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Li et al. Ageing Neur Dis 2022;2:12 https://dx.doi.org/10.20517/and.2022.14 Page 5 of 13

[88]
bodies or Lewy neurites . Similar to AD, most PD cases are sporadic, but roughly 10% of cases are familial,
[89]
caused by mutations in the genes encoding α-Synuclein, PINK1, Parkin, LRRK2, and DJ-1 . Accordingly,
many genetically modified mouse models were developed to carry the PD gene mutations, but none of them
[90]
can recapitulate the dopamine neuron degeneration in PD .

PD has long been considered as a human-specific disease; however, recent studies revealed that aged NHPs
[92]
[91]
display prominent synucleinopathy , and a monkey naturally showing PD symptoms was identified ,
underlining the great potential of NHPs for PD research. Due to technical difficulties in generating
transgenic NHP PD models, most NHP models for PD were generated using chemical toxins. 6-
hydroxydopamine (6-OHDA) (a hydroxylated analog of dopamine) or 1-methyl-4-phenyl-1,2,3,6-
tetrahydropyridine (MPTP) can selectively target SN dopaminergic neurons [93,94] , inducing typical behavioral
manifestations despite the absence of Lewy pathology. Since the 1980s, the MTPT-induced monkey model
has been prevalent for drug and therapy tests . However, phenotypes of this toxin-induced model are
[94]
unstable and do not exactly mirror slow disease progression. Recently developed protocols attempt to
address this issue by stereotaxic injection of MPTP into multiple SN sites . Interestingly, in a monkey
[95]
model with repeated MPTP administrations, alpha synucleinopathy was detected , which opens up a new
[96]
avenue for researchers to establish a better PD animal model.

Since α-Synuclein is the core component of Lewy bodies, many attempts have been made to generate NHP
models by overexpressing human mutant α-Synuclein in the monkey SN . Highly efficient expression of
[97]
A53T α-Synuclein in SN can be achieved by stereotaxic brain injection, giving rise to pathological hallmark
Lewy neurites that can be detected four months post-surgery and loss of over 50% dopaminergic
neurons . However, no motor dysfunction and α-Synuclein spreading are displayed . A longitudinal
[97]
[97]
study of disease progression with this model in the future may bring novel insights. An α-Synuclein
transgenic PD monkey model carrying a clinical mutation was generated by expressing mutant transgenes
in fertilized embryos with lentivirus, and these monkeys showed impaired finger coordination and cognitive
dysfunction at three years of age . Because PD is an age-dependent disease, Yang et al. performed
[98]
intracerebral injection of mutant α-Synuclein into the SN in monkeys at different ages and found that aging
greatly promotes the accumulation of α-Synuclein and alpha synucleinopathy .
[99]

As described above, all genetically modified mouse models of PD are unable to develop degeneration of
dopamine neurons, a key pathological hallmark [90,100] . However, recent studies on establishing PINK1-
targeted monkeys via CRISPR/Cas9 indicate that severe neuronal loss occurs in the monkey brains when
PINK1 is lost [101,102] . PINK1 mutations cause autosomal recessive PD with early-onset manifestation. Because
of the loss of function mechanism for PD with PINK1 mutations, several groups tried to create PD monkey
models by CRISPR/Cas9-mediated PINK1 knockout in fertilized monkey embryos [101,102] . It seems that a
large DNA fragment deletion in the PINK1 gene mediated by CRISPR/Cas9 can lead to a severe loss of
neurons in the monkey brain [101,103] . However, most PINK1 mutations in humans are point mutations that
may partially affect PINK1’s function to cause age-dependent neurodegeneration. Thus, CRISPR/Cas9-
mediated deletion of PINK1 may completely eliminate the function of PINK1, allowing identification of its
critical role in neuronal survival in the primate brain . Indeed, Chen et al. used a single strand cutting
[104]
enzyme D10A nickases to target monkey PINK1 but did not find PD symptoms in the generated monkey
model . Conversely, focal disruption of the PINK1 and DJ-1 genes in the adult monkey brain was able to
[102]
mimic some PD pathology , suggesting that both types of genetic mutations and aging could be important
[105]
for developing PD phenotypes in monkeys.