The Journal of Clinical Endocrinology & Metabolism Vol. 83, No. 7 2255-2259
Copyright © 1998 by The Endocrine Society
Vitamin D Receptor (VDR) and Parathyroid Hormone Messenger Ribonucleic Acid Levels Correspond to Polymorphic VDR Alleles in Human Parathyroid Tumors1
Tobias Carling,
Jonas Rastad,
Göran Åkerström and
Gunnar Westin
Department of Surgery, Endocrine Unit, Uppsala University Hospital,
S-751 85 Uppsala, Sweden
Address all correspondence and requests for reprints to: Tobias Carling, Ph.D., Department of Surgery, Endocrine Unit, Uppsala University Hospital, S-751 85 Uppsala, Sweden. E-mail:
tobias.carling{at}kirurgi.uu.se
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Abstract
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Calcitriol, via its receptor (VDR) is a main regulator of PTH secretion
and parathyroid cell proliferation. Recently, marked overrepresentation
of the polymorphic VDR alleles b, a, and
T was found in patients with primary hyperparathyroidism
(pHPT), which suggests pathogenic importance in the disease. Using the
ribonuclease protection assay, relative VDR and PTH messenger
ribonucleic acid (mRNA) levels of parathyroid adenomas from 42 patients
with sporadic pHPT were related to these VDR polymorphisms. The tumors
of patients homozygous for the b, a, or
T alleles demonstrated significantly lower VDR and
higher PTH mRNA levels than those exhibiting the BB,
AA, or tt genotypes
(P < 0.00010.02), whereas heterozygotes had
intermediate values. A similar discrepancy was found when comparing the
baT and non-baT haplotypes (0.042 ±
0.005 vs. 0.064 ± 0.004 for VDR; 34.4 ± 3.7
vs. 21.6 ± 2.2 for PTH; both P
< 0.005). The lower VDR mRNA levels associated with the
b, a, and T alleles may
affect the calcitriol-mediated control of parathyroid function and
thereby contribute to the development of sporadic pHPT.
 |
Introduction
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CALCITRIOL has been shown to be a
main inhibitor of both PTH secretion and parathyroid cell proliferation
(1, 2, 3). Genetic association studies using polymorphisms in intron 8
(B/b and A/a alleles) and
exon 9 (T/t alleles) of the vitamin D receptor
(VDR) gene have been conducted in patients with primary
hyperparathyroidism (pHPT). The increased prevalence of the VDR alleles
b, a, and T in sporadic pHPT indicated
that these genetic variants might predispose to parathyroid
tumorigenesis (4, 5, 6), although one Japanese study failed to demonstrate
any association between VDR alleles and pHPT (7). The VDR
baT haplotype has also been related to enhanced abnormality
in the calcium regulation of the PTH secretion from adenomatous
parathyroid cells of primary HPT (8). These findings could relate to
alterations in VDR expression, as the baT haplotype has been
coupled to lower expression of a reporter gene as well as lower VDR
messenger ribonucleic acid (mRNA) levels from heterozygotic cell lines
compared to those of the BAt haplotype (9). Recently,
studies on VDR mRNA expression in human peripheral blood mononuclear
cells failed to demonstrate correlation to the VDR
B/b alleles (10). Possible influences of the VDR
polymorphisms on VDR mRNA in human parathyroid tissue have not been
elucidated, however.
 |
Subjects and Methods
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Subjects
Forty-two Caucasian patients (31 females and 11 males; mean age,
69.1 ± 2.0 yr) with primary HPT due to operatively verified
parathyroid adenoma were included in the study and randomly selected to
create comparable numbers (n = 1118) of the VDR genotype groups.
None of them had a history of familial hypercalcemia, signs of multiple
endocrine neoplasia syndromes, or substantially raised serum creatinine
values. Preoperative mean values for serum calcium (determined by
atomic absorption and corrected for albumin; reference range,
2.202.60 mmol/L) and serum PTH (Nichols Institute, San Juan
Capistrano, CA; reference range, 1255 ng/L) were 2.91 ± 0.02
mmol/L and 95 ± 3.8 ng/L, respectively. The weight of the
parathyroid adenomas averaged 882 ± 117 mg.
DNA analysis
High mol wt DNA was prepared from leukocytes or parathyroid
tissue (4). The PCR and restriction enzymes BsmI,
ApaI, and TaqI were used to identify the
polymorphic alleles B/b,
A/a, and T/t, as previously
described (4, 8, 9, 11). The BsmI site in intron 8 of the
VDR gene is linked to the presence of the ApaI (intron 8)
and absence of the TaqI site (exon 9), concordance between
them being approximately 7580% and 9599%, respectively (9, 11, 12). No differences in genotype frequencies between males and females
have been observed (12). The distribution of the VDR genotypes in the
examined primary HPT patients were: BB = 11,
Bb = 18, bb = 13; AA =
12, Aa = 15, aa = 15;
TT = 11, Tt = 17, and
tt = 14.
RNA isolation
Biopsies (50150 mg) of the parathyroid adenomas, without a
macroscopically identified rim of normal tissue, were intraoperatively
snap-frozen and stored at -70 C. The tissue was pulverized in liquid
nitrogen, and total RNA was isolated by standard procedures (13) .
Riboprobes for VDR, PTH, and glyceraldehyde-3-phosphate
dehydrogenase (GAPDH)
A PstI/BamHI fragment of the human VDR
complementary DNA (cDNA) was subcloned into pBluescript II KS
(Stratagene, La Jolla, CA). This region corresponds to bases 16742006
of the published sequence and is located downstream of the examined VDR
polymorphisms (14). A 253-bp DNA fragment of PTH m124, a prepro-PTH
cDNA clone (15) (provided by Dr. H. M. Kronenberg), was amplified
by PCR primers (5'-gtagaatggctgcgtaagaagctgc-3' and
5'-catgtattgttgccctacactgtctag-3') and Pfu polymerase
(Stratagene). The 206-bp PstI/XbaI fragment was
subcloned into pBluescript II KS (Stratagene). Radiolabeled antisense
RNA for VDR, PTH, and GAPDH (Ambion, Austin, TX) were produced by
in vitro transcription from linearized plasmids using
[
-32P]UTP (Amersham, Aylesbury, UK) and T3 or T7 RNA
polymerase (Stratagene). The RNA probes were purified on a 6%/7 mol/L
urea polyacrylamide gel and eluted overnight in a buffer containing 0.5
mol/L NaAc (pH 7.0), 1 mmol/L ethylenediamine tetraacetate, and 0.2%
SDS. The sizes of the RNA probes and protected fragments were 471 and
332 nucleotides for VDR, 248 and 206 nucleotides for PTH, and 383 and
316 nucleotides for GAPDH. Several protected PTH fragments were
obtained due to heterogeneous initiation of the PTH transcription
(16).
Ribonuclease (RNase) protection assay
Total RNA from the parathyroid adenomas or HeLa cells were
analyzed by RNase protection assay with RNase A (8 µg/mL) and RNase
T1 (16 U/mL), essentially as previously described (17). Briefly, excess
32P-labeled VDR and GAPDH or PTH and GAPDH antisense RNA
were hybridized overnight to 10 µg total RNA. After RNase digestion,
proteinase K treatment, phenol-chloroform extraction, and ethanol
precipitation, the samples were run every second slot on a 6%/7 mol/L
urea polyacrylamide gel. After overnight exposure, the bands were
quantified by PhosphorImager (Molecular Dynamics, Sunnyvale, CA)
analysis. All parathyroid tumors were subjected to measurement of VDR
mRNA levels, whereas PTH mRNA levels could only be determined in 34
adenomas due to the lack of parathyroid tissue. The individual VDR and
PTH mRNA values was corrected by the GAPDH mRNA level, assuming that
the amount of GAPDH mRNA in each sample could be used as an internal
standard, not influenced by the polymorphic VDR alleles.
Statistical analysis
Differences in mean values were calculated by t test
and ANOVA using a P < 0.05 significance level.
Regression analysis was performed with StatView 4.0.2 (Abacus,
Berkeley, CA).
 |
Results
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No significant differences in mean values for serum calcium, serum
PTH, age, or parathyroid adenoma weight were found among the VDR
genotype groups (not shown). The VDR and GAPDH mRNA levels of the 42
parathyroid adenomas were determined by RNase protection assay (Fig. 1a
). The VDR probe used was located
downstream of the examined VDR polymorphisms. PTH mRNA levels adjusted
for GAPDH were determined in 34 of the adenomas (Fig. 1b
). Homozygosity
for the b, a, and T alleles were
associated with a 4143% lower VDR/GAPDH mRNA ratio vs.
B, A, and t, respectively (Fig. 2a
). The values were 0.042 ± 0.005
and 0.072 ± 0.005 for bb vs. BB
(P < 0.001), 0.043 ± 0.005 and 0.075 ±
0.005 for aa vs. AA (P <
0.0001), and 0.043 ± 0.006 and 0.072 ± 0.005 for
TT vs. tt (P < 0.002). The
heterozygotes in each allele group demonstrated intermediate values.
The same relationships between VDR alleles and VDR mRNA levels could be
detected when the analyses were restricted to those parathyroid
adenomas available for PTH mRNA measurement. When combining the
genotypes, parathyroid adenomas from patients exhibiting the
baT haplotype (n = 13) demonstrated a relative VDR mRNA
level of 0.042 ± 0.005 vs. 0.064 ± 0.004 for
those with non-baT (n = 29; P <
0.005). These associations persisted when using an antisense probe
spanning a different portion of the VDR message (not shown). The
relative PTH mRNA levels for the bb, aa, and
TT genotypes were 4145% higher than those for
BB, AA, and tt (Fig. 2b
). The levels
were 34.4 ± 3.7 vs. 19.1 ± 3.1 for bb
vs. BB (P < 0.001), 33.8 ±
3.5 vs. 20.0 ± 2.8 for aa vs.
AA (P < 0.01), and 32.8 ± 3.8
vs. 19.1 ± 3.1 for TT vs.
tt (P < 0.02). A similar discrepancy was
noted between the baT and non-baT haplotypes
(34.4 ± 3.7 vs. 21.6 ± 2.2; P <
0.005). Linear regression analysis substantiated inverse correlations
between VDR gene expression and serum calcium levels (r2 =
-0.13; P < 0.02) and between PTH mRNA levels and
adenoma weight (r2 = -0.26; P < 0.002),
whereas the VDR and PTH mRNA levels were not related to each other
(r2 = -0.02; P > 0.05). No other
correlations were seen between VDR mRNA levels and serum PTH, adenoma
weight, or age of the patients or between PTH mRNA levels and serum
calcium, serum PTH, or age of the patients (data not shown).

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Figure 1. The RNase protection assay was used to
quantify the VDR (a) and PTH (b) mRNA levels in the parathyroid
adenomas. GAPDH was used as an internal standard. Ten micrograms of
total RNA from eight representative parathyroid adenomas (PAd 18) and
HeLa cells (H) were analyzed. V, G, and P are free probes for VDR,
GAPDH, and PTH, respectively, and M is a 123-bp DNA ladder marker (Life
Technologies, Gaithersburg, MD).
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Figure 2. Results of the quantification of VDR and PTH
mRNA levels in the parathyroid adenomas in relation to VDR
genotypes/haplotypes. The data are presented as a ratio of VDR/GAPDH
mRNA levels of 42 parathyroid adenomas (a) and as a ratio of PTH/GAPDH
mRNA levels of 34 parathyroid adenomas (b). Values are the mean ±
SEM for the indicated number of adenomas in each allele
group, and P values were calculated using
t test and ANOVA.
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Discussion
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Calcitriol is known to inhibit both PTH secretion and
proliferation of parathyroid cells (1, 3, 18, 19), and reduced
parathyroid VDR expression or function has been implicated in the
pathogenesis of both primary and secondary HPT (20, 21). Recently
described overrepresentation of the polymorphic VDR alleles
b, a, and T in Caucasian patients with
primary HPT in Sweden and the USA has suggested that these alleles may
be important for the development of the disease (4, 5, 6). Such
overrepresentation was not found in a Japanese study on limited HPT
patient material (7). This discrepancy might be ethnically related, as
VDR allelic frequencies differ between populations (4, 7, 22, 23, 24).
The present study demonstrates that homozygosity for the physically
related alleles b, a, and T is
associated with lower VDR mRNA levels in parathyroid adenomas of
patients with primary HPT. This is consistent with studies in which the
baT haplotype was associated with lower reporter gene
expression and VDR mRNA levels in heterozygotic cell lines (9). It is
not known whether the different VDR mRNA levels in the present and
previous studies are due to altered VDR transcription or stability of
its mRNA, nor is it known whether the effects are direct or dependent
on linkage to other polymorphisms in the VDR gene or other genes
regulating parathyroid VDR expression.
Reduced VDR expression in human parathyroid tumors could conceivably
interfere with the inhibitory actions of calcitriol on PTH synthesis
and parathyroid cell proliferation and may thereby contribute to the
development of primary HPT. Consistent with this assumption, the
baT haplotype seems to be associated with higher PTH mRNA
levels and, as previously demonstrated, with a more pronounced
functional derangement in the calcium-regulated PTH secretion (8). It
is unlikely that the relation of VDR genotypes with VDR and PTH mRNA
levels mainly resulted from differences in the clinical expression of
HPT, as biochemical variables of HPT were similar in the allelic
groups. Circulating calcitriol levels could be regarded as a confounder
in this respect, although influences of VDR alleles on these levels are
controversial (9, 10, 25, 26, 27).
It seems evident that calcium up-regulates parathyroid VDR in normal
rat and avian parathyroids (28, 29). However, in the parathyroid
adenomas, VDR mRNA levels were negatively related to serum calcium.
Parathyroid tumors consequently may differ from their normal
equivalents, as serum calcium is abnormally high and positively
correlated to serum PTH and tumor size in HPT (30). The reduced
expression of calcium-sensing receptors on the parathyroid cell surface
in parathyroid tumors could also impair the effects of calcium on VDR
expression (31, 32, 33). Tissue-related heterogeneity exists in the complex
regulation of VDR expression and may explain the lack of correlation
between VDR alleles and VDR mRNA levels in human peripheral blood
mononuclear cells (10). Regulation of VDR is mainly performed by
ligand-induced stabilization, which is seen in most target tissues (34, 35), and by increasing VDR mRNA levels, which tends to be more cell
type specific (29). In the parathyroid, calcitriol seems to up-regulate
VDR mRNA levels (28, 29, 36), whereas one study showed no such effects
in the kidney (34). The precise role of the VDR polymorphisms in this
complex regulation of VDR expression in the parathyroid and other
target organs remains to be elucidated. We conclude that the lower
level of parathyroid VDR mRNA expression in patients with the VDR
baT haplotype supports the hypothesis that VDR alleles are
important for the development of primary HPT.
 |
Acknowledgments
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Peter Lillhager is acknowledged for expert technical assistance,
and Dr. Steven D. Lucas for linguistic revision of the manuscript.
 |
Footnotes
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1 This work was supported by the Swedish Medical Research Council, the
Swedish Cancer Society, and the Swedish Society for Medical
Research. 
Received December 11, 1997.
Revised February 18, 1998.
Accepted February 25, 1998.
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[Abstract]
[Full Text]
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S. Ferrari, D. Manen, J.-P. Bonjour, D. Slosman, and R. Rizzoli
Bone Mineral Mass and Calcium and Phosphate Metabolism in Young Men: Relationships with Vitamin D Receptor Allelic Polymorphisms
J. Clin. Endocrinol. Metab.,
June 1, 1999;
84(6):
2043 - 2048.
[Abstract]
[Full Text]
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P. Correa, J. Rastad, P. Schwarz, G. Westin, A. Kindmark, E. Lundgren, G. Åkerström, and T. Carling
The Vitamin D Receptor (VDR) Start Codon Polymorphism in Primary Hyperparathyroidism and Parathyroid VDR Messenger Ribonucleic Acid Levels
J. Clin. Endocrinol. Metab.,
May 1, 1999;
84(5):
1690 - 1694.
[Abstract]
[Full Text]
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