Hemophilia A
by: Julian Raffoul
Hemophilia A is an X-linked recessive, hereditary blood clotting disorder,
that affects approximately 20,000 Americans (Hoyer, 1994). Because of its
genetic makeup, hemophilia A is carried by females, but those affected are
almost always males. There is no cure; people with hemophilia require lifelong
treatment. Contrary to popular belief, people with hemophilia do not bleed
to death from minor cuts or injuries, nor do they bleed faster than what
is considered normal. People with hemophilia bleed longer, because their
blood cannot develop a firm clot. Often bleeding is internal, into joints,
and results in arthritis or crippling.
In some cases hemophilia A is ìhiddenî for many generations
if no affected male children are born. The gene for hemophilia in such cases
is carried through several generations of females who, because they have
a second X chromosome that is normal, do not suffer from the disease themselves.
Other cases may have no family history, meaning that the change in the X
chromosome is a new one. This would be a mutation of the gene. Most mothers
of children with hemophilia, however, have fathers, grandfathers, brothers,
or other male relatives on the maternal side of their families who were
born with hemophilia. Very rarely, a female with hemophilia is born if her
mother is a carrier and her father has hemophilia.
It is important to realize that hemophilia A is a clinically heterogeneous
disorder. This is expected because of the large number of different molecular
defects in the factor VIII gene. It is also important to distinguish patients
with severe hemophilia A, whose plasma has no detectable factor VIII, from
patients who have moderate (1 to 4 percent of the normal factor VIII level)
or mild (5 to 25 percent of the normal level) hemophilia. The distinction
of severe disease from disease that is not is relevant since mild or moderate
hemophilia A is rarely complicated by episodes of apparently unprovoked
bleeding (Hoyer, 1994). The level of coagulation factors present in a patientís
plasma aids in clinical determination of the severity of the disease.
Blood coagulation normally proceeds through a series of sequential enzymatic
reactions in which protein cofactors (i.e. factor VIII) have an essential
role (Hoyer, 1994). A very low concentration of factor VIII (0.2 micrograms
per milliliter of plasma) ensures adequate coagulant function in normal
persons; a substantial (>80 percent) reduction in or absence of the factor
leads to a bleeding disorder (Hoyer, 1994).
A person with hemophilia A has either an inactive or inadequate supply of
the factor VIII protein which is needed for blood to clot normally. Sequences
within intron 22 of the factor VIII gene have been implicated in the cause
of hemophilia in almost 50 percent of severely affected patients (Goodeve
et al., 1994). The changes result from intrachromosomal rearrangements of
the tip of the long arm of the X chromosome, one break-point being within
the intron 22 of the factor VIII gene (Goodeve et al., 1994).
Carrier detection and prenatal diagnosis are important aspects of care in
hemophilia A and are based on intragenic or extragenic polymorphism factor
VIII gene tracking or by detection of the causative mutation in potential
carriers or in the unborn male at risk (Peake et al., 1993). The size and
complexity of the factor VIII gene makes comprehensive analysis difficult.
The gene is located at the tip of the long arm of the X chromosome. It compromises
nearly 186 kilobases (kb) and constitutes nearly 0.1 percent of the X chromosome.
The coding DNA is distributed in 26 exons, approximately 9 kb in all (Hoyer,
1994). The techniques used to analyze the complete coding region of the
factor VIII gene come from genomic DNA and are used to detect mutations
in the gene.
The molecular characterization of mutations in hemophilia A can be carried
out by using PCR-SSCP, Southern blotting, and reverse transcribed-PCR (Pieneman,
1995). A multiplex PCR in which four to eight exons are co-amplified is
used to reduce the time needed for screening the coding region of the factor
VIII gene. PCR-SSCP is used to screen small molecular defects, and reverse
transcriptase PCR combined with Southern blotting is used to screen DNA
for the inversions that occur frequently in intron 22 of the factor VIII
gene. These methods of detection assist in identifying deletions or insertions
that result in a frameshift in the coding DNA sequence, inversions that
are a result in a disruption of the gene, and ultimately elucidating an
abnormality in the factor VIII gene in the patient being tested (Pieneman,
1995).
Current products used to treat hemophilia are either manufactured from fresh
frozen plasma and cryoprecipitate, which are from single blood donors and
require special freezing, or are ìfreeze driedî factor VIII
concentrates. These concentrates are made in large lots, come in small bottles
and may be kept at room temperature or in the regular refrigerator allowing
for a patient to use this method of treatment at home. Although this type
of treatment may prove effective, it may also prove harmful. The possibility
of viral transmission by plasma-derived products has spurred efforts to
produce factor VIII by recombinant DNA technology. Following pre-clinical
evaluations, clinical trials demonstrated that factor VIII produced by recombinant
technology has no immediate side effects, is comparable to plasma-derived
factor VIII in its recovery and half-life characteristics, and is effective
in treating bleeding episodes (White et al., 1989). Ongoing clinical studies
are monitoring patients receiving factor VIII for increasingly long periods
and special attention is being placed on the possibility that these products
might cause a higher incidence of antibodies to factor VIII than do plasma-derived
factor VIII concentrates (Lusher et al., 1993).
Recombinant blood coagulant factor VIII appears to be an effective, well-
tolerated treatment for patients with hemophilia A. Use of plasma-derived
clotting factor in hemophiliacs risks the transmission of blood-borne viral
diseases such as hepatitis B, hepatitis C, HIV, or any other viruses that
could be present in human plasma (Lusher et al., 1993). Recombinant DNA-derived
clotting factor, however, contains no virus and is thus potentially advantageous
to hemophiliacs. Another advantage of recombinant factor VIII would be the
unlimited supply, however, the cost may be prohibitive.
Regardless of the risk involved, if any, in the treatment of hemophilia
A, not treating can be very painful and may lead to other serious medical
complications. Physicians caring for patients with hemophilia A recognize
that improved care is increasingly available. Factor VIII concentrates,
effectively treated to inactivate viruses, can prevent or control bleeding
in most patients, comprehensive care programs make available the range of
necessary resources, and the availability of recombinant factor VIII will
free patients from the possibility of infection by human-derived viruses.
Recombinant factor VIII will not cure the disease, however, and inhibitor
formation will be an ongoing concern. Moreover, this potential will only
be reached if cost issues are addressed so that concentrates can be used
as medically indicated and access to care is broadened to include all patients
with hemophilia.
References
Goodeve AC, Preston FE, Oeake IR (1994). Factor VIII gene rearrangements
in patients with severe hemophilia A. The Lancet . v 343. p329 (2).
Hoyer LW (1994). Hemophilia A (review article). N Engl J Med. v 330. p38
(10).
Hughes-Jones NC (1995). Risk Assessment and factor VIII concentrates. The
Lancet. v 345. p 502 (2).
Klug WS & Cummings MR (1994). Concepts of Genetics. NewJersey Prentice-Hall.
Lusher JM, Arkin S, Abilgaard CF, and Schwartz RS (1993). Recombinant factor
VIII for the treatment of previously untreated patients with hemophilia
A: safety, efficacy, and development of inhibitors. N Engl J Med. v 328.
p453 (7).
Pieneman WC (1995). Screening for mutations in hemophilia A patients by
multiplex PCR-SSCP, Southern blotting and RNA analysis: the detection of
a genetic abnormality in the factor VIII gene in 30 out of 35 patients.
Br J Haematol. v 90. p442 (8).
White GC, McMillan CW, Kingdon HS, and Shoemaker CB (1989). Use of recombinant
antihemophilic factor in the treatment of two patients with classic hemophilia.
N Eng J Med. v 320. p166 (4).
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