High levels of vitamin D are hypothesized to reduce the risk of breast cancer. In order to be biologically active, dietary vitamin D must be converted to its biologically active form 1,25(OH)₂D₃. Cyp2r1 is a major vitamin D hydroxylase that catalyzes the first step of this activation producing 25(OH)D₃. Cyp2r1 is located within SuprMam1, a mammary tumour susceptibility locus identified in the BALB/c-Trp53+/- mouse model of spontaneous breast cancer (Blackburn et al, Am J Path, 2007). We have examined the vitamin D pathway in SM09 congenic mice, which contain the BALB/c SuprMam1 locus on a C57BL/6 background.

qPCR and western blotting for Cyp2r1 in tissues from SM09 and control mice revealed a significant 2-3-fold reduction in Cyp2r1 expression in mammary glands and liver (female but not male) of SM09 mice, however differences in plasma 25(OH)D_3, calcium or phosphate levels were not found. Instead, 3-fold higher levels of plasma parathyroid hormone (PTH), a major vitamin D / calcium regulator, were present in female (but not male) mice carrying the BALB/c allele of the SuprMam1 locus. Affymetrix expression profiling of mammary glands found differential expression of many genes of the vitamin D pathway, consistent with disruption of the pathway. Increasing dietary calcium or vitamin D returned PTH levels to normal in BALB/c and SM09 mice. We are currently characterizing several polymorphisms in the Cyp2r1 promoter which may alter promoter function.

Thus, chronically elevated PTH levels due to an interaction between low calcium / vitamin D intake and reduced Cyp2r1 expression from the BALB/c allele of Cyp2r1 may contribute to increased breast cancer susceptibility.  The SM09 congenic mice may serve as a valuable model for studying the role of gene-environment interactions of the vitamin D pathway in cancer and other diseases.

This work was supported by NHMRC, NBCF and Cancer Australia.