Background
Colorectal cancer (CRC) is one of the four most prevalent cancers in Western countries due to a low recovery rate in advanced stages, when micrometastases may be already present. It develops from precancerous lesions (adenomas) that are easily removed by polypectomy, a procedure that reduces CRC incidence by 75-90% [
1]; moreover, treatment of early diagnosed tumours has a good prognosis [
2]. Furthermore, according to the MISCAN-COLON simulation model, a 23% reduction in CRC mortality would be achieved with the screening of at least 70% of the target population [
3]. Therefore screening for CRC aims to reduce mortality rates by detection and removal of early-stage tumours, and incidence rates by identification and resection of polyps [
4].
There is a great variety of methods for the detection of CRC. A review of the ongoing screening initiatives worldwide was recently published [
5]. The latest Joint Colorectal Cancer Screening Guidelines [
6] divide the available tests into two categories: those primarily aimed to detect cancer (blood and DNA stool tests), and those directed to detect also advanced lesions (endoscopic and radiographic methods).
Within the available invasive tests are the flexible sigmoidoscopy, the double-contrast barium enema and the colonoscopy. The latter is recommended nowadays as the gold standard, though its miss rates have been estimated as 22% for adenomas and 2-6% for CRC [
7]. However, bowel preparation constitutes the most objectionable aspect of the procedure, fundamental for a proper screening colonoscopy [
8]. Other limitations of the colonoscopy are the risk of complications, the costs, and access. Regarding non-invasive tests, the most common method, nowadays recommended for CRC screening, is the guaiac faecal occult blood test (gFOBT), with highly variable and brand-dependent sensitivities and specificities [
6,
9] and requiring dietary restrictions.
Thus there is an imperative need for developing non-invasive screening tests for the detection of CRC and adenomas, and hence many current studies are evaluating serum markers. Examples of these are the α-defensins [
10], the nicotinamide N-methyltransferase [
11], the α-L-fucosidase [
12], or the colon cancer-specific antigens (CCSA) -2, -3 and -4 [
13,
14]. A major drawback in these studies is they are limited to discriminating between CRC patients and healthy individuals, leaving aside precursor lesions and not including in the control cohort individuals with benign pathologies.
Previous studies of our group detected that soluble serum CD26 (sCD26) levels were diminished in CRC patients as compared to healthy donors [
15,
16]. In those studies, a sCD26 cut-off of 410 ng/mL demonstrated a 90% diagnostic efficiency, with a specificity and sensitivity of 90% as well [
15]. This high diagnostic efficiency, even in early tumour stages, suggested its potential utility for diagnosis. Thus we considered of interest to extend the validation of the sCD26 as an early biomarker for CRC, including also precancerous lesions (adenomas). One of the novelties of the study is the use of a colonoscopically healthy cohort instead of a blood donor cohort as the control population. Moreover, this is the first time the sCD26 is analyzed regarding the clinicopathological characteristics of colorectal polyps. Thus, we aim to analyse the relationship of the serum CD26 levels with the colonoscopic findings, in order to validate the utility of this protein as a marker for CRC diagnosis.
Discussion
The glycoprotein CD26 or dipeptidyl peptidase IV (DPPIV, E.C. 3.4.14.5) is an exopeptidase of the plasma membrane able to release dipeptides from the
N-terminal end of peptides/proteins bearing proline or alanine in the penultimate position [
19]. Biological fluids contain relatively high levels of sCD26, which is presumably shed by proteolytic cleavage from any cell expressing transmembrane CD26 [
20]. Although its origin is still unclear, the liver and the T cells are cited as the most likely sources [
21].
The measurement of the sCD26 levels was performed in serum from individuals whom, due to different medical indications, had undergone colonoscopy; most of them referred abdominal, colon or rectal symptoms, or familial/personal history of polyps or CCR. The individuals without colorectal findings after the colonoscopy were considered as the control cohort; the remaining were classified as: non-IBD, IBD, colorectal polyps or CRC patients. The mean sCD26 concentration decreased, although non-significantly, as the pathology diagnosed was more severe (as seen in table
1), this is, from no colorectal pathology to CRC, with a noticeable decrease in the group with IBD.
In our previous study, the control cohort was formed exclusively by healthy donors [
15], whereas in the present study it was formed by individuals with confirmed no colorectal pathology but bearing symptoms, or with history of polyps or cancer. Thus we calculated a new cut-off of 460 ng/mL, higher than the value of 410 ng/mL reported for healthy donors [
15].
According to this cut-off, within the no colorectal pathology group, individuals with anaemia showed a substantially elevated positivity rate (71.4%) as expected from their mean levels. Non-IBD exhibited a low positivity rate (28.1%), whereas the IBD group reached 73.1%. This pathology is associated with at least a 5-fold increased risk for CRC, representing one of the highest risk groups based on the inflammation-dysplasia-carcinoma sequence [
22]. In compliance with this sequence, the sCD26 positivity rate increased from no colorectal pathology to hyperplastic polyps and non-advanced adenomas, with a further increase in advanced adenomas and CRC.
The capability of colorectal polyps to develop into cancer is related to the size of the lesion, the proportion of villous component and the grade of dysplasia. In relation to dysplasia, a morphological marker of neoplastic lesions, we observed a direct (positive) trend between the grade of dysplasia and the positivity of the biomarker, though there was no significant correlation between both parameters. Concerning advanced adenomas, a term commonly used to group adenomas that have an increased likelihood of malignant transformation, the sCD26 positivity resulted statistically significant.
Recent works also studied other potential markers in relation to polyp characteristics: for the serum sulfatase activity, differences regarding the number of adenomas (single or multiple) were significant [
23]; serum leptin, adiponectin and resistin also differed between controls and patients with adenomas or CRC, though there was no relationship with dysplasia, histopathology or polyp localization [
24].
In our experimental setting we have also evaluated the diagnostic parameters for the sCD26. At the 460 ng/mL cut-off, the sensitivity and specificity for CRC versus non-cancer groups were 81.8% and 72.3%, respectively. This specificity is measured in the framework of symptomatic and asymptomatic patients bearing intermediate benign pathologies including non-IBD and IBD as well as polyps. When considering only asymptomatic individuals specificity increases to 90%, which agrees with the results previously published by our group [
15]; nevertheless a decrease in sensitivity will be expected in this context.
In an asymptomatic high-risk cohort of individuals with familial history of CRC or personal history of CRC or adenoma, Hazazi
et al. [
25] suggested the use of a quantitative immunochemical FOBT (iFOBT) for screening of high-risk individuals. Excluding individuals with anaemia, rectal bleeding and IBD, they reported a sensitivity and specificity for CRC of 100% and 85.3%, respectively.
In the classical CRC screening studies in average-risk individuals, the gFOBT (guaiac-based) is extensively used, despite its wide range of sensitivity and specificity. The most common tests are the Hemoccult II
® (rehydrated or unrehydrated) and the Hemoccult SENSA
®, though the unrehydrated gFOBT is the one recommended for screening [
26]. For the unrehydrated Hemoccult II
®, the most accurate diagnostic parameters have been estimated with one-time testing on a Chinese [
27] and an American populations [
28]; in both, colonoscopy was performed to all individuals with positive or negative gFOBT results, reporting a sensitivity for CRC of 25 and 12.9%, and a specificity of 80 and 95.2%, respectively [
27,
28]. These studies reflect a poor sensitivity, although it is slightly improved with repeated annual or biennial testing (54-80%), reaching up to 97.7% specificity [
29,
30]. On the other hand, a 50% sensitivity was obtained for a one-time rehydrated testing combined with sigmoidoscopy, though no specificity was reported, perhaps owing to an increase in the number of false positives due to rehydration [
31].
When a highly sensitive test like the Hemoccult SENSA
® is used, a 71-79% sensitivity is reached with single testing, and about 85% with multiple testing, with corresponding specificities of 86% and 95% [
32,
33]. However, these parameters are probably overestimated as these studies lacked colonoscopic examination of the negative cases.
Besides guaic-based FOBT, iFOBT has been recently offered as an alternative for average-risk screening. The studies reported up to date have shown that the iFOBT is more adequate for screening because of its high specificity since it detects human globin [
26].
Throughout our study, evidence was gathered regarding the utility of the sCD26 in the detection of advanced adenomas. In separating CRC and advanced adenomas from all other groups, the sCD26 exhibited a 58.0% sensitivity and a 75.5% specificity. For the same pathologies (CRC and advanced adenomas) in an asymptomatic high-risk cohort, Hazazi
et al. [
25] reported for a quantitative iFOBT a sensitivity and specificity of 65.3% and 87.5%, respectively.
Regarding studies performed in average-risk individuals for the detection of CRC and advanced adenomas, gFOBT has shown a sensitivity of 10.8-14.3% [
27,
28] and a specificity of 79.2% [
27], and consequently is not recommended for the detection of advanced lesions [
4,
6]. On the other hand, iFOBT showed a sensitivity of 33.1% and a specificity of 97.5%, though these parameters were given for only distal advanced neoplasm as compared to flexible sigmoidoscopy [
25].
In relation to other experimental serum biomarkers for advanced adenomas, the CCSA-2 has shown a 97.3% sensitivity with a 78.4% specificity considering normal colonoscopy, hyperplastic polyps and non-advanced adenomas [
13], whereas for CCSA-3 and -4 a combined sensitivity of 91.3% and a specificity of 78.7% was reported [
14]. While sCD26 shows lower sensitivity, the specificity is equivalent even including patients with confounding pathologies such as IBD, which were not present in other studies.
Although sCD26 seems to perform adequately as a blood biomarker for CRC and advanced adenomas, independent of the frequent but intermittent bleeding unlike gFOBT or iFOBT, our study presents some limitations that should be considered: i) the symptomatic population included is at high-risk for CRC, with an elevated prevalence of colorectal pathology; ii) although no differences regarding age were detected, the age range of the patients differs from that recommended for screening; iii) the classification of the patients into the categories proposed resulted in several sub-groups with a small number of patients. Therefore, further research in a large population and under a screening context is desirable, along with the comparison to a sensitive gFOBT or iFOBT, and other experimental non-invasive methods.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
LD: measurement of the sCD26 levels, data analysis, statistical evaluation, manuscript preparation. AMRP: data analysis, statistical evaluation, manuscript preparation. FJRB: Study design, coordination of the study. OJC: data analysis, manuscript preparation. DMA: Patient recruitment, collection of samples and clinical data. MPD: Study design, coordination of the study, final revision of the manuscript. All authors read and approved the final manuscript.