Patients with persistent APC-resistance without factor V Leiden mutation.

BACKGROUND: Activated protein C (APC) resistance and factor V Leiden mutation are major risk factors for deep venous thrombosis. Previous work has led to the view that the coagulation phenotype and the genetic defect are associated in almost all patients. It has been reported about single APC-resistant patients without associated factor V Leiden, but significance and thrombotic risk of this constellation have not yet been established. PATIENTS AND METHODS: We tested 486 consecutive patients with deep venous thrombosis, arterial disease or other than vascular disease for APC-resistance with a factor VIII based assay. RESULTS: 149 patients (31%) showed a pathological APC-ratio. Sensitivity and specificity for detection of factor V Leiden were 100% and 40%, respectively. At 6 months follow-up APC-ratio returned to normal in 55% of the patients with initial pathological APC-resistance. At 12 months follow-up 91% of the patients with persistent APC-resistance showed a pathological ratio as well. CONCLUSIONS: Patients with APC-resistance not due to factor V Leiden can be attributed to one subset with reversible APC-resistance--possibly due to a hypercoagulable state in an acute thrombotic situation, and to another with persistent APC-resistance.

Identification of an intronic regulatory element in the human protein C (PROC) gene.

Regulatory DNA elements responsible for human protein C (PROC) gene expression have previously been identified in the upstream promoter region and first (untranslated) exon of the gene. Here we show that an additional sequence element located more than 500 bp downstream of the core promoter within intron 1 further enhances PROC promoter-driven reporter gene expression in human hepatoma cells. In common with core promoter constructs used in previous studies, the activity of this 3'-extended regulatory region is diminished by a naturally occurring promoter mutation. However, in contrast to constructs lacking intronic sequence, the promoter/intron regulatory region is repressed rather than activated by the transcription factor HNF-1. Using both conventional alignment procedures and complexity analysis to study the human and canine PROC sequences, we identified two conserved intronic regions, which were tested for their involvement in gene regulation. High-level gene expression from the intron-coupled promoter was dependent upon the integrity of a 142 bp sequence element, a duplicate copy of which is located in an upstream region of the PROC gene that possesses enhancer activity. These findings emphasise the potential importance of intragenic sequences for gene regulation and serve to illustrate that the results of PROC promoter/reporter gene experiments are critically dependent upon the sequence context. The identification of such intragenic elements is relevant to the analysis of human genetic disease since it will facilitate the detection and functional evaluation of regulatory mutations and polymorphisms.

Expression of human TRPC genes in the megakaryocytic cell lines MEG01, DAMI and HEL.

Store-regulated Ca2+ entry represents a major mechanism for Ca2+ influx in non-excitable cells although many details remain to be evaluated including the identification of cation entry channels. Recently human homologues of the Drosophila proteins TRP and TRPL, have been described (TRPC1, TRPC1A, HTRP1) and suggested as candidate cation channels. In this study we sought to examine if the producers of blood platelets, megakaryocytic cells (using the cell lines MEG01, DAMI, HEL), expressed these genes. RNA was prepared from the cell lines and platelets and converted to cDNA. The cDNA was then subjected to 30-35 cycles of PCR using gene specific primers for TRPC1-3. PCR products of the expected sizes were observed for all three TRPC genes in the three cell lines. Direct sequencing confirmed their identity. Additionally for TRPC1, a larger species, and for TRPC2, a smaller species was detected in all three cell lines with sequencing revealing the fragments to contain TRPC sequence, suggesting that they were either products of alternative splicing events or from closely related genes. These results suggest that TRPC genes are expressed in megakaryocytic cell lines and that the TRPC proteins may play a role in mediating cation influx in both megakaryocytes and platelets.

Aberrant RNA splicing of the protein C and protein S genes in healthy individuals.

RNA-based studies are an important tool for the identification and functional characterization of mutations underlying inherited disease. These studies could in principle be compromised by 'aberrant splicing' (the generation of alternatively spliced transcripts lacking any obvious function) during normal expression of the genes under investigation. Using a highly sensitive RT-PCR assay, we show here that aberrant splicing is a frequent occurrence during expression of the protein C (PROC) and protein S (PROS) genes. Aberrantly spliced transcripts were present in different cell types including liver, the main expressing tissue for both protein C and protein S. In an attempt to compare individual mRNA splicing patterns, PROC and PROS RNA from easily accessible cells of different healthy control individuals was studied. However, variation between different RT-PCR assays from the same individual precluded both the relative quantitation of the aberrant transcripts and the analysis of interindividual differences. Our findings are consistent with the notion that a low level of aberrantly spliced transcripts are routinely generated during PROC and PROS gene expression. The possibility that these transcripts may complicate the RT-PCR analysis of pathological transcripts must be taken into account when RNA-based strategies of disease analysis are considered.

Ectopic transcript analysis indicates that allelic exclusion is an important cause of type I protein C deficiency in patients with nonsense and frameshift mutations in the PROC gene.

Nonsense mutations, deletions and splice site mutations are a common cause of type I protein C deficiency. Either directly or indirectly by altering the reading frame, these lesions generate or may generate premature stop codons and could therefore be expected to result in premature termination of translation. In this study, the possibility that such mutations could instead exert their pathological effects at an earlier stage in the expression pathway, through "allelic exclusion" at the RNA level, was investigated. Protein C (PROC) mRNA was analysed in seven Spanish type I protein C deficient patients heterozygous for two nonsense mutations, a 7bp deletion, a 2bp insertion and three splice site mutations. Ectopic RNA transcripts from patient and control lymphocytes were analysed by RT-PCR and direct sequencing of amplified PROC cDNA fragments. The nonsense mutations and the deletion were absent from the cDNAs indicating that only mRNA derived from the normal allele had been expressed. Similarly for the splice site mutations, only normal PROC cDNAs were obtained. In one case, exclusion of the mutated allele could be confirmed by polymorphism analysis. In contrast to these six mutations, the 2 bp insertion was not associated with loss of mRNA from the mutated allele. In this case, cDNA analysis revealed the absence of 19 bases from the PROC mRNA consistent with the generation and utilization of a cryptic splice site 3' to the site of mutation, which would result in a frameshift and a premature stop codon. It is concluded that allelic exclusion is a common causative mechanism in those cases of type I protein C deficiency which result from mutations that introduce premature stop codons.

Detection and characterization of seven novel protein S (PROS) gene lesions: evaluation of reverse transcript-polymerase chain reaction as a mutation screening strategy.

The molecular genetic analysis of protein S deficiency has been hampered by the complexity of the protein S (PROS) gene and by the existence of a homologous pseudogene. In an attempt to overcome these problems, a reverse transcript-polymerase chain reaction (RT-PCR) mutation screening procedure was developed. However, the application of this mRNA-based strategy to the detection of gene lesions causing heterozygous type I protein S deficiency appears limited owing to the high proportion of patients exhibiting absence of mRNA derived from the mutation-bearing allele ("allelic exclusion"). Nevertheless, this strategy remains extremely effective for rapid mutation detection in type II/III protein S deficiency. Using the RT-PCR technique, a G-to-A transition was detected at position +1 of the exon IV donor splice site, which was associated with type I deficiency and resulted in both exon skipping and cryptic splice site utilization. No abnormal protein S was detected in plasma from this patient. A missense mutation (Asn 217 to Ser), which may interfere with calcium binding, was also detected in exon VIII in a patient with type III protein S deficiency. A further three PROS gene lesions were detected in three patients with type I deficiency by direct sequencing of exon-containing genomic PCR fragments: a single base-pair (bp) deletion in exon XIV, a 2-bp deletion in exon VIII, and a G0to-A transition at position -1 of the exon X donor splice site all resulted in the absence of mRNA expressed from the disease allele. Thus, the RT-PCR methodology proved effective for further analysis of the resulting protein S-deficient phenotypes. A missense mutation (Met570 to Thr) in exon XIV of a further type III-deficient proband was subsequently detected in this patient's cDNA. No PROS gene abnormalities were found in the remaining four subjects, three of whom exhibited allelic exclusion. However, the father of one such patient exhibiting allelic exclusion was subsequently shown to carry a nonsense mutation (Gly448 to Term) within exon XII.

Functional analysis of an unusual length polymorphism in the human antithrombin III (AT3) gene promoter.

The prevalence of the alternative alleles of an unusual length polymorphism in the promoter of the human antithrombin III (AT3) gene was determined in a sample of 155 unrelated individuals from the Northern Irish population. The 108bp L allele and the 32bp S allele occurred at frequencies of 0.21 and 0.79 respectively. Some homology was noted between the L-specific sequence and the region immediately downstream. Residual homology was also evident between the L and S sequences, suggesting that the S allele was derived from the L allele during evolution by partial deletion followed by sequence divergence. The functional significance of the polymorphism was investigated by transient transfection of AT3 promoter/luciferase reporter gene constructs into two human hepatoma cell lines in vitro. The promoter strength of the L allele was found to be 1.6-fold higher than the S allele in HepG2 cells whereas in Hep3B cells, the strength of the S allele was 1.7-fold higher than that of the L allele. In order to evaluate the phenotypic consequences of the AT3 promoter polymorphism in vivo, plasma samples from the 155 control individuals were assayed for antithrombin III (ATIII) activity. Mean activities of the different promoter polymorphism genotypes (SS, LL, SL) were not significantly different. These results suggest that the AT3 promoter polymorphism does not contribute to the variation in plasma ATIII activity that occurs in the general population.

Evaluation of the use of the luciferase-reporter-gene system for gene-regulation studies involving cyclic AMP-elevating agents.

The effects of cyclic AMP (cAMP)-elevating agents on the activity of cis-acting gene promoter sequences are frequently studied using the luciferase-reporter-gene system. The aim of the present study was to assess whether cAMP-elevating agents induce any changes in the level of luciferase activity independently of a transcriptional activation of promoter elements. Chloramphenicol acteyltransferase (CAT) and luciferase reporter genes under the control of the same promoter elements were transiently expressed in primary cultures of human vascular smooth-muscle cells. Transfected cells were treated with a cell-permeable and non-hydrolysable cAMP analogue, 2'-O-monobutyryl-8-bromo cAMP, or with the cAMP-elevating agents forskolin and prostaglandin E1 (PGE1). Forskolin and PGE1 induced a significant increase in the level of luciferase activity, but had no effect on CAT activity. Conclusions based solely on the use of the luciferase-reporter-gene system in studies involving promoter activation by cAMP-elevating agents could therefore be misleading.

Polymorphic variation in the human protein C (PROC) gene promoter can influence transcriptional efficiency in vitro.

The haplotype frequencies of two closely linked diallelic polymorphisms in the protein C(PROC) gene promoter were determined in the British population. In principle, differences in transcription efficiency between PROC promoter haplotypes could represent an additional risk factor in determining whether or not an individual already predisposed to venous thrombosis will come to clinical attention. In order to explore this postulate, transient transfection of human hepatoma cells with PROC promoter-luciferase reporter gene constructs was performed in vitro. In HepG2 cells, the T.....A haplotype exhibited at least a two-fold higher transcription efficiency than the C.....G haplotype. A sample of British protein C deficiency patients with recurrent venous thrombosis was then allelotyped by a combination of oligonucleotide discriminant hybridization and DNA sequencing. The frequency of the low expressing C.....G haplotype was found to be slightly yet not significantly higher in these patients (0.43) than in controls (0.35).

Disruption of a binding site for hepatocyte nuclear factor 1 in the protein C gene promoter is associated with hereditary thrombophilia.

A heterozygous T-->C transition was detected in the putative promoter region of the protein C (PROC) gene in a patient with type I protein C deficiency and a history of recurrent venous thrombosis. This mutation occurred 14 bp upstream of the transcription initiation site and within a sequence strongly homologous to the consensus binding site for the liver-enriched transcription factor, hepatocyte nuclear factor 1 (HNF-1). Transfection experiments demonstrated that a CAT reporter gene construct containing 626 bp of the putative PROC gene promoter was capable of driving CAT expression in HepG2 hepatoma cells. Levels of CAT expression from constructs bearing the mutation were found to be drastically reduced by comparison with the wild-type, consistent with the reduced plasma protein C antigen levels observed in the patient. Gel retardation and cotransfection experiments demonstrated that the mutation abolished both the binding and the transactivating ability of HNF-1 observed with the wild-type PROC gene promoter. Further, the ability of the mutation to disrupt HNF-1 binding appears to be a function not only of the nature of the nucleotide substitution and its position within the recognition sequence, but also of the relative affinity of the wild-type binding site for HNF-1. This analysis is therefore indicative of a vital role for HNF-1 in the expression of the PROC gene in vivo. Taken together with the identification of a human hepatoma cell line which contains HNF-1 but which does not express protein C, these findings are consistent with the view that HNF-1 is necessary although not sufficient for PROC gene expression in the liver.

Combined factor VIII/factor XI deficiency may cause intra-familial clinical variability in haemophilia A among Ashkenazi Jews.

Heterozygous factor XI deficiency occurs very frequently among Ashkenazi Jews. To investigate the potential influence of a co-inherited factor XI deficiency state on the clinical phenotype of mild/moderate haemophilia A, 28 unrelated haemophiliacs of Jewish origin were screened for the two most common factor XI gene mutations. Gene lesions were identified in two out of 14 patients of Ashkenazi origin. In the one family analysed further, co-inheritance of both factor XI and factor VIII deficiencies was associated with a bleeding tendency that was more severe than that associated with either deficiency alone.

Ectopic (illegitimate) transcription: new possibilities for the analysis and diagnosis of human genetic disease.

By means of the Polymerase Chain Reaction (PCR), 'ectopic' or 'illegitimate' transcripts from any gene may be amplified from any tissue or cell type. RNA transcript analysis is therefore no longer dependent upon possession of the often inaccessible 'expressing' tissue. We review here the applications of ectopic transcript analysis to mutation detection and characterization, analysis of RNA splicing and the study of the genotype-phenotype relationship.

De novo splice site mutation in the antithrombin III (AT3) gene causing recurrent venous thrombosis: demonstration of exon skipping by ectopic transcript analysis.

A single basepair substitution at conserved position -1 in the exon 3a donor splice site of the liver-expressed antithrombin III (AT3) gene was detected by PCR/direct sequencing in a patient with sporadic type 1 ATIII deficiency and recurrent venous thrombosis. The lesion, a heterozygous silent AAG----AAA transition at Lys 176 occurred de novo in the proposita. Ectopic transcript analysis of lymphocyte mRNA demonstrated the presence of an abnormally sized mRNA specific to the patient which was shown by cDNA sequencing to lack exon 3a. Oligonucleotide discriminant hybridization demonstrated the absence of any detectable transcript of normal length derived from the disease allele. These findings demonstrate the utility of ectopic transcript analysis in the characterization of defects of mRNA splicing.

Omission of exon 12 in cystic fibrosis transmembrane conductance regulator (CFTR) gene transcripts.

Cystic fibrosis transmembrane conductance regulator (CFTR) mRNA transcripts isolated from both expressing and "non-expressing" cell types of normal individuals exhibit differential splicing to a variable extent in a region encoding the putative nucleotide binding fold of the CFTR polypeptide. Sequence analysis of the aberrant fragments obtained after cDNA polymerase chain reaction amplification confirmed the in-frame joining of exons 11 and 13. The proportion of alternative splicing is reproducible and constant in a given individual. The omission of exon 12 in a significant proportion of transcripts supports the hypothesis that a minimal amount of correctly expressed CFTR is sufficient for the maintenance of a clinically normal phenotype.

Characterization of pathological dystrophin transcripts from the lymphocytes of a muscular dystrophy carrier.

Both normal and pathological transcripts of tissue-specific genes may be detected by polymerase chain reaction (PCR) amplification in tissues not normally considered to express the gene product. The exploitation of constitutive basal mRNA levels ("ectopic" transcription) would be a major boon to diagnostic medicine since it promises both to simplify the analysis of complex genes and to avoid the requirement for an expressing tissue that is sometimes obtainable only by biopsy. We have demonstrated the feasibility of this novel strategy by characterizing a mutation in the X-chromosomal Duchenne (or Becker) muscular dystrophy (DMD/BMD) gene encoding dystrophin. The massive size of this gene has in the past often hindered carrier detection due to the high frequency of recombination and the high proportion of new mutations. In this study a deletion was identified in both a BMD patient and a heterozygous carrier using only a minimal volume of peripheral blood. Following specifically primed reverse transcription of lymphocyte RNA, the relevant region of the pathological cDNA was PCR-amplified. Sequence analysis indicated an in-frame deletion of exons 45 to 47.

Detection of a novel point mutation causing haemophilia A by PCR/direct sequencing of ectopically-transcribed factor VIII mRNA.

Specifically-primed reverse transcripts of lymphocyte-derived factor VIII (FVIII) mRNA were successfully amplified by means of the polymerase chain reaction (PCR) thus further extending the phenomenon of ectopic ("illegitimate") transcription of tissue-specific genes. The diagnostic potential of a basal rate of transcription in non-expressing tissues was then demonstrated by the detection of a novel point mutation in the FVIII gene causing haemophilia A by PCR/direct sequencing of ectopically transcribed mRNA derived from patient lymphocytes.