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Original Studies |
Section on Pediatric Endocrinology, DEB, NICHD, NIH, (A.V., C.A.S., C.A.L., M.K., G.P.C.) Bethesda, Maryland 20892; Department of Pediatrics (C.A.S.), Georgetown University, Washington, DC 20007; and Department of Pediatrics (L.G.), University of Parma, Italy
Address all correspondence and requests for reprints to: Alessandra Vottero, M.D., DEB, NICHD, NIH, Building 10, Room 10N262, 10 Center Drive, MSC 1862, Bethesda, Maryland 20892-1862. E-mail: votteroa{at}cc1.nichd.nih.gov
| Abstract |
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| Introduction |
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Methylation of deoxycytosine residues is another factor involved in the modulation of gene expression. Belmont et al. (11) found that the methylation of HpaII and HhaI sites near the polymorphic CAG repeats in the first exon of the human AR (HUMARA) locus correlated with X-inactivation. Most women with idiopathic hirsutism have normal circulating adrenal and gonadal androgens; thus, increased target tissue sensitivity to androgens has been considered as a potential mechanism for this condition. To examine this hypothesis, we analyzed the polymorphic CAG microsatellite and evaluated X-chromosome inactivation in patients with significant hirsutism but normal levels of circulating androgens.
| Subjects and Methods |
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Sixteen patients with nonhyperandrogenic hirsutism and 10
age-matched normal controls (hirsute group: age 21.7 yr ± 6.5;
control group: age 25.7 yr ± 5.1; mean ± SD)
were studied. All women were Caucasian, of Italian ethnicity. The
Ferriman score for hirsutism was 16 ± 4.7 (mean ±
SD) in the patients and 3 ± 1.4 in the control group
(12, 13). There were no differences between the hormonal profiles
(including measurements of plasma LH, FSH, testosterone,
free-testosterone, androstenedione, dehydroepiandrosterone, and
dehydroepiandrosterone-sulfate) of the two groups (Table 1
). Late onset congenital adrenal
hyperplasia, caused by 21-hydroxylase deficiency, was ruled out by
measuring plasma 17-hydroxyprogesterone response to ACTH 124.
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Genomic DNA was extracted from peripheral blood samples obtained
from a total of 26 individuals, using a rapid extraction protocol
(QIAamp Blood kit, Qiagen, Chatsworth, CA), according to
the instructions provided by the manufacturer, or by the standard
phenol/chloroform method (14). PCR amplification of the AR CAG repeat
was carried out using primers designed to flank the repeat region of
interest (8). The sense (5'-AGAGGCCGCGAGCGCAGCACCTC-3') and the
antisense (5'- GCTGTGAAGGTTGCTGTTCCTCAT-3') primers correspond,
respectively, to nucleotides 204226 and 404427 of the AR sequence
(15). After labeling the antisense primer by
[
33P]-dATP (Amersham, Arlington Heights, IL) through
the use of T4 polinucleotide kinase (New England Biolabs, Inc., Beverly, MA), about 100 ng of DNA was amplified through 30
cycles in a 10-µl vol. Amplification conditions consisted of an
initial denaturating step at 95 C for 5 min, followed by 30 cycles at
95 C for 1 min, at 57 C for 1 min, and at 72 C for 1 min. Extension was
carried out at 72 C for 5 min. The PCR final products were denatured at
95 C for 5 min and then analyzed by electrophoresis on a 6% sequencing
gel. The gels were subsequently subjected to autoradiography, and the
number of CAG repeats was calculated from the size of the PCR products,
in relation to a series of previously determined (by direct sequencing
of PCR products) CAG repeat size standards (8).
X-chromosome inactivation analysis
Assessment of clonality at the human AR (HUMARA) locus was performed by PCR amplification, according to the procedure of Allen et al. (11), with minor modifications. Human genomic DNA, isolated from peripheral blood by standard procedures (see Microsatellite Size Determination: CAG repeat), was employed.
For each DNA sample, two parallel reactions were set: in the first, 2
µg DNA was digested with 20 U HpaII; in the second, 2 µg
DNA was incubated with the enzyme digestion buffer containing no
enzyme. All the reactions were allowed to take place in 20 µl total
vol at 37 C overnight. After digestion, the mixture was incubated at 95
C for 5 min to terminate the enzymatic reaction and to denature the
DNA. An aliquot of 2 µl was then amplified by PCR, using the primers
5'-TCCAGAATCTGTTCCAGAGCGTGC-3' and 5'-GCTGTGAAGGTTGCTGTTCCTCAT-3'
labeled with [
-33P]-dATP. An initial denaturation at
95 C for 5 min was followed by 30 PCR cycles (1 min at 95 C, 1 min at
57 C, and 1 min at 72 C). PCR products were then run in a 6%
acrylamide gel; autoradiography was performed, and the DNA bands
corresponding to the different alleles were measured by optical
densitometry (NIH 1.611 program). HpaII digests only the
unmethylated DNA, allowing thus PCR amplification of only the remaining
methylated DNA.
Statistical analyses
The Software SigmaStat for Windows Version 1.0 (Jandel Scientific Co., San Rafael, CA) and Statistica for Windows Release 4.5 (StatSoft Inc., Tulsa, OK) were used to perform the statistical analyses. After performing a normality test, Students t test was employed to compare the same variable between the two groups. Variables without normal distribution were analyzed by the Mann-Whitney rank sum test. When data from the same individual were analyzed before and after HpaII digestion, a paired Students t test was employed. The z-test was used to compare the proportion of patients and controls presenting a mean decrease in OD greater or less than 0.4. The correlation between different variables was tested by the Pearson product moment correlation. The level of significance throughout was set at 0.05.
| Results |
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The number of repeats ranged from 1020 in the control group and
from 819 in the hirsute group (Table 2
). There was no difference between
hirsute patients (mean ± SE: 13.66 ± 0.60) and
controls (13.85 ± 0.64). Also, there was no correlation between
the number of repeats and Ferriman score or hormonal values. It is
noteworthy, however, that the shortest alleles (less than 10 repeats)
were recorded only in hirsute patients.
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Before digestion with HpaII, no difference was found in
the OD of the DNA bands between the hirsute and control groups
(P = 0.4) (Fig. 1
, Table 2
). After digestion, however, a significant decrease in the OD of the
DNA bands (corresponding to a lower degree of methylation) was observed
in the hirsute group (P < 0.0001), whereas there was a
smaller decrease in the control group (P = 0.0003)
(Fig. 2A
). When the two groups were
compared after digestion, the OD of the bands was significantly lower
in hirsute patients than in controls (P = 0.005). The
percentage of decrease in the OD after digestion (representing the
unmethylated DNA) was significantly higher in patients than in controls
(P = 0.003) (Fig. 2B
).
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| Discussion |
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Agonist-activated ARs stimulate the expression of androgen target genes, including those related to the growth and function of the hair follicle and sebaceous glands. Part of the transactivation activity of the AR resides in the N-terminal domain of the protein, which is encoded by exon 1 and contains a polyglutamine chain of variable length. Differences in the size of triplet repeat regions have clinical consequences in several diseases (2, 3, 4, 9, 17, 18, 19, 20) and alter the biological function of the AR in transactivation assays in vitro (10). Recently, the size of the CAG-repeat/polyglutamine chain was inversely correlated with the incidence of prostate cancer, suggesting that men prone to develop this neoplasm have androgen hypersensitivity (21). These observations support the idea that there is an optimal repeat length, which varies in the population (average size: 21 ± 2; range: 1131) (5).
In our study, we found no differences in the number of repeats between hirsute patients and normal controls. Interestingly, however, the shortest (and hence, most active) alleles were recorded only in hirsute patients. This tendency of smaller size alleles to be more frequent in IH patients than in controls may contribute to hirsutism in some patients and may become significant in a larger study.
More interesting were our findings from the X-chromosome inactivation analysis of the peripheral lymphocyte DNA. Because methylation can silence gene expression and can be replicated during mitosis, DNA methylation has long been considered an important mechanism in the inactivation of the X-chromosome (22). In the present study, hirsute patients had skewing of their X-chromosome inactivation, as shown by the methylation status of the HUMARA locus. Specifically, and in contrast to Lyons hypothesis of random X-chromosome inactivation (23), patients with hirsutism, but not normal controls, had preferential methylation of their longer AR allele and, thus, inactivation of the functionally weaker gene. This skewing could have allowed the shorter, more active AR allele (6, 10) to be expressed in patients with hirsutism, and this might explain their peripheral hypersensitivity to androgens. In this study, we selected patients whose main complaint was hirsutism; we believe that similar findings may be obtained in patients whose predominant manifestations are postadolescent cystic acne or male pattern baldness, depending on the genetics and constitution of the specific end-organ (24, 25).
Although skewing of the X-chromosome inactivation is a novel finding for hirsute patients, this phenomenon, favoring the expression of specific alleles of various X-linked genes, has been shown before (23, 26). It was hypothesized that hemizygous selection of favorable genes was influenced by X-chromosome loci and that this was the mechanism behind skewing of the X-chromosome inactivation process in peripheral blood (27). We speculate that similar skewing of X-chromosome inactivation exists in patients with hirsutism, which results in an increase of tissue sensitivity to androgens. Although skewing of X-chromosome may occur because deleterious genes are present in one of the X-chromosomes, other potential mechanisms may also be present (28, 29, 30). It is thus possible that somatic, epigenetic changes create conditions that render certain lymphocytic cell lines more fit for survival than others. We speculate that decreased methylation of the AR may lead to increased presence of its messenger RNA and protein, which in turn may be in favor of the survival of certain cell clones. It is also possible that other modifier genes may be differentially expressed in women with idiopathic hirsutism that favor the expansion of clones that have the less methylated AR allele.
Direct studies on androgen target tissues need to be performed to
confirm our hypothesis that skewing of X-chromosome inactivation may
lead to higher expression of more potent AR molecules, conferring
hypersensitivity to the skin of hirsute women. Increased
5
-reductase-mediated conversion of testosterone to
dihydrotestosterone in the skin has also been considered as a potential
pathophysiologic mechanism of tissue hypersensitivity to androgens
(31). One may additionally speculate that increased AR activity in the
skin may also lead to tissue hypersensitivity to androgens.
| Acknowledgments |
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| Footnotes |
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Received July 30, 1998.
Revised November 11, 1998.
Accepted December 7, 1998.
| References |
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