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Original Studies |
2nd Department of Pediatrics (A.F., M.G., E.K., M.P., J.S., G.F.) and Department of Medical Chemistry (M.S.-S., C.B., M.S.), Molecular Biology and Pathobiochemistry, Semmelweis University of Medicine, Budapest, H-1094 Hungary
Address all correspondence and requests for reprints to: Miklós Garami, 2nd Department of Paediatrics, Semmelweis University of Medicine, Tuzoltó utca 79., Budapest, H-1094 Hungary. E-mail: miklos{at}gyer2.sote.hu
| Abstract |
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| Introduction |
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CAH, resulting from 21-OHD, is a classical monogenic disorder with a very wide range of disease manifestation. This wide spectrum is characterized by severe classical forms, with prenatal virilization with or without salt loss, and milder nonclassical forms (NC) that may be asymptomatic or associated with symptoms of androgen excess developing during childhood or at puberty. Affected females should be treated in utero, to reduce virilizing malformations of external genitalia, but this treatment is controversial (6, 7). Thus, prenatal diagnosis of 21-OHD is highly desired. Patients with CAH caused by 21-OHD have traditionally been divided into three types: salt-wasting (SW); nonsalt losing or simple virilizing (SV), and late-onset or NC forms. SW form presents with neonatal electrolyte disturbances, together with virilization of external genitalia, at birth in girls, and sexual precocity (because puberty and gonadotropins are not involved in boys). The SV form is similar but without electrolyte disturbances. The NC form lacks the neonatal symptoms and presents with late-onset androgen excess ranging from sexual precocity in childhood to menstrual disturbances, infertility, and hirsutism in adult women. This clinical diversity is shown to result from different mutations in the CYP21 gene, causing varying degrees of enzyme inactivation. These mutations can be deletions, large gene conversions, or point mutations. The adrenal steroid 21-hydroxylase enzyme (also called P450c21) is encoded by two genes: CYP21 and CYP21P (8, 9, 10). These genes are located within the human leukocyte antigen class III gene region on the short arm of chromosome 6; both are 3.4-kb long and consist of 10 exons. The CYP21P gene is a so-called pseudogene, with no evident function (11). The majority of the mutations in the active steroid 21-hydroxylase gene originate from this highly homologous adjacent pseudogene. The establishment of genotyping of CYP21 gene has been especially challenging, because of the complex structure of the locus.
There has not been any information available on the frequencies of different types of mutations of the active CYP21 gene in the Hungarian population. It is possible that, as a result of several centuries of intermingling foreign populations, the pattern of alterations of the CYP21 gene in Hungary is different from the European pattern. The knowledge of relative frequencies of the point mutations might be useful to delineate appropriate strategies for prenatal diagnoses.
To analyze the mutational spectrum and genotype-phenotype correlation in Hungarian patients with 21-OHD, we investigated the CYP21 gene for the most frequent mutations.
| Subjects and Methods |
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We analyzed the CYP21 active gene in 167 Hungarian patients (patients from 153 families, 14 sibling-pairs, 306 unrelated chromosomes) with steroid 21-hydroxylase deficiency registered in the 2nd Department of Pediatrics, Semmelweis University of Medicine. Clinical classification was based on genital anomalies, the age at presentation, and the hormonal and electrolyte abnormalities.
DNA
Samples were extracted from peripheral blood leukocytes, as described by Madisen (12). The eight most common mutations were analyzed in the CYP21 gene: Del-apparent large gene conversion, I2 Splice, Ile172Asn, Cluster E6, Leu307insT, Gln318Stop, Arg356Trp, and Val281Leu.
PCR
Allele-specific PCR (ASA) was performed according to the protocol described by Wedell et al.(13), with slight modifications. Initially, samples were analyzed for the presence of an active CYP21 gene, using an allele-specific primer in exon 3. The samples containing the active gene were further analyzed for the seven most frequent disease-causing, known mutation sites amplifying CYP21 in two separate fragments, i.e. downstream and upstream of the 8-bp deletion site in exon 3. In the first round of PCR, the fragment was amplified; then, in the second round, ASA was performed on the nonpseudogene sequence targeted at the mutation sites. After initial denaturation at 96 C for 3 min, 30 cycles of PCR were done at 96 C for 1 min, 5456 C or 60 C (depending on the primers used) for 30 sec, and 72 C for 3 min, in a reaction mixture containing 10 mM Tris-HCl (pH 9.0 at room temperature), 1.5 mmol/L MgCl2, 50 mmol/L KCl, 5% glycerol, 0.2 mmol/L deoxynucleotide triphosphate, 1 U of Taq polymerase (Pharmacia Biotech, Uppsala, Sweden), and 0.5 µmol/L of primers except the primer in limiting concentration (0.15 µmol/L). The oligonucleotide primers were as described by Wedell et al. (13) and synthesized by Pharmacia Biosystems. We used 1% agarose gel electrophoresis (30 min, 12 V/cm) for separation of PCR products, which were visualized by ethidium bromide staining.
Because of the nature of this method, deletions and apparent large gene conversions were not distinguished from each other and are included in the same group. In cases in which no evidence of deletions was detected (we could not obtain samples from the parents of the patients), the samples were designated, for example, In2/In2 (In2 being the intron 2 splice mutation) or Del. In other words, homozygosity could not be distinguished from hemizygosity.
| Results |
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Screening for 8 mutations allows the identification of most of the
mutations in 282 (92.1%) out of 306 unrelated chromosomes. Table 2
contains the distribution of different
genotypes and the relationships of genotype and phenotype in the
Hungarian 21-OHD patients. There are only 12 patients (7%) whose
mutations could not be characterized and 17 patients (10%) in whom
only one mutated allele was characterized. The most frequent genotypes
are the In2/In2 or Del (30.5%), the I172N/I172N or Del (10%), and the
Del/Del (9%).
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| Discussion |
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We screened 167 identified CAH patients in Hungary. This number includes 62.3% of the living patients of the Hungarian CAH registry. In this present work, we screened 7 of the most common point mutations and deletion (in other words, the absence of the active CYP21 gene) (16). Identifying these mutations, we were able to determine 155 (92.8%) CAH genotypes. However, 12 (7.2%) genotypes remained unknown. This is not surprising if we consider that the 8 mutations that we analyzed represent 9092% of all mutations in the CYP21 gene, according to the literature (13, 16). To determine the unknown genotypes, we are performing sequence analysis on their CYP21 genes.
Genetic diagnosis is more complicated for steroid 21-hydroxylase deficiency than for many other monogenic disorders, because of the high variability of the locus. If genotyping is performed without segregation of mutations in families, the linkage phase of mutations in compound heterozygotes can be determined by allele-specific PCR, and the positions of all pseudogene-derived mutations should be analyzed (17). Our present study agrees with the previous results of other nation-wide screening programs (13, 18, 19, 20, 21, 22, 23).
In some other studies, variation has been detected, and phenotypes are not always correlating with allelic variations in CYP21 gene (24). For example, the In2 mutation manifests with variable clinical symptoms (25, 26), whereas all of our patients with In2 as the determining mutation, presented with severe mineralocorticoid deficiency. On the other hand, we found that patients with the I172N mutation revealed clinical variation. This study agrees with the previous results of Jääskeläinen et al. in 1997 (19). The I172N mutation is known to result in clearly reduced enzymatic activity, to about 12% of that found in the wild-type, and it is usually associated with the SV form, although patients with the I172N/deletion genotype with the SW form have been reported (27). In the present study, the severity of both (SW and SV) mineralocorticoid deficiencies varied excessively; although we formally cannot rule out some additional mutations in CYP21 that might explain the observed clinical heterogeneity.
Certain mutations in CYP21 cause a mild form of the disease that is hard to distinguish from similar, common conditions of unknown etiology. Thus, knowledge about the molecular pathology of 21-OHD may provide a first step toward the understanding of more complex hormonal disorders. The use of PCR advances our knowledge of the mutations causing SW, SV, and NC forms of CAH and is useful in prenatal diagnosis of this disease.
In this study, we screened the entire accessible Hungarian CAH population, using the ASA technique that was proven to be effective for the rapid identification of the most common mutations in the CYP21 gene (13). However, the present work brought our attention to the limits of this unique technique. Using the ASA method alone, we were not able to determine the exact genotype of the patients without knowing the parents genotype. To eliminate this problem, we are extending our screening program to the index patients family, involving mothers and fathers. Having these data at hand, we shall be able to determine the exact genotype-phenotype correlation.
| Acknowledgments |
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| Footnotes |
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Received April 27, 1998.
Revised August 27, 1998.
Revised March 22, 1999.
Accepted March 29, 1999.
| References |
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