Cyp2r1, located at 11p15.2, is the principal hepatic 25-hydroxylase. It is crucial in the vitamin D metabolic pathway, responsible for the hydroxylation of parent vitamin D to 25-hydroxyvitamin D [25(OH)D], which reflects vitamin D status as the major circulating metabolite of vitamin D [1,2,3].

In Cyp2r1-/-knockout mice, serum 25(OH)D levels were reduced by more than 50% compared to wild-type mice, indicating its essential role in maintaining normal vitamin D metabolite levels [1]. In humans, inactivating mutations in CYP2R1 can lead to a novel form of vitamin D-deficiency rickets (classified as vitamin D dependent rickets type 1B, VDDR1B) due to impaired 25-hydroxylation of vitamin D [1]. Missense mutations (L99P and K242N) in affected members of some Nigerian families with rickets were found. In vitro studies of the mutant CYP2R1 proteins in HEK293 cells showed normal expression levels but completely absent or markedly reduced 25-hydroxylase activity by the L99P and K242N mutations, respectively [1].

In conclusion, Cyp2r1 is vital for the 25-hydroxylation of vitamin D in the vitamin D metabolic pathway. The study of Cyp2r1 knockout mouse models has revealed its key role in maintaining normal vitamin D metabolite levels and its association with vitamin D-deficiency rickets, providing important insights into the mechanism of this disease.