It was found in 2016 that mutations that
cause early translation termination account for 50% of inherited disorders.
Transcripts bearing aberrant termination codons are most likely and are
identified by an evolutionarily conserved posttranscriptional pathway known as
nonsense-mediated mRNA decay (NMD). There are many pieces of evidence of decay
among coagulation factor genes. However, the hemophilia gene (F8) does not seem
to be exposed to NMD. Since the F8 gene is located on the X chromosomes, a
connection between X-linked traits and mRNA decay could be assumed (18).
Multiple mutations in the same gene within haemophilia have been reported. In
case of haemophilia A, out of 2740 entries, ten are double mutants. Among the
2891 patient entries in the Haemophilia B mutation database, there are 34
double mutants and one triple mutant (19).

 The formation of inhibitors
which inhibit the function of
factors VIII and IX is a big
challenge for the treatment of patients with congenital haemophilia. Genetic
factors are involved in the formation of
antibodies (7). Drugs that have a negative influence on blood clotting, such as
NSAIDs, can lead to life-threatening bleeding in haemophilia patients (9).
Current treatments for haemophilia consist of injections with plasma-derived or
recombinant clotting factors. These do not constitute a cure and patients are
still at risk of bleeding. Significant progress has recently been made in the
development of gene therapy for hemophilia. This has been mainly due to the
technical improvements of existing vector systems and the development of new
gene delivery methods. Therapeutic and sometimes physiological levels of FVIII
and FIX could be achieved in FVIII and FIX-deficient mice and hemophilic dogs
using different types of viral vectors. However, in addition to the induction
of neutralizing antibodies; viral promoter inactivation often results. A number
of gene therapy phase I clinical trials have been started in patients suffering
from acute Hemophilia A or B. The results of extensive pre-clinical studies and
preliminary clinical data are encouraging. Successful gene therapy for
hemophilia will become a reality in future (24).

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                          Haemostasis prevents blood loss following
vascular injury. It depends on the single concert of events involving platelets
and specific blood proteins, known as coagulation factors. The clotting system
requires precise and consistent reactions to maintain the integrity of the
vasculature. Clotting insufficiency mostly occurs due to the genetically
inherited coagulation factor deficiencies such as hemophilia. Haemophilia is
congenital lifelong disorder. It results from the decreased level of clotting factor
VIII which causes hemophilia A and of
factor IX which causes hemophilia B. Haemophilia A results in joint and muscle
hemorrhage, easy bruising, and prolonged bleeding from wounds. Blood loss from
minor cuts and abrasions is not excessive. Most probably the mutation, which causes haemophilia A is an inversion
of factor VIII that accounts for nearly 45% of patients with severe hemophilia.
More than 300 mutants have been identified in case of Haemophilia B which
causes mutations at multiple sites. These include complete and partial
deletions, rare insertions, and point mutations. It has been observed that
amino acid substitutions at the site of activation lead to inactive factor IX
with decreased clotting activity. The release of the peptide activation is
necessary for optimal interaction of factor IX with its cofactors and
substrates. Hemophilia B symptoms include prolonged bleeding. Bleeding into
joints and sometimes muscles can occur, accompanied by bread and swelling.
Bleeding can be from the nose, can result from cuts, tooth extraction, surgery,
or trauma, can continue for a longer period, and can restart after stopping. It
has been found that haemophilia is caused by multiple mutations. Double and
triple mutants have been found in cases of haemophilia A and B respectively.
One of the main complications of haemophilia treatment is the formation of
inhibiting antibodies that inactivates FVIII or FIX. Genetic risk factors are
known to be of importance in the development of inhibitors, whereas the impact
of non-genetic factors is less clear.

Treatment of hemophilia
was done by injections of factor VIII and IX, prepared by recombinant DNA
technology, or sometimes prepared as concentrates from human plasma. However
the half-life of such preparations is just 12 h. Such therapy is effective in
most cases. Gene transfer holds promise as a therapeutic approach for the
treatment of haemophilia. Different viral vectors have been developed which
mediate gene transfer. AAV mediated gene transfer has been proved useful. 

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