Transcript LehmanSpr11

Dominant Characteristics of Osteogenesis
Imperfecta:
Brent Lehman  Molecular Biology
Classical approaches of treatment
Abstract
Osteogenesis imperfecta (OI) has been characterized as both an autosomal dominant and recessive genetic disease. OI is
caused by abnormalities in collagen I fibers. In the dominant form of the disease, two genes COL1A1 or COL1A2 can be
mutated to form the faulty collagen I fibers. Individuals with these mutations generally experience mild forms of OI. 90% of
patients who exhibit the OI phenotype have the dominant form of the disease 7.
An orthopedic treatment involves placing a rod along the medulla of the bone5. This helps in support and also minimizes
fractures. (Figure 6). A Pharmacological approach would be threatment by bisphosphates. Bisphosphates increase bone
density while reducing the number of fractures (Figure 7)6.
Background
OI is an autosomal dominant negative disorder that varies from mild to prenatal lethal symptoms. OI has an incidence of
approximately 1/10,000 and can be separated into 4 main types (Figure 1)1,4. Common phenotypes include osteoporosis,
blue sclera, skeletal deformities, brittle bones, and hearing loss. These phenotypes are a result of an alteration in the type I
collagen fiber, which make up about 90% of bone tissue4.
Figure 7. Treatment of patients with pamidronate, a bisphosphate. At both time points, 45
patients were tested. The p-value indicates the significance of the two scored6.
Figure 6. X-ray of patient with type III OI.
Intramedullary rod is visible in the right
femur5.
Cell and gene therapy
Figure 1. Classification and characteristics of four distinct types of OI5.
In cell therapy, mesecnhymal stem cells, are implanted that would not
contain the proα1 or proα2 mutations and would produce normal collagen
fibers2. Also, because the implemented cells are stem cells and will selfrenew and will provide treatment for life5. Patients who received this
treatment saw an increase in bone mineral content (Figure 8))2.
Introduction these cells can be measured by GFP (Figure 9).
Discovery
In 1983, a RFLP was discovered for one of the proteins that make up the collagen fiber,
COL1A2 (Figure 2)9. In 1986, a genetic marker was identified for COL1A1 and no other
markers suggesting only COL1A1 and COL1A2 are the genes that are responsible for OI.
Subsequent pedigree analysis have also yielded similar results. COL1A1 and COL1A2 have
been marked at 17q 21.31-22.15 and 7q 21.3-22.1 respectively8.
Gene therapy is more difficult to use as an effective mean for curing OI
because of the repetitive nature of the Gly-X-Y peptide. However, in
heterozygote transgenic mice, the lethality of type II OI dropped from 92
to 27% 5 . Ribozymes have also been used to recognize specific cleavage
sites in abnormal mRNA5.
Figure 8. Changes in total body bone mineral content (TBBMC) after bone
marrow transplantation (BMT) of patients with type III OI. Arrows indicate time of
BMT. (A) measurements made by x-ray absorptiometry to determine TBBMC (g)
against time. (B) TBBMC (g) as measure of total body weight (kg). The shaded
portion represents normal range of healthy patients of identical weight2.
The overall strategy of these methods illustrated in figure 10.
Figure 2. RFLP detected with the probe for the
5'-half of the gene (NJ-3' and NJ-3v) after
digestion of genomic DNA with EcoRI. (A)
Parent who is homozygous for absence of the
polymorphic site (-/-); (B) parent who is
homozygous for the site (+/+); (C) offspring
who is heterozygous for the site (+/-)9.
Makeup of collagen I fiber
COL1A1 and COL1A2 code for proα1 and proα2 respectively, which will combine to form the collagen fiber. In the collagen fiber there
are two α1 chains and one α2 chain. Each fiber forms a helical structure and then get twisted around the other two fibers in a triple helix
(Figure 3). There is a three amino acid repeat of Gly-X-Y that makes up the helix4. The peptide sequences are then further modified to
form cross bridges (Figure 4). The vast majority of mutations that lead to OI are in this Gly-X-Y region of the protein where glycine is
substituted for a bulkier amino acid (Figure 5)3,4.
Figure 10. Schematic diagram showing approaches for the treatment of OI
Figure 9. Distribution of the GFP-positive cells in different tissues of mice infused with GFP- dominant-negative mutations using gene therapy or cell therapy.5
positive cells at neonatal stage. (a) Mouse bone marrow stromal cells transduced with DFGe-GFP retrovirus and selected in a medium containing zeocin. (b) Whole-body image of a
neonatal mouse 5 days after cell infusion of GFP-positive cells via the superficial temporal
vein. The GFP expression is seen in a larger number of the neonatal mouse tissues
including the mouse paws. (c) GFP-positive cells in pieces of lung tissue in culture; the lung
was harvested from a neonatal mouse at 7days after infusion of the GFPpositive stromal
cells. (d) GFP-positive cells in fragments of a femur harvested from a recipient mouse and
placed in culture; the femur was harvested from a mouse that was infused with the e-GFPpositive cells at neonatal stage, and harvested at 25 days after cell infusion. Large numbers
of GFP-positive cells are evident in the bone fragment.
Literature Cited
Figure 3. The Gly-X-Y repeat of a
collagen fiber. Figure taken from
NCBI database.
Figure 4. Schematic diagram showing steps
in biosynthesis and assembly usually of type
I collagen into fibrils. Type I collagen is
composed of two polypeptide chains namely
proa1(I) and proa 2(I). Each chain contains a
helical domain with a repeating triplet of
GLY–X–Y sequence, where X is most often
proline and Y is usually hydroxyproline.5
Figure 5. Nucleotide sequence of 730 bp of normal proa2(I) collagen gene
extending from last 318 bp of intervening sequence 5 to first 358 bp of
intervening sequence 6. Horizontal arrows denote primers used to amplify
genomic DNA to obtain sequences from both alleles; vertical arrowhead
denotes site of mutation; HindIII denotes cleavage site for restriction
endonuclease HindIII (5' cloning site). Bottom, Nucleotide sequence
of genomic DNA clones of normal and mutant allele from proband. The
sequences are in the antisense orientation. The arrow and asterisk indicate
the G-to-A mutation of the first nucleotide of intervening sequence 6.10
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