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Genetic defects of the 1,25-dihydroxyvitamin D3 receptor
M R Hughes1, P J Malloy, B W O'Malley
1Department of Molecular Genetics, Baylor College of Medicine, Houston, Texas.
Abstract:
Target organ resistance to steroid hormone action is known to produce clinical disorders ranging from testicular feminization in the case of androgen resistance to hypocalcemic vitamin D-resistant ricets (HVDRR) in the case of 1,25-dihydroxyvitamin D3. The etiologic basis of these disorders is thought to be genetic mutations in the gene encoding receptors for these hormones. We investigated this possibility by analyzing the vitamin D receptor (VDR) protein, mRNA, and DNA from patients with HVDRR. This autosomal recessive disease of children is characterized by early onset rickets, hypocalcemia, hyperparathyroidism, and elevated levels of 1,25-(OH)2D3. Cells from patients fall into three general classes of molecular defects: (i) decreased or absent hormone binding; (ii) decreased affinity of VDR for DNA, or; (iii) defective nuclear translocation or retention. Analysis of the DNA and/or mRNA from these cells has identified missense mutations in the DNA binding (zinc finger) domain and a nonsense mutation in the steroid binding domain of VDR. The mutations were individually recreated in wild type VDR and the expressed mutant protein behaved biochemically identically to the patient receptor. Further studies have shown that the receptor is unable to interact with the specific hormone response element (HRE) of the osteocalcin gene and activate appropriate transcription. Rapid diagnostic genotyping of these mutations is possible with either restriction digestion or allele-specific oligonucleotide hybridization. Analysis of these naturally occurring, disease producing mutations of a gene regulatory protein should provide insight into the key amino acid residues of the protein and the mechanism by which steroids modulate gene transcription.
Insights
Genetic mutations in the vitamin D receptor (VDR) cause hypocalcemic vitamin D-resistant rickets (HVDRR). Researchers identified specific VDR mutations affecting hormone binding, DNA binding, and nuclear translocation, impacting gene regulation.
Area of Science:
- Endocrinology
- Molecular Biology
- Genetics
Background:
- Steroid hormone resistance can cause genetic disorders like hypocalcemic vitamin D-resistant rickets (HVDRR).
- HVDRR is an autosomal recessive childhood disease characterized by rickets, hypocalcemia, and elevated 1,25-dihydroxyvitamin D3 levels.
- Genetic mutations in the vitamin D receptor (VDR) gene are suspected causes of HVDRR.
Purpose of the Study:
- To investigate the role of genetic mutations in the vitamin D receptor (VDR) in patients with HVDRR.
- To characterize the molecular defects in VDR protein, mRNA, and DNA from HVDRR patients.
- To understand how identified VDR mutations affect hormone binding, DNA interaction, and gene transcription.
Main Methods:
- Analysis of VDR protein, mRNA, and DNA from HVDRR patients.
- Categorization of molecular defects into impaired hormone binding, reduced DNA affinity, or defective nuclear translocation.
- Identification and recreation of specific VDR mutations (missense in DNA binding domain, nonsense in steroid binding domain) in vitro.
- Assessment of mutant VDR's interaction with the osteocalcin gene's hormone response element (HRE) and transcriptional activity.
Main Results:
- Identified three classes of molecular defects in HVDRR patients: altered hormone binding, reduced DNA binding affinity, or impaired nuclear translocation.
- Discovered missense mutations in the VDR DNA binding domain and a nonsense mutation in the steroid binding domain.
- Recreated mutations confirmed their biochemical impact, showing mutant VDR's inability to bind the osteocalcin HRE and activate transcription.
Conclusions:
- Genetic mutations in the VDR gene are the cause of HVDRR.
- Specific mutations disrupt VDR function by affecting hormone binding, DNA interaction, or nuclear localization.
- Understanding these mutations provides insights into VDR structure-function relationships and steroid hormone-mediated gene regulation.