Carrier detection strategy in haemophilia A: the benefits of combined DNA marker analysis and coagulation testing in sporadic haemophilic families

Clinical Application of Factor VIII:C to VWF:Ag Ratio for the Screening of Haemophilia A Carriers

Analyses of factor VIII procoagulant activity (FVIII:C) and the FVIII:C to VWF:Ag ratio (FVIII:C/VWF:Ag ratio) have been investigated as screening bioassays to detect haemophilia carriers. This study aimed to determine the validity of the FVIII:C/VWF:Ag ratio and FVIII:C analyses as screening tests. We reviewed the medical records of 137 genetically confirmed, proband haemophilia A patients and 179 of their familial females who had undergone carrier testing. The collected data included the severity and mutation type of F8 gene from probands and age, ABO blood type, FVIII:C, VWF:Ag, and the result of targeted gene analysis in females. We diagnosed 110 females as carriers, and their FVIII:C and FVIII:C/VWF:Ag ratio were lower than those in 69 non-carriers (FVIII:C: 59.3 IU/dL vs. 106.1 IU/dL, p = 0.000; FVIII:C/VWF:Ag ratio: 0.62 vs. 1.08, p = 0.000). In receiver operating characteristic analysis, the areas under the curve (AUC) of the FVIII:C/VWF:Ag ratio and FVIII:C were 0.936 and 0.876, respectively. The cut-off value of FVIII:C/VWF:Ag ratio (0.81) at the maximum Youden J index provided a sensitivity of 82.8% and specificity of 96.6%. The cut-off value of FVIII:C (83.8 IU/dL) showed a sensitivity of 81.8% and specificity of 79.7%. Considering the AUC, the FVIII:C/VWF:Ag ratio is a good screening test to detect haemophilia A carriers, as evidenced by its specificity of 96.6%; however, it may also induce false-negative results.

Clinical significance of gene-diagnosis for defects in coagulation factors and inhibitors

cDNA sequences of all known coagulation factors and inhibitors of coagulation have been described and an enormous number of disease generating mutations in these factors has been found by genetic analysis of affected families. The vast majority of these defects have severe clinical consequences such as spontaneous bleeding or predisposition to venous thrombosis and pulmonary embolism. While all the genetic defects described so far cause disease, or at least represent a risk factor for diseases such as bleeding or thrombosis, only a minority of these conditions actually need DNA analysis to be detected and/or treated properly. The purpose of this review is therefore to describe clinical situations in which the knowledge of the underlying genetic defect is important for decision making in patients with inherited hemophilia or thrombophilia.

Detection of carriers of haemophilia A: use of bioassays and restriction fragment length polymorphisms (RFLP)

BACKGROUND

Haemophilia A is a sex-linked bleeding disorder carried by unaffected females. Currently, the two main methods used for the determination of carrier status in women from families with haemophilia A are bioassays and DNA-based assays using restriction fragment length polymorphisms (RFLP).

AIM

The aim of this paper was to assess the current usefulness of these two methods.

METHODS

Bioassays measured factor VIII coagulation activity by a two-stage coagulation assay and von Willebrand antigen by immunoelectrophoresis. RFLP were determined with two intragenic probes (p114 and p486) and two linked probes (St14 and DX13). Data were analysed using a Bayesian analysis to allow for all possible recombination events. We also incorporated an estimate for the risk of mosaicism into calculations in isolated haemophilia families. Both bioassays and RFLP were used to determine carrier status in 63 women, 31 from known haemophilia families and 32 from families of isolated cases. The techniques were assessed for their ability to classify the patients as normal (p < 0.2) or carrier (p > 0.7). Where applicable, intron 22 inversion was also tested.

RESULTS

In the known families, six women could not be classified after bioassay, but all could be classified by RFLP. Of the 32 women from families of isolated cases, eight were unclassified by bioassay and 12 were not definitely classified using RFLP. However, RFLP was useful in determining that a recent mutation had occurred in six of the eight families in which DNA from the grandparents was available.

CONCLUSION

For diagnosis of carriers of haemophilia, RFLP is the preferred method in familial haemophilia, but is less useful in isolated haemophilia.

Multiplex analysis of two intragenic microsatellite repeat polymorphisms in the genetic diagnosis of haemophilia A

Use of molecular genetic studies for carrier detection and prenatal diagnosis of haemophilia A will be informative in approximately 75% of kindreds with a prior history of the disorder if previously reported bi-allelic intragenic polymorphisms are used. In this study we report the use of two multi-allelic, microsatellite repeat polymorphisms within the factor VIII gene in genetic testing for this disease. These two, dinucleotide repeat, polymorphisms have been analysed using a multiplex polymerase chain reaction (PCR) protocol, and results can be available within 3 d of receipt of samples. At the intron 13 polymorphic locus we have confirmed the original observation of eight alleles, whilst at the intron 22 locus, in contrast to a preliminary report of this polymorphism, we have seen five alleles. We have analysed 32 families (174 subjects) with the two microsatellite repeat markers and have found that using these two polymorphisms alone, 81% of families are informative for linkage analysis. The intron 13 repeat was informative in 22/31 families tested (71%) and the intron 22 polymorphism was informative in 12/17 families (71%). We have also found that in 11/32 families tested (34%) the microsatellite repeats were the only informative intragenic markers. In view of these observations, we believe that the most effective strategy for initiating haemophilia A genetic testing is to combine the multiplex PCR analysis of the intron 13 and 22 dinucleotide repeats with the PCR analysis of the intron 18 BclI marker. In our population this protocol provided informative results in approximately 88% of families and results can be available within 3 d of receiving the samples for testing.

Value of DNA analysis with multiple DNA probes for the detection of hemophilia A carriers

Detection of hemophilia carriers is an important issue and should be addressed with great care. The allelic frequencies of three intragenic probes (Bcl I for probe p114.12, Xba I for probe p482.6, and Bgl I for probe C) and one linked probe (Bgl II for probe DX 13) are reported, together with their diagnostic yield singly and in combination. In this series, 725 individuals (405 females) in 156 families were analyzed for restriction fragment-length polymorphisms. A total of 255 females (63%) were found to be informative for their carrier state with one or more probes. The most informative intragenic probe was p482.6 (useful in 49% of informative females). The most informative probe was DX 13 (useful in 59% of informative females), but this is a linked probe that carries a 5% risk of cross-over. By the use of probes p114.12, p482.6, and DX 13, almost 98% of all the informative females could be detected. In about 71% of families with a family history and a known carrier, prenatal diagnosis was feasible.

The use of preventive health care services: carrier testing for the genetic disorder haemophilia

A retrospective study was performed to explore carrier testing among women who were possible or obligate carriers of the haemophilia gene. Knowledge of the possibility of carrier testing and use of carrier testing were studied separately. In our exploration we were guided by the diffusion theory and the Health Belief Model. Logistic regression analysis showed that four factors were statistically significant related to knowledge of carrier testing: information via mass media, a haemophilic relative in the nuclear family rather than in the extended family, medical severity of haemophilia, and information from the patients' organization. For those women acquainted with carrier testing two of the four factors just mentioned were significantly related to the utilization of carrier testing i.e. having a haemophilic relative in the nuclear family and the medical severity of the haemophilia. In addition the following factors were associated: attitude towards abortion because of haemophilia, educational level, and marital status. Notwithstanding the prominent function of the mass media and the patients' organization, the respondents themselves stated that relatives, especially parents and sisters, were the most important source of information on genetic counselling and carrier testing.

The contribution of DNA analysis to carrier detection and prenatal diagnosis of hemophilia A and B

Developments in DNA technology have provided a novel means of carrier detection and prenatal diagnosis of hemophilia A and B. The collection of a large set of data has enabled us to evaluate the present feasibility and reliability of a diagnosis at the gene level and its contribution to methods already available. Since 1984, 533 potential and obligate carriers belonging to 170 families with hemophilia have been referred to us. By the combined use of pedigree analysis, coagulation assays, and DNA (RFLP) analysis, certainty about the carrier status has been markedly increased for the potential carriers. Although RFLP analysis revealed the possible origin of the mutation in many families with an isolated patient, uncertainty remained for quite a number of their female relatives because of the possible occurrence of germline mosaicism. Forty-one women requested prenatal diagnosis during one or more pregnancies. The short time interval between pregnancies, even after abortion of an affected fetus, proved that first-trimester prenatal diagnosis has become an acceptable option for women at risk. Recently, efficient methods for direct identification of mutations have been developed, and they may allow a definite diagnosis for all families with hemophilia in the near future.