Table 4.

Gene therapy limitations

Limitation	Description	Potential solution(s)
Insertional mutagenesis	Incorporation of viral vector DNA into a host gene driving malignant transformation or cell death	Identification of vectors less prone to high risk sites of DNA insertion (ex. Lentiviral vectors) Use of gene editing methods (ex. Meganucleases, CRISPR-Cas, Transcription activator-like effector-based nucleases (TALENs), Zinc finger nucleases)
Genetic heterogeneity	21 known gene mutations leading to FA	Target highest frequency complementation group, FANCA (accounts for 60% of cases) [6]
Non-hematopoietic disease manifestations	Somatic cells continue to express FA mutations and contribute to risk of solid tumor development	Correction of FA HSCs may improve immune surveillance for transformation of somatic cells Avoid exacerbating exposures (chronic GVHD, mutagens, UV radiation)
FA HSPCs:
- Acquired mutations	FA HSPCs accumulate mutations over time resulting at times in MDS or leukemia	Derive healthy HSPCs from somatic induced pluripotent stem (iPS) cells (not yet demonstrated scientifically) [83–86]
- Sensitivity to apoptosis	DNA damage prompts apoptosis
- Low HSPC numbers	FA patients with 2-fold reduction in overall cellularity and 6-fold fewer CD34+ HSPCs compared to healthy controls [82]	Utilize ex vivo HSPC expansion strategies such as aryl hydrocarbon receptor antagonist StemRegenin1 [87], Notch ligand [88, 89], nicotinamide analogs [90], copper chelators [91, 92]

FA, Fanconi anemia; HSCs, hematopoietic stem cells; GVHD, graft-versus-host disease; HSPCs, hematopoietic stem and progenitor cells; MDS, myelodysplastic syndrome