A new strategy for the genetic counselling of diseases of marked mutational heterogeneity: haemophilia B as a model

Chemical cleavage of heteroduplex DNA to identify mutations

Mutation screening by the chemical-cleavage method is based on the fact that mismatched cytosine (C) and thymidine (T) are more reactive with the compounds hydroxylamine and osmium tetroxide, respectively, than are Watson and Crick-paired cytosine and thymidine bases. In this protocol, an excess of unlabeled target DNA is hybridized with labeled wild-type DNA probe and heteroduplexes are formed. One aliquot is treated with hydroxylamine, which reacts with mismatched C bases. Another aliquot is treated with osmium tetroxide, which reacts with mismatched T bases. The reactions are mixed with piperidine; the strands are then cleaved at the sites where hydroxylamine and osmium tetroxide react. Cleaved fragments are then electrophoresed and sized on polyacrylamide gels, identifying the point of cleavage (and hence the position of the mutation). Then only a small portion of the mutant gene needs to be sequenced to define a single change between two DNA sequences.

Identification of factor IX mutations in Iranian haemophilia B patients by SSCP and sequencing

Different kinds of mutations, mostly point mutations, in the coagulation factor IX (FIX) gene F9 result in a recessive X-linked bleeding disorder known as haemophilia B. In this study, molecular analysis of 76 unrelated Iranian haemophilia B patients was performed by PCR, single strand conformational polymorphism (SSCP) on important functional regions of the F9 gene followed by sequencing on samples with different migration pattern. Using this approach we found mutation in 52 out of 76 patients. Our data showed that the pathologic mechanisms are heterogeneous as recorded for patients in haemophilia B mutation database and seven of the mutations are previously undescribed.

Carrier testing and prenatal diagnosis of haemophilia B by SSCP in an Iranian family

Haemophilia B is an X-linked recessive bleeding disorder caused by multiple molecular defects in factor IX gene. During our genotyping programme of haemophilia B patients, a family from the west of Iran was referred to us. We first detected the mutation in the index case of the family by single-strand conformation polymorphism (SSCP) technique. This technique was then applied in carrier detection and prenatal diagnosis for females of this family. Sequencing later revealed the mutation as being G to A at 20519.

Identification of twenty-one new mutations in the factor IX gene by SSCP analysis

In this study we have analyzed the factor IX gene from 84 hemophilia B patients of Spanish origin. It included single-strand conformation polymorphism (SSCP) analysis of all functional regions of the gene and further sequencing of all fragments showing abnormal migration. In 76 patients (90.4%), it was possible to identify molecular alterations leading to the appearance of the disease. Twenty-one new mutations were identified, including 13 missense mutations, two nonsense mutations, three splice-site mutations, one frameshift deletion, one frameshift insertion, and one non-frameshift deletion. The approach appears to be very suitable for molecular diagnosis of hemophilia B.

Allele frequencies and molecular diagnosis in haemophilia A and B patients from Russia and from some Asian republics of the former U.S.S.R

RFLP analysis of some intra- and extra-genic polymorphic sites of Factor VIII (FVIII) and Factor IX (FIX) genes with relevant DNA probes or by polymerase chain reaction (PCR) was carried out in Slavic populations from the European part of Russia and also in the native ethnic groups of Uzbekistan and Kazahstan. The allele frequencies for the HindIII (intron 19) and XbaI (intron 22) polymorphic sites (PSs) in the FVIII gene were very similar in the two populations studied, but different for the intron 13 (CA)n repeat. Significant variations in the TaqI (intron d) and DdeI (intron a) polymorphisms of the FIX gene were evident between the Russian and Asian populations. Two unusual alleles (4.35 and 4.2 kb) for the extragenic PS St14/TaqI were registered in Slavs and one new allele (380 bp) for the DdeI polymorphic site of FIX was discovered in both Asian populations. Altogether, 210 haemophilia A (HA) and 24 haemophilia B (HB) families were subjected to molecular studies. So far, 160 HA and 12 HB families have been found to be informative for DNA analysis. Carrier status was ascertained in 42 HA and 6 HB female relatives, and rejected in 52 and 10 of them, respectively. The origin of some HA and HB mutations was traced with relevant polymorphic markers in several at-risk families. Prenatal diagnosis was accomplished in 28 HA and three HB families, resulting in the identification of 20 affected male fetuses.

Carrier detection and prenatal diagnosis of hemophilia B with more advanced techniques

We used the PCR to amplify three polymorphic regions of Factor IX gene on 35 Italian families: DdeI intron 1, Mn1I exon f, and the polymorphism HhaI located 8 kb at the 3' end of FIX gene. We analyzed the Mn1I and HhaI markers on DGGE and DdeI polymorphism on agarose gel. We reached an informativity of 78% and we found one mutation at codon 145 (exon f) during the screening for Mn1I polymorphism. Furthermore, we performed 16 prenatal diagnoses on chorionic villus samples; five were female and 11 male. Four were uninformative three healthy and one affected male fetus were recognized by PCR techniques, two healthy and one affected fetus by Southern analysis. In three pregnant women examined for the first time during pregnancy, the PCR technique allowed us to perform a rapid diagnosis of noncarrier status, avoiding the fetal sampling procedures.

Haemophilia B caused by a missense mutation in the prepeptide sequence of factor IX

In the course of analysing mutation in the factor IX gene from 200 haemophilia B patients in Sweden and the UK, we have identified one patient with a prepeptide missense mutation. He has severe, antigen negative haemophilia, and complete analysis of his coding sequence reveals a single base transversion (A-->T) causing substitution of isoleucine by asparagine at position -30. This change disrupts the hydrophobic core of the prepeptide, a feature which is required for secretion. Thus, haemophilia in this patient is caused by a failure to secrete factor IX from the hepatocytes.