**5.2 Gene therapy for AD**

*Geriatric Medicine and Gerontology*

disease treatment are summarized in **Table 4**.

**5.1 Genetics of Alzheimer's disease**

**5. Gene and stem cell therapy in Alzheimer disease**

are thought as associated with neuropathological conditions [95].

of EOAD could be enlightened by dominant APP gene mutations [97].

Another protein that is strictly associated with the progression of AD is PSEN1 as it is the principal component of γ-secretase complex. Since neurotoxic fragments are formed by proteolytic function of γ-secretase on APP, PSEN1 gene mutations give rise to abnormal activity of the proteolytic enzyme leading to abnormal or longer Aβ fragments and, therefore this contributes to development of EOAD [95]. More than 180 autosomal dominant PSEN1 mutations associated with AD have been reported, which makes PSEN1 significantly important protein in the occurrence of EOAD [98]. Disease-causing PSEN1 gene mutations, showing complete penetrance, accounts for majority of EOAD (approximately 80%) and these mutations are defined as the most common cause of the disease [99]. Lastly, the gene PSEN2 is also coding for one subunit of γ-secretase, the aspartyl protease generates Aβ. Missense mutations are reported in PSEN2, which are rarely genetic basis of EOAD [100]. In total, as mentioned in Zou's review article in 2013, majority of the diseasecausing mutations identified for the EOAD have been reported in PSEN1 gene (approximately 78%), followed by APP mutations (17%) then with rare PSEN2

Technological advances in sequencing methods over the past decade allow researchers to investigate AD thoroughly, especially genetic fundamentals of the

Early onset AD (Familial/EOAD) represent <5% of all cases of AD. APP (Amyloid beta (A4) precursor protein), PSEN1 (Presenilin 1), and PSEN2 (Presenilin 2) genes mutations are exclusively considered as a basis for EOAD in most cases [94]. APP, a transmembrane protein in neuron cells, is cleaved by β-secretase and γ-secretase, respectively, to produce β-amyloids (Aβ) and some other side products [96]. Since neurotoxic consequences of altered Aβ ratios like neurodegeneration resulting from aberrant synaptic function take place in brain, APP mutations have continuously been investigated. Yet, only approximately 15%

NCT03249688) [88–92].

exercise and diet are the focuses for primary prevention of AD (NCT01767909,

Deep brain stimulation is a novel therapeutic strategy for AD. One trial is ongoing in patients with mild AD (NCT03622905). Other strategies of Alzheimer's

Both age and family history are important risk factors for AD. The risk of developing AD increases for one who has a first-degree relative with AD when compared to the general population. AD can be grouped into two subtypes with respect to age of onset. Most of the AD cases (>95%) are late-onset AD (sporadic/LOAD) (above age 65) that is considered to be multifactorial [93]. Many susceptibility genes for LOAD have been defined thanks to genome-wide association studies (GWAS) and several other sequencing analyzes. For instance, one of the well-studied genetic risk factors for LOAD is an alteration in Apolipoprotein E (APOE) coded by the gene localized to 19q13 [94]. APOE is a multifunctional protein which serves a number of functions in neuronal activities. In brain tissue, there are three main isoforms that are diversified by each other by different one amino acid, which are APOEε2 (Cys112, Cys158), APOEε3 (Cys112, Arg158) and APOEε4 (Arg112, Arg158). The differences between these three APOE isoforms have a significant impact on the structure and function of APOE at molecular and cellular levels. Therefore, those

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gene mutations (approximately 5%) [94].

Discovering risk loci by GWAS studies may help to enlighten the biological mechanisms underlying AD because the reported genes might have been target for medicines, thereby this issue promises further investigation in order to improve gene therapy strategies and thus precision medicine concept for AD [101].

Over time, gene delivery of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), APOE, ECE (endothelin-converting enzyme) have been investigated in several animal models of AD. Endothelin-converting enzyme (ECE) is protease involved in the degradation of Aβ peptides. Intracranial administration of five recombinant adeno-associated viral vector (rAAV) containing the ECE-1 synthetic gene showed reduced Aβ in the anterior cortex and hippocampus in APP-PS1 transgenic mice. Use of AAV vector encoding anti-Aβ Ab in Tg2576 mice results in a significant decrease in Aβ level in the brain of subjects. These results support its use for the prevention and treatment of AD [102].

The first clinical trial using Adeno-Associated Virus delivery of NGF has been accomplished and the results indicate amelioration of AD pathogenesis. Clinical trials were conducted using CERE-110 that is an AAV2/2 vector containing full length NGF transgene for the treatment of AD patients. These trials confirmed that AAV2- NGF delivery was well tolerated with a high level of safety and no systemic toxicity but did not affect clinical outcomes or selected AD biomarkers (NCT00087789, NCT00876863) [103].

### **5.3 Stem cell treatment for AD**

Stem cells (SCs) are continuously capable of self-renewing and differentiating into specialized cells. Accordingly, SC therapy is surely becoming a promising strategy in the treatment of neurodegenerative diseases including AD owing to the capacity of SCs to migrate and reach areas of the brain. SCs are classified into four groups; embryonic stem cells (ESCs), induced pluripotent stem cells (iPSCs), mesenchymal stem cell, and neural stem cells [104].

### *5.3.1 Embryonic stem cells (ESCs)*

ESCs, called as pluripotent, are derived from the inner cell mass of blastocyst because they have the ability to develop cell types from the ectoderm, mesoderm, and endoderm germ layers [105]. ESCs may an excellent cell replacement therapy approaches for transplantation in AD [104]. In vitro studies have been successful to differentiate ESCs into specific neuronal cell types like dopaminergic neurons and these studies show that the role of ESCs and their derivatives reduce AD pathology in rodent models [106, 107].

Several studies reveal that ESC-derived NSCs can be safely transplanted without tumorigenesis despite the fact that undifferentiated ESCs have risks of tumor formation, transplantation rejection and immune responses [106, 108, 109]. Experiments conducted on human ESCs have been able to generate dopaminergic neurons, spinal motor neurons and astroglial cells [110]. Some studies demonstrated use of retinoic acid (RA) induce direct differentiation of human ESCs into basal forebrain cholinergic neurons (BFCNs). Tang et al. showed that ESC-derived NPC transplantation into an Aβ-injured rat model improves memory impairment compared to sham controls [106].

#### *5.3.2 Induced pluripotent stem cells (IPSCs)*

Induced pluripotent stem cells could be generated from adult cells by the overexpression of key transcription factors (OCT4, SOX2, KLF4, LIN28, and NANOG) [111, 112]. iPSCs are in general similar to embryonic stem cells (ESCs) in morphology, gene expression profile and potential of differentiation [113].

Human iPSCs derived from AD patients' somatic cells can provide a new perspective to develop new strategies for disease modeling. Yagi et al. showed that fAD-iPSC-derived differentiated neurons have increased amyloid β42 secretion, responds to γ-secretase inhibitors and modulators, indicating the potential for identification and validation of candidate drugs [114]. Takamatsu et al. used iPSCs to derive macrophage-like myeloid lineage cells that could express neprilysin which is a protease with Aβ-degrading activity [115].

Recent studies have shown reprogramming structural chromosomal abnormalities and aberrant DNA methylation patterns in hiPSCs [116]. iPSCs can be edited by gene editing technologies like recombinant homologous, transcription activatorlike effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR-cas9) and can function as more suitable for cell transplantation.

#### *5.3.3 Mesenchymal stem cells (MSCs)*

Mesenchymal stem cells (MSCs) are adult multipotent progenitors and can be obtained from various adult tissues including bone marrow, peripheral blood, umbilical cord, adipose tissue, amniotic fluid. MSCs are most favored cell types in the treatment of AD due to their accessibility, relative ease of handling, secretion of a wide range of cytokines, easily transplanted intravenously into patients, and lack of ethical issues.

Most important features of ESCs is a wide range of differentiation potentials including neuronal cells [110]. Park et al. reported that transplanted human adipose tissue derived mesenchymal stem cells (ADMSCs) differentiate into neural cells in the brain and these cells can restore cognitive functions of mice by increasing acetylcholine synthesis, brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and restoring neuronal integrity [117]. In addition, MSC transplantation has been shown to inhibit Aβ and tau-related cell death, and to reduce Aβ residues and plaque formation by modulating neuroinflammation [118, 119]. It has been reported that bone marrow-derived mesenchymal stem cells provide a reduction in Aβ deposits and facilitate changes in key proteins required for synaptic transmissions such as dynamin 1 and synapsin 1 [120].

#### *5.3.4 Neural stem cells (NSCs)*

Transplantation of growth factor-secreting NSC was reported to increase neurogenesis and cognitive function in a rodent AD model [121]. And the overexpression of NSC derived cholinergic neurons restored cognitive performance and synaptic integrity in a rodent model [122].

#### **6. Conclusion**

Alzheimer's disease is a progressive neurodegenerative disease that affects the central nervous system. Many complex pathological and genetic features have been

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provided the original work is properly cited.

*Future Treatment of Alzheimer Disease DOI: http://dx.doi.org/10.5772/intechopen.85096*

effective strategies for AD prevention.

**Author details**

Zerrin Yıldırım<sup>5</sup>

Turkey

Turkey

Ahmet Onur Keskin1

, Nazlı Durmaz2

3 Neurology Department, Eskisehir City Hospital, Turkey

1 Neurology Department, Adana Hospital, Baskent University, Turkey

, Nese Tuncer6

, Gülgün Uncu3

described in the disease. Aβ aggregation, tau aggregation, metal dyshomeostasis, oxidative stress, cholinergic dysfunction, inflammation and downregulation of autophagy based on pathophysiological changes occur during the onset and progression of AD have been proposed. There is no effective treatment currently, however, at present, current drug treatments of AD, such as cholinesterase inhibitors or NMDA antagonists, mainly help to manage symptoms hereby obviating the need for new approaches to deal with AD underlying mechanisms. Ongoing advances in the knowledge of pathogenesis, in the identification of novel targets, in improved outcome measures, and in identification and validation of biomarkers may lead to

2 Neurology Department, Medical Faculty, Eskisehir Osmangazi University, Turkey

4 Medical Genetic Department, Medical Faculty, Eskisehir Osmangazi University,

5 Neurology Department, Istanbul Bagcilar Education and Research Hospital,

6 Neurology Department, Medical Faculty, Marmara University, Turkey

7 Neurology Department, Acibadem Eskisehir Hospital, Turkey

\*Address all correspondence to: demetozbabalik@gmail.com

and Demet Özbabalık Adapınar7

, Ebru Erzurumluoglu4

\*

,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

#### *Future Treatment of Alzheimer Disease DOI: http://dx.doi.org/10.5772/intechopen.85096*

*Geriatric Medicine and Gerontology*

compared to sham controls [106].

*5.3.2 Induced pluripotent stem cells (IPSCs)*

is a protease with Aβ-degrading activity [115].

transmissions such as dynamin 1 and synapsin 1 [120].

*5.3.4 Neural stem cells (NSCs)*

integrity in a rodent model [122].

*5.3.3 Mesenchymal stem cells (MSCs)*

NPC transplantation into an Aβ-injured rat model improves memory impairment

Induced pluripotent stem cells could be generated from adult cells by the overexpression of key transcription factors (OCT4, SOX2, KLF4, LIN28, and NANOG) [111, 112]. iPSCs are in general similar to embryonic stem cells (ESCs) in morphol-

Human iPSCs derived from AD patients' somatic cells can provide a new perspective to develop new strategies for disease modeling. Yagi et al. showed that fAD-iPSC-derived differentiated neurons have increased amyloid β42 secretion, responds to γ-secretase inhibitors and modulators, indicating the potential for identification and validation of candidate drugs [114]. Takamatsu et al. used iPSCs to derive macrophage-like myeloid lineage cells that could express neprilysin which

Recent studies have shown reprogramming structural chromosomal abnormalities and aberrant DNA methylation patterns in hiPSCs [116]. iPSCs can be edited by gene editing technologies like recombinant homologous, transcription activatorlike effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR-cas9) and can function as more suitable for cell transplantation.

Mesenchymal stem cells (MSCs) are adult multipotent progenitors and can be obtained from various adult tissues including bone marrow, peripheral blood, umbilical cord, adipose tissue, amniotic fluid. MSCs are most favored cell types in the treatment of AD due to their accessibility, relative ease of handling, secretion of a wide range of cytokines, easily transplanted intravenously into patients, and lack of ethical issues. Most important features of ESCs is a wide range of differentiation potentials including neuronal cells [110]. Park et al. reported that transplanted human adipose tissue derived mesenchymal stem cells (ADMSCs) differentiate into neural cells in the brain and these cells can restore cognitive functions of mice by increasing acetylcholine synthesis, brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and restoring neuronal integrity [117]. In addition, MSC transplantation has been shown to inhibit Aβ and tau-related cell death, and to reduce Aβ residues and plaque formation by modulating neuroinflammation [118, 119]. It has been reported that bone marrow-derived mesenchymal stem cells provide a reduction in Aβ deposits and facilitate changes in key proteins required for synaptic

Transplantation of growth factor-secreting NSC was reported to increase neurogenesis and cognitive function in a rodent AD model [121]. And the overexpression of NSC derived cholinergic neurons restored cognitive performance and synaptic

Alzheimer's disease is a progressive neurodegenerative disease that affects the central nervous system. Many complex pathological and genetic features have been

ogy, gene expression profile and potential of differentiation [113].

**100**

**6. Conclusion**

described in the disease. Aβ aggregation, tau aggregation, metal dyshomeostasis, oxidative stress, cholinergic dysfunction, inflammation and downregulation of autophagy based on pathophysiological changes occur during the onset and progression of AD have been proposed. There is no effective treatment currently, however, at present, current drug treatments of AD, such as cholinesterase inhibitors or NMDA antagonists, mainly help to manage symptoms hereby obviating the need for new approaches to deal with AD underlying mechanisms. Ongoing advances in the knowledge of pathogenesis, in the identification of novel targets, in improved outcome measures, and in identification and validation of biomarkers may lead to effective strategies for AD prevention.
