Study of biallelic variants in CYP2R1 causing vitamin D hydroxylation deficiency rickets type 1B

Vitamin D hydroxylase deficiency rickets type 1B (VDDR1B) is a rare hereditary bone disease, due to a defect in vitamin D-25-hydroxylation. It is caused by mutations in CYP2R1, consistent with an autosomal recessive pattern. We aim to access the phenotype and investigate the genetic etiology of a pedigree with suspected rickets. The probands were a pair of identical twin girls, who had leg curvature and delayed independent ambulation with abnormal gait. X-rays showed valgus ribs and genu varum. The probands had taken vitamin D and calcium supplements since the age of 6 months. Their serum 25(OH)D levels were low but in a state of sufficiency, and decreased to mild deficiency one week after drug withdrawal. Whole genome sequencing revealed compound heterozygous variants of CYP2R1 (NM_024514.5), c.75_76insA (p.G26Rfs*68) and c.1115G > C (p.C372S), which were inherited from their parents, respectively. In vitro studies showed that CYP2R1 p.G26Rfs*68, the novel frameshift variant, was not expressed and completely lost its catalytic function, while CYP2R1 p.C372S showed decreased expression due to the activation of unfolded protein response signaling and partial loss of catalytic function. In conclusion, the biallelic variants of CYP2R1 were the genetic cause of rickets in this pedigree, expanding the mutation spectrum of CYP2R1, and facilitating the diagnosis of VDDR1B as well as subsequent treatment and efficacy surveillance of the probands.