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Li et al. Ageing Neur Dis 2022;2:13 https://dx.doi.org/10.20517/and.2022.13 Page 5 of 13

progressive loss of upper and lower MNs, muscle atrophy, and eventually paralysis, and they usually die
within 3-5 years after the onset of symptoms [36,37] . Currently, the pathophysiological mechanism of ALS
remains to be fully understood. Genetic studies have identified more than 30 gene mutations that are highly
associated with the etiology of ALS, including copper/zinc superoxide dismutase 1 (SOD1) and TAR DNA-
binding protein 43 (TDP-43). Mutations of these genes affect many cellular and molecular processes,
leading to increased oxidative stress, mitochondrial dysfunction, excitatory toxicity, neuroinflammation,
protein aggregation, and abnormal RNA metabolism. The neuropathology of ALS is characterized by
protein aggregation and accumulation of ubiquitinated protein inclusion bodies in the neuronal cytoplasm.
In most ALS patients, SOD1 and TDP-43 are the main components of these inclusion bodies, suggesting
that SOD1 and TDP-43 are causative factors for the occurrence and development of ALS. Therefore, several
studies have generated pig models that express mutant SOD1 or TDP-43 and showed ALS-like phenotypes.

Chieppa et al. produced an ALS pig model using SCNT in combination with transfected somatic cells
[38]
expressing the G93A mutation of human SOD1 . In 2014, Yang et al. used similar techniques to generate
transgenic pigs that express the same SOD1 mutation. The transgenic pigs developed age-dependent
neuropathology and movement disorders, which recapitulate the features of the early disease symptoms
seen in human ALS . Moreover, transgenic mutant SOD1 pigs show intranuclear inclusions and an
[9]
[9]
association of SOD1 with the nuclear protein PCBP1, which were not seen in mouse brains . In addition to
SOD1, researchers also established transgenic miniature pigs expressing mutant TDP-43. They found that
transgenic TDP-43 was also distributed in the cytoplasm of neuronal cells resembling the pathology seen in
human ALS brain tissues , which was not found in many transgenic TDP-43 mouse models [40-42] . Therefore,
[39]
these pig models of ALS have a great value in studying the pathogenesis mediated by cytoplasmic mutant
TDP-43 or intranuclear SOD1.

Pig models of Huntington’s disease
Huntington’s disease (HD) is an autosomal dominant and age-dependent neurological disorder
characterized by motor dysfunction, cognitive decline, and psychological disturbance. Pathologically, HD is
characterized by selective neurodegeneration, which preferentially occurs in the striatum. Most HD patients
develop symptoms in middle age, and the symptoms worsen with age with patients usually dying 10-15
[43]
years after symptom onset . HD results from a monogenetic mutation of a CAG repeat expansion in the
exon1 of the gene Huntingtin (HTT). HTT is a multifaceted protein that is expressed ubiquitously and has
[44]
numerous roles . CAG repeat expansion (> 36 CAGs) in the HTT gene is translated to a polyglutamine
(polyQ) expansion that causes HTT to misfold and aggregate in the brain. HD transgenic mice and HD-KI
mice have been widely used, but their brains do not display the selective and striking neuronal loss seen in
human HD patients .
[45]

In 2001, a transgenic pig model for HD was produced by pronuclear microinjection. However, the
development of behavioral and neuropathological symptoms of HD in this transgenic pig model remains
unclear [46,47] . In 2010, researchers used SCNT to successfully establish a transgenic HD pig model expressing
N-terminal mutant HTT (1-208 amino acids) with 105Q. This pig model showed apoptosis in the brain and
died postnatally. However, mice expressing the same transgene did not produce the brain pathology seen in
pigs . Later, another group used lentiviral transduction of pig embryos to establish a transgenic minipig
[48]
model of HD expressing N-terminal mutant huntingtin (1-548 aa) under the control of human HTT
promoter. However, this pig model did not develop motor deficits at up to 40 months of age, although
[49]
mutant HTT mRNA and protein fragments were detected in the brain and peripheral tissues .