Multiple, large deletions in rat mitochondrial DNA: evidence for a major hot spot.

This study identified 33 different deletions in mitochondrial DNA from four aging Fischer-344 rat brains and from a cultured rat lymphoma cell line (Nb2 cells). The deletions were located in the longer arc between the heavy and light strand origins of replication. PCR products that spanned across the deleted regions were sequenced, and deletions ranging between 6548 bp and 9977 bp in length were identified. Short direct repeats of < or = 8 bp were present at the end points of all but one of the deletions. The remaining deletion contained, instead, a near-perfect direct repeat (9/10 bp) within two base pairs of its end points. In 24 of the deletions, a sequence equivalent to one member of the paired direct repeats was lost with the deleted segment. In the remaining nine, either more or less of the base pairs of a single repeat were lost. Twelve of the 33 different deletions terminated on one side at a common locus (major hot spot) of 5 bp in length, located at the 5' end of the tRNAThr gene. The opposite ends of these 12 deletions were at different sites. The hot spot was located in a region of the mtDNA with strong potential for secondary structure and was flanked by a pair of AT-rich sequences. The utilization of the hot spot as an end point for deletions appeared to be widespread in that it was represented in 1/3-1/2 of the deletions characterized in each of the five mtDNA sources examined. In addition, several minor hot spots, where one end of two or three different deletions coincided, were also identified.