Measurement of vitamin D levels in inflammatory bowel disease patients reveals a subset of Crohn's disease patients with elevated 1,25-dihydroxyvitamin D and low bone mineral density

Abstract

Objectives: Many patients with Crohn's disease (CD) have low bone mineral density (BMD) that may not be solely attributable to glucocorticoid use. We hypothesised that low BMD in patients with CD is associated with elevated circulating levels of the active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)(2)D). We further hypothesised that this was secondary to increased synthesis of 1,25(OH)(2)D by inflammatory cells in the intestine. The aim of this study was to examine the relationship between 1,25(OH)(2)D levels and BMD in patients with CD.

Methods: An IRB approved retrospective review of medical records from patients with CD (n = 138) or ulcerative colitis (UC, n = 29). Measurements of vitamin D metabolites and immunoreactive parathyroid hormone (iPTH) were carried out. BMD results were available for 88 CD and 20 UC patients. Immunohistochemistry or real time reverse transcription-polymerase chain reaction (RT-PCR) for the enzyme 1alpha-hydroxylase was performed on colonic biopsies from patients with CD (14) or UC (12) and normal colons (4).

Results: Inappropriately high levels of serum 1,25(OH)(2)D (>60 pg/ml) were observed in 42% of patients with CD compared with only 7% in UC, despite no differences in mean iPTH. Serum 1,25(OH)(2)D levels were higher in CD (57 pg/ml) versus UC (41 pg/ml) (p = 0.0001). In patients with CD, there was a negative correlation between 1,25(OH)(2)D levels and lumbar BMD (r = -0.301, p = 0.005) independent of therapeutic glucocorticoid use. 1,25(OH)(2)D levels also correlated with CD activity. Lastly, immunohistochemistry and RT-PCR demonstrated increased expression of intestinal 1alpha-hydroxylase in patients with CD.

Conclusions: These data demonstrate that elevated 1,25(OH)(2)D is more common in CD than previously appreciated and is independently associated with low bone mineral density. The source of the active vitamin D may be the inflamed intestine. Treatment of the underlying inflammation may improve metabolic bone disease in this subgroup of patients.

Figure 1

Crohn’s disease (CD) patients have lower bone mineral density than ulcerative colitis (UC) patients. Bone mineral density, determined as age matched Z scores, was assessed in patients with CD or UC. Median scores are shown for the lumbar spine (n = 20 for UC; n = 96 for CD) and the left hip (n = 18 for UC; n = 94 for CD) with standard error bars. p values show statistically significant differences in CD values compared with equivalent UC analyses.

Figure 2

Circulating levels of 1,25-dihydroxyvitamin D (1,25(OH)₂D) are significantly higher in patients with Crohn’s disease (CD) compared with ulcerative colitis (UC). The normal range of 1,25(OH)₂D is indicated by the shaded area (15–60 pg/ml) and mean 1,25(OH)₂D levels are indicated by the horizontal lines (57.77 pg/ml for CD and 41.3 pg/ml for UC; p = 0.0001). Overall, 42% of patients with CD had elevated 1,25(OH)₂D compared with only 7% of UC patients.

Figure 3

Circulating levels of 1,25-dihydroxyvitamin D (1,25(OH)₂D) are inversely correlated with bone mineral density in patients with Crohn’s disease. The correlation between lumbar Z scores and 1,25(OH)₂D levels is shown. The r value indicates Spearman’s correlation coefficient.

Figure 4

Expression of 1α-hydroxylase in normal colon and Crohn’s disease. (A) Positive control demonstrating characteristic anti-1α-hydroxylase staining in human renal tubular epithelial cells (left, 250× magnification). Non-specificity control of human kidney stained with antiserum preincubated with immunising peptide (right, 250× magnification). (B–D) Immunostaining for 1α-hydroxylase in colon from a normal subject (B) and subjects with quiescent (C) and active Crohn’s disease (D), respectively (350× magnification). (E, F) Comparison of immunostaining of 1α-hydroxylase in granulomas from the colon of a patient with Crohn’s disease and elevated serum 1,25-dihydroxyvitamin D (E) and the skin of a patient with active sarcoidosis (F) (350× magnification).

Figure 5

Expression of 1α-hydroxylase and vitamin D receptor (VDR) in normal, Crohn’s disease (CD), and ulcerative colitis (UC) colons. 1α-Hydroxylase (A) and VDR (B) mRNA expression was assessed by quantitative real time polymerase chain reaction. These data represent an average of 10 mucosal biopsy samples each from patients with CD or UC.

Figure 6

Model of elevated 1,25-dihydroxyvitamin D (1,25(OH)₂D) in patients with Crohn’s disease. The inflamed intestine in Crohn’s disease is characterised by an infiltrate of mononuclear cells, including antigen presenting cells (APC) and macrophages (mΦ). These cells express the 1α-hydroxylase enzyme resulting in conversion of 25-hydroxyvitamin D to 1,25(OH)₂D. T cells and APC/mΦ express vitamin D receptors (VDR). Thus 1,25(OH)₂D may have both paracrine and autocrine effects to suppress Th1-type differentiation. Excess local production of 1,25(OH)₂D may lead to spillage of the hormone into the systemic circulation and distant osteoclast activation and bone resorption. IFN-γ, interferon γ; IL-2, interleukin 2.