Page 1196 - Clinical Immunology_ Principles and Practice ( PDFDrive )

CHaPTEr 85 Gene Therapy for Primary Immune Deficiency Diseases 1161

These results are promising and of significant clinical benefit deficiency caused by hypogammaglobulinemia, the high rate of
and safety, but would be better if more normal levels of platelets other clinical complications that CVID patients may experience
could be achieved. It is possible that the level of expression of necessitate new treatments. However, gene therapy requires
WASP from the WAS gene promoter in the lentiviral vector is knowing the responsible pathogenic gene, and has been limited
inadequate to support normal platelet production or survival. to monogenic disorders. To date, a single gene defect has not
If so, introducing more than one copy of the vector per cell been identified in the majority of patients with CVID. There are
could lead to higher levels of WASP and higher platelet counts. some known CVID genes, including TNFRSF13B (encoding TACI
The WAS gene promoter that has been used as a regulatory 8–10%), TNFRSF13C (encoding BAFF-R), COS, CD19, and
element (a 1.6 kb fragment from the 5′ end of the gene) may MSH5, which in total may be responsible for 10–15% of all
be sub-optimal to drive sufficient expression; there may be other CVID patients. A separate gene therapy project would be needed
regulatory elements from the WAS genomic locus not included to develop treatment for each causal gene, through the full
in the vectors that are needed for some essential aspect of expres- spectrum from pre-clinical activities to clinical trial performance.
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sion (lineage, level, longevity). Astrakhan et al reported that a And, because most of these known CVID-causing genes are
lentiviral vector using a stronger internal promoter (a retroviral involved in cell stimulation and signaling, they may require
LTR) to drive the WASP cDNA led to better restoration of T regulated, rather than ubiquitous, constitutive expression for
and B cell activity in the WAS gene knock-out mouse model. safety. Because of these constraints, it is not currently possible
Alternatively, the absolute number of gene-corrected HSCs to apply gene therapy for the majority of patients with CVID.
engrafted may mediate the platelet levels and a higher percentage There has been growing recognition of numerous immune
of transduced cells or higher numbers of gene-corrected cells dysregulation and auto-inflammatory syndromes due to auto-
may support higher platelet levels. Relative risks of multiple somal dominant gain-of-function mutations (e.g. STAT3, MEFV,
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integrants versus a stronger promoter are not known. As for the IL-1, NFkB, interferon pathways). The pathology attributed to
other disorders discussed here, direct gene correction should blood cells (as opposed to those effects on other tissues) may
yield normal, physiologic expression of the WAS gene. be corrected by HSCT, and here too autologous transplant/gene
therapy may have advantages over allogeneic. Gene modification
GENE THERAPY CONSIDERATIONS FOR OTHER PID of a patient’s HSC by addition of a shRNA cassette or gene disrup-
tion using site-specific endonucleases may suppress expression
Currently, a new development project is needed to bring gene of the dominant gene. And, direct correction of the pathogenic
therapy to clinical application for each individual PID-causing mutation could also be beneficial, as discussed below. It is likely
locus, such as the more than 20 human genes that may cause that the gene modification would need to be efficient to yield a
SCID, 5 or more CGD loci, several for Hemophagocytic Lym- high fractional correction of the engrafting stem cells. In this
phoistiocytosis (HLP), X-linked lymphoproliferative disease setting, some pre-transplant immune suppression may be needed
(XLP), etc. Each gene and disease setting poses different challenges to ablate pre-existing auto-immunity. And, perhaps most chal-
in terms of necessary gene transfer efficiency, level of expression, lenging for gene therapy are the PIDs that also have major somatic
need for regulation of the transferred gene, safety considerations, or developmental problems, such as chromosomal abnormalities,
and measurable end-points. ataxia-telangiectasia, and others. Here the gene therapy with the
X-linked agammaglobulinemia (XLA; Chapter 34) is another HSC may benefit that component of the disorder but would not
logical disease to consider treating by autologous transplant with address the others; systemic delivery of genes or delivery to the
gene therapy, since normal B cell development from HSC with CNS is under study, but it is not yet sufficiently efficient for
a normal Bruton tyrosine kinase (BTK) gene should correct the most clinical needs.
immune deficiency. Because of the good clinical effects from
immunoglobulin replacement therapy for XLA and the toxicities GENE CORRECTION (EDITING) FOR GENE
from HSCT, especially from chemotherapy and GVHD, HSCT THERAPY OF PID
is rarely done for XLA patients. A few XLA patients have had
allogeneic transplants from healthy donors and have developed A major paradigm shift in gene therapy is under way as methods
B cell reconstitution. to perform precise edits of cells’ genomes are being developed.
Gene therapy studies in BTK gene knock-out mice have shown As an alternative to the semi-random insertion of normal copies
that it can lead to immune reconstitution, using lentiviral vectors of the relevant gene delivered by a viral vector, as discussed for
with B lymphoid-specific promoters. 24,25 While no adverse effects all of the studies above, techniques are being established to either
were seen from constitutive expression of the BTK gene in these correct specific bases in the DNA, or to insert (or remove) gene
murine studies, these do not constitute formal toxicology studies sequences at specific sites by harnessing cellular DNA repair
which would be needed before clinical applications. It is likely pathways. These DNA repair mechanisms normally correct the
that regulated expression of BTK, rather than constitutive, many double-stranded DNA breaks that occur during DNA
ubiquitous expression, is needed for highest efficacy and safety. replication or from environmental genotoxic agents (ionizing
In theory, lentiviral viral vectors using components of the BTK radiation, chemicals). To simplify a highly complex topic, there
gene transcriptional regulatory sequences could yield vectors are two major DNA repair pathways for rejoining the sequences
with the desired expression specificity. Alternatively, gene cor- that flank a double-stranded DNA break: non-homologous end
rection of the BTK gene, using the methods discussed below, joining (NHEJ) and homologous recombination (HR). NHEJ
could restore precise BTK expression regulation and may have reconnects the broken ends of the chromosomes in a way that
the greatest safety profile. often leads to insertion or deletion of DNA bases (indels) at the
Common Variable Immune deficiency (CVID) comprises the junctional site. This is, in essence, a mutagenic process and
most common severe human PID (Chapter 34). While immu- may be used to disrupt genes to knock-out their activity,
noglobulin replacement therapy can ameliorate the immune examples being the HIV-1 co-receptor CCR5, dominant-active