Variant ABO Blood Group Alleles, Secretor Status and Risk of Pancreatic Cancer: Results from the Pancreatic Cancer Cohort Consortium

Abstract

Background

Subjects with non-O ABO blood group alleles have increased risk of pancreatic cancer. Glycosyltransferase activity is greater for the A₁ versus A₂ variant, while O01 and O02 variants are nonfunctioning. We hypothesized: (1) A¹ allele would confer greater risk than A² allele, (2) protective effect of the O allele would be equivalent for O01 and O02 variants, (3) secretor phenotype would modify the association with risk.

Methods

We determined ABO variants and secretor phenotype from single nucleotide polymorphisms in ABO and FUT2 genes in 1533 cases and 1582 controls from 12 prospective cohort studies. Adjusted odds ratios (ORs) for pancreatic cancer were calculated using logistic regression.

Results

An increased risk was observed in participants with A¹, but not A² alleles. Compared to subjects with genotype O/O, genotypes A²/O, A²/A¹, A¹/O, and A¹/A¹ had ORs of 0.96 (95% confidence interval [CI], 0.72–1.26), 1.46 (95%CI, 0.98–2.17), 1.48 (95%CI, 1.23–1.78), and 1.71 (95%CI, 1.18–2.47). Risk was similar for O01 and O02 variant O alleles. Compared to O01/O01, the ORs for each additional allele of O02, A¹, and A² were 1.00 (95%CI, 0.87–1.14), 1.38 (95%CI, 1.20–1.58), and 0.96 (95%CI, 0.77–1.20); P-value, O01 versus O02=0.94, A¹ versus A²=0.004. Secretor phenotype was not an effect modifier (P-interaction=0.63).

Conclusions

Among participants in a large prospective cohort consortium, ABO allele subtypes corresponding to increased glycosyltransferase activity were associated with increased pancreatic cancer risk.

Impact

These data support the hypothesis that ABO glycosyltransferase activity influences pancreatic cancer risk, rather than actions of other nearby genes on chromosome 9q34.

INTRODUCTION

The ABO gene encodes a glycosyltransferase with three main alleles (A, B, and O), with different substrate specificities (1). The A, B and O glycosyltransferases transfer N-acetyl-D-galactosamine (GalNAc), D-galactose (Gal), and no sugar residue, respectively, to an oligosaccharide acceptor, known as the H histo-blood group antigen, which is expressed on the surface of red blood cells, endothelial cells, and epithelial cells, including the gastrointestinal mucosa (2).

Recent studies have demonstrated an increased risk of pancreatic cancer in individuals with non-O blood type (A, AB, and B) (3–5). In addition, a gene-dose effect has been noted, whereby each additional A or B allele is associated with a further increase in risk, compared to the O allele (6). In the current study, we examined variants in the ABO and FUT2 genes to further investigate a possible role for the ABO glycosyltransferase and ABO antigen expression in pancreatic cancer pathogenesis.

The A₁ glycosyltransferase is the predominant transferase underlying the A blood group. However, a variant transferase, known as A₂, is present in approximately 20% of White individuals. The A₂ phenotype is characterized by altered acceptor preference and a large reduction in transferase activity (7, 8). Therefore, the A² (or A201) allele results in an intermediate phenotype, with approximately 5-fold fewer and less complex A antigens on the cell surface, between the “full” enzymatic activity defined by the A¹ (or A101) allele and the non-functioning enzyme defined by the O allele. We hypothesized that the risk of pancreatic cancer in individuals with the A² allele would be intermediate between those with the A¹ allele and those with the O allele.

The O allele has two main variants, O01 and O02, also known as O¹ and O^1v, respectively. These two variants share the same single nucleotide deletion at position 261 of the ABO gene, but they are dissimilar at numerous other positions, represent different phylogenetic lineages, and are thought to have arisen independently at distinct time points in evolution (9, 10). Therefore, the truncated proteins encoded by the O01 and O02 alleles are functionally the same (i.e. non-functional transferases), but exist on unique genetic backgrounds. We hypothesized that the risk of pancreatic cancer would be the same for individuals carrying O01 and O02 alleles, based on the assumption that the ABO gene product, not the genetic background, was the main factor leading to the association of ABO polymorphisms with pancreatic cancer risk.

The secretor phenotype is defined by the FUT2 gene, a fucosyltransferase that catalyzes the addition of terminal α(1,2)fucose residues to produce the H antigen, an acceptor to which the ABO transferase adds its glycosyl groups (11). A functioning FUT2 enzyme allows for the secretion of ABO antigens into gastrointestinal secretions; however, homozygous inactivating mutations in FUT2 occur in approximately 20% of individuals (“non-secretors”) (12, 13). We hypothesized that the association between ABO blood group alleles and pancreatic cancer risk was modified by secretor status, such that the association was stronger among secretors.

To investigate our three main hypotheses, we utilized genotype data from over 3000 subjects participating in 12 prospective cohort studies involved in the PanScan genome wide association study (4, 6).

MATERIALS AND METHODS

Study Population

The Pancreatic Cancer Cohort Consortium (PanScan) genome wide association study (GWAS) has been described previously, in detail (4, 6). It includes nested case-control studies from 12 prospective cohorts: Alpha-Tocopherol Beta-Carotene Cancer Prevention Study (ATBC), CLUE II, American Cancer Society Cancer Prevention Study-II (CPS II); European Prospective Investigation into Cancer and Nutrition Study (EPIC); Health Professional's Follow-up Study (HPFS); New York University Women's Health Study (NYUWHS); Nurses' Health Study (NHS); Physicians' Health Study I (PHS I); Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial (PLCO); Shanghai Men's and Women's Health Study (SMWHS); Women's Health Initiative (WHI); and Women's Health Study (WHS). In each cohort, a defined population of subjects was followed prospectively with assessments of lifestyle factors and ascertainment of cancer diagnoses. Cases included subjects with incident primary pancreatic adenocarcinoma (ICD-O-3 code C250–C259 or C25.0–C25.3, C25.7–C25.9). All subjects with non-exocrine pancreatic tumors (C25.4, histology type, 8150, 8151, 8153, 8155, 8240, 8246) were excluded. Each cohort study selected participants with blood or buccal cells collected prior to cancer diagnosis. Incident pancreatic cancer cases identified by self-report, report of next-of-kin or through national death indices were typically confirmed by subsequent medical record review, linkage with a cancer registry, or both, without prior knowledge of genetic data.

One control was selected per case within each cohort. Controls were matched on year of birth (+/−5 years), gender, self-reported race/ethnicity, and source of DNA (peripheral blood or buccal cells). Controls were alive without pancreatic cancer on the incidence date of the matched case. Four cohorts (HPFS, NHS, PHS, and WHS) were additionally matched on smoking status (never, former, current), and some cohorts were also matched on age at baseline (+/−5 years), age at blood draw (+/−5 years), date/time of day of blood draw, or fasting status at blood draw. Each cohort obtained informed consent from study participants and approval from its Institutional Review Board (IRB). The Special Studies Institutional Review Board (SSIRB) of the National Cancer Institute approved the pooled PanScan study.

Assessment of ABO Blood Group Alleles

Detailed methods and quality control procedures for genotyping by PanScan can be found elsewhere (4). We utilized single nucleotide polymorphism (SNP) data collected in the PanScan GWAS using the Illumina 550 HumanHap assay to define blood group subtype alleles and secretor phenotype. If the causative SNP was not available as part of the GWAS, we used SNPs in high linkage disequilibrium with the causative SNP to mark the appropriate phenotype. As described previously, rs505922 marked the O allele and rs8176746 marked the B allele (6, 14). Additionally, we used rs8176704 to mark the A² allele, and rs574347 to mark the O02 allele. The SNP rs601338 (G>A) defines secretor status in Whites (11), and individuals with the homozygote A/A genotype are non-secretors. Because rs601338 is not genotyped by the Illumina 550 HumanHap array, we used the AC haplotype of rs602662 and rs281377 to infer secretor status; this haplotype is in perfect linkage disequilibrium (r²=1) with the rs601338 A allele. Of note, secretor status is recessive, such that two non-secretor alleles are necessary to manifest the non-secretor phenotype. Using haplotypes of the above SNPs for each participant, we determined ABO alleles (O01, O02, A¹, A², and B) and secretor status (secretor, non-secretor), as shown in Table 1. All haplotypes could be inferred with greater than 99% posterior probability for each subject using an EM algorithm to impute missing phase (14).

Table 1.

ABO blood group subtype alleles (A) and secretor status (B) among pancreatic cancer cases and nested controls in 12 prospective cohort studies^*

(A)	Cases No. (%)	Controls No. (%)	rs505922	rs8176746	rs8176704	rs574347
No. of participants	1533	1582
A blood group alleles	943 (31)	847 (27)
A1 alleles	729 (77)	611 (72)	C	C	G	T
A2 alleles	214 (23)	236 (28)	C	C	A	T
O blood group alleles†	1773 (58)	2018 (64)
O01 alleles	1108 (63)	1268 (63)	T	C	G	T
O02 alleles	643 (37)	733 (37)	T	C	G	C
B blood group alleles	350 (11)	299 (9)	C	A	G	T

(B)	Cases No. (%)	Controls No. (%)	rs601338
Secretor status‡
Secretor	1103 (81)	1145 (79)	G/G or G/A
Non-secretor	261 (19)	297 (21)	A/A

^*Single nucleotide polymorphism (SNP) data collected in the PanScan genome-wide association study were used to define blood group subtype alleles and secretor phenotype. As shown, the single nucleotide changes at rs505922, rs8176746, rs8176704, rs574347, and rs601338 were used to mark the relevant alleles.

^†The number of O01 and O02 alleles do not sum to the number of O alleles, as blood group O subtype could not be accurately imputed for thirty-nine O alleles.

^‡Secretor status is recessive, such that two non-secretor alleles are necessary to manifest the non-secretor phenotype. Secretor status was successfully imputed for 2,806 of the 2,840 White subjects.

Assessment of Covariates

Across all 12 participating cohorts, covariates were collected though written questionnaires or in-person interviews. Detailed descriptions of data collection methods have been published previously (6). We obtained data from each cohort on participants' age, gender, race/ethnicity (White, Asian, African, other), body-mass index (BMI) smoking status (current, past, never), and history of diabetes (yes, no).

Statistical Analyses

ABO blood group subtype alleles and secretor status were examined for cases and controls, and the distribution of blood type alleles in our study was compared with that seen in other comparable populations. We used unconditional logistic regression to calculate odds ratios (ORs) and 95% confidence intervals (CI) for pancreatic cancer by ABO blood group subtype alleles, adjusted for age, gender, race/ethnicity, cohort, smoking status, BMI, and history of diabetes. We repeated our analyses limited to White subjects. In addition, our previous studies (4, 6) demonstrated little change in our results after adjusting for five principal components of population substructure determined using the entire set of the nearly 550,000 SNPs with the EIGENSTRAT program (4, 15). We assessed effect-measure modification by secretor phenotype (secretor versus non-secretor); the test for modification was assessed by entering the cross-product of blood group (non-O versus O blood group) and secretor status into the model. We demonstrated previously the lack of heterogeneity across the 12 cohorts for the association between blood group and pancreatic cancer risk using Cochran's Q-statistic (P = 0.91 for the comparison of non-O blood group [i.e. A, AB or B] to O blood group across cohorts) (6, 16). All statistical analyses were performed using the SAS 9.1 statistical package (SAS Institute, Cary, North Carolina), and all P-values were two-sided.

RESULTS

From the 12 participating cohorts, 1533 pancreatic cancer cases and 1582 controls were available for analysis. Clinical characteristics of the cases and controls were described previously (6). The frequency distributions of ABO subtype alleles and secretor status were highly similar among our control participants and subjects in previous studies (11, 17–20). Among controls, 72% of A alleles were A¹ and 28% were A², 63% of O alleles were O01 and 37% were O02, and 79% were secretors and 21% were non-secretors (Table 1).

To address the hypothesis that inheritance of an A¹ allele would confer a greater risk of pancreatic cancer than an A² allele, we estimated odds ratios for combinations of O, A¹, and A² alleles, compared to O/O as the referent (Table 2). Compared to subjects with genotype O/O, those with genotype A²/O had OR of 0.96 (95% confidence interval [CI], 0.72–1.26), genotype A²/A¹ had OR 1.46 (95% CI, 0.98–2.17), genotype A¹/O had OR 1.48 (95% CI, 1.23–1.78), and genotype A¹/A¹ had OR 1.71 (95% CI, 1.18–2.47). Only 12 cases and 8 controls inherited an A²/A² genotype, limiting our ability to accurately assess risk for subjects with this genotype. These data suggested that risk associated with inheritance of an A² allele was less than that of inheriting an A¹ allele and similar to that of inheriting an O allele. We evaluated this further by classifying cases and controls by the number of inherited A¹ and A² alleles (0, 1 or 2) (Table 3). For inheritance of each additional A¹ allele, the OR for pancreatic cancer was 1.38 (95% CI, 1.20–1.58). In contrast, for inheritance of each additional A² allele, the OR for pancreatic cancer was 0.96 (0.77–1.20; P-value for comparison=0.004), again suggesting that the risk for pancreatic cancer with inheritance of an A² allele was less than that with inheritance of an A¹ allele.

Table 2.

Age-adjusted and multivariable-adjusted odds ratios (95% confidence intervals) for incident pancreatic cancer by blood group A subtype alleles

Second Allele		First Allele
		O	A1	A2
O	No. of cases / controls	511 / 657	441 / 381	113 / 145
	Age-adjusted OR	1.0	1.48 (1.24–1.77)	1.00 (0.76–1.31)
	Multivariable-adjusted OR*	1.0	1.48 (1.23–1.78)	0.96 (0.72–1.26)
	Multivariable-adjusted OR†	1.0	1.47 (1.22–1.78)	0.94 (0.71–1.25)
A1	No. of cases / controls	-	74 / 57	60 / 51
	Age-adjusted OR		1.68 (1.17–2.42)	1.48 (1.00–2.19)
	Multivariable-adjusted OR*		1.71 (1.18–2.47)	1.46 (0.98–2.17)
	Multivariable-adjusted OR†		1.89 (1.27–2.81)	1.46 (0.97–2.18)
A2	No. of cases / controls	-	-	12 / 8
	Age-adjusted OR			1.92 (0.78–4.74)
	Multivariable-adjusted OR*			1.89 (0.76–4.73)
	Multivariable-adjusted OR†			1.90 (0.76–4.76)

Abbreviations: OR, odds ratio (95% confidence interval)

^*Multivariable adjustment by age, gender, race/ethnicity, cohort, smoking status, body-mass index, and history of diabetes mellitus, in the entire study population.

^†Multivariable adjustment by age, gender, cohort, smoking status, body-mass index, and history of diabetes mellitus, in White participants only.

Table 3.

Age-adjusted and multivariable-adjusted odds ratios (95% confidence intervals) for incident pancreatic cancer by number of blood group subtype alleles

Allele		OR for each additional allele (0, 1, 2)	P-value (O01 vs. O02)	P-value (A1 vs. A2)
O01	Age-adjusted OR	1.0	0.84	-
	Multivariable-adjusted OR*	1.0	0.94
	Multivariable-adjusted OR†	1.0	0.72
O02	Age-adjusted OR	1.01 (0.89–1.16)	-	-
	Multivariable-adjusted OR*	1.00 (0.87–1.14)
	Multivariable-adjusted OR†	1.03 (0.89–1.18)
A1	Age-adjusted OR	1.37 (1.20–1.58)	-	0.009
	Multivariable-adjusted OR*	1.38 (1.20–1.58)		0.004
	Multivariable-adjusted OR†	1.42 (1.23–1.64)		0.003
A2	Age-adjusted OR	1.00 (0.80–1.24)	-	-
	Multivariable-adjusted OR*	0.96 (0.77–1.20)
	Multivariable-adjusted OR†	0.96 (0.77–1.20)

Abbreviations: OR, odds ratio (95% confidence interval)

^*Multivariable adjustment by age, gender, race/ethnicity, cohort, smoking status, body-mass index, and history of diabetes mellitus, in the entire study population.

^†Multivariable adjustment by age, gender, cohort, smoking status, body-mass index, and history of diabetes mellitus, in White participants only.

To address the hypothesis that inheritance of an O01 allele would confer a similar protective effect to inheriting an O02 allele, we estimated odds ratios for combinations of O01 and O02 alleles, compared to non-O blood groups (A, AB, and B) as the referent (Table 4). For each combination of O allele subtypes, i.e. O01/O01, O01/O02, and O02/O02, ORs were highly similar to each other and to the OR for the O/O genotype. We evaluated this further by classifying cases and controls by the number of inherited O01 and O02 alleles (0, 1 or 2) (Table 3). With O01/O01 as the referent, the inheritance of each additional O02 allele resulted in an OR for pancreatic cancer of 1.00 (95% CI, 0.87–1.14; P-value for comparison=0.94), indicating a similar protective effect for inheritance of either O01 or O02 alleles.

Table 4.

Age-adjusted and multivariable-adjusted odds ratios (95% confidence intervals) for incident pancreatic cancer by blood group O subtype alleles

O Alleles		Non-O Blood Group (A, AB, and B)	O Blood Group
O / O	No. of cases / controls	1016 / 918	497 / 648
	Age-adjusted OR	1.0	0.69 (0.60–0.80)
	Multivariable-adjusted OR*	1.0	0.69 (0.60–0.81)
	Multivariable-adjusted OR†	1.0	0.70 (0.60–0.82)
O01 / O01	No. of cases / controls	1016 / 918	192 / 250
	Age-adjusted OR	1.0	0.70 (0.56–0.86)
	Multivariable-adjusted OR*	1.0	0.71 (0.57–0.88)
	Multivariable-adjusted OR†	1.0	0.72 (0.58–0.90)
O01 / O02	No. of cases / controls	1016 / 918	240 / 303
	Age-adjusted OR	1.0	0.71 (0.59–0.87)
	Multivariable-adjusted OR*	1.0	0.71 (0.58–0.86)
	Multivariable-adjusted OR†	1.0	0.76 (0.62–0.93)
O02 / O02	No. of cases / controls	1016 / 918	65 / 95
	Age-adjusted OR	1.0	0.62 (0.45–0.86)
	Multivariable-adjusted OR*	1.0	0.60 (0.43–0.84)
	Multivariable-adjusted OR†	1.0	0.65 (0.46–0.93)

Abbreviations: OR, odds ratio (95% confidence interval)

^*Multivariable adjustment by age, gender, race/ethnicity, cohort, smoking status, body-mass index, and history of diabetes mellitus, in the entire study population.

^†Multivariable adjustment by age, gender, cohort, smoking status, body-mass index, and history of diabetes mellitus, in White participants only.

To address the hypothesis that secretor status is an effect modifier of ABO blood type and pancreatic cancer risk, we estimated odds ratios for participants with secretor/O blood type, non-secretor/non-O blood type, and secretor/non-O blood type, compared to non-secretor/O blood type as the referent in Whites (Table 5). An increased risk of pancreatic cancer was observed in participants with non-O blood type, irrespective of secretor status (OR 1.33, 95% CI, 0.94–1.87, among non-secretors; OR 1.51, 95% CI, 1.13–2.01, among secretors; P-interaction=0.63).

Table 5.

Age-adjusted and multivariable-adjusted odds ratios (95% confidence intervals) for incident pancreatic cancer by ABO blood group and secretor status in Whites

Secretor Status	O Blood Group	Non-O Blood Group	P-interaction
Non-secretor
No. of cases / controls	100 / 134	161 / 163
Age-adjusted OR	1.0	1.33 (0.95–1.86)	0.63
Multivariable-adjusted OR†	1.0	1.33 (0.94–1.87)	0.63
Secretor
No. of cases / controls	354 / 468	749 / 677
Age-adjusted OR	1.02 (0.76–1.36)	1.48 (1.12–1.96)
Multivariable-adjusted OR†	1.03 (0.77–1.39)	1.51 (1.13–2.01)

Abbreviations: OR, odds ratio (95% confidence interval)

^†Multivariable adjustment by age, gender, cohort, smoking status, body-mass index, and history of diabetes mellitus.

DISCUSSION

We investigated three hypotheses related to ABO glycosyltransferase activity and pancreatic cancer risk among more than 3,000 cases and controls from 12 large prospective cohort studies. First, the association of cancer risk with the A allele appears predominantly due to the A₁ glycosyltransferase, which has greater enzymatic activity than the A₂ glycosyltransferase. In our analysis, the less active A₂ glycosyltransferase did not increase risk of pancreatic cancer in comparison to the nonfunctioning O glycosyltransferase, despite giving rise to blood group A. Second, the protective effect of the O allele was essentially identical for the two main O allele variants, which share the same point mutation that abolishes enzyme activity but otherwise exist on distinct genetic backgrounds. Third, the association of pancreatic cancer risk with blood group was not statistically significantly modified by secretor status.

Older studies examining serologically determined ABO blood group and pancreatic cancer risk were inconsistent (21–25). Recently, epidemiologic studies and the PanScan GWAS demonstrated an increased risk of pancreatic cancer in subjects with non-O blood group (A, AB, and B), compared to those with blood group O (3–5). Of note, the most statistically significant SNPs identified in the PanScan GWAS were located in the first intron of the ABO gene, in high linkage disequilibrium with the single base deletion that defines blood group O (4). A follow-up study noted a remarkably consistent increase in risk for non-O/O genotypes across 12 separate prospective cohorts and a gene-dose effect, whereby each additional non-O allele further increased a subject's risk for pancreatic cancer (6).

The nucleotide sequence of the ABO gene was elucidated in 1990 (26, 27), and numerous allelic variants have been identified since that time (8). The human ancestral ABO gene is thought to have encoded the A₁ glycosyltransferase, with accumulation of subsequent nucleotide alterations in this consensus sequence leading to generation of the A², B, O01, and O02 alleles (10, 28). The A² allele is characterized by a single-base deletion in a stretch of three consecutive cytosine residues at nucleotides 1059 to 1061. This deletion shifts the open reading frame of ABO, resulting in 21 extra amino acids in the C-terminal domain and a protein with 30–50 fold lower transferase activity than the parent A₁ glycosyltransferase (7).

Actual biologic consequences have been suggested previously in humans for the reduced enzymatic activity of the A₂ glycosyltransferase, most clearly in relation to circulating levels of von Willebrand factor (vWF) and the risk of venous thromboembolism (VTE). The ABO glycosyltransferase attaches sugar residues to vWF, which is subsequently secreted into the circulation. In subjects with blood group O, vWF is cleared more quickly (29, 30), resulting in approximately 25% lower levels of circulating vWF than in subjects with blood groups A or B (31) and a lower risk of VTE (32, 33). The A₂ glycosyltransferase loads a lesser amount of A antigen onto vWF, compared with the A₁ transferase (34), and the A² allele is associated with lower circulating vWF levels than the A¹ or B alleles. Furthermore, the risk of VTE with the A² allele is comparable to that with the O allele, and reduced in comparison to the A¹ and B alleles, which code for more active transferases (32, 33). Therefore, the differing glycosyltransferase activity resulting from the A¹ and A² alleles is biologically relevant to a disease outcome (i.e., VTE) and appears to be directly related to the efficiency of glcosyl group transfer to a target molecule (i.e., vWF). Similarly, we noted a statistically significant increase in the risk of pancreatic cancer in subjects with the A¹ allele, but not with the A² allele, which we believe further implicates ABO glycosyltransferase activity in the pathogenesis of pancreatic cancer.

The O allele is characterized by a single base deletion in the ABO gene at nucleotide 261. This deletion shifts the open reading frame of ABO, resulting in a premature stop codon and a truncated, non-functional protein (35). The O allele has two main variants, O01 and O02, which share the 261 deletion. However, they are dissimilar at numerous other positions in exonic, intronic, and upstream and downstream regulatory regions (35–37). In fact, phylogenetic studies have suggested that the O01 and O02 alleles have distinct genetic backgrounds and acquired the 261 deletion as separate mutational events (10, 38). In the current study, the protective effect of the O allele on pancreatic cancer risk was nearly identical for the O01 and O02 alleles, further supporting ABO glycosyltransferase activity, as opposed to genetic background, as the predisposing factor for pancreatic cancer risk.

The secretor phenotype is defined by the FUT2 gene, a fucosyltransferase that catalyzes the addition of terminal α(1,2)fucose residues to produce the H antigen, the primary precursor to which the ABO enzyme adds its glycosyl groups. Across diverse populations, approximately 20% of individuals have homozygous loss of function mutations in FUT2 due to creation of premature stop codons (11). These individuals have no ABO antigens in their gastrointestinal secretions, and such “non-secretors” have reduced susceptibility to multiple pathogens, including noroviruses (39), Helicobacter pylori (40), and Campylobacter jejuni (41), due to decreased pathogen adherence to the mucous layer and epithelial cells lining the gastrointestinal tract (42). Furthermore, serum levels of vitamin B₁₂ have been associated with secretor status, possibly as a consequence of susceptibility to chronic infection by H. pylori (43, 44). We hypothesized that secretor status might also modify the association of non-O blood groups with an increased risk of pancreatic cancer. Although a slightly higher risk was noted among secretors, this did not approach statistical significance. Interestingly, data are inconsistent for effect modification of the association between non-O blood groups and risk of VTE by secretor status, and this remains an area of continued investigation (45, 46).

These data suggest that ABO glycosyltransferase activity may play an important role in pancreatic tumorigenesis. Nevertheless, the underlying mechanisms linking the ABO glycosyltransferase with pancreatic cancer risk are not currently clear. One obvious hypothesis would be that the enzymatic activity of the ABO transferase impacts the processing and clearance of molecules that promote tumorigenesis. This would be analogous to the role of the ABO glycosyltransferase in determining levels of circulating vWF and risk of VTE. Interestingly, in five recent studies, SNPs at the ABO gene locus were found to be genetic determinants of circulating levels of soluble E-selectin, soluble P-selectin, soluble intercellular adhesion molecule-1 (ICAM-1), and tumor necrosis factor-alpha (TNF-α) (18, 47–50). In each of these studies, the most statistically significant SNPs were proxies for defining the O versus non-O allele. These molecules are important mediators of chronic inflammation and immune cell recruitment, and suggest interesting pathways for interrogation, particularly in light of the importance of stroma and tumor-host interactions in pancreatic cancer development (51).

Our study has several possible limitations. Using genetic data from a previously completed GWAS, we determined ABO alleles and secretor status using SNPs in high linkage disequilibrium with the causative nucleotide changes. Therefore, some degree of exposure misclassification remains a possibility. However, we have demonstrated previously that ABO genotypes determined in this manner are highly accurate (6). In addition, the genetic map of the ABO locus has been investigated for over 20 years, and numerous previous studies have genetically determined ABO alleles with high accuracy using a variety of methodologies (17, 19, 20). Moreover, any resultant misclassification due to measurement error is likely to be non-differential in nature, and therefore attenuate, rather than exaggerate our findings.

Our study population was composed primarily of White participants, which somewhat limits the generalizability of our results. However, other risk factors for pancreatic cancer do not appear to differ substantially by race/ethnicity, and the associations did not differ materially between the White and non-White subjects in this study. Nonetheless, further investigations that include more diverse study populations are warranted. Our power was somewhat limited to detect a statistically significant difference in the association between ABO blood group and pancreatic cancer risk among secretors compared to non-secretors, given that only 20% of subjects were non-secretors. Further analyses may be warranted of secretor status and pancreatic cancer risk in larger patient populations.

Our study has several notable strengths. The Pancreatic Cancer Cohort Consortium provided a large number of pancreatic cancer cases from 12 cohort studies, and the prospective design of these cohorts minimized the potential for survival or selection biases. The risk of detecting a false association due to population stratification was relatively low, given the inclusion of prospective cohorts with homogeneous ethnic compositions, the primarily non-Hispanic European ancestry of the full study population, and the paucity of evidence for variation in pancreatic cancer risk in the ancestral population (52, 53). In addition, we demonstrated previously that our results did not change after adjusting for potential population stratification bias by including the top 5 principal components of genetic variation as covariates in our logistic regression models (4, 6).

There remains only a limited understanding of the genetic determinants and initiating molecular events for pancreatic cancer. Our results suggest that the ABO glycosyltransferases may play an important role in pancreatic tumorigenesis. Further work is necessary to better understand the mechanisms which may underlie the association of ABO glycosyltransferase activity and resultant ABO carbohydrate phenotype with pancreatic cancer risk. The use of model systems of pancreatic cancer to further this work deserves particular consideration.