The frequency of heteroplasmy in the HVII region of mtDNA differs across tissue types and increases with age

An immobilized sequence-specific oligonucleotide (SSO) probe system consist ing of 16 SSO probes that detect sequence polymorphisms within five regions of the mtDNA control region was used to investigate the frequency of heter oplasmy in human mtDNA. Five regions of hypervariable region II (HVII) of t he control region were studied in blood-, muscle-, heart-, and brain-tissue samples collected from 43 individuals during autopsy. An initial search fo r heteroplasmy was conducted by use of the SSO probe system. Samples in whi ch multiple probe signals were detected within a region were sequenced for the HVII region, to verify the typing-strip results. The frequency of heter oplasmy was 5 of 43 individuals, or 1.1.6%. The frequency of heteroplasmy d iffered across tissue types, being higher in muscle tissue. The difference in the frequency of heteroplasmy across different age groups was statistica lly significant, which suggests that heteroplasmy increases with age. As a test for contamination and to confirm heteroplasmy, the samples were sequen ced for the HVI region and were typed by use of a panel of five polymorphic nuclear markers. Portions of the tissues that appeared to be heteroplasmic were extracted at least one additional time; all gave identical results. T he results from these tests indicate that the multiple sequences present in individual samples result from heteroplasmy and not from contamination.