Supplementation studies with dAMP/dGMP in primary cultures of human myoblasts and myotubes of patients with mitochondrial DNA depletion caused by mutations in the DGUOK and POLG1 genes

Mitochondrial DNA depletion syndrome (MDS), a frequent cause of severe childhood (hepato)encephalomyopathies, is defined as a reduction of mitochondrial DNA copy number related to nuclear DNA in different tissues which leads to insufficient synthesis of respiratory chain complexes. MDS is responsible for approximately 30–40% of combined RC deficiencies in children. Mutations of eight nuclear genes (DGUOK, POLG1, MVP17. ECGF1, TK2, SUCLA2, SUCLG1 and RRM2B), all involved in the synthesis or maintenance of mitochondrial nucleotide pools, were identified in approximately 40% of all MDS cases implying further genetic heterogeneity. It was previously shown that mtDNA depletion can be prevented by dGMP and dAMP supplementation in culture of deoxyguanosine kinase-deficient fibroblasts. Since then, similar experiments were not published.

We performed experiments on human primary myoblasts of patients carrying pathogenic mutations in DGUOK and POLG1. After supplementation with dGMP and dAMP alone and in combination, mtDNA copy number and biochemical analysis of the cytochrome c oxidase (COX) were tested.

Serum deprivation and myotube formation triggered a decrease in the mtDNA copy number in primary myotubes of patients carrying pathogenic mutations in DGUOK or POLG1. MtDNA copy number decreased significantly in myotubes of patients with both gene defects but not in controls. Supplementation of the cell culture medium with dGMP and dAMP alone and in combinations rescued the mtDNA depletion in DGUOK deficient cells, but not in myotubes carrying compound heterozygous mutations in POLG1. The effect of the administration of dGMP and dAMP were significant and reproducable.

Our results show that supplementation with dGMP/dAMP can increase mtDNA copy number in DGUOK deficient myotubes. The lack of an improvement of mtDNA depletion in POLG1 deficiency reflects a heterogeneous pathomechanism of mtDNA depletion. No adverse effect was observed on high dose supplementation. Further studies are needed to decide about the possible therapeutic implications of dAMP/dGMP supplementation in DGUOK deficiency.