What is the role of genetics in the etiology of hemophilia A?

Updated: Jun 05, 2020
  • Author: Douglass A Drelich, MD; Chief Editor: Srikanth Nagalla, MBBS, MS, FACP  more...
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Hemophilia A is inherited in an X-linked recessive pattern. The gene for FVIII is located on the long arm of the X chromosome in band q28. The factor VIII gene is one of the largest genes, comprising approximately 0.1% of the DNA in the X chromosome; it is 186 kilobases (kb) long and has a 9-kb coding region that contains 26 exons. The mature protein contains 2332 amino acids and has a molecular weight of 300 kd. It includes 3 A domains, 1 B domain, and 2 C domains.

Intron 22 of the factor VIII gene, uniquely, contains two other genes. The first gene, F8A, is transcribed in a direction opposite to that of the factor VIII gene itself. The second gene, F8B, is transcribed in the 3' (normal) direction similar to the factor VIII gene. Sequences called A2 and A3, homologous to the F8A sequence, are present on the X chromosome, 300 kb telomeric to the factor VIII gene.

Homologous recombination of the factor VIII gene, with inversion and crossover involving the F8A sequence in intron 22 and the homologous distal sequence on the X chromosome, results in a split in the factor VIII gene with the two parts aligned in opposite directions. This causes a disruption in the normal factor VIII coding sequence, with an inability to transcribe the complete, normal factor VIII protein, resulting in a loss of function.

The mutation in intron 22 occurs during spermatogenesis and is a common cause of severe factor VIII deficiency; it is present in approximately 40% of patients. It is easily detected using a Southern blot analysis of the patient's DNA. These patients are more likely to develop an inhibitor to factor VIII.

In one study, all detected inversions originated in a maternal grandparent during male meiosis (spermatogenesis), supporting the hypothesis that an unpaired Xq, rather than a paired X chromosome, is more likely to undergo an intrachromosomal inversion. The majority of mothers of persons with the sporadic, inversion-related severe hemophilia are carriers. [17]

The knowledge of the parental origin of the inversion mutation has important implications for genetic counseling.

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