Introduction of heteroplasmic mitochondrial DNA (mtDNA) from a patient with NARP into two human rho degrees cell lines is associated either with selection and maintenance of NARP mutant mtDNA or failure to maintain mtDNA

Mitochondria from a patient heteroplasmic at nucleotide position 8993 of mi tochondrial DNA (mtDNA) were introduced into two human tumour cell lines la cking mtDNA. The donor mitochondria contained between 85 and 95% 8993G:C mt DNA, All detectable mtDNA in the mitochondrially transformed cells containe d the pathological 8993G:C mutation 3 months after transformation. These re sults suggest that 8993G:C mtDNA had a selective advantage over 8993T:A mtD NA in both lung carcinoma and osteosarcoma cell backgrounds. In contrast, t wo other presumed pathological mtDNA variants were lost in favour of 'wild- type' mtDNA molecules in the same lung carcinoma cell background. Taken tog ether, these findings suggest that the transmission bias of mtDNA variants is dependent upon a combination of nuclear background and mtDNA genotype, A second phenomenon observed was a marked decrease in the growth rate of man y putative transformed cell lines after 6 weeks of culturing in selective m edium, and in these cell lines mtDNA was not readily detectable by Southern blotting, Restriction endonuclease analysis and sequencing of amplified mt DNA demonstrated that the slow growing cells contained little or no mtDNA, It is concluded that these cells represented transient mitochondrial transf ormants.