1,2,3Department of General Medicine, Institute of Medical Science (IMS), Banaras Hindu University, BHU Varanasi-221005.
4,5Centre for Genetic Disorder, Institute of Science, Varanasi- 221005.
Hemophilia B is a congenital X-linked recessive bleeding disorder caused by mutations in the F9 gene, leading to a deficiency in clotting factor IX. Mutational analysis identifies genetic mutations affecting factor IX production, aiding diagnosis, genetic counseling, and personalized treatment. Techniques like Sanger sequencing and next-generation sequencing (NGS) are used to detect these mutations. Sanger offers detailed single-gene analysis, while NGS provides broader, high-throughput screening. Identifying mutations helps predict disease severity, guide carrier detection, and enable prenatal diagnosis, supporting family planning. Challenges in mutational analysis include detecting complex mutations, high costs, and interpreting rare variants' clinical significance. Hemophilia B shows clinical variability due to multiple mutations and genetic modifiers, complicating severity predictions and treatment outcomes. Understanding genotype-phenotype correlations aids in tailoring factor IX replacement therapy and other interventions. Gene therapy and emerging treatments, like bypassing agents, offer promise for long-term management by targeting specific F9 mutations. Genetic counseling is crucial for families, particularly carrier females, providing options like prenatal diagnosis. Mutational analysis has advanced personalized care strategies, though managing inhibitor formation from therapy mismatches remains challenging. Future advancements in detection techniques may enhance treatment precision, prevent inhibitor formation, and improve outcomes for Hemophilia B patients.
An X-linked recessive congenital bleeding disorder is hemophilia. The causes of hemophilia A (classical) and B (Christmas illness), respectively, are deficiencies in coagulation factors VIII (FVIII) or IX (FIX). Of all cases of hemophilia, hemophilia A accounts for 80% and hemophilia B for 20% [1]. It is representative of the majority of hereditary clotting issues. Hemophilia A and B prevalence in male neonates is approximately 5% and 3%, respectively. Hemophilia severity is classified based on measurable clotting activity, with normal levels at 100% (100 IU/dL). Three phenotypes are recognized: mild (>5% to <40%), moderate (1%?5%), and severe (<1%). However, these classifications don’t always match clinical presentation, as other hemostatic factors may influence bleeding risk. Recent interest has grown in moderate-to-mild phenotypes, particularly with gene therapy and extended half-life (EHL) medications targeting these factor levels. In cases with unknown family history, hemophilia is often diagnosed after initial bleeding episodes, a scenario that is increasingly common, affecting around 50% of cases [2].
Mutational Analysis in Hemophilia B
Hemophilia B (HB) is a genetic disorder caused by a deficiency or dysfunction of coagulation factor IX (FIX), a protein encoded by the FIX gene located on the long arm of the X chromosome (q27 region). This gene spans 34 kb of DNA, consisting of eight exons. HB affects approximately 1 in 30,000 males and is classified based on FIX activity into severe, moderate, or mild forms. Mutations in the FIX gene are the primary cause of HB, with point mutations being the most common. While 19 recurrent mutations account for about half of the cases, de novo mutations also frequently occur. Genetic counseling has become a critical aspect of HB management, providing essential information to at-risk families regarding inheritance patterns and prenatal diagnosis. Advances in gene analysis, such as those performed on chorionic villus samples, have improved the accessibility of prenatal diagnosis for carriers, leading to better family planning decisions [3].
Molecular Studies
In a linkage study of the Italian population using intragenic polymorphisms De I, Mnl I, and Ha I, a heterozygosity rate of over 70% was observed. Among 14 mothers of sporadic hemophilia B (HB) patients, 8 were heterozygous for at least one marker, while 4 were homozygous for all markers. For 7 female relatives at risk, unknown polymorphisms left their carrier status uncertain. In familial HB, obligatory carriers were homozygous for all markers. Conformation-Sensitive Gel Electrophoresis (CSGE) identified distinct mutations, enhancing genetic counseling and facilitating prenatal diagnosis for uninformative female relatives in HB families [4].
Purpose and significance of mutational analysis in hemophilia B
Heterozygous Females (Carriers)
Females heterozygous for F9 mutations causing hemophilia B (HB) exhibit a wide range of factor IX (FIX) levels due to random X-chromosome inactivation (XCI), which equalizes X-linked gene dosage between males and females. XCI results in the paternal or maternal X being inactivated in different cell clusters. Some heterozygous females may show typical hemophilia symptoms, such as hematuria and muscle hemorrhaging, and should be considered hemophiliacs. Others may have normal FIX levels, making identification through FIX measurement alone impossible. Gene sequencing has improved the identification of HB heterozygotes, who experience higher bleeding risks after surgery, tooth extraction, and childbirth than non-heterozygotes. The genetic analysis is started straight away in individuals with mild to moderate HA and HB to identify any molecular flaws by direct sequencing. [Figure No. 1].
Figure no. 1 An outline for diagnosing hemophilia B. Abbreviations include prothrombin time (PT), activated partial thromboplastin time (APTT), thrombin time (TT), vitamin K-dependent clotting factors-1 (VKCFD1), and bleeding assessment tool (BAT). Insights into the Molecular Genetic of Hemophilia A and Hemophilia B Pezeshkpoor et al. (2022).
Menorrhagia is not always present however, it is common [5,6]. Furthermore, a substantial inverse association was found between the bleeding score and factor level [7]. The International Society on Thrombosis and Haemostasis developed a bleeding assessment tool, showing that women heterozygous for hemophilia A (HA) or hemophilia B (HB) score higher in bleeding categories such as menorrhagia, hemarthrosis, and postpartum hemorrhage. Women with multiple HB sons, daughters of HB men, or those with HB relatives are considered obligatory heterozygotes. Potential heterozygotes include other female relatives of HB males. F9 gene sequencing identifies most HB heterozygotes, though deletions and duplications may require multiplex ligation-dependent probe amplification (MLPA). Heterozygous women should have their factor IX levels and bleeding history evaluated, receive appropriate treatment, and undergo genetic counseling for reproductive planning [8].
Females with Hemophilia B
Despite the rarity of mild hemophilia B (HB) in women, 24% of severe or moderate HB cases at U.S. Hemophilia Treatment Centers involved females. Genetic studies revealed compound heterozygosity (one hemophilia-causing variant and one normal allele), homozygosity (two copies of the same variant), and transzygosity (two distinct variants). A 9-year-old girl with FIX <1% had a homozygous F9 mutation (c.484C>T), inherited from her heterozygous mother and homozygous father [9]. Another case involved a mother and daughter with moderate HB (FIX 1%) due to somatic mosaicism. Additionally, two girls showed hemizygosity with X-chromosome deletions, and Turner syndrome was present in three of five cases. In 43 families, defective X-chromosome inactivation led to heterozygosity [10].
Different techniques used for mutational analysis (Sanger sequencing, next-generation sequencing, etc.)
Hemophilia A, one of the most common severe inherited bleeding disorders, is caused by mutations in the coagulation factor VIII (F8) gene. Understanding genetic variants is essential for individualized treatment. Since the F8 gene was cloned in 1984, over 3,000 variants have been cataloged in global databases. Three main techniques are used for F8 mutation detection: (i) direct sequencing of the F8 coding region and regulatory elements, (ii) PCR and other methods to investigate Intron 22 inversions, and (iii) MLPA for detecting copy number variants. Some patients (2%) show no F8 mutations, suggesting deep intron variants. Recent studies identified mutations in introns 9 and 10, causing abnormal splicing. Next-generation sequencing (NGS) is now being developed to detect hidden intronic variants in the F8 sequence [11]
Challenges and limitations of mutational analysis in Hemophilia B
Hemophilia patients often lack awareness of their condition, and around 30% have no family history of the disorder. Acute bleeds should be treated within two hours, but children often fear injections and may hide injuries. Families frequently underestimate the serious consequences of untreated bleeding. A major obstacle to effective care is a lack of understanding about the urgency of treating bleeds. Parents may delay administering clotting factors, fearing they might harm their child, experiencing anxiety over injections, or lacking confidence in giving them. Home-based care brings additional challenges, including concerns about long-term prevention, fear of blood product contamination, and reluctance to use medication unnecessarily. In hemophilia B, multiple mutations are underrepresented in genetic databases due to incomplete screening. The smaller, compact factor IX gene is more thoroughly screened in developed nations, leading to better detection rates. In underdeveloped regions, testing is inconsistent, often halting after one abnormality is found, ignoring moderate mutations like missense or splice-site variants [12].
The substantial gene sizes and mutation heterogeneity in hemophilia create challenges in accurately identifying mutations and their effects. Notably, discussions about how phenotypically significant mutations may interact with discrepancies in clinical manifestations are lacking. Many missense mutations in databases lack thorough functional testing, leaving their pathomechanisms unknown. Organizations often report mutations without verifying proper segregation, increasing the risk of false positives, particularly in individuals with multiple mutations. Recent findings highlight double mutations in hemophilia A and B, exemplified by a family where one brother had a hemizygous c.1094A>G mutation, while the other had both c.1094A>G and c.3870delA mutations linked to severe hemophilia [13]. Thorough family screening for segregation analysis is essential for understanding the clinical significance of genetic variants in hemophilia. identified a 6.8% incidence of multiple mutations among hemophilia B patients in western India, emphasizing the importance of comprehensive genetic analysis for effective treatment strategies [14,15]. Numerous other genes, such includes those that code for chronic granulomatous disease (CGD), glucose 6 phosphate dehydrogenase (G6PD), and a few other inherited diseases, have also been shown to have mutations [16,17]. Through the creation of several mutant pairs, It has been studied how Escherichia coli double mutations interact with one another. While synergism is evident in some situations, other examples did not show this impact, it has been discovered [18, 19].
Hemophilia: multiple mutations and clinical heterogeneity
When two or more mutations are identified in an affected individual, it signals a need for risk stratification, indicating a subgroup requiring more intensive care in hemophilia management. The effects of these mutations can be additive, partially additive, antagonistic, neutral, or synergistic. An analysis of 201 missense mutations in the hemophilia B mutation database revealed that only 3% of mutations in discrepant phenotypes were CpG site transitions. For example, the common g.G10430A mutation in exon 4 of the factor IX gene, prevalent in the Gujarati community, results in a wide range of factor IX levels from <1% to 36%, raising questions about additional mutations' roles. While it remains unproven that multiple mutations lead to more severe phenotypes, this is a crucial area for future research, particularly regarding prenatal diagnosis and prognosis. Factors such as thrombophilia and fibrinolytic factor polymorphisms also influence clinical phenotypes. Additionally, variability in factor test results can create apparent phenotypic differences for the same genetic mutation, complicating clinical assessments [20,21,22,23,24,25].
Identification of specific mutations and their correlation with disease severity
The severity of hemophilia is often determined by gene mutations affecting FVIII/IX protein production. Non-null mutations, such as missense and splice-site mutations, as well as deletions and insertions, typically result in residual factor activity, reflecting underlying factor levels [26].
Genotype-Phenotype Correlations
A deficiency or anomaly in coagulation factor IX (FIX) results from mutations in the factor IX (F9) gene, located at Xq27.1, and is responsible for hemophilia B (HB), an X-linked recessive disorder affecting approximately 1 in 30 male live births. The F9 gene contains eight exons that encode a 2.8 Kb mRNA, of which 1.4 Kb is translated, spanning about 34 kilobases of genomic DNA. The gene was fully cloned in 1982 and sequenced in 1985. FIX is a single glycoprotein consisting of 415 amino acids, activated by factor VIIa and factor XIa. Diagnosis of HB is confirmed by low plasma FIX activity, while female carriers usually show no symptoms, exhibiting variable FIX levels. Carrier status is determined through indirect linkage analysis or direct mutation search. Although DNA sequencing is the standard method, high costs limit its widespread use, leading to the adoption of alternative screening techniques such as CSGE, SSCP, DGGE, and dHPLC [26,27,28,29,30].
Relationship between certain mutations and hemophilia B's severity
According to Giannelli et al.'s 1998 study, the majority of F9 mutations in the Hemophilia B Mutation Database occur at CG dinucleotides, particularly in the first and second codon positions, where G transitions are predominant. Point mutations, especially those altering arginine residues, are most frequently reported, with six arginine-coding codons showing high CG concentration. This analysis indicated a higher percentage of severe cases (72%) compared to the 48% reported in 1998, where severity is defined as <1% FIX activity. Notably, mutations at highly conserved residues correlate with more severe phenotypes, highlighting the importance of cysteine residues for proper FIX structure and function.
Thirty percent of hemophilia cases arise from spontaneous mutations, but most are inherited. The severity of illness correlates with the lowest free energy of mRNA structures, particularly in the altered 25-nucleotide sequences compared to wild-type. Mutations associated with severe illness destabilize mRNA significantly more. Furthermore, local translation rates, reflected in the relative synonymous codon usage (RSCU), also play a role in disease severity. Understanding mutation effects on translation rates, rather than focusing solely on individual codon rarity, may provide deeper insights into the pathophysiology of hemophilia B [31,32,33,34,35].
Identification of genetic modifiers and their influence on disease manifestation
Individuals with severe hemophilia experience bleeding even after minimal injury, while those with mild hemophilia typically bleed significantly only after serious trauma, surgeries, or dental extractions. Severe cases are characterized by symptoms such as hematuria, soft-tissue hematomas, and life-threatening bleeding following surgery, often requiring orthopedic intervention due to complications like severe arthropathy and muscular contractures. Preventive regimens, including routine intravenous infusions of recombinant or plasma-derived products, can effectively manage these issues in affluent countries. Notably, 10-15% of patients with the same severe FVIII deficiency exhibit milder bleeding phenotypes, displaying lower frequencies of spontaneous bleeding and reduced factor concentrate consumption. The reasons for this variability in bleeding severity among individuals with severe hemophilia remain unclear, prompting investigations into the roles of environmental and genetic factors in shaping clinical profiles [36,37].
Role of F9 gene Mutations:
An underlying gene mutation causes the remaining FVIII:C/FIX: C levels. Therefore, Non-null variants that allow for some protein synthesis, like missense, splice site, and small deletions/insertions, are linked to undetectable FVIII or FIX activity, but non-null mutations that block protein synthesis, such nonsense, big, and inversion mutations, are linked to undetectable levels of these processes. and explain residual factor activity is typically linked to undetectable FVIII or FIX activity [38,39].
Role of Thrombophilic mutations:
All examples of phenotypic variance in severe hemophilia, however, are not explained by the causal F8 and F9 mutations. Along with the type of mutations in F8/F9, other factors could be involved, as evidenced by the varying bleeding tendencies observed even in people with identical molecular abnormalities [40].
Other Non-Genetic and Genetic Elements:
Other genetic variables are thought to be involved in regulating the clinical manifestation of severe hemophilia, along with thrombophilic and F8/F9 mutations. Haemophiliacs' development of joint damage appears to be significantly influenced by polymorphisms associated with decreased levels of pro-inflammatory cytokines (like tumor necrosis factor [TNF]-α) and higher levels of anti-inflammatory cytokines (like interleukin [IL]-10) [41].
Role of genotype information in predicting clinical outcomes and treatment response
Evaluation of treatment response and clinical heterogeneity in hemophilia B
Coagulation factor testing is crucial for the clinical management of hemophilia, aiding in accurate diagnosis, severity assessment, and treatment safety. The one-stage assay (OSA) based on aPTT is the most commonly used method for evaluating factor IX (FIX) activity, while chromogenic tests are less frequent. Although OSA is cost-effective and quick, it exhibits significant variability depending on the reagents and equipment used. Recent advancements have shifted focus toward global hemostatic testing, which measures thrombin production and clot development through real-time monitoring. These tests offer a more comprehensive understanding of hemophilia's clinical heterogeneity, especially in differentiating between severe and mild cases. Notably, recent studies suggest that thrombin generation post-high-dose rFIX administration correlates well with FIX coagulation activity assay results, potentially guiding treatment optimization [42].
A technique requires more consideration for widespread interpretations
Thrombin production tests, using platelet-rich or platelet-poor plasma, measure thrombin levels and clot formation. Although assay sensitivity may limit the differentiation between hemophilia carriers with varying bleeding tendencies, thrombin generation can help assess clinical variability and distinguish between milder disease phenotypes. Notably, a recent study indicated that thrombin production data after high-dose rFIX treatment correlated well with FIX coagulation activity assay results, though further research is needed for validation. Additionally, thromboelastography/thromboelastometry (TEG/ROTEM) provides insights into clot viscosity and elasticity, offering valuable information on clot stability, flexibility, and rigidity, thus enhancing the evaluation of hemostatic capacity in hemophilia patients [43].
Genetic Counseling and Family Planning
Advancements in DNA analysis have enhanced the identification of heterozygotes in hemophilia B (HB) families, revealing the genetic complexity of the disorder, including multiple pathogenic variants and a higher incidence of affected females. Genetic counseling is essential for women undergoing testing and their identified heterozygotes. Pedigree analysis, combined with measured FIX levels, helps assess bleeding risk. Consulting a qualified genetic counselor is crucial for effectively conveying this intricate information. Various reproductive options are available, including preimplantation genetic diagnosis in some countries, prenatal diagnosis through amniocentesis or chorionic villus sampling, and non-invasive testing of fetal cells in maternal plasma as early as 10 weeks into pregnancy, particularly useful for assessing HB risk in potential future grandchildren [44,45].
Importance of genetic counseling for families affected by hemophilia B
Hemophilia A (HA) and B (HB) are hereditary disorders that significantly affect the mental, emotional, and physical well-being of those impacted and their families. The possibility of passing on inherited disorders can lead to feelings of guilt and shame, profoundly influencing reproductive decision-making. Comprehensive hemophilia care now emphasizes genetic counseling, which educates individuals and families about the inheritance of these disorders and assists in making informed choices. This support is particularly crucial for women and girls, who often experience heightened mental distress when considering reproductive options. Genetic counseling for hemophilia typically focuses on three main components: identifying women at risk of carrying the gene, educating families about the genetic aspects of hemophilia, and providing information on genetic diagnosis during pregnancy. Research indicates that women's attitudes toward pregnancy are shaped by their familial experiences with hemophilia. Those from families that successfully manage the condition are generally more inclined to pursue parenthood compared to those from families that struggle with the disorder [46,47,48,49,50].
Discussion of reproductive options and family planning for carrier females and affected males
The World Federation of Haemophilia has issued a monograph that discusses genetic counseling for hemophilia. This study defines a genetic counselor as a skilled communicator who has a thorough awareness of prenatal diagnostic methods, hemophilia, genetics, and molecular biology. Genetic counseling for hemophilia primarily addresses the medical condition, emphasizing how advancements in therapy have mitigated the grave consequences of infections previously spread by blood transfusions. It's crucial to clarify the inheritance as well. Counseling ought to be a collaborative process that gives both parties the chance to discuss the implications of the material shared. Conflicts may arise between the interests of the hemophiliac and those of their spouse, sibling, or offspring; genetic counseling must address these matters. Entire Recommendations on prenatal diagnosis and carrier identification have been released by the UK Haemophilia Doctors Organisation. Regarding hemophilia, genetic testing consent and counseling [adapted from Ludlam et al.].
1. Confirm that there is a family history of hemophilia and ascertain the kind and severity of the condition.
2. Create a family tree or pedigree to weed out carriers and find potential or required carriers.
3. Give a thorough discussion of the possible clinical consequences of having hemophilia or being a male impacted by the disease.
4. Give details regarding the condition's ramifications and available treatments.
5. Give a thorough explanation of how hemophilia is inherited.
6. Talk about the reasoning behind determining the genetic abnormality in hemophiliac individuals.
7. Explain the evaluation of carrier status.
8. Talk about the components of genetic testing (consent, sample gathering, data transmission and storage, research initiatives involving material that has been stored, error risk).
9. If applicable, offer advice on antenatal testing procedures.
10. Give people a chance to ask questions
11. Give the person being consulted a chance to summarise the material that has been discussed.
12. Offer a patient information form and the chance to schedule a follow-up visit.
Prenatal and preimplantation genetic diagnosis for hemophilia B
Hemophilia severity is determined by the activity levels of the F8 (FVIII) or F9 (FIX) genes. Males with hemophilia experience a range of severity, often linked to age at diagnosis and frequency of bleeding episodes. A carrier mother has a 50% chance of passing the X-linked mutation to her children—sons will exhibit hemophilia, while daughters will be carriers. Fathers with hemophilia pass the mutation to all daughters, making them carriers. Severe hemophilia A is usually caused by intron 22-A inversions, while mild cases often result from point mutations. Hemophilia B severity is linked to specific mutations, with larger, disruptive mutations typically leading to severe disease. Families can consider adoption, spontaneous conception with prenatal diagnosis, or preimplantation genetic diagnosis (PGD) to reduce the risk of having an affected child. PGD involves genetic counseling, DNA testing, and health evaluations, with potential risks including multiple pregnancies and birth defects [51].
Therapeutic Implications
Impact of mutational analysis on personalized treatment strategies for hemophilia B
The notion that different individuals respond to the same treatment in various ways is not new; in fact, it has existed almost since the inception of Western medicine. It was discovered in the last century that hereditary variables affect the variability of patient reactions to drugs in terms of both toxicity and efficacy. However, because biological systems are inherently complex, it has proven difficult to identify biomarkers, making the integration of pharmacogenetic techniques into ordinary medical practice. Here, we address how pharmacogenetics particularly lends itself to the prediction and avoidance of immunogenicity, a significant treatment barrier for hemophilia patients. We also review recent developments in individualized hemophilia treatment in clinical settings [52].
Figure No- 2. Shown based on demographic reactions, comparing medical practice and personalized medicine, Pezeshkpoor et al. (2022).
Protein therapies and pharmacogenetics
When developing new drugs, the general population's response is typically used to assess a treatment's safety and effectiveness, despite individual variations in reactions. Although drug development processes and licensing standards have evolved little, they generally ensure that newly launched medications are safe and effective for most people. However, a minority may use these drugs in ways that are unsafe or ineffective. Research shows that patient response rates to medications can vary significantly, with around 25% for cancer treatments and about 80% for analgesics. The advent of pharmacogenomic-driven personalized medicine aims to address this issue by tailoring treatments based on individuals' genetic profiles, which influence disease susceptibility and treatment responses. Genetic polymorphisms, especially in genes related to human leukocyte antigen (HLA), drug targets, transporters, and metabolizing enzymes, can significantly impact drug efficacy and the likelihood of adverse drug reactions (ADRs) [Figure No. 2] [52,53,54].
Since their introduction in the 1980s, over 200 protein-based therapeutics have been developed across various therapeutic domains. However, the emergence of neutralizing antibodies (nADAs) significantly limits their effectiveness by complicating the interpretation of toxicological, pharmacokinetic (PK), and pharmacodynamic data. nADAs can inhibit the therapeutic protein's function and may lead to serious adverse drug reactions (ADRs) by cross-reacting with endogenous substances. While the risk of developing nADAs cannot be eliminated, it can be managed based on years of experience with therapeutic proteins. Regulatory bodies now mandate immunogenicity testing for all biologicals and advocate for risk-reduction strategies, emphasizing the importance of quality by design (QbD) to improve product quality, even as patient diversity remains a challenge [55].
When a therapeutic protein containing an endogenous component is injected, the patient's immune system recognizes it as "self," leading to tolerance. To elicit an immune response, discrepancies between the infused and native proteins are required. Antigen-presenting cells (APCs) present foreign peptides through their MHC-II proteins, which are recognized by T-cell receptors (TCRs), potentially triggering an immune reaction. Significant pharmacogenetic factors have been used to predict immunogenicity by sequencing both the therapeutic and endogenous proteins to identify and quantify foreign peptides. HLA typing helps determine MHC-II variants, allowing for the calculation of binding affinities between MHC-II and foreign peptides. Research indicates that foreign peptides with 15 mers accurately measure immunogenicity, particularly when identifying at-risk individuals through peptide–MHC–II binding [56].
Sequence mismatch as an indicator of hemophilia immunogenicity
Infusions of recombinant (r)-FVIII or FIX, or plasma-derived therapies, are used to treat hemophilia A (HA) and B (HB), respectively. Approximately 15-20% of HA patients and 1-3% of HB patients develop neutralizing antibodies (nADAs). While HA has a higher incidence of nADAs, FIX may provoke more severe allergic reactions. Genetic factors can indicate susceptibility to developing anti-drug antibodies (ADAs). Although mutations occur at similar frequencies in both HA and HB, missense mutations are more prevalent in HB. Patients with significant gene structural alterations, like large exon deletions, are less able to produce endogenous FVIII, increasing the likelihood of adverse reactions to infused therapies [57].
B. Consideration of specific mutations in selecting appropriate treatment options (factor replacement therapy, gene therapy)
Recombinant Fix (F9 gene)
After the cloning of the FIX gene and cDNA in 1982, Chinese hamster ovary (CHO) cells were used to produce recombinant FIX (rFIX), which entered clinical trials before its licensure in 1995. The FDA approved BeneFIX in February 1997. The CHO cell line is co-transfected with a human recombinant FIX cDNA expression plasmid and an engineered paired amino acid cleaving enzyme (PACE), enhancing pro-factor IX processing. BeneFIX is produced and purified without using human or animal plasma, minimizing the risks of transmitting spongiform agents and blood-borne viruses. Although it has a strong safety record and efficacy in preventing bleeding, a higher dose is often recommended due to reduced recovery values, especially in young children and infants. Variations in rFIX recovery are attributed to differences in post-translational modifications, particularly the sulfation of tyrosine 155 and phosphorylation of serine 158, which affect FIX clearance [58,59].
FVIII & FIX Recombinant Concentrates, First and Second Generation
Over the past decade, viral reduction techniques for clotting factors derived from plasma have improved significantly, yet concerns remain about blood-borne virus transmission, such as Human parvovirus B19 and hepatitis A. Supply issues arose in the U.S. due to the withdrawal of plasma-derived products linked to donors later identified with CJD and factory closures. The FDA licensed recombinant FVIII (rFVIII) and FIX (rFIX) concentrates in the early 1990s. By 1999, rFVIII constituted approximately 78% of FVIII use in the U.S., while rFIX accounted for 80% of FIX use, reflecting their enhanced safety. Pre-licensure trials demonstrated the safety and efficacy of these products for various applications, although about 30–35% of previously untreated patients developed inhibitory antibodies to FVIII, with some achieving spontaneous resolution. Those who underwent immunological tolerance induction regimens generally fared well [60,61]. These findings are consistent with prospective studies involving plasma-derived products, highlighting the significant influence of genetic factors, such as FVIII gene deficiency and racial background, on the development of FVIII inhibitors. Immunologic variables also play a role, though no clear association with either rFVIII or rFIX has been established. While comprehensive rFVIII formulations have maintained an excellent safety record over 12–13 years, concerns arose from cases in Europe where plasma-derived FVIII concentrates, even those inactivated for two viruses, led to higher inhibitor rates. In response, pharmaceutical companies developed alternative stabilization strategies for rFVIII, such as using saccharides instead of human serum albumin, exemplified by Bayer's Kogenate FS, which was licensed in 2000 and shown to be safe and effective without increased immunogenicity compared to other FVIII preparations.
Gene Therapy:
Ten years ago, two young girls with ADA deficiency-related SCID were successfully treated with gene therapy, marking the start of over 4,000 gene transfer treatments in the following decade. Recently, Fischer et al. reported successful treatment of two boys with X-linked SCID using a retroviral vector to transduce hematopoietic stem cells, showcased at the ASH meeting and later published. Additionally, other groups have observed therapeutic responses in cancer patients using plasmid-based or adenoviral vectors in phase III trials, suggesting that gene transfer products could soon be integrated into treatment options. Clinical trials are underway to explore gene transfer for hemophilia, leveraging insights from over 30 years of experience in maintaining clotting factor levels to reduce disease morbidity and mortality [62,63].
Development of novel therapies targeting specific mutations
Emerging by passing agents
Paul Ehrlich first described antibodies as "silver bullets" for targeting specific proteins in 1908. The Nobel Prize in 1984 was awarded for the discovery of monoclonal antibodies. Kitazawa's 2012 studies introduced bispecific antibodies, such as ACE910, which connect factors IX and X to mimic factor VIII, demonstrating effective hemostatic correction in hemophilia models. Emicizumab (HemLibra) emerged as a game-changer for hemophilia A, offering subcutaneous administration, extended half-life, and superior bleed prevention. Clinical trials showed a significant reduction in annualized bleed rates (ABR), with 73% of participants experiencing no bleeding. Notably, 63% of prophylactic emicizumab had no bleeding episodes, leading to ongoing studies evaluating its efficacy and safety in surgical setting [64].
Enhanced prevention and elimination of inhibitors
The discovery that 30% of hemophilia A patients and 3% of hemophilia B patients develop neutralizing antibodies against replacement factors poses a significant challenge in treatment, leading to increased morbidity and mortality. Inhibitor allo-antibodies complicate the use of recombinant or plasma-derived factors, necessitating bypass therapies like activated prothrombin complexes and recombinant FVIIa, which are costly and have variable hemostatic efficacy. High-dose FVIII therapy can induce immune tolerance in about 70% of patients with inhibitors, though the underlying mechanisms remain unclear. Studies like Exposed Toddlers show that plasma-derived products significantly reduce inhibitor incidence, and modern recombinant therapies with longer half-lives or human cell lines may offer lower immunogenicity. Research continues on the effectiveness of new therapies like emicizumab in promoting tolerance while minimizing FVIII or FIX exposure [65,66,67,68].
VIII. Future Directions and Conclusion
Potential advancements in mutational analysis techniques for hemophilia B
Beyond conventional therapy, innovative treatment strategies for hereditary coagulation factor deficits focus on molecular techniques to enhance the production and processing of coagulation factors. Key approaches include (i) gene editing, which allows precise intervention at the DNA level to recognize, cleave, correct, or activate mutated genes; (ii) restoration of pre-mRNA processing through essential spliceosome components that improve missense mutations' impact on splicing elements, facilitating the production and secretion of functional coagulation factors. These strategies target molecular mechanisms at the DNA, mRNA, and protein levels to address underlying deficiencies [69].
Gene Replacement
Several initiatives have demonstrated the potential of genome editing as a treatment for hemophilia B. Both ZFNs and CRISPR/Cas9 techniques have been employed in animal models and patient-derived induced pluripotent stem cells (iPSCs). AAV8-ZFNs successfully repaired clotting times in mice by stimulating gene repair through homology-directed repair (HDR). The corrected cDNA contained promoter-less F9 exons 2–8, with homology arms, delivered alongside AAV8-ZFNs. Durable genome modification was noted after partial hepatectomy, although AAV delivery of F9 transgenes dropped to background levels post-surgery. In initial CRISPR/Cas9 attempts, hydrodynamic injection of linearized DNA edited the aberrant allele, resulting in a coagulation deficiency cure with average FIX concentrations 3.39 times higher than control mice [70].
Innovative Methods for Point Mutations
Point mutations are the most prevalent mutations associated with hereditary disorders in humans, presenting significant therapeutic opportunities for correction. A recent study demonstrated the potential to repair point mutations in the endogenous F9 gene, responsible for hemophilia B, using a Cas9-based approach. By injecting donor DNA and Cas9 nuclease into liver tissue via hydrodynamic force, HDR efficiencies of 1.55% for plasmid donors and 0.56% for single-stranded DNA oligonucleotides (ssODNs) were achieved. Notably, even with reduced HDR efficiency, hemostasis was restored in hemophilic mice, indicating the technique's therapeutic promise, though it may not be suitable for human applications [71].
Methods for Transactivating Promoters
Another innovative approach for restoring gene expression at the DNA level involves utilizing genetically modified transcription factors. This strategy employs a transcriptional activator domain (VP64) fused to TALE DNA-binding domains, forming a TALE-TF. In a study simulating severe coagulation FVII deficiency, mutations c.-94C>G and c.-61T>G impaired F7 promoter activity. Remarkably, the TALE-TF module restored reporter gene expression by over 100-fold in hepatoma HepG2 cells. Additionally, it enhanced endogenous F7 mRNA and protein expression by two to three times without off-target effects. A recent CRISPR activation (CRISPRa) method using deactivated Cas9 and transcriptional inhibitors also effectively transactivated the F7 and F8 promoters, boosting FVII secretion and activity while minimizing off-target risks. Overall, genome engineering offers advantages over viral vector-based gene therapy, including precise target modification and reduced insertional mutagenesis, despite challenges related to donor DNA delivery and off-target effects [71].
Importance of continued research in understanding the genetic basis of hemophilia B
Hemophilia B is a recessive X-linked bleeding disorder caused by a deficiency in vitamin K-dependent coagulation factor IX (FIX). While FIX activity levels are used to define disease severity, they do not correlate directly with bleeding phenotypes due to the complex regulation of coagulation processes. With advancements in gene therapy and long-acting medications, understanding the factors influencing hemostatic outcomes and therapeutic responses is crucial. Notably, FIX operates at the platelet procoagulant membrane and interacts with vascular collagen. Investigating the roles of magnesium in FIX activation and platelet adhesion will enhance our understanding and lead to the development of relevant tests for assessing treatment efficacy [72].
FIX-A ASSOCIATED ABNORMALITIES PATHOBIOLOGY
Liver Synthesis of FIX
Vitamin K-dependent factor IX (FIX) is produced by hepatocytes, with over a thousand mutations identified that affect its functionality. Blood Type B manifests as Leyden, characterized by altered FIX regulation, leading to irregular bleeding shortly after birth. However, spontaneous recovery of FIX expression occurs around puberty, resulting in a clinically normal state by midlife. Gene transfer of the F9 gene, particularly utilizing the FIX Padua variant—known for its eightfold increase in activity—shows promise in therapy. While liver-targeted adeno-associated viral gene delivery is being studied, the mechanisms behind FIX Padua's enhanced activity remain unclear. The initial discovery highlighted the need for clinical laboratory collaboration to understand coagulation mechanisms further [72,73].
Vitamin K
The synthesis of vitamin K-dependent factor IX (FIX) in the liver relies on the availability of vitamin K for the utilization and absorption processes, including the four gamma-glutamyl acid (Gla) domains. FIX works alongside factor VII (FVII), another vitamin K-dependent factor, and hemostatic breakdown in hemophilia B may be linked to lower baseline levels of FVIIa in severe cases compared to healthy controls. FIX activity prolongs the prothrombin time (PT) without affecting the activated partial thromboplastin time (aPTT), differing from other vitamin K-dependent coagulation factors, as measured by reagent-based techniques like Quick or Owren.Phenotypic heterogeneity [74]. The phenotypic variability and treatment responses in hemophilia B patients may stem from additional coagulation factors and their regulation on platelet procoagulant surfaces. Although disease severity is classified by FIX activity levels, the bleeding phenotype does not always correlate with the absence of a coagulation factor, leading to differing bleeding tendencies among patients with the same gene deficiency. This variability may arise from altered genotypes affecting hemostatic balance, alongside factors influencing coagulation phases and internal feedback loops, such as extrinsic FXI's role in the tenase complex and procoagulant phospholipids interacting with receptors like glycoprotein VI on platelets [75,76].
Immune and Allergic Reactions
Although significant gene deletions causing the absence of FIX are linked to certain disorders, the underlying pathogenetic pathways remain unclear. Factors such as immune tolerance induction (ITI), the rapid extravascular spread of FIX, and the substantial amounts of FIX needed for replacement therapy (with plasma concentrations typically 50 times higher than FVIII) may play a role. Additionally, unlike antibodies against FVIII, those targeting FIX can form circulating immune complexes, potentially leading to anaphylaxis upon re-exposure. Activated mast cells further complicate hemostasis in hemophilia B by releasing tissue plasminogen activator (tPA), thrombin-inactivating proteases, and platelet-inhibiting heparin proteoglycans [77].
Summary of the role of mutational analysis in improving diagnosis, treatment, and genetic counseling for hemophilia B
Hemophilia B Treatment Response
Coagulation factor testing is crucial for the clinical management of hemophilia patients, as it aids in accurate diagnosis, severity assessment, and therapy safety. The chromogenic assay is the most commonly used method for evaluating FIX activity, while the one-stage assay based on activated partial thromboplastin time (aPTT) is used less frequently. Both tests may be needed to diagnose and classify hemophilia A accurately. The one-stage assay, approved by the European Pharmacopoeia for labeling FIX concentrate potency, is quick, cost-effective, and easily automated; however, it is associated with significant variability depending on the aPTT reagent used [78].
Ethics approval and consent to participate: Institutional ethical approval was taken from the ethical committee (No: Dean/2022/EC/3611) Institute of Medical Science, Banaras Hindu University.
Consent for publication: The written informed consent was obtained from the patient/participant for publication.
Availability of data and material: The datasets used or analyzed during the current study are available from the corresponding author upon reasonable request.
Competing interests: NA
Funding Interest: None
Authors' contributions:
1Chanda Hemaliya, , 1Arun Kumar Singh,1Lalit Prashant Meena, 2Ajeet Kumar, 2Akhtar Ali,
1Chanda Hemaliya, M.Sc. Ph.D. Scholar, Department of General Medicine, Institute of Medical Science, Banaras Hindu University, Varanasi, U.P., India
Email Id: hemaliya.chanda@gmail.com
Contributions to the manuscript: Data collection, reviewing the literature, Data analysis, and manuscript writing.
1Arun Kumar Singh, Associate Professor, Department of General Medicine, Institute of Medical Science, Banaras Hindu University, Varanasi, U.P, India
Contributions to the manuscript: Supervised the study, conceived and designed the experiments, and performed the data analysis.
1 Lalit Prashant Meena, Professor Department of General Medicine, Institute of Medical Science, Banaras Hindu University, Varanasi, U.P, India
Contributions to the manuscript: Supervised the study, conceived and designed the experiments, data analysis, and manuscript writing.
2Ajeet Kumar, Centre for Genetic Disorders, Institute of Science, Banaras Hindu University, Varanasi, U.P., India
Email ID: ajeetambuj@gmail.com
Contributions to the manuscript: Sample collection, Data analysis, manuscript writing
2Akhtar Ali, Ph.D. Assistant Professor, Centre for Genetic Disorders, Institute of Science, Banaras Hindu University, Varanasi, U.P., India
Email Id: akhtar_genetics@yahoo.co.in
Contributions to the manuscript: data analysis and reviewing the literature.
Address for Correspondence: Chanda Hemaliya, Ph.D Scholar, Department of General Medicine, Institute of Medical Science (IMS), Banaras Hindu University, BHU Varanasi-221005
Email id: hemaliya.chanda@gmail.com
Acknowledgments statement: I thank CSIR, New Delhi, India, for the fellowship, CH. Mr. Deepak Kumar, Ph.D. Research Scholars are thanked for their contribution to this paper.
REFERENCE
Chanda Hemaliya, Arun Kumar Singh, Lalit Prashant Meena, Ajeet Kumar, Akhtar Ali, A Comprehensive Narrative Review of Hemophilia A and B: Screening, Counselling, and Therapeutic Advances, Int. J. of Pharm. Sci., 2025, Vol 3, Issue 9, 3143-3162 https://doi.org/10.5281/zenodo.17211902
10.5281/zenodo.17211902
