Scenario 5 (English) Tutorial 7th Block : Hemophilia B

Scenario 5 (English) Tutorial 7^th Block versi III

Author : Didit

Hemophilia B

Hemophilia B is characterized by deficiency in factor IX clotting activity that results in prolonged oozing after injuries, tooth extractions, or surgery, and delayed or recurrent bleeding prior to complete wound healing. The age of diagnosis and frequency of bleeding episodes are related to the level of factor IX clotting activity. In severe hemophilia B, spontaneous joint or deep-muscle bleeding is the most frequent symptom. Individuals with severe hemophilia B are usually diagnosed during the first two years of life; without prophylactic treatment, they may average up to two to five spontaneous bleeding episodes each month.

Individuals with moderate hemophilia B seldom have spontaneous bleeding; however, they do have prolonged or delayed oozing after relatively minor trauma and are usually diagnosed before age five to six years; the frequency of bleeding episodes varies from once a month to once a year.

Individuals with mild hemophilia B do not have spontaneous bleeding episodes; however, without pre- and post-operative treatment, abnormal bleeding occurs with surgery or tooth extractions; the frequency of bleeding may vary from once a year to once every ten years. Individuals with mild hemophilia B are often not diagnosed until later in life.

Other. In any individual with hemophilia B, bleeding episodes may be more frequent in childhood and adolescence than in adulthood. Approximately 10% of carrier females are at risk for bleeding (even if the affected family member has mild hemophilia B) and are thus symptomatic carriers, although symptoms are usually mild. After major trauma or invasive procedures, prolonged or excessive bleeding usually occurs, regardless of severity.

Diagnosis/testing.

The diagnosis of hemophilia B is established in individuals with low factor IX clotting activity. Molecular genetic testing of F9, the gene encoding factor IX, identifies disease-causing mutations in more than 99% of individuals with hemophilia B. Such testing is available clinically.

Management.

Treatment of manifestations: Referral to one of the approximately 140 federally funded hemophilia treatment centers (HTCs) in the USA or others worldwide for assessment, education, genetic counseling, and to facilitate management. Training and home infusions administered by parents followed by patient self-infusion are critical components, especially for those with severe disease, where recombinant or plasma-derived factor IX concentrate is most effective when infused within one hour of the onset of bleeding.

Prevention of primary manifestations: For those with severe disease, prophylactic infusions of factor IX concentrate two to three times a week usually maintains a “trough” factor IX clotting activity higher than 1% and prevents spontaneous bleeding.

Prevention of secondary complications: Reduction of chronic joint disease by prompt effective treatment of bleeding, including home therapy.

Surveillance: For individuals with severe or moderate hemophilia B, annual assessments at an HTC are recommended; for individuals with mild hemophilia B, every two to three years; monitor carrier mothers for delayed bleeding post partum unless it is known that their baseline factor IX clotting activity is normal.

Agents/circumstances to avoid: Circumcision of at-risk males until hemophilia B is either excluded or treated with factor IX concentrate regardless of severity; intramuscular injections; activities with a high risk of trauma, particularly head injury; aspirin and all aspirin-containing products.

Testing of relatives at risk: To clarify genetic status of females at risk before pregnancy or early in pregnancy, to facilitate management.

Other: Vitamin K does not prevent or control bleeding in hemophilia B. Clinical trials for gene therapy in hemophilia B are currently in progress and several approaches to improve safety and efficacy are in pre-clinical testing.

Genetic counseling.

Hemophilia B is inherited in an X-linked manner. The risk to sibs of a proband depends on the carrier status of the mother. Carrier females have a 50% chance of transmitting the F9 mutation in each pregnancy. Sons who inherit the mutation will be affected; daughters who inherit the mutation are carriers. Affected males transmit the mutation to all of their daughters and none of their sons. Carrier testing for family members at risk and prenatal testing for pregnancies at increased risk are possible if the F9 disease-causing mutation has been identified in a family member or if informative intragenic, linked markers have been identified.

Diagnosis

Clinical Diagnosis

The diagnosis of hemophilia B cannot be made on clinical findings. A coagulation disorder is suspected in individuals with any of the following:

• Hemarthrosis, especially with mild or no antecedent trauma
• Deep-muscle hematomas
• Intracranial bleeding in the absence of major trauma
• Neonatal cephalohematoma or intracranial bleeding
• Prolonged oozing or renewed bleeding after initial bleeding stops following tooth extractions, mouth injury, or circumcision *
• Prolonged or delayed bleeding or poor wound healing following surgery or trauma *
• Unexplained GI bleeding or hematuria *
• Menorrhagia, especially with onset at menarche (in symptomatic carriers) *
• Prolonged nosebleeds, especially recurrent and bilateral *
• Excessive bruising, especially with firm, subcutaneous hematomas

* Any severity, or especially in more severely affected persons

Testing

Coagulation screening tests. Evaluation of an individual with a suspected bleeding disorder includes: platelet count and bleeding time or platelet function analysis (PFA closure times), activated partial thromboplastin time (APTT), and prothrombin time (PT). Thrombin time and/or plasma concentration of fibrinogen can be useful for rare disorders.

In individuals with hemophilia B, the above screening tests are normal, with the following exceptions:

• The APTT is prolonged in severe and moderate hemophilia B. Prolongations that correct on mixing with an equal volume of normal plasma indicate an intrinsic system clotting factor deficiency, including factor IX, without an inhibitor.
  
  Note: It is important to confirm the diagnosis of hemophilia B and to exclude other deficiencies with a specific factor IX clotting activity which is available in most hospital laboratories or coagulation reference laboratories.
• The APTT may be normal but is usually mildly prolonged n mild hemophilia B.
• The prothrombin time (PT), a screen for the extrinsic clotting system, should be normal except with some reagents and certain Crm+ missense genotypes, unless there is another hemostatic defect such as acquired liver disease.

Note: In some clinical laboratories, the APTT is not sensitive enough to diagnose a mild bleeding disorder.

Coagulation factor assays. Individuals with a history of a lifelong bleeding tendency should have specific coagulation factor assays performed, even if all the coagulation screening tests are in the normal range:

• The normal range for factor IX clotting activity is approximately 50% to 150% [Khachidze et al 2006].
• Individuals with factor IX clotting activity higher than 30% usually have normal coagulation in vivo. However, some increased bleeding can occur with low to low-normal factor IX clotting activity in hemophilia B carrier females [Plug et al 2006].
• In hemophilia B, the factor IX clotting activity is usually less than 30%.
• Classification of hemophilia B based on in vitro clotting activity:
• Severe hemophilia B: <1% factor IX
• Moderate hemophilia B: 1%-5% factor IX
• Mild hemophilia B: >5%-30% factor IX

Carrier females

Coagulation factor assays. Approximately 10% of carrier females have a factor IX clotting activity below 30%, regardless of the severity of hemophilia B in their family. Bleeding may also be more severe in those with low-normal factor IX activity [Plug et al 2006].

Note: The majority of obligate carriers, even of severe hemophilia B, have normal factor IX clotting activities.

Molecular Genetic Testing

Gene. F9 is the only gene in which mutations are known to cause hemophilia B.

Clinical testing

• Sequence analysis of amplified fragments that include all eight exons of F9 can identify a mutation in more than 99% of individuals with hemophilia B. Direct sequencing is the usual approach, although several screening strategies have been used [Mitchell et al 2005].
  
  In some instances, it is appropriate to include F9 promoter mutations that cause hemophilia B Leyden, an uncommon variant that results from one of several mutations from base pairs -23 through +13 (numbered according to Yoshitake et al 1985 [see Bajaj & Thompson 2006]; see also Genotype-Phenotype Correlations; Table A, Locus-Specific Databases).
  
  In males, a presumptive diagnosis of a large or partial-gene deletion occurs from failure to amplify one or more exon fragments.
• Deletion/duplication analysis
• In affected males, deletion/duplication analysis detects and/or confirms exonic, multiexonic, or complete F9 deletions suspected on sequence analysis in males with severe hemophilia B.
• In carrier females deletion duplication analysis can detect gene deletions and rearrangements not detectable by sequence analysis.

Table 1. Summary of Molecular Genetic Testing Used in Hemophilia B

Gene Symbol	Test Method	Mutations Detected	Mutation Detection Frequency by Test Method 1		Test Availability
			Males	Carrier Females
F9	Sequence analysis	Sequence variants 2, 3	~100% 4, 5, 6	97% 7	Clinical
	Deletion / duplication analysis 8	Deletion / duplication of one or more exons or the whole gene	3%	3%

Test Availability refers to availability in the GeneTests Laboratory Directory. GeneReviews designates a molecular genetic test as clinically available only if the test is listed in the GeneTests Laboratory Directory by either a US CLIA-licensed laboratory or a non-US clinical laboratory. GeneTests does not verify laboratory-submitted information or warrant any aspect of a laboratory's licensure or performance. Clinicians must communicate directly with the laboratories to verify information.

1.      The ability of the test method used to detect a mutation that is present in the indicated gene;

2.      Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions, missense, nonsense, and splice site mutations, and changes in the 5’ or 3’ untranslated portions of F9;

3.      Includes ~1% with hemophilia B Leyden with specific 5' base substitutions in an "androgen-responsive element" in which the bleeding tendency becomes milder after puberty;

4.      Lack of amplification by PCRs prior to sequence analysis suggests a deletion of one or more exons or the entire X-linked gene in a male; confirmation may require additional testing by deletion/duplication analysis including use of additional sets of amplification primers;

5.      Includes the mutation detection frequency using deletion/duplication analysis;

6.      Somatic mosaicism occurs and could lower the mutation detection frequency in males with hemophilia B [Ketterling et al 1999];

7.      Sequence analysis of genomic DNA cannot detect deletion or duplication of one or more exons or the entire X-linked gene in a carrier female;

8.      Testing that identifies deletions/duplications not readily detectable by sequence analysis of the coding and flanking intronic regions of genomic DNA; a variety of methods including quantitative PCR, long-range PCR, multiplex ligation-dependent probe amplification (MLPA), or targeted array GH (gene/segment-specific) may be used. A full array GH analysis that detects deletions/duplications across the genome may also include this gene/segment. See array GH.

Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.

Linkage analysis can be used to track an unidentified F9 disease-causing allele in a family and to identify the origin of de novo mutations:

• Tracking an unidentified F9 mutation. When a disease-causing mutation of F9 is not identified in an affected family member by direct DNA testing, linkage analysis can be considered to obtain information for genetic counseling in families in which more than one family member has the unequivocal diagnosis of hemophilia B. Linkage studies are always based on accurate clinical diagnosis of hemophilia B in the affected family members and accurate understanding of the genetic relationships in the family. In addition, linkage analysis depends on the availability and willingness of family members to be tested and on the presence of informative heterozygous polymorphic markers. The markers used for hemophilia B linkage are intragenic and are informative with greater than 99% accuracy in approximately 95% of African American families, 85%-90% of families of European origin, and 60% of Asian/Native American families with hemophilia B [Bajaj & Thompson 2006].
• Identifying the origin of a de novo mutation. Among the nearly 50% of families with a simplex case of hemophilia B (i.e., occurrence in one family member only), the origin of a de novo mutation can often be identified by performing molecular genetic testing in conjunction with linkage analysis. The presence of the mutation on the affected individual's factor IX haplotype is tracked back through the parents and, if necessary, through maternal grandparents to identify the individual in whom the mutation originated.

Testing Strategy

To confirm/establish the diagnosis in a proband requires measurement of factor IX clotting activity.

Molecular genetic testing is performed on a proband to detect the family-specific mutation in F9 in order to obtain information for genetic counseling of at-risk family members. If an affected individual is not available, an obligate carrier female can be tested.

In individuals who represent a simplex case, identification of the specific F9 mutation can help predict the clinical phenotype and assess the risk of developing a factor IX inhibitor. (See Genotype-Phenotype Correlations).

For individuals with (a) hemophilia B or (b) females with a family history of hemophilia B in whom the family-specific mutation is not known, molecular genetic testing is generally performed in the following sequence until a mutation (or linkage) is identified:

• Sequence analysis of the eight exons in F9
• Deletion/duplication analysis
• Linkage analysis

Note: When carrier testing is performed on an at-risk relative without previous identification of the F9 mutation in the family, a negative result does not necessarily exclude a potential carrier.

Carrier testing for at-risk relatives is most informative after identification of the disease-causing mutation in the family. See above for testing of at-risk females when the family specific mutation is not known.

Note: Carriers are heterozygotes for this X-linked disorder and may develop clinical findings related to the disorder.

Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutation in the family.

Note: It is the policy of GeneReviews to include clinical uses of testing available from laboratories listed in the GeneTests Laboratory Directory; inclusion does not necessarily reflect the endorsement of such uses by the author(s), editor(s), or reviewer(s).

Genetically Related (Allelic) Disorders

Certain missense mutations within the propeptide portion of factor IX enhance sensitivity to warfarin by altering the binding of a gamma-carboxylase responsible for post-translational Gla residue formation [Bajaj & Thompson 2006].

One family has been described in which a missense change, p.Arg338Leu, is associated with markedly elevated circulating levels of factor IX and venous thrombosis at a young age [Simioni et al 2009].

Clinical Description

Natural History

Hemophilia B in the untreated individual is characterized by prolonged oozing after injuries, tooth extractions, or surgery or renewed bleeding after initial bleeding has stopped [Kessler & Mariani 2006]. Muscle hematomas or intracranial bleeding can occur immediately or up to four to five days after the original injury. Intermittent oozing may last for days or weeks after tooth extraction. Prolonged or delayed bleeding or wound hematoma formation after surgery is common. After circumcision, males with hemophilia B of any severity may have prolonged oozing, or they may heal normally. In severe hemophilia B, spontaneous joint bleeding is the most frequent symptom.

The age of diagnosis and frequency of bleeding episodes are generally related to the factor IX clotting activity (see Table 2). In any affected individual, bleeding episodes may be more frequent in childhood and adolescence than in adulthood. To some extent, this greater frequency is a function of both physical activity levels and vulnerability during more rapid growth.

Individuals with severe hemophilia B are usually diagnosed during the first two years of life. On rare occasions, infants with severe hemophilia have extra- or intracranial bleeding following birth. In untreated toddlers, bleeding from minor mouth injuries and large "goose eggs" from minor head bumps are common; these are the most frequent presenting symptoms of severe hemophilia B. Intracranial bleeding may also result from head injuries. The untreated child almost always has subcutaneous hematomas; some have been referred for evaluation of possible non-accidental trauma.

As the child grows and becomes more active, spontaneous joint bleeds occur with increasing frequency unless the child is on a prophylactic treatment program. Spontaneous joint bleeds or deep-muscle hematomas initially cause pain or limping before swelling appears. Children and young adults with severe hemophilia B who are not treated have an average of two to five spontaneous bleeding episodes each month. Joints are the most common sites of spontaneous bleeding; other sites include the muscles, kidneys, gastrointestinal tract, brain, and nose. Without prophylactic treatment, individuals with hemophilia B have prolonged bleeding or excessive pain and swelling from minor injuries, surgery, and tooth extractions.

Individuals with moderate hemophilia B seldom have spontaneous bleeding but bleeding episodes may be precipitated by relatively minor trauma. Without pretreatment (as for elective invasive procedures) they do have prolonged or delayed oozing after relatively minor trauma and are usually diagnosed before age five to six years. The frequency of bleeding episodes requiring treatment with factor IX concentrates varies from once a month to once a year. Signs and symptoms of bleeding are otherwise similar to those found in severe hemophilia B.

Individuals with mild hemophilia B do not have spontaneous bleeding. However, without treatment, abnormal bleeding occurs with surgery, tooth extractions, and major injuries. The frequency of bleeding may vary from once a year to once every ten years. Individuals with mild hemophilia B are often not diagnosed until later in life when they undergo surgery or tooth extraction or experience major trauma.

Carrier females with a factor IX clotting activity level lower than 30% are at risk for bleeding that is usually comparable to that seen in males with mild hemophilia. However, more subtle abnormal bleeding may occur with baseline factor IX clotting activities between 30% and 60% [Plug et al 2006].

Table 2. Symptoms Related to Severity of Untreated Hemophilia B

Clinical Severity	Factor IX Clotting Activity 1	Symptoms	Usual Age of Diagnosis
Severe	<1%	Frequent spontaneous bleeding; excessive and/or prolonged bleeding after minor injuries, surgery, or tooth extractions	Age ≤2 years
Moderate	1%-5%	Spontaneous bleeding rare; excessive and/or prolonged bleeding after minor injuries, surgery, or tooth extractions	Age <5-6 years
Mild	>5%-30%	No spontaneous bleeding; excessive and/or prolonged bleeding after major injuries, surgery, or tooth extractions	Often later in life, depending on hemostatic challenges

1. Clinical severity does not always correlate with the in vitro assay result.

Complications of untreated bleeding. The leading cause of death related to bleeding is intracranial hemorrhage. The major cause of disability from bleeding is chronic joint disease [Luck et al 2004]. Currently available treatment with clotting factor concentrates is normalizing life expectancy and reducing chronic joint disease for children with hemophilia B. Prior to the availability of such treatment, the median life expectancy for individuals with severe hemophilia B was 11 years (the current life expectancy for affected individuals in several developing countries). Excluding death from HIV, life expectancy for those severely affected individuals receiving adequate treatment is 63 years [Darby et al 2007], having been greatly improved with factor replacement therapy [Tagliaferri et al 2010].

Other. Since the late1960s, the mainstay of treatment of bleeding episodes has been factor IX concentrates that initially were derived solely from donor plasma. By the late 1970s, more purified preparations became available, reducing a risk of thrombogenicity. Viral inactivation methods and donor screening of plasmas were introduced by 1990 and a recombinant factor IX concentrate became available shortly thereafter [Monahan & Di Paola 2010]. HIV transmission from concentrates essentially occurred between 1979 and 1985. Approximately half of these individuals died of AIDS prior to the advent of effective HIV therapy.

Hepatitis B transmission from earlier plasma-derived concentrates was eliminated with donor screening and then vaccination introduced in the 1970s. Most individuals exposed to plasma-derived concentrates prior to the late 1980s became chronic carriers of the hepatitis C virus. Viral inactivation methods implemented in concentrate preparation and donor screening assays developed by 1990 have essentially eliminated hepatitis C transmission from plasma-derived concentrates.

Alloimmune inhibitors occur much less frequently than in hemophilia A. Approximately 3% of individuals with severe hemophilia B develop alloimmune inhibitors to factor IX. These individuals usually have partial or complete gene deletions or certain nonsense mutations (see Genotype-Phenotype Correlations and Table A, Locus-Specific Databases). At times, the onset of an alloimmune response has been associated with anaphylaxis to transfused factor IX or development of nephrotic syndrome [DiMichele 2007, Chitlur et al 2009].

Genotype-Phenotype Correlations

Disease severity

• Large gene deletions, nonsense mutations, and most frameshift mutations cause severe disease.
• Missense mutations can cause severe, moderate, or mild disease depending on their location and the specific substitutions involved.

Alloimmune inhibitors

• Alloimmune inhibitors occur with the greatest frequency (~20%) in individuals with large partial- or whole-gene deletions.
• Among individuals with the p.Arg29X mutation approximately 20% have developed inhibitors and/or anaphylaxis in response to factor IX infusion.
• Missense mutations are rarely associated with inhibitors.

Unlike hemophilia A, severe hemophilia B is often caused by a missense mutation and several of these are associated with normal CRM (factor IX antigen) levels (see Table A, Locus-Specific Databases).

Uncommon variants within the carboxylase-binding domain of the propeptide cause increased sensitivity to warfarin anticoagulation in individuals without any baseline bleeding tendency [Bajaj & Thompson 2006] (see Management).

In hemophilia B Leyden (caused by mutations in a restricted 5’ UT promoter region of F9) the severity of disease decreases after puberty; mild disease disappears and severe disease becomes mild, depending on the specific mutation.

Penetrance

All males with an F9 disease-causing mutation are affected and will have hemophilia B of approximately the same severity as all other affected males in the family; however, other genetic and environmental effects may modify the clinical severity to some extent.

Approximately 10% of females with one F9 disease-causing mutation and one normal allele have a factor IX clotting activity of approximately 30% and a bleeding disorder; mild bleeding can occur in carriers with low-normal factor IX activities [Plug et al 2006].

Anticipation

Anticipation is not observed.

Prevalence

The birth prevalence of hemophilia B is approximately one in 20,000 live male births worldwide.

The birth prevalence is the same in all countries and all races, presumably because of the high spontaneous mutation rate in F9 and its presence on the X chromosome.

Prevalence is approximately one in 25,000 males in the US (about one fifth as prevalent as hemophilia A) and in other countries in which optimum treatment with clotting factor concentrates is available [Kessler & Mariani 2006].

Differential Diagnosis

When an individual presents with bleeding or the history of being a "bleeder," the first task is to determine if he/she truly has abnormal bleeding. "Bleeding a lot" during or immediately after major trauma, after a tonsillectomy, or for a few hours following tooth extraction may not be significant. In contrast, prolonged or intermittent oozing that lasts several days following tooth extraction or mouth injury, renewed bleeding or increased pain and swelling several days after an injury, or development of a wound hematoma several days after surgery almost always indicates a coagulation problem. A careful history of bleeding episodes can help determine if the individual has a lifelong, inherited bleeding disorder or an acquired (often transient) bleeding disorder.

Physical examination provides few specific diagnostic clues. An older individual with severe or moderate hemophilia B may have joint deformities and muscle contractures. Large bruises and subcutaneous hematomas for which no trauma can be identified may be present, but individuals with a mild bleeding disorder usually have no outward signs except during an acute bleeding episode. Petechial hemorrhages indicate severe thrombocytopenia and are not a feature of hemophilia B.

A family history with a pattern of autosomal dominant, autosomal recessive, or X-linked inheritance provides clues to the diagnosis but is not definitive. At least 10% of women who carry hemophilia B of any severity have low enough factor IX activity levels to have mild bleeding themselves, which in some families may erroneously suggest autosomal rather than X-linked inheritance.

Hemophilia B is only one of several lifelong bleeding disorders, and coagulation factor assays are the main tools for determining the specific diagnosis. Other bleeding disorders associated with a low factor IX clotting activity include the following:

• Combined vitamin K-dependent factor deficiencies (prothrombin and factors VII, IX, and X and proteins C and S), usually caused by a γ-carboxylase or epoxide reductase deficiency [Weston & Monahan 2008]
• Common acquired deficiencies of these factors in individuals with vitamin K disorders, including warfarin treatment or liver disease

Other bleeding disorders with normal factor IX levels include the following:

• Hemophilia A is clinically indistinguishable from hemophilia B. Diagnosis is based on a factor VIII clotting activity level lower than 35% in the presence of a normal von Willebrand factor (VWF) level. Mutations in F8 are causative. Inheritance is X-linked.
• von Willebrand disease (VWD) type 1 or type 2 is characterized predominantly by mucous membrane bleeding. Eighty percent of individuals with VWD have a quantitative deficiency of von Willebrand factor (low VWF antigen, factor VIII activity, and ristocetin cofactor activity). Essentially all individuals with hemophilia B have a normal VWF level and a normal factor VIII activity. VWD types 2A and 2B are characterized by a qualitative deficiency of VWF, with a decrease of the high molecular-weight multimers. Type 2B VWD is caused by a gain of function in platelet binding and is often accompanied by thrombocytopenia. Type 2M VWD is caused by a similar gain of function in platelet binding as with type 2B although it is associated with a normal multimer pattern. Molecular genetic testing can aid in the diagnosis. VWF antigen and factor VIII clotting activity may be low-normal to mildly decreased. Functional VWF level is low in a ristocetin cofactor assay. Inheritance of VWD is autosomal dominant with the exception of some variants including 2N, which is autosomal recessive.
• Severe, type 3 VWD is characterized by frequent episodes of mucous membrane bleeding and joint and muscle bleeding similar to that seen in individuals with hemophilia B. The VWF level is lower than 1% and the factor VIII clotting activity level is 2%-8%. Inheritance is autosomal recessive.
• Factor XI deficiency [Thompson 2006] is inherited in an autosomal recessive manner with heterozygotes showing a factor XI coagulant activity level of 25% to 75% of normal, while homozygotes have activity of less than 1% to 15%, depending on their genotype. Two mutations are common among individuals of Ashkenazi Jewish descent. Both compound heterozygotes and homozygotes may exhibit bleeding similar to that seen in mild or moderate hemophilia B. Specific factor assays establish the diagnosis.
• Factor XII, prekallekrein, or high-molecular-weight kininogen deficiencies do not cause clinical bleeding, but can cause a long activated partial thromboplastin time (APTT).
• Prothrombin (factor II), factor V, factor X, and factor VII deficiency are rare bleeding disorders inherited in an autosomal recessive manner. Affected individuals may display easy bruising and hematoma formation, epistaxis, menorrhagia, and bleeding after trauma and surgery. Hemarthroses are uncommon. Spontaneous intracranial bleeding can occur. Factor VII deficiency should be suspected if the PT is prolonged with a normal APTT. Individuals with factors II, V, or X deficiency usually have prolonged PT and APTT, but specific factor assays establish the diagnosis of these rare bleeding tendencies.
• Fibrinogen disorders include severe, mild, and asymptomatic variants [Thompson 2006]:
• Congenital afibrinogenemia is a rare disorder inherited in an autosomal recessive manner with manifestations similar to those observed in hemophilia B, except that bleeding from minor cuts is prolonged because of the lack of fibrinogen to support platelet aggregation.
• Hypofibrinogenemia can be inherited in either an autosomal dominant or autosomal recessive manner and is usually asymptomatic but may be combined with dysfibrinogenemia.
• Dysfibrinogenemia is inherited in an autosomal dominant manner. Individuals with hypofibrinogenemia or dysfibrinogenemia have mild-to-moderate bleeding symptoms or may be asymptomatic; some individuals with dysfibrinogenemia are at risk for thrombosis. Diagnosis is based on kinetic and antigenic protein levels, although the thrombin time is usually prolonged and is a simple screening test.
• Factor XIII deficiency is a rare autosomal recessive disorder [Thompson 2006]. Umbilical stump bleeding is common (>80% of individuals). Intracranial bleeding that occurs spontaneously or following minor trauma occurs in 30% of affected individuals. Subcutaneous hematomas, muscle hematomas, defective wound healing, and recurrent spontaneous abortion are also seen. Joint bleeding is rare. All the kinetic coagulation screening tests are normal; a specific test for clot solubility must be performed.
• Platelet function disorders cause bleeding problems similar to those seen in individuals with thrombocytopenia. Affected individuals have skin and mucous membrane bleeding, recurring epistaxis, gastrointestinal bleeding, menorrhagia, and excessive bleeding during or immediately after trauma and surgery. Joint, muscle, and intracranial bleeding are rare. Diagnosis is made utilizing platelet aggregation assays and flow cytometry.
• Bernard-Soulier syndrome, inherited in an autosomal recessive manner, involves the VWF receptor, the platelet membrane GPIb-IX complex.
• Glanzmann's thrombasthenia, also autosomal recessive, involves the GPIIb-IIIa receptor necessary for platelet aggregation. Abnormal platelet function is usually associated with a prolonged bleeding time or prolonged closure times on platelet function analysis.

• Hemophilia B: males
• Hemophilia B: female heterozygotes

Management

Evaluations Following Initial Diagnosis

To establish the extent of disease in an individual diagnosed with hemophilia B, the following evaluations are recommended:

Identification of the specific F9 mutation in an individual to aid in determining: disease severity; the likelihood of inhibitor development; and the risk of anaphylaxis if an inhibitor does develop [Chitlur et al 2009]

A personal and family history of bleeding to help predict severity

A joint and muscle evaluation, particularly if the individual describes a past hemarthrosis or muscle hematoma

Screening for hepatitis A, B, and C and HIV, particularly if blood products or plasma-derived clotting factors were administered prior to 1985

Baseline CBC and platelet count, especially if there is a history of nose bleeds, GI bleeding, mouth bleeding, or, in women, menorrhagia or postpartum hemorrhage

Treatment of Manifestations

In developed countries, life expectancy for individuals with hemophilia B has greatly increased over the past four decades [Darby et al 2007]; disability has decreased with the intravenous infusion of factor IX concentrates, home infusion programs, prophylactic treatment, and improved patient education.

Individuals with hemophilia B benefit from referral for assessment, education, and genetic counseling at one of the approximately 140 federally funded hemophilia treatment centers (HTCs) in the USA that can be located through the National Hemophilia Foundation. Worldwide, treatment centers can be found through the World Federation of Haemophilia. The treatment centers establish appropriate treatment plans and referrals or direct care for individuals with inherited bleeding disorders. They are also a resource for current information on new treatment modalities for hemophilia. An assessment at one of these centers usually includes extensive patient education, genetic counseling, and laboratory testing.

Intravenous infusion of factor IX concentrate. A recombinant factor IX concentrate that has no human- or animal-derived proteins is available [Kessler & Mariani 2006]. Additional preparations including fusion proteins (to prolong half-life) are undergoing clinical trials [Monahan & Di Paola 2010]. Virucidal treatment of plasma-derived concentrates has eliminated the risk of HIV transmission since 1985, and of hepatitis B and C viruses since 1990.

Bleeding episodes are controlled rapidly after intravenous infusions of factor IX concentrate. Fast, effective treatment of bleeding episodes prevents pain, disability, and reduces the risk of chronic joint disease. Ideally, the affected individual should receive clotting factor within an hour of noticing symptoms or trauma. Knowing the previous in vivo recovery of a patient with hemophilia B helps estimate the proper dose [Björkman et al 2007]:

• Arranging efficient, effective treatment for infants and toddlers is especially challenging. Because frequent venipunctures may be necessary, it is important to identify staff members who are expert in performing venipunctures in small children.
• It is recommended that the parents of children age two to five years with severe hemophilia B be trained to administer the infusions as soon as is feasible. Home treatment allows for prompt treatment after symptoms occur and facilitates prophylactic therapy.

Pediatric issues. Special considerations for care of infants and children with hemophilia B include the following [Chalmers et al 2005]:

• Infant males with a family history of hemophilia B should not be circumcised unless hemophilia B is either excluded or, if present, treated with factor IX concentrate directly before and after the procedure to prevent delayed oozing and poor wound healing.
• Intramuscular injections should be avoided; immunizations should be administered subcutaneously.
• Effective dosing of factor IX requires an understanding of different pharmacokinetics in young children.

Inhibitors. Alloimmune inhibitors to factor IX, seen in 1%-3% of persons with severe hemophilia B, greatly compromise the ability to manage bleeding episodes [Hay et al 2006]. Their onset can be associated with anaphylactic reactions to factor IX infusion and nephrotic syndrome [DiMichele 2007, Chitlur et al 2009].

Prevention of Primary Manifestations

Children with severe hemophilia B are often given "primary" prophylactic infusions of factor IX concentrate two to three times a week to maintain factor IX clotting activity above 1%; these infusions prevent spontaneous bleeding and decrease the number of bleeding episodes. As shown for hemophilia A [Manco-Johnson et al 2007], prophylactic infusions almost completely eliminate spontaneous joint bleeding, decreasing chronic joint disease, although complications of venous access ports in young children can occur.

Prevention of Secondary Complications

Prevention of chronic joint disease is a major concern. Controversy still exists as to indications for beginning primary prophylaxis in individuals with severe hemophilia B, especially whether the benefits of primary prophylaxis justify the risk of an indwelling venous catheter in a young child.

"Secondary" prophylaxis is often used for several weeks, even in adults, if recurrent bleeding in a "target" joint or synovitis occurs, or for longer periods in adults with frequent bleeding.

Surveillance

Persons with hemophilia followed at hemophilia treatment centers (HTCs) (see Resources) have lower mortality than those who are not [Soucie et al 2000].

It is recommended that young children with severe or moderate hemophilia B have assessments at an HTC (accompanied by the parents) every six to 12 months to review and evaluate signs and symptoms of possible bleeding episodes and to adjust treatment as needed. The assessment should also include a joint and muscle evaluation, an inhibitor screen, viral testing if indicated, and a discussion of any other problems related to the individual's hemophilia and family and community support.

Screening for alloimmune inhibitors is usually done in those with severe hemophilia B after treatment with factor IX concentrates has been initiated either for bleeding or prophylaxis; additional screening is usually performed up to a few years of age when the genotype is a large partial or complete F9 deletion or a nonsense mutation at p.Arg29X (c.85C>T) (see Genotype-Phenotype Correlations; see Molecular Genetics: Normal allelic variants and Normal gene product for reference sequences). Testing for inhibitors should also be performed in any individual with hemophilia whenever a suboptimal clinical response to treatment is suspected, regardless of disease severity; with hemophilia B, the onset may be heralded by an allergic reaction to infused factor IX concentrate.

Older children and adults with severe or moderate hemophilia B benefit from contact with an HTC (see Resources) and periodic assessments to review bleeding episodes and treatment plans, evaluate joints and muscles, screen for an inhibitor, perform viral testing if indicated, provide education, and discuss other issues relevant to the individual's hemophilia.

Individuals with mild hemophilia B can benefit by maintaining a relationship with an HTC and having regular assessments every two to three years.

Agents/Circumstances to Avoid

Avoid the following:

• Activities that involve a high risk of trauma, particularly head injury
• Aspirin and all aspirin-containing products

Cautious use of other medications and herbal remedies that affect platelet function is indicated.

Older, intermediate purity plasma-derived “prothrombin complex” concentrates should be used cautiously (if at all) in hemophilia B because of their thrombogenic potential.

Testing of Relatives at Risk

Identification of at-risk relatives. A thorough family history may identify other male relatives who are at risk but have not been tested (particularly in families with mild hemophilia B).

Early determination of the genetic status of males at risk. Either assay of factor IX clotting activity from a cord blood sample obtained by venipuncture of the umbilical vein (to avoid contamination by amniotic fluid or placenta tissue) or molecular genetic testing for the family-specific F9 mutation can establish or exclude the diagnosis of hemophilia B in newborn males at risk. Infants with a family history of hemophilia B should not be circumcised unless hemophilia B is either excluded or, if present, factor IX concentrate is administered immediately before and after the procedure to prevent delayed oozing and poor wound healing.

Note: (1) The cord blood for factor IX clotting activity assay should be drawn into a syringe containing one-tenth volume of sodium citrate to avoid clotting and to provide an optimal mixing of the sample with the anticoagulant. (2) Factor IX clotting activity in cord blood in a normal-term newborn is lower than in adults (mean: ~30%; range: 15%-50%); thus, the diagnosis of hemophilia B can be established in an infant with activity lower than 1%, but is equivocal in an infant with moderately low (15%-20%) activity.

Determination of genetic status of females at risk. Approximately 10% of carriers have factor IX clotting activity lower than 30% and may have abnormal bleeding themselves. In a recent Dutch survey of hemophilia carriers, bleeding symptoms correlated with baseline factor clotting activity; there was suggestion of a very mild increase in bleeding even in those with 40% to 60% factor IX clotting activity [Plug et al 2006]. Therefore, all daughters and mothers of an affected male and other at-risk females should have a baseline factor IX clotting activity assay to determine if they are at increased risk for bleeding unless they are known on the basis of molecular genetic testing to be non-carriers. Very occasionally, a woman will have particularly low factor IX clotting activity that may result from heterozygosity for an F9 mutation associated with skewed X-chromosome inactivation or, on rare occasion, compound heterozygosity for two F9 mutations.

It is recommended that the carrier status of a woman at risk be established prior to pregnancy or as early in a pregnancy as possible.

Pregnancy Management

Obstetric issues. It is recommended that the carrier status of a woman at risk be established prior to pregnancy or as early in a pregnancy as possible [Lee et al 2006].

In some carriers, postpartum hemorrhage has been a prominent feature, despite the absence of menorrhagia [Yang & Ragni 2004].

If the mother is a symptomatic carrier (i.e., has a baseline factor IX clotting activity below ~30%), she may be at risk for excessive bleeding, particularly post partum, and may require therapy with factor IX concentrate [Yang & Ragni 2004].

Newborn males. Controversy remains as to indications for Cesarean section versus vaginal delivery [James & Hoots 2010, Ljung 2010]. For elective deliveries, the relative risks of Cesarean section versus vaginal delivery should be considered, especially if a male has been diagnosed with hemophilia B prenatally.

At birth or in the early neonatal period, intracranial hemorrhage is uncommon (<1%-2%), even in males with severe hemophilia B who are delivered vaginally.

Therapies Under Investigation

Additional recombinant factor IX proteins show promise in improving treatment [Monahan & Di Paola 2010].

• One recombinant factor IX, with a higher yield from cultured cells, will hopefully lower the cost of therapy.
• Another preparation in which recombinant factor IX is fused to a portion of the immunoglobulin Fc protein shows prolonged survival and efficacy in animal models [Peters & Bitonti 2007, Shapiro et al 2011] as is a factor IX that is N-glycoPEGylated [Ostergaard et al 2011]. Phase III clinical trials are in progress.
• A factor IX fused to albumin also appears to have prolonged survival.

Other

Clinical trials for gene therapy in hemophilia B were discontinued because of complications and failure to achieve significant factor IX expression in humans with hemophilia B. The hemophilia community remains hopeful, but many obstacles remain [Pierce et al 2007]. As recently reviewed, two clinical trials have been initiated using AAV vectors with strategies to avoid immune responses to capsid proteins that limited success in previous trials [Mingozzi & High 2011].

Vitamin K does not prevent or control bleeding caused by hemophilia B.

Genetics clinics, staffed by genetics professionals, provide information for individuals and families regarding the natural history, treatment, mode of inheritance, and genetic risks to other family members as well as information about available consumer-oriented resources. See the GeneTests Clinic Directory.

Genetic Counseling

Mode of Inheritance

Hemophilia B is inherited in an X-linked manner.

Risk to Family Members

Parents of a male proband

• The father of an affected male will not have the disease nor will he be a carrier of the mutation.
• Women who have an affected son and one other affected relative in the maternal line are obligate carriers.
• If a woman has more than one affected son and the disease-causing mutation cannot be detected in her DNA, she has germline mosaicism.
• Approximately 50% of affected males have no family history of hemophilia B. If an affected male represents a simplex case (an affected male with no family history of hemophilia), several possibilities regarding his mother's carrier status and the carrier risks of extended family members need to be considered:
• The mother is not a carrier and the affected male has a de novo disease-causing mutation
• The mother is a carrier of a de novo disease-causing mutation that occurred in one of the following ways:
• As a "germline mutation" (i.e., in the egg or sperm at the time of her conception, and thus present in every cell of her body and detectable in her DNA)
• As a somatic mutation (i.e., a change that occurred very early in embryogenesis, resulting in somatic mosaicism, in which the mutation is present in some but not all cells and may be detectable in leukocyte DNA in ≤11% of families) [Ketterling et al 1999]
• As "germline mosaicism" (in which some germ cells have the mutation and some do not, and the mutation is not detectable in DNA from her leukocytes)
• The mother is a carrier and has inherited the disease-causing mutation either from her mother who has a de novo disease-causing mutation or from her asymptomatic father who is mosaic for the mutation.
• The mother is a carrier of a mutation arising in a previous generation that has been passed on through the family without manifesting symptoms in female carriers.
• Molecular genetic testing combined with linkage analysis can determine the point of origin of a de novo mutation in up to half of the families with newly diagnosed, affected members [Sommer et al 2001]. Determining the point of origin of a de novo mutation is important for determining which branches of the family are at risk for hemophilia B.

Sibs of a male proband

• The risk to the sibs depends on the mother's carrier status. If the proband's mother is a carrier, each male sib has a 50% chance of having hemophilia B and each female sib is at a 50% risk of being a carrier.
• Germline mosaicism is possible, albeit uncommon. Thus, if an affected male represents a simplex case and if his mother has normal factor IX clotting activity and no evidence of her son's F9 disease-causing mutation in DNA from her leukocytes, she is still at a theoretically increased (but low) risk of having additional affected children.
• All sibs should have factor IX clotting activity assayed unless mutation analysis confirms that they have not inherited the F9 mutation present in their family.

Offspring of a male proband

• All daughters will be carriers of the F9 mutation causing hemophilia B of the same severity as their father's hemophilia.
• No sons will inherit the mutant allele, have hemophilia B, or pass it on to their offspring.

Other family members of a proband. The proband's maternal aunts and their offspring may be at risk of being carriers or being affected (depending on their gender, family relationship, and the carrier status of the proband's mother).

Carrier Detection

Carrier testing by molecular genetic testing is clinically available for most at-risk females if the mutation has been identified in the family.

Factor IX clotting activity is not a reliable test for determining carrier status: it can only be suggestive if low.

Related Genetic Counseling Issues

See Management, Testing of Relatives at Risk for information on testing at-risk relatives for the purpose of early diagnosis and treatment.

Family planning

• The optimal time for determination of genetic risk, clarification of carrier status, and discussion of the availability of prenatal testing is before pregnancy.
• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected, are carriers, or are at risk of being carriers.

Prenatal Testing

Prenatal testing is available for pregnancies of women who are carriers if the mutation has been identified in a family member or if linkage has been established in the family. The usual procedure is to determine fetal sex by performing chromosome analysis of fetal cells obtained by chorionic villus sampling (CVS) at approximately ten to 12 weeks' gestation or by amniocentesis usually performed at approximately 15-18 weeks' gestation. If the karyotype is 46,XY, DNA extracted from fetal cells can be analyzed for the known F9 disease-causing mutation or for the informative markers.

Note: Gestational age is expressed as menstrual weeks calculated either from the first day of the last normal menstrual period or by ultrasound measurements.

Percutaneous umbilical blood sampling (PUBS). If the disease-causing F9 mutation is not known and if linkage is not informative, prenatal diagnosis is possible using a fetal blood sample obtained by PUBS in citrate anticoagulant at approximately 18 to 21 weeks' gestation for assay of factor IX clotting activity. Factor IX clotting activity in a 20-week fetus averages 10% (range 6%-14%); thus, the diagnosis of hemophilia B can be established if factor IX clotting activity is less than 1%, but is equivocal when activity is moderately low.

Requests for prenatal testing for conditions which (like hemophilia B) do not affect intellect and for which treatment is available are not common. Differences in perspective may exist among medical professionals and within families regarding the use of prenatal testing, particularly if the testing is being considered for the purpose of pregnancy termination rather than early diagnosis. Although most centers would consider decisions about prenatal testing to be the choice of the parents, discussion of these issues is appropriate.

Preimplantation genetic diagnosis (PGD) may be available for families in which the disease-causing mutation has been identified. For laboratories offering PGD, see .

Note: It is the policy of GeneReviews to include clinical uses of testing available from laboratories listed in the GeneTests Laboratory Directory; inclusion does not necessarily reflect the endorsement of such uses by the author(s), editor(s), or reviewer(s).

Table A. Hemophilia B: Genes and Databases

Gene Symbol	Chromosomal Locus	Protein Name	Locus Specific	HGMD
F9	Xq27​.1	Coagulation factor IX	Hemobase: Hemophilia B mutation registry Haemophilia B mutation database F9 @ LOVD	F9

Data are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.

Table B. OMIM Entries for Hemophilia B (View All in OMIM)

300746	COAGULATION FACTOR IX; F9
306900	HEMOPHILIA B; HEMB

Normal allelic variants. F9 (reference sequence NM_000133.3) is 34 kb in length and comprises eight exons. Normal variants are uncommon in F9, but several have been identified [Mitchell et al 2005, Bajaj & Thompson 2006, Khachidze et al 2006].

Normal allelic variants (and their dbSNP identifier) that are useful for linkage analysis include an MseI site 5' (rs378815) and an HhaI site (rs3117459) 3' to the gene in all populations, a 50-bp intron 1 insert and an exon 6 MnlI site (rs6048) in blacks and whites, a 5' BamHI (rs4149657) and an intron 4 MspI (rs408567) site in blacks, and an intron 1 transition in Asians and Native Americans. The MnlI site is the only known exonic polymorphism and codes for Thr at codon 148 (see Note) or (less frequently) Ala, and is in strong linkage disequilibrium with a TaqI site (rs398101) in intron 4; however, it is not polymorphic in East Asians or Native Americans. See Table A, Locus-Specific Databases; Bajaj & Thompson [2006]; Khachidze et al [2006].

Note: Assuming that the initiating Met is the first of three within the first seven codons of the signal peptide, this would be residue 194 in the translated protein.

Pathologic allelic variants. Severe hemophilia B is caused by gross gene alterations, frameshift or splice junction changes, or nonsense or missense mutations. Mild or moderate hemophilia B is predominantly associated with missense changes (see Table A, Locus-Specific Databases). Occasionally, individuals with severe hemophilia B have exonic, multiexonic, or complete F9 deletions. Mild to moderate hemophilia is most often caused by missense mutations. Approximately half of the missense mutations are recurrent, and some clearly represent founder effects (see Table A, Locus-Specific Databases).

Normal gene product. The factor IX gene product (reference sequence NP_000124.1) includes several distinct domains [Bajaj & Thompson 2006]. The first and second domains are a signal peptide and a propeptide (respectively) that are cleaved to yield the mature protein, which is secreted as a single-chain peptide with 415 amino acid residues. Post-translational modifications include glycosylation, sulfation, phosphorylation, β-hydroxylation, and γ-carboxylation. A γ-carboxylase binds to the propeptide before cleavage and, in a vitamin K-dependent step, converts the first 12 glutamic acid residues (near the amino-terminus) to γ-carboxyglutamic residues or Gla. This Gla domain then binds calcium ions and adopts a conformation capable of binding to a phospholipid surface where the clotting cascade occurs. Adjacent to the Gla domain are two domains homologous with epidermal growth factor. The next domains are a connecting sequence that includes the activation peptide, and finally the catalytic domain. The latter is typical of serine proteases. Crystal structures are consistent with other data that show the catalytic domain elevated above a lipid surface. Factor IX is homologous with clotting factors VII and X and protein C.

Factor IX is synthesized in hepatocytes and circulates as a zymogen at 90 nmol/L (5 µg/mL). During coagulation in vivo, it is activated by factor VIIa tissue factor in a reaction in which the activation peptide is cleaved. Activated factor IX is the intrinsic factor X activator, requiring its cofactor, activated factor VIII, a lipid surface, and calcium. Sites of interaction of the active enzyme and cofactor are being identified [Bajaj & Thompson 2006]. Factor X activation is a critical early step that can regulate the overall rate of thrombin generation in coagulation.

Abnormal gene product. Different genotypes are associated with either absolute or relative lack of factor IX protein. Several missense mutations are associated with dysfunctional protein (see Table A, Locus-Specific Databases).

Author : Didit

Source : http://www.ncbi.nlm.nih.gov