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From the Clinical Research Centers |
Division of Endocrinology, Diabetes, and Metabolism, Department of Medicine, Cornell University Medical College, New York, New York 10021.
Address all correspondence and requests for reprints to: Dr. Julianne Imperato-McGinley, Division of Endocrinology, Diabetes, and Metabolism, Department of Medicine, New York Hospital-Cornell Medical Center, 1300 York Avenue, Box-149, Room F-263, New York, New York 10021. E-mail: jimperat{at}mail.med.cornell.edu
| Abstract |
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| Introduction |
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The AR is a member of the nuclear steroid receptor superfamily (2, 5). It consists of 910919 amino acids and is encoded by a gene with 8 exons located in Xq1112. Like other steroid receptors, the AR is a single polypeptide comprised of relatively distinct domains: an amino-terminal domain, a DNA binding domain, a hinge region, and a steroid-binding domain. The large amino-terminal domain encoded by exon 1 is the least conserved region among the steroid receptors and is involved in transcriptional activation of target genes. The DNA binding domain encoded by exon 2 and 3 contains two zinc finger motifs (6) and is the most highly conserved region and is responsible for specific binding to its cognate DNA, i.e. androgen response element of target genes. The carboxyl-terminal of the AR contains the steroid-binding domain, encoded by the 3' portion of exon 4, and exons 58. It is responsible for the specific high-affinity ligand binding. The carboxyl-terminal region also contains the subdomains involved in dimerization and transcriptional activation (7, 8, 9). Between the DNA-binding domain and the steroid-binding domain is the hinge region, which is encoded by the 5' portion of exon 4 and which contains the nuclear translocation signal (7, 8).
We have previously reported the largest known kindred with complete
androgen insensitivity (1, 3, 10), consisting of 17 affected subjects
(Fig. 1
). The clinical phenotype and the
hormone profiles of the affected subjects from this kindred were
previously published (3). Pedigree analysis (Fig. 1
) demonstrated a
maternal transmission, with the defect manifested in the fourth, fifth
and sixth generations. Plasma testosterone levels were high normal or
elevated. Dihydrotestosterone (DHT) levels were decreased and
the ratio of testosterone/DHT elevated, as a consequence of a secondary
5
-reductase deficiency caused by androgen resistance (3). Analysis
of AR binding in genital skin fibroblasts from CAIS subjects
demonstrated a complete absence of receptor binding (3). DNA linkage
analysis indicated that the absence of receptor binding was not caused
by an entire AR gene deletion (10).
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| Materials and Methods |
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Peripheral blood was drawn from two affected subjects (indicated
by an asterisk in Fig. 1
), into EDTA-containing tubes; and
genomic DNA from white blood cells was isolated by a genomic DNA
isolation kit (Qiagen, Chatsworth, CA), according to the
manufactures instruction. The concentrations of DNA were determined
by ultraviolet absorbance. This study was approved by the Institutional
Review Board of Cornell University Medical College. Exons 18 of the
AR gene (11, 12) were amplified by PCR using primers and conditions
shown in Table 1
. Six sets of inner
primers were used for exon 1. The reaction mixture contained 0.18 µg
genomic DNA, 200 µmol/L of each of four deoxyribonucleotide
triphosphates, 10 mmol/L Tris-HCl (pH 8.3), 50 mmol/L KCl, 0.1% Triton
X-100, 2.5 U thermostable DNA polymerase (Promega Corp.,
Madison, WI), and the amounts of primers and MgCl indicated in Table 1
.
For hot PCR, 10 µCi [
-32P]deoxy-ATP was added. The
samples were denatured at 95 C for 2 min and then sequentially
denatured at 95 C for 15 sec, annealed at the temperature shown in
Table 1
for 30 sec, and extended at 72 C for 30 sec for a total of 35
cycles. A final extension cycle consisted of 72 C for 10 min.
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SSCP analysis was performed as described earlier (13, 14). Briefly, exon DNA was amplified and radiolabeled as described above. One microliter of the PCR product was added to 9 µL formamide denaturing dye (98% formamide, 20 mmol/L EDTA, 10 mmol/L NaOH, and 0.05% each of xylene cyanol and bromophenol blue), denatured at 100 C for 6 min and immediately cooled on ice. Three microliters of this solution were loaded onto a 0.5 x Hydrolink MDE gel (J. T. Baker Inc., Phillipsburg, NJ), containing 10% glycerol, and electrophoresed at 350 V at room temperature overnight in 0.6 x TBE buffer (54 mmol/L Tris-borate (pH 8.3) and 2.4 mmol/L EDTA). An aliquot of hot PCR sample, without denaturation, was loaded in an adjacent lane to determine the position of migration of the double-stranded DNA fragment. After electrophoresis, the gel was dried and exposed to Kodak BioMAX film (Eastman Kodak Co., Rochester, NY) at room temperature.
For DGGE, GC-clamp primers were synthesized for exons 48 of the AR gene, and PCR amplification was carried out as described above. Amplified DNA fragments were denatured at 95 C for 10 min and reannealed by cooling down slowly to room temperature, and electrophoresed in an 8% denaturing gradient polyacrylamide (19:1 acrylamide:bisacrylamide) gel in 0.5 x TBE buffer. The 100% denaturant was 7 mol/L urea plus formamide (60:40 by vol). Intermediate denaturants were prepared by diluting 100% denaturant with 8% acrylamide in 0.5 x TBE. Gradients were prepared bottom-up by gravity flow in a gradient maker. A Miniprotein apparatus (Bio-Rad Laboratories, Hercules, CA.) with 1.5 mm spacers was used for all of the experiments. The results were visualized by ethidium bromide staining, as described previously (15, 16).
DNA sequencing
DNA fragments of the AR gene were amplified by PCR, as described above. The PCR product was purified by PAGE (17). The concentration of purified DNA fragment was estimated using DNA QuikSTRIP (Eastman Kodak Co.). DNA sequencing was carried out using fmol DNA sequencing kit (Promega Corp.) with 32P end-labeled primer (13, 14). Both strands of DNA were sequenced.
| Results |
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The identified mutation in the CAG triplet repeats of exon 1 of the AR gene was not detected in a normal 46,XY brother, suggesting that this new mutation is responsible for complete androgen insensitivity in this large pedigree.
| Discussion |
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Our previous studies of affected subjects from this pedigree demonstrated absent DHT-binding in genital skin (3), consistent with the fact that a ligand-binding domain is absent in the mutant receptor. Also, a large deletion of the AR gene was not detected by previous studies of DNA linkage analysis (10), because of the fact that only a single base insertion is present in exon 1. Because this mutation leads to the synthesis of a nonfunctional truncated AR fragment in the affected subjects, it is not surprising that affected subjects from this kindred present with the phenotype of complete androgen insensitivity.
Different frameshift mutations in exon 1 of the AR
gene have been previously reported by others, in affected CAIS subjects
(2, 18, 19) and in the Tfm mouse (20). It has been shown that in the
Tfm mouse, reinitiation of protein translation from an internal
methionine codon downstream of the abnormal premature stop codon
produces a low level of truncated receptor (20), containing both the
DNA binding domain and the ligand binding domain. This explains why a
small amount of DNA binding and androgen binding activity is detected
in Tfm mice (21). A similar phenomenon has been reported in CAIS
affected subjects with a nonsense mutation
(CAG
TAG) at amino acid position 60 (Q60stop)
of exon 1 of the AR gene (22). Reinitiation of AR translation distal to
the abnormal stop codon results in synthesis of a small amount of
truncated AR and a low level of androgen binding activity. This
truncated AR is named as an A form of AR, and has recently been shown
to possess similar functional activities as the wild-type, B form (23).
Theoretically, reinitiation of AR translation downstream of the
mutation could also occur in our currently identified mutation.
However, because we did not detect any AR binding activity in the
genital skin fibroblasts from the CAIS affected subjects of this
kindred (3), it is likely that the reinitiation of truncated AR
translation is either absent or negligible. Although reinitiation of
truncated AR translation has been reported (20, 21, 22), most AR exon 1
mutations identified in CAIS subjects did not report downstream
reinitiation. The reason for this difference is unclear. Translation
efficiency may be one explanation, as McPhauls group reported that
the replacement of the upstream sequences of the translation
reinitiation site by the upstream sequences of the normal translation
initiation site could significantly increase mutant receptor expression
(23).
To date, more than 245 different mutations, involving all 8 exons in the AR gene, have been reported (2, 19). These mutations range from a single point mutation to an entire gene deletion and result in various degrees of functional impairment of the AR and a wide phenotypic spectrum of the syndrome of androgen insensitivity. However, there is no consistent relationship between the clinical phenotypes and the molecular defects in the AR gene. Most of the reported mutations are single point missense mutations located in exons 28 of the AR gene. Interestingly, mutations in exon 1 of the AR gene are found less frequently than found in other exons. Seventeen mutations of exon 1 have been reported in androgen insensitivity. Six are attributable to small deletions or insertions (2, 19), resulting in CAIS. The other mutations include 8 nonsense and 3 missense mutations.
Our reported mutation has never been previously reported. It is unique
in terms of its location within the region of polymorphic CAG
trinucleotide repeats of exon 1 and its feature as either a single
adenine insertion or a GC-dinucleotide deletion (see Fig. 3
). The
region of CAG triplet repeats in exon 1 of the AR gene is genetically
unstable, and this instability is greater in male than in female
meiosis (2). An increase in CAG repeats in this region is associated
with Kennedy disease, i.e. X-linked spinal and bulbar
muscular atrophy (2, 24). A shortening of the number of CAG repeats may
also be related to prostate cancer risk (25). Thus, all previously
reported variants in this region are variations in the numbers of
triplet repeats. Our currently reported mutation represents the first
example of small insertion or deletion in this CAG triplet region. The
mechanism of this genetic instability is unclear, although slippage of
DNA polymerase during DNA replication is possible (2).
In summary, we have identified a novel new mutation in exon 1 of the AR gene, responsible for the CAIS in the largest known kindred with this syndrome in the world.
| Footnotes |
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Received August 31, 1998.
Revised February 3, 1999.
Accepted February 9, 1999.
| References |
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-reductase
deficiency. J Clin Endocrinol Metab. 54:931941.[Medline]
-reductase-2 gene mutation in the Dominican Republic. J Clin
Endocrinol Metab. 81:17301735.[Abstract]
-reductase-2 and 17ß -hydroxysteroid
dehydrogenase-3 gene defects in male pseudohermaphrodites from a
Turkish kindred. J Clin Endocrinol Metab. 83:560569.This article has been cited by other articles:
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J. K. Lee, Y.-S. Zhu, J. J. Cordero, L.-Q. Cai, I. Labour, C. Herrera, and J. Imperato-McGinley Long-Term Growth Hormone Therapy in Adulthood Results in Significant Linear Growth in Siblings with a PROP-1 Gene Mutation J. Clin. Endocrinol. Metab., October 1, 2004; 89(10): 4850 - 4856. [Abstract] [Full Text] [PDF] |
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K. Muroya, I. Sasagawa, Y. Suzuki, T. Nakada, T. Ishii, and T. Ogata Hypospadias and the androgen receptor gene: mutation screening and CAG repeat length analysis Mol. Hum. Reprod., May 1, 2001; 7(5): 409 - 413. [Abstract] [Full Text] [PDF] |
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B. D. Sullivan, J. E. Evans, K. L. Krenzer, M. Reza Dana, and D. A. Sullivan Impact of Antiandrogen Treatment on the Fatty Acid Profile of Neutral Lipids in Human Meibomian Gland Secretions J. Clin. Endocrinol. Metab., December 1, 2000; 85(12): 4866 - 4873. [Abstract] [Full Text] |
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K. L. Krenzer, M. Reza Dana, M. D. Ullman, J. M. Cermak, D. B. Tolls, J. E. Evans, and D. A. Sullivan Effect of Androgen Deficiency on the Human Meibomian Gland and Ocular Surface J. Clin. Endocrinol. Metab., December 1, 2000; 85(12): 4874 - 4882. [Abstract] [Full Text] |
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