Haematologica 2000;85:supplement to no. 10 - Supplements ...

8
J. Oldenburg et al.
Tuddenham et al., 7 a clear correlation between
type of FVIII gene mutation and inhibitor prevalence
could be demonstrated (Table 1). Two
groups of mutations, the missense mutations
and the small deletions, showed a low inhibitor
prevalence of about 5%. In contrast, the other
mutation types, the prevalent intron 22 inversions,
the large deletions and the nonsense
mutations, exhibited a 7-10 fold higher inhibitor
prevalence of about 35%. The data of the Bonn
patients were in very good agreement with those
of the patients listed in the mutation database.
The observation that in non-severe hemophiliacs
with a low inhibitor prevalence almost all of
the patients had missense mutations corresponded
well with these findings.
Concerning the pathomechanism it is conceivable
that patients with missense mutations
have some – however non-functional – endogenous
FVIII protein, which is sufficient to induce
immune tolerance to substituted FVIII. In contrast,
no endogenous FVIII is synthesized in
patients with the more severe molecular defects
such as intron 22 inversions, large deletions and
nonsense mutations. Thus, substituted FVIII represents
a foreign protein, leading to the immune
response of FVIII antibodies.
A mutation type profile for patients with severe
hemophilia A was published by Becker et al. in
1996 8 (Table 2). Intron 22 inversions account for
37.4% of mutations in severe hemophilia A, nonsense
mutations for 13.6%, missense mutations
for 18.4%, small deletions for 9.5%, large deletions
for 5.4% and insertions for 1.4%. In 11.6%
of the patients the mutation could not been identified.
These data show that in severe hemophilia
A about two third of the patients have high risk
mutations and one third of the patients low risk
mutations for inhibitor formation.
Since 1995 more information about the various
mutation types and the risk of inhibitor formation
has been gathered resulting in at least
ten groups of mutation types with different characteristics
concerning the risk of inhibitor formation.
An overview of these groups of mutations
is shown in Figure 1 and described in more
detail in the following sections of text.
Different inhibitor risks associated with
the various types of null mutations
Interestingly, the risk of inhibitor formation is
not homogenous within the types of nonsense
mutations and large deletions. Data taken from
the HAMSTeRS mutation register (http://europium.csc.mrc.ac.uk)
9 showed that (i) nonsense mutations
affecting the light chain of the FVIII molecule
were more frequently associated with
inhibitors (29 inhibitors at nine different codons)
than nonsense mutations affecting the heavy
chain (3 inhibitors at 3 different codons) and
that (ii) patients with large deletions affecting
more than one domain of the FVIII molecule have
a higher risk of developing an inhibitor than those
in whom a single domain is affected (74%
inhibitors in multi domain deletions vs. 21%
inhibitors in single domain deletions). The
inhibitor risk associated with the intron 22 inversion
may have been overestimated in the first
studies 3 because patients with an inhibitor may
have been tested for the causative mutation with
higher priority. Antonarakis et al. 10 reported a
lower inhibitor prevalence in patients with an
intron 22 inversion. Nevertheless, this mutation
type is the most prevalent in severe hemophilia A
and especially in the subgroup of patients who
developed an inhibitor.
The reasons for the different inhibitor risks
associated with the various types of null mutations
are still not understood, however they raise
two important questions i) Why do not all
patients with a single type of null mutation
develop an inhibitor and ii) Why do patients
with different types of null mutations have different
risks for inhibitor development The first
question likely addresses other factors than the
nature of the mutation while the second question
must be related to the nature of the genetic
defect in the FVIII gene.
Inhibitor prevalence in patients with small deletion/insertion
mutations
The inhibitor prevalence found in patients with
small deletions was unexpectedly low, as most of
the deletions led to a frame shift with a stop
codon. However, in contrast to that associated
with nonsense mutations, the inhibitor prevalence
was much lower. The explanation of this
phenomenon was given recently in a paper by
Young et al. 11 and a paper by our group. 12
Young et al. 11 described a mildly affected
hemophiliac with a T-deletion within an A8TA2-
sequence at codons 1439-1441 of exon 14, that
led to a run of 10 adenines (A10). Investigating
the mRNA transcripts of this patient he found
not only the expected transcript of 10 adenines
but also small proportions of transcripts of 7, 8,
9 and 11 adenines. The few mRNA transcripts of
8 and 11 adenines restored the reading frame
and led to the synthesis of some residual
endogenous FVIII protein. This FVIII protein was
fully active, probably because the mutation was
located in a functionally non-decisive region of
the B-domain. The pathomechanism behind the
variable length of the mRNA transcripts is a slippage
of the DNA/RNA-polymerases within the
adenine run during DNA replication and RNA
transcription.
Encouraged by the study of Young et al. 11 we
Haematologica vol. 85(supplement to n. 10):October 2000