Vitamin D receptor gene polymorphism detected by digestion with Apa I influences the parathyroid response to extracellular calcium in Japanese chronic dialysis patients

Pharmacogenetic analysis of cinacalcet response in secondary hyperparathyroidism patients

Background

Secondary hyperparathyroidism (SHPT) is one of the major risk factors of morbidity and mortality in end-stage renal disease. Cinacalcet effectively controls SHPT without causing hypercalcemia and hyperphosphatemia. However, there is significant inter-individual response variance to cinacalcet treatment. Therefore, we aimed to evaluate the genetic effects related with parathyroid hormone regulation as factors for cinacalcet response variance.

Methods

Patients with a diagnosis of SHPT based on intact parathyroid hormone (iPTH) >300 pg/mL on dialysis were included in this study. They were over 18 years and have been treated by cinacalcet for more than 3 months. Responders and nonresponders were grouped by the serum iPTH changes. Twenty-four single nucleotide polymorphisms of CASR, VDR, FGFR1, KL, ALPL, RGS14, NR4A2, and PTHLH genes were selected for the pharmacogenetic analysis.

Results

After adjusting for age, sex, and calcium level, CASR rs1042636 (odds ratio [OR]: 0.066, P=0.027) and rs1802757 (OR: 10.532, P=0.042) were associated with cinacalcet response. The association of haplotypes of CASR rs1042636, rs10190, and rs1802757; GCC (OR: 0.355, P=0.015); and ATT (OR: 2.769, P=0.014) with cinacalcet response was also significant.

Conclusion

We obtained supporting information of the associations between cinacalcet response and CASR polymorphisms. CASR single nucleotide polymorphisms (SNPs) rs1802757, rs1042636, and haplotypes of rs1042636, rs10190, and rs1802757 were significantly associated with cinacalcet response variance.

Genomic and metabolomic patterns segregate with responses to calcium and vitamin D supplementation

Inter-individual response differences to vitamin D and Ca supplementation may be under genetic control through vitamin D and oestrogen receptor genes, which may influence their absorption and/or metabolism. Metabolomic studies on blood and urine from subjects supplemented with Ca and vitamin D reveal different metabolic profiles that segregate with genotype. Genotyping was performed for oestrogen receptor 1 gene (ESR1) and vitamin D receptor gene (VDR) in fifty-six postmenopausal women. Thirty-six women were classified as low bone density as determined by a heel ultrasound scan and twenty women had normal bone density acting as ‘controls’. Those with low bone density (LBD) were supplemented with oral Ca and vitamin D and were classified according to whether they were ‘responders’ or ‘non-responders’ according to biochemical results before and after therapy compared to controls receiving no supplementation. Metabolomic studies on serum and urine were done for the three groups at 0 and 3 months of therapy using NMR spectroscopy with pattern recognition. The ‘non-responder’ group showed a higher frequency of polymorphisms in the ESR1 (codons 10 and 325) and VDR (Bsm1 and Taq1), compared with to the ‘responders’. The wild-type genotype for Fok1 was more frequent in those with LBD (70 %) compared with the control group (10 %). Distinctive patterns of metabolites were displayed by NMR studies at baseline and 3 months of post-treatment, segregating responders from non-responders and controls. Identification of potential ‘non-responders’ to vitamin D and Ca, before therapy, based on a genomic and/or metabolomic profile would allow targeted selection of optimal therapy on an individual basis.

Gene polymorphism association studies in dialysis: bone and mineral metabolism

Abnormalities in bone and mineral metabolism have a significant impact on morbidity and mortality among patients with end-stage renal disease (ESRD). In addition to confounding environmental factors, genetic susceptibility factors may also influence the occurrence and severity of these abnormalities and account for interindividual variability among patients. Indeed, polymorphisms involving genes of the calcium/parathyroid hormone/calcitriol axis have been associated with bone and mineral metabolism abnormalities. This review summarizes studies involving polymorphisms of candidate genes and their effects on the development of complications related to bone and mineral metabolism abnormalities among patients with ESRD.

Vitamin D receptor polymorphisms and diseases

The vitamin D endocrine system is central to the control of bone and calcium homeostasis. Thus, alterations in the vitamin D pathway lead to disturbances in mineral metabolism. Furthermore, a role for vitamin D has been suggested in other diseases, like cancer, diabetes and cardiovascular disease. Expression and nuclear activation of the vitamin D receptor (VDR) are necessary for the effects of vitamin D. Several genetic variations have been identified in the VDR. DNA sequence variations, which occur frequently in the population, are referred to as "polymorphisms" and can have biological effects. To test whether there is a linkage between VDR polymorphisms and diseases, epidemiological studies are performed. In these studies, the presence of a variation of the gene is studied in a population of patients, and then compared to a control group. Thus, association studies are performed, and a link among gene polymorphisms and diseases can be established. Since the discovery of VDR polymorphisms a number of papers have been published studying its role in bone biology, renal diseases, diabetes, etc. The purpose of this review is to summarize the vast amount of information regarding vitamin D receptor polymorphisms and human diseases, and discuss its possible role as diagnostic tools.

Single nucleotide polymorphism screening and association analysis--exclusion of integrin beta 7 and vitamin D receptor (chromosome 12q) as candidate genes for asthma

BACKGROUND

The human genes coding for integrin beta 7 (ITGB7) and vitamin D receptor (VDR) are two of the several candidate genes for asthma and related phenotypes found in a promising candidate region on chromosome 12q that has been identified in multiple genomewide screens and candidate gene approaches.

METHODS

All exons, including parts of the neighbouring introns, and the predicted promoter region of the ITGB7 gene were screened for common polymorphisms in 32 independent asthmatic and healthy probands, resulting in the detection of two single nucleotide polymorphisms (SNPs) unknown so far. In addition to these SNPs, five already described SNPs of the ITGB7 and one in the human VDR gene were analysed in a Caucasian sib pair study of 176 families with at least two affected children, using matrix assisted laser desorption/ionization time of flight mass spectrometry. All confirmed SNPs were tested for linkage/association with asthma and related traits (total serum IgE level, eosinophil cell count and slope of the dose-response curve after bronchial challenge).

RESULTS

Two new variations in the ITGB7 gene were identified. The coding SNP in exon 4 causes a substitution of the amino acid GLU by VAL, whereas the other variation is non-coding (intron 3). None of the eight analysed SNPs, of either the ITGB7 or the VDR genes, showed significant linkage/association with asthma or related phenotypes in the family study.

CONCLUSIONS

These findings indicate that neither the human ITGB7 nor the VDR gene seem to be associated with the pathogenesis of asthma or the expression of related allergic phenotypes such as eosinophilia and changes in total IgE level.

Model for gene-environment interaction: the case for dialysis

Organ complications in end-stage renal disease (ESRD) such as cardiovascular disease, anemia, bone disease, malnutrition, inflammation, and infections occur in many organ systems and are caused by a multitude of underlying disease-, uremia-, and therapy-related factors, and with a wide range of manifestations and severity. Interindividual variability in the pathophysiologic response of the uremic host to environmental factors, including renal replacement therapy, may be governed to a significant degree by genetic susceptibility factors. Specific genes regulate the pathophysiologic responses of organ systems affected by ESRD and can serve as candidate genes for the host-environment interaction. This review summarizes emerging clinical and translational research work in the field of genetic polymorphism of candidate genes and their effects on the development of organ complications in ESRD. Methodologic limitations of the existing published data, the need for large prospective cohort studies, and potential future risks for the use of genomic information in resource allocation are also highlighted.

The negative Ca(2+) balance is involved in the stimulation of PTH secretion

The low calcium (Ca(2+)) dialysate have been developed to diminish the risk of hypercalcemia with the administration of active vitamin D and Ca(2+) carbonate as phosphate binder. Today, increasing numbers of hemodialysis (HD) patients have been on the low Ca(2+) dialysate (Ca(2+) = 2.5 mEq/l). However, the clinical consequences of a negative calcium net-balance which may be induced by the use of low Ca dialysate are not well evaluated. In the present study, we explored the effects of low Ca(2+) dialysate on the calcium balance and the PTH secretion. Eighty one chronic HD patients (male/female: 47/34; mean age: 60.2 +/- 1.5 years; mean HD periods: 11.1 +/- 0.8 years) who had been dialyzed with 3.0 mEq/l Ca(2+) dialysate were studied. All patients were transferred to the low Ca dialysate, which actually brought about a negative net-balance in Ca (mean: -94.5 mg) and an increase in serum intact PTH levels (mean: +23.7%: p = 0.03) during a single HD session. However, no changes in serum ionized Ca(2+) were found in spite of negative Ca(2+) balance. One month after change to the low Ca(2+) dialysate (total 12 sessions in each case), serum intact PTH levels increased significantly (186.7 +/- 19.5 vs. 216.2 +/- 21.9 pg/ml: p = 0.01) in spite of the fact that no changes were found in serum ionized Ca(2+), Pi and Mg. This result indicates that the negative Ca(2+) balance during low-Ca(2+) hemodialysis-stimulated PTH secretion, which offset the decrease of serum Ca(2+); a trade-off phenomenon between negative Ca balance and PTH. This suggests that low Ca(2+) dialysate may exaggerate the progression of secondary hyperparathyroidism.

Human calcium-sensing receptor gene. Vitamin D response elements in promoters P1 and P2 confer transcriptional responsiveness to 1,25-dihydroxyvitamin D

The calcium-sensing receptor (CASR), expressed in parathyroid chief cells, thyroid C-cells, and cells of the kidney tubule, is essential for maintenance of calcium homeostasis. Here we show parathyroid, thyroid, and kidney CASR mRNA levels increased 2-fold at 15 h after intraperitoneal injection of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in rats. Human thyroid C-cell (TT) and kidney proximal tubule cell (HKC) CASR gene transcription increased approximately 2-fold at 8 and 12 h after 1,25(OH)2D3 treatment. The human CASR gene has two promoters yielding alternative transcripts containing either exon 1A or exon 1B 5'-untranslated region sequences that splice to exon 2 some 242 bp before the ATG translation start site. Transcriptional start sites were identified in parathyroid gland and TT cells; that for promoter P1 lies 27 bp downstream of a TATA box, whereas that for promoter P2, which lacks a TATA box, lies in a GC-rich region. In HKC cells, transcriptional activity of a P1 reporter gene construct was 11-fold and of P2 was 33-fold above basal levels. 10(-8) m 1,25(OH)2D3 stimulated P1 activity 2-fold and P2 activity 2.5-fold. Vitamin D response elements (VDREs), in which half-sites (6 bp) are separated by three nucleotides, were identified in both promoters and shown to confer 1,25(OH)2D3 responsiveness to a heterologous promoter. This responsiveness was lost when the VDREs were mutated. In electrophoretic mobility shift assays with either in vitro transcribed/translated vitamin D receptor and retinoid X receptor-alpha, or HKC nuclear extract, specific protein-DNA complexes were formed in the presence of 1,25(OH)2D3 on oligonucleotides representing the P1 and P2 VDREs. In summary, functional VDREs have been identified in the CASR gene and provide the mechanism whereby 1,25(OH)2D up-regulates parathyroid, thyroid C-cell, and kidney CASR expression.