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TABLE 57.1
Applying the Appropriate Gene Correction
Strategy for the Specific Type of OI
the mutant allele is able to produce at least some normal
collagen chains. The extent of these modifiers probably
explains the clinical heterogeneity of type I OI
29
and
needs to be considered in the design of a gene therapy
approach to type I OI.
The best understood recessive forms of OI, types VII,
VIII and IX, arise from mutations in non-collagen genes
that participate in forming the molecular complex that
guides the nascent collagen chain through the intracel-
lular steps of chain assembly and secretion
17-19
or serve
as molecular chaperones.
20
While it may be possible to
develop pharmacologic mimetics of the missing activity,
even a partial increase of the activity by gene replace-
ment within the osteogenic lineage would have a major
impact on the ability of the collagen-producing cell to
function normally without having the potential off-
target effects of drug-based strategy.
Even more diverse molecular defects in non-collagen
genes are being uncovered by exome sequencing in asso-
ciation with an OI phenotype, reflecting the complex
biological controls required for normal skeletal growth,
modeling and repair. Potentially a number of these dis-
orders may be approached by pharmacological strategies
that impact the process of matrix mineralization (type V),
while others may require intervention at the genetic
level utilizing the same rationale of either mutant gene
inactivation (
IFITM5
30
) or gene replacement (
WNT1
or
TMEM38B
). A better understanding of the molecular
and physiological processes that are impacted by the
mutations and the cell types where the genes have a sig-
nificant function should provide direction for the most
promising strategy for therapeutic intervention.
OI
Types
Molecular
Basis
Corrective Steps Within the
Affected Osteoblast
Inh.
Type I OI
Null type I
collagen allele
D
Haploid insufficiency of type I
collagen is responsible for the
phenotype. Restore regulated
activity of the allele by gene
correction or enhance activity of the
normal allele.
Type II,
III and
IV
Glycine
substitution
and exon skip
D
Inactivate the mutant allele
required,
13
but the step induces
haploid insufficiency; a second step
to enhance activity of the normal
allele would be needed. Thus gene
correction is the most direct strategy.
Function of
IFITM5
gene
14,15
is not
well understood; at least inactivation
of mutant gene would be required.
Type V
Mineralization
D
Type VI
Uncertain
R
SERPINF1
encodes an extracellular
factor that circulates.
16
Possible role
for protein replacement therapy.
Type VII
and VIII
Post-
translational
modification
R
Disrupted post-translational
processing affecting 3-hydroxylation
and chain assembly.
17,18
Introduction
of some level of normal activity of the
deficient enzyme or cofactor function
within osteoblasts will be required.
Type IX
Intracellular
trafficking
R
Chaperone proteins needed for
guiding collagen molecules from the
Golgi to secretion.
19,20
Osteoblast-
directed replacement would be
required.
Not yet
classified
Differentiation
R
Oster
,
Wnt1
21
and
TMEM38B
.
22,23
Gene correction within the early
osteoblast lineage would be needed
to maintain proper regulation.
MOLECULAR APPROACHES AND SAFETY
CONSI
DERATIONS FOR GENE TH
ERAPY
Dramatic advances in the molecular tools available for
gene therapy have been achieved in the past 5 years that
provide a wide variety of options to alter the impact of a
mutant gene on cell function. As with any intervention,
the potential benefits of a particular approach have to be
weighed against unintended off-target or safety concerns
that could arise from a particular strategy, as will be dis-
cussed below.
misfolded collagen chains may improve the ability of
OI osteoblasts to function,
26
ultimately the activity from
the mutant collagen gene must be eliminated.
Type I OI, although also inherited as a Mendelian
dominant trait, most commonly results from a muta-
tion that severely reduces the output of a type I colla-
gen allele such as a premature stop codon or an error
in splicing that results in a similar outcome.
27,28
Unlike
most null mutations which are asymptomatic in the
heterozygous state because sufficient activity for nor-
mal function can be provided by the remaining normal
allele, both collagen alleles are necessary during times of
rapid skeletal growth or high bone turnover. This type
of dominant mutation is classified as haploid insuf-
ficiency and the impact of the marginal level of type I
collagen that can be produced from a single functional
allele is probably significantly affected by the genetic
heterogeneity of the individual as well as the extent that
Augment Gene Activity
In many recessively inherited disorders, even a mod-
est increase in activity (25% of normal) can result in a sig-
nificant improvement in biological function. If the missing
activity acts outside of the cell that produces it, providing
the factor as a protein product that acts in the circulation
(clotting factor
31
), cell surface (alkaline phosphatase
32,33
) or
is internalized (mucopolysaccharidosis
34,35
) is an effective,
