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Page 8 of 11 Agresti et al. J Transl Genet Genom 2018;2:9 I http://dx.doi.org/10.20517/jtgg.2018.05

Table 3. Corrective gene therapy approaches for MERRF and MELAS
Gene therapy approach Mitochondrial disease
MERRF MELAS
Nucleic acid delivery Kolesnikova et al. [43] , 2004 Karicheva et al. [44] , 2011
Mahata et al. [41] , 2005
Mahata et al. [42] , 2006
Peptide-mediated therapy Chang et al. [50] , 2013 Chang et al. [52] , 2017
[51]
Chang et al. , 2013 Li and Guan [47] , 2010
Muratovska et al. [53] , 2001 Park et al. [46] , 2008
Perli et al. [49] , 2016 Perli et al. [48] , 2014
Taylor et al. [54] , 1997 Perli et al. [49] , 2016
Sasarman et al. [45] , 2008
mitoTALENs Bacman et al. [55] , 2015 Bacman et al. [55] , 2015
Hashimoto et al. [56] , 2015 Hashimoto et al. [56] , 2015

Moreover, peptide-mediated therapy uses peptides to facilitate restoring mitochondrial function.
Overexpression of mitochondrial translation elongation factors, namely EFTu or EFG2, results in the partial
suppression of respiratory chain deficiency in MELAS myoblasts, but not in MERRF myoblasts . Additionally,
[45]
researchers determined that overexpression of leucyl-tRNA synthetase corrected mitochondrial dysfunction
in human mitochondria carrying either the nearly homoplasmic A3243G or A83443G mutation [46-49] .
Specifically, Perli et al. demonstrated the carboxy-terminal domain of leucyl-tRNA synthetase is sufficient
[48]
for cell rescue, and is even more efficient than the whole enzyme at doing so. Researchers were able to further
isolate the rescuing activity of the carboxy-terminal domain of leucyl-tRNA synthetase; with this knowledge,
they designed short peptide sequences capable of cell rescue that effectively bind with high affinity to both
wild-type and mutant human mt-tRNALeu(UUR) and mt-tRNALys . Chang et al. [50,51] conducted several
[49]
studies suggesting that a Pep-1 peptide delivery system is capable of rescuing mitochondria containing the
MERRF mutation. Pep-1, a cell-penetrating peptide, can translocate mtDNA from wild-type mitochondria
into cybrid cell models of MERRF [50,51] . After three days of treatment with Pep-1-mediated mitochondrial
delivery, mitochondrial function was recovered and cells were maintained for twenty-one days . Recently,
[50]
[52]
Chang et al. demonstrated that Pep-1-mediated mitochondrial delivery can ameliorate mitochondrial
function in cells containing the pathogenic MELAS mutation. Further, Muratovska et al. and Taylor et al.
[54]
[53]
independently synthesized peptide nucleic acids (PNA) complementary to the sequence containing the MERRF
mutation; upon entry into the mitochondria, PNA oligomers are able to selectively inhibit replication and
translation of the mutant mtDNA template using in vitro and cell-based studies, respectively.

Lastly, mitochondrial-targeted transcription activator-like effector nucleases (mitoTALENS) are designed to
cleave specific sequences of mtDNA that contain a pathogenic mutation [55,56] . When tested on heteroplasmic
cybrid cells harboring MERRF and MELAS, respectively, mitoTALENS permanently reduced the mutant
load and resulted in the return to normal mitochondrial respiratory activity [55,56] .

CONCLUDING REMARKS AND FUTURE DIRECTIONS
A review of gene therapy strategies targeting the mtDNA disease’s, MERRF and MELAS, presents a series
of realistic possibilities for their potential correction. Understanding the backstory of mitochondrial biology
sets the tone for further understanding the path of mitochondrial dysfunction when considering an mtDNA-
causing mitochondrial disease. While multiple forms of care exist for patients living with either MERRF or
MELAS, none are capable of permanently correcting its associated mitochondrial mutation. Even if a certain
approach is capable of restoring mitochondrial function, one must be prepared to answer several general
arguments: (1) does the treatment approach lead to permanent or transient correction; (2) if the treatment
was initially conducted in an in vitro setting, does it have the potential to be translated to an in vivo setting;
and (3) if it can be adapted to an in vivo environment, what type of an immunogenicity is it met with?
Rationally speaking, a researcher targeting MERRF or MELAS diseases must be both practiced and prudent