Unusual mtDNA Control Region Length Heteroplasmy in the COS-7 Cell Line

The COS-7 cell line is a workhorse of virology research. To expand this cell line’s utility and to enable studies on mitochondrial DNA (mtDNA) transcription and replication, we determined the complete nucleotide sequence of its mitochondrial genome by Sanger sequencing. In contrast to other available mtDNA sequences from Chlorocebus aethiops, the mtDNA of the COS-7 cell line was found to contain a variable number of perfect copies of a 108 bp unit tandemly repeated in the control region. We established that COS-7 cells are heteroplasmic with at least two variants being present: with four and five repeat units. The analysis of the mitochondrial genome sequences from other primates revealed that tandem repeats are absent from examined mtDNA control regions of humans and great apes, but appear in lower primates, where they are present in a homoplasmic state. To our knowledge, this is the first report of mtDNA length heteroplasmy in primates.

Length heteroplasmy in the predominate mitochondrial DNA haplogroups in the Croatian population

Mitochondrial control region represents the most variable segment of the mitochondrial genome. The frequency and pattern of heteroplasmy has been described in several studies; however, none of the reports documented the Croatian population. In the present study, we screened the control region (1122 bp) of 95 individuals belonging to two predominant mitochondrial phylogenetic branches in the Croatian population, haplogroups H and U. Length heteroplasmy occurred in polycytosine (poly-C) tracts within three hypervariable segments of the control region with the following frequencies: HVSI - 26.3%, HVSII - 52.6% and HVSIII - 7.4%. Furthermore, the association between certain polymorphisms in HVSI and length heteroplasmy was investigated. Our results indicate that only polymorphisms located in the poly-C tract are associated with HVSI length heteroplasmy. The T to C transition at np 16189 is significantly associated with the occurrence of length heteroplasmy (p<0.0001). This effect was even stronger if the C insertion was present in the position 16193. The data support the hypothesis that an uninterrupted poly-C tract of more than eight cytosines leads to length heteroplasmy. Length heteroplasmy associated with the T to C substitution in np 16189 was predominantly found in haplogroup U.