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Lower prediagnostic circulating 25‐hydroxyvitamin D (25[OH]D)—considered the best marker of total vitamin D exposure—is associated with higher mortality risk among colorectal cancer (CRC) patients. However, it is unknown whether this association differs by the vitamin D‐binding protein (GC) isoform Gc2 (encoded by GC rs4588*C>A, Thr436Lys), which may substantially affect vitamin D metabolism and modify associations of 25(OH)D with colorectal neoplasm risk. Prediagnostic 25(OH)D‐mortality associations according to Gc2 isoform were estimated using multivariable Cox proportional hazards regression among 1281 CRC cases (635 deaths, 483 from CRC) from two large prospective cohorts conducted in the United States (Cancer Prevention Study‐II) and Europe (European Prospective Investigation into Cancer and Nutrition). 25(OH)D measurements were calibrated to a single assay, season standardized, and categorized using Institute of Medicine recommendations (deficient [<30], insufficient [30 ‐ <50], sufficient [≥50 nmol/L]). In the pooled analysis, multivariable‐adjusted hazard ratios (HRs) for CRC‐specific mortality associated with deficient relative to sufficient 25(OH)D concentrations were 2.24 (95% CI 1.44‐3.49) among cases with the Gc2 isoform, and 0.94 (95% CI 0.68‐1.22) among cases without Gc2 (P interaction = .0002). The corresponding HRs for all‐cause mortality were 1.80 (95% CI 1.24‐2.60) among those with Gc2, and 1.12 (95% CI 0.84‐1.51) among those without Gc2 (P interaction = .004). Our findings suggest that the association of prediagnostic vitamin D status with mortality among CRC patients may differ by functional GC isoforms, and patients who inherit the Gc2 isoform (GC rs4588*A) may particularly benefit from higher circulating 25(OH)D for improved CRC prognosis.

Background: Higher circulating 25-hydroxyvitamin-D [25(OH)D] concentrations are consistently inversely associated with colorectal cancer (CRC) risk in observational studies. However, it is unknown whether this association depends on the functional GC-rs4588*A (Thr436Lys) variant encoding the vitamin D-binding protein-2 (DBP2) isoform, which may affect vitamin D status and bioavailability. Methods: We analyzed data from 1710 incident CRC cases and 1649 incidence-density-matched controls nested within three prospective cohorts of mostly Caucasians. Study-specific incidence rate ratios (RRs) for associations of prediagnostic, seasonstandardized 25(OH)D concentrations according to DBP2 isoform with CRC were estimated using multivariable unconditional logistic regression and were pooled using fixed-effects models. All statistical significance tests were two-sided. Results: The odds of having 25(OH)D concentrations less than 50 nmol/L (considered insufficient by the Institute of Medicine) were 43% higher for each DBP2-encoding variant (rs4588*A) inherited (per DBP2 odds ratio [OR] = 1.43, 95% confidence interval [CI] = 1.27 to 1.62, P-trend = 1.2 x 10(-8)). The association of 25(OH)D concentrations with CRC risk differed by DBP2: 25(OH)D concentrations considered sufficient (> 50 nmol/L), relative to deficient (< 30 nmol/L), were associated with a 53% lower CRC risk among individuals with the DBP2 isoform (RR = 0.47, 95% CI = 0.33 to 0.67), but with a non-statistically significant 12% lower risk among individuals without it (RR = 0.88, 95% CI = 0.61 to 1.27) (P-heterogeneity = .01). Conclusions: Our results suggest that the 25(OH)D-CRC association may differ by DBP isoform, and those with a DBP2-encoding genotype linked to vitamin D insufficiency may particularly benefit from adequate 25(OH)D for CRC prevention.