Table 2.
In vitro and in vivo models of MNGIE.
| Cell type | Investigation | Summary of findings | References |
|---|---|---|---|
| In vitro models | |||
| Healthy control and MNGIE fibroblasts | Contribution of thymidine phosphorylase deficiency to nucleotide pool imbalance | Decline in thymidine concentration in culture medium of healthy cells. MNGIE fibroblasts incapable of metabolising thymidine but released it | Spinazzola et al., 2002 |
| MNGIE fibroblasts | Role of thymidine phosphorylase deficiency and deoxynucleotide pool accumulation in mtDNA damage | Identification of 36 mtDNA point mutations, a TT to AA substitution and single nucleotide deletion in MNGIE cell lines. COX activity reduced and ROS production increased contributing to mtDNA mutations | Nishigaki et al., 2003 |
| HeLa cell line | Perturbation of deoxynucleoside pools in cultured cells to evaluate mtDNA damage | Cells cultured in 50μM thymidine showed expansion of TTP and dGTP pools and depletion of dCTP and dATP pools. Several mtDNA deletions observed | Song et al., 2003 |
| Healthy skin and lung quiescent fibroblasts | Association of mtDNA depletions with post-mitotic cells | Thymidine phosphorylated via mitochondrial TK2 in quiescent cells and via cytosolic TK1 in cycling cells. Absence of TK1 in quiescent creates a bias in TTP pools, contributing to mtDNA depletions | Ferraro et al., 2005 |
| Murine hepatocytes | Murine hepatocyte mitochondria as an in organello model to demonstrate mtDNA depletion is a result of deoxynucleoside depletion | Excess thymidine resulted in increased dTTP and consequent depletion of dCTP, due to competition of thymidine and cytidine for TK2, resulting in mtDNA depletion. Supplementation of dCTP restored mtDNA depletions | González-Vioque et al., 2011 |
| MNGIE-derived iPSCs | Differentiation of patient derived iPSCs into cerebral organoids as an in vitro model of the CNS | MNGIE cerebral organoids expressed neuronal progenitors, neurons, differentiated astroglial cells and myelinating oligodendrocytes. No difference in myelination patterns observed between MNGIE and healthy control organoids | Pacitti and Bax, in press |
| In vivo models | |||
| Murine KO (Tymp−/−/Upp1−/−) | Physiological function of thymidine phosphorylase. Ascertain if pathogenesis of MNGIE and mtDNA depletion and replication error were attributable to aberrant thymidine metabolism | 10-fold increase in plasma deoxyuridine and thymidine. Development of cerebral oedema and hyperintense T2 MRI regions, with dilation in axonal myelin fibers but no demyelination. No peripheral neuropathy observed. Lack of mtDNA abnormality in brain and muscle | Haraguchi et al., 2002 |
| Murine KO (Tymp−/−/Upp1−/−) | Characterization of the biochemical, genetic and histological features of MNGIE and specific tissues involved | Undetectable thymidine phosphorylase in all tissue except liver. Thymidine elevated by 4-65-fold in all tissues. MRI showed cerebral oedema and T2 hyperintensities, with late onset cerebral and cerebellar white matter vacuoles without demyelination or axonal loss. Detection of mtDNA depletion and histological abnormalities in the brain but without skeletal muscle and gastrointestinal system involvement | López et al., 2009 |
| Murine KO (Tymp−/−/Upp1−/−) | Role of deoxynucleoside accumulation in the pathogenesis of MNGIE. Recreation of the gastrointestinal phenotype by dietary supplementation with thymidine and deoxyuridine | 100-fold increase in thymidine concentrations. Acquisition of mtDNA depletion and histologically evident COX deficiency in brain and small intestine cells. Treated mice had reduced body masses and intestinal smooth muscle cells, and increased fibrosis, muscle weakness, leukoencephalopathy, and decreased survival | Garcia-Diaz et al., 2014 |