what-when-how
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FIGURE 57.1
Molecular strategy for site-directed homologous recombination for gene correction. (A) Using an engineered amino acid
sequence that specifically binds a DNA sequence, a DNA nuclease activity induces a double-stranded break which draws host repair enzymes
to the cleavage site. When the nucleus is flooded with a fragment that has flanking sequences homologous to the sequences near the cleavage
site, the chance for site-specific insertion of the fragment is greatly enhanced. (B) Detail of the introduced fragment with its homologous ends
will insert the correct fragment (blue) into the host DNA. (C) The genomic organization of the OI mutation in one of the collagen alleles produc-
ing mutant RNA and collagen chains. (D) Correction of the OI mutation leads to normal collagen chains from both alleles.
insufficient disorder, gene correction would be preferred
because it restores the activity to normal and the activity
will be appropriately regulated.
In summary, while the molecular engineering aspect
of a gene therapy for OI has become available for investi-
gators to develop, all of the studies have been performed
in cells derived from affected individuals in a cell culture
dish. The challenge now is how to return the corrected
cells to the patient in a therapeutically meaningful way.
a sporadic new mutation in a germ cell of a parent. With
the mutation arising after the embryo has begun develop-
ment, only a portion of the cells that will form the com-
pleted individual will carry the mutation.
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If a child of
the mosaic parent is conceived from one of the mutation-
bearing germ cells, then the child will have OI because
all of the cells that formed the child carry the mutation.
However, the parent is phenotypically normal because
only a proportion of their tissues harbors the mutation.
The absence of skeletal findings in the subject that is
mosaic for an OI mutation suggests that the presence of
normal cells in the context of OI cells can suppress the
effect of the OI mutation. While the cellular basis for
this observation has not been directly tested, the most
likely explanation is the selective advantage of the nor-
mal osteoblasts relative to the OI-bearing osteoblast that
are inefficient at type I collagen secretion and matrix
production.
In vitro
, OI cells proliferate less frequently
RATIONALE AND CHALLENGES FOR A
CE
LL-BASED THERAPY FOR
OI
The possibility that a cell-based therapy for OI could be
beneficial came from the clinical observation that approxi-
mately 5-10% of new cases of type II and III OI are due to
somatic mosaicism in one of the parents,
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as opposed to
