Stomach-specific Biomarkers (GastroPanel) Can Predict the Development of Gastric Cancer in a Caucasian Population: A Longitudinal Nested Case-Control Study in Siberia

Discussion

Screening for GC remains under debate outside East Asian countries. Non-invasive tests including PGI, PGII, G-17 and HP Ab biomarkers are attractive (6, 11-14), but still further confirmatory studies are required before biomarker tests (e.g. GastroPanel) can be recommended for implementation in population-based GC screening programs (24). Most importantly, the encouraging results reported from Asia on the value of PG testing as a predictor of GC (16, 17) have not been reproduced in a Caucasian population to date. The present study is the first to provide such evidence, by demonstrating that testing for PGI, PGII and PGI/PGII ratio is helpful in predicting the development of incident GC in a Caucasian population, living in the high-risk territory for GC (18).

In the present longitudinal matched case-control setting, we provided evidence that the baseline PGI and PGI/PGII levels below the agreed cut-off values (6, 11-15) were associated with a significantly increased risk of developing GC among 45-69-year-old individuals prospectively followed-up of for 7 to 10 years (2003-2012). In conditional logistic regression, the OR for developing GC was 2.9 (95%CI=1.3-6.4) for decreased PGI and OR=3.3 (95%CI=1.5-7.3) for PGI/PGII ratio (Table III). These findings are consistent with the results obtained in similar type of follow-up studies originating from Japan (17, 19).

Accordingly, Yanaoka et al. conducted a 10-year follow-up of 5,209 asymptomatic middle-aged Japanese subjects, showing that the risk of incident GC increased in the presence of elevated HP antibodies (HR=3.48, 95% CI=1.26-9.64) and below cut-off levels of PGI (HR=3.54, 95% CI=1.95-6.40) or PGI/PGII ratio (HR=4.25, 95% CI=2.47-7.32) (17). In a more recent cohort study in rural villages of the Kyoto prefecture, the baseline examination of 2,859 subjects included testing of HP Ab and PGI and PGII levels (19). During a 10-year follow-up, a multivariate proportional hazards (Cox) regression model, adjusted for age and sex, HR for GC was 4.2 (95% CI=0.96-18.4) for HP Ab even without mucosal atrophy. The risk increased markedly for cases with HP Ab was associated with AG, HR=11.2; 95%CI=2.71-46.51. The subjects with AG but without elevated HP Ab showed the highest risk for incident GC: HR=14.81; 95%CI=2.47-88.8 (19). The present study is fully comparable with the Japanese studies regarding its cohort size (n=9,360) as well as the number of incident GC cases diagnosed during the follow-up that extends to an almost 10-year period (2003-2012) (18).

Despite the fact that PG testing has been considered a useful non-invasive test in identification of populations at-risk for developing GC (6,12-14) either by Asian-Pacific (15, 20) or European guidelines (11, 21-22) biomarker testing is not yet implemented in organized screening for GC in any geographic region (23). So far, the key limiting step has been the low sensitivity of PGs in GC prediction (24). Although PGI and PGI/PGII levels are usually clearly decreased in subjects who were diagnosed with GC during the follow-up compared to the group of controls, only the minority of GC cases usually present with decreased levels of these biomarkers at baseline, i.e., 34.6% had decreased PGI levels, and 39.2% showed below-cut-off PGI/PGII levels (24). Therefore, if used as a diagnostic test in population-based screening, there is a danger that a substantial proportion of individuals at increased GC risk would be missed (23, 24).

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Figure 1.

Cases and controls classified according to their Atrophy Index (AI).

In contrast to the two Japanese studies (17,19), the present analysis did not disclose any longitudinal predictive value of GC for HP Ab-positivity, other than a non-significant trend of the GC group being HP-infected less frequently than the controls. The failure to establish HP Ab status as a predictive factor for incident GC is most likely due to the fact that the prevalence of the HP-infection was very high in both study groups, i.e., 80.0% in the GC group and 90.1% in the control group, being substantially higher than even in the high-risk GC areas in Asia (25-27).

It is well-established that in the late stages of AG development, HP-infection may disappear spontaneously (6-8, 12, 14). This does not, however, decrease the risk of GC, but vice versa, this risk is even increased, making these people the group at the highest risk of all. Indeed, this was convincingly demonstrated in the present study as well (Table IV). When study subjects were stratified by the ABCD classification (10) on the basis of their baseline samples, 19.4% of the subjects in the GC group fell into Group D (low PG levels but testing HP Ab-negative). This is in perfect alignment with the results from Japan, confirming Group D subjects being the group at the highest risk for incident GC (28). In the present study, however, the data were missing whether the subjects in Group D have undergone previous HP eradication therapy, which might cause some inherent bias in their stratification between Group C and D (23).

The full GastroPanel test includes G-17 as the fourth biomarker to complement the panel of PGI, PGII and HP Ab (13). G-17 is a well-established biomarker of the G-cells in the gastric antrum, providing information e.g. on mucosal atrophy in that topographic location (11). Used in combination, the 4-biomarker panel provides comprehensive information on the stomach health and disease (both structure and function) (6, 11-14). Compared to PGI and PGII, the physiological regulation of G-17 is more complex, however. As recently pointed out, low levels of G-17 are not exclusively inherent to antral AG, but may also reflect high gastric acid output (13). On the other way round, G-17 is up-regulated (through a negative feedback loop) by a low acid content of the corpus, caused by either (i) AG of the corpus, or (ii) more frequently, by a prolonged use of proton pump inhibitor (PPI) medication (11, 13, 14). Consequently, any biomarker being regulated by more than one trigger cannot be a highly accurate indicator of only one of these. In the case of fasting G-17, the below-cut-off values can be due to either AG of the antrum or high acid output of the corpus (11, 13). Accordingly, a distinction between antral AG and high acid output as the cause of low (fasting) G-17 levels should always be made in GastroPanel examination by measuring the G-17 levels after a protein-rich meal stimulation (11, 13, 29). Failure to increase G-17 output after such a stimulation is a specific indicator of antral AG (11, 13). By definition, however, antral AG shall be diagnosed only in the presence of HP infection, while in HP-negative subjects, even the fasting G-17 output below cut-off level is considered as an indicator of high acid output of the corpus (11, 13).

In the present study, only the levels of basal G-17 (G-17b) were measured (18, 23), precluding the possibility for making the distinction between antral AG and high acid output as the cause of low G-17b levels. Not unexpectedly then, we failed to reveal any significant difference in G-17b levels between GC and the control groups in their baseline samples (Table II). The same was true with the frequency of the below-cut-off levels in the two study groups, being more frequent (19.6%) in the GC than (11.5%) in the control group (NS) (Table III). This situation could not be amended by using different cut-off values (2, 3 or 5 pmol/l) for G-17b, that were also tested for this.

In accordance with the above-mentioned facts on the complex physiology of G-17, several previous studies have reported low sensitivity for G-17b as a marker of antral AG. In a Caucasian population from Latvia and Lithuania, the sensitivity of G-17b to diagnose AG in the antrum was only 15.4%, but increased to 30.8% following protein stimulation (G-17s) (29). A recent study from Japan (30) suggested that adding G-17b to PG testing is a valuable approach in discriminating between multifocal AG and other types of AG, which is not possible with PG testing alone. However, also in their study, G-17b levels below 1 pmol/l were present only in 15.6% of the GC patients, which did not differ from the non-GC group (30). As recently pointed out, however, the decline of G-17b output due to antral AG is a gradual process where values below the cut-off are reached only in moderate-severe AG, and because of this, G-17b levels can remain within normal range during several years preceding the GC diagnosis (13, 15).

Finally, we also tested a novel atrophy index (AI), calculated as a sum of any abnormal values of PGI, PGI/PGII ratio and G-17. In the group with most severe atrophy (AI 3; all biomarkers below cut-off), the vast majority of subjects (77.8%) were GC patients, and only a minority were healthy controls. This suggests a “dose-dependent” relationship between AG and GC risk, also illustrated in Figure 1, where AI score is directly related to GC and inversely related to healthy stomach.

Alltogether, the present study is the first to demonstrate that low (below cut-off) PGI and PGI/PGII serum levels could serve as a predictive marker for incident GC in a Caucasian population as well, followed-up for almost 10 years. Because PGs are biomarkers of gastric mucosal atrophy, timely identification of AG by these biomarkers (GastroPanel) may allow for adequate preventive measures, e.g. H. pylori eradication and endoscopic surveillance to the subjects at increased risk who would clearly benefit by such interventions. Failure to accurately distinguish between antral AG and high acid output as the cause of low G-17 levels (the 4th biomarker of GastroPanel) in this study, precludes the assessment of this marker as a longitudinal predictor of GC.