Clinical Aspects of and Therapy for Hemophilia A. Incidence. Clinical Severity, страница 14

Other female relatives are considered possible carriers of the gene for hemophilia. This would include women who have one son with hemophilia and no other affected relatives. In these isolated cases, the hemophilia may result from (1) transmission through asymptomatic females, (2) a new mutation in the mother, (3) a new mutation in the individual with hemophilia (a true de novo mutation), or (4) as a result of somatic or germline mosaicism in the mother. MLID89126635  90 The probability for carriership for a mother of an isolated case is estimated to be 0.85. MLID93271395  91

Carrier Detection

Potential hemophilia A carriers should be offered genetic testing for determination of carrier status. Current methods of carrier detection include standard phenotypic clotting assays, as well as more accurate genotypic analysis. Probability of carriership should first be determined from pedigree data. Information anterior to the proband is used to determine genetic risk. This figure can be modified by Bayesean analysis if the individual has any sons without hemophilia.

Phenotypic assessment of carrier status for hemophilia A is determined by specific assays for factor VIII activity, factor VIII antigen, and von Willebrand factor antigen. In general, women who are carriers of hemophilia A have approximately 50% of the normal level of factor VIII. These values may be affected by various physiologic conditions or medications, or both. Of particular note are the effect of pregnancy (especially after the 22nd week of gestation) and estrogen-containing drugs such as birth control pills, which elevate factor VIII levels. MLID83049576 MLID75189904  92,93 The age of the individual being tested and the ABO blood type must also be noted; blood type O is associated with decreased factor VIII levels. MLID86216617  94 Factor VIII concentrations are also influenced by X-chromosome inactivation (the Lyon hypothesis), which may cause false-negative results. 95 Laboratory data are used to determine probability of carriership. The laboratory values can be combined with the pedigree data to give a final probability. For determination of odds ratios favoring carriership in hemophilia A, bivariate linear discriminant analysis using factor VIII, von Willebrand antigen, age, and ABO blood type is recommended.

Determination of carrier status for hemophilia A may also be performed using molecular diagnostic methods. The factor VIII gene is large (see Ch. 105), and the mutations identified to date are heterogeneous. Thus, direct mutational analysis is not available for routine screening. Most genotypic testing employs an indirect marker, restriction fragment length polymorphism (RFLP). Which marker to use for polymorphism analysis is determined by the mutation's location (intragenic versus extragenic), the degree of heterozygosity, and the ethnic origin of the family. Southern blot analysis and polymerase chain reaction are the techniques used for identification of these polymorphisms. For hemophilia A, >95% of females are informative, with the following intragenic markers: the intron 13 CA repeat, the intron 22 CA repeat, BclI, and XbaI. The BglI and the intron 7 polymorphism may be informative in other families. Indirect testing using intragenic markers is >99% accurate. However, there are limitations. For the study to be informative, the marker must be heterozygous. Heterozygosity differs significantly in various ethnic groups, and this must be accounted for when considering which polymorphisms to use in a particular family. Blood samples from several key family members are required for genotypic testing. Blood from an affected male is required. Since many older hemophiliacs are infected with HIV-1 and thus have shortened survival, blood sampling to obtain DNA that can then be frozen for future use in genetic analysis should be encouraged. All family members must agree to genotypic testing, and all family relationships reported must be correct. In some families linked extragenic markers may be informative, but accuracy of the results is decreased due to possible genetic recombination.