Application of prostate-specific antigen in prostate cancer

Oct. 1, 2008

Prostate cancer is the sixth most common cancer in the
world and the most commonly diagnosed visceral cancer in the United States,
representing about 29% of all cancers diagnosed in men each year. In the
United States, approximately 218,890 cases are diagnosed annually, and
approximately 27,050 men die from prostate cancer.1 It is the
second leading cause of death overall and cancer death in men, next to
non-melanoma skin cancer and lung cancer.

For an American male, the lifetime risk of developing
prostate cancer is one in six, but the risk of dying from prostate cancer is
only one in 33.1,2 Many more cases of prostate cancer do not
become clinically apparent, as demonstrated in autopsy series, where
prostate cancer is identified in one-third of men under the age of 80 and in
two-thirds of men older than 80.3 These data suggest that
prostate cancer often progresses so slowly that most men die of other causes
before the disease becomes clinically advanced.

Prostate cancer has been detected with increasing
frequency, even before the introduction of serum prostate-specific antigen
(PSA) testing. The incidence peaked in 1992, declined between 1992 and 1995,
and has been rising about 1.1% annually since 1995.1 The reasons
for the increasing incidence are not known; both genetic and environmental
factors have been implicated. Of the several known risk factors, the most
important are age, ethnicity, genetic factors, and, possibly, dietary
factors. Prostate cancer demonstrates one of the strongest relationships
between age and any human malignancy. Prostate cancer rarely occurs before
the age of 40, but the incidence rises rapidly thereafter.1,2
Prostate cancer is more common in black than in white or Hispanic men,
perhaps related to a combination of dietary and/or genetic factors.
Nevertheless, despite evidence that prostate cancer has a strong genetic
component, identifying the genetic and inherited risk factors that underlie
the disease has proven more challenging than initially anticipated. Some
intriguing work has highlighted certain genes, but results have been
inconsistent across studies. A diet high in animal fat could also be a risk
factor, while studies have shown that dietary supplement of antioxidants
(e.g., lycopene, vitamin E, selenium) may reduce the risk.1-3
Currently, however, there is no scientific consensus on effective strategies
to reduce the risk of prostate cancer.

Prostate-cancer survival is related to many factors,
especially the advance of tumor at the time of diagnosis. The 10-year
survival among men with cancer confined to the prostate (localized) is 75%,
compared with 55% and 15% among those with regional extension and distant
metastases, respectively.4 Bone lesion is the most common
metastatic prostate cancer. While men with advanced-stage disease may
benefit from palliative treatment, their tumors are generally not curable.
As a result, a screening program that could identify asymptomatic men with
aggressive localized tumors might be expected to considerably reduce
prostate-cancer morbidity, including urinary obstruction and painful
metastases, and, hopefully, mortality.

Before PSA was introduced, the diagnosis of prostate
cancer was first detected by digital rectal examination (DRE) or because of
urinary symptoms (e.g., urinary urgency, nocturia, frequency, and
hesitancy). Today, the diagnosis of prostate cancer is most often suspected
after finding an elevated serum PSA as a screening test at a routine
physical examination. Less commonly, a diagnostic assessment is introduced
as a result of abnormal findings on DRE. Prostate biopsy is still considered
the gold standard to establish the diagnosis of prostate cancer.

The introduction of PSA testing in the 1980s
revolutionized prostate-cancer screening. Although PSA was originally
employed as a tumor marker for detecting cancer recurrence or monitoring
disease progression following treatment, it became extensively implemented
for cancer screening by the early 1990s. Subsequently, professional
societies established guidelines supporting prostate-cancer screening with
PSA.5,6 The application of widespread PSA screening and earlier
detection can potentially lead to decrease in prostate-cancer mortality
associated with a decline in metastatic disease. PSA-screening testing led
to a significant increase in the incidence of prostate cancer, peaking in
1992.2? The majority of these newly diagnosed cancers were
clinically localized, which led to an increase in radical prostatectomy and
radiation therapy — aggressive treatments intended to cure these early-stage
cancers.7-10

PSA

PSA is a 27-kD glycoprotein that is produced by prostate
epithelial cells and with chymotrypsin-like serine protease activity. Its
major function in vivo
is to activate the major in-gel proteins, seminogelin I, II, and fibronectin,
in freshly ejaculated semen, which then liquefies semen through proteolytic
fragmentation and releases progressively motile sperm. Extensive study in
this field over the years has revealed that PSA in serum is present in
several molecular forms and that the understanding of the proportions of
these PSA forms can aid in the diagnosis of the status for cancer from
benign disease. Under normal conditions, PSA is produced as a precursor form
(pPSA) by the secretory cells that line the prostate glands (acini) and
secreted into the lumen, where the 7-amino acid propeptide is then cleaved
by the enzyme — human kallikrein 2 — to generate active PSA. This molecule,
in turn, undergoes a proteolytic process and become inactive PSA, which then
enters the bloodstream and circulates in an unbound state (free PSA). A
small amount of active PSA diffuses into the circulation and is covalently
bound by protease inhibitors, including alpha-1-antichymotrypsin (ACT) (complexed
PSA) and, to a lesser extent, with alpha-2-macroglobulin.11,12 A
higher percentage of free PSA in the serum is correlated with a lower risk
of prostate cancer. The ratio of free to total PSA in serum has been
demonstrated to significantly improve the differential of prostate cancer
from benign prostatic hyperplasia (BPH).13

Free (uncomplexed) PSA in serum is now identified as
being composed of at least three distinct forms of inactive PSA. One form
has been identified as the precursor form of PSA, or pPSA, and is associated
with cancer.12 A second form of PSA, called benign PSA (BPSA), is
an internally cleaved or degraded form of the active PSA that is more highly
associated with BPH.12 The third PSA form is known to be an
intact, denatured PSA that is similar to native, active PSA, except for
changes in structure or conformation that render the molecule enzymatically
inactive. The association of this form with prostate cancer is still largely
unknown.

pPSA.
Studies have revealed that there are a number of minor variants for pPSA. In
addition to the intact pPSA, there are also significant levels of truncated
pPSA, which refer to pPSA in which any length of the first seven amino acids
in the proleader peptide have been removed. The truncated pPSA forms
containing proleader peptides of four and two amino acids, [-4]pPSA and [-2]pPSA,
respectively, are of particular interest. Truncated pPSA forms are more
resistant to activation to mature PSA than the intact pPSA with the 7-aa
proleader peptide.13 The truncated pPSA forms are, therefore,
more stable since they cannot be converted to active PSA. The sum or all
pPSA forms represents about a third of the free PSA typically present in
cancer serum.13

BPSA.
Studies have also demonstrated that there is another isoform of PSA in BPH
tissues and seminal plasma, which has been demonstrated to have higher
degree of internal peptide bond cleavages and is more enzymatically
inactive.13 The distinct degraded form of PSA, BPSA, has been
identified in BPH tissue. BPSA concentrations were relatively lower in
cancer tissue from the same prostate.13 BPSA is highly associated
with the presence of BPH nodules in the prostate, the primary pathological
feature of BPH.13 BPSA contains two internal peptide bond
cleavages at Lys145 and Lys182, while native PSA and pPSA contain no
internal cleavages. BPSA has no known natural function in the prostate and
is considered to result from post-translational cleavage by proteases in the
hyperplastic BPH tissue. BPSA-specific immunoassays have been developed, and
BPSA has been shown to be significantly elevated in the serum of
biopsy-negative men with elevated PSA.13 BPSA is detected in
seminal plasma13 and can range from 0% to 50% in individual seminal-plasma
specimens, which suggests that BPSA formation changes as a function of
biochemical processes in the transition zone of the prostate gland.

Although producing less PSA per cell than normal tissue,
prostate cancer causes the disruption of the basement membrane, basal cells,
and normal lumen architecture. This mechanism has caused the secreted pPSA
and several truncated forms to have direct access to the circulation; and as
a result, a larger fraction of the PSA generated by malignant cells escapes
the proteolytic processing (i.e., activation of pPSA to active PSA and
degradation of active PSA to inactive PSA).

In men with a normal prostate, the majority of free PSA
in the serum reflects the mature protein that has been inactivated by
internal proteolytic cleavage. In contrast, this cleaved fraction is
comparatively decreased in prostate cancer. Therefore, the percentage of
free or unbound PSA is lower in the serum of men with prostate cancer (and,
conversely, the amount of complexed PSA is higher) compared with those who
have a normal prostate or BPH.14-17 This finding has been
exploited in the use of the ratio of free to total PSA and complexed PSA as
a means of distinguishing between prostate cancer and BPH as a cause of an
elevated total PSA.

Problems

Although serum total PSA is a prostate-specific marker,
elevations can be caused by both cancer and benign conditions such as BPH.
Malignant prostate tissue generates more PSA than normal or hyperplastic
tissue, probably because of the increased cellularity associated with
cancer. Moreover, malignant prostate tissue may disrupt the prostate-blood
barrier, causing more PSA to leak into the circulation and further
increasing the serum concentration of total PSA.

The traditional cutoff for an abnormal total PSA level in
the major screening studies has been 4.0 ng/mL.18-21 At this
level, the sensitivity of PSA has been estimated to be about 70% to 80%,
while the specificity is anticipated to be about 60% to 70%.22
PSA has poorer discriminating ability in men with prostate cancer and
symptomatic BPH, of which both would have elevated total PSA.23

The test performance statistic that has been best
characterized by screening studies is the positive predictive value: the
proportion of men with an elevated PSA who have prostate cancer. Overall,
the positive predictive value for a PSA level >4.0 ng/mL is approximately
30%, meaning that slightly less than one in three men with an elevated PSA
will have prostate cancer detected on biopsy — the gold standard of
diagnosis.24-26 The positive predictive value is 42% to 64% for
PSA levels >10 ng/mL.25 Prostate biopsy is regularly recommended
in men whose serum PSA is >10.0 ng/mL, since the chance of finding prostate
cancer is over 50%. It is also true, however, that more than 50% of these
men will have malignancy that is no longer organ-confined, and therefore,
not amenable to cure.24 For PSA levels between 4.0 ng/mL to 10.0
ng/mL, the positive predictive value is about 25%25; however,
nearly 75% of cancers detected within the “gray zone,” PSA values between
4.0 ng/mL to 10.0 ng/mL, are organ confined and potentially curable.24
Thus, detecting the curable cancers in men with PSA levels less than 10.0 ng/mL
presents a diagnostic challenge because the high false-positive rate,
identified by biopsy result, leads to many unnecessary biopsies and
generates anxieties.

PSA is not an ideal biomarker for prostate cancer, so
numerous strategies have been proposed to improve the diagnostic performance
of PSA when levels are less than 10.0 ng/mL, including lowering PSA cutoffs,
serial measurements, and alternatively processing PSA data (e.g., PSA
velocity [change in PSA over time], PSA density [PSA per unit volume of
prostate]). None of them, however, can resolve the controversy of PSA as a
screening test and fulfill the significant current clinical diagnostic needs
due to its low specificity for prostate cancer. Therefore, the scientific
community is still searching for the ideal and novel biomarkers for early
detection in prostate cancer.

Solutions

The use of multiple-marker panels has been proposed for
many years in the search for improved clinical utility of tumor markers.
With the rapid development of mass spectrometry, it is possible that
proteomic technologies may also help to lead to a new frontier of multiple-analyte
testing or multiplexed assays, to achieve significant new insights into
prostate-disease management.

Shu-Ling Liang, PhD, DABCC, is an instructor in pathology
at Harvard Medical School, and the assistant director of clinical chemistry
in the department of pathology at Beth Israel Deaconess Medical Center in
Boston, MA.
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