Gene therapy of X-linked severe combined immunodeficiency

Genome editing in human hematopoietic stem and progenitor cells via CRISPR-Cas9-mediated homology-independent targeted integration

Ex vivo gene correction of hematopoietic stem and progenitor cells (HSPCs) has emerged as a promising therapeutic approach for treatment of inherited human blood disorders. Use of engineered nucleases to target therapeutic transgenes to their endogenous genetic loci addresses many of the limitations associated with viral vector-based gene replacement strategies, such as insertional mutagenesis, variable gene dosage, and ectopic expression. Common methods of nuclease-mediated site-specific integration utilize the homology-directed repair (HDR) pathway. However, these approaches are inefficient in HSPCs, where non-homologous end joining (NHEJ) is the primary DNA repair mechanism. Recently, a novel NHEJ-based approach to CRISPR-Cas9-mediated transgene knockin, known as homology-independent targeted integration (HITI), has demonstrated improved site-specific integration frequencies in non-dividing cells. Here we utilize a HITI-based approach to achieve robust site-specific transgene integration in human mobilized peripheral blood CD34+ HSPCs. As proof of concept, a reporter gene was targeted to a clinically relevant genetic locus using a recombinant adeno-associated virus serotype 6 vector and single guide RNA/Cas9 ribonucleoprotein complexes. We demonstrate high levels of stable HITI-mediated genome editing (∼21%) in repopulating HSPCs after transplantation into immunodeficient mice. Our study demonstrates that HITI-mediated genome editing provides an effective alternative to HDR-based transgene integration in CD34+ HSPCs.

A reversion of an IL2RG mutation in combined immunodeficiency providing competitive advantage to the majority of CD8+ T cells

Mutations in the common gamma chain (γc, CD132, encoded by the IL2RG gene) can lead to B(+)T(-)NK(-) X-linked severe combined immunodeficiency, as a consequence of unresponsiveness to γc-cytokines such as interleukins-2, -7 and -15. Hypomorphic mutations in CD132 may cause combined immunodeficiencies with a variety of clinical presentations. We analyzed peripheral blood mononuclear cells of a 6-year-old boy with normal lymphocyte counts, who suffered from recurrent pneumonia and disseminated mollusca contagiosa. Since proliferative responses of T cells and NK cells to γc -cytokines were severely impaired, we performed IL2RG gene analysis, showing a heterozygous mutation in the presence of a single X-chromosome. Interestingly, an IL2RG reversion to normal predominated in both naïve and antigen-primed CD8(+) T cells and increased over time. Only the revertant CD8(+) T cells showed normal expression of CD132 and the various CD8(+) T cell populations had a different T-cell receptor repertoire. Finally, a fraction of γδ(+) T cells and differentiated CD4(+)CD27(-) effector-memory T cells carried the reversion, whereas NK or B cells were repeatedly negative. In conclusion, in a patient with a novel IL2RG mutation, gene-reverted CD8(+) T cells accumulated over time. Our data indicate that selective outgrowth of particular T-cell subsets may occur following reversion at the level of committed T progenitor cells.

The influence of semen-derived enhancer of virus infection on the efficiency of retroviral gene transfer

BACKGROUND

An improvement of retroviral infection has been postulated using a naturally occurring fragment of the abundant semen marker prostatic acidic phosphatase. This peptide, termed semen-derived enhancer of virus infection (SEVI), promotes HIV attachment to the target cells.

METHODS

In the present study, we examined whether SEVI would also enhance the infectivity of other viruses with different envelope proteins. We focused on retroviruses pseudotyped with envelopes that are commonly used for the genetic modification of cells, in particular, T cells and hematopoietic progenitor cells. Because the effect of SEVI is considered to be a result of its cationic properties, we compared SEVI with other cationic agents such as protamine sulfate and Polybrene.

RESULTS

We found that SEVI increases the efficiency of gene transfer for lentiviral and gammaretroviral vector constructs pseudotyped with VSV-G, GALV, RD114 or foamy viral envelopes on hematopoietic and nonhematopoietic cell lines. On T cells, the transduction efficiency of GALV and RD114 pseudotyped vectors was significantly increased by SEVI. A significant increase of the gene transfer rate was also detected for foamy virally pseudotyped lentivirus on murine hematopoietic progenitor cells. No toxic effect of SEVI treatment was detected on any cell type tested, including human and murine hematopoietic stem/progenitor cells. When directly comparing the effect of SEVI with Polybrene or protamine sulfate, we show that the semen-derived protein is more efficient in increasing the gene transfer rate.

CONCLUSIONS

SEVI is a promising agent for promoting and improving gene transfer and may also be useful for clinical gene therapy studies.

Transduction of murine hematopoietic stem cells and in vivo selection of gene-modified cells

Hematopoietic stem cells (HSCs) were among the first targets of genetic manipulation for the purpose of treating human diseases. The translational aspects of the first human clinical trials were based on results obtained using the mouse as an experimental model. Murine studies have shown that the major limitations of HSC gene therapy are similar to those encountered when using non-hematopoietic cells as targets and include (1) an inability to genetically modify sufficient numbers of target cells, (2) the loss of transgene function over time, and (3) potential complications due to vector integration. With continued improvements in transduction protocols, murine HSC transduction and transplantation are now routine with transduction efficiencies >50% easily achievable and even >90% feasible. However, attaining high-level engraftment of gene-modified cells after transplantation is still problematic. Basic transduction conditions entail cytokine stimulation of HSC populations, such as stem cell antigen-1 positive (Sca-1(+)) cells isolated from bone marrow, in serum-free media followed by multiple additions of recombinant retrovirus. Analysis of peripheral blood 12 weeks post transplantation of transduced cells into lethally irradiated recipients shows genetic marking in all hematopoietic lineages. Transduction of HSCs is then confirmed by transplanting bone marrow cells harvested from primary transplant recipients into lethally irradiated secondary recipients. Analysis of these mice shows that recombinant retroviruses transduce murine HSCs efficiently and stably and that the genetically modified cells are capable of completely repopulating the hematopoietic system.

Perspectives of gene therapy for primary immunodeficiencies

PURPOSE OF REVIEW

Standard therapies for patients with severe primary immunodeficiencies include bone marrow transplantation and, for adenosine deaminase deficiency, enzyme replacement. In the last decade, gene therapy has been developed as an alternative for these conditions. We summarize the recent advances in gene therapy for primary immunodeficiencies and discuss the unexpected occurrence of leukemia in a gene therapy trial for X-linked severe combined immunodeficiency.

RECENT FINDINGS

Eight of 10 infants with X-linked severe combined immunodeficiency who received autologous hematopoietic stem cells transduced with a retroviral vector carrying the IL2RG complementary DNA achieved immune reconstitution. However, the two youngest patients developed leukemic expansions of gene-corrected cells. The first case had proliferation of a gamma delta T cell clone, and the second case had three alpha beta T cell clones derived from a single transduced progenitor. Leukemic cells in both patients aberrantly expressed the LIM domain only-2 transcription factor due to retroviral vector insertions in this locus. After receiving anti-leukemic treatment one patient achieved a lasting remission, but the other relapsed. Four adenosine deaminase deficient severe combined immunodeficiency patients also developed functional immunity after receiving autologous hematopoietic stem cells transduced with the adenosine deaminase gene complementary DNA following submyeloablative chemotherapy. Chronic granulomatous disease, Wiskott-Aldrich syndrome, JAK3 deficiency and RAG2 deficiency are other immunodeficiencies being studied as candidates for gene therapy.

SUMMARY

Gene therapy is a promising therapeutic option for some primary immunodeficiencies, especially when cells expressing the correct gene have a selective advantage. More clinical trials with closer patient monitoring are under way to define which patients may benefit from this approach, and strategies are being developed to understand and ultimately reduce the risk of leukemia secondary to retroviral vector insertion.

Therapeutic protein transduction of mammalian cells and mice by nucleic acid-free lentiviral nanoparticles

The straightforward production and dose-controlled administration of protein therapeutics remain major challenges for the biopharmaceutical manufacturing and gene therapy communities. Transgenes linked to HIV-1-derived vpr and pol-based protease cleavage (PC) sequences were co-produced as chimeric fusion proteins in a lentivirus production setting, encapsidated and processed to fusion peptide-free native protein in pseudotyped lentivirions for intracellular delivery and therapeutic action in target cells. Devoid of viral genome sequences, protein-transducing nanoparticles (PTNs) enabled transient and dose-dependent delivery of therapeutic proteins at functional quantities into a variety of mammalian cells in the absence of host chromosome modifications. PTNs delivering Manihot esculenta linamarase into rodent or human, tumor cell lines and spheroids mediated hydrolysis of the innocuous natural prodrug linamarin to cyanide and resulted in efficient cell killing. Following linamarin injection into nude mice, linamarase-transducing nanoparticles impacted solid tumor development through the bystander effect of cyanide.