Deficiency of the ADP-forming succinyl-CoA synthase activity is associated with encephalomyopathy and mitochondrial DNA depletion

Abstract

The mitochondrial DNA (mtDNA) depletion syndrome is a quantitative defect of mtDNA resulting from dysfunction of one of several nuclear-encoded factors responsible for maintenance of mitochondrial deoxyribonucleoside triphosphate (dNTP) pools or replication of mtDNA. Markedly decreased succinyl-CoA synthetase activity due to a deleterious mutation in SUCLA2, the gene encoding the beta subunit of the ADP-forming succinyl-CoA synthetase ligase, was found in muscle mitochondria of patients with encephalomyopathy and mtDNA depletion. Succinyl-CoA synthetase is invariably in a complex with mitochondrial nucleotide diphosphate kinase; hence, we propose that a defect in the last step of mitochondrial dNTP salvage is a novel cause of the mtDNA depletion syndrome.

Figure 1

Family tree, with allele sizes of the microsatellite markers in the critical region on chromosome 13. Only those individuals whose DNA was available for analysis were annotated. The region of shared homozygosity is shaded. Markers are listed in the left column, and their physical locations on the chromosome are given (in Mb) in the right column.

Figure 2

A, Genomic sequence of the exon 6–IVS6 junction of the SUCLA2 gene in patient II-7 and her mother, I-4 (upper and lower chromatograms, respectively). The deleted 43 nt are shown in italics, and the 14 exonic nucleotides are also capitalized. The deletion start site is indicated by an arrow. The inserted 5 nt are shown in bold and are followed by the normal IVS6 sequence. B, SUCLA2 cDNA sequence of patient II-7. A schematic representation (g) shows the exon-intron organization around the mutation site, which is shown in black. With the use of primers cF1 and cR1, two amplification products (c1 and c2) were seen, both of which skip exon 6, and transcript c2 also lacks the first 74 bp of exon 7. The splicing site of exon 5 with mid-exon 7 is indicated by an arrow.