Deficiency of human complement protein C4 due to identical frameshift mutations in the C4A and C4B genes

The complement protein C4, encoded by two genes (C4A and C4B) on chromosome 6p, is the most polymorphic among the MHC III gene products. We investigated the molecular basis of C4 deficiency in a Finnish woman with systemic lupus erythematosus. C4-specific mRNA was present at low concentrations in C4-deficient (C4D) patient fibroblasts, but no pro-C4 protein was detected. This defect in C4 expression was specific in that synthesis of two other complement proteins was normal. Analysis of genomic DNA showed that the proposita had both deleted and nonexpressed C4 genes. Each of her nonexpressed genes, a C4A null gene inherited from the mother, a C4A null gene, and a C4B null gene inherited from the father, all contained an identical 2-bp insertion (TC) after nucleotide 5880 in exon 29, providing the first confirmatory proof of the C4B pseudogene. This mutation has been previously found only in C4A null genes. Although the exon 29/30 junction is spliced accurately, this frameshift mutation generates a premature stop at codon 3 in exon 30. These truncated C4A and C4B gene products were confirmed through RT-PCR and sequence analysis. Among the possible genetic mechanisms that produce identical mutations is both genes, the most likely is a mutation in C4A followed by a gene conversion to generate the mutated C4B allele.     

Complete complement components C4A and C4B deficiencies in human kidney diseases and systemic lupus erythematosus

Although a heterozygous deficiency of either complement component C4A or C4B is common, and each has a frequency of approximately 20% in a Caucasian population, complete deficiencies of both C4A and C4B proteins are extremely rare. In this paper the clinical courses for seven complete C4 deficiency patients are described in detail, and the molecular defects for complete C4 deficiencies are elucidated. Three patients with homozygous HLA A24 Cw7 B38 DR13 had systemic lupus erythematosus, mesangial glomerulonephritis, and severe skin lesions or membranous nephropathy. Immunofixation, genomic restriction fragment length polymorphisms, and pulsed field gel electrophoresis experiments revealed the presence of monomodular RP-C4-CYP21-TNX (RCCX) modules, each containing a solitary, long C4A mutant gene. Sequencing of the mutant C4A genes revealed a 2-bp, GT deletion in exon 13 that leads to protein truncation. The other four patients with homozygous HLA A30 B18 DR7 had SLE, severe kidney disorders including mesangial or membranoproliferative glomerulonephritis, and/or Henoch Schoenlein purpura. Molecular genetic analyses revealed an unusual RCCX structure with two short C4B mutant genes, each followed by an intact gene for steroid 21-hydroxylase. Nine identical, intronic mutations were found in each mutant C4B. In particular, the 8127 g-->a mutation present at the donor site of intron 28 may cause an RNA splice defect. Analyses of 12 complete C4 deficiency patients revealed two hot spots of deleterious mutations: one is located at exon 13, the others within a 2.6-kb genomic region spanning exons 20-29. Screening of these mutations may facilitate epidemiologic studies of C4 in infectious, autoimmune, and kidney diseases.     

Nonsense mutation in exon 4 of human complement C9 gene is the major cause of Japanese complement C9 deficiency

Deficiency of the ninth component of human complement (C9) is the most common complement deficiency in Japan but is rare in other countries. We studied the molecular basis of C9 deficiency in four Japanese C9-deficient patients who had suffered from meningococcal meningitis. Direct sequencing of amplified C9 cDNA and DNA revealed a nonsense substitution (CGA→TGA) at codon 95 in exon 4 in the four C9-deficient individuals. An allele-specific polymerase chain reaction system designed to detect exclusively only one of the normal and mutant alleles indicated that all the four patients were homozygous for the mutation in exon 4 and that the parents of patient 2 were heterozygous. The common mutation at codon 95 in exon 4 might be responsible for most Japanese C9 deficiency. Received: 28 December 1997 / Accepted: 25 February 1998     

Substitution of Ile-172 to Asn in the steroid 21-hydroxylase B (P450c21B) gene in a Finnish patient with the simple virilizing form of congenital adrenal hyperplasia

Summary The steroid 21-hydroxylase enzyme (P450c21) is a member of the cytochrome P450 gene superfamily and is essential in the synthesis of cortisol and aldosterone. Defects in the P450c21B gene cause congenital adrenal hyperplasia (CAH), a common genetic disorder leading to virilization of newborn females. To avoid the standard cloning of mutant P450c21 genes from genomic libraries, we amplified the full-length genomic P450c21 genes by polymerase chain reaction (PCR). The amplification was followed by cloning and sequencing of a defective P450c21B gene. The strategy described here is generally applicable, thus making a simple characterization of the complete P450c21B gene possible. The method was tested in one patient suffering from the simple virilizing form of CAH. The sequence of three independent clones originating from the defective P450c21B showed that Ile at position 172 in exon 4 was substituted by Asn. The identical mutation also has been found in other patients with CAH.     

Genetically determined partial complement C4 deficiency states are not independent risk factors for SLE in UK and Spanish populations

Systemic lupus erythematosus (SLE) is a chronic, multisystem autoimmune disease. Complete deficiency of complement component C4 confers strong genetic risk for SLE. Partial C4 deficiency states have also shown association with SLE, but despite much effort over the last 30 years, it has not been established whether this association is primarily causal or secondary to long-range linkage disequilibrium. The complement C4 locus, located in the major histocompatibility complex (MHC) class III region, exhibits copy-number variation (CNV) and C4 itself exists as two paralogs, C4A and C4B. In order to determine whether partial C4 deficiency is an independent genetic risk factor for SLE, we investigated C4 CNV in the context of HLA-DRB1 and MHC region SNP polymorphism in the largest and most comprehensive complement C4 study to date. Specifically, we genotyped 2,207 subjects of northern and southern European ancestry (1,028 SLE cases and 1,179 controls) for total C4, C4A, and C4B gene copy numbers, and the loss-of-function C4 exon 29 CT indel. We used multiple logistic regression to determine the independence of C4 CNV from known SNP and HLA-DRB1 associations. We clearly demonstrate that genetically determined partial C4 deficiency states are not independent risk factors for SLE in UK and Spanish populations. These results are further corroborated by the lack of association shown by the C4A exon 29 CT insertion in either cohort. Thus, although complete homozygous deficiency of complement C4 is one of the strongest genetic risk factors for SLE, partial C4 deficiency states do not independently predispose to the disease.     

Molecular analysis of a novel hereditary C3 deficiency with systemic lupus erythematosus

A case of inherited homozygous complement C3 deficiency (C3D) in a patient with systemic lupus erythematosus (SLE) and the molecular basis for this deficiency are reported. A 22-year-old Japanese male was diagnosed as having SLE and his medical history revealed recurrent tonsillitis and pneumonia. He was diagnosed as having C3D because of undetectable serum C3 level. His parents were consanguineous. Sequence analysis of C3D cDNA revealed a homozygous deletion of exon 39 (84bp). A single base substitution (AG to GG) in the 3'-splice acceptor site of intron 38 was identified by sequencing the genomic DNA. Expression of C3Delta(ex39) cDNA, the C3cDNA lacking exon 39, in COS-7 cells revealed that C3Delta(ex39) was retained in endoplasmic reticulum-Golgi intermediate compartment because of defective secretion. These data indicate that a novel AG-->GG 3'-splice acceptor site mutation in intron 38 caused aberrant splicing of exon 39, resulting in defective secretion of C3.     

Acute intermittent porphyria caused by a single base insertion of C in exon 15 of the porphobilinogen deaminase gene that results in a frame shift and premature stopping of translation

A single base insertion of C in exon 15 of the porphobilinogen deaminase (PBG-D) gene was observed in a patient with acute intermittent porphyria (AIP) by polymerase chain reaction (PCR)-direct sequencing analysis. The insertion locates between positions -22 and -21 from the translation termination codon TAA, causes a frame shift, and results in a stop codon located 4 codons downstream from the insertion (premature stopping of translation). The mutation generates an MspI recognition site, which can be used, in turn, to detect the mutant allele. Analysis of the cDNA fragments amplified by PCR revealed the existence of the abnormal PBG-D mRNA from the mutant allele in the patient.     

Molecular basis of a selective C1s deficiency associated with early onset multiple autoimmune diseases

We have investigated the molecular basis of selective and complete C1s deficiency in 2-year-old girl with complex autoimmune diseases including lupus-like syndrome, Hashimoto's thyroiditis, and autoimmune hepatitis. This patient's complement profile was characterized by the absence of CH50 activity, C1 functional activity <10%, and undetectable levels of C1s Ag associated with normal levels of C1r and C1q Ags. Exon-specific amplification of genomic DNA by PCR followed by direct sequence analysis revealed a homozygous nonsense mutation in the C1s gene exon XII at codon 534, caused by a nucleotide substitution from C (CGA for arginine) to T (TGA for stop codon). Both parents were heterozygous for this mutation. We used the new restriction site for endonuclease Fok-1 created by the mutation to detect this mutation in the genomic DNA of seven healthy family members. Four additional heterozygotes for the mutation were identified in two generations. Our data characterize for the first time the genetic defect of a selective and complete C1s deficiency in a Caucasian patient.     

Determination of the loss of function complement C4 exon 29 CT insertion using a novel paralog-specific assay in healthy UK and Spanish populations

Genetic variants resulting in non-expression of complement C4A and C4B genes are common in healthy European populations and have shown association with a number of diseases, most notably the autoimmune disease, systemic lupus erythematosus. The most frequent cause of a C4 "null" allele, following that of C4 gene copy number variation (CNV), is a non-sense mutation arising from a 2 bp CT insertion into codon 1232 of exon 29. Previous attempts to accurately genotype this polymorphism have not been amenable to high-throughput typing, and have been confounded by failure to account for CNV at this locus, as well as by inability to distinguish between paralogs. We have developed a novel, high-throughput, paralog-specific assay to detect the presence and copy number of this polymorphism. We have genotyped healthy cohorts from the United Kingdom (UK) and Spain. Overall, 30/719 (4.17%) individuals from the UK cohort and 8/449 (1.78%) individuals from the Spanish cohort harboured the CT insertion in a C4A gene. A single Spanish individual possessed a C4B CT insertion. There is weak correlation between the C4 CT insertion and flanking MHC polymorphism. Therefore it is important to note that, as with C4 gene CNV, disease-association due to this variant will be missed by current SNP-based genome-wide association strategies.     

Molecular characterization of genetic mutations in human lactate dehydrogenase (LDH) B (H) variant

Summary We have previously detected a single base substitution of G by A at the Arg codon CGC in exon 4 of the mutant lactate dehydrogenase (LDH) gene, an unstable LDH-B variant (case 1). Here, we use the polymerase chain reaction (PCR) to amplify genomic DNA of two cases (the original case 1 and a new patient, case 2). We were able to confirm that case 1 is homozygous for the mutation, causing a replacement of the conserved Arg by His at residue 173. The resulting LDH-B variant subunit is unstable in vivo. Whereas the mutation in exon 4 was not observed in case 2, a different single base substitution of A by C was detected at the Ser codon AGT in exon 3. This mutation causes a replacement of the conserved Ser by Arg at residue 131. Genomic analysis of the family of case 2 by mismatched PCR showed that the missense mutation was consistent with their biochemical phenotypes. The replacement results in a conformational change of the residues near the Ser, probably because the side chain of Arg is much more bulky than that of Ser. The change may affect the arrangement of the cofactor binding site and result in the loss of enzyme activity. The experimental observations are consistent with computer graphics analyses.     

Direct analysis of CYP21B genes in 21-hydroxylase deficiency using polymerase chain reaction amplification

Steroid 21-hydroxylase deficiency is the leading cause of impaired cortisol synthesis in congenital adrenal hyperplasia (CAH). We have studied the structure of the CYP21B gene in 30 unrelated CAH patients using the polymerase chain reaction (PCR) to differentiate the active CYP21B gene from its highly related CYP21A pseudogene. The PCR approach obviates the need to distinguish the CYP21A and CYP21B genes by restriction endonuclease digestion and electrophoresis before analysis with labeled probes. Furthermore, direct nucleotide sequence analysis of CYP21B genes is demonstrated on the PCR-amplified DNA. Gene deletion of CYP21B, gene conversion of the entire CYP21B gene to CYP21A, frame shift mutations in exon 3, an intron 2 mutation that causes abnormal RNA splicing, and a mutation leading to a stop codon in exon 8 appear to be the major abnormalities of the CYP21B gene in our patients. These mutations appear to account for 21-hydroxylase deficiency in 22 of 26 of our salt-wasting CAH patients.     

Two mutations within the coding sequence of the phenylalanine hydroxylase gene

Summary Two previously unidentified mutations at the phenylalanine hydroxylase locus were found during a study of the relationship between genotype and phenotype in phenylketonuria and hyperphenylalaninemia. One mutation eliminates the BamHI site in exon 7 and the other eliminates the HindIII site in exon 11 of the phenylalanine hydroxylase gene. They were suspected because of deviating restriction fragment patterns and confirmed by amplification, via the polymerase chain reaction, of exon 7 and exon 11, respectively, followed by digestion with the appropriate restriction enzyme. Direct sequencing of amplified mutant exon 7 revealed a G/C to T/A transversion at the first base of codon 272, substituting a GGA glycine codon for a UGA stop codon. Direct sequencing of amplified mutant exon 11 revealed a deletion of codon 364, a CTT leucine codon. The exon 7 mutation can be expected to result in a truncated protein and the exon 11 mutation in the elimination of an amino acid in the catalytic region of the enzyme. A patient who is a compound heterozygote for these two mutations has classical phenylketonuria. It is concluded that each of the two mutations leads to a profound loss of enzymatic activity. The segregation of these mutations with disease alleles in 4 and 2 families, respectively, supports the hypothesis that multiple mutations at the phenylalanine hydroxylase locus explain the variable phenylalanine tolerance in patients with phenylalanine hydroxylase deficiency.     

A frameshift mutation results in a truncated alpha 1-antitrypsin that is retained within the rough endoplasmic reticulum

The major physiological role of the serine protease inhibitor alpha 1-antitrypsin (alpha 1-AT) is to protect elastic fibers in the lung from excessive hydrolysis by neutrophil elastase. Genetic deficiency of alpha 1-AT predisposes individuals toward the development of emphysema. We have cloned and characterized a mutant alpha 1-AT gene from an individual exhibiting a total absence of immunoreactive alpha 1-AT in serum. Nucleotide sequence analysis of this "null" allele has demonstrated a TC dinucleotide deletion within the codon for Leu318 in exon IV. This frame-shift mutation results in the generation of a premature termination codon at residue 334, which is upstream of the active inhibitory site. To determine the biochemical basis of the null phenotype, the mutant and normal genes were transferred into mouse hepatoma cells for expression analysis. Pulse-chase experiments demonstrated that the mutant gene is expressed into a truncated protein of 45 kDa, which is retained within the rough endoplasmic reticulum. The complete lack of secretion of the truncated protein is consistent with the absence of immunoreactive alpha 1-AT in the patient's serum. In addition, a G to A transition was identified in exon II of the mutant gene, changing the codon for Arg101 to His101. Finally, an A to C transversion was identified in exon V changing the codon for Glu376 to Asp376. Since the latter conservative amino acid substitution has previously been identified in the common PiM2 variant, the frame-shift mutation might have occurred on a PiM2 background chromosome. Using the birthplace of this index case, this mutant alpha 1-AT allele has been designated "nullHong Kong."     

A novel missense mutation in exon 4 of the human coproporphyrinogen oxidase gene in two patients with hereditary coproporphyria

Hereditary coproporphyria (HCP) is an autosomal dominant disease characterized by a deficiency of coproporphyrinogen oxidase. To date, four mutations of the gene have been reported. We report here another mutation in two Japanese families with HCP, which was revealed by analysis of polymerase chain reaction (PCR)-amplified DNA fragments of the gene by a direct-sequencing method. A point mutation, G to A, was found in exon 4 of the gene at position 538 of the cDNA from the reported putative translation initiation codon ATG. This mutation results in a glycine to arginine substitution at amino acid 180. Two carriers in the family were successfully diagnosed by detecting the mutation using restriction analysis of the PCR products. Received: 23 April 1996 / Revised: 15 July 1996     

New compound heterozygous mutation in the CYP17 gene in a 46,XY girl with 17 alpha-hydroxylase/17,20-lyase deficiency

BACKGROUND: 17alpha-Hydroxylase/17,20-lyase deficiency is caused by a defect of P450c17 which catalyzes both 17alpha-hydroxylase and 17,20-lyase reactions in adrenal glands and gonads. RESULTS: In the present study, we analyzed the CYP17 gene in a Japanese patient with 17alpha-hydroxylase/17,20-lyase deficiency. The patient was a phenotypic girl and referred to us for right-sided inguinal hernia at the age of 4 years. Biopsy of the herniated gonad showed testicular tissue. The karyotype was 46,XY. At 6 years of age, hypertension was clearly recognized and the patient was diagnosed as having 17alpha-hydroxylase/17,20-lyase deficiency based on the clinical and laboratory findings. Analysis of the CYP17 gene revealed a compound heterozygous mutation. One mutation was an undescribed single nucleotide deletion at codon 247 in exon 4 (CTT to CT: 247delT) and the other was a missense mutation resulting in a substitution of His to Leu at codon 373 in exon 6 (CAC to CTC: H373L), which has been previously shown to abolish both 17alpha-hydroxylase and 17,20-lyase activities. The functional expression study of the 247delT mutant showed that this 247delT mutation completely eliminates both 17alpha-hydroxylase and 17,20-lyase activities. CONCLUSIONS: Together, these results indicate that the patient is a compound heterozygote for the mutation of the CYP17 gene (247delT and H373L) and that these mutations inactivate both 17alpha-hydroxylase and 17,20-lyase activities and give rise to clinically manifest 17alpha-hydroxylase/17,20-lyase deficiency.     

Copy number analysis of complement C4A, C4B and C4A silencing mutation by real-time quantitative polymerase chain reaction

Low protein levels and copy number variation (CNV) of the fourth component of human complement (C4A and C4B) have been associated with various diseases. High-throughput methods for analysing C4 CNV are available, but they commonly do not detect the most common C4A mutation, a silencing CT insertion (CTins) leading to low protein levels. We developed a SYBR? Green labelled real-time quantitative polymerase chain reaction (qPCR) with a novel concentration range approach to address C4 CNV and deficiencies due to CTins. This method was validated in three sample sets and applied to over 1600 patient samples. CTins caused C4A deficiency in more than 70% (76/105) of the carriers. Twenty per cent (76/381) of patients with a C4A deficiency would have been erroneously recorded as having none, if the CTins had not been assessed. C4A deficiency was more common in patients than a healthy reference population, (OR?=?1.60, 95%CI?=?1.02-2.52, p?=?0.039). The number of functional C4 genes can be straightforwardly analyzed by real-time qPCR, also with SYBR? Green labelling. Determination of CTins increases the frequency of C4A deficiency and thus helps to elucidate the genotypic versus phenotypic disease associations.     

Selective complement C1s deficiency caused by homozygous four-base deletion in the C1s gene

The complement system plays an important role in defense mechanisms by promoting the adherence of microorganisms to phagocytic cells and lysis of foreign organisms. Deficiencies of the first complement components, C1r/C1s, often cause systemic lupus erythema-tosus-like syndromes and severe pyogenic infections. Up to now no genetic analysis of the C1r/C1s deficiencies has been carried out. In the present work, we report the first genetic analysis of selective C1s deficiency, the patient having a normal amount of C1r. C1s RNA with a normal size was detected in patient’s subcutaneous fibroblasts (YKF) by RNA blot analysis and RT-PCR. The amount of C1s RNA was approximately one-tenth of the RNA from the human chondrosarcoma cell line, HCS2/8. In contrast, the levels of C1r and β-actin RNA of YKF were similar to that of HCS2/8. Sequence analysis of C1s cDNA revealed a deletion at nucleotides 1087–1090 (TTTG), creating a stop codon (TGA) at position 94 downstream of the mutation site. Direct sequencing of the gene between the primers designed on intron 9 and exon 10 indicated the presence of the deletion on exon 10 of the gene. Quantitative Southern blot hybridization suggested the mutation was homozygous. The 4-bp deletion on exon 10 was also found in the patient’s heterozygous mother who had normal hemolytic activity. Received: 6 July 1998 / Accepted: 1 August 1998     

Molecular and endocrine characterization of a mutation involving a recombination between the steroid 21-hydroxylase functional gene and pseudogene

The gene encoding steroid 21-hydroxylase activity, P450c21B, is located in the major histocompatibility complex (MHC) class III region, in close proximity to a highly homologous pseudogene, P450c21A. Recombinations between P450c21B and P450c21A have been shown to result in deficiency of 21-hydroxylase activity, the usual cause of congenital adrenal hyperplasia (CAH). A mutant P450c21 gene from a patient with simple virilizing CAH was identified and shown to be consistent with a recombination between P450c21A and P450c21B. Sequence analysis of the mutant gene showed the recombination site to be located between the first exon and the second intron. The mutant gene encodes a leucine instead of the normal proline at codon 31. This mutation resides on a chromosome bearing the HLA-B44 serotype. A comparison of mutation associated with HLA-B44 and that normally found with the HLA-Bw47 serotype suggests that the HLA-B44 mutations are of more ancient origin. The patient's homologous chromosome has a deletion of P450c21B. Endocrinological testing therefore allows for testing of the mutant gene in genetic isolation. Such testing demonstrated that the patient was capable of producing aldosterone and retaining sodium in response to a low-sodium diet, indicating that the mutant gene encodes an enzyme with partial 21-hydroxylase activity.