• Intrachromosomal recombination
• Transposable element insertion
• Variable expressivity
• Protein replacement therapy
Major Phenotypic Features
• Age at onset: Infancy to adulthood
• Bleeding diathesis
History and Physical Findings
S.T., a healthy 38-year-old woman, scheduled an appointment for counseling regarding her risk for having a child with hemophilia. She had a maternal uncle who had died in childhood from hemophilia and a brother who had had bleeding problems as a child. Her brother's bleeding problems had resolved during adolescence. No other family members had bleeding disorders. The geneticist explained to S.T. that her family history was suggestive of an X-linked abnormality of coagulation such as hemophilia A or B and that her brother's improvement was particularly suggestive of the hemophilia B variant factor IX Leyden. To confirm the diagnosis of hemophilia, the geneticist told S.T. that her brother should be evaluated first because identification of an isolated carrier is difficult. S.T. talked to her brother, and he agreed to an evaluation. Review of his records showed that he had been diagnosed with factor IX deficiency as a child but now had nearly normal plasma levels of factor IX. DNA mutation analysis confirmed that he had a mutation in the F9 gene promoter, consistent with factor IX Leyden. Subsequent testing of S.T. showed that she did not carry the mutation identified in her brother.
Disease Etiology and Incidence
Hemophilia A (MIM 307600) and hemophilia B (MIM 306900) are X-linked disorders of coagulation caused by mutations in the F8 and F9 genes, respectively. Mutations of F8 cause deficiency or dysfunction of clotting factor VIII; mutations of F9 cause deficiency or dysfunction of clotting factor IX.
Hemophilia is a panethnic disorder without racial predilection. Hemophilia A has an incidence of 1 in 5000 to 10,000 newborn males. Hemophilia B is far more rare, with an incidence of 1 in 100,000.
The coagulation system maintains the integrity of the vasculature through a delicate balance of clot formation and inhibition. The proteases and protein cofactors composing the clotting cascade are present in the circulation as inactive precursors and must be sequentially activated at the site of injury to form a fibrin clot. Timely and efficient formation of a clot requires exponential activation or amplification of the protease cascade. Clotting factors VIII and IX, which complex together, are key to this amplification; they activate clotting factor X, and active factor X, in turn, activates more factor IX and factor VIII (see Figure 8-8). Factor IX functions as a protease and factor VIII as a cofactor. Deficiency or dysfunction of either factor IX or factor VIII causes hemophilia.
Mutations of F8 include deletions, insertions, inversions, and point mutations. The most common mutation is an inversion deleting the carboxyl terminus of factor VIII; it accounts for 25% of all hemophilia A and for 40% to 50% of severe hemophilia A. This inversion results from an intrachromosomal recombination between sequences in intron 22 of F8 and homologous sequences telomeric to F8. Another intriguing class of mutation involves retrotransposition of L1 repeats into the gene. For all F8 mutations, the residual enzymatic activity of the factor VIII–factor IX complex correlates with the severity of clinical disease (see Table).
Many different F9 mutations have been identified in patients with hemophilia B; but in contrast to the frequent partial inversion of F8 in hemophilia A, a common F9 mutation has not been identified for hemophilia B. Factor IX Leyden is an unusual F9 variant caused by point mutations in the F9 promoter; it is associated with very low levels of factor IX and severe hemophilia during childhood, but spontaneous resolution of hemophilia occurs at puberty as factor IX levels nearly normalize. For each F9 mutation, the residual enzymatic activity of the factor VIII–factor IX complex correlates with the severity of clinical disease (see Table).
Phenotype and Natural History
Hemophilia is classically a male disease, although rarely females can be affected because of skewed X chromosome inactivation. Clinically, hemophilia A and hemophilia B are indistinguishable. Both are characterized by bleeding into soft tissues, muscles, and weight-bearing joints (Fig. C-21). Bleeding occurs within hours to days after trauma and often continues for days or weeks. Those with severe disease are usually diagnosed as newborns because of excessive cephalohematomas or prolonged bleeding from umbilical or circumcision wounds. Patients with moderate disease often do not develop hematomas or hemarthroses until they begin to crawl or walk and therefore escape diagnosis until that time. Patients with mild disease frequently present in adolescence or adulthood with hemarthroses or prolonged bleeding after surgery or trauma.
FIGURE C-21 Subcutaneous hematoma of the forehead in a young boy with hemophilia. The photograph was taken 4 days after a minor contusion. The appearance of the forehead returned to normal in 6 months. See Sources & Acknowledgments.
Clinical Classification and Clotting Factor Levels
% Activity (Factor VIII or IX)
Hemophilia A and hemophilia B are diagnosed and distinguished by measurement of factor VIII and IX activity levels. For both hemophilia A and hemophilia B, the level of factor VIII or IX activity predicts the clinical severity.
The diagnosis of hemophilia A is established by identifying low factor VIII clotting activity in the presence of a normal von Willebrand factor level. Molecular genetic testing of F8, the gene encoding factor VIII, identifies disease-causing mutations in as many as 98% of individuals with hemophilia A. The diagnosis of hemophilia B is established by identifying 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. Both tests are available clinically.
Although current gene therapy trials show promise, no curative treatments are available for hemophilia A and hemophilia B except for liver transplantation (see Chapter 13). Currently the standard of care is intravenous replacement of the deficient factor. Factor replacement therapy has increased life expectancy from an average of 1.4 years in the early 1900s to approximately 65 years today.
If a woman has a family history of hemophilia, her carrier status can be determined by linkage analysis or by identification of the F8 or F9 mutation segregating in the family. Routine mutation identification used to be available only for the common F8 inversion, but advances in DNA sequencing have made targeted exome sequencing much more effective. Carrier detection by enzyme assay is difficult and not widely available.
If a mother is a carrier, each son has a 50% risk for hemophilia, and each daughter has a 50% risk for inheriting the F8 or F9 mutation. Reflecting the low frequency of clinically significant skewing of X chromosome inactivation, daughters inheriting an F8 or F9 mutation have a low risk for hemophilia.
If a mother has a son with hemophilia but no other affected relatives, her a priori risk for being a carrier depends on the type of mutation. Point mutations and the common F8 inversions almost always arise in male meiosis; as a result, 98% of mothers of a male with one of these mutations are carriers due to a new mutation in their father (the affected male's maternal grandfather). In contrast, deletion mutations usually arise during female meiosis. If there is no knowledge of the mutation type, then approximately one third of patients are assumed to have a new mutation in F8 or F9. Through the application of Bayes's theorem, this risk can be modified by considering the number of unaffected sons in the family (see Chapter 16).
Questions for Small Group Discussion
1. What other diseases are caused by recombination between repeated genome sequences? Compare and contrast the recombination mechanism observed with hemophilia A with that observed with Smith-Magenis syndrome and with familial hypercholesterolemia.
2. One of the more unusual mutations in F8 is insertion of an L1 element into exon 14. What are transposable elements? How do transposable elements move within a genome? Name another disease caused by movement of transposable elements.
3. In patients with hemophilia B due to factor IX Leyden, why does the deficiency of factor IX resolve during puberty?
4. Compare and contrast protein replacement for hemophilia to that for Gaucher disease. Approximately 10% of patients with hemophilia develop a clinically significant antibody titer against factor VIII or IX. Why? Is there a genetic predisposition to development of antibodies against the replacement factors? How could this immune reaction be circumvented? Would gene therapy be helpful for patients with antibodies?
5. Discuss current approaches to gene therapy in hemophilia B.
Konkle BA, Josephson NC, Nakaya Fletcher S. Hemophilia A. [Available from] http://www.ncbi.nlm.nih.gov/books/NBK1404/.
Konkle BA, Josephson NC, Nakaya Fletcher S. Hemophilia B. [Available from] http://www.ncbi.nlm.nih.gov/books/NBK1495/.
Santagostino E, Fasulo MR. Hemophilia A and hemophilia B: different types of diseases? Semin Thromb Hemost. 2013;39:697–701.