what-when-how
In Depth Tutorials and Information
had only marginal effects on the folding rate of the colla-
gen triple helix.
66,115
Since FKBP65 does not play a major
role in the cis-trans isomerization of proline residues in
collagen, but still binds to the gelatin Sepharose (
Figure
6.3
), another function for it in relation to collagen was
expected. Recent work by Ishikawa et al
.
clearly dem-
onstrated that FKBP65 is a molecular chaperone.
94
The
chaperone activity of FKBP65 was described as compara-
ble to that of PDI in
in vitro
assays. FKBP65 also interacts
with triple helical type I collagen and stabilizes the triple
helical structure.
A first report describing the involvement of FKBP65 in
OI was published in 2010 by Alanay et al.
102
Five Turkish
families from the same region were identified and rep-
resented moderately severe phenotypes. The ERs of the
cultured patient fibroblasts were dilated compared to the
control, indicating accumulation of unfolded proteins.
Furthermore, the secretion rate of type I collagen was
delayed, similarly to what has been observed in OI cases
with mutations in the prolyl 3-hydroxylation complex
components. All affected individuals described in this
publication had an additional recessive disorder, epider-
molysis bullosa (EB) simplex. EB simplex was attributed
to the mutation in keratin.
A second group reported on six more individu-
als with simultaneous OI and Bruck syndrome which
resulted from mutations in FKBP65.
8
Bruck syndrome is
an OI-like syndrome consisting of osteopenia, recurrent
fractures, and congenital joint contractures.
116
However,
it is currently considered a separate disease. It has been
previously shown that Bruck syndrome type I is caused
by mutations in PLOD2 which codes for lysyl hydroxy-
lase 2 (LH2).
117
Kelley et al.
8
argued that inter- and
intrafamilial variability in presentation in the cases they
reported suggests that Bruck syndrome falls within the
spectrum of OI phenotypes and is not a distinct biochem-
ical disorder. No biochemical characterization of collagen
has been done in these cases.
More cases have been added which demonstrate that
mutations in
FKBP10
(gene for FKBP65) can cause both
OI and Bruck syndrome, even within one family with
the same mutation.
6,118-120
Very recently, a novel splic-
ing mutation in FKBP65 has been characterized.
103
This
mutation leads to a complete absence of FKBP65, but
type I collagen was shown to run normally on SDS gels.
Finally, Barnes et al. published a case of a deletion muta-
tion in FKBP65, where type I collagen was only slightly
overmodified with 10% excessive hydroxylysines and
hydroxyprolines in the triple helix.
4
The secretion rate of
type I collagen was normal. However, the collagen fibrils
deposited in the extracellular matrix were disorganized.
Interestingly they found a dramatic decrease in hydroxy-
lysine content in the collagen telopeptide. Consequently,
collagen crosslinks were reduced. The authors discussed
that FKBP65 might be required for proper functioning
of lysyl hydroxylase 2, which is responsible for lysine
hydroxylation in the type I collagen telopeptide
domains. This would be a similar complex as that dis-
covered with CypB and LH1.
71
They also suggested that
sparse collagen fibrils deposited into the matrix might be
a consequence of the reduced collagen crosslinks.
The combination of all currently available data about
OI causative mutations in FKBP65 does not allow for
the identification of a single molecular mechanism that
would account for the disease. Complexity of the phe-
notypes and different collagen modifications in differ-
ent cases suggest a multifunctional role for FKBP65 in
the collagen maturation process. Some mutations might
alter its chaperone activity; others might result in longer
folding times of type I collagen as was shown in the first
five individuals.
102
Other mutations might have a greater
effect on FKBP65-LH2 binding (which is still to be deter-
mined) and result in Bruck syndrome with fewer colla-
gen crosslinks as discussed.
4
CONCLUSIONS
In summary, the discovery of OI causative genes,
which are involved in collagen folding and post-trans-
lational modifications, has opened a new avenue in
the understanding of the phenotypes and pathologi-
cal mechanisms leading to these phenotypes. These are
not simple mechanisms and currently some data seem
to contradict one another. However, our understand-
ing of collagen folding and its effect in human diseases
will likely improve from a combination of biochemical
and clinical data. There are certainly new genes involved
in collagen folding and processing awaiting discovery
as novel OI genes. Some of them are already discov-
ered.
120-123
Other likely candidates, two unknown pro-
teins eluted from the gelatin Sepharose (
Figure 6.3
), are
yet to be identified. The identification of new cogs in the
collagen folding machinery will lay the base for a better
understanding of how this delicate machine works.
References
