Gene Therapy for the Enhancement of Fracture Healing - Engineered Bone: Tissue Engineering Intelligence Unit - page 127

Gene therapy vectors for bone fracture healing that have shown promise are retroviral,
adenoviral and adeno-associated viral vectors. These vectors will be discussed below. Lentivirus
might show promise for the development as a vector in gene therapy. However, this is still not
yet ready to be used in a clinical setting. 7,8
Viral Gene Delivery
Retroviral Vectors
Retroviruses are viral vectors that have been used most extensively for gene transfer. 4,9-14
These were the first viral vectors used in human clinical trials. 15 In the year 2000, it was re-
ported that 40% of clinical gene therapy protocols involved recombinant retrovirus particles or
retrovirus producing cells for gene transfer. 16 Retroviruses are small RNA viruses that infect
target cells through a specific interaction between the viral envelope protein and a cell surface
“receptor” on the target cell.
For many types of retroviruses such as derivatives of Moloney murine leukaemia virus
(MoMuLV), mitosis, and thus, breakdown of the nuclear envelope is required for the viral
genome to reach the nucleus. 4,17 Furthermore, derivatives of MoMuLV are able to carry eight
kilobases of additional genetic information which is a rather small capacity. 3
Retroviruses, in contrast to adenovirus, integrate their genomes into the chromosomal DNA
of the host. Hence, a potential advantage is that theoretically, this additional genetic informa-
tion is preserved throughout every cell division and propagated to the next generation of cells.
To demonstrate the feasibility of treating arthritis and other joint disorders with gene therapy
Bandara and coworkers transduced rabbit synoviocytes in culture by a retrovirus encoding for
the human interleukin-1 receptor antagonist (IL-1ra). The genetically modified synovial cells
were then transplanted by intraarticular injection into the knee joints of rabbits, where they
efficiently colonized the synovium. Assay of joint lavages confirmed the in vivo expression of
biologically active human IL-1ra. However, the detectable amount of the transgene decreased
and lasted for only 6 weeks. 18
Inflammatory response is a particular concern for adenoviral vectors due to persistent viral
protein expression. 3,19,20 Vector immunogenicity is an important consideration for adenoviral
based strategies since viral proteins are present on the surfaces of host cells after digestion in the
cell's cytoplasm. In contrast, retrovirally infected cells do not express viral proteins and thus are
not immunogenic. 18
Last but not least there is concern about the site in the host's DNA where the transferred
genetic information integrates. Because integration of the retroviral genome happens at ran-
dom locations in the host's DNA it is unlikely, but possible, that functional deletion of a
tumor-suppressor gene or activation of an oncogene could occur, due to interactions with
house-keeping genes, leading to uncontrolled and malignant cell growth. 3
Retroviral Vectors in Gene Therapy
Retroviral vectors currently used for gene therapy are predominantly derived from MoMuLV
and possess the classical properties of a retrovirus. 10,16 MoMuLV derivatives can be generated
that are amphotropic and therefore, are able to infect several species of cells including murine
and human cells. According to Walther and Stein more than 150 clinical studies were under-
way in the year 2000, using retroviral vectors for gene transfer and more than 20 studies using
virus-producing cells for gene transfer. These clinical studies included more than 1500 pa-
tients. 16
Retroviral gene transfer as a therapeutic approach was effectively performed in several dis-
eases, including adenosine deaminase (ADA) deficiency, 21,22 Gaucher's disease, a lysosomal
storage disorder affecting the glucocerebrosidase enzyme, 23 plus many more, especially in the
field of oncology. In a clinical study, gene therapy strategies are being used to treat rheumatoid
arthritis. This is the first non-life-threatening disease in the usual sense, which was treated by
gene therapy in a clinical study. A retroviral vector was used in an ex vivo approach. 24-27