Microsatellite markers in the indirect analysis of the steroid 21-hydroxylase gene

Molecular Diagnosis of Steroid 21-Hydroxylase Deficiency: A Practical Approach

Adrenal insufficiency in paediatric patients is mostly due to congenital adrenal hyperplasia (CAH), a severe monogenic disease caused by steroid 21-hydroxylase deficiency (21-OHD, encoded by the CYP21A2 gene) in 95% of cases. CYP21A2 genotyping requires careful analyses that guaranty gene-specific PCR, accurate definition of pseudogene-gene chimeras, gene duplications and allele dropout avoidance. A small panel of well-established disease-causing alterations enables a high diagnostic yield in confirming/discarding the disorder not only in symptomatic patients but also in those asymptomatic with borderline/positive results of 17-hydroxyprogesterone. Unfortunately, the complexity of this locus makes it today reluctant to high throughput techniques of massive sequencing. The strong relationship existing between the molecular alterations and the degree of enzymatic deficiency has allowed genetic studies to demonstrate its usefulness in predicting/classifying the clinical form of the disease. Other aspects of interest regarding molecular studies include its independence of physiological variations and analytical interferences, its usefulness in the diagnosis of simple virilizing forms in males and its inherent contribution to the genetic counseling, an aspect of great importance taking into account the high carrier frequency of CAH in the general population. Genetic testing of CYP21A2 constitutes an irreplaceable tool to detect severe alleles not just in family members of classical forms but also in mild late-onset forms of the disease and couples. It is also helpful in areas such as assisted reproduction and preimplantation diagnosis. Molecular diagnosis of 21-OHD under expert knowledge definitely contributes to a better management of the disease in every step of the clinical course.

Carrier detection and prenatal diagnosis of congenital adrenal hyperplasia must identify 'apparently mild' CYP21A2 alleles which associate neonatal salt-wasting disease

OBJECTIVE

Couples at risk of severe congenital adrenal hyperplasia (CAH) may be offered prenatal treatment or preimplantation diagnosis. However, proper genetic counselling requires the accurate identification of apparently 'mild alleles' in partners of CAH-carriers/patients.

METHODS

CYP21A2 gene analyses were performed in 255 patients with severe 21-hydroxylase deficiency (21-OHD), 94 with mild 21-OHD, 752 parental samples, 233 clinically unaffected partners and 253 historic DNA samples. GENSCAN and ClustalX2.0 software were used for in silico analyses, and Epidat 3.1 software for statistical calculations.

RESULTS

Twenty-seven partners were carriers of p.Val282Leu (alias p.Val281Leu, allele frequency 11.7%, 7.4-16.1). 'Val282Leu alleles' were detected in 30 patients with salt-wasting (SW) disease, 21 with other pseudogene-derived and rare coding cis severe mutations, and 9 without. These CYP21A2 genes were compared to those of 94 fully characterized patients with mild deficiency carrying p.Val282Leu in compound heterozygosity with a severe allele. A rare intronic variant, c.292+5G>A, was detected in the nine 'severe Val282Leu alleles' and that was not seen in mild p.Val282Leu alleles, in other deficient alleles or in normal chromosomes. The in silico documented effect on splicing and the clinical association (p < 0.0001) confirmed p.Val282Leu; c.292+5G>A as a severe allele.

CONCLUSION

As only severe alleles require clinical intervention, CAH-carrier detection of p.Val282Leu should be followed by the analysis of c.292+5G>A.

Should 21-hydroxylase deficiency genotyping be considered in assisted reproductive technology programs?

OBJECTIVE

To describe and discuss our experience of patients with congenital adrenal hyperplasia (CAH) conceived via assisted reproduction techniques (ART).

DESIGN

Case reports.

SETTING

Tertiary hospitals and a CAH molecular diagnosis reference laboratory belonging to one of these.

PATIENT(S)

Five patients with CAH (with 21-hydroxylase deficiency [21OHD]) conceived via homo/heterologous ART with egg or sperm donations.

INTERVENTION(S)

Molecular diagnosis following DNA analysis of patients, parents, and siblings, including direct analysis of the gene (polymerase chain reaction/allele-specific hybridization), Southern analysis (for gene deletions and duplications), semiquantitative primer extension (to confirm duplications), complementary sequencing, and microsatellite analysis to confirm allele segregation.

MAIN OUTCOME MEASURE(S)

Genotype identification and segregation analysis of alleles. Clinical evaluation of patients.

RESULT(S)

Three children (two girls, one boy) with classic neonatal forms of CAH (salt wasting and severe virilization) and two with nonclassic forms (two girls, one compound heterozygous with a severe mutation who showed clinical signs at 3.5 years of age) were born to parents who used ART. All showed segregated 21OH gene mutations. The respective genotypes were: 655G/655G, Q318X/Q318X,R356W, gene deletion hybrid with break point at exons 3-4/8 bp deletion at exon 3, 655G/V281L, and V281L/V281L. The severe mutations in the donated gametes were 655G and Q318X-R356W.

CONCLUSION(S)

As a common, infertility-related and prenatal treatment-susceptible recessive genetic disease, 21OHD genotyping should be considered in ART.

Gene duplications in 21-hydroxylase deficiency: the importance of accurate molecular diagnosis in carrier detection and prenatal diagnosis

BACKGROUND

The detection of 21-OH deficiency (21OHD) carriers in the general population requires that misinterpretations of apparently severe mutations in alleles carrying duplicated genes be avoided. Prenatal treatment prevents virilization in female fetuses and genetic counseling may be offered to couples in which one partner is either a patient or a carrier. This paper proposes a semiquantitative PCR method involving primer extension that distinguishes the severe point mutation Q318X in single gene copy alleles from the normal/nondeficient variant in gene-duplicated alleles.

SAMPLES AND METHODS

DNA from 65 individuals carrying Q318X variants, that of 85 partners of 21OHD carriers or patients, and one fetal sample (as well as the DNA of his family) were analyzed. 21OHD alleles were studied by gene-specific PCR/allele-specific oligonucleotides hybridization for common mutations, Southern analysis, complementary direct sequencing and microsatellite typing. Primer extension analysis of the Q318X variants using fluorescent dideoxynucleotides was performed on CYP21A2 gene-specific PCR-amplified DNA samples from controls, patients, potential carriers and prenatal samples.

RESULTS

Different fluorescence patterns were seen for the severe mutation (single gene copy) and the nondeficient (gene-duplicated) alleles carrying Q318X. The normal/mutant fluorescence peak (N/M) ratio was < 1 in all heterozygous carriers (mean 0.83; min. 0.70; max. 0.95). In all normal individuals carrying the gene-duplicated Q318X normal variant, the N/M ratio was > 1 (mean 1.69; min. 1.44; max. 2.02).

CONCLUSION

The proposed method discriminated between the severe Q318X mutation and the normal Q318X variant in gene duplication, and could be a useful complementary tool in prenatal diagnosis and carrier detection.

Three novel mutations in CYP21 gene in Brazilian patients with the classical form of 21-hydroxylase deficiency due to a founder effect

Three different new mutations were found after CYP21 gene sequencing in three unrelated patients with the classical form of the 21-hydroxylase deficiency. These mutations were also screened in their affected relatives. In one patient and her brother, both affected with the simple virilizing form and in their aunt, with the nonclassical form, an AG>GG transition was found in the acceptor site of intron 2. In another patient with the salt wasting form, we found a 1003 1004 insA, in exon 4, that altered the reading frame and created a stop codon in codon 297. In the third patient and his sister, we found a C>T transition in codon 408. This transition led to the substitution of arginine by cysteine (R408C) in a conserved region where arginine is conserved in at least four different species. These siblings with the R408C mutation, both affected with the salt wasting form, have the IVS2-13A/C>G mutation in the other allele, suggesting that the R408C should lead to complete impairment of enzymatic activity. To rule out the possibility of polymorphism, R408C was screened through allele specific PCR, and it was not found in 100 normal alleles. The screening of these three new mutations by allele-specific PCR or enzymatic restriction in 212 CAH patients disclosed their presence in 2.3% (9/387) of the alleles. All three new mutations were found in compound heterozygous state with previously known mutations. Microsatellite studies, using markers flanking CYP21 gene, revealed that each new mutation presents the same haplotype, suggesting a gene founder effect, similar to what was previously observed with the G424S mutation also described in our population. Although microconversion events are the main cause of mutations in the CYP21 gene, random mutations with a common origin can also be the cause of 21-hydroxylase deficiency.

Gene conversion (655G splicing mutation) and the founder effect (Gln318Stop) contribute to the most frequent severe point mutations in congenital adrenal hyperplasia (21-hydroxylase deficiency) in the Spanish population

This study addresses the contributions of gene conversion and a founder effect to the distribution of the two most frequent severe point mutations of the 21-hydroxylase (21OH) gene causing congenital adrenal hyperplasia: the 655G splicing mutation at intron 2, and Gln318Stop in a Spanish population. Direct and indirect analyses of segregated mutant and normal 21OH genes in 200 Spanish families (classic and nonclassic 21OH deficiency) were performed. Both mechanisms were found to contribute to different degrees to the defective investigated alleles. The 655G splicing mutation (62 alleles, 15.5%) seemed to be almost exclusively related to recent conversion events, whereas Gln318Stop (33 alleles, 8.3%) is more likely to be due to the dissemination of remotely generated mutant alleles. Other severe defective alleles, 8 bp-deletion (13 alleles, 3.3%), 306insT (5 alleles, 1.3%), and gene deletions (43 alleles, 11%), as well as the mild mutation Val281Leu (120 alleles, 30%), also appear to be strongly associated with particular D6S273 alleles. Although gene conversion contributes to the generation of severe 21OH alleles, the high frequency of some severe mutations in different geographic areas is consistent with a founder effect.

Congenital adrenal hyperplasia due to 21-hydroxylase deficiency

More than 90% of cases of congenital adrenal hyperplasia (CAH, the inherited inability to synthesize cortisol) are caused by 21-hydroxylase deficiency. Females with severe, classic 21-hydroxylase deficiency are exposed to excess androgens prenatally and are born with virilized external genitalia. Most patients cannot synthesize sufficient aldosterone to maintain sodium balance and may develop potentially fatal "salt wasting" crises if not treated. The disease is caused by mutations in the CYP21 gene encoding the steroid 21-hydroxylase enzyme. More than 90% of these mutations result from intergenic recombinations between CYP21 and the closely linked CYP21P pseudogene. Approximately 20% are gene deletions due to unequal crossing over during meiosis, whereas the remainder are gene conversions--transfers to CYP21 of deleterious mutations normally present in CYP21P. The degree to which each mutation compromises enzymatic activity is strongly correlated with the clinical severity of the disease in patients carrying it. Prenatal diagnosis by direct mutation detection permits prenatal treatment of affected females to minimize genital virilization. Neonatal screening by hormonal methods identifies affected children before salt wasting crises develop, reducing mortality from this condition. Glucocorticoid and mineralocorticoid replacement are the mainstays of treatment, but more rational dosing and additional therapies are being developed.

Microsatellites in the HLA region: 1998 update

In order to update the review published in Tissue Antigens in 1997, we present here a new overview on microsatellites in the HLA region, including additional information, with focus on the following points: * Description of 103 microsatellite characteristics in the HLA region, 50 markers having been newly described since 1996. * An integrated map of the HLA region, including microsatellites and some HLA genes, revealing an important microsatellite density in the MHC (Class I, Class II and Class III regions). * A synthesis of microsatellite analysis in disease studies, summarizing results of microsatellite approaches in 24 pathologies, including autoimmune diseases, HLA-associated or HLA-linked diseases and cancers. * Other applications of HLA region microsatellites in population or transplantation studies.

D6S273 microsatellite polymorphism and susceptibility to rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic articular inflammatory disease associated with HLA-DR genes that share a five amino acid sequence motif, QKRAA or QRRAA, from position 70 to 74 in the third hypervariable region of the DRB1 molecule. Since these associations between DRB1 genes and susceptibility to RA are incomplete, we examined the role of a CA repeat polymorphic microsatellite marker, D6S273, located between HSP70 and Bat2 genes in the class III region of MHC, in susceptibility to RA. Ninety-seven adult patients with seropositive RA and 100 normal healthy subjects were studied. Two D6S273 alleles (132 and 138) showed significant differences in their prevalence in RA patients as compared to normal controls; allele 132 was significantly higher in total patients and in DRB1 QKRAA/QRRAA epitope-positive patients, and allele 138 was significantly higher in QKRAA/QRRAA-negative patients. Analysis of data suggested that the association of D6S273 132 allele with RA was secondary to that of DRB1 genes. On the other hand, D6S273 138 allele showed primary association with RA susceptibility in QKRAA/QRRAA epitope-negative patients. The D6S273 138 allele thus provides an additional risk in RA susceptibility. The results in the present study therefore suggest that two regions in MHC, DRB1 and D6S273, contribute to susceptibility to RA.