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collagen protein,
30
which is usually visible on biochemi-
cal analysis as a reduced intensity of the bands represent-
ing type I collagen produced by skin fibroblasts of the
affected patient. Types II-IV OI are usually caused by
defects in the type I collagen structure, most commonly
glycine substitutions (80%) in the triple-helical domain
of either the pro-α1(I) or the pro-α2(I) collagen chain or,
less frequently, splice site mutations (20%).
31,32
Glycine
substitutions delay helix propagation, which permits
prolonged access of modifying enzymes to the chain's
N-terminal to the alteration, whereas the sequence altera-
tions themselves alter the helical structure
33,34
and reduce
molecular thermal stability.
35,36
Mutations in the C-propeptide regions of both type I
procollagen α chains are infrequent, and account for only
~4% of autosomal dominant OI, but the phenotypic out-
come of these mutations reflects the critical role of this
region in directing chain assembly, trimerization and
the initiation of helix formation. In other fibrillar procol-
lagens (types II, III and V procollagen), mutations in the
C-propeptide domains also account for a small minority
of mutations, but are associated with severe, often lethal
disorders.
8
Hitherto 61 mutations in the pro-α1 and pro-α2
C-propeptides of type I procollagen have been reported in
the Osteogenesis Imperfecta Variant Database (Dalgleish
with phenotypes ranging from mild to lethal OI. We add 13
pro-α1(I) and five pro-α2(I) C-propeptide mutations (per-
sonal observation) (
Tables 13.1 and 13.2 and Figure 13.1
).
then propagated vectorially in a C- to N-terminal direc-
tion,
18,19
and until the chains become inaccessible, the
helical portions of the collagen chains undergo extensive
post-translational modifications, such as lysyl hydroxyl-
ation and hydroxylysyl glycosylation.
Assembled procollagen trimers are translocated from
the rough endoplasmic reticulum (ER) to the Golgi appa-
ratus, further modified, packaged and secreted into
the extracellular spaces. During or following secretion
of procollagen molecules into the extracellular matrix
(ECM), propeptides are removed by procollagen N- and
C-proteinases, thereby triggering spontaneous self-assem-
bly of collagen molecules into fibrils.20,21
20,21
The procollagen
type I N-propeptide is removed by ADAMTS-2,
22
and
mutations in the gene encoding this enzyme result in the
Ehlers-Danlos syndrome dermatosparaxis type (EDS type
VIIC).
23
Processing of the procollagen type I C-terminal
propeptide is accomplished by a number of tolloid-
like proteinases, including bone morphogenic protein 1
(BMP1), which is the major procollagen C-proteinase,
24-26
mammalian tolloid (mTLD; an alternatively spliced form
of
BMP1
), and two closely related homologs, known as
mammalian tolloid-like 1 and 2 (mTll1 and 2).
In concert with enhancer proteins (PCPE1 and 2), PCP
cleavage of the C-propeptides of types I-III procollagen by
BMP1 is increased up to ten-fold.
24,27
The cleavage reac-
tion occurs at the Ala-Asp [pro-α1(I)_p.Ala1218Asp1219
and pro-α2(I)_p.Ala1119Asp1120] bonds that separate
the C-telopeptide from the C-propeptide. Besides its
intracellular function in molecular trimerization, the
C-propeptide plays a crucial role in collagen fibril forma-
tion.
20,28
As long as the C-propeptide domain remains
attached to the triple helical domain of the molecule, sol-
ubility of the procollagen molecule remains high.
3
Thus
C-propeptide cleavage is the rate-limiting step in collagen
fibril assembly. Once cleaved, collagen trimers are further
assembled into fibrils, which serve as the main source of
mechanical strength in connective tissue and the template
for matrix deposition and mineralization in the bone.
Defects in the Pro-
α
1(I) Collagen C-Propeptide
Domain
The first mutation in the C-propeptide domain of the
proα1(I) collagen chain, reported in 1989 by Bateman
and coworkers, is a single basepair insertion in exon 49
of the
COL1A1
gene in a baby with perinatal lethal OI.
This mutation was shown to result in the production of
a truncated protein with an altered amino acid sequence
from Val
1146
to the C-terminus of the pro-α1(I) chain. The
mutant protein is 37 residues shorter, more basic, lacks
Asn
1187
, which normally carries N-linked oligosaccha-
rides, and has an altered distribution of cysteine residues
when compared to the normal sequence. Although the
mutant pro-α1(I) chains were not able to form normal
interchain disulfide bonds, some of the mutant chains
were shown to be incorporated into the procollagen
type I heterotrimers, which resulted in abnormal triple-
helix formation, severe overmodification, intracellular
retention and increased degradation of these type I pro-
collagen molecules. Only normal type I collagen was
incorporated into the ECM
in vivo
, resulting in a tissue
type I collagen content of only 20% of that of control.
70
In 1990 Willing et al. reported a heterozygous
COL1A1
5 bp deletion in a three-generation family with OI type I,
MUTATIONS THAT LEAD TO
ALTERATIONS IN THE TYPE I
PROCO
LLAGEN C-PROPEPTIDE D
OMAIN
Mutations in the
COL1A1
and
COL1A2
genes gen-
erally cause autosomal dominant osteogenesis imper-
fecta (OI). Mild OI (OI type I) usually results from a
heterozygous mutation in the helix-encoding region of
the
COL1A1
gene that introduces a premature termina-
tion codon (PTC), causing the mutant mRNA to be rap-
idly degraded by a cellular surveillance mechanism,
termed “nonsense-mediated decay” (NMD).
29
This pro-
cess reduces mutant, unstable mRNA transcripts, finally
leading to the synthesis of half the amount of normal
