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Tissue resistance to 1,25-dihydroxyvitamin D without a mutation of the vitamin D receptor gene
M Hewison1, A R Rut, K Kristjansson
1Department of Medicine, UCL Medical School, London, UK.
Objective:
Hereditary vitamin D resistant rickets (HVDRR) is characterized by severe rickets and is often accompanied by alopecia. Mutations in the gene encoding the vitamin D receptor have been found in this condition. In a patient with the characteristic phenotype we have investigated the functional defect and sequenced the gene to seek a mutation.
Design:
Patient and control cell lines prepared from skin fibroblasts and peripheral blood lymphocytes were used to measure binding of 1,25(OH)2D3 and to isolate vitamin D receptor mRNA. VDR cDNA was sequenced and transfected into receptor defective cells.
Patient:
A child with alopecia diagnosed as having rickets due to resistance to 1,25(OH)2D3.
Measurements:
Cytosolic binding and nuclear association of 1,25(OH)2D3 were determined in patient and control cells, and functional response to 1,25(OH)2D3 assessed by measurement of 24-hydroxylase activity. VDR mRNA was prepared, reverse transcribed, and cDNA sequenced. VDR cDNA was also transfected into VDR-deficient CV-1 cells and functional response to 1,25(OH)2D3 assessed by co-transfection with a chloramphenicol acetyltransferase (CAT) reporter plasmid.
Results:
VDR from the patient were able to bind 1,25(OH)2D3 but showed no nuclear localization resulting in an absence of functional response to 1,25(OH)2D3. Sequencing revealed that the VDR coding region was normal. Expression studies of the patient's VDR showed functionally normal VDR as evidenced by normal transactivation in the presence of 1,25(OH)2D3.
Conclusion:
These data indicate a new cause of tissue resistance to 1,25(OH)2D3 which occurs in the absence of mutations in the coding region of VDR gene and which is characterized by defective nuclear localization of this receptor.
Insights
Hereditary vitamin D resistant rickets (HVDRR) in a patient was linked to defective nuclear localization of the vitamin D receptor (VDR), not a VDR gene mutation. This finding reveals a new cause of VDR resistance.
Area of Science:
- Endocrinology
- Genetics
- Molecular Biology
Background:
- Hereditary vitamin D resistant rickets (HVDRR) presents with severe rickets and alopecia.
- Mutations in the vitamin D receptor (VDR) gene are a known cause of HVDRR.
- Investigating a patient with characteristic HVDRR to identify the functional defect and genetic cause.
Observation:
- The patient exhibited rickets and alopecia, consistent with HVDRR.
- Patient-derived cells showed normal binding of 1,25(OH)2D3 to the VDR.
- However, VDR failed to localize to the nucleus, resulting in a lack of functional response.
Findings:
- Sequencing revealed no mutations in the VDR coding region.
- Functional studies demonstrated that the patient's VDR, when expressed, was functionally normal in transactivation assays.
- The primary defect identified was the defective nuclear localization of the VDR.
Implications:
- Identifies a novel mechanism of 1,25(OH)2D3 tissue resistance.
- Highlights defective VDR nuclear localization as a cause of HVDRR, independent of VDR gene mutations.
- Suggests potential for targeted therapies addressing VDR transport or localization in specific cases of rickets.