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which altered the 12 terminal amino acids of the normal
α1(I) C-propeptide, deleting a cysteine residue involved
in intrachain disulfide bonding and extending the chain
by 84 amino acids. Biochemical analysis of type I collagen
produced by skin fibroblasts from an affected individual
showed decreased production of type I collagen with-
out signs of post-translational overmodification or intra-
cellular retention. The mutant chains failed to associate
with other chains and, although the mRNA was readily
detectable and the protein could be synthesized by cell-
free translation, the abnormal chain could not be detected
within the cell. Presumably the product was very rapidly
degraded within the rough endoplasmic reticulum.
60
Subsequently a number of other mutations altering
the structure of the pro-α1(I) C-propeptide were reported
in association with OI phenotypes of varying severity.
These included two amino acid substitutions [p.
(Asp1277His), p.(Leu1388Arg)] and a deletion of two
amino acids [p.(Glu1337_Tyr1338del)] in three infants with
perinatal lethal OI;
45
a missense mutation resulting in the
substitution of the highly conserved most C-terminal leu-
cine residue by proline [p.(Leu1464Pro)] in a child with
OI type III, with a history of at least 200 bone fractures
before the age of 4 years, scoliosis, and severe pectus cari-
natum,
61
and a missense mutation that substitutes a cys-
teine residue, involved in intrachain disulfide bonding, by
tryptophan [p.(Cys1299Trp) ] in a family with mild OI (OI
type I).
71
Biochemical studies on skin fibroblasts of these
patients showed evidence of slowed formation of procol-
lagen molecules, which were extensively overmodified
by post-translational modifying enzymes and which were
poorly secreted, yet which displayed normal thermal sta-
bility. Unlike helical defects, these pro-α1(I) C-propeptide
alterations impede the initial oligomerization event, and
allow modifying enzymes access to the entire helical
domain for a prolonged period of time, resulting in uni-
form overmodification. Once chain association does occur,
helix formation, however, proceeds without interruption,
hence thermal stability of the trimer is normal.
In 2002 Pace et al. reported a p.(Asp1441Tyr) substitu-
tion in an infant with a perinatal lethal OI phenotype that
differed from the usual OI type II phenotype in the pres-
ence of regions of increased bone density. Biochemical
findings on skin fibroblasts from the affected proband
were comparable with those observed with the previ-
ously reported missense mutations, with evidence for
slow assembly of overmodified procollagen chains and
intracellular retention of these chains, yet with normal
thermal stability.
57
Recently two other heterozygous
C-propeptide mutations in
COL1A1
, p.(Ala1387Val)
and p.(Thr1416Argfs*11), were reported in association
with perinatal lethal OI and features of dense bone dis-
ease.
53
Pace et al. claimed that the combined phenotype
of OI and dense bone disease reflects both a diminished
amount of secreted procollagen type I molecules and
the presence of a population of stable and overmodified
molecules that might support increased mineralization
or interfere with degradation of bone during remodel-
ing. However, this alone cannot explain the observed
phenotype, in light of the fact that other C-propeptide
defects lead to similar biochemical findings, yet lack
the dense bone regions. Pace et al. hypothesize that the
C-propeptide of type I procollagen, in addition to its func-
tion as mediator of collagen assembly, acts as a signaling
molecule in the ECM, and that structural defects hamper
this activity and contribute as such to the pathogenesis
of OI. Studies showing that the C-propeptide suppresses
collagen synthesis of fibroblasts and of osteoblastic cells
at the early stage of differentiation C-propeptide, and that
it is a physiological modulator of TGF-β in bone metabo-
lism, corroborate this hypothesis.
72-74
Table 13.1
gives an overview of hitherto reported
COL1A1
mutations that affect the pro-α1(I) C-propeptide
domain, and their associated phenotype. The majority of
these lead to either mild or lethal OI phenotypes.
Defects in the Pro-
α
2(I) Collagen C-Propeptide
Domain
The earliest report of a pro-α2(I) C-propeptide muta-
tion dates from 1984 and involves a
homozygous COL1A2
frameshift mutation in exon 52 (c.4001-4004del) in a
patient with OI type III, characterized by severe defor-
mation of the limbs, osteoporosis, multiple fractures
and short stature. Both parents were heterozygous for
the mutation, yet they were phenotypically normal.
Biochemical analysis of type I collagen, produced by the
patient's skin fibroblasts, revealed complete absence of the
α2(I) collagen chains with the formation of α1(I) homotri-
mers which were slightly overmodified. The mutant
COL1A2
mRNA was, however, shown to be stable and
to result in synthesis of mutant pro-α2(I) chains, in which
the terminal 33 amino acids of the C-propeptide were
altered. This new amino acid sequence had an over-
all different charge and lacked the last cysteine residue
involved in intrachain disulfide bonding. Instead it gen-
erated a new cysteine residue, 25 amino acids upstream
of the normal cysteine, which interfered with normal
folding of the α2(I) C-propeptide and eliminated the abil-
ity of the chains to associate with pro-α1(I) chains. These
mutant chains were presumably very rapidly degraded.
As a consequence only pro-α1(I) trimers were formed,
resulting in a moderately severe form of OI.
67,75
Similar
molecular and biochemical findings were observed in
two naturally occurring animal models for OI, display-
ing both features compatible with human OI type III. The
oim/oim mouse harbors a homozygous
col1a2
c.3983delG
mutation, leading to a frameshift which alters the last 48
amino acids of the pro-α2(I) C-propeptide,
76
whereas a
canine model (beagle) was shown to harbor a frameshift
