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. 2014 Apr;22(4):725-33.
doi: 10.1038/mt.2013.290. Epub 2014 Jan 6.

Somatic Correction of Junctional Epidermolysis Bullosa by a Highly Recombinogenic AAV Variant

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Free PMC article

Somatic Correction of Junctional Epidermolysis Bullosa by a Highly Recombinogenic AAV Variant

Sandra P Melo et al. Mol Ther. .
Free PMC article

Abstract

Definitive correction of disease causing mutations in somatic cells by homologous recombination (HR) is an attractive therapeutic approach for the treatment of genetic diseases. However, HR-based somatic gene therapy is limited by the low efficiency of gene targeting in mammalian cells and replicative senescence of primary cells ex vivo, forcing investigators to explore alternative strategies such as retro- and lentiviral gene transfer, or genome editing in induced pluripotent stem cells. Here, we report correction of mutations at the LAMA3 locus in primary keratinocytes derived from a patient affected by recessive inherited Herlitz junctional epidermolysis bullosa (H-JEB) disorder using recombinant adenoassociated virus (rAAV)-mediated HR. We identified a highly recombinogenic AAV serotype, AAV-DJ, that mediates efficient gene targeting in keratinocytes at clinically relevant frequencies with a low rate of random integration. Targeted H-JEB patient cells were selected based on restoration of adhesion phenotype, which eliminated the need for foreign sequences in repaired cells, enhancing the clinical use and safety profile of our approach. Corrected pools of primary cells assembled functional laminin-332 heterotrimer and fully reversed the blistering phenotype both in vitro and in skin grafts. The efficient targeting of the LAMA3 locus by AAV-DJ using phenotypic selection, together with the observed low frequency of off-target events, makes AAV-DJ based somatic cell targeting a promising strategy for ex vivo therapy for this severe and often lethal epithelial disorder.

Figures

Figure 1
Figure 1
AAV-DJ is a highly recombinogenic virus. (a) Relative colony number, a measure of homologous recombination (in red), and transduction titer as determined by green fluorescent protein (GFP)-based fluorescence-activated cell sorting (FACS) (in blue) of naturally occurring and hybrid adenoassociated virus (AAV) capsids in keratinocytes. Targeting was measured as the number of colonies stained with crystal violet after 10 days of adhesion selection on plastic and expressed relative to AAV3, which is the standard AAV with the highest homology to AAV-LK19, the most efficient capsid at functionally transducing keratinocytes. (b) Diagram depicting the mutation in the LAMA3 locus. The mutation causes incorrect splicing of exon 44, leading to a frame shift and an early termination codon. At right, light microscopy images of cells grown in culture. Mutant cells do not proliferate to plastic tissue culture dishes and attach normally. (c) Schematic overview of the targeting strategy based on adhesion selection for the LAMA3 locus. The targeting vector is a 4.2 Kb wild-type (WT) LAMA3 fragment encompassing exons 43 and 44. Southern probe is shown as a black line. ITR, inverse terminal repeats.
Figure 2
Figure 2
AAV-DJ-LAMA3 efficiently corrects somatic junctional EB cells with a low random integration frequency and leads to mosaicism. (a) Diagram depicting the restriction fragment length polymorphism (RFLP) assay based on the presence of SfcI restriction site. (b) RFLP assay on 13 clones selected based on restoration of adhesion. (c) Quantification of percentage of targeted and untargeted allele by densitometry on the SfcI produced bands on 70 clones. The presence of untargeted cells in the selected clones likely comes from rescue coming from deposited and diffusable laminin-332 from corrected cells (diagram on the right in a). (d,e) Detection of off-target events by (d) Southern blot and (e) inverse PCR. WT, wild-type.
Figure 3
Figure 3
AAV-DJ-LAMA3 targeted primary keratinocytes correctly assemble laminin-332 and reverse the blistering phenotype. (a) Quantification of percentage of targeted allele by SfcI digest. Densitometry is shown below the gel. (b) Correction of the LAMA3 mutation in primary cell pools by DNA sequencing. (c) Restoration of laminin-332 assembly by native electrophoresis followed by western blot on the secreted media. (d,e) Detection of off-targeted events by (d) inverse PCR and (e) Southern blot. (f) Immunofluorescence staining on deposited laminin-α3 and laminin-332. (g) H&E and immunofluorescence staining of 2-week old organotypic cultures derived from wild-type (WT), mutant and targeted primary cells.
Figure 4
Figure 4
Targeted primary cells have long-term graftability in vivo. (a) Immunofluorescence staining of xenografts from wild-type (WT) (5 weeks), mutant (3 weeks) and targeted (5 weeks) cells. (b) Immunoelectron micrograph on ultrathin sections of a 5-week xenograft from targeted cells using an antibody against human laminin-α3. (c) A higher resolution micrograph. Immunostaining is pointed by the black arrows. Scale bar corresponds to 100 nm. HD, hemidemosome, LL, lamina lucida, LD, lamina densa.

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