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Bax. J Transl Genet Genom 2020;4:1-16 I http://dx.doi.org/10.20517/jtgg.2020.08 Page 11

the high expression of thymidine phosphorylase in hepatic tissues of unaffected individuals, this being six
times higher than activities found in the bone marrow [105] . The procedure was first applied to the treatment
of a severely affected patient; 13 months post OLT there were mild improvements in the neurological
features, a reduction in the cerebral lactate concentration and a normalisation of the plasma metabolites.
No clear changes in gastrointestinal function were recorded and the patient continued to receive parenteral
[85]
nutrition . Two further patients have since received OLT, although it is understood that there are other
unreported cases of patients with MNGIE who have been successfully transplanted [104,106] . Both patients
demonstrated normalisation of metabolites soon after OLT, with one patient having a reported increased
body mass index and quality of life 90 days post-transplant and the other patient having an improved
walking ability and motor strength, although still requiring total parenteral nutrition at one-year post
transplant. Unlike AHSCT, OLT does not require a conditioning treatment. Patients with MNGIE often
have disease-related hepatopathies, and, considering that OLT has a high survival rate, this approach
presents an ideal solution to simultaneously rescue thymidine phosphorylase activity and replace a
diseased liver. However, the administration of parenteral nutritional support post-transplant should be
[79]
carefully considered due to the association of its use with hepatic steatosis and elevated transaminases .
OLT is limited by the availability of matched donors and can potentially present risks from transplantation
complications and the use of long-term immunosuppression therapy.

Pre-clinical experimental therapeutic approaches
Enzyme replacement with polymeric non-reactors
Another approach to enzyme replacement that has been investigated is the encapsulation of thymidine
phosphorylase in polymeric nanoreactors [107] . The nanoreactors were designed to have nucleoside
specific porin Tsx as channel forming proteins integrated in the walls of the nanoreactor, thus permitting
the efficient transport of the enzyme’s substrates and products. In vitro studies demonstrated that the
nanoreactors were stable, showing no leakage of protein when incubated for four days at 37 °C in mouse
serum. However, thymidine phosphorylase activity declined by 50% after three days. The nanoreactors
produced no cytotoxic effects on hepatocytes isolated from rats, and no inflammatory responses were
observed in mice that received intra-peritoneal administration [107] .

Gene therapy
Gene therapy offers a potential curative option for MNGIE and two promising gene therapy approaches
have been investigated using the Tymp-/-Upp1-/- double knockout mouse, a disease model which
recapitulates the metabolic imbalances of MNGIE [108] . The first approach employed a lentiviral vector which
was targeted to the haematopoietic progenitor cells with the aim of restoring the activity of thymidine
phosphorylase in the blood cells. Although metabolic correction of the biochemical abnormalities was
achieved, the survival time of the treated mice was reduced, which was a consequence of the procedure
requiring myelosuppression [109-111] . In the second approach, mice were treated with an adeno-associated
virus vector containing the TYMP coding sequence which was targeted to the liver. These studies revealed a
normalisation of the biochemical perturbations without any adverse effects [112,113] . These studies support the
feasibility of gene therapy for the treatment of MNGIE.

CONCLUSION
Patients with MNGIE present with a heterogenous array of symptoms, making diagnosis challenging
for non-specialist healthcare professionals unfamiliar with the condition. Patients are invariably
underdiagnosed and undergo a number of invasive clinical procedures before obtaining a correct diagnosis
by laboratory testing. Several experimental therapies have been developed over the past two decades which
have successfully achieved metabolic correction. Clinical responses to treatment have been particularly
positive in those patients who are at early stages of the disease trajectory, thus reinforcing the need to