what-when-how
In Depth Tutorials and Information
ADVANCES IN COLLAGEN
PROTEIN BIOCHEMISTRY LED TO
IDENTIFICATION OF THE FIRST OI
MUTATIONS
Enormous advances in our understanding of colla-
gen protein structure, biosynthesis and assembly were
made during the 1960s and 1970s (for detailed reviews
see
11-13
). By the end of the 1970s, four structurally and
genetically distinct collagen types were known, types
I-IV, and all the major steps in collagen biosynthesis
had been identified. Collagen type I, the main collagen
in skin, tendon, ligament and bone, had been studied
most intensively and following reports of altered type
I to type III collagen ratios in OI skin,
14
and synthesis of
type I collagen by OI fibroblasts that was more sensitive
than usual to pepsin digestion,
15
was already the prime
OI candidate.
That collagen type I chains were synthesized as pre-
cursors with both N- and C-terminal non-collagenous
propeptides
16
was long anticipated and the propeptides
were proposed to be important for chain association
and alignment. Detailed studies on collagen type III
confirmed that procollagen chains associated via their
C-propeptides and the helix folded at a uniform rate
in a zipper-like manner from the C-terminus.
17,18
These
experiments also revealed that the rate of cis-trans
isomerization of peptide bonds determined the rate of
triple helix folding,
17,19
a critical observation that would
later inform our understanding of recessive forms of OI.
The collagen triple helix has a characteristic molecu-
lar formula, the Gly-X-Y triplet repeat, which was deter-
mined by amino acid analyses of collagens purified from
various tissues, and later amino acid sequencing. Glycine
in every third position is crucial for triple helix confor-
mation; it is the smallest amino acid and is found in the
center of the helix where the three chains come together.
Most, but not all, proline residues in the Y position are
4-hydroxylated. Evidence had been mounting that prolyl
hydroxylation occurred co-translationally on free chains
and that triple helix conformation prevented hydroxy-
lation and this was confirmed in 1973.
20
Why collagen
had so much hydroxyproline, an amino acid found in
only a few other proteins, was debated for many years
until it was shown to be essential for thermal stability of
the triple helix and required for efficient collagen secre-
tion.
21,22
Collagens contain another unusual amino acid,
hydroxylysine, albeit in smaller quantities than hydroxy-
proline. Hydroxylysine is also found in the Y position of
Gly-X-Y triplets; O-linked galactose then glucose carbo-
hydrate groups are attached to hydroxylysine residues
and certain hydroxylysines are essential for intermolecu-
lar covalent crosslinks.
In 1962, 3-hydroxyproline was discovered in colla-
gen type I,
23
and we now know that α1(I) chains have a
FIGURE 1.1
Frontispiece O.J. Ekman Doctoral Thesis.
(From
Seedorf K. Osteogenesis imperfecta: a study of clinical features and heredity
based on 55 Danish families comprising 180 affected members. Copenhagen:
Universitetsforlaget I Arhus; 1949.)
of mutations in different genes might therefore be
reflected in a different prevalence of various types of
OI in separate human populations. As early as 1993,
Wallis, Beighton and colleagues had concluded that
autosomal recessive forms of OI were the most preva-
lent forms of OI in Southern Africa and segregation of
OI in these families was not linked to type I collagen
genes.
8
In 1975, such was the state of knowledge of matrix
biology including collagen (type I) biochemistry that
very little light was shed on this subject of genetic
heterogeneity, but the knowledge base was rapidly
changing. After 30 years of intense study of collagen
(1952-1982), which in retrospect was collagen type I,
and recognition that there were other fibrillar collagens
in cartilage and blood vessels distinguishable at the
biochemical, biophysical and ultrastructural level,
9,10
the conditions were present by 1975 for a major leap
forward in discovery of the molecular pathology of OI.
Progress in the molecular biology of the gene and tran-
scription and translation of collagen proteins proceeded
in parallel with the advances in clinical and genetic
delineation of OI.
