Serum properdin consumption as a biomarker of C5 convertase dysregulation in C3 glomerulopathy

Properdin (P) stabilizes the alternative pathway (AP) convertases, being the only known positive regulator of the complement system. In addition, P is a pattern recognition molecule able to initiate directly the AP on non-self surfaces. Although P deficiencies have long been known to be associated with Neisseria infections and P is often found deposited at sites of AP activation and tissue injury, the potential role of P in the pathogenesis of complement dysregulation-associated disorders has not been studied extensively. Serum P levels were measured in 49 patients with histological and clinical evidence of C3 glomerulopathy (C3G). Patients were divided into two groups according to the presence or absence of C3 nephritic factor (C3NeF), an autoantibody that stabilizes the AP C3 convertase. The presence of this autoantibody results in a significant reduction in circulating C3 (P < 0·001) and C5 levels (P < 0·05), but does not alter factor B, P and sC5b-9 levels. Interestingly, in our cohort, serum P levels were low in 17 of the 32 C3NeF-negative patients. This group exhibited significant reduction of C3 (P < 0·001) and C5 (P < 0·001) and increase of sC5b-9 (P < 0·001) plasma levels compared to the control group. Also, P consumption was correlated significantly with C3 (r = 0·798, P = 0·0001), C5 (r = 0·806, P < 0·0001), sC5b-9 (r = -0·683, P = 0·043) and a higher degree of proteinuria (r = -0·862, P = 0·013). These results illustrate further the heterogeneity among C3G patients and suggest that P serum levels could be a reliable clinical biomarker to identify patients with underlying surface AP C5 convertase dysregulation.

Successful renal transplantation in a patient with atypical hemolytic uremic syndrome carrying mutations in both factor I and MCP

Kidney transplantation in patients with atypical hemolytic uremic syndrome (aHUS) carrying mutations in the soluble complement regulators factor H (CFH) or factor I (CFI) is associated with elevated risk of disease recurrence and almost certain graft loss. In contrast, recurrence is unusual in patients with mutations in the membrane-associated complement regulator membrane cofactor protein (MCP) (CD46). Therefore, a panel of experts recently recommended the combined liver-kidney transplantation to minimize aHUS recurrence in patients with mutations in CFH or CFI. There was, however, very limited information regarding transplantation in patients carrying mutations in both soluble and membrane-associated complement regulators to support a recommendation. Here, we report the case of an aHUS patient with a heterozygous mutation in both CFI and MCP who received an isolated kidney transplant expressing normal MCP levels. Critically, the patient suffered from a severe antibody-mediated rejection that was successfully treated with plasmapheresis and IvIgG. Most important, despite the complement activation in the allograft, there was no evidence of thrombotic microangiopathy, suggesting that the normal MCP levels in the grafted kidney were sufficient to prevent the aHUS recurrence. Our results suggest that isolated kidney transplantation may be a good first option for care in aHUS patients carrying CFI/MCP combined heterozygous mutations.

Lack of association between polymorphisms in C4b-binding protein and atypical haemolytic uraemic syndrome in the Spanish population

Dysregulation of the alternative pathway of complement activation, caused by mutations or polymorphisms in the genes encoding factor H, membrane co-factor protein, factor I or factor B, is associated strongly with predisposition to atypical haemolytic uraemic syndrome (aHUS). C4b-binding protein (C4BP), a major regulator of the classical pathway of complement activation, also has capacity to regulate the alternative pathway. Interestingly, the C4BP polymorphism p.Arg240His has been associated recently with predisposition to aHUS and the risk allele His240 showed decreased capacity to regulate the alternative pathway. Identification of novel aHUS predisposition factors has important implications for diagnosis and treatment in a significant number of aHUS patients; thus, we sought to replicate these association studies in an independent cohort of aHUS patients. In this study we show that the C4BP His240 allele corresponds to the C4BP*2 allele identified previously by isoelectric focusing in heterozygosis in 1.9-3.7% of unrelated Caucasians. Crucially, we found no differences between 102 unrelated Spanish aHUS patients and 128 healthy age-matched Spanish controls for the frequency of carriers of the His240 C4BP allele. This did not support an association between the p.Arg240His C4BP polymorphism and predisposition to aHUS in the Spanish population. In a similar study, we also failed to sustain an association between C4BP polymorphisms and predisposition to age-related macular degeneration, another disorder which is associated strongly with polymorphisms in factor H, and is thought to involve alternative pathway dysregulation.

Translational mini-review series on complement factor H: genetics and disease associations of human complement factor H

Factor H is an abundant plasma glycoprotein that plays a critical role in the regulation of the complement system in plasma and in the protection of host cells and tissues from damage by complement activation. Several recent studies have described the association of genetic variations of the complement factor H gene (CFH) with atypical haemolytic uraemic syndrome (aHUS), age-related macular degeneration (AMD) and membranoproliferative glomerulonephritis (MPGN). This review summarizes our current knowledge of CFH genetics and examines the CFH genotype-phenotype correlations that are helping to understand the molecular basis underlying these renal and ocular pathologies.

Hepatic disease as the first manifestation of progressive myoclonus epilepsy of Lafora

BACKGROUND

Lafora disease (LD; progressive myoclonus epilepsy type 2; EPM2) is an autosomal recessive disorder caused by mutations in the EPM2A and EPM2B genes. LD is characterized by the presence of strongly PAS-positive intracellular inclusions (Lafora bodies) in several tissues. Glycogen storage disease type IV (GSD-IV; Andersen disease) is an autosomal recessive disorder characterized by cirrhosis leading to severe liver failure. GSD-IV has been associated with mutations in the glycogen branching enzyme gene (GBE). Histopathologic changes of the liver in both diseases show an identical appearance, although cirrhosis has never been described in patients with LD. We report a LD family in which the proband presented severe liver failure at onset of the disease.

METHODS

Clinical histories, physical and neurologic examination, laboratory tests, EEGs, MRI of the brain, and liver or axillary skin biopsies were performed in the two affected siblings. The diagnosis was confirmed by molecular genetic analysis of the EPM2A, EPM2B, and GBE genes and loci.

RESULTS

During the first decade of life, abnormalities in liver function tests were detected in the two affected siblings. The proband's liver dysfunction was severe enough to require liver transplantation. Subsequently, both sibs developed LD. Mutation analysis of EPM2A revealed a homozygous Arg241stop mutation in both patients.

CONCLUSIONS

This is the first description of severe hepatic dysfunction as the initial clinical manifestation of LD. The phenotypic differences between the two affected siblings suggest that modifier genes must condition clinical expression of the disease outside the CNS.

Lafora disease due to EPM2B mutations: a clinical and genetic study

OBJECTIVE

To study EPM2B gene mutations and genotype-phenotype correlations in patients with Lafora disease.

METHODS

The authors performed a clinical and mutational analysis of 25 patients, from 23 families, diagnosed with Lafora disease who had not shown mutations in the EPM2A gene.

RESULTS

The authors identified 18 mutations in EPM2B, including 12 novel mutations: 4 nonsense mutations (R265X, C26X, W219X, and E67X), a 6-base pair (bp) microdeletion resulting in a two amino acid deletion (V294_K295del), a 4-bp insertion resulting in a frameshift mutation (S339fs12), and 6 missense mutations (D308A, I198N, C68Y, E67Q, P264H, and D233A). In our data set of 77 families with Lafora disease, 54 (70.1%) tested probands have mutations in EPM2A, 21 (27.3%) in EPM2B, and 2 (2.6%) have no mutations in either gene. The course of the disease was longer in patients with EPM2B mutations vs patients with EPM2A mutations.

CONCLUSIONS

Genetic allelic heterogeneity is present in Lafora disease associated with mutations in EPM2B. Patients with mutations in EPM2A and EPM2B express similar clinical manifestation, although patients with EPM2B-associated Lafora disease seem to have a slightly milder clinical course. The lack of mutations in EPM2A and EPM2B in two families could be because of the presence of mutations in noncoding, nontested regions or the existence of an additional gene associated with Lafora disease.

Functional analysis in serum from atypical Hemolytic Uremic Syndrome patients reveals impaired protection of host cells associated with mutations in factor H

A subgroup of patients with the most severe form of the Hemolytic Uremic Syndrome (HUS) presents mutations in the complement regulatory protein factor H. The functional analyses of the factor H mutant proteins purified from some of these patients have shown a specific defect in the capacity to control complement activation on cellular surfaces. Here, we show that these factor H-related complement regulatory defects can be detected in the patients' serum with a simple hemolytic assay. Data obtained from HUS patients and control individuals indicate that this assay is a useful tool for the molecular diagnosis of factor H-related HUS.

Clustering of missense mutations in the C-terminal region of factor H in atypical hemolytic uremic syndrome

Hemolytic-uremic syndrome (HUS) is a microvasculature disorder leading to microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure. Most cases of HUS are associated with epidemics of diarrhea caused by verocytotoxin-producing bacteria, but atypical cases of HUS not associated with diarrhea (aHUS) also occur. Early studies describing the association of aHUS with deficiencies of factor H suggested a role for this complement regulator in aHUS. Molecular evidence of factor H involvement in aHUS was first provided by Warwicker et al., who demonstrated that aHUS segregated with the chromosome 1q region containing the factor H gene (HF1) and who identified a mutation in HF1 in a case of familial aHUS with normal levels of factor H. We have performed the mutational screening of the HF1 gene in a novel series of 13 Spanish patients with aHUS who present normal complement profiles and whose plasma levels of factor H are, with one exception, within the normal range. These studies have resulted in the identification of five novel HF1 mutations in four of the patients. Allele HF1 Delta exon2, a genomic deletion of exon 2, produces a null HF1 allele and results in plasma levels of factor H that are 50% of normal. T956M, W1183L, L1189R, and V1197A are missense mutations that alter amino acid residues in the C-terminal portion of factor H, within a region--SCR16-SCR20--that is involved in the binding to solid-phase C3b and to negatively charged cellular structures. This remarkable clustering of mutations in HF1 suggests that a specific dysfunction in the protection of cellular surfaces by factor H is a major pathogenic condition underlying aHUS.

Assessment of the interaction of human complement regulatory proteins with group A Streptococcus. Identification of a high-affinity group A Streptococcus binding site in FHL-1

Group A Streptococcus (GAS), the most frequent bacterial cause of suppurative infections in humans, expresses on the cell surface M proteins with capacity to bind factor H, FHL-1 and C4b binding protein (C4BP). This has been interpreted as a mechanism developed by this pathogen to decrease phagocytosis by macrophages and polymorphonuclear cells. We report the analysis of the capacity to bind factor H, FHL-1 and C4BP of 69 clinical isolates from 19 different serotypes. We show that strains binding complement regulators (30/69) belong to specific M serotypes. Of these, M18 strains are relatively frequent and interact with all three complement regulators simultaneously. However, the most virulent M1 and M3 strains did not bind complement regulators in our assays. The relevance of the interaction between complement regulators and S. pyogenes was analyzed using different approaches with the conclusion that under physiological conditions only FHL-1 and C4BP bind to streptococci. We show that FHL-1 presents a higher binding affinity for S. pyogenes than factor H because it carries a hydrophobic, high-affinity, GAS binding site in addition to the heparin binding site in SCR7. Using synthetic peptides we provide evidence that the high-affinity GAS binding site in FHL-1 involves the hydrophobic tail (Ser-Phe-Thr-Leu) that distinguishes FHL-1 from factor H.

Mutational analysis of the HGO gene in Finnish alkaptonuria patients

Alkaptonuria (AKU), the prototypic inborn error of metabolism, has recently been shown to be caused by loss of function mutations in the homogentisate-1,2-dioxygenase gene (HGO). So far 17 mutations have been characterised in AKU patients of different ethnic origin. We describe three novel mutations (R58fs, R330S, and H371R) and one common AKU mutation (M368V), detected by mutational and polymorphism analysis of the HGO gene in five Finnish AKU pedigrees. The three novel AKU mutations are most likely specific for the Finnish population and have originated recently.

Genomic cloning and characterization of the human homeobox gene SIX6 reveals a cluster of SIX genes in chromosome 14 and associates SIX6 hemizygosity with bilateral anophthalmia and pituitary anomalies

The Drosophila gene sine oculis (so), a nuclear homeoprotein that is required for eye development, has several homologues in vertebrates (the SIX gene family). Among them, SIX3 is considered to be the functional orthologue of so because it is strongly expressed in the developing eye. However, embryonic SIX3 expression is not limited to the eye field, and SIX3 has been found to be mutated in some patients with holoprosencephaly type 2 (HPE2), suggesting that SIX3 has wide implications in head development. We report here the cloning and characterization of SIX6, a novel human SIX gene that is the homologue of the chick Six6(Optx2) gene. SIX6 is closely related to SIX3 and is expressed in the developing and adult human retina. Data from chick and mouse suggest that the human SIX6 gene is also expressed in the hypothalamic and the pituitary regions. SIX6 spans 2567 bp of genomic DNA and is split in two exons that are transcribed into a 1393-nucleotide-long mRNA. Chromosomal mapping of SIX6 revealed that it is closely linked to SIX1 and SIX4 in human chromosome 14q22.3-q23, which provides clues about the origin and evolution of the vertebrate SIX family. Recently three independent reports have associated interstitial deletions at 14q22.3-q23 with bilateral anophthalmia and pituitary anomalies. Genomic analyses of one of these cases demonstrated SIX6 hemizygosity, strongly suggesting that SIX6 haploinsufficiency is responsible for these developmental disorders.

Expression of the beta-chain of the complement regulator C4b-binding protein in human ovary

Human C4b-binding protein (C4BP) is an important regulator of the complement system that also binds and inactivates the anticoagulant vitamin K-dependent protein S. These two activities are performed by two distinct polypeptides of 70 kDa and 45 kDa known as alpha and beta chains, respectively. C4BP is present in plasma in various isoforms with different alpha/beta composition. We report here that C4BPbeta, but not C4BPalpha, is expressed in adult human ovary. Expression of C4BPbeta was detected in all ovarian biopsies analyzed (n = 15), independently of age and phase of the menstrual cycle. In situ hybridization and immunostaining analyses on cryostat sections demonstrated expression of C4BPbeta in both regressing corpus luteum and corpus albicans, but not in the follicles, the corpus luteum, the ovary stroma or the vascular cells. In addition, we noted that the expression pattern of the C4BPbeta mRNA resembles that described for the connective tissue that invades the degenerating corpus luteum and causes a progressive fibrosis that gradually converts it into a scar, the corpus albicans. RT-PCR and immunostaining analyses of primary cultures derived from human ovaries demonstrated the presence of fibroblast-like cells that express C4BPbeta. As a whole, these data suggest a role for the C4BPbeta in human ovary during the healing and scar resorption processes that leads to the formation of the corpus albicans and its replacement by ovarian stroma.

Analysis of alkaptonuria (AKU) mutations and polymorphisms reveals that the CCC sequence motif is a mutational hot spot in the homogentisate 1,2 dioxygenase gene (HGO)

We recently showed that alkaptonuria (AKU) is caused by loss-of-function mutations in the homogentisate 1,2 dioxygenase gene (HGO). Herein we describe haplotype and mutational analyses of HGO in seven new AKU pedigrees. These analyses identified two novel single-nucleotide polymorphisms (INV4+31A-->G and INV11+18A-->G) and six novel AKU mutations (INV1-1G-->A, W60G, Y62C, A122D, P230T, and D291E), which further illustrates the remarkable allelic heterogeneity found in AKU. Reexamination of all 29 mutations and polymorphisms thus far described in HGO shows that these nucleotide changes are not randomly distributed; the CCC sequence motif and its inverted complement, GGG, are preferentially mutated. These analyses also demonstrated that the nucleotide substitutions in HGO do not involve CpG dinucleotides, which illustrates important differences between HGO and other genes for the occurrence of mutation at specific short-sequence motifs. Because the CCC sequence motifs comprise a significant proportion (34.5%) of all mutated bases that have been observed in HGO, we conclude that the CCC triplet is a mutational hot spot in HGO.

Genomic cloning, structure, expression pattern, and chromosomal location of the human SIX3 gene

The Drosophila gene sine oculis (so) is a nuclear homeoprotein that is required for eye development. Homologous genes to so, denoted SIX genes, have been found in vertebrates. Among the SIX genes, SIX3 is considered to be the functional homologue of so. To provide insight into the potential implications of SIX3 in human ocular malformations, we have cloned and characterized the human SIX3 gene. In human eye, SIX3 produces a 3-kb transcript that codes for a 332-amino-acid polypeptide that is virtually identical to its mouse and chick homologues. Expression of SIX3 was detected in human embryos as early as 5-7 weeks of gestation and found to be maintained in the eye throughout the entire period of fetal development. At 20 weeks of gestation, expression of SIX3 in the human retina was detected in the ganglion cells and in cells of the inner nuclear layer. The human SIX3 gene spans 4.4 kb of genomic DNA and is split in two exons separated by a 1659-bp intron. SIX3 was mapped to human chromosome 2p16-p21, between the genetic markers D2S119 and D2S288. Interestingly, the map position of human SIX3 overlaps the locations of two dominant disorders with ocular phenotypes that have been assigned to this chromosomal region, holoprosencephaly type 2 and Malattia Leventinese.

Sequence and structure of the human 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase heart isoform gene (PFKFB2)

6-Phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2/FBPase-2) is a bifunctional enzyme that catalyzes the synthesis and degradation of Fru-2,6-P2, a key regulator of glycolysis. In mammals, several genes have been found to code for different PFK-2/FBPase-2 isoforms that differ in tissue distribution and enzymatic activities. In the present study, we report the characterization of the PFK-2/FBPase-2 heart isoform gene in humans (PFKFB2), including a full analysis of repetitive sequences and potential transcription binding sites. The genomic sequence of the PFKFB2 gene spans 22,485 bp and contains 15 exons. Heart cDNA analysis shows that PFKFB2 codes for a protein of 505 amino acids with a deduced molecular mass of 58,849 Da. Comparison of the human PFKFB2 gene to the homologous genes in rat and ox outlines a significant conservation of the intron-exon structure, sequence of 5' and 3' flanking regions, and simple sequence repetitive element positions. Most important, the human heart PFK-2/ FBPase-2 protein was found to retain all the important regulatory sites, as well as the catalytic and substrate binding sites identified in the rat and bovine heart isoforms, suggesting that the human enzyme is regulated in a manner similar to that observed in these organisms.

A region of allelic imbalance in 1q31-32 in primary breast cancer coincides with a recombination hot spot

Previous studies have shown that the 1q31-32 region frequently presents allelic imbalance (AI) in various neoplastic diseases, such as breast cancer, medulloblastoma, male germ cell tumors, and renal collecting duct carcinoma, suggesting the presence of a tumor suppressor gene in this location. We used 19 informative microsatellite markers to analyze 33 primary breast tumors for AI in the 1q31-32 region. Our results demonstrate a 10-cM critical region of AI that is present in more than 60% of the tumors. This region is located proximal to the REN locus and is flanked by the CACNL1A3 and D1S2655 markers. Most important, the critical region of AI coincides with a female hot spot of recombination, suggesting a possible correlation between the two regions.

The human homogentisate 1,2-dioxygenase (HGO) gene

Alkaptonuria (AKU; McKusick No. 203500), a rare hereditary disorder of the phenylalanine catabolism, was the first disease to be interpreted as an inborn error of metabolism (A. E. Garrod, 1902, Lancet 2: 1616-1620). AKU patients are deficient for homogentisate 1,2-dioxygenase (HGO; EC 1.13.11.5). This enzymatic deficiency causes homogentisic aciduria, ochronosis, and arthritis. Recently we cloned the human HGO gene and showed that AKU patients carry two copies of a loss-of-function HGO allele. Here we describe the complete nucleotide sequence of the human HGO gene and the identification of its promoter region. The human HGO gene spans 54,363 bp and codes for a 1715-nt-long transcript that is split into 14 exons ranging from 35 to 360 bp. The HGO introns, 605 to 17,687 bp in length, contain representatives of the major classes of repetitive elements, including several simple sequence repeats (SSR). Two of these SSRs, a (CT)n repeat in intron 4 and a (CA)n repeat in intron 13, were found to be polymorphic in a Spanish population sample. The HGO transcription start site was determined by primer extension. We report that sequences from -1074 to +89 bp (relative to the HGO transcription start site) are sufficient to promote transcription of a CAT reporter gene in human liver cells and that this fragment contains putative binding sites for liver-enriched transcription factors that might be involved in the regulation of HGO expression in liver.

Modulation of C4b-binding protein isoforms during the acute phase response caused by orthopedic surgery

Orthopedic surgery is described as an event with a high risk of thromboembolic diseases. This is probably a consequence of a synergistic combination of different risk factors in the patients subjected to this type of surgery, including age, immobilization, anesthesia and different hypercoagulable states. After surgery patients develop an acute-phase response that leads to changes in several plasma proteins. One of these proteins is the complement regulator C4b-binding protein (C4BP). We have recently shown that in some acute-phase patients C4BP is incorrectly controlled (with elevation of the C4BP beta-containing isoforms), leading to a potential hypercoagulable state by decreasing the plasma levels of free (active) protein S. Here we have studied whether patients subjected to orthopedic surgery have an appropriate modulation of the C4BP isoforms during their postoperative acute-phase responses. We have analyzed the evolution of the C4BP isoforms in serial samples from 11 patients who have undergone knee (or hip) prosthesis surgery (mean age 70 years), or scoliosis surgery (mean age 18 years). Our data suggest a similar evolution of C4BP isoforms in all these patients, with an almost exclusive increase of C4BP isoforms lacking C4BP beta polypeptides and steady levels of free protein S.

A high-resolution map of the regulator of the complement activation gene cluster on 1q32 that integrates new genes and markers

Sixteen microsatellite markers, including two described here, were used to construct a high-resolution map of the 1q32 region encompassing the regulator of the complement activation (RCA) gene cluster. The RCA genes are a group of related genes coding for plasma and membrane associated proteins that collectively control activation of the complement component C3. We provide here the location of two new genes within the RCA gene cluster. These genes are PFKFB2 that maps 15 kilobases (kb) upstream of the C4BPB gene, and a gene located 4 kb downstream of C4BPA, which seems to code for the 72 000 Mr component of the signal recognition particle (SRP72). Neither of these two genes is related structurally or functionally to the RCA genes. In addition, our map shows the centromere-telomere orientation of the C4BPB/MCP linkage group, which is: centromere-PFKFB2-C4BPB-C4BPA-SRP72-C4BPAL1++ +-C4BPAL2-telomere, and outlines an interval with a significant female-male recombination difference which suggests the presence of a female-specific hotspot(s) of recombination.

Binding of human complement component C4b-binding protein (C4BP) to Streptococcus pyogenes involves the C4b-binding site

A key step in the elimination of invading pathogens from the body is the covalent binding of complement proteins C3b and C4b to their surface. However, many pathogens have evolved mechanisms to avoid the complement system of the host. Understanding how these mechanisms work may lead to more efficacious forms of therapy. Here we provide an insight into the molecular basis of how Streptococcus pyogenes binds human plasma C4b-binding protein (hC4BP), a complement regulatory molecule that may decrease C3b and C4b deposition on the streptococcal surface. We show that streptococcal surface molecules bind to a site on hC4BP that is indistinguishable from the C4b binding site. This site involves multiple binding surfaces that span short consensus repeats 1 to 3 of the alpha-chain of hC4BP. We propose that hC4BP is bound to the bacterial surface because the streptococcal surface molecules involved in the interaction mimic human C4b epitopes.

Binding of human complement component C4b-binding protein (C4BP) to Streptococcus pyogenes involves the C4b-binding site

A key step in the elimination of invading pathogens from the body is the covalent binding of complement proteins C3b and C4b to their surface. However, many pathogens have evolved mechanisms to avoid the complement system of the host. Understanding how these mechanisms work may lead to more efficacious forms of therapy. Here we provide an insight into the molecular basis of how Streptococcus pyogenes binds human plasma C4b-binding protein (hC4BP), a complement regulatory molecule that may decrease C3b and C4b deposition on the streptococcal surface. We show that streptococcal surface molecules bind to a site on hC4BP that is indistinguishable from the C4b binding site. This site involves multiple binding surfaces that span short consensus repeats 1 to 3 of the alpha-chain of hC4BP. We propose that hC4BP is bound to the bacterial surface because the streptococcal surface molecules involved in the interaction mimic human C4b epitopes.

The molecular basis of alkaptonuria

Alkaptonuria (AKU) occupies a unique place in the history of human genetics because it was the first disease to be interpreted as a mendelian recessive trait by Garrod in 1902. Alkaptonuria is a rare metabolic disorder resulting from loss of homogentisate 1,2 dioxygenase (HGO) activity. Affected individuals accumulate large quantities of homogentisic acid, an intermediary product of the catabolism of tyrosine and phenylalanine, which darkens the urine and deposits in connective tissues causing a debilitating arthritis. Here we report the cloning of the human HGO gene and establish that it is the AKU gene. We show that HGO maps to the same location described for AKU, illustrate that HGO harbours missense mutations that cosegregate with the disease, and provide biochemical evidence that at least one of these missense mutations is a loss-of-function mutation.

Promoter region of the human gene coding for beta-chain of C4b binding protein. Hepatocyte nuclear factor-3 and nuclear factor-I/CTF transcription factors are required for efficient expression of C4BPB in HepG2 cells

Differential expression of the human genes coding for the alpha and beta polypeptides of the human C component C4b binding protein (C4BP) modulates the levels of C4BP molecules containing C4BP beta polypeptides, providing a mechanism to avoid the potential harmful effects of elevated concentrations of C4BP beta in plasma. To understand how the expression of the C4BPB gene is controlled, we have examined, in the major promoter of the human C4BP B gene, potential regulatory elements. A region from nucleotide -126 to +25 was able to drive high expression of a reporter gene in the human hepatoma cell line HepG2. A small subfragment of this region (from -126 to -90) is responsible for more than 90% of the promoter activity. Electrophoretic mobility shift assays revealed that transcription factors of the hepatocyte nuclear factor-3 (HNF-3) and nuclear factor-I (NFI/CTF) families were able to bind to this region in a sequence-specific manner. We have characterized binding sites for these transcription factors and determined their relative contribution to the activity of the C4BPB promoter. The results suggest that cooperative interaction between HNF-3 and NF-I/CTF is required to obtain a full C4BPB promoter activity. Comparison of the structures of the C4BPA and C4BPB promoters reveals significant differences that could explain the differential transcription of the C4BP alpha and C4BP beta polypeptides during the acute phase response.

Promoter region of the human gene coding for beta-chain of C4b binding protein. Hepatocyte nuclear factor-3 and nuclear factor-I/CTF transcription factors are required for efficient expression of C4BPB in HepG2 cells

Differential expression of the human genes coding for the alpha and beta polypeptides of the human C component C4b binding protein (C4BP) modulates the levels of C4BP molecules containing C4BP beta polypeptides, providing a mechanism to avoid the potential harmful effects of elevated concentrations of C4BP beta in plasma. To understand how the expression of the C4BPB gene is controlled, we have examined, in the major promoter of the human C4BP B gene, potential regulatory elements. A region from nucleotide -126 to +25 was able to drive high expression of a reporter gene in the human hepatoma cell line HepG2. A small subfragment of this region (from -126 to -90) is responsible for more than 90% of the promoter activity. Electrophoretic mobility shift assays revealed that transcription factors of the hepatocyte nuclear factor-3 (HNF-3) and nuclear factor-I (NFI/CTF) families were able to bind to this region in a sequence-specific manner. We have characterized binding sites for these transcription factors and determined their relative contribution to the activity of the C4BPB promoter. The results suggest that cooperative interaction between HNF-3 and NF-I/CTF is required to obtain a full C4BPB promoter activity. Comparison of the structures of the C4BPA and C4BPB promoters reveals significant differences that could explain the differential transcription of the C4BP alpha and C4BP beta polypeptides during the acute phase response.

Isoforms of human C4b-binding protein. I. Molecular basis for the C4BP isoform pattern and its variations in human plasma

Human C4b-binding protein (C4BP) is an important regulator of the complement system that also binds and inactivates the anticoagulant vitamin K-dependent protein S. These two activities are performed by two distinct polypeptides of 70 kDa and 45 kDa known as alpha- and beta-chains, respectively. C4BP is present in plasma in various isoforms with different alpha beta composition. Here we report multiple discrete variations of the relative levels of the C4BP isoforms among normal individuals and provide evidence that they are determined by genetic factors that segregate with the regulator of complement activation gene cluster. We also report the characterization of the C4BP molecules secreted by HepG2 and Hep3B cells, as well as transfection experiments in COS cells, to illustrate that the relative levels of expression of the C4BPA and C4BPB genes play a major role in determining the proportion in which the different C4BP isoforms are synthesized. Altogether, the data indicate that the human C4BP isoform pattern is genetically determined, but can be modified by factors with a differential effect on the expression of the C4BPA and C4BPB genes. These observations provide a new way to explore the possible association between elevated levels of C4BP and an increased risk to thromboembolic disorders.

Isoforms of human C4b-binding protein. II. Differential modulation of the C4BPA and C4BPB genes by acute phase cytokines

Human C4b-binding protein (C4BP) controls activation of the complement system and inactivates the anticoagulant vitamin K-dependent protein S using two distinct polypeptides known as C4BP alpha and C4BP beta, respectively. C4BP presents three isoforms, alpha 7 beta 1, alpha 7 beta 0, and alpha 6 beta 1, the proportion of which depends on the relative levels of C4BP alpha and C4BP beta. To better understand the regulation of C4BP during the acute phase response we analyzed the C4BP isoforms in 23 serial samples of acute phase patients and characterized the effect of various acute phase cytokines on the expression of the C4BPA and C4BPB genes using Hep3B cells. We show that the elevation of C4BP during acute phase response leads to changes in the proportion of the C4BP isoforms. However, there are striking differences among acute phase individuals. Some of them present a pattern of induction that primarily affects the alpha 7 beta 0 isoform, whereas others present the opposite situation, increasing the C4BP beta-containing isoforms. In vitro studies demonstrate that IL-6, IL-1 beta, and INF-gamma increase the levels of both C4BP alpha- and C4BP beta-mRNAs, whereas TNF-alpha down-regulates these mRNAs. INF-gamma shows, in addition, a differential effect on the C4BP alpha- and C4BP beta-mRNAs. Differential modulation of the C4BPA and C4BPB genes has been postulated as an efficient mechanism to maintain steady concentrations of C4BP beta when C4BP is induced. A synergistic 10-fold induction of C4BP alpha-mRNA, but a marginal increase of C4BP beta-mRNA, was observed when INF-gamma was used together with TNF-alpha, suggesting that association of these cytokines is critical to avoid elevation of C4BP beta during the acute phase induction of C4BP.

Isoforms of human C4b-binding protein. II. Differential modulation of the C4BPA and C4BPB genes by acute phase cytokines

Human C4b-binding protein (C4BP) controls activation of the complement system and inactivates the anticoagulant vitamin K-dependent protein S using two distinct polypeptides known as C4BP alpha and C4BP beta, respectively. C4BP presents three isoforms, alpha 7 beta 1, alpha 7 beta 0, and alpha 6 beta 1, the proportion of which depends on the relative levels of C4BP alpha and C4BP beta. To better understand the regulation of C4BP during the acute phase response we analyzed the C4BP isoforms in 23 serial samples of acute phase patients and characterized the effect of various acute phase cytokines on the expression of the C4BPA and C4BPB genes using Hep3B cells. We show that the elevation of C4BP during acute phase response leads to changes in the proportion of the C4BP isoforms. However, there are striking differences among acute phase individuals. Some of them present a pattern of induction that primarily affects the alpha 7 beta 0 isoform, whereas others present the opposite situation, increasing the C4BP beta-containing isoforms. In vitro studies demonstrate that IL-6, IL-1 beta, and INF-gamma increase the levels of both C4BP alpha- and C4BP beta-mRNAs, whereas TNF-alpha down-regulates these mRNAs. INF-gamma shows, in addition, a differential effect on the C4BP alpha- and C4BP beta-mRNAs. Differential modulation of the C4BPA and C4BPB genes has been postulated as an efficient mechanism to maintain steady concentrations of C4BP beta when C4BP is induced. A synergistic 10-fold induction of C4BP alpha-mRNA, but a marginal increase of C4BP beta-mRNA, was observed when INF-gamma was used together with TNF-alpha, suggesting that association of these cytokines is critical to avoid elevation of C4BP beta during the acute phase induction of C4BP.

Isoforms of human C4b-binding protein. I. Molecular basis for the C4BP isoform pattern and its variations in human plasma

Human C4b-binding protein (C4BP) is an important regulator of the complement system that also binds and inactivates the anticoagulant vitamin K-dependent protein S. These two activities are performed by two distinct polypeptides of 70 kDa and 45 kDa known as alpha- and beta-chains, respectively. C4BP is present in plasma in various isoforms with different alpha beta composition. Here we report multiple discrete variations of the relative levels of the C4BP isoforms among normal individuals and provide evidence that they are determined by genetic factors that segregate with the regulator of complement activation gene cluster. We also report the characterization of the C4BP molecules secreted by HepG2 and Hep3B cells, as well as transfection experiments in COS cells, to illustrate that the relative levels of expression of the C4BPA and C4BPB genes play a major role in determining the proportion in which the different C4BP isoforms are synthesized. Altogether, the data indicate that the human C4BP isoform pattern is genetically determined, but can be modified by factors with a differential effect on the expression of the C4BPA and C4BPB genes. These observations provide a new way to explore the possible association between elevated levels of C4BP and an increased risk to thromboembolic disorders.

Expression of the human gene coding for the alpha-chain of C4b-binding protein, C4BPA, is controlled by an HNF1-dependent hepatic-specific promoter

C4b-binding protein (C4BP) is an abundant oligomeric plasma glycoprotein which controls the activation of the complement cascade through the classical pathway. In humans, the majority form of C4BP is composed of seven alpha-chains and one beta-chain, covalently linked by their C-termini. C4BP is mainly expressed in the liver. We have previously cloned and characterized the structure of the genes encoding the alpha and beta chains, C4BPA and C4BPB, respectively. Here we addressed the characterization of the mechanisms controlling the hepatic restricted expression of the C4BPA gene. We found that the C4BPA promoter is contained within the first 369 bp upstream of the transcription start site. The activity of this promoter is restricted to hepatic cells in transfection experiments. The hepatic transcription factor HNF1 interacts with a region of this promoter at -38 bp. This region is absolutely required for the activity of this promoter, suggesting that HNF1 is essential for the hepatic activity of the C4BPA promoter. We speculate that this extreme requirement of HNF1 for the activity of the human C4BPA promoter is related to the fact that this promoter lacks a TATA box.

The gene coding for the beta-chain of C4b-binding protein (C4BPB) has become a pseudogene in the mouse

C4BP beta is one of the two polypeptides that in humans compose the plasma glycoprotein C4b-binding protein (C4BP). C4BP beta binds the anticoagulant vitamin K-dependent protein S. Two, nonmutually exclusive, roles have been proposed for the C4BP-protein S interaction. It has been suggested to play a role in the control of the protein C anticoagulatory pathway. In addition, it may serve an important role in localizing C4BP to the surface of injured or activated cells. While the physiological significance of C4BP-protein S interaction is unclear, it has clinical relevance because elevated plasma levels of C4BP are associated with increased risk for thromboembolic disorders in humans, due to an inactivation of the protein C anticoagulatory pathway. Using a human C4BP beta cDNA probe, we have isolated and characterized a genomic DNA fragment that includes the murine C4BPB gene. Murine C4BPB is a single-copy gene that maps close to the C4BPA gene in chromosome 1. It contains two exons homologous to the exons coding for the SCR-1 and SCR-2 repeats of the human C4BP beta polypeptide chain. Sequence analysis of the C4BPB exons in the Mus musculus inbred strains CBA, Balb/c, and C57BL/6, in pen-bred Swiss mice, and in Mus spretus demonstrated the presence of two in-phase stop codons that are incompatible with the expression of a functional C4BP beta polypeptide. Thus, the characterization of the murine C4BPB gene documents the peculiar situation of a single-copy gene that is functional in humans but has become a pseudogene in the mouse.(ABSTRACT TRUNCATED AT 250 WORDS)

C4BPAL1, a member of the human regulator of complement activation (RCA) gene cluster that resulted from the duplication of the gene coding for the alpha-chain of C4b-binding protein

The regulator of complement activation (RCA) gene cluster evolved by multiple gene duplications to produce a family of genes coding for proteins that collectively control the activation of the complement system. We report here the characterization of C4BPAL1, a member of the human RCA gene cluster that arose from the duplication of the C4BPA gene after the separation of the rodent and primate lineages. C4BPAL1 maps 20 kb downstream of the C4BPA gene and is the same 5' to 3' orientation found for all RCA genes characterized thus far. It includes nine exon-like regions homologous to exons 2-8, 11, and 12 of the C4BPA gene. Analysis of the C4BPAL1 sequence suggests that it is currently a pseudogene in humans. However, comparisons between C4BPAL1 and the human and murine C4BPA genes show sequence conservation, which strongly suggests that, for a long period of time, C4BPAL1 has been a functional gene coding for a protein with structural requirements similar to those of the alpha-chain of C4b-binding protein.

Regional localization of the human vitronectin receptor alpha subunit gene (VNRA) to chromosome 2q31-->q32

The vitronectin receptor (av:beta 3; CD51/CD61), a member of the beta 3 integrin subfamily (cytoadhesins), functions as a receptor for a group of proteins that includes vitronectin, fibrinogen, thrombospondin, and von Willebrand factor. The human locus for the av gene (VNRA) was previously mapped to the long arm of chromosome 2 by DNA analysis of somatic cell hybrids. By using fluorescence in situ hybridization, coupled with GTG-banding, we have regionally mapped the human av gene to chromosome 2q31-->q32. An identical location was previously reported for the human gene coding for the integrin VLA-alpha 4 subunit (CD49D). These data, therefore, suggest the existence of a cluster of integrin genes at this chromosomal location.

Does the mouse C4-binding protein gene (C4BP) map in the H-2 region?

A previous study on the genetics of mouse C4-binding protein (C4-bp) indicated the existence of a genetic polymorphism. Two genetic variants were reported and their segregation used to determine the mapping position of the C4BP locus to the H-2D-Qa interval of the mouse H-2 system. We show here, however, that purified C4-bp does not display the previously reported polymorphism. The mapping position of C4BP in the mouse therefore remains undetermined.

Genetic polymorphism of human factor H (beta 1H)

Human Factor H (beta 1H) was found to be polymorphic after neuraminidase treatment and isoelectric focusing (IEF) under completely denaturing conditions. Three variants, FH 1, FH 2, and FH 3, were identified in a sample population of 81 unrelated caucasoid individuals. Family studies demonstrated correct mendelian segregation of FH 1, FH 2, and FH 3. Our data indicate that these genetic variants of human Factor H are encoded by three codominant alleles, FH*1, FH*2, and FH*3, at a single autosomal locus FH. In the sample analyzed, the gene frequencies of FH*1, FH*2, and FH*3 were, respectively, 0.691, 0.302, and 0.006.

Genetic polymorphism of human factor H (beta 1H)

Human Factor H (beta 1H) was found to be polymorphic after neuraminidase treatment and isoelectric focusing (IEF) under completely denaturing conditions. Three variants, FH 1, FH 2, and FH 3, were identified in a sample population of 81 unrelated caucasoid individuals. Family studies demonstrated correct mendelian segregation of FH 1, FH 2, and FH 3. Our data indicate that these genetic variants of human Factor H are encoded by three codominant alleles, FH*1, FH*2, and FH*3, at a single autosomal locus FH. In the sample analyzed, the gene frequencies of FH*1, FH*2, and FH*3 were, respectively, 0.691, 0.302, and 0.006.