Page 2254 - Hematology_ Basic Principles and Practice ( PDFDrive )

C H A P T E R 135

HEMOPHILIA A AND B

Manuel Carcao, Paul Moorehead, and David Lillicrap

EPIDEMIOLOGY is described later. The size of F8 introns varies from 207 base pairs
(intron 17) to 32.4 kb (intron 22).
Hemophilia is the most common severe inherited bleeding disorder In addition to the 9-kb FVIII transcript, this locus also encodes
recognized in humans. The hereditary and sex-linked nature of the two additional messenger RNAs (mRNAs) that, unlike FVIII, are
disease has been appreciated since prebiblical times, and the previous expressed ubiquitously. Within intron 22 of the F8 gene there are
occurrence of the disease in the European Royal family has added two additional coding elements, F8A and F8B. The F8A transcript
further interest in this condition. is made up entirely of intronic sequence from intron 22, and the F8A
The prevalence of hemophilia is worldwide, with no major geo- mRNA is transcribed in the opposite direction to F8. Although the
graphic variances aside from rare clusters of disease in areas where function of the F8A transcript/protein is unknown, there is the
founder mutations have been propagated (e.g., Twillingate, New- potential that this mRNA could act as an antisense regulator of FVIII
foundland). In many parts of the developing world the true prevalence expression. In addition, two (or more) other copies of the F8A
of the condition is unknown because of inadequate diagnostic facili- sequence are located further telomeric to F8 and are involved in a
ties, but from experiences in the developed world, we can probably frequent intrachromosomal recombination event in approximately
assume that the prevalence of hemophilia A is approximately one in 45% of patients with severe hemophilia A. The F8B transcript is
5000 males and for hemophilia B, one in 30,000 males These preva- expressed in the same orientation as the native F8 mRNA, and this
lence figures do show some variances among countries but except transcript comprises an initial intron 22 sequence that is spliced onto
where local founder effects are important, these differences likely exons 23 to 26 of F8. As with F8A, the F8B transcript is also ubiq-
represent variable disease ascertainment and diagnosis. Because of the uitously expressed. Although the function of the protein is unknown,
X-linked recessive inheritance of the disease, most affected subjects because it contains the phospholipid binding region of FVIII it may
are male. However, for every male with hemophilia we estimate that play a role in membrane binding.
there are on average at least two carrier females (in most cases the
mother of the affected male as well as potentially some sisters of the
male, his daughters and potentially his aunts). Hence the hemophilia Factor VIII Expression
carrier state is almost certainly much more common although there
are no good data for this. Many carriers may not be diagnosed mainly Transcription of the F8 gene is regulated by a promoter that contains
because they never have affected sons. Although most hemophilia binding sites for both tissue-specific and ubiquitous transcription
carrier women do not experience significant bleeding, some will have factors. The mRNA transcribed from F8 is 9 kb in length and con-
clotting factor levels low enough to cause symptoms. tains 7053 nucleotides of coding (i.e., translated) sequence and short
International studies in the past decade indicate that the numbers 5′ and long 3′ untranslated sequences. The cellular site of FVIII
of persons with hemophilia in the population are increasing by expression has long been a matter of controversy. The situation is
approximately 2% each year. There are likely several reasons for this complicated by the very low abundance and instability of the FVIII
trend, including the overall increase in population numbers, increas- transcript and protein. Nevertheless, F8 mRNA has been found to
ing longevity of persons with hemophilia, particularly because human be expressed in a variety of human tissues, including liver, spleen,
immunodeficiency virus (HIV) and hepatitis C are having less impact kidney cells, and muscle. It has long been known that hemophilia
on mortality than they did in the past, and increasing worldwide can be cured by liver transplantation. The cell types within the liver
awareness and diagnosis of patients with milder forms of responsible for FVIII synthesis have previously been unclear, but
hemophilia. there is now strong evidence that the liver sinusoidal endothelium is
1
The clinical signs and symptoms and inheritance patterns for the source of FVIII production. Nonetheless, FVIII levels are main-
hemophilia A and B are essentially identical, and it was not until the tained during episodes of severe liver failure, suggesting the existence
early 1950s that the two forms of hemophilia were differentiated. of extrahepatic sites of FVIII expression. Recently, two reports using
Subsequently, in the early 1980s, the two genes encoding factor VIII different conditional knock-out approaches in mice have shown that
(FVIII) and factor IX (FIX) were cloned, and the specific mutations the vascular endothelium is the predominant site of FVIII expression
responsible for hemophilia began to be determined. in this animal model. 2,3
FVIII is an acute phase reactant, and studies have shown that
FACTOR VIII BIOLOGY: GENETICS, STRUCTURE, FVIII expression can be induced through a nuclear factor kappa-B
(NFκB). FVIII is an acute phase protein whose levels increase with
FUNCTION, AND PATHOPHYSIOLOGY hormone induction (estrogen use), but the mechanistic basis for this
increase is not known.
The Factor VIII Gene

The FVIII gene (F8) encoding the FVIII protein is located at Xq28, The Biosynthesis of Factor VIII
the most distal band of the long arm of the X chromosome (Fig.
135.1). It is a large and complex structure, 186 kb in length, consist- FVIII biosynthesis has been extensively investigated in vitro using cell
ing of 9 kb of exonic DNA arranged into 26 exons and 177 kb of types that do not normally express this protein, such as baby hamster
intronic DNA in 25 introns. Most of the exons are small, ranging in kidney (BHK) or Chinese hamster ovary (CHO). Therefore conclu-
size from 69 base pairs (bp) (exon 5; the smallest exon) to 3106 bp sions about the details of this process in a physiologic context should
(exon 14; the largest exon—encoding the central B domain). The be made with caution. After production of the primary polypeptide
correspondence of these exons with the domains of the FVIII protein chain and cleavage of the 19-amino-acid signal peptide, the protein

2001