CHAPTER 15: Inherited Coagulation Disorders

No hematologic disorder has more historical import and portent than hemophilia. Figure 15-1 shows the descendants of Queen Victoria and Prince Albert. One of their three sons, Leopold, Duke of Albany, suffered from a life-long bleeding -disorder, as did nine grandsons and great grandsons, including members of the Russian, Prussian, and Spanish royal families. Particularly foreboding was the affliction of young Prince Alexie, the only son of Czar Nicholas and Czarina Alexandra. His illness was but one of many travails that beset the Russian royal family and led to their dethronement and execution during the Bolshevik revolution.

Hemophilia A is the inherited bleeding disorder most often associated with severe morbidity, frequently requiring hospitalization. It is due to deficiency of factor VIII. When activated, factor VIII forms a complex with activated factor IX, enabling the efficient proteolytic activation of factor X (see bottom of sidebar and Chapter 13 for more information about factor VIII). Hemophilia A occurs in about 1 in 5000 male births. The gene encoding factor VIII is located near the tip of the long arm of the X chromosome. Thus, hemophilia A is inherited in an X-linked manner, with female heterozygous carriers passing the disease on to half of their sons. The royal families depicted in Figure 15-1 comprise a typical kindred demonstrating X-linked transmission. About 30% of patients have no family history of abnormal bleeding. Most of these cases are due to spontaneous mutations. Although hemophilia A can occur in homozygous females, often owing to consanguinity, most females who bleed abnormally are heterozygous carriers of the hemophilia gene. Although the mechanism for their low factor VIII levels is not known, skewed X-inactivation (lyonization) has been hypothesized.

CLINICAL PRESENTATION

Patients with hemophilia A tend to have deep bleeding into joints (hemarthroses) or muscle beds, rather than bleeding from mucosal surfaces. Patients with repeated joint hemorrhage develop chronic disability due to swelling, deformity, severe pain, limitation of motion, and contractures that can be corrected only by joint replacement. Less often patients have bouts of gastrointestinal or genitourinary bleeding and, rarely, intracerebral hemorrhage—a catastrophic event. Patients are assigned to one of three levels of severity according to plasma levels of factor VIII. These levels generally predict the phenotype of the patient. Those with 1% or less factor VIII (severe hemophilia) have an average of 20 to 30 episodes per year of either spontaneous bleeding or excessive bleeding after minor trauma. Patients with 1% to 4% factor VIII levels have moderate hemophilia, whereas individuals with greater than 4% factor VIII levels generally have bleeding episodes only after trauma or surgery.

GENETICS

As might be surmised from the range of phenotypes, hemophilia A is a genetically heterogeneous disease. A wide variety of mutations of the factor VIII gene have been identified in hemophilia patients, including missense mutations, nonsense mutations leading to premature termination of translation, frame shifts, deletions, and rearrangements. The most commonly encountered mutation, depicted in Figure 15-2A, is an inversion that reverses the orientation of the 3' end of the gene relative to the 5' promoter and transcriptional start site. Alleles bearing this rearrangement, observed in about 50% of severe hemophilia A patients, make no factor VIII. As shown in Figure 15-2B, the great majority of mutations in patients with severe hemophilia A have either inversions, large deletions, nonsense mutations, or frame shifts, all of which preclude synthesis of full-length factor VIII. In contrast, those with moderate or mild hemophilia A usually have missense mutations in which full-length protein is produced but has impaired stability or function.

DIAGNOSIS

Because factor VIII is part of the intrinsic pathway of coagulation, the partial thromboplastin time in a patient with hemophilia A is prolonged, whereas the prothrombin time is normal. Moreover, there are no abnormalities in the platelet count or in tests of platelet function such as the bleeding time. Assay of factor VIII coagulant activity is used not only in establishing the diagnosis but also in monitoring patients with hemophilia A and assessing their need for therapy. This test entails measurement of the clotting time after the patient's plasma is mixed with factor VIII–deficient plasma. Molecular genetic testing is now available to diagnose hemophilia A during early fetal life for purposes of genetic counseling.

THERAPY

Currently, patients with hemophilia A are treated with infusions of either recombinant factor VIII or highly purified plasma-derived factor VIII prepared from human donor plasma. Both are equally effective and free of viral pathogens. The recombinant product costs 2 to 3 times as much as the plasma concentrates. Factor VIII has a plasma half-life of 8 to 12 hours. Thus, twice daily infusions are required to maintain levels adequate for hemostasis in patients with serious active bleeding (eg, intracranial, intraperitoneal, and gastrointestinal bleeds) or facing major surgery. Isolated joint hemorrhage is usually successfully treated with a single infusion sufficient to raise the plasma level of factor VIII to over 30%. Many children in the United States and in other developed countries are treated with prophylactic self-infusions of factor VIII three times a week in order to prevent damage to joints. Raising the factor VIII level from a baseline of 1% or less to 3% to 4% alters the phenotype of the patient dramatically. In the absence of human immunodeficiency virus (HIV) infection, patients with severe hemophilia treated with factor replacement have a life expectancy of about 65 years, whereas life expectancy in those with mild or moderate disease is near normal. The limiting factor is cost. Prophylactic treatment of children with factor VIII deficiency costs over $100,000 per year. In less developed countries, the high cost of factor VIII precludes most patients from being adequately treated, and many suffer a life of severe pain and disability. In patients with mild or moderate hemophilia A who produce some factor VIII, treatment with desmopressin releases von Willebrand -factor (vWF) from stores in endothelial cells and results in a significant increase in plasma levels of factor VIII, due to the ability of vWF to bind and stabilize factor VIII. As a general rule, desmopressin infusion raises factor VIII levels 3-fold to 4-fold when given daily for up to 3 days.

Patients with hemophilia A are ideal candidates for gene therapy. Hemophilia A is a monogenetic disease that, in contrast to sickle cell disease, is not significantly confounded by concurrent genetic modifiers. Moreover, tight regulation of factor VIII gene expression is not necessary for safe and effective treatment of hemophilia A, because factor VIII levels from 10% to 150% of normal are adequate to achieve hemostasis and carry no apparent risk of thrombosis on the high end of the range. Contrast this with the situation in sickle cell anemia and thalassemia, in which erythroid-specific, precisely balanced and controlled globin gene expression is critical. Despite these inherent advantages, progress in this endeavor has been disappointing to date.

COMPLICATIONS

In addition to the consequences of repetitive hemarthroses mentioned earlier, two other major problems threaten the well-being and life expectancy of patients with hemophilia A. Prior to 1990, patients were treated with concentrates of pooled plasma that were contaminated with pathogenic viruses—in particular, hepatitis B and C as well as HIV. As a result, nearly all patients with hemophilia A severe enough to require infusions became infected with hepatitis B or C, and of these, nearly 20% developed hepatic cirrhosis. About two-thirds of these patients were also infected with HIV. The development of AIDS in hemophilia patients enormously complicates their management and for the last two decades has been the leading cause of death among adult patients with severe disease.

A second major complication is the development of antibodies that neutralize factor VIII. These inhibitors arise primarily in about 10% to 15% of severe hemophiliacs who lack the ability to produce even small amounts of endogenous factor VIII. The immune system in these patients sees the exogenously administered protein as "foreign" and mounts an antibody response. In some patients, inhibitor titers remain low enough to be overridden by infusion of large amounts of factor VIII. Moreover, the titer of low-level inhibitors generally does not rise with factor VIII infusions (non-inducible). Unfortunately, in others the titer of anti-factor VIII antibodies builds up to extremely high levels—enough to neutralize the total amount of factor VIII normally present in the circulation and all of the exogenous factor VIII that can reasonably be infused. Moreover, infusions of factor VIII in such patients usually result in an anamnestic antibody response. These patients are at very high risk of catastrophic bleeding. If there is minimal cross-reactivity between the anti-factor VIII antibody and porcine factor VIII, an effective therapeutic option is to switch to porcine factor VIII infusions. Alternatively, patients with high-titer inhibitors that neutralize porcine factor VIII can be treated with recombinant activated factor VII or by preparations that bypass the intrinsic pathway and directly trigger the activation of factor X to factor Xa.

The description in the beginning of this chapter of hemophilia among progeny of Queen Victoria makes no mention of whether afflicted family members suffered from factor VIII deficiency (hemophilia A) or factor IX deficiency (hemophilia B). It might have been anticipated that this family suffered from hemophilia A because its incidence is about 6-fold higher than that of hemophilia B. However, recent sequencing evidence has demonstrated that the mutation in this family involves the factor IX gene and produces severe hemophilia B. As the side bar on the first page of this chapter indicates, the two disorders are remarkably similar genetically, clinically, and molecularly. Both involve defects in the "Xase" complex in which factors VIIIa and IXa interact cooperatively to activate factor X. Both have an X-linked inheritance pattern. Indeed, the gene encoding factor IX is located quite close to the factor VIII gene on the long arm of the X chromosome. Both disorders have identical clinical presentations, a wide spectrum of diverse mutations spread throughout their respective genes, similar ranges of clinical severity due to variation in factor levels, and shared risks for development of therapy-induced viral infections and neutralizing antibodies.

Recombinant factor IX and ultrapure plasma-derived factor IX are used for treatment of patients with hemophilia B. Both are free of contamination with viruses and activated coagulation factors. The guidelines for treatment with factor IX infusions are very similar to those mentioned earlier for factor VIII. Because factor IX has a half-life of about 24 hours, it requires less frequent infusions to maintain adequate levels in the plasma. However, because of its smaller size, factor IX rapidly equilibrates between the intravascular and extravascular space, so that, on the basis of units administered, twice as much factor IX must be given compared with factor VIII. Patients who develop high-titer inhibitors to factor IX—a very rare occurrence—are treated with recombinant activated factor VII. The prospect of gene therapy for hemophilia B is every bit as promising and as frustrating as that for hemophilia A.

von Willebrand disease is the most common inherited bleeding disorder, with a prevalence in the United States of about 1 in 90, more than than 50 fold higher than that of hemophilia A. Thus, about 3 million Americans are affected. However, the majority of these individuals escape diagnosis because the defect is so mild it does not come to medical attention. von Willebrand disease can be distinguished from the hemophilias in three major respects: 1) it is inherited in an autosomal dominant manner, 2) bleeding is primarily from mucous membranes, and 3) the bleeding time is prolonged. Additional differences between von Willebrand disease and hemophilia A, as well as important similarities, are listed in Table 15-1.

von Willebrand disease is caused by mutations spread throughout the gene encoding von Willebrand factor (vWF). This protein serves two very different functions: it is the chaperone in the plasma for factor VIII, protecting factor VIII from rapid clearance and degradation. Also, through its binding domains for both collagen and for platelets, vWF acts as a molecular glue that enables platelets to adhere to the wall of injured blood vessels. Further information on the structure and function of this protein is provided in Chapter 13. Because vWF is so large and contains numerous functionally important domains, it has been a challenge to link individual missense mutations to the clinical phenotype in any particular patient. Two other considerations also complicate the pathogenesis of the disease. First, a substantial fraction of individuals with the clinical phenotype of von Willebrand disease lack any detectable abnormality in either vWF allele. Second, the levels of vWF protein and the clinical severity can be significantly affected by genetic factors such as the co-inheritance of modifying genes unlinked to vWF as well as environmental factors including hormones and drugs that affect the synthesis of vWF. Humans with group O blood have lower levels of plasma vWF and are commonly misdiagnosed as having von Willebrand disease.

DIAGNOSIS

A panel of three laboratory tests is used in both the diagnosis and monitoring of von Willebrand disease. Factor VIII activity is measured with the same assay used to monitor patients with hemophilia A. vWF antigen is measured with an immunologic assay that detects both normal and mutant vWF proteins. The ristocetin cofactor assay provides a functional assessment of vWF protein. This test measures the agglutination of formalin-fixed normal platelets by the patient's plasma in the presence of ristocetin, which enables vWF multimers to bind to platelet GPIb complex.

von Willebrand disease is divided into three major subtypes, based on a combination of salient clinical features and molecular pathogenesis. Types 1 and 3 are caused by vWF protein deficiency, as measured by vWF antigen levels. Accordingly, the levels of factor VIII activity and ristocetin cofactor activity are decreased in parallel with the vWF antigen level. Type 1 is by far the most common form, accounting for over 70% of affected individuals. The levels of vWF antigen vary widely from 15% to 60% of normal, principally owing to environmental and genetic factors mentioned earlier. As shown in Figure 15-3, the distribution of vWF multimers is normal. In contrast to those of type 1 disease, levels of vWF antigen are less than 5% of normal in type 3 von Willebrand disease (Fig. 15-3). Unlike type 1 and type 2 individuals, patients with type 3 have inherited mutant vWF genes from both parents. Since levels of vWF protein are so low in type 3 patients, they have much more severe bleeding, and indeed, because their factor VIII levels are comparably low, they may develop hemarthroses and other deep tissue bleeding similar to patients with hemophilia A.

Type 2 disease, like type 1, is inherited in an autosomal dominant manner and accounts for about 25% of patients with von Willebrand disease. In these patients, the vWF antigen and factor VIII levels are generally normal, but the vWF -protein is qualitatively abnormal, leading to a low ristocetin co-factor activity. Type 2 disease includes several subtypes. Most individuals with type 2A disease have defective multimerization of vWF protein, as illustrated in Figure 15-3. Patients with type 2A bleed because of a failure to form large multimers, an abnormality that greatly impairs the ability of vWF to tether platelets to injured endothelium. Two other less common variants of von Willebrand disease merit comment because of their pathophysiologic interest. In type 2B, there is also reduction in the very large multimers (Fig. 15-3), but in this case due to structural mutations that increase the binding of vWF to platelet GPIb/IX. This results in clearance of large multimers from the plasma and sometimes causes a mild thrombocytopenia. Owing to the increased binding of vWF to circulating platelets, platelet agglutination in patients with type 2B disease is unusually sensitive to ristocetin. Also of pathophysiologic interest are rare patients with autosomal recessive type 2N disease, in which patients inherit two defective copies of vWF with mutations within the N-terminal domain that abolish vWF binding to factor VIII. As a result, the vWF antigen and ristocetin cofactor levels are normal, but the factor VIII levels are very low, and the patient has a clinical phenotype very similar to that of hemophilia A.

THERAPY

Patients with severe von Willebrand disease and those requiring optimal hemostasis during and following major surgery should be treated with intravenous infusion of factor VIII concentrates that are enriched in vWF. Patients with less severe disease are often treated quite effectively with desmopressin, a vasopressin analog, which triggers the release into the plasma of vWF protein stored in endothelial cells. However, desmopressin is contraindicated in type 2B von Willebrand disease because it induces severe thrombocytopenia.

Genetic deficiencies of fibrinogen, prothrombin, and factors V, VII, X, XI, and XIII have been shown to cause abnormal bleeding. All but factor XIII can be diagnosed by a panel of coagulation studies that includes specific factor assays. Factor XIII catalyzes the covalent crosslinking of fibrin, which gives clots greater tensile strength. Deficiency of factor XIII can be demonstrated by testing the stability of the fibrin clot in a chaotropic solution such as 5 M urea. All of these disorders are very rare, except for factor XI deficiency, which is encountered primarily among Ashkenazi Jews. Individuals with factor XI deficiency, even when severe, seldom bleed abnormally unless challenged by trauma or major surgery. Because factor XI has a relatively long half-life, patients can be effectively treated with infusions of fresh frozen plasma.