Gene therapy is now bringing a revolutionary treatment options to most of genetic related diseases including cancers, offering the theoretical advantage of the possibility of achieving the therapeutic goal by a single treatment. Clustered regularly interspaced short palindromic repeat is a recent genome-editing approach have attracted significant attention among the researchers due to their potential to cure all genetic related diseases including cancer by editing the genome in a way based on RNA-guided nuclease

2. Gene therapy in treatment of cancer
By
Romissaa Aly Esmail
Assistant lecturer of Oral
Medicine, Periodontology,
Diagnosis and Dental Radiology
(Al-Azhar Univerisity)

4.  In recent years, cancer has become one of the most important health
issues and the second top leading cause of mortality after heart disease
[1,2].
 Cancer is characterized by uncontrolled proliferation of cells, resulting in
formation of tumor [3]. It occurs either by genetic lesions in any gene that
encode cell cycle proteins or as a result of somatic mutations in upstream
cell signaling pathways.

5. Despite the increasing
availability of conventional
cancer therapeutic options,
million patients suffered from
the cancer-induced death
every year [4].
Due to metastasis of cancer
cells, recurrence,
heterogeneity, and their
resistance to chemotherapy
and radiotherapy, many
traditional treatment
methods become ineffective
against many malignant
tumors [5].
The ability of cancer
cells to avoid the
immune response is
another reason has
been reported for
failing the therapy

6. Gene therapy is now bringing a
revolutionary treatment options to most of
genetic related diseases including cancers,
offering the theoretical advantage of the
possibility of achieving the therapeutic goal
by a single treatment.
Clustered regularly interspaced short
palindromic repeat is a recent genome-
editing approach have attracted
significant attention among the
researchers due to their potential to
cure all genetic related diseases
including cancer by editing the genome
in a way based on RNA-guided
nuclease

10. Gene therapy has been
supported with the advances
in bioinformatics science and
invention of various high-
throughput genomic
approaches, that able to
identify particular mutations
for targeting gene delivery
[8].
Liu et al. [83] reported 4
major strategies in gene
therapy, including:
replacement of the targeted
gene, modifying it, gene
augmentation; and blockage
of defected gene.
However, more than 2200
clinical trials based on gene
therapy had been worldwide
conducted by July 2015 [84].
About 60% of these trails
are associated with cancer
gene therapy, which indicate
the effectiveness of gene
therapy in cancer treatment
and it is not only limited to
hereditary diseases.Various
gene therapy strategies
have been developed for
cancer treatment including:
anti-angiogenic gene
therapy,
immunomodulatory gene
therapy, suicide gene
therapy, siRNA therapy,
oncolytic gene therapy and
pro-apoptotic gene therapy
[85,86].

21. Gene amplification, which is used in
the treatment of various human
diseases, was put forward by Cusack
andTanabe in 1998[9].
Gene therapy is defined as gene
transfer for the purpose of treating
human diseases effectively (Cusack
andTanabe, 1998)[9,10], which
includes both the transfer of new
genetic material and manipulation of
the existing genetic material [9].
The first successful treatment was of
X-linked SevereCombined
Immunodeficiency (X-SCID) by ex vivo
gene replacement therapy [11].

23.  In somatic gene therapy, the therapeutic genes are introduced into
somatic cells, which restricts the effects of the individual and are not
passed on to the next generation.
 In germ line gene therapy, either the
sperm or egg can be altered by introducing
the therapeutic gene, which gets integrated
into the genome

24. The other various gene therapies are
suicide gene therapy, immunologic gene
therapy, excision gene therapy, etc.
Suicide gene therapy is the introduction
of a gene into the cell, which converts a
non-toxic pro-drug into a toxic substance
and it is also called genetic pro-drug
activation therapy .

25. Immunologic gene therapy aims at increasing the
immunologic potential of the tumor cells, thereby increasing
the patient’s immune response to the tumor.
 Excision gene therapy is the removal of oncogenes, which
inhibits the growth of the tumor cells.

30. Gene transfer into tumor cells has
been demonstrated via
administration of therapeutic
nucleic acids (TNAs) ex vivo and/or
in vivo (Figure 1)
ex vivo approach, patient-derived
tumor cells are collected, propagated
usually as 2D monolayers,
manipulated genetically and then
introduced back into the host [20].
Pharmaceutics 2020, 12(3), 233; https://doi.org/10.3390/pharmaceutics12030233

31. In the in vivo approach, TNAs may be introduced in loco into the
tumor cells, systemically via intravenous administration, or in a
pre-systemic manner through oral, ocular, transdermal or nasal
delivery routes, depending of the specific localization of tumors and
disease progression.

35. •The guide RNA hybridizes with the
target DNA. Cas-9 recognizes this
complex and should mediate
cleavage of the DNA double strand
and reparation in the presence of a
(homologous) donor DNA.The result
of this process is the integration of an
exogenous sequence into the
genome (knock-in) or allele
substitution.

36. In the case of systemic and pre-systemic deliveries, the
administration of nakedTNAs is hindered by biological
barriers, nuclease susceptibility, phagocyte uptake, renal
clearance and/or immune response stimulation.

37. Hence, the use of stable
carriers/vectors that protect the
nucleic acid cargo from circulatory
nucleases, avoid the immune system,
and ensure the efficient targeting
of the therapeutic vector into the
tumor cells,
without dissipation in the
body through lymphatic and
blood systems and avoiding
non-target cells is required
[21].

39. VIRAL VECTORS

40. Genetic material is delivered into the
host cells through viruses or bacteria
[14].
Gene therapy is concerned with DNA which
can be delivered into cells by various
methods.
All viruses such as retroviruses, adenoviruses,
lentiviruses, herpes simplex virus,vaccinia, pox
virus, and adeno-associated virus bind to their
hosts by introducing their genetic materials
into the host cells.

41. In gene therapy, the viral DNA can be removed, while the
viruses can act as vehicles to deliver the therapeutic DNA into the
host cells.
The viruses which are used as vectors in gene therapy include
retroviruses, adenoviruses, adeno associated viruses and Herpes
simplex virus [1].

42. The methods of non-viral gene therapy include the injection of naked
DNA, electroporation, the gene gun and the use of oligonucleotides,
dendrimers and inorganic nanoparticles.
However, the non-viral vectors which are
inhibited by the serum components, limit the
efficiency of the gene delivery in vivo [15].
 Despite the use of several non viral
methods, viruses provide a more efficient
mode in gene therapy [16].

50. NON-VIRAL VECTORS

51. The success of cancer gene therapy
relies on a safe, effective and
controllable vector.Viral vectors were
the first platform proposed for gene
therapy.
Indeed, the nature and properties of
viruses made them tempting vectors
for RNA and DNA delivery to human
cells, with multiple clinical trials that
ended-up in clinical approved of some
gene therapy drugs.

52.  However, the immunogenicity, limited genetic-load, cancer
risk due to therapeutic payload insertion near genes that
control cell growth, and constrained mass-production of viral
vectors prompted the development and engineering of non-
viral vectors, supported by nanomedicine .

53. Nanotechnology refers to the area of science focused
on the study of the synthesis, characterization and
application of materials and functional systems of particles
whose size is between 1-100 nm .

54. Nowadays, the interest in these materials is not only due to their
small size, but also to their unique physical (electric, optical,
magnetic) and chemical properties at these dimensions (in
comparison to the same material at the macroscopic scale), conveying
a more scalable interaction with cells and biomolecules.

55. The application of nanoparticles as carriers in gene
therapy is one of the most promising technologies in
biomedical research due to its ease and straightforward
synthesis and functionalization with different moieties, low
immunogenicity and toxicity.

56. One of the interests is focused on the development of
biocompatible and more effective transfection systems [36,37] to
vectorizeTNAs to cells and tissues, such as DNA (e.g., plasmid DNA,
antisense oligonucleotides (ASO)) or RNA (e.g., microRNA (miRNA),
short hairpin RNA (shRNA), small interfering RNA (siRNA)) into cells

57. Some of the limitations
in the efficiency of
transfection of naked
plasmid DNA (pDNA) or
siRNAs can be improved
by the application of
functionalized
nanoparticles.
However, this technology
still faces several
shortcomings that need
to be addressed,
including lower
transfection efficiency
when compared to viral
vectors, or blood
clearance before
reaching the target site in
the case of systemic
administration.

58.  EffectiveTNA agents require a vector that can travel through
the circulatory system, accumulate in the tumor, enter target cells
via endosome pathway and be able to escape the endosome to
efficiently accomplish cargo delivery .

64. The therapies that express gene products, which result in
the death of cancer cells, include, gene addition therapy,
gene excision therapy, antisense RNA therapy,
immunotherapy,
The main objective of gene therapy is to introduce new genetic
material into the target cells without causing any sort of damage to
the surrounding normal cells.

65. suicide gene therapy, gene
therapy with the use of oncolytic
viruses, the introduction of
genes to inhibit tumor
angiogenesis and the delivery
of drug resistance genes into
normal tissues for protection
against chemotherapy [6].
According to the results of
animal studies which were done
in mice, combination gene
therapy which uses several
genes, showed significant tumor
regression in mice [17,18].

66. Gene AdditionTherapy

67. Genetic alterations include
mutations of p53, the
RetinoblastomaGene (RB1),
p16 and p21.
Among which, the tumor
suppressor gene which is
commonly used in gene therapy
is the p53 gene [19] and about
60% of the human tumors are
associated with mutations at the
p53 locus [20].
In this technique, the tumor
growth is controlled by the
introduction of tumor
suppressor genes which
inactivate the carcinogenic cells
[6].

68. In this technique, the defective oncogenes are removed, as a result of
which, there is an inhibition in the growth of the tumor cells [6].
Journal of Clinical and Diagnostic Research. 2013 June, Vol-7(6): 1261-1263 1261
Gene ExcisionTherapy

69. The Antisense RNA checks the tumor growth by inhibiting the
RNA which is complementary to the strands of the DNA which
expresses that particular gene [6].

70. Immunotherapy

75. Suicide GeneTherapy
Many studies have been done on the
gene delivery system with retrovirusor
adenovirus vectors [21-24].
This therapy involves enzymes, the
expression of which transforms the non-
toxic producing drug into an active cytotoxic
substance [6].
It is the most commonly used gene therapy
which uses thimidine kinase or other chemo
sensitizing genes [25].

78. GeneTherapy with the Use of Oncolytic Viruses
 In this therapy, a vector (virus) is genetically modified,
which replicates and lyses the tumor cells [6].
 For example, adenovirus mediated gene therapy is used
for advanced cancers than traditional therapies [26].

79. The Delivery of Drug Resistance Gene(s) to Normal
Tissues for Protection from Chemotherapy
The drug resistance genes
protect the normal tissues which
are vulnerable to destruction.
The drug resistance gene in
humans is the Multidrug
Resistance-1 (MDR-1) gene.
The other drug resistance genes
include the bacterial
nitroreductase gene and the
dihydrofolate reductase
mutants which protect against
methotrexate [6].

80. Advantages of gene therapy

81. Advantages of gene
therapy as well;
i. A functional gene
has the ability to
replace a defective
gene.
ii. Gene therapy aids
in the prevention
against the
potentially toxic
effects in the body,
which can be caused
by other therapies.
iii. It decreases the
cost of various
therapies and
improves the patient’s
life style for a longer
period [27].

82. Some Disadvantages of GeneTherapy

83. Even after the therapeutic DNA is
integrated into the genome, some
cells prevent the gene therapy
for long-term effects, for which
patients may undergo multiple
rounds of the gene therapy.
ii.There is a possibility that
the host’s immune system
and its response may
reduce the effectiveness of
the gene therapy.
iii. The viral vectors can
present a variety of
potential problems to the
patient, such as toxicity and
immune and inflammatory
responses.

84. iv. Single gene disorders are the best candidates for gene therapy.
But, some of the most commonly occurring diseases, such as heart
disease, high blood pressure,Alzheimer’s disease, arthritis, and
diabetes are multi-gene or multi-factorial disorders which are
difficult to treat effectively with the use of gene therapy.
v. If the DNA is introduced into a wrong place in the genome, for
example, into a tumor suppressor gene, it can induce aTumor.

86.  Oral cancer is associated with genetic mutations which
occur due to the exposure to tobacco, alcohol, betel quid,
etc [3].
 Oral cancer, namely, Oral Squamous Cell Carcinoma (OSCC)
is one of the commonly seen malignant lesions in the oral
cavity, which is seen globally [1] and it is about the 6th most
common cancer world-wide [2].

87. It occurs in people who are aged 50 years or over. However, about 6%
of the cases occur in young people who are under the age of 45 years.
 It is a malignant disorder in which the genes that
control cell growth and apoptosis are mutated and this
results in an uncontrolled proliferation of the cells in
the tumor [1].
 With an increase in the total cell mass in the tumor,
it has the ability to invade and undergo metastasis.

88. The patients with cancer
usually remain resistant
to the standard therapies
which are used readily.
But, there may be
chances of acute and
chronic toxicities, as well
as secondary
malignancies.
Hence, to improve the
treatment modality and
the over- all survival
rates, gene therapy has
emerged in the field of
bio-medicine, which
replaces the defective
gene and this is repaired
by a therapeutic gene [6].

89. The applications of gene
therapy in cancer are
associated with bio-
engineering and clinical
development [7].
In the dental field, it is also
applied in bone repair, auto
immune diseases, pain,
caries and periodontal
diseases [8].