Spectrum of F9 mutations in Chinese haemophilia B patients: identification of 20 novel mutations

The genetic analysis of eight families with hemophilia B in Mongolia: Identification of two novel mutation

BACKGROUND

This study aimed to conduct molecular diagnostics among individuals with hemophilia B (HB) and carriers of hemophilia in Mongolia.

METHODS

Eight patients (six severe, two mild) with HB and their 12 female relatives were enrolled from eight families. Sanger sequence was performed for mutation identification. The questionnaire survey was conducted to evaluate carrier symptoms in female relatives.

RESULTS

Two families had a history of HB. A total of five different variants (c.223C > T; c.344A > G; c.464G > C; c.187_188del; and c.1314_1314delA) were identified in six patients with severe HB. Of these, two (c.187_188del and c.1314_1314delA) were novel. No variant in the entire F9 was found in two patients with mild HB. Nonsense c.223C > T (p.Arg75*) mutation was detected in two unrelated patients. Carrier testing identified five mothers as carriers, while one younger sister was a non-carrier. The carrier status of six female relatives of the two mild patients remained undetermined. By questionnaire survey, only one of the five genetically identified carriers displayed noticeable symptoms of being a carrier.

CONCLUSION

The novel variants c.187_188del and c.1314_1314delA can cause severe hemophilia B. This study did not observe a significant association between symptoms and carrier status in the five carriers.

The Molecular Basis of FIX Deficiency in Hemophilia B

Coagulation factor IX (FIX) is a vitamin K dependent protein and its deficiency causes hemophilia B, an X-linked recessive bleeding disorder. More than 1000 mutations in the F9 gene have been identified in hemophilia B patients. Here, we systematically summarize the structural and functional characteristics of FIX and the pathogenic mechanisms of the mutations that have been identified to date. The mechanisms of FIX deficiency are diverse in these mutations. Deletions, insertions, duplications, and indels generally lead to severe hemophilia B. Those in the exon regions generate either frame shift or inframe mutations, and those in the introns usually cause aberrant splicing. Regarding point mutations, the bleeding phenotypes vary from severe to mild in hemophilia B patients. Generally speaking, point mutations in the F9 promoter region result in hemophilia B Leyden, and those in the introns cause aberrant splicing. Point mutations in the coding sequence can be missense, nonsense, or silent mutations. Nonsense mutations generate truncated FIX that usually loses function, causing severe hemophilia B. Silent mutations may lead to aberrant splicing or affect FIX translation. The mechanisms of missense mutation, however, have not been fully understood. They lead to FIX deficiency, often by affecting FIX's translation, protein folding, protein stability, posttranslational modifications, activation to FIXa, or the ability to form functional Xase complex. Understanding the molecular mechanisms of FIX deficiency will provide significant insight for patient diagnosis and treatment.

Mutation analysis and characterisation of F9 gene in haemophilia- B population of India

Background

Hemophilia B (HB) is an X-linked bleeding disorder resulting from coagulation factor IX defects. Over 3,000 pathogenic, HB-associated mutations in the F9 gene have been identified. We aimed to investigate the role of F9 variants in 150 HB patients using sequencing technology.

Methods

F9 gene sequences were amplified from peripheral blood-derived DNA and sequenced on an Applied Biosystems (ABI) 3500 Sanger sequencing platform. Functional and structural predictions of mutant FIX were analyzed.

Results

Among 150 HB patients, 102 (68%), 30 (20%), and 18 (12%) suffered from severe, moderate, and mild HB, respectively. Genetic analysis identified 16 mutations, including 3 novel mutations. Nine mutations (7 missense and 2 stop-gain) were found to be pathogenic. Only 3 mutations (c.127C>T, c.470G>A, and c.1070G>A) were associated with different severities. While 2 mutations were associated with mild HB cases (c.304C>T and c.580A>G), 2 (c.195G>A and c.1385A>G) and 3 mutations (c.223C>T, c.1187G>A, and c.1232G>A) resulted in moderate and severe disease, respectively. Additionally, 1 mutation each was associated with mild-moderate (c.*1110A>G) and mild-severe HB disease (c.197A>T), 4 mutations were associated with moderate-severe HB cases (c.314A>G, c.198A>T, c.676C>T, and c.1094C>A). FIX concentrations were lower in the mutated group (5.5±2.5% vs. 8.0±2.5%). Novel p.E66D and p.S365 mutations were predicted to be pathogenic based on changes in FIX structure and function.

Conclusion

Novel single nucleotide polymorphisms (SNPs) largely contributed to the pathogenesis of HB. Our study strongly suggests that population-based genetic screening will be particularly helpful to identify risk prediction and carrier detection tools for Indian HB patients.

Molecular analysis of severe hemophilia B in Indian families: Identification of mutational hotspot and novel variants

INTRODUCTION

Hemophilia B is associated with molecular heterogeneity, with more than 1200 unique variants in the F9 gene. We hereby describe the mutational spectrum of severe hemophilia B patients presenting in a tertiary-care center in India.

METHOD

DNA was extracted from peripheral blood samples of 35 diagnosed severe hemophilia B patients belonging to 32 families, and were subjected to Sanger sequencing. Determination of the effect of novel variants on the protein structure and correlation between genotype and phenotype was attempted using in-silico tools.

RESULTS

Twenty-seven different mutations were detected in 30 probands, including 20 known and 7 novel variants. Also, we found one suspected case of whole gene deletion. The serine peptidase domain harbored most of the variants (48.1%). Inhibitory antibodies were found in two patients.

CONCLUSIONS

This study provides a comprehensive mutational spectrum and mutation screening strategy by Sanger sequencing of F9 gene in severe hemophilia B patients, in a resource-constraint setting.

Defining the molecular pathology and consequent phenotypes in Egyptian HB patients

BACKGROUND

Hemophilia B (HB) (also known as Christmas disease) is a rare X-linked recessive disorder characterized by spontaneous or prolonged hemorrhages caused by mutations in Factor 9 (F9) gene leading to deficient or defective coagulation F9. Our study aimed at identifying the causative mutations within a sample of HB Egyptian patients. The present study comprised clinical data of eleven HB patients descending from six unrelated families and a seventh family including a carrier mother with a history of deceased HB sibling. Sequencing of F9 gene was performed.

RESULTS

The study revealed four mutations; two missense NM_000133.3:c.676C>G, (P.Arg226Gly) and NM_000133.3:c.1305T>G, (p.Cys435Trp), and two nonsense mutations NM_000133.3:c.880C>T, (p.Arg294*) and NM_000133.3:c.1150C>T, (p.Arg384*), identified mutations spanned exons 6 and 8 of which a total of three mutations are located in hotspot exon 8 of F9 gene.

CONCLUSIONS

Reviewing the literature, this is the first molecular analysis of F9 gene in HB Egyptian patients. Consistent genotype/phenotypic severity correlation could be concluded, helping proper genetic counseling and prenatal decision taking.

Heterogeneity in Bleeding Tendency and Arthropathy Development in Individuals with Hemophilia

People with hemophilia (PWH) have an increased tendency to bleed, often into their joints, causing debilitating joint disease if left untreated. To reduce the incidence of bleeding events, PWH receive prophylactic replacement therapy with recombinant factor VIII (FVIII) or FIX. Bleeding events in PWH are typically proportional to their plasma FVIII or IX levels; however, in many PWH, bleeding tendency and the likelihood of developing arthropathy often varies independently of endogenous factor levels. Consequently, many PWH suffer repeated bleeding events before correct dosing of replacement factor can be established. Diagnostic approaches to define an individual's bleeding tendency remain limited. Multiple modulators of bleeding phenotype in PWH have been proposed, including the type of disease-causing variant, age of onset of bleeding episodes, plasma modifiers of blood coagulation or clot fibrinolysis pathway activity, interindividual differences in platelet reactivity, and endothelial anticoagulant activity. In this review, we summarize current knowledge of established factors modulating bleeding tendency and discuss emerging concepts of additional biological elements that may contribute to variable bleeding tendency in PWH. Finally, we consider how variance in responses to new gene therapies may also necessitate consideration of patient-specific tailoring of treatment. Cumulatively, these studies highlight the need to reconsider the current "one size fits all" approach to treatment regimens for PWH and consider therapies guided by the bleeding phenotype of each individual PWH at the onset of therapy. Further characterization of the biological bases of bleeding heterogeneity in PWH, combined with the development of novel diagnostic assays to identify those factors that modulate bleeding risk in PWH, will be required to meet these aspirations.

Molecular Insights into Determinants of Translational Readthrough and Implications for Nonsense Suppression Approaches

The fidelity of protein synthesis, a process shaped by several mechanisms involving specialized ribosome regions and external factors, ensures the precise reading of sense and stop codons. However, premature termination codons (PTCs) arising from mutations may, at low frequency, be misrecognized and result in PTC suppression, named ribosome readthrough, with production of full-length proteins through the insertion of a subset of amino acids. Since some drugs have been identified as readthrough inducers, this fidelity drawback has been explored as a therapeutic approach in several models of human diseases caused by nonsense mutations. Here, we focus on the mechanisms driving translation in normal and aberrant conditions, the potential fates of mRNA in the presence of a PTC, as well as on the results obtained in the research of efficient readthrough-inducing compounds. In particular, we describe the molecular determinants shaping the outcome of readthrough, namely the nucleotide and protein context, with the latter being pivotal to produce functional full-length proteins. Through the interpretation of experimental and mechanistic findings, mainly obtained in lysosomal and coagulation disorders, we also propose a scenario of potential readthrough-favorable features to achieve relevant rescue profiles, representing the main issue for the potential translatability of readthrough as a therapeutic strategy.

Characterization of missense mutations in the signal peptide and propeptide of FIX in hemophilia B by a cell-based assay

Many mutations in the signal peptide and propeptide of factor IX (FIX) cause hemophilia B. A FIX variants database reports 28 unique missense mutations in these regions that lead to FIX deficiency, but the underlying mechanism is known only for the mutations on R43 that interfere with propeptide cleavage. It remains unclear how other mutations result in FIX deficiency and why patients carrying the same mutation have different bleeding tendencies. Here, we modify a cell-based reporter assay to characterize the missense mutations in the signal peptide and propeptide of FIX. The results show that the level of secreted conformation-specific reporter (SCSR), which has a functional γ-carboxyglutamate (Gla) domain of FIX, decreases significantly in most mutations. The decreased SCSR level is consistent with FIX deficiency in hemophilia B patients. Moreover, we find that the decrease in the SCSR level is caused by several distinct mechanisms, including interfering with cotranslational translocation into the endoplasmic reticulum, protein secretion, γ-carboxylation of the Gla domain, and cleavage of the signal peptide or propeptide. Importantly, our results also show that the SCSR levels of most signal peptide and propeptide mutations increase with vitamin K concentration, suggesting that the heterogeneity of bleeding tendencies may be related to vitamin K levels in the body. Thus, oral administration of vitamin K may alleviate the severity of bleeding tendencies in patients with missense mutations in the FIX signal peptide and propeptide regions.

Microfluidic hemophilia models using blood from healthy donors

BACKGROUND

Microfluidic clotting assays permit drug action studies for hemophilia therapeutics under flow. However, limited availability of patient samples and Inter-donor variability limit the application of such assays, especially with many patients on prophylaxis.

OBJECTIVE

To develop approaches to phenocopy hemophilia using modified healthy blood in microfluidic assays.

METHODS

Corn trypsin inhibitor (4 µg/mL)-treated healthy blood was dosed with either anti-factor VIII (FVIII; hemophilia A model) or a recombinant factor IX (FIX) missense variant (FIX-V181T; hemophilia B model). Treated blood was perfused at 100 s-1 wall shear rate over collagen/tissue factor (TF) or collagen/factor XIa (FXIa).

RESULTS

Anti-FVIII partially blocked fibrin production on collagen/TF, but completely blocked fibrin production on collagen/FXIa, a phenotype reversed with 1 µmol/L bispecific antibody (emicizumab), which binds FIXa and factor X. As expected, emicizumab had no significant effect on healthy blood (no anti-FVIII present) perfused over collagen/FXIa. The efficacy of emicizumab in anti-FVIII-treated healthy blood phenocopied the action of emicizumab in the blood of a patient with hemophilia A perfused over collagen/FXIa. Interestingly, a patient-derived FVIII-neutralizing antibody reduced fibrin production when added to healthy blood perfused over collagen/FXIa. For low TF surfaces, reFIX-V181T (50 µg/mL) fully blocked platelet and fibrin deposition, a phenotype fully reversed with anti-TFPI.

CONCLUSION

Two new microfluidic hemophilia A and B models demonstrate the potency of anti-TF pathway inhibitor, emicizumab, and a patient-derived inhibitory antibody. Using collagen/FXIa-coated surfaces resulted in reliable and highly sensitive hemophilia models.

The characteristics and spectrum of F9 mutations in Chinese sporadic haemophilia B pedigrees

INTRODUCTION

Sporadic haemophilia B (HB) without obvious familial history poses challenges for genetic diagnosis and counselling.

AIM

To identify the F9 variants in sporadic HB patients and probe the origin of these de novo mutations.

METHOD

A total of 294 unrelated HB pedigrees sought genetic diagnosis were analysed in this single-centre study. The F9 gene was analysed by direct sequencing, and AccuCopy technique was adopted to screen for gene copy number variations. Six short tandem repeats approximal or within F9 gene were applied for linkage analysis. Mosaicism of sequence variant was determined by ddNTP Primer Extension method.

RESULTS

Sporadic HB patients constituted 36% (61/294) of cases enrolled in current study. The sporadic and familial HB patients shared similar spectrum of F9 variants, with single nucleotide substitution as predominant form of disease-causing mutation and no mutation prone hotspot sites, including CpG dinucleotide sequences, had been identified. Majority of the mothers of sporadic HB patients were F9 mutation carriers (70%, 43/61), and most of them (95%, 41/43) had the inherited bleeding trait traced back to maternal grandfathers. Although most de novo mutations occur in germ cells, 2 maternal grandfathers, who had somatic mosaic mutations of F9, were also revealed to be the source of genetic variations identified in patients. In our cohort, FIX inhibitor incidence was 1%, developed only in patients carrying null mutations.

CONCLUSION

The diversity of F9 genetic variants and possible mosaicism of de novo mutation demand extensive study and more cautious in genetic counselling of sporadic HB.

Mutational Profiles of F8 and F9 in a Cohort of Haemophilia A and Haemophilia B Patients in the Multi-ethnic Malaysian Population

Background

Haemophilia A (HA) and Haemophilia B (HB) are X-linked blood disorders that are caused by various mutations in the factor VIII (F8) and factor IX (F9) genes respectively. Identification of mutations is essential as some of the mutations are associated with the development of inhibitors. This study is the first comprehensive study of the F8 mutational profile in Malaysia.

Materials and methods

We analysed 100 unrelated HA and 15 unrelated HB patients for genetic alterations in the F8 and F9 genes by using the long-range PCR, DNA sequencing, and the multiplex-ligation-dependent probe amplification assays. The prediction software was used to confirm the effects of these mutations on factor VIII and IX proteins.

Results

44 (53%) of the severe HA patients were positive for F8 intron 22 inversion, and three (3.6%) were positive for intron one inversion. There were 22 novel mutations in F8, including missense (8), frameshift (9), splice site (3), large deletion (1) and nonsense (1) mutations. In HB patients, four novel mutations were identified including the splice site (1), small deletion (1), large deletion (1) and missense (1) mutation.

Discussion

The mutational spectrum of F8 in Malaysian patients is heterogeneous, with a slightly higher frequency of intron 22 inversion in these severe HA patients when compared to other Asian populations. Identification of these mutational profiles in F8 and F9 genes among Malaysian patients will provide a useful reference for the early detection and diagnosis of HA and HB in the Malaysian population.

Expression and Characterization of Gly-317 Variants of Factor IX Causing Variable Bleeding in Hemophilia B Patients

We recently identified two hemophilia B patients who carried Gly-317 to Arg (FIX-G317R) or Gly-317 to Glu (FIX-G317E) substitutions in their FIX gene. The former mutation caused severe and the latter moderate bleeding in afflicted patients. To understand the molecular basis for the variable clinical manifestation of Gly-317 mutations, we prepared recombinant G317R and G317E derivatives of FIX and compared their kinetic properties to those of recombinant wild-type FIX in appropriate assay systems. Both physiological activators, factor XIa and extrinsic Tenase (factor VIIa-tissue factor), activated both zymogen variants with an ∼1.5-fold elevated K(m); however, extrinsic Tenase activated FIX-G317E with an ∼2-fold improved k(cat). By contrast to zymogen activation, the catalytic activities of both FIXa-G317R and FIXa-G317E enzymes toward the natural substrate, factor X, were dramatically (>4 orders of magnitude) impaired, but their apparent affinity for interaction with factor VIIIa was only slightly (<2-fold) decreased. Further studies revealed that the reactivity of FIXa-G317R and FIXa-G317E with antithrombin has been impaired 10- and 13-fold, respectively, in the absence and 166- and 500-fold, respectively, in the presence of pentasaccharide. As expected, the clotting activities of FIX variants could not be measured by the aPTT assay. These results implicate a critical role for Gly-317 in maintaining normal catalytic function for FIX/FIXa in the clotting cascade. The results further suggest that improved k(cat) of FIX-G317E activation in the extrinsic pathway together with dramatically impaired reactivity of FIXa-G317E with antithrombin may account for the less severe bleeding phenotype of a hemophilia B patient carrying the FIX-G317E mutation.

Translational data from adeno-associated virus-mediated gene therapy of hemophilia B in dogs

Preclinical testing of new therapeutic strategies in relevant animal models is an essential part of drug development. The choice of animal models of disease that are used in these studies is driven by the strength of the translational data for informing about safety, efficacy, and success or failure of human clinical trials. Hemophilia B is a monogenic, X-linked, inherited bleeding disorder that results from absent or dysfunctional coagulation factor IX (FIX). Regarding preclinical studies of adeno-associated virus (AAV)-mediated gene therapy for hemophilia B, dogs with severe hemophilia B (<1% FIX) provide well-characterized phenotypes and genotypes in which a species-specific transgene can be expressed in a mixed genetic background. Correction of the hemophilic coagulopathy by sustained expression of FIX, reduction of bleeding events, and a comprehensive assessment of the humoral and cell-mediated immune responses to the expressed transgene and recombinant AAV vector are all feasible end points in these dogs. This review compares the preclinical studies of AAV vectors used to treat dogs with hemophilia B with the results obtained in subsequent human clinical trials using muscle- and liver-based approaches.

Mutation analysis of a cohort of US patients with hemophilia B

Hemophilia B (HB) is a disorder resulting from genetic mutations in the Factor 9 gene (F9). Genotyping of HB patients is important for genetic counseling and patient management. Here we report a study of mutations identified in a large sample of HB patients in the US. Patients were enrolled through an inhibitor surveillance study at 17 hemophilia treatment centers. A total of 87 unique mutations were identified from 225 of the 226 patients, including deletions, insertions, and point mutations. Point mutations were distributed throughout the F9 gene and were found in 86% of the patients. Of these mutations, 24 were recurrent in the population, and 3 of them (c.316G>A, c.1025C>T, and c.1328T>A) accounted for 84 patients (37.1%). Haplotype analysis revealed that the high recurrence arose from a founder effect. The severity of HB was found to correlate with the type of mutation. Inhibitors developed only in severe cases with large deletions and nonsense mutations. None of the mild or moderate patients developed inhibitors. Our results provide a resource describing F9 mutations in US HB patients and confirm previous findings that patients bearing large deletions and nonsense mutations are at high risk of developing inhibitors.

Efficient detection of factor IX mutations by denaturing high-performance liquid chromatography in Taiwanese hemophilia B patients, and the identification of two novel mutations

Hemophilia B (HB) is an X-linked recessive disorder characterized by mutations in the clotting factor IX (FIX) gene that result in FIX deficiency. Previous studies have shown a wide variation of FIX gene mutations in HB. Although the quality of life in HB has greatly improved mainly because of prophylactic replacement therapy with FIX concentrates, there exists a significant burden on affected families and the medical care system. Accurate detection of FIX gene mutations is critical for genetic counseling and disease prevention in HB. In this study, we used denaturing high-performance liquid chromatography (DHPLC), which has proved to be a highly informative and practical means of detecting mutations, for the molecular diagnosis of our patients with HB. Ten Taiwanese families affected by HB were enrolled. We used the DHPLC technique followed by direct sequencing of suspected segments to detect FIX gene mutations. In all, 11 FIX gene mutations (8 point mutations, 2 small deletions/insertions, and 1 large deletion), including two novel mutations (exon6 c.687-695, del 9 mer and c.460-461, ins T) were found. According to the HB pedigrees, 25% and 75% of our patients were defined as familial and sporadic HB cases, respectively. We show that DHPLC is a highly sensitive and cost-effective method for FIX gene analysis and can be used as a convenient system for disease prevention.