Application of intron 9 and intron 25 dinucleotide repeats of the factor VIII gene for carrier diagnosis in haemophilia A

Summary.  We describe the usefulness of two dinucleotide repeats located in intron 9 and in intron 25 of the factor VIII gene for carrier diagnosis of haemophilia A. We analyzed 100 unrelated Spanish women and 34 women from haemophilia A (HA) families in whom known intragenic markers were unhelpful in determining their carrier status. The heterozygosity rate of intron 9 and intron 25 markers in the 100 control women was lower (0.28 and 0.38, respectively) than the values obtained with common markers routinely used in our laboratory. However, the application of intron 9 and intron 25 markers was effective in identifying the at-risk X chromosome in 11 of 34 (32%) of the uninformative women from HA families. The combined use of these repeats with current markers may facilitate the identification of the X chromosome in HA families for application in carrier, prenatal and pre-implantation diagnoses.     

Variation of the non-factor VIII sequences detected by a probe from intron 22 of the factor VIII gene

A severe hemophilia A family has been studied with the factor VIII (F.VIII) intragenic XbaI polymorphism. During this investigation, a new variant hybridization pattern was observed with important implications concerning the non-F.VIII DNA sequences detected by the probe from intron 22, p482.6. Both Southern hybridization studies and direct analysis of amplified DNA demonstrated a variant form of the non-F.VIII sequences. This variant DNA sequence has not been responsible for any detectable phenotypic abnormalities, and likely represents a polymorphic change. In conclusion, this study has shown that the non-F.VIII sequences detected with the probe p482.6 are situated on the X chromosome, they seem to be present in two copies, and either or both copies infrequently possess a polymorphic XbaI site or a partial deletion.     

A semi-automated method for analysis of intron 13 and intron 22 dinucleotide repeat polymorphisms of the factor VIII gene

A semi-automated method has been developed for the simultaneous analysis of two microsatellite repeat polymorphisms located in intron 13 and 22 of the Factor VIII gene. The fluorescent dyes, 6-FAM- and HEX-phosphoramidites were used to 5′-end label the respective 5′ primers of these two microsatellite repeats and a multiplex polymerase chain reaction (PCR) devised for amplification. The PCR product was loaded onto the gel with DNA size marker labelled with ROX. A total of 24 samples could be analysed simultaneously on an automated DNA sequencer. The results were computed using a dedicated software, with assignment of PCR product size in basepair. This method compares well with the conventional manual procedure using radio-labelled primers, but at the same time overcomes many of the inherent disadvantages associated with the latter method.     

Carrier detection and prenatal diagnosis by intron 22 inversion analysis of the factor VIII gene

Summary In approximately 50% of severe haemophilia A patients the mutation is present in the form of a large chromosomal disruption in the factor VIII gene; this disruption is described as an inversion. It results in the physical breakage and separation of exons 1-2 and exons 23-26 of the factor VIII gene.     

Analysis of intron 13 microsatellite repeat polymorphism of the factor VIII gene by capillary electrophoresis

To establish a rapid and automatic gene analysis method, we used capillary electrophoresis (CE) for the analysis of the intron 13 microsatellite repeat polymorphism (MRP) of the coagulation factor VIII gene for the diagnosis of hemophilia A. In the analysis of a 20-bp DNA ladder marker, the reproducibility evaluated using the relative standard deviation (RSD) of the relative migration time for one fragment of each fragment were less than 0.01%, whereas the RSDs of the actual migration time of each fragment were 0.1-0.3%. Thus, the appropriate internal standard should be mixed with the sample when CE resolves polymerase chain reaction (PCR) products. We next analyzed the intron 13 MRP evaluated with (CA)n repeats of the factor VIII gene using a 200-bp DNA fragment as the internal standard. The results showed that the PCR products from the intron 13 MRP could be resolved using CE, even with the repeat numbers of 20 and 21, which differ by only 2 bp. These results suggest that CE is a suitable method for analyzing PCR products for gene diagnosis.     

Screening of intron 1 inversion and three intragenic factor VIII gene polymorphisms in Pakistani hemophilia A families

Indirect linkage analysis using highly informative polymorphic markers is the method of choice for carrier detection of hemophilia A in developing countries because direct DNA or mRNA sequence analysis is manifold costly and difficult than indirect gene tracking. Worldwide populations have revealed marked variation in the informativeness of polymorphic markers because of which each country has to select its own panel of markers for linkage analysis in hemophilia A families. The present study aimed at determining the informativeness of three factor VIII gene polymorphisms [intron 13(CA)n repeats, HindIII and AlwNI] in the Pakistani population. One hundred and forty-three individuals from 32 hemophilia A families and 68 unrelated anonymous females from the general population were screened for these polymorphisms using PCR and RFLP techniques. An inversion in intron 1 of the factor VIII gene causing 2-5% of severe hemophilia A cases was also screened in 128 Pakistani hemophilia A patients. None of the affected individuals carried the intron 1 inversion at least in peripheral blood leucocytes. The informativeness of intron 13 repeats, HindIII and AlwNI was 59.1% (13/22 hemophilia A families revealing five different alleles), 40.6% (13/32 hemophilia A families) and 6.25% (2/32 hemophilia A families), respectively. The cumulative informativeness of intron 13 repeats and HindIII was 63.6% (14/22 hemophilia A families), revealing strong linkage disequilibrium between these two polymorphic markers. These results suggest that there is a need to determine the informativeness of other polymorphic markers of the factor VIII gene to achieve 100% success rate for carrier detection of hemophilia A in Pakistan.     

Specific analysis of the intron 22 XbaI polymorphism of the human factor VIII gene using long-distance PCR

A rapid, non-radioactive, PCR-based method to genotype the XbaI restriction fragment length polymorphism of the human factor VIII gene is described. The method uses long-distance PCR followed by XbaI restriction digestion and agarose gel electrophoresis. The 6.6 kb amplification product includes a constant XbaI site, which provides a digestion control. The specificity of the method was challenged by a blind experiment with 16 genomic DNA samples previously genotyped by Southern blot analysis: a perfect correlation was obtained between genotypes determined using Southern blot and PCR.     

Analysis of factor VIII gene intron 1 inversion in Argentinian families with severe haemophilia A and a review of the literature.

Besides intron 22 factor VIII gene inversion (Inv22), intron 1 inversion (Inv1) has recently been reported as a further recurrent mutation that causes approximately 5% of severe haemophilia A (HA) cases. We analysed the presence of the Inv1 in a group of 64 severe HA-affected families from Argentina, and found only one positive case. This Inv1 patient has not developed a factor VIII inhibitor, and the screening for small mutations in the coding sequences of the factor VIII gene did not detect any additional defect in this case. The Inv1 genotyping was further applied to analyse the haemophilia carrier status of the proband's sister. In addition, we studied the accuracy of the current polymerase chain reaction-based method to investigate the Inv1, and confirmed the absence of amplimer length polymorphisms associated to the Inv1-specific polymerase chain reaction amplifications in 101 X-chromosome haplotypes from unrelated Argentinian healthy males. In order to discuss Inv1 mutation frequency in severe HA and the risk of inhibitor formation, a review of the literature was included. Our data highlight the importance of analysis of the Inv1 in Inv22-negative severe HA cases. This will benefit both genetic counselling and the study of the relationship between genotype and inhibitor development.     

Analysis of intron 22 inversions of the factor VIII gene in severe hemophilia A: implications for genetic counseling

Intrachromosomal recombinations involving F8A, in intron 22 of the factor VIII gene, and one of two homologous regions 500 kb 5' of the factor VIII gene result in large inversions of DNA at the tip of the X chromosome. The gene is disrupted, causing severe hemophilia A. Two inversions are possible, distal and proximal, depending on which homologous region is involved in the recombination event. A simple Southern blotting technique was used to identify patients and carriers of these inversions. In a group of 85 severe hemophilia A patients, 47% had an inversion, of which 80% were of the distal type. There was no association with restriction fragment length polymorphism (RFLP) haplotypes. The technique has identified a definitive genetic marker in families previously uninformative on RFLP analysis and provided valuable information for genetic counselling information may now be provided for carriers without the need to study intervening family members and the diagnosis of severe hemophilia A made in families with only a nonspecific history of bleeding. Analysis of intron 22 inversion should now be the first-line test for carrier diagnosis and genetic counselling for severe hemophilia A and may be particularly useful when there is no affected male family member or when intervening family members are unavailable for testing.     

Characterization of four novel molecular changes in the promoter region of the factor VIII gene

Haemophilia A (HA) is an X‐linked recessive bleeding disorder, caused by a wide variety of mutations in the factor VIII (F8) gene, leading to deficiency in the activity of coagulation FVIII. These mutations can affect all the F8 exons from the initiation codon to the termination codon, however, only few molecular changes in the promoter region of the F8 gene were reported so far. Here, we describe six nucleotide variations (c.‐51G>A, c.‐218T>C, c.‐219C>T, c.‐219delC, c.‐221T>A and c.‐664G>A) detected in the F8 promoter and their correlation with clinical phenotype of the patients. Potential role of these mutations in HA was also assessed. Causality was demonstrated with transient transfection experiments using luciferase reporter gene plasmids and computational analysis. Two molecular changes (c.‐51G>A and c.‐664G>A) did not seem to affect the promoter function of the F8 gene whereas c.‐218T>C, c.‐219C>T, c.‐219delC, c.‐221T>A mutations had an impact on the F8 promoter function and were responsible for HA. Furthermore, these mutations were associated with resistance to 1‐deamino‐8‐d ‐argininevasopressin (desmopressin) therapy when they were causative. When molecular variation was detected in F8 promoter, we propose to use prediction software and to verify predictions by reporter gene analysis. If the mutation is causative, it will be probably associated with a lack of therapeutic response to desmopressin and this clinical implication should be considered by clinicians.     

De novo factor VIII gene intron 22 inversion in a female carrier presents as a somatic mosaicism

The intron 22 inversion represents the most prevalent factor VIII gene defect in severe hemophilia A, accounting for about 40% of all mutations. It is hypothesized that the inversion mutations occur almost exclusively in germ cells during meiotic cell division by intrachromosomal recombination between 1 of 2 telomeric copies of the Int22h region and its intragenic homologue. The majority of inversion mutations originate in male germ cells, where the lack of bivalent formation may facilitate flipping of the telomeric end of the single X chromosome. This is the first intron 22 inversion that presents as a somatic mosaicism in a female, affecting only about 50% of lymphocyte and fibroblast cells of the proposita. Supposing a post-zygotic de novo mutation as the usual cause of somatic mosaicism, the finding would imply that the intron 22 inversion mutation is not restricted to meiotic cell divisions but can also occur during mitotic cell divisions, either in germ cell precursors or in somatic cells. (Blood. 2000;96:2905-2906)     

Genotyping the factor VIII intron 22 inversion locus using fluorescent in situ hybridization

The factor VIII intron 22 inversion is the most common cause of hemophilia A, accounting for approximately 40% of all severe cases of the disease. Southern hybridization and multiplex long distance PCR are the most commonly used techniques to detect the inversion in a diagnostic setting, although both have significant limitations. Here we describe our experience establishing a multicolor fluorescent in situ hybridization (FISH) based assay as an alternative to existing methods for genetic diagnosis of the inversion. Our assay was designed to apply three differentially labelled BAC DNA probes that when hybridized to interphase nuclei would exhibit signal patterns that are consistent with the normal or the inversion locus. When the FISH assay was applied to five normal and five inversion male samples, the correct genotype was assignable with p<0.001 for all samples. When applied to carrier female samples the assay could not assign a genotype to all female samples, probably due to a lower proportion of informative nuclei in female samples caused by the added complexity of a second X chromosome. Despite this complication, these pilot findings show that the assay performs favourably compared to the commonly used methods.