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Corresponding Author: Stefanie S. Jeffrey
1
Molecular Profiling of Breast Cancer
Vincent A. Funari, Ph.D. 1 and Stefanie S. Jeffrey, M.D.2
1Department of Surgery, Stanford University School of Medicine, Medical School Lab-SurgeBldg. Room P229, 1201 Welch Road, Stanford, California 94305-5494, USAphone (650) 724-3519, fax (650) 724-3229
2Department of Surgery, Stanford University School of Medicine, Medical School Lab-SurgeBldg. Room P214, 1201 Welch Road, Stanford, California 94305-5494, USAphone (650) 723-0799, fax (650) 724-3229
Email addresses for authors:Vincent A. Funari - [email protected] S. Jeffrey - [email protected]
Corresponding author: Stefanie S. Jeffrey (email: [email protected])
Corresponding Author: Stefanie S. Jeffrey
2
The human genome project and the development of high throughput technology over the
last 5-10 years have thrust biology and medicine into a new era. Characterizing, diagnosing, and
treating breast cancer using new molecular profiling techniques is a powerful patient-specific
approach to treating and even preventing breast cancer. The technology is advancing rapidly and
changes in the field occur often. This chapter will focus on the promises, progress and problems
of molecular profiling in breast cancer.
PROBLEMS WITH CURRENT METHODS
At the present time, only a limited set of tumor parameters are used to estimate prognosis
for a patient with breast cancer. In general, these include tumor type (ductal, lobular, medullary,
mucinous, etc), size of invasive component, grade of the invasive component, the expression of
hormone receptors including Estrogen Receptor (ER) and Progesterone Receptor (PR), the
expression of the growth factor receptor HER2/neu (ERBB2), the presence and number of lymph
node metastases, and any evidence of distant disease. In many areas of the U.S., measures of
tumor proliferation, such as S-phase analysis or Ki67 expression are also determined. From these
data, risk of distant relapse is assessed and recommendations for systemic therapy are given.
Generally, almost all patients with lymph node metastases and the great majority of patients with
lymph node negative invasive tumors greater than one centimeter will be candidates for systemic
therapy.1 As a result, many women with stage I and II breast cancer, who may be cured by
surgery and/or radiation alone, are over treated by systemic therapy. Other women are treated
with systemic therapies that are ineffective against their specific tumor type. Further, many
chemotherapeutic agents are non-specific, killing rapidly dividing cells in other organs (eg bone
marrow or the GI tract). In general, currently used tumor parameters do not provide sufficient
tumor-specific predictions for survival, need for systemic therapy, and drug response.
Corresponding Author: Stefanie S. Jeffrey
3
Although individual gene measurements (such as ER, PR, HER2/neu) have provided
insightful information, it is now possible to measure global genetic changes using new
technologies that provide a unique molecular profile or a fingerprint of the tumor. These multiple
gene measurements represent a more comprehensive tumor signature that should provide more
precise insights into a tumor’s clinical behavior, response to systemic therapy, or offer possible
targets for the development of novel tumor-specific therapeutics.
CONSTRUCTING A MOLECULAR PROFILE
For precise characterization, breast tumors must be analyzed at all molecular levels:
DNA, RNA, and protein. The goal is to identify tumor-specific features that molecularly subtype
a tumor and then to correlate clinical outcome with molecular features. This would enable a
patient and her physician to make specific decisions as to whether systemic therapy is indicated,
and if it is, to use targeted therapies for treatment specifically aimed to kill or immobilize the
molecular type-specific tumor cells.
There are four major steps in achieving accurate molecular profiling data. The first step is
to obtain samples (from cell cultures, human tissues, blood, or body fluids) and purify the
molecules of interest (DNA, RNA, protein). In the second step the DNA, RNA, or protein from
the sample is measured. This usually involves constructing or purchasing a high throughput
assay device, such as a microarray or protein chip, that can measure the presence or absence of
hundreds to tens of thousands of genes, expressed genes, or proteins in a single sample. The third
step involves data analysis using bioinformatics tools. This entails information storage and
application of data processing algorithms to analyze and visualize the complex data. Finally,
conclusions must be reached, validated, and translated into clinical applications.
Corresponding Author: Stefanie S. Jeffrey
4
DNA MOLECULAR PROFILES
Changes in chromosomal DNA occur in breast cancer. Identifying specific sites of DNA
copy number change may identify candidate oncogenes or tumor suppressor genes. In contrast to
methods such as loss of heterozygosity (LOH) and sequencing that traditionally have measured
genetic modifications in specific genes or loci, comparative genomic hybridization (CGH)2 and
array based CGH3-6 map chromosomal or gene copy number changes on a global genomic scale.
In CGH, tumor DNA and control DNA (isolated from peripheral blood lymphocytes from a
healthy donor) are differentially labeled with fluorescent dyes and cohybridized onto normal
metaphase chromosomes, also obtained from peripheral blood lymphocytes stimulated in vitro.
The image is digitized and bioinformatic tools calculate the fluorescence ratio of tumor to normal
genomic DNA. The ratio of fluorescence along the chromosome identifies regions of
amplifications (gains) and deletions (losses) in the tumor DNA.
Chromosomal imbalances in breast cancer
CGH has identified multiple regions of chromosomal gains and losses in breast cancer. In
primary breast cancers, chromosomal gains have been most frequently identified as whole arm
gains in 1q and 8q and regional copy number increases at 17q and 20q.7 These data are, for
example, consistent with known breast cancer oncogenes on chromosomes 8q (MYC) and 17q
(HER2/neu [ERBB2]). In DCIS, chromosomal gains are observed in 1q, 8q, and 17q, whereas
losses are most common in 8p, 11q, 13q, 14q, and 16q.8 In invasive breast cancer, gains of 1q,
6p, 8q, 11q, 16p, 17q, and 20q are most common. Chromosomal losses have been identified in
1p, 8p, 11q, 16q, 18q, and 22.9 Using CGH, Forozan and colleagues10 compared 38 established
tumor cell lines to a meta analysis of CGH results from 698 primary tumors. In addition to the
Corresponding Author: Stefanie S. Jeffrey
5
chromosomal gains and losses mentioned for invasive tumors above, gains at 3q, 5p, 7p, 7q, 20p
and losses at 4p, 18p, Xp, Xq were also found.
CGH may also be used to study tumor biology. Jain and colleagues11 studied the
statistical relationship between CGH loci ratios and survival. Alterations in two loci, a gain at
8q24 and loss at 9q13, were associated with poor survival and were also associated with
mutations in TP53, the tumor suppressor gene that codes for p53 protein. To study tamoxifen
resistance, CGH has been used to compare a tamoxifen sensitive breast cancer cell line (MCF-7)
and a tamoxifen resistant clone (CL-9).12 CGH findings revealed differential gains on
chromosomes 2p, 2q, 3p, 12q, 13q, 17q, 20q, 21q and differential losses on chromosomes 6p, 7q,
11p, 13q, 17p, 18q, 19p, 22q. Neither ER-alpha on 6q25.1 nor ER-beta on 14q were involved in
the differences. The authors suggest that this technique may be useful for identifying candidate
genes involved in tamoxifen resistance.
Characterizing cancer cell progression
Beginning with usual ductal hyperplasia, there is evidence of accumulation of
chromosomal aberrations that lead to invasive breast cancer.13-15 Progression from hyperplasia to
atypical hyperplasia to DCIS and finally to invasive breast cancer is thought to occur in a multi-
step fashion.16 Consistent with this linear progression theory is that higher grade DCIS lesions
demonstrate increased chromosomal aberrations with loss of differentiation.17
However, others have argued against this linear continuum, and instead suggest
alternative differentiation pathways of progenitor cells in the glandular tissue.18, 19 In support of
non-linear, independent pathways of genetic evolution in breast cancer, Buerger8 used CGH to
study DCIS samples including all differentiation grades and some with associated invasive breast
cancer. All cases showed chromosomal imbalances, identifying DCIS as a genetically advanced
Corresponding Author: Stefanie S. Jeffrey
6
lesion, with identical genetic lesions between the DCIS and invasive components in 83% of the
cases. The most frequent chromosomal changes in well-differentiated DCIS were losses at 16q
and gains at 1q. In contrast, high grade DCIS demonstrated losses at 8p, 11q, 13q, 14q and gains
at 1q, 8q, 17q. Moreover, in 30% of DCIS cases with an invasive component, a gain of 11q13
was identified which was not present in pure DCIS. CGH was then performed on a larger
population of intermediate and high grade invasive cancer.20 Chromosomal gains of 1q and 8q
were seen in all invasive tumor grades. The loss of 16q, seen in well-differentiated DCIS, was
not observed in the majority of poorly differentiated invasive cancers whereas more that half of
intermediate grade DCIS showed this loss, suggesting that a subset evolved from well-
differentiated DCIS and another subset evolved from poorly differentiated DCIS. Other
chromosomal alterations, including gains at 8q, 17q and 20q and losses of 13q were found to be
associated with poorly differentiated invasive carcinoma. Overall, this data suggests that invasive
carcinoma recapitulates the genetic differentiation pattern of its precursor DCIS (low grade DCIS
progresses to low grade invasive cancer and high grade DCIS progresses to high grade invasive
cancer). Intermediate grade carcinoma may represent a mixture of DCIS subtypes evolving along
different genetic pathways.
Array-based CGH analysis
While CGH provides a genome-wide view of chromosomal changes, its resolution is
limited to measuring chromosomal imbalances of 10-20 megabases or more. Assuming about 10
genes per megabase, the resolution of conventional CGH spans about a 100-200 gene range.
Array-based CGH is a high resolution alternative that can measure DNA copy number changes at
the kilobase or gene level. For array CGH, tumor and normal genomic DNA are labeled with two
different fluorescent dyes. The differentially labeled DNA is cohybridized to a microarray which
Corresponding Author: Stefanie S. Jeffrey
7
is a glass slide containing thousands of DNA elements. These elements can include either
cDNAs (individual genes) or larger chromosomal segments that contain one or more genes with
known chromosomal location, such as bacterial artificial chromosomes. The fluorescence ratio of
tumor to normal DNA at each gene represents the copy number ratio between the two samples.
Since gene expression studies may also be performed on similarly configured microarrays (see
below), it is possible to directly correlate DNA copy number change and gene expression. 5
Array-based CGH has been used to investigate previously recognized areas of
amplification, such as chromosome 20q13, which had been characterized extensively by other
techniques. The increased resolution of array CGH was able to identify two potential oncogenes,
CYP24 and ZNF217, the former not previously associated with breast cancer.21 Pollack and
colleagues22 used array CGH to study gene copy number changes and their correlation to gene
expression. Interrogating 6,691 mapped human genes in locally advanced primary breast tumors
and ten breast cancer cell lines, DNA chromosomal alterations were found in all samples with
aberrations found in every chromosome. Gains were identified within 1q, 8q, 17q and 20q in a
large proportion of tumors and cell lines; losses were observed within 1p, 3p, 8p, and 13q. A
strong relationship between DNA copy number and gene expression was found, well exemplified
by chromosome 17. Although gene amplification does not always yield an increase in gene
expression, for highly amplified DNA regions, 42% were associated with high gene expression
and 62% were associated with moderately high gene expression. This suggests that a tumor’s
molecular phenotype is in large part impacted by underlying variation in DNA copy number. The
authors estimate that overall 7-12% of variation in gene expression in breast tumors is due to
variation in gene copy number. A study by Kallioneimi and colleagues23 had similar findings.
Comparing DNA copy number and mRNA expression levels of 13,824 genes in 14 breast cancer
Corresponding Author: Stefanie S. Jeffrey
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cell lines, they showed that 44% of highly amplified genes were overexpressed and 10.5% of the
genes with high-level expression were amplified.
RNA MOLECULAR PROFILES
Since only a fraction of genes in a cell are expressed at any given time, the set of
expressed genes (the gene expression profile) provides a snapshot reflecting that cell’s
physiology and response to environmental influences. Differences in gene expression profiles
can be used to define different molecular phenotypes of breast cancer, to predict the need for and
responsiveness to systemic therapies, and to identify novel targets for tumor-specific therapies.
There are a number of reasons why RNA expression profiling has dominated the
molecular profiling arena: (1) RNA is the product of an expressed gene and usually contains
more functional significance than DNA, (2) protein assays are still in their infancy and
sensitivity and precision require further optimization and validation, (3) classical RNA
technologies were easily adapted to high throughput systems, and (4) conserved RNA properties
facilitate amplification and measurement of minute amounts.
Before the genome project began, scientific methodology was candidate gene dependent,
discovering and identifying one gene at a time was knowledge driven. High throughput
technologies developed as part of the Human Genome Project changed this systematic
methodology. Using these technologies, global gene expression profiles for thousands of known
and unknown genes were determined in tissues before the genome was even sequenced.24, 25 The
initial gene discovery methods, included Expressed Sequence Tags (ESTs, explained below),
subtractive hybridization,26, 27 serial analysis of gene expression (SAGE),28 and differential
display (DD),29 were developed based on universal RNA properties and available laboratory
techniques without needing prior knowledge of an expressed gene’s function, sequence, or
Corresponding Author: Stefanie S. Jeffrey
9
chromosomal location. Using this technology, novel genes were identified at a more rapid pace
than functions could be assigned. Today, approximately half of the expressed sequences (ie,
genes) still have no assigned function, yet the abundance of gene sequence knowledge available
from these techniques has enabled scientific focus to change from gene discovery to gene
function. While these methods are powerful, they are technically difficult, require large-scale
robotic sequencing instruments, and only allow study of a few different biological samples at one
time. In contrast, DNA microarrays were developed in the mid-1990’s and have been used to
measure RNA expression of thousands of genes from multiple samples at one time.30, 31 They
represent the quickest, easiest, and least expensive method to relate expressed genes to clinical
data.
ESTs
An EST is a sequence of nucleotides that represents a portion of an expressed gene. It is
obtained from automated sequencing of a cDNA library. A cDNA library is constructed by first
isolating mRNA from a tissue sample of interest. The mRNA is reverse transcribed into
complementary DNA (cDNA), which is then inserted into plasmids that are replicated in E. coli
colonies on a nutrient-enriched plate. The colonies are randomly picked and the amplified cDNA
is isolated and sequenced using an automated sequencer. A set of sequences from the same tissue
sample is called an EST library.
If every cDNA clone is picked and sequenced, the entire transcript population of the cell
(called a transcriptome) will be represented quantitatively and qualitatively in the EST library.
The ESTs are matched by sequence identity to a database of known genes to determine if the
expressed sequences have been previously identified. Thousands of unidentified genes have been
discovered using EST technology. ESTs were the first successful functional molecular profiling
Corresponding Author: Stefanie S. Jeffrey
10
project of the human genome era. They represented a paradigm shift in scientific methodology
because huge sums of expression data were collected without having any prior information about
genes. EST technology yielded the publication of many transcriptomes, and as of mid-2003, 17.8
million ESTs were deposited in GenBank (with many times this number available in the private
sector). New functional genomic technologies such as microarrays depend on EST sequences.
DNA Microarrays
Microarrays produce a gene expression profile by simultaneously measuring gene
expression of hundreds to thousands of genes from a single sample. Known gene sequences are
attached to membrane-based or glass arrays. Although more expensive than membrane-based
arrays, glass arrays are smaller, easier to use, and allow a higher density of gene spots.
There are two types of glass arrays. One type is constructed using short 20-80 nucleotide
fragments (oligonucleotides) to represent each gene. The oligonucleotides are synthesized in situ
on the glass slide, using special lithographic32 or ink-jet printing33 technologies that were
developed by Affymetrix Corporation or Agilent Technologies. Oligonucleotides can also be
synthesized in batches prior to immobilization onto an array, which can reduce the cost. The
second type of array, cDNA microarrays,30 contains partial to full length cDNAs, 500-5,000
nucleotides in length that are “spotted” on histological slides using robotics and fine print tips
and then immobilized. The cDNAs consist of known and unknown genes, identified using EST
technology. Oligonucleotide array technology is more expensive, but in general, demonstrated to
be more precise and sensitive.
Total RNA (approximately 50 µg) or mRNA (3 µg) is used to measure expressed
transcripts on cDNA microarrays. In general, total RNA or mRNA is isolated and reverse
transcribed with fluorescently-tagged nucleotides to label the cDNA. For samples that do not
Corresponding Author: Stefanie S. Jeffrey
11
contain sufficient amounts of RNA for microarray hybridization, RNA amplification techniques
can be employed.34
Each spot (or feature) on a microarray corresponds to a specific gene or EST. Labeled
cDNA from an experimental sample (eg, cDNA prepared from breast cancers and containing
unknown quantities of specific genes, such as HER2/neu) is hybridized to the microarray. Excess
or non-hybridized cDNA is washed off. Because of the specificity of base pairing at each feature,
the abundance of a gene in the sample is measured. It is difficult to measure an absolute gene
expression value on cDNA microarrays due to systematic differences in gene printing and
hybridization kinetics. Therefore, reference RNA is used to generate a relative abundance ratio
between the sample and a reference that allows gene-to-gene comparisons between different
samples. Sample and reference RNA are labeled with different fluorophores (usually Cy5, which
fluoresces red at 635 nm, and Cy3, which fluoresces green at 525 nm) and cohybridized to the
microarray (Figure 86-1). The hybridized fluorescence signals can be read with an optical
scanner. Using bioinformatics software, a fluorescence signal intensity ratio between the sample
and the reference is computed. Signal intensity ratios provide a relative measure of gene
abundance. Correlations can be made based on the gene expression similarity between
independent samples.35 Genes or samples that demonstrate similar expression patterns are called
clusters (Figure 86-2). Statistical analyses36-38 can be performed and related to pathological and
clinical data to define samples or reactions to treatments.
Characterizing breast cancer subtypes
In 1999, human breast cancers were the first solid tumor to undergo global transcription
analysis using microarrays.39, 40 Before these studies, it was not known whether the genetic and
cellular diversity of solid tumors would preclude identifying gene expression patterns in breast
Corresponding Author: Stefanie S. Jeffrey
12
cancer. Despite the limited number of tumor samples consisting of different breast cancer types
and grades, the small number of genes assayed, and lack of usual breast cancer-associated genes
on the array (eg, HER2/neu and ER), Perou and colleagues39 identified multiple genes that were
similarly expressed and implicated in the molecular phenotype of solid tumors. In a follow-up
study, cDNA microarrays were used to molecularly subtype normal, benign and malignant breast
tumors.41 Variations in growth rate, activity of specific signaling pathways, and cellular
composition of the tumors were all reflected in gene expression profiles. This and follow-up
studies42, 43 identified genes that divided the tumors into distinct molecular subtypes: two ER-
overexpressing subtypes (denoted “Luminal A and B” due to presence of luminal epithelial
cytokeratin markers) and three ER-negative subtypes: “basal-like” tumors that expressed
cytokeratin markers characteristic of basal epithelial cells, “ERBB2 (HER2/neu)-
overexpressing” tumors, and “normal-like” tumors that showed relatively high expression of
genes characteristic of basal epithelial cells and adipocytes which clustered with normal breast
tissue samples. The expression of known luminal and basal cytokeratin epithelial cell markers
suggests that breast cancers may arise from at least two progenitor cell types through different
mechanisms. Other studies44-48 have since demonstrated that ER and ER co-regulated gene
expression (or lack thereof) provides a pervasive molecular signature marked by an abundant and
robust gene expression. c-myc is amplified in 15% of breast cancers and is highly expressed in
“basal-like” tumors, possibly regulating the expression of genes that play a role in the behavior
of these tumors.48 Overall, these data suggest that groups of genes better characterize and refine
tumor subtypes than single gene markers, like ER or HER2/neu.
Using 43,000 feature cDNA microarrays to profile histologically varied tumors from
more racially diverse patient populations, our lab has identified additional molecular subtypes of
Corresponding Author: Stefanie S. Jeffrey
13
breast cancer. We have also shown that invasive lobular carcinomas may be classified into
“typical” and “ductal-like” lobular tumors by their expression profiles.
Subtype profiling of hereditary breast cancers
Mutations in the breast cancer susceptibility genes, BRCA1 and BRCA2, influence DNA
repair and transcriptional regulation differently. Using microarrays, multiple genes were
identified that distinguished BRCA1 from BRCA2 subtypes.49 Interestingly, a patient without a
BRCA1 mutation whose tumor expressed a BRCA1 molecular phenotype, had DNA
hypermethylation of the BRCA1 promoter, silencing its expression. In another expression
profiling study,46 16 of 18 BRCA1 tumors from lymph node negative patients under age 55 were
characterized by downregulation of ER co-regulated genes and upregulation of lymphocytic
genes, including those primarily expressed by B and T cells. All the BRCA1 tumors from this
study were also demonstrated in a different study43 to have a “basal-like” gene-expression
phenotype, consistent with classical studies characterizing BRCA1 tumors as mostly high grade
ER, PR, and HER2/neu negative tumors that stain positive for basal cytokeratins and are often
associated with a lymphocytic infiltrate.50-52 Tumors from patients with BRCA2 mutations,
however, appeared to have a luminal estrogen receptor positive expression profile,43 consistent
with ER positive status and luminal keratin overexpression also found in another study.49
Prophylactic tamoxifen therapy significantly reduces the incidence of breast cancer in patients
with BRCA2 mutations and only modestly, if at all, in patients with BRCA1 mutations,53, 54
further supporting the hypothesis that these tumors arise from different epithelial origins, luminal
ER-expressing and basal ER-negative cell types. Global profiling studies are also being used to
evaluate familial non-BRCA1/2 breast cancers, with preliminary studies suggesting a partition
Corresponding Author: Stefanie S. Jeffrey
14
into at least two subtypes that do not share gene expression profiles with BRCA1 or BRCA2
tumors.55 In summary, molecular profiling data suggest that BRCA1 and BRCA2 hereditary
breast cancers originate from different progenitor cell populations, with independent malignant
mechanisms, different prognosis, and different response to prophylactic tamoxifen treatment.
Characterizing cancer cell progression
Specific changes in DCIS, atypical hyperplasias, usual hyperplasias, normal lobules or
ducts, can be measured by isolating these cell populations from neighboring cells by
microdissection. This can be done manually with a dissecting microscope13 or with newer
techniques that, under microscopic guidance, apply laser energy to excise the cells of interest
(laser microdissection, LMD) or melt a polymer onto the cells to be captured and extract only the
targeted cells from the surrounding tissue (laser capture microdissection, LCM).56
LCM has been used to extract pure populations of epithelial cells from normal lobules
from reduction mammoplasties or breasts with associated cancer, atypical ductal hyperplasia
(ADH), DCIS, and invasive ductal carcinoma (IDC)57 for microarray analysis. Expression
profiling demonstrated that normal epithelial cells distant from cancers had similar
transcriptional signatures to normal epithelial cells from reduction mammoplasties. Significant
expression changes were observed in ADH and persisted in DCIS and IDC dissected from the
same patient, showing patient-specific phenotypes and suggesting that ADH and DCIS are
precursors to IDC. The authors found that Grade I expression signatures generally differed from
Grade III signatures, but intermediate grade lesions shared either a hybrid signature or a distinct
low grade or high grade signature.
Corresponding Author: Stefanie S. Jeffrey
15
In sum, global RNA profiling studies at the invasive41, 43 and preinvasive57 stages suggest
that breast cancers originate from progenitor cells with specific molecular subtypes. This
corroborates earlier studies by Warnberg58 with traditional immunohistochemical (IHC)
techniques and by Buerger8, 59 who used CGH, fluorescence in situ hybridization (FISH), and
IHC analyses.
Molecular profiling in clinical use
Tailoring patient treatments using microarrays
Several groups have shown that molecular profiling can be performed on minimally
invasive breast biopsies taken prior to primary chemotherapy or from non-palpable lesions
identified by breast imaging. Fine needle aspiration (FNA) biopsies and core needle biopsies
have been used47, 60-63 to successfully isolate RNA for microarray studies. In a small pilot study
using core needle biopsies taken before and within the first 48 hours of different regimens of
neoadjuvant chemotherapy, Buchholz and colleagues60 showed that expression profiles of tumors
with and without a good pathological response clustered distinctly. Sotiriou and colleagues61
used FNA biopsies performed on ten patients before and during neoadjuvant chemotherapy to
monitor patient response to doxorubicin and cyclophosphamide. Candidate gene expression
profiles were identified that distinguished responders from nonresponders. Interestingly, the
responders also showed expression changes in ten times the number of genes than the
nonresponders after the first cycle of chemotherapy. Chang and colleagues63 performed core
needle biopsies on 24 patients with locally advanced breast cancer. Using microarray analysis,
they were able to define a set of 92 differentially expressed genes that characterized docetaxel
sensitive tumors, defined as those that had 25% or less residual disease following treatment. This
Corresponding Author: Stefanie S. Jeffrey
16
gene set showed a positive predictive value of 92% and negative predictive value of 83% and is
currently being applied in a larger clinical trial.
Prognosis Profiling
Currently, most lymph node negative breast cancer patients with tumors over 1 cm and
all lymph node positive patients are candidates for adjuvant systemic treatment,64 yet only 2-15%
will benefit.65 Better diagnostic methods are necessary to successfully identify the patients that
require treatment, predict who will benefit from specific therapies, and discover targets to serve
as the basis for new therapies. Patients whose breast cancers are stratified by expression profiling
into five molecular subtypes (ER-positive “luminal A and B”; ER-negative “basal”, “ERBB2
over-expressing” and “normal” breast subtypes) or simply into luminal and basal phenotypes
demonstrate independent relapse-free survival curves.42, 43, 48 Of the five major subtypes, the
basal-like and ERBB2 subtypes reveal the poorest prognosis. Although luminal A and B
subtypes share gene expression similarities and both overexpress ER co-regulated genes, luminal
A tumors show the best prognosis of all the subtypes, even in patients with locally advanced
breast cancer, while luminal B tumors demonstrate poorer survival. Luminal B tumors express
groups of known and unknown genes that are also expressed in ERBB2+ and basal-like tumors,
and like these other subtypes, also exhibit TP53 mutations, possibly influencing the poorer
prognosis of these three subtypes in initial studies. Since long-term survival in locally advanced
breast cancer patients treated with 16 weeks of doxorubicin and tamoxifen was better for the
luminal A phenotype, these results suggest that either the tumors possessed a favorable biology
or it reflected their responsiveness to doxorubicin and/or tamoxifen treatment.
Breast cancer staging criteria is based on tumor size and the presence of lymph node
metastases. Recent data, however, suggests that current staging criteria may need reevaluation. In
Corresponding Author: Stefanie S. Jeffrey
17
expression profiling studies, nodal status and tumor size appear to have less impact on gene
expression and survival than tumor biology. Hormone receptor status and grade, however, appear
to strongly impact gene expression.46, 48, 66 Metastatic potential may be pre-programmed in the
biology of the tumor.46, 67, 68
Using a 70 gene expression profile, van’t Veer and colleagues46 were able to successfully
predict outcome in 81% of women aged less than 55 years with lymph node negative Stage I and
II breast cancers, most of whom did not receive systemic therapy: 91% of the good prognosis
group and only 27% of the poor prognosis group were disease-free at five years. In a follow-up
study by van de Vijver and colleagues,66 the 70 gene profile was retrospectively tested on tumors
from patients less than 53 years of age, but this time with lymph node negative and positive
Stage I and II disease, many of whom received treatment. Lymph node positive patients were
evenly divided between good- and poor-prognosis signatures, suggesting that lymph node
metastasis may be an independent event distinct from systemic metastasis. After ten years, 85%
in the good prognosis set remained distant metastases free compared to 51% in the poor
prognosis group, offering improvement over St. Gallen69 and NIH70 criteria. A clinical trial is
now underway in Europe to prospectively compare this 70 gene profile to standard classification
criteria as the basis for treatment decisions.
Although ER status of the tumors was not an independent prognostic factor in the van de
Vijver study, it has been shown to be the most important clinico-pathological discriminator of
expression subtype by Sotiriou and colleagues,48 who also showed that lymph node status has a
minimal influence on expression profiling. Using overlapping gene expression data from the
van’t Veer study, Sorlie43 showed that basal-like tumors were a prominent subtype with rapid
development of metastases within five years. It is possible that the relatively homogeneous
Corresponding Author: Stefanie S. Jeffrey
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expression pattern shared by basal-like tumors strongly influenced the 70 gene poor-prognosis
signature.
Validation
The advantage of high throughput global gene expression is the precision afforded in
measuring thousands of genes simultaneously; precision at the individual gene level, however,
can sometimes be sacrificed to perform global assessments. Therefore, validation must be
performed to confirm gene expression and, if desired, to identify the cell type expressing the
gene.
A high throughput validation technology is the tissue microarray (TMA). This is a
paraffin block made up of hundreds of cores from paraffin-embedded tissues from different
patients.71 When the TMA is sectioned, placed on a slide and combined with traditional
validation technologies like IHC and RNA in situ hybridization, gene expression can be
validated over hundreds of samples at one time. Another validation tool is real-time quantitative
polymerase chain reaction (qPCR) (also called TaqMan PCR). In this technique, RNA from a
tissue sample is purified and amplified under optimized gene-specific conditions. Fluorescence
molecules are discharged with each amplification cycle and the amount of fluorescence released
is dependent on the abundance of RNA in the sample.72 Using this technology with plates
containing multiple sample wells, hundreds of genes can be rapidly measured with high precision
and sensitivity.
Proteomics
Corresponding Author: Stefanie S. Jeffrey
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Proteomics is the study of expressed proteins from a genome. The proteome is potentially the
most important molecular profile because proteins are the actuators of the genome and a cell’s proteins
should determine its phenotype at a given moment. Like DNA and RNA, comprehensive proteomics can
be studied in a quantitative (abundance) and qualitative (presence or absence) manner. Unlike DNA and
RNA, protein function is also influenced by other factors that shape protein activity, such as protein-
protein interactions, subcellular location, conformational changes, half-life changes, and post-
translational modifications. To resolve these changes, proteomic techniques include separation and
identification techniques.73 Due to the increased biochemical and structural diversity of proteins relative
to DNA and RNA, these two tasks are difficult. Current techniques are still in development and have not
been able to construct a genome-wide proteome to describe breast cancer phenotypes. However,
proteomic patterns in the breast cancer serum and ductal fluid already show promise for clinical use in
early diagnosis of breast cancer.
Proteomic Techniques—Protein separation and identification
Two-dimensional gel electrophoresis (2-DE) sequentially separates proteins by their charge and
mass. The separation on a single gel can show thousands of proteins, including proteins that may
undergo post-translational modification (such as by phosphorylation, glycosylation, lipid attachment, or
peptide cleavage) and be represented by multiple spots on a gel. 2-DE can be utilized to identify protein
patterns or to separate proteins prior to identification by mass spectrometry.
Mass spectrometry (MS) is a sensitive and precise approach to identify proteins that are first
separated, digested into peptides, and then ionized. Protein separation can be accomplished with 2-DE or
other methods such as high performance liquid chromatography (HPLC), 2-D liquid chromatography
(2D-LC or LC/LC), capillary electrophoresis, or by biochip chromatography. Proteins are then
individually ionized into a protonated gas phase using multiple techniques. Electrospray ionization (ESI)
Corresponding Author: Stefanie S. Jeffrey
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creates a fine spray of charged droplets from a liquid sample that evaporates, producing gaseous ionized
molecules. For samples in a solid state, matrix-assisted laser desorption/ionization (MALDI) is a
technique that mixes proteins digested by sequence-specific proteases with a light-absorbing organic
acid matrix that catapults the peptides into an ionized form when irradiated by an ultraviolet laser.
Surface-enhanced laser desorption/ionization (SELDI) uses resin biochips with different
chromatographic properties on their surface to fractionate and isolate proteins through affinity capture.
After washing, retained proteins are mixed with energy absorbing molecules and ionized by laser
pulsation. A newer modification places the energy absorbing molecules directly on the chip. After
ionization by any of these methods, protein fragments are propelled and accelerated by magnetic or
electrostatic forces through a time of flight (TOF) mass spectrometer, which separates them by their
specific mass to charge (m/z) ratio, forming a peptide mass fingerprint. For MALDI, protein
identification is typically accomplished by searching large protein databases and comparing the masses
of collections of peptides (peptide mass fingerprint) to those predicted from digestion of protein
sequences. For LC-ESI analysis, tandem mass spectrometry (MS/MS), in which individual peptides are
fragmented in the mass spectrometer, is utilized to determine the identity of proteins by their amino acid
sequences. For SELDI, in which proteins are analyzed in intact form, there is as of yet no
straightforward method to identify proteins from mass spectra.
Characterizing breast tissues proteomes and identifying biomarkers and targets
2-DE has been used to differentiate protein patterns in normal breast tissue, benign breast tissue,
and breast cancer.74 A 2-DE technique called difference gel electrophoresis (DIGE), which compares
samples from multiple sources differentially labeled with fluorescent dyes by using post-run fluorescent
imaging, has been used to differentiate lysates of breast cancer cell lines to identify proteins associated
with ERBB2 overexpression.75 Bergman and colleagues76 used 2-DE combined with ESI-MS and
Corresponding Author: Stefanie S. Jeffrey
21
MALDI-MS to identify polypeptides differentially expressed in solid tumor cell extracts made from
scrapings of benign and malignant breast tumors. Some of the overexpressed proteins in breast cancer
included nuclear matrix proteins, cytoskeletal and redox proteins, while the known oncogene product
DJ-1 was identified in a breast fibroadenomas, not malignant tissue. Truncated forms of overexpressed
proteins were also identified, suggesting proteolytic processing in both benign and malignant tissue.
Cell type heterogeneity in breast tissue adds complexity to the characterization of protein
populations. Page and colleagues77 grew primary epithelial cell cultures derived from reduction
mammoplasties and used cell sorting techniques to separate luminal and myoepithelial cells. Protein
differences were studied with 2-DE, MALDI and MS/MS technology; a fraction of the differentially
expressed proteins were annotated. Many of these corresponded to known cytokeratin markers that
distinguish the two cell types. Luminal and myoepithelial cell types also demonstrated significant global
homology in their protein profiles, which the authors believed was consistent with derivation from a
common stem cell. Several groups have purified epithelial cells from breast cancers and normal tissue
using LCM and then performed comparative proteomic analyses.78, 79 Wulfkuhle and colleagues80
isolated DCIS and normal ductal epithelium by LCM and identified proteins in DCIS involved in
intracellular trafficking of lipids, vesicles, and membranes. They also found changes in proteins
involved in cell motility and genomic instability, suggesting that DCIS is an already advanced
preinvasive lesion.
In the future, sets of cancer-associated biomarkers identified in nipple aspirate fluid and serum
may prove useful as clinical diagnostic tools. Varnum and colleagues81 collected nipple aspirate fluid
(NAF) in healthy women and identified 64 proteins, showing that NAF is a highly concentrated source
of biomarkers. Paweletz and colleagues82 used SELDI-TOF to analyze NAF, and found protein profiles
that appeared to distinguish women with breast cancer from healthy controls. Reasoning that the breast
Corresponding Author: Stefanie S. Jeffrey
22
is a paired organ, Kuerer and colleagues83 found much higher spot variation comparing protein profiles
by 2-DE of paired NAF samples between matched malignant and normal breasts in women with
unilateral breast cancer. Applying SELDI-TOF technology to NAF, Sauter and colleagues84 identified
five proteins differentially expressed in women with and without breast cancer that are now being tested
in a prospective clinical trial.
At present, investigators are searching for accurate blood tests to diagnose breast cancer. They
are hoping that serum protein profiles may be eventually applied to clinical practice. Using SELDI
technology, Li and colleagues85 identified three biomarkers in breast cancer serum. Together these
markers can differentiate over 90% of serum samples obtained from women with and without breast
cancer. This test did not, however, discriminate serum samples on the basis of tumor size or lymph node
metastases. Following up on studies suggesting distinct serum markers in women with ovarian cancer,86
Petricoin, Liotta and colleagues are using serum protein profiles to develop a blood test to screen for
early breast cancer.
New techniques to more accurately characterize subpopulations of the proteome
Since current technologies are not able to measure the entire proteome, scientists have also
focused on developing proteomic technologies to analyze protein subpopulations. These techniques
promise a more detailed and complete view of interesting proteins (membrane proteins or biomarkers) or
protein characteristics (protein activity).
A protein microarray measuring comparative fluorescence can be constructed based on protein
(eg, antibody) and ligand interactions, analogous to a high throughput enzyme-linked immunosorbant
assay (ELISA).87, 88 Used as an antibody array to detect antigens or an autoantigen array to detect
antibodies,89 this high density array can separate and identify proteins related to breast cancer in
complex solutions such as serum90 or NAF81 in a fast, efficient, and cost effective manner.
Corresponding Author: Stefanie S. Jeffrey
23
Adam and colleagues91 combined membrane isolation techniques, gel electrophoresis and mass
spectrometry to gain insight into the enriched membrane protein fractions of breast cancer cell lines,
which traditionally have been poorly defined by current global proteomic techniques because of their
hydrophobic properties. In addition to many membrane proteins with known significance in breast
cancer, such as MUC1 and the HER2/neu and EGF receptors, three novel genes were identified:
BCMP11, BCMP84, BCMP101. Protein and mRNA expression of BCMP101 was low in normal tissues
in contrast to high levels in many breast cancers confirming BCMP101 as a potential breast cancer
marker.
Le Naour and colleagues92 used a novel proteomics approach to identify secreted breast cancer
proteins in serum using antibodies from patients’ serum. Antibodies in breast cancer serum identified a
reactive protein in lysates of human breast cancer tissues and cell lines spotted on a 2-D gel. MALDI-
TOF was then used to identify the protein as RS/DJ-1, which was detected at high levels in the sera of
37% of patients diagnosed with breast cancer. The combined use of autoantibodies and proteomics to
discover and identify secreted proteins in cancer remains a promising methodology.
In contrast to other proteomic techniques that measure protein abundance, Jessani and
colleagues93 used a technique called activity-based protein profiling (ABPP) that detects enzymes only
in their active states. Specific active site-directed probes that covalently labeled serine hydrolases, a
large enzyme superfamily that comprises approximately 1% of all proteins in the human proteome,
allowed detection of activity in different subcellular locations and glycosylation states in various cancer
cell lines. The authors identified proteases, lipases and esterases differentially regulated specific to tissue
origin, including breast cancer. The most invasive cell lines, as demonstrated by matrigel assay, showed
downregulation of these enzyme activities while a different set of secreted and membrane-associated
serine hydrolases showed activation, possibly representing new markers of tumor aggressiveness.
Corresponding Author: Stefanie S. Jeffrey
24
Conclusions:
The progress achieved through molecular profiling tools has allowed us to reevaluate
concepts involved in breast tumor evolution, diagnosis, and treatment. DNA molecular profiles
have shown tumor progression is associated with accumulating genetic alterations and have
exposed DCIS as an advanced lesion; one model suggests specific genetic lesions in DCIS can
determine progression of invasive carcinomas; ie, that the differentiation status of the invasive
cancer recapitulates that of the in situ lesion. In breast tumors, when RNA expression was
compared to changes in the DNA, gene expression signatures were most often related to
increases in DNA copy number. Furthermore, single mutations or events are probably not
entirely culpable for carcinogenesis since global DNA profiling shows that among multiple
breast cancers, a wide range of tumor genotypes (different chromosomal amplifications and
deletions) exist. RNA molecular profiles are not quite as diverse, and at least five different
expressed phenotypes exist, each with independent survival characteristics. This evidence
suggests breast cancer treatments may need to be tailored to different tumor biologies.
RNA expression profiles indicate breast cancers may arise from progenitor cells that
occur along basal or luminal differentiation pathways, with basal-like tumors associated with a
worse prognosis. BRCA1 breast cancers exclusively carry a basal-like expression signature that is
easily identified using molecular profiling. The profiling also takes into account BRCA1
methylation, which is not measured by mutation analysis. Importantly, expression profiling also
shows that a tumor’s ability to metastasize may not be reliably measured by lymph node
metastasis or size. This is in contradistinction to hormone receptor status and grade that play
greater roles in distinguishing expression phenotypes.
Corresponding Author: Stefanie S. Jeffrey
25
Promising proteomic studies have utilized nipple aspirate fluid and serum to identify
several breast cancer biomarkers. These non-invasive approaches are being tested in clinical
trials. Functional proteomics, a new field that measures protein activity within tumor specimens,
may identify biomarkers and therapeutic targets not discoverable by other techniques.
Despite the clear impact molecular profiling has made to improving our understanding of
breast cancer, there is still a great deal of work ahead. It is important to note that nucleotide
mutations in many key genes associated with breast cancer (eg TP53) are not distinguished using
the global DNA, RNA, or protein molecular profiling methodologies discussed here, but are
being studied using other techniques such as single nucleotide polymorphism (SNP) arrays. SNP
arrays may also augment our understanding of the affects of chromosomal vs. nucleotide
instability on tumor evolution and progression. Furthermore, other areas that may strongly
impact breast cancer biology, such as racial/ethnic differences or stromal-epithelial cell
interactions, are now being explored.
RNA expression profiling currently holds the most translational promise in breast cancer,
but may ultimately be superceded by proteomic techniques. This technique appears to predict
clinical outcome and response to systemic therapy better than classical staging criteria in initial
studies. Recruitment for large prospective clinical trials to better assess molecular prognostic and
predictive gene lists is now underway. It is anticipated that as new global profiling technologies
are applied to clinical care, breast cancer diagnosis and care will be more precise and
individualized than current methods and will lead to the development of novel tumor-specific
therapeutics.
Corresponding Author: Stefanie S. Jeffrey
26
Figure Legends
Figure 1. A general illustration of a cDNA microarray protocol. A cDNA microarray can be
used to determine either gene expression (RNA) or gene copy number (DNA) changes. After
purification, the tumor and reference samples are labeled with Cy5 and Cy3, respectively. The
mixture is hybridized to a microarray and scanned with two wavelengths to measure the relative
intensities of red and green fluorescence at each feature. The relative intensities of features can
be compared among tumors to identify changes in expression associated with a tumor subtype.
Reproduced with permission from the American Society for Pharmacology and Experimental
Therapeutics (ASPET) 94
Figure 2. Gene expression patterns of 85 breast samples. Seventy-eight carcinomas, three
benign tumors, and four normal breast tissues cluster into 5 subtypes: Luminal A (ER positive,
favorable survival); Luminal B (ER positive, poor survival); Normal breast-like; ERBB2
(HER2/Neu) amplicon; Basal epithelial-like cluster. (A) Tumors clusters are represented by
branched dendrograms at the upper figure which indicate degree of similarity between samples.
Genes are clustered by rows with genes that are expressed most similarly clustered together. Red
indicates high relative gene expression compared to reference; green indicates more expression
in reference RNA than in tumor sample (low relative expression). Representataive gene clusters
expressed by the five tumor subtypes above are shown: (B) the ERBB2 amplicon cluster; (C)
genes coexpressed by the Luminal B tumors and the basal and ERBB2 tumors; (D) basal
epithelial cluster containing keratins 5, 17; (E) normal breast-like cluster; and (F) Luminal A
cluster containing ER-associated genes with lower relative expression of these genes by the
Luminal B tumors.
Permission requested by the Proceedings of the National Academy of Sciences 42
Corresponding Author: Stefanie S. Jeffrey
27
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