The factor VIII Structure and Mutation Resource Site: HAMSTeRS version 4

Mutational Profile in Romanian Patients with Hemophilia A

Hemophilia A (HA) is an X-linked recessive bleeding disorder caused by mutations in the F8 gene, resulting in deficient or dysfunctional factor VIII (FVIII). This study aimed to characterize the mutational profile of HA in Romanian patients using next-generation sequencing (NGS) and multiplex ligation-dependent probe amplification (MLPA). A total of 107 patients were analyzed, revealing pathogenic or likely pathogenic variants in 96.3% of cases. The identified mutations included missense (30.5%), nonsense (9.1%), small deletions (6.4%), small insertions (2.1%), splice-site variants (4.3%), large deletions (1.6%), and large duplications (1.1%). Large intron inversion was previously found in 37.5% of the patients. Novel variants accounted for 21.5% of identified mutations, expanding the spectrum of F8 variants in this population. This study underscores the genetic heterogeneity of HA and provides insights into genotype-phenotype correlations, aiding in clinical management and prenatal diagnosis.

F8 variants and their genotype-phenotype correlations in Thai patients with haemophilia A: a nationwide multicentre study

AIMS

Analysis of the F8 gene helps predict the risk of developing factor VIII (FVIII) inhibitors and the depth of phenotype in haemophilia A (HA) patients. Since data in Southeast Asian countries remain scarce, we aim to study F8 variation correlated with HA phenotypes in Thailand.

METHODS

Thai patients with HA were enrolled from seven haemophilia treatment centres during 2022-2023. Using peripheral blood DNA, inverse shifting-polymerase chain reaction (IS-PCR) for F8-intron 22 inversion (Inv22) and F8-intron 1 inversion (Inv1) was performed. Whole exome sequencing (WES) was explored in cases without Inv22/Inv1.

RESULTS

Of 124 patients with HA, 91.9% were detected with a causative F8 variant, including Inv22 (30.6%), Inv1 (1.6%), missense (23.4%), nonsense (16.9%) and small insertion/deletion (16.1%) mutations. Inv22, small insertion/deletion and nonsense were associated with severe HA, compared with missense variants, by the ORs of 13.9 (95% CI, 4.2 to 56.7), 14.7 (95% CI, 3.4 to 104.7) and 15.6 (95% CI, 3.6 to 110.2), respectively. While nonsense variants affecting the light chain increased the risk of developing FVIII inhibitors (OR, 6.8; 95% CI, 1.5 to 32.6) compared with the low-risk (small insertion/deletion, missense and splice-site) variants. Twelve patients (9.7%) harboured novel F8 variants, comprising five missense (p.Pro540Leu, p.Ser564Pro, p.Leu668Pro, p.Ala1721Glu, p.His2024Pro), five small insertion/deletion (p.Val502SerfsTer13, p.Ile522PhefsTer13, p.Phe992LysfsTer11, p.Leu1223PhefsTer18, c.6427_6429+3delATGGTA) and one nonsense mutations (p.Glu1292Ter).

CONCLUSIONS

IS-PCR followed by WES successfully assesses F8 alterations in most HA cases. With several unique variants, severe HA in Thailand is considerably caused by Inv22, small insertion/deletion and nonsense, whereas missense variants are more responsible for nonsevere HA phenotypes.

Unearthing the genotype-inhibitor phenotype association in severe haemophilia A: A north Indian cohort study

INTRODUCTION

Various risk factors for inhibitor development in haemophilia A (HA) have been described but Indian data remains scanty.

AIM

We aimed to evaluate the genetic changes in Indian HA-patients that are associated with the development of inhibitors.

METHODS

All HA-patients with inhibitors who availed coagulation-laboratory services from January-2015 till December-2021 and had their samples preserved for DNA extraction were included in this study. An equal number of severity-matched HA patients without inhibitors were also included as controls. Intron 22 and intron 1 inversions in Factor VIII gene were identified using inverse-shifting-PCR. Inversion-negative patients were further assessed by targeted NGS, MLPA.

RESULTS

Thirty HA-patients with inhibitors were identified. All had severe-HA. Thirty severe-HA-patients without inhibitors were also included as controls. Intron 22 inversion (63.3%) and large deletions (15%) were the commonest variants identified. There was no difference in genetic variants in patients with low and high titre inhibitors. A3, A2 and C2 were the most common domains involved in inversion-negative patients with inhibitors. However, there was no significant difference in domain involvement among inversion-negative patients with and without inhibitors. Seven novel-variants were identified, including three large deletions, one large duplication and two nonsense variants in inhibitor-positive patients, and one frameshift variant in inhibitor-negative patient. After adjusting for clinical risk-factors, large deletions were independently associated with the presence of inhibitors [aOR:6.1 (1.41-56.3)].

CONCLUSION

Intron 22 inversions are the commonest variant in Indian patients with severe-HA. Large deletions predispose to inhibitor development independent of clinical risk factors.

Comprehensive Analysis of Hemophilia A (CAHEA): Towards Full Characterization of the F8 Gene Variants by Long-Read Sequencing

BACKGROUND

 Hemophilia A (HA) is the most frequently occurring X-linked bleeding disorder caused by heterogeneous variants in the F8 gene, one of the largest genes known. Conventional molecular analysis of F8 requires a combination of assays, usually including long-range polymerase chain reaction (LR-PCR) or inverse-PCR for inversions, Sanger sequencing or next-generation sequencing for single-nucleotide variants (SNVs) and indels, and multiplex ligation-dependent probe amplification for large deletions or duplications.

MATERIALS AND METHODS

 This study aimed to develop a LR-PCR and long-read sequencing-based assay termed comprehensive analysis of hemophilia A (CAHEA) for full characterization of F8 variants. The performance of CAHEA was evaluated in 272 samples from 131 HA pedigrees with a wide spectrum of F8 variants by comparing to conventional molecular assays.

RESULTS

 CAHEA identified F8 variants in all the 131 pedigrees, including 35 intron 22-related gene rearrangements, 3 intron 1 inversion (Inv1), 85 SNVs and indels, 1 large insertion, and 7 large deletions. The accuracy of CAHEA was also confirmed in another set of 14 HA pedigrees. Compared with the conventional methods combined altogether, CAHEA assay demonstrated 100% sensitivity and specificity for identifying various types of F8 variants and had the advantages of directly determining the break regions/points of large inversions, insertions, and deletions, which enabled analyzing the mechanisms of recombination at the junction sites and pathogenicity of the variants.

CONCLUSION

 CAHEA represents a comprehensive assay toward full characterization of F8 variants including intron 22 and intron 1 inversions, SNVs/indels, and large insertions and deletions, greatly improving the genetic screening and diagnosis for HA.

Development of a novel fully functional coagulation factor VIII with reduced immunogenicity utilizing an in silico prediction and deimmunization approach

BACKGROUND

Up to 30% of hemophilia A patients develop inhibitory antibodies against the infused factor VIII (FVIII). The development of a deimmunized FVIII is an unmet high medical need. Although improved recombinant FVIII (rFVIII) products evolved within the last years, the immunogenicity has not been solved. A deimmunized FVIII could reduce the probability of inhibitor development, providing safer therapy.

OBJECTIVE

To develop a deimmunized FVIII molecule by modifying major histocompatibility complex (MHC) class II presentation, leading to a functional but less immunogenic molecule.

METHODS

We performed (1) in silico prediction of potentially immunogenic T cell epitopes and their modification by amino acid substitutions in the FVIII sequence, (2) evaluation of functional and structural similarity of the modified rFVIII to unmodified FVIII and registered products, and (3) confirmation of the reduced immunogenicity by in vitro testing.

RESULTS

A partially deimmunized fully functional FVIII molecule incorporating 19 amino acid substitutions was generated. The substitutions led to a reduction of the immunogenicity score, indicating a reduced immunogenicity based on in silico calculations. This was confirmed in an in vitro dendritic cell (DC)--T cell assay. Using this assay, cells from healthy donors proved the significantly reduced immunogenicity of the modified FVIII variant by revealing less proliferation of T helper cells to this variant than to the unmodified FVIII.

CONCLUSION

In silico predictions resulted in a partially deimmunized FVIII. This FVIII is fully functional and was demonstrated to be less immunogenic in in vitro testing. This approach may result in a reduction of the inhibitor risk for patients with hemophilia A.

Genomic alterations in the F8 gene correlating with severe hemophilia A in Egyptian patients

Background

Hemophilia A (HA) is an inherited X‐linked recessive coagulation disorder caused by factor VIII (F8) deficiency. F8 rearrangements involving intron 22 (int22) and intron 1 (int1) account for almost half of severe HA phenotype also a hotspot exon 14 provides numerous mutational patterns. This study aims to identify F8 gene mutations among Egyptian HA patients.

Methods

DNA samples from 60 HA patients were screened for int22 and int1 rearrangements using simplified inverse shifting PCR (IS‐PCR) followed by exon 14 sequencing. Also, four uncharacterized patients were studied by targeted exome sequencing.

Results

In 33.3% of the studied patients, we identified three int22 rearrangements, three exon 14 mutations (two frameshift; one novel (NM_000132.3:c.2734_2735delAA, p.(N912Ffs*6)), a second reported mutation (NM_000132.3:c.3091_3094delAGAA, p.(K1031Lfs*9)), and one nonsense mutation (NM_000132.3:c.2440C>T, p.(R814*)). All identified mutations were detected in patients with severe HA phenotype. Targeted exome sequencing could not detect any known pathogenic variants.

Conclusion

Intron 22 rearrangement and exon 14 mutations correlate with most severe hemophilia A Egyptian patients.

Identification of the Intron 22 and Intron 1 Inversions of the Factor VIII Gene in Iraqi Kurdish Patients With Hemophilia A

Hemophilia A (HA) is a severe coagulation disorder affecting 1 in 5000 to 10 000 male births. In severe cases, the most deleterious large DNA rearrangements are inversions of intron 22 (Inv22) and intron 1 (Inv1) of the factor VIII (FVIII) gene. These account for 40% to 50% and 1% to 5% of all causative mutations, respectively. Nevertheless, no genetic analysis to identify the actual causative mutation of FVIII, particularly Inv22 and Inv1, among Iraqi Kurdish hemophiliacs has been performed. In this study, we aimed to genotype Inv22 and Inv1 of the FVIII gene in our patients with HA and reveal the genotype/phenotype correlation with the inversion mutations and their role as a risk factor for the development of inhibitors. Analyses of the Inv22 and Inv1 mutations in 80 Iraqi Kurdish patients with HA (60 severe, 18 moderate, and 2 mild) were performed using the inverse shifting–polymerase chain reaction (IS-PCR) method. In severe cases, 46.7% (28/60) had Inv22 and 3.3% (2/60) had Inv1. The genotype/phenotype relation of Inv22 and Inv1 illustrated a statistically significant association (P = .012) between disease severity and inversion mutations. Slightly more patients with Inv22 (39%) developed inhibitors than those without Inv22 (28%; odds ratio = 1.65, 95% confidence interval = 0.56-4.87, P = .361). Inv22 is a major cause of severe HA in Iraqi Kurdish patients, and IS-PCR is a rapid, robust, and effective method that can be applied for carrier detection and prenatal diagnosis of HA in developing countries.

The EAHAD blood coagulation factor VII variant database

Hereditary blood coagulation factor VII (FVII) deficiency is a rare autosomal recessive bleeding disorder resulting from variants in the gene encoding FVII (F7). Integration of genetic variation with functional consequences on protein function is essential for the interpretation of the pathogenicity of novel variants. Here, we describe the integration of previous locus-specific databases for F7 into a single curated database with enhanced features. The database provides access to in silico analyses that may be useful in the prediction of variant pathogenicity as well as cross-species sequence alignments, structural information, and functional and clinical severity described for each variant, where appropriate. The variant data is shared with the F7 Leiden Open Variation Database. The updated database now includes 221 unique variants, representing gene variants identified in 728 individuals. Single nucleotide variants are the most common type (88%) with missense representing 74% of these variants. A number of variants are found with relatively high minor allele frequencies that are not pathogenic but contribute significantly to the likely pathogenicity of coinherited variants due to their effect on FVII plasma levels. This comprehensive collection of curated information significantly aids the assessment of pathogenicity.

The European Association for Haemophilia and Allied Disorders (EAHAD) Coagulation Factor Variant Databases: Important resources for haemostasis clinicians and researchers

INTRODUCTION

Advances in genomic sequencing have facilitated the sequencing of genes associated with disorders of haemostasis. The identification of variants within genes and access to curated data incorporating structural, functional, evolutionary as well as phenotypic data has become increasingly important in order to ascribe pathogenicity.

AIM

The European Association for Haemophilia and Allied Disorders (EAHAD) Coagulation Factor Variant Database Project aims to provide a single port of entry to a web-accessible resource for variants in genes involved in clinical bleeding disorders.

RESULTS

New databases have evolved from previously developed single gene variant coagulation database projects, incorporating new data, new analysis tools and a new common database architecture with new interfaces and filters. These new databases currently present information about the genotype, phenotype (laboratory and clinical) and structural and functional effects of variants described in the genes of factor (F) VII (F7), FVIII (F8), FIX (F9) and von Willebrand factor (VWF).

CONCLUSION

The project has improved the quality and quantity of information available to the haemostasis research and clinical communities, thereby enabling accurate classification of disease severity in order to make assessments of likely pathogenicity.

Systematic molecular analysis of hemophilia A patients from Colombia

Hemophilia A (HA) is an X-linked recessive disorder and the second most common coagulation disorder with an incidence of 1 in 5,000 live born males. Worldwide, there are 178,500 affected individuals, 60% with the severe form of the disease. Intron 22 and 1 inversions (Inv22 and Inv1) are the most frequent molecular alterations found in severe HA patients with a frequency of 45-50% and 0.5-5%, respectively. We have implemented a systematic cost-effective strategy for the identification of the molecular alteration in HA patients using Inverse shifting-PCR for Inv22 and Inv1, followed by the analysis of the F8 gene coding region by means of high resolution melting (HRM) PCR and Sanger sequencing in Inv22 and Inv1 negative patients. A total of 33 male HA patients and 6 women were analyzed. Inversion 22 was detected in 14/33 male patients (42.4%), 3/33 (9.1%) had Inv1, 3/33 (9.1%) had large structural variants, and 11/33 (33.3%) single nucleotide/ small frameshift variants. No genetic variant was found in 2/33 patients (6%). With this systematic approach we detected pathogenic variants in 31 out of 33 male affected individuals (94%) tested for the first time.in a cohort of patients from Colombia.

Mechanisms of human neutrophil elastase-catalysed inactivation of factor VIII(a)

Mechanisms of inflammation and coagulation are linked through various pathways. Human neutrophil elastase (HNE), can bind to activated platelets, might be localised on platelet membranes that provide negatively-charged phospholipid essential for the optimum function of tenase complex. In this study, we examined the effect of HNE on factor (F)VIII. FVIII activity was rapidly diminished in the presence of HNE and was undetectable within 10 minutes. The inactivation rate waŝ8-fold greater than that of activated protein C (APC). This time-dependent inactivation was moderately affected by von Willebrand factor. HNE proteolysed the heavy chain (HCh) of FVIII into two terminal products, A11–358 and A2375–708, by limited proteolysis at Val358, Val374, and Val708. Cleavage at Val708 was much slower than that at Val358 in the >90-kDa A1-A2-B compared to the 90-kDa A1-A2. The 80-kDa light chain (LCh) was proteolysed to 75-kDa product by cleavage at Val1670. HNE-cata- lysed FVIIIa inactivation was markedly slower than that of native FVIII (by ~25-fold), due to delayed cleavage at Val708 in FVIIIa. The inactivation rate mediated by HNE was ~8-fold lower than that by APC. Cleavages at Val358 and Val708 were regulated by the presence of LCh and HCh, respectively. In conclusion, HNE-catalysed FVIII inactivation was associated with the limited-proteolysis that led to A11–358, A2375–708, and A3-C1-C21671–2332, and subsequently to critical cleavage at Val708. HNE-related FVIII(a) reaction might play a role in inactivation of HNE-induced coagulation process, and appeared to depend on the amounts of inactivated FVIII and active FVIIIa which is predominantly resistant to HNE inactivation.

Note: An account of this work was presented at the 51st annual meeting of the American Society of Hematology, December 10, 2009, New Orleans, LA, USA.

Molecular simulation studies of human coagulation factor VIII C domain-mediated membrane binding

The C-terminal C domains of activated coagulation factor VIII (FVIIIa) are essential to membrane binding of this crucial coagulation cofactor protein. To provide an overall membrane binding mechanism for FVIII, we performed simulations of membrane binding through coarsegrained molecular dynamics simulations of the C1 and C2 domain, and the combined C-domains (C1+C2). We found that the C1 and C2 domain have different membrane binding properties. The C1 domain uses hydrophobic spikes 3 and 4, of its total of four spikes, as major loops to bind the membrane, whereas all four of its hydrophobic loops of the C2 domain appear essential for membrane binding. Interestingly, in the C1+C2 system, we observed cooperative binding of the C1 and C2 domains such that all four C2 domain spikes bound first, after which all four loops of the C1 domain inserted into the membrane, while the net binding energy was higher than that of the sum of the isolated C domains. Several residues, mutations of which are known to cause haemophilia A, were identified as key residues for membrane binding. In addition to these known residues, we identified residues from the C1 and C2 domains, which are involved in the membrane binding process, that have not been reported before as a cause for haemophilia A, but which contribute to overall membrane binding and which are likely candidates for novel causative missense mutations in haemophilia A.

The mild phenotype in severe hemophilia A with Arg1781His mutation is associated with enhanced binding affinity of factor VIII for factor X

The clinical severity in some patients with haemophilia A appears to be unrelated to the levels of factor (F)VIII activity (FVIII:C), but mechanisms are poorly understood. We have investigated a patient with a FVIII gene mutation at Arg1781 to His (R1781H) presenting with a mild phenotype despite FVIII:C of 0.9 IU/dl. Rotational thromboelastometry using the patient’s whole blood demonstrated that the clot time and clot firmness were comparable to those usually observed at FVIII:C 5–10 IU/dl. Thrombin and FXa assays using plasma samples also showed that the peak levels of thrombin formation and the initial rate of FXa generation were comparable to those observed at FVIII:C 5–10 IU/dl. The results suggested a significantly greater haemostatic potential in this individual than in those with severe phenotype. The addition of incremental amounts of FX to control plasma with FVIII:C 0.9 IU/dl in clot waveform analyses suggested that the enhanced functional tenase assembly might have been related to changes in association between FVIII and FX. To further investigate this mechanism, we prepared a stably expressed, recombinant, B-domainless FVIII R1781H mutant. Thrombin generation assays using mixtures of control plasma and FVIII revealed that the coagulation function observed with the R1781H mutant (0.9 IU/dl) was comparable to that seen with wild-type FVIII:C at ∼5 IU/dl. In addition, the R1781H mutant demonstrated an ∼1.9-fold decrease in K m for FX compared to wild type. These results indicated that relatively enhanced binding affinity of FVIII R1781H for FX appeared to moderate the severity of the haemophilia A phenotype.

Note: An account of this work was presented, in part, at the 23rd Congress of the International Society of Thrombosis and Haemostasis, July 27, 2011, Kyoto, Japan.

Treatment burden, haemostatic strategies and real world inhibitor screening practice in non-severe haemophilia A

Inhibitor formation in non-severe haemophilia A is a life-long risk and associated with morbidity and mortality. There is a paucity of data to understand real-world inhibitor screening practice. We evaluated the treatment burden, haemostatic strategies, F8 genotyping and inhibitor screening practices in non-severe haemophilia A in seven London haemophilia centres. In the 2-year study period, 44% (377/853) patients received at least one haemostatic treatment. Seventy-nine percent of those treated (296/377) received factor VIII (FVIII) concentrate. F8 genotype was known in 88% (331/377) of individuals. Eighteen per cent (58/331) had 'high-risk' F8 genotypes. In patients with 'standard-risk' F8 genotypes treated on-demand with FVIII concentrate, 51·3% episodes (243/474) were screened within 1 year. However, poor screening compliance was observed after 'high-risk' treatment episodes. In patients with 'standard-risk' F8 genotypes, 12·3% (28/227) of treatment episodes were screened in the subsequent 6 weeks after surgery or a bleed requiring ≥5 exposure days. Similarly, in the context of 'high-risk' F8 genotypes after any FVIII exposure, only 13·6% (12/88) of episodes were screened within 6 weeks. Further study is required to assess optimal practice of inhibitor screening in non-severe haemophilia A to inform subsequent clinical decisions and provide more robust prevalence data to further understand the underlying immunological mechanism.

Immunogenicity of Biotherapeutics: Causes and Association with Posttranslational Modifications

Today, potential immunogenicity can be better evaluated during the drug development process, and we have rational approaches to manage the clinical consequences of immunogenicity. The focus of the scientific community should be on developing sensitive diagnostics that can predict immunogenicity-mediated adverse events in the small fraction of subjects that develop clinically relevant anti-drug antibodies. Here, we discuss the causes of immunogenicity which could be product-related (inherent property of the product or might be picked up during the manufacturing process), patient-related (genetic profile or eating habits), or linked to the route of administration. We describe various posttranslational modifications (PTMs) and how they may influence immunogenicity. Over the last three decades, we have significantly improved our understanding about the types of PTMs of biotherapeutic proteins and their association with immunogenicity. It is also now clear that all PTMs do not lead to clinical immunogenicity. We also discuss the mechanisms of immunogenicity (which include T cell-dependent and T cell-independent responses) and immunological tolerance. We further elaborate on the management of immunogenicity in preclinical and clinical setting and the unique challenges raised by biosimilars, which may have different immunogenic potential from their parent biotherapeutics.

Experience of Preimplantation Genetic Diagnosis for Hemophilia at the University Hospital Virgen Del Rocío in Spain: Technical and Clinical Overview

Hemophilia A and B are the most common hereditary hemorrhagic disorders, with an X-linked mode of inheritance. Reproductive options for the families affected with hemophilia, aiming at the prevention of the birth of children with severe coagulation disorders, include preimplantation genetic diagnosis (PGD). Here we present the results of our PGD Program applied to hemophilia, at the Department of Genetics, Reproduction and Fetal Medicine of the University Hospital Virgen del Rocío in Seville. A total of 34 couples have been included in our program since 2005 (30 for hemophilia A and 4 for hemophilia B). Overall, 60 cycles were performed, providing a total of 508 embryos. The overall percentage of transfers per cycle was 81.7% and the live birth rate per cycle ranged from 10.3 to 24.1% depending on the methodological approach applied. Although PGD for hemophilia can be focused on gender selection of female embryos, our results demonstrate that methodological approaches that allow the diagnosis of the hemophilia status of every embryo have notorious advantages. Our PGD Program resulted in the birth of 12 healthy babies for 10 out of the 34 couples (29.4%), constituting a relevant achievement for the Spanish Public Health System within the field of haematological disorders.

A large-scale computational study of inhibitor risk in non-severe haemophilia A

Over 500 missense F8 mutations have been reported to cause non-severe haemophilia A. Some F8 genotypes appear to confer a higher risk of inhibitor formation than others and individuals with the same F8 genotype may have differing risks of inhibitor formation. We present an in silico strategy demonstrating the heterogeneity of factor VIII (FVIII)-derived antigen presentation whilst identifying patterns of human leucocyte antigen (HLA) peptide binding that might predict future inhibitor risk. A well-validated computational tool, NetMHCII, enabled large-scale comparison of predicted antigen presentation between endogenous, mutated FVIII-derived peptides and wild-type, therapeutic FVIII-derived peptides spanning all F8 missense mutation positions reported to The Haemophilia A Mutation, Structure and Resource Site (HADB). We identify 40 F8 genotypes to be 'low risk' at a 50% inhibitory concentration (IC50 )-binding threshold of 300 nmol/l (P = 0·00005), defined as absence of novel peptide-major histocompatibility complex (MHC) surfaces for all 14 common HLA-DR alleles assessed. Analysing each of the possible 7280 F8 genotype/HLA-DR permutations individually at an IC50 threshold of 300 nmol/l, 65% are predicted to not generate a novel peptide-MHC surface that would be necessary to engage T cell help for subsequent anti-FVIII antibody generation. This study demonstrates the future importance of interpreting F8 genotype in the context of an individual's HLA profile to personalize inhibitor risk prediction.

Role of hydrophobic mutations on the binding affinity and stability of blood coagulation factor VIIIa: a computational molecular dynamics and free-energy analysis

Factor VIIIa is a non-covalently bound hetero-trimer among A1, A2 and A3-C1-C2 domains and an essential co-factor for factor IXa enzyme during proteolytic activation of factor X zymogen. The relatively weak interactions between A2 and the interface A1/A3 domains dampen the functional stability of FVIIIa in plasma and results in rapid degradation. We studied the mutational effect of three charged residues (Asp519, Glu665 and Asp666) to several hydrophobic residues by molecular dynamics simulations. Analysis of the binding free energy by MM-PBSA and MM-GBSA methods shows that the mutation of Asp519 and Glu665 residues to either Val or Ala enhance the A2 domain binding affinity in agreement with the experimental site-specific mutagenesis data. Mutation of Asp666 to Val, Tyr, Met and Phe showed largest improvement in the A2-domain binding among the eight hydrophobic mutants studied. Our studies suggest that the enrichment of hydrophobic interactions in the buried surface regions of A2 domain plays crucial role in improving the overall stability of FVIIIa.

In silico calculated affinity of FVIII-derived peptides for HLA class II alleles predicts inhibitor development in haemophilia A patients with missense mutations in the F8 gene

Forty per cent of haemophilia A (HA) patients have missense mutations in the F8 gene. Yet, all patients with identical mutations are not at the same risk of developing factor VIII (FVIII) inhibitors. In severe HA patients, human leucocyte antigen (HLA) haplotype was identified as a risk factor for onset of FVIII inhibitors. We hypothesized that missense mutations in endogenous FVIII alter the affinity of the mutated peptides for HLA class II, thus skewing FVIII-specific T-cell tolerance and increasing the risk that the corresponding wild-type FVIII-derived peptides induce an anti-FVIII immune response during replacement therapy. Here, we investigated whether affinity for HLA class II of wild-type FVIII-derived peptides that correspond to missense mutations described in the Haemophilia A Mutation, Structure, Test and Resource database is associated with inhibitor development. We predicted the mean affinity for 10 major HLA class II alleles of wild-type FVIII-derived peptides that corresponded to 1456 reported cases of missense mutations. Linear regression analysis confirmed a significant association between the predicted mean peptide affinity and the mutation inhibitory status (P = 0.006). Significance was lost after adjustment on mutation position on FVIII domains. Although analysis of the A1-A2-A3-C1 domains yielded a positive correlation between predicted HLA-binding affinity and inhibitory status (OR = 0.29 [95% CI: 0.14-0.60] for the high affinity tertile, P = 0.002), the C2 domain-restricted analysis indicated an inverse correlation (OR = 3.56 [1.10-11.52], P = 0.03). Our data validate the importance of the affinity of FVIII peptides for HLA alleles to the immunogenicity of therapeutic FVIII in patients with missense mutations.

Domain organization of membrane-bound factor VIII

Factor VIII (FVIII) is the blood coagulation protein which when defective or deficient causes for hemophilia A, a severe hereditary bleeding disorder. Activated FVIII (FVIIIa) is the cofactor to the serine protease factor IXa (FIXa) within the membrane-bound Tenase complex, responsible for amplifying its proteolytic activity more than 100,000 times, necessary for normal clot formation. FVIII is composed of two noncovalently linked peptide chains: a light chain (LC) holding the membrane interaction sites and a heavy chain (HC) holding the main FIXa interaction sites. The interplay between the light and heavy chains (HCs) in the membrane-bound state is critical for the biological efficiency of FVIII. Here, we present our cryo-electron microscopy (EM) and structure analysis studies of human FVIII-LC, when helically assembled onto negatively charged single lipid bilayer nanotubes. The resolved FVIII-LC membrane-bound structure supports aspects of our previously proposed FVIII structure from membrane-bound two-dimensional (2D) crystals, such as only the C2 domain interacts directly with the membrane. The LC is oriented differently in the FVIII membrane-bound helical and 2D crystal structures based on EM data, and the existing X-ray structures. This flexibility of the FVIII-LC domain organization in different states is discussed in the light of the FVIIIa-FIXa complex assembly and function.

In vitro and In vivo Model Systems for Hemophilia A Gene Therapy

Hemophilia A is a hereditary disorder caused by various mutations in factor VIII gene resulting in either a severe deficit or total lack of the corresponding activity. Recent success in gene therapy of a related disease, hemophilia B, gives new hope that similar success can be achieved for hemophilia A as well. To develop a gene therapy strategy for the latter, a variety of model systems are needed to evaluate molecular engineering of the factor VIII gene, vector delivery efficacy and safety-related issues. Typically, a tissue culture cell line is the most convenient way to get a preliminary glimpse of the potential of a vector delivery strategy. It is then followed by extensive testing in hemophilia A mouse and dog models. Newly developed hemophilia A sheep may provide yet another tool for evaluation of factor VIII gene delivery vectors. Hemophilia models based on other species may also be developed since hemophiliac animals have been identified or generated in rat, pig, cattle and horse. Although a genetic nonhuman primate hemophilia A model has yet to be developed, the non-genetic hemophilia A model can also be used for special purposes when specific questions need to be addressed that cannot not be answered in other model systems. Hemophilia A is caused by a functional deficiency in the factor VIII gene. This X-linked, recessive bleeding disorder affects approximately 1 in 5000 males [1–3]. Clinically, it is characterized by frequent and spontaneous joint hemorrhages, easy bruising and prolonged bleeding time. The coagulation activity of FVIII dictates severity of the clinical symptoms. Approximately 50% of all cases are classified as severe with less than 1% of normal levels of factor VIII detected [4]. This deficiency may lead to spontaneous joint hemorrhages or life-threatening bleeding. In contrast, patients with 5–30% of normal factor VIII activity exhibit mild clinical manifestations.

Study of intron 22 inversion mutation in north India with review

The present study assessed the frequency of intron 22 inversion mutation (Inv 22) in north Indian population with a cost analysis of different methods used for Inv 22 detection. We assessed the frequency of intron 22 inversion mutation in a series of 181 cases with hemophilia A and also compared methods used for detection of the mutation including the long-distance PCR, Southern blot analysis, and inverse PCR in terms of cost, infrastructure, and technical input as well as turnaround time. The study group comprised 102 severe cases and 79 moderate cases of hemophilia A from a north Indian population of which 77 cases tested positive for Inv 22. The observed frequency of Inv22 mutation was 42.5%. Inv 22 resulted in a more severe phenotype and lower FVIII bioassay levels as compared to Inv 22 negative cases. Inv 22 positive cases also frequently presented with bleeding episodes at birth and the mean age for commencement of bleeding was lower (19 months) as compared to Inv-negative cases (50 months). The mean frequency of Inv 22 in cases with hemophilia A in a worldwide review is 44.25% of hemophilia A. Inv 22 can be conveniently detected by using the inverse PCR method. This technique is easy to standardize and lowest in cost.

A novel mutation (4040-4045 nt. delA) in exon 14 of the factor VIII gene causing severe hemophilia A

Hemophilia A is an X-linked congenital bleeding disorder caused by Factor VIII deficiency. Different mutations including point mutations, deletions, insertions and inversions have been reported in the FVIII gene, which cause hemophilia A. In the current study, with the use of conformational sensitive gel electrophoresis (CSGE) analysis, we report a novel 1-nt deletion in the A6 sequence at codons 1328-1330 (4040-4045 nt delA) occurring in exon 14 of the FVIII gene in a seven-year-old Iranian boy with severe hemophilia A. This mutation that causes frameshift and premature stop-codon at 1331 has not previously been reported in the F8 Hemophilia A Mutation, Structure, Test and Resource Site (HAMSTeRS) database.

Ribosome readthrough accounts for secreted full-length factor IX in hemophilia B patients with nonsense mutations

We investigated the spontaneous ribosome readthrough, virtually unexplored in genes encoding secreted proteins, over coagulation F9 nonsense mutations. Expression of recombinant factor IX (FIX) in eukaryotic cells demonstrated appreciable levels of secreted FIX molecules for the mutations p.R162* (5 ± 0.3% of rFIX-wt antigen levels), p.R294* (3.1 ± 1.1%) and p.R298* (2.5 ± 0.7%), but not for the p.L103*. Western blotting revealed a large proportion of truncated molecules, which correlated with small amounts of full-length FIX (rFIX-162*, ∼0.5%; rFIX-294*; and rFIX-298*, ∼0.2%). Western blotting of plasma from FIX deficient (Hemophilia B) patients revealed traces of full-length FIX for the p.R294* and p.R298* mutations, but not for the p.L103* mutation that triggered major FIX mRNA decay. The detection of full-length proteins has clinical implication, particularly for post-therapeutic immunological complications in Hemophilia. Data in patients' plasma and in vitro, obtained in the proper protein context, support a ribosome readthrough gradient, consistent with its predicted determinants of efficiency.

P3-P3' residues flanking scissile bonds in factor VIII modulate rates of substrate cleavage and procofactor activation by thrombin

Thrombin-catalyzed activation of factor VIII (FVIII) occurs through proteolysis at three P1 Arg residues: Arg(372) and Arg(740) in the FVIII heavy chain and Arg(1689) in the FVIII light chain. Cleavage at the latter two sites is relatively fast compared with cleavage at Arg(372), which appears to be rate-limiting. Examination of the P3-P3' residues flanking each P1 site revealed that those sequences at Arg(740) and Arg(1689) are more optimal for thrombin cleavage than at Arg(372), suggesting these sequences may impact reaction rates. Recombinant FVIII variants were prepared with mutations swapping scissile bond flanking sequences in the heavy chain individually and in combination with a second swap or with a P1 point mutation. Rates of generation of A1 and A3-C1-C2 subunits were determined by Western blotting and correlated with rates of cleavage at Arg(372) and Arg(1689), respectively. Rates of thrombin cleavage at Arg(372) were increased ~10- and ~3-fold compared with that of wild-type FVIII when it was replaced with P3-P3' residues flanking Arg(740) and Arg(1689), respectively, and these values paralleled increased rates of A2 subunit generation and procofactor activation. Positioning of more optimal residues flanking Arg(372) abrogated the need for initial cleavage at Arg(740) to facilitate this step. These results show marked changes in cleavage rates correlate with the extent of cleavage-optimal residues flanking the scissile bond and modulate the mechanism for procofactor activation.

Prevalence of intron 1 inversion of cases with hemophilia A in North Indian population

Hemophilia A is an X-linked bleeding disorder resulting from heterogeneous mutations in the factor VIII (FVIII) gene. The disease is frequently caused by intron 22 inversion mutation in FVIII gene in approximately 50% cases and by intron 1 inversion (Inv1) in 2% to 5% cases with severe hemophilia A. Both inversion mutations occur due to intrachromosomal recombination between inverted repeats found in intronic sequences in upstream regions located telomeric to the FVIII gene and result in a severe phenotype. Frequency of Inv1 mutation varies in different populations. We have analyzed the frequency of the Inv1 in a group of 110 cases with Hemophilia A from North Indian Population and found 4 inversion-positive cases (3.6%). The Inv1 genotyping was further applied to analyze the hemophilia carrier status of the mother and proband's sister. Direct mutation detection strategy is more perfective in diagnosis of cases, carriers, and in prenatal diagnosis of hemophilia A.

Mass spectrometry-assisted study reveals that lysine residues 1967 and 1968 have opposite contribution to stability of activated factor VIII

The A2 domain rapidly dissociates from activated factor VIII (FVIIIa) resulting in a dampening of the activity of the activated factor X-generating complex. The amino acid residues that affect A2 domain dissociation are therefore critical for FVIII cofactor function. We have now employed chemical footprinting in conjunction with mass spectrometry to identify lysine residues that contribute to the stability of activated FVIII. We hypothesized that lysine residues, which are buried in FVIII and surface-exposed in dissociated activated FVIII (dis-FVIIIa), may contribute to interdomain interactions. Mass spectrometry analysis revealed that residues Lys(1967) and Lys(1968) of region Thr(1964)-Tyr(1971) are buried in FVIII and exposed to the surface in dis-FVIIIa. This result, combined with the observation that the FVIII variant K1967I is associated with hemophilia A, suggests that these residues contribute to the stability of activated FVIII. Kinetic analysis revealed that the FVIII variants K1967A and K1967I exhibit an almost normal cofactor activity. However, these variants also showed an increased loss in cofactor activity over time compared with that of FVIII WT. Remarkably, the cofactor activity of a K1968A variant was enhanced and sustained for a prolonged time relative to that of FVIII WT. Surface plasmon resonance analysis demonstrated that A2 domain dissociation from activated FVIII was reduced for K1968A and enhanced for K1967A. In conclusion, mass spectrometry analysis combined with site-directed mutagenesis studies revealed that the lysine couple Lys(1967)-Lys(1968) within region Thr(1964)-Tyr(1971) has an opposite contribution to the stability of FVIIIa.

Long-term expression of human coagulation factor VIII in a tolerant mouse model using the φC31 integrase system

We generated a mouse model for hemophilia A that combines a homozygous knockout for murine factor VIII (FVIII) and a homozygous addition of a mutant human FVIII (hFVIII). The resulting mouse, having no detectable FVIII protein or activity and tolerant to hFVIII, is useful for evaluating FVIII gene-therapy protocols. This model was used to develop an effective gene-therapy strategy using the φC31 integrase to mediate permanent genomic integration of an hFVIII cDNA deleted for the B-domain. Various plasmids encoding φC31 integrase and hFVIII were delivered to the livers of these mice by using hydrodynamic tail-vein injection. Long-term expression of therapeutic levels of hFVIII was observed over a 6-month time course when an intron was included in the hFVIII expression cassette and wild-type φC31 integrase was used. A second dose of the hFVIII and integrase plasmids resulted in higher long-term hFVIII levels, indicating that incremental doses were beneficial and that a second dose of φC31 integrase was tolerated. We observed a significant decrease in the bleeding time after a tail-clip challenge in mice treated with plasmids expressing hFVIII and φC31 integrase. Genomic integration of the hFVIII expression plasmid was demonstrated by junction PCR at a known hotspot for integration in mouse liver. The φC31 integrase system provided a nonviral method to achieve long-term FVIII gene therapy in a relevant mouse model of hemophilia A.

F8 gene mutation type and inhibitor development in patients with severe hemophilia A: systematic review and meta-analysis

This systematic review was designed to provide more precise effect estimates of inhibitor development for the various types of F8 gene mutations in patients with severe hemophilia A. The primary outcome was inhibitor development and the secondary outcome was high-titer-inhibitor development. A systematic literature search was performed to include cohort studies published in peer-reviewed journals with data on inhibitor incidences in the various F8 gene mutation types and a mutation detection rate of at least 80%. Pooled odds ratios (ORs) of inhibitor development for different types of F8 gene mutations were calculated with intron 22 inversion as the reference. Data were included from 30 studies on 5383 patients, including 1029 inhibitor patients. The inhibitor risk in large deletions and nonsense mutations was higher than in intron 22 inversions (pooled OR = 3.6, 95% confidence interval [95% CI], 2.3-5.7 and OR = 1.4, 95% CI, 1.1-1.8, respectively), the risk in intron 1 inversions and splice-site mutations was equal (pooled OR = 0.9; 95% CI, 0.6-1.5 and OR = 1.0; 95% CI, 0.6-1.5), and the risk in small deletions/insertions and missense mutations was lower (pooled OR = 0.5; 95% CI, 0.4-0.6 and OR = 0.3; 95% CI, 0.2-0.4, respectively). The relative risks for developing high titer inhibitors were similar.

RNA-based therapeutic approaches for coagulation factor deficiencies

Substitutive therapy has significantly ameliorated the quality of life of patients with coagulation factor deficiencies. However, there are some limitations that support research towards alternative therapeutic approaches. Here we focus on the rescue of coagulation factor biosynthesis by targeting the RNA processing and translation, which would permit restoration of the altered gene expression while maintaining the gene regulation in the physiological tissues. The essential prerequisite of the three reported RNA-based correction approaches (i-iii), which rely on mutation types and are applicable even to large size mRNAs, is the presence in cells of the precursor (pre-mRNA) or mature mRNA forms. (i) In the F7 gene, modification of the small nuclear RNA U1 (U1 snRNA), the key component of the spliceosomal U1 ribonucleoprotein, re-directs correct usage of a mutated exon-intron junction, triggering synthesis of correct mRNA and secretion of functional factor (F)VII. (ii) Spliceosome-mediated RNA trans-splicing (SMaRT) between mutated and engineered pre-mRNAs produces normal FVIII mRNA and secretion of functional protein. (iii) Aminoglycoside drugs induce ribosome readthrough and suppress premature translation termination caused by nonsense mutations in FVII, VIII and IX. The rescued expression levels ranged from very low (aminoglycosides) to moderate (U1 snRNA and SMaRT), which could result in amelioration of the disease phenotypes. These findings prompt further studies aimed at demonstrating the clinical translatability of RNA-based strategies, which might open new avenues in the treatment of coagulation factor deficiencies.

Eighteen years of molecular genotyping the hemophilia inversion hotspot: from southern blot to inverse shifting-PCR

The factor VIII gene (F8) intron 22 inversion (Inv22) is a paradigmatic duplicon-mediated rearrangement, found in about one half of patients with severe hemophilia A worldwide. The identification of this prevalent cause of hemophilia was delayed for nine years after the F8 characterization in 1984. The aim of this review is to present the wide diversity of practical approaches that have been developed for genotyping the Inv22 (and related int22h rearrangements) since discovery in 1993. The sequence- Southern blot, long distance-PCR and inverse shifting-PCR-for Inv22 genotyping is an interesting example of scientific ingenuity and evolution in order to resolve challenging molecular diagnostic problems.

Use of affinity-directed liquid chromatography-mass spectrometry to map the epitopes of a factor VIII inhibitor antibody fraction

BACKGROUND

Neutralizing factor (F) VIII antibodies develop in approximately 30% of individuals with hemophilia A and show specificity to multiple sites in the FVIII protein.

METHODS

Reactive epitopes to an immobilized IgG fraction prepared from a high-titer, FVIII inhibitor plasma were determined after immuno-precipitation (IP) of tryptic and chymotryptic peptides derived from digests of the A1 and A2 subunits of FVIIIa and FVIII light chain. Peptides were detected and identified using highly sensitive liquid chromatography-mass spectrometry (LC-MS).

RESULTS

Coverage maps of the A1 subunit, A2 subunit and light chain represented 79%, 69% and 90%, respectively, of the protein sequences. Dot blots indicated that the inhibitor IgG reacted with epitopes contained within each subunit of FVIIIa. IP coupled with LC-MS identified 19 peptides representing epitopes from all FVIII A and C domains. The majority of peptides (10) were derived from the A2 domain. Three peptides mapped to the C2 domain, while two mapped to the A1 and A3 domains, and single peptides mapped to the a1 segment and C1 domain. Epitopes were typically defined by peptide sequences of < 12 residues.

CONCLUSIONS

IP coupled with LC-MS identified extensive antibody reactivity at high resolution over the entire functional FVIII molecule and yielded sequence lengths of < 15 residues. A number of the peptides identified mapped to known sequences involved in functionally important protein-protein and protein-membrane interactions.

Haemophilia management: time to get personal?

The possibility of alloimmunization in patients receiving protein replacement therapy depends on (at least) three risk factors, which are necessary concomitantly but insufficient alone. The first is the degree of structural difference between the therapeutic protein and the patient's own endogenous protein, if expressed. Such differences depend on the nature of the disease mutation and the pre-mutation endogenous protein structure as well as on post-translational changes and sequence-engineered alterations in the therapeutic protein. Genetic variations in the recipients' immune systems comprise the second set of risk determinants for deleterious immune responses. For example, the limited repertoire of MHC class II isomers encoded by a given person's collection of HLA genes may or may not be able to present a 'foreign' peptide(s) produced from the therapeutic protein - following its internalization and proteolytic processing - on the surface of their antigen-presenting cells (APCs). The third (and least characterized) variable is the presence or absence of immunologic 'danger signals' during the display of foreign-peptide/MHC-complexes on APCs. A choice between existing therapeutic products or the manufacture of new proteins, which may be less immunogenic in some patients or patient populations, may require prior definition of the first two of these variables. This leads then to the possibility of developing personalized therapies for disorders due to genetic deficiencies in endogenous proteins, such as haemophilia A and B. [Correction made after online publication 11 July 2011: several critical corrections have been made to the abstract].

Most factor VIII B domain missense mutations are unlikely to be causative mutations for severe hemophilia A: implications for genotyping

BACKGROUND & OBJECTIVE

 The factor VIII (FVIII) B domain shares very little amino acid homology with other known proteins and is not directly necessary for procoagulant activity. Despite this, missense mutations within the B domain have been reported in patients with hemophilia A. Given that the B domain is dispensable for secretion and function of FVIII, we hypothesized that these mutations should not be causative of hemophilia A in these patients.

METHODS

 Plasmid vectors containing B domain missense mutations that were reported to be associated with moderate/severe hemophilia A (T751S, D826E, V993L, H1047Y, T1353A, N1441K, L1462P, E1579D, A1591S, P1641L and S1669L) were analyzed for their effect on synthesis and secretion compared with FVIII wild-type (WT) following transient transfection into COS-1 and CHO cells in vitro. Further, H1047Y, N1441K and E1579D mutants were expressed in vivo in a hemophilia A mouse model by hydrodynamic tail-vein injection.

RESULTS

 FVIII activity and antigen levels for all mutants expressed into the conditioned media of COS-1 and CHO cells were similar to FVIII WT. Also, plasma expression of these mutants was similar to FVIII WT in hemophilia A mice. An in vivo tail clip bleeding assay also demonstrated that blood loss from hemophilia A mice expressing FVIII WT, H1047Y, N1441K and E1579D was similar.

CONCLUSIONS

 We conclude that most missense mutations within the FVIII B domain would be unlikely to lead to severe hemophilia A and that the majority of such missense mutations represent polymorphisms or non-pathologic mutations.

Informativeness of a novel multiallelic marker-set comprising an F8 intron 21 and three tightly linked loci for haemophilia A carriership analysis

The extraordinary heterogeneous nature of the deleterious mutations in the F8 gene that lead to functional deficiency of clotting factor VIII in haemophilia A makes routine direct mutation profiling difficult. When direct mutation analysis cannot be performed or a causative/candidate mutation is not found, a second-line approach to track the defective F8 gene within at-risk families is linkage genetic analysis with, tried-and-tested, F8-intragenic and/or extragenic non-recombining multiallelic short tandem repeats (STR). Although several typing STR loci within and around F8 have been described, there is need for improving assessment, because the combined informativeness of available assays rarely reaches 100%. Here, we characterized a newly identified 0.28 cM-resolution marker-set, consisting of a dinucleotide STR located on F8 intron 21 (F8Int21; [AC](n)) and three extragenic tetranucleotide STR located on GAB3 intron 1 (GAB3Int1; [TAAA](n)) and TMLHE intron 1 (TMLHEInt1.1; [GAAA](n) and TMLHEInt1.3; [ATTC](n)). Heterozygosity rates determined in 100 unrelated females ranged from 0.25 (GAB3Int1) to 0.63 (F8Int21). The set rendered a combined informativeness of 0.91 for at least one marker and 0.60 for a minimum of two loci, with at least one F8-intragenic. Multiallelic interlocus non-random association analysis revealed that GAB3Int1 is not in significant gametic disequilibrium (GD) with F8Int21, F8Int9.2, TMLHEInt1.3 or TMLHEInt1.1. Gametic disequilibrium breakdown attests historical recombination between GAB3Int1 and the F8 gene. Through computational analysis of reference assembly sequence data, we note in the GD breakdown region and in the F8 gene a higher than average density of the 13-mer CCNCCNTNNCCNC consensus motif, commonly associated with recombination hotspots.

In non-severe hemophilia A the risk of inhibitor after intensive factor treatment is greater in older patients: a case-control study

BACKGROUND

 Twenty-five percent of new anti-factor VIII (FVIII) antibodies (inhibitors) that complicate hemophilia A occur in those with mild and moderate disease. Although intensive FVIII treatment has long been considered a risk factor for inhibitor development in those with non-severe disease, its strength of association and the influence of other factors have remained undefined.

OBJECTIVE

 To evaluate risk factors for inhibitor development in patients with non-severe hemophilia A.

METHODS

 Information on clinical and demographic variables and FVIII genotype was collected on 36 subjects with mild or moderate hemophilia A and an inhibitor and 62 controls also with mild or moderate hemophilia A but without an inhibitor.

RESULTS

 Treatment with FVIII for six or more consecutive days during the prior year was more strongly associated with inhibitor development in those ≥30years of age compared with those <30years of age [adjusted odds ratio (OR) 12.62; 95% confidence interval (CI), 2.76-57.81 vs. OR 2.54; 95% CI, 0.61-10.68]. Having previously received <50days of FVIII was also not statistically associated with inhibitor development on univariate or multivariate analysis.

CONCLUSIONS

 These findings suggest that inhibitor development in mild and moderate hemophilia A varies with age, but does not vary significantly with lifetime FVIII exposure days: two features distinct from severe hemophilia A.

Mutation analysis of factor VIII in Korean patients with severe hemophilia A

Hemophilia A is an X-linked recessive disorder caused by mutations of the factor VIII gene. The mutation spectrum has been reported in various populations, but not in Koreans. Mutation analysis of the factor VIII gene was performed in 22 unrelated Korean patients with severe hemophilia A. We extracted genomic DNA from their blood, and assessed intron inversions, deletions, and point mutations by direct DNA sequencing. A multiplex ligation-dependent probe amplification gene dosage assay was also performed to identify exon deletions. Disease-causing mutations were identified in all patients, of which four cases were previously unreported. Seven intron 22 inversions, nine point mutations (6 nonsense mutations and 3 missense mutations), and four small rearrangements were identified. One multi-exon deletion and one 5'-donor splicing site mutation were also observed. Four novel mutations (one small deletion, one multiple exon deletion, one missense, and one splice site mutation) were detected, and point mutations were predominant (40.9%), followed by intron 22 inversions (31.8%). Further studies are required in order to establish a solid conclusion regarding the prevalence of various mutations in the Korean population.