Analysis of haemophilia B database and strategies for identification of common point mutations in the factor IX gene

Hemophilia B: molecular pathogenesis and mutation analysis

Hemophilia B is an X-chromosome-linked inherited bleeding disorder primarily affecting males, but those carrier females with reduced factor IX activity (FIX:C) levels may also experience some bleeding. Genetic analysis has been undertaken for hemophilia B since the mid-1980s, through linkage analysis to track inheritance of an affected allele, and to enable determination of the familial mutation. Mutation analysis using PCR and Sanger sequencing along with dosage analysis for detection of large deletions/duplications enables mutation detection in > 97% of patients with hemophilia B. The risk of the development of inhibitory antibodies, which are reported in ~ 2% of patients with hemophilia B, can be predicted, especially in patients with large deletions, and these individuals are also at risk of anaphylaxis, and nephrotic syndrome if they receive immune tolerance induction. Inhibitors also occur in patients with nonsense mutations, occasionally in patients with small insertions/deletions or splice mutations, and rarely in patients with missense mutations (p.Gln237Lys and p.Gln241His). Hemophilia B results from several different mechanisms, and those associated with hemophilia B Leyden, ribosome readthrough of nonsense mutations and apparently 'silent' changes that do not alter amino acid coding are explored. Large databases of genetic variants in healthy individuals and patients with a range of disorders, including hemophilia B, are yielding useful information on sequence variant frequency to help establish possible variant pathogenicity, and a growing range of algorithms are available to help predict pathogenicity for previously unreported variants.

Evolution of prenatal diagnostic techniques from phenotypic diagnosis to gene arrays: its likely impact on prenatal diagnosis of hemophilia

Prenatal diagnostic techniques in hemophilia have evolved through the early sex-determination techniques of offering a nonspecific diagnosis in case of a male fetus through the various mutation screening techniques to the more recent gene array techniques. Each of these techniques has specific advantages and disadvantages. The sampling techniques have evolved simultaneously to suit the requirements of each technique and also the different gestation periods. The DNA-based testing methods provide a range of aberrations detected with different levels of genomic resolution. The more recent gene array analysis is poised to have substantial impact on prenatal diagnosis of hemophilia not only in studying the highly heterogeneous mutations but may also be useful in studying the effect of various ameliorating or epistatic genetic mutations/ polymorphisms simultaneously, providing a wide range of options to the prenatal diagnosis experts, the genetic counselors, and the couples opting for prenatal diagnosis.

The Italian haemophilia B mutation database: a tool for genetic counselling, carrier detection and prenatal diagnosis

INTRODUCTION

The Italian database of factor IX gene (F9) mutations has been built since 2001 and is, so far, the most practical instrument for comprehensive genetic counselling, carrier detection and prenatal diagnosis. Over time the haemophilia B database has been enriched by entries on a larger number of patients and molecular genetic data identifying heterogeneous mutations spanning the entire F9.

METHODS

Conformation sensitive gel electrophoresis is a variant of heteroduplex analysis, which has been applied for screening F9 for mutations, which are further fully characterised by direct sequencing of the amplified mutated regions. This project has involved 29 Italian haemophilia centres and provides data concerning the analysis of a cohort of 306 unrelated patients with haemophilia B (191 with severe, 67 with moderate and 48 with mild disease, including 8 patients with severe haemophilia B with inhibitors). The recorded data include levels of factor IX clotting activity, inhibitor status and clinical severity.

RESULTS

Detailed analysis of the mutations revealed 164 different mutations, that are considered as unique molecular events (8 large deletions, 11 small deletions, 1 combined deletion/ insertion, 2 insertions, 104 missense, 20 nonsense, 14 mutations in a splicing site, 3 in the promoter and 1 silent). The data recorded in the Italian F9 mutation database provided the basis to study 85 families with haemophilia B, involving 180 females (20 obligate carriers, 106 carriers and 54 non-carriers) and enabled 14 prenatal diagnoses to be made in 12 females.

CONCLUSIONS

Genetic analysis is required to determine female carrier status reliably. Female relatives may request carrier analysis, when a male relative is first diagnosed as having haemophilia or when they are pregnant. At present, the data collected in the Italian national register of mutations in haemophilia B provide the opportunity to perform prompt and precise determination of carrier status and prenatal diagnosis by specific mutation analysis.

Haemophilia A: from mutation analysis to new therapies

Haemophilia is caused by hundreds of different mutations and manifests itself in clinical conditions of varying severity. Despite being inherited in monogenic form, the clinical features of haemophilia can be influenced by other genetic factors, thereby confounding the boundary between monogenic and multifactorial disease. Unlike sufferers of other genetic diseases, haemophiliacs can be treated successfully by intravenous substitution of coagulation factors. Haemophilia is also the most attractive model for developing gene-therapy protocols, as the normal life expectancy of haemophiliacs allows the side effects of gene therapy, as well as its efficiency, to be monitored over long periods.

Molecular pathology of haemophilia B: identification of five novel mutations including a LINE 1 insertion in Indian patients

Heterogeneous mutations in factor IX (FIX) gene cause haemophilia B and a large number of mutations have been characterized. However, reports on gene defects among Indian haemophilia B patients are rare despite a high estimate of such patients in the country. We report identification of 22 independent mutations including five novel mutations in 24 unrelated patients. The novel gene defects include two point mutations, two deletions and one insertion of a LINE 1 element. Majority of the mutations (14 of 24) occurred on the same haplotype background, but do not suggest any founder effect. Direct identification of mutations can be utilized to perform the carrier detection and prenatal diagnosis, especially in families with isolated patients.