 5, Sec. 5.3.4 and Chap. 7, Sec. 7.6.3). Only 5% of all breast cancer incidence can be attributed to inherited mutations in these genes and less than 10% of all breast cancer incidence can be attributed to an inherited predisposition. The importance of environmental factors is highlighted by the observation that the risk of breast cancer in Asian women rises over a few generations following migration to Western countries (Ziegler et al, 1993). Most breast cancers are sporadic, although polymorphisms in genes associated with the biosynthesis of estrogens (Thompson and Ambrosone, 2000) and factors that regulate ER activity such as AR (Giguere et al, 2001), may influence breast cancer development. Breast cancer risk is complex, probably because of the influence of multiple genes with the environment.
The primary role of estrogen in breast cancer is thought to be because of its proliferative effect on breast epithelium. A complex interplay of steroid hormones, growth factors, extracellular matrix, and their respective receptors is likely involved. Genotoxic effects of steroids cannot be excluded, although the relative roles of the different mechanisms remain unclear (Lin et al, 2009; Pauklin et al, 2009).
20.3.2 Development of Breast Cancer
Some of the cellular events associated with the natural history of breast cancer are illustrated in Figure 20–8A. Most invasive breast cancers arise from the epithelial cells of the terminal duct lobular unit (TDLU). Histopathological studies have identified a series of premalignant breast lesions referred to as hyperplasia without atypia, usual ductal hyperplasia (UDH), atypical hyperplasia (AH), and ductal carcinoma in situ (DCIS). These lesions are associated with increasing risks of developing invasive breast cancer. For example, AH is associated with a 5-fold increased risk, and DCIS is associated with a 10-fold increased risk (Page et al, 2000). Comparisons between premalignant and/or preinvasive lesions in the same biopsy sample as invasive breast cancers have identified common genetic abnormalities, suggesting that the malignant lesions are clonally derived from the earlier lesions (Allred and Mohsin, 2000; Gong et al, 2001).
An overview of the natural history of cancers of the breast and the prostate. A) Normal breast epithelium can undergo a stepwise transition from a series of premalignant breast lesions referred to as hyperplasia without atypia/usual ductal hyperplasia (UDH), atypical hyperplasia (AH), and ductal carcinoma in situ (DCIS) leading to invasive carcinoma and metastasis (adapted from Myal et al, 2010). The relative hormonal dependency and receptor status at the various stages are indicated. B) A normal prostate epithelial cell can develop into a premalignant tumor cell that can give rise to prostatic intraepithelial neoplasia (PIN), and then become an invasive carcinoma.
Although normal development of the mammary gland is dependent on the presence of ERα and only a rudimentary ductal remnant is present in "knockout" mice that do not have the ERα gene (Korach, 1994), only 7% to 17% of normal breast epithelial cells express ERα (Clarke et al, 1997). In contrast more than 70% of human breast tumors are ERα+, and often the level of ERα expression in tumor cells is higher than that found in normal breast luminal epithelial cells. Most hyperplastic lesions, with or without atypia, show increased expression of ERα and increased frequency of ER+ cells compared to normal epithelium, and more than 70% of DCIS are ERα+, similar to invasive breast cancer (Allred and Mohsin, 2000). In normal breast epithelial cells, ERα and Ki67, a marker of cell proliferation, are rarely, if ever, coexpressed (Anderson et al, 1998), suggesting that either ERα-expressing cells are incapable of proliferating or that ERα must be downregulated before normal breast epithelial cells can proliferate. This inverse relationship is maintained in UDH, but is lost in AH, DCIS, and ERα+ invasive breast cancer cells (Shoker et al, 1999). Thus an alteration in estrogen responsiveness and/or mechanism of estrogen action occurs during the development of breast cancer.
The second ER, ERβ, is also expressed in both normal and neoplastic human breast tissues (Leygue et al, 1998; Roger et al, 2001). Unlike ERα, ERβ does not play a pivotal role in development of the mammary gland as "knockout" mice for ERβ have normal development of the mammary gland (Couse and Korach, 1999). Expression of ERβ is much more frequent than ERα in normal human and rodent mammary glands, and its expression generally declines during breast cancer development (Leygue et al, 1998; Roger et al, 2001). Altered expression of several coregulators of ERα occurs during breast tumorigenesis (Murphy et al, 2000; Gojis et al, 2010) with a general trend toward increased expression of known coactivators and decreased expression of known corepressors; this observation suggests that a marked upregulation of ERα signaling occurs during breast tumorigenesis.
Growth suppression pathways associated with transforming growth factor (TGF)-β (see Chap. 8, Sec. 8.4.4) are altered in some early lesions. For example TGF-β receptor type II is highly expressed in normal breast epithelium, but is downregulated in some UDH lesions and identifies a group of women at higher risk of developing breast cancer (Gobbi et al, 1999). Altered growth factor pathways that stimulate the cell cycle, for example overexpression of human epidermal growth receptor 2 (HER2) (neu/erbB-2) and cyclin D1, and/or inactivation of tumor-suppressor genes such as p53, can be detected in some premalignant lesions such as DCIS (Allred and Mohsin, 2000; see Chap. 7, Secs. 7.5.3 and 7.6.1).
Breast cancer is extremely heterogeneous, as defined by the great variability that is seen in morphology, gene expression patterns, and behavior of individual tumor cells within any given tumor (Campbell and Polyak, 2007). The origin of this heterogeneity and the target cell(s) of origin of breast cancer are unknown, but 2 hypotheses that are not mutually exclusive have been suggested: the cancer stem or initiating cell and the clonal evolution hypotheses (see Chap. 13, Sec. 13.2). Substantial progress in defining the normal human mammary stem cell and the mammary epithelial hierarchy has been made at the molecular and functional levels (Eirew et al, 2008; Visvader, 2009). Despite the importance of ERα signaling in human breast cancer and in the normal development of the mammary gland, normal mammary stem cells and/or populations enriched for stem cell repopulating characteristics, appear not to express ERα or PR, supporting the idea that at least in the normal mammary gland the effect of steroid hormones on proliferation is indirect. Expression of ERα seems to appear within the mammary epithelial hierarchy at the committed luminal progenitor stage (Visvader, 2009) and the expression of PR may occur earlier than this stage in the common-bipotent epithelial progenitor cells (Raouf et al, 2008). Support for the existence of a population of human breast cancer cells with cancer initiating/stem cell-like characteristics is emerging (Al-Hajj et al, 2003; Eirew et al, 2008; Lim et al, 2009; Korkaya et al, 2011; see Chap. 13, Sec. 13.4.5), but whether the target cell for breast cancer is the normal mammary stem cell or other cells along the mammary epithelial hierarchy is unknown. The use of molecular signatures from molecular expression analysis is helping to resolve such issues, as there are now data linking newly identified breast cancer subtypes (Fig. 20–9, discussed below) with their closest normal mammary epithelial counterpart based on similarities of gene expression profiles (Visvader, 2009). However, it is also possible that cancer-initiating cells could be derived from mutation of tissue-specific progenitors or more differentiated cells which, because of mutation, acquire self-renewal capacity (Hershkowitz, 2010).
Molecular subtypes of breast tumors. A) Gene expression microarray analyses where expression of more than 500 genes, at the RNA level, has been measured in 122 different breast tissue samples (115 tumors, 7 non-malignant). Each column represents one breast sample, and each row represents the expression level of one gene. The scale bar represents the fold-change for any given gene relative to the median level of expression of all samples. Green is decreased and red is increased expression. B) Dendrogram of the heirarchical clustering of the tissues with respect to similarities in their gene expression patterns, into 5 subgroups shown by the different colors (dark blue, luminal A; aqua-blue, luminal B; purple, ERBB2+; orange, basal like; green, normal breast-like). Tumors with low correlations to these 5 subtypes are shown in gray. C) Gene cluster showing ERBB2 oncogene and co-expressed genes. D) Gene cluster associated with luminal subtype B. E) Gene cluster associated with the basal subtype. F) Gene cluster relevant for the normal breast-like group. G) Cluster of genes including the estrogen receptor (ESR1) highly expressed in luminal subtype A tumors. This classification is associated with different clinical outcomes as illustrated by Kaplan-Meier analyses from 2 different datasets shown in H and J where the colors of the survival curves correspond to those of the molecular subtypes. H) Overall survival for 72 patients with locally advanced breast cancer. J) Time to development of distal recurrence in 97 patients in another study (Adapted from Sorlie et al, 2003).
20.3.3 Progression of Invasive Breast Cancer
Amplification and upregulation of expression of several oncogenes, including those encoding growth factors such as EGF, TGF-α, IGF, and their receptor tyrosine kinases, for example, EGFR, HER2 (neu/erbB-2), and IGFR, together with inactivation or downregulation of tumor-suppressor genes, such as p53, Rb, or BRCA1, have been associated with breast cancer progression. The introduction of comparative genomic hybridization (CGH), fluorescence in situ hybridization (FISH), spectral karyotyping (SKY), DNA microarrays, and next-generation sequencing (Shah et al, 2009; see Chap. 2) is allowing a more global analysis of the molecular genetic alterations that occur during cancer progression.
Prognosis of patients with invasive breast cancer is related to lymph node involvement, tumor size, histological grade, and steroid receptor status. Approximately 70% of all primary invasive breast tumors are ER+, and in general, these tumors are more differentiated, less aggressive, and have lower levels of growth factor receptors compared to ER– tumors. In contrast, consistent evidence has accumulated to show that young patients (younger than age 40 years) with ER+ tumors have a worse prognosis than those with ER– tumors (Aebi, 2005).
Not only is ER status a prognostic factor, but it also is a marker of response to treatment, as is discussed subsequently. Until recently, the only molecular classifiers of breast cancers were ERα, PR, and HER2/ErbB2. ERα and overexpressed/amplified HER2 are the targets of endocrine therapies and the humanized antibody trastuzumab, respectively. HER2/ErbB2 is amplified and/or overexpressed in up to 20% of all breast tumors, and these tumors are aggressive with generally poor outcome, but the development of trastuzumab targeted to HER2 overexpressed protein has improved the outcome for these patients (Di Cosimo and Baselga, 2010). The advent of global molecular profiling technologies has led to a new way of classifying human breast cancer and is the basis of a more individualized approach to breast cancer prognostication and therapy. Most effort has focused on classification according to gene-expression profiling at the mRNA levels using microarray chip technology (Perou et al, 2000). However, a combination of transcriptomic, epigenetic, proteomic, and genomic analyses may be necessary to appreciate the true heterogeneity of breast cancer (Weigelt and Reis-Filho, 2009).
Despite the limitations outlined above, gene expression analysis has resulted in human breast tumors being classified into 5 different subtypes, with potentially a sixth recently identified (Visvader, 2009). The subtypes are called luminal A, luminal B, HER2 overexpressing, basal-like, normal breast-like (Sorlie, 2004) and, most recently, a claudin-low subtype has been described (Creighton et al, 2010). The different subtypes are clinically relevant as some are associated with different clinical outcomes (see Fig. 20–9). ERα and PR are only expressed in the luminal A and B subtypes, although about half of the HER2 overexpressing subtype also can express ERα. Furthermore, although alternative hypotheses cannot be excluded (Stingl and Caldas, 2007), the similarity of expression signatures to those represented in the mammary epithelial hierarchy may provide information concerning the cell of origin of the subtypes and identify the nature of the cancer-initiating cells such that new treatments can be designed to target them. However, these molecular classifications are research tools and are not used routinely in clinical practice, in contrast to the determination of ER, PR, and HER2 expression.
The triple negative (ERα-negative, PR-negative, and HER2-negative) group of tumors represents a challenge in breast cancer because of their lack of responsiveness to endocrine therapies and trastuzumab. These tumors are aggressive with increased risk of metastasis, but in some cases respond well to chemotherapy. The triple negative or basal-like subtype is itself heterogeneous, and a better understanding of the molecular heterogeneity of this subgroup is necessary to improve clinical outcome (Di Cosimo and Baselga, 2010). Most breast cancers arising from BRCA1 mutation carriers are triple negative (Podo et al, 2010), whereas most breast cancers arising from BRCA2 mutation carriers are ER+ and have a luminal molecular profile (Bane et al, 2007).
A proportion of all tumor subgroups, despite originally responding to treatment, will recur and progress, having acquired resistance to targeted therapies. Most ER+ tumors treated with endocrine therapies develop resistance despite the continued expression of ERα. Mechanisms of resistance to endocrine therapies are discussed below, but a challenge is to identify biomarkers that predict early resistance to endocrine therapy so that either more aggressive therapy can be used sooner, or a rational combination of therapies can be given. The use of new technologies that allow a more global analysis of the molecular and genetic alterations occurring in tumors holds promise with respect to achieving this goal (Wood et al, 2007).
20.3.4 Risk Factors for Prostate Cancer
Carcinogenesis of the prostate involves genetic and environmental influences with no obvious etiological agent. Risk factors include family history, age, and race (Hsing and Chokkalingam, 2006). First-degree male relatives of prostate cancer patients have an approximately 2.5-fold increase in risk, and the risk of prostate cancer appears to be higher for relatives of women with breast cancer. Men carrying a germline BRCA2 mutation have increased risk of developing a more aggressive prostate cancer by 3.2-fold compared with noncarriers (Gallagher et al, 2010). However, hereditary factors most commonly affect men with early onset disease and are responsible for relatively few cases (<10%). Diet is probably important, with the predominantly vegetarian diet of Asians providing a protective influence, whereas the high intake of red meat associated with a typical American diet is likely related to increased risk (Denis et al, 1999). Epidemiological studies have linked obesity with a range of cancer types, although its role in the development and progression of prostate cancer has not been elucidated. The association of obesity with numerous hormonal changes that influence endocrine pathways may contribute to prostate cancer development and progression (Calle and Kaaks, 2004).
Prostate cancer is a disease of the elderly, with more than 75% of cancers diagnosed in men older than 65 years of age. However, microfoci of high-grade prostatic intraepithelial neoplasia (PIN), the presumed precursor of the disease, can be found in men in their third and fourth decade of life (see Fig. 20–8B). Most of the early tumors are microscopic, generally well to moderately differentiated, and tend to be multifocal. The frequency with which these neoplastic lesions are seen in autopsy material is similar among African Americans, white Americans, and Japanese men, but the incidence of clinical disease is higher in African American men and lower in Japanese men. In Japanese immigrants, the incidence of prostate cancer rises to levels near those of white Americans within 2 generations, suggesting the involvement of environmental factors. Collectively these observations suggest that the critical event in the natural history of prostate cancer is tumor promotion rather than tumor initiation, and that promotion and progression of this cancer are strongly influenced by epigenetic or adaptive processes.
20.3.5 Development of Prostate Cancer
Two pathological conditions that frequently coexist with latent and clinical prostate cancer are benign prostatic hyperplasia (BPH) and PIN. BPH shares many biological properties with prostate cancer, including androgen regulation of growth and increasing prevalence with advancing age. However, BPH is neither a premalignant lesion nor a precursor of invasive prostate cancer. A more likely candidate for this role is PIN, which is characterized by cytological atypia of proliferating luminal epithelium within preexisting acini and ducts with no penetration of the basement membrane (see Fig. 20–8B). Histologically, PIN is generally subdivided into low or high grade and autopsy studies reveal that high-grade PIN is found in association with cancer in 60% to 95% of malignant prostates and that a wide spectrum of molecular/genetic abnormalities are common to both high grade PIN and prostate cancer (Sakr and Partin, 2001). Specific chromosomal alterations (eg, loss of 8p, 10q, 16q, 18q, and gain of 7q31, 8q), amplification of the c-myc gene, along with changes in telomerase activity, cell-cycle status, and proliferative indices, suggest, collectively, that high-grade PIN is intermediate between benign epithelium and prostatic carcinoma (Sakr and Partin, 2001). A related staining profile for growth factors and for the AR has been demonstrated in the luminal epithelium of high-grade PIN and in carcinoma, with a tendency to higher expression of membrane EGFR, c-erbB-2, and cytoplasmic TGF-α, and lower levels of fibroblast growth factor (FGF)-2, than in glands with low-grade PIN or BPH (Harper et al, 1998). In addition to being a precursor for prostate cancer, it is likely that PIN predates invasive cancer by at least a decade and thus may serve as a predictive marker for the disease.
Although uncertain, prostate and other cancers may arise as a consequence of genetic alterations in a stem cell population (Lawson and Witte, 2007). Indeed, their longevity makes stem cells more likely to develop genetic alterations over time that may eventually culminate in cancer. Moreover, unlike mature prostate cells, primitive stem cells can thrive under androgen depletion, and it has been shown that these cells can subsequently regenerate prostate tissues with androgen stimulation. These castration-resistant cells were found originally to be of basal origin (Mulholland et al, 2009); however, the recent finding in mice of a luminal stem cell population that displays castration-resistant characteristics during prostate regeneration (Wang et al, 2009) suggests that there may be at least 2 different cell types that play a role in the development of castration-resistant phenotypes. However, only a small proportion of cancer cells within a tumor may possess stem cell properties (see Chap. 13, Sec. 13.4). These findings may be of critical importance to identify novel therapeutic approaches for future clinical management.
20.3.6 Progression of Invasive Prostate Cancer
When organ confined, prostate cancer is potentially curable by prostatectomy or radiation therapy. The problem here is selecting patients who need such treatment, as many will never develop symptomatic disease, whereas others will have occult metastases. In men with locally advanced or metastatic disease, treatment is largely palliative with androgen withdrawal as first-line treatment. The clinical approaches used for androgen withdrawal are described below. Despite high initial response rates on the order of 80%, patients will inevitably progress to hormone-independent disease in a manner analogous to that seen following hormonal therapy for metastatic breast cancer. Response to androgen ablation therapies depends on the degree of retention by the tumor of the capacity for activation of apoptosis after androgen withdrawal. However, many so-called hormone-independent cancers may in fact still be dependent on androgens, but resistant to medical or surgical castration, hence the term "castration-resistant" prostate cancer (CRPC).