A de novo pathological point mutation at the 21-hydroxylase locus: implications for gene conversion in the human genome

Challenges of CYP21A2 genotyping in children with 21-hydroxylase deficiency: determination of genotype-phenotype correlation using next generation sequencing in Southeastern Anatolia

BACKGROUND/PURPOSE

Although it is known that there is generally a good correlation between genotypes and phenotypes, the number of studies reporting discrepancies has recently increased, exclusively between milder genotypes and their phenotypes due to the complex nature of the CYP21A2 gene and methodological pitfalls. This study aimed to assess CYP21A2 genotyping in children with 21-hydroxylase deficiency (21-OHD) and establish their predictive genotype-phenotype correlation features using a large cohort in Southeastern Anatolia's ethnically diverse population.

METHODS

The patients were classified into three groups: salt-wasting (SW), simple virilizing (SV) and non-classical (NC). The genotypes were categorized into six groups due to residual enzyme activity: null-A-B-C-D-E. CYP21A2 genotyping was performed by sequence-specific primer and sequenced with next generation sequencing (NGS), and the expected phenotypes were compared to the observed phenotypes.

RESULTS

A total of 118 unrelated children with 21-OHD were included in this study (61% SW, 24.5% SV and 14.5% NC). The pathogenic variants were found in 79.5% of 171 mutated alleles (60.2%, 22.2%, and 17.6% in SW, SV and NC, respectively). Patient distribution based on genotype groups was as follows: null-16.1%, A-41.4%, B-6.0%, C-14.4%, E-22%). In2G was the most common pathogenic variant (33.9% of all alleles) and the most common variant in the three phenotype groups (SW-38.8%, SV-22.2% and NC-23.3%). The total genotype-phenotype correlation was 81.5%. The correlations of the null and A groups were 100% and 76.1%, respectively, while it was lower in group B and poor in group C (71.4% and 23.5%, respectively).

CONCLUSION

This study revealed that the concordance rates of the severe genotypes with their phenotypes were good, while those of the milder genotypes were poor. The discrepancies could have resulted from the complex characteristics of 21-OHD genotyping and the limitations of using NGS alone without integrating with other comprehensive methods.

Analysis of phenotypes and genotypes in 84 patients with 21-Hydroxylase deficiency in southern China

OBJECTIVE

21-hydroxylase deficiency (21-OHD) caused by mutation in CYP21A2 gene is the most common form of Congenital adrenal hyperplasia (CAH). This study aimed to analyze the gene mutation frequency and the phenotype-genotype correlation of 21-OHD patients from southern China.

METHOD

The clinical features, laboratory tests and gene mutational analysis of 84 patients with 21-OHD were retrospectively investigated. Subsequently, the correlation between phenotypes and genotypes of these patients was analyzed.

RESULTS

59 of 84 cases of 21-OHD (70.2%) were classified as salt-wasting (SW) forms presenting adrenal crisis or other signs of salt loss at the age between neonatal period and 2 months, and other 25 cases were classified as simple virilizing (SV) forms. Mutations of CYP21A2 gene on both alleles were found in all 84 patients (168 alleles). The most common types of mutations included micro-conversions (129/168, 76.8%), large gene conversions and deletions (23/168, 13.7%), and bona fide point mutations (16/168, 9.5%). In increasing order of frequency, the most common micro-conversions were I2G (41.1%), p.I172N (13.1%), p.R356W (7.7%), p.Q318* (7.7%) and E6 Cluster (3.0%). Genotypes and phenotypes correlated in 86.1% of the patients analyzed.

CONCLUSION

Micro-conversions were the most common types of CYP21A2 gene mutations in our study, and the frequency of the identified mutations was not significantly different compared with most other Chinese areas and different ethnic regions. However, fewer large gene conversions and deletions were found compared to studies in other ethnic populations. Genotype-phenotype correlation was found in patients with the SW and SV forms of 21-OHD. This study expanded the number of mutations affecting CYP21A2 gene in Chinese patients with 21-OHD, providing additional information for a precise clinical diagnosis and genetic counseling.

A de novo mutation in CYP21A2 gene in a case of in vitro fertilization

Congenital adrenal hyperplasia, one of the most frequent autosome recessive disorders, is caused by defects in steroidogenic enzymes involved in the cortisol biosynthesis. Approximately 95% of the cases are caused by abnormal function of the 21-hydroxylase enzyme. This deficiency leads to androgen excess, consequently, to virilization and rapid somatic growth with accelerated skeletal maturation. Mutations in CYP21A2 are responsible for different forms of 21-hydroxylase deficiency. Mild impairment in the enzymatic activity causes the non-classic or late-onset congenital adrenal hyperplasia that is observed with a prevalence of 1 in 1000 subjects in different populations. The present paper describes a de novo mutation that occurred in the paternal meiosis. The child, who was conceived by in vitro fertilization, presented with precocious puberty and diagnosed with non-classical 21-hydroxylase deficiency. DNA sequencing showed the compound heterozygosis for a de novo CYP21A1P/A2 chimeric gene and the p.Val281Leu mutation inherited from her mother, who was heterozygous for the mutation. The chimeric gene showed pseudogene-derived sequence from 5′-end to intron 3 and CYP21A2 sequences from intron 3 to 3′-end of the gene. Sequencing analysis of the father did not show any mutation. The multiplex ligation-dependent probe amplification (MLPA) assay did not indicate loss of DNA discarding gene deletion but confirmed the chimeric gene. In addition, supernumerary copies of CYP21A1P were observed for both parents and for the affect child. Since paternity has been confirmed, those results suggest that a de novo large gene conversion in the paternal meiosis could have occurred by misalignment of alleles bearing different copy numbers of genes in CYP21 locus.

Intraspecific evolution of human RCCX copy number variation traced by haplotypes of the CYP21A2 gene

The RCCX region is a complex, multiallelic, tandem copy number variation (CNV). Two complete genes, complement component 4 (C4) and steroid 21-hydroxylase (CYP21A2, formerly CYP21B), reside in its variable region. RCCX is prone to nonallelic homologous recombination (NAHR) such as unequal crossover, generating duplications and deletions of RCCX modules, and gene conversion. A series of allele-specific long-range polymerase chain reaction coupled to the whole-gene sequencing of CYP21A2 was developed for molecular haplotyping. By means of the developed techniques, 35 different kinds of CYP21A2 haplotype variant were experimentally determined from 112 unrelated European subjects. The number of the resolved CYP21A2 haplotype variants was increased to 61 by bioinformatic haplotype reconstruction. The CYP21A2 haplotype variants could be assigned to the haplotypic RCCX CNV structures (the copy number of RCCX modules) in most cases. The genealogy network constructed from the CYP21A2 haplotype variants delineated the origin of RCCX structures. The different RCCX structures were located in tight groups. The minority of groups with identical RCCX structure occurred once in the network, implying monophyletic origin, but the majority of groups occurred several times and in different locations, indicating polyphyletic origin. The monophyletic groups were often created by single unequal crossover, whereas recurrent unequal crossover events generated some of the polyphyletic groups. As a result of recurrent NAHR events, more CYP21A2 haplotype variants with different allele patterns belonged to the same RCCX structure. The intraspecific evolution of RCCX CNV described here has provided a reasonable expectation for that of complex, multiallelic, tandem CNVs in humans.

Novel PKD1 and PKD2 mutations in autosomal dominant polycystic kidney disease (ADPKD)

BACKGROUND

Autosomal dominant polycystic kidney disease (ADPKD) is a common genetic renal disorder with an incidence of 1:1000. Mutations in two genes (PKD1 and PKD2) have been identified as causative. Eighty-five percent of patients with ADPKD carry their mutation in the PKD1 gene. So far, > 500 mutations for PKD1 and > 120 mutations for PKD2, respectively, are known.

METHODS

In this study, we performed mutation analysis of PKD1 and PKD2 by exon sequencing in patients during routine molecular diagnostics for ADPKD.

RESULTS

In total, 60 mutations were identified in 93 patients representing a mutation detection efficiency of 64.5%. Fifty-two mutations were identified in PKD1 (86.7%) and 8 in PKD2 (13.3%). These include 41 novel mutations detected in PKD1 and 5 novel mutations in PKD2. Accordingly, our data expand the spectrum of known PKD mutations by 8% for PKD1 (41/513) and 4.2% for PKD2 (5/120). These results are in agreement with the detection ranges of 42%, 63% and 64% for definitive disease-causing mutations, and 78%, 86% and 89% for all identified variants reported in several comprehensive mutation screening reports.

CONCLUSIONS

The increased number of known mutations will facilitate future studies into genotype-phenotype correlations.

The power of the methods for detecting interlocus gene conversion

Interlocus gene conversion can homogenize DNA sequences of duplicated regions with high homology. Such nonvertical events sometimes cause a misleading evolutionary interpretation of data when the effect of gene conversion is ignored. To avoid this problem, it is crucial to test the data for the presence of gene conversion. Here, we performed extensive simulations to compare four major methods to detect gene conversion. One might expect that the power increases with increase of the gene conversion rate. However, we found this is true for only two methods. For the other two, limited power is expected when gene conversion is too frequent. We suggest using multiple methods to minimize the chance of missing the footprint of gene conversion.

Predisposition for de novo gene aberrations in the offspring of mothers with a duplicated CYP21A2 gene

CONTEXT

Although CYP21A2 de novo mutations are assumed to account for 1 to 2% of congenital adrenal hyperplasia (CAH) alleles and CYP21 genotyping has been done worldwide, there are only a few well-documented cases of CYP21A2 de novo mutations. The majority of these are deletions resulting from unequal crossings over owing to misalignment of homologous chromosomes during meiosis. Whereas so far, only heterozygous deletions of the CYP21A1P pseudogene were seen as premutations for de novo aberrations, the present report addresses such a predisposing role for parental duplicated CYP21A2 genes.

SUBJECTS AND METHODS

As part of routine diagnostic procedures, CYP21 genotyping has been performed in two unrelated female CAH index patients and in their clinically asymptomatic parents and siblings.

RESULTS

Both patients have inherited the paternal Intron2splice mutation and have harbored a de novo gene aberration (large deletion and I271N/exon 4) on their maternal haplotype. Surprisingly, both mothers were carriers of rare duplicated CYP21A2 haplotypes carrying CAH alleles, which were not detected in the daughters. Among 133 CAH alleles that were detected in patients and that could be traced to the respective family members by genotyping, these two de novo aberrations (representing 1.5% of 133 traced CAH alleles) were the only ones identified.

CONCLUSION

Because both de novo CYP21A2 gene aberrations so far identified in our laboratory occurred in the gametes of mothers carrying rare duplicated CYP21A2 haplotypes, we hypothesize that duplicated CYP21A2 genes could predispose for de novo mutations in the offspring, which is of relevance for prenatal CYP21 genotyping and genetic counseling.

Divergent phenotype of two siblings human leukocyte antigen identical, affected by nonclassical and classical congenital adrenal hyperplasia caused by 21-hydroxylase deficiency

CONTEXT

Congenital adrenal hyperplasia (CAH) is a group of autosomal recessive disorders most often caused by enzyme 21-hydroxylase deficiency. Most mutations causing enzymatic deficiency are generated by recombinations between the active gene CYP21 and the pseudogene CYP21P. Only 1-2% of affected alleles result from spontaneous mutations. The phenotype of CAH varies greatly, usually classified as classical or nonclassical, depending on variable degree in 21-hydroxylase activity. Here we report a divergent phenotype of two human leukocyte antigen identical siblings, affected by nonclassical and classical CAH caused by 21-hydroxylase deficiency due to different genotype.

PATIENTS AND METHODS

Using direct sequencing method and Southern blot, we studied two children (one male and one female), affected, respectively, by nonclassical and classical CAH and their parents.

RESULTS

The mother was heterozygous for the Q318X mutation, and the father was heterozygous for the V281L mutation. The brother was a compound heterozygote for the mutations V281L and Q318X, whereas the proband was compound heterozygote for the Q318X mutation and a large conversion. The two children are human leukocyte antigen identical (A*02;B*14;DRB1*01/A*33;B*14;DRB1*03).

CONCLUSIONS

Different phenotype of the proband is the result of compound heterozygosity for the maternal mutation Q318X and a de novo large conversion.

How homologous recombination generates a mutable genome

Recombination and mutation have traditionally been regarded as independent evolutionary processes: the latter generates variation, which the former reshuffles. Recent studies, however, have suggested that allelic recombination influences the underlying mutation rate, as high mutation rates are inferred in regions of high recombination. Furthermore, recombination between duplicated sequences introduces structural variation into the human genome and facilitates the formation of clustered gene families. Comparisons of wholegenome sequences reveal the expansion of gene family clusters to be an important mode of genome evolution. The negative aspect of this genomic dynamism is the contribution of these rearrangements to genetic diseases.

Gene conversion between functional trypsinogen genes PRSS1 and PRSS2 associated with chronic pancreatitis in a six-year-old girl

Gene conversion--the substitution of genetic material from another gene--is recognized as the underlying cause of a growing number of genetic diseases. While in most cases conversion takes place between a normal gene and its pseudogene, here we report an occurrence of disease-associated gene conversion between two functional genes. Chronic pancreatitis in childhood is frequently associated with mutations of the cationic trypsinogen gene (serine protease 1; PRSS1). We have analyzed PRSS1 in 1106 patients with chronic pancreatitis, and identified a novel conversion event affecting exon 2 and the subsequent intron. The recombination replaced at least 289 nucleotides with the paralogous sequence from the anionic trypsinogen gene (serine protease 2; PRSS2), and resulted in the PRSS1 mutations c.86A > T and c.161A > G, causing the amino acid substitutions N29I and N54S, respectively. Analysis of the recombinant N29I-N54S double mutant cationic trypsinogen revealed increased autocatalytic activation, which was solely due to the N29I mutation. In conclusion, we have demonstrated that gene conversion between two functional paralogous trypsinogen genes can occur and cause genetically determined chronic pancreatitis.

Duplication and positive selection among hominin-specific PRAME genes

Background

The physiological and phenotypic differences between human and chimpanzee are largely specified by our genomic differences. We have been particularly interested in recent duplications in the human genome as examples of relatively large-scale changes to our genome. We performed an in-depth evolutionary analysis of a region of chromosome 1, which is copy number polymorphic among humans, and that contains at least 32 PRAME (Preferentially expressed antigen of melanoma) genes and pseudogenes. PRAME-like genes are expressed in the testis and in a large number of tumours, and are thought to possess roles in spermatogenesis and oogenesis.

Results

Using nucleotide substitution rate estimates for exons and introns, we show that two large segmental duplications, of six and seven human PRAME genes respectively, occurred in the last 3 million years. These duplicated genes are thus hominin-specific, having arisen in our genome since the divergence from chimpanzee. This cluster of PRAME genes appears to have arisen initially from a translocation approximately 95–85 million years ago. We identified multiple sites within human or mouse PRAME sequences which exhibit strong evidence of positive selection. These form a pronounced cluster on one face of the predicted PRAME protein structure.

Conclusion

We predict that PRAME genes evolved adaptively due to strong competition between rapidly-dividing cells during spermatogenesis and oogenesis. We suggest that as PRAME gene copy number is polymorphic among individuals, positive selection of PRAME alleles may still prevail within the human population.

A homozygous nonsense mutation in SOX9 in the dominant disorder campomelic dysplasia: a case of mitotic gene conversion

Campomelic dysplasia (CD; MIM 114290), an autosomal dominant skeletal malformation syndrome with XY sex reversal, is caused by heterozygous de novo mutations in and around the SOX9 gene on 17q. We report a patient with typical signs of CD, including sex reversal, who was, surprisingly, homozygous for the nonsense mutation Y440X. Since neither parent carried the Y440X mutation, possible mechanisms explaining the homozygous situation were a de novo mutation followed by uniparental isodisomy, somatic crossing over, or gene conversion. As the patient was heterozygous for six microsatellite markers flanking SOX9, uniparental isodisomy and somatic crossing over were excluded. Analysis of intragenic single-nucleotide polymorphisms suggested that the homozygous mutation arose by a mitotic gene conversion event involving exchange of at least 440 nucleotides and at most 2,208 nucleotides between a de novo mutant maternal allele and a wild-type paternal allele. Analysis of cloned alleles showed that homozygous mutant cells constituted about 80% of the leukocyte cell population of the patient, whereas about 20% were heterozygous mutant cells. Heterozygous Y440X mutations, previously described in three CD cases, have been identified in seven additional cases, thus constituting the most frequent recurrent mutations in SOX9. These patients frequently have a milder phenotype with longer survival, possibly because of the retention of some transactivation activity of the mutant protein on SOX9 target genes, as shown by cell transfection experiments. The fact that the patient survived for 3 months may thus be explained by homozygosity for a hypomorphic rather than a complete loss-of-function allele, in combination with somatic mosaicism. This is, to our knowledge, the first report of mitotic gene conversion of a wild-type allele by a de novo mutant allele in humans.

Estimating the frequency of events that cause multiple-nucleotide changes

Existing mathematical models of DNA sequence evolution assume that all substitutions derive from point mutations. There is, however, increasing evidence that larger-scale events, involving two or more consecutive sites, may also be important. We describe a model, denoted SDT, that allows for single-nucleotide, doublet, and triplet mutations. Applied to protein-coding DNA, the SDT model allows doublet and triplet mutations to overlap codon boundaries but still permits data to be analyzed using the simplifying assumption of independence of sites. We have implemented the SDT model for maximum-likelihood phylogenetic inference and have applied it to an alignment of mammalian globin sequences and to 258 other protein-coding sequence alignments from the Pandit database. We find the SDT model's inclusion of doublet and triplet mutations to be overwhelmingly successful in giving statistically significant improvements in fit of model to data, indicating that larger-scale mutation events do occur. Distributions of inferred parameter values over all alignments analyzed suggest that these events are far more prevalent than previously thought. Detailed consideration of our results and the absence of any known mechanism causing three adjacent nucleotides to be substituted simultaneously, however, leads us to suggest that the actual evolutionary events occurring may include still-larger-scale events, such as gene conversion, inversion, or recombination, or a series of rapid compensatory changes.

Dynamics of a human interparalog gene conversion hotspot

Gene conversion between paralogs can alter their patterns of sequence identity, thus obscuring their evolutionary relationships and affecting their propensity to sponsor genomic rearrangements. The details of this important process are poorly understood in the human genome because allelic diversity complicates the interpretation of interparalog sequence differences. Here we exploit the haploid nature of the Y chromosome, which obviates complicating interallelic processes, together with its known phylogeny, to understand the dynamics of conversion between two directly repeated HERVs flanking the 780-kb AZFa region on Yq. Sequence analysis of a 787-bp segment of each of the HERVs in 36 Y chromosomes revealed one of the highest nucleotide diversities in the human genome, as well as evidence of a complex patchwork of highly directional gene conversion events. The rate of proximal-to-distal conversion events was estimated as 2.4 x 10(-4) to 1.2 x 10(-3) per generation (3.9 x 10(-7) to 1.9 x 10(-6) per base per generation), and the distal-to-proximal rate as about one-twentieth of this. Minimum observed conversion tract lengths ranged from 1 to 158 bp and maximum lengths from 19 to 1365 bp, with an estimated mean of 31 bp. Analysis of great ape homologs shows that conversion in this hotspot has a deep evolutionary history.

Justified chauvinism: advances in defining meiotic recombination through sperm typing

Sperm typing offers an efficient means of studying the quantitative and qualitative aspects of meiotic recombination that are virtually unapproachable by pedigree analysis. Since the initial development of the technique >10 years ago, several salient findings based on empirically derived recombination data have been described. The precise rates and distributions of recombination have been reported for specific regions of the genome, serving as the prototype for high-resolution genome-wide recombination patterns. Identification and characterization of molecular genetic events, such as unequal crossing over, gene conversion and crossover asymmetry, are under close inspection for the first time as a result of this technology. The influence of these phenomena on the evolution of the genome is of primary interest from a scientific and medical perspective. In this article, we review the novel discoveries in mammalian meiotic recombination that have been revealed through sperm typing.

Intense and highly localized gene conversion activity in human meiotic crossover hot spots

Meiotic gene conversion has an important role in allele diversification and in the homogenization of gene and other repeat DNA sequence families, sometimes with pathological consequences. But little is known about the dynamics of gene conversion in humans and its relationship to meiotic crossover. We therefore developed screening and selection methods to characterize sperm conversions in two meiotic crossover hot spots in the major histocompatibility complex (MHC) and one in the sex chromosomal pseudoautosomal pairing region PAR1 (ref. 9). All three hot spots are active in gene conversion and crossover. Conversion tracts are short and define a steep bidirectional gradient centered at the peak of crossover activity, consistent with crossovers and conversions being produced by the same recombination-initiating events. These initiations seem to be spread over a narrow zone, rather than occurring at a single site, and seem preferentially to yield conversions rather than crossovers. Crossover breakpoints are more broadly diffused than conversion breakpoints, suggesting either differences between conversion and crossover processing after initiation or the existence of a quality control checkpoint at which short interactions between homologous chromosomes are preferentially aborted as conversions.

Characterization of a de novo conversion in human complement C4 gene producing a C4B5-like protein

Complement C4 is a highly polymorphic protein essential for the activation of the classical complement pathway. Most of the allelic variation of C4 resides in the C4d region. Four polymorphic amino acid residues specify the isotype and an additional four specify the Rodgers and Chido determinants of the protein. Rare C4 allotypes have been postulated to originate from recombination between highly homologous C4 genes through gene conversions. Here we describe the development of a de novo C4 hybrid protein with allotypic and antigenic diversity resulting from nonhomologous intra or interchromosomal recombination of the maternal chromosomes. A conversion was observed between maternal C4A3a and C4B1b genes producing a functional hybrid gene in one of the children. The codons determining the isotype, Asp(1054), Leu(1101), Ser(1102), Ile(1105) and His(1106), were characteristic of C4B gene, whereas the polymorphic sites in exon and intron 28 were indicative of C4A3a sequence. The protein produced by this hybrid gene was electrophoretically similar to C4B5 allotype. It also possesses reversed antigenicity being Rodgers 1, 2, 3 and Chido-1, -2, -3, 4, -5, and -6. Our case describes the development of a rare bimodular C4B-C4B haplotype containing a functional de novo C4 hybrid gene arisen through gene conversion from C4A to C4B. Overall the data supports the hypothesis of gene conversions as an ongoing process increasing allelic diversity in the C4 locus.

Genetic variation at the 22q11 PRODH2/DGCR6 locus presents an unusual pattern and increases susceptibility to schizophrenia

The location of a schizophrenia susceptibility locus at chromosome 22q11 has been suggested by genome-wide linkage studies. Additional support was provided by the observation of a higher-than-expected frequency of 22q11 microdeletions in patients with schizophrenia and the demonstration that approximately 20-30% of individuals with 22q11 microdeletions develop schizophrenia or schizoaffective disorder in adolescence and adulthood. Analysis of the extent of these microdeletions by using polymorphic markers afforded further refinement of this locus to a region of approximately 1.5 Mb. Recently, a high rate of 22q11 microdeletions was also reported for a cohort of 47 patients with Childhood Onset Schizophrenia, a rare and severe form of schizophrenia with onset by age 13. It is therefore likely that this 1.5-Mb region contains one or more genes that predispose to schizophrenia. In three independent samples, we provide evidence for a contribution of the PRODH2/DGCR6 locus in 22q11-associated schizophrenia. We also uncover an unusual pattern of PRODH2 gene variation that mimics the sequence of a linked pseudogene. Several of the pseudogene-like variants we identified result in missense changes at conserved residues and may prevent synthesis of a fully functional enzyme. Our results have implications for understanding the genetic basis of the 22q11-associated psychiatric phenotypes and provide further insights into the genomic instability of this region.

Novel mutations in the human CYP21 gene

The great majority of genetic defects underlying steroid 21-hydroxylase deficiency appear to result from intergenic recombinations between the homologous CYP21 and CYP21P genes. For a minority, novel sporadic point mutations have been detected. De novo mutations in CYP21 have also been reported, but only a few studies have systematically screened their occurrence. We here describe a population-based patient sample in order to estimate the rate of single-family (i.e. sporadic) and de novo germline mutations in the human CYP21 locus. Among 76 Finnish families were observed three single-family mutations and two de novo mutations in CYP21. The rates obtained, approximately 5% and approximately 2% for novel and de novo mutations, respectively, indicate that they are not rare and that their occurrence should not be ignored in genetic diagnostics of this disorder.

Molecular genetics and pathogenesis of autosomal dominant polycystic kidney disease

Autosomal dominant polycystic kidney disease (ADPKD) is a common and systemic disease characterized by formation of focal cysts. Of the three potential causes of cysts, downstream obstruction, compositional changes in extracellular matrix, and proliferation of partially dedifferentiated cells, evidence strongly supports the latter as the primary abnormality. In the vast majority of cases, the disease is caused by mutations in PKD1 or PKD2, and appears to be recessive at the cellular level. Somatic second hits in the normal allele of cells containing the germ line mutation initiate or accelerate formation of cysts. The intrinsically high frequency of somatic second hits in epithelia appears to be sufficient to explain the frequent occurrence of somatic second hits in the disease-causing genes. PKD1 and PKD2 encode a putative adhesive/ion channel regulatory protein and an ion channel, respectively. The two proteins interact directly in vitro. Their cellular and subcellular localization suggest that they may also function independently in a common signaling pathway that may involve the membrane skeleton and that links cell-cell and cell-matrix adhesion to the development of cell polarity.

Novel mutations in the duplicated region of the polycystic kidney disease 1 (PKD1) gene provides supporting evidence for gene conversion

Autosomal dominant polycystic kidney disease (ADPKD) is one of the most common human single-gene disorders, and is the most common inherited form of cystic kidney disease. It is estimated that approximately 85% of ADPKD is due to mutations in the PKD1 gene, which is located on chromosome 16p13.3. Mutation analysis in this gene is difficult, because more than two-thirds of reiterated several times at 16p13.1. In this study, mutation screening in 90 ADPKD patients was carried out on exons in the duplicated region of the PKD1 gene (23-34), using genomic long-range PCR followed by nested PCR and single-strand conformation polymorphism (SSCP), and finally cycle sequencing. Two nonconservative missense mutations were detected in exons 25 and 31, and two conservative mutations were found in exons 24 and 29. A novel splicing mutation, which is expected to cause skipping of exon 30, was detected in one case. Moreover, six intronic variants, three silent variants, and one polymorphic variant were detected in this study. Comparison between some of these changes and published sequences from the homologous genes on 16p13.1, revealed supporting evidence for the gene conversion theory as a mechanism responsible for some of the mutations in the PKD1 gene. Factors likely to facilitate gene conversion in this region of the PKD1 gene are discussed.

Mutational analysis of patients with p47-phox-deficient chronic granulomatous disease: The significance of recombination events between the p47-phox gene (NCF1) and its highly homologous pseudogenes

OBJECTIVE

The aim of this study was to determine the molecular basis of p47-phox-deficient chronic granulomatous disease (CGD), the most common autosomal recessive form of the disease. CGD is an inherited condition characterized by defective oxygen radical production due to defects in the phagocyte nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Mutational analysis of p47-phox-deficient CGD patients previously demonstrated that the majority of patients have a GT dinucleotide (Delta GT) deletion at the start of exon 2, a signature sequence also observed in the highly homologous pseudogenes of NCF1.

MATERIALS AND METHODS

We performed genetic analysis of NCF1 and its pseudogenes using genomic DNA in 29 p47-phox-deficient CGD patients from 22 separate families. First-strand cDNA analysis was performed in 17 of the 29 patients.

RESULTS

We confirmed the significance of the Delta GT mutation; in 27 of 29 patients, only the Delta GT sequence was detectable. All but one of the 27 had at least one additional signature sequence, specific to the pseudogene, in either intron 1 and/or intron 2. We extended our analysis to look at signature sequence differences in exons 6 and 9 and detected both the wild-type and pseudogene sequences in all patients tested.

CONCLUSIONS

Although detection of only Delta GT sequence accounts for over 85% of affected patients, the molecular basis is most likely due to partial cross-over events between the wild-type and pseudogene(s) of p47-phox at different recombination sites. Our results suggest that complete gene conversion or deletion of the p47-phox gene (NCF1) occurs rarely, if it all.

Gene conversion-like missense mutations in the human cationic trypsinogen gene and insights into the molecular evolution of the human trypsinogen family

Over the past decade, gene conversion has been shown increasingly to be a cause of human disease. Through this process, a functional gene is converted into a mutant by a homologous, nonfunctional one. In this article, we demonstrate that gene conversion is a likely cause of the mutations of the human cationic trypsinogen (PRSS1) gene that are associated with hereditary or sporadic pancreatitis, including the R122H (CGC>CAT: c.365-366 GC>AT), N29I (AAC>ATC: c.86A>T), and A16V (GCC>GTC: c.47C>T) missense mutations. This hypothesis is strongly supported by four lines of observation. First, human group I trypsinogen genes are tandemly repeated and share a high sequence homology between them. Secondly, a possible donor sequence for each variant is present in the PRSS1 gene's paralog(s). Thirdly, there exist uninterrupted sequence tracts ranging from 30 to 114 bp in the putatively converted regions. Finally, Chi-like and palindromic sequences are found in the vicinity of these missense mutations. This theory, if correct, will make the pancreatitis-associated PRSS1 mutations a unique example, as it shows that a functional gene may be converted by several paralogs, and that such an event may even occur between two functional genes (i.e., the N29I mutation), resulting in disease. This adds further to the diversity of genetic mechanisms underlying human disease. In addition, this genetic finding provides, for the first time, concrete evidence of the contribution made by gene conversion to the molecular evolution of the human trypsinogen family.

The rearrangement of the human alpha(1)-acid glycoprotein/orosomucoid gene: evidence for tandemly triplicated genes consisting of two AGP1 and one AGP2

The human alpha(1)-acid glycoprotein (AGP) or orosomucoid (ORM) is controlled by the two tandemly arranged genes, AGP1 and AGP2. The further duplication of the AGP1 gene has been suggested by a few duplicated ORM1 locus haplotypes including ORM1*F1. S and ORM1*B9. S, detected by isoelectric focusing. To clarify the triplication of the AGP gene, 39 DNA samples from Japanese subjects were studied by the long-range PCR of intergenic regions. The analysis of PCR products showed that the tandemly triplicated genes, AGP1A-AGP1B-AGP2, occurred on about 20% of chromosomes. These composites were divided into ORM1A*F1-ORM1B*S-ORM2*M and ORM1A*B9-ORM1B*S-ORM2*M by allelic variations. Furthermore, the former was classified into a few haplotypes by three synonymous sequence variations, which might have arisen through gene conversion-like events. The recombination breakpoints existed between the 5' flanking region and intron 2 of the AGP1B gene. Thus, it is likely that the rearrangement of the AGP gene has often occurred.

Steroid 21-hydroxylase deficiency: mutational spectrum in Denmark, three novel mutations, and in vitro expression analysis

We have investigated 68 unrelated 21-hydroxylase deficient Danish patients, representing 136 alleles, and determined the mutational spectrum of the CYP21 gene. The most frequent mutations detected were deletion of CYP21 and the splice mutation in intron 2 (I2-splice). Segregation analysis showed evidence of a de novo mutation in each of two patients. Three novel mutations were detected: G64E in exon 1, Q262X in exon 7, and A362V in exon 8. G64E and A362V were introduced in the CYP21 cDNA by in vitro site-directed mutagenesis, and the two corresponding proteins were transiently expressed in COS-7 cells. The activity of 21-hydroxylase was determined using the two hormone substrates 17-hydroxyprogesterone and progesterone. The analysis showed no enzyme activity for any of the substrates, a result that correlates well with the severity of the patients' disease.

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.

Recombination events between the p47-phoxgene and its highly homologous pseudogenes are the main cause of autosomal recessive chronic granulomatous disease

Chronic granulomatous disease (CGD) is an inherited disease caused by defects in the superoxide-generating nicotinamide adenine dinucleotide phosphate (NADPH) oxidase of phagocytes. Genetic lesions in any of 4 components of this antimicrobial enzyme have been detected. Family-specific mutations are found in 3 of 4 forms of CGD due to deficiencies of the gp91-phox, p22-phox, andp67-phox genes. In p47-phox–deficient CGD (autosomal recessive form A47°) patients, a GT deletion (▵GT) at the beginning of exon 2 of the p47-phox gene has been reported in 19 of 20 alleles. This GT deletion is also characteristic for the recently identified p47-phox pseudogenes. To explore a possible link between these findings, a sequence analysis of 28 unrelated, racially diverse A47° CGD patients and 37 healthy individuals was performed. The GT deletion in exon 2 was present on all alleles in 25 patients. Only 3 patients but all healthy individuals contained the GTGT and ▵GT sequences. A total of 22 patients carried additional pseudogene-specific intronic sequences on all alleles, either only in intron 1 or in intron 1 and intron 2, which lead to different types of chimeric DNA strands. It is concluded that recombination events between the p47-phox gene and its highly homologous pseudogenes result in the incorporation of ▵GT into the p47-phox gene, thereby leading to the high frequency of GT deletion in A47° CGD patients.

Gene conversion events contribute to the polymorphic variation of the surrogate light chain gene lambda 5/14.1

Normally occurring and experimentally induced models of immunodeficiency indicate that B cell development and antibody production are influenced by genetic factors. It is highly likely that polymorphic variants in genes that encode receptors for growth and differentiation factors, signal transduction molecules, and components of the B cell and pre-B-cell receptor complex contribute to this genetic control. We have identified a surprisingly large number of polymorphic variants in lambda5/14.1. Together with VpreB, lambda5/14.1 forms the surrogate light chain in the pre-B-cell receptor complex. Thirteen variant alleles of lambda5/14.1 were found in 134 unrelated individuals. Nine of these variants result in changes in the amino acid sequence of this small protein. The majority of the single base pair substitutions in lambda5/14.1 could be attributed to gene conversion events in which donor sequences from the lambda5 pseudogenes, 16.1, 16.2, and Glambda1, replace the wild-type sequence in the lambda5/14.1 functional gene. These findings indicate that gene conversion events play a major role in generating diversity that could affect stability or expression of the pre-B-cell receptor complex.

Cloning, expression, and physical mapping of the 3beta-hydroxysteroid dehydrogenase gene cluster (HSD3BP1-HSD3BP5) in human

Seven members of the human 3beta-hydroxysteroid dehydrogenase (3beta-HSD) gene family (HGMW-approved symbols HSD3BP1-HSD3BP5) have been cloned and physically mapped. HSD3B1 and 2 express 3beta-HSD enzymes; HSD3Bpsi1-5 are unprocessed pseudogenes that are closely related to HSD3B1 and 2 but contain no corresponding open reading frames. mRNA is expressed from psi4 and psi5 in several tissues, but with altered splice sites that disrupt reading frames. A 0.5-Mb contig of 3 yeast artificial chromosome and 32 bacterial artificial chromosome genomic clones contained no additional members of the gene family. The seven genes and pseudogenes mapped within 230 kb in the order HSD3Bpsi5-psi4-psi3-HSD3B1-psi1-psi2 -HSD3B2. HSD3B1 and 2 are in direct repeat, 100 kb apart. Six HSD3B2 mutations involve substitutions that are present in several of the pseudogenes. In four cases, mutations arose in CpG sites that are conserved within the gene cluster. The tendency for CpG sites to mutate by transition provides an adequate explanation for these HSD3B2 mutations, which are unlikely to be due to recombination or conversion within the gene family.

Quantitative analysis of survival motor neuron copies: identification of subtle SMN1 mutations in patients with spinal muscular atrophy, genotype-phenotype correlation, and implications for genetic counseling

Problems with diagnosis and genetic counseling occur for patients with autosomal recessive proximal spinal muscular atrophy (SMA) who do not show the most common mutation: homozygous absence of at least exon 7 of the telomeric survival motor neuron gene (SMN1). Here we present molecular genetic data for 42 independent nondeleted SMA patients. A nonradioactive quantitative PCR test showed one SMN1 copy in 19 patients (45%). By sequencing cloned reverse-transcription (RT) PCR products or genomic fragments of SMN1, we identified nine different mutations in 18 of the 19 patients, six described for the first time: three missense mutations (Y272C, T274I, S262I), three frameshift mutations in exons 2a, 2b, and 4 (124insT, 241-242ins4, 591delA), one nonsense mutation in exon 1 (Q15X), one Alu-mediated deletion from intron 4 to intron 6, and one donor splice site mutation in intron 7 (c.922+6T-->G). The most frequent mutation, Y272C, was found in 6 (33%) of 18 patients. Each intragenic mutation found in at least two patients occurred on the same haplotype background, indicating founder mutations. Genotype-phenotype correlation allowed inference of the effect of each mutation on the function of the SMN1 protein and the role of the SMN2 copy number in modulating the SMA phenotype. In 14 of 23 SMA patients with two SMN1 copies, at least one intact SMN1 copy was sequenced, which excludes a 5q-SMA and suggests the existence of further gene(s) responsible for approximately 4%-5% of phenotypes indistinguishable from SMA. We determined the validity of the test, and we discuss its practical implications and limitations.

A p47-phox pseudogene carries the most common mutation causing p47-phox- deficient chronic granulomatous disease

The predominant genetic defect causing p47-phox-deficient chronic granulomatous disease (A47 degrees CGD) is a GT deletion (DeltaGT) at the beginning of exon 2. No explanation exists to account for the high incidence of this single mutation causing a rare disease in an unrelated, racially diverse population. In each of 34 consecutive unrelated normal individuals, both the normal and mutant DeltaGT sequences were present in genomic DNA, suggesting that a p47-phox related sequence carrying DeltaGT exists in the normal population. Screening of genomic bacteriophage and YAC libraries identified 13 p47-phox bacteriophage and 19 YAC clones. The GT deletion was found in 11 bacteriophage and 15 YAC clones. Only 5 exonic and 33 intronic differences distinguished all DeltaGT clones from all wild-type clones. The most striking differences were a 30-bp deletion in intron 1 and a 20-bp duplication in intron 2. These results provide good evidence for the existence of at least one highly homologous p47-phox pseudogene containing the DeltaGT mutation. The p47-phox gene and pseudogene(s) colocalize to chromosome 7q11.23. This close linkage, together with the presence within each gene of multiple recombination hot spots, suggests that the predominance of the DeltaGT mutation in A47 degrees CGD is caused by recombination events between the wild-type gene and the pseudogene(s).

Prenatal diagnosis of steroid 21-hydroxylase deficiency by analysis of polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) profiles

The polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP) profile analysis could be applied to the prenatal diagnosis of steroid 21-hydroxylase deficiency. We designed PCR primers to amplify most of the 21-hydroxylase gene, including all the mutations previously reported. PCR-SSCP analysis in eight patients showed at least one polymorphic site in each case. We confirmed that the mobility shifts in SSCP in an affected kindred were transmitted as a Mendelian trait. As these results indicated that PCR-SSCP profiles could be used for DNA-based diagnosis, we attempted to use this technique for prenatal diagnosis. DNA was obtained by chorionic villus sampling of a fetus and PCR-SSCP profiles were analysed in the PCR-amplified fragments in which the mobility shifts had been observed in the SSCP of the proband. We concluded that the fetus was a carrier. Direct nucleotide sequencing and allele-specific oligonucleotide hybridization confirmed that the fetus was heterozygous. At birth, the infant showed no signs of virilization or of abnormal endocrine findings on laboratory study. The results suggest that this new application of PCR-SSCP has advantages over conventional RFLP analysis and is useful in making a prenatal diagnosis of steroid 21-hydroxylase deficiency both rapidly and accurately.

Molecular basis of human 3 beta-hydroxysteroid dehydrogenase deficiency

The enzyme 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) catalyses an essential step in the biosynthesis of all classes of steroid hormones. Classical 3 beta-HSD deficiency is responsible for CAHII, a severe form of congenital adrenal hyperplasia (CAH) that impairs steroidogenesis in both the adrenals and gonads. Newborns affected by 3 beta-HSD deficiency exhibit signs and symptoms of adrenal insufficiency of varying degrees associated with pseudohermaphroditism in males, whereas females exhibit normal sexual differentiation or mild virilization. Elevated ratios of 5-ene-to 4-ene-steroids appear as the best biological parameter for the diagnosis of 3 beta-HSD deficiency. The nonclassical form has been suggested to be related to an allelic variant of the classical form of 3 beta-HSD as described for steroid 21-hydroxylase deficiency. To elucidate the molecular basis of the classical form of 3 beta-HSD deficiency, we have analysed the structure of the highly homologous type I and II 3 beta-HSD genes in 12 male pseudohermaphrodite 3 beta-HSD deficient patients as well as in four female patients. The 14 different point mutations characterized were all detected in the type II 3 beta-HSD gene, which is the gene predominantly expressed in the adrenals and gonads, while no mutation was detected in the type I 3 beta-HSD gene predominantly expressed in the placenta and peripheral tissues. The finding of a normal type I 3 beta-HSD gene provides the explanation for the intact peripheral intracrine steroidogenesis in these patients and increased androgen manifestations at puberty. The influence of the detected mutations on enzymatic activity was assessed by in vitro expression analysis of mutant enzymes generated by site-directed mutagenesis in COS-1 cells. The mutant type II 3 beta-HSD enzymes carrying mutations detected in patients affected by the salt-losing form exhibit no detectable activity in intact transfected cells, whereas those with mutations found in nonsalt-loser index cases have some residual activity ranging from approximately 1-10% compared to the wild-type enzyme. Although in general, our findings provide a molecular explanation for the enzymatic heterogeneity ranging from the severe salt-losing form to the clinically inapparent salt-wasting form of the disease, we have observed that the mutant L108W or P186L enzymes found in a compound heterozygote male presenting the salt-wasting form of the disease, has some residual activity (approximately 1%) similar to that observed for the mutant N100S enzyme detected in a homozygous male patient suffering from a nonsalt-losing form of this disorder.(ABSTRACT TRUNCATED AT 400 WORDS)

Expressed cadherin pseudogenes are localized to the critical region of the spinal muscular atrophy gene

Low-copy repeats have been associated with genomic rearrangements and have been implicated in the generation of mutations in several diseases. Here we characterize a subset of low-copy repeats in the spinal muscular atrophy (SMA) region in human chromosome 5q13. We show that this repeated sequence, named c41-cad, is a highly expressed pseudogene derived from an intact neuronal cadherin gene, Br-cadherin, situated on 5p13-14. Br-cadherin is expressed specifically in the brain, whereas the c41-cad transcripts are 10-15 times more abundant and are present in all tissues examined. We speculate that the c41-cad repeats, separately or in concert with other repeats in the SMA region, are involved in the pathogenesis of SMA by promoting rearrangements and deletions.

The gene for neuronal apoptosis inhibitory protein is partially deleted in individuals with spinal muscular atrophy

The spinal muscular atrophies (SMAs), characterized by spinal cord motor neuron depletion, are among the most common autosomal recessive disorders. One model of SMA pathogenesis invokes an inappropriate persistence of normally occurring motor neuron apoptosis. Consistent with this hypothesis, the novel gene for neuronal apoptosis inhibitory protein (NAIP) has been mapped to the SMA region of chromosome 5q13.1 and is homologous with baculoviral apoptosis inhibitor proteins. The two first coding exons of this gene are deleted in approximately 67% of type I SMA chromosomes compared with 2% of non-SMA chromosomes. Furthermore, RT-PCR analysis reveals internally deleted and mutated forms of the NAIP transcript in type I SMA individuals and not in unaffected individuals. These findings suggest that mutations in the NAIP locus may lead to a failure of a normally occurring inhibition of motor neuron apoptosis resulting in or contributing to the SMA phenotype.

Ceruloplasmin gene defect associated with epilepsy in EL mice

Epilepsy is a dominant trait in EL mice, a model for human complex partial seizures. We recently mapped the major gene, El-1, to chromosome 9 near the predicted location for the ceruloplasmin (Cp) gene. We now present evidence for a partial duplication in the Cp gene in EL mice. This Cp duplication is coinherited with seizures in backcross generations and is associated with enhanced expression of Cp mRNA and increased Cp oxidase activity. Moreover, the duplication is associated with an enhanced frequency of double recombinants, simulating negative interference. The findings are relevant to the basic mechanisms of epilepsy and to theories of genetic recombination and gene mapping.

Prenatal diagnosis of congenital adrenal hyperplasia due to 21-hydroxylase deficiency by allele-specific hybridization and Southern blot

The feasibility and accuracy of gene-specific molecular genetic diagnosis for congenital adrenal hyperplasia due to 21-hydroxylase deficiency was studied in a group of 24 pregnancies at 25% risk of carrying an affected fetus. Chorionic villus sampling was performed at 9-10 weeks' gestation. Southern analysis and polymerase chain reaction, followed by allele-specific hybridization for a panel of nine known mutations, were performed for each family. Mutations were identified in 95% of chromosomes examined; the molecular diagnosis was accurate in 96% of infants as confirmed by postnatal examination. The most common mutation identified was an A-to-G transition at base 656 in the second intron, the result of an apparent gene conversion. In one family, there had been a de novo mutation in intron 2, which was detected in the proband, but not in the mother or in the fetus. We conclude that first trimester prenatal diagnosis of congenital adrenal hyperplasia due to 21-hydroxylase deficiency is feasible and accurate employing CYP21-specific probes.

Molecular pathology of 21-hydroxylase deficiency

Steroid 21-hydroxylase deficiency is a recessively inherited disorder of adrenal steroidogenesis. Different clinical variants map to a single gene CYP21B, which maps within the HLA complex and is located about 30 kb proximal to a very closely related 21-hydroxylase pseudogene, CYP21A. The two CYP21 genes are located on highly homologous tandemly repeated 30kb units, facilitating interlocus sequence exchanges. One type of exchange, unequal crossover, can result in CYP21B gene deletion or replacement of a large segment of the CYP21B gene by the analogous segment of the CYP21A gene. Gene conversion-like mechanisms can result in replacement of a very small segment of CYP21B by the analogous CYP21A sequence, thereby introducing a deleterious CYP21A-specific mutation. The vast majority of point mutation alleles seem to be accounted for by only a few of the mutations copied from CYP21A and can be assayed by ASO hybridization or allele-specific amplification assays of selectively amplified CYP21B gene sequences. Genotype-phenotype correlations are largely as expected: mutations resulting in no or severely curtailed gene expression are associated with severe clinical phenotypes; those resulting in significant residual enzyme activity are associated with milder clinical phenotypes.

Molecular basis of neurofibromatosis type 1 (NF1): mutation analysis and polymorphisms in the NF1 gene

Neurobromatosis type 1 (NF1) is one of the commonest genetic disorders in humans. The gene for NF1 was cloned in 1990. The protein encoded by the gene (neurofibromin) has extensive sequence homology with GTPase-activating protein (GAP). Despite screening the whole coding region of the gene for large and medium size rearrangements and approximately 40% of the coding region of the gene for small alterations, only 45 germ-line mutations have been reported in more than 500 unrelated patients. Of these, 25 mutations involve small changes in the gene, of which 17 (68%) result in the formation of an inappropriate stop codon. A "hot spot" for mutations has not been identified. The high mutation rate at this locus and the general difficulty in identifying mutations are discussed. A complete understanding of the structure and function of the NF1 gene awaits further detailed studies of both naturally occurring and in vitro-generated mutations.

Genetic diseases of steroid metabolism

All major classes of biologically active steroid hormones (progestins, mineralocorticoids, glucocorticoids, and sex steroids) are synthesized from cholesterol through 11 different bioconversions. With the exception of 5 alpha-reductase, all the enzymes mediating these reactions fall into two classes, cytochromes P450 and short-chain dehydrogenases. Cytochromes P450 are heme-containing membrane-bound proteins with molecular weights of approximately 50,000 that utilize molecular oxygen and electrons from NADPH-dependent accessory proteins to hydroxylate substrates. Short-chain dehydrogenases have molecular weights of 30,000-40,000, have tyrosine and lysine residues at the active site, and remove a hydride from the substrate, transferring the electrons of the hydride to NAD+ or NADP+. In most cases, this reaction is reversible so that the dehydrogenase can also function as a reductase under appropriate conditions. Inherited disorders in enzymes required for steroid biosynthesis have varying effects. Defects that prevent cortisol from being synthesized are referred to collectively as congenital adrenal hyperplasia. Because the enzymes required for cortisol biosynthesis in the adrenal cortex are in many cases required for the synthesis of mineralocorticoids and/or sex steroids, these classes of steroids may also not be synthesized normally. Thus, cholesterol desmolase and 3 beta-hydroxysteroid dehydrogenase deficiencies affect synthesis of all classes of steroids in both the adrenals and gonads. Steroid 21-hydroxylase deficiency, the most common cause (> 90% of cases) of congenital adrenal hyperplasia, can affect both mineralocorticoid and glucocorticoid synthesis, but androgen secretion is usually abnormally high due to shunting of accumulated precursors into this pathway. Excessive secretion of androgens and mineralocorticoids occurs in 11 beta-hydroxylase deficiency (the second most frequent form of congenital adrenal hyperplasia). Mineralocorticoid excess is also seen in 17 alpha-hydroxylase deficiency, but in this disorder sex steroid synthesis is defective. All defects that affect estrogen synthesis (deficiencies of cholesterol desmolase, 3 beta-hydroxysteroid dehydrogenase, 17 alpha-hydroxylase, aromatase, and 17 beta-hydroxysteroid dehydrogenase) are very rare, suggesting that the inability to synthesize placental estrogens may adversely affect fetal survival. A number of enzymes are expressed at sites of steroid action and regulate the amount of active steroid available to steroid receptors. Steroid 5 alpha-reductase converts testosterone to the more active dihydrotestosterone. Deficiency of this activity leads to incomplete development of male genitalia; 17 beta-hydroxysteroid dehydrogenase deficiency has similar phenotypic effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Mutations in steroid 21-hydroxylase (CYP21)

The inherited inability to synthesize cortisol is termed congenital adrenal hyperplasia. More than 90% of cases are caused by 21-hydroxylase deficiency. This syndrome is characterized by signs of androgen excess and often mineralocorticoid deficiency. Steroid 21-hydroxylase (P450c21) is a microsomal enzyme expressed in the adrenal gland that catalyzes conversion of 17-hydroxyprogesterone and progesterone to 11-deoxycortisol and deoxycorticosterone respectively. In man, this enzyme is encoded by the CYP21 (CYP21B) gene which is located in the HLA major histocompatibility complex along with a pseudogene, CYP21P (CYP21A). Mutations in CYP21 causing congenital adrenal hyperplasia are almost all generated by recombinations between CYP21 and CYP21P. These recombinations either delete CYP21 or transfer deleterious mutations from CYP21P to CYP21, a process termed apparent gene conversion. The degree of enzymatic compromise caused by each mutation is correlated with the clinical severity of the deficiency observed in patients carrying that mutation.

Molecular analysis of patient and carrier genes with congenital steroid 21-hydroxylase deficiency by using polymerase chain reaction and single strand conformation polymorphism

Steroid 21-hydroxylase deficiency is a major cause of congenital adrenal hyperplasia and is caused by genetic impairment of this enzyme. Since approximately 80% of cases are caused by point mutations of the CYP21B (CYP21A2) gene, whereas the remaining 20% are due to deletion of this gene, we used the polymerase chain reaction single strand conformation polymorphism technique for rapid and accurate diagnosis of this disease. Of 23 patients examined, 1 had a hemizygous CYP21B gene. 18 patient's genes localized their harmful mutations or deletion on both the alleles, while 4 of them found their causative mutations on one of the two alleles, and 1 failed to find any responsible mutation. All the mutations (four nucleotide substitutions) detected are also found in the CYP21A (CYP21A1) pseudogene. A mutation at the intron 2 site is most prevalent in both salt-wasting and simple virilizing forms of the disease, and accounts for 37% of the patient's genes (17/46). Pedigree analysis of these mutations revealed that the mutations (at least four of them) occurred de novo at a considerable frequency on both the paternally and maternally inherited chromosomes. This result could explain occasional discordance of the diagnosis using HLA typing with the clinical symptoms.