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mouse and the neomycin cassette removed, the homo-
zygous mouse (
G610C/G610C
Neo−
) appeared to have a
lethal phenotype. Heterozygote +/
G610C
Neo−
breeding
pairs produced 1/3 wild-type and 2/3 +/
G610C
Neo−
mice
offspring consistent with Mendelian recessive lethal-
ity. One proposed explanation for the viability of
G610C/
G610C
Neo+
mice is the unique production of homotri-
meric type I collagen, α1(I)
3,
in addition to mutant G610C
collagen [α1(I)
2
α2
G610C
(I)], which was not present in wild-
type or
+/G610C
Neo−
mice. The composition of the type I
collagen in the viable
G610C/G610C
Neo+
mouse tail ten-
don is 67% α1(I)
3
, suggesting the aberrant production
of α1(I)
3
provided a protective or compensatory affect.
Disulfide-bonded α2
G610C
(I) containing type I collagen
molecules incorporates at a low frequency (approxi-
mately 1% of
+/G610C
Neo−
and 9% of
G610C/G610C
Neo+
collagen) into mouse tail tendon.
97
The abnormal disul-
fide bonds disrupt normal fibril formation.
+/G610C
Neo−
mice genotypically and phenotypically
model the human +/
G610C
OI population.
+/G610C
Neo−
mice, like the Amish G610C OI population, exhibit phe-
notypic variation despite carrying an identical mutation.
This was demonstrated by breeding the mutation onto
four strains of mice (A/J, BALB/cByJ, C3H/HeJ or FVB/
NJ).
97
Consistent with the human population, all genetic
+/G610C
Neo−
strains exhibited reduced body mass, BMC,
BMD, collagen content, bone volume and bone strength
with hypermineralization.
97
In addition, this model
allows for more direct translation from murine to human
as fibroblasts are available for study from both human
G610C patients and the mice.
but provides a system for investigating collagen process-
ing, ER to Golgi trafficking and ER UPR. Weaknesses of
this model include embryonic lethality of homozygote
mice, the question of potentially unidentified mutations
generated by the ENU strategy which may account or
contribute to the clinical outcome, and the absence of an
equivalent mutation in human OI patients.
Canine Models: G208A
COL1A1
and
COL1A2
Frameshift
Two independent naturally occurring autosomal dom-
inant OI canine models have been described.
99-101
The
G208A
COL1A1
mutation (glycine-to-alanine substitution
at position 208) was identified in a 12-week-old golden
retriever puppy with dentiogenesis imperfecta, small
stature and multiple rib and long bone fractures.
99,101
The
COL1A2
frameshift mutation was identified in a 10-week-
old beagle reported to have multiple fractures in “almost
every long bone.”
100
The frameshift results from a unique
4-bp deletion with a 9-bp insertion in
COL1A2
gene in
the carboxy-terminal end coding region.
100
Both of these
canine models are representative of severe human OI
type III. Dogs as large animal models promise to better
reflect the mechanoforces and pathophysiology of human
OI than mice; however, they are more expensive to main-
tain and dog breeder's interests are typically to eradicate
lethal or debilitating alleles.
PROLYL HYDROXYLATION (CRTAP/P3H1/
CYPB COMPLEX)
Aga2
Mouse (
Col1a1
)
The
Aga2
(abnormal gait 2) mouse generated by the
Munich ENU (N-ethyl-N-nitrosourea) mutagenesis strat-
egy was originally identified by its abnormal gait result-
ing from hindlimb bone deformities.
98
The heterozygous
Aga2
/+
mice are smaller in size and range clinically from
severe OI with bone deformities and frequent fractures to
perinatal lethal.
98
Homozygous
Aga2
−/−
mice are embry-
onic lethal with gestational arrest occurring by 9.5 days
postcoitus. The mutation in the
Aga2
mouse is a splice-
site mutation within intron 50 of the
Col1a1
gene.
98
The
resultant transcripts have 16 extra bases, generating a
frameshift, which results in 90 additional amino acids,
including five new cysteine residues, past the normal ter-
mination signal.
98
This disrupts the chain recognition sig-
nal and abnormal pro-α1(I) collagen chains accumulate
intracellularly leading to ER stress and osteoblast apopto-
sis via unfolded protein response (UPR), increased bone
turnover and altered collagen matrix.
98
Aspects of this
model are similar to the
oim
mouse whose mutation in the
Col1a2
gene also disrupts carboxy-terminal chain recog-
nition.
45,98
The
Aga2
mouse represents a rare cause of OI,
Mutations in the genes encoding the collagen prolyl
3-hydroxylation complex [cartilage associated protein
(CRTAP), prolyl 3-hydroxylase 1 (P3H1) and cyclophilin
B (CypB)] can cause severe, recessive forms of OI.
3,5
P3H1 and CRTAP form stable complexes with CypB, and
P3H1 then catalyzes the 3-hydroxylation of specific pro-
line residues of types I, II and V collagen and may act as
a general chaperone.
102,103
CRTAP
−/−
Mouse
The CRTAP knockout (
CRTAP
−/−
) mouse was gener-
ated by classical targeting inactivation strategy using
homologous recombination.
104,105
CRTAP
−/−
mice develop
severe progressive kyphoscoliosis as well as shorten-
ing of the proximal limb bones (rhizomelia), generalized
osteopenia and mild cartilage dysplasia. The
CRTAP
−/−
mouse model is analogous to loss-of-function mutation in
human patients with CRTAP deficiency, resulting in the
loss of 3-hydroxylation of the P986 residue in α1(I) and
α1(II) collagen chains.
102,105
OI patients with 10% CRTAP
activity have recessive rhizomelic form of OI type VII.
