Clinical Aspects of and Therapy for Hemophilia A. Incidence. Clinical Severity, страница 15

When sporadic cases of hemophilia occur in a family, polymorphism testing can only be used to exclude transmission of the mutation because the mutation is not identified by these methods, and therefore its origin is not known. In these families, direct mutation analysis can be considered. Direct screening methods for mutations in these large genes have been developed. Three screening methods are utilized for hemophilia A: denaturing gradient gel electrophoresis, single-stranded conformational polymorphism analysis, and direct sequencing of amplified DNA. MLID90169988 MLID89323457 MLID90152691 MLID94100976  96–100

Prenatal Diagnosis

Several techniques are available for the prenatal diagnosis of hemophilia. The basic strategy is to determine fetal sex first, and then to determine whether the fetus has the gene for hemophilia. Fetal sex can be determined by ultrasonography and fetal chromosome analysis. Experienced sonographers can determine fetal sex reliably at 16–20 weeks of gestation. Fetal sex, however, is more accurately determined by examination of the fetal chromosomes. Chromosomes can be obtained by amniocentesis or chorionic villus sampling (CVS). Amniocentesis is performed at 15–20 weeks' gestation or at 12–14 weeks' gestation ("early amniocentesis"—risk approximately 1%). MLID92373696  101,102 CVS, by either the transcervical or the transabdominal route, is done at 9–12 weeks' gestation. MLID89143658 MLID92350194  103,104 In general, whether amniocentesis or a CVS procedure is chosen depends on patient preferences regarding timing of the procedure, risk to the fetus, and reliability of the results. Reports suggesting an association between CVS and limb reduction defects have raised questions about the risks of this procedure. These risks can be lessened if the CVS procedure is performed after 10 weeks' gestation, and if placental trauma is minimized. The risk of loss of pregnancy may be somewhat higher for CVS compared with amniocentesis.

The diagnosis of hemophilia in a fetus can be accomplished by genotypic analysis of fetal DNA or by phenotypic analysis of fetal blood. Fetal DNA can be extracted from both fetal amniocytes and chorionic villi and analyzed by RFLP of the DNA, as above. If prenatal diagnosis is not possible by DNA analysis, then fetal blood sampling can be done by a cordocentesis. This technique is done from 20–21 weeks' gestation and involves the ultrasound-guided puncture of an umbilical cord vessel. MLID86047995  105 The fetal blood is analyzed by phenotypic clotting assays.

Preimplantation diagnosis after in vitro fertilization has recently been proposed as a method of prenatal testing. Amplification of informative regions of the factor VIII gene by polymerase chain reaction from a single cell has been reported. All women who undergo carrier testing or prenatal diagnosis, or both, for hemophilia should have genetic counseling, preferably before choices regarding carrier testing and prenatal diagnosis must be made. The aims of counseling in these cases are to provide information about genetic risk, determination of carrier status, and prenatal diagnosis as well as to provide psychological and emotional support during the processes.