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Keywords:

  • gene therapy;
  • mesenchymal stem cells;
  • bone;
  • tissue engineering;
  • osteoblast differentiation

Abstract

  1. Top of page
  2. Abstract
  3. APPLICABLE GENES FOR MSCs MODIFICATION
  4. POTENTIAL CANDIDATE GENES
  5. SELECTION OF VECTORS
  6. THE ADMINISTRATION ROUTES
  7. THE FATES OF DONOR MSCs IN VIVO
  8. POTENTIAL CLINICAL APPLICATIONS
  9. SUMMARY
  10. Acknowledgements
  11. LITERATURE CITED

Bone grafting is crucial in the surgical treatment of bone defects and nonunion fractures. Autogenous bone, allogenous bone, and biomaterial scaffold are three main sources of bone grafts. The biomaterial scaffold, both natural and synthetic, is widely accessible but weak in osteogenic potential. One approach to solve this problem is cell-based bone tissue engineering (BTE), established by growing living osteogenic cells on scaffold in vitro to build up its osteoinducitive capability. Mesenchymal stem cell (MSC) is suitable for use in cell-based BTE, but it remains a considerable challenge to induce MSCs to form solely bone and while preventing MSCs from differentiating into fats, muscles, and possibly neural elements in vivo. Recently, there is a drastic rise in use of genetically engineered MSCs, which can secrete growth factors or alter the transcription level, leading to osteoblast lineage commitment, bone formation, fracture repair, and spinal fusion. In this article, we reviewed the literatures regarding applications of genetically engineered MSCs in BTE. We addressed the currently applicable genes and candidate genes for MSCs modification, transduction efficiency and safety issues of the transfect vectors, and administration routes, and we briefly described in vivo tracking and potential clinical application of the genetically modified MSCs in BTE. Anat Rec, 2010. © 2009 Wiley-Liss, Inc.

Bone grafting is crucial in the surgical treatment of bone defects and nonunion fractures that arise from trauma, tumor, or other pathological conditions. Autogenous bone, allogenous bone, and biomaterial scaffold are three main sources of bone grafts. The autogenous bone works effectively in bone restoration because of its superior osteoinductive and osteoconductive capability. However, the surgical harvest process is traumatic, increasing the risk of bleeding, infection, and even morbidity in donor site in patients (Banwart et al.,1995). The allogenous bone requires pretreatment to eliminate its immunogenicity and pathogenicity, and therefore its osteogenic potential is also diminished. The biomaterial scaffold, both natural and synthetic, is widely accessible but weak in osteogenic potential.

Two strategies have been developed to improve the osteogenic potential of biomaterial scaffold. The first one, named growth factors-based bone tissue engineering (BTE), attempts to enhance bone formation by incorporating bone-favor growth factors into the scaffold. However, it is difficult to control the local concentration of the growth factors because they could be rapidly absorbed by adjacent tissues. In this regard, it is not cost-effective because a large amount of growth factors is usually required (Boden,1999). The second one is cell-based BTE, established by growing living osteogenic cells on scaffold in vitro to build up its osteoinducitive capability (Meijer et al.,2007).

Mesenchymal stem cell (MSC), a type of adult stem cells, has the capability to differentiate into osteoblasts and other lineages under certain conditions (Prockop,1997). As the harvest and expansion in vitro are relatively easy (Bianco and Robey,2001), MSC is widely applied in cell-based BTE. The autologous bone marrow (BM)-MSCs have been successfully implanted in BTE in animal models to heal bone defects (Ohgushi et al.,1989; Bruder et al.,1994; Shang et al.,2001; Mankani et al.,2006; Clarke et al.,2007). Clinical case report also demonstrates that autologous MSCs, grown on biomaterial scaffold, are beneficial for treatment of large bone defects in patients (Quarto et al.,2001). However, it remains a considerable challenge as the MSCs may differentiate into fat, muscle, and possibly neural tissues in vivo (Prockop,1997). Recently, there is a trend in which the MSCs are engineered with specific genes, enabling MSCs to commit osteoblast lineage only and improve bone formation.

In this article, we reviewed the literatures regarding genetically engineered MSCs and their use in BTE. We addressed the currently applicable genes and candidate genes for MSCs modification, transduction efficiency and safety issues of the transfect vectors, and administration routes, and we briefly described in vivo tracking and potential clinical application of the genetically modified MSCs in BTE.

APPLICABLE GENES FOR MSCs MODIFICATION

  1. Top of page
  2. Abstract
  3. APPLICABLE GENES FOR MSCs MODIFICATION
  4. POTENTIAL CANDIDATE GENES
  5. SELECTION OF VECTORS
  6. THE ADMINISTRATION ROUTES
  7. THE FATES OF DONOR MSCs IN VIVO
  8. POTENTIAL CLINICAL APPLICATIONS
  9. SUMMARY
  10. Acknowledgements
  11. LITERATURE CITED

The modification by bone-specific genes enables MSCs to secrete certain growth factors, cytokines, or hormones and to change intracellular transcription level by regulating the expression of transcription factors and transcription coactivators (Table 1). The engineered MSCs are then expanded in vitro and used for BTE.

Table 1. List of applicable and potential genes for MSCs-based BTE
  1. bFGF, basic fibroblast growth factor; IL-1, interleukin-1; IL-6, interleukin-6; PTHrP, parathyroid hormone-related protein; PTH, parathyroid hormone; TNF-α, tumor necrosis factor alpha.

Applicable genesGrowth factors or cytokinesBMP-2, BMP-4, BMP-6, BMP-7, BMP-9
HormoneGH
Transcriptional factorRunx2, Osterix
Potential genesGrowth factors or cytokinesBMP-14, VEGF, IGF, Osteoactivin, PDGF, bFGF, IL-1, IL-6. TNF-α, Osteoprotegerin (OPG)
Transcriptional factor and coactivatorTAZ, Sox9
HormonesPTHrP, PTH
 Receptorssoluble IL-1R, and soluble BMP receptors

Many members of the whole bone morphogenetic proteins (BMPs) superfamily correlate with bone, cartilage, and joint development (Table 2). BMP-2, approved by Food and Drug Administration (FDA) for clinical practice, is the most potent member in promoting bone and cartilage development and therefore wins a popular choice for MSCs-based BTE (Lieberman et al.,1998; Lieberman et al.,1999; Lou et al.,1999; Turgeman et al.,2001; Olmsted-Davis et al.,2002; Blum et al.,2003; Gugala et al.,2003; Park et al.,2003; Riew et al.,2003; Tsuda et al.,2003; Kumar et al.,2004; Hasharoni et al.,2005; Egermann et al.,2006; Feeley et al.,2006). The BMP-2-modified MSCs are proven to increase the alkaline phosphatase (ALP) activity, mineralization, and cell proliferation in vitro and induce ectopic bone formation, heal critical size bone defect, repair fracture, and trigger spinal fusion in vivo (Lou et al.,1999; Moutsatsos et al.,2001; Turgeman et al.,2001; Blum et al.,2003; Park et al.,2003; Riew et al.,2003; Tsuda et al.,2003; Hasharoni et al.,2005; Egermann et al.,2006; Feeley et al.,2006). BMP-7 plays a key role in osteoblast differentiation, and there is only one study in which MSCs are engineered with BMP-7. Engineered MSCs are seeded in the distraction gaps of the mandibles, and the study demonstrates accelerated callus formation in distraction osteogenesis (Hu et al.,2007). BMP-4 is associated with bone and cartilage development and fracture repair. BMP-4-modified MSCs increase trabecular bone mineral density (BMD) and can heal critical sized femur defects in adult rats (Rose et al.,2003; Zhang et al.,2004). BMP-6 is characterized by maintaining joint integrity in adults. Interestingly, BMP-6-modified bone marrow MSCs (BM-MSCs) demonstrate accelerated osteogenic differentiation and mineralization in vitro, but the result is less robust than BMP-2-modified BM-MSCs (Zachos et al.,2006). BMP-9 only has a moderate osteoinductive capability (Celeste et al.,1990). However, BMP-9-engineered MSCs stimulate ALP activity in vitro and induce ectopic bone formation in vivo (Ploemacher et al.,1999; Helm et al.,2000; Varady et al.,2001; Dumont et al.,2002; Dayoub et al.,2003; Li et al.,2003), promoting its future use in BTE.

Table 2. Overview of BMPs
BMPsReported skeleton-related characteristics
BMP-1Involved in cartilage development
BMP-2Plays a key role in osteoblast differentiation, inducing bone and cartilage formation
BMP-3An antagonist to other BMPs in the differentiation of osteogenic progenitors
BMP-4Regulates the formation of teeth, limbs, and bone from mesoderm, also playing a role in fracture repair
BMP-5Performs functions in cartilage development
BMP-6Plays a role in joint integrity in adults
BMP-7 (OP1)Plays a key role in osteoblast differentiation. It also induces the production of SMAD1. Also key in renal development and repair
BMP-8a (OP2)Involved in bone and cartilage development
BMP-8b (OP2)Expressed in the hippocampus
BMP-9Moderate osteoinduce effect, high doses of rhBMP-9 have been shown to induce ectopic bone formation in vivo
BMP-10May play a role in the trabeculation of the embryonic heart
BMP-11 (GDF11)Expressed in terminally differentiated odontoblasts might stimulate differentiation of the pulp stem cells into odontoblasts in vitro
BMP-12 (GDF7)Formation of tendon-like tissue
BMP-13 (GDF6)Heals tears and avulsion injuries of tendon and ligament, regulating patterning of the ectoderm by interacting with other bone morphogenetic proteins
BMP-14 (GDF5)Promotes the initial stages of chondrogenesis by promoting cell adhesion, increases the size of the skeletal elements, and plays a critical role in limb bud and joint development
BMP-15 (GDF9B)May play a role in oocyte and follicular development

The genes encoding systemic hormone may not regularly be considered as candidates for BTE because the main biological function of the hormone will only be reached while distributed systemically. However, when the gene of human growth factor (hGH) is engineered into dog and mouse MSCs in vitro, and reintroduced into the foreleg vein, the iliac crest marrow, and the femur marrow, higher level of hGH is detected in the blood of the recipient animal a few days later (Hurwitz et al.,1997; Suzuki et al.,2000). In this regard, BM-MSCs may be used for the delivery of GH or other systemic hormones targeting at bone to treat systemic bone disorders such as osteoporosis.

The genes of transcription factors accounting for osteoblast differentiation of MSCs are also on the list. Overexpression of Runx2 and osterix in engineered MSCs may direct osteoblastic lineage differentiation and suppress other lineage differentiation (Merriman et al.,1995; Nakashima et al.,2002). Runx2-engineered MSCs directly enhance the repair of critical-sized calvarial defects when implanted solely (Zheng et al.,2004; Zhao et al.,2007) and form more bone when used in BTE (Zhao et al.,2005; Byers et al.,2006). Despite this, Runx2 and BMP2 also produce a strong synergistic effect after both are engineered into MSCs (Yang et al.,2003). Osterix may act downstream of Runx2 (Akiyama et al.,2005), and neither endochondral nor intramembranous bone formation occurs in osterix-null mutant mice (Nakashima et al.,2002). In one study, osterix-engineered BM-MSCs in vivo form five times the amount of new bone than control in healing of the calvarial bone defects in mouse (Tu et al.,2007).

POTENTIAL CANDIDATE GENES

  1. Top of page
  2. Abstract
  3. APPLICABLE GENES FOR MSCs MODIFICATION
  4. POTENTIAL CANDIDATE GENES
  5. SELECTION OF VECTORS
  6. THE ADMINISTRATION ROUTES
  7. THE FATES OF DONOR MSCs IN VIVO
  8. POTENTIAL CLINICAL APPLICATIONS
  9. SUMMARY
  10. Acknowledgements
  11. LITERATURE CITED

There are other growth factors, cytokines, or transcription factors that contribute to osteoblast differentiation, vascular reconstruction, and eventual bone formation. Their coding genes may be candidates for MSCs modification in future cell-based BTE (Table 1).

BMP-14 is primarily considered because it correlates with the skeletal development and fracture repair (Buxton et al.,2001; Chhabra et al.,2005), and BMP-14-deficient mice show a delay in long bone fracture repair (Chhabra et al.,2005). The insulin-like growth factor 1 (IGF-1) has been proven to regulate skeletal development and homeostasis and to accelerate repair of intramembranous bone defects (Thaller et al.,1993; Conover,2000; Rosen,2000; Zofkova,2003; Fisher et al.,2005; Conover,2008). In a transgenic mouse model, IGF-I is transduced and expressed at high levels in osteoblasts, and the bone formation and BMD in mouse femur are significantly increased (Zhao et al.,2000). The vascular endothelial growth factor (VEGF) has an essential role in angiogenesis and bone regeneration (Carano and Filvaroff,2003). Muscle-derived stem cells modified with VEGF show no effect, whereas with both rhBMP-4 and VEGF, the stem cells augment cartilage formation in the early stages of endochondral bone formation (Peng et al.,2002), indicating the necessary coexistence of bone-specific growth factor in VEGF gene therapy. Osteoactivin gene is an osteoblast-specific gene in bone, positively regulating osteoblast differentiation (Owen et al.,2003; Selim et al.,2003; Abdelmagid et al.,2007,2008). One isoform of osteoactivin is a secretion protein (Abdelmagid et al.,2008), and such supporting osteoactivin is a potential candidate for gene therapy. Transcriptional coactivator with PDZ-binding motif (TAZ) modulates MSC to differentiate into osteogenic and adipogenic lineages and is considered to be the key factor between BMP-2 and Runx-2 (Hong et al.,2005), validating the use of TAZ gene in MSC modification. Sex determining region Y-box 9 (Sox9), a transcription factor, is required for chondrocyte cell fate determination and marks early chondrogenic differentiation of mesenchymal progenitors. Inactivation of Sox9 before mesenchymal condensation abolishes not only cartilage formation but also later endochondral bone formation (Bi et al.,1999; Akiyama et al.,2002). Therefore, Sox9 modification in MSCs may be considered because endochondral bone formation is required the early stage of bone healing.

SELECTION OF VECTORS

  1. Top of page
  2. Abstract
  3. APPLICABLE GENES FOR MSCs MODIFICATION
  4. POTENTIAL CANDIDATE GENES
  5. SELECTION OF VECTORS
  6. THE ADMINISTRATION ROUTES
  7. THE FATES OF DONOR MSCs IN VIVO
  8. POTENTIAL CLINICAL APPLICATIONS
  9. SUMMARY
  10. Acknowledgements
  11. LITERATURE CITED

The ideal vectors used for MSCs gene therapy would be those that are not toxic to surrounding tissues and organs, have higher transfect efficiency, and enable target genes to be expressed for an appropriate length of time. The vectors used for MSCs genetical modification in cell-based BTE are liposome, adenovirus, and retrovirus (Table 3).

Table 3. Vectors for MSCs genetic modification
VectorsIntegration of genomePathogenic potentialTransfection efficiency and other features
LiposomeNoVery low or noLess than viral vectors
AdenovirusNoElicit robust immune responseTransient transduction of the target genes
OncoretrovirusesYesInsertional mutagenesisHigh efficiency and stable transfection
SpumavirusesYesVery low or noHigh efficiency but resistant to packaging by heterologous envelope proteins
LentivirusYesHIV-related safety issuesBetter in transduction of many types of noncycling cells

Liposome, a nonviral delivery system, is nonpathogenic. Although there are remarkable improvements in its transfect efficiency in recent years, the nonviral vectors remain less efficient in delivery than viral vectors. Although BMP-7 and BMP-2 gene have been successfully engineered into MSCs via liposomes (Park et al.,2003; Hu et al.,2007), further optimization of liposome or other nonviral formulations is still needed.

Adenovirus vector is one of the most popular vectors for MSC gene therapy aimed at BTE (Turgeman et al.,2001; Dumont et al.,2002; Olmsted-Davis et al.,2002; Blum et al.,2003; Dayoub et al.,2003; Gugala et al.,2003; Li et al.,2003; Park et al.,2003; Riew et al.,2003; Sugiyama et al.,2003; Tsuda et al.,2003; Yang et al.,2003; Kumar et al.,2004; Zheng et al.,2004; Zhao et al.,2005; Egermann et al.,2006; Feeley et al.,2006; Zachos et al.,2006). Adenovirus vector is characterized by efficient transduction of quiescent cells and a relatively high capacity for transgene insertion. However, adenovirus does not incorporate the target gene DNA into the genome of host cells, leading to a transient transduction. Additionally, it is virtually impossible to redose with adenovirus vector because of the robust immune response (Nixon et al.,2007).

An advantage of retroviral vectors over adenovirus vectors is their ability to stably incorporate the target genes into the cell genome. This virus family includes oncoretroviruses, spumaviruses, and lentiviruses, and recent decade has seen a dramatic rise in the use of retroviral vectors for bone-related gene therapy in MSCs (Peng et al.,2001; Blum et al.,2003; Rundle et al.,2003; Byers and Garcia,2004; Zhang et al.,2004; Byers et al.,2006). However, its pathogenic potentials, such as production of a replication competent retrovirus (RCR) and insertional mutagenesis, remain a major hurdle during the transfection (Donahue et al.,1992; Li et al.,2002; Hacein-Bey-Abina et al.,2003; Anson,2004). The spumaviruses, such as the human foamy virus (HFV), do not appear to be linked to any specific pathogenic state (Meiering and Linial,2001) but are weak in pseudotyping and packaging of heterologous envelope proteins, leading to inefficient transduction of several clinically relevant cell types (Mergia and Heinkelein,2003). Lentivirus vector, a kind of nononcogenic retrovirus, supposedly performs better in transducing noncycling cells, such as MSCs. However, lentivirus has been shown to contribute to several slow progressive diseases, including arthritis, encephalitis, leukemia, anaemia, and immunodeficiency, in animals and humans (Anson,2004). In addition, as lentivirus is derived from the human immunodeficiency virus type 1 (HIV), the inadvertent production of RCR remains a principal safety issue for the use in human clinical practices.

THE ADMINISTRATION ROUTES

  1. Top of page
  2. Abstract
  3. APPLICABLE GENES FOR MSCs MODIFICATION
  4. POTENTIAL CANDIDATE GENES
  5. SELECTION OF VECTORS
  6. THE ADMINISTRATION ROUTES
  7. THE FATES OF DONOR MSCs IN VIVO
  8. POTENTIAL CLINICAL APPLICATIONS
  9. SUMMARY
  10. Acknowledgements
  11. LITERATURE CITED

Venous Route

The venous administration attempts to indirectly introduce engineered MSCs into bone marrows through vascular route, and it may be helpful in gene therapy for treatment of systemic skeletal disorders such as osteoporosis and osteogenesis imperfecta. However, the efficient homing of systemically distributed MSCs to bone marrow remains a major dilemma in applying the venous route. In one study, after tail vein injection in mouse, only a few marked BMP-4-modified mouse MSCs are found in recipient mouse bone marrows, whereas a large number of marked cells initially observed in lung, liver, and spleen are cleared rapidly within a few days (Zhang et al.,2004).

Skeletal Muscle Injection

Skeletal muscles provide superior vascular conditions for harboring modified MSCs and promoting ectopic bone formation. When engineered MSCs are implanted to thigh muscles (Lou et al.,1999; Dayoub et al.,2003; Li et al.,2003) or paraspinal muscles (Dumont et al.,2002; Hasharoni et al.,2005), ectopic bone formation is observed. Indeed, skeletal muscle injection is an optimal method in testing the viability and function of genetically engineered MSCs in vivo. Considering the growth factors or cytokines secreted by MSCs might be absorbed quickly by muscle and distributed systemically, direct skeletal muscle injection should be considered for some systemic bone disorders such as osteoporosis and osteogenesis imperfecta.

Bone Marrow Injection

Bone marrow is a small niche in rodents for implantation of genetically modified MSCs, and the donor MSCs in host bone marrow may survive for significantly longer time than expected (Hou et al.,1999; Nilsson et al.,1999). MSCs are usually injected into femur with larger bone marrow cavities of rodent by the knee approach (Zhang et al.,2004). In a study, the trabecular BMD of femur is significantly increased after bone marrow injection of BMP-4-engineered MSCs (Zhang et al.,2004).

Cell-Based BTE

In cell-based BTE, the MSCs are expanded and genetically engineered in vitro, participating with a scaffold to form a three-dimensional tissue structure (Turgeman et al.,2001; Zhao et al.,2005; Feeley et al.,2006). An optimal scaffold should provide genetically modified MSCs with the appropriate environment for attachment, proliferation, and differentiation, preventing them from migrating into other places of the body. The scaffold may be soluble for direct injection or solid for surgical implantation (Peng et al.,2001; Turgeman et al.,2001; Blum et al.,2003; Park et al.,2003; Rose et al.,2003; Tsuda et al.,2003; Feeley et al.,2006). Genetically engineered MSCs have the ability to induce bone regeneration not only through paracrine secretion of growth factors to adjacent cells but also by an autocrine effect on MSCs themselves to promote their osteogenic differentiation (Gazit et al.,1999).

THE FATES OF DONOR MSCs IN VIVO

  1. Top of page
  2. Abstract
  3. APPLICABLE GENES FOR MSCs MODIFICATION
  4. POTENTIAL CANDIDATE GENES
  5. SELECTION OF VECTORS
  6. THE ADMINISTRATION ROUTES
  7. THE FATES OF DONOR MSCs IN VIVO
  8. POTENTIAL CLINICAL APPLICATIONS
  9. SUMMARY
  10. Acknowledgements
  11. LITERATURE CITED

Ideally, the genetically engineered MSCs should survive on biomaterial scaffold in vivo for a certain period of time, completing differentiation into osteoblast lineage, and secretion of specific growth factors that trigger bone formation. However, little is known about the real fates of donor MSCs in vivo. The donor MSCs are usually labeled with a specific transgenic reporter, such as chloramphenicol acetyltransferase (CAT) or eGFP, with an attempt to reveal their distribution and ultimate fates in vivo (Niyibizi et al.,2004; Zhao et al.,2005). The donor cells can also be visualized and tracked through noninvasive magnetic resonance imaging (MRI) when they are labeled with MR contrast agents (Bulte et al.,2002; Terrovitis et al.,2006). However, in vivo magnetic resonance tracking is not endowed with the capacity to identify the actual biological status of these implanted donor cells.

POTENTIAL CLINICAL APPLICATIONS

  1. Top of page
  2. Abstract
  3. APPLICABLE GENES FOR MSCs MODIFICATION
  4. POTENTIAL CANDIDATE GENES
  5. SELECTION OF VECTORS
  6. THE ADMINISTRATION ROUTES
  7. THE FATES OF DONOR MSCs IN VIVO
  8. POTENTIAL CLINICAL APPLICATIONS
  9. SUMMARY
  10. Acknowledgements
  11. LITERATURE CITED

To date, the autologous MSCs have been used in BTE because they are comparatively safe. In a noncontrolled clinical trial, Quarto et al. first reported that implantation of a porous ceramic scaffold seeded with in vitro expanded autologous BM-MSCs repaired massive bone defects in patients (Quarto et al.,2001; Marcacci et al.,2007). Bajada et al reported that autologous BM-MSCs were expanded for 3 weeks in vitro and combined with calcium sulfate to cure a 9-year tibial nonunion in a patient (Bajada et al.,2007). Gan et al. demonstrated the efficacy of using enriched preparations of autologous BM-MSCs combined with beta-tricalcium phosphate for posterior spinal fusion in 41 patients (Gan et al.,2008). However, there is no clinical case reporting the use of genetically engineered MSCs in BTE. The clinical application is largely restrained by the safety concern of vectors, especially retrovirus vectors, even though the retrovirus has been safely used in other clinical disciplines (Levine et al.,2006; Schwarzwaelder et al.,2007; Bauer et al.,2008).

SUMMARY

  1. Top of page
  2. Abstract
  3. APPLICABLE GENES FOR MSCs MODIFICATION
  4. POTENTIAL CANDIDATE GENES
  5. SELECTION OF VECTORS
  6. THE ADMINISTRATION ROUTES
  7. THE FATES OF DONOR MSCs IN VIVO
  8. POTENTIAL CLINICAL APPLICATIONS
  9. SUMMARY
  10. Acknowledgements
  11. LITERATURE CITED

The potential therapeutic application of genetic-engineered MSCs in BTE represents a promising approach for the treatment of bone defects and nonunion fractures. A variety of specific genes have been successfully used for MSC modification, and genetically engineered MSCs have shown efficacy in promoting bone formation both through direct implantation or through cell-based BTE. In addition, more candidate genes favoring bone formation and vascular reconstruction need further consideration. There are many aspects should be considered for MSCs genetical modification, such as the safety issues of vectors, the transduce efficiency, and the duration of transduction. In addition, a more compatible scaffold can provide appropriate environment for MSCs attachment, proliferation, and differentiation. However, the fate of transgenic MSCs in vivo remains unclear, and the use of genetically engineered MSCs in cell-based BTE is still largely restrained by the safety issue of vectors.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. APPLICABLE GENES FOR MSCs MODIFICATION
  4. POTENTIAL CANDIDATE GENES
  5. SELECTION OF VECTORS
  6. THE ADMINISTRATION ROUTES
  7. THE FATES OF DONOR MSCs IN VIVO
  8. POTENTIAL CLINICAL APPLICATIONS
  9. SUMMARY
  10. Acknowledgements
  11. LITERATURE CITED
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